THRESHOLD ESTIMATION IN SINGLE-PHOTON EMISSION COMPUTED-TOMOGRAPHY AND PLANAR IMAGING FOR CLINICAL RADIOIMMUNOTHERAPY

Thresholding is the most widely used organ or tumor segmentation techn ique used in single photon emission computed tomography (SPECT) and pl anar imaging for monoclonal antibodies. Selecting the optimal threshol d requires a priori knowledge (volumes from CT or magnetic resonance) for the size and contrast level of the organ in question, Failure to s elect an optimal threshold leads to overestimation or underestimation of the volume and, subsequently, the organ-absorbed dose value in radi oimmunotherapy. To investigate this threshold selection problem, we pe rformed a phantom experiment using six lucite spheres ranging from 1 t o 117 ml and filled with a uniform activity of 1 mu Ci/ml Tc-99m. Thes e spheres were placed at the center and off-center locations of a Jasc zsak phantom and scanned with a three-headed gamma camera in SPECT and planar modes. Target:nontarget (T:NT) ratios were changed by adding t he appropriate activity to the background. A threshold search algorith m with an interpolative background correction was applied to sphere im ages, This algorithm selects a threshold that minimizes the difference between the true and measured volumes (SPECT) or areas (planar), It w as found that for spheres equal to or larger than 20 ml [diameter (D) > 38 mm] and T:NT ratios higher than 5:1, mean thresholds at 42% for S PECT and 38% for planar imaging yielded minimum image segmentation err ors, which is in agreement with current literature, However, for small T:NT ratios (<5:1), the threshold values as high as 71% for SPECT and 85% for planar imaging were substantially different than those fixed thresholds for large spheres (D > 38 mm), Hence, the use of fixed thre sholds in low contrasts and with tumor and organ sizes of clinical int erest (25 less than or equal to D less than or equal to 50 mm) may res ult in limited volume estimation accuracy, Therefore, we have provided the investigator a method to obtain the threshold values in which the proper threshold can be selected based on the organ and tumor size an d image contrast, By measuring and calibrating the proper threshold va lue derived through machine-specific phantom measurements, a more accu rate volume and activity quantitation can be performed. This, in turn, will provide tumor-absorbed dose optimization and greater accuracy in the measurement of potentially subacute, toxic absorbed doses to norm al organs for patients undergoing radioimmunotherapy.