Quantifying cerebral microbleeds using quantitative susceptibility mapping from magnetization-prepared rapid
Nashwan Naji1, Myrlene Gee2, Glen C Jickling2
1Department of Biomedical Engineering, University of Alberta, Edmonton, Alberta, Canada.
Abstract:
T1-weighted magnetization-prepared rapid gradient-echo (MPRAGE) is commonly included in brain studies for structural imaging using magnitude images; however, its phase images can provide an opportunity to assess microbleed burden using quantitative susceptibility mapping (QSM). This potential application for MPRAGE-based QSM was evaluated using in vivo and simulated measurements. Possible factors affecting image quality were also explored. Detection sensitivity was evaluated against standard multiecho gradient echo (MEGE) QSM using 3-T in vivo data of 15 subjects with a combined total of 108 confirmed microbleeds. The two methods were compared based on the microbleed size and susceptibility measurements. In addition, simulations explored the detection sensitivity of MPRAGE-QSM at different representative magnetic field strengths and echo times using microbleeds of different size, susceptibility, and location. Results showed that in vivo microbleeds appeared to be smaller (× 0.54) and of higher mean susceptibility (× 1.9) on MPRAGE-QSM than on MEGE-QSM, but total susceptibility estimates were in closer agreement (slope: 0.97, r2: 0.94), and detection sensitivity was comparable. In simulations, QSM at 1.5 T had a low contrast-to-noise ratio that obscured the detection of many microbleeds. Signal-to-noise ratio (SNR) levels at 3 T and above resulted in better contrast and increased detection. The detection rates for microbleeds of minimum one-voxel diameter and 0.4-ppm susceptibility were 0.55, 0.80, and 0.88 at SNR levels of 1.5, 3, and 7 T, respectively. Size and total susceptibility estimates were more consistent than mean susceptibility estimates, which showed size-dependent underestimation. MPRAGE-QSM provides an opportunity to detect and quantify the size and susceptibility of microbleeds of at least one-voxel diameter at B0 of 3 T or higher with no additional time cost, when standard T2*-weighted images are not available or have inadequate spatial resolution. The total susceptibility measure is more robust against sequence variations and might allow combining data from different protocols.
Insights
Magnetization-prepared rapid gradient-echo quantitative susceptibility mapping (MPRAGE-QSM) can detect microbleeds in brain imaging. This method offers comparable sensitivity to standard techniques at 3 Tesla and above, with no extra time cost.
Area of Science:
- Neuroimaging
- Medical Physics
- Radiology
Background:
- T1-weighted magnetization-prepared rapid gradient-echo (MPRAGE) is standard for brain structural imaging.
- MPRAGE phase images offer potential for microbleed assessment via quantitative susceptibility mapping (QSM).
- Microbleeds are critical markers in various neurological conditions.
Purpose of the Study:
- To evaluate the potential of MPRAGE-based QSM for assessing microbleed burden.
- To compare the detection sensitivity and accuracy of MPRAGE-QSM against standard multiecho gradient echo QSM (MEGE-QSM).
- To explore factors influencing MPRAGE-QSM image quality and detection performance.
Main Methods:
- In vivo QSM analysis of 108 microbleeds in 15 subjects using 3-T MPRAGE and MEGE sequences.
- Simulations to assess MPRAGE-QSM sensitivity across varying magnetic field strengths (1.5T, 3T, 7T), echo times, and microbleed characteristics (size, susceptibility, location).
- Comparison of microbleed size, mean susceptibility, and total susceptibility estimates between MPRAGE-QSM and MEGE-QSM.
Main Results:
- In vivo MPRAGE-QSM detected microbleeds with comparable sensitivity to MEGE-QSM, although appearing smaller and with higher mean susceptibility.
- Total susceptibility estimates from MPRAGE-QSM closely agreed with MEGE-QSM (slope: 0.97, r²: 0.94).
- Simulations showed MPRAGE-QSM detection sensitivity improved significantly at 3T and 7T compared to 1.5T, with detection rates of 0.80 and 0.88 respectively for microbleeds of at least one-voxel diameter and 0.4-ppm susceptibility.
Conclusions:
- MPRAGE-QSM is a viable method for detecting and quantifying microbleeds (≥1 voxel) at 3T or higher magnetic fields without additional scan time.
- Total susceptibility measurements are more robust to sequence variations, potentially enabling data harmonization across different imaging protocols.
- MPRAGE-QSM is particularly valuable when standard T2*-weighted images are unavailable or lack sufficient spatial resolution for microbleed detection.
