WO2014007817A1 - Method and apparatus for providing a cryogenic gas-cooled coil system for magnetic resonance imaging (mri) - Google Patents

Abstract

A cryogenic gas-cooled radiofrequency (RF) coil system is capable of providing a signal to noise ratio (SNR) gain from at least about 200% (two-fold) to about 300% (three-fold) over room temperature coils. The disclosed system cools RF coils to LN₂ temperature without freezing one or more target samples on the imaging surface thereby providing a minimal distance between the coils and the target samples. A phased array coil is housed in a casing that can be placed in close proximity to a target to be imaged. The LN₂ gas - cooled array disclosed herein minimizes freezing hazards for patients and operators and enables high-resolution clinical imaging of joints and other relatively hard to image target areas including those of small animals. The disclosed device is a portable diagnostic tool that is adaptable to various MRI devices, is easy to set up, easy to use, easy to disengage and/or dismantle, and which provides for increased patient safety.

Description

TITLE

Method and Apparatus for Providing a Cryogenic Gas-Cooled Coil System for Magnetic

Resonance Imaging (MRI) FIELD OF ART

The disclosed device relates generally to Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS) systems, and more specifically to a system of thermal cooling and insulation of RF coils to increase a signal-to-noise ratio (SNR) which thereby provides for improved magnetic resonance (MR) images, safer clinical use and ease of configurability, i.e. volume coils.

BACKGROUND

Magnetic Resonance systems are well known in the art and widely used in medicine to produce diagnostically important/useful images of the inside of a patient's body. MRI systems utilize principles of nuclear magnetic resonance (NMR) and typically include: 1) a large imaging magnet for generating a steady, strong magnetic field, Bo; 2) a gradient system of water cooled coils within the magnet and powerful, stable current amplifiers for generating gradient fields in the Bo in the X, Y, and Z directions for imaging; and 3) generally a resonant structure, such as a coil, that provides a radiofrequency (RF) energy for generating a Bi field orthogonal to Bo to excite the nuclei precessing in Bo and once the RF energy is applied the same coil detects the nuclear induction signal. The resonant structure can be a combined transmit/receive coil (Tx/Rx) or it can be split into dual structures of separate Tx and Rx coils.

In addition to these components, a computer system is typically used to coordinate signal generation, data acquisition and image formation and display. The computer system can be used to control all components of the imaging system. The operator of the imager gives input to the computer system through a control console. An imaging sequence is selected and customized from the console. The operator can see the images on a video display located on the console or can make hard copies of the images on a film printer. The computer system is linked to means for storing image data, files and programs and communicates with other system control means through high speed serial or system bus links.

In operation, a patient is positioned within a magnet by a computer-controlled patient table. The hydrogen nuclei spins become temporarily more polarized. This magnetized state is achieved when the preponderance of the hydrogen nuclei in the patient align with the magnetic field according to a Boltzmann distribution. When polarized, the hydrogen atoms in a patient's body respond to the application of RF "energy". Spatial information is provided by the application of weaker gradients provided by non-RF-gradient "coils" or in the case of SENSE, SMASH or GRAPPA -like acquisitions by use of a combination of RF coils and gradients within the magnet system. These, in turn, cause small variations in Larmor frequencies as a function of their location in the body which can be detected as small variations in the received RF signal most often using a "spin echo" acquisition. This phenomenon only occurs at the Larmor frequency corresponding to the specific strength of the magnetic field. The spin echo signal is composed of multiple resonance characteristics which reflect different positions along the superimposed magnetic field gradient. The signal is first received as an envelope of many frequencies in the spin echo; this is then separated and transformed into its resonance components by a mathematical algorithm technique called a "Fourier Transform". The magnitude of the signal at each resonant component is complexly proportional to the hydrogen density and the longitudinal (Tl) and transverse spin magnetization (T2 and T2*) detected, e.g., dependent on the tissue type at that location which allows for a grayscale MR image to be constructed. Thus, spatial and tissue contrast information in MRI is contained in the magnetic resonance characteristic of the signal.

The SNR of a tissue in an image is the ratio of the average signal for the tissue to the standard deviation of the noise. Poor SNR can result in the requirement for long scan times to capture spectroscopic data or to build up MRI images. Thus, the RF coil is recognized as being of critical importance for obtaining maximum SNR in MRI. A RF coil typically achieves a higher SNR when it is closer to the specific part of the patient being imaged, which is often termed "the filling factor" of the coil. Due to the "near field" nature of the MRI and NMR signal detection, the receiving sensitivity of the RF receiver coil decreases with increasing distance from the coil wires.

Noise in the MR image is caused by the thermally driven Brownian motion of atoms and molecules within the body's conducting tissue and within the receiving coil itself. A ratio higher than 1: 1 indicates more signal than noise.

The basic formulae for noise temperature, noise figure, and noise factor are:

Noise temperature (NT) = 290 * (10^Λ (Noise Figure/10)-1) K Equation 1

Noise Figure (NF) = 10 * log₁₀ (Noise factor) dB Equation 2 TABLE 1 which is set forth below can be useful for converting Noise Figure (NF) to Noise

Temperature (NT).

To be diagnostically useful, a SNR of at least 2 (twice as much signal as noise) is considered by many to be required. No absolute minimum for SNR has been set although receiver operator characteristic (ROC) curves would support higher SNR as being diagnostically important. In addition, ROC curves demonstrate that higher SNR yields fewer false positives and negatives and better lesion detection. See e.g., Yip et al, ROC Curve Analysis of Lesion Detectability on Phantoms: Comparison of Digital Spot Mammography with Conventional Spot Mammography, Brit. J. of Radiology. 74: 621-628 (2001). For many reasons it is complicated to set a minimum SNR in MRI.

TABLE 1

In volume coils, noise can come from any source including the anatomic region that such a larger coil covers. Because clinical MRI quality depends crucially on the SNR available from the receiving coil, there have been many attempts to increase SNR to generate a less noisy image. Improvements include surface coils which optimize SNR over a small superficial localized volume of interest followed by innovations in RF coil design which improved sensitivity. Though arrays do not improve on image SNR per se, some solutions sought to use several single-loop RF coil elements arranged in a manner similar to a radar phased array which are capable of acquiring multiple channels of data in parallel. Others included circularly polarized (CP) or quadrature coils and highly homogenous RF field CP "birdcage" designs. Still other improvements claim to cryogenically cool the radiation detector itself. Other methods use microfluidic channels through which a cryogenic fluid is pumped to achieve localized cooling. And still others have submersed copper coils directly into liquid nitrogen (LN₂). Methods which attempt to overcome the alleged shortcomings of cryogenically-cooled copper coils propose the use of high-critical temperature superconducting (HTS) materials. In addition, within limits, the SNR can be improved by increasing the applied, static strength of the magnetic field used by virtue of the Boltzmann distribution and its increase as the Bo field increases.

Despite the improvements made, the prior art has its disadvantages. Many of the devices are cumbersome and hard to use while others subject the patient to hazardous conditions. None of these previous attempts have been able to increase SNR and in combination provide for a portable diagnostic tool that is adaptable to various conventional MRI scanners, is easy to set up, easy to use, easy to disengage and/or dismantle, and which provides for necessary patient safety. The disclosed device is a diagnostic apparatus that houses the coil elements in an insulated, coolable environment. Not only does the device improve the overall design of the RF receiving coil, it provides a reliable method of preventing injury to a patient and/or an operator while providing high spatial resolution/SNR. It has been demonstrated that the quality of the images produceable by the disclosed device can be superior to prior images by at least two times. In addition, the disclosed device provides for an instrument that can be placed in very close proximity to a target area being imaged without causing freezing burns to the target.

While it is apparent that a higher SNR can result in a less noisy, greater resolution and/or higher contrast image, it should be also be recognized that a higher SNR can result in faster imaging times which can decrease overall scan time, increase patient throughput and increased patient compliance via faster scanning. This reduces per patient test/exam costs and can improve patient safety. By providing a decrease in overall scan time, the disclosed device enables for expansion into the area of imaging pediatric patients. In addition, an optimized MRI processing time has many other advantages and can translate into reduced equipment cost, reduced operational costs, increased utilization of available MRI equipment, shorter patient queues, improved patient comfort and confidence, etc. as suggested by the simple illustration set forth above.

