Diffusion-Weighted Imaging of the Liver

Abstract

SYNOPSIS Diffusion weighted MRI (DWI) is a technique that assesses the cellularity, tortuosity of the extracellular/extravascular space and cell membrane density based upon differences in water proton mobility in tissues. The strength of the diffusion weighting is reflected by the b-value. DWI using several b-values enables quantification of the apparent diffusion coefficient (ADC). DWI is increasingly employed in liver imaging for multiple reasons: it can add useful qualitative and quantitative information to conventional imaging sequences, it is acquired relatively quickly, it is easily incorporated into existing clinical protocols, and it is a non-contrast technique. DWI is useful for focal liver lesion detection and characterization, for the assessment of post-treatment tumor response and for evaluation of diffuse liver disease. ADC quantification can be used to characterize lesions as cystic/necrotic or solid and for predicting tumor response to therapy. Advanced diffusion methods such as IVIM (intravoxel incoherent motion) may have potential for detection, staging and evaluation of the progression of liver fibrosis and for liver lesion characterization. The lack of standardization of DWI technique including choice of b-values and sequence parameters has somewhat limited its widespread adoption.

Liver MRI: From basic protocol to advanced techniques

“…Measures can be taken to avoid voluntary movements but physiologic motion is inherent to any liver imaging protocol, whether it results from breathing or cardiac motion, blood flow and vessel pulsation or even gastrointestinal peristalsis [6,11,12]. Movement during image acquisition leads to blurring and ghosting, image duplicates from misplaced signal that may hamper image interpretation ( Fig.…”

“…The use of navigator echoes for respiratory triggering is among the most popular and well succeeded techniques [4,6,12].…”

“…Cardiac motion affects mostly the left liver lobe and can be overcome by ECG or pulse triggering techniques, but again with an acquisition time penalty [12].…”

“…Acceleration factors, or the number of lines of k-space acquired in parallel, are typically limited by the development of residual artefacts and severe signal loss, thus, a factor higher than 2 is rarely used [7,11,12]. Acronyms of parallel imaging techniques include GRAPPA, SENSE and mSENSE [11].…”

“…This movement is restricted by cell membranes, resulting in tissues with different cellularity having differences in water mobility [2,12]. A specialized MR tool, diffusion weighted imaging (DWI) derives image contrast from these differences [5,11].…”