Application of the SUSHI method to the design of gradient coils.

An approach to potential improvements in magnetic field shielding for a gradient coil system with cylindrical geometry is presented, utilizing "supershielding" conditions for the currents on both the primary and the secondary coils. It is demonstrated that the field can be strongly suppressed everywhere outside a cylindrical shield coil radius, even though the finite-length active shield only partially surrounds a primary coil. The supershielding method, which is aimed at controlling eddy currents, still has sufficient freedom to maintain the desired magnetic field behavior inside the imaging volume. The trade-off is an additional primary current oscillation and increased current peaks and field energy. This method has been applied to design short transverse and axial gradient coils, giving substantially improved shielding compared to an apodization method. Magn Reson Med 45:147-155, 2001.     

Maximum efficiency radiofrequency shimming: Theory and initial application for hip imaging at 7 tesla

Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high‐frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power‐demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal‐to‐noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Hemispherical gradient coils for magnetic resonance imaging

Hemispherical gradient coils offer an open geometry that is well suited to imaging of the human brain. The windings of a hemispherical gradient coil are on average closer to the target region than those of a comparable cylindrical coil, and consequently hemispherical coils can produce higher efficiency at fixed inductance. The mathematical formalism needed for the design of hemispherical gradient coils is described here, including expressions relating the current distribution on the hemisphere to the magnetic field generated, as well as the stored energy and power dissipation. In addition, expressions for the torque experienced by the current distribution in the presence of the main magnetic field have been derived and used to develop an approach allowing the design of torque‐balanced, hemispherical transverse gradient coils. Hemispherical coil designs suitable for brain imaging are presented and shown to have improved performance compared with their cylindrical counterparts. Small, prototype, hemispherical z‐ and x‐gradient coils have been constructed and tested in phase‐mapping experiments at 3 T. The experimental results show good agreement with theoretical predictions, validating the mathematical expressions used in the coil design process. The formalism also allows the design of coils wound on a complete spherical surface, and the performance of such coils is additionally described here. Magn Reson Med, 2005. © 2005 Wiley‐Liss, Inc.     

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.     

A fuse for magnetic resonance imaging probes

In a research environment, a magnetic resonance imaging system's manufacturer-supplied protection against excessive rf power deposition in the sample is often compromised. This paper presents a widely applicable scheme, based on a commonly available fuse, that simply and effectively limits the average rf power into a magnetic resonance imaging probe.     

Determination of absorbed dose for photon and electron beams from a linear accelerator by comparing various protocols on different phantoms.

Protocols developed for high-energy dosimetry IAEA (Technical Reports Series No. 277, 1997), AAPM (Med. Phys. 10 (1983) 741: Med. Phys. 18 (1991) 73: Med. Phys. 21 (1994) 1251), IPEMB (Phys. Med. Biol. 41 (1996) 2557), and HPA (Phys. Med. Biol. 28 (1983) 1097) have continued to enhance precision in dose measurements and the optimization of radiotherapy procedures. While recent dosimetry protocols, including those due to the IAEA and IPEMB, have made a number of improvements compared with previous protocols, it is further desirable to develop absolute dosimetry methods of dose measurements. Measurements based on careful implementation of procedures contained within the various protocols have been carried out in an effort to determine the extent to which discrepancies exist among the protocols. Dose in water at dmax was measured using cylindrical and parallel-plate ionization chambers for 6 MV photon beams and 5 and 12 MeV electron beams. Results obtained from the use of the AAPM and HPA protocols for 6 MV photon beams were found to be 0.9% larger and 0.1% smaller, respectively, than those measured following the IAEA protocol. Calibration dose measurements for 5 and 12 MeV electron beams in water phantoms were found to agree to within 1%, this being well within recommendations from the ICRU and other sources regarding the accuracy of dose delivery.     

A prototype RF dosimeter for independent measurement of the average specific absorption rate (SAR) during MRI

PURPOSE: To develop a scanner-independent dosimeter for measuring the average radio frequency (RF) power deposition and specific absorption rates (SAR) for human MRI exposure. MATERIALS AND METHODS: A prototype dosimeter has a transducer with orthogonal conducting loops surrounding a small signal-generating MRI sample. The loops contain resistors whose values are adjusted to load the scanner's MRI coils equivalent to an average head or body during MRI. The scanner adjusts its power output to normal levels during setup, using the MRI sample. Following calibration, the total power and average SAR deposited in the transducer are measured from the root-mean-square (rms) power induced in the transducer during MRI. RESULTS: A 1.5 Tesla head transducer was adjusted to elicit the same load as the average of nine adult volunteers. Once adjusted, the transducer loads other head coils the same as the head does. The dosimeter is calibrated at up to 20 W total deposited power and 4.5 W/kg SAR in the average head, with about 5% accuracy. CONCLUSION: This dosimeter provides a simple portable means of measuring the power deposited in a body-equivalent sample load, independent of the scanner. Further work will develop SAR dosimetry for the torso and for higher fields.     

