Dynamic Susceptibility Contrast
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Dynamic Susceptibility Contrast
MRI perfusion showing a delayed time-to-maximum flow (Tmax) in the penumbra in a case of occlusion of the left middle cerebral artery.

Perfusion MRI or perfusion-weighted imaging (PWI) is perfusion scanning by the use of a particular MRI sequence. The acquired data are then postprocessed to obtain perfusion maps with different parameters, such as BV (blood volume), BF (blood flow), MTT (mean transit time) and TTP (time to peak).

Clinical use

In cerebral infarction, the penumbra has decreased perfusion.[1] Another MRI sequence, diffusion weighted MRI, estimates the amount of tissue that is already necrotic, and the combination of those sequences can therefore be used to estimate the amount of brain tissue that is salvageable by thrombolysis and/or thrombectomy.[1]

Sequences

There are 3 main techniques for perfusion MRI:

  • Dynamic susceptibility contrast (DSC): Gadolinium contrast is injected, and rapid repeated imaging (generally gradient-echo echo-planar T2 weighted) quantifies susceptibility-induced signal loss.[2]
  • Dynamic contrast enhanced (DCE): Measuring shortening of the spin-lattice relaxation (T1) induced by a gadolinium contrast bolus.[3]
  • Arterial spin labelling (ASL): Magnetic labeling of arterial blood below the imaging slab, without the need of gadolinium contrast.[4]

Dynamic contrast-enhanced imaging

Dynamic contrast-enhanced (DCE) imaging gives information about physiological tissue characteristics. For example, it enables analysis of blood vessels generated by a brain tumor. The contrast agent is blocked by the regular blood-brain barrier but not in the blood vessels generated by the tumor. The concentration of the contrast agent is measured as it passes from the blood vessels to the extracellular space of the tissue (it does not pass the membranes of cells) and as it goes back to the blood vessels.[5][6]

The contrast agents used for DCE-MRI are often gadolinium-based. Gadolinium injection causes the relaxation time to decrease, and therefore images done after gadolinium injection have higher signal. First a regular T1-weighted MRI scan is done (with no gadolinium), then gadolinium is injected (usually at a dose of 0.05-0.1 mmol/kg) and another T1-weighted scan is done. By comparing the values of T1 in both scans, for each voxel, it is possible to identify permeable blood vessels and tumor tissue. In tissues with healthy cells or a high cell density, gadolinium re-enters the vessels faster since it cannot pass the cell membranes. In damaged tissues or tissues with a lower cell density, the gadolinium stays in the extracellular space longer.

References

  1. ^ a b Chen, Feng (2012). "Magnetic resonance diffusion-perfusion mismatch in acute ischemic stroke: An update". World Journal of Radiology. 4 (3): 63. doi:10.4329/wjr.v4.i3.63. ISSN 1949-8470. 
  2. ^ Frank Gaillard; et al. "Dynamic susceptibility contrast (DSC) MR perfusion". Radiopaedia. Retrieved . 
  3. ^ Prof Frank Gaillard; et al. "Dynamic contrast enhanced (DCE) MR perfusion". Radiopaedia. Retrieved . 
  4. ^ Prof Frank Gaillard; et al. "Arterial spin labelling (ASL) MR perfusion". Radiopaedia. Retrieved . 
  5. ^ Paul S. Tofts. "T1-weighted DCE Imaging Concepts: Modelling, Acquisition and Analysis" (PDF). paul-tofts-phd.org.uk. Retrieved 2013. 
  6. ^ Buckley, D.L., Sourbron, S.P. (2013). "Classic models for dynamic contrast enhanced MRI". NMR in Biomedicine. 26 (8): 1004-1027. doi:10.1002/nbm.2940. PMID 23674304. 

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