METHOD OF UTILITZATION OF HIGH DIELECTRIC CONSTANT (HDC) MATERIALS FOR REDUCING SAR AND ENHANCING SNR IN MRI

Abstract

A method for enhancing the performance of an imaging system that includes providing a magnetic resonance imaging system; providing an object to be imaged, wherein the object has a shape, includes a region of interest therein, and has a first dielectric constant; providing a pad that conforms to the shape of the object and contains a material having a second dielectric constant that is higher than the first dielectric constant; surrounding the pad and the object being imaged with at least one radio frequency coil; using the at least one coil to generate a magnetic field having an intensity of 3 T that extends through the material of the pad and into the region of interest of the object being imaged, and wherein positioning the at least one coil to surround the pad and the object increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object; using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged and to convert the detected magnetic field into detectable electrical signals; and increasing signal to noise ratio of detected electrical signals and decreasing required power input to the at least one coil through the placement of the at least one coil around the pad.

Claims

1. A method for enhancing the performance of an imaging system, comprising: (a) providing an imaging system, wherein the imaging system is a magnetic resonance system; (b) providing an object, (i) wherein the object is imaged by the imaging system, (ii) wherein the object being imaged has a shape, (iii) wherein the object being imaged includes a region of interest therein, and (iv) wherein the object being imaged has a first dielectric constant; (c) providing a pad, (i) wherein the pad conforms to the shape of the object being imaged, (ii) wherein the pad contains a material having a second dielectric constant, and (iii) wherein the second dielectric constant is higher than the first dielectric constant: (d) surrounding the pad and the object being imaged with at least one radio frequency coil; (e) using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; (f) using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; (g) using the at least one coil to convert the detected magnetic field into detectable electrical signals; and (h) increasing signal to noise ratio of detected electrical signals and decreasing required power input to the at least one coil through the placement of the at least one coil around the pad.

2. The method of claim 1, further comprising selecting the material in the pad from the group consisting of ceramics, deuterium-based gel, water-based gel, suspensions including dielectric additives, and combinations thereof.

3. The method of claim 1, further comprising providing the pad in the form of a helmet that conforms to a portion of the object being imaged.

4. The method of claim 1, further comprising providing the material contained within the pad in a uniform thickness.

5. The method of claim 1, further comprising adapting the pad to receive different volumes of high dielectric constant material therein.

6. The method of claim 1, further comprising reducing the required power input by 50%.

7. The method of claim 1, further comprising increasing the signal to noise ratio by 27%.

8. A method for enhancing the performance of an imaging system, comprising: (a) providing an imaging system, wherein the imaging system is a magnetic resonance system; (b) providing an object, (i) wherein the object is imaged by the imaging system, (ii) wherein the object being imaged has a shape, (iii) wherein the object being imaged includes a region of interest therein, and (iv) wherein the object being imaged has a first dielectric constant; (c) providing a pad in the form of a helmet adapted to receive different volumes of material, (i) wherein the pad conforms to the shape of the object being imaged, (ii) wherein the pad contains a material having a second dielectric constant, and (iii) wherein the second dielectric constant is higher than the first dielectric constant; (d) surrounding the pad and the object being imaged with at least one radio frequency coil; (e) using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; (f) using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; (g) using the at least one coil to convert the detected magnetic field into detectable electrical signals; and (h) increasing signal to noise ratio of detected electrical signals and decreasing required power input to the at least one coil through the placement of the at least one coil around the pad.

9. The method of claim 8, further comprising selecting the material in the pad from the group consisting of ceramics, deuterium-based gel, water-based gel, suspensions including dielectric additives, and combinations thereof.

10. The method of claim 8, further comprising providing the material contained within the pad in a uniform thickness.

11. The method of claim 8, further comprising reducing the required power input by 50% and increasing the signal to noise ratio by 27%.

