WO2023177915A1 - Insert de tomographie par émission de positrons (pet) destiné à un système d'imagerie par résonance magnétique (irm) - Google Patents

Insert de tomographie par émission de positrons (pet) destiné à un système d'imagerie par résonance magnétique (irm) Download PDF

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WO2023177915A1
WO2023177915A1 PCT/US2023/015586 US2023015586W WO2023177915A1 WO 2023177915 A1 WO2023177915 A1 WO 2023177915A1 US 2023015586 W US2023015586 W US 2023015586W WO 2023177915 A1 WO2023177915 A1 WO 2023177915A1
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pet
detector
bed
section
mri
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PCT/US2023/015586
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English (en)
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Chenming CHANG
Craig S. Levin
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The Board Of Trustees Of The Leland Stanford Junior University
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computed tomography [CT]
    • A61B6/037—Emission tomography
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/50—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment specially adapted for specific body parts; specially adapted for specific clinical applications
    • A61B6/501—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment specially adapted for specific body parts; specially adapted for specific clinical applications for diagnosis of the head, e.g. neuroimaging or craniography
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/52—Devices using data or image processing specially adapted for radiation diagnosis
    • A61B6/5211—Devices using data or image processing specially adapted for radiation diagnosis involving processing of medical diagnostic data
    • A61B6/5229—Devices using data or image processing specially adapted for radiation diagnosis involving processing of medical diagnostic data combining image data of a patient, e.g. combining a functional image with an anatomical image
    • A61B6/5247—Devices using data or image processing specially adapted for radiation diagnosis involving processing of medical diagnostic data combining image data of a patient, e.g. combining a functional image with an anatomical image combining images from an ionising-radiation diagnostic technique and a non-ionising radiation diagnostic technique, e.g. X-ray and ultrasound
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16—Measuring radiation intensity
    • G01T1/1603—Measuring radiation intensity with a combination of at least two different types of detector
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/29—Measurement performed on radiation beams, e.g. position or section of the beam; Measurement of spatial distribution of radiation
    • G01T1/2914—Measurement of spatial distribution of radiation
    • G01T1/2985—In depth localisation, e.g. using positron emitters; Tomographic imaging (longitudinal and transverse section imaging; apparatus for radiation diagnosis sequentially in different planes, steroscopic radiation diagnosis)

Definitions

  • PET + MRI magnetic resonance imaging
  • insert approaches proposed to date have limited the PET system diameter to a diameter that fits on top of a patient bed within a bore of the MRI scanner, which significantly limits the PET system diameter.
  • no PET insert approaches for achieving combined PET+MRI have been appropriate for imaging the adult body, and they are mainly only an appropriate size to image the head of a patient.
  • a positron emission tomography (PET) apparatus for insertion into a magnetic resonance imaging (MRI) system.
  • An arc detector section comprises a plurality of first detector modules arranged in an arc for fixable insertion within a bore of the MRI system.
  • Each of the first detector modules comprises detectors configured for luminescent emissions in response to incoming annihilation photons.
  • a bed detector section is provided for disposing on a bed of the MRI system. The bed detector section is disposed underneath the arc detector section and axially movable with respect to the arc detector section.
  • the bed detector section comprises a plurality of second detector modules arranged in a substantially flat surface. Each of the second detector modules comprises detectors configured to measure luminescent emissions in response to incoming annihilation photons.
  • the first and second detector modules are coupled to a data acquisition system.
  • the data acquisition system is configured for receiving signals from the detector modules and passing digitized signals to a computer system for processing the received signals to reconstruct a three-dimensional image.
  • a system for simultaneous magnetic resonance imaging (MRI) and positron emission tomography (PET) comprises a magnetic resonance imaging system having a bore and a bed, and the PET apparatus provided above.
  • a method for PET imaging within a magnetic resonance imaging (MRI) bore of an MRI system is provided.
  • a bed detector section is positioned underneath an arc detector section that is fixed within the MRI bore.
  • the bed detector section is disposed on a bed of the MRI system and is axially movable with respect to said arc detector section.
  • the bed detector section moves with a patient as the bed moves through the MRI bore.
  • the arc detector section comprises a plurality of first detector modules arranged in an arc, each of the first detector modules comprising detectors configured for measuring luminescent emission in response to incoming annihilation photons emitted from the patient.
  • the bed detector section comprises a plurality of second detector modules arranged in a substantially flat surface, each of the second detector modules comprising detectors configured for measuring luminescent emission in response to incoming annihilation photons emitted from the patient. Signals are received that are generated from the first and second detector modules by a data acquisition system. The received signals can be processed to reconstruct an image.
  • FIG. 1 shows a PET insert according to example embodiments.
  • Figures 2A-2C show an example PET insert for a first generation PET device for comparison and for illustrating various features.
  • Figure 3 shows a simulated electromagnetic field comparison between a standard PET insert approach for PET/MR that is grounded with respect to the RF transmitter of the MR system and the example first generation electrically floating PET insert.
  • Figure 4A shows a 3D printed resolution phantom.
  • Figure 4B shows reconstructed PET and MR image slices from an acquired Hoffman brain phantom.
  • Figure 5 shows resulting reconstructed images from resolution phantom experiments for the first generation insert and for a PET component of an PET/MR system.
  • Figure 6A shows an example second generation RF-penetrable PET insert for comparison and for illustrating various features.
  • Figures 6B-6E show features of an example detector module.
  • Figure 7A shows an example truncated detector geometry including arc and bed detector sections according to an example embodiment.
  • Figure 7B shows a simulated 30 cm diameter cylindrical image quality phantom.
  • Figure 7C shows a reconstructed slice through the system center transaxial plane for the phantom oriented with the 3 and 13 mm diameter spheres closest to the bed detector section.
  • Figure 8 shows coincidence time spectrum results from two coincident modules.
  • Figures 9A-9B show front and side views, respectively, of a PET insert system (PET insert) and RF receiver coils according to an example embodiment, inserted into a wide-bore 3T MR system.
  • Figures 9C-9D show another example embodiment of a PET insert system and RF receiver coils.
  • Figure 10 shows an additional view of a bed detector section and bed cushion.
  • Figure 11 shows an example sensitive, non-magnetic front-end readout system.
  • FIG. 12 shows an example data acquisition (DAQ) system and processor coupled to detector modules.
  • Figure 13 shows an example imaging operation.
  • DETAILED DESCRIPTION [0030] Combined PET/MRI systems have been established over the past decade since their release, and there are now a significant, but relatively small number of such systems worldwide. Combined PET/MRI is uniquely capable of providing excellent anatomical soft tissue contrast and multi-parameter information in a single scan and, as a result, PET/MRI is now commonly used for characterizing disease in regions such as the brain, head and neck, breast, liver, abdomen, and pelvis. The resulting significant reduction of ionizing radiation dose also makes PET/MRI an attractive modality for those requiring recurring PET studies as well as pediatric patients.
  • the high cost of an integrated PET/MRI system is due at least in part to the fact that the conventional PET system design blocks the radiofrequency (RF) fields generated by the MRI system’s built-in RF transmitter (Tx) (a.k.a. the “body coil”); and therefore, the Tx body coil of the MRI system must be re-engineered to reside inside the PET ring in order to acquire MRI images.
  • RF radiofrequency
  • Figure 1 shows a PET insert 100 according to example embodiments herein, which can employ an arc detector portion or section (arc detector section) 102 and a bed detector portion or section (bed detector section) 104.
  • the arc detector section 102 is embodied in a fixed circular arc-shaped arrangement of detector modules 106 that is fixably inserted into the top of an MRI bore 108.
  • An example detector geometry for the PET insert 100 can include, for instance, an arc detector section providing a 16 cm (for example) axial field-of-view (FOV) arc portion of the insert.
  • the arc detector section 102 is inserted into and fixed with respect to the MR system bore 108.
  • the bed detector portion 104 is embodied in a separate (from the arc detector system 102), preferably longer bottom arrangement of detector modules 109 that is placed under the patient 110, such as on top of a patient bed 112.
  • the bed detector system 104 can be an unattached flat or generally flat portion or section.
  • the bed detector section 104 can move with the bed 112 (e.g., along an axial direction within the bore 108) and the patient 110 with respect to the (fixed) arc detector section 102 as the MRI system scans the patient’s body.
  • the bed detector section 104 can be generally flat (e.g., to make it more easily insertable), but can also be contoured and/or made flexible.
  • Example detector modules 106, 109 are configured for luminescent emissions, e.g., configured for scintillation or Cherenkov radiation.
  • Example detector modules 106, 108 can include crystal materials such as but not limited to bismuth germinate (BGO), lutetium, or other appropriate crystal materials.
