Use of 123I and 14C in a double radionuclide autoradiographic technique for simultaneous measurement of LCBF and LCMRgl. Theory and method.

We have developed an autoradiographic technique for the simultaneous measurement of local cerebral blood flow (LCBF) and local cerebral metabolic rate for glucose (LCMRgl) using 123I-iodoantipyrine and 14C-2-deoxyglucose respectively. By exploiting the different half-lives of 123I and 14C, 13.0 hours versus 5730 years, we produce 2 autoradiographic images. The first is predominantly a result of the 123I and the second is predominantly from the 14C. Because of the impracticality of making 123I standards, it was necessary to determine the constant which relates the ability of 123I, with respect to 14C, to expose Kodak SB-54 film, so that the 123I exposure could be quantified using 14C standards. Subtraction equations can then be used to solve for the local 123I and 14C concentrations. The technique was validated in conditions simulating a 12-fold decoupling of flow and metabolism and the standard deviation of error in measuring tracer concentrations was less than 6%. It was then used to measure these parameters in the normal awake rats and values obtained agreed well with published values from single radionuclide studies. The technique is expected to be useful in the simultaneous measurement of LCBF and LCMRgl in various physiologic or pathologic states, including those with significant decoupling of flow and metabolism. In addition, by using other tracers labelled with 123I and 14C, other parameters can be measured concurrently.