WO2022126227A1 - Carbon nanotubes based composites with high shielding efficiency and method of production thereof - Google Patents

Abstract

The present disclosure provides a composite and method for synthesis of thereof for electromagnetic shielding. The method comprising adding conductive carbon nanotubes (CNTs) having an average aspect ratio of at least 800 to a solvent to make a mixture; mixing said mixture to provide energy higher than binding energy of the CNTs to each other and lower than or near an energy required to fracture the CNTs to cause dispersion of the CNTs in said solvent without agglomeration; adding a polymer matrix material to said mixture; mixing said mixture and said polymer matrix material while substantially maintaining said dispersion and said average aspect ratio of at least 800 in remaining CNTs; and curing said mixture to obtain the composite material.

Description

CARBON NANOTUBES BASED COMPOSITES WITH HIGH SHIELDING EFFICIENCY AND METHOD OF PRODUCTION THEREOF

Technical Field

[001] The present application relates generally to carbon nanotube composites and more specifically to use of carbon nanotubes composites in electromagnetic shielding, for example in shielding of Positron Emission Tomography (PET) electronics within a Magnetic Resonance Imaging (MRI) System.

Background

[002] This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.

[003] In the presence of electromagnetic fields, the performance of electronic devices may fail because of electromagnetic interferences (EMI). Electromagnetic fields at high frequency can be efficiently shielded using metallic materials such as copper, aluminum, or conductive composites. Typically, a sheet of these materials with a thickness of several micrometers or a mesh with the hole size designed for the desired frequency range, is used to achieve the required shielding effectiveness (SE) at high frequencies. For low-frequency shielding, however, a thicker layer of metal must be employed due to the higher skin depth of the material, hence the weight and the cost of such an interface increases.

[004] Moreover, in the case of static or slowly changing magnetic fields, a Faraday cage is ineffective; instead, high-magnetic permeability materials, such as Permalloy and Mu-Metal sheets, or ferromagnetic metal coatings with nano-crystalline grain structure are commonly used. [005] The advance in electronic technologies and the necessity to integrate electronic components with high packaging density, to reach the compact and complex electronic devices, form a new challenge in terms of electromagnetic interferences. These interactions cause system malfunctions or errors. Thus, effective shielding, by considering different system requirements, is essential. The recent growth in medical imaging toward dual-modality scanners augment the necessity of efficient shielding at different frequency ranges. The significant aspect in medical imaging is that, due to its sensitive nature, as a shield, it requires a material with higher shielding effectiveness than the one used in other electronic systems such as telecommunication or aerospace applications.

[006] High-magnetic permeability materials redirect magnetic fields and protect sensitive devices. Some solutions suggested in the art use magnetic nanoparticles embedded in a polymer matrix to shield low frequencies at several hundred kHz. This approach is not very effective and cannot be used for most imaging machineries such as positron emission tomography/magnetic resonance imaging (PET/MRI) scanners. In the design of a bimodal PET/MRI scanner, it would be necessary that all electromagnetic interferences between the two modalities be eliminated without degrading the overall performance of each scanner. However, the use of shielding layers based on metallic or high-magnetic permeability materials inside MRI scanners is impractical, since these materials distort the MR image quality. In this respect, the gradient coils of the MRI are the primary source of low-frequency interferences. In fact, the amplitude of the gradient field is smaller than that of the main magnetic field. Nonetheless, it switches very rapidly and induces Foucault currents (eddy currents) in any closed electric and conductive path, as explained by Faraday’s law. These Foucault currents may affect the performance of an electronic device by generating heat. Besides, based on Lenz’s law, they produce reverse magnetic fields that distort the gradient field and cause MR image artifacts. Regarding the PET scanner, the accuracy and integrity of data acquisition is of great importance to obtain high contrast images, which necessitates implementing a compatible shielding layer in the PET/MRI scanner to eliminate the effect of the gradient coils low-frequency electromagnetic fields on the PET electronics. Otherwise, low-frequency EMIs increase the temperature of the PET electronics, changing the photodetector characteristics such as the noise and amplification gain, and thus disturbing the detection module performance. Consequently, data may be lost or distorted, and the PET image quality may be degraded.

[007] Another effect of the gradient field on PET detectors is the instability of the detector bias voltage during gradient switching. Essentially, the noise from the gradient pulses, injected via the bias controller and amplified by the feedback loop, causes fluctuations in voltage regulators, which in turn add noise in the electronic signals of the detectors. This increase in the noise level could ultimately deteriorate the PET detector energy resolution, resulting in less efficient scatter radiation rejection and deteriorated contrast-to-noise ratio in PET images.

[008] To date, two main types of shielding materials have been reported for PET/MRI applications. The first type is the metallic shielding made of copper or aluminum that has been commonly used in commercial PET/MRI systems. The second one is the carbon fiber-based shielding composites, which have been introduced by research groups from UC Davis [B. J. Peng, Y. Wu, S. R. Cherry, and J. H. Walton, "New shielding configurations for a simultaneous PET/MRI scanner at 7T," Journal of Magnetic Resonance (San Diego, Calif. : 1997), vol. 239, pp. 50-56, 2014] and Aachen University [Peter M. Duppenbecker et al., "Gradient Transparent RF Housing for Simultaneous PET/MRI Using Carbon Fiber Composites," in IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC) Anaheim, California, 2012.] [009] Some studies explored use of metallic shielding with both plate and mesh structures. They reported that the plate configuration, with only 30 pm thickness, provided sufficient SE for one PET module. However, when increasing the area of the copper plate inside the MRI bore to cover the whole system, both signal-to-noise ratio (SNR) and homogeneity of MRI were deteriorated and the temperature of the PET module was augmented. Alternatively, the mesh structure offered a better homogeneity and SNR, while it delivered a lower SE compared to the plate configuration. [0010] One study was conducted on a chemical shift artifact protocol to determine the Foucault current effects in the MR image. By comparing different thicknesses of the copper plate, they demonstrated that the chemical shift artifact increases with the thickness of the metallic materials located inside the MR bore. As a result of the thicker metallic materials, the induction of the Foucault current increased. In 2014, the same research group investigated the shielding effectiveness of carbon fiber. It was shown that, although the carbon fiber provided enough SE against RF coil signals, the gradient effects could not be eliminated without inserting a thin copper foil between PET detectors and gradient coils [B. J. Peng, Y. Wu, S. R. Cherry, and J. H. Walton, "New shielding configurations for a simultaneous PET/MRI scanner at 7T," Journal of Magnetic Resonance (San Diego, Calif.: 1997)]. Concurrently, some Foucault current was also induced in the MRI field-of-view, as confirmed by a slight frequency shift in the chemical shift artifact. However, such artifacts were only significant in the case of fast imaging sequences.

[0011] Since previous studies established that metallic materials inevitably deteriorate the performance of PET/MRI, research effort was directed toward other non-metallic shielding layers. For instance, Pari et al. [C. Pari et al., "A novel optically transparent RF shielding for fully integrated PET/MRI systems," Physics in Medicine & Biology, vol. 62, no. 18, pp. 7357-7378, 2017/09/01 2017] used polymer-based conductive foils as an optically transparent RF shielding. In their study, two commercially available conductive foils (ITO and 9900) were compared with a copper-wire mesh, and the results were in favor of the mesh structure. The preceding discussion underlines that, despite the interesting outcomes of previous works, finding an effective shielding layer without inducing Foucault currents is still an ongoing challenge.

[0012] Therefore, there exists a need for shielding materials with high shielding efficiency with low Foucault currents induction which may affect the performance of an electronic device by generating heat.

[0013] Additionally, in aerospace industry electromagnetic shielding for electronics is essential as some data may be lost without proper shielding. However, most shielding materials and methods known in the art, with the required shielding efficiency, are quite heavy which is a big disadvantage in the aerospace industry.

[0014] Therefore, there also exists a need for a shielding material with required shielding efficiency which is not heavy like most metals used for this purpose.

Summary

[0015] The present disclosure provides, among others, solutions addressing the above-mentioned problems with the existing shielding materials. This patent application provides complementary improvements that may be applied separately or in combination.

[0016] In one broad aspect, the present disclosure provides a method for synthesis of a composite material for electromagnetic shielding. The disclosed method includes adding conductive carbon nanotubes (CNTs) having an average aspect ratio of at least 800 to a solvent to make a mixture; mixing said mixture to provide energy higher than binding energy of the CNTs to each other and lower than or near an energy required to fracture the CNTs to cause dispersion of the CNTs in said solvent without agglomeration; adding a polymer matrix material to said mixture, mixing said mixture and said polymer matrix material while substantially maintaining said dispersion and said average aspect ratio of at least 800 in remaining CNTs; and removing the solvent from said mixture and curing said mixture to obtain the composite material.

[0017] It will be appreciated by those skilled in the art that different types of mixing methods such as shear mixing and sonification and any combination of different methods can be used at any stage of the process as long as the required dispersion of the CNTs in the solvent and composite material is achieved without agglomeration.

[0018] Accordingly, in one example, the mixing said mixture and said polymer matrix material comprises applying ultra- sonication or applying shear mixing or a combination of both mixing methods as disclosed herein.

[0019] Likewise, the mixing of the mixture may include ultra-sonication, shear mixing or a combination of ultra-sonication and shear mixing.

[0020] In some embodiments, the CNT may include functionalized or non-functionalized singlewall CNTs, double-wall CNTs or multi-wall CNTs or a combination of different types of CNTs.

[0021] In some examples, it may be required to maintain the temperature of the mixture stable during the mixing of the CNTs with the solvent. In some embodiments, a temperature between 20- 50 degree Celsius may be required.

[0022] In some examples, the temperature of the mixture may have to be maintained below the curing temperature of the polymer added to the mixture.

[0023] In some examples, the conductivity of CNTs may be higher than 100 S/cm.

