Whole-body imaging at 7T: preliminary results

Abstract

The objective of this study was to investigate the feasibility of whole-body imaging at 7T. To achieve this objective, new technology and methods were developed. Radio frequency (RF) field distribution and specific absorption rate (SAR) were first explored through numerical modeling. A body coil was then designed and built. Multichannel transmit and receive coils were also developed and implemented. With this new technology in hand, an imaging survey of the "landscape" of the human body at 7T was conducted. Cardiac imaging at 7T appeared to be possible. The potential for breast imaging and spectroscopy was demonstrated. Preliminary results of the first human body imaging at 7T suggest both promise and directions for further development.

Figure 1

7 T TEM body and surface coils (14). Figure 1a. shows a schematic diagram of the TEM body coil connected for four-port transmit and receive (1). Figure 1b shows the body coil mounted on the Siemens bore tube for insertion into the Siemens Sonata gradients. The transmission line “rungs” of the coil occupy the middle third (33 cm) of the one-meter coil as shown. The cavity or shield component of the coil is slotted for gradient induced eddy current attenuation and TEM element segregation. Figure 1c shows a schematic of a section of the TEM surface coil. The capacitances C_T, C_M, and C_D are for tuning, matching, and decoupling the independent elements respectively. Figure 1d shows an eight element TEM surface coil (32 cm × 15 cm) built of eight sets of two parallel copper strips separted by a 2 cm thick polytetrafluoroethylene dielectric, with eight independent drive cables connected.

Figure 2

Finite difference time domain models of B₁ and SAR contours generated by a body coil in the human body at 7 T. Figures 2a and 2b show relative B₁ magnitude isocontours, T/m (dB) in centerline transverse and sagittal planes. Figures 2c and 2d show SAR, W/kg (dB) isocontours in the same planes, respectively.

Figure 3

Whole body abdominal images at 7 T. The coronal, sagittal, and transaxial, multi-slice image sets were acquired with body coil transmit and receive using low-tip, low power, low resolution gradient echo sequences. No intensity correction has been applied to the data.

Figure 4

Whole body thorax images at 7 T. Using the same imaging parameters as in Figure 4, the heart is visible in coronal, sagittal and transaxial, multi-slice planes.

Figure 5

Breast imaging and spectroscopy at 7 T. Figure 5a shows a crossed-pair surface coil transceiver used to acquire breast images (5b), and proton spectra (5c). Figure 5b shows a sagittal slice from a fat-suppressed, T₁-weighted 3D FLASH image acquired in a normal subject. The box indicates voxel placement in the fibro-glandular tissue. A single voxel spectrum acquired from the voxel indicated in 5b appears in 5c. Peaks from taurine and total choline are visible.