12. A method for enhancing the performance of an imaging system, comprising: (a) providing an imaging system, wherein the imaging system is a magnetic resonance system; (b) providing an object, (i) wherein the object is imaged by the imaging system, (ii) wherein the object being imaged has a shape, (iii) wherein the object being imaged includes a region of interest therein, and (iii) wherein the object being imaged has a first dielectric constant; (c) providing a pad, (i) wherein the pad conforms to the shape of the object being imaged, (ii) wherein the pad contains a material having a second dielectric constant, and (iii) wherein the second dielectric constant is higher than the first dielectric constant; (d) surrounding the pad and the object being imaged with at least one radio frequency coil; (e) using the at least one coil to generate a magnetic field that extends through the material of the pad and into the region of interest of the object being imaged, wherein the magnetic field has an intensity of 3 T, and wherein positioning the at least one coil to surround the pad and the object being imaged increases the intensity of the magnetic field produced by the at least one coil within the region of interest of the object being imaged; (f) using the at least one coil to detect the magnetic field generated in the region of interest of the object being imaged; (g) using the at least one coil to convert the detected magnetic field into detectable electrical signals; and (h) increasing signal to noise ratio of detected electrical signals by 27% and decreasing required power input to the at least one coil by 50% through the placement of the at least one coil around the pad.

13. The method of claim 12, further comprising selecting the material in the pad from the group consisting of ceramics, deuterium-based gel, water-based gel, suspensions including dielectric additives, and combinations thereof.

14. The method of claim 12, further comprising providing the pad in the form of a helmet that conforms to a portion of the object being imaged.

15. The method of claim 12, further comprising providing the material contained within the pad in a uniform thickness.

16. The method of claim 12, further comprising adapting the pad to receive different volumes of high dielectric constant material therein.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings, which are incorporated into and form a part of the specification, schematically illustrate one or more exemplary embodiments of the invention and, together with the general description given above and detailed description given below, serve to explain the principles of the invention, and wherein:

[0021] The foregoing and other novel features and advantages of the invention will become more apparent and more readily appreciated by those skilled in the art after consideration of the following description in conjunction with the associated drawings, of which:

[0022] FIG. 1A provides an illustration of a computer model of a bird-cage coil.

[0023] FIG. 1B provides an illustration of a bird-cage coil surrounding a human head with an HDC-pad.

[0024] FIGS. 2A-2C provide images of elliptical ROI used to evaluate T2-weighted image intensity for SNR measurement and uniformity assessment on mid-axial, sagittal, and coronal planes, wherein FIG. 2A is an image of the mid-axial plane, wherein FIG. 2B is an image of the sagittal plane, and wherein FIG. 2C is an image of the coronal plane.

[0025] FIGS. 3A-J provide images illustrating that the addition of an HDC-pad resulted in dark regions in central regions of all brain images due to over-tipping with the RF power level optimized without the HDC-pad. The images shown in FIGS. 3K-3T were acquired using input RF power with the attenuation values listed below. The images shown in FIGS. 3P-3T were obtained with the final power attenuation for the 90 flip angle being 14.5 dB, which was 3-dB more attenuation than without the HDC-pad.

[0026] FIGS. 4A-4C are axial (FIG. 4A), sagittal (FIG. 4B), and coronal images (FIG. 4C) through the brain with an HDC-pad, and FIGS. 4D-4E are axial (FIG. 4D), sagittal (FIG. 4E), and coronal images (FIG. 4F) through the brain without an HDC-pad. Measured SNR values are given under each image.

[0027] FIG. 5A is an image of a calculated B1+ map at 128 MHz in a first orthogonal plane with an HDC-pad, FIG. 5B is an image of a calculated B1+ map at 128 MHz in a first orthogonal plane without an HDC-pad, and FIG. 5C is the B1+ distribution plotted along the dotted lines in FIGS. 5A and 5B. FIG. 5D is an image of a calculated B1+ map at 128 MHz in a second orthogonal plane with an HDC-pad, FIG. 5E is an image of a calculated B1+ map at 128 MHz in a second orthogonal plane without an HDC-pad, and FIG. 5F is the B1+ distribution plotted along the dotted lines in FIGS. 5D and 5E. FIG. 5G is an image of a calculated B1+ map at 128 MHz in a third orthogonal plane with an HDC-pad, FIG. 5H is an image of a calculated B1+ map at 128 MHz in a third orthogonal plane without an HDC-pad, and FIG. 5I is the B1+ distribution plotted along the dotted lines in FIGS. 5G and 5H. All the B1+ maps are scaled to 1 W input power.