High-order coils as transmitters for NMR imaging

High-order radiofrequency transmitter coils are described that permit, in the context of NMR imaging, a knowledgeable trade-off between coil complexity and the degree of inhomogeneity over a specified volume of space. A mathematical formulation of high-order coils is developed for an axially symmetric polarizing dc field and transverse rf field. The field is created by a number of longitudinal current filaments with equal angular separations on the wall of a cylinder. In its most general form, each current can be independently specified. Circularly or linearly polarized fields can be created. Quadrature and saddleshaped coils emerge as special cases of the general formulation. The development has been tested experimentally and good agreement found. High-order coils can also be used as receivers, permitting more equal weighting of the sensitivities of each pixel.     

Spatial distribution of RF‐induced E‐fields and implant heating in MRI

The purpose of this study was to assess the distribution of RF‐induced E‐fields inside a gel‐filled phantom of the human head and torso and compare the results with the RF‐induced temperature rise at the tip of a straight conductive implant, specifically examining the dependence of the temperature rise on the position of the implant inside the gel. MRI experiments were performed in two different 1.5T MR systems of the same manufacturer. E‐field distribution inside the liquid was assessed using a custom measurement system. The temperature rise at the implant tip was measured in various implant positions and orientations using fluoroptic thermometry. The results show that local E‐field strength in the direction of the implant is a critical factor in RF‐related tissue heating. The actual E‐field distribution, which is dependent on phantom/body properties and the MR‐system employed, must be considered when assessing the effects of RF power deposition in implant safety investigations. Magn Reson Med 60:312–319, 2008. © 2008 Wiley‐Liss, Inc.     

Performance of Philips Gemini TF PET/CT scanner with special consideration for its time-of-flight imaging capabilities.

Results from a new PET/CT scanner using lutetium-yttrium oxyorthosilicate (LYSO) crystals for the PET component are presented. This scanner, which operates in a fully 3-dimensional mode, has a diameter of 90 cm and an axial field of view of 18 cm. It uses 4 x 4 x 22 mm(3) LYSO crystals arranged in a pixelated Anger-logic detector design. This scanner was designed to perform as a high-performance conventional PET scanner as well as provide good timing resolution to operate as a time-of-flight (TOF) PET scanner. METHODS: Performance measurements on the scanner were made using the National Electrical Manufacturers Association (NEMA) NU2-2001 procedures to benchmark its conventional imaging capabilities. The scatter fraction and noise equivalent count (NEC) measurements with the NEMA cylinder (20-cm diameter) were repeated for 2 larger cylinders (27-cm and 35-cm diameter), which better represent average and heavy patients. New measurements were designed to characterize its intrinsic timing resolution capability, which defines its TOF performance. Additional measurements to study the impact of pulse pileup at high counting rates on timing, as well as energy and spatial, resolution were also performed. Finally, to characterize the effect of TOF reconstruction on lesion contrast and noise, the standard NEMA/International Electrotechnical Commission torso phantom as well as a large 35-cm-diameter phantom with both hot and cold spheres were imaged for varying scan times. RESULTS: The transverse and axial resolution near the center is 4.8 mm. The absolute sensitivity of this scanner measured with a 70-cm-long line source is 6.6 cps/kBq, whereas scatter fraction is 27% measured with a 70-cm-long line source in a 20-cm-diameter cylinder. For the same line source cylinder, the peak NEC rate is measured to be 125 kcps at an activity concentration of 17.4 kBq/mL (0.47 microCi/mL). The 2 larger cylinders showed a decrease in the peak NEC due to increased attenuation, scatter, and random coincidences, and the peak occurs at lower activity concentrations. The system coincidence timing resolution was measured to be 585 ps. The timing resolution changes as a function of the singles rate due to pulse pileup and could impact TOF image reconstruction. Image-quality measurements with the torso phantom show that very high quality images can be obtained with short scan times (1-2 min per bed position). However, the benefit of TOF is more apparent with the large 35-cm-diameter phantom, where small spheres are detectable only with TOF information for short scan times. CONCLUSION: The Gemini TF whole-body scanner represents the first commercially available fully 3-dimensional PET scanner that achieves TOF capability as well as conventional imaging capabilities. The timing resolution is also stable over a long duration, indicating the practicality of this device. Excellent image quality is achieved for whole-body studies in 10-30 min, depending on patient size. The most significant improvement with TOF is seen for the heaviest patients.     

