Attenuation correction using level set method for application specific PET scanners

Attenuation correction is essential to obtain quantitative positron emission tomography (PET) images. Typically, attenuation map is calculated from corresponding X-ray CT images or by using transmission scan data acquired with an external radionuclide source. However, in a PET scanner dedicated for a specific organ, such as brain or breast, surrounded closely by radiation detector modules, it is difficult to implement transmission scan mechanism. Although some techniques have been developed to calculate uniform attenuation maps from emission sinograms or images based on intensity thresholding, it is generally difficult to decide an optimal threshold value especially due to complicated target shapes, nonuniform activity distribution, or large statistical noise. In this paper, we propose a new method to generate the attenuation map from the attenuation-uncorrected reconstructed image using the level set method, which is one of the active contour models to solve the problem of image segmentation. We evaluated the accuracy of attenuation maps generated from various patient images with our dedicated breast PET scanner. Experimental results showed that the proposed method can provide more accurate and robust attenuation map in comparison with the conventional thresholding method.