SNR increases by the Fellgett's Theorem (also called the Fellgett Advantage or the multiplex advantage) which states that SNR increases as the square root of the number of acquisitions. So to improve a room temperature coil's SNR by a factor of 2 one would have to do 4 times as many "averages" or signal acquisitions [4x1 = 4 and 4 = 2]. If a cooled coil has an inherent increase of SNR of 2, then it has the same SNR for one signal acquisition as the room temperature coil does for 4. Hence, there is a gain in imaging time (shortening) of three -fold or for the same time twice the voxel resolution compared to a 'room' or ambient temperature coil, which is significant for lesion detection, biomarker development and the use of MRI in clinical applications.

As stated above, the disclosed device is easy to set up, easy to use, portable and can be easily adapted to any number of known MRI devices, e.g., GE, Siemens, Philips, etc. It has been demonstrated that the start-up to imaging step with the disclosed device can take as few as 15 minutes. In addition, safety is embodied in the method of insulation which enables an operator's to easily position (and re-position) the coils of the disclosed device as needed during screening. The device can be retrofitted to be used with existing MRIs; thus it can improve the image quality significantly in the installed base of MRIs - improving performance without any need to swap out a perfectly functional and useful MRI unit. SUMMARY OF THE DISCLOSURE

The disclosed system cools RF coils to near liquid nitrogen (LN₂) temperature (typically about 105° K to about 110° K as measured in the coil) without freezing the imaging surface of a target sample thereby providing an opportunity to minimize the distance between the coils and a target sample. A phased array coil is housed in a casing that can be placed in very close proximity to a target to be imaged. The disclosed device minimizes the freezing risk to human and/or animal patients as well as to MRI operators while enabling high-resolution clinical imaging of joints, extremities and other relatively hard to image target areas.

The disclosed device is a portable diagnostic tool that is adaptable to various conventional

MRI devices, easy to set up, easy to use, easy to disengage and/or dismantle, and which provides for increased patient safety. The casing is connectable to a gas flow control device and a control box by means of a set of umbilical hoses. The gas flow device is mountable to a dewar housing a cryogenic liquid agent that, when boiled, enters the casing where it may cool the coil. An internal vacuum is used to further insulate the coil to promote cooling and to minimize freezing burns to patients (human/animal) or MRI operators. Not only can the control box be used to initiate boiling of the liquid agent, it can be used to monitor temperatures at any point in the system by means of one or more sensors. In addition, the control box could convert coil temperatures to SNR improvement values up to one or more programmed maximum values if desired.

An aspect of the disclosed device is to provide an apparatus which by its construction increases the SNR of image signals received by RF coils.

Another aspect of the disclosed device is to effectively cool the device' s RF coils to increase the SNR (reduce noise) over that detected by conventionally constructed coils, thereby providing improved magnetic resonance images.

Another aspect of the disclosed device is to effectively cool the device' s RF coils to increase the SNR (reduce noise in the coil) while simultaneously providing effective insulation of the cooling gas so as to provide safety to the patient.

Another aspect of the disclosed device is to provide an RF coil that can achieve a higher receiving sensitivity than a comparable room temperature operating coil by virtue of its being cooled to approximately 120°K.

Another aspect of the disclosed device is to provide an assembly for a cryogenically coolable phased array coil which allows for increased field of view coverage at a higher SNR.

Another aspect of the disclosed device is to provide a cryogenically coolable phased array coil that is positionable at a minimal distance from the part of the patient being imaged.

Another aspect of the disclosed device is to provide a method of localized cooling which affects only the coil regions but not the target sample. Another aspect of the disclosed device utilizes vacuum technology to provide increased patient and technician safety.

Another aspect of the disclosed device is to cool the device's coils by controlling a cryogenic gas flow in communication with one or more coil arrays.

Another aspect of the disclosed device is to provide for more efficient usage of the cryogenic fluid by means of effective system insulation.

Another aspect is to provide a means for monitor internal temperatures the housing of one or more coil arrays.

Another aspect of the disclosed device is to provide a device that is easy to set up, easy to use, easy to dismantle, and easy to remove from the magnet.

Other aspects of the disclosed device are its portability and adaptability to currently available MRI machines.

Another aspect of the disclosed device is to provide a system that can produce images having a higher resolution than prior art images.

Another aspect of the disclosed device is to provide a system that can produce images with similar resolution of prior art images but in a shorter period of time.

Another aspect of the disclosed device is to safely vent a gas cooling agent to atmosphere after the cooling process.

Yet another aspect of the disclosed device is to increase the resolution of MRI imaging when desirable thereby improving diagnostic accuracy and diagnostic utility.

Another aspect of the present invention is to improve patient compliance, that is, reduce the time a patient must be still while undergoing the MRI screening process.

Another aspect of the present invention is to increase patient throughput and/or reduce patient test costs.

These and other advantages of the disclosed device will appear from the following description and/or appended claims, reference being made to the accompanying drawings that form a part of this specification wherein like reference characters designate corresponding parts in the several views. BRIEF DES CRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of one embodiment of the disclosed device in an MRI lab environment. FIG. 2 is a perspective view of one embodiment of the SNR enhancement apparatus disclosed herein.

FIG. 2A is a cross section of a hose assembly shown in FIG. 2, taken along line 2A-2A. FIG. 3 is a top view of one embodiment of a paddle subassembly, shown in an opened position. FIG. 4 is an exploded view of the paddle subassembly embodiment shown in FIG. 3. FIG. 5 is a cross section of the coil subassembly embodiment shown in FIG. 3, taken along line 5-5.

FIG. 6 is a top view of one embodiment of a gas flow assembly with the cover removed.

FIG. 7 is a flow chart depicting a typical system start-up procedure.

FIG. 8 is a flow chart depicting a typical system shut down procedure.

FIGS. 9, 10, 11 are schematics of the temperature control circuitry utilized in one embodiment of the disclosed device.

FIG. 12 is an exploded view of another embodiment of the SNR enhancement apparatus disclosed herein, the embodiment shown from a top perspective.

FIG. 13 is a bottom perspective of the embodiment shown in FIG. 12.

FIGS. 14-17 depict the coil subassembly of the embodiment shown in FIGS. 12-13.

FIG. 18 shows the embodiment of FIGS. 12 and 13 in an assembled mode.

FIG. 19 depicts a control box capable of monitoring and controlling the system disclosed herein.

FIG. 20 is a cross section of the coil subassembly embodiment shown in FIG. 18, taken along line 20-20.

FIG. 21 depicts another embodiment of a coil subassembly.

Before explaining the disclosed embodiments of the disclosed device in detail, it is to be understood that the device is not limited in its application to the details of the particular arrangements shown, since the device is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.

DESCRIPTION OF THE DISCLOSED EMBODIMENTS

The following description is provided to enable any person skilled in the art to make and use the disclosed apparatus. Various modifications, however, will remain readily apparent to those skilled in the art, since the generic principles of the present apparatus have been defined herein specifically to provide for a method and apparatus for providing a cryogenically cooled RF phased array coil system to enhance MRI image resolution.

The cryogenically cooled RF coil structure of the disclosed device comprises a sealed assembly constructed of non-conducting materials and which is adapted for containing a cooling agent and at least one coil disposed therein. When the sealed assembly and the coil disposed therein are placed on an object to be imaged, cooling affects primarily the coil inside of the sealed assembly and not the surface of the object being imaged thereby maintaining patient and/or operator safety and comfort. Thus, the disclosed assembly enables the coil to be placed as close to the object to be imaged as possible. The insulation allows the coils to be kept cold and the patient or subject to remain warm. Not only does the cooling step improve the SNR which results in improved MRI images, the disclosed device can decrease imaging times and increase patient throughput and machine utilization. In addition, an absence of "icing" on the system's component can be used as an indicator so show that the disclosed device can be positioned in very close proximity to a target area being imaged without causing freezing burns in the area being imaged.