  • An example bed detector portion 104 may have, for instance, an axial extent of ⁇ 50 cm, and may lie flat on the MR system bed 112, under the patient 110, and move with the bed. This can provide a multi-bed position body PET scan covering ⁇ 50 cm (for example) along the axis of the patient’s 110 body.
  • the arc detection section 102 and bed detector section 104 can be provided to in combination define a PET imaging region 120 within the MRI environment that includes a non-standard, e.g., non-cylindrical geometry, such as a truncated cylinder as shown in cross-section in Fig. 1.
  • the geometry can be defined, for instance, by a volume generally enclosed by an arc and chord in cross-section (as opposed to a circle in cross-section).
  • Using an insert approach to achieve combined PET + MRI studies implies that the PET insert must fit within the physical constraints inside the MR system bore 108.
  • the non-standard geometry of embodiment PET inserts 100 herein allows such inserts to fit into the geometrical and physical constraints imposed by the MR system bed 112 and overall bore 108 of an MRI system, while promoting a relatively large aperture size fitting the body of most adult patients. This in turn allows a multi-bed position scanning PET + MRI study acquisition that can be used to cover a sufficient portion along the axis of the body.
  • Previously disclosed insertable PET/MRI systems at best provide a cylindrical imaging region that is unsuitable for full body imaging of a typical adult body, due at least to the required smaller size.
  • Such conventional systems are usually suitable only for imaging a patient’s head.
  • Example removable PET insert geometries provided herein allow a stand-alone MRI system to achieve large-field-of-view (FOV) scanning for multi-bed position body PET+MR studies appropriate for imaging any part of the body (such as but not limited to the thorax) with large axial coverage, and thus example PET insert systems can be employed as body PET systems.
  • FOV large-field-of-view
  • example PET insert systems 100 can be “RF-penetrable”, enabling simultaneous MRI data acquisition using the MRI system’s built-in body coil RF transmitter.
  • PET inserts can be electrically isolated (or floating) with respect to the RF transmitter of the MR system using an electro-optical signal transmission technology, e.g., as disclosed in Olcott, P.D., H. Peng, and C.S. Levin, Novel Electro-Optical Coupling Technique for Magnetic Resonance-Compatible Positron Emission Tomography Detectors. Molecular Imaging, 2009. 8(2): p. 74-86; Bieniosek, M.F., P.D. Olcott, and C.S.
  • an RF receiver coil can reside inside of the PET insert 100 during combined PET+MRI data acquisition.
  • Example devices and systems can improve on earlier RF-penetrable ToF-PET inserts, such as but not limited to earlier generations of inserts described in more detail herein.
  • RF-penetrable for ToF PET refers to an insert approach that can acquire simultaneous ToF-PET+MR data by transmitting the RF field of the MR system’s built-in body coil through the PET ring into the patient.
  • Rx RF receiver coil
  • the example bed detector section 104 is axially movable (i.e., along the direction of the patient’s spine) with respect to the MRI bore 108 and the (e.g., stationary) arc detector section 102.
  • the bed detector section 104 can, for instance, be axially movable with the bed 112.
  • the bed detector section 104 can be separately movable with respect to, and can be separate from, the arc detector section 102 (though both may be coupled to a control and/or data acquisition system, as disclosed in further detail herein).
  • Providing relative movement between the bed detector system 104 and the arc detector system 102 allows axial repositioning of the resulting PET imaging region within the MRI bore, and thus can provide full-body simultaneous PET/MRI imaging, e.g., by scanning the patient’s 110 body.
  • some prior disclosed systems for simultaneous PET/MRI imaging have been provided by a fixed cylinder of detectors that wraps around and under the bed. The portions of such a cylinder that are above and below the bed are permanently integrated and have the same axial length.
  • Data acquisition electronics e.g., a PC
  • a processor e.g., a PC
  • DAQ data acquisition
  • An example DAQ system can be separate from or combined with (e.g., integrated with) a control system for powering and/or operating the PET system, or the combined PET/MRI imaging system.
  • a bed 1300 on which the bed detector section (e.g., 1302) is disposed (“on” may refer to being over or under the bed) is axially movable (i.e., along the direction of a patient’s 1304 spine, indicated by an arrow in Fig. 13) with respect to the MRI bore and the (e.g., stationary) arc detector section 1306 disposed within the MRI bore.
  • the PET imaging region 1310 provided at least by a truncated cylinder formed by the (e.g., aligned) arc and bed detector section is axially repositioned with respect to the bed 1300 and the patient 1310. This scans the patient 1310 within the MRI bore. It will be appreciated that based on the field of view (FOV) of the detectors, the available PET imaging region can extend beyond the volume (shown in dark green) disposed directly between the detector modules in the arc detector section 1306 and the detector modules in the bed detector section 1302, as illustrated by the larger PET imaging region 1310 shown in light green.
  • FOV field of view
  • the detector modules in the arc detector and bed detector sections 1306, 1302 receive emitted photons and produce energy signals, which are sent to the DAQ system for processing, e.g., for imaging of photon pairs.
  • Example processing methods can account for the changing PET imaging regions as the patient is scanned, e.g., by tracking the axial position of the bed 1300 (or bed detector section 1302) with respect to the arc detector section 1306.
  • example systems herein can be installed by a user procuring the body PET insert components and inserting them into a stand-alone MRI system already installed on-site, e.g., at an institution (the latter being quite common at most medical centers).
  • Example PET inserts can also be removable and/or portable, and thus can potentially be used in conjunction with any other MRI system available at the user’s institution. Furthermore, example PET insert designs can provide higher spatial resolution and photon sensitivity owing to example detector configurations and system geometries (e.g., a fixed arc + long scanning panel detector that moves with a patient bed).
  • Example body PET insert to be RF-penetrable enables the body PET+MRI data acquisitions using the MRI system’s built-in body RF transmitter coil; for instance, only an RF receiver coil need reside inside the PET ring.
  • Example systems and methods can meet the need for a PET insert design that can achieve multi-bed position body PET acquisition inside of an MRI system while also acquiring MRI data.
  • example inserts provided herein enable body PET + MRI as well as multi-bed position body PET + MRI to image a large axial portion of the patient’s body.
  • Example systems, devices, and methods provide benefits for medical devices and systems such as but not limited to PET systems, MRI systems, and PET/MR systems.
  • example systems, devices, and methods can also benefit researchers and clinicians who currently have an MRI system (e.g., to acquire anatomical information) but aim to also acquire body PET data (e.g., to acquire biochemical information) in their research studies, but currently lack an integrated PET/MRI product at their disposal (e.g., due to the high cost to procure and install a permanently integrated PET/MR system).
  • a low-cost solution to achieving body PET + MRI provided by example embodiments can also make PET/MR accessible to more patients worldwide.
  • Example embodiments provide, among other things, a photon time-of-flight (ToF) detector module from which example PET inserts can be built.
  • An example detector module can be made from BGO, a less expensive scintillation crystal than is conventionally used, to further reduce costs.
  • Example ToF detector modules can be operated using computer-executable instructions (e.g., software code) to provide image reconstruction and/or to provide for data corrections to improve quality and accuracy of image data.
  • example body PET insert geometry comprising an arc + panel section.
  • Performed image reconstruction has demonstrated that an example non-standard PET system geometry can provide high quality images compared to the standard cylindrical geometry used for commercially available PET systems.
  • Computer simulated PET scans of different imaging “phantoms” as well as various attempts at tomographic image reconstruction have been performed. Images of simulated data of example non- standard PET insert geometries produced image quality equivalent to that from simulated data that was also generated of a standard cylindrical geometry PET system using the same imaging phantom.
  • Example devices, systems, and methods can also improve existing RF- penetrable time of flight-positron emission tomography (ToF-PET) insert approaches to achieve simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI).
  • Embodiments provided herein include inventive features that allow ToF PET methods for insertable PET systems to work for a body size geometry.
  • Example body insert devices can be placed within any magnetic resonance imaging (MRI) system to achieve simultaneous body ToF-PET/MRI.
  • MRI magnetic resonance imaging
  • the larger aperture provided by example body P E T inserts 100 including the separate, longer bed detector section 104 allows full tomographic angular sampling by accommodating the geometrical constraints imposed by the MR system bed 112, while allowing a multi-bed position acquisition (e.g., via axial movement of the bed) that is required to cover a sufficient portion along the axis of the body.
  • example embodiments can employ bismuth germanate (BGO) scintillation crystals for the entire detector system, which is roughly 3-5x less expensive than state-of-the-art lutetium-based alternatives.
  • Excellent coincidence time resolution (CTR) ( ⁇ 300 ps FWHM) required for ToF-PET can be achieved using the prompt component of the luminescence, mainly comprising Cherenkov radiation.
