Detection of the Ischemic Penumbra Using pH-Weighted MRI

TLDR

The ischemic penumbra is a hypoperfused region with impaired metabolism yet preserved cellular polarization, and while perfusion‑ and diffusion‑weighted MRI can detect perfusion deficits and cellular depolarization, it remains unclear whether a PWI–DWI mismatch reflects benign oligemia or true penumbra. The study hypothesizes that pH‑weighted MRI can differentiate benign oligemia from true penumbra within the PWI–DWI mismatch. Twenty‑one rats underwent permanent middle cerebral artery occlusion and were imaged with multiparametric MRI over 3.5 h, with infarct size defined by 24‑h T2 hyperintensity; acute pH deficits were consistently larger than DWI deficits and smaller than PWI deficits. Acute pH‑weighted MRI deficits predicted the final infarct size, being smaller than PWI deficits (65–90 % of the mismatch) and larger than DWI deficits, indicating that the pH‑decreased but DWI‑normal border marks the penumbra while normal‑pH hypoperfused tissue represents benign oligemia, thereby demonstrating that pHWI adds complementary information to PWI and DWI for ischemic tissue delineation.

Abstract

The classic definition of the ischemic penumbra is a hypoperfused region in which metabolism is impaired, but still sufficient to maintain cellular polarization. Perfusion- and diffusion-weighted MRI (PWI, DWI) can identify regions of reduced perfusion and cellular depolarization, respectively, but it often remains unclear whether a PWI—DWI mismatch corresponds to benign oligemia or a true penumbra. We hypothesized that pH-weighted MRI (pHWI) can subdivide the PWI—DWI mismatch into these regions. Twenty-one rats underwent permanent middle cerebral artery occlusion and ischemic evolution over the first 3.5 h post-occlusion was studied using multiparametric MRI. End point was the stroke area defined by T 2 -hyperintensity at 24 h. In the acute phase, areas of reduced pH were always larger than or equal to DWI deficits and smaller than or equal to PWI deficits. Group analysis showed that pHWI deficits during this phase coincided with the resulting infarct area at endpoint. Final infarcts were smaller than PWI deficits (range 65% to 90%, depending on the severity of the occlusion) and much larger than acute DWI deficits. These data suggest that the outer boundary of the hypoperfused area showing a decrease in pH without DWI abnormality may correspond to the outer boundary of the ischemic penumbra, while the hypoperfused region at normal pH may correspond to benign oligemia. These first results show that pHWI can provide information complementary to PWI and DWI in the delineation of ischemic tissue.