Abdominal and Skeletal Imaging

Radiology — Abdominal and Skeletal Imaging, NMC MBBS licence examination syllabus (Nepal Medical Council).

Abdominal and Skeletal Imaging

A radiograph answers the question you asked it. The commonest error is not misreading the film — it is reading it as though it could answer a question it was never able to.

NMCRD01 covered choosing a modality and reading a chest film. This chapter covers the abdominal and skeletal films a candidate is handed most often, and the three findings that change what happens next.

Gas outside the bowel

Gas belongs inside bowel. A rim of gas outside it has escaped through a hole, and in a patient with sudden severe abdominal pain and a rigid abdomen that is a surgical emergency. NMCSU01 covers the clinical picture.

The practical point is about how the film is taken. Free gas rises, so it collects under the diaphragm — and it is only visible there if the patient has been upright long enough for it to gather. An erect chest film shows it; a supine abdominal film can miss it entirely.

Which gives the rule that matters: a normal supine film does not exclude a perforation. If the clinical suspicion is real, the imaging must be the kind that could show the finding — or the question has not actually been asked.

Obstruction: which bowel, and where

Dilated bowel on a plain film can usually be assigned to small or large bowel by two features: where the loops sit, and how far the lines across them extend.

Small bowel lies centrally, with many loops and with lines that cross the full width of the bowel. Beyond the obstruction the colon is typically empty of gas.

Large bowel sits around the periphery of the film, with fewer and larger loops, and the lines cross only part of the width.

💡 The film suggests the level; the history and examination suggest the cause. Previous abdominal surgery, a hernia, or a change in bowel habit each point somewhere different — and examining the hernial orifices in a patient with obstruction is the step most often skipped and most likely to give the answer at the bedside. NMCSU04 covers hernia.

A normal film is not a normal patient

Some conditions are invisible early, and knowing which ones prevents false reassurance.

Some fractures do not appear on the first film. The scaphoid is the classic example: a wrist injury with tenderness in the anatomical snuffbox is treated as a fracture despite a normal radiograph, because missing it risks the blood supply to the bone. NMCOR01 covers the consequence.

Early bone infection shows nothing for some time. Bone must lose a considerable amount of mineral before the change is visible on a plain film, so an early normal film in a child with bone pain and fever does not exclude osteomyelitis.

The general rule: if the patient and the film disagree, believe the patient. A normal image excludes what that image is capable of showing, at the time it was taken — no more. The next step is usually a different modality or a repeat later, not reassurance.

Who needs particular care before irradiation

Ask every woman of reproductive age whether she could be pregnant before abdominal or pelvic imaging. It is a question that takes seconds and it is the one most often omitted in a busy department.

Children are more sensitive to radiation and have longer to live with its effects, so ultrasound is preferred wherever it can answer the question. NMCRD01 covers modality choice in more detail.

🩺 The balancing point matters as much as the caution: an urgent indication still outweighs the risk. Delaying necessary imaging in a seriously ill patient — including a pregnant one — is itself a harm, and often a larger and more immediate one. The purpose of asking is to choose the right test and to shield appropriately, not to avoid imaging that is genuinely needed.

Clinical reasoning: four situations

Sudden severe abdominal pain with a rigid abdomen; the supine film looks normal. The film cannot exclude free gas. An erect chest film, or cross