  • excellent spatial resolution can be achieved using, for instance 3x3 mm 2 cross-section crystal elements.
  • Example 3D image reconstruction methods can be used to create images from a range of useful detector element pair lines-of-response (LORs) traversing the insert’s arc + flat (bed) detector sections.
  • Such image reconstruction method can include data corrections and iterative image reconstruction approaches for multi- bed position data acquired with embodiment BGO-based body ToF-PET detector insert systems.
  • LORs lines-of-response
  • Example PET inserts can provide a lower cost approach to achieving simultaneous body ToF-PET/MR imaging.
  • Embodiments herein provide, among other things, a novel, relatively inexpensive, insertable/removable body PET system that can be integrated with a high-performance RF receiver coil.
  • Computer-implemented computational methods e.g., implemented by a processor executing computer-executable instructions in software, firmware, or hardware, can be employed for quantitative PET imaging.
  • existing MRI sites would be able to achieve integrated PET/MRI capability by procuring a removable, ‘PET/MR receiver coil’ and associated software. Since MRI is widely available and in production, the cost of example body PET inserts provided herein can be about 5- 10-fold less than the cost of a permanently integrated PET/MR system plus required room renovations and annual service costs.
  • Table 1 shows example components that may be used in PET inserts including comparative PET inserts and PET inserts according to example embodiments herein (right column). It is believed that such cost reduction would make PET/MRI more widely available, thus increasing its global impact. This can enable widespread adoption and impact of this useful multi-modality, multi-parameter imaging technology, which is suitable for various applications, nonlimiting examples of which are described herein.
  • BGO which, per volume, is roughly 3-5x less expensive than LYSO or LSO, the scintillation crystal materials used in state-of-the-art PET/MR and PET/CT systems.
  • BGO higher effective atomic number Z (74 vs.66) leads to substantially higher 511 keV photon intrinsic detection efficiency per volume, which can be exploited for higher PET system sensitivity using the same crystal element length, or further cost reductions using shorter crystal element lengths (e.g., 15 vs.20 mm length) for the same scanner diameter and axial coverage.
  • System electronics can include a novel, very low time jitter FPGA-based TDC.
  • Additional features and benefits of example systems and methods will be described with respect to comparative PET insert systems.
  • First Generation PET Device For further explanation and illustration of example features and benefits of example embodiments, a head-sized RF- penetrable PET device (referred to herein as a first generation PET device) and a second generation PET device will first be described.
  • Figs. 2A-2C show the example PET insert 200 for a first generation PET device, which is penetrable or transmissive to the RF field produced by an MRI system’s body Tx coil.
  • a PET system With such an RF-transmissive PET system technology, it is possible for a PET system to be inserted into an existing MR system, such as MR system 202, to acquire simultaneous PET and high-quality MRI data, e.g., without requiring modifications to the MR system’s hardware (such as bringing the Tx inside the PET insert), and the MR system’s built-in body coil can be used for generating the RF transmit pulses.
  • Additional features of the first generation PET device 200 are disclosed in: Lee, Brian J., Ronald D. Watkins, Keum Sil Lee, Chen-Ming Chang, and Craig S.
  • the first generation PET device 200 includes, among other features: (a) electro-optical PET detector modules 204 with compressed sensing multiplexed readout, (b) a 16 Faraday cage arrangement 206 engineered into an RF-transmissive form factor (Figs.2A, 2B), (c) ‘PET-compatible’ (low photon attenuation) RF receiver coils 208 (Fig.2C), and (d) a 256 channel optical data acquisition (DAQ) system for supporting the 16 detector modules.
  • electro-optical PET detector modules 204 with compressed sensing multiplexed readout
  • a 16 Faraday cage arrangement 206 engineered into an RF-transmissive form factor (Figs.2A, 2B)
  • Figs.2C ‘PET-compatible’ (low photon attenuation) RF receiver coils 208
  • DAQ optical data acquisition
  • Promoting RF penetrability of the first generation PET ring involves electrical isolation of the PET electronics from the MR system 202.
  • This isolation includes: (1) electro-optical coupling, where scintillation pulses are converted to near infrared light that transmits coherently, with minimal dispersion, down 20 m length fibers to a photodiode receiver followed by an optical DAQ. A digital version of this electro- optical readout is provided in the second generation brain PET insert system described in further detail below.
  • LV low voltage
  • HV high voltage
  • the example RF-penetrable PET insert 200 includes 16 Faraday cages assembled into a 32 cm inner diameter ring with 1 mm inter-module gaps (Figs.2A, 2B). Rather than grounding the PET insert 200 to the MR system’s Tx ground, the example PET detectors 204 were powered with (isolated) batteries 210 so that, together with the fiber optical signal coupling, the PET detectors are electrically floating with respect to the MR system ground.
  • the ring of detector Faraday cages resembles a series of capacitors; the side plates of two adjacent modules form a capacitor. Roughly 30% RF field attenuation occurs due to this capacitive power dissipation, depending on the equivalent capacitance between the detector module Faraday cages, but this can be compensated for by increasing the RF transmitter gain, and therefore minimal MRI SNR loss occurs.
  • Fig.3 shows a simulated electromagnetic field comparison between a standard PET insert approach for PET/MR that is grounded with respect to the RF transmitter of the MR system and the example first generation electrically floating PET insert 200.
  • the grounded PET insert (Fig.3, right) blocks the RF field from penetrating inside the FOV
  • the electrically floating PET insert Fig.3, middle
  • the external RF field e.g., from the MR system’s 202 built-in body coil
  • each PET detector module 204 is shielded from RF fields via the Faraday cages (no field lines entering inside of modules).
  • the magnitude of the RF field within the FOV of the electrically floating PET insert 200 is roughly 30% lower than the case without the PET insert; however, surprisingly, that ‘B1’ field retains a high level of homogeneity inside the floating insert, which is useful for good MRI performance.
  • the ⁇ 30% attenuation of the RF field can be compensated for by increasing the RF transmitter power, e.g., by ⁇ 30%, to preserve the desired magnetization “flip angle” required for good MR image SNR, with an acceptable increase in the SAR of RF power in the rest of the body.
  • the second generation head insert (described further below) provides even lower RF attenuation, and thus less needed increase in the RF transmit power, and thus lower SAR to the rest of the body.
  • PET system performance outside and inside the MRI environment Figure 2A depicts the first generation insert system 200 on the bed of the 3T MR system 200 just before it is inserted.
  • the performance of the PET insert system 200 was evaluated for energy resolution, photopeak position, CTR, and count rate performance outside and inside the 3T MRI system 202.
  • the global energy and coincidence time resolutions were, respectively, 16.2 % and 5.3 ns FWHM.
  • MRI system performance with First generation PET insert Experimental studies of the effects of the RF-penetrable PET insert performance on MR performance were also conducted using a 3-Tesla GE MR750 with a 60-cm diameter bore (Fig. 2A). In a first set of experiments, to test effects of RF transmission into and out of the PET insert, the built-in MRI system body coil was used for both RF signal transmitter and receiver. The insert was placed at the MR isocenter. [0074] Effects on B 0 : Results of “B 0 static field” analysis showed that the powered-on PET insert configuration had a negligible effect on the MR system static B-field .
  • RF noise spectrum with operating PET The example PET system was placed inside the MR bore and the MRI RF noise spectra were acquired to assess the effect of the PET electronics on the MR receive signals. These RF noise “blank scans” were performed by receiving the MRI signal without any RF excitation or gradient pulses running. The noise scan made 14 measurements in ⁇ 62.5 kHz frequency centered at the Larmour frequency (127.8 MHz) for the 3T scanner used. In assessing the signal amplitude across the scanned frequencies without and with the PET system operating, no significant differences were observed between the two cases, indicating that any RF noise generated by the PET electronics was effectively shielded by the PET Faraday cages.
  • Fig.4A The reconstructed phantom PET images are shown in Fig.4A, where the top row depicts the case of the PET system outside the MR system, and within the MR system without and with GRE and FSE MR pulse sequences. PET image data are significantly the same for all four conditions, and the small hot rods as well as the cold rods are clearly resolved.
  • Fig.4A shows the reconstructed MR images for GRE and FSE pulse sequences with and without the PET insert present, again with the built-in MRI body coil used as both RF transmitter and receiver.
  • the MR image data with and without PET insert are significantly the same. Note that for the MR images labeled “with PET”, indicating that they were acquired with the PET insert operating, the RF transmit field from the MR body coil transmitted through the PET ring, excited the proton spins of the phantom sitting inside the PET FOV, those spins relaxed, and the resulting RF receive signal passed back out of the PET ring and were received by the body coil.
