The value of positron emission tomography magnetic resonance imaging (PET/MRI) and positron emission tomography computed tomography (PET/CT) in oncological imaging is based on the fundamental principle that abnormal alterations in intra-cellular biochemical reactions result in the development of malignancy. Positron-emitting analogue tracers may be synthesised from organic matter and utilised to demonstrate these intra-cellular sequential processes. The principle aim of this study was to prospectively evaluate if PET/MRI may be considered to be a more accurate imaging modality than PET/CT or an equivalent or complimentary imaging modality to conventional imaging (CT or MRI) for the complete staging of patients with primary colon and rectal cancers. The purpose was to conduct investigations to enable evaluation of the diagnostic accuracy of PET/MRI and PET/CT for T, N and M colon and rectal cancer staging in a clinical setting. The work in this thesis may help to evaluate PET/MRI as a new imaging modality by providing high-resolution anatomical, molecular and functional information, which in combination may allow the comprehensive assessment of tumour location and stage in a single imaging examination. PET/MRI may also potentially provide loco-regional and systemic staging of CRC disease. Through incorporation into CRC protocols, patient pathways may be streamlined and provide an effective algorithm for more appropriate and timely treatment, resulting in improved long-term survival.