Xinran Zhong^{1,2}, Dapeng Liu^{1}, James Sayre^{1}, Holden H Wu^{1,2}, and Kyunghyun Sung^{1,2}

Accurate T_{1} estimation is critical for quantitative prostate DCE MRI. B_{1}^{+} inhomogeneity can introduce significant error into the T_{1} quantification, especially for variable flip angle method. Reference region variable flip angle (RR-VFA) method is a promising B_{1}^{+} and T_{1} estimation technique, which requires no separate scans for B_{1}^{+ }mapping and can reduce slice profile and position mismatch between B_{1}^{+} and T_{1} maps. In this study, we investigated both intra-scanner repeatability and inter-scanner reproducibility regarding B_{1}^{+} corrected T_{1} on two 3.0 T scanners to compare RR-VFA to a commercially available B_{1}^{+} estimation technique. RR-VFA showed comparable _{1}^{+} estimation technique.

With IRB approval, ten healthy male subjects with mean age of 29.3±4.2 years old were prospectively recruited in this study with written informed consent. Each subject was scanned on two different 3.0T systems Prisma (‘Scanner 1”) and Skyra (“Scanner 2”) (Siemens Healthcare, Erlangen, Germany) respectively. On each scanner, each subject was scanned twice in the feet-first supine position, with repositioning between the two scans. The general workflow is shown in Fig. 1. The protocol included 2D T_{2} weighted Turbo Spin Echo (T2W TSE), 2D saturated turbo FLASH (satTFL) B_{1}^{+} sequence and 3D VFA imaging with a dual echo bipolar readout (TEs=1.23/2.46 ms). The body coil was used for RF transmission and a receive-only phased array coil was used for signal reception. “TrueForm” B_{1}^{+} shimming mode was active for both scanners.^{7}

Relative flip angle maps (obtained flip angle/prescribed flip angle×100%) were generated using both RR-VFA and satTFL. Three T_{1} maps (T_{1} without B_{1}^{+} correction, satTFL corrected T_{1} and RR-VFA corrected T_{1}) were calculated for each scan. Regions of interests (ROI) were positioned on three representative slices (base, mid and apex) in the prostate, as well as the left and right obturator internus muscles, on each T2W TSE scan. We matched the slices manually between scans. For each scan, two B_{1}^{+} maps and three T_{1} maps were estimated, and both B_{1}^{+} and T_{1} maps were linearly interpolated to the same dimension as the 2D T2W TSE acquisition so that the ROI drawn on T2W TSE scans could be transferred to those maps directly. All post-processing was performed using in-house scripts written in Matlab (Mathworks, Natick, Mass).

B_{1}^{+} estimations from two methods within each ROI were compared using linear regression and Pearson’s correlation. The Pearson’s correlation was also used to compare T_{1} between scans for intra- and inter-scanner variability evaluation. For intra-scanner variability, the average T_{1} between the two scans on the same scanner for the same volunteer were compared using paired t-test, Linear Regression and Bland Altman plot. Similarly, inter-scanner variability was assessed by determining the correlation and agreement of the average T_{1} between the scans of two different systems for the same volunteer.

Representative ROI positioning for three T_{1} maps from the same scan is shown in Fig. 2. The uncorrected T_{1} shows inconsistent T_{1} values between left and right obturator internus muscles, and the inconsistency is reduced in both B_{1}^{+} corrected T_{1} maps.

Linear regression between average B_{1}^{+} from RR-VFA and satTFL gives a slope of 1.2 when comparing RR-VFA to satTFL B_{1}^{+}. The squared Pearson correlation coefficient (r^{2}) between two B_{1}^{+} estimation techniques is 0.859, showing good linear correlation.

Evaluation of T_{1} using paired t-test is shown in Fig 3. T_{1} from different scans before B_{1}^{+} correction is significantly different from each other, while not significantly different after B_{1}^{+} correction using both B_{1}^{+} estimation methods. Also, the intra- and inter-scanner comparison of linear regression is shown in Fig. 4. A higher squared Pearson Correlation (r^{2}) is observed in RR-VFA corrected T_{1} compared to uncorrected T_{1} and satTFL corrected T_{1}. The intra-and inter-scanner comparison of Bland-Altman plot is shown in Fig. 5. The 95% limits of agreement for RR-VFA corrected T_{1} is smaller than that of uncorrected T_{1} as well as satTFL corrected T_{1}.

