The analysis of intravascular indicator dynamics is important for cardiovascular diagnostics as well as for the assessment of tissue perfusion, aimed at the detection of ischemic regions or cancer hypervascularization. To this end, indicator dilution curves are measured after the intravenous injection of an indicator bolus and fitted by parametric models for the estimation of the hemodynamic parameters of interest. Based on heuristic reasoning, the dilution process is often modeled by a gamma variate. In this paper, we provide both a physical and stochastic interpretation of the gamma variate model. The accuracy of the model is compared with the local density random walk model, a known model based on physics principles. Dilution curves were measured by contrast ultrasonography both in vitro and in vivo (20 patients). Blood volume measurements were used to test the accuracy and clinical relevance of the estimated parameters. Both models provided accurate curve fits and volume estimates. In conclusion, the proposed interpretations of the gamma variate model describe physics aspects of the dilution process and lead to a better understanding of the observed parameters, increasing the value and credibility of the model, and possibly expanding its diagnostic applications.

To measure cortical and medullary MR renograms, regions of interest (ROIs) are placed on the kidney in images acquired using dynamic MRI. Since native kidneys move with breathing, and breath-holding techniques are not feasible, movement correction is necessary. In this contribution we compare three correction methods, based on image matching, phase difference movement detection (PDMD), and cross-correlation, respectively. The PDMDbased method showed the best performance and was able to determine kidney movement in our test series in 68% of the scans with no visible deviation, and in 88% of the scans if a one-pixel deviation is considered acceptable.

We evaluated a method of semiautomated analysis of dynamic MR image series in renal transplants. Nine patients were studied twice, with an average time interval of 7 days. MR examination consisted of a run of 256 T1-weighted coronal scans (GE; TR/TE/flip: ‫؍‬ 11/3.4/60°; slice thickness ‫؍‬ 6 mm; temporal resolution ‫؍‬ 2 seconds). Gadolinium-DTPA (0.05 mmol/kg) was injected with an injector pump (5 ml/seconds). MR renographs of the cortex and medulla were obtained by segmentation of the renal transplant and placement of two regions of interest (ROIs) MR RENOGRAPHY (MRR) is a dynamic MR examination that provides functional information of the kidney and renal transplant in terms of changes in signal intensity of the renal parenchyma after gadolinium-DTPA injection (1-3). The high spatial resolution of the MR image allows cortical and medullary structures to be separated, a feature that is not possible with scintigraphic techniques and that may be of importance in separating disease processes affecting different parts of the kidney or renal transplant (1-12). Although biopsy is still the standard of reference for diagnosis of diseases affecting the kidney and renal transplant, early reports indicate the possible usefulness of MRR in discriminating disease processes such as acute rejection and acute tubular necrosis (8,10,11).The MR renograph is the time-dependent signal intensity of suitably placed regions of interest (ROIs) over cortical and medullary tissue (1-12). ROIs are usually hand-placed by the operator, who selects cortical and medullary tissue by visual inspection of the early-enhanced images on which contrast between cortical, medullary, and perinephric tissue is optimal.In cases in which contrast between cortex and medulla is suboptimal, however, ROI positioning may be more difficult and time consuming, thereby requiring skill and experience from the operator.In this work we present and evaluate a new method to derive cortical and medullary MR renographs of renal transplants by means of semiautomated ROI placement. The obtained renographs were tested nonparametrically on reproducibility. MATERIALS AND METHODS PatientsNine renal transplant recipients (three men and six women, 46 -66 years old, mean age 53.6 years) were included in this study. These patients were selected from a population of renal transplant recipients who took part in a larger clinical MRI study. Consecutive patients entering this study were asked if they were willing to participate in the reproducibility study and have a second MR session. All patients were in good clinical condition and had a stable renal function for a period of at least 3 months prior to the investigations (creatinine clearances ranging between 22.3 ml/min and 94.2 ml/min, median 62.5 ml/min). Patients with diseased renal transplants, who usually do not have stable renal function, were not included. None of the patients were on diuretics. Immunosuppressive drug regimes consisted of Azathioprine and Prednisone in one patient, Cyclosporine in three patients, and Tac...

The intra-thoracic blood volume (ITBV) is a cardiovascular parameter related to the cardiac preload and left ventricular function. Its assessment is, therefore, important for diagnosis and follow-up of several cardiovascular dysfunctions. Nowadays, the ITBV can be accurately measured only by invasive indicator dilution techniques, which require a double catheterization of the patient. In this study, a novel technique is presented for ITBV assessment by dynamic magnetic resonance imaging after intravenous injection of a small bolus of gadolinium chelate.