Pre-Procedure Planning
The referring physician must provide a detailed clinical history that meets American College of Radiology (ACR) appropriateness criteria and discuss with the patient use of alternative modalities, taking into consideration age-based risk estimates for radiation exposure. Females of childbearing age should be screened for pregnancy. [14]
Although the CT scan itself may take under a minute to complete, preparation may require an interval of 1-1.5 hours during which oral contrast is administered.
Before prescribing intravenous contrast, the radiologist must ensure that the medical record has been screened for contrast-related risks and discuss any concerns or contraindications with the referring clinician with the most recent data at hand. The screening includes the following:
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Allergy to contrast or other severe allergies - Risk of anaphylaxis to iodinated contrast
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Renal function, hydration - Risk of contrast-induced nephropathy
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Metformin use - Risk of lactic acidosis [15]
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Recent food ingestion - Risk of aspiration
Equipment
A state-of-the-art multidetector CT scanner is constructed around a ring-shaped gantry, which houses one or more spinning x-ray sources and multiple detector elements (ranging from 64-320). The more detector elements, the more tissue volume can be sampled during the tube rotation without advancing the couch position. A motorized height-adjustable scanning table can optimize patient centering in the gantry and advances the patient through the scanner at a specified rate. Iodinated contrast is introduced intravenously during the scan via a programmable power injector to ensure a stable injection rate.
Dual-source tubes suitable for cardiac imaging have been developed, which also have applications in abdominal imaging, allowing chemical analysis of renal calculi, for example.
A wide multidetector array (such as is found in 320-detector scanners) allows the tube to be rotated while multiple image sequences are acquired over time (perfusion imaging), which is being investigated for tumor characterization.
The integrated hardware consists of a high-performance computing system that can reconstruct the familiar CT images according to filtered back projection using x-ray attenuation and spatial and temporal data that also apply complex, selectable, and proprietary algorithms to enhance image quality. This allows reduction of radiation exposure, for example, through iterative reconstruction techniques. The scans are viewed at a console operated by the technologists and ae then transmitted into a PACS (picture archive and communication system) for interpretation by the radiologist.
Image quality is negatively affected in very cachectic or obese patients or in patients who are unable to remain motionless or hold their breath because of underlying pain, pulmonary disease, or other comorbid conditions (eg, congestive heart failure or mental-status change). Metallic hardware, such as spinal-stabilization or hip-arthroplasty devices, as well as bone itself, can cause streak artifacts due to “beam hardening.”
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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.
