Rapid Assessment of Longitudinal Relaxation Time in Materials and Tissues With Extremely Fast Signal Decay Using UTE Sequences and the Variable Flip Angle Method

Abstract

: To develop suitable strategies for quantification of longitudinal relaxation time (T1) by means of ultrashort echo time (UTE) sequences and the variable flip-angle approach in materials and tissues with extremely fast signal decay. : A recently published modified Ernst equation, which correctly accounts for in-pulse relaxation of transverse magnetization, was used to numerically determine optimal flip angles for reliable assessment of T1 in case of extremely short effective transverse relaxation time (T2*). Various ratios of repetition time (TR) to T1 and radiofrequency (RF) pulse duration (TRF) to T2* were evaluated. Theoretical considerations were applied to solid polymeric material (T2* = 0.295 milliseconds), and T1 quantification was performed using various optimized flip-angle approaches at different RF pulse durations (TRF = 0.1-0.4 milliseconds). Furthermore, in vivo measurement of T1 in cortical bone was exemplarily performed in 3 healthy volunteers to test the applicability of the proposed method in vivo. For in vitro and in vivo studies, MR imaging was performed on a 3 T whole-body MR system using a 3D UTE sequence with a rectangular excitation pulse and centric radial readout. : Optimal flip angles were shown to be strongly dependent on TR/T1 and TRF/T2* ratios. Exemplarily, longitudinal relaxation time of the investigated solid polymeric material was determined to T1 = 223.1 ± 3.1 milliseconds with RF pulse duration of TRF = 0.2 milliseconds, and 12 acquired flip angles ranging from 5 to 60 degrees. Using only 2 optimized flip angles (8 degrees, 44 degrees), T1 of the same material was determined to T1 = 223.8 ± 4.2 milliseconds in a markedly less acquisition time. In vivo evaluation of cortical bone was feasible and showed T1 values of 80.4 ± 25.1 milliseconds, exemplarily. : Using the modified Ernst equation, it seems possible to rapidly evaluate 3D distribution of longitudinal relaxation time in materials and tissues with extremely fast signal decay by means of UTE sequences and only 2 measurements with optimized flip angles.