Recognizing the Normal Abdomen and Pelvis: Computed Tomography

Introduction To abdominal and Pelvic CT

It is estimated that around 90 million CT scans of all kinds were performed in the United States in 2019. Almost 10% of all visits to the emergency department are for abdominal pain of a nontraumatic nature. Many of those patients undergo a CT scan of the abdomen and pelvis performed to detect or clarify their clinical findings. Since the advent of CT, exploratory surgery has become rare and the need for emergency surgery has dramatically decreased.

• As with all imaging studies, one of the guiding principles in CT scanning of the abdomen and pelvis is to maximize the differences in density between tissues to best demonstrate their unique anatomy. Toward that end, abdominal CT studies make extensive use of intravenous and oral contrast agents.

Intravenous Contrast in CT Scanning

• CT scans can be performed with and/or without the intravenous administration of iodinated contrast material but, in general, they yield more diagnostic information that is more easily detectable when intravenous contrast can be used. CT scans done with intravenous contrast are called contrast enhanced or simply enhanced . Typically, the radiologist will choose the scanning parameters to optimize the CT study for the patient’s particular clinical issues. For example, different rates of contrast administration and timing of the scan will allow diagnostic enhancement of hepatic vessels versus the liver parenchyma.

• Although it might sound like a great idea to give everyone contrast, keep in mind that iodinated contrast can have adverse effects and produce serious reactions in susceptible individuals ( Box 13.1 ).

  BOX 13.1

  Contrast Reactions

  • Intravenous contrast materials available today are nonionic, low-osmolar solutions containing a high concentration of iodine that circulate through the bloodstream, opacify those tissues and organs with high blood flow, are absorbed by x-rays (and therefore appear “whiter” on images), and are finally excreted in the urine by the kidneys.

  • In some patients who have compromised renal function suggested by an estimated glomerular filtration rate (eGFR) less than 30 mL/min/1.73 m ² or a serum creatinine 1.5 mg/dL or higher who are not undergoing maintenance dialysis, prophylaxis with intravenous normal saline may reduce the risk for contrast-associated acute kidney injury in such patients. Although usually reversible, in a small number of patients with underlying renal insufficiency, renal dysfunction may permanently worsen.

  • Iodinated contrast agents can sometimes produce mild side effects including a feeling of warmth, nausea and vomiting, and local irritation (itching and hives) at the site of injection. These side effects usually require no treatment. Occasional idiosyncratic, allergic-like reactions may also include itching, hives, and laryngeal irritation.

  • Asthmatics and those with a history of severe allergies or prior reactions to intravenous contrast have a higher likelihood of contrast reactions (but still very low overall) and may benefit from steroids, diphenhydramine (Benadryl®), and cimetidine (e.g., Tagamet®) administered prior to and/or after injection. Prior shellfish allergy bears absolutely NO relationship to iodinated contrast reactions.

  • In about 0.01 to 0.04% of all patients, severe and idiosyncratic reactions to contrast can occur that can produce intense bronchospasm, laryngeal edema, circulatory collapse, and, very rarely, death (1 in 200,000 to 300,000).

  • In patients with a history of intravenous contrast reactions, oral contrast can be safely administered without the need for premedication.

Oral Contrast in CT Scanning

• For abdominal and pelvic CT imaging, oral contrast may also be administered to define the bowel, although its use has diminished as the quality of CT images has improved. Oral contrast is usually not employed in chest CT scanning unless there is a particular question concerning the esophagus.

• Orally administered contrast, frequently given in doses divided over time to allow earlier contrast to reach the colon while later contrast opacifies the stomach, is utilized for many abdominal CT scans except those performed for trauma , the stone search study , and studies specifically directed toward evaluating vascular structures like the aorta.

• One of two different types of oral contrast may be used. The most widely used is a dilute solution of barium sulfate , the same contrast agent employed in upper gastrointestinal studies and barium enemas. If there is concern for bowel perforation and the possibility that contrast may exit from the lumen of the bowel, an iodine-based, water-soluble contrast is sometimes used (e.g., Gastrografin ®). Contrast may also be introduced rectally to opacify the distal colon more quickly than it would take for orally administered contrast to reach the large bowel or through a Foley catheter to quickly opacify the urinary bladder.

• You will probably not be required to make the decision of when or if to use contrast as the radiologist will usually tailor the examination to best answer the clinical question being asked. That means it is always important to provide as much clinical information as possible when requesting a study.

• Table 13.1 summarizes, in general, when intravenous and oral contrast are utilized for particular problems.

