Observation of B1 Inhomogeneities on Large Biological Samples at 11.1 Tesla (2003)

B. L. Beck, K. A. Jenkins, K. Padgett, J. R. Fitzsimmons, S. J. Blackband University of Florida, Gainesville, Florida, United States Introduction A major drive in the development of MR has been towards stronger magnetic field strengths so as to increase the signal to noise ratio, which in turn can be traded for faster imaging and/or improved spatial resolution. However there are caveats that may compromise the gains high magnetic fields provide including increased potential magnetic field hazard, image inhomogeneities arising from magnetic susceptibilities, decreased rf penetration and increased standing wave effects. The University of Florida has an 11.1 Tesla, 40 cm Magnex clear bore magnet interfaced with a Bruker Biospec imaging/spectroscopy console for which we have constructed large rf resonators for proton imaging on moderately large samples, including a fixed human brain and a large piece of fresh beef. Here, we report the observation of large image distortions arising from wave behavior within the sample and interactions between the coil and sample. Methods All samples were placed within a quadrature ReCav coil 1,2 of inner diameter 20.5 cm, window extent 20.5 cm, shield diameter 24.2 cm and shield length 35.5 cm. The resonant frequency of 470.7 MHz yields a free space wavelength (λ) of 63.7 cm, making the ReCav a large wavelength structure (inner diameter and window extent ~ λ/3). Three large samples were imaged in the coil; a fixed human brain (~ 16 x 13 x 10 cm), a lean piece of fresh beef (~ 20 x 18 x 10 cm), and an oil phantom in a gallon plastic container. The complex relative permittivity of the samples was measured with an HP85070B coaxial probe/software and HP8752C network analyzer, from which the dielectric constant and conductivity were determined. The dielectric constant of the oil was measured to be 2.5 and the conductivity 1.7x10-3 S/m. The values measured for the beef and the brain were higher leading to a significant shortening of the electrical wavelength in the tissues. The dielectric constant of the beef was 62.9 and the conductivity 0.8 S/m (~ that of muscle at 470 MHz). The dielectric constant of the brain was dominated by the fixative (4% paraformaldehyde, 96% phosphate buffered saline) and was measured to be 77.5 (~ that of water) and the conductivity 0.5 S/m. Target values for permittivity and conductivity at 470 MHz are 48.8 and 0.6 for average brain, respectively, and 57.5 and 0.8 for average muscle, respectively . Results A coronal spin echo image (FOV 34 cm, 512*512 matrix, TR 500 ms, TE 19ms, 1 avg., 2 mm thick slice, sweep width 50 kHz, rf pw 8000 μs) of the oil phantom is shown in Figure 1a and indicates good field homogeneity. The rf power settings were determined by increasing the power until a signal null was seen at the center of a slice projection. This setting was chosen to be the power for a 180 degree pulse and the 90 degree pulse power was set 6 dB lower. The brain was drained of excess liquid and triple wrapped in 0.32 mil clear plastic bags. A spin echo image was acquired using a FOV 25 cm, 128*128 matrix, TR 671ms, TE 17ms, 1 average, 3 mm thick slice, sweep width 50 kHz, rf pw 8000 μs. The rf power settings were difficult to determine due to the inhomogeneity of the B1 field in this sample. Dark banding is seen on the coronal and sagittal images (Fig. 1b-c). Low power settings were investigated to be sure that the inhomogeneities were not the result of overtipping the spins. Finally, the beef was imaged using the following spin echo parameters; FOV 34 cm, 128*128 matrix , TR 500, TE 17, 1 average, 3 mm thick slice, sweep width 50 kHz, rf pw 8000 μs. The images show (Fig. 1d-h) a quite striking pattern of banding. The rf power settings were difficult to determine, but as a baseline, a setting was chosen that indicated maximum signal from the slice. This baseline was approached from low power settings where the signal intensities were near the level of the noise to again ensure that the image inhomogeneities were not simply caused by overtipping of spins. Figure 1d is the image with the baseline power settings. Further images were acquired where the rf power settings were changed in increments of 6 dB (Fig 1e-h) and it can be seen that the location of signal void does not change. However, the areas of signal intensity are influenced by the power settings to the point of overtipping in some areas as seen in Fig. 1h.