Dynamic contrast enhanced–MRI in head and neck cancer patients: Variability of the precontrast longitudinal relaxation time ( T 10 )

Purpose: Calculation of the precontrast longitudinal relaxation times( T 10)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 ofT 10in head and neck primary tumors and involved nodes and to determine the medianT 10for primary and nodes( T 10p , n) . The authors also looked at the implication of using voxel‐basedT 10values versus region of interest (ROI)‐basedT 10on the calculated values for vascular permeability( K trans)and extracellular volume fraction( v e ) . Methods: Twenty head and neck cancer patients receiving concurrent chemoradiation and molecularly targeted agents on a prospective trial comprised the study population. Voxel‐basedT 10'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‐basedT 10,K trans, andv ewere calculated using iCAD's ® (Nashua, NH) software. The meanT 10's in muscle and fat ROIs were calculated( T 10m , f) . To assess reliability of ROI drawing,T 10p , nvalues from ROIs delineated by 2 users (A and B) were calculated as the average of theT 10's for 14 patients. For a subset of three patients, theT 10variability from baseline to end of treatment was also investigated. TheK transandv efrom primary and node ROIs were calculated using voxel‐basedT 10values andT 10p , nand differences reported. Results: The calculatedT 10values for fat and muscle are within the range of values reported in literature for 1.5 T, i.e.,T 10m = 0.958 s andT 10f = 0.303 s . The average over 14 patients of theT 10's based on drawings by users A and B wereT 10p A = 0.804 s ,T 10n A = 0.760 s ,T 10p B = 0.849 s , andT 10n B = 0.810 s . The absolute percentage difference betweenK transandv ecalculated with voxel‐basedT 10versusT 10p , nranged from 6% to 81% and from 2% to 24%, respectively. Conclusions: There is a certain amount of variability in the medianT 10values between patients, but the differences are not significant. There were also no statistically significant differences between theT 10values for primary and nodes at baseline and the subsequent time points ( p = 0.94 Friedman test). Voxel‐basedT 10calculations are essential when quantitative Tofts‐based PK analysis in heterogeneous tumors is needed. In the absence ofT 10mapping capability, when a relative, qualitative analysis is deemed sufficient, a value ofT 10p , n = 0.800 s can be used as an estimate forT 10for both the primary tumor and the affected nodes in head and neck cancers at all the time points considered.