Patent Application
20160095552
The disclosure relates to a non-invasive, radiofrequency (RF) coil array, a Magnetic Resonance Imaging (MRI) system and methods of using the MRI system. In magnetic resonance imaging (MRI), the detection sensitivity of a radiofrequency (RF) loop coil dependents on the distance of the organ to the coil, as well as the coil's shape, size, and orientation. The closer the RF coil is placed to the region-of-interest (ROI), the better the signal-to-noise ratio (SNR) of the resulting MR image (Hayes and Axel 1985). Thus instead of using a general-purpose RF coil (e.g. the cylindrical birdcage coil), dramatic improvements in image quality can be achieved by utilizing an array of loop coils configured for the specific anatomy of interest. The loop coils can be designed and positioned to be as close to the ROI as possible to achieve the maximum SNR.
The torso-pelvic receiver array coil is one such example of tailoring the design for a specific anatomy, in this case the overall pelvic region which includes the prostate. However, the internal position of the prostate, and the distance to the external coils limits its detection sensitivity. As such, prostate MRI using only the torso-pelvic coil lacks sufficient SNR to visualize transcapsular tumor spread or seminal vesicle involvement, resulting in a decrease in staging accuracy and specificity (Fütterer et al 2007, Heijmink et al 2007). To address the SNR requirements for prostate MRI, an endorectal coil can be utilized instead. This coil is placed internally in close proximity to the prostate, resulting in a substantial increase in the SNR from the prostate. However due to its limited field-of-view (FOV) and non-uniform sensitivity, the endorectal coil can be used in combination with the torso-pelvic coil to provide full coverage of the pelvic region. This approach provides the high SNR of the prostate along with a large FOV to evaluate the pelvic lymph nodes and pelvic bones for metastatic disease. The combination of endorectal and torso-pelvic coils is the current state-of-the-art approach for prostate MRI (Turkbey et al 2009).
Some embodiments relate to a non-invasive, radiofrequency (RF) coil array including a plurality of loop coils configured to wrap in close proximity around a specific anatomical region of a subject.
In some embodiments, the specific anatomical region is the anteroposterior region of the inferior pelvic abdomen and rectum of a subject.
In some embodiments, the plurality of receiver coils comprises a central coil loop and a one or more loop coil(s) positioned adjacent to and on each side of the central loop.
In some embodiments, the RF coil array includes five coils, wherein two pairs coils are positioned adjacent to and on each side of a central coil loop.
In some embodiments, the coils include one or more copper printed circuit board(s) (PCB), wherein circuit patterns for the loop coils are etched on the PCB.
In some embodiments, the RF coil array includes a central, butterfly shaped loop coil.
In some embodiments, non-adjacent coil elements in the plurality of coils are decoupled by low noise amplifiers (LNAs).
Some embodiments relate to a magnetic Resonance Imaging (MRI) system including an RF coil array disclosed herein, wherein the plurality of receiver coils are connected to the MRI system.
In some systems, the RF coil array is connected to the MRI system by one or more coaxial cables.
Some embodiments relate to a method of magnetic resonance imaging including: placing an RF coil array as disclosed herein in close proximity to a specific anatomical region of a subject, and scanning the specific anatomical region of the subject by magnetic resonance imaging.
In some embodiments, the method further includes providing a medical diagnosis pertaining to said subject.
In some embodiments of magnetic resonance imaging, the specific anatomical region of the subject is the anteroposterior region of the inferior pelvic abdomen and rectum of a subject.
Some embodiments of the disclosed method include a step of visualizing a prostate region of said subject.
The combined use of a torso-pelvic RF array coil and endorectal RF coil is the current state-of-the-art in prostate MRI. The endorectal coil provides high detection sensitivity to acquire high-spatial resolution images and spectroscopic data, while the torso-pelvic coil provides large coverage to assess pelvic lymph nodes and pelvic bones for metastatic disease. However, the use of an endorectal coil is an invasive procedure that presents difficulties for both patients and technicians. We disclose here a novel, non-invasive RF coil design that provides both image SNR and FOV coverage comparable to the combined torso-pelvic and endorectal coil configuration. A prototype coil was constructed and tested using a pelvic phantom. The results demonstrate that this new design is a viable alternative for prostate MRI.
