Can a MR Imaging Scanner Accurately Measure Hematocrit to Determine ECV Fraction?

F ocal fibrosis, most due to myocardial infarction, is readily identified by cardiac magnetic resonance (CMR) using widely validated late gadolinium enhancement methods. However, many common conditions of the heart, ranging from hypertrophic cardiomyopathy to idiopathic dilated cardiomyopathy or myocarditis, are histologically characterized by diffuse, interstitial myocardial fibrosis. In these conditions, “T1 mapping” may be used to determine abnormalities in the T1 relaxivity of the myocardium either before or after administration of a gadolinium-based CMR contrast agent (1). The term T1 mapping involves measuring the T1 time, or spin-lattice relaxation time, on a pixel-bypixel basis over the entire image field of view. The T1 relaxation time is the time (in milliseconds) for a tissue to obtain approximately 63% of its longitudinal relaxation (parallel to the main magnetic field). Most current magnetic resonance imaging (MRI) scanners include software that allows routine measurement of T1 time for each pixel in a slice of tissue, thus providing a “map” of myocardial T1 times. For reference, the normal T1 time of the heart is approximately 1,000 ms at 1.5-T (2). For comparison, the T1 time of fat (250 ms) is much shorter than myocardium, whereas the T1 time of blood is approximately 1,400 ms. The T1 time of blood depends on temperature,