Magnetic resonance imaging Assignment Example | Topics and Well Written Essays - 2750 words

Magnetic resonance imaging - Assignment Example 2001). MRI is used to measure tissue perfusion through the use of different techniques such as arterial sin labelling (ASL) and dynamic susceptibility contrasting imaging (DSC-MRI). DSC is based on injected contrast agent use that changes the blood’s magnetic susceptibility, thus, producing a MR signal continually measured throughout the bolus passage (Petrella & Provenzale 2000). ASL, on the other hand, is whereby before the arterial blood enters into the tissue to be assessed, it is magnetically tagged, and consequently, the labelling amount is measured and compared to a blank recording achieved without spin labelling. Currently, MRI is a powerful tool in a clinical setting for evaluation of brain anatomy, which is achieved via a number of metabolic or functional assessments. MRI perfusion is a technique used to measure cerebral perfusion non-invasively via several hemodynamic measurements assessments including cerebral blood flow, cerebral blood, volume, and meant transit t ime. This technique plays an important role in the diagnosis and treatment cerebrovascular disease patients, as well as patients with other brain disorders (Petrella & Provenzale 2000). ... Exogenous Tracers Exogenous is a model of MR perfusion, which assumes that the tracer does not diffuse into the outer cellular space because it is constrained in the intravascular compartment. In this model imaging can either be performed dynamically or in a steady state. Dynamic imaging utilizes transient fluctuations in local magnetic field of the tissues in the surrounding that are induced by paramagnetic tracer bolus passing through the capillary network of the organ. The local magnetic field changes can be measured as signal fluctuations on MR imaging. Accurate measurements are enabled by Ultrafast imaging methods such as spiral MR and echoplanar imaging, which measures differing signal changes that occur rapidly (Ostergaard, et al. 1996). Data from the signal-time course is then changed to relative tracer tissue data from the concentration-time course. This results in tracer concentration-time curve that can be evaluated to establish different parameters of hemodynamic tissues such as blood flow, transit time, tissue blood volume and bolus arrival time. The hemodynamic parameters mentioned above are influenced by features of the bolus injection such as the injection rate, contrast agent paramagnetic properties, the amount of injected contrast material among others. Furthermore, these parameters rely on variables inside the subject under imaging, which are cardiac output and vascular volume of total-body (Buxton, et al. 1996). Therefore, it is not possible to compare the parameters between varied subjects, and at different times they may even cause variation on examination of the same subject. Nevertheless, there is an internal standard of reference,