The intra- and inter-scanner variability of T_{1} estimation has been significantly reduced using both RR-VFA and satTFL. The RR-VFA corrected T_{1} had similar intra- and inter-scanner variability to the satTFL corrected T_{1, }and RR-VFA provides B_{ 1} ^{ +} estimation highly correlated to satTFL (r^{2} = 0.859). Considering other advantages of RR-VFA such as no requirement for a separate scan, RR-VFA shows great potential in improving the prostate quantitative DCE-MRI.

1. Alonzi R, Padhani AR, Allen C. Dynamic contrast enhanced MRI in prostate cancer. European Journal of Radiology. 2007;63(3):335–350. doi:10.1016/j.ejrad.2007.06.028 2

2. Gupta RK. A new look at the method of variable nutation angle for the measurement of spin-lattice relaxation times using fourier transform NMR. Journal of Magnetic Resonance. 1977;25(1):231–235. doi:10.1016/0022-2364(77)90138-X

3. Sung K, Daniel BL, Hargreaves BA. Transmit B1+ field inhomogeneity and T1 estimation errors in breast DCE-MRI at 3 tesla. Journal of magnetic resonance imagingâ€Ż: JMRI. 2013;38(2):454–9. doi:10.1002/jmri.23996

4. Sung K, Saranathan M, Daniel BL, Hargreaves BA. Simultaneous T 1 and B 1 + Mapping Using Reference Region Variable Flip Angle Imaging. Magnetic Resonance in Medicine. 2013;70(4):954–961. http://doi.wiley.com/10.1002/mrm.24904. doi:10.1002/mrm.24904

5. Rangwala NA, Dregely I, Wu HH, Sung K. Optimization and evaluation of reference region variable flip angle (RR-VFA) B1+ and T1 Mapping in the Prostate at 3T. Journal of Magnetic Resonance Imaging. 2017;45(3):751–760. doi:10.1002/jmri.25410

6. Chung S, Kim D, Breton E, Axel L. Rapid B1+ mapping using a preconditioning RF pulse with turboFLASH readout. Magnetic Resonance in Medicine. 2010;64(2):439–446. http://doi.wiley.com/10.1002/mrm.22423. doi:10.1002/mrm.22423

7. Blasche M, Riffel P, Matthias L. TimTX TrueShape and syngo ZOOMit technical and practical aspects. MAGNETOM Flash 2012:74–84. 39.

Figure 1. Workflow and protocols for
the experiment. Each volunteer was scanned four times on two 3.0 T scanners.

Figure 2. T_{1} value within ROIs overlaid on T2W-TSE image. Three ROIs for obturator internus muscles and the prostate region were positioned on three selected slices respectively for each scan. The inconsistent T_{1} between left and right muscles is reduced after B_{1}^{+} correction using either method (marked by white arrow).

Figure 3. Paired t-test results between average T_{1} within ROIs for both intra-and inter scanner comparison. The T_{1}s between different scans are significantly different (marked by *) before B_{1}^{+} correction and become not significantly different after correction using B_{1}^{+} correction with two B_{1}^{+} estimation techniques.

Figure
4. Linear Regression and squared Pearson’s
Correlation (r^{2}) for both intra-scanner comparison (a-f) and
inter-scanner comparison (g-i) between average T_{1} within each ROI
between scans. Each encoded color indicates one volunteer. RR-VFA corrected T_{1} has the
highest r^{2}.

Figure 5. Bland-Altman plots for both intra-scanner comparison (a-f) and inter-scanner comparison (g-i) to show the agreement of average T_{1} within each ROI between scans. Each encoded color indicates one volunteer. RR-VFA corrected T_{1} has the smallest 95% limits of agreement.