17.1 Contrast Studies

Contrast Studies of the GI and GU Systems

Contrast media turn hollow and vascular structures, which are otherwise nearly invisible on plain film, into diagnostic images by filling their lumen with a radiopaque agent. This section teaches the procedures of gastrointestinal (GI) and genitourinary (GU) contrast examinations: which study answers which clinical question, which agent is safe, and the positioning-and-timing sequence for each. The pharmacology of contrast agents (ionic versus nonionic, osmolality, patient screening, and reaction management) belongs to Chapter 4 and is only referenced here. Likewise, routine abdominal (KUB) positioning is taught in Chapter 14; this section builds on it rather than repeating it.

The Barium-versus-Water-Soluble Decision

The most heavily tested contrast decision is barium sulfate versus water-soluble iodinated contrast. Barium sulfate is an inert, non-absorbed suspension that coats mucosa beautifully and is the agent of choice for the intact GI tract. Its danger is leakage: barium that escapes through a perforation or anastomotic leak into the peritoneum causes barium peritonitis and granulomas, a potentially fatal complication.

Therefore, whenever perforation, an acute post-operative leak, or a tracheoesophageal fistula is suspected, a water-soluble iodinated agent (diatrizoate/Gastrografin) is substituted because it is resorbed by the body. The one caveat is aspiration: high-osmolar water-soluble contrast can draw fluid into the lungs and cause pulmonary edema, so a low-osmolar agent is preferred when aspiration is the concern. Screen the chart for prior contrast reactions and renal function before any iodinated study; a documented shellfish allergy is a myth, not a true contraindication.

Esophagram and Upper GI (UGI)

An esophagram (barium swallow) evaluates the pharynx and esophagus for strictures, varices, reflux, and swallowing dysfunction. Thin barium demonstrates the lumen and mucosal relief; thick barium and marshmallow/barium tablets localize obstruction. The key projection is the RAO 35-40 degrees, which throws the barium-filled esophagus between the thoracic vertebral column and the heart where it is free of superimposition; AP, lateral, and LPO complete the series. Reflux is provoked with the Valsalva maneuver, the water test, or the Trendelenburg position.

The upper GI (UGI) series follows barium through the stomach and duodenum after an 8-hour NPO fast. On the RAO of the stomach, the pylorus and duodenal bulb fill with barium in the average (sthenic) patient, making RAO the single most useful stomach projection. PA demonstrates the barium-filled body and greater curvature; AP places air in the fundus and barium retrograde into the fundus for double-contrast; the LPO opens the duodenal bulb and C-loop. Body habitus shifts stomach position and shape (high and transverse in the hypersthenic patient, low and J-shaped in the asthenic patient), so centering must be adjusted accordingly.

Small Bowel and Barium Enema

A small bowel follow-through (SBFT) continues after the UGI: PA images are taken at timed intervals of roughly 15-30 minutes until barium reaches the ileocecal valve, usually within about two hours. An enteroclysis (a more invasive double-contrast small-bowel study) instills barium and methylcellulose through a duodenal tube for higher detail.

The barium enema (BE) studies the colon retrograde through a rectal tip. A single-contrast BE (SBE) fills the lumen with barium; a double-contrast BE (DBE) uses high-density barium plus air to coat the mucosa for polyp detection. Thorough cleansing prep is essential. The flexure rule is high-yield: the LAO opens the right (hepatic) colic flexure and the RAO opens the left (splenic) colic flexure; lateral decubitus projections with a horizontal beam demonstrate the up-side wall in double-contrast studies.

Genitourinary (GU) Studies

The intravenous urogram (IVU/IVP), also called excretory urography, evaluates renal function and the collecting system after an IV bolus of iodinated contrast. Contraindications include acute renal failure, an elevated creatinine/low eGFR, multiple myeloma, dehydration, and known severe contrast reaction; the risk is contrast-induced nephrotoxicity. The sequence is: a scout KUB centered at the iliac crests to check technique and bowel prep, a nephrogram at about 30 seconds to 1 minute (contrast in the functional renal parenchyma), then timed images of the renal pelves and ureters (commonly 5, 10, and 15 minutes), a post-void image, and sometimes upright or ureteric-compression views.

Retrograde pyelography is a non-functional urologic study: the urologist advances a catheter through the ureters at cystoscopy and injects contrast directly, so no IV agent and no renal function are required. Cystography fills the bladder retrograde by catheter for AP and oblique images, while the voiding cystourethrogram (VCUG) images the bladder and urethra during voiding to demonstrate the urethra and vesicoureteral reflux, a common pediatric indication.

Arthrography and Myelography

Arthrography injects the joint (most often knee or shoulder) under fluoroscopic guidance with iodinated contrast, sometimes combined with air for a double-contrast study, to show menisci, ligaments, and the joint capsule; MRI has largely replaced it. Myelography injects nonionic water-soluble contrast into the subarachnoid space by lumbar or cervical puncture to demonstrate cord and nerve-root compression; cross-table lateral and decubitus images are taken with careful tilt-table (Trendelenburg) control to steer contrast, and CT usually follows. Hysterosalpingography (HSG) instills contrast through the cervix to evaluate the uterine cavity and fallopian-tube patency.

Contrast Study Reference Table

Common Traps

Do not choose barium when the stem says "perforation," "free air," or "post-operative leak"; the answer is water-soluble iodinated contrast every time. Do not confuse the flexure obliques (LAO = hepatic/right, RAO = splenic/left). And remember that a VCUG images during voiding while a plain cystogram images the filled, static bladder.