Synopsis

Keywords: Quantitative Imaging, Brain

Recently, strategically acquired gradient echo (STAGE) imaging has been developed as a potential standardized brain imaging protocol. STAGE is designed to use two 3D fully flow compensated multi-echo SWI sequences with 2 different flip angles (FAs). We evaluated the inter-scanner consistency of T1 values and proton spin density (PSD) values derived from STAGE imaging for elderly healthy volunteers. We conducted VOI analysis both in Native space and “Mutual space” using AAL3 VOIs. Parcellation was automatically performed with few manipulations, and good ICC and minimal biases were demonstrated between the two scanners.

Introduction

Multi-contrast imaging provides both qualitative and quantitative images at the same time, which is suitable for serving as a standardized protocol for various brain diseases (1, 2). Recently, strategically acquired gradient echo (STAGE) imaging has been developed as a potential standardized brain imaging protocol. STAGE is designed to use two fully flow compensated multi-echo SWI sequences with 2 different flip angles (FAs) (3). The 2 FAs are used to compensate the transmit and receiver field effects associated with both RF penetration artifacts and slice profile effects, thus various uniform quantitative and qualitative images are created including T1, proton spin density (PSD), R2*, and magnetic susceptibility values (4, 5).
Although previous papers have studied the reproducibility of T1 and PSD maps (6), inter-scanner consistency has not been studied well, especially for elderly subjects. We focused on inter-scanner consistency of T1 and PSD maps derived from STAGE imaging. The purpose of this study was to evaluate the inter-scanner consistency of T1 and PSD values for elderly healthy volunteers.

Methods

Subjects
Fourteen subjects (average: 64 years [range: 40-84 years old]; 9 males; 5 females) were enrolled. This prospective study was approved by the institutional review board. Written informed consent was obtained from each subject.

Image Acquisition
Image acquisition was conducted using two 3T MR scanners (MAGNETOM Prisma and Vida, Siemens Healthineers, Erlangen, Germany) with a 64-channel head/neck coil and a 32-channel head coil, respectively. Subjects underwent STAGE acquisition (T1-weighted SWI and PD-weighted SWI) on both scanners. Imaging parameters were as follows: T1-weighted SWI (T1-SWI): TR, 29 ms; TEs, 7.5/15/22.5 ms; FA, 27°; FOV, 256 × 192 mm²; voxel size, 0.67 × 1 × 1.34 mm³; number of slices, 112; bandwidth, 210/160/160 Hz/pixel; acceleration factor, GRAPPA 2; acquisition time, 4 min 48 sec. PD-weighted SWI (PD-DWI): same as T1-SWI except FA of 6°. 3D T1-weighted MRI (MPRAGE) was also obtained: TR, 2300 ms, TE, 3.52 ms; TI, 961 ms; FA, 9°; FOV, 230 × 230; voxel size 0.9 × 0.9 × 0.9 mm³; number of slices, 240; bandwidth 197 Hz/pixel; acceleration factor, Wave-Caipi 3 × 2, acquisition time, 1 min 46 sec.

Image Analysis
STAGE analysis
T1-SWI and PD-SWI data acquired from the scanner were transferred to the STAGE module for post-processing (SpinTech, Inc., Bingham Farms, MI, USA). Various quantitative and qualitative STAGE output data including T1 and PSD maps were automatically sent back to the MR scanners.

Native space
3D T1WI was registered to the T1 and PSD maps using SPM12 Software (https://www.fil.ion.ucl.ac.uk) followed by brainstorm (https://neuroimage.usc.edu/brainstorm/) with CAT12 (https://neuro-jena.github.io/cat/) to segment each registered 3D T1WI (7). Brain masks were created using tissue segmentation results to avoid including CSF segments, and AAL3 VOIs were applied to measure each value.

Mutual space
Mean images of the T1 and PSD maps were created for each subject by the realign function of SPM12. Next, each of the T1 and PSD maps was registered to the corresponding mean image. Then, AAL3 VOIs were applied (Figure 1).

Statistical Analysis
Intraclass coefficients (ICC) for T1 and PSD maps were evaluated between the two scanners. Statistical analysis was performed using MedCalc version 16.200 (MedCalc Software Ltd, Ostend, Belgium). Asymmetry index was also calculated between the two scanners.

Results

T1 and PSD maps created from the STAGE protocol showed similar quantitative values between the two MR scanners (Figure 2, Table 1). ICCs of for the T1 and PSD maps are shown in Table 2. ICCs of native space and mutual space were comparable. The asymmetry index of the T1 map in Native space was 3.89 [1.84-7.37] and in Mutual space was 2.67 [1.33-5.09] while that of the PSD map in Native space was 1.05 [0.47-1.94] and in Mutual space was 0.97 [0.44-1.83], respectively (Table 2). Bland-Altman plots are shown in Figure 3, and most of the VOIs of Native space and Mutual space fall within the 1.96 standard deviation lines.

Discussion

We conducted VOI analysis both in Native space and “Mutual space” using AAL3 VOIs. Parcellation was automatically performed with few manipulations, and good ICC and minimal biases were demonstrated between two scanners. Elderly subjects are prone to a relatively large CSF space in the brain scan. The CSF component shows high T1 values (e.g. > 2000ms) and high PSD values (nearly 100%), and the partial volume effect of CSF affected the results even though we used a brain mask to avoid CSF contamination.

Limitations: Only a limited number of subjects were enrolled in this study. We have not conducted intrascanner reproducibility of T1 and PSD maps in this study nor were R2* maps or QSM data from STAGE analyzed in this paper.

Conclusion

This study showed inter-scanner consistency of 1 to 4% asymmetry in the T1 and proton spin density maps derived from strategically acquired gradient echo (STAGE) imaging in our elderly subjects.

Acknowledgements

No acknowledgement found.

References

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