Abdominal Computed Tomography Scanning 

Updated: Sep 13, 2017
Author: Caroline R Taylor, MD; Chief Editor: Mahan Mathur, MD 

Overview

Background

Abdominal computed tomography (CT) scanning has revolutionized patient care in the past two decades. The introduction of CT technology is widely viewed by medical practitioners as one of the major medical advances.[1] Abdominal CT scanning is used in the evaluation of trauma victims for visceral injury[2] and in the evaluation of acute abdominal pain, with a major role in the evaluation of renal calculi,[3] acute appendicitis,[4] and complex abdominal pathology (see the images below).[5] It is also the gold standard in identifying abdominal injury in children.[6, 7]  Appropriate precautions must be taken prior to the scan to ensure that the patient has adequate renal function and no history of allergy before prescribing intravenous iodinated contrast.[8, 9]   The technologist will have a direct discussion with the patient to review the risks and refer any issues identified to the radiologist. Informed consent and counseling is required for patients categorized as high risk according to the clinic or institutional policy. Technologists are usually certified to obtain verbal consent for uncomplicated cases, but, for written consent, a physician must review risks and benefits with the patient directly.

CT of the abdomen and pelvis using oral and IV con 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).
CT of the abdomen and pelvis without contrast; a r 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).
CT of the abdomen and pelvis using PO and IV contr 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.
CT colonography with oral contrast tagging is show 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.

Abdominal CT scanning is used in the emergency setting to diagnose complex intra-abdominal conditions, to differentiate causes of bowel obstruction, and to evaluate complications of hernia, pancreatitis, biliary obstruction, acute vascular compromise, and abdominal aneurysm. Multidetector CT (MDCT) has a major role in characterization and staging of tumors of the liver, pancreas, kidneys, bowel, reproductive organs, and lymphatic system. It also has a role in screening for hepatocellular carcinoma[10] and colon carcinoma using virtual colonoscopy.[11, 12, 13]  It is also used in surgical treatment planning and in the diagnosis of postoperative complications.

Scan protocols are adapted for the specific diagnostic problem, and the images are reformatted for viewing on picture archiving and communication system (PACS) workstations. Additional manipulation can be done on PACS or dedicated workstations, providing for multiplanar and 3-dimensional views of organs, vessels, and bones. With the use of appropriate luminal and intravenous contrast agents, information can be obtained on lesion characteristics based on tissue attenuation changes, detailed 3-dimensional arterial anatomy, and/or luminal surface contour (eg, in virtual colonoscopy).

The patient is optimally positioned on the CT scanner table. As the patient is advanced into the scanner, he or she is coached by the technologists, who have direct visualization and bidirectional auditory communication with the patient as the study is performed. Typically, the patient is warned to anticipate the effects of contrast injection is informed about breath-holding requirements during the scan. The technologists select the correct protocol for the prescribed examination and select exposure parameters, taking into consideration factors such as the patient’s body habitus, in order to optimize image quality while limiting radiation exposure.

 

Periprocedural Care

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:

  • Allergy to contrast or other severe allergies - Risk of anaphylaxis to iodinated contrast

  • Renal function, hydration - Risk of contrast-induced nephropathy

  • Metformin use - Risk of lactic acidosis[15]

  • 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.”

 

Technique

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.

 

Medication

Medication Summary

It is generally accepted that contrast is not indicated in cases of suspected retroperitoneal bleeding or renal stones. Otherwise, in patients who are tolerant, contrast agents are usually used during routine abdominal CT scans to help identify the internal contents of hollow organs, to visualize spaces in the abdomen, and/or to delineate surrounding blood vessels. These agents can be broadly categorized in terms of their route of administration, either through the gastrointestinal tract or intravenously, and further described by their relative density to the surrounding tissues.

Proprietary names of contrast agents used and protocols detailing the specifics of their administration vary from institution to institution. The radiologist reviews the indications for the study and prescribe appropriate contrast with consultation as needed with the referring clinician. It is crucial that the radiologist tailors the use of contrast when dealing with special or high-risk populations (ie, children, uremic patients, patients with documented allergies). The radiologist must know the possible complications of administering contrast and how to treat them as they arise.[21, 22]

Gastrointestinal Contrast Agents

GI contrast agents are most commonly administered orally but may also be given per rectum, bladder, or support tube. Oral contrast agents have the advantage of being quite dilute compared with their intravenous counterparts; as such, the risk of contrast reactions is lower, precluding the need for patient consent. The only major contraindication to their use is in patients who are at risk for aspiration, in whom they can still be administered via an enteric tube.