[0024] In one example, the amount of energy per volume required for the mixing of the CNTs with the solvent is on average 5000 Joules per 5 ml of the mixture.

[0025] In some embodiments, the polymer is chosen from polymers having electrical conductivity higher than 10 -12 S/m. In some examples, the polymer is chosen from a group epoxy resin, PDMS, PMMA, silica, Polystyrene or any other polymer with the required conductivity and/or flexibility requirement.

[0026] In one aspect, the present disclosure provides a method for synthesis of a composite material for electromagnetic field shielding. The method comprises adding carbon nanotubes (CNTs) having an aspect ratio of at least 800 to a solvent to make a mixture, followed by applying ultra-sonication mixing, at a first frequency and for a first number of mixing cycles, to the mixture, wherein said frequency and said period provides energy higher than binding energy of the CNTs to each other and lower than or near an energy required to fracture the CNTs, adding a polymer matrix material to said mixture and applying ultra-sonication at a second frequency and for a second number of cycles and applying shear mixing to said mixture as to obtain dispersion of the CNTs in the solvent substantially uniformly and without agglomeration while substantially maintaining said aspect ratio of at least 800 in remaining CNTs. Finally, the solvent is removed and curing the mixture to obtain the composite material.

[0027] In some examples, the adding carbon nanotubes (CNTs) may comprise adding a specific weight percentage of CNTs ranging between 5% to 25%. [0028] It would be appreciated by those skilled in the art that the higher CNT weight percentage can be equally used but it would not be a good option as it would not provide higher conductance at a considerable rate and increases both the cost of the composite and stiffness of it.

[0029] In some embodiments, the CNT may be a mix of single wall nanotubes and multiwall nanotubes each providing shielding effect at certain frequencies. The combination can provide the required shielding effect at a wide range of frequencies.

[0030] In some examples, said adding CNTs may comprise adding CNTs having an aspect ratio of length to diameter of about 500 to about 2000.

[0031] In some examples, said applying ultra-sonication mixing to the mixture comprises applying ultra-sonication mixing with each one of the cycles providing energy ranging between 4,000-6,000 J (with solvent) and 8,000-10,000 J (with polymer).

[0032] In some examples, said applying ultra-sonication mixing may comprise applying ultrasonication mixing at a first frequency comprises applying ultra- sonication in range of 18 kHz to 22 kHz, for said first number of cycles equal to 10 to 20 minutes.

[0033] In some examples, the method further comprises maintaining a stable temperature ranging between 20 and 50 degree Celsius. In one example, the maintaining a stable temperature comprises performing the mixing in an ice bath.

[0034] In some examples, the adding said polymer matrix material to said mixture comprises mixing said polymer matrix material to said solvent and adding said solved polymer matrix material to said mixture.

[0035] In some examples of the method disclosed herein, the method may further comprise molding said material to a desired shape. In one example, the method may comprise applying the composite material as a coating layer before said removing the solvent from said mixture and curing said mixture to obtain the composite material.

[0036] In one example, the adding carbon nanotubes (CNTs) step of the method may comprise adding CNTs having an aspect ratio of about 1250.

[0037] In some other examples, said adding single-wall carbon nanotubes (CNTs) may comprise adding a concentration of about 7% to about 12% of said CNTs by weight in said polymer matrix. [0038] In some examples, the applying shear mixing to said mixture comprises applying shear mixing to said mixture at 400 to 600 rpm for 6 to 12 hours, maximum screw rotation speed of 3000 rpm depending on viscosity of mixture. [0039] In some embodiments, the time required to remove more than 90% of solvent from the mixture could be about 6-12 hours. In one example, the mixture is subsequently put under fume hood to remove excess liquid or curing on hot plate mentioned.

[0040] It will be appreciated that in some examples, the removing of solvent can be done as a separate step, but it would be possible to remove a part of the solvent as a part of the curing or during the mixing of the polymer with the mixture of CNT and solvent.

[0041] It will be appreciated by those skilled in the art that the timing for curing is dependent on many factors such as type of polymer, solvent or the thickness of the sample and can be shorter or longer as known in the art.

[0042] In one example of the method, the curing of the mixture comprises curing said mixture at 50-70 °C for 2 to 4 h.

[0043] In another broad aspect, the present disclosure provides an electromagnetic shielding composite material which includes a polymer matrix material; and carbon nanotubes (CNTs) having an average aspect ratio of length to diameter of about 800 at a concentration of about 5% to 15% by weight in said polymer matrix.

[0044] In this embodiment, the CNTs are dispersed in the polymer matrix substantially uniformly and without agglomeration; and said composite material at a thickness of 1 micron has a shielding efficiency greater than about 20 dB in the 20 kHz to 1 MHz frequency range, and has a Foucault current loss less than about 2%. When the thickness is about 1 mm the shielding efficiency may be about 80 dB in the said frequency range.

[0045] In some aspects of present disclosure, the CNTs may have an aspect ratio of about 1000 to about 1400. In one embodiment, the CNTs have an aspect ratio of 1250 on average.

[0046] In some embodiments, the concentration of CNTs may be about 7% to about 12% by weight in said polymer matrix.

[0047] In some embodiments, the concentration of CNTs may be of about 10% by weight in said polymer matrix.

[0048] In some embodiments of the present disclosure, the composite material may have a thickness of 1 micrometer and a shielding efficiency greater than about 35 dB in the 10 kHz to 100 kHz frequency range.

[0049] The composite material may have a thickness of 1 mm and a shielding efficiency greater than around 80 dB in the 10 kHz to 100 kHz frequency range and in 50-500 MHz frequency range. [0050] In some embodiments, the composite material may provide a Foucault current loss that is less than about 0.5%.

[0051] In one example, the polymer matrix material is poly dimethyl siloxane (PDMS).

[0052] In one broad aspect of the present disclosure, a simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) apparatus is provided which includes an MRI apparatus having a bore defined by an elongated annular housing, a PET subsystem mounted within said housing surround at least a portion of said bore; and electromagnetic shielding placed between said PET subsystem and gradient coils and RF coils of said MRI apparatus, wherein said electromagnetic shielding comprises said composite material disclosed herein.

[0053] In one example of the PET and MRI apparatus as defined, the electromagnetic shielding provides greater than 60 dB of shielding with less than 1.5% of Foucault current induction loss.

[0054] In one example of the PET and MRI apparatus, the electromagnetic shielding provides greater than 65 dB of shielding with less than about 0.75% of Foucault current induction loss.

[0055] In another broad aspect, the present disclosure provides a vehicle with electromagnetic shielding comprising electronics used within said vehicles placed within housings a coating substantially covering said housings and providing electromagnetic shielding to said electronics, wherein said coating comprises at least one layer of said composite material.

[0056] In one broad aspect, the present disclosure provides an aerospace and/or telecommunications equipment having electronics with electromagnetic shielding. The electromagnetic shielding comprises the composite material as provided by different embodiments as disclosed in this application. This may be advantageous in other industries where electronic shielding is required because of the lighter weight the CNT composite material has in comparison to other shielding materials known in the art. With a flexible polymer matrix material and a suitably low percentage of CNTs, desired flexibility or elastomeric properties can be provided in the shielding material. It can be equally useful in other cases where weight of the shielding material is an important factor.

[0057] In some examples, the electronics may include a field-programmable gate array (FPGA).

[0058] In some examples, implants such as a pacemaker may need electromagnetic shielding. In such examples, the composite material can be used instead of metal shielding. This can provide possibility of using different devices such as an MRI machine despite having an implant. Brief Description of the Drawings

[0059] The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which:

[0060] Figures la, lb and 1c show schematic views of three different methods to combine PET/MRI, (a) sequential, (b) insert, and (c) fully integrated PET/MRI devices.

[0061] Figure 2 is schematic view of the mechanisms for electromagnetic wave attenuation.

[0062] Figures 3a and 3b show shielding efficiency (SE) as a function of the thickness: (a) a copper (Cu) plate for two different frequencies, (b) a Cu mesh with various hole sizes for RF frequency of 100 MHz.

[0063] Figure 4 shows shielding effectiveness of CNT based composites in megahertz to gigahertz range.

[0064] Figure 5 is an example of experimental set-up for EMI measurements showing a gradient coil placed around one detector module with a radioactive rod source also shown between the two detector modules.

[0065] Figure 6a shows the conductivity of SWCNT in different solvents using electrochemical impedance spectroscopy (EIS).

[0066] Figure 6b shows the conductivity of MWCNT in different solvents using EIS.

[0067] Figure 6c shows UV-Vis spectra of SWCNT in isopropyl alcohol and chloroform. [0068] Figure 6d shows UV-Vis spectra of MWCNT in isopropyl alcohol and chloroform.

[0069] Figure 6e shows Dispersion level of SWCNT in four different solvents after a week at rest.

[0070] Figure 6f shows Dispersion level of MWCNT in four different solvents after a week at rest.

[0071] Figure 7a illustrates real conductivity versus frequency for SWCNT-IPA-PDMS composite with 2 wt.% and 10 wt.%. To provide adequate EMI shielding, the conductivity of a material should be above the threshold level, indicated by the green dashed line.

[0072] Fig. 7b illustrates DC conductivity versus SWCNT wt.%.

[0073] Fig. 8a is a SEM image of network generation in SWCNT -PDMS of the surface of 10 wt.% sample obtained with 5 kV energy and 30 pm aperture size.

[0074] Fig. 8b is a SEM image of network generation in SWCNT -PDMS of the cross-sectional SEM obtained with 2 kV energy and 20 pm aperture size of 10 wt.% sample. [0075] Fig. 9a illustrates coating of FR4 with CNT composite (10 wt.%) with the shape of the sample specimens required for ASTM-D4935-99 standard.