DETAILED DESCRIPTION OF THE INVENTION

[0028] Exemplary embodiments of the present invention are now described with reference to the Figures. Reference numerals are used throughout the detailed description to refer to the various elements and structures. Although the following detailed description contains many specifics for the purposes of illustration, a person of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following embodiments of the invention are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.

[0029] A preferred embodiment of the present invention will now be described in detail with reference to the Figures. Those skilled in the art will appreciate that the description given herein with respect to those figures is for exemplary purposes only and is not intended in any way to limit the scope of the invention. All questions regarding the scope of the invention may be resolved by referring to the appended claims.

Theoretical Considerations

[0030] For conductive dielectric materials such as human brain tissues, the RF field inside the sample is perturbed by conductive current (Jc) and displacement current (Jd) according to Ampere's Law with Maxwell's correction,

B=Jc+Jd=E+iroE

where B is magnetic flux density, E is electric field, is angular frequency, r is relative electric permittivity (dielectric constant), o is the electric permittivity in vacuum, is electrical conductivity, m is magnetic permeability, and i=is the complex unit which introduces a 90-degree phase difference between conductive current and displacement current (Johnk 1988). For plane waves traveling in a homogeneous medium, the conductive current leads to decay of the RF field in the direction of propagation, while the displacement current with a 90 phase shift acts as a secondary field source facilitating RF wave propagation. In this case, the opposing contributions of the two sources to B1 can be considered using ratio Jc and Jd given by

Jc/Jd=/or.

In principle, materials with low and high r can enhance the local B1 field strength for an RF field frequency range high enough to induce the displacement current to a much stronger conducting current. This equation describes the relationship within the dielectric materials. Subsequently, the RF wave propagates into the sample with stronger amplitude enhanced by the HDC materials. Thus, in general, placement of HDC-pads near an ROI in MRI should result in enhanced local B1 field strength with concomitant improvement of SNR and reduction of overall SAR.

HDC-Pad Design for Human Head Imaging

[0031] FIG. 1B illustrates geometry and configuration related to the RF coil and the human head of the preferred embodiment of present invention. The embodiment of the present invention is an HDC-pad in a form of a helmet, containing approximately 6 liters of distilled water, was conformed around the superior portion of the head, extending to just above the level of the eyes in front, just above the ears on the sides, and just below the end of the skull in the back of the head. The thickness of the HDC-pad is 3 mm.

Computer Modeling

[0032] The quantitative evaluation on how placement of an HDC-pad changes the B1 field distribution depends on the detailed geometries and sizes of the coil and sample in this embodiment of the present invention, and thus, must be determined numerically with computer modeling. In the following a computer modeling on an HDC-helmet is used to demonstrate the efficacy of the invention.

[0033] A numerical model with finite difference time domain (FDTD) method was used to calculate the RF field distribution in the sample and coil model shown in FIG. 1A at 128 MHz corresponding to a static magnetic field strength of 3 Tesla. All the FDTD calculations were performed with commercially available software (XFDTD; Remcom, Inc, State College, Pa.), and post-processing of the simulation results was performed with home-built programs in MATLAB (The Mathworks, Inc., Natick, Mass.). A 30 mesh with isometric 2 mm resolution was created within a region of 575042 cm3. The calculation was performed with 35 dB convergence to ensure that the steady state was reached. A Liao boundary condition was used for the outer boundary truncation of the grid (12). The coil was modeled after that used in experiment: a copper 12-rung high-pass birdcage coil (32.8 cm i.d. and 25.4 cm length, shield diameter of 40.0 cm). The coil model was driven using 24 current sources spaced evenly on the two end rings, at the locations of capacitors in the actual coil. The human head model used for the FDTD calculation included 16 types of tissue and the corresponding electric properties at 128 MHz were HDC-pad was modeled with a 3 cm thick uniform layer of water (=0.0047 S/m2, r=78) over the head excluding the face as shown in FIG. 1B (13).

