Abstract Body: Introduction
The use of ultra low-field portable Magnetic Resonance Imaging (pMRI) in acute ischemic stroke (AIS) is gaining interest, as pMRI is FDA approved and has practical advantages over high-field MRI brain (hMRI). The ACTION PMR study is a prospective cohort study enrolling AIS and perform pMRI within 24 hours of last known normal (LKN). We sought to report patients enrolled in the study from our institution to evaluate the performance of pMRI compared to hMRI.

Methods
Among the 10 patients enrolled in our center, 1 was excluded for technically nondiagnostic imaging and 1 was excluded due to no infarct visible on hMRI, leaving 8 AIS patients in the cohort for analysis. All 8 patients had both pMRI and hMRI during their hospitalization. Our institutional pMRI (Hyperfine Inc. Swoop®) is a 0.064 Tesla MRI machine, and the protocol includes diffusion-weighted imaging, apparent diffusion coefficient, and fluid-attenuated inversion recovery (scan time 18 minutes) sequences. Infarct size was measured based upon maximum longitudinal axis on DWI.

Results
Among the 8 AIS patients with radiographic infarcts on hMRI, the mean age was 61.0 years (range, 47.2 to 76.2) and 2 were female. Thrombolysis was administered in 7 (87.5%) patients, and the mean NIHSS on presentation was 3.8 (range, 2 to 7). The mean time from LKN to pMRI was 15.9 hours (range, 10.9 to 20.3) and from pMRI to hMRI was 7.3 hours (range 1.2 to 17.6). All 8 AIS patients had infarcts visible on pMRI. On the 8 hMRI scans, 10 infarct lesions were demonstrated. Infarct distribution was as follows: subcortical (6), cerebellar (1), and cortical MCA/PCA (3). Among 10 infarct lesions measured, mean size was 15.2 mm (range 3.6 to 37.6), and 9 (90%) of hMRI infarcts were visible on pMRI, with the smallest infarct visible on pMRI being 4.5 mm. In contrast, the hMRI visible infarct that was not visible on pMRI measured 3.6 mm. Case 1 (Figure 1) demonstrated DWI-FLAIR mismatch of a right subcortical infarct on hMRI and on pMRI as well.

Conclusions
In our series, pMRI performed reasonably well on AIS patients and was able to detect the majority (90%) of acute infarcts seen on hMRI. pMRI was able to detect infarcts above 4.5 mm and was also able to demonstrate DWI-FLAIR mismatch. Clinicians need to be aware of the strengths and limitations of pMRI to maximize patient selection for use in the AIS population. Future improvements in DWI imaging at low field may improve sensitivity for small infarcts in the future.