[0076] Fig. 9b and 9c show self-standing CNT-PDMS film fabricated by the Doctor blade technique.

[0077] Figure 10 shows shielding effectiveness of SWCNT-PDMS using a modified IEEE Std 299.1 2013 standard for the low-frequency range.

[0078] Figure 11 shows shielding effectiveness of SWCNT-PDMS with 10% and 2% CNT weight percentages in Megahertz frequencies.

[0079] Figure 12a shows baseline voltage level for 64 channels of a PET detection module in the presence of RF coil signals at 127.7 MHz (3 T) and 298 MHz (7 T). The results for the shielded module are also displayed.

[0080] Figure 12b shows RMS noise for 64 channels of the PET detection module in the presence of RF coil signals at 127.7 MHz (3 T) and 298 MHz (7 T). The results for the shielded module are also displayed.

[0081] Figure 13 shows Gaussian fits of the 511 keV photopeak in the pulse height spectra measured by the Time-Over- Threshold (TOT) technique for one typical pixel (here pixel 10) without and with RF coil switching at 127 MHz and 298 MHz, when the PET detector module is not shielded and shielded with the composite layer.

[0082] Figure 14 shows Gaussian fits of the 511 keV photopeak in the pulse height spectra measured by the Time-Over- Threshold (TOT) technique for one typical pixel (here pixel 10) without and with gradient switching at 10 kHz, 50 kHz and 100 kHz, when the PET detector module is not shielded and shielded with the CNT composite layer.

[0083] Figure 15 shows a flowchart of the synthesis process of CNT based composite in accordance with one embodiment of the present disclosure.

Detailed Description

[0084] Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[0085] Moreover, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. Reference will now be made in detail to the preferred embodiments of the invention.

[0086] In one improvement, the present disclosure provides composites based on pristine CNTs with polymers such as polydimethylsiloxane (PDMS) with high shielding efficiency and with low Foucault current induction.

[0087] In another improvement provided by the present disclosure is a synthesis method providing CNT dispensability to fabricate a homogeneous composite through use of a hybrid method based on ultra- sonication and shear mixing of CNT mixtures. The impedance measurements on thin layers confirmed that a well-processed composite provides the required conductivity (>10³ S/m) for electromagnetic interference shielding in the desired frequency range. Such composite layers achieved a shielding effectiveness of more than 68 dB in the MRI gradient range (10-100 kHz) and exceeded 80 dB in the MRI RF range (50-500 MHz). The results demonstrated that using these CNT-composite shielding layers, the artifacts due to the electromagnetic interferences of gradient and RF coils in the MRI environment can be avoided for PET imaging.

[0088] As described above, carbon nanotubes (CNT) have shown promising properties, as a filler, for increasing the electrical conductivity of composites entering in the fabrication high-frequency shielding materials. However, up to now, their features at low-frequency range have received little attention.

[0089] In one improvement, the present disclosure provides method of producing composite layers for example composites based on pristine CNT and polydimethyl siloxane (PDMS). The present disclosure is based on single-wall CNTs and how they can be used for providing electromagnetic shielding for different purposes including eliminating the impact of low-frequency electromagnetic interferences in positron emission tomography/magnetic resonance imaging (PET/MRI) scanners.

[0090] In another improvement, the present disclosure provides a CNT dispensability to fabricate a homogeneous composite through use of a hybrid method based on ultra-sonication and shear mixing of CNT mixtures. The details of these improvements have been disclosed herein. [0091] The advance in electronic technologies and the necessity to integrate electronic components with high packaging density, to reach the compact and complex electronic devices, form a new challenge in terms of electromagnetic interferences. These interactions cause system malfunctions or errors. Thus, effective shielding, by considering different system requirements, is essential. The recent growth in medical imaging toward dual-modality scanners augment the necessity of efficient shielding at different frequency ranges. The significant aspect in medical imaging is that, due to its sensitive nature, as a shield, it requires a material with higher shielding effectiveness than the one used in other electronic systems such as telecommunication or aerospace applications. In the telecommunication system and aerospace research, nanocomposite materials grabbed the attention of researchers as an appealing candidate to substitute metallic shielding in the GHz range. As for their advantages over metals, nanocomposites offer flexibility, resistance to corrosion, lightweight, cost effectiveness, and simple synthesis process. In this regard, the unique structure and specific properties of carbon nanotubes (CNT), namely their high aspect ratio, high thermal/electrical conductivity, and excellent tensile strength, make them a viable filling for composites. Besides, CNTs have unique electronic-transport properties and optoelectronic- response characteristics,

[0092] Several reasons make the PET/MRI scanner an appropriate medical imaging system for clinical and preclinical studies. First, large varieties of PET tracers are available and the sensitivity of PET is in the pico-molar range. Second, MRI delivers high resolution and high soft-tissue contrast images in comparison with other molecular imaging modalities. Third, the PET image can be complemented by the MR image. From another point of view, simultaneous imaging of PET and MRI could save total acquisition time and create images with multiple dynamic processes. Besides, MRI can be used to correct motion in PET data. PET/MRI overcomes some of the limitations of PET/CT such as limited soft-tissue contrast and high radiation doses. In PET/CT scanners, while both systems share a common patient bed, they are hard-wired back-to-back thus impeding simultaneous data acquisition. However, it is possible to achieve simultaneous PET and MRI scanners. Although, PET/CT is a fast and cost-effective method in comparison to PET/MRI since MRI requires a longer time to take an image (Krug et al., 2010, Sher et al., 2010), PET and MRI images can be collected simultaneously in a PET/MRI scanner. In addition, a CT image is used to perform the attenuation correction of PET, which is a challenging topic in the available PET/MRI systems. [0093] Academic research and commercial works on PET/MRI scanners are categorized into three different types shown in Figure 1 (a) to (c) as the sequential, insert, and hybrid scanners, respectively (Pichler et al., 2008); among them, the last two perform simultaneous acquisition of PET and MRI data. The sequential PET/MRI system (Figure 1 (a)), such as the combined HRRT- PET/MRI (Cho et al., 2008, Cho et al., 2007), scans the PET and MRI images using two separate devices to avoid the interaction of the two modalities, which is the main advantage of the sequential PET/MRI. This system is flexible according to the application and workload. However, it is difficult to guarantee that the posture and metabolic state of the patient are the same, since the two modalities are scanning at different times. Two stand-alone systems, allowing only sequential data acquisition and subsequent image fusion, cannot provide complementary dynamic information. Moreover, post-acquisition image fusion by software, especially in high-resolution abdominal or thoracic studies, is complicated and inaccurate. Equally, sequential imaging of functional parameters similar to fMRI and PET necessitates the reproducibility of operational processes to allow a temporal correlation after the acquisition.

[0094] In simultaneous PET/MRI, the two modalities are combined in one system and this bimodal scanner overcomes the limitations of each individual scanner and guarantees temporal and spatial registration of the two datasets. In fact, PET scanner, using tracers, determines how organs and tissues are functioning, which highlight abnormalities to indicate disease even before any structural modification of tissue happened. MRI scan uses a strong magnetic field to produce detailed images of soft tissues, organs, and other internal structures of the body. Therefore, combining MRI and PET provides an excellent opportunity to understand tissue metabolism with anatomical precision. Simultaneous PET/MRI requires redesigning of PET detectors to be MR compatible (Hu et al., 2014) from a material point of view and eliminating the interferences between the two subsystems. The PET-insert is an MR-compatible scanner that is introduced inside an MRI gantry whenever it is required (Figure 1 (b)), while a hybrid PET/MRI represents a scanner where PET detectors are permanently integrated within the MRI bore (Figure 1 (c)).

[0095] The inhomogeneity within the MRI bore is one of the significant problems requires to be addressed before realizing a simultaneous PET/MRI. Moreover, magnetic effects necessitate the use of photodetectors that are insensitive to magnetic fields and require front-end electronics with non-magnetic material and minimum heat radiation. The effect of APD- or SiPM-based PET modules on MRI mainly arises from the PET electronic interactions with the RF and gradient coils. These interferences must be minimized especially when high-frequency digital clocks are used for the PET front-end electronics and Foucault currents are produced in the electronic boards.

[0096] MRI scanner has three main parts: Main magnet, gradient coils and RF coil. Each of these parts causes specific interaction with PET electronics. The main magnet generates a strong magnetic field that changes the path of electrons in long-length PMTs and makes them practically useless as a photodetector for PET-insert. Thus, in early version of PET/MRI based on PMT- detector, long optical fibers were used to transfer light from crystals placed inside MRI Field of View (FoV) to PMTs located outside MRI FoV. However, in the recent PET/MRI scanner, those PMTs were replaced by APDs or SiPMs to eliminate electron path distortion. Although this approach is very efficient, it requires more electronic read-out channels, more interaction sources and higher cost. As indicated before, to develop a PET/MRI scanner, the critical challenge is to eliminate electromagnetic interferences. The sources of EMIs from MRI are gradient coils switching at kilohertz range and RF coil working at Larmor frequency of MRI in the megahertz range. As the scope of this work is the EMI shielding, these two sources are explained in more detail in the following paragraphs.

[0097] Gradient coils: The first source of interferences in the integrated PET/MRI is fast switching gradient coils. Typically, 1.5 T to 3.0 T whole body MRI scanners have maximum gradient strengths of 30-45 mT/m, while the gradient amplitude for lower field (<0.5 T) scanners is in the range of 15-25 mT/m. Although the amplitude of the gradient fields is smaller than that of the main magnetic field, it switches very fast with frequency range of 10 kHz to 200 kHz. It generates the slew rate, that is the speed rate of ascent or descent of a gradient from zero to its maximum amplitude, which could reach to up to 200 T/m/s. Based on Faraday’s law; this high slew rate induces Foucault currents in any closed electric circuits and conductive paths. The Foucault currents have negative impacts on electronic components and cause heating and vibration, which in turn change the performance of each element. Additionally, Foucault currents generate reverse magnetic fields, based on Lenz’s law, which distort the intended gradient field and produce image artifacts. To minimize Foucault currents and their associated distortions, the volume of conductive structures has to be reduced inside the MRI bore, the electronic circuits must be redesigned and the temperature ought to be controlled.