It is known that cryogenic cooling of RF coils reduces the coil noise by lowering the coil resistance and the coil temperature. Applicants have utilized mathematical relationships for predicting SNR increase and corroborated these predictions in the laboratory, the results of a few trials being described herein. Assume that current at 63 MHz is essentially direct current (DC). The calculated results show that the SNR can be expected to increase about 2 times to about 3.35 times the value of the established SNR. To illustrate:

At room temperature (20° C) as measured in the coil, copper has a resistivity (p₀) of 1.712 x 10^~8 Ω m, a temperature coefficient (a) of 3.93 x 10^~3/ K, and an electrical conductivity of 5.69 * 10⁸ S/m (annealed).

Δρ = α ΔΤ po Equation 3

If the coils are cooled from room temperature (T_rt) where T_rt = 298° K or 20° C to a temperature (T_lt) of 120° K, this represents a temperature difference (ΔΤ) of 178° K. Applying these values to Equation 3

Δρ = (3.93 x 10^"3/ K) * 178 K * 1.712 x 10^"8 Ω m

= 1.197 x 10^"8 D m

Resistivity p at 120 K, or p₂

= (1.712 x 10^"8 D m) - (1.197 x 10^"8 Ω m)

= 0.515 x 10^"8 D m

The resistivity ratio, RR, can be calculated to be

RR = 1.712 x 10^"8 Ω ιη ÷ 0.515 x 10^"8 D m

= 3.3

This value represents a theoretical 3.3-fold SNR increase over SNR values achieved by room temperature coils.

Calculating the anticipated SNR increase using the known temperature coefficient (a) of 3.93 x 10^"3/K and from a conductivity standpoint provides for an expected SNR gain of 3.32 when the coils are cooled to LN₂ temperature. However, since the biological 'system' being imaged has noise - Brownian motion - of its own at magnetic fields commonly used for MRI, the tissue imaged produces noise of its own and the theoretical SNR improvement is not realized.

Depending on the coil type, it is anticipated that the SNR increases can range from just below 2.0 to about 3.35 when the coils are cooled to LN₂ temperature. During various imaging tests conducted (phantom and animal) using the embodiments disclosed herein, the measured SNR gains of about 250% are in agreement with the calculated SNR gains discussed above. An experimental using a frequency of 63.5 MHz and a 1.5 Tesla Bo yielded a two- to three -fold increase in SNR as a result of cooling the coil as compared to not cooling the coil. In another case, testing at 63.5 MHz and 1.5 Tesla resulted in a five-fold increase. Other tests run at 2.9 Tesla (123.5MHz for protons) using 2.0 cm diameter linear coils have also shown a consistent 2.5-fold increase in SNR. In these studies, each of the devices was placed in a lab-built vacuum dewar which was placed directly in contact with the target area to be imaged. The result was a cooled linear coil displaced by about 2 millimeters from the actual object imaged.

Shown in FIG. 1 is an illustration of the disclosed device in an MRI lab environment. A patient 120 is situated on a movable slab of MRI machine (magnet) 110. In this example, it will be assumed that the patient's knee will be imaged. A paddle subassembly 201, 202 is placed on either side of the patient's knee (or other extremity, e.g., arm, elbow, wrist, breast, foot, ankle). Each paddle subassembly 201, 202 is connectable to an end of a vacuum line 145. An opposite end of vacuum line 145 is connectable to a vacuum control pump 140 (oiless for noise reduction purposes or equivalents). Pump 140 provides a vacuum insulation between RF coils housed in paddle subassembly 201, 202 and the exterior casing of each paddle subassembly 201, 202, the configuration of which are described below in FIGS. 3, 4 and 5. A continuous connection between vacuum control pump 140 and vacuum line 145 can be maintained if desired. In this way, a vacuum may be able to be maintained over, for example, several days of usage depending on the thermal cycles and total time of usage. The length of time that the vacuum can be maintained is an engineering and/or design matter and will depend on the particular application. In some cases, a constant vacuum is desirable. Vacuum pump 140 and vacuum line 145 are shown in dotted line format to indicate that each can be removed from the MRI environment as desired. One having skill in the vacuum pump art would recognize that a high quality vacuum seal could allow for the drawing of a vacuum to be done along with the periodic removal of the connection lines whereas a lower quality vacuum seal would more than likely require a constant connection to a vacuum pump.

Each paddle subassembly 201, 202 is also connected to an end of a gas hose assembly or "umbilical" 300, 301. Each hose assembly 300, 301 is connectable to a gas flow assembly 5000, which is also known as a heater block or "stinger". Each hose assembly 300, 301 serves as a sheath for an internal gas output hose and an internal gas return hose. The "A" suffix represents flow from gas flow assembly 5000 toward paddle subassembly 201, 202, while the "B" suffix represents a return flow from paddle subassembly 201, 202 to gas flow assembly 5000. Thus, hose 300A extends through the interior of hose assembly 300 and serves to deliver gas to paddle subassembly 201, 202. Hose 300B also extends through the interior of hose assembly 300 and serves to return gas from paddle subassembly 201, 202. Similarly, hose 301A extends through the interior of hose assembly 301 to deliver gas to paddle subassembly 201, 202. Hose 301B also extends through the interior of hose assembly 301 and serves to return gas from paddle subassembly 201, 202. FIGS. 3, 4 and the accompanying text provide additional details. In the embodiment shown, each hose assembly comprises two internal hoses. It should be recognized that each hose assembly could comprise a single hose that can be used for delivery and return purposes.

Gas flow assembly 5000 is mountable into dewar vessel (vacuum canister) 130 used to store a gas cooling agent such as LN₂. A dewar is commonly constructed of aluminum, the inner and outer vessels forming the dewar being cylindrical and the evacuated space between the walls being a practical nonconductor of heat. Stainless steel dewars are also known, wherein one or more of surfaces of the inner and outer vessels can be highly polished to reduce the emissivity of these surfaces. The inner vessel is usually suspended from the top of the outer vessel by a short, thick, fiberglass/epoxy tube which can be cemented at its junctions with the inner and outer vessels by means of epoxy resin. The tube provides thermal isolation between the inner and outer vessels while permitting liquid cooling agent to be poured into the inner vessel through a hole in the top of the outer vessel. To further reduce transfer of heat to the inner vessel, the inner vessel can be wrapped in aluminized polyester such as Mylar™ or other types of superinsulation. Here, the dewar comprises fiber-reinforced resin. A gas receiver tube 500 and a liquid heating rod 540 are configured so as to extend down into the inner vessel of dewar 130 (refer to FIG. 2).

One problem which is commonly encountered in standard dewar-type devices is the formation of ice crystals on the inner surfaces of the vacuum chamber or on the exterior of the hoses and/or couplings. In one test conducted under vacuum, the LN₂ was heated to its boiling point (about >77°K or about -196°C). As the LN₂ boiled off (with the gaseous nitrogen being discharged to the ambient atmosphere), the level of the LN₂ in the dewar dropped. A small amount of ice crystal formation was observed utilizing the apparatus disclosed herein, indicating that the coil assembly may not have been adequately sealed or insulated. In a later study, the system was placed under a continuous vacuum. The study began with a full 30 liter dewar of LN₂ which typically lasts for 36 to 48 hours. After about 5 hours, only about 4-5 liters of the LN₂ gas cooling agent had been used, generating a boil-off to cool the coil elements and maintain them at 115°K. In this later study no ice crystal formation was observed on the exterior of the hoses and/or couplings running to the coil assembly which appears to demonstrate the effectiveness of the insulation of the coil assembly.

Another problem is that only flat coils can be used in conjunction with a dewar and these cannot conform to a 3-dimensional object, such as a body part. In addition, there are difficulties associated with getting optimal filling factor of the object being imaged - namely getting the coil close enough to the sample without cooling or freezing the sample. The necessity of using an insulating material makes the distance from the coil in a dewar to the object imaged farther than is necessary to obtain the optimal improvement in SNR. In operation, gas flows from dewar 130 through hoses 300A, 301A and into each paddle subassembly 201, 202. Control box 600 is connectable to gas flow assembly 5000 to control the gas boiling process and to monitor internal temperatures within each paddle subassembly 201, 202. It should be noted that other cryogenic cooling agents can be used. Nitrogen can be a natural 5 choice due to its abundance in the earth's atmosphere (about 78% by volume). If used, nitrogen can be safely exhausted into the atmosphere after it flows through paddle subassemblies 201, 202 and exits gas flow assembly 5000. Safety in terms of temperature and oxygen levels in the patient/magnet room can be monitored via alarms. Most MRI suites have ample and rapid air exchanges to ensure adequate ambient oxygen levels so N₂ gas from boil off can be vented in situ

10 or closer to the magnet. Thus, gas delivered via return hose 300B, 301B is vented by means of exhaust exit ports 521B, 520B, respectively. However, if needed the N₂ gas from boil-off can be vented outside the MRI suite as well. In the many tests conducted, no "quenching" or rapid boil off of LN₂ has ever been observed in the embodiment disclosed herein. In addition, none are anticipated since no appreciable RF energy is applied to the coils or to the LN₂ gas source. In this

15 embodiment the cold LN₂ boil-off is well isolated from the coils.

