No abstract

In this work absolute values of regional renal blood volume (rRBV) and flow (rRBF) are assessed by means of contrastenhanced (CE) MRI using an intravascular superparamagnetic contrast agent. In an animal study, eight foxhounds underwent dynamic susceptibility-weighted MRI upon injection of contrast agent. Using principles of indicator dilution theory and deconvolution analysis, parametric images of rRBV, rRBF, and mean transit time (MTT) were computed. For comparison, wholeorgan blood flow was determined invasively by means of an implanted flow probe, and the weight of the kidneys was evaluated postmortem. A mean rBV value of 28 ml/100 g was found in the renal cortex, with a corresponding mean rBF value of 524 ml/100 g/min and an average MTT of about 3.4 s. Although there was a systematic difference between the absolute blood flow values determined by MRI and the ultrasonic probe, a significant correlation (r s ‫؍‬ 0.72, P < 0.05) was established. The influence of the arterial input function (AIF), T 1 relaxation effects, and repeated measurements on the precision of the perfusion quantitation is discussed.Magn Key words: kidney; perfusion; susceptibility contrast; intravascular contrast agent; dynamic T* 2 -weighted imaging; time series deconvolutionIt has been shown that absolute values of regional cerebral blood volume and flow can be determined from T* 2 -weighted dynamic MR images acquired during the passage of a paramagnetic contrast agent (1-4). However, with the use of a conventional diffusible MR contrast agent (such as Gd-DTPA), this method is limited to tissue regions with an intact blood-brain barrier, wherein the contrast agent remains within the intravascular space (5). Recently, ultrasmall superparamagnetic iron oxide (USPIO) particles, which can be employed as intravascular contrast agents, have become available (6). These agents generate a strong transient decrease in signal intensity when delivered as a bolus, and enable determination of both blood flow and volume in extracranial tissues (7).Absolute quantification of tissue perfusion by dynamic MRI appears to be highly promising in the evaluation of diseases of the kidney, since MRI has already been established as a valuable tool for the assessment of renovascular disease through the combination of morphologic and functional imaging (8,9). As renoparenchymal damage is often caused by a long-standing renal artery stenosis, the challenge for an MR perfusion technique is to differentiate hemodynamically significant stenoses from nonsignificant stenoses. Renoparenchymal perfusion would provide a valuable parameter for deciding whether a stenosis is hemodynamically relevant, in the context of a reduction in renal blood flow.The primary objective of this study was to investigate whether quantification of regional renal blood flow (rRBF) and regional renal blood volume (rRBV) is feasible by means of T* 2 -weighted dynamic MRI with an intravascular contrast agent. To this end, eight foxhounds under physiologic conditions underwent dynamic MRI upon in...

The interrelation between the morphologic degree of renal artery stenosis and changes in parenchymal perfusion is assessed using an intravascular contrast agent. In seven adult foxhounds, different degrees of renal artery stenosis were created with an inflatable clamp implanted around the renal artery. Dynamic susceptibility-weighted gradient-echo imaging was used to measure signal-time curves in the renal artery and the renal parenchyma during administration of 1.5 mg/kg BW of an intravascular ultrasmall particle iron oxide (USPIO) contrast agent. From the dynamic series, regional renal blood volume (rRBV), regional renal blood flow (rRBF), and mean transit time (MTT) were calculated. The morphologic degree of stenosis was measured in the steady state using a high-resolution 3D contrast-enhanced (CE) MR angiography (MRA) sequence (voxel size ‫؍‬ 0.7 ؋ 0.7 ؋ 1 mm 3 ). Five patients with renoparenchymal damage due to long-standing renal artery stenosis were evaluated. In the animal stenosis model, cortical perfusion remained unchanged for degrees of renal artery stenosis up to 80%. With degrees of stenoses > 80%, cortical perfusion dropped to 151 ؎ 54 ml/100 g of tissue per minute as compared to a baseline of 513 ؎ 76 ml/100 g/min. In the patients, a substantial difference in the cortical perfusion of more than