[0098] RF coil: Another critical source of interferences in a simultaneous PET/MRI is EMIs due to the RF coil and the PET dynamic signals controlled by a clock signal. PET electronics, especially its front end, are sensitive to RF interferences, and a robust RF transmission from MRI deteriorates the PET signals. The RF coil frequency is linearly related to main magnetic field by the gyromagnetic constant of hydrogen atoms (42.6 MHz/T); this frequency is called Larmor frequency and defined as m = YB₀ (1) where a> is Larmor frequency, y is gyromagnetic constant and Bo is the magnetic field strength. Therefore, for a 3 T MRI, the Larmor frequency is ~ 128 MHz, for 7 T it is about 300 MHz, etc. To minimize the RF effects, the PET clock should be chosen in a way that neither its fundamental frequency nor any of its harmonics are located in the bandwidth of RF coil, that is a narrow band. In addition, to preserve the PET signal accuracy using a shielding layer is essential.

[0099] To control electromagnetic interferences (EMIs), shielding is the best option. Shields work by reflecting, absorbing or redirecting electric and/or magnetic fields. Electromagnetic waves consist of two perpendicular components, a magnetic field (H) and an electric field (E). The ratio of E to H is called wave impedance. When an electromagnetic wave with initial energy encounters a shielding layer, some of the wave energy is reflected and the rest is transmitted into the new material.

[00100] The efficiency of a material to reduce the electromagnetic signal is defined by a parameter called shielding effectiveness (SE), which is the ratio of the field before and after its attenuation and can be expressed as:

SE(dB) = 20 log log ® (2)

where E [V/m] and H [A/m] are electric and magnetic fields and the subscripts t and i refer to the transmitted and incident waves. The electromagnetic wave attenuation occurs by three mechanisms shown in Figure 2: absorption (A), reflection (R), and multiple internal reflections (B). Thus, shielding effectiveness is defined based on the sum of these three terms: SE = A + R + B.

[00101] Some shielding materials are metals (including non-synthetic metallic materials, metallic graphite), embedded metal in plastics, intrinsically conducting polymers (such as Grafted Nylon, PAn-PTSA, highly doped PPY, PAn/ PXSA), optically transparent conductive foils (such as indium tin oxide (ITO)) and polymers-based nanocomposites including carbon-nanotube based composites, carbon nanofibers-based composites, graphene-based composites and nanoparticle filled composites. Also, a multi-shell configuration of any material can be used for shielding purposes. To date, two main types of shielding materials have been reported for PET/MRI applications. The first type is the metallic shielding made of copper or aluminum that has been commonly used in commercial PET/MRI systems. The second one is the carbon fiber-based shielding composites.

[00102] Metallic layers commonly used in two forms as shielding: a plate film or a mesh layer. As the plate layer is a flat conductive layer, while the mesh one has several void spaces, they provide different shielding properties. In fact, the plate layer has higher conductivity than the mesh one, whilst the mesh structure, owing to its empty space, could decrease the Foucault current induction and consequently the errors due to the heat and vibrations inside MRI scanner.

[00103] For a plate of conductive material, the SE is defined as:

SEpi_at(dB) = 201og— + 201oge^t/⁵ + 201og e^-2t/^CT (3) where t is the thickness of the material used for the shielding, 7/ i s the intrinsic impedance of free space which is equal to 377 Q, T] is the intrinsic impedance of the shielding material defined as

(usually a few mm). The c is the electrical conductivity of the material [S/m], f represents the frequency, and p is the magnetic permeability of material.

[00104] Mesh shielding structure is expected to improve the MR image quality because of the reduction in the area of the conductive material in the MRI bore. However, the shielding effectiveness degrades due to the presence of holes and subsequently lower electrical conductivity. For a mesh designed with multiple circular apertures, the SE is described as:

SE_mesh(dB = 201og^ - lO logn + 32 ^ (4) where the 1 is the wavelength of the electromagnetic source, d is the diameter of a circular hole, n is the number of aperture in a distance of 1/2 and t is the thickness of the mesh layer. Figure 3 (a)and (b) show a comparison between SE of the copper plate and mesh shielding. As the curves display, the required thickness for a copper mesh configuration is almost one order of magnitude higher than that of a copper plate.

[00105] In clinical imaging, a magnetic field of 3 T is commonly used. This magnetic field generates a Larmor frequency of 127.7 MHz. To shield this electromagnetic source using a copper plate, and assuming the criterion of 5 skin depths, based on equation (3) a thickness of about 30 pm of copper is sufficient to have SE of -100 dB. However, due to the conductivity of copper, the gradient induces Foucault currents leading to a rise in the temperature of the layer. Consequently, using plate shielding is not a satisfactory technique in the presence of changing electromagnetic fields. As mentioned, the other useful alternative is mesh shielding. In a changing magnetic field, mesh performance is better than plate shielding since it involves less usage of conductive materials in the MRI bore. It has been experimentally shown that using copper mesh with 4% of hole area provides good SNR for MR images and keeps the homogeneity of the magnetic field. Although using copper mesh, the conductivity was reduced, the Foucault currents still create electromagnetic signals and errors in MR images for fast switching sequences, which highlights the necessity of adopting an alternative method.

[00106] Carbon nanotubes (CNT), in either single-walled (SWCNT) or multi -walled (MWCNT) form, have a lower diameter, and less structural defects than carbon fiber. Carbon fibers are bulky (diameters of a few microns), disordered, and they have a disoriented structure with lots of defects. Therefore, the electrical and thermal conductivity of CNTs are better than carbon fibers (see Table 1).

[00107] Additionally, the unique structure and specific properties of carbon nanotubes (CNTs), namely high aspect ratio, high thermal/electrical conductivity and excellent tensile strength, make it a viable filling for shielding composite. Besides, CNT has unique electronic- transport properties and optoelectronic-response characteristics

[00108] These characteristics prompted the usage of CNT-based composite to reach high shielding effectiveness and to reduce temperature instability.

[00109] TABLE 1

Material Thermal Electrical Specific Density

Conductivity Conductivity (g/cm³)

(W/m.K) (S/cm)

CNT > 3000 10⁶ - 10⁷ 1.3-2

Copper 400 6 x l0⁷ 8.9

Carbon Fiber 1000 2 - 8.5 x 10⁶ 2 - 2.2

[00110] It should be noted that SE values of composites based on single-wall CNT (SWNT) and multi-wall CNT (MWNT) for shielding of electromagnetic interferences at different frequencies have been measured. The reports indicate that that the SWNT-composite provides proper shielding at low frequency, whereas MWNT-composite has high conductivity at high frequencies up to 14 GHz. Compared to carbon black and carbon fiber, CNT provided a higher electrical conductivity and a better SE. However, the SE of the CNT based composites known in the art is not sufficient for shielding EMI in PET/MRI scanners.

[00111] Some approaches were applied, unsuccessfully, for improving the SE of the CNT based composites, for instance using higher weight percent of CNT, employing CNT with higher aspect ratio, annealing process to decrease wall defects and functionalizing CNT. Increasing the weight percentage of CNT to 15%, enhanced the SE up to 49 dB, while an actual EMI shielding for PET/MRI uses must provide at least 60 dB of SE. Despite the promising potential of CNT based composites for shielding interferences up to GHz range frequencies, it has never been applied for PET/MRI applications, primarily due to the low conductivity of current nanocomposites at low frequencies.

[00112] In order to improve the conductivity of composites, some parameters such as CNT ’ s weight percent, quality of CNT, its aspect ratio (length to diameter), and the dispersion uniformity have to be considered. Also, a combination of single wall, double wall and multi wall composite may be used to provide shielding in a larger range of frequencies.

[00113] When preparing a composite having a combination of SWCNTs and MWCNTs, the dispersion of the SWCNTs and the dispersion of the MWCNTs be done separately to reduce the risk of CNT breakage. The shear mixing of each dispersed CNT mixture into the polymer matrix can then done. Shielding materials using MWCNTs in the 8 to 12 GHz range are known in the art, see for example the article “Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range”, http://dx.doi.org/10.5772/62508, where Table 1 gives examples. By including about 7% to 12% SWCNTs having an average aspect ratio of at least about 800 in a composite also having about 10% to 30% MWCNTs, good SE can be expected in the range from about 10 to 20 kHz up to the GHz range (about 1 to 18 GHz).

[00114] Furthermore, there is a possibility that the interfacial couplings between the CNT as a filler and the polymer matrix stimulate the diamagnetic response from CNTs and decreases the total magnetization of the composite. The demagnetization can be used to compensate the effects of ferromagnetic materials of the PET electronic components on the MR image quality. Carbon nanotubes (unlike the carbon black and carbon fiber) have demonstrated promising performance as a filling material due to their unique characteristics such as small diameter, mechanical strength, high aspect ratio (length/diameter) and high conductivity. Moreover, the electrical percolation is achieved with a small amount of CNT because of its tunable electrical conductivity. By comparing this low percolation threshold around 1-5 wt.% of CNT as filler to 10 to 40 wt.% of nickel or aluminum filler, the advantage of CNT filler is obvious. Note that the percolation threshold is defined as the lowest concentration of filler where an insulating material is converted to a conductive material. Furthermore, the measurement of electrical conductivity of CNT-polymer confirms that by introducing a small fraction of CNT, the polymer converts to conductive material and its electrical conductivity increases exponentially. For example, the conductivity of functionalized SWCNT -Reactive Ethylene Terpolymer composites increases from 10-12 S/cm to 10-3 S/cm by adding 3 wt.% of filler. Figure 4 illustrates the shielding effectiveness of SWCNT-epoxy (curves A-D) and MWCNT- polymer (curves E-H).

