Importance of the choice of the collimator for the detection of small lesions in scintimammography: a phantom study

Tc-99m methoxyisobutylisonitrile planar scintimammography (SMM) is mostly p erformed using low-energy high-resolution (LEHR) parallel collimators. We s tudied whether using a different collimator could improve the detection of small (<1.5 cm) lesions for which SMM sensitivity is poor. Thirty four brea st phantom configurations were considered, either with hot spheres simulati ng lesions or without any spheres. For each configuration, four planar acqu isitions were performed using LEHR, low-energy ultra high-resolution (LEUHR ), high-resolution fan-beam (HRFB) and ultra high-resolution fan-beam (UHRF B) collimators. Images corresponding to the 20% and 10% energy windows and to the Jaszczak subtraction were calculated. A database including 156 borde rline images was derived. After training, 10 observers scored the images fo r the presence of a sphere. The performances in sphere detection were studi ed using receiver operating characteristic (ROC) analysis. For all types of image, the area under the ROC curve was highest with the UHRFB collimator and lowest with either the LEUHR or the HRFB collimator. For the 10% energy window images conventionally used in SMM, the detection sensitivities aver aged 91%, 73%, 60% and 55% for the UHRFB, LEHR, HRFB and LEUHR collimators respectively, for the same specificity of 64%. We conclude that detection o f small tumours in planar SMM might be significantly improved by using a UH RFB collimator instead of an LEHR collimator.