**2.4 Positron Emission Tomography**

FDG PET is a highly sensitive and specific imaging study detecting hepatic metastases from CRC (92–100% and 85–100% respectively), although for some authors the strength of these data is moderate (Lucey et al. 2006). Several studies have also shown the utility of FDG PET in identifying additional metastatic lesions when initial CT showed single hepatic metastases and, thus, changed the management strategy. Nevertheless, false negative and false positive findings in FDG PET for hepatic metastases are not negligible (Udayasankar et al. 2008) and its positive predictive value (PPV) is not high, leading to some authors to confirm histologically the FDG PET findings suggesting non-resectability (Valls et al. 2009).

Two meta-analyses have demonstrated high diagnostic values of PET in the evaluation of hepatic metastases (Bipat et al. 2005; Wiering et al. 2005), as well as a recent review (Patel et al. 2011) confirming the superior sensitivity of FDG PET for detecting liver metastases on a per patient basis, but not on a per lesion basis. Other papers have shown FDG PET/CT to be slightly less sensitive than MRI with liver-specific contrast agents or dedicated sequences for small lesions (Coenegrachts et al. 2009), but more sensitive than MDCT alone (Kong et al. 2008; Selzner et al. 2004), although its role is not yet clear owing to the small number of studies (Niekel et al. 2010). In the context of CRC metastases, the role of FDG PET/CT is to avoid unnecessary surgery, based on its ability to detect extrahepatic foci of disease (nodal metastases, lung nodules) that are not depicted or characterized as malignant by other imaging methods (Sørensen et al. 2007). In addition, this technology is not suitable for liver resection planning. In patients evaluated with FDG PET prior to surgery, a lower risk of "non-therapeutic laparotomy" (Pawlik et al. 2009) and improved survival (Fernandez et al. 2004) has been observed, reflecting better patient selection.

A recent meta-analysis reviewing more than 3,000 patients found that sensitivity of CT, MR imaging and FDG PET on a per lesion basis were 74.4%, 80.3% and 81.4%, respectively, while on a per patient basis, the sensitivities were 83.6%, 88.2% and 94.1%, respectively. Specificity estimates were comparable. No differences were seen for lesions measuring at least 10 mm. Data about FDG PET/CT were too limited for comparisons with other modalities (Niekel et al. 2010).

In brief, although every modality has benefited from advances in technology, MDCT scanning remains a dominant imaging modality not only for lesion detection and preoperative planning, but also for treatment monitoring and post-treatment surveillance. High-resolution CT with contrast combined with FDG PET/CT may obviate the need for additional studies and may improve patient management (Bipat et al. 2007; Doan et al. 2010; Vauthey 2006). Dynamic gadolinium-based contrast-enhanced MRI should be reserved for problem solving. MRI has the highest sensitivity for lesion detection, but because of its low sensitivity in detecting extrahepatic disease in the peritoneum and chest, it is not a desirable primary imaging modality (Vauthey 2006) except for evaluating patients who have not previously undergone therapy (Lucey et al. 2006; Niekel et al. 2010). Ultimately, the modality used must be tailored not only to the patient and the clinical situation, but also to the imaging expertise within the institution.