  • Fig.4B shows reconstructed PET and MR image slices from an acquired Hoffman brain phantom.
  • Fig. 5 presents resulting reconstructed images from resolution phantom studies in the first generation insert 200 and PET component of the Signa PET/MR system; data for the phantom studies were summed along the axial direction. A line profile was taken through the 2.8 mm diameter rods in the images to assess the achievable resolution in each system. Owing to the ⁇ 5-fold better volumetric resolution (i.e.
  • Second generation PET insert shows an example second generation PET insert 600, features of which are also described in C. Chang, B. J. Lee, I. Sacco, R. D. Watkins, Q. Dong and C. S.
  • Figures 6B-6E illustrate an example detector module 602 for the second generation PET insert 600, including a Faraday cage 604 (FIG. 6B), thermal regulation system 606 (FIG. 6C), module readout board 608, and one of six detector sub-modules 610 (FIG. 6D) including a 128 pixel LYSO-SiPM array on one side and front-end readout electronics on the back).
  • An example second generation PET insert 600 has an inner diameter of 33 cm and an outer diameter of 42 cm, vs. 32 cm and 40 cm, respectively, for the example first generation insert 200.
  • Table 1, above summarizes example differences between commercially available permanently- integrated PET/MR systems and the example first and second 200, 600 generation insert systems, as well as an example body PET insert 100 according to additional embodiments herein, including example in-production cost estimates and annual service fees.
  • a system including the example second generation PET insert 600 can achieve a peak sensitivity of 6%, spatial resolution of 2.7 mm, a CTR of ⁇ 250 ps, and an energy resolution of 13%.
  • the example second generation system 600 When compared to other brain inserts, such as MINDView (Gonzalez, Antonio J., Andrea Gonzalez-Montoro, Luis F. Vidal, Julio Barbera, Sebastian Aussenhofer, Liczandro Hernandez, Laura Moliner et al. "Initial results of the MINDView PET insert inside the 3T mMR.” IEEE Transactions on Radiation and Plasma Medical Sciences 3, no.3 (2016): 343-351), the example second generation system 600 has comparable sensitivity and spatial resolution, but better CTR & energy resolution, and is RF-penetrable.
  • Example body insert PET System A PET insert for simultaneous body PET/MR imaging according to example embodiments will now be described with reference to Figs.9A-9B.
  • Figs.9A-9B depict front and side views, respectively, of a PET insert system (PET insert) 900 and RF receiver coils 902, 903 according to an example embodiment, shown inserted into a wide-bore 3T MR system 906, depicted with some partially transparent layers for ease of visualization.
  • Example embodiments can provide a removable, large FOV body PET insert for PET/MRI.
  • brain PET inserts for PET/MR are too small to fit an adult body.
  • Each Faraday cage of the example arc detector section 910 has dimensions of roughly 20x6x4 cm 3 , with 16 cm crystal axial field of view (FOV), e.g., similar to that for the second generation head insert described above.
  • FOV crystal axial field of view
  • the example cage dimensions are 30x6x3 cm 3 , each with 26 cm crystal axial FOV (thus, in the example embodiment shown in Fig.9B, 52 cm total active axial FOV for the two columns of bed modules is provided).
  • a part of the patient body cushion 920 covering the bed detector section 912 can include a thin, cut-out section to spread the patient’s upper body weight over a large area, and a thick portion covering the bottom half of the body. Lines for power, signal, and cooling (not shown in Figs.
  • FIG. 9A-9B can be, for example, drawn out a back end (toward the feet) of the Faraday cages for the arc detector section 910, and lines 924 may be drawn out of both ends (in opposite directions) for the bed section.
  • Figures 9C-9D show another embodiment PET insert 930, illustrating alternate configurations of RF receiver coils 932, 934.
  • Table 3 shows amounts of components of an example system + data acquisition (DAQ), though it will be appreciated that one of more of these components and/or amounts can vary.
  • Table 3 [0089] Detector modules: Functions of example detector modules 914, 916 can include, for instance, communication with a control system including a processor (e.g., a PC 1200) (see FIG.
  • a processor e.g., a PC 1200
  • processing circuitry embodied in a field- programmable gate array can be employed to configure all on-board chips, collect data from the front-end readout circuit, and communicate with a data acquisition (DAQ) system 1202.
  • FPGA field- programmable gate array
  • DAQ data acquisition
  • An example FPGA for instance in conjunction with a jitter-attenuating clock multiplier and a clock buffer, can also provide intra- and inter-module synchronization.
  • sub-module 610 such as shown in Figs.6C and 6E
  • the crystal element dimensions can measure 3.2x3.2x15 mm 3 and 3.2x3.2x10 mm 3 , respectively, formed into sub-modules including 128 crystal elements with ESR (enhanced specular reflector) inter-crystal reflectors.
  • Each element can be coupled, for instance, one-to-one via optical epoxy to matching area silicon photomultipliers (SiPMs), e.g., from Broadcom (device area: 3.14x3.14 mm 2 , active area 3.0x3.0 mm 2 ), which have enhanced ultra-violet (UV) detection sensitivity useful for Cherenkov photons, as disclosed in Cates, JW; Levin, CS.
  • SiPMs silicon photomultipliers
  • the surface-mounted SiPM devices can be, for example, soldered directly to the detector PCB to minimize detector module height.
  • 10 mm instead of a significantly larger (say, 15 mm) crystal element length in the bed detector section 912 allows the example bed Faraday cage for the bed detectors 916 to be low profile—e.g., 3 cm high, facilitating its insertion on top of the bed, and beneath the patient body cushion 930 and bed RF coil array 903 (Figs.9A, 9B).
  • the 10 cm crystal length still enables good sensitivity (e.g., as illustrated in Table 1 above), while mitigating spatial resolution-degrading effects due to variable 511 keV photon interaction depth in the bed detector crystals 916.
  • the timing resolution can also be improved, since variations in Cherenkov light collection efficiency and transit time to the photodetector are lower in a 10 vs.15 mm length crystal (e.g., see Fig.8).
  • Detector Faraday cage design An example detector Faraday cage shielding design for the detectors 914, 916 can include, e.g., one or more of the following functions: shielding PET detector electronics from the RF fields from the MR system and vice versa, while minimizing the induction of Eddy currents from the gradient field; housing connections for the power/signal cables and thermal regulation cooling lines; and allowing RF fields to pass through the small gaps between adjacent cages (in addition to through the open ends of the insert). [0094] To construct example Faraday cages, an approach as described in Lee, Brian J., Ronald D. Watkins, Chen-Ming Chang, and Craig S. Levin.
  • the patient’s lower body, including the buttocks, can be supported by the thick portion of the bed cushion 920 in an example embodiment, as opposed to being supported by the bed detector section 912.
  • the head can be supported by a head cushion 940, as opposed to being supported by the bed detector section 912.
  • Feedthrough configuration The connections for data, powering, and cooling lines in example embodiments can be provided, for instance, through one end face of each of the Faraday cages.
  • the feedthrough configuration can be, for instance the same (or similar) as for the second generation head insert 600 (e.g., as shown in Fig.6B and 6C; feedthroughs for arc section are not illustrated in Figs.9A-9B), and these lines can be taken out in the direction toward the patient’s feet.
  • the connections 924 are shown coming out of each Faraday cage on the opposing ends of the two cage columns as seen in Figure 9B (not shown in Fig. 9A).
  • Figure 10 shows an additional view of the bed detector section 1000 and bed cushion 1002, illustrating a combined power/signal connector 1004 and cooling inlet/outlet fittings 1006.
  • Interlinking bed detector Faraday cages To mechanically link the Faraday cages for the bed detectors 1018 together in the example bed detector section 1000, and to allow it to comply to a bed shape (i.e., a curved bed contour), connectors such as but not limited to low profile, plastic hinges 1020 can be added on both sides of each cage. Coup le rs such as bu t no t l im i ted to plastic pins (not shown) can go through the interlocked hinges, linking the cages together, while preserving gaps for RF penetrability.
  • a bed shape i.e., a curved bed contour
  • plastic hinges 1020 can be added on both sides of each cage. Coup le rs such as bu t no t l im i ted to plastic pins (not shown) can go through the interlocked hinges, linking the cages together, while preserving gaps for RF penetrability.
  • a total radial footprint of the Faraday cages in the example arc detector section 910 can be, for instance 4 cm, which is the same height of the Faraday cage used in the first generation PET insert 200 described above.
  • the inlet/outlet cooling lines 1006 can enter/exit through the opposite ends of the two columns of Faraday cages (see Figs.
