DIFFUSION ACQUISITION TECHNIQUE FOR LIVER DWI
sequences of the liver typically include at least two b-values: a low b-value (range of 0–100 s/mm2 ) and a high b-value (range of 400–800 s/mm2 ). Low b-values (50–100 s/mm2 ) can suppress intravascular signal, producing black-blood T2-weighted images with good signal-to-noise ratio (SNR) and tissue contrast, enhancing lesion detectability particularly in small lesions that are close to vessels. High b-values have a lower SNR because of signal loss from diffusion, as well as artifacts, but provide better lesion characterization by enhancing differences in signal intensity between liver parenchyma and lesions. The most widely used strategy for DWI is echoplanar imaging (EPI), which allows acquisition of a full slice in a single shot. A typical protocol involves the use of fat-saturated single-shot diffusion-weighted EPI played in an interleaved multislice fashion to allow volume coverage. However, the EPI readout is also subject to “ghosting” and susceptibility artifacts (Le Bihan et al. 2006). Turbo spin-echo and steadystate free-precession imaging (Lu et al. 2012) may offer reduced susceptibility artifacts compared with EPI readouts. However, this usually leads to increased echo trains and to higher-power deposition of the RF pulse train. Periodically rotated overlapping parallel lines with enhanced reconstruction (Deng et al. 2006) is a modified segmented EPI technique whereby successive segments are acquired in a radial fashion. Compared with segmented EPI, this technique may be more immune to motion artifacts, with options for robust motion correction (Deng et al. 2006). Physiologic motion is inherent to any liver imaging protocol, with breathing and cardiac motion resulting in subject- and acquisition-dependent imaging artifacts. Single-shot EPI is robust to motion because its acquisition time is faster than physiologic processes. However, in a typical DWI acquisition with multiple b-values and signal averaging, residual fluctuations exist between successively acquired EPI images. Breath-hold, freebreathing, or respiratory-triggering technique may be used. Breath-hold techniques result in the shortest image acquisition time, with a limitation in the number of bvalues that can be used. Free-breathing protocols typically result in blurred images. Respiratory-triggering acquisition (with the use of navigator echoes or bellows) may improve DWI data quality (Dyvorne et al. 2013) at the cost of increased imaging time. Cardiac motion artifacts usually appear in the left liver lobe as signal loss at a high b-value as a result of strong dephasing of the coherently moving spins under the influence of diffusion gradients. This artifact can be overcome by performing DWI acquisitions at diastole by using an electrocardiogram or pulse trigger (Mürtz et al. 2002), but this approach results in significantly increased scan time. Another promising approach to mitigate signal loss caused by cardiac motion is to use motion-compensated diffusion gradients (Ozaki et al. 2013) to cancel the dephasing of coherently moving tissues while maintaining diffusion weighting.