One point of debate among different radiologists is the length of the interval between initial administration of oral contrast and the beginning of the CT scan. In general, longer wait times allow for increased transit through the gastrointestinal tract, improving the quality of the scan. However, this may not always be clinically feasible in the emergency setting.

Positive GI agents such as barium or iodinated water-soluble contrast are used when distention and opacification of the gastrointestinal lumen is needed. These agents aid in the evaluation of GI tumors, viscus perforation and/or extraluminal fluid collections. Iodinated water soluble contrast agents are advantageous when compared to barium because they cause minimal GI distress, are absorbed from the peritoneum in cases of viscus perforation, and rapidly dissipate from the lungs if aspirated. They have lower overall viscosity than barium, allowing for uniform distribution. In patients with known severe reactions to intravenous iodinated contrast, however, barium should be used instead.

Neutral GI contrast agents are used to distend the bowel without obscuring evaluation of the mucosa. They include water, CO2, and Volumen.

These agents are especially helpful in cases of suspected bowel ischemia, in which mucosal enhancement (or lack thereof) helps point to the diagnosis. Studies have shown that milk may have a promising role as a cost-effective and tolerable alternative.[23]

Contrast agents can also be administered rectally to evaluate for rectosigmoid cancer, perirectal abscesses, and fistulas. Neutral contrast agents such as CO2 help distend the colon when performing virtual colonoscopy.

Water-soluble contrast agents can be instilled into the bladder via catheter during CT cystography for evaluation of suspected bladder neoplasms or in cases of suspected traumatic bladder perforation.

Intravenous Contrast Agents

Iodine-containing water-soluble, nonionic contrast agents dramatically increase the attenuation within blood vessels while within the blood pool; when they are subsequently distributed within extracellular spaces prior to renal excretion, they cause tissue enhancement within normal and abnormal organs. Ischemic or inflammatory changes or neoplastic processes are associated with distinctive patterns of enhancement during various phases of contrast administration, which can be documented on CT during multiphase acquisitions, aiding in the detection of specific lesions, for example in the liver, kidney, or pancreas. However, in many instances, a single-phase acquisition is adequate for diagnostic purposes.

Intravenous contrast agents are generally described in terms of their iodine concentration (ie, Omnipaque-350 has 350 mg of iodine per mL). It should be noted, however, that higher iodine concentrations do not necessarily correlate with improved image quality and usually lead to more toxic side effects. Because of the risk of contrast-induced nephropathy and allergies with intravenous agents (discussed below), all patients must be appropriately counseled and give consent prior to intravenous contrast administration.

Intravenous contrast is cleared via glomerular filtration in the kidneys, so estimation of renal function must be performed with either serum creatinine or glomerular filtration rate prior to administering the agent. The latter has demonstrated to be more accurate in older patients, as it takes into account age, sex, race, and weight in addition to serum creatinine levels.

Patients who are at high risk for contrast-induced nephropathy include those with chronic renal disease, diabetes, congestive heart failure, and multiple myeloma. Also, patients who are taking nephrotoxic medications or those who have received more than 100 mL of intravenous contrast in the previous 24 hours are considered at risk. Since dehydration is the most common risk factor for contrast-induced nephropathy, especially in those predisposed, patients can be pretreated with bicarbonate or intravenous fluids for volume expansion. Other measures include using isoosmolar or nonionic agents, limiting the volume of contrast used, and avoiding other nephrotoxic drugs.

Patients receiving intravenous contrast agents can have idiosyncratic and nonidiosyncratic reactions. For a detailed discussion of these reactions, please refer to Contrast Medium Reactions.

Diagnostic Imaging Agents

Class Summary

These agents are used for the radiographic visualization of anatomical structures.

Barium Sulfate (Bar-Test, E-Z-Dose, Maxibar, Tagitol V, Readi-Cat, VoLumen)

Diluted to 1%-3% concentration

Provides good visualization of GI mucosa

More resistant to dilution than water-soluble iodinated contrast

Cheap

Omnipaque (Iohexol)

Low-osmolar water-soluble contrast agent with multiple uses, administration routes, and concentrations (ranging from 140-350 mg/mL iodine). It is the only contrast agent that can be administered PO, intraperitoneally, intrathecally. Can also be used intravenous or intra-arterially. Does not cause irritation to mucosa or serosa. Suitable for preoperative or postsurgical patients or those with suspected bowel perforation or obstruction.