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

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 = 1000 and 3000 msec; B) seven consecutive shots at b-values = 0, 1000, 750, 500, 250, 100, 0 seconds/mm(2) (in that order) with TR = 3500 msec; and C) seven consecutive shots (as in B) with TR = 1000, 1750, and 7000 msec.

RESULTS

For protocol A, signal intensity decreased significantly from the first to second shot (P<0.0001) and thereafter remained constant. For protocol B, the ADC depended on which b = 0 seconds/mm(2) image was used. Using the first b = 0 seconds/mm(2), the mean ADC was 15% higher than using the second b = 0 seconds/mm(2) (P<0.0001). For protocol C, the difference between ADC using the first b = 0 seconds/mm(2) and the second b = 0 seconds/mm(2) decreased as the TR increased.

CONCLUSION

The signal intensity can vary with shot number in SE EPI. For TR > or = 3000 msec, steady-state is attained after one shot. Using data acquired prior to steady-state confounds the calculation of ADC values.