Six human subjects (four exhibiting early symptoms of dementia) were scanned at 7T using both standard echo planar and the proposed reduced FOV diffusion tensor imaging (DTI) protocols. Imaging experiments were conducted on a 7T research scanner (GE Healthcare, Waukesha, WI), equipped with an eight-channel phased array head coil positioned within an insert volume transmitter coil (Nova Medical, Wilmington, MA). Prior to the DTI acquisitions, higher order shimming with multichannel B0 mapping was performed using in-house software [18
]. The standard 7T single shot spin echo EPI DTI protocol includes state of the art methods of axial, full brain coverage at 1.8-mm isotropic spatial resolution (FOV= 23cm, matrix= 128×128, 40 slices), with b
. With R=2 ASSET parallel imaging and 62% fractional k
-space phase encoding (40 lines acquired), a minimum TE of 62ms and readout duration of 25ms were achieved. For the reduced FOV acquisition, the number of acquired lines was further reduced by halving the phase FOV (anterior-posterior direction) to a 11.5-cm region covering both temporal lobes (24 lines). The readout duration was thus shortened to 16 ms, while the minimum TE remained unchanged, constrained mainly by limited gradient strength for diffusion encoding. The nominal spatial resolution was identical for both scans.
Outer volume suppression was achieved using a pair of custom designed quadratic phase 90° RF pulses [14
] that were integrated into the DTI pulse sequence with graphic prescription capability and placed anteriorly and posteriorly around the temporal lobes. The pulse () was designed for low peak power and high bandwidth, for reduced sensitivity to decreased RF coil efficiency and increased B0
variation at 7T, respectively. The pulse was designed in MATLAB (Mathworks Inc., Natick, MA) using an extended version [17
] of previously described routines [19
], which allows specification of a quadratic phase, parameterized by a linear delay and a quadratic shift, over the passband of the desired frequency response function. The RF waveform was then back-calculated from the resulting A and B polynomials (B derived using the Parks-McClellan algorithm), using the usual piecewise constant approximation [19
]. Finally, the magnetization response was simulated as a function of frequency offset, and a 90° pulse with the desired bandwidth and peak B1
amplitude was designed by iteration over the parameter set.
Quadratic phase RF pulse waveform amplitude (bottom) and phase (top).
The final chosen suppression pulse () had a time-bandwidth product of 16, and a peak B1 of 0.111 Gauss (G) for a 90° pulse with 5 ms duration. The effectiveness of suppression was compared to a standard linear phase SLR pulse optimized for 1.5T (time-bandwidth product = 6.4, peak B1 = 0.089 G with 4 ms duration). The uni-axial crusher gradient area was 3000 G μsec / cm. For simpler evaluation of the effectiveness of suppression, the pulses were initially tested on a high dielectric head phantom (17-cm sphere, doped water) in a standard 2D gradient recalled echo (GRE) pulse sequence, using the quadrature transmitter coil in transmit-receive mode (i.e. without phased array receive).
The imaging time for each DTI protocol was 4 min, 12 sec (TR= 9 s, 6 directions, NEX=4). Rapid multi-slice low flip angle 2D GRE scans of the whole brain were acquired for image domain parallel imaging calibration (by the array spatial sensitivity encoding technique, or ASSET). Maps of the apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were generated from images acquired using both protocols by processing of the diffusion weighted images in FSL [20