Andreas Wetscherek1, Bram Stieltjes2, Frederik Bernd Laun1,2. 1. Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany. 2. Quantitative Imaging-Based Disease Characterization, German Cancer Research Center (DKFZ), Heidelberg, Germany.
Abstract
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.
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.
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