Handout - Extracranial Diffusion - weighted Imaging Page 1 of 4 Diffuse liver and kidney disease : value of Diffusion - weighted imaging ( DWI )

Diffusion acquisition parameters for imaging the upper abdomen Breath-hold or free breathing or respiratory triggered (using a navigator echo) SS EPI DWI sequence can be acquired before contrast injection, with the suggested following parameters: axial or coronal acquisition, fat suppression, tridirectional gradients using the following b-values: 0 (used as reference), 50 (black-blood sequence-useful for lesion detection in the liver)-400 to 500 (intermediate) and 800 to 1000 (high) sec/mm. Lower b-values will generate higher ADC values, owing to the contribution of intravoxel incoherent motion effects other than diffusion (eg, perfusion or flow phenomena), as opposed to higher b-values, which will enable “pure” diffusion-weighting, at the expense of lower residual signal. To reduce the effects of intravoxel incoherent motion, maximum b-values ≥ 800 sec/mm are suggested whenever possible. We suggest a baseline b=0 sec/mm image (used as a reference), and intermediate b-value (for example, 400-500 sec/mm) which provides intermediate diffusion-weighting with acceptable image quality, and a higher b-value (for example, b=800-1000 sec/mm) which provides higher diffusion-weighting, free from perfusion and flow contamination. In addition, the use of 3 b-values provides a more precise ADC fit. The other parameters used in our protocol are as follows: TR 1800-2300 msec., TE min., matrix 180x192, slice thickness/gap 7/1.4 mm, number of averages 2 (breath-hold) or 4 (respiratory triggered acquisition), parallel imaging (acceleration factor 2), EPI factor 144-192, bandwidth 1302 Hz/pixel, acquisition time approximately 2 x 20 sec. to cover the whole kidneys (for the breath-hold acquisition), and at least 2 min. for the respiratory triggered acquisition. We obtain pixelbased ADC maps (integrating the 3 b-values) using a commercial workstation, ADC is calculated with a linear regression analysis of the function S = S0 x exp (-b x ADC), where S is the SI after application of the diffusion gradient, and S0 is the SI at b = 0 sec/mm.