Perfusion is defined as the passage of fluid through the lymphatic system or blood vessels to an organ or a tissue. The practice of Perfusion scanning, aims to ‘see’ the passage of fluids through body tissues.
Being able to observe and quantify perfusion in the human body has been an invaluable step forward in medicine.
There are two methods used for producing what are termed perfusion maps of the brain, i.e. maps of the distribution of blood in the tissue, these are Arterial Spin labelling (an extension of normal MR Angiography technique) and Contrast Bolus Tracking (assessing the speed a contrast agent goes through vessels).
In Contrast Bolus Tracking (sometimes referred to as dynamic susceptibility contrast imaging (DSC-MRI)) a Gadolinium contrast agent is injected and a time series of fast T2*-weighted images is acquired. As the Gadolinium passes through the tissues, it produces a reduction of T2* intensity depending on the local concentration. The acquired data is then processed to obtain perfusion maps with different parameters, such as:
- CBV (cerebral blood volume)
- CBF (cerebral blood flow)
- MTT (mean transit time)
- TTP (time to peak)
Figure 1. A movie of contrast arriving in the brain, notice that the presence of contrast causes the MR signal to reduce, in signal processing terms this image is difficult to analyse. For this reason the contrast concentration is calculated in each voxel as shown in the next figure.
Figure 2. A movie showing the change of contrast concentration in the same patient as in the previous movie. Notice that as the concentration of contrast increases that the image becomes brighter. Calculation of contrast agent concentration is the first stage in estimating CBF, CBV and MTT.