Assessment of MRI contrast agent concentration by quantitative susceptibility mapping (QSM): application to estimation of cerebral blood volume during steady state

Emelie Lind,Robin Kämpe,Freddy Ståhlberg,Linda Knutsson,Ronnie Wirestam

doi:10.1007/s10334-017-0637-9

Abstract

ObjectiveOne major issue in dynamic susceptibility contrast MRI (DSC-MRI) is to accurately determine contrast agent (CA) concentration, since T2* relaxivity in vivo is generally unknown and varies between blood and tissue. In this study, quantitative susceptibility mapping (QSM) was used for quantification of CA concentration.Materials and methodsA DSC-MRI protocol, including phase data acquisition, was applied to 20 healthy volunteers in a test–retest study. By selecting a CSF reference region of interest (ROI), the values of all QSM images were shifted to show no CA-induced change in CSF. CA concentration and cerebral blood volume (CBV) were estimated using shifted QSM data. CSF reference ROI optimization was evaluated by investigation of CBV repeatability. The CBV age dependence was analysed and tissue T2* relaxivity was estimated.ResultsThe best repeatability of CBV, using an optimal CSF reference ROI, showed test-versus-retest correlations of r = 0.81 and r = 0.91 for white and grey matter, respectively. A slight CBV decrease with age was observed, and the estimated in vivo T2* relaxivity was 85 mM−1s−1.ConclusionProvided that a carefully selected CSF reference ROI is used to shift QSM image values, susceptibility information can be used to estimate concentration of contrast agent and to calculate CBV.

Highlights

Provided that the relaxivities of an MRI contrast agent (CA) are known, its concentration can, in principle, be determined from the observed change in relaxation rate (ΔR1, ΔR2, or ΔR2*)
A slight cerebral blood volume (CBV) decrease with age was observed, and the estimated in vivo T2* relaxivity was 85 mM−1s−1
As shown by the intra-class correlation coefficient (ICC)-based optimization, a reasonably low value of threshrange is beneficial, and a low value implies that only pixels showing a very small effect of the CA in the magnitude images are included in the CSF reference, and it is not unreasonable to select as low a threshold as possible

Summary

Introduction

Provided that the relaxivities of an MRI contrast agent (CA) are known, its concentration can, in principle, be determined from the observed change in relaxation rate (ΔR1, ΔR2, or ΔR2*). The relationship between the change in relaxation rate and the concentration of CA is usually assumed to be linear, but this is not always correct [2, 3]. For ΔR2*, measured by gradient-echo imaging, Kjølby et al predicted that the change in transverse relaxation rate is proportional to the CA concentration in tissue, while the relationship between ΔR2* and CA concentration in whole blood is non-linear [1, 2]. For structures with simple geometries, for example, a large blood vessel (approximated by an infinite cylinder) with known angulations relative to the main magnetic field, the geometry dependence can be accounted for, and the underlying magnetic susceptibility of the structure can be determined. The difference in susceptibility between tissue with and without CA is proportional to the CA concentration, and the proportionality constant is described by the molar susceptibility

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Conclusion