Most studies use a low dose technique, without additional use of IV contrast. The reasons are the care of radiation dose in a technique requiring prone-supine imaging, and the fear of adverse reaction using IV contrast.
The low dose technique capitalizes on the high contrast that exists between the air-filled lumen and the soft tissue density wall. Polyps protruding into air-filled lumen can be detected using these low-dose technique.
Solid organ contrast however requires higher radiation dose and correct diagnosis frequently necessitates IV contrast. As a result, it is likely that a c
Fig. 10.3a-e. Sixty-six-year-old women referred after incomplete colonoscopy due to carcinoma of the sigmoid: a unen-hanced CT colonography shows the sigmoidal tumor (arrows); b abdominal window settings slightly suggest the presence of two focal liver lesions (arrows); c lung window settings show a solitary small pulmonary nodule in the right lower lobe (arrows); d bone window settings shows Pagets disease in the right hemi-pelvis.(arrows); e axial CT scans after IV contrast clearly shows the presence of two liver metastasis (arrows). This case clearly illustrates the need for different window settings, and the difficulty of recognizing solid liver lesions using low dose unenhanced CT scans b a c e solid organ lesions will remain undetected using low dose CTC without IV contrast (Fig. 10.3). This is nicely reflected when we compare the results of the population evaluated in the study by Spreng et al., using high dose and IV contrast (Spreng et al. 2005): compared with other studies, this study has the highest number of patients with extracolonic findings.
In our institution, we use a low dose technique (140 KV, with 10 mAs for supine, and 30 mAs for prone scanning), without IV contrast. For the evaluation of the colon, 0.6-mm slices are reconstructed at every 0.3 mm; for the evaluation of extracolonic findings, 3 mm thick slices are reconstructed at every 1.5 mm. The latter reduces noise, enabling visualisation of solid organ lesions (Fig. 10.4).
Since the use of IV contrast and normal dose might influence diagnosis of extracolonic findings, an active online physician monitoring could be considered to identify immediately patients who need IV contrast (Hara et al. 2000).
same holds true for focal fatty infiltration or normal variants in renal or pancreatic morphology.
Low dose scanning without IV contrast also causes false negatives, as reported by Hara et al. (Hara et al. 2000), and Gluecker et al. (GlueckeR et al. 2003). Hara reported one missed gastric carcinoma, one psoas metastasis, and one invasive transitional cell carcinoma of the bladder. Gluecker reported one missed adenocarcinoma of the pancreas, one splenic and one ovarian mass.
It can thus be concluded that low dose technique without IV contrast results in possible, but low (<1.5%) false negative diagnosis (Hara 2005).
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