As will be discussed below, each paddle subassembly 201, 202 houses two RF coils 224A, 224B. The RF coils comprise high purity copper but any suitable electrically conductive material of construction may be used depending on the application. Output signal cables 230A, 230B, 230C, 230D are connectable to the RF coils in a respective paddle subassembly 201, 202. Cables

20 230A, 230B, 230C, 230D are also connectable to processor 150 by means of receiver signal cable

230. Processor 150 can be used to coordinate signal generation, data acquisition and image formation and display and acts on the emitted signals via Fourier transform to form images of the patient's image target (a knee, in this example).

With the disclosed device, the RF coils within each paddle subassembly 201, 202 can be

25 cooled to extremely low temperatures, e.g., 110°K, which can result in a large SNR gain as

compared to similarly configured room temperature copper coils. It has been shown that SNR can be increased by a factor of three in imaging time where 4 NEX are needed. Such an improvement offers double or more resolution in the same amount of time as done by prior art coils operating at room temperature. The improved SNR can also be used to increase processing speed by at least a

30 factor of two or more (in lieu of improving resolution).

It should be noted that prior to starting the MRI process a cool-down period of about 15 to about 25 minutes may be required and sufficient time is recommended so that paddle

subassemblies 201, 202 can be cooled from room temperature (about 298K) (variable with the insulation and the size of the coil) to an operating temperature of about 110°K to about 115°K. 35 Also, prior to cool-down the vacuum pump(s) should be activated if the unit has not been used for an extended period of time depending on the quality of the vacuum desired. FIG. 2 is a perspective view of one embodiment of the enhancement apparatus disclosed herein. In embodiment 1000, gas flow assembly 5000 is shown in an inverted position. Gas flow assembly handle 590 which is used to position the device in a dewar is shown at the bottom. Gas flow assembly 5000 comprises gas receiver tube 500, and liquid heating rod 540 having a heating element 542. Heating element 542 may be attached to heat control cable 546 and series heating diode 544 which connect to temperature control box 600 via gas flow control cable 580. Control box 600 controls heating element 540 and regulates boiling of the liquid within dewar 130.

Control box 600 displays coil temperatures within each paddle subassembly 201, 202 on LED readouts 601, 602.

Gas (under slight pressure) from gas receiver tube 500 flows through hose connectors

512A, 512B and gas output hoses 300A, 301A to RF coils 224A, 224B in paddle subassembly 201, 202. Gas returns through hose assemblies 300, 301 via return hoses 300B, 301B and hose connectors 524A, 524B. Returned gas flows through gas flow assembly 5000 and out into the atmosphere via exhaust exit ports 520B, 521B. Sensor cable plugs 560A, 560B facilitate the connection of input cables 564A, 564B to a circuit within gas flow assembly 5000 Sensor input cables 564A, 564B are connectable to temperature control box 600 via gas flow control cable plug 570 (see also FIG. 6) and gas flow control cable 580.

Hose assemblies 300, 301 used herein can be typical rubber hydraulic hoses, however, one having skill in the art would recognize that other suitable constructions should be selected based on pressure, temperature, flexibility and fluid compatibility of the specific application. For example, it is contemplated that gas flow hoses 300A, 301 A, 300B, 301B could be constructed with Teflon^® but other materials capable of maintaining flexibility under cold temperature conditions could be used. In addition, it will be recognized that the connection of the hose to the various fittings should be adequately engineered to avoid hose failures at the connection points.

Paddle assembly 2000 comprises paddle subassemblies 201, 202 and hinge assembly 250.

See also FIGS. 3, 4 and the accompanying text. Casing 200 of paddle subassemblies 201, 202 can be constructed of a polymeric material. Vacuum hose connectors 202A, 202B allow a vacuum hose (not shown) to be connected to each paddle subassembly 201, 202. Vacuum control valves 204A, 204B can be used to control external vacuum lines (not shown). A sealant can be used to ensure vacuum reliability.

FIG. 2A is a cross section of the hose assembly 301 shown in FIG. 2. Directional arrows 310, 320 indicate the direction of gas flowing to and from paddle subassembly 202. For example, arrow 310 indicates gas flow through gas output hose 301A towards paddle subassembly 202. Opposing arrow 320 indicates gas flow through gas return hose 301B away from paddle subassembly 202. Hose assemblies 300, 301 are also capable of carrying temperature sensor input cables 564A, 564B. Temperature sensor input cable 564A is positioned adjacent gas output hose 301A. It has been demonstrated that the coils of the disclosed device can be cooled to temperatures lower than 113K. At such temperature, it is not inconceivable that a cooled coil or any of the adjoining hoses, when placed in direct or indirect contact with a patient, could cause injury to the patient. Thus, insulation 324 can provide for an effective safety feature for the disclosed device. It helps to ensure that the gas flowing to and from paddle subassembly 202 can be properly isolated so as to minimize potential injury in the event of skin contact. In one embodiment, insulating foam was used. An air pump could be also used to provide warmed air to external tubing/ surfaces to control blackbody heat transfer which causes the cooling and frosting of surfaces that might make contact with a patient or technician. In addition, the air could be dried to ensure that no condensation would be formed in its feed lines.

Additional safety systems such as the installation of temperature gauges, flow stoppage devices/valves, overpressure sensors and/or 0₂ monitors can also help minimize the possibility of system defects, such as leaks. For example, vacuum and/or pressure actuated valves could be placed in parallel with each gas output hose 300A, 301A to stop the flow of cold gas in the event of an unexpected temperature rise or an excess of gas flow. Other redundant systems could be implemented. In addition, sealants could be used to ensure vacuum reliability. One having skill in the art of vacuum insulation would recognize that a variety of methods of providing patient protection and proper system operation would fall into the scope of the disclosure.

FIG. 3 is a top view of paddle subassembly 201 with its covers removed to expose the internal components housed in casing 200. Hose assembly 300 comprises input hose 300A and return hose 300B. In this embodiment, the hose is one and the same and numbered differently to distinguish its respective gas flow to and from paddle subassembly 201. Other configurations are possible and would depend on the desired application. A temperature sensing circuit comprising resistor 236, thermistor 238 and temperature sensor 240 can be attached to temperature sensor cable 564A. RF coils 224A and 224B form a criss-cross pattern in coil subassembly 220 adjacent gas input hose 300A and gas return hose 300B. In one embodiment, gas input hose 300A and gas return hose 300B are corrugated tubing. Coil subassembly 220 can also be referred to as a thermal transfer block. The RF coils comprise high purity copper but any suitable electrically conductive pr superconductive material of construction may be used depending on the application.

As can be seen more clearly illustrated in FIG. 5, coil subassembly 220 comprises preformed channels that serve to receive the RF coils and gas hoses in the device's paddle subassembly. Coil subassembly 220 comprises routing channels 228A, 228B, 228C, 228D for receiving and containment of a portion of RF coils 224A, 224B. Routing channels 226A, 226B, 226 C, 226D receive and contain a portion of gas input hose 300A and gas return hose 300B. RF coils 224A, 224B and gas hose 300A are affixed within the delineated channels of coil subassembly 220 by epoxy (or other adhesives or bonding agents). Coil subassembly 220 can comprise a clam-shell arrangement and may be assembled so as to be vacuum-tight. Epoxy is used herein as an example and not a limitation. One having skill in the art would recognize that other mounting and fixing methods and materials are possible and would depend on the desired application. Flexible copper shims may be used to prevent failures due to cryogenic fatigue.

