Effect of shot number on the calculated apparent diffusion coefficient in phantoms and in human liver in diffusion‐weighted echo‐planar imaging

Abstract

Purpose:To show the signal intensity varies with shot number in diffusion-weighted (DW) echo-planar imaging (EPI) and affects apparent diffusion coefficient (ADC) calculation. Materials and Methods:This prospective study was performed on 35 adult patients and 20 volunteers. Measurements were made on a 3T scanner using a breathhold DW spin-echo EPI (SE EPI) sequence. Three protocols were used: A) eight consecutive shots at a fixed b-value of 0 seconds/mm 2 with TR ϭ 1000 and 3000 msec; B) seven consecutive shots at b-values ϭ 0, 1000, 750, 500, 250, 100, 0 seconds/mm 2 (in that order) with TR ϭ 3500 msec; and C) seven consecutive shots (as in B) with TR ϭ 1000, 1750, and 7000 msec.Results: For protocol A, signal intensity decreased significantly from the first to second shot (PϽ0.0001) and thereafter remained constant. For protocol B, the ADC depended on which b ϭ 0 seconds/mm 2 image was used. Using the first b ϭ 0 seconds/mm 2 , the mean ADC was 15% higher than using the second b ϭ 0 seconds/mm 2 (PϽ0.0001). For protocol C, the difference between ADC using the first b ϭ 0 seconds/mm 2 and the second b ϭ 0 seconds/mm 2 decreased as the TR increased. Conclusion:The signal intensity can vary with shot number in SE EPI. For TR Ն 3000 msec, steady-state is attained after one shot. Using data acquired prior to steady-state confounds the calculation of ADC values.

Solid focal liver lesion characterisation with apparent diffusion coefficient ratios

“…Most other investigators have attempted to determine an ADC threshold that can be used to differentiate benign and malignant lesions. However, the ADC value of normal liver is variable and is effected by the b values used, the acquisition technique (respiratory triggered, vs. free breathing vs. breath hold), shot number, pulse triggering, the anatomical location, portal venous blood flow related to post‐prandial state, artefacts and hepatic fibrosis . These factors make the application of a single ADC threshold for characterisation unreliable.…”

Liver diffusivity in healthy volunteers and patients with chronic liver disease: Comparison of breathhold and free‐breathing techniques

“…When considering causal effects of ADC differences between sequences, it is important to consider the impact of the sequence parameters. In a study of 35 patients and 20 volunteers, Shiehmorteza et al (30), reported that shot number in a SE EPI sequence may influence ADC, such that the application of a “dummy” b = 0 scan or scans may need to be acquired to attain a steady‐state before further DWI acquisitions. Our data were acquired at 1.5T, whereas the data from Shiehmorteza et al, (30) were acquired at 3T with its concomitant higher T1 and greater difference between fully relaxed signals and steady‐state signals.…”

“…In a study of 35 patients and 20 volunteers, Shiehmorteza et al (30), reported that shot number in a SE EPI sequence may influence ADC, such that the application of a “dummy” b = 0 scan or scans may need to be acquired to attain a steady‐state before further DWI acquisitions. Our data were acquired at 1.5T, whereas the data from Shiehmorteza et al, (30) were acquired at 3T with its concomitant higher T1 and greater difference between fully relaxed signals and steady‐state signals. Also, our data were acquired with three signal averages and one dummy scan (BH) or six signal averages (FB) compared with their single acquisition.…”

Cardiac motion in diffusion‐weighted MRI of the liver: artifact and a method of correction

“…Breathhold DW images were obtained using a customized DW spin-echo echo-planar-imaging (SE EPI) sequence (14). A pair of trapezoidal gradient pulses was applied along a single diffusion direction, which was the vector sum of the three physical gradients.…”