Single Scan T1 and T2* Mapping without Flip Angle Correction

Introduction: Quantitative magnetic resonance imaging is a powerful tool for investigating a large variety of biological phenomena. Thus, big efforts have been made during the recent years in order to develop fast methods for the quantitative assessment of various MR parameters. Two very important parameters are the T1 and T2* relaxation times. Here a new method for the simultaneous measurement of T1 and T2* maps is presented. Various approaches towards the simultaneous acquisition of T1 and T2* have been made [1, 2]. But all of these methods need the computation or measurement of flip angle maps in order to correct for B1 inhomogeneities. The presented method is robust against variations in flip angles due to B1 inhomogeneities of the resonator. Moreover, this sequence is time efficient since both parameters can be measured in the time a single conventional T1 measurement [3] would take. Another advantage is that there are no misregistration artifacts due to motion which can occur during the consecutive measurement of the single parameters T1 and T2*. Material and Methods: The proposed method uses the large dynamic range of the inversion recovery experiment for the T1 sampling and the robustness of single echo acquisition for the T2* acquisition. We implemented this using an inversion recovery snapshot FLASH sequence [3, 4] including a T2* measurement using single echo acquisition with exponentially increasing echo times. The conventional inversion recovery snapshot FLASH sequence applies a 180° inversion pulse followed by a series of FLASH modules for sampling the T1 relaxation curve, which relaxes towards M0* instead of M0 due to the subsequent FLASH excitation pulses. In the method introduced here the echo times within these FLASH modules (and with it the repetition times of the FLASH modules) are increased exponentially with increasing time between the inversion pulse and the FLASH module. This results in a signal decay with the time constant T2* during the sampling of the inversion recovery T1 curve (solid line in Figure 1). The exponential increase is adjusted such that nearly the complete dynamic range of the inversion recovery curve is covered. This is achieved by keeping the echo time close to its minimal value at the first half of the acquisition. During the steady state tail of the T1 signal, the echo time of the FLASH modules then rapidly increases, which leads to the T2* decay and allows the calculation of T2* values (Figure 1). Since the consecutive excitation pulses of the single FLASH sequences are no longer equidistant and thus the signal does not relax towards a steady state, the analytic equations introduced by Deichmann [4], which implement a correction for these effects, do not apply for this model. We therefore implemented a fitting algorithm which simulates the pulse sequence with the known timings and thus allows the calculation of the resulting signal of the sequence. This enables the fitting of the parameters M0, T1, and T2* by minimizing the mean square error of the measured data and the simulated signal.