[00115] The graph shown in Figure 3 indicates for the low frequencies, the SWCNT composites provided better SE while, however, for the higher frequency, some of the MWCNT composites delivered better SE. Note that, the MWCNT composites were surveyed for high frequency and the lack of study in low frequency, make it impossible to drive a conclusion. - There are several techniques to increase the conductivity of CNT-composite such as increasing the weight percentage of CNT, annealing CNT, and using larger SWCNT with a smaller diameter (high aspect ratio). Increasing weight percentage is a straightforward way to improve conductivity; however, its effect is saturated after a specific amount of the filler and makes the composite inflexible and fragile. Another way to increase the polymer conductivity is to increase the aspect ratio of CNTs, which has a significant exponential effect on improving the SE. That is to say, the larger the aspect ratio, the better the SE. Although, the aspect ratio should be carefully selected as the longer CNTs are more vulnerable to break in the mixing process with polymer.

[00116] Besides, annealing is used to deliver high-quality CNT. In fact, the annealing process removes wall defects and improves conductivity. Although annealing reduces the aspect ratio, it makes noteworthy increase in SE (Li et al., 2006). Moreover, adding acceptor material, such as phosphor, also increases the conductivity of CNT [ISHII, M. & YAMAZAKI, Y. A study on measurement method of shielding effectiveness using loop antenna in low-frequency is reported in: 2014 International Symposium on Electromagnetic Compatibility, Tokyo, 12-16 May 2014 2014. 749-752], However, the P-doped CNTs are not stable. The long SWCNT provides a higher amount of SE and annealing the short one improves the SE in a way that, at the higher frequencies ~1 GHz, it reaches to the SE level of long SWCNT (50 % increase in SE). [00117] The maximum shielding effectiveness of CNT-composite, reported in the literature, is 49 dB at 10 MHz for SWCNT in epoxy resin [LI, N., HUANG, Y., DU, F., et al. 2006. Electromagnetic Interference (EMI) Shielding of Single- Walled Carbon Nanotube Epoxy Composites. Nano Letters, 6, 1141-1145] and 65 dB in kilohertz range (from 10-100 kHz) for SWCNT in Polydimethylsiloxane (PDMS) [MOGHADAM, N., ESPAGNET, R., BOUCHARD, J., et al. 2019. Studying the effects of metallic components of PET-insert on PET and MRI performance due to gradient switching. Physics in Medicine & Biology, 64, 075003.] This shielding property of CNT composite could be used in aerospace and telecommunication applications due to its good SE and lightweight. For PET/MRI application, at least 60 dB of shielding effectiveness is required to eliminate MRI interferences efficiently on the PET data and vice versa. In fact, the signals passing through the shielding layer must be decreased by 6-order of magnitude.

[00118] As discussed before, typically, a sheet of these materials with a thickness of several micrometers or a mesh with the hole size designed for the desired frequency range, is used to achieve the required shielding effectiveness (SE) at high frequencies. However, for low-frequency shielding, a thicker layer of metal must be employed due to the higher skin depth of the material, hence the weight and the cost of such an interface increases. In the case of static or slowly changing magnetic fields, a Faraday cage is ineffective; instead, high-magnetic permeability materials, such as Permalloy and Mu-Metal sheets, or ferromagnetic metal coatings with nano-crystalline grain structure are commonly used. High-magnetic permeability materials redirect magnetic fields and protect sensitive devices. On this subject, Livesey et al. synthesized a composite made of magnetic nanoparticles embedded in a polymer matrix to shield low frequencies at several hundred kHz [5], [00119] Designing a bimodal PET/MRI scanner necessitates all electromagnetic interferences between the two modalities to be eliminated without degrading the overall performance of each scanner. However, the use of shielding layers based on metallic or high- magnetic permeability materials inside MRI scanners is impractical, since these materials distort the MR image quality. In this respect, the gradient coil of the MRI is the primary source of low- frequency interferences. In fact, the amplitude of the gradient field is smaller than that of the main magnetic field. Nonetheless, it switches very rapidly and induces Foucault currents in any closed electric and conductive path, as explained by Faraday’s law. These Foucault currents may affect the performance of an electronic device by generating heat. Besides, based on Lenz law, they produce reverse magnetic fields that distort the gradient field and cause MR image artifacts.

[00120] Regarding the PET scanner, the accuracy and integrity of data acquisition is of great importance to obtain high contrast images, which necessitates implementing a compatible shielding layer in the PET/MRI scanner to eliminate the effect of the gradient coils low-frequency electromagnetic field on the PET electronics. Otherwise, low-frequency EMIs increase the temperature of the PET electronics, changing the photodetector characteristics such as the noise and amplification gain, and thus disturbing the detection module performance. Consequently, data may be lost or distorted, and the PET image quality may be degraded.

[00121] Another effect of the gradient field on PET detectors is the instability of the detector bias voltage during gradient switching. Essentially, the noise from the gradient pulses, injected via the bias controller and amplified by the feedback loop, causes fluctuations in regulated power supplies, which in turn add noise in the electronic signals of the detectors. This increase in the noise level could ultimately deteriorates the PET energy resolution. To date, two main types of shielding materials have been reported for PET/MRI applications. The first type is the metallic shielding made of copper or aluminum that has been commonly used in commercial PET/MRI systems. The second one is the carbon fiber-based shielding composites.

[00122] As explained, metallic shielding with both plate and mesh structures may provide, for the plate configuration with only 30 pm thickness, sufficient SE for one PET module. However, when increasing the area of the copper plate inside the MRI bore to cover the whole system, both signal-to-noise ratio (SNR) and homogeneity of MRI were deteriorated and the temperature of the PET module was augmented. Alternatively, the mesh structure offered a better homogeneity and SNR, while it delivered a lower SE compared to the plate configuration.

[00123] When a chemical shift artifact protocol was performed to determine the Foucault current effects in the MR image by comparing different thicknesses of the copper plate, they demonstrated that the chemical shift artifact increases with the thickness of the metallic materials located inside the MR bore and, as a result, the induction of the Foucault current increased. Also, it has been shown that, although the carbon fiber provided enough SE against RF coil signals, the gradient effects could not be eliminated without inserting a thin copper foil between PET detectors and gradient coils. Concurrently, some Foucault currents was also induced in the MRI field-of- view, as confirmed by a slight frequency shift. However, such artifacts were only significant in the case of fast imaging sequences. [00124] For example, two commercially available conductive foils were compared with a copper wire mesh were studies, and the results were in favor of the mesh structure. The preceding discussion underlines that, despite the interesting outcomes of previous works, finding an effective shielding layer without inducing Foucault currents is still an ongoing challenge.

[00125] As discussed in this disclosure, nanocomposite materials provide a good alternative to metal shielding as nanocomposites offer flexibility, resistance to corrosion, lightweight, cost effectiveness, and simple synthesis process. In this regard, the unique structure and specific properties of carbon nanotubes (CNT), namely their high aspect ratio, high thermal/electrical conductivity, and excellent tensile strength, make them a viable filling for composites.

[00126] Additionally, CNTs have unique electronic-transport properties and optoelectronic- response characteristics and can be used to improve their thermal and electrical properties. Moreover, SE values of composites based on single-wall CNT (SWCNT) and multi-wall CNT (MWCNT) for shielding of electromagnetic interferences at different frequencies have shown that the SWCNT-composite provides proper shielding in the megahertz frequency range, whereas MWCNT-composite has a high conductivity at high frequencies up to 14 GHz. Compared to the carbon black and the carbon fiber, CNTs provided a higher electrical conductivity and a better SE. [00127] Nevertheless, the SE of these nanocomposites is not sufficient for shielding EMI in PET/MRI scanners. Some approaches were applied for improving the SE of the CNT -based composites, for instance, using a higher weight percentage of CNTs, employing CNTs with higher aspect ratio, annealing CNTs to decrease wall defects, and functionalizing CNTs. Increasing the weight percentage of CNT to 15% enhanced the SE up to 49 dB, but an actual EMI SE of at least 60 dB must be provided for the PET/MRI application. Despite the promising potential of CNT- based composites for shielding interferences up to the GHz frequency range, it has never been applied for PET/MRI applications, primarily due to the low conductivity of current nanocomposites at low frequencies.

[00128] The present disclosure provides a synthesis process for the fabrication of new CNT composites for, among others, eliminating the interferences originating mainly from low- frequency switching signals of MRI gradients affecting PET electronics. To obtain the required SE, the high dispersion level of CNTs in the solvent is crucial.

[00129] An exemplary test with the specific goal was to eliminate impacts of both EMIs and Foucault currents effects on the performance of the LabPET II detection technology was conducted. In an MRI-like environment, the LabPET II detection module performance was investigated with and without CNT-composite shielding.

Materials

[00130] In an exemplary test, MWCNT with an outer diameter of 10-20 nm, length of 10- 30 pm and purity of 95%, as well as SWCNT with a diameter of 1^1 nm, length of 5-30 pm and purity of 60% were purchased from Cheap Tubes Inc. [Grafton, VT, USA], The elastomer of a 2- component Polydimethylsiloxane (PDMS) from Dow Corning (SYLGARD 184) with a 10: 1 ratio of base to curing agent mixing was used. Various solvents, including chlorobenzene [J.T. Baker], isopropyl alcohol (IP A), chloroform and toluene [Fisher Scientific], were investigated to determine the level of dispersion of the carbon nanotubes and the best solvent for the synthesis process of the nanocomposite. It will be appreciated that other elastomeric polymer materials may be substituted, and in certain applications, non-elastomeric polymers can be suitable.