It is well known that variations in the size and tissue composition of the anatomy placed in 5 an imaging coil will affect the amount of RF energy absorbed by the imaged anatomy as well as the amount of signal detected from the imaged anatomy. For these reasons the RF coil should be tuned based on the composition of the anatomy or changes in the material in the coil. Tuning is the process of adjusting the transmitter and receiver circuitry so that it provides optimal signal performance at the Larmor (resonant) frequency. Tuning the probe usually entails 10 adjusting the matching capacitor and the tuning capacitor on the RF probe. A properly tuned scanner will produce images with a higher SNR, and therefore improved diagnostic versatility. Thus, for human tissue, the coils would be tuned to the Larmor frequency for hydrogen.

RF coil 224A comprises tuning capacitors 222A, 222B and tuning resistor 223A; RF coil 15 224B comprises tuning capacitors 222C, 222D and tuning resistor 223B. The tuning capacitor changes the resonance frequency of the RF coil. Thus, RF coils 224A, 224B should be tuned to the Larmor frequency of the hydrogen nucleus. The frequency signals are transmitted via output coil receiver signal lines 230A, 230B that are sent to an imaging computer.

RF coils 224A, 224B act as two small phased arrays which overlap for optimum 20 decoupling. Phased array coils are multiple small coils arranged to cover a specific anatomic region so as to obtain high-resolution, high-SNR images of a larger volume and provide. In addition, potential benefits of phased array designs include faster imaging via methods such as generalized auto-calibrating partially parallel acquisitions (GRAPPA) technique and sensitivity encoding (SENSE). In order to obtain optimal SNR from a phased array coil, the design should ensure that

25 the noise from coil to coil is largely uncorrected. Methods for determining the optimal

arrangement and number of coils in a phased array are still a subject of research. However, it is contemplated that the design of the disclosed coil, namely, the shape and the arrangement of the coil elements, provide a desirable electromagnetic interaction such that the noise is largely uncorrected. Other configurations such as a Helmholtz coil pair can be used but can be less

30 desirable.

Although a two-coil array in each paddle in shown, other phased array geometries are also possible. For example, it is contemplated that as few as one or as many as four coils (quadrature) in each paddle can be used. It is contemplated that each paddle contains a mirror image array of the other paddle. Arrangements are being made for an embodiment having up to eight coils. It 35 should also be noted that although the coils shown are transmit and receive coils, one having skill in the art would recognize that other coil configurations and arrangements could be used in the transmitting and receiving of information. As shown herein, a dual paddle system has been disclosed. However, it is possible that any number of paddles could be used, the arrangement dependent on the specific application.

A vacuum line 145 (see FIG. 1) is connectable to vacuum connector 202A and allows a vacuum to be pulled from inner coil cavity 206. Vacuum control valve 204A controls the vacuum (on/off) into the paddle subassembly 201. When the vacuum unit is "on", the vacuum within inner coil cavity 206 insulates the inner environment of coil cavity 206 from the external surfaces of paddle subassembly 201 so that gas passing through the coil cavity 206 to provide localized cooling RF coils 224A, 224B will not become a hazard to a patient or a technician. The insulation provided by the vacuum is a safety feature of the disclosed device. It helps to ensure that the gas flowing to and from paddle subassembly 201 can be properly isolated so as to minimize potential injury in the event of skin contact. It should be noted that paddle subassembly 201 and its components are described herein by way of example and not of limitation. For example, coil cavity 206 could comprise a thin coat of insulation. In addition, a thin coat of insulation could be applied to the external surfaces of each paddle subassembly. Numerous modifications and variations are possible and are dependent on engineering and the specific application. Alterations can be made and still the result will come within the scope of the disclosure.

The disclosed system utilizes separate transmit (typically supplied with the MRI unit) and receive coils. The room temperature transmit coil serves as the source of the applied Bi field and the receive only coil detects the RF energy from the imaged object. In other words, the cooled RF coils of the disclosed system create and detect the Bi field which results when the net

magnetization in a pulse sequence is generated following the application of RF energy and only functions to detect the energy given off by magnetic induction from the precessing of the hydrogen atoms. It is feasible to make a transmit and receive coil in one unit although the RF energy applied in the transmit portion of the "pulse" sequence will create power which will produce its own heating and potentially heat the coil to some degree.

The RF coils 224A, 224B of the disclosed device detect the energy given off by magnetic induction from the precessing of the hydrogen atoms. The magnetic moments of nuclei in normal matter will result in a nuclear paramagnetic polarization upon establishment of equilibrium in a constant magnetic field. It is shown that a radiofrequency field at right angles to the constant field causes a forced precession of the total polarization around the constant field with decreasing latitude as the Larmor frequency approaches adiabatically the frequency of the r-f field. Thus there results a component of the nuclear polarization at right angles to both the constant and the r-f field and it is shown that under normal laboratory conditions this component can induce observable voltages. See, F. Bloch, Nuclear Induction, Physical Rev. 70: 460-474 (1946).

When energy is released by the excited nuclei in the target, the signal produced is an emitted radio wave at a specific frequency relative to hydrogen. This signal is carried through output signal cables 230A, 230B, 230C, 230D and through cable 230 to an imaging computer for processing. The interface of cable 230 can be customized to any standard MRI processing device. The imaging computer stores the data (digitized) for further Fast Fourier Transforms to form the processed/ displayed images . For MRI systems having an analog to digital converter (ADC), the detected analog signal should be boosted enough to run through the ADC and Fast Fourier- 5 Transformed (FFT'ed) (twice) in a computer system to produce the image or a slice from an image set.

Some typical result of analyses are given here: Analysis using E-film of comparison of a human wrist coil room temperature array showed a 1.85 increase in SNR with the cooled coil array for both a banana and a human wrist when compared to the SNR of the room wrist volume array

10 coil. This demonstrates both a lower filling factor sample (the banana) and a routine sample

(human wrist) and when used on a dog stifle (knee). Independent analysis using tools applied by VirtualScopics staff demonstrated an at least 2.0 fold increase in SNR compared to a four channel custom - built dog knee coil of a different geometry. Arrays are even more complicated to calculate the potential increases in SNR than linear coils but it is anticipated that one should see at

15 least the same gain in SNR at a minimum as a linear coil.

FIG. 4 is an exploded view of paddle subassembly 201. The disclosed embodiment comprises a paddle having a flat top and bottom surface. From a manufacturing perspective, it may be desirable that paddle subassemblies 201 and 202 be constructed to be identical in structure. However, it is not required. In some configurations, it may be useful to form the paddle to closely

20 follow a contour of a patient's body. Not only could the paddle be sized for varying body shapes, e.g., child, dog, small test animal, etc., the paddle configuration could vary with relation to its mate, e.g., in sizes and/or shapes. Other configurations and methods and materials of construction could be incorporated and would still fall within the scope of the disclosed device.

Top outer cover 270 is mountable to casing 200 via outer cover subassembly screws 262A,

25 262B, 262C. Top outer cover 270 is hingedly connected to paddle hinge assembly 250 when

location hole 276 is mated with swing pin 252. Recess 208 is sized to fit vacuum connector 202A and vacuum control valve 204A. Vacuum access hole 210 is in communication with inner coil cavity 206. Access holes 214, 216 allow an input/return component of hose assembly 301 to communicate with inner coil cavity 206. Inner cavity cover 260 is mountable to casing 200 via

30 inner cover assembly screws 272A to 272H. O-ring 274 provides a seal for inner cavity cover 260 and casing 200.

Coil subassembly 220 comprises channels to receive and contain RF coils 224A, 224B and gas hose assembly 301. Gas, e.g., nitrogen, flowing through gas hose assembly 301 comes into close proximity with RF coils 224A, 224B so that localized cooling can take place. As stated 35 herein, this cooling effect results in a significant improvement in the SNR and thus provides a better resolution MR image. A technician can pull a vacuum in inner cavity 206 to insulate the outside of paddle subassembly 201 from the effects of the gas flowing through gas hose assembly 301, resulting in a safer environment for the patient and the technician handling the device.

