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.
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.
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%.
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.