In vivo assessment of optimal b-value range for perfusion-insensitive apparent diffusion coefficient imaging

Abstract

Purpose: To assess the optimal b-values range for perfusion-insensitive apparent diffusion coefficient (ADC) imaging of abdominal organs using short-duration DW-MRI acquisitions with currently available ADC estimation methods.

Methods: DW-MRI data of 15 subjects were acquired with eight b-values in the range of 5-800 s∕mm(2). The reference-standard, a perfusion insensitive, ADC value (ADC(IVIM)), was computed using an intravoxel incoherent motion (IVIM) model with all acquired diffusion-weighted images. Simulated DW-MRI data was generated using an IVIM model with b-values in the range of 0-1200 s∕mm(2). Monoexponential ADC estimates were calculated using: (1) Two-point estimator (ADC(2)); (2) least squares three-point (ADC(3)) estimator and; (3) Rician noise model estimator (ADC(R)). The authors found the optimal b-values for perfusion-insensitive ADC calculations by minimizing the relative root mean square error (RRMS) between the ADC(IVIM) and the monoexponential ADC values for each estimation method and organ.

Results: Low b-value = 300 s∕mm(2) and high b-value = 1200 s∕mm(2) minimized the RRMS between the estimated ADC and the reference-standard ADC(IVIM) to less than 5% using the ADC(3) estimator. By considering only the in vivo DW-MRI data, the combination of low b-value = 270 s∕mm(2) and high b-value of 800 s∕mm(2) minimized the RRMS between the estimated ADC and the reference-standard ADC(IVIM) to <7% using the ADC(3) estimator. For all estimators, the RRMS between the estimated ADC and the reference standard ADC correlated strongly with the perfusion-fraction parameter of the IVIM model (r = [0.78-0.83], p ≤ 0.003).

Conclusions: The perfusion compartment in DW-MRI signal decay correlates strongly with the RRMS in ADC estimates from short-duration DW-MRI. The impact of the perfusion compartment on ADC estimations depends, however, on the choice of b-values and estimation method utilized. Likewise, perfusion-related errors can be reduced to <7% by carefully selecting the b-values used for ADC calculations and method of estimation.

Figure 1

Representative ADC maps (first row) and liver (encircled) signal-decay plots (second row) organized according to the minimal b-value used to calculate the ADC. The ADC maps were calculated using the ADC₂ estimator with fixed b_max = 800 s/mm² and varying b_min. The discrepancy between the ADC₂ and the slow-diffusion component of the IVIM model (ADC_IVIM) decreases as the b_min increases.

Figure 2

The relative root mean squared error surface between the ADC estimates from the simulated DW-MRI data and the reference standard ADC_IVIM as a function of the b_min and b_max. The surfaces are organized according to the organ (rows) and the ADC estimator (columns).

Figure 3

The relative root mean squared error surface between the ADC estimates from the in vivo DW-MRI data and the reference standard ADC_IVIM as a function of the b_min and b_max. The surfaces are organized according to the organ (rows) and the ADC estimator (columns).