Purpose: Calculation of the precontrast longitudinal relaxation times is an integral part of the Tofts-based pharmacokinetic (PK) analysis of dynamic contrast enhanced–magnetic resonance images. The purpose of this study was to investigate the interpatient and over time variability of in head and neck primary tumors and involved nodes and to determine the median for primary and nodes . The authors also looked at the implication of using voxel-based values versus region of interest (ROI)-based on the calculated values for vascular permeability and extracellular volume fraction .
Methods: Twenty head and neck cancer patients receiving concurrent chemoradiation and molecularly targeted agents on a prospective trial comprised the study population. Voxel-based 's were generated using a gradient echo sequence on a 1.5 T MR scanner using the variable flip angle method with two flip angles [J. A. Brookes et al., “Measurement of spin-lattice relaxation times with FLASH for dynamic MRI of the breast,” Br. J. Radiol. 69, 206–214 (1996)]. The voxel-based , , and were calculated using iCAD's® (Nashua, NH) software. The mean 's in muscle and fat ROIs were calculated . To assess reliability of ROI drawing, values from ROIs delineated by 2 users (A and B) were calculated as the average of the 's for 14 patients. For a subset of three patients, the variability from baseline to end of treatment was also investigated. The and from primary and node ROIs were calculated using voxel-based values and and differences reported.
Results: The calculated values for fat and muscle are within the range of values reported in literature for 1.5 T, i.e., and . The average over 14 patients of the 's based on drawings by users A and B were , , , and . The absolute percentage difference between and calculated with voxel-based versus ranged from 6% to 81% and from 2% to 24%, respectively.
Conclusions: There is a certain amount of variability in the median values between patients, but the differences are not significant. There were also no statistically significant differences between the values for primary and nodes at baseline and the subsequent time points ( Friedman test). Voxel-based calculations are essential when quantitative Tofts-based PK analysis in heterogeneous tumors is needed. In the absence of mapping capability, when a relative, qualitative analysis is deemed sufficient, a value of can be used as an estimate for for both the primary tumor and the affected nodes in head and neck cancers at all the time points considered.