  • System electronics For readout of the example PET detector system 900, a scalable electronic readout analogous to that of the second generation MR- compatible TOF-PET insert 600 can be provided (e.g., see Dong, Qian, Ilaria Sacco, Chen-Ming Chang, Brian J. Lee, and Craig S. Levin. "Performance evaluation of an advanced detector module for an RF-penetrable TOF-PET insert for simultaneous PET/MRI.” In 2018 IEEE Nuclear Science Symposium and Medical Imaging Conference Proceedings (NSS/MIC), pp.1-3.
  • Figs.11 and 12 depict an example front-end readout 1100 for each example detector channel and an example readout 1200 for an example DAQ system 1202, respectively.
  • an SiPM anode 1102 can be connected to an amplifier and directly sent to an FPGA-based time-to-digital converter (TDC) (e.g., see S. Pourashraf, Shirin; A. Gonzalez-Montoro; J.Y. Won, M.S. Lee, J.W. Cates, Z. Zhao, J.S. Lee, C.S. Levin. Scalable electronic readout design for a 100 ps coincidence time resolution TOF-PET system.
  • TDC time-to-digital converter
  • a first energy threshold is applied.
  • a common FPGA time-over-threshold (TOT) energy threshold can be applied to the negative input of low-voltage differential signaling (LVDS) interfaces for all energy channels.
  • the Eth-TOTs can be evaluated to determine an optimal threshold value for these interfaces versus SiPM bias.
  • TOT values can be measured using the TDC, and converted into an energy value.
  • a second threshold can be applied during data processing, for instance, to only include those events that fell within the photopeak FWTM.
  • Extracting timing For the timing signal, both the SiPM anode and cathode signals are used in the example circuit 1100. The cathode and anode outputs can be connected to a bootstrapping circuit 1110, e . g . , a s d i s c l o s e d i n Cates, JW; Levin, CS. Electronics method to advance the coincidence time resolution with bismuth germanate.
  • the SiPM cathode and anode signals may be connected to, for instance, a low magnetic susceptibility micro-balun transformer, e.g., in a ‘balanced-to-unbalanced’ configuration.
  • the resulting signal is connected to two (for example) RF amplifiers 1104 in cascade.
  • the amplified timing signal is then sent to the FPGA to apply a two- threshold level scheme, in which the lower threshold is set, for instance, below one light photon above noise equivalent level, and the upper threshold is set, for instance above the signal amplitude of two single photon avalanche detectors (SPADs) of each SiPM.
  • a two- threshold level scheme in which the lower threshold is set, for instance, below one light photon above noise equivalent level, and the upper threshold is set, for instance above the signal amplitude of two single photon avalanche detectors (SPADs) of each SiPM.
  • SSDs single photon avalanche detectors
  • These values can be used to estimate the rise time of the signal.
  • a multiplexing scheme can be employed that only adds one channel per module while providing greatly improved CTR performance.
  • an example FPGA can provide the time stamp for each event.
  • an example FPGA can provide, for instance, 2 pairs (1 for each hit detector) of rise-time + time-stamp per event.
  • the arc and bed detector sections 910, 912 can be powered by, for example, isolated and filtered power supplies, which can supply high voltage (as a nonlimiting example, ⁇ 31V) to the SiPMs, as well as low voltage to the readout electronics board.
  • the digital, analog and SiPM bias voltage of the detector modules 914, 916 can be supplied separately in an example embodiment to avoid cross talk.
  • Synchronization In embodiments, a daisy-chain synchronization method can be employed such as that provided for the second generation head insert 600 (e.g., see Dong, Qian, Ilaria Sacco, Chen-Ming Chang, Brian J. Lee, and Craig S. Levin.
  • each detector module 1210 can include two clock sources: an external clock from an optical transceiver and a local clock from an oscillator.
  • the first detector module (e.g., detector 0) can be configured, for instance, as master mode, which utilizes the local clock and sends it to the next detector module, e.g., through lines such as made from plastic optical fiber (POF), while the remaining detector modules (e.g., detectors 0-20, and 21-36 in example arc and bed detector sections 910, 912, respectively) can be configured in slave mode, accepting the external clock from the optical transceiver and passing the clock signal to the next detector.
  • the FPGA on the example detector module can also serve the function of clock switching and phase-locking.
  • the output clock from the FPGA can be fed, for instance, into a jitter attenuator to reduce the jitter and multiply the clock frequency.
  • the low-jitter clock can be copied at the fanout clock buffer and sent to, for instance, either 6 or 9 submodules of each detector module for the arc or bed detector sections, respectively.
  • Data communication out of MR bore To promote signal integrity of the energy and time measurement, as well as MR-compatibility and RF-penetrability, the digital energy and timing data can be sent, for instance, via an optical transceiver link 1212 (e.g., see Dong, Qian, Ilaria Sacco, Chen-Ming Chang, Brian J. Lee, and Craig S. Levin.
  • Example Back-end DAQ design An example back end data acquisition (DAQ) design 1202 (Fig.12) can be embodied in, for instance, a scaled-up version of the 16-channel TOF-PET DAQ design provided for the second generation head insert system described above for PET/MR (e.g., see Dong, Qian, Ilaria Sacco, Chen- Ming Chang, Brian J. Lee, and Craig S. Levin. "Performance evaluation of an advanced detector module for an RF-penetrable TOF-PET insert for simultaneous PET/MRI.” In 2018 IEEE Nuclear Science Symposium and Medical Imaging Conference Proceedings (NSS/MIC), pp. 1-3. IEEE, 2018; Dong, Qian, Brian J.
  • DAQ back end data acquisition
  • the example DAQ simultaneously reads events on all (e.g., 37) optical link channels 1212 coming from the PET insert (e.g., including detector modules 1210).
  • An example DAQ system 1202 can be configured similarly to an off-the-shelf evaluation board, a nonlimiting example being HTG-K800: Xilinx Kintex UltraScaleTM PCI Express Development Platform, HiTech Global. Extension boards (e.g., two) including a total of 37 high-speed small form-factor pluggable (SFP) transceivers 1220, 1222 can be plugged in the DAQ board 1202 to receive data packages from 21+16 detector modules through optical fibers, e.g., using the Aurora protocol.
  • SFP small form-factor pluggable
  • An FPGA 1204 (e.g., Xilinx) can be used for single event extraction and sorting, coincidence and random filtering, and data repackaging. The repacked data can be sent in a compact format to the computer 1200, e.g., via a PCIe x4 interface 1230.
  • An example optical link e.g., 2 Gbit/sec
  • Post-processing The energy and timing information can then be sent to the processor (e.g., PC) 1200 for data analysis and post-processing.
  • a second energy filter e.g., to consider the events that fell within the FWTM of the photopeak
  • Each event category can be fitted using a double Gaussian.
  • the time bias for each category can be corrected using the centroid of the fast component of the distribution.
  • the image reconstruction process can incorporate different timing kernels based on this classification. This information splitting in many TOF- kernels can be seen as additional information to enhance the overall CTR performance of BGO (for instance). Although a significant fraction of events have the best time resolution (as illustrated in Fig.8), the very good sensitivity of BGO can be preserved, as the remaining events with moderate time resolution can be used as well. [0113] Some degradation of the overall system time resolution may be expected as compared to single array results of ⁇ 290 ps.
  • the readout 1100 and DAQ electronics 1200, calibrations, and/or postprocessing may be further optimized to provide a system CTR of ⁇ 300 ps, e.g., 250 ps (for instance).
  • an approach to thermal regulation can be used as provided for the second generation head insert system (see Dong, Qian, Ilaria Sacco, Chen-Ming Chang, Brian J. Lee, and Craig S. Levin.
  • the temperature of the system can be regulated, for instance, by liquid cooling tubes and a chiller system.
  • An outlet temperature of an example chiller is 15 degrees Celsius.
  • Figs.9A-9B illustrate example RF receiver coils 902, 903 that reside above and below a patient.
  • the example flat phased array coil structure 903 underneath the patient shown in Figs.9A and 9B (through the transparent bed cushion) includes (for instance) 16 loops and sits below the thinned bed cushion 920 and on top of the bed detector section 912 Faraday cages, again facilitating the spread of the patient’s upper-body weight across a large area.
  • the curved surface phased array coil structure 902 (visible in Fig.9B) rests on top of the patient 940 and also includes (for instance) 16 loops.
  • Both the 16- channel Rx-only curved surface and the 16-channel bed phased-array coils in the example structure measure, for instance, 55 cm axially and cover 55 cm in width transaxially.
  • the coil structure may include a greater number of loops (e.g., 40 loops for the flat phased array coil structure 932 and 27 loops for the curved surface phased array coil structure 934) or a fewer number of loops.
  • the example RF receiver coils 902, 903 can be configured similarly to the phased array coils used in the first and second generation head insert systems 200, 600, and as disclosed by example in Lee, Brian J., Ronald D. Watkins, Keum Sil Lee, Chen-Ming Chang, and Craig S. Levin.
