The effect of gate width on thallium 201 scintigraphy of the myocardium

Myocardial scanning with 201T1 is now a well established technique. Quantification of thallium studies has been attempted by several workers (Maseri et al., 1976; Sawayama et al., 1977; Meade et al., 1977; Wainwright et al., 1977; Lenaers et al., 1977; Rouleau et al., 1975) but there is not yet an accepted method of analysis. Because of the high renal radiation dose 0.104cGy/MBq (3.85 rad/mCi) and the high cost of the radionuclide, thallium is usually administered in quantities of only 30–60 MBq (1.0–2.0 mCi). Thus recorded counts are at a premium, and one would like to make the gamma camera as sensitive as possible, especially for quantitative studies where good counting statistics are essential. As an adjunct to our studies on the quantification of thallium scans, we have investigated the use of various gate widths on the resolution and sensitivity of the camera. Groch and Lewis (1977) have shown that the X-ray emissions from the mercury daughter of thallium give superior imaging properties to the gamma photons at 135 and 167 keV purely because of their greater abundance. The X-ray emissions have a peak at about 75 keV and these authors used a 25% gate width, which is equivalent to 18.7 keV. Atkins et al. (1977) used gate widths of 10% and 20% in their investigation of the imaging properties of thallium, and Lenaers et al. (1977) used a 25% window in their segmental quantitative analysis. We have investigated the use of gate widths between 5 and 40 keV centred on the mercury X-ray photopeak. The 40 keV gate completely straddles this peak.