Radiofrequency heating in porcine models with a “large” 32 cm internal diameter, 7 T (296 MHz) head coil

Temperatures were measured in vivo in four pigs (mean animal weight = 110.75 kg and standard deviation = 6.13 kg) due to a continuous wave radiofrequency (RF) power irradiation with a 31.75 cm internal diameter and a 15.24 cm long, 7 T (296 MHz), eight channel, transverse electromagnetic head coil. The temperatures were measured in the subcutaneous layer of the scalp, 5, 10, 15, and 20 mm deep in the brain, and rectum using fluoroptic temperature probes. The RF power was delivered to the pig's head for ～3 h (mean deposition time = 3.14 h and standard deviation = 0.06 h) at the whole head average specific absorption rate of ～3 W kg^?1 (mean average specific absorption rate = 3.08 W kg^?1 and standard deviation = 0.09 W kg^?1). Next, simple bioheat transfer models were used to simulate the RF power induced temperature changes. Results show that the RF power produced uniform temperature changes in the pigs' heads (mean temperature change = 1.68°C and standard deviation = 0.13°C) with no plateau achieved during the heating. No thermoregulatory alterations were detected due to the heating because the temperature responses of the pre‐RF and post‐RF epochs were not statistically significantly different. Simple, validated bioheat models may provide accurate temperature changes. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Novel RF coil geometry for lower extremity imaging.

A receive-only phased-array coil was designed to image the lower extremities. The array consists of four volume coils placed on two cylindrical formers. The coil array has the ability to image both legs simultaneously over a 40 cm longitudinal field of view (FOV). Experiments using phantoms show an increase in signal-to-noise ratio (SNR) in regions of interest through the center of the coil by an average factor of 2.8 over the body coil and 1.5 over the GE 4-channel torso array. In vivo data acquired from 10 subjects show that the X array provided similar SNR improvement in spin-echo images and more vascular details in angiographic images compared to the torso array.     

Short echo spectroscopic imaging of the human brain at 7T using transceiver arrays

Recent advances in magnet technology have enabled the construction of ultrahigh‐field magnets (7T and higher) that can accommodate the human head and body. Despite the intrinsic advantages of performing spectroscopic imaging at 7T, increased signal‐to‐noise ratio (SNR), and spectral resolution, few studies have been reported to date. This limitation is largely due to increased power deposition and B₁ inhomogeneity. To overcome these limitations, we used an 8‐channel transceiver array with a short TE (15 ms) spectroscopic imaging sequence. Utilizing phase and amplitude mapping and optimization schemes, the 8‐element transceiver array provided both improved efficiency (17% less power for equivalent peak B₁) and homogeneity (SD(B₁) = ±10% versus ±22%) in comparison to a transverse electromagnetic (TEM) volume coil. To minimize the echo time to measure J‐modulating compounds such as glutamate, we developed a short TE sequence utilizing a single‐slice selective excitation pulse followed by a broadband semiselective refocusing pulse. Extracerebral lipid resonances were suppressed with an inversion recovery pulse and delay. The short TE sequence enabled visualization of a variety of resonances, including glutamate, in both a control subject and a patient with a Grade II oligodendroglioma. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Increased rf power absorption in MR imaging due to rf coupling between body coil and surface coil

Fears have been voiced that excessive tissue heating could occur in the event that first, a surface coil is placed with its axis parallel to the transmitting rf field leading to a maximal coupling of the two coils and second, the decoupling circuit of the surface coil breaks down. To avoid an rf coupling of the transmitting body coil to the receive-only surface coil, conventionally applied surface coils are equipped with an active electronic rf decoupling circuit. In extensive worst-case experiments on phantoms we have shown that no tissue heating occurs for surface coils which are equipped with semiconductor varicap diodes for tuning and matching. These coils should be safe for patient applications even if the decoupling circuit fails. Surface coils equipped with mechanically variable capacitors are generally passively decoupled. To simulate the worst-case situation phantom experiments were performed in which a surface coil of this type having no passive decoupling circuit was coupled to the transmitter coil by its geometric position. Theoretical calculations, in agreement with the experimental results achieved during a 15-min measurement in a 1.5-T MRI whole-body imager, show that a significant rf power deposition in the tissue underneath the coil wire occurs, leading typically to a local specific absorption rate of 24 W/kg and a local temperature rise of 5.2 degrees C.     

