RF Body Coil Research Articles

MRI compatibility study of a prototype radiofrequency penetrable oval PET insert at 3T.

To perform an MRI compatibility study of an RF field-penetrable oval-shaped PET insert that implements an MRI built-in body RF coil both as a transmitter and a receiver. Twelve electrically floating RF shielded PET detector modules were used to construct the prototype oval PET insert with a major axis of 440mm, a minor axis of 350mm, and an axial length of 225mm. The electric floating of the PET detector modules was accomplished by isolating the cable shield from the detector shield using plastic tape. Studies were conducted on the transmit (B1) RF field, the image signal-to-noise ratio (SNR), and the RF pulse amplitude for a homogeneous cylindrical (diameter: 160mm and length: 260mm) phantom (NaCl + NiSO4 solution) in a 3T clinical MRI system (Verio, Siemens, Erlangen, Germany). The B1 maps for the oval insert were similar to the MRI-only field responses. Compared to the MRI-only values, SNR reductions of 51%, 45%, and 59% were seen, respectively, for the spin echo (SE), gradient echo (GE), and echo planar (EPI) images for the case of oval PET insert. Moreover, the required RF pulse amplitudes for the SE, GE, and EPI sequences were, respectively, 1.93, 1.85, and 1.36 times larger. However, a 30% reduction in the average RF reception sensitivity was observed for the oval insert. The prototype floating PET insert was a safety concern for the clinical MRI system, and this compatibility study provided clearance for developing a large body size floating PET insert for the existing MRI system. Because of the RF shield of the insert, relatively large RF powers compared to the MRI-only case were required. Because of this and also due to low RF sensitivity of the body coil, the SNRs reduced largely.

Study on the radiofrequency transparency of partial-ring oval-shaped prototype PET inserts in a 3T clinical MRI system.

The purpose of this study is to evaluate the RF field responses of partial-ring RF-shielded oval-shaped positron emission tomography (PET) inserts that are used in combination with an MRI body RF coil. Partial-ring PET insert is particularly suitable for interventional investigation (e.g., trimodal PET/MRI/ultrasound imaging) and intraoperative (e.g., robotic surgery) PET/MRI studies. In this study, we used electrically floating Faraday RF shield cages to construct different partial-ring configurations of oval and cylindrical PET inserts and performed experiments on the RF field, spin echo and gradient echo images for a homogeneous phantom in a 3T clinical MRI system. For each geometry, partial-ring configurations were studied by removing an opposing pair or a single shield cage from different positions of the PET ring. Compared to the MRI-only case, reduction in mean RF homogeneity, flip angle, and SNR for the detector opening in the first and third quadrants was approximately 13%, 15%, and 43%, respectively, whereas the values were 8%, 23%, and 48%, respectively, for the detector openings in the second and fourth quadrants. The RF field distribution also varied for different partial-ring configurations. It can be concluded that the field penetration was high for the detector openings in the first and third quadrants of both the inserts.

Body composition profiling at 0.55T: Feasibility and precision.

Body composition MRI captures the distribution of fat and lean tissues throughout the body, and provides valuable biomarkers of obesity, metabolic disease, and muscle disorders, as well as risk assessment. Highly reproducible protocols have been developed for 1.5T and 3T MRI. The purpose of this work was to demonstrate the feasibility and test-retest repeatability of MRI body composition profiling on a 0.55T whole-body system. Healthy adult volunteers were scanned on a whole-body 0.55T MRI system using the integrated body RF coil. Experiments were performed to refine parameter settings such as TEs, resolution, flip angle, bandwidth, acceleration, and oversampling factors. The final protocol was evaluated using a test-retest study with subject removal and replacement in 10 adult volunteers (5 M/5F, age 25-60, body mass index 20-30). Compared to 1.5T and 3T, the optimal flip angle at 0.55T was higher (15°), due to the shorter T1 times, and the optimal echo spacing was larger, due to smaller chemical shift between water and fat. Overall image quality was comparable to conventional field strengths, with no significant issues with fat/water swapping or inadequate SNR. Repeatability coefficient of visceral fat, subcutaneous fat, total thigh muscle volume, muscle fat infiltration, and liver fat were 11.8 cL (2.2%), 46.9 cL (1.9%), 14.6 cL (0.5%), 0.1 pp (2%), and 0.2 pp (5%), respectively (coefficient of variation in parenthesis). We demonstrate that 0.55T body composition MRI is feasible and present optimized scan parameters. The resulting images provide satisfactory quality for automated post-processing and produce repeatable results.

Transcranial Magnetic Resonance Imaging-Guided Focused Ultrasound with a 1.5 Tesla Scanner: A Prospective Intraindividual Comparison Study of Intraoperative Imaging.

Background: High-quality intraoperative imaging is needed for optimal monitoring of patients undergoing transcranial MR-guided Focused Ultrasound (tcMRgFUS) thalamotomy. In this paper, we compare the intraoperative imaging obtained with dedicated FUS-Head coil and standard body radiofrequency coil in tcMRgFUS thalamotomy using 1.5-T MR scanner. Methods: This prospective study included adult patients undergoing tcMRgFUS for treatment of essential tremor. Intraoperative T2-weighted FRFSE sequences were acquired after the last high-energy sonication using a dedicated two-channel FUS-Head (2ch-FUS) coil and body radiofrequency (body-RF) coil. Postoperative follow-ups were performed at 48 h using an eight-channel phased-array (8ch-HEAD) coil. Two readers independently assessed the signal-to-noise ratio (SNR) and evaluated the presence of concentric lesional zones (zone I, II and III). Intraindividual differences in SNR and lesional findings were compared using the Wilcoxon signed rank sum test and McNemar test. Results: Eight patients underwent tcMRgFUS thalamotomy. Intraoperative T2-weighted FRFSE images acquired using the 2ch-FUS coil demonstrated significantly higher SNR (R1 median SNR: 10.54; R2: 9.52) compared to the body-RF coil (R1: 2.96, p < 0.001; R2: 2.99, p < 0.001). The SNR was lower compared to the 48-h follow-up (p < 0.001 for both readers). Intraoperative zone I and zone II were more commonly visualized using the 2ch-FUS coil (R1, p = 0.031 and p = 0.008, R2, p = 0.016, p = 0.008), without significant differences with 48-h follow-up (p ≥ 0.063). The inter-reader agreement was almost perfect for both SNR (ICC: 0.85) and lesional findings (k: 0.82–0.91). Conclusions: In the study population, the dedicated 2ch-FUS coil significantly improved the SNR and visualization of lesional zones on intraoperative imaging during tcMRgFUS performed with a 1.5-T MR scanner.

246. Analysis of signal-to-noise ratio for a 2-channels coil developed to enable transcranial Magnetic Resonance-guided Focused Ultrasound Surgery (tcMRgFUS) with 1.5 T MRI scanners

PurposeIn recent years trans-cranial Magnetic Resonance-guided Focused Ultrasound Surgery (tcMRgFUS) treatments have aroused large scientific and medical interest [1]. Usually, tcMRgFUS systems are integrated with 3 T systems which allow the use of the body RF coil for both real time imaging and MR-thermometry. At University Hospital of Palermo there is the world-first installation of a tcMRgFUS system integrated with a 1.5 T scanner. Here we present the characterization in terms of signal-to-noise ratio (SNR) of a dedicated RF coils suitably developed (by InSightec) to enable tcMRgFUS treatments with 1.5 T MR units. Methods and materialsThe focused ultrasound equipment (ExAblate 4000, InSightec) integrates with an MRI unit operating at 1.5 T (Signa HDxt, GE) and consists of a hemispheric 1024-element phased-array transducer. T2w-FRFSE images were obtained from a dedicated Dose Quality Assurance phantom using the same MRI protocol used for real treatments. ResultsThe dedicated flexible coil resulted in larger SNR values with respect to that achieved using MRI’s body RF coil. In particular, we obtained for the axial plane a SNR equal to 27.5 using dedicated coil whereas it was equal to 2.5 with RF body coil (SNR gain more than 10 times higher). ConclusionWe can conclude that, even though the SNR linearly increases with magnetic field strength, the use of a dedicated coil on a 1.5 T scanner enabled us to obtain images with a SNR roughly 5 times larger than those acquired on 3T scanners using MRI’s body RF coil. This could facilitate the spread of this cutting-edge technology worldwide [2].

Changes in the specific absorption rate (SAR) of radiofrequency energy in patients with retained cardiac leads during MRI at 1.5T and 3T.

To evaluate the local specific absorption rate (SAR) and heating around retained cardiac leads during MRI at 64 MHz (1.5T) and 127 MHz (3T) as a function of RF coil type and imaging landmark. Numerical models of retained cardiac leads were built from CT and X-ray images of 6 patients with retained cardiac leads. Electromagnetic simulations and bio-heat modeling were performed with MRI RF body and head coils tuned to 64 MHz and 127 MHz and positioned at 9 different imaging landmarks covering an area from the head to the lower limbs. For all patients and at both 1.5T and 3T, local transmit head coils produced negligible temperature rise ( ) for . For body imaging with quadrature-driven coils at 1.5T, during a 10-min scan remained < 3°C at all imaging landmarks for and <6°C for . For body imaging at 3T, during a 10-min scan remained < 6°C at all imaging landmarks for . For shorter pulse sequences up to 2 min, remained < 6°C for . For the models based on 6 patients studied, simulations suggest that MRI could be performed safely using a local head coil at both 1.5T and 3T, and with a body coil at 1.5T with pulses that produced . MRI at 3T could be performed safely in these patients using pulses with .

Investigating the effect of coil model losses on computational electromagnetic exposure of an ASTM phantom at 64 MHz MRI.

The goal of this work is to investigate the effect of coil losses on the electromagnetic field generated in an ASTM phantom by a birdcage coil. The study was based on different numerical implementations of an RF body coil at 64 MHz, using the same 3D EM and RF circuit co-simulation procedure. The coil quality factor was evaluated with respect to losses due to power feed mismatch and to resistive losses of the coil components. The results of the study showed that the magnetic field at the coil iso-center, normalized to the square root of the whole body specific absorption rate, depends on the coil quality factor.

Local Multi-Channel RF Surface Coil versus Body RF Coil Transmission for Cardiac Magnetic Resonance at 3 Tesla: Which Configuration Is Winning the Game?

IntroductionThe purpose of this study was to demonstrate the feasibility and efficiency of cardiac MR at 3 Tesla using local four-channel RF coil transmission and benchmark it against large volume body RF coil excitation.MethodsElectromagnetic field simulations are conducted to detail RF power deposition, transmission field uniformity and efficiency for local and body RF coil transmission. For both excitation regimes transmission field maps are acquired in a human torso phantom. For each transmission regime flip angle distributions and blood-myocardium contrast are examined in a volunteer study of 12 subjects. The feasibility of the local transceiver RF coil array for cardiac chamber quantification at 3 Tesla is demonstrated.ResultsOur simulations and experiments demonstrate that cardiac MR at 3 Tesla using four-channel surface RF coil transmission is competitive versus current clinical CMR practice of large volume body RF coil transmission. The efficiency advantage of the 4TX/4RX setup facilitates shorter repetition times governed by local SAR limits versus body RF coil transmission at whole-body SAR limit. No statistically significant difference was found for cardiac chamber quantification derived with body RF coil versus four-channel surface RF coil transmission. Our simulation also show that the body RF coil exceeds local SAR limits by a factor of ~2 when driven at maximum applicable input power to reach the whole-body SAR limit.ConclusionPursuing local surface RF coil arrays for transmission in cardiac MR is a conceptually appealing alternative to body RF coil transmission, especially for patients with implants.

Subject‐loaded quadrifilar helical‐antenna RF coil with high B1+ field uniformity and large FOV for 3‐T MRI

AbstractA novel method for excitation of RF B1 field in high‐field (3‐T) magnetic resonance imaging (MRI) systems using a subject‐loaded quadrifilar helical antenna as an RF coil is proposed, evaluated, and demonstrated. Design, analysis, characterization, and evaluation of the novel coil when situated in a 3‐T MRI bore and loaded with different phantoms are performed and cross‐validated by extensive numerical simulations using multiple computational electromagnetics techniques. The results for the quadrifilar helical‐antenna RF body coil show (a) strong field penetration in the entire phantoms; (b) excellent right‐hand circular polarization (RCP); (c) high spatial uniformity of RCP RF magnetic field, B1+, throughout the phantoms; (d) large field of view (FOV); (e) good transmit efficiency; and (f) low local specific absorption rate (SAR). The examples show that the new RF coil provides substantially better B1+‐field uniformity and much larger FOV than any of the previously reported numerical and experimental results for the existing RF coil designs at 3 T in literature that enable comparison. In addition, helical RF body coils of different lengths can, for instance, easily provide an excellent RCP and highly uniform B1+‐field within the MRI maximum FOV length of 50 cm, and even 100 cm. The proposed MRI RF coil yields a remarkable improvement in the field uniformity in the longitudinal direction, for various phantoms, with comparable efficiency and SAR levels.

An adaptive diffusion-weighted whole-body magnetic resonance imaging scheme using the multistation approach

Whole-body diffusion-weighted imaging (DWI) is a useful tool in oncology, which enables fast screening of disseminated tumors, lymph nodes or abscesses in the body. Multistation magnetic resonance imaging (MRI) or continuously moving table (CMT) MRI can be performed to overcome the limited field of view (FOV) of the magnet bore in whole-body DWI. Although CMT-MRI is regarded as a more advanced form of whole-body MRI, it cannot be widely used because most of the available MR systems are not equipped with the required hardware/software to perform CMT. Thus, optimizing the multistation approach for whole-body DWI, which is more widely available and easier to perform with the existing MR systems, is worthwhile. To improve the quality of DW images acquired with the multistation approach, we used different combinations of the built-in body RF coil and the phased-array surface RF coils for reception of the signals in whole-body DWI in this work. If different coils are selectively used in the extended FOV and appropriate reconstruction algorithms are exploited, the screening ability of whole-body DWI can be improved while minimizing the patient’s discomfort and the artifacts due to physiological motions.

In vitro assessment of MRI issues at 3-Tesla for a breast tissue expander with a remote port

ObjectivesA patient with a breast tissue expander may require a diagnostic assessment using magnetic resonance imaging (MRI). To ensure patient safety, this type of implant must undergo in vitro MRI testing using proper techniques. Therefore, this investigation evaluated MRI issues (i.e., magnetic field interactions, heating, and artifacts) at 3-Tesla for a breast tissue expander with a remote port. MethodsA breast tissue expander with a remote port (Integra Breast Tissue Expander, Model 3612-06 with Standard Remote Port, PMT Corporation, Chanhassen, MN) underwent evaluation for magnetic field interactions (translational attraction and torque), MRI-related heating, and artifacts using standardized techniques. Heating was evaluated by placing the implant in a gelled-saline-filled phantom and MRI was performed using a transmit/receive RF body coil at an MR system reported, whole body averaged specific absorption rate of 2.9-W/kg. Artifacts were characterized using T1-weighted and GRE pulse sequences. ResultsMagnetic field interactions were not substantial and, thus, will not pose a hazard to a patient in a 3-Tesla or less MRI environment. The highest temperature rise was 1.7°C, which is physiologically inconsequential. Artifacts were large in relation to the remote port and metal connector of the implant but will only present problems if the MR imaging area of interest is where these components are located. ConclusionsA patient with this breast tissue expander with a remote port may safely undergo MRI at 3-Tesla or less under the conditions used for this investigation. These findings are the first reported at 3-Tesla for a tissue expander.

SU-E-J-90: MRI-Guided Targeted Prostate Cancer Radiation Therapy: Imaging, Diagnosis, and Image Co-Registration

Purpose: This study investigates using MR imaging for prostate cancer targeted radiation therapy, including using MR imaging to localize tumor in prostate for focused radiation therapy, co‐registering MR images to CT or ultrasound images to guide or evaluate cancer radiation therapy, and determining tumor stage and its invasion to locoregional lymph nodes. Methods: Either endorectal (ER) or body RF coil has been used to acquire MR images. To determine the localization and volume of the prostate tumor from the MR images and to understand their correlation with the pathology slices, both pre‐surgery ER coil MR imaging and post‐surgery multi‐slice pathological analysis of the surgically excised tumor were studied (n=8). To avoid prostate dislocation caused by ER coil, multiple MR imaging protocols using the body coil were implemented to depict the structure of prostate and its surrounding tissues, pelvic lymph nodes, and obtain dynamics of contrast enhancement (n=37). MIM™ software was used to co‐register MR images to CT or ultrasound images for guiding treatment planning and evaluating prostate cancer brachytherapy. Results: The MR diagnosed tumor was proven by prostate pathology, which shows the rationale of using MRI guidance for focused treatment of prostate cancer to decrease toxicity and improve tumor control. It has been demonstrated that the capability of tumor localization and volume determination based on MR images are associated with tumor cell density. CT/MRI and ultrasound/MRI co‐registration provides excellent prostate micro‐structure, and surrounding tissue information for improved treatment planning, as well as post‐treatment evaluation. By using multi‐MR image synergistic co‐registration, T2‐weighted MR images can be accurately matched to CT image with <2 mm alignment error. Conclusion: MR image guidance is excellent for targeted prostate cancer radiation therapy, and it has been successfully used in our clinical practice.

Assessment of MRI issues at 3-Tesla for metallic surgical implants: findings applied to 61 additional skin closure staples and vessel ligation clips.

PurposeMetallic skin closure staples and vessel ligation clips should be tested at 3-Tesla to characterize MRI issues in order to ensure patient safety. Therefore, metallic surgical implants were assessed at 3-Tesla for magnetic field interactions, MRI-related heating, and artifacts.MethodsA skin closure staple (Visistat Skin Stapler, staple, Polytetrafluoroethylene, PTFE, coated 316L/316LVM stainless steel; Teleflex Medical, Durham, NC) and a vessel ligation clip (Hemoclip Traditional, stainless steel; Teleflex Medical, Durham, NC) that represented the largest metallic sizes made from materials with the highest magnetic susceptibilities (i.e., based on material information) among 61 other surgical implants (52 metallic implants, 9 nonmetallic implants) underwent evaluation for magnetic field interactions, MRI-related heating, and artifacts using standardized techniques. MRI-related heating was assessed by placing each implant in a gelled-saline-filled phantom with MRI performed using a transmit/receive RF body coil at an MR system reported, whole body averaged SAR of 2.9-W/kg for 15-min. Artifacts were characterized using T1-weighted, SE and GRE pulse sequences.ResultsEach surgical implant showed minor magnetic field interactions (20- and 27-degrees, which is acceptable from a safety consideration). Heating was not substantial (highest temperature change, ≤ 1.6°C). Artifacts may create issues if the area of interest is in the same area or close to the respective surgical implant.ConclusionsThe results demonstrated that it would be acceptable for patients with these metallic surgical implants to undergo MRI at 3-Tesla or less. Because of the materials and dimensions of the surgical implants that underwent testing, these findings pertain to 61 additional similar implants.

The design of a body RF coil for low-field open MRI using pseudo electric dipole radiation and simulated annealing

The paper presents a new body RF coil design scheme for a low-field open MRI system. The RF coil is composed of four rectangular loops which are made of wide copper strips located near the surfaces of the bottom and top pole faces of the permanent magnet. The body RF coil has been designed by using the pseudo electric dipole radiation (PEDPR) method with the Metropolis algorithm. In the calculation of the RF fields via the finite difference time domain (FDTD) method, the computational time increases as the RF frequency becomes lower. Moreover, the computational process using the FDTD method takes a very long time when the RF coil is optimized. The optimization requires varying the configuration of the RF coil system and performing successive calculations of field strength and field homogeneity. When we perform these successive calculations, the computational time can be reduced by using the PEDPR method, where the segmented current elements of the RF coil are treated as pseudo electric dipole radiation sources. Because the RF coil is made of wide strips, the variation of the current density on the strip has been considered in the B 1-field calculation. For each configuration of the RF coil system, the current distribution is calculated via circuit analysis, where each copper strip is considered as a parallel combination of current element lines. The preliminary field calculation study by the FDTD method verifies both the circuit analysis method for the current distribution and the PEDPR method for the radiation field strength. The optimization of the RF coil configuration is performed by the Simulated Annealing (SA) process using the Metropolis algorithm. Simulations have been performed for a 10 MHz RF frequency. The optimized RF coil has four rectangular loops of 37 cm × 100 cm with 6.5 cm wide strips which are separated vertically 49 cm and horizontally center-to-center 63 cm. In the 25 cm diameter of spherical volume (DSV), the design results show a good field inhomogeneity of the B 1-field below 0.49 dB (5.8%).

Ventricular assist device implant (AB 5000) prototype cannula: in vitro assessment of MRI issues at 3-Tesla.

PurposeTo evaluate MRI issues at 3-Tesla for a ventricular assist device (VAD).MethodsThe AB5000 Ventricle with a prototype Nitinol wire-reinforced In-Flow Cannula and Out-Flow Cannula attached (Abiomed, Inc., Danvers, MA) was evaluated for magnetic field interactions, heating, and artifacts at 3-Tesla. MRI-related heating was assessed with the device in a gelled-saline-filled, head/torso phantom using a transmit/received RF body coil while performing MRI at a whole body averaged SAR of 3-W/kg for 15-min. Artifacts were assessed for the main metallic component of this VAD (atrial cannula) using T1-weighted, spin echo and gradient echo pulse sequences.ResultsThe AB5000 Ventricle with the prototype In-Flow Cannula and Out-Flow Cannula attached showed relatively minor magnetic field interactions that will not cause movement in situ. Heating was not excessive (highest temperature change, +0.8°C). Artifacts may create issues for diagnostic imaging if the area of interest is in the same area or close to the implanted metallic component of this VAD (i.e., the venous cannula).ConclusionThe results of this investigation demonstrated that it would be acceptable for a patient with this VAD (AB5000 Ventricle with a prototype Nitinol wire-reinforced In-Flow Cannula and Out-Flow Cannula attached) to undergo MRI at 3-Tesla or less. Notably, it is likely that the operation console for this device requires positioning a suitable distance (beyond the 100 Gauss line or in the MR control room) from the 3-Tesla MR system to ensure proper function of the VAD.

Drug Eluting Coronary Stent: In Vitro Evaluation of Magnet Resonance Safety at 3 Tesla

To evaluate MR safety at 3 Tesla for a drug eluting coronary stent. A drug eluting coronary stent (Endeavor, cobalt alloy, Medtronic Vascular, Santa Rosa, CA) was evaluated for magnetic field interactions, heating, and artifacts at 3 Tesla. MRI-related heating was assessed with the stent in a gelled saline-filled phantom using a transmit/received RF body coil with a whole body averaged SAR of 2.0 W/kg. Artifacts were characterized using T1-weighted, spin echo, and gradient echo pulse sequences. The stent exhibited minor magnetic field interactions that will not cause migration. Heating was not substantial (+0.5 degrees C). Artifacts may create a problem if the area of interest is in the same area or close to the stent (e.g., for a T1-weighted, spin echo pulse sequence, within approximately 16 mm; for a gradient echo pulse sequence, within approximately 23 mm). Conclusion. The findings indicated that it would be safe for a patient with this cobalt alloy-based, drug-eluting coronary stent to undergo MRI at 3 Tesla or less. Importantly, because of the relative lack of magnetic field interactions, MRI may be performed immediately after implantation.

Optimization of MR-Relaxometry for BMD-measurements and its Correlation with DEXA.

