Using contrast agents to obtain maps of regional perfusion and capillary wall permeability

The term perfusion imaging refers, traditionally, to the use of a medical imaging modality for measurements of tissue capillary blood flow [1], although it has become increasingly common to include other characteristics of tissue microcirculation and microvasculature in the perfusion imaging framework. In quantitative approaches, the regional perfusion is normally visualized as a parametric map calculated on the basis of an appropriate tracer-kinetic model. Perfusion imaging is of obvious importance for assessment of cerebral blood flow (CBF) and myocardial blood flow, although investigations of numerous other organs or tissue types are also highly relevant. Quantitative or semi-quantitative estimates of perfusion or regional blood flow can serve as important markers of tissue function and viability. Brain perfusion imaging has found applications in ischemic stroke, dementia and trauma, among others. Perfusion imaging in oncology is used extensively throughout the body [2], often in combination with assessment of capillary wall permeability, and extracted parameters are applied to tumor characterization and grading [3], as well as in the assessment of pharmacological efficiency and follow-up and outcome of treatment [4]. Perfusion measurements using radioisotopes have been available for many years, using SPECT and PET. Stable xenon-CT is another reference method for CBF measurements. This article is primarily dedicated to perfusion and permeability imaging by using non-radioactive contrast agents (CAs), developed for general diagnostic use in combination with an appropriate medical imaging technique in a clinical environment. It should be noted that the term ‘indicator’ would, formally, be a better choice of terminology than ‘tracer’ for such substances, since a tracer is, in the classic literature, defined as an indicator molecule that is identical to (and thereby ‘traces’) the corresponding systemic carrier substance [5]. However, it has always been very common not to firmly distinguish between the two terms. This overview will primarily focus on MRI methods, but the application of similar concepts to CT and ultrasound (US) will also be briefly introduced. CT offers excellent availability also in the acute setting, and US has the additional advantage of being portable as well as readily available. MRI offers the combined advantages of quite reasonable availability and the possibility to obtain additional information about softtissue morphology, diffusion, large vessels and metabolism during a single imaging session.