Perfusion-Weighted Imaging in MRI Table of Contents I. Introduction 3 II. Exogenous Tracers 3 III. Endogenous Tracers 9 IV. Clinical Applications 13 References 17 I. Introduction This paper explores the use of endogenous and exogenous tracers in perfusion-weighted magnetic resonance imaging, focusing on a comparison and contrast of the two tracers and answering the question with regard to how the final information on perfusion is procured…
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Aside from strokes, perfusion-weighted imaging are useful in the diagnosis of tumors of the brain and the pituitary gland, multiple sclerosis, disease of the pituitary system, and abnormalities of the cerebellum and of the brain stem. Two kinds of perfusion-weighted imaging make use of either endogenous or exogenous tracers, and in both kinds the result is the accurate determination of mean transit times or MTT, blood flow, blood volume and other quantitative haemodynamic characteristics (Stony Brook Medicine 2013; Neumann-Haefelin et al. 1999; Huisman and Sorensen 2004; Petrella and Provenzale 2000; Bochar 2001; Cutrer et al. 2004; Luypaert et al. 2001). II. Exogenous Tracers Endogenous and exogenous tracers refer to whether substances that are native to the human body or non-native to the human body respectively, and are both in use in perfusion-weighted imaging to determine the quantities tied to the hemodynamics of the sample being investigated. These quantities relate to the flow of blood, the volume of blood, and the time of transit of blood through the tracking of the tracer substance as it passes through the sample tissue. This latter time is also known as the mean transit time (Module 4: Perfusion Imaging 2013; Luypaert et al. 2001). In perfusion-weighted imaging in general, the tracer used is the agent that sensitizes the way blood microcirculates through a tissue or an organ’s network of capillaries and which also sensitizes the images formed with perfusion-weighted images themselves. The image intensity morphs as the tracer goes through the tissue capillary bed, so that the changes in intensity can be utilized for the determination for the metrics for blood that have been described above. The use of exogenous tracers allow for a comprehensive determination of the numerical numbers associated with the blood dynamics. This includes mean transit time, volume of blood, and flow of blood. In strokes, lesions translate to reduced volume and flow, while increasing mean transit times for the affected tissues in comparison to healthy, unaffected tissues. The image below typifies the three kinds of images that can be had from perfusion-weighted image protocols (Module 5: Applications of Perfusion and Diffusion 2013; Bochar 2001): Image source: Module 5 2013 In the images above, image (a) is for mean transit time, image (b) is for cerebral flow of blood, and image (c) is cerebral volume of blood. In all three images, it is noted that the right side of the brain exhibits abnormal perfusion (Module 5 2013, p. 4). Haemodynamic information is gathered in perfusion-weighted imaging making use of exogenous tracers via a reliance on inflow effects as well as susceptibility from a magnetic point of view. The exogenous tracer is sometimes introduced into the venous system via an injection, such as in the case of the exogenous tracer gadolinium. For this purposes, to be exact, the form that gadolinium takes is gadolinium dieethyltriamine pentaacetate or the Gd-DTPA compound. As the perfusion of the exogenous tracer occurs through the subject tissues, the signal undergoes a transient loss, which can then be subjected to MRI following. The paramagnetic characteristics of this exogenous tracer translate to differences in how susceptible different capillaries are
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