Tuning capacitors 222A, 222B and tuning resistor 223A are shown mounted on RF coil 224A; tuning capacitors 222C, 222D and tuning resistor 223B are shown mounted on RF coil 224B.

FIG. 6 is a top view of gas flow assembly 5000 with the cover removed. Gas receiver tube 500 is capable of residing in the contents of dewar 130 (see FIG. 1). When the gas is heated to boiling, it flows through gas receiver tube 500 into T-connector 502 and out of gas output hoses 300A, 301A which are connected to gas flow assembly 5000 via gas hose output connectors 524A, 524B. Gas returns via gas return hoses 300B, 301B which are connected to gas flow assembly 5000 via connectors 512A, 512B respectively. Return gas flows through gas flow assembly 5000 and is exhausted into the atmosphere via gas return exit ports 520B, 521B. Temperature sensor input cables 564A, 564B, which monitor paddle subassembly temperatures, are connectable to temperature sensor cable plugs 560A, 560B respectively whereby signals can be transmitted to resistor network 550 and temperature control/monitor box 600 (see FIG. 1) via gas flow control cable 580.

FIG. 7 is a flow chart of a typical system start-up procedure 700. Steps 702 through 732 comprise recommended start-up steps. Once a cool down procedure has commenced, the SNR enhancement device can take approximately 35 - 45 minutes to cool from room temperature (about 298K) to operating temperatures ranging from about 125K to aboutl30K. If a continuous vacuum is not used or if the system or coils have not been used for about 3-5 days, a vacuum should be pulled in both halves of the coil. Step 702 recommends the checking of all connections on the heater block/stinger module and the hand-tightening of any loose connections. After the system is cooled to within the recommended operating temperature range, the connections can be optionally tool-tightened using a wrench or pliers (non-sparking or non-magnetic).

The vacuum pump may then be started up while the primary vacuum valve is in a closed mode (see Step 704). The tubing can be connected to one of the ports on the pump, observing that the larger Tygon^® tubing (or other acceptable tubing such as Teflon^® tubing) is connectable to the pump. Step 706 advises the connecting of the other end of the tubing to one of the vacuum connectors on a paddle subassembly 201 or 202. The plastic nut fastener on the port of vacuum connector 202A, 202B can be disconnected by hand to facilitate the inserting of the tubing into the port and hand-tightening the nut in place. The vacuum control valve 204A, 204B on the paddle subassembly 201 or 202 is opened (see Step 708). The valve may be rotated about one half turn to about a full turn. Step 710 recommends that the main valve on the vacuum pump be opened for about 5 - 10 minutes. The vacuum control valve 204A, 204B on the paddle subassembly 201 or 202 should then be carefully closed so as not to be over- tightened (see Step 712). The primary vacuum valve can be shut on the vacuum pump whereupon the tubing can be removed from the paddle subassembly vacuum port. Step 714 involves the repeating of steps 702 to 712 for the second paddle subassembly. When Step 714 is completed, the vacuum pump may be shut off. The tubing may be removed from the coil and disconnected from the vacuum pump (see Step 716). If necessary, the device may be removed from the MRI environment. It should be noted that the pump-down portion of the start-up procedure (Steps 702 through 716) could be done outside the MRI room.

In Step 718 the room temperature components and the control box 600 are connected to a source of 110V power. The gas flow control cable 580 is connected to the gas flow assembly control plug 570. The paddle subassembly is located and positioned on the patient table. The coil can be centered in relation to the MRI magnet or otherwise positioned in a desired location (see Step 720). Step 724 advises the placement of the gas flow assembly 5000 on the lip of the dewar. Since cold gas will start to vent from around the gas flow assembly, care should be taken so as not to position the heater unit in the top opening of the dewar until gas boil-off subsides. Since the cryogenic fluid can cause freezing burns if it makes contact with tissues such as skin, eyes, etc. at normal temperatures, it is recommended that the proper safety equipment, e.g., face shields, safety masks, insulating gloves, lab aprons, etc., be utilized during handling operations.

Once the gas boil-off slows, the gas flow assembly may be securely mounted into the dewar so that the gas receiver tube and liquid heating rod make good contact with the container's contents (see Step 726). If the gas flow assembly is mounted correctly and the O-rings cannot be seen, the device will likely form a good seal. Step 728 recommends that the heater control box dial be set to "0" before turning on the controller unit. A green indicator LED should be visible. The heater controller dial is adjusted to "750" whereby a yellow indicator LED becomes visible. When liquid starts dripping from the umbilical ends nearest the heater block, the heater dial can be adjusted to " 100" as set forth in Step 730. The coil temperature readouts are monitored. When the coil temperature ranges from about 125K to about 130K (about 25minutes to about 30 minutes), which indicates that the RF coils within the paddle subassemblies are sufficiently cooled, the imaging process can begin and a patient can be positioned to start the imaging process (see Step

732). The control box generates RF noise that will likely contaminate the images. In addition, it will pick up RF energy and gradient currents which can distort the readings on the digital displays. Therefore, during the image acquisition period, the temperature control/monitor unit is turned off. When not scanning, it is suggested that control box be periodically turned on to monitor the coil temperatures. It is usually normal for the inner surface of the coil to become cool/frosty during cool-down due to blackbody heat transfer. Should the coil remain cold once a patient is loaded, the subject should be removed and the system checked for a vacuum leak. A maintenance technician should be notified immediately if a vacuum leak and/or faulty insulation is suspected.

FIG. 8 is a flow chart of a typical system shut down procedure 800. When scanning is concluded, the control box can be turned on (see Step 802). Step 804 then recommends the gentle lifting of the gas flow assembly from the dewar. For example, using a rocking motion, the assembly may be partially removed from the top of the dewar. The bottom block of the assembly can rest on the top of the opening in a cocked manner so the O-rings are just visible. This provides time for the umbilical hoses to warm up and regain flexibility before the unit is completely disengaged from the dewar. When the umbilicals (hoses) have warmed-up and are flexible the gas flow assembly can be removed from the dewar (see Step 806). According to Step 808, the entire apparatus (paddle assembly, umbilical hoses, and gas flow assembly) can be removed from the MRI room. When the assembly has reached room temperature, it is recommended that condensation from the cooling process be wiped off the inner surfaces of the coils in each paddle subassembly after each use. This will allow any surface moisture that might accumulate to be removed and help prolong equipment life

Maintenance of the disclosed device can comprise maintaining liquid levels in the dewar and pumping the vacuum spaces of the coils as needed. The vacuum should be regularly pumped about once every two weeks even if the coil is used infrequently. Oil in the vacuum pump should be checked to make sure it is at the required level contacting maintenance technicians for replacement should it become low or no longer clear. There are no user serviceable parts in the coil. Thus, the coil should be returned to the manufacturer in the event of any component breakage or failure to produce images

FIGS. 9, 10, 11 are schematic diagrams of the temperature control circuitry. Circuit 910 in FIG. 9 depicts the control circuit for heater element 542 (see also FIG. 2). Potentiometer 912 acts as a variac to control the heating element and thus boil the liquid gas, e.g., nitrogen.

Circuit 920 in FIG. 10 depicts the temperature monitor circuit for displaying each paddle coil temperature on LED readout 601. Circuit 920 is repeated twice, once for each paddle assembly. One circuit pertains to LED readout 601 and another pertains to LED readout 602 (see also FIG. 2).

Circuit 930 as shown in FIG. 11 depicts a simple voltage regulator having 12V regulator VR1 and 5V regulator VR2. The circuits shown are by way of example and not of limitation.

Other circuits can be designed to accomplish the same results as shown in FIGS. 9, 10, 11.

In one example, two different LN₂ gas - cooled phase arrays were built and tested on specimens and volunteers in 1.5 Tesla field. The results were compared with those of two similar phased arrays at room temperature. The LN₂ gas - cooled arrays were shown to have significant SNR improvement over the room temperature versions. Also the SNR results in the MR images matched well with the theoretical calculations using measured RF properties of the phased arrays.

