Decreased oxygen saturation in asymmetrically prominent cortical veins in patients with cerebral ischemic stroke

Abstract

Decreased oxygen saturation in asymmetrically prominent cortical veins (APCV) seen in ischemic stroke has been hypothesized to correlate with an increase of de-oxygenated hemoglobin. Our goal is to quantify magnetic susceptibility to define APCV by establishing a cutoff above which the deoxyhemoglobin levels are considered abnormal. A retrospective study was conducted on 26 patients with acute ischemic stroke in one cerebral hemisphere that exhibited APCV with 30 age- and sex-matched healthy controls. Quantitative susceptibility mapping (QSM) was used to calculate the magnetic susceptibility of the cortical veins. A paired t-test was used to compare the susceptibility of the cortical veins in the left and right hemispheres for healthy controls as well as in the contralateral hemisphere for stroke patients with APCV. The change in oxygen saturation in the APCV relative to the contralateral side was calculated after thresholding the susceptibility using the mean plus two standard deviations of the contralateral side for each individual. The thresholded susceptibility value of the APCVs in the stroke hemisphere was 254 ± 48 ppb which was significantly higher (p < 0.05) than that in the contralateral hemisphere (123 ± 12 ppb) and in healthy controls (125 ± 8 ppb). There was a decrease of oxygen saturation in the APCV ranging from 16% to 44% relative to the veins of the contralateral hemisphere. In conclusion, APCV seen in SWI correspond to reduced levels of oxygen saturation and these abnormal veins can be identified using a susceptibility threshold on the QSM data.

Introduction

Asymmetrically prominent cortical veins (APCV) seen in ischemic stroke have been introduced in several papers [1], [2], [3], [4], [5], with many presented as case reports [2], [3], [4], [6], [7]. In these observations, APCVs were typically identified if: 1) the diameter of the cortical veins in the ischemic hemisphere was larger than those in the contralateral hemisphere and/or 2) the length and visibility of veins was increased compared to those in the contralateral hemisphere. It has been proposed that APCV in the area of ischemia represent salvageable tissue [1], [2], [3], [4], [5], [8]. The presence of APCV has been hypothesized to be related to increased deoxyhemoglobin (DHb) which is closely related with oxygen saturation and oxygen extraction fraction [4], [9].

Understanding the difference in blood oxygen saturation between arteries and veins (Ya-Yv) could provide key information about cerebral oxygen demand and supply [10], [11] of normal and diseased tissues. Previous MR studies to measure oxygen saturation have included T2 [12], T2* [13] and phase based methods [11], [14], [15]. Recently, phase based methods have had a resurgence in studying oxygen saturation of major veins which has been referred to as magnetic susceptometry [11], [14], [15]. The phase based method has been used to measure relative changes in oxygen saturation in traumatic brain injury in animals [16]. Susceptibility mapping (SM), using susceptibility weighted imaging (SWI), for measuring oxygen saturation was introduced by Haacke et al. [17] and has the potential to provide quantitative oxygen saturation estimations of cortical veins.

Ideally, to understand the changes of the brain’s hemodynamics after stroke, perfusion measurements should be performed and then correlated with possible changes of oxygen saturation. For example, if the cerebral blood flow decreases while the brain metabolism remains stable then it should be possible to see that oxygen saturation of the veins will decrease due to increased oxygen extraction fraction on blood with an already depleted supply. However, even without this direct measure, the presence of APCV can be used to infer such changes in perfusion. In this study, we hypothesize that it is possible to establish a susceptibility threshold above which one can infer that the level of deoxyhemoglobin in the cortical veins is abnormally high.