  • Additional possible features of the example coil preparation including, for instance, tuning to the Larmor frequency (127.8 MHz), impedance matching, feedline cables, coil interface, element decoupling, preamplifier decoupling, and passive and active RF transmit field blocking are disclosed, for instance, in Lee et al., 2019.
  • the 6 mm thick coil structure can slightly flex as needed to fit the (slightly curved) shape of the bed detector panel that can be enabled by the hinges 1020 interlocking its Faraday cages (e.g., as shown in Fig.10).
  • an example PET insert system 900 can employ two 3D printed, thin, removable fillable structures: an arc, fitting just inside the contours of the arc detector section 910 Faraday cages, and a flat section that fits just above the RF coil 903 of the bed detector section 912 (Figs. 9A-9B); these arc and flat sections can be filled, for instance, with uniform activity of F-18 that exposes each system LOR to an equal amount of activity. “Fully-3D Casey” variance reduction can be applied to reduce noise in the normalization data (e.g., as disclosed in Groll, A., and C. S. Levin.
  • An example PET system can apply processor- implemented machine learning methods for such correction.
  • volume-to-volume translation neural networks implemented by a processor can be trained to generate (a) emission-based scatter+attenuation corrected PET images, and (b) co- registered pseudoCT images from unattenuated and scatter corrected PET images.
  • An example approach employs conditional generative adversarial networks (cGANs), e.g., as disclosed in L. Tao, J. Fisher, E. Anaya, X. Li, C.S. Levin.
  • Example input training data to each model can be composed of, for instance, energy discriminated sub-images derived from PET simulation data.
  • Radiotracer biodistribution and attenuation maps used as the basis of example simulation data can be derived, for instance, from clinically available PET/CT data, e.g., obtained from a nuclear medicine clinic. For instance, a number (e.g., 50) of patient data sets can be obtained for example GATE simulations. The number of energy windows, and therefore number of network input channels, and the energy window range per channel can be varied to optimize the correction. Corrected PET emission data, and CT can serve as the ground truth for the network training. The resulting PET and pseudoCT images from each condition can be compared to the PET simulation data reconstructed with only true coincidences and the respective attenuation maps using mean percentage error (MPE) with an aim of ⁇ 5% difference, for instance.
  • MPE mean percentage error
  • Random coincidences Using a coarse coincidence window (e.g., 30 ns), multiple sub-windows can be generated to estimate prompt and delayed data as disclosed by example in Groll, A., and C. S. Levin. "Analysis of Data Corrections for the First-Generation Radiofrequency-Penetrable PET Insert for Simultaneous PET/MR.” In 2019 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), pp. 1-3. IEEE. For instance, random estimates can be placed into a LOR histogram for subtraction from prompt data. Use of a dual window method for random coincidence rate estimation can produce LOR bins with few events and therefore high statistical noise.
  • a coarse coincidence window e.g. 30 ns
  • multiple sub-windows can be generated to estimate prompt and delayed data as disclosed by example in Groll, A., and C. S. Levin. "Analysis of Data Corrections for the First-Generation Radiofrequency-Penetrable PET Insert for Simultaneous PET/MR
  • the random coincidence data stored in LOR bins can undergo variance reduction using the “single-plane Casey” method extended to the “Fully-3D Casey” method, which offers an exact per LOR variance reduced random estimate after application, e.g., as disclosed in Badawi, Ramsey D., M. A. Lodge, and P. K. Marsden. "Algorithms for calculating detector efficiency normalization coefficients for true coincidences in 3D PET.” Physics in medicine & biology 43, no.1 (1998): 189, and just requires knowledge of the PET system geometry.
  • Positioning and recovery of multi-interaction and multi-photon events Owing to the higher probability for 511 keV photons to Compton scatter compared to photoabsorbtion in the crystals (including, for instance, BGO), there is a high chance of inter-crystal Compton scatter, yielding multi-interaction photon events.
  • the same sub-module or module at high activity, it is possible for the same sub-module or module to receive interactions from more than one incoming 511 keV photon due to the relatively large area detector modules that can be provided in embodiments.
  • Employing one- to-one crystal to SiPM coupling allows an example system to resolve and correctly position such multi-interaction and multi-photon events since the energy and positioning information is available for each hit crystal.
  • Timing calibrations Timing resolution of the acquired data can be optimized, for instance, with a convex optimization algorithm that corrects for the different delays that occur between detector pairs in the system.
  • An example algorithm can be similar to the method disclosed in P.D. Reynolds, P.D. Olcott, G. Pratx, F.W.Y. Lau, C.S. Levin. Convex Optimization of Coincidence Time Resolution for a High Resolution PET System. IEEE Trans. Med. Imag., vol 30(2), pp. 391-400, Sept 2010.
  • 3D image reconstruction for multi-bed acquisition can be similar to the method disclosed in P.D. Reynolds, P.D. Olcott, G. Pratx, F.W.Y. Lau, C.S. Levin. Convex Optimization of Coincidence Time Resolution for a High Resolution PET System. IEEE Trans. Med. Imag., vol 30(2), pp. 391-400, Sept 2010.
  • Image reconstruction can be performed using, for example, a 3D iterative list-mode MLEM (maximum- likelihood expectation-maximization) algorithm formulated to run on a graphics processing unit (GPU) such as disclosed in Cui, J.Y., et al., Fully 3D list-mode time- of-flight PET image reconstruction on GPUs using CUDA. Medical Physics, 2011. 38(12): p.6775-6786; and G. Pratx, P.D. Olcott, G. Chinn, and C.S. Levin. Fast, Accurate and Shift-Varying Line Projections for Iterative Reconstruction Using the GPU.
  • MLEM maximum- likelihood expectation-maximization
  • Each coincidence event can be processed with a non-Gaussian time-of-flight (TOF) kernel to address the tails shown by example in Fig. 8, such as disclosed in N. Efthimiou et al., "TOF-PET Image Reconstruction With Multiple Timing Kernels Applied on Cherenkov Radiation in BGO,” in IEEE Transactions on Radiation and Plasma Medical Sciences, vol.5, no.5, pp.703-711, Sept.2021. Normalization, attenuation and scatter, and random coincidence estimation can be configured to maximize reconstructed image SNR.
  • TOF time-of-flight
  • An example GPU can be installed in a PC for generating 3D PET images in approximately one minute (for example) following data acquisition.
  • Experimental data demonstrates that the example geometry seen in Fig. 1 can provide high quality and accurate tomographic images.
  • GATE e.g., Strulab, D., Santin, G., Lazaro, D., Breton, V., & Morel, C. (2003).
  • GATE geant4 application for tomographic emission: a PET/SPECT general-purpose simulation platform.
  • a PET reconstruction volume was used that included just a 16 cm axial FOV section from the flat (bed) portion of the detector system, matching an example 16 cm axial FOV of the arc section (an expanded reconstruction volume is described below).
  • TOF reconstruction was employed with 280 ps FWHM CTR in a GPU-based list mode 3D OSEM reconstruction, e.g., as disclosed in Cui, J.Y., et al., Fully 3D list-mode time-of-flight PET image reconstruction on GPUs using CUDA. Medical Physics, 2011.
  • GATE geant4 application for tomographic emission: a PET/SPECT general-purpose simulation platform. Nuclear Physics B - Proceedings Supplements, 125, 75–79.
  • Figure 7C shows a reconstructed slice through the system center transaxial plane for the phantom oriented with the 3 and 13 mm diameter spheres closest to the bed detector section. This 1 mm thick image slice contains ⁇ 600k counts and shows that excellent tomographic imaging results are possible with this non-standard PET system geometry.
  • the example image reconstruction data depicted in Figs. 7A-7C used coincidences between the 16 cm active axial FOV of the arc detector section and just 16 cm from the bed detector section, as indicated in the dark green transparent section depicted in Fig.13.
  • the 52 cm active axial FOV of the bed detector section can move with the MR system bed (the arc section can remain stationary with respect to the MR system), enabling a wider fan of coincidences (depicted in light green) in addition to the dark green section for the example three-bed position acquisition shown in Fig.13. This wider fan of coincidences can significantly improve sensitivity.
  • Body insert sensitivity By simulating the source configuration for measuring absolute sensitivity, such as disclosed in Grant, Alexander M., Timothy W. Deller, Mohammad Mehdi Khalighi, Sri Harsha Maramraju, Gaspar Delso, and Craig S. Levin. "NEMA NU 2-2012 performance studies for the SiPM-based ToF-PET component of the GE SIGNA PET/MR system.” Medical physics 43, no.5 (2016): 2334-2343, the result was estimated to be >3%, which is an improvement over that measured in commercial body PET/MR systems (See Table 1). This better sensitivity is due to the higher intrinsic detection efficiency of BGO vs.
