Approach Considerations
Scan protocols are adapted to the specific diagnostic problem. Noncontrast scans are typically used to evaluate renal stones or retroperitoneal hematoma or is used in patients in whom iodinated contrast is contraindicated.
Contrast scans can be classified as single-phase, multiphase, or special. Single-phase scans are typically used to evaluate acute abdomen or suspected abdominal infections, with imaging usually in the portal venous phase. It is usually combined with administration of oral contrast. Oral or intraluminal contrast enhances the evaluation of the bowel. Multiphase scans consist of precontrast and combinations of arterial phase, portal venous phase, and delayed imaging, depending on the organ of interest. [16]
CT cystography uses water-soluble dilute iodinated contrast introduced via a catheter into the bladder, typically to evaluate for bladder rupture or leak. [17] CT enterography combines a negative contrast agent (Volumen) and multiphase scanning to evaluate bowel, arterial supply, and mesenteric and portal venous integrity. [18] CT colonography uses a noncontrast protocol in supine and prone positions after insufflation of the prepared large bowel with gaseous carbon dioxide at a controlled pressure. [19, 20]
Radiation exposure during a CT scan depends on the parameters used for the scan, such as the kilovoltage (kVp) and milliamperage (mA) selected, as well as factors such as speed of tube rotation, speed of table advancement, and the volume of tissue scanned. The dose is modulated to allow a tradeoff of noise (which becomes greater as the dose decreases) versus exposure. Exposure is measured in CTDI and DLP indexes, which can be converted to an estimate of absorbed dose in millisieverts (mSv).
A patient with a large body habitus represents a challenge, as the exposure parameters need to be increased to generate images of adequate diagnostic quality, with a resultant increase in absorbed dose. Exposure parameters for body CT examinations range on average from 3-25 mSv (annual background exposure in the United States estimated at 3.6 mSv), depending on factors such as body habitus, increasing if multiphase examination is needed. Manufacturers are actively developing iterative reconstruction techniques that allow further reduction in radiation exposure.
Contrast reactions are relatively rare, and usually minor, but the healthcare facility must have an emergency plan in place to deal with contrast reactions, including remedial medications (checked and regularly updated), physiologic monitoring apparatus, and staff certified in emergency life-support procedures. If cardiovascular collapse occurs, timely support should be available from emergency services or code teams, while, in the interim, cardiopulmonary resuscitation is administered by the trained CT staff.
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CT of the abdomen and pelvis using oral and IV contrast; a reformatted coronal image is shown. A 67-year-old male presented to the emergency department with sharp right lower quadrant abdominal pain . The appendix, seen here as an enlarged tubular structure in the right lower quadrant (small arrow), demonstrates diffuse wall thickening and contains a central appendicolith. There is associated periappendiceal fat stranding (*). The adjacent cecum appears thickened and demonstrates characteristic wall enhancement (large arrow). Findings were consistent with acute appendicitis with a surrounding inflammatory response. Diffuse abdominal aortic calcifications are incidentally noted (A).
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CT of the abdomen and pelvis without contrast; a reformatted 5-mm coronal image shown. An 81-year-old male presented with acute left flank pain and macroscopic hematuria. A large, 1.5-cm partially obstructing stone is seen at the level of the left ureteropelvic junction (large arrow). There is associated periureteral stranding (*). Scarring is noted in the lower pole of the left kidney (small arrow). Incidental note is made of colonic diverticulosis without diverticulitis (D).
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CT of the abdomen and pelvis using PO and IV contrast. A reformatted 5-mm coronal image is shown. A 66-year-old male presented with a history of coronary artery disease, chronic renal insufficiency, ostomyelitis of the foot, and 6 days of constant abdominal pain. Contrast is seen extending beyond the calcified abdominal aortic lumen at, and below, the level of the origin of the renal arteries, with a contained saccular portion measuring 3.4 X 1.7 X 2.7 cm (representing a pseudoaneurysm or a contained dissection secondary to an atherosclerotic ulcer). The patient's condition worsened, and he had MRSA sepsis. At surgery a mycotic aneurysm was found.
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CT colonography with oral contrast tagging is shown. A coronal CT reformat is shown on the left; a companion 3D-reconstruction colonography image of the same patient is shown on the right. An 89-year-old male with iron deficiency anemia presented for colorectal cancer screening. The patient was felt to be high risk for optical colonoscopy secondary to sedation risk. On both the coronal CT image and the reformatted image, a 4-cm mass is seen protruding into the lumen of the cecum, consistent with cecal carcinoma beyond the valve (white arrows). Multiple polyps were also identified during the procedure (not shown). The findings were confirmed by optical colonoscopy.
