Distortion; Spatial Image Shifts - Siemens fMRI User Manual

fMRI User Guide
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5.2 Distortion

(a) (b) (c)
Figure 8. Distortion in an EPI sequence. Image (a) (reference) was acquired after shimming and frequency
adjustments were performed. The shim was intentionally disturbed in the y-direction to demonstrate global image
distortion (stretching) in (b). Despite shimming, there are large susceptibility induced gradients near the sinuses
due to air-tissue interfaces that cause local distortions in the frontal lobe (c).
Despite shimming, different areas of the brain may have different resonant frequencies due to susceptibility
differences. These effects are pronounced especially near the sinuses due to the large susceptibility gradients
induced by air-tissue interfaces. Due to spatially varying resonant frequencies, spins are incorrectly localized in
the image, leading to distortion artifacts. A common method to reduce distortion is to use parallel imaging or
partial Fourier acquisitions to shorten the echo train. A small level of distortion near the sinuses and frontal lobe
is normal.

5.3 Spatial image shifts

(a)
(b)
Figure 9. Spatial image shift in EPI sequence. Image (a) (reference) was acquired using an EPI sequence after
shimming and frequency adjustment. For image (b), the synthesizer frequency was intentionally shifted by
150 Hz to demonstrate the effect of mismatch between the Larmor frequency of the spins and the synthesizer
frequency as a result of B0 drift.
Note that image (b) is spatially shifted with respect to image (a).
Spatial shifts occur in EPI images due to B0 drifts, resulting in a mismatch between the Larmor frequency and the
synthesizer frequency. The ep2d sequences have an built-in B0 drift correction that is applied during a scan. To
minimize spatial shifts, it is important to perform frequency adjustments before each EPI scan, which is also done
automatically by the EPI sequence.