Experimental Measurements

[0034] Human brain images were acquired on a 3 T whole body system (Bruker, Biospin, Ettlingen, Germany) using a quadrature 12-element high-pass birdcage coil with 26 cm inner diameter and 29 cm length. Axial brain images were acquired with identical imaging parameters with and without an HDC-pad placed around the head and after the coil was tuned and matched, and RF power was calibrated for each condition. The subject remained in the magnet during the process of placing and/or removing the HDC-pad, re-tuning the coil and adjustment of RF power for 90/180 flip angle. Input power for the flip angle was adjusted manually with and without the HDC-pad while maximizing the total signal on 5 axial slices covering a 2.5 cm slab through the center of the brain. Fast spin-echo (RARE) images with slice thickness=5 mm, matrix=128128, FA=180, and FOV=30 cm were acquired on five axial, sagittal, and coronal planes spaced 5 mm apart through the cerebrum. The experiment was repeated four times with two human subjects. All of the subjects provided written informed consent prior to participation, in accord with the requirements of the Institutional Review Board of the Pennsylvania State University College of Medicine.

[0035] Signal-to-noise ratio (SNR) was measured using the magnitude images acquired under the above two conditions. The average signal intensity was calculated in an elliptical ROI covering most of the cerebrum in each of the 15 images acquired with and without the HDC-pad. Examples of the elliptical region in each orientation are shown in FIGS. 2A-2C. The noise was measured by calculating the standard deviation in an ROI covering 29 cm2 in a region of no visible signal or artifact across the top of each image.

Results

[0036] FIGS. 3A-3T show a set of axial images of human brain after addition of the HDC-pad and retuning of the coil, but before re-optimization of the RF power levels (using power values optimized without the HDC-pad) on the left columns, and images with the HDC-pad using the RF power levels indicated at the bottom of columns of the images. After addition of the pad, the RF power levels that previously generated 90/180 excitation/refocusing pulses in the brain created much larger flip angles due to enhancement of the B1 field. A 3-dB reduction of power for both the excitation and refocusing pulses was required to maximize the signal and images were acquired shown on the far-right column (FIGS. 3P-3T).

[0037] FIGS. 4A-4F show images acquired with and without the HDC-pad on axial, sagittal, and coronal planes. The SNR in the brain measured in the elliptical regions shown in the Figures is listed below each corresponding image. For all 15 planes acquired, the SNR increased by 20 to 40% (in average, 27%) with addition of the HDC-pad while the overall uniformity of image intensities appears to be similar in both cases. The SNR of repeated experiments are within the same range.

[0038] FIGS. 5A-5I show the calculated B1+ distributions with and without the HDC-pad in three orthogonal planes arranged as in the experimental results. All the fields correspond to 1 W of power delivered to the coil. The calculated B1+ distributions under the two conditions presented similar characteristics as the in vivo images in FIGS. 4A-4F. The RF field appears to be greater inside the brain regions surrounded by the HDC-pad but lower in the regions in the face and neck outside of the padded region. B1+ outside the upper portion of the head and pad is lower when the HDC-pad is present, and the same input power is maintained. This can be seen in the line plots at the bottom of the figure, which present B1+ along the dotted lines in the maps above in FIGS. 5A-5I. The black and gray lines indicate B1+ for cases with and without the HDC-pad respectively. The RF field around the coil elements and in between the coil element and shield (seen most clearly in the middle line plot) was reduced more than 70%, indicating less current is required in the coil to maintain the same RF power level when the pad is present. In ROIs similar to those used in the in vivo results, the average B1+ in the simulation is seen to increase by 12-20% with HDC-pad while the source current in the coil is reduced. These computer modeling results demonstrated that the HDC-pad improves the efficiency of generation of B1 field by a given RF coil, resulting in an increase in image SNR and decrease of RF power required for a given flip angle.

[0039] Addition of an HDC-pad surrounding the head resulted in a reduction of required RF power by approximately 50% and an increase in image SNR by approximately 27% with a transmit/receive volume coil at 3 T. No obvious local bias field induced by HDC-pad in the entire cerebrum was observed in the images in FIGS. 4A-4F. In addition, image uniformity within the cerebrum appeared to be somewhat improved. The standard deviation of the signal intensity distribution in the elliptical regions on all 15 images acquired from approximately the same brain volume decreased by an average of 12% with addition of the HDC-pad. The uniformity in this brain area that is normally interference by the so-called bright center spot due to the RF wave effect was reduced markedly.

