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A Magnetic Resonance Imaging Protocol for Stroke Onset Time Estimation in Permanent Cerebral Ischemia
Published on: September 16, 2017
Enhanced Detection of Acute Ischemic Stroke With Low-Field MRI
Annabel Sorby-Adams1, Nandor K Pinter2,3, Amelia Demopoulos1
1Department of Neurology and the Center for Genomic Medicine (A.S.-A., A.D., S.B., J.G., W.T.K.), Massachusetts General Hospital and Harvard Medical School, Boston.
Background:
Portable, low-field magnetic resonance imaging (MRI) has the potential to expand access to neuroimaging in environments where conventional MRI is limited. However, diffusion-weighted imaging at low magnetic field is challenged by a low signal-to-noise ratio and gradient strength, which may limit diagnostic confidence in acute ischemic stroke evaluation, particularly for very small strokes. In this study, we evaluated a combination of novel pulse sequences and low-field MRI hardware to enhance lesion detection.
Methods:
Patients with a suspected diagnosis of acute ischemic stroke were prospectively enrolled at 3 centers. Diffusion-weighted imaging was performed using a single-direction (SD) or a custom multi-direction (MD) sequence comprising 3 orthogonal directions. Imaging was acquired on 2 0.064 T hardware versions: a first-generation C-arm system (Swoop v1) and a next-generation H-arm system featuring optimized gradient amplifiers, form factor, and cooling system (Swoop v2). Diagnostic accuracy and the lower limit of lesion detection were calculated for both SD and MD images on each system compared with ground-truth MRI (1.5-3 T).
Results:
A total of 95 patients (n=62 confirmed acute ischemic stroke; n=33 stroke mimics) were included. On SD images, agreement between assessors regarding lesion detection was κ=0.72, and κ=0.84 on MD images. The positive predictive value for differentiating acute ischemic stroke from stroke mimics was 78.2% on SD and 95% on MD images. For SD images, a lesion volume cut point of 0.6 mL yielded a sensitivity of 89% and specificity of 88%. For MD images, the lesion volume cut point was 0.4 mL, with a corresponding sensitivity of 86% and specificity of 83%. MD imaging on the next-generation v2 system improved image uniformity (P<0.05), reduced scan time by ≈30%, and enabled the detection of lesions as small as 0.15 mL (2.8 mm maximum diameter).
Conclusions:
Implementation of diffusion-weighted imaging optimization strategies on low-field MRI improves detection of very small strokes in a clinically feasible time frame.
Insights
Low-field MRI with optimized diffusion-weighted imaging (DWI) sequences significantly improves the detection of small acute ischemic strokes. This advancement enhances diagnostic confidence in stroke evaluation, making MRI more accessible.
Area of Science:
- Medical Imaging
- Neurology
- Biomedical Engineering
Background:
- Portable, low-field magnetic resonance imaging (MRI) offers expanded neuroimaging access.
- Low-field diffusion-weighted imaging (DWI) faces challenges in signal-to-noise ratio and gradient strength for acute ischemic stroke evaluation.
- Optimized pulse sequences and hardware are explored to enhance lesion detection in low-field MRI.
Purpose of the Study:
- To evaluate novel pulse sequences and low-field MRI hardware for enhanced lesion detection in acute ischemic stroke.
- To assess the diagnostic accuracy and lesion detection limits of single-direction (SD) and multi-direction (MD) DWI sequences on different low-field MRI systems.
- To compare low-field MRI performance against conventional high-field MRI as the ground truth.
Main Methods:
- Prospective enrollment of 95 patients with suspected acute ischemic stroke across 3 centers.
- Acquisition of DWI using SD or custom MD sequences on two 0.064 T MRI systems (Swoop v1 and v2).
- Calculation of diagnostic accuracy, positive predictive value, and lesion detection limits, comparing results to 1.5-3 T MRI.
Main Results:
- Multi-direction (MD) DWI demonstrated higher inter-rater agreement (κ=0.84) and positive predictive value (95%) compared to single-direction (SD) DWI (κ=0.72, 78.2%).
- MD imaging enabled detection of smaller lesions (0.4 mL cut-off, sensitivity 86%) than SD imaging (0.6 mL cut-off, sensitivity 89%).
- Next-generation v2 system with MD imaging improved uniformity, reduced scan time by ~30%, and detected lesions as small as 0.15 mL.
Conclusions:
- Optimized diffusion-weighted imaging strategies on low-field MRI significantly improve the detection of very small acute ischemic strokes.
- The enhanced detection capabilities are achieved within a clinically feasible timeframe.
- This advancement holds promise for expanding access to critical stroke diagnostics.
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