Low-dose CT Using Iterative Reconstruction in Patients With Inflammatory Bowel Disease

The increasing use of CT has prompted the development of new scanning protocols which reduce radiation doses to patients and minimise the likelihood of radiation related morbidity. The use of disease specific low-dose CT examinations is an emerging method of limiting radiation doses.

Research conducted in Cork University Hospital(CUH) by the current authors has identified a pressing need to reduce radiation doses in patients with IBD. A retrospective study of radiation doses in patients with Crohn's disease demonstrated that increasing numbers of CT exams are performed with average cumulative effective doses rising from 7.9 to 25mSv when the first 5-years of the 15 year study period were compared with the final 5-years. Eight-five percent of the dose during the final 5-year period was due to CT. Younger patients with more severe disease requiring surgery or steroids were more likely to undergo an increased number of exams. 15.5% of patients received cumulative effective doses of greater than 75mSv. This quantity of radiation exposure is associated with a 7.3% increase in mortality from cancer. In addition, patients with Crohn's disease are inherently predisposed to gastrointestinal and hepatobiliary carcinoma and small bowel lymphoma.

As an alternative to CT, IBD patients are frequently imaging with conventional abdominal radiography. The effective dose of a conventional abdominal radiograph (CAR) is approximately 10% that of a standard abdominal CT varying between 0.7 -0.1mSv. The current authors have also investigated the value of CAR. We retrospectively examined over 500 CAR's performed over a 16 year period in patients with IBD. Patients had an average of 3.5 CAR's performed but there were positive findings in less than 30% of exams. Many of these findings were non-specific requiring further investigation. For example separation of bowel loops on a plain radiograph has a wide differential diagnosis including abscess formation, presence of a phlegmonous mass, fibrofatty proliferation, bowel wall thickening and lymphadenopathy.

Patients with inflammatory bowel disease referred to CUH will undergo a modified abdominal CT protocol. The radiation dose of a standard CT abdomen and pelvis will be divided into 2 quotients. Patients will have a low-dose CT scan requiring approximately 10% of the dose of a standard abdominal CT. This equates to the radiation dose of a conventional abdominal radiograph. Patients will be imaged with a second CT exam using 90% of the standard abdominal CT dose ensuring a diagnostic study is acquired. Patients will be given oral and intravenous contrast agents as for a standard CT. Patients will have a C-reactive protein measured on the day of CT and will have their heights and weights also measured at the time of scanning. Patients will have a plain film of abdomen performed prior to CT.