The analysis of multidetector CT images requires the use of a workstation, a sound knowledge of anatomy and familiarity with potential artefacts. The reader should be familiar with the techniques that have been employed in obtaining the images, and in particular each phase of the study should be clearly labelled, e.g. arterial, portal, to avoid any confusion both at the time of reporting and in later clinical situations.
CT data sets are usually interpreted on 3–5 mm axial sections. Additional coronal or sagittal multiplanar reconstructions (MPR), curved planar reformations (CPR), maximum (or minimum) intensity projection (MIP/MinIP) and volume-rendered (VR) images can be reconstructed from thin-slice data sets. These additional image reconstructions may be available routinely or on demand, or may have to be created interactively on a workstation, all depending on the clinical indication and the chosen workflow in a Department of Radiology.
When an abnormality is recognised, the site and size of the lesion should be recorded; this is often easier with smaller lesions than larger ones invading several structures. The density (or CT number/Hounsfield units) may be helpful in some situations, particularly in determining the presence of fat, or evaluating the degree to which a structure has enhanced, but caution should be exercised in this regard with very small lesions or regions of interest, which may be misleading.
Air may be easily identified by its very low attenuation (−1000 HU). The presence of free air within the abdomen or mediastinum may be more easily determined using lung windows than standard soft-tissue windows. In addition, the distribution of air should be analysed; pockets of air that have not risen to the most superior point of the cavity must be constrained by septae or walls that may be beyond the resolution of the CT system.
Fat has a density on the low negative range (around −100 HU) and may be useful in identifying adrenal adenomas, angiomyolipomas, dermoid cysts or extramedullary haematopoeisis, for example.
Water has a density of approximately 0 HU, but the presence of proteins within many bodily fluids will increase this. Acute haemorrhage will often be of high density (up to 80 HU), reducing over time. As haemorrhage matures, the dependent portion will often be denser than the non-dependent fluid. Many soft tissues have similar density, and are best evaluated following contrast medium.
A high-density structure (>150 HU) generally implies calcification or the presence of contrast medium. Metallic structures, e.g. prostheses, have a density of several thousand Hounsfield units and lead to streak artefacts. Other foreign bodies will have a variable density and it is important to be aware of these in reporting such cases.
The effect of a lesion on neighbouring structures should also be described: is the lesion invading/displacing/causing retraction of an adjacent structure or vessel? When measuring lesions, it is important to be consistent between studies. Pulmonary lesions should always be measured using lung windows, whereas for abdominal lesions and lymph nodes, soft-tissue windows should be used. The latter will underestimate the size of pulmonary lesions. For oncology clinical trials, it is imperative that the reader is familiar with the various response assessment criteria (e.g. RECIST) and is aware of which criteria are being used in any particular case.
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