Synthesis Process of Nanocomposite

[00131] Synthesis of the CNT-based composite materials requires careful formulation of the fabrication procedures to produce composites with appropriate characteristics. Due to the CNT’s tendency to aggregate, attributed to the strong van der Waals binding, uniform dispersion of CNTs in the solvent is a complicated processing step. To prepare the composite, we used the ultra-sonication method along with the shear mixing technique. For each sample, a specific weight percentage of CNT was added to the solvent and mixed for 15 min at a frequency of 20 kHz and power of 100 W. By supplying energy higher than the binding energy of the aggregated mixture, this process provides a uniform CNT solution without any agglomeration. At the same time, the energy transferred to the solution must remain lower than that required to fracture CNTs by selecting a short period of sonication. Then, PDMS was diluted in the desired solvent and the mixture was added to the CNT solution. Afterward, they were mixed for 15 min using the ultrasonication probe, followed by shear mixing at 500 rpm for 6 h. The sample was placed under a fume hood overnight to evaporate the extra solvent. In the next step, the part B of the PDMS kit was added and mixed rigorously using a laboratory glass rod. The composite was placed into a vacuum chamber for 10 min, to remove air bubbles. At this step, the sample was ready to be cast over a substrate of glass or FR4 for characterization.

[00132] To characterize the network structure of a thin nanocomposite layer, a spin coating system was used at 200 rpm for 30 sec., followed by 500 rpm for 60 sec. The sample was then dried for 60 min at 70°C. To create thicker samples of the final shielding layer, the Doctor blade (tape casting) technique was used to have a 1-mm thick layer. This thickness was selected as it is equal to five skin depths of copper at the frequency of 10 kHz. Subsequently, the film was cured at 50-70°C for 2 h to 4 h, depending on the thickness, and removed from the glass substrate to obtain a self-standing film. The thickness of each layers was measured using an ellipsometer or a digital caliper depending on its dimension.

Nanocomposite Characterization

[00133] Dispersibility of CNT-solvent: Electrochemical impedance spectroscopy (EIS) was employed to examine the dispensability of CNT-solvent mixtures. Both SWCNT and MWCNT were dispersed in chlorobenzene, chloroform, toluene and isopropyl alcohol using an ultrasonication mixer for 15 min, and their conductivity was measured by means of a CH Instruments, Model 640 C, electrochemical analyzer [Austin, TX], For this purpose, a Zensor screen-printed electrode, TE100 [Katy, TX] was immersed in 0.05 g/L of each CNT-solvent. The impedance of each sample was analyzed at different frequencies. As neither Hansen nor Hildebrand solubility parameters could predict the dispersion behavior, the UV-Vis spectrum was measured to confirm the EIS data [36], The stability of all suspensions was examined visually, one week after sonication.

[00134] Electrical conductivity: The percolation threshold and the AC electrical conductivity of samples were measured. For low-frequency conductivity measurements, particularly evaluating the conductivity of layers versus frequency, an impedance Gain-phase analyzer [Schlumberger, Solatron 1260] was employed. The frequency range of the impedance analyzer is 10 pHz to 32 MHz. Two small pads of silver were deposited on the thin CNT composite surface to create an ohmic contact. The frequency dependency of the real part of the complex electrical conductivity was analyzed. The DC conductivity of SWCNT-PDMS was examined using 4-point probe measurement [Wentworth, MP400] to determine the percolation threshold.

[00135] Morphological studies: The structure of the CNT composite with different concentrations of filler was observed using a scanning electron microscope (SEM) [Zeiss — LEO, 1530VP CrossBeam Workstation], To eliminate the unwanted charging effects on SEM images, a very thin layer of silver was deposited on the composite surface using a sputter coater in a plasma environment [Emitech, K550], Samples were cleaved to observe the CNT networks.

[00136] Shielding effectiveness: To measure the shielding effectiveness of composite layers, specimens were fabricated according to the modified version of ASTM-D4935-99 standard described in. The load and reference specimens were cut from the synthesized CNT-composites using an LPKF Protolaser U3, to have a precise size and preserve impedance matches. Given that the low-frequency cut-off of this standard is 30 MHz, the SE was measured for the frequency range of 50 MHz to 450 MHz using S-Parmeter Network analyzer [Agilent Technologies, 8720ES], [00137] The IEEE Std 299.1-2013 is the reference for SE measurements at frequencies from 9 kHz to 18 GHz with the smallest enclosure size of 0.1 m in the GHz range. Correspondingly, a scale up of dimensions for the kHz range imposes fabrication of a bulky enclosure, which can reach up to tens of meters. Thus, characterizing CNT-based composites with this dimension becomes very expensive. Here, the shielding capability of the CNT-composite for the low- frequency range was interpreted from a modified version of the IEEE standard [39] and the performance of the LabPET II detector module.

[00138] Foucault currents: The Foucault currents induction was measured using two circular coils. The primary coil was triggered with a sinusoidal signal at 100 kHz, and the voltage from the secondary coil was monitored using an oscilloscope. The variance between initial and secondary voltage is linearly correlated to the induced Foucault current in the shielding layers — the lower the difference, the lower the Foucault current.

Performance Measurement of LabPET II detector module

[00139] The LabPET II technology is an avalanche photodiode (APD) based detection platform for positron emission tomography allowing scanners with an adjustable FoV to be designed. Its sub-millimeter spatial resolution and digital data acquisition system are unique features that make this technology suitable for integration into PET/MRI scanners. The LabPET II electronic system uses a one-to-one coupling of pixels to photodetectors and it digitizes the signal at the first stage using an application-specific integrated circuit (ASIC), which makes it less vulnerable to EMI. Each detector module has two ASICs and four APDs, as well each ASIC supports 64 pixels and two APDs. Further information on LabPET II electronics and the set-up employed for PET -insert measurements can be found in [N. Moghadam et al., "Performance investigation of LabPET II detector technology in an MRI-like environment," Physics in Medicine & Biology, 2019.]

[00140] The set-up for gradient interference measurements is displayed in Fig.5. The same set-up was used for RF interference tests in which the gradient coil was removed and replaced by an RF coil placed at the right side of the same DM. Three electronic parameters were measured with and without the CNT-composite layer wrapped around the LabPET II DM to assess its SE in the PET/MRI application: the average baseline voltage and the RMS noise level at the analog output of the shaper amplifiers, along with the energy resolution of 511 keV radiation in the energy spectra of the LabPET II pixelated detectors. The RF coil was excited at 127.74 MHz and 298 MHz, corresponding to 3 T and 7 T MRI, to measure the shielding efficiency of the new composite sheet in the megahertz frequency range. The gradient coil was used with switching frequencies of 10 kHz, 50 kHz and 100 kHz. The data acquisitions for energy resolution measurements were made with an 18.5 MBq 68Ge radioactive rod source. Energy spectra were collected for 5 min. It is worth pointing out that energy spectra were digitized by the dual Time- Over- Threshold (dTOT) technique yielding a nonlinear energy dependence; therefore, the TOT energy resolution reported here do not represent the actual energy resolution of the detectors [8, 40] but it is suitable for the comparison of setups with and without composite shielding. A quad fan was used to cool down the entire set-up and the temperature of the detector module read out from an embedded sensor was registered for each measurement.

RESULTS AND DISCUSSION

[00141] The properties of the CNT-PDMS composites have been investigated and the most suitable material composition was carefully chosen to obtain the required SE for a PET/MRI scanner. Then, the selected composite was used as a shielding layer, while the performance of the LabPET II DM was examined in the presence of pulses from customized RF and gradient coils. [00142] Nanocomposite Characterization

[00143] Dispersibility of CNT-solvent: Figures 6 (a) and (b) show the impedance variation versus frequency of the SWCNT-solvent and MWCNT-solvent mixtures, respectively, measured with the EIS set-up. As these data show, isopropyl alcohol (IP A) yields the highest conductivity while toluene provides the lowest conductivity for both SWCNT and MWCNT solutions over almost all the frequency range. The SWCNT-chloroform mixture also exhibits an appropriate conductivity. Based on these results, IPA and chloroform were selected for further investigation. [00144] Figures 6 (c) and (d) present the UV-Vis spectroscopy data for SWCNTs and MWCNTs dispersed in chloroform and IPA. In the UV-Vis spectrum region, only individual CNTs are active; thus, increasing the dispersion level of CNT results in the elevation of absorbance. In good agreement with EIS data, the intensity value of UV-Vis measurements indicate that the absorbance of CNT-IPA was generally higher than that of CNT-chloroform, suggesting that IPA is a better solvent for dispersing CNTs. IPA is also safer than chloroform and chlorobenzene, which are toxic if inhaled and may cause corrosion. The visual examination of the dispersion level of the SWCNTs and MW CNTs in the four solvents after resting for a week is depicted in Fig. 2 (e). IPA and chloroform solvents produce a uniform black-colored suspension without agglomerations. In the CNT -toluene and CNT-chlorobenzene mixtures, swollen nanotubes were observed, and clear transparent supernatant was present above the precipitated MWCNTs. In the case of SWCNT, the CNTs were not even dissolved well in these two solvents. Thus, toluene and chlorobenzene do not appear as suitable options for having better conductivity. As both the conductivity and the dispersion level of SWCNT -IPA was found to be higher than that of the other samples, the rest of the experiments were conducted based on this solvent only.

