A comparison of two methods for assessing the performance of the display system of a clinical radioisotope imaging device.

Two ways of measuring the performance of the display system of a clinical radioisotope imaging device are considered: (i) using the method of constant stimuli; and (ii) using signal detection theory. The adjustment of a rectilinear scanner with multicolour, equal bandwidth, linear ribbon for better abnormality detection is discussed. A comparison was made between setting the maximum counts at the edge and at the middle of a colour band using the two methods. It was found that setting up the maximum counts at the middle of the band rather than at the edge favours easy detection by both methods independently.

The influence of size and radiopharmaceutical concentration ratio on the detection of abnormalities in clinical radionuclide imaging.

The effect of the size of an abnormality on its detectability was measured. The cylindrical abnormalities studied, with diameters ranging from 3 mm to 4 cm, were superimposed on a uniform background. To avoid artefacts produced by the imaging devices, the images were simulated by computer. The method of constant stimulus was used to determine the detectability of the abnormalities. By using a Perspex phantom filled with 99Tcm, measurements were made of the concentration ratio between abnormality and background which permitted detection of the abnormality. This allowed evaluation of the effectiveness of 99Tcm pertechnetate in the detection of small abnormalities. Assuming a concentration ratio between abnormal and normal tissue of 22.5:1, then at a depth of 50 mm in the brain a cylindrical abnormality of about 8 mm diameter was the smallest detectable under the chosen scanning conditions.

Technical note: thyratron performance--dependence on mounting in a medical linear accelerator.

A thyratron is an important component of a medical linear accelerator. The working life of a thyratron has been found to be dependent on its physical orientation in the accelerator. This observation is a result of maintaining accelerators for over 15 years.

Transmission of 4-MeV electron beam through lead shielding.

The relative reduction in percentage transmission through lead for 4-MeV electrons produced from a linear accelerator for different field sizes has been measured both with and without buildup. For our study a parallel plate chamber with Farmer dosimeter in polymethylmethacrylate phantom was used. Lead strips were used as a shielding material. The result of this study shows that as (i) the depth of measurement in phantom, (ii) the gap width between lead shielding material and phantom increases, the lead thickness required for the same percentage of reduction in transmission decreases due to energy loss of scattered electrons and decreased scattered electrons reaching the detector. The 95% and 98% reduction in transmission was evaluated and found to increase with increasing field size due to higher lateral scattering. From our study it was concluded that the reduction in transmission with lead was found to depend upon the field size, the depth at which measurements are made, the gap width between lead shielding and phantom, and the focus to skin distances.