Effect of inaccurate b-values from imaging gradients on intravoxel incoherent motion.

PURPOSE: Neglecting imaging gradients in b-value calculations has been a documented issue for decades and remains unaccounted for in the current postprocessing pipelines. This omission may introduce inaccuracies that propagate into diffusion parameter estimates, such as in ADC and DTI analysis. Because intravoxel incoherent motion (IVIM) makes use of low b-values, these inaccuracies may be of greater importance. This study examines the impact of biased b-values on IVIM analysis in simulations and in vivo.

METHODS

In simulations, b-values were calculated for two pulsed gradient spin-echo sequence designs: one with large cross-terms between imaging and diffusion gradients, and one with minimal cross-terms. Biased and unbiased b-values were calculated from sequences with 200 diffusion directions. These b-values were used to generate IVIM signal curves for parameter estimation. Simulations were repeated with varying in-plane resolutions (1-4 mm) and slice thicknesses (2-10 mm). Additionally, 15 prostate exams were analyzed with scanner-provided b-values and actual b-values derived from the gradient waveforms of the full pulse sequence.

RESULTS

The magnitude and direction of errors in IVIM parameters depended on pulse sequence design. Errors persisted until the full contribution of imaging gradients was considered. Errors in the in vivo data were coherent with the simulations, showing errors of -0.7% in f, 0.8 μm2/ms in D*, and 0.07 μm2/ms in D.

CONCLUSION

Ignoring imaging gradients in b-value calculations introduces unnecessary inaccuracies, making IVIM results spurious and highly dependent on specific pulse sequence design and imaging parameters. These inaccuracies can be corrected by adjusting the b-value calculations, without additional measurements.