To demonstrate the viability of using interleaved EPI to carry out fMRI at 3.0 Tesla, three experiments were performed.
A two shot multislice interleaved EPI technique was used to acquire a 256 x 256 matrix size image, using a switching rate of 1.04 kHz. Eight slices of thickness 4.5 mm, covering the visual cortex were acquired in eight seconds, during periods of visual stimulation from the pattern reversal (4 Hz) of a checkerboard pattern. There were 32 seconds of stimulation followed by 32 seconds of rest, repeated 16 times. The in-plane resolution was 0.75 x 0.91 mm2. A zeroth order phase correction was applied to one interleave, until the Fourier transform of the central line of k-space had the maximum ratio of signal in the inner points to signal in the outer points. The images were re-registered, but no spatial or temporal smoothing applied. The fMRI data was correlated to a square wave of the same period and phase as the stimulus and p-values calculated on the basis on peak height and spatial extent of the correlation maps using the theory of Gaussian random fields (see Chapter 6).
The resulting activation map is shown in Figure 5.13. The coloured regions represent the regions that correlate well to the stimulus (p>0.05). The background images are the average of the actual fMRI data set, so that the resolution seen in these images is the same as that of the activation maps. Regions of activation can be seen in the primary visual cortex and visual association areas. The change in the primary visual cortex upon activation was approximately 20%.
Low distortion EPI images were acquired using a two shot multislice interleaved EPI technique with a matrix size of 128 x 128 and a switching rate of 1.9 kHz. These were compared with images from a single shot experiment with the same matrix size. The activation paradigm consisted of 32 seconds of the subject observing the pattern reversal (4 Hz) of a checkerboard display whilst finger tapping, followed by 32 seconds of rest, repeated eight times. The whole brain, in each experiment, was scanned in 16 slices, in a period of 8 seconds. The images were registered and spatially smoothed (FWHM 4 mm), and correlated to a square wave of the same period and phase as the stimulus. Activation maps were thresholded, using the theory of Gaussian random fields, based on peak height and spatial extent. The activation maps were overlaid on white matter inversion recovery images (TI 1200 ms) acquired using either the single shot or two shot technique.
The resulting images are shown in Figure 5.14. Both methods produced very similar activation maps, with slightly more activation detected in the single shot technique. This is because the shot-to-shot signal to noise was not quite as good in the two shot experiment, presumably due to slight subject movement between interleaves. The level of distortion is slightly less in the two shot case and these results demonstrate the viability of carrying out low distortion interleaved EPI at 3.0 T.
One final application of interleaved EPI is to produce high resolution anatomical images to overlay the activation data on, with the level of distortion being the same in each case, allowing direct superposition.
A single shot EPI, functional imaging experiment was performed using a matrix size of 128 x 64, and a switching rate of 1.04 kHz. The activation paradigm consisted of 16 seconds of viewing a flashing LED display, and pressing a hand held button at the same rate, followed by 16 seconds of rest, repeated 16 times. The visual cortex was scanned in eight slices, every 2 seconds (resolution 3 x 3 x 9 mm3). Following the activation experiment, the switched and broadening gradient strengths were doubled and four volume images of the same slices were obtained using a two shot interleaved EPI sequence, having twice the in-plane resolution (1.5 x 1.5 mm2). These were used as background images for the activation maps to be overlaid upon. The activation maps were interpolated up to twice their size, and registered to the high resolution images. The activation images are shown in Figure 5.15.
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