One of the two room temperature coils was a commercially available phased array wrist coil having four channels. The other was a custom-built four coil array used in MRI of dog knees. In both cases, software was used to analyze images acquired under identical conditions. Two curved vacuum-insulated coil assemblies (coils) were used. The coils were set up to plug into a standard GE Horizon LX interface. The coils were cooled to approximately 138°K (or about - 160°C) by means of a stream of vapor from the boil-off of LN₂ provided from a manifold fitted to a stainless steel LN₂ container. The hold time of the cryogenic fluid container was set to be at least 4 hours. The temperature at the exterior of the two coils was measured to be at about 296°K (or about -2°C). The exterior surfaces of the two coils were coated with a thin layer (about 0.125" thick) of Styrofoam insulation. An increase in SNR ranging from about two to three was obtained 5 at about 63.5 MHz or about 1.5 Tesla. When small coils (linear surface) were used, they were placed directly into LN2 in a specially designed dewar. Tests yielded about a 2.5 times increase in SNR at about 2.9 Tesla (about 123.5 MHz).

FIGS. 12-18, 20 depict another embodiment of the SNR enhancement device disclosed herein. In this embodiment, a cryogenically cooled RF coil assembly is housed in a sealed vacuum 10 assembly adapted to contain a cooling medium and at least one RF coil disposed therein and

wherein the sealed vacuum assembly is housed in a plate assembly which can be disposed on a target to be imaged. Vacuum is used to insulate the coil elements 1607 from the outer shells of coil assembly 3000.

As can be seen in FIGS. 12 and 13, plate assembly 3000 comprises outer shells 1200, 15 1201 and inner vacuum shells 1300, 1301. Mounting plate 1500 and coil assembly 1600 form coil subassembly 3001. Coil assembly 1600 comprises one or more RF coils 1607 mounted in an insulator 1625. Cold plate housing 1400 mates with mounting plate 1500 via holes 1405, 1505 to form cold plate subassembly 3050 which resides between inner vacuum shells 1300, 1301 in an assembled mode which will be referred to herein as sealed vacuum assembly 3080. Standoff ribs 20 1302 provide for spacing/insulation between the adjacent surfaces of vacuum shell 1300 and coil assembly 1600. In this example, vacuum shell 1300 comprises receiving holes 1303 which align with receiving holes 1313 in outer shell 1301 to facilitate closure. The mating of vacuum shells 1300, 1301 can be achieved with conventional closure and/or securing means including clamps, fasteners, epoxies, etc. To ensure sealed vacuum assembly 3080 will contain a cooling medium,

25 such as gas or liquid, the assembly comprises a sealing means to maintain a sealed environment and vacuum reliability. Channels 1410 permit flow of the medium through cold plate housing 1400 for cryogenic cooling of coil assembly 1600.

In this embodiment, outer shells 1200, 1201 and inner vacuum shells 1300, 1301 comprise channels 1210, 1310 as a consequence of the particular manufacturing process employed. It is

30 contemplated, however, that these inner surfaces could be substantially smooth or have a variety of textures and appearances other than that shown. The outer shell and inner shell components of plate assembly 3000 can be constructed of a polymeric material. Outer shell 1200 comprises receiving holes 1203 which can be aligned with receiving holes 1213 in outer shell 1201 to facilitate closure. The mating of outer shells 1200, 1201 can be achieved with conventional

35 closure and/or securing means including clamps, fasteners, epoxies, etc. See FIG. 18 which

depicts plate assembly 3000 in an assembled mode. FIG. 20 depicts a cross section of the plate assembly 3000 shown in FIG. 18. While these examples illustrate a planar, substantially rigid embodiment, it is contemplated that the various components of the disclosed device can be adapted with a curvature or shape to fit a particular part of a target's anatomy. For example, it is contemplated that a cylindrical embodiment that is hingedly openable in the form of clam shell will constructed and tested. Flexible embodiments can also be employed as long as a sealed environment and vacuum can be maintained.

In the disclosed embodiment, coil assembly 1600 comprises two halves or sections 1601, 1602, the adjacent edges of which may overlap by means of an edge strip 1620. FIG. 16 illustrates one of the coil assembly halves, namely section 1602. Section 1602 comprises a plurality of RF coils 1607 housed in an insulator 1625 and arranged in an array for imaging. Insulator 1625 comprises high thermally conductive and electrically non-conductive materials and may be selected from a group comprising ceramic, glass, aerogels or other finely divided non- metal insulator compositions. Imaging studies using a LN₂ gas - cooled array comprising aerogel and low level vacuum insulation showed significant SNR gains over its room temperature counterpart. Coils 1607 are formed into a rounded rectangular shape. Decoupling of coils 1607 can be achieved through partial overlapping of the coil elements. Holes/slots 1605 facilitate the mounting of section 1602 to a corresponding hole 1405, 1505 on cold plate housing 1400 and mounting plate 1500, respectively. See also FIGS. 12, 13. Holes/slots 1605 can also serve to provide a mechanism by which coil sections 1601, 1602 can be repositioned in relation to one another, thereby allowing for adjustment of the degree of coil overlap.

FIG. 14 depicts a coil assembly 1600 positioned on mounting plate 1500 (see FIGS. 12-

13 and 17) and secured thereto by means of nylon screws 1403. Holes/slots 1605 can be aligned with holes 1405 on cold plate housing 1400 and holes 1505 on mounting plate 1500, respectively. Coil assembly sections 1601, 1602 each comprise four RF coils 1607 housed in an insulator 1625 and together form an eight (8) coil array. Individual coils 1607 in the phased array may be constructed from elongated copper rod (6 gauge AWG copper wire) having a known resistance.

The size of the coil elements may be sized as needed and broken into smaller pieces to avoid self resonance. Coil assembly 1600 is shown residing in cold plate housing 1400 which is positionable in vacuum shell 1301 and inner shell 1201. An innermost edge of section 1601 is shown overlapping and innermost edge of section 1602.

As shown, each of the coils 1607 comprises at least two capacitors 1654, 1653 for tuning and matching the coil. See also FIG. 15. This enables the four (4) identical linear phased arrays to be separately tuned and matched at room temperature and at LN₂ temperature. Coaxial cable (not shown) is in electrical connection with end 1649 of balun 1650 whereby the RF current flowing on the outer surface of the coaxial cable can be attenuated. Trap 1648 manages any frequencies in the coil. Pin diode 1651 allows current to pass in the forward direction while it blocks current in the reversed direction and helps with preventing system resonance and potentially monitoring temperature/vacuum system. Capacitor 1652 can be added to improve coil decoupling.

FIG. 17 illustrates a partial assembly of cold plate subassembly 3050. Mounting plate 1500 is positioned on cold plate housing 1400 (as referenced by channels 1410 shown in dotted line format). Cold plate housing 1400 is positionable in vacuum shell 1301 and inner shell 1201. Hose assemblies (not shown) carry a cooling medium to inlet port 1706 of coil assembly 3000. Gas flows through channels 1410 of cold plate housing 1400 in sealed vacuum assembly 3080 whereby coil assembly 1600 can undergo cooling. The cooling medium of the disclosed device can comprise a cryogenically cooled gas or liquid that can be supplied by a tank or dewar 130 (see FIGS. 1, 2) or in a sealed system with a cryocooler to remove the heat. The cooling medium can also be a gas that is circulated by means of a pump.

Gas flows through sealed vacuum assembly 3080 of coil assembly 3000 and is returned to gas flow assembly 5000 (see FIGS. 1, 2) via outlet port 1704 (see also FIG. 18) where it is then exhausted to atmosphere. Tubing 1703, 1705 can be used to direct cooling medium to and from channels 410. In this embodiment, Teflon^® tubing was utilized; however other materials and/or types may be more suitable. Vacuum hose connector 1708 allows for a vacuum hose (not shown) to be connected to coil assembly 3000. The vacuum hose (not shown) may also be connectable a vacuum pump (not shown) capable of removing gas molecules from the sealed environment so as to leave behind at least a partial vacuum. Control box 3003 enables an operator to have a means of desk top control. See FIG. 19. Control box 3003 is connectable to gas flow assembly 5000 to control the gas boiling process. Control box 3003 can be useful for monitoring coil temperature in coil assembly 3000.

