TU-H-206-03: Characterizing B1 Inhomogeneities in DCE MRI

Authors


Abstract

Purpose:

Dynamic Contrast Enhanced (DCE) MRI is a valuable technique for measuring perfusion and permeability characteristics of tumors. Exogenous contrast concentrations are calculated based on changes in T1 measured using fast 3D gradient echo (FLASH) sequences. However, the slab selective pulses used in 3D MRI may result in B1 inhomogeneities across the volume of interest that can lead to errors in T1 and thus the estimated gadolinium concentration. We compared three FLASH DCE sequences (GRE, TWIST, and VIBE) to determine their signal homogeneity across slices and the accuracy in calculating T1 using acquisitions with variable flip angles.

Methods:

The sequences were tested at 3 T on a Siemens mMR (VB20P) using a doped water phantom 3.75 g/L NiSO4 - 6H2O + 5 g/L NaCl (T1 = 104 ms) and a 2% agar, 0.67% NaCl phantom (T1= 1.71 s). 2D EPI B1 maps and inversion recovery T1maps were acquired for ground truth. 3D MRI was acquired at different flip angles to generate a T1 map. Regions of interest were drawn to measure signal inside the phantoms as a function of slice position. The T1 for each slice ROI was fit to the FLASH steady-state model of magnetization.

Results:

Based on the data, GRE gave the most uniform signal homogeneity and T1 values in the middle slices of the 3D volume. The 3D VIBE sequence had the largest region of signal inhomogeneity compared to the 3D GRE and TWIST sequences. VIBE's B1 inhomogeneity is inconsistent at low flip angles. However, VIBE resulted in more slices with T1 values similar to the ground truth.

Conclusion:

The central 1/3 of the slices yielded signals that result in T1 fits consistent with the ground truth. However, the remaining slices required some form of B1 inhomogeneity correction for quantitative DCE analysis.

The research was supported in part by NIH NCI Grant R01CA159471.

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