12.1 Abdomen & Acute Abdominal Series
Key Takeaways
- Supine AP abdomen (KUB) is typically centered at the iliac crest to include kidneys through the pubic symphysis on expiration.
- Upright AP or left lateral decubitus uses a horizontal beam and is centered about 5 cm (2 in) above the crest to include the diaphragm for free intraperitoneal air.
- Left lateral decubitus (left side down) is preferred for free air in non-ambulatory patients so gas rises over the liver at the right flank.
- Acceptable exposure should demonstrate psoas margins and kidney outlines when habitus allows, plus sharp gas–soft-tissue interfaces without motion.
- An acute abdominal series pairs supine abdomen with upright or decubitus views (and often chest) to evaluate obstruction, levels, and pneumoperitoneum.
12.1 Abdomen & Acute Abdominal Series
Quick Answer: A supine AP abdomen (KUB) is centered at the iliac crest to include kidneys through bladder/pubic symphysis. Upright AP or left lateral decubitus uses a horizontal beam and is typically centered about 5 cm (2 in) above the crest to include the diaphragm for free intraperitoneal air. An acute abdominal series pairs chest and/or upright/decubitus abdomen with the supine view so gas patterns, fluid levels, and pneumoperitoneum can be evaluated.
General radiography of the digestive system is a medium secondary-weight clinical procedure area on the CAMRT Radiological Technology blueprint (May 2024). Urinary plain-film work and pathology signs (obstruction, free air, ascites) appear in both RTR.4 procedure and RTR.6 image-analysis items. Fluoro-heavy GI volume has declined in many Canadian departments as CT and endoscopy absorb work, but plain abdomen technique, acute series rationale, and critique criteria remain high-yield for entry-to-practice RTRs.
Scope of the Abdominal Radiograph
The plain abdomen radiograph (often still called a KUB—kidneys, ureters, bladder—when ordered for urinary stone workup) demonstrates:
- Soft-tissue outlines of liver, spleen, kidneys, and psoas muscles when technique and patient habitus allow
- Gas pattern in stomach, small bowel, and colon
- Calcifications, surgical clips, foreign bodies, and many urinary calculi (some stones are non-opaque)
- Bony pelvis and lumbar spine included in the field
- Support devices (NG tubes, enteric tubes, surgical drains) in anatomic context
It is not a substitute for CT when the clinical question needs organ-level soft-tissue contrast, but it remains a first-line, low-cost tool for many emergency and inpatient pathways.
Standard Supine AP Abdomen (KUB)
Patient position and setup
| Element | Typical practice |
|---|---|
| Position | Supine, MSP perpendicular to IR; arms away from field; legs extended or slightly flexed for comfort if protocol allows |
| SID | Typically 100–115 cm (department Bucky/table standard) |
| CR | Perpendicular to IR at the level of the iliac crest, mid-sagittal |
| Collimation | Lateral skin margins; include pubic symphysis inferiorly and as much of the upper abdomen as the IR allows; kidneys must be included when the order is a true KUB |
| Markers | Correct R/L; recumbent annotation if required |
| Respiration | Expiration hold (diaphragm rises, abdominal contents less compressed by inspiration) |
| Shielding | Gonadal shielding per department policy and exam intent—never compromise diagnostic anatomy needed for the ordered study |
Why crest centering for supine KUB? Centering at the iliac crest balances inclusion of the kidneys (upper poles roughly T12–L1) with the bladder and pubic symphysis on a standard 35 × 43 cm IR for average adults. Very tall patients may need a second image (upper and lower abdomen) rather than elongating the field with poor collimation habits.
Evaluation criteria (supine AP)
| Criterion | Acceptable appearance |
|---|---|
| Coverage | Diaphragm may not be fully included on a crest-centered KUB; symphysis and lateral abdominal walls included; kidneys included when ordered as KUB |
| Rotation | Spinous processes midline in vertebral bodies; symmetric iliac wings/obturator foramina; pedicles equidistant |
| Motion | Sharp bowel gas margins and bone trabeculae |
| Exposure | Visualize psoas margins, kidney outlines (when body habitus allows), lumbar spine, and soft-tissue–gas interfaces without clipping extremes |
| Artifacts | Remove removable objects (ECG leads, buttons, thick folds of gown) |
Exposure critique tip (RTR.6): Underexposure loses soft-tissue and stone contrast; overexposure may blacken gas-filled loops and hide free air under the diaphragm on uprights. Digital systems still require correct technique and processing awareness—bright windows cannot invent anatomy that was never recorded with adequate SNR.
Upright AP Abdomen
The upright abdomen is the workhorse view for air–fluid levels and many free-air assessments when the patient can stand or sit.
| Element | Typical practice |
|---|---|
| Position | Erect AP (or PA if protocol/department preference and patient can face IR) |
| CR | Horizontal beam; centered approximately 5 cm (2 in) above the iliac crest, mid-sagittal, to include diaphragm |
| Respiration | Expiration hold after coaching |
| Time upright | Allow the patient to remain upright several minutes before exposure when free air is the clinical question so gas can rise under the diaphragm |
Why 2 inches above the crest? The diagnostic goal shifts from “full KUB coverage of kidneys-to-bladder” to upper abdomen and hemidiaphragms. Free intraperitoneal air collects under the diaphragm; if the image is crest-centered like a KUB, the critical free-air space may be cut off.
