Fast model-based T₂ mapping using SAR-reduced simultaneous multislice excitation.

To obtain whole-brain high-resolution T <sub>2</sub> maps in 2 minutes by combining simultaneous multislice excitation and low-power PINS (power independent of number of slices) refocusing pulses with undersampling and a model-based reconstruction. A multi-echo spin-echo sequence was modified to acquire multiple slices simultaneously, ensuring low specific absorption rate requirements. In addition, the acquisition was undersampled to achieve further acceleration. Data were reconstructed by subsequently applying parallel imaging to separate signals from different slices, and a model-based reconstruction to estimate quantitative T <sub>2</sub> from the undersampled data. The signal model used is based on extended phase graph simulations that also account for nonideal slice profiles and B <sub>1</sub> inhomogeneity. In vivo experiments with 3 healthy subjects were performed to compare accelerated T <sub>2</sub> maps to fully sampled single-slice acquisitions. The accuracy of the T <sub>2</sub> values was assessed with phantom experiments by comparing the T <sub>2</sub> values to single-echo spin-echo measurements. In vivo results showed that conventional multi-echo spin-echo, simultaneous multislice, and undersampling result in similar mean T <sub>2</sub> values within regions of interest. However, combining simultaneous multislice and undersampling results in higher SDs (about 7 ms) in comparison to a conventional sequence (about 3 ms). The T <sub>2</sub> values were reproducible between scan and rescan (SD < 1.2 ms) within subjects and were in similar ranges across subjects (SD < 4.5 ms). The proposed method is a fast T <sub>2</sub> mapping technique that enables whole-brain acquisitions at 0.7-mm in-plane resolution, 3-mm slice thickness, and low specific absorption rate in 2 minutes