[0040] Comparing images in FIGS. 4A-4F acquired with and without the HDC-pad showed that the image intensity in the cerebrum relative to the neck and face is much greater with the HDC-pad than without it. It is likely that this is due to the enhancement of the B1 field for a given input power in the regions that were surrounded by the HDC-pad. B1 field distribution of the coil is altered by the strong displacement currents in the region of the pad, resulting in stronger B1 fields in the vicinity of the HDC-pad. These experimental data suggested that the regional enhancement of the B1 fields in the ROI can be extended to the entire cerebrum with proper coverage by the HDC-pad while the B1 field outside the sample is decreased. As a direct result, average SAR levels for the brain imaging will be lowered with the HDC-pad, and because SNR is proportional to the ratio of the B1 field strength to the square root of the corresponding RF power, SNR is increased with addition of the HDC-pad. The numerical modeling results summarized in FIGS. 5A-5I are consistent with the experimental data and clearly demonstrate the enhancement of the B1 field in the brain by the HDC-pad. The B1 field is increased in the entire cerebrum region while outside the brain appears to be reduced by the placement of the HDC-pad when the B1+ maps are normalized to unit input power. Under this condition, B1+ is increased in the cerebrum with reduced input current in the coil. This is apparent in the middle line plot, where the two ends of the line pass through coil current elements. Since the magnetic field in the upper portion of the head is relatively high compared to elsewhere, a greater percentage of the input power is also dissipated in that region. To maintain the same total input power as is the case in FIGS. 5A-5I, the coil requires less current and produces lower B1+ in regions outside the HDC-pad compared to the case when the HDC-pad is not present. Thus, overall, the presence of the HDC-pad improves efficiency of delivery of both B1 field and RF energy to the section of the head surrounded by the HDC-pad.

[0041] Those skilled in the art will appreciate that further development of the present invention will lead to an even greater improvement in reducing SAR and improving regional SNR in MRI. The in vivo data presented here at 3 T suggested that HDC-pads around the head or other parts of the anatomy could be used to enhance performance of an RF coil in a variety of cases. An HDC-pad with adjustable volume could be used to enhance RF coil performance while simultaneously providing comfort and reduction of patient motion. This could be particularly beneficial for pediatric patients since most RF coils are designed to accommodate larger adult anatomies. In some cases, there may also be advantages to incorporating dielectric material directly into RF coil constructions. Further developments of the various embodiments of this invention include determination of the locations, dimensions, geometries and permittivity distributions of the material for optimal B1 enhancement.

[0042] Water was used in this embodiment as a dielectric medium to demonstrate the desired effect of the present invention as water has relatively high dielectric constant and low conductivity, is readily available, inexpensive, and nontoxic. From a technical point of view, however, water is unlikely to be the most suitable dielectric material for many intended applications in the art because it produces strong signal that saturates the receiver and decreases the dynamic range of the digitizer and its movements and geometry are difficult to control. Deuterium (D2O) and high dielectric constant material such as barium titanites slurry suspension in the deuterium can be used to replace the water. It is known to the art also that certain ceramic materials have a dielectric constant as high as a few thousand, which can be used for the embodiments of the present invention.

[0043] Those skilled in the art will appreciate that strategic placement of HDC-pads around the head within a given RF coil at 3 T can result in reduced RF transmission power and improved image SNR throughout the cerebrum, and that with further exploration and development, use of HDC-pads may provide a relatively simple and low-cost method for improving quality and safety of MRI in a variety of applications.

[0044] Those skilled in the art will also appreciate that numerous other modifications to the preferred embodiment and other embodiments of the present invention are possible within the scope of the invention. These include further developments in optimization of the size, shape, thickness and volume of HDC-pads for specific applications to given body parts or organs for MRI systems with various static magnetic field strengths; in formulation and processing of high dielectric materials used for the HDC-pads; in selection of the values of dielectric constant (permittivity) and in incorporation of HDC material in RF coil constructions. Other developments would be implementing HDC-pads and other embodiments of the present invention to the MRI systems with different static magnetic field strengths available.

[0045] While the present invention has been illustrated by the description of exemplary embodiments thereof, and while the embodiments have been described in certain detail, there is no intention to restrict or in any way limit the scope of the claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to any of the specific details, representative devices and methods, and/or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept.