Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements

High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B(1) (B(1) (+)) shimming to minimize B(1) (+) losses resulting from destructive interferences. B(1) (+) shimming was performed by altering the input phase of individual RF channels based on relative B(1) (+) phase maps rapidly obtained in vivo for each channel of an eight-element stripline coil. The relative transmit phases needed to maximize B(1) (+) coherence within a limited region around the prostate greatly differed from those dictated by coil geometry and were highly subject-dependent. A set of transmit phases determined by B(1) (+) shimming provided a gain in transmit efficiency of 4.2 +/- 2.7 in the prostate when compared to the standard transmit phases determined by coil geometry. This increased efficiency resulted in large reductions in required RF power for a given flip angle in the prostate which, when accounted for in modeling studies, resulted in significant reductions of local specific absorption rates. Additionally, B(1) (+) shimming decreased B(1) (+) nonuniformity within the prostate from (24 +/- 9%) to (5 +/- 4%). This study demonstrates the tremendous impact of fast local B(1) (+) phase shimming on ultrahigh magnetic field body imaging.     

An MR transceive phased array designed for spinal cord imaging at 3 Tesla: preliminary investigations of spinal cord imaging at 3 T

A 3 Tesla transceive phased array has been developed that demonstrates the feasibility of spinal cord imaging at high fields. The phased array includes transmit/receive switches, a power distribution network, and 4 coil elements arranged for specific anatomies. Images demonstrating anatomy of the spinal cord and posterior spine were presented. Simulations were performed to predict B(1) field and SAR, with SAR values found to be within Food and Drug Administration limits for the pulse sequences that were used.     

Neurostimulation systems for deep brain stimulation: in vitro evaluation of magnetic resonance imaging-related heating at 1.5 tesla

PURPOSE: To assess magnetic resonance imaging (MRI)-related heating for a neurostimulation system (Activa Tremor Control System, Medtronic, Minneapolis, MN) used for chronic deep brain stimulation (DBS). MATERIALS AND METHODS: Different configurations were evaluated for bilateral neurostimulators (Soletra Model 7426), extensions, and leads to assess worst-case and clinically relevant positioning scenarios. In vitro testing was performed using a 1.5-T/64-MHz MR system and a gel-filled phantom designed to approximate the head and upper torso of a human subject. MRI was conducted using the transmit/receive body and transmit/receive head radio frequency (RF) coils. Various levels of RF energy were applied with the transmit/receive body (whole-body averaged specific absorption rate (SAR); range, 0.98-3.90 W/kg) and transmit/receive head (whole-body averaged SAR; range, 0.07-0.24 W/kg) coils. A fluoroptic thermometry system was used to record temperatures at multiple locations before (1 minute) and during (15 minutes) MRI. RESULTS: Using the body RF coil, the highest temperature changes ranged from 2.5 degrees-25.3 degrees C. Using the head RF coil, the highest temperature changes ranged from 2.3 degrees-7.1 degrees C.Thus, these findings indicated that substantial heating occurs under certain conditions, while others produce relatively minor, physiologically inconsequential temperature increases. CONCLUSION: The temperature increases were dependent on the type of RF coil, level of SAR used, and how the lead wires were positioned. Notably, the use of clinically relevant positioning techniques for the neurostimulation system and low SARs commonly used for imaging the brain generated little heating. Based on this information, MR safety guidelines are provided. These observations are restricted to the tested neurostimulation system.     

4 T actively detunable transmit/receive transverse electromagnetic coil and 4-channel receive-only phased array for (1)H human brain studies.

The design and construction of a 4 T transverse electromagnetic (TEM) transmit/receive head coil and a four-channel phased array receive-only RF system are described. To enable both high-resolution imaging of the entire brain and high-resolution spectroscopic imaging, active PIN diode decoupling was used in both the TEM resonator and each surface coil in the array. This configuration allows for both transmission and reception from the volume coil as well as reception from the phased array. The surface coils were decoupled by overlapping the coils and using preamplifier decoupling. Since at high frequencies construction of a lumped element matching quarter wavelength transformer, an important component of the preamplifier decoupling, becomes difficult, a transmission line approach was used. The system was tested and compared to a TEM volume transmit/receive head coil. A four- to sixfold improvement in signal-to-noise ratio from the sensitive volume of the array was achieved.     

SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

Purpose

To determine the best radiofrequency (RF) shimming method for 7 T body imaging that provides sufficient B₁⁺ excitation inside the target region while energy deposition (SAR) and power demands are as low as possible and that does not incorporate anatomy specific electric field information inside the patient models, as this information is not available in practice.

Materials and Methods

Finite difference time domain (FDTD) simulations were used to evaluate five RF shimming strategies for the pelvis inside a body coil. The results were compared to the theoretical best solution that could be achieved if the electric field inside the patient was known.

Results

Most of the RF shimming strategies were successful. However, between the different strategies a factor of two difference in average SAR reduction, a factor of three difference in local maximum SAR reduction, and a factor of 20 difference in power efficiency was observed. Phase matching was found to be the most promising RF shimming method for the body coil used and patient models.

Conclusion

RF shimming can reduce the SAR and improve power efficiency in an accurate patient model without knowing the electric field. However, choosing the right method is critical to prevent unexpected behavior in local SAR deposition. J. Magn. Reson. Imaging 2009;30:194–202. © 2009 Wiley‐Liss, Inc.     

Open half-volume quadrature transverse electromagnetic coil for high-field magnetic resonance imaging.

A half-volume quadrature head transverse electromagnetic (TEM) coil has been constructed for 4 T imaging applications. This coil produces a sufficiently large homogeneous B(1) field region for the use as a volume coil. It provides superior transmission efficiency, resulting in significantly lower power deposition, as well as greater sensitivity and improved patient comfort and accessibility compared with conventional full-volume coils. Additionally, this coil suppresses the RF penetration artifact that distorts the RF magnetic field profile and alters the intensity in high-field images recorded with linear surface and volume coils. These advantages make it possible to apply this device as an efficient transmit/receive coil for high-field imaging with a restricted field of view.     

Improved homogeneity of the transmit field by simultaneous transmission with phased array and volume coil

Purpose:

To improve the homogeneity of transmit volume coils at high magnetic fields (≥4 T). Due to radiofrequency (RF) field/tissue interactions at high fields, 4 T to 8 T, the transmit profile from head‐sized volume coils shows a distinctive pattern with relatively strong RF magnetic field B₁ in the center of the brain.

Materials and Methods:

In contrast to conventional volume coils at high field strengths, surface coil phased arrays can provide increased RF field strength peripherally. In theory, simultaneous transmission from these two devices could produce a more homogeneous transmission field. To minimize interactions between the phased array and the volume coil, counter rotating current (CRC) surface coils consisting of two parallel rings carrying opposite currents were used for the phased array.

Results:

Numerical simulations and experimental data demonstrate that substantial improvements in transmit field homogeneity can be obtained.

Conclusion:

We have demonstrated the feasibility of using simultaneous transmission with human head‐sized volume coils and CRC phased arrays to improve homogeneity of the transmit RF B₁ field for high‐field MRI systems. J. Magn. Reson. Imaging 2010;32:476–481. © 2010 Wiley‐Liss, Inc.     

Patient safety concept for multichannel transmit coils

PURPOSE: To propose and illustrate a safety concept for multichannel transmit coils in MRI based on finite-differences time-domain (FDTD) simulations and validated by measurements. MATERIALS AND METHODS: FDTD simulations of specific absorption rate (SAR) distributions in a cylindrical agarose phantom were carried out for various radio frequency (RF) driving conditions of a four-element coil array. Additionally, maps of transmit amplitude, signal phase, and temperature rise following RF heating were measured by MRI. RESULTS: Quantitative agreement was achieved between simulated and measured field distributions, thus validating the numerical modeling. When applying the same RF power to each element of the coil array but systematically varying the RF phase between its elements, the maximum of the SAR distribution was found to vary by a factor of about 15. CONCLUSION: Our results demonstrate that current RF safety approaches are inadequate to deal with the new challenge of multichannel transmit coils. We propose a new concept based on a systematic investigation of the parameter space for RF phases and amplitudes. In this way the driving conditions generating the highest local SAR values per unit power can be identified and appropriately considered in the RF safety concept of a given MRI system.     

Thermal effects of MR imaging: worst-case studies on sheep

The objective of this study was to provide a worst-case estimate of thermal effects of MR imaging by subjecting anesthetized unshorn sheep to power deposition at specific absorption rates (SARs) well above approved standards for periods of time in excess of normal clinical imaging protocols. A control period with no RF power was followed by 20-105 min of RF power application. Afterward, there was a 20-min or longer recovery period with no RF power. Eight sheep were given whole-body RF exposure (1.5- to 4-W/kg SAR) while rectal and skin temperatures were monitored. Four sheep were subjected to 4-W/kg head scans for an average of 75 min while temperatures of the cornea, vitreous humor, head skin, jugular vein, and rectum were measured. In head scanning experiments, skin and eye temperatures increased about 1.5 degrees C. Jugular vein temperature rose a maximum of 0.4 degrees C after an average exposure of 75 min. In whole-body exposures, elevation of rectal temperature was correlated with energy input. Deep-body temperature rises in excess of 2.0 degrees C were attained for 4-W/kg whole-body exposure periods greater than 82 min. Animals exposed for 40 min to 4 W/kg in either body coil (three sheep) or head coil (two sheep) were recovered and observed to be in good health for 10 weeks; no cataracts were found. MR power deposition at SAR levels well above typical clinical imaging protocols caused body temperature to increase. For exposure periods in excess of standard clinical imaging protocols the temperature increase was insufficient to cause adverse thermal effects. Studies in healthy humans are needed to determine whether enhanced heat-loss effector mechanisms are likely to cause deep-body temperatures to plateau at an acceptable level, and to elucidate mechanisms that determine subcutaneous temperature.     

Comparison between eight- and sixteen-channel TEM transceive arrays for body imaging at 7 T

Eight- and sixteen-channel transceive stripline/TEM body arrays were compared at 7 T (297 MHz) both in simulation and experiment. Despite previous demonstrations of similar arrays for use in body applications, a quantitative comparison of the two configurations has not been undertaken to date. Results were obtained on a male pelvis for assessing transmit, signal to noise ratio, and parallel imaging performance and to evaluate local power deposition versus transmit B(1) (B(1) (+) ). All measurements and simulations were conducted after performing local B(1) (+) phase shimming in the region of the prostate. Despite the additional challenges of decoupling immediately adjacent coils, the sixteen-channel array demonstrated improved or nearly equivalent performance to the eight-channel array based on the evaluation criteria. Experimentally, transmit performance and signal to noise ratio were 22% higher for the sixteen-channel array while significantly increased reduction factors were achievable in the left-right direction for parallel imaging. Finite difference time domain simulations demonstrated similar results with respect to transmit and parallel imaging performance, however, a higher transmit efficiency advantage of 33% was predicted. Simulations at both 3 and 7 T verified the expected parallel imaging improvements with increasing field strength and showed that, for a specific B(1) (+) shimming strategy used, the sixteen-channel array exhibited lower local and global specific absorption rate for a given B(1) (+) .     

Two-Dimensional sixteen channel transmit/receive coil array for cardiac MRI at 7.0 T: Design, evaluation, and application

Purpose:

To design, evaluate, and apply a 2D 16‐channel transmit/receive (TX/RX) coil array tailored for cardiac magnetic resonance imaging (MRI) at 7.0 T.

Materials and Methods:

The cardiac coil array consists of two sections each using eight elements arranged in a 2 × 4 array. Radiofrequency (RF) safety was validated by specific absorption rate (SAR) simulations. Cardiac imaging was performed using 2D CINE FLASH imaging, T mapping, and fat–water separation imaging. The characteristics of the coil array were analyzed including parallel imaging performance, left ventricular chamber quantification, and overall image quality.

Results:

RF characteristics were found to be appropriate for all subjects included in the study. The SAR values derived from the simulations fall well within the limits of legal guidelines. The baseline signal‐to‐noise ratio (SNR) advantage at 7.0 T was put to use to acquire 2D CINE images of the heart with a very high spatial resolution of (1 × 1 × 4) mm³. The proposed coil array supports 1D acceleration factors of up to R = 4 without significantly impairing image quality.

Conclusion:

The 16‐channel TX/RX coil has the capability to acquire high contrast and high spatial resolution images of the heart at 7.0 T. J. Magn. Reson. Imaging 2012;36:847–857. © 2012 Wiley Periodicals, Inc.     

Effect of body coil electric field distribution on receive-only surface coil heating

Although in the design of transmit RF coils, B(1) homogeneity is crucial for good image quality, discussion of electric field (E-field) distribution in the literature has been mostly limited to specific absorption rate (SAR) and patient loading (dielectric) effects. In this work, we report on a different aspect of E-field: the receive-only surface coil heating resulting from the voltage drop across the blocking (decoupling) networks and cable traps that are used to minimize the transmit field distortion. The results show that the z-component (parallel to the coil cable) of the E-field has a significant effect on the temperature rise in the surface coil. Therefore, in the receive-only coil designs, it is not sufficient to consider only the induced voltage on the coil loop due to the B(1) field, as is generally done in blocking network analysis calculations. The body coil E-field distribution must be considered as well.     

X-ray compatible radiofrequency coil for magnetic resonance imaging.

The range of RF coils that can be used in combined X-ray/MR (XMR) systems is limited because many conventional coils contain highly X-ray attenuating materials that are visible in the X-ray images and potentially obscure patient anatomy. In this study, an X-ray compatible coil design that has minimal X-ray attenuation in the field of view (FOV) of the X-ray image is presented. In this design, aluminum is used for the loop conductor and discrete elements of the coil are eliminated from the X-ray FOV. A surface coil and an abdominal phased array coil were built using the X-ray compatible design. X-ray attenuation and MR imaging properties of the coils were evaluated and compared to conventional coils. The X-ray compatible phased array coil was used to image patients during two interventional procedures in the XMR system. The X-ray compatible coils allowed for fluoroscopic X-ray image acquisition, without degradation by the coil, while maintaining excellent MR imaging qualities.     

Design,evaluation and application of an eight channel transmit/receive coil array for cardiac MRI at 7.0 T

The objective of this work is to design, examine and apply an eight channel transmit/receive coil array tailored for cardiac magnetic resonance imaging at 7.0 T that provides image quality suitable for clinical use, patient comfort, and ease of use. The cardiac coil array was designed to consist of a planar posterior section and a modestly curved anterior section. For radio frequency (RF) safety validation, numerical computations of the electromagnetic field (EMF) and the specific absorption rate (SAR) distribution were conducted. In vivo cardiac imaging was performed using a 2D CINE FLASH technique. For signal-to-noise ratio (SNR) assessment reconstructed images were scaled in SNR units. The parallel imaging capabilities of the coil were examined using GRAPPA and SENSE reconstruction with reduction factors of up to R = 4. The assessment of the RF characteristics yielded a maximum noise correlation of 0.33. The baseline SNR advantage at 7.0 T was put to use to acquire 2D CINE images of the heart with a spatial resolution of 1 mm × 1 mm × 4 mm. The coil array supports 1D acceleration factors of up to R = 3 without impairing image quality significantly. For un-accelerated 2D CINE FLASH acquisitions the results revealed an SNR of approximately 140 for the left ventricular blood pool. Blood/myocardium contrast was found to be approximately 90 for un-accelerated 2D CINE FLASH acquisitions. The proposed 8 channel cardiac transceiver surface coil has the capability to acquire high contrast, high spatial and temporal resolution in vivo images of the heart at 7.0 T.     

Geometrical optimization of a phased array coil for high-resolution MR imaging of the carotid arteries.

The geometry of an RF phased-array receiving coil for high-resolution MRI of the carotid artery, particularly the bifurcation, was optimized with respect to signal-to-noise ratio (SNR). A simulation tool was developed to determine homogeneity, sensitivity, and SNR for a given imaging situation. The algorithm takes into account the coil geometry, the parameters of the measured object, and the imaging parameters of the pulse sequence. The coil with the optimum geometry was implemented as a receive-only coil for 1.5 T and comparative SNR measurements with different coils were performed. The experimental SNR measurements verified the simulations. The optimized carotid artery phased array offered the best SNR over the desired field of view. In vivo high-resolution MRI of the carotid arteries of healthy volunteers and patients with known stenosis was conducted with the optimized phased array coil. The capability of the phased array coil for resolving components within the carotid artery walls is demonstrated. Magn Reson Med 50:439-443, 2003.     

Adiabatic turbo spin echo in human applications at 7 T

Nonuniform B(1) fields in ultrahigh-field MR imaging cause severe image artifacts, when conventional radiofrequency (RF) pulses are used. Particularly in MR sequences that encompass multiple RF pulses, e.g., turbo spin echo (TSE) sequences, complete signal loss may occur in certain areas. When using a surface coil for transmitting the RF pulses, these problems become even more challenging, as the spatial B(1) field variance is substantial. As an alternative to conventional TSE sequences, adiabatic TSE sequences can be applied, which have the benefit that these sequences are insensitive to B(1) nonuniformity. In this study, we investigate the potential of using adiabatic TSE at 7 T with surface coil transceivers in human applications. The adiabatic RF pulses were tuned to deal with the constraints in B(1) strength and RF power deposition, but remained in the superadiabatic regime. As a consequence, the dynamic range in B(1) is compromised, and signal modulation is obtained over the echo train. Multidimensional Bloch simulations over the echo train and phantom measurements were obtained to assess these limitations. Still, using proper k-space sampling, we demonstrate improved image quality of the adiabatic TSE versus conventional TSE in the brain, the neck (carotid artery) and in the pelvis (prostate) at 7 T.     

A phased array coil for human cardiac imaging

A prototype cardiac phased array receiver coil was constructed that comprised a cylindrical array and a separate planar array. Both arrays had two coil loops with the same coil dimensions. Data acquisition with the cylindrical array placed on the human chest, and the planar array placed under the back, yielded an overall enhancement of the signal-to-noise ratio (SNR) over the entire heart by a factor of 1.1–2.85 over a commercially available flexible coil and a commercially available four-loop planar phased array coil. This improvement in SNR can be exploited in cardiac imaging to increase the spatial resolution and reduce the image acquisition time.     

High-sensitivity coil array for head and neck imaging: technical note.

The purpose of this study was to develop coils for MR imaging of the head and neck region, with the aim of improving sensitivity and coverage. A head and neck phased array coil was constructed and compared with volume and temporomandibular joint surface coils for sensitivity and coverage in phantom studies. An algorithm was implemented to correct for the nonuniformity in the surface coil reception profile. Its application to high-resolution T2-weighted imaging in healthy volunteers was investigated.     

Parallel excitation with an array of transmit coils.

Theoretical and experimental results are presented that establish the value of parallel excitation with a transmit coil array in accelerating excitation and managing RF power deposition. While a 2D or 3D excitation pulse can be used to induce a multidimensional transverse magnetization pattern for a variety of applications (e.g., a 2D localized pattern for accelerating spatial encoding during signal acquisition), it often involves the use of prolonged RF and gradient pulses. Given a parallel system that is composed of multiple transmit coils with corresponding RF pulse synthesizers and amplifiers, the results suggest that by exploiting the localization characteristics of the coils, an orchestrated play of shorter RF pulses can achieve desired excitation profiles faster without adding strains to gradients. A closed-form design for accelerated multidimensional excitations is described for the small-tip-angle regime, and its suppression of interfering aliasing lobes from coarse excitation k-space sampling is interpreted based on an analogy to sensitivity encoding (SENSE). With or without acceleration, the results also suggest that by taking advantage of the extra degrees of freedom inherent in a parallel system, parallel excitation provides better management of RF power deposition while facilitating the faithful production of desired excitation profiles. Sample accelerated and specific absorption rate (SAR)-reduced excitation pulses were designed in this study, and evaluated in experiments.     

Dependence of RF heating on SAR and implant position in a 1.5T MR system

PURPOSE: We evaluated radiofrequency (RF) heating of a humerus implant embedded in a gel phantom during magnetic resonance (MR) imaging for the specific absorption rate (SAR), angle between the implant and static magnetic field (B(0)), and position of the implant in the irradiation coil. METHODS: We embedded a stainless steel humerus implant 2 cm deep in tissue-equivalent loop and mass phantoms, placed it parallel to the static magnetic field of a 1.5T MR scanner, and recorded the temperatures of the implant surface with RF-transparent fiberoptic sensors. We measured rises in temperature at the tips of the implant by varying the SAR from 0.2 to 4.0 W/kg and evaluated RF heating of the implant for its angle to B(0) and its displacement along B(0) from the center of the RF irradiation coil. RESULTS: RF heating was similar for the loop and mass phantoms because the eddy current flows through the periphery of both. As the SAR increased, the temperature at the implant tip increased, and there was a linear relationship between the SAR and temperature rise. The values were 6.4 degrees C at 2.0 W/kg and 12.7 degrees C at 4.0 W/kg. Rise in temperature decreased steeply as the angle between the implant and B(0) surpassed 45 degrees . In addition, as the implant was displaced from the center of the RF coil to both ends, the rise in temperature decreased. CONCLUSION: The rise in temperature in deep tissue was estimated to be higher than 1.0 degrees C for SAR above 0.4 W/kg. RF heating was greatest when the implant was set parallel to B(0). In MR imaging of patients with implants, there is a risk of RF heating when the loop of the eddy current is formed inside the body.