Role of dual PET/CT scanning in abdominal malignancies

Modern cross-sectional structural imaging techniques like ultrasonography, computed tomography (CT) and magnetic resonance imaging (MRI) provide high resolution images that aid in accurate detection, delineation and anatomic localization of abdominal malignancies. However, characterization of lesions into benign and malignant abdominal etiologies is often not possible from structural imaging techniques alone. Although functional imaging techniques like positron emission tomography (PET) with radiolabeled 18F labeled 2-fluoro-2-deoxyD-glucose (18F-FDG) often provide critical information pertaining to a benign or malignant etiology, accurate anatomic localization of abnormal regions of uptake is often problematic due to inadequate spatial resolution. These circumstances make the combination of PET with CT appealing. It has the potential of offering a comprehensive ‘one-stop’ examination by providing information about lesion etiology based on functional activity on PET scanning along with precise anatomic localization and other morphological features of the abnormality with CT scanning [1–3]. Attempts at combining PET and CT data from different machines with software image fusion are facilitated by extracorporeal (fiducial) points and line markers fixed on the patient’s skin in the same position for each imaging study. This software fusion permits evaluation of two modalities in one integrated image dataset but results in less satisfactory fusion due to differences in patient positioning and involuntary movement of abdominal organs between scans [4,5]. Although true hardware fusion of PET and multidetector CT does not exist, more precise projection of the PET image over the CT image can be obtained with the currently available hybrid PET/CT scanners, which consist of separate scanners that are positioned in line at a fixed distance within a single gantry assembly [6]. The CT images are used for more precise and rapid attenuation correction of the PET data and as anatomic reference of the radiotracer uptake patterns evaluated with PET. They also provide some valuable information regarding morphological features and attenuation values of lesions. In addition to reducing the PET imaging time per patient from 45 to 60 min with a conventional dedicated PET scanner to 15–30 min, the hybrid PET/CT scanners also reduce the number of equivocal PET interpretations. The introduction of CT-based attenuation correction and its integration with PET necessitates different PET/CT scanning protocols. In general, the two approaches adopted for PET/CT scanning are using the CT to perform faster attenuation correction with little emphasis on anatomic co-registration or using the CT not just for attenuation correction but for diagnosis and co-registration as well [7]. Whereas the initial approach mandates that the CT be performed with the lowest permissible radiation dose without affecting attenuation correction, in the latter approach CT is performed with standard radiation dose to attain diagnostic image quality. Regardless of the approach, prior to PET scanning, CT images are acquired to optimize patient positioning and perform attenuation correction for PET images. Although recent studies have shown that oral and intravenous contrast media can be administered for the diagnostic CT to aid lesion localization and support characterization, modifications are necessary to