Diagnostic efficacy of single-pass abdominal multidetector-row CT: prospective evaluation of a low dose protocol

To evaluate the diagnostic efficacy of single-pass contrast-enhanced multidetector CT (CE-MDCT) performed with a low-radiation high-contrast (LR-HC) dose protocol in selected patients with non-traumatic acute bowel disease.

65 (32 males, 33 females; aged 20-67 years) consecutive patients with non-traumatic acute bowel disease underwent single-pass CE-MDCT performed 70-100 s after i.v. bolus injection of a non-ionic iodinated contrast medium (CM) (370 mgI ml^-1). In 46 (70%) patients with a clinical and/or ultrasonographic suspicion of inflammatory bowel disease, up to 1.2-1.4 l of a 7% polyethylene-glycol solution was orally administered 45-60 mins prior to the CT examination. Patients were then divided into two groups according to age: Group A (20-44 years; n = 34) and Group B (45-70 years; n = 31). Noise index (NI) and CM dose were selected as follows: Group A (NI = 15; 2.5 ml kg^-1) and Group B (NI = 12.5; 2 ml kg^-1). All patients of Group A underwent thyroid functional tests at 4-6 weeks. Final diagnoses were obtained by open (n = 12) or laparoscopic surgery (n = 4), endoscopy w/without biopsy (n = 24) and clinical (n = 19) and/or instrumental (ultrasonography) (n = 6) follow-up at 11 ± 4 months (range 6-18 mo.). Statistical analysis was performed by χ² and Student's t-test for categorical and continuous variables, respectively.

Sensitivity and specificity were 91.3 vs 95.4% (p = 0.905) and 90.9 vs 88.8% (p = 0.998) with an overall diagnostic accuracy of 91.1 vs 93.5% (p = 0.756), whereas the radiation (in millisievert) and CM dose (in millilitre) were 7.5 ± 2.8 mSv and 155 ± 30 ml for Group A and 14.1 ± 5.3 mSv and 130 ± 24 ml for Group B (p < 0.001), respectively. No patients of Group A showed laboratory signs of thyrotoxicosis at follow-up.

The LR-HC has proved to be a safe and a dose-effective protocol in the evaluation of selected young patients with non-traumatic acute bowel disease. Advances in knowledge: (1) As reaching the highest diagnostic benefit to risk ratio (AHARA) appears to be the current principle of MDCT imaging, an increased amount of iodinated CM (0.7-0.9 gI ml^-1) can be safely administered to young patients (<40 years) with normal thyroid and renal function to compensate for the lower image quality resulting from low-dose CT protocols performed with the standard filter back-projection algorithm. Such an approach will result in a significant reduction of the radiation dose, which could be otherwise achieved only using iterative reconstruction algorithms combined with either low tube voltage and/or low tube current protocols. (2) An optimal scan delay (T_delay) for a venous phase caudocranial acquisition can be calculated by the following formula: T_delay = CI + 25 - T_SD, where CI is the duration of the contrast injection, 25 is the average of the sum of abdominal aortic and peak hepatic arrival times and T_SD is the scan duration. With such an approach, the radiation exposure resulting from bolus tracking, albeit performed with low-dose scans, can be spared in patients with normal transit times.