A novel method is presented to simultaneously measure the permeability surface area product of water (PS), also known as capillary diffusion capacity, and the regional blood volume (RBV). It is based on magnetic resonance imaging of the longitudinal relaxation times of tissue and blood at different concentrations of an intravascular MR contrast agent. PS and RBV were measured in vivo in different regions of the brain and the skeletal muscle of the rat. The average PS values (n = 5) obtained in cerebral cortex, corpus callosum, hippocampus, thalamus, jaw muscle, and tongue muscle were 3.31 +/- 0.20, 1.81 +/- 0.25, 3.37 +/- 0.36, 3.68 +/- 0.44, 10.6 +/- 1.1, and 14.1 +/- 2.51 ml x min(-1) x g(-1), respectively. The corresponding average RBV values were 1.63 +/- 0.18, 1.22 +/- 0.25, 3.30 +/- 0.37, 3.03 +/- 0.36, 1.66 +/- 0.30, and 1.38 +/- 0.33 ml x 100 g(-1). These results are in good agreement with previously reported literature values obtained by means of autoradiography.

Interest in techniques yielding quantitative information about brain tissue proton densities is increasing. In general, all parameters influencing the signal amplitude are mapped in several acquisitions and then eliminated from the image data to obtain pure proton density weighting. Particularly, the measurement of the receiver coil sensitivity profile is problematic. Several methods published so far are based on the reciprocity theorem, assuming that receive and transmit sensitivities are identical. Goals of this study were (1) to determine quantitative proton density maps using an optimized variable flip angle method for T 1 mapping at 3 T, (2) to investigate if systematic errors can arise from insufficient spoiling of transverse magnetization, and (3) to compare two methods for mapping the receiver coil sensitivity, based on either the reciprocity theorem or bias field correction. Results show that insufficient spoiling yields systematic errors in absolute proton density of about 3-4 pu. A correction algorithm is proposed. It is shown that receiver coil sensitivity mapping based on the reciprocity theorem yields erroneous proton density values, whereas reliable data are obtained with bias field correction. Key words: proton density; water content; receiver sensitivity profile; variable flip angle; reciprocity theorem; bias field Over the last years, the number of studies based on quantitative magnetic resonance imaging (MRI) techniques has increased a lot. The purpose of quantitative MRI is the direct measurement of tissue parameters. Although magnetic relaxation times are the most frequently mapped parameters, several methods for mapping the brain tissue water content have been described. In general, water contents are directly derived from the measured proton density (r), as the majority of visible protons reside in water (1). Clinical applications of quantitative r-mapping comprise the investigation of patients suffering from hepatic encephalopathy (2), ischemia (3,4), multiple sclerosis (5), and brain tumors (6).However, r-mapping is challenging, and great care has to be taken to correct for any image intensity bias that may impair the accuracy of measured results (1), in particular any bias due to variations of the longitudinal (T 1 ) and transverse (T 2 and T 2 *) relaxation times, inhomogeneities of the transmitted radiofrequency (RF) field B 1 , distortions of the static magnetic field B 0 , and intensity nonuniformities imposed by spatial variations of the receiver coil sensitivity profile (RP). Thus, r-mapping methods usually comprise several individual acquisitions for mapping all parameters influencing signal amplitudes, followed by correction of measured image intensities (6-8). The intensity-corrected images are converted into quantitative r-maps by suitable normalization, a value of 100 percent units (pu) corresponding to r in pure water at 37 C. Reference values for water are obtained either from an external reference, requiring correction for temperature differences (1,7), or directly from ce...

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