Quantitative dynamic contrast-enhanced (DCE-) MRI, including the estimation of T₁ relaxation times, has shown promise in tumor detection and characterization in prostate cancer^1,2. Pre-contrast T₁ relaxation times are usually measured using variable flip-angle³ (VFA) imaging that is highly susceptible to B₁+ inhomogeneity⁴. Existing methods to map and correct B₁+ field inhomogeneity⁵ can be added, but their clinical usability remains limited due to increased scan time, slice profile and position mismatch, and availability based on the scanner manufacturer. The reference region variable flip angle (RR-VFA) method was proposed for simultaneous B₁+ and T₁ mapping⁶ and recently evaluated in the prostate of healthy volunteers⁷. The purpose of this study was to demonstrate improved T₁ estimation in the prostate using RR-VFA B₁+ mapping and correction across multiple MRI scanners with different RF transmission modes.

Our optimized prostate RR-VFA method⁷ utilized VFA images with two-echo Dixon fat-water separation to simultaneously estimate both B₁+ inhomogeneity and T₁ relaxation time. The fat reference tissue was identified using Otsu’s method⁸ and signal fat fraction⁹, and fat T₁ was characterized using a population-based effective value of 320 ms. Using the VFA signal equation, the B₁+ inhomogeneity was initially calculated in fat and interpolated to the entire FOV (including prostate), as a unit of relative flip angle (rFA = obtained flip angle/prescribed flip angle×100 %).

With IRB approval, four healthy male volunteers (age = 29±3.2 years, weight = 75±10 kgs) were scanned on three Siemens 3T scanners (Skyra (“S1”), Trio (“S2”), and Prisma (“S3”), Erlangen, Germany), using the body coil for RF transmission and receive-only phased-array coil for signal reception. RF transmission modes differed between scanners: S1 and S3 were operated with “TrueForm” RF transmission and S2 operated with circular polarization¹⁰. The VFA imaging protocol was acquired with four flip angles: 2°, 5°, 10°, 15°, and the following common imaging parameters: TR/TE1/TE2 = 4.17/1.23/2.46 ms, FOV = 26 cm, acquisition matrix = 160×160, 20 partitions with partition thickness of 3.6 mm. The total scan duration for all flip angles was under four minutes.

For evaluation, three-dimensional regions of interest (ROIs) were selected manually to cover the entire prostate. Mean rFA (denoted by A_RR-VFA) and T₁ measurements were calculated from the ROIs before (denoted by T1_non) and after (denoted by T1_RR-VFA) RR-VFA correction.

Figure 1 shows a representative example of rFA and T₁ maps from all three scanners. The rFA maps show differences in the prostate ROI within this volunteer (Fig. 1a-c), and with the following group-averaged A_RR-VFA in the prostate for each scanner: S1: 98.3±2.5 %, S2: 90.5±4.4 %; S3: 97.6±5.5 %. T₁ maps without RR-VFA correction (Fig. 1d-f) show differences in the prostate and areas of severe shading due to B₁+ inhomogeneity, with a large range of T1_non values for different volunteers and scanners (1760±278 ms, range: 1260 ms to 2119 ms, Fig. 2a). Corrected T₁ maps indicate better agreement of T1_RR-VFA measurement in the three scanners within the prostate (Fig. 1g-i) in this volunteer, and for all subjects in the group (Fig. 2b), with lower standard deviation in T1_RR-VFA (1921±131 ms, range: 1646 – 2072 ms) compared with T1_non. A comparison of Figs. 2a and 2b demonstrate reduced range of T1_RR-VFA compared with T1_non, within each scanner. Most importantly, for the same volunteer, T₁ measurement comparisons on the three scanners show variations in T1_RR-VFA reduced by an average of 61% compared with the corresponding T1_non, to an average of 10% across scanners, reflecting improved consistency of the T₁ measurement in the prostate after RR-VFA correction.The multi-scanner comparison demonstrates a decreased range of B₁+ inhomogeneity-corrected T₁ values on a per-volunteer basis and reflects the improved consistency of T₁ measurements after B₁+ correction using RR-VFA. The application of RR-VFA B₁+ correction has great potential to improve T₁ quantification, resulting in improved quantitative prostate DCE-MRI.