The aim of this study was to optimize MRI conventional protocols for BMD measurements using MR-Relaxometry in systems not facilitated with special multi echo protocols. Since, cortical and trabecular bone separation can not be performed in DEXA, so the results might lead to erroneous interpretation of BMD values. One method for bone quality determination is MR relaxometry that derives R2(=1/T2), R2*(=1/T2*) and R2'(=R2*-R2). This study was performed by 1.5T MRI system(Picker Vista-Q800), an uniformity phantom(1.25gr/l CuSO4, with T2=200ms for calibration), a body RF-Coil, 7 normal, 7 osteopenia, 7 osteoporosis volunteers and Lunar DEXA system(DPX-MD). To determine R2*and R2, multi GE and SE protocols with different TE/TR were used. Then in phantom and in coronal section of femoral-neck, relaxation rates were compared with BMD. The slope of neperian-logarithm of signal vs. TE in GE as -R2*used for protocol optimization. Therefore, for phantom calibration, optimized GE parameters of TE=13.42/18/26.8 ms, TR=800ms and ST=8mm used for the measurement of R2*, while, the measurement of R2 required the optimized SE parameters of TE=30/60/90/120ms, TR=800ms and ST=8mm, with CV(R2*)=2.96%, CV(R2)=3%, respectively. In volunteers for SE, TE of 36/54/63/72ms and TR=800ms were used, while, for GE the TEs/TR were the same as those of phantom study. R2*and R2' showed a significant positive correlation with BMD, r=0.62(p<0.05) & r=0.62(p<0.05) respectively. Finally, in accordance with DEXA values, the results showed that MR-Relaxometry is a proper tool for BMD-measurements in femoral-neck. Also it may be used as a complement method for DEXA failure in BMD-assessments.

Helical MR: continuously moving table axial imaging with radial acquisitions.

A technique for extended field of view MRI is presented. Similar to helical computed tomography, the method utilizes a continuously moving patient table, a 2D axial slice that remains fixed relative to the MRI magnet, and a radial k-space trajectory. A fully refocused SSFP acquisition enables spatial resolution comparable to current clinical protocols in scan times that are sufficiently short to allow a reasonable breathhold duration. RF transmission and signal reception are performed using the RF body coil and the images are reconstructed in real time. Experimental results are presented that illustrate the technique's ability to resolve small structures in the table-motion direction. Simulation experiments to study the steady-state response of the fully refocused SSFP acquisition during continuous table motion are also presented. Finally, whole body images of healthy volunteers demonstrate the high image quality achieved using the helical MRI approach.

Making MRI Quieter

We have mitigated acoustic noise in a 1.5 T cylindrical MRI scanner equipped with epoxy-potted, shielded gradients. It has been widely assumed that MRI acoustic noise comes overwhelmingly from vibrations of the gradient assembly. However, with vibration-isolated gradients contained in an airtight enclosure, we found the primary sources of acoustic noise to be eddy-current-induced vibrations of metal structures such as the cryostat inner bore and the rf body coil. We have elucidated the relative strengths of source-pathways of acoustic noise and assembled a reduced-acoustic-noise demonstration MRI system. This scanner employed a number of acoustic noise reduction measures including a vacuum enclosure of a vibrationally isolated gradient assembly, a low-eddy-current rf coil and a non-conducting inner bore cryostat. The demonstration scanner reduced, by about 20 dBA, the acoustic noise levels in the patient bore to 85 dBA and below for several typical noisy pulse sequences. The noise level standing near the patient bore is 71 dBA and below. We have applied Statistical Energy Analysis to develop a vibroacoustic model of the MR system. Our model includes vibrational sources and acoustic pathways to predict acoustic noise and provides a good spectral match above 400 Hz to experimentally measured sound levels. This tool enables us to factor acoustics into the design parameters of new MRI systems.

An analytical model for the design of RF resonators for MR body imaging

A closed form, analytical solution describing the RF fields generated by an RF body coil resonator for MR imaging at 1.5 and 4.0 T is presented. This solution extends the results of earlier studies of RF penetration in the body by explicitly including the RF coil, the RF shield, and the field variation along the z axis for high-pass bird cage coils. A salient feature of this treatment is the calculation of the axial propagation constant, kz, which determines the z dependence of the RF field. We have determined the relative power deposition in the body, the B1 field homogeneity, and coil losses, which are functions of the coil-to-shield separation and body size. The relative power deposition in the body has been calculated to vary as the 1.58 power of the body radius. The calculations have also predicted that the field homogeneity in the z direction exhibits greater degradation at higher frequencies in a high-pass coil than in a low-pass coil. The model predicts an increase in coil losses by a factor of 2.8 as the coil-to-shield separation is reduced from 5 to 2 cm in a standard body resonator. Although the results for only a homogeneous cylindrical object or body are presented, the theory can be extended to a multilayered heterogeneous object of varying permittivity and conductivity.