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The Channel Ratio Method of Scatter Correction for Radionuclide Image Quantitation

Biophysics Department, University of the Orange Free State, Bloemfontein, South Africa

Correspondence: For correspondence and reprints contact: Mattheus G. Lötter, PhD, Biophysics Department, University of the Orange Free State, PO Box 339, Bloemfontein 9300, South Africa.

ABSTRACT

The accuracy of quantitation of radionuclide distributions in human tissue with the scintillation camera is decreased by attenuation and scatter of photons. If scatter correction is applied satisfactorily, narrow beam attenuation can be applied. In this article, a scatter correction technique, the channel ratio (CR) method, is introduced. The CR scatter correction method is proposed for quantitation of the radionuclide distribution in organs. The improvement in the geometrical resolution was measured and examples of clinical images are presented. In this method, the change in the ratio of counts from two symmetrical adjacent energy windows straddling the energy photopeak was used to eliminate the contribution of scattered photons during imaging with ^99mTc. The theory and methods for the empirical affirmation are described. To apply the CR scatter correction method, two constants, the ratio of primary photons G and the ratio of scattered photons H in the same windows, were determined. Different sized sources in varying depths of water were imaged. When the source activities were quantified after scatter correction with the CR method, the measurements ranged from 96%–108% in comparison to the reference value in 100 mm water. The scatter fraction increased from 0.20 in 10 mm water to 1.44 in 200 mm water. The geometrical resolution expressed as full width at tenth maximum in 150 mm water improved by 30.4% and was restored to the value of the geometrical resolution in air. The CR scatter correction method is a simple method to correct for scatter in order to facilitate accurate quantitation of the radionuclide distribution during imaging with a scintillation camera.