  TABLE 13.1

  CT Scans: When Contrast Is Used

  IV Contrast Used	IV Contrast Usually Not Used
  Chest
  CT-pulmonary angiogram (CT-PA) for pulmonary embolism Evaluation of the mediastinum or hila for mass or adenopathy Detect aortic aneurysm or dissection Evaluation of blunt or penetrating trauma Characterization of pleural disease (metastases, empyema) CT densitometry of pulmonary masses Evaluation of the coronary arteries	Evaluation of diffuse infiltrative lung diseases using HRCT Confirmation of the presence of a lung nodule suspected from conventional radiographs Detect pneumothorax/pneumomediastinum Calcium scoring for the coronary arteries Known allergies to contrast or renal failure

  IV Contrast Used	IV Contrast Usually Not Used
  Abdomen and Pelvis
  Evaluate for the presence of and/or to characterize a mass and to stage or follow-up malignancies Trauma Abdominal pain (e.g., appendicitis) Detect aortic aneurysm or dissection	CT colonography, unless staging a suspected cancer detected by colonoscopy Search for a ureteral calculus
  When Oral Contrast is Used
  Most cases of nontraumatic abdominal pain Inflammatory bowel disease Abdominal or pelvic abscess Locate the site of bowel perforation, including fistulae

  HRCT, High-resolution computed tomography; IV, intravenous.

• Table 13.2 outlines some of the common patient preparations that are generally suggested for a variety of imaging studies. Preparation instructions may vary depending on the facility and individual patient needs. In general, a patient is permitted to take their oral medications with a small sip of water, even when instructions are nothing to eat or drink before the study.

  TABLE 13.2

  Preparations for Imaging Studies

  Study	Before Order Is Placed	Before Start of the Study	After Study Is Done
  CT
  Head CT with or without contrast	Solicit history of previous contrast reaction	Assessment of renal function may be needed before contrast injections	Nothing
  Body CT without contrast	Nothing	No preparation needed	Nothing
  Body CT with oral contrast and/or IV contrast	Solicit history of previous contrast reaction	Assessment of renal function may be needed before contrast injections; oral contrast is given just before study	Nothing
  US
  Upper abdomen , general survey study: aorta, gall bladder, inferior vena cava, liver, pancreas, renal stenosis, retroperitoneal, spleen	Nothing	Nothing to eat or drink for several hours prior to examination	Nothing
  Renal or kidney	Nothing	Patient may be asked to drink a prescribed amount of water to distend bladder 1–2 hours before procedure; patient should not empty bladder	Nothing
  Male or female pelvis or lower abdomen; obstetric/gynecologic US	Nothing	Patient may be asked to drink a prescribed amount of water to distend bladder 1–2 hours before procedure; patient should not empty bladder	Nothing
  Renal transplant , thyroid, and vascular studies	Nothing	No preparation needed	Nothing
  MRI
  Without contrast	Solicit history of working with metal, grinding, welding, or possible metal in eyes (patient may need an orbital x-ray); solicit history of pacemaker, aneurysm clips, neural stimulators, IUD, permanent makeup, cochlear implants, artificial heart valves, pregnancy, metallic fragments, claustrophobia	No preparation needed	Nothing
  With contrast	Solicit history of working with metal, grinding, welding, or possible metal in eyes (patient may need an orbital x-ray); solicit history of pacemaker, aneurysm clips, neural stimulators, IUD, permanent makeup; cochlear implants, artificial heart valves, pregnancy, metallic fragments, claustrophobia	Assessment of renal function may be needed before contrast injections	Nothing
  Barium Study
  Esophagram or video fluoroscopic swallowing examination	Nothing	No preparation needed	Nothing
  Upper gastrointestinal series /small bowel series	Nothing	Nothing to eat or drink for several hours prior to study	Nothing
  Barium enema ; virtual colonography	Nothing	Bowel preparation to cleanse colon before study may consist of oral laxatives, suppositories, fluids	Mild laxative if desired
  Mammography
  Mammogram	Nothing	Patient should not use any deodorant, perfume, powder, ointment, or any other skin products on chest, breast, or under arms on the day of appointment	Nothing
  Nuclear Medicine
  Thyroid uptake and scan	No intravenous, iodinated contrast studies within previous 4–8 weeks	Stop thyroid medications or foods high in iodine content	Nothing
  Bone density	Is the patient pregnant?	No contrast or barium studies for 48 hours prior to the procedure. No food restriction	Nothing
  Bone scan	Is the patient pregnant?	No food restriction	Nothing
  Cardiac treadmill and pharmacologic stress test	Is the patient pregnant?	Nothing to eat or drink for several hours prior to the examination; no caffeine several hours prior to the examination	Nothing