Magnetic resonance perfusion imaging

Blood perfusion is a crucial and fundamental physiologic process that controls delivery of nutrients to tissue [l] and thus a very important parameter in the assessment of tissue viability and function. Perfusion is usually expressed as so many milliliters of blood per minute per lOOg of tissue. Conventional in vivo perfusion measurements are based on the administration of exogenous tracers [2] detected by several different imaging techniques such as X-ray computerized tomography (CT) [3] and positron emission tomography (PET) [4]. All these methods are based on the indicator-dilution methods originally introduced a century ago by Stewart [5] and further developed by Zierler [6]. In practice, this method involves introduction of a known quantity of indicator into the system under study and measurement of the concentration of the indicator as a function of time at one or more points downstream from the location of the injection. Recently, there have been a number of applications of nuclear magnetic resonance (NMR) techniques proposed to image and measure tissue perfusion. Some are based on the indicator-dilution principle and others depend on inherent magnetic resonance (MR) mechanisms. Even though P E T is still considered to be the “gold standard” for in vivo blood flow measurements, MR perfusion imaging is attractive because M R scanners are more readily available. Also, there is no ionizing radiation involved, and spatial resolution with M R is much superior to that obtained with PET. Further, magnetic resonance imaging (MRI) offers the unique advantage of obtaining both anatomic and functional information with a single modality. In this review, we introduce some of the different concepts involved in assessing tissue blood flow using MRI and provide some illustrative examples. Before starting a discussion of the different types of techniques that have been proposed to perform perfusion MRI, it is worthwhile to make it clear that many of these techniques d o not measure perfusion in classical terms. M R is sensitive to several physiologic parameters that may be related to perfusion and could be used as indicators of local blood flow. Following is a brief description of some of these parameters that could directly or indirectly influence the MRI signal and thus potentially be deduced from the M R measurements. A. Perfusion ( f ) . If a volume of tissue ( V ) is supplied with arterial blood at a rate of F (ml/min), then perfusionf= FIV, the milliliters of arterial blood delivered per minute per milliliter of tissue. Even though this is slightly different from the classical description of perfusion, it is more applicable for imaging studies. Perfusion f is the fundamental rate constant determining delivery of metabolic substrates to the local tissue and clearance of products of metabolism.