[00145] Electrical conductivity: Figure 7 (a) displays the real part of the AC conductivity as a function of the frequency measured for SWCNT-IPA-PDMS composites with different CNT concentrations. With a CNT concentration of 10 wt.%, this composite reaches a suitable electrical conductivity for EMI shielding in the range of 100 Hz to 100 kHz, which is the typical gradient frequency range for PET/MRI scanner. The dashed line in the figure represents the threshold level above which materials have EMI shielding properties [41], In Fig. 7 (a), the slope of the conductivity curve versus frequency for PDMS alone is one, which is typical of insulating materials. The two curves corresponding to the CNT -PDMS show a similar behavior. The conductivity exhibits a plateau that is related to the percolation threshold value (Fig. 7(b)) of the SWCNT -PDMS. This frequency independency is a characteristic of conductive materials [23] and is extendable in the whole frequency range. For this composite, the percolation threshold (see Fig. 7 (b)) is observed between 0.5 and 1 wt.% SWCNT, followed by a significant increase in the DC conductivity. For this composite, the conductivity continues increasing even after 10 wt.%, and it still has the capability to provide higher conductivity before reaching a saturation level.

[00146] Morphological studies: Figure 8 shows the surface topography of the 10 wt.% SWCNT -PDMS and a cross-section image of a cleaved SWCNT -PDMS 10 wt.% sample obtained by the SEM technique. The SEM image of the composite surface displays a uniform layer of PDMS with CNT boundaries between polymer (Fig. 8 (a)). The SEM image in Fig. 8 (b) revealed the generation of conductive networks in the cleaved sample. Although a small amount of CNT agglomeration was observed as bright long filament, the CNT distributions were generally uniform.

[00147] Shielding effectiveness: Figure 9(a) displays the FR4 substrates that were spin- coated with 1-pm thick SWCNT-IPA-PDMS composite for shielding measurements using the ASTM-D4935-99 standard. Controlling the speed of spin-coating provides a reproducible method to deposit films with a constant thickness from sample to sample. By increasing the rotation speed, the thickness of the layer can be decreased to less than 1 pm. The thickness of each coated layer was determined by ellipsometry and the thinnest sample layer had a thickness of 1 pm. Figures 9 (b) and (c) show a self-standing flexible layer of SWCNT-PDMS fabricated by the Doctor blade technique. The thickness of these layers was measured by digital calipers, and the thickest one had a depth of 1 mm. These two thicknesses were considered as the two extreme cases for determining the SE of the SWCNT-IPA-PDMS composite.

[00148] A modified IEEE Std 299.1-2013 standard was used to measure the SE of the nanocomposite layers in the low-frequency range. As can be seen in Fig. 10, the 1-pm thick SWCNT-IPA-PDMS (10 wt.%) composite provided a SE of nearly 40 dB in the 10 kHz to 100 kHz frequency range, while the 1-mm thick layer reached up to 80 dB at 20 kHz slowly dropping to 74 dB at 100 kHz.

[00149] Using a vector network analyzer, the SE in the megahertz frequency range was measured according to the modified ASTM-D4935-99 standard. The SE was plotted in Fig. 7 for 10 wt.% SWCNT-PDMS and 2 wt.% SWCNT-PDMS films, both with 1 pm thickness. The 10 wt.% sample demonstrated SE of 82 dB and higher for frequencies from 50 MHz to 500 MHz.

[00150] Foucault current: As the length of CNTs was in the micrometer range and their diameter was in the nanometer range, conductive networks of the composite could not make large loops, as evidenced in the SEM images in Fig. 8. Thus, each loop can generate a minimal quantity of Foucault currents. These Foucault current inductions can be canceled out by the effect of other layers or by the randomness of conductive loops’ locations and directions inside the composite. Hence, based on SEM data, CNT-composite networks appear to be a compelling reason for the reduction of the effect of gradients on the electronic circuits. Besides, considering shielding effectiveness, the 1 mm thick layer is an appropriate option to reject EMI between the PET and MR. I systems.

[00151] The Foucault current measurements of 1-mm thick sample were conducted for SWCNT-PDMS composites with different concentrations. The results shown in Table 2 indicate that the deviation of secondary voltage was less than 0.5% for the 10 wt.% CNT-PDMS layer, whereas a 1-mm thick copper layer resulted in 8.75% loss of initial voltage. Consequently, it can be concluded that gradient-induced Foucault currents will be insignificant with this new composite shielding layer.

Table 2. Eddy current induction on SWCNT-PDMS layers with different concentrations of CNT. wt. % 1% 2% 5% 7% 10%

Loss ratio (%) 0 0.01 0.04 0.1 0.5

[00152] Performance Measurement of LabPET II detector module

[00153] RF impacts: To verify the shielding capability in a PET/MRI environment, the SWCNT-IPA-PDMS composite was used as a shielding layer for the LabPET II DM. The baseline voltage and RMS noise level of 64 detection channels are displayed in Fig. 12 (a) and (b), respectively, in the presence of the RF coil, with and without shielding. As depicted in Fig. 12 (a) the baseline voltage of all the pixels was almost constant whether RF coil was on or off, with or without shielding. However, the RMS noise level increased significantly in the presence of RF pulses but was essentially restored to its initial value when shielding with the SWCNT-PDMS composite inserted as seen in Fig. 12 (b). The average of baseline voltage and the RMS noise level for all the 128 channels of a LabPET II DM are summarized in Table 3.

[00154] The Gaussian fit of the TOT energy spectra of one typical pixel obtained with and without shielding composite and in the presence of RF signals are compared in Fig. 13. It is relevant to mention that the photopeak in the energy spectrum of each pixel randomly shifts to either left or right side. The shift of the photopeak toward the left side can be attributed to the temperature increase resulting from Foucault current effects. For the selected pixel, i.e., pixel 5, the photopeak was shifted to the right for 127.74 MHz, corresponding to 3 T RF coil’s signal, and was shifted to the left for 298 MHz, equivalent to the Larmor frequency of 7 T MRI. The TOT energy resolution was also slightly degraded. By adding the shielding layer and connecting it to the ground of the DM, the photopeak position and TOT energy resolution were restored to their original values. Table 3 Average baseline voltage. PAIS noise level, pliotopeak position shift and TOT energy resolution of all 128 pixels of a LabPET II DM for three repetitions in the presence of RE signals with and without shielding layer

Condition Baseline Noise level Photopeak TOT energy

(mV) (mV) position shift resolution (%)

(bin)

0 T 430 3.5 ± 1.5 0 = 5 9.90 ± 0.1

[00155] For the DM containing 128 pixels, on average the TOT energy spectrum was shifted to the left for both frequencies, and inserting the shielding layer restored the photopeak to its initial position (Table 2). The average TOT energy resolution measured on each individual spectrum increased from 9.9% (RF off) to 11.2% (3 T) and 12.1% (7 T) for non-shielded DM. The fabricated shielding layer eliminates all the electromagnetic interferences from RF coils and the energy resolution was restored to its original value.

[00156] Gradient impacts: The same tests were repeated with the gradient coil at the frequencies of 10 kHz, 50 kHz, and 100 kHz. The baseline voltage remained almost constant for each individual pixel as expected. The average of the baseline and the RMS noise level for the 128 pixels of the DM, in the presence of different gradient pulses, are summarized in Table 3. The average noise level without the gradient is 3.7 mV, but when the gradient is switching at 100 kHz, the noise level increases to 21.7 mV, and in the presence of gradient at 100 kHz by inserting the CNT-PDMS layer, the average noise is restored to its initial value (Off case). The results for 50 kHz gradient switching follow the same behavior to a lesser extent.

[00157] The Gaussian fit to the TOT photopeak of one typical pixel of a LabPET II DM, with and without the gradient, in the presence or not of the shielding layer, are displayed in Fig. 10. As with RF interference, there are effects on the photopeak position and energy resolution. The average of TOT energy resolution and photopeak position shift are summarized in Table 3. The average TOT energy resolution changes from 10.1% (Gradient off) to 13.7% (switching the gradient on at 100 kHz), while it is completely restored by inserting the composite shielding layer. Table 4 Average of baseline voltage. RMS noise level, pliotopeak position shift and TOT energy resolution of all 128 pixels of a LabPET II DM in the presence of the gradient switching, with and without CNT-PDMS shielding.

Condition Baseline Noise level Pliotopeak TOT energy

(mV) (mV) position shift resolution (%)

(bin)

Off 430 3.7 = 1.5 0 ± 5 10.1 ± 0.1

100 kHz shielded 430 3.7 = 1 5 2 ± 5 10.1 ± 0.2

[00158] Therefore, as disclosed in the present disclosure, by using the hybrid mixing method, a highly conductive composite was fabricated with only 10 wt.% of SWCNT. Further this new CNT -based composite has excellent SE in both the kilohertz and megahertz ranges, which turns out to be significantly higher than the SE of other similar composite materials reported in the literature. In fact, the highest reported SE was 80 dB for a CNT-polyurethane composite with 76 wt.% SWCNT.

