Blood Oxygen Level–Dependent MRI Allows Metabolic Description of Tissue at Risk in Acute Stroke Patients

Abstract

Background and Purpose-The delineation of the "penumbra" is of particular interest in acute stroke imaging. The "mismatch concept" applying perfusion-weighted imaging (PWI) and diffusion-weighted imaging (DWI) appears to be an oversimplification of the underlying electrophysiological tissue status. An additional parameter reflecting the metabolic state of the threatened brain tissue would improve our ability to describe the penumbra. One candidate is deoxyhemoglobin (deoxy-Hb) as an indicator of the oxygen extraction fraction that can be visualized by T2*-based blood oxygen level-dependent (BOLD) imaging. Methods-We analyzed data from 32 patients with acute stroke in the territory of the middle cerebral artery. MRI included fluid-attenuated inversion recovery, DWI, PWI, time-of-flight angiography, and quantitative T2 and T2* (qT2, qT2*) imaging. Follow-up was performed on day 1 and days 5 to 8. We calculated 1/T2Јϭ1/qT2*Ϫ1/qT2. Changes of T2Ј, representing the deoxy-Hb effect, were analyzed by 3D regions of interest (ROIs): apparent diffusion coefficient lesion day 0 (L0), time-to-peak-lesion day 0 (T0), final infarct size days 5 to 8 (F5-8), lesion growth (LG; F5-8ϪL0), and surviving tissue (ST; T0ϪF5-8). Results-We observed a clear decrease of T2Ј in the infarcted hemisphere compared with the unaffected control ROIs. The mean value showed the most pronounced loss of T2Ј signal intensity in L0 (Ϫ15.7%), followed by LG (Ϫ10.5%) and ST (Ϫ8.0%). Conclusions-The

Quantitative T2′-Mapping in Acute Ischemic Stroke

“…In contrast to previous studies, 15 we were able to detect significantly lower T2′ values within ADC lesions when compared with surrounding perfusion-restricted tissue (Figure 3), with an approximate decrease of 6%. Although this difference was numerically small, the finding was robust, attaining statistical significance for all TTP delays and TTP ranges under investigation.…”

“…15 Again, the T2′ reduction is best explained by an increase of deoxygenated haemoglobine because of an OEF increase within the area of impaired perfusion. A study correlating MRI-derived TTP abnormalities with PET-based OEF measurement demonstrated elevated OEF values, indicating a metabolic compensation in hypoperfused tissue.…”

“…A previous study using T2′ mapping in acute stroke also found a more pronounced shortening of T2′ in DWIpositive areas than in surrounding penumbral tissue. 15 Nevertheless, this finding seems contraintuitive given the fact that DWI-positive tissue is commonly regarded as the ischemic core, characterized by a reduced cerebral metabolic rate of oxygen, which in turn should lead to reduced oxygen demands, a lowered OEF and therefore increased T2′ values. 16 But tissue viability within DWI lesions is more heterogeneous than previously thought, containing penumbral and infarcted tissue, both exhibiting substantially different metabolic demands.…”

Quantitative BOLD: The effect of diffusion

“…Before we are able to fit these components to the GESSE signal, T 2 decay must be allowed for in the model. Equations [3] and [4] are symmetric functions of t because magnetic field inhomogeneities around blood vessels are static. Conversely, due to the rapid fluctuation of spin-spin interaction, T 2 decay is an asymmetric function of t. The two mechanisms may therefore be fitted simultaneously without any risk of parameter coupling.…”

Detecting Misery Perfusion in Unilateral Steno-Occlusive Disease of the Internal Carotid Artery or Middle Cerebral Artery by MR Imaging

“…Some researchers were able to provide a better estimation of the real penumbra with the changes in signal intensity from deoxyhemoglobin. 12 With the current understanding of the underlying mechanism of BOLD contrast, an absolute quantification of cerebral oxygenation is possible by measuring the signalintensity alteration induced by deoxyhemoglobin. On the basis of an analytic model, An and Lin 13 proposed some specifically designed sequences to obtain quantitative measures of the OEF; results consistent with those of PET studies were obtained in humans during both normocapnia and hypercapnia.…”