T1 Quantification: Variable Flip Angle Method vs Use of Reference Phantom

G. Litjens, M. Heisen, J. Buurman, A. Wood, M. Medved, G. Karczmar and B. Haar-Romeny

Annual Meeting of the Radiological Society of North America 2009.

PURPOSE For standardized interpretation of DCEMRI curves, calculation of contrast agent (CA) concentration from signal intensity over time is desired. Accurate measurement of tissue T1 before and after CA administration is thus necessary. Current T1 measurement methods are time-consuming. We propose the use of the ?reference tissue? method for fast T1 measurements concurrent with DCEMRI data acquisition, but with use of a reference phantom. METHOD AND MATERIALS The ?reference tissue? method is based on the approximation that in T1-weighted gradient echo images, signal intensity is proportional to 1/T1 ? thus signal intensity can be referenced to a tissue or a phantom with a known T1. We compared this method to the ?variable flip angle? method, most commonly used in clinical practice. We compared the ?reference tissue? method (TR/TE = 25/1.1 ms, a=40?) to the ?variable flip angle? method (TR/TE = 25/1.1 ms, a=3/5/10/15/20/25/30/35/40?), using the Eurospin T05 phantom, in which 10 out of 18 vials containing agar with varying concentrations of Gd-DTPA (T1 range: 281 ? 1384 ms) were used. RESULTS With the ?reference tissue? method, using 9 tubes successively as a reference for the remaining tube, the average error in the estimation of T1 was 8.5%, with a standard deviation of 6.1%, and was random. Using the ?variable flip angle? method, the average error was 5.7 % with a standard deviation of 3.8%. Using a two-sided Student?s t-test we found no statistically significant differences in the performance of the two methods (p-value = 0.52). CONCLUSION As DCEMRI imaging is done in a heavily T1-weighted regime, the ?reference tissue? method can be used to concurrently measure T1, and quantify CA concentration. Use of an agar phantom will require a correction for lower proton density in biological tissue. Proton density can be measured prior to CA administration, or tabulated values can be used. We demonstrated the feasibility of fast T1 measurements using a reference phantom, providing T1 maps without additional scanning time. This will allow quantification of the CA concentration throughout the DCEMRI scan, which cannot be achieved using the current clinical method. CLINICAL RELEVANCE/APPLICATION The use of a reference phantom for determining T1 can lead to a drastic reduction in scanning time and thus patient discomfort when compared to a regular clinical ?variable flip angle? scan.