[00159] The proposed composite achieves the same level of SE using a seven times lower weight percentage of SWCNT. These outstanding characteristics were obtained by ensuring an optimal dispersion of SWCNTs in a properly selected solvent, making randomly oriented networks of small conductive loops in the material. One critical condition to synthesize a high conductive composite based on a CNT mixture is to increase the dispensability of individual CNTs without fracturing them. This can be achieved by providing an energy level sufficiently higher than the binding energy of the aggregated mixture, while keeping the energy low enough to prevent fracturing of CNTs. Reaching a uniform mixture with no agglomeration is the key benefit that was obtained using both shear mixing and ultra- sonication techniques. A flexible thin layer of the composite was shown to have an adequate SE to eliminate the electromagnetic interferences from the MRI environment. The composites by nature have a higher resistivity than metallic materials, hence they carry a lower amount of Foucault currents when placed in a varying magnetic field such as MRI gradients. Finding a threshold of conductivity where the composite not only provides enough SE but also carries the lowest amount of Foucault current is of importance for selecting a shielding layer for electronic devices working near electromagnetic fields. The advantages of using the proposed approach over the ferromagnetic and magnetic nanoparticle materials for shielding detection modules inside an MRI, are (i) the proposed composite is flexible and transparent to the positron annihilation radiation; (ii) it causes less distortion of the MR image; and (iii) it has diamagnetic properties that can compensate the effect of remaining ferromagnetic materials in the PET-insert. In fact, the first advantage was clearly proven when the performance of a PET detector was fully retrieved to its initial value by applying a SWCNT-PDMS composite shielding layer around the detection module. In comparison, using a copper-based shielding caused a temperature increase and changed PET DM performance. As for the second advantage, it could be verified considering the lower conductivity of the new CNT-composite compared to the commonly used metallic shielding. The measurements presented in Table 1 confirmed that the induced Foucault current in this composite material is insignificant. According to Faraday’s law, Foucault currents generate a magnetic field in the opposite direction of the original varying magnetic field, here gradient fields. The lower Foucault currents induced by the gradient fields will thus cause less distortion of the magnetic fields inside the MRI and preserve the image quality. The argument for supporting the third advantage is that, primarily, most materials including polymers are weakly diamagnetic [43]; secondarily, the interfacial coupling between the CNTs as a filler and the PDMS as the polymer matrix stimulates the diamagnetic response from CNTs and decreases the total magnetization of the composite as demonstrated by Sun et al. for CNT -polyimide composite. Therefore, by making a proper balance between ferromagnetic-based components in the electronic devices and the diamagnetic shielding layer, one could largely avoid the unwanted effects of both types of materials. This is an aspect that deserves to be dealt with in more detail in future research. [00160] The SWCNT composites, for example CNT -PDMS composite, described in this description, are alternative shielding material that successfully achieved practical shielding effectiveness in the kilohertz and megahertz frequency ranges. Consistent with electrochemical impedance spectroscopy (EIS) data, UV-Vis measurements confirmed that isopropyl alcohol (IP A) offers the best condition for the dispersion of CNT in comparison to chlorobenzene, chloroform, and toluene. Consequently, a reasonable shielding effectiveness of 68 dB and almost 80 dB were achieved at low and high frequencies, respectively. This new material can potentially eliminate the issues associated with ferromagnetic and metallic materials inside MRI scanners. Since a composite network carries an insignificant amount of current, it decreases the effect of Foucault currents and temperature elevation on PET electronics, while it is transparent to annihilation photon emissions. The RMS noise level of the baseline voltage and the photopeak energy resolution of a LabPET II detection module were fully restored to their initial values using this shielding configuration without inserting metallic shielding layer inside the MRI-like environment. Hence, we believe that the proposed composite has the potential to prevent the susceptibility artifacts in a real PET/MRI system.

[00161] For the DM containing 128 pixels, on average the TOT energy spectrum was shifted to the left for both frequencies, and inserting the shielding layer restored the photopeak to its initial position (Table 2). The average TOT energy resolution measured on each individual spectrum increased from 9.9% (RF off) to 11.2% (3 T) and 12.1% (7 T) for non-shielded DM. The fabricated shielding layer eliminates all the electromagnetic interferences from RF coils and the energy resolution was restored to its original value.

[00162] Figure 15 shows a flowchart of the different steps involved in the synthesis process of CNT based composite described herein. Including different mixing steps and curing steps which may be used for some of the embodiments described herein.

[00163] For example, the composite may be applied to a surface as coat of paint or covering before curing and then cured to provide the required shielding. This may be more practical if it is used as a part of multilayer coating so other layers protect the CNT composite. Alternatively, the composite mixture may be molded or formed in different shapes before the curing.

[00164] It will be appreciated that an exemplary combination is described in this description, different solvents and polymers can be used alternatively as long as they meet the requirement as described herein.

Claims

What is claimed is:

1. A method for synthesis of a composite material for electromagnetic shielding, the method comprising: adding conductive carbon nanotubes (CNTs) having an average aspect ratio of at least 800 to a solvent to make a mixture; mixing said mixture to provide energy higher than binding energy of the CNTs to each other and lower than or near an energy required to fracture the CNTs to cause dispersion of the CNTs in said solvent without agglomeration; adding a polymer matrix material to said mixture; mixing said mixture and said polymer matrix material while substantially maintaining said dispersion and said average aspect ratio of at least 800 in remaining CNTs; and curing said mixture to obtain the composite material.

2. The method of claim 1, further comprising removing the solvent from said mixture before curing the mixture to obtain the composite material.

3. The method of claim 1 or 2, wherein said mixing said mixture and said polymer matrix material comprises applying ultra-sonication.

4. The method of any one of claims 1 to 3, wherein said mixing said mixture and said polymer matrix material comprises applying shear mixing.

5. The method of any one of claims 1 to 4, wherein said mixing said mixture comprises ultrasonication.

6. The method of any one of claims 1 to 4, wherein said mixing said mixture comprises shear mixing.

7. The method of any one of claims 1 to 4, wherein said mixing said mixture comprises ultrasonication and shear mixing.

8. The method of any one of claims 1 to 7, wherein said CNTs comprise single-wall CNTs.

9. The method of any one of claims 1 to 7, wherein said CNTs comprise double-wall CNTs.

10. The method of any one of claims 1 to 7, wherein said CNTs comprise multi -wall CNTs.

11. The method of any one of claims 1 to7, wherein said CNTs comprise a combination of single-wall CNTs, double- or multi-wall CNTs.

34

12. The method of claim 8, 9, 10 or 11 wherein said aspect ratio of said CNTs is about 1200.

13. The method of any one of claims 1 to 11, wherein said CNTs comprises adding a specific weight percentage of CNTs ranging between 5% to 15% in said polymer matrix.

14. The method of any one of claims 1 to 13, further comprising cooling said mixture and said polymer matrix material during said mixing to maintain temperature below a curing temperature of said polymer matrix material.

15. The method of any one of claims 1 to 14, wherein said adding said polymer matrix material to said mixture comprises mixing said polymer matrix material into said solvent and adding said solvent and said polymer matrix material to said mixture.

16. The method of any one of claims 1 to 15, further comprising molding said material to a desired shape.

17. The method of any one of claims 1 to 16, further comprising applying the composite material as a coating layer before curing said mixture and then curing said coating layer to obtain the shielding layer.

18. The method in any one of claims 1 to 17, wherein said mixing said mixture to provide energy higher than binding energy of the CNTs to each other and lower than or near an energy required to fracture the CNTs in said solvent without agglomeration comprises providing energy to said mixture at an average of 4000 to 6000 J per 5 ml per cycle.

19. The method in any one of claims 1 to 18, wherein said adding conductive carbon nanotubes (CNTs) having an average aspect ratio of at least 800 to a solvent to make a mixture comprises adding CNTs having conductivity higher than 100 S/cm.

20. The method in any one of claims 1 to 18, wherein said adding conductive carbon nanotubes (CNTs) having an average aspect ratio of at least 800 to a polymer to make a mixture comprises choosing a polymer having electrical conductivity higher than 10 ² S/m.

21. An electromagnetic shielding composite material comprising: a polymer matrix material; and conductive carbon nanotubes (CNTs) having an average aspect ratio of length to diameter greater than about 800 in said polymer matrix; wherein: said CNTs are dispersed in said polymer matrix substantially uniformly and without agglomeration; and

35 said composite material at a thickness of 1 micron has a shielding efficiency greater than about 20 dB in the 20 kHz to 1 MHz frequency range, and has a Foucault current loss less than about 2%.

22. The composite material as defined in claim 21, wherein said composite material at a thickness of 1 mm has a shielding efficiency greater than about 80 dB in the 20 kHz to 500 MHz frequency range.

23. The composite material as defined in claim 21 or 22, wherein said CNTs comprise a mixture of single-wall or double-wall and multi -wall CNTs, and said composite material has said shielding efficiency in the 20 kHz to 10 GHz frequency range.

24. The composite material as defined in claim 21, 22 or 23, wherein said composite material has a conductivity greater than 1000 S/m.

25. The composite material as defined in claim 21, 22 or 23, wherein said CNTs have an aspect ratio of about 1000 to about 1400.

26. The composite material as defined in claim 25, wherein said CNTs have an aspect ratio of about 1200.

27. The composite material as defined in any one of claims 21 to 26, wherein said CNTs have a concentration of about 7% to about 12% by weight in said polymer matrix.

28. The composite material as defined in claim 27, wherein said concentration is about 10% by weight in said polymer matrix

29. The composite material as defined in any one of claims 21 to 28 wherein said composite material at a thickness of 1 micron has a shielding efficiency greater than about 35 dB in the 10 kHz to 100 kHz frequency range.

30. The composite material as defined in any one of claims 21 to 29, wherein said Foucault current loss is less than about 0.5%.

31. The composite material as defined in any one of claims 21 to 30, wherein said polymer matrix material is flexible or elastomeric.

32. The composite material as defined in claim 31, wherein said polymer matrix material is polydimethylsiloxane (PDMS).

33. An aerospace equipment having electronics with electromagnetic shielding, wherein said electromagnetic shielding comprises said composite material as defined in any one of claims 21 to 32.

34. An implant having electronics, wherein said electronic has shielding electromagnetic comprising said composite material as defined in any one of claims 21 to 32.

35. A simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) apparatus comprising: an MRI apparatus having a bore defined by an elongated annular housing; a PET subsystem mounted within said housing surround at least a portion of said bore or integrated within said MRI; and electromagnetic shielding placed between said PET subsystem and gradient coils and RF coils of said MRI apparatus, wherein said electromagnetic shielding comprises said composite material as defined in any one of claims 21 to 32.

36. The simultaneous PET and MRI apparatus as defined in claim 35 wherein said electromagnetic shielding provides greater than 60 dB of shielding with less than 1.5% of Foucault current induction loss.

37. The simultaneous PET and MRI apparatus as defined in claim 35 wherein said electromagnetic shielding provides greater than 65 dB of shielding with less than about 0.75% of Foucault current induction loss.