  • This example setup employed non-optimized readout electronics configured to boost the detection SNR for extracting timing information from the prompt luminescence component, generally including ⁇ 20 photons from Cherenkov radiation, such as disclosed in Cates, JW; Levin, CS. Electronics method to advance the coincidence time resolution with bismuth germanate.
  • a housing structure e.g., a sturdy plastic skeleton structure, can be provided to house the example arc detector section and facilitate its insertion and removal as well as reliable alignment with the MR system, generally similar to the structures provided for the first and second generation head insert systems 200, 600.
  • An example housing structure can be rigidly fixed, for instance, to a portable, non-magnetic cart (on sturdy wheels) that will facilitate transport and insertion/removal of the example system into/from a 3T wide bore MR system.
  • An example cart can be wide open underneath and tall and wide enough, for instance to slide over the back of the MR system bed, which is the side for insertion/removal of the arc detector section.
  • the example housing structure can also hold in place a removable (e.g., truncated cylinder) annulus for sensitivity normalization, which can reside, for instance, just inside the Faraday cages.
  • the bed detector portion integrated with the bed RF receiver coil, can be manually placed on the bed.
  • Removable structures can be used to facilitate mechanical alignment of the bed detector section so that it will be parallel to and centered with respect to the MR bed, and therefore aligned with the arc detector portion.
  • a short scout scan can be performed, for instance, to confirm this alignment, which in embodiments can be designed to be within 2 mm; this scout scan can also be used to calibrate the positions of all crystal elements in both detector sections.
  • Weights of the arc and panel detector sections may be, as nonlimiting examples, approximately 35 kg (77 lbs.) and 30 kg (66 lbs.), respectively.
  • An example system insertion into or removal from the MR system can include, for instance, positioning the arc and bed detectors, making all power, signal, and cooling connections, and routing all cabling through a wall panel, for instance, that houses all other cablings going into the MR magnet room.
  • Example calibrations include temperature stabilization and gain correction, and DAQ system initialization.
  • An example system may be calibrated, for instance, once each time it is installed and powered on, due to thermal regulation and the ability to re-calibrate data post-acquisition.
  • PET insert and MR image registration The PET and MRI components of currently available PET/MR imaging systems are permanently integrated with a fixed system geometry. As a result, the images acquired from both modalities can be easily registered and fused.
  • PET inserts provided herein which may be used for any clinical MR system
  • registration between the PET insert and MRI data can be performed, e.g., at least once, for the specific PET insert and MR system combination provided.
  • a co-registration phantom can be provided including multiple compartments filled with water and FDG to be imaged by the MRI and PET insert systems, respectively.
  • the phantom can be imaged simultaneously by the MRI and PET insert, and the difference in orientation of the phantom between the images in each system of coordinates can be used to derive the required translational and rotational parameters for aligning both images.
  • the images can be processed using the calibrated translational and rotational parameters for registration and a final fused image can be presented.
  • Accuracy of co-registration of the PET and MR images may be configured to be (for instance) less than 3% error for all instances of co-registration. [0140] Performance of the PET insert outside & inside MR system.
  • the example PET detectors and full insert can be optimized for parameters such as but not limited to energy and coincidence time resolution, single and coincidence count rate, spatial resolution, photon sensitivity, image quality and quantification, noise equivalent count rate (NECR), and thermal stability both outside and inside a 3T MR system.
  • Example testing procedures are disclosed in Grant, Alexander M., Timothy W. Deller, Mohammad Mehdi Khalighi, Sri Harsha Maramraju, Gaspar Delso, and Craig S. Levin. "NEMA NU 2-2012 performance studies for the SiPM-based ToF- PET component of the GE SIGNA PET/MR system.” Medical physics 43, no. 5 (2016): 2334-2343; Chang, Chen-Ming, Brian J. Lee, Alexander M. Grant, Andrew N.
  • the built-in body coil can generate the RF transmit pulses (spin-echo, gradient-echo and EPI), using pulse sequence parameters described in Lee, Brian J., Alexander M. Grant, Chen-Ming Chang, Ronald D. Watkins, Gary H. Glover, and Craig S. Levin. "MR performance in the presence of a radio frequency-penetrable positron emission tomography (PET) insert for simultaneous PET/MRI.” IEEE Transactions on Medical Imaging 37, no. 9 (2016): 2060-2069. These different pulses can be alternately (2-minute period) turned on and off to assess their effect on the insert performance.
  • PET radio frequency-penetrable positron emission tomography
  • Testing can progress, for instance, from sub-modules in Faraday cages, then to modules, until the full system is tested.
  • Nonlimiting example parameters are shown in Table 1.
  • Faraday cages, materials, RF filters, power, cablings, etc. can be optimized such that, for instance, measurements outside the magnet are within a threshold, e.g., within 5%, of those inside.
  • MR performance can be characterized with and without an example PET + RF coil insert in place, e.g., by measuring parameters such as MR susceptibility ( B 0 map), receiver noise, RF transmissivity of insert, TG, B 1 map, and MR image SNR and homogeneity, for instance as disclosed in Lee et al., 2018; Dong et al., 2018; Dong et al., 2019; and Dong et al., 2020.
  • Faraday cages, materials, RF filters, power, cablings, etc. can be optimized such that measurements with the powered PET insert present inside the MR system are within a threshold, e.g., within 5%, of those without the insert present.
  • example PET insert geometry is the first disclosed solution for a removable PET insert for body PET/MR.
  • example PET inserts can employ an innovative long bed detector section that lies on the bed, underneath the patient, and moves with the bed. Since this bed section of the detector is closer to the patient and has a long axial extent, higher geometric detection efficiency and thus higher sensitivity is possible compared to the cylindrical geometry in the permanently integrated systems (which is not viable for a body-sized insert owing to the presence of the bed).
  • PET- compatible, multi-loop, phased array receiver coils e.g., one that rests on top of the patient, and the other underneath, can be integrated with the bed detector in example embodiments.
  • the arc detector section in an example ToF- PET insert detector can include a geometry with active axial FOV of, e.g., 16 cm and a flat portion with active axial FOV of, e.g., ⁇ 50 cm.
  • Such an example 3D image reconstruction approach can address LORs that fall within the 16 cm axial FOV (arc-to-arc or arc-to-bed section coincidences) as well as those ‘very oblique’ arc-to-bed section LORs that fall outside the arc section’s 16 cm axial FOV, and can add significant sensitivity.
  • the best-performing MR configuration has been to use the built- in body coil for RF transmission, and a multi-channel Rx- only coil; see J. H. Duyn, “The future of ultra-high field MRI and fMRI for study of the human brain,” 20 YEARS fMRI, vol. 62, no. 2, pp. 1241–1248, Aug. 2012.
  • RF-penetrable insert technology can enable this optimal MR configuration in an example body PET insert, which technology has been shown (e.g., see Lee, Brian J., Ronald D. Watkins, Keum Sil Lee, Chen-Ming Chang, and Craig S. Levin. "Performance evaluation of RF coils integrated with an RF-penetrable PET insert for simultaneous PET/MRI.” Magnetic resonance in medicine 81, no.2 (2019): 1434- 1446) to yield better MRI performance in a PET/MR study compared to the combined Tx/Rx birdcage approach that other insert designs employ. [0146] Additionally, data corrections are useful for quantitative accuracy of the PET data.
  • example data correction methods can employ a machine- learning-based approach that simultaneously corrects both photon attenuation and scatter using emission data alone.
  • Example Applications can be provided between an example TOF PET insert system and, for instance, a host 3T MRI system. Such mutual compatibility can be reflected in, for instance, MR image quality using the built-in body coil for RF transmission through the PET insert. However, in such environments, it is important to consider the effects of the MR system’s harsh electromagnetic environment on PET performance.
  • Diagnostic and theranostics applications of body PET/MR PET/MRI exhibits numerous complementary features compared to PET/CT.
  • MRI provides high soft tissue contrast for imaging regions such as the brain, head and neck, breast, liver and pelvis, facilitating significant utility of PET/MR studies of those regions, such as disclosed in Ole Martin, Benedikt M. Schaarschmidt, Julian Kirchner et al., PET/MRI Versus PET/CT for Whole-Body Staging: Results from a Single-Center Observational Study on 1,003 Sequential Examinations. J Nucl Med 2020; 61:1131–1136; and von Schulthess, G.K., et al., Clinical Positron Emission Tomography/Magnetic Resonance Imaging Applications. Seminars in Nuclear Medicine, 2013.43(1): p.3-10.