Measuring the axial rotation of lumbar vertebrae in vivo with MR imaging

BACKGROUND AND PURPOSE: Flexion-extension radiography is neither sensitive nor specific in the diagnosis of degenerative spinal instability, a presumed cause of back pain and an indication for spinal fusion. We tested the hypothesis that with MR imaging and a device to rotate the torso, axial rotations of lumbar vertebrae can be measured with sufficient accuracy and that significantly different rotations can be detected between lumbar segments with degenerated disks and those with normal disks. METHODS: We studied five volunteers without back pain (group 1), five patients who underwent MR imaging because of back pain but were not considered candidates for fusion (group 2), and five patients in whom diskography identified one or more disks with concordant pain (group 3). Each participant was placed on a specially built table that provided separate supports for the torso and for the hips and legs. Series of sagittal images were acquired with a T2-weighted fast spin-echo sequence, with the torso rotated clockwise and then counterclockwise. The amount of rotation was calculated from axial images with use of an automated program. RESULTS: In the five volunteers, rotations of the lumbar motion segments varied between -1.8 degrees and 5.7 degrees, with an average of 0.8 degrees. The abnormal disks in five patients in group 2 rotated from -0.9 degrees to 5.6 degrees, with an average of 3.2 degrees. In group 3, the disks in which concordant pain was elicited rotated from 0.8 degrees to 4.4 degrees, with an average of 2.2 degrees. Difference in rotation between abnormal and normal disks was statistically significant. CONCLUSION: Measurements of rotations of lumbar vertebrae with MR imaging may have value for determining levels that move abnormally in axial rotation.     

Imaging characteristics of a 3-dimensional GSO whole-body PET camera.

A whole-body 3-dimensional PET scanner using gadolinium oxyorthosilicate (GSO) crystals has been designed to achieve high sensitivity and reduced patient scanning time. This scanner has a diameter of 82.0 cm and an axial field of view of 18 cm without interplane septa. The detector comprises of 4 x 6 x 20 mm(3) GSO crystals coupled via an optically continuous light guide to an array of 420 photomultiplier tubes (39-mm diameter) in a hexagonal arrangement. The patient port diameter is 56 cm, and 2.86-cm (1.125 in.) thick lead shielding is used to fill in the region up to the detector ring. METHODS: Performance measurements on the scanner were made using the National Electrical Manufactures Association (NEMA) NU 2-2001 procedures. Additional counting rate measurements with a large phantom were performed to evaluate imaging characteristics for heavier patients. The image-quality torso phantom with hot or cold spheres was also measured as a function of counting rate to evaluate different techniques for randoms and scatter subtraction as well as to determine an optimum imaging time. RESULTS: The transverse and axial resolutions near the center are 5.5 and 5.6 mm, respectively. The absolute sensitivity of this scanner measured with a 70-cm-long line source is 4.36 cps/kBq, whereas the scatter fraction is 40% with a 20 x 70 cm line source cylinder. For the same cylinder, the peak noise equivalent count (NEC) rate of 30 kcps at an activity concentration of 9.25 kBq/mL (0.25 micro Ci/mL) leads to a 7% increase in the peak NEC value. A significant reduction in the peak NEC is observed with a larger 35 x 70 cm line source cylinder. Image-quality measurements show that the small 10-mm sphere in the NEMA NU 2-2001 image-quality phantom is clearly visible in a scan time of 3 min, and there is no noticeable degradation in image contrast at high activity levels. CONCLUSION: This whole-body scanner represents a new generation of 3D, high-sensitivity, and high-performance PET cameras capable of producing high-quality images in <30 min for a full patient scan. The use of a pixelated GSO Anger-logic detector leads to a high-sensitivity scanner design with good counting rate capability due to the reduced light spread in the detector and fast decay time of GSO. The light collection over the detector is fairly uniform, leading to a good energy resolution and, thus, reduced scatter in the collected data due to a tight energy gate.     

Accuracy and precision of MR blood oximetry based on the long paramagnetic cylinder approximation of large vessels

An accurate noninvasive method to measure the hemoglobin oxygen saturation (%HbO₂) of deep‐lying vessels without catheterization would have many clinical applications. Quantitative MRI may be the only imaging modality that can address this difficult and important problem. MR susceptometry–based oximetry for measuring blood oxygen saturation in large vessels models the vessel as a long paramagnetic cylinder immersed in an external field. The intravascular magnetic susceptibility relative to surrounding muscle tissue is a function of oxygenated hemoglobin (HbO₂) and can be quantified with a field‐mapping pulse sequence. In this work, the method's accuracy and precision was investigated theoretically on the basis of an analytical expression for the arbitrarily oriented cylinder, as well as experimentally in phantoms and in vivo in the femoral artery and vein at 3T field strength. Errors resulting from vessel tilt, noncircularity of vessel cross‐section, and induced magnetic field gradients were evaluated and methods for correction were designed and implemented. Hemoglobin saturation was measured at successive vessel segments, differing in geometry, such as eccentricity and vessel tilt, but constant blood oxygen saturation levels, as a means to evaluate measurement consistency. The average standard error and coefficient of variation of measurements in phantoms were <2% with tilt correction alone, in agreement with theory, suggesting that high accuracy and reproducibility can be achieved while ignoring noncircularity for tilt angles up to about 30°. In vivo, repeated measurements of %HbO₂ in the femoral vessels yielded a coefficient of variation of less than 5%. In conclusion, the data suggest that %HbO₂ can be measured reproducibly in vivo in large vessels of the peripheral circulation on the basis of the paramagnetic cylinder approximation of the incremental field. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Broadband proton decoupling for in vivo brain spectroscopy in humans.

A new decoupling sequence, PBAR, is described for broadband heteronuclear decoupling in vivo in humans at 1.5T. The sequence uses non-adiabatic, frequency- and amplitude-modulated inversion pulses designed to minimize decoupling sidebands at low applied gammaB(2) RF field levels and to cover only the narrow range of resonance offsets encountered in practice. The offset dependence of the decoupling efficiency of PBAR is demonstrated and compared to the conventional WALTZ-4 sequence. At the same average power levels, PBAR had slightly reduced bandwidth but significantly less intense decoupling sidebands. Applications of PBAR are shown in vivo in the human brain both for (31)P and natural abundance (13)C spectroscopy using volume decoupling coils. The PBAR sequence allows whole brain [(1)H]-[13]C decoupling to be performed at 1.5T with a standard head coil within FDA guidelines for RF power deposition. Magn Reson Med 45:226-232, 2001.     

Single‐shot T1 mapping using simultaneous acquisitions of spin‐ and stimulated‐echo‐planar imaging (2D ss‐SESTEPI)

The conventional stimulated‐echo NMR sequence only measures the longitudinal component while discarding the transverse component, after tipping up the prepared magnetization. This transverse magnetization can be used to measure a spin echo, in addition to the stimulated echo. Two‐dimensional single‐shot spin‐ and stimulated‐echo‐planar imaging (ss‐SESTEPI) is an echo‐planar‐imaging‐based single‐shot imaging technique that simultaneously acquires a spin‐echo‐planar image and a stimulated‐echo‐planar image after a single radiofrequency excitation. The magnitudes of the spin‐echo‐planar image and stimulated‐echo‐planar image differ by T₁ decay and diffusion weighting for perfect 90° radiofrequency and thus can be used to rapidly measure T₁. However, the spatial variation of amplitude of radiofrequency field induces uneven splitting of the transverse magnetization for the spin‐echo‐planar image and stimulated‐echo‐planar image within the imaging field of view. Correction for amplitude of radiofrequency field inhomogeneity is therefore critical for two‐dimensional ss‐SESTEPI to be used for T₁ measurement. We developed a method for amplitude of radiofrequency field inhomogeneity correction by acquiring an additional stimulated‐echo‐planar image with minimal mixing time, calculating the difference between the spin echo and the stimulated echo and multiplying the stimulated‐echo‐planar image by the inverse functional map. Diffusion‐induced decay is corrected by measuring the average diffusivity during the prescanning. Rapid single‐shot T₁ mapping may be useful for various applications, such as dynamic T₁ mapping for real‐time estimation of the concentration of contrast agent in dynamic contrast enhancement MRI. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.