Direct, Longitudinal Comparison of ¹ H and ²³ Na MRI After Transient Focal Cerebral Ischemia

Abstract

Background and Purpose — 23 Na MRI may offer new insight into the evaluation of tissue injury. We performed a direct, longitudinal, morphological comparison of 1 H T2 relaxation, 1 H apparent diffusion coefficient (ADC), 23 Na content, and histopathology after cerebral ischemia to address the hypotheses that (a) 23 Na MRI is unique in comparison to 1 H MRI, and (b) accumulation of 23 Na is an unambiguous marker for dead tissue. Methods —Rats underwent 30 minutes of focal ischemia. MRIs of 1 H T2, 1 H ADC, and 23 Na content were acquired from 12 hours up to 1, 2, or 14 days after reperfusion. On excision, brains were stained with triphenyltetrazolium chloride (TTC). Results —In all cases, the region of abnormality increased in size for 2 days. On day 5, both 1 H T2 and ADC temporarily appeared normal despite the presence of TTC-defined infarction. By comparison, the volume of tissue exhibiting abnormally intense 23 Na signal mirrored the TTC-defined infarct at all time points. Conclusions —Regions of high 23 Na content correlate well with the TTC-defined infarct and may be a quantitative in vivo marker for dead tissue. In contrast, the dynamics of the 1 H T2 and ADC make it difficult to interpret these images without additional information because they may appear normal despite infarction. Neither type of 1 H image delineates dead tissue, and none of these methods predicts the potential infarct size at early time points.

Sodium T2*-weighted MR imaging of acute focal cerebral ischemia in rabbits

“…These regions were sporadically distributed and mostly associated with tissue/CSF boundaries. Although many studies have shown changes in T 2 -weighted 1 H 2 O images in tissue after 24 h of ischemia [14], changes were not consistently detectable in this study after 4 h, however, a trend towards increased ipsilateral hemispheric volume was observed, suggestive of edema. In contrast, 23 Na signal showed an increase of 25% Ϯ 14% from baseline in ischemic tissue during the same time interval.…”

“…This rate of acute 23 Na increase is also consistent with chronic measurements of tissue sodium concentration made by atomic absorption spectroscopy [38] and flame photometry [39,40] in infarcted tissue that have shown continuous increases during the first 24 -72 h of ischemia followed by a gradual reduction in the days and weeks that follow. Increased 23 Na concentration in the infarct region is likely due to the diffusion of 23 Na ions from surrounding tissue [24], which occurs at a slower rate than the diffusion of water [41], diffusion from collateral circulation, or diffusion from residual flow through the MCA, and may follow a different time course in necrotic regions compared to other MR indicators of tissue damage such as 1 H 2 O diffusion [14]. Increased 23 Na signal in ipsilateral regions that did not correspond to necrotic tissue according to the TTC staining ( Fig.…”

“…This pattern of signal change is notably different than that observed with recent DWI studies [7,23], which show a large initial change followed by small changes in signal intensity during the first 6 h after the focal ischemia insult. Furthermore, Lin et al [14] have shown distinctly different signal time courses between 23 Na and diffusion images in a rat model of slowly evolving transient ischemia, suggesting that 23 Na MRI and DWI measure different phenomenon.…”

Untitled

“…The hypothesis was that altered sodium signal intensities do correlate with immunohistology and cytopathologic biomarkers indicating different tumor features. Step down multivariate statistical analysis was performed to determine the order of immunohistolgical and pathologic parameters that correlate with the intracellular sodium (IC-Na) MRI signal [15]. …”

Untitled

“…Specifically, high-resolution, three-dimensional MRI can be used to assess noninvasively differences in brain tissue sodium concentration, which is a highly sensitive marker of tissue viability that highlights areas that traditional MRI can miss [82-86]. The method is based on sodium ion homeostasis, which is tightly regulated in the body and is a major energy consuming process.…”