X-ray Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) coupled to the injection of contrast agents are now common tools for imaging of the human body. Other techniques such as Ultra Sound imaging and Nuclear Medicine are also widely used despite a lower spatial resolution. Nowadays, efforts are aimed at extending the usefulness of these techniques beyond anatomic mapping to obtain functional information. Assessment of this information, i.e. physio-pathological parameters, is a key issue for the prediction of the evolution of a given pathology and for guidance in therapeutic choices. In this context, the Synchrotron Radiation Computed Tomography (SRCT) technique available on ID17 appears as a complementary approach to MRI and CT to obtain quantitative information about physio-pathology. In particular, the technique is based on the subtraction of two images taken above the K edge of a particular contrast agent such as iodine (33.169 keV) or gadolinium (50.239 keV) before and after the injection of the contrast agent. The logarithmic subtraction of these two measurements leads to a precise quantification of the contrast agent in the tissue, i.e. to a fingerprint of the physio-pathology [1,2].

Brain circulation parameters have been widely studied to improve the understanding of various pathological states such as ischemia and tumour development. It is a well-known feature that vasculature is highly involved in the tumour growth mechanism. The purpose of these new experiments was to design a protocol enabling the measurement of two parameters derivable from the vascular function; the cerebral blood volume (CBV) and the cerebral blood flow (CBF). An anaesthetised rat bearing a C6 glioma was installed on a rotating stereotactic frame (1 turn in 2 sec) allowing computerised tomography attenuation profiles to be acquired continuously over a 34 second period. Each pixel of the CT images corresponds to a 0.043 mm3 cube of brain tissue. After a delay of two turns, a bolus of iodine based contrast agent was infused using a remote controlled injector (0.1 ml of a 350 mg/ml iodine solution). The image obtained before the bolus infusion was subtracted from the following images to obtain CT maps of the absolute iodine concentration with an initial temporal resolution equal to 2 sec. Nevertheless, reconstruction of images starting each 0.5 sec. led to an over-sampling of the iodine concentration curves. These curves provided the evolution of the absolute contrast agent concentration inside the rat brain. The observed curves were fitted with gamma functions. When choosing any arterial input, the CBV percentage in any given voxel of the slice is equal to the ratio of the time integral of the fitted concentration curves (voxel observed / arterial input) [3], see Figure 23. Deriving the cerebral flow is theoretically possible provided the two following conditions are verified; reproducibility and linearity in terms of the vascular response to the bolus infusion (work in progress).

These studies will contribute towards the design of tools to observe vasculature changes in brain tumours.

[1] H. Elleaume, A.M. Charvet, G. Le Duc, F. Estève, B. Bertrand, S. Corde, R. Farion, J.L. Lefaix, J.J. Leplat, P. Berkvens, G. Berruyer, T. Brochard, Y. Dabin, A. Draperi, S. Fiedler, C. Nemoz, M. Perez, M. Renier, P. Suortti, W. Thomlinson, J.F. Le Bas, Cell. Mol. Biol., (2000) in press.
[2] G. Le Duc, A.M. Charvet, H. Elleaume, F. Estève, S. Corde, S. Fiedler, A. Collomb, J.F. Le Bas, European Radiology, (2000) in press.
[3] N.A. Lassen, W. Perl, Tracers Kinetics Methods in Medical Physiology, Raven Press, New York, (1979).

J.F. Adam, H. Elleaume, G. Le Duc, S. Corde, A-M. Charvet, J.F. Le Bas, F. Estève.
Equipe d'accueil "Rayonnement Synchrotron et Recherche Médicale", Unité IRM, CHU Grenoble (France).