Despite the improved image quality, the use of an endorectal coil is an invasive, highly uncomfortable procedure that requires a specialized technician to properly position the device and carefully monitor RF heating. Its discomfort for patients and difficulty for technicians poses a significant limitation that impedes the widespread use of prostate MRI. The development of a non-invasive RF coil that provides comparable SNR to the endorectal coil greatly contributes to the full clinical potential of prostate MRI. To this end, a new diaper-shaped array coil (henceforth referred to as the “diaper coil”) was designed and tested for significantly improving the image SNR and uniformity of the prostate region. A prototype coil was constructed on an acrylic frame and evaluated through 3 T MR imaging of a pelvic phantom. Imaging was also performed using a commercial 6-channel torso-pelvic array coil and single-loop endorectal coil for comparison.
At first, the proposed RF coil and dielectric phantom were modeled using a full wave electromagnetic field simulation program (SEMCAD X, Ver. 14.2.1; Schmid & Partner Engineering AG, Zurich, Switzerland) to visualize the B1-field maps for individual loop coils. For the dielectric phantom, the dielectric properties of the abdomen (relative permittivity ∈r=69.0, electric conductivity σ=1.50 S/m), bladder (∈r=21.9, σ=0.30), and prostate (∈r=72.2, σ=0.93) were selected to mimic mean tissue values at 127.7 MHz (Gabriel et al 1996).
To eliminate the use of the invasive endorectal coil while still obtaining high SNR within the prostate region and coverage across the pelvis, we developed a new design consisting of a diaper-shaped array of receiver coils placed around the anteroposterior region of the inferior pelvic abdomen and rectum. To test the efficacy of this design concept, we constructed and tested a five-segment prototype as shown in
The performance of this diaper coil was compared to a commercial 6-channel torso-pelvic array coil (USA Instruments, Inc.; Aurora, Ohio) and endorectal coil (Medrad, Inc.; Indianola, Pa.) though the imaging of an in-house-built pelvic phantom. The torso-pelvic coil consists of 6 non-overlapping rectangular loops (width=125 mm, length=235 mm). Three loops are placed above the patient, while 3 loops are positioned below the patient, providing coverage of 395 mm along the x-axis and 180 mm along the y-axis. The endorectal coil consists of a single rectangular loop (width=30 mm, height=80 mm) formed by a thin copper strip (width=2 mm, thickness=34 μm). The pelvic phantom was constructed using acrylic for the oval cylinder (width=300 mm, length=300 mm, height=190 mm) to mimic the abdomen and hollow polypropylene balls for the bladder and prostate regions. A hollow polypropylene ball to model the bladder (diameter=100 mm) and a smaller hollow polypropylene ball to model the prostate (diameter=35 mm) were positioned within the cylinder to mimic human anatomy (Schulte et al 2006, Standring et al 2008, Yokochi et al 1978).
To mimic the dielectric properties of the human body, we followed the approach of Yang et al (2004), where the concentration of NaCl in solution relative to the phantom volume was adjusted so that the ratio σ/ω∈0∈r was the same as in human tissue, where w is the angular (Larmor) frequency and ∈0 is the permittivity of free space. The phantom was filled with 120 mM, 20 mM, and 70 mM of NaCl solution in the abdomen, bladder and prostate regions respectively. 10 mM of CuSO4 was also added to each region to improve image quality in T1-weighted imaging.
The diaper coil was positioned across the curved face of phantom. The torso-pelvic coil was wrapped on the top and bottom faces of the phantom. The endorectal coil was positioned within the phantom directly underneath the sphere representing the prostate.