  • PET/MR does not introduce ionizing radiation
  • PET/MR enables a lower dose multimodality molecular imaging study compared to PET/CT
  • PET/CT such as disclosed in B. Huang, M. W.-M. Law, and P.-L. Khong, “Whole-Body PET/CT Scanning: Estimation of Radiation Dose and Cancer Risk,” Radiology, vol. 251, no.1, pp.166–174, Apr.2009, making PET/MRI a desired modality for those requiring repeat/recurring PET studies as well as pediatric patients, such as disclosed in J. F. Schfer, S. Gatidis, H. Schmidt, B. Gckel, I. Bezrukov, C. A. Pfannenberg, M.
  • PET/MRI Versus PET/CT for Whole-Body Staging Results from a Single-Center Observational Study on 1,003 Sequential Examinations. J Nucl Med 2020; 61:1131–1136; and von Schulthess, G.K., et al., Clinical Positron Emission Tomography/Magnetic Resonance Imaging Applications. Seminars in Nuclear Medicine, 2013.43(1): p.3-10.
  • a quickly emerging application very well-suited to body PET/MR is in the area of theranostics for prostate cancer, such as disclosed in Elisabeth O'Dwyer, Lisa Bodei, Michael J. Morris.
  • a targeted diagnostic agent for example, targeting prostate specific membrane antigen (PSMA) (e.g., 68 Ga-PSMA-11 or more recently 18 F-Piflufolastat) is paired with a therapeutic agent (e.g., 177 Lu-PSMA-617).
  • PSMA prostate specific membrane antigen
  • a therapeutic agent e.g., 177 Lu-PSMA-617.
  • the former confirms the agent adequately targets the disease, and a therapeutic dose of the latter irradiates the prostate cancer cells with short range beta emissions.
  • PET/MR thus far has had limited impact on the field, largely due to the high initial investment for purchasing the system + room renovations, plus additional service fees.
  • PET/MR is currently inaccessible to most patients in the world.
  • One reason for the high system cost is due to the fact that the PET insert design in commercially available systems blocks the RF transmission field from the MR system body coil, as explained above, so it has been necessary to re-engineer the RF transmitter to reside inside the PET ring, locking the two systems together.
  • the PET and MR systems come permanently integrated, and a new customer must purchase both systems to achieve combined PET/MR.
  • PET inserts As most medical centers of all sizes have MR systems, PET inserts have been proposed that can be installed into an MR bore to achieve PET/MR imaging at a lower cost.
  • these PET insert designs require the RF transmitter (Tx) coil to reside inside the PET ring, almost always in the form of a combined ‘bird-cage’ Tx/receiver (Rx) coil.
  • This combined birdcage Tx/Rx ‘transceiver’ coil configuration has worse MR imaging performance compared to using the MR system’s built-in- body Tx coil (e.g., see J. H.
  • a positron emission tomography (PET) apparatus for insertion into a magnetic resonance imaging (MRI) system comprising: an arc detector section comprising a plurality of first detector modules arranged in an arc for fixable insertion within a bore of the MRI system, each of the first detector modules comprising detectors configured to measure luminescent emissions in response to incoming annihilation photons; a bed detector section for disposing on a bed of the MRI system, the bed detector section being disposed underneath said arc detector section and axially movable with respect to said arc detector section, said bed detector section comprising a plurality of second detector modules arranged in a substantially flat surface, each of the second detector modules comprising detectors configured to measure lumi
  • MRI magnetic resonance imaging
  • the arc detector section and the bed detector section may be RF-penetrable.
  • the detectors in the first and second detector modules may be disposed within a Faraday cage.
  • said bed detector section and arc detector sections together may generally enclose in volume a truncated cylinder generally having in cross-section an arc section of a circle truncated by a chord.
  • the detectors may comprise scintillation crystals.
  • the detectors may comprise bismuth germinate (BGO), lutetium, or other appropriate crystal materials.
  • the detector readout electronics may be configured to process emitted Cherenkov radiation.
  • the system may be configured for processing the received signals, wherein the signals contain spatial, energy, and temporal information, and wherein the system is further configured to use time-of-flight (TOF) processing to enhance signal-to-noise ratio (SNR) of the reconstructed image.
  • TOF time-of-flight
  • a system for simultaneous magnetic resonance imaging (MRI) and positron emission tomography (PET comprising: a magnetic resonance imaging system having a bore and a bed; an arc detector section comprising a plurality of first detector modules arranged in an arc for fixable insertion within the bore of the MRI system, each of the first detector modules comprising detectors configured to measure luminescent emissions in response to incoming annihilation photons; a bed detector section for disposing on the bed of the MRI system, the bed detector section being disposed underneath said arc detector section and axially movable with respect to said arc detector section, said bed detector section comprising a plurality of second detector modules arranged in a substantially flat surface, each of the second detector modules comprising detectors configured for measuring luminescent emission in response to incoming annihilation photons, said first and second detector modules being coupled to readout electronics and a data acquisition system, said data acquisition system being configured for receiving and processing signals from the first and second detector
  • said bed detector section and arc detector sections together may generally enclose in volume a truncated cylinder generally having in cross-section an arc section of a circle truncated by a chord.
  • the arc detector section and the bed detector section may be RF-penetrable.
  • the system may further comprise: an RF receiver coil disposed between the arc section and the bed section.
  • said magnetic resonance imaging system may be configured for generating MRI images.
  • a method for PET imaging within a magnetic resonance imaging (MRI) bore of an MRI system comprising: positioning a bed detector section underneath an arc detector section that is fixed within the MRI bore, the bed detector section being disposed on a bed of the MRI system and axially movable with respect to said arc detector section, the bed detector section moving with a patient as the bed moves through the MRI bore; said arc detector section comprising a plurality of first detector modules arranged in an arc, each of the first detector modules comprising detectors configured for measuring luminescent emission in response to incoming annihilation photons emitted from the patient; said bed detector section comprising a plurality of second detector modules arranged in a substantially flat surface, each of the second detector modules comprising detectors configured for measuring luminescent emissions in response to incoming annihilation photons emitted from the patient; receiving signals generated from the first and second detector modules by a data acquisition system; and processing the received signals by a processing computer
  • said bed detector section and arc detector sections together may generally enclose in volume a truncated cylinder generally having in cross-section an arc section of a circle truncated by a chord to provide a PET imaging region.
  • the method may further comprise: repositioning the bed detector section underneath the arc detector section to provide a new PET imaging region; receiving additional signals generated from the first and second detector modules by the data acquisition system; and processing the received signals to further reconstruct an image or reconstruct an additional image.
  • said axially positioning a bed detector section may axially position a first portion of a body of a patient disposed on the bed to scan the first portion, and said repositioning the bed detector section underneath the arc detector section may axially position a second portion of a body of a patient disposed on the bed to scan the second portion.
  • the arc detector section and the bed detector section may be RF-penetrable.
  • the method may further comprise: processing signals from the magnetic resonance imaging system for simultaneous magnetic resonance imaging and PET imaging.

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Abstract

L'invention concerne un appareil PET destiné à être inséré dans un système d'imagerie par résonance magnétique (IRM). Une section de détecteur d'arc comprend une pluralité de premiers modules de détecteur agencés dans un arc pour une insertion pouvant être fixée dans un tunnel du système d'IRM. Une section de détecteur de lit destinée à être disposée sur le lit du système d'IRM est mobile axialement par rapport à la section de détecteur d'arc.
PCT/US2023/015586 2022-03-18 2023-03-18 Insert de tomographie par émission de positrons (pet) destiné à un système d'imagerie par résonance magnétique (irm) WO2023177915A1 (fr)

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Citations (5)

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US20170135656A1 (en) * 2011-04-22 2017-05-18 Washington University Insert device for enhancing pet and mri images
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US20210059629A1 (en) * 2019-08-26 2021-03-04 Siemens Medical Solutions Usa, Inc. Energy-Based Scatter Correction for PET Sinograms
US20220011451A1 (en) * 2018-10-31 2022-01-13 Universitat Politècnica De Valéncia Device for the Detection of Gamma Rays with Active Partitions

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US20170135656A1 (en) * 2011-04-22 2017-05-18 Washington University Insert device for enhancing pet and mri images
US20160063741A1 (en) * 2014-08-28 2016-03-03 Kabushiki Kaisha Toshiba Method and Apparatus for Estimating Scatter in a Positron Emission Tomography Scan at Multiple Bed Positions
US20170164915A1 (en) * 2015-08-05 2017-06-15 Shanghai United Imaging Healthcare Co., Ltd. A pet/mri insert system
US20220011451A1 (en) * 2018-10-31 2022-01-13 Universitat Politècnica De Valéncia Device for the Detection of Gamma Rays with Active Partitions
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