Evaluation criteria (upright)
- Both hemidiaphragms included when free air is the indication
- Air–fluid levels demonstrated in dilated loops when obstruction is present
- No rotation; markers and erect annotation correct
- Soft-tissue and gas interfaces visible under the diaphragm
Left Lateral Decubitus Abdomen
When the patient cannot stand or sit, free intraperitoneal air is evaluated with a left lateral decubitus abdomen (patient lying on the left side, horizontal CR).
| Element | Typical practice |
|---|---|
| Position | Left side down; right side elevated; arms up; knees flexed for stability; radiolucent pad under hip/chest as needed |
| Why left side down | Free air rises to the right flank over the lateral aspect of the liver, producing a clean gas–soft-tissue interface; right-side-down would superimpose free air over gastric/fundal gas and confuse interpretation |
| CR | Horizontal beam, mid-coronal plane of the abdomen; include the right hemidiaphragm/flank and as much abdomen as the IR allows (often centered slightly above crest similar to upright goals) |
| Time on side | Prefer several minutes on the left side before exposure so free air migrates |
Decubitus language check: Decubitus means the patient is recumbent with a horizontal beam. Naming is by the side down (left lateral decubitus = left side down). Do not confuse with a recumbent lateral abdomen using a vertical beam, which will not show free air rising against a flank.
Acute Abdominal Series — Rationale
An acute abdominal series (department names vary: “three-view abdomen,” “obstruction series”) is ordered when the clinical question includes bowel obstruction, perforation, or acute abdomen rather than isolated stone workup.
Typical components (confirm local protocol):
- PA or AP chest (upright if possible) — screens for free air under the diaphragm, pneumonia mimicking abdominal pain, and thoracic causes of symptoms
- Upright AP abdomen (or left lateral decubitus if non-ambulatory) — air–fluid levels, free air, gas pattern
- Supine AP abdomen — overall gas distribution, bowel calibre, calcifications, foreign bodies, baseline anatomy
RTR decision-making: Match the series to the order and clinical note. A “KUB for renal colic” is not automatically an acute series. A “rule out free air / obstruction” order is incomplete if only a supine film is obtained when the patient can be positioned for a horizontal-beam view.
Imaging Signs You Must Recognize
Pneumoperitoneum (free intraperitoneal air)
- Upright: Crescent of free air under one or both hemidiaphragms (right side often clearer)
- Left lateral decubitus: Free air between liver and right lateral abdominal wall
- Supine signs (less sensitive, still examinable): Rigler’s (double-wall) sign, football sign in infants, air outlining the falciform ligament — know that horizontal-beam views are preferred for detection
Bowel obstruction / ileus pattern
- Dilated loops with air–fluid levels on upright/decubitus views
- Small-bowel vs large-bowel pattern: central vs peripheral gas, valvulae conniventes vs haustral markings (image analysis language)
- String-of-pearls sign and differential air–fluid levels support mechanical small-bowel obstruction in the right clinical context
- Transition points are often better defined on CT; plain film still triage-relevant
Ascites
- Generalized ground-glass density, central bowel loops, poorly defined solid organ margins, bulging flanks
- Exposure may need adjustment; soft-tissue contrast is reduced compared with a gas-filled abdomen
Other findings
- Calcified gallstones, renal/ureteric calculi, phleboliths, vascular calcification — location and morphology matter for critique comments
- Abnormal gas in biliary tree, portal vein, or bowel wall (pneumatosis) — flag for urgent review pathways per department policy
Bowel Preparation (When Relevant)
Plain acute series usually has no bowel prep — patients are often NPO for surgical reasons, not cathartics. Prep appears more often for elective contrast enema, IVU (where still performed), or barium studies:
| Context | Typical prep themes |
|---|---|
| Acute/emergent abdomen | No cathartic prep; time-critical positioning and horizontal-beam views |
| Elective barium enema / colon studies | Dietary restriction, cathartics, hydration per protocol (see Section 12.2) |
| KUB for stones | Often no prep; avoid obscuring opacities when possible |
| Outpatient “flat plate” | Follow order-specific instructions; clarify with ordering provider if prep would delay urgent care |
Canadian RTR Practice Notes
- Use metric landmarks in documentation when your site does (crest; 5 cm above crest) while remaining fluent with the classic “2 inches above crest” exam phrasing
- Pregnancy: screen per policy before abdomen radiography; shield and justify; document. Prefer ultrasound/MRI pathways when clinically appropriate, but do not refuse a justified, ordered exam without the care team
- Mobile abdomen: same geometric principles—horizontal beam for free air if the patient cannot leave the bed; communicate with nursing about lines, drains, and pain limits
- Communication: explain breath holds, why the patient must stay on the left side, and why an upright chest may be part of an “abdomen” order
Common Errors That Trigger Repeats
- Crest-centering an upright abdomen ordered for free air → diaphragm cut off
- Right lateral decubitus for free air → gas over stomach confuses interpretation
- Insufficient time upright/on side → free air not migrated
- Heavy rotation mimicking asymmetric gas or “mass effect”
- Failure to include pubic symphysis on a KUB ordered for distal ureteric stone
- Exposure that obliterates soft-tissue interfaces under the diaphragm
Master the why of each projection. CAMRT-style items rarely ask only for a centering point in isolation—they pair centering with clinical intent (free air vs KUB coverage) and acceptance criteria (psoas, kidneys, diaphragms, levels).
For a standard supine AP abdomen ordered as a KUB in an average adult, where is the central ray typically centered?
A non-ambulatory patient needs evaluation for free intraperitoneal air. Which projection best applies free-air geometry?
Why is an upright AP abdomen usually centered approximately 5 cm (2 in) above the iliac crest when free air is the clinical question?
Which set of findings best supports acceptable technical quality on a supine abdominal radiograph?