Abstract Body: Introduction: Portable, low-field (LF) magnetic resonance imaging (MRI) has the potential to improve access to brain imaging and facilitate diagnosis of acute ischemic stroke (AIS). However, due to longer acquisition time, diffusion-weighted imaging (DWI) at LF is often restricted to a single diffusion direction, which may reduce sensitivity of detection. We developed a multi-direction DWI sequence and evaluated its ability to detect stroke lesions compared with a single-direction sequence.

Methods: Patients with a suspected diagnosis of AIS were prospectively enrolled at three US centers and underwent single-direction b0 and b900 s/mm² DWI and generation of corresponding apparent diffusion coefficient (ADC) maps on a 0.064 T MRI (Hyperfine Inc.). A subset of patients underwent additional multi-direction DWI (3 diffusion directions in x, y, and z planes) and generation of trace and ADC maps. Two independent assessors evaluated DWI and ADC scans and recorded the presence or absence of stroke lesions and their location (left/right hemisphere). The agreement, positive predictive value (PPV), negative predictive value (NPV), sensitivity, and specificity were calculated for both single- and multi-directional images as compared to the ground-truth clinical diagnosis.

Results: A total of N=66 patients were included, with N=18 undergoing additional multi-directional acquisition. Of these patients, N=36 had a confirmed ischemic stroke, and N=29 had a stroke mimic with no evidence of infarction or diffusion restriction abnormality on conventional high-field imaging. Agreement between assessors regarding lesion detection on single-direction DWI was κ=0.88 and κ=0.84 on ADC, with a κ=0.92 agreement on lesion location. On multi-directional LF-MRI, agreement increased to κ=1.0 for DWI, ADC, and lesion location. Predictive values for detecting AIS on single-direction DWI revealed a PPV of 83.9% and NPV of 71.4% which was associated with a 72.2% sensitivity and 83.3% specificity. NPV, PPV, sensitivity and specificity all increased to 100% when multi-directional DWI was assessed.

Conclusion: LF single-direction diffusion is sensitive and specific for the detection of acute ischemic infarcts, except for small lesions residing in white matter tracts that align with diffusion gradient direction. A novel LF multi-directional DWI sequence can overcome this limitation and potentially improve qualitative lesion conspicuity. Validation in a larger cohort is warranted.