For constructing the diaper array coil, the circuit pattern for each loop coil was etched on a separate printed circuit board (PCB) composed of flame-retardant G-10 plastic. Discrete capacitors (ATC 100B Series Porcelain Superchip Multilayer Capacitor; American Technical Ceramics Corporation, Huntington Station, USA) and a trimmer capacitor (NMAF30; Voltronics Corporation, Denville, USA) were soldered onto each PCB as shown in
To interface the array coil with the MRI scanner, we prepared five coaxial cable (length=λ/2) and connector assemblies each containing a cable balun tuned to 127.7 MHz. For each assembly, one cable end was soldered to the capacitive matching circuit of one coil element and the other end was connected to the coil interface box. Eight LNAs with 25 dB gain, 0.4 dB noise figure, and 5±1Ω input impedance tuned to 127.7 MHz were also mounted in the coil interface box. Identical LNAs were employed for the torso-pelvic and endorectal coils. The low input impedance of the LNAs effectively work in conjunction with an individual coil's matching/decoupling circuit to eliminate residual magnetic fields induced in neighboring coil elements, thus further reducing mutual coupling. The coil interface box also provided the voltages (−5V) and currents (150 mA) that drive the PIN diode in each active decoupling circuit.
MR images of the phantom were acquired using all three coils (diaper, torso-pelvic, endorectal) with a 3 T Philips Achieva system (Philips Medical Systems) with the following scan parameters: sequence=T1-weighted gradient echo (T1W-FFE), repetition time (TR)=161 ms, echo time (TE)=4.6 ms, flip angle=80°, matrix=528×528, FOV=300 mm×300 mm, slice thickness=3 mm and number of excitations (NEX)=2. Acquired individual coil loop images were combined by most commonly used sum-of-squares algorithm (Roemer et al 1990). The axial slice covering the middle of the prostate region was used to calculate the integral uniformity (IU) and the SNR within a local FOV. The SNR of the local FOV was computed by Eq. [1]. The mean signal (Savg.) was defined as the mean pixel intensity value in a 50 mm ROI covering the prostate phantom. The mean noise (Navg.) as well as the standard deviation (σ) is measured on the image background.
SNR=(Savg−Navg)/σ Eq. [1]
The percentage integral uniformity (IU) was calculated as Eq. [2] where the maximum and minimum values were taken from pixels within the prostate region.
MR imaging of the prostate phantom, shown in
Our results demonstrate that the diaper coil can serve as a noninvasive alternative to the standard torso-pelvic and endorectal coil combination currently used in clinical prostate MRI by providing both comparable image SNR and FOV coverage. While the mean image SNR using the diaper and endorectal coils are similar, the endorectal coil provides superior detection sensitivity in regions closest to the coil.
Our original 5-channel prototype was designed with the concept of positioning the coil between a patient's legs immediately adjacent to the crotch. The shape of the coil was formed with this configuration in mind and tested using phantoms. The ergonomics of the coil design are relevant to in vivo studies. Advantageously, the coil is designed to fit as comfortable as possible, while maintaining its performance quality. Specifically, in some embodiments, edges and sharp bends are shaped to smooth contours, particularly around the patent's crotch. In some embodiments, electronic components are fully insulated from contact with the patient. The rigid cylindrical acrylic shell used to mount the coil loops can be replaced with flexible, folding elements that can form to each patient's unique body shape.
We disclose a new RF coil design (e.g., for prostate MRI, consisting of a diaper-shaped array of receiver (or loop) coils placed around the anteroposterior region of the inferior pelvic abdomen and rectum). The placement of the new RF coil array may be adjacent to, near or in close proximity to (e.g., at a distance no greater than that which provides adequate imaging and no less than non-invasive contact with the subject), around (e.g., at least partially surrounding) the anatomical region of interest. MRI experiments using a phantom demonstrated that the non-invasive diaper coil can provide image SNR and FOV coverage comparable to the torso-pelvic and invasive endorectal coil combination currently used in clinical practice. These results provide a basis for imaging on human subjects.
In one embodiment, after placing the RF coil array adjacent to a specific anatomical region of a subject, and scanning the specific anatomical region of the subject by magnetic resonance imaging, an image is visualized by conventional equipment/methods, and optionally, a medical diagnosis is rendered based on the visualized image.
The above references are hereby incorporated in their entireties by reference thereto.
This application claims priority U.S. Provisional Application No. 61/988,480 filed on May 5, 2014. Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57.
| Number | Date | Country | |
|---|---|---|---|
| 61988480 | May 2014 | US |
