WE‐G‐18C‐02: Estimation of Optimal B‐Value Set for Obtaining Apparent Diffusion Coefficient Free From Perfusion in Non‐Small Cell Lung Cancer

Abstract

Purpose:Diffusion‐weighted MRI (DW‐MRI) is increasingly being investigated for radiotherapy planning and response assessment. Selection of a limited number of b‐values in DW‐MRI is important to keep geometrical variations low and imaging time short. We investigated various b‐value sets to determine an optimal set for obtaining monoexponential apparent diffusion coefficient (ADC) close to perfusion‐insensitive intravoxel incoherent motion (IVIM) model ADC (ADC<sub>IVIM</sub>) in nonsmall cell lung cancer.Methods:Seven patients had 27 DW‐MRI scans before and during radiotherapy in a 1.5T scanner. Respiratory triggering was applied to the echo‐planar DW‐MRI with TR=4500ms approximately, TE=74ms, pixel size=1.98×1.98mm2, slice thickness=4–6mm and 7 axial slices. Diffusion gradients were applied to all three axes producing traceweighted images with eight b‐values of 0–1000μs/μm2. Monoexponential model ADC values using various b‐value sets were compared to ADC<sub>IVIM</sub> using all b‐values. To compare the relative noise in ADC maps, intra‐scan coefficient of variation (CV) of active tumor volumes was computed.Results:ADC<sub>IVIM</sub>, perfusion coefficient and perfusion fraction for tumor volumes were in the range of 880‐1622 μm2/s, 8119‐33834 μm2/s and 0.104–0.349, respectively. ADC values using sets of 250, 800 and 1000; 250, 650 and 1000; and 250–1000μs/μm2 only were not significantly different from ADC<sub>IVIM</sub>(p>0.05, paired t‐test). Error in ADC values for 0–1000, 50–1000, 100–1000, 250–1000, 500–1000, and three b‐value sets‐ 250, 500 and 1000; 250, 650 and 1000; and 250, 800 and 1000μs/μm2 were 15.0, 9.4, 5.6, 1.4, 11.7, 3.7, 2.0 and 0.2% relative to the reference‐standard ADC<sub>IVIM</sub>, respectively. Mean intrascan CV was 20.2, 20.9, 21.9, 24.9, 32.6, 25.8, 25.4 and 24.8%, respectively, whereas that for ADC<sub>IVIM</sub> was 23.3%.Conclusion:ADC values of two 3 b‐value sets (250, 650 and 1000; and 250, 800 and 1000μs/μm2) approached ADC<sub>IVIM</sub>, with relative noise comparable to that of ADC<sub>IVIM</sub>. These sets may be used to obtain perfusion‐insensitive ADC values in lung tumors.E. Weiss: Funding through Varian Medical Systems and Philips Oncology Systems, UpToDate royalties.G. Hugo: NIH R01CA166119, P01 CA116602, NHMRC Project Grant.