Susceptibility‐based time‐resolved whole‐organ and regional tissue oximetry

Abstract

The magnetism of hemoglobin – being paramagnetic in its deoxy and diamagnetic in its oxy state – offers unique opportunities to probe oxygen metabolism in blood and tissues. The magnetic susceptibility χ of blood scales linearly with blood oxygen saturation, which can be obtained by measuring the magnetic field ΔB of the intravascular MR signal relative to tissue. In contrast to χ the induced field ΔB is non-local. Therefore, to obtain the intravascular susceptibility Δχ relative to adjoining tissue from the measured ΔB demands solution of an inverse problem. Fortunately, for ellipsoidal structures, to which a straight, cylindrically shaped blood vessel segment conforms, the solution is trivial. The article reviews the principle of MR susceptometry-based blood oximetry. It then discusses applications for quantification of whole-brain oxygen extraction – typically on the basis of a measurement in the superior sagittal sinus – and, in conjunction with total cerebral blood flow, the cerebral metabolic rate of oxygen (CMRO2). By simultaneously measuring flow and venous oxygen saturation (SvO2) a temporal resolution of a few seconds can be achieved, allowing the study of the response to nonsteady-state challenges such as volitional apnea. Extensions to regional measurements in smaller cerebral veins are also possible, as well as voxelwise quantification of venous blood saturation in cerebral veins accomplished by quantitative susceptibility mapping (QSM) techniques. Applications of susceptometry-based oximetry to studies of metabolic and degenerative disorders of the brain are reviewed. Lastly, the technique is shown to be applicable to other organ systems such as the extremities using SvO2 as a dynamic tracer to monitor the kinetics of the microvascular response to induced ischemia.

Long‐term sequelae of hippocampal lesions in patients with transient global amnesia: A multiparametric MRI study

“…An acute increase in magnetic susceptibility during the onset of TGA may be due to the increase of blood deoxyhemoglobin. Deoxyhemoglobin is paramagnetic . Paramagnetic substances in tissues cause local susceptibility changes.…”

“…Deoxyhemoglobin is paramagnetic. 34 Paramagnetic substances in tissues cause local susceptibility changes. A recent study showed that the susceptibility value in the prominent cortical veins in the stroke hemisphere was significantly higher than that in the contralateral hemisphere and in healthy controls and in the meantime, the oxygen saturation in the prominent cortical veins was decreased, ranging from 16% to 44% relative to the veins of the contralateral hemisphere.…”

The clinical utility of QSM: disease diagnosis, medical management, and surgical planning

“…Flow‐compensated QSM sequences can be used not only to visualize veins, but also to measure venous susceptibility , which can be used to noninvasively calculate venous oxygen saturation level, SvO 2 . SWI is very useful for its detailed depiction of veins, but the vein signal intensity on SWI (as well as on T 2 and T 2 *) varies both with the vein orientation and with echo time, and the underlying venous oxygenation cannot be derived .…”

Cerebral metabolic rate of oxygen (CMRO2) mapping by combining quantitative susceptibility mapping (QSM) and quantitative blood oxygenation level‐dependent imaging (qBOLD)

“…Magnitude modeling methods include quantitative imaging of extraction of oxygen and tissue consumption (QUIXOTIC), calibrated fMRI, and quantitative BOLD (qBOLD) . Phase modeling methods have been used for whole brain CMRO 2 measurements . Building on this, quantitative susceptibility mapping (QSM) enables generation of CMRO 2 maps .…”