DSC-MRI: Analysis
Linda Knutsson1
1Lund University, Lund, Sweden

Synopsis

Perfusion is the term applied to capillary blood flow in tissue. The study of brain perfusion has clinical applications due to the changes in perfusion associated with several neurological diseases. Dynamic susceptibility contrast (DSC) MRI is a method for retrieving perfusion and perfusion-related parameters using an exogenous contrast agent. The quantification of the perfusion and perfusion-related parameters from DSC-MRI is a two-step procedure. In the first step, the signal intensities are converted into contrast agent concentrations by employing MR signal theory. The second step aims to derive the relevant parameters from the time-resolved concentrations by means of tracer-kinetic theory.

Dynamic Susceptibility Contrast MRI

Perfusion is the term applied to capillary blood flow in tissue. Since the blood carries oxygen and nutrition to the tissue through the capillaries, perfusion is important in maintaining tissue viability. The study of brain perfusion has clinical applications due to the changes in perfusion associated with several neurological diseases. Diagnosis, lesion characterisation and follow- up of treatment in oncology, depression, dementia and acute ischaemic stroke are examples where assessment of perfusion is of value. A widespread approach of cerebral perfusion measurements using MRI in combination with an exogenous contrast agent is the dynamic susceptibility contrast MRI (DSC-MRI) method [1,2]. By tracking the first passage of the contrast agent/tracer through the blood vessels in the brain using rapid imaging and by applying kinetic models for intravascular tracers [3-4] perfusion parameters such as cerebral blood flow (CBF) in ml/(min 100 g), cerebral blood volume (CBV) in ml/100 g and mean transit time (MTT) in seconds can be calculated.

This approach relies on the fact that the exogenous paramagnetic contrast agent produces local magnetic field gradients that extend from the vascular compartment into the surrounding tissue, even if the contrast agent is not present in the extravascular space. The contrast agent remains in the vascular compartment and this compartmentalization creates susceptibility gradients. The gradients cause local dephasing of the spins, leading to signal loss in T2*- weighted magnetic resonance (MR) images during the passage of the contrast-agent bolus. Signal loss is also seen in spin echo pulse sequences due to intravascular T2 shortening in combination with spin diffusion in the contrast-agent-induced magnetic field gradients. Normally it is assumed that the tissue concentration of the contrast agent, C, is proportional to the change in T2/T2* relaxation rate.
The so-called tissue impulse residue function, R(t) describes the retention of a tracer in the tissue [3,5]. Thus, R(t) is the fraction of the injected tracer still present in the vasculature at time t after an infinitely short injection of tracer into the tissue-feeding artery.

In practice the arterial tracer bolus will not arrive to the tissue as a delta function due to the duration of the injection of the tracer, and the transport of the tracer from the injection site, through the vasculature to the brain. Consequently, the measured concentration-versus-time curve does not reflect the response to an infinitely short arterial bolus. Instead, the concentration curve is the convolution of CBF×R(t), and the concentration-versus-time curve in the tissue-feeding artery, i.e., the arterial input function (AIF) [5-7].
By measuring the arterial input function, the CBF can be determined by deconvolution, as the initial height of the product of CBF and R(t), due to the fact that R(0)=1. Furthermore, the MTT can be determined by Zierler’s area-to-height relationship [8] where the area of R(t) is divided with the height of R(t).
The cerebral blood volume can then be calculated using the central volume theorem by taking the product of the cerebral blood flow and the mean transit time [3,9].

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)