FIG. 21 depicts an embodiment of a coil assembly having three (3) coil elements. In the disclosed embodiment, coil assembly 1800 comprises sections 1801 (not shown), 1802. Similar to the installation and operation described above, section 1802 comprises a plurality of RF coils 1807 housed in an insulator 1825. Insulator 1825 comprises high thermally conductive and electrically non-conductive materials and may be selected from a group comprising ceramic, glass, aerogels or other finely divided metal insulator compositions. Coils 1807 are formed into a rounded rectangular shape. Decoupling of coils 1807 can be achieved through partial overlapping of the coil elements and various electrical decoupling means.

MRI equipment varies in cost, depending on the strength of the scanner. Scanners with more strength produce more detailed images; therefore, these scanners cost more. According to recent literature, MRI machines can range in cost between $1 and $3 million. Construction of MRI suites can easily add another $500,000 to the total cost. An extremity MRI machine alone costs $300,000 or more, and can only be used to scan hands, feet and knees. Purchasing a used- extremity MRI scanner can cost as much as $150,000. Aside from the initial cost of purchasing the MRI equipment, there is the additional cost of $800,000 each year on average to operate the scanner, including the expensive process used to chill the superconducting magnet. Another associated cost can include the cost of hiring employees with the technical skills to operate and repair these sensitive machines. In addition, if an MRI exam is not conducted properly, another MRI study may need to be ordered. Another consideration is the life cycle of a new MRI machine, which is typically assessed at about seven years. Even if a machine lasts longer, as it gets older, a scanner is likely to require repairs more often. The overall cost of MRI makes it a huge investment to health-care providers. Thus, the improved utilization of MRI machines can result in considerably more profit and/or reduced examination costs.

To illustrate the potential savings, assume fixed costs to be $724,000 (equipment, space, maintenance, salaries, utilities, overhead) and variable costs to be $115 (supplies, billings, collection, upgrades, etc). At a rate of eight MRI examinations per day over a 250-day year (2000 total), the cost per examination could be calculated as:

Cost/Exam = (Annual Fixed Costs/ Annual Exam Volume) + Variable Cost/Exam

= [$724,000/ 2000 exams] + $115/exam

= $477/exam

If the annual volume could be increased by 1000 (3000 total), the cost becomes

$356/exam. Doubling the number of exams to 4000 would result in a cost of $295/exam.

Operating MRI equipment can generate up to $10,000 of income an hour. By leveraging throughput, the MRI facility could opt to increase profit or decrease its charges. In any case, each facility will be able to decide this issue based upon referrals and what the referring practice instructs.

As stated herein, this increase in SNR results in the enhancement of the quality of MRI images or conversely, the same quality of images in less time elapsed to acquire the image data. Higher resolution can improve diagnostic utility and accuracy in some cases. If shorter times are desired (less time in the magnet) and the same quality of images can be tolerated (being acceptable for diagnosis), this could allow for increased patient compliance and throughput. Not only does the disclosed device enable expansion into the area of imaging pediatric patients, the shortened imaging times can provide for improved overall machine utilization and patient tolerance of the procedure (better compliance). It is contemplated that the disclosed device can be applied to the development of cryogenically phased arrays using other cryogens and other magnetic strengths.

The disclosed device pertains to an MRI enhancement apparatus comprising one or more RF coils housed in a coil casing. The RF coils are capable of receiving a plurality of spin/field echo signals during an MR pulsing sequence and can be cooled by a cryogen for noise reducing purposes. The coil casing comprises a vacuum system capable of insulating the cooled RF coil from an external surface of the coil casing. The coil casing can be positioned at a minimal distance from a part of a patient or target being imaged.

The disclosed device pertains to an MRI enhancement apparatus comprising a coil casing having a plurality of RF coils housed in an insulating material and which is capable of being cooled by a cryogen for noise reducing purposes. The coil casing further comprises a sealed vacuum assembly capable of insulating the plurality of coolable RF coils from an external surface of the coil casing. A cooling of the plurality of coolable RF coils affects the RF coils and not a target placed in contact with said coil casing.

The disclosed device pertains to an MRI enhancement apparatus comprising a coil housed in a casing and which is capable of receiving a plurality of spin/field echo signals during an MR pulsing sequence. The system comprises a hose assembly having a first end connectable to the casing and a second end connectable to a flow assembly. The flow assembly has heating element and a receiver tube that are each mountable in a dewar housing a cryogen. A controller is connectable to the flow assembly and is capable of heating the cryogen whereby a low temperature gas flows through a receiver tube and the hose assembly to contact the coil so a cooling of the coil may occur. The controller is also capable of monitoring and controlling a temperature of the coil. The hose assembly further comprises a set of signal cables capable of delivering temperature information from the coil to the flow assembly and the controller. A vacuum system is in communication with the casing and can insulate the coil from an external surface of the casing. A portion of the low temperature gas returns through the hose assembly to the flow assembly to be exhausted therefrom. The spin/field echo signals detected by the coil are transmitted to a computer wherein the spin/field echo signals detected by the coil are transmitted to a computer whereby the signal data is Fourier transformed into MR image data.

The enhancement apparatus can comprise a plurality of coils housed in the casing. The plurality of coils can also be configured as a phased array. The enhancement apparatus can also comprise a plurality of casings. The vacuum system further comprises a vacuum control pump that can produce a vacuum on an intermittent or on a continuous basis depending on the application. The coils can be housed in an insulator such as Aerogel™. The enhancement apparatus comprises an improved signal-to-noise (SNR) as a result of localized cooling of the coil thereby allowing the computer to process an improved image resolution or generate data for a reduced imaging time.

Although the disclosed device and method have been described with reference to disclosed embodiments, numerous modifications and variations can be made and still the result will come within the scope of the disclosure. No limitation with respect to the specific embodiments disclosed herein is intended or should be inferred.

Claims

WE CLAIM:

1. A cryogen-cooled radiofrequency (RF) coil assembly comprising:

one or more RF coils housed in a coil casing and capable of receiving a plurality of

spin/field echo signals during a magnetic resonance pulsing sequence;

said one or more RF coils capable of being cooled by a cryogen for noise reducing

purposes;

said coil casing comprising a vacuum system capable of insulating said one or more

cooled RF coils from an external surface of said coil casing; and wherein said coil casing is positionable at a minimal distance from a part of a patient being imaged.

2. A cryogen-cooled radiofrequency (RF) coil assembly comprising:

a coil casing having a plurality of RF coils housed in an insulating material;

said plurality of RF coils capable of being cooled by a cryogen for noise reducing

purposes;

said coil casing further comprising a sealed vacuum assembly capable of insulating said plurality of coolable RF coils from an external surface of said coil casing; and wherein a cooling of said plurality of coolable RF coils affects said RF coils and not a target placed in contact with said coil casing.

3. The assembly of claim 2, wherein said plurality of RF coils are configured in a phased array.

4. The assembly of claim 2 further comprising an intermittent vacuum or continuous vacuum.

5. The assembly of claim 2, wherein said insulating material is selected from a group comprising ceramic, glass, aerogels or other non-metal insulator compositions.

6. A system comprising:

a coil housed in a casing and capable of receiving a plurality of spin/field echo signals during an MR pulsing sequence;

a hose assembly having a first end connectable to said casing and a second end connectable to a flow assembly, said flow assembly having a heating element and a receiver tube each mountable in a dewar housing a cryogen;

a controller connectable to said flow assembly, said controller capable of heating said cryogen whereby a low temperature gas flows through a receiver tube and said hose assembly to contact said coil so a cooling of said coil may occur, said controller further being capable of monitoring and controlling a temperature of said coil; said hose assembly further comprising a set of signal cables capable of delivering temperature information from said coil to said flow assembly and said controller;

a vacuum system in communication with said casing and capable of insulating said coil from an external surface of said casing; and

wherein a portion of the low temperature gas returns through the hose assembly to the flow assembly to be exhausted therefrom; and

wherein the spin/field echo signals detected by the coil are transmitted to a computer whereby the signal data is Fourier transformed into MR image data.

7. The system of claim 6, wherein said coil further comprises a plurality of coil elements.

8. The system of claim 6, wherein said coil further comprises a phased array.

9. The system of claim 6, wherein said coil comprises a plurality.

10. The system of claim 6, wherein said vacuum system further comprises a vacuum control pump that can produce a vacuum on an intermittent or on a continuous basis.