Diffusion-weighted MRI sequences (DW-MRI) of the kidney: normal findings, influence of hydration state and repeatability of results.

PURPOSE: This study was performed to determine the apparent diffusion coefficient (ADC) of the normal kidney using diffusion-weighted magnetic resonance imaging (DW-MRI) sequences and to analyse both the changes due to hydration state and results repeatability. MATERIALS AND METHODS: Ten volunteers underwent DW-MRI imaging of the kidneys with a breath-hold single-shot spin-echo planar imaging (SE-EPI) sequence in the axial and coronal planes with b values of 300, 500, 800 s/mm(2), in different states of hydration. Urine osmolarity (OsmU) and sodium excretion (NaU) were measured at the time of each examination. ADC maps were created for all b values, and ADC values were calculated and compared between different states of hydration. In five subjects, the protocol was conducted twice to test data repeatability. RESULTS: ADC values were lower with higher b values (3.00 vs. 2.47 vs. 1.99 x 10(-3) mm(2)/s with b values of 300, 500, 800 s/mm(2), respectively). ADC values in different hydration states were not statistically different. Measurements were reproducible. OsmU and NaU were statistically different in the different states of hydration (p<0.01). CONCLUSIONS: ADC values significantly decrease with higher b values. Average ADC values in the normal kidney are reproducible. Hydration state does not significantly influence mean ADC values.

Prediction of coronary blood flow with a numerical model based on collapsible tube dynamics.

We present a theoretical, hydrodynamic model of the vascular system feeding the left ventricle from which the inflow and outflow waveforms can be predicted given the waveforms of aortic and left ventricular pressure. The main feature of the model is that the central portion of the tubes representing intramyocardial vessels is subjected to an external pressure equal to left ventricular pressure, and they therefore collapse (and empty) when that pressure exceeds the internal pressure. The model is a one-dimensional model, so that the propagation of the collapse waves into the vessels can be properly described; this process takes a finite time, and volume change is not in phase with transmural pressure change. Parameters of the model are assessed from independent physiological data. The predicted inflow waveform is compared with experimental data, and the model is shown to reproduce all the main features, in particular the second minimum of flow rate in late systole as well as the first minimum in early systole. The corresponding lumped-parameter model, which cannot take account of wave propagation, is shown not to agree with experiments and in particular to predict unphysiological spikes in the inflow waveform.