Consistent T1 Quantification in a Multiscanner Setting using Reference Region Variable Flip Angle B1+ Mapping
Novena Rangwala1, Isabel Dregely1, Holden Wu1, and Kyunghyun Sung1

1Department of Radiological Sciences, David Geffen School of Medicine, University of California Los Angeles, Los Angeles, CA, United States

Synopsis

The purpose of this study was to demonstrate improved T1 estimation in the prostate using the Reference Region Variable Flip Angle (RR-VFA) B1+ mapping and correction across multiple MRI scanners with different RF transmission modes. Prostate T1 measurements were compared in four volunteers on three MRI scanners before and after B1+ correction. The results showed that, for each volunteer, T1 variations across scanners decreased by 62% after RR-VFA correction, to an average of 10% among scanners, highlighting the need for B1+ correction and the ability to effectively yield precise T1 estimations in a multi-scanner setting using RR-VFA.

Purpose

Quantitative dynamic contrast-enhanced (DCE-) MRI, including the estimation of T1 relaxation times, has shown promise in tumor detection and characterization in prostate cancer1,2. 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.

Methods

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 ARR-VFA) and T1 measurements were calculated from the ROIs before (denoted by T1non) and after (denoted by T1RR-VFA) RR-VFA correction.

Results

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 ARR-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 T1non values for different volunteers and scanners (1760±278 ms, range: 1260 ms to 2119 ms, Fig. 2a). Corrected T1 maps indicate better agreement of T1RR-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 T1RR-VFA (1921±131 ms, range: 1646 – 2072 ms) compared with T1non. A comparison of Figs. 2a and 2b demonstrate reduced range of T1RR-VFA compared with T1non, within each scanner. Most importantly, for the same volunteer, T1 measurement comparisons on the three scanners show variations in T1RR-VFA reduced by an average of 61% compared with the corresponding T1non, to an average of 10% across scanners, reflecting improved consistency of the T1 measurement in the prostate after RR-VFA correction.

Conclusions

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.

Acknowledgements

This research was supported, in part, by Siemens Medical Solutions.

References

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Figures

FIGURE 1: Example images from one volunteer show different B1+ inhomogeneity patterns on the three scanners (a-c). Shading is seen on (d-f) before correction, illustrating differences in T1 values due to B1+ inhomogeneity. After RR-VFA correction, shading is reduced (g-i) and the corrected prostate T1 is more consistent across scanners.

FIGURE 2: Scatter plots show T1 measurements before (a) and after (b) RR-VFA correction, illustrating more consistent T1 values across all scanners for each volunteer and lower T1 ranges after correction than before.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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