Andreas Potthast

Comparison of physiological noise at 1.5 T, 3 T and 7 T and optimization of fMRI acquisition parameters

Previous studies have shown that under some conditions, noise fluctuations in an fMRI time-course are dominated by physiological modulations of the image intensity with secondary contributions from thermal image noise and that these two sources scale differently with signal intensity, susceptibility weighting (TE) and field strength. The SNR of the fMRI time-course was found to be near its asymptotic limit for moderate spatial resolution measurements at 3 T with only marginal gains expected from acquisition at higher field strengths. In this study, we investigate the amplitude of image intensity fluctuations in the fMRI time-course at magnetic field strengths of 1.5 T, 3 T, and 7 T as a function of image resolution, flip angle and TE. The time-course SNR was a similar function of the image SNR regardless of whether the image SNR was modulated by flip angle, image resolution, or field strength. For spatial resolutions typical of those currently used in fMRI (e.g., 3 x 3 x 3 mm(3)), increases in image SNR obtained from 7 T acquisition produced only modest increases in time-course SNR. At this spatial resolution, the ratio of physiological noise to thermal image noise was 0.61, 0.89, and 2.23 for 1.5 T, 3 T, and 7 T. At a resolution of 1 x 1 x 3 mm(3), however, the physiological to thermal noise ratio was 0.34, 0.57, and 0.91 for 1.5 T, 3 T and 7 T for TE near T2*. Thus, by reducing the signal strength using higher image resolution, the ratio of physiologic to image noise could be reduced to a regime where increased sensitivity afforded by higher field strength still translated to improved SNR in the fMRI time-series.

32-channel 3 Tesla receive-only phased-array head coil with soccer-ball element geometry

A 32‐channel 3T receive‐only phased‐array head coil was developed for human brain imaging. The helmet‐shaped array was designed to closely fit the head with individual overlapping circular elements arranged in patterns of hexagonal and pentagonal symmetry similar to that of a soccer ball. The signal‐to‐noise ratio (SNR) and noise amplification (g‐factor) in accelerated imaging applications were quantitatively evaluated in phantom and human images and compared with commercially available head coils. The 32‐channel coil showed SNR gains of up to 3.5‐fold in the cortex and 1.4‐fold in the corpus callosum compared to a (larger) commercial eight‐channel head coil. The experimentally measured g‐factor performance of the helmet array showed significant improvement compared to the eight‐channel array (peak g‐factor 59% and 26% of the eight‐channel values for four‐ and fivefold acceleration). The performance of the arrays is demonstrated in high‐resolution and highly accelerated brain images. Magn Reson Med, 2006.

96‐Channel receive‐only head coil for 3 Tesla: Design optimization and evaluation

The benefits and challenges of highly parallel array coils for head imaging were investigated through the development of a 3T receive‐only phased‐array head coil with 96 receive elements constructed on a close‐fitting helmet‐shaped former. We evaluated several designs for the coil elements and matching circuitry, with particular attention to sources of signal‐to‐noise ratio (SNR) loss, including various sources of coil loading and coupling between the array elements. The SNR and noise amplification (g‐factor) in accelerated imaging were quantitatively evaluated in phantom and human imaging and compared to a 32‐channel array built on an identical helmet‐shaped former and to a larger commercial 12‐channel head coil. The 96‐channel coil provided substantial SNR gains in the distal cortex compared to the 12‐ and 32‐channel coils. The central SNR for the 96‐channel coil was similar to the 32‐channel coil for optimum SNR combination and 20% lower for root‐sum‐of‐squares combination. There was a significant reduction in the maximum g‐factor for 96 channels compared to 32; for example, the 96‐channel maximum g‐factor was 65% of the 32‐channel value for acceleration rate 4. The performance of the array is demonstrated in highly accelerated brain images. Magn Reson Med, 2009.

Accurate prediction of V1 location from cortical folds in a surface coordinate system

Previous studies demonstrated substantial variability of the location of primary visual cortex (V1) in stereotaxic coordinates when linear volume-based registration is used to match volumetric image intensities [Amunts, K., Malikovic, A., Mohlberg, H., Schormann, T., and Zilles, K. (2000). Brodmanns areas 17 and 18 brought into stereotaxic space-where and how variable? Neuroimage, 11(1):66-84]. However, other qualitative reports of V1 location [Smith, G. (1904). The morphology of the occipital region of the cerebral hemisphere in man and the apes. Anatomischer Anzeiger, 24:436-451; Stensaas, S.S., Eddington, D.K., and Dobelle, W.H. (1974). The topography and variability of the primary visual cortex in man. J Neurosurg, 40(6):747-755; Rademacher, J., Caviness, V.S., Steinmetz, H., and Galaburda, A.M. (1993). Topographical variation of the human primary cortices: implications for neuroimaging, brain mapping, and neurobiology. Cereb Cortex, 3(4):313-329] suggested a consistent relationship between V1 and the surrounding cortical folds. Here, the relationship between folds and the location of V1 is quantified using surface-based analysis to generate a probabilistic atlas of human V1. High-resolution (about 200 microm) magnetic resonance imaging (MRI) at 7 T of ex vivo human cerebral hemispheres allowed identification of the full area via the stria of Gennari: a myeloarchitectonic feature specific to V1. Separate, whole-brain scans were acquired using MRI at 1.5 T to allow segmentation and mesh reconstruction of the cortical gray matter. For each individual, V1 was manually identified in the high-resolution volume and projected onto the cortical surface. Surface-based intersubject registration [Fischl, B., Sereno, M.I., Tootell, R.B., and Dale, A.M. (1999b). High-resolution intersubject averaging and a coordinate system for the cortical surface. Hum Brain Mapp, 8(4):272-84] was performed to align the primary cortical folds of individual hemispheres to those of a reference template representing the average folding pattern. An atlas of V1 location was constructed by computing the probability of V1 inclusion for each cortical location in the template space. This probabilistic atlas of V1 exhibits low prediction error compared to previous V1 probabilistic atlases built in volumetric coordinates. The increased predictability observed under surface-based registration suggests that the location of V1 is more accurately predicted by the cortical folds than by the shape of the brain embedded in the volume of the skull. In addition, the high quality of this atlas provides direct evidence that surface-based intersubject registration methods are superior to volume-based methods at superimposing functional areas of cortex and therefore are better suited to support multisubject averaging for functional imaging experiments targeting the cerebral cortex.

Slice-Selective RF pulses for In-vivo B1+ Inhomogeneity Mitigation at 7 Tesla using Parallel RF Excitation with a 16-Element Coil

Slice‐selective RF waveforms that mitigate severe B  1+ inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16‐element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least‐squares (MLS) criteria with an optimized k‐space excitation trajectory to significantly improve profile uniformity compared to conventional least‐squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in‐plane flip angle (FA) with slice selection in the z‐direction were demonstrated and compared with conventional sinc‐pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B  1+ on the excitation FA. The optimized parallel RF pulses for human B  1+ mitigation were only 67% longer than a conventional sinc‐based excitation, but significantly outperformed RF shimming. For example the standard deviations (SDs) of the in‐plane FA (averaged over six human studies) were 16.7% for conventional sinc excitation, 13.3% for RF shimming, and 7.6% for parallel excitation. This work demonstrates that excitations with parallel RF systems can provide slice selection with spatially uniform FAs at high field strengths with only a small pulse‐duration penalty. Magn Reson Med 60:1422–1432, 2008.

A 128-channel receive-only cardiac coil for highly accelerated cardiac MRI at 3 Tesla

A 128‐channel receive‐only array coil is described and tested for cardiac imaging at 3T. The coil is closely contoured to the body with a “clam‐shell” geometry with 68 posterior and 60 anterior elements, each 75 mm in diameter, and arranged in a continuous overlapped array of hexagonal symmetry to minimize nearest neighbor coupling. Signal‐to‐noise ratio (SNR) and noise amplification for parallel imaging (G‐factor) were evaluated in phantom and volunteer experiments. These results were compared to those of commercially available 24‐channel and 32‐channel coils in routine use for cardiac imaging. The in vivo measurements with the 128‐channel coil resulted in SNR gains compared to the 24‐channel coil (up to 2.2‐fold in the apex). The 128‐ and 32‐channel coils showed similar SNR in the heart, likely dominated by the similar element diameters of these coils. The maximum G‐factor values were up to seven times better for a seven‐fold acceleration factor (R = 7) compared to the 24‐channel coil and up to two‐fold improved compared to the 32‐channel coil. The ability of the 128‐channel coil to facilitate highly accelerated cardiac imaging was demonstrated in four volunteers using acceleration factors up to seven‐fold (R = 7) in a single spatial dimension. Magn Reson Med 59:1431–1439, 2008.

Eight-channel phased array coil and detunable TEM volume coil for 7 T brain imaging

An eight‐channel receive‐only brain coil and table‐top detunable volume transmit coil were developed and tested at 7 T for human imaging. Optimization of this device required attention to sources of interaction between the array elements, between the transmit and receive coils and minimization of common mode currents on the coaxial cables. Circular receive coils (85 mm dia.) were designed on a flexible former to fit tightly around the head and within a 270‐mm diameter TEM transmit volume coil. In the near cortex, the array provided a fivefold increase in SNR compared to a TEM transmit‐receive coil, a gain larger than that seen in comparable coils at 3 T. The higher SNR gain is likely due to strong dielectric effects, which cause the volume coil to perform poorly in the cortex compared to centrally. The sensitivity and coverage of the array is demonstrated with high‐resolution images of the brain cortex. Magn Reson Med 54:235–240, 2005.

Broadband slab selection with B1+ mitigation at 7T via parallel spectral-spatial excitation.

Chemical shift imaging benefits from signal‐to‐noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B  1+ mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B  1+ mitigation methods, which are based on a type of echovolumnar trajectory referred to as “spokes” or “fast‐kz”. Although such pulses, in conjunction with parallel excitation methodology, can efficiently mitigate large B  1+ inhomogeneities and achieve relatively short pulse durations with slice‐selective excitations, they exhibit a narrow‐band off‐resonance response and may not be suitable for applications that require B  1+ mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectral‐spatial excitation that achieves a target‐error minimization simultaneously over a bandwidth of frequencies and a specified spatial‐domain. The technique is demonstrated for slab‐selective excitation with in‐plane B  1+ mitigation over a 600‐Hz bandwidth. The pulse design method is validated in a water phantom at 7T using an eight‐channel transmit array system. The results show significant increases in the pulses spectral bandwidth, with no additional pulse duration penalty and only a minor tradeoff in spatial B  1+ mitigation compared to the standard spoke‐based parallel RF design. Magn Reson Med 61:493–500, 2009.

High-flip-angle slice-selective parallel RF transmission with 8 channels at 7 T.

At high magnetic field, B(1)(+) non-uniformity causes undesired inhomogeneity in SNR and image contrast. Parallel RF transmission using tailored 3D k-space trajectory design has been shown to correct for this problem and produce highly uniform in-plane magnetization with good slice selection profile within a relatively short excitation duration. However, at large flip angles the excitation k-space based design method fails. Consequently, several large-flip-angle parallel transmission designs have recently been suggested. In this work, we propose and demonstrate a large-flip-angle parallel excitation design for 90 degrees and 180 degrees spin-echo slice-selective excitations that mitigate severe B(1)(+) inhomogeneity. The method was validated on an 8-channel transmit array at 7T using a water phantom with B(1)(+) inhomogeneity similar to that seen in human brain in vivo. Slice-selective excitations with parallel RF systems offer means to implement conventional high-flip excitation sequences without a severe pulse-duration penalty, even at very high B(0) field strengths where large B(1)(+) inhomogeneity is present.

Design considerations and coil comparisons for 7 T brain imaging

The development of 300 MHz radio-frequency (RF) head coils analogous to those used at field strengths of 1.5 and 3 T is complicated by increased dissipative losses in conductive tissue, effects arising from the short RF wavelength in biological tissue (about 13 cm at 300 MHz), and the constraints imposed by the use of head gradient sets desirable for mitigating increased static field susceptibility effects. In this study, five RF head coils were constructed and tested on a 7 T scanner including 2 TEM designs, 2 birdcage designs and a local receive-only array. Signal-to-noise ratio, coil reception profiles and interactions between the coil and dielectric head were examined. Particular attention was placed on the coil’s reception in the neck and shoulders, where the head gradient is unable to spatially encode the image. With the use of conductive shields and distributed capacitance, all of the coil designs could be made to image effectively at high field, but each design was found to have subtle differences in field distribution, interaction with the dielectric boundary conditions of the head and fringe fields in the neck and shoulders. In particular, the birdcage and array coils were found to have reducedB1 reception field profiles in the neck and shoulders which helped reduce signal detection outside the linear region of the head gradient coil. Although the TEM coils exhibited higher signal detection in the neck and shoulders, all the coils picked up enough signal from these regions to produce artifacts in the brain. These artifacts could be mitigated through use of a conductive shield or by small local dephasing shims sewn into the shoulders of a jacket worn by the subject. Although homogeneous in low-dielectric-constant phantoms, the volume coil’sB1 profile was strongly peaked in the center of the head, rendering them spatially complementary to that observed in the surface coil array. The image profile of the surface coil was found to be less dramatically changed from patterns observed at lower field strength. Its dielectric brightening pattern was found to depend on the orientation of the coil with respect to the head.