MODELING DUAL-ISOTOPE PARATHYROID SCANS - OBJECTIVE DECISION-MAKING FOR TUMOR-DETECTION

Parathyroid subtraction imaging using the radioisotopes thallium-201 a nd technetium-99m was first described in 1983. Since then it has becom e a routine method for the pre-operative localization of parathyroid t umours. Despite its widespread use, the interpretation of the subtract ion images remains very subjective. This paper describes and evaluates an automatic objective method for the analysis of parathyroid scans. The method is based on the calculation of a likelihood ratio (the prob ability of obtaining a count in a given pixel if a tumour is present d ivided by the probability of obtaining that same count if a tumour is not present). The likelihood ratio is calculated for each pixel in a s ubtraction image. The calculated value is compared with a threshold. P ixels with likelihood ratios greater than the threshold are classified as belonging to tumour sites. Pixels with likelihood ratios less than , or equal to, the threshold are classified as belonging to 'no tumour ' sites. The probability density functions required were obtained from computer simulated images. The likelihood ratio technique has a simil ar sensitivity and specificity to that of experienced human observers analysing computer simulated images. Furthermore, it is a completely a utomatic method which can provide the basis of a decision support syst em.