André Keyeux

Assessment of hepatic perfusion parameters with dynamic MRI

Quantification of hepatic perfusion parameters greatly contributes to the assessment of liver function. The purpose of this study was to describe and validate the use of dynamic MRI for the noninvasive assessment of hepatic perfusion parameters. The signal from a fast T1‐weighted spoiled gradient‐echo sequence preceded by a nonslice‐selective 90° pulse and a spoiler gradient was calibrated in vitro with tubes filled with various gadolinium concentrations. Dynamic images of the liver were obtained after intravenous bolus administration of 0.05 mmol/kg of Gd‐DOTA in rabbits with normal liver function. Hepatic, aortic, and portal venous signal intensities were converted to Gd‐DOTA concentrations according to the in vitro calibration curve and fitted with a dual‐input one‐compartmental model. With MRI, hepatic blood flow was 100 ± 35 mL min‐1 100 mL‐1, the arterial fraction 24 ± 11%, the distribution volume 13.0 ± 3.7%, and the mean transit time 8.9 ± 4.1 sec. A linear relationship was observed between perfusion values obtained with MRI and with radiolabeled microspheres (r = 0.93 for hepatic blood flow [P < 0.001], r = 0.79 for arterial blood flow [P = 0.01], and r = 0.91 for portal blood flow [P < 0.001]). Our results indicate that hepatic perfusion parameters can be assessed with dynamic MRI and compartmental modeling. Magn Reson Med 47:135–142, 2002.

Late Functional and Circulatory Changes in Rats after Local Irradiation

SummaryLate circulatory disturbances of the lungs, brain, liver and skin and late modifications of the respiratory and hepatic functions after local x-irradiation, are assessed by appropriate quantitative in vivo radiotracer tests in rats.Except for the central nervous system, which shows an increase of its blood flow, a decrease of the organic functions and their circulatory supply is observed. These late changes become statistically demonstrable above a dose-level of 1000 R.

Induced response to hypercapnia in the two-compartment total cerebral blood volume: influence on brain vascular reserve and flow efficiency.

This study was undertaken to investigate the mechanisms of CBF increase as induced by hypercapnia. It was achieved in anesthetized rats by determining total cerebral blood volume (TCBV), parenchymal blood (CBV), plasma (CPV), erythrocyte (CEV) volumes and cerebral hematocrit (CHct) as well as CBF at about 40, 60, and 80 mm Hg Paco2. TCBV was measured by a noninvasive blood dilution method using [99mTc]pertechnetate. CBV, CPV, and CEV were measured on isolated brain by 125I-serum albumin and 51Cr-erythrocytes. CBF was measured by both [131I/14C]iodoantipyrine and 57Comicrosphere extractions. The extraparenchymal blood volume (ECBV) was evaluated by subtracting CBV from TCBV. Under normocapnia, ECBV was 2.8 times larger than CBV. Under moderate hypercapnia; ECBV increased by 44%, CBV was not modified, and CBF increased by 52%. These results demonstrate that the main site of vasodilation is located in the extraparenchymal vasculature, which thus acts as a vascular reserve. By contrast, under severe hypercapnia, ECBV remained unchanged, whereas CBV then increased by 17%; CBF simultaneously showed an additional augmentation of either 52 or 309% when diffusible tracer or microspheres were used. This important increase in CBF cannot be explained either by capillary recruitment of closed capillaries or by active diameter lengthening of already open capillaries. The concomitant and great increase in capillary blood velocity was also shown to reduce cerebral flow efficiency, a situation consistent with a “luxury perfusion.”

MR-monitored remitting-relapsing pattern of cortical involvement in Rasmussen syndrome: comparative evaluation of serial MR and PET/SPECT features.

We report the serial MR and PET/SPECT findings in a 2 1/2-year-old boy presenting with Rasmussen syndrome and highlight the close qualitative correlations between the results of the imaging modality and the functional isotopic techniques. The latter demonstrated a wider field of extension of the disease process. Routine MRI demonstrated its ability to detect brain changes matching the more sensitive PET and SPECT data and correlated well with the clinical evolution.

Early behaviour of 99mTc-pertechnetate in the head after intravenous bolus injection: Its relevance to the cerebral blood circulation

The relative differences between the behavior of 99mTc-pertechnetate (Tc) and both, non-diffusible and diffusible reference tracers in the head were evaluated by a statistical comparison of their time-activity curves in blood, brain and some tissues underlying the brain, after IV injection in the rat. This study showed that the particular cephalic behaviour of Tc was neither similar to that of diffusible tracers (even with restricted diffusion) nor equivalent to that of a non-diffusible tracer in the whole head. Although Tc is not an intravascular tracer in the entire cephalic volume, it was demonstrated that the initial peak characterizing the dilution of this tracer in the head is exclusively generated by its first passage in the cerebral circulation, even if the blood flow rate is changed.To extract from this initial peak a first dilution curve relevant to the cerebral circulation, Tc kinetics in the head were considered as a two compartmental model. Assuming that the maximum uptake of tracer was reached at the same time in both compartments of this model, the disappearance of Tc from the fast compartment approximates the first dilution curve of Tc in the fast cerebral circulation, if the slope of the Tc disappearance curve from the slow compartment is assimilated to a plateau.

Blood-brain-barrier impairment after irradiation: implication in boron neutron capture therapy.

The rationale of boron neutron capture therapy (BNCT) in brain tumors rests on the assumption that the boronated compounds will not be incorporated in the normal CNS protected by the tight blood-brain barrier (BBB) but will enter the tumor at the level at which the BBB is impaired.l

Validation of automated brain contour determination in normal and abnormal cerebral single-photon emission tomography.

A contour detection algorithm for cerebral studies, using the method of Tomitani, has been implemented on a single-photon emission tomographic (SPET) system. It is based on the detetion by threshold of the brain edge in the sinogram and does not depend on the reconstruction algorithm. Thirteen normal subjects underwent an examination on both computed tomography (CT) and SPET using a head holder to ensure the reproducibility of the positioning. The CT scan contour of the brain was drawn manually according to the brain parenchyma limits. The SPET brain contour was obtained by use of the Tomitani algorithm after the threshold had been determined on an active cylindrical phantom. Using a threshold of 37% of the maximum uptake, the length of the contour as well as the area obtained with SPET and CT were not found to be statistically different. The method of Tomitani, which is simpler and faster then previous methods, provides contours which superimpose very well with CT scan images. Application to patients with unilateral pathological defects is possible by requiring that the contour is symmetrical.

A quantitative radioisotope method for the In Vivo evaluation of Cephalic circulation in the rat

/sup 99m/Tc-pertechnetate can be used as a vascular tracer in spite of its diffusibility. Indices characterizing cerebral (/b V/ /sub H/) and extracerebral (/b F//sub H/) circulation in the rat are derived from parameters extracted from the dilution curve obtained by external radioactivity counting over the head, using the time course of the tracer in the cephalic circulation and the cardiac output (/b CO/). /b V//sub H/ and /b F//sub H/ values do not vary with sex. /b CO/ and /b F//sub H/, but not /b V //sub H/, tend to decrease with age.