Flow-compensated intravoxel incoherent motion diffusion imaging.

PURPOSE

The pseudo-diffusion coefficient D* in intravoxel incoherent motion (IVIM) imaging was found difficult to seize. Flow-compensated diffusion gradients were used to test the validity of the commonly assumed biexponential limit and to determine not only D*, but also characteristic timescale τ and velocity v of the incoherent motion.

THEORY AND METHODS

Bipolar and flow-compensated diffusion gradients were inserted into a flow-compensated single-shot EPI sequence. Images were obtained from a pipe-shaped flow phantom and from healthy volunteers. To calculate the IVIM signal outside the biexponential limit, a formalism based on normalized phase distributions was developed.

RESULTS

The flow-compensated diffusion gradients caused less signal attenuation than the bipolar ones. A signal dependence on the duration of the flow-compensated gradients was found at low b-values in the volunteer datasets. The characteristic IVIM parameters were estimated to be v = 4.60 ± 0.34 mm/s and τ = 144 ± 10 ms for liver and v = 3.91 ± 0.54 mm/s and τ = 224 ± 47 ms for pancreas.

CONCLUSION

Our results strongly indicate that the biexponential limit does not adequately model the diffusion signal in liver and pancreas. By using both bipolar and flow-compensated diffusion gradients of different duration, the characteristic timescale and velocity of the incoherent motion can be determined.