Gamma Spectrum Subtraction Technique for Measurement of Activity in Body Organs and its Use for Cerebral Blood Flow Studies

With nearly all practical detector systems geometrical factors and radiation absorption result in the highest response being observed to activity in superficial tissues. This can result in substantial errors when it is necessary to record the activity in a deeper organ or tissue. The spectrum subtraction technique can be used to modify the scintillation counter/collimator characteristics so that the detection efficiency is maximum for deeper tissues and zero at the body surface, without the prohibitive loss of sensitivity found with the more restricted coincidence techniques. The technique has been used to eliminate the effects of activity in the skull and scalp when measurements of 133Xe uptake and washout from the brain are made for blood-flow studies by the inhalation technique. The activity/time curves for the 80-keV and 30-keV radiations of 133Xe are recorded separately. The 30-keV count-rate is multiplied by a normalizing factor and subtracted from the 80-keV count-rate. The normalizing factor which results in zero sensitivity at the surface is determined by placing a small source of 133Xe on the scalp. Water-filled phantom measurements show that the region corresponding to maximum detector sensitivity is now about 2.5 cm below the surface. In a series of cerebral blood flow studies the data derived from the two-component curve usually obtained with the spectrum subtraction technique were compared with those from the more usual three-component analysis of the total activity curve. The latter method assumes that the extracerebral activity is reflected by the slowest component and that this is monoexponential. No significant difference was found between the values for cerebral perfusion rates obtained by the two methods. The subtraction technique has many potential applications, using different radiations from the same nuclide or from different isotopes of the same element. The dual ratemeter/analogue subtract unit used in renography is convenient for this type of measurement, alternatively a dual digital multiscaler data acquisition system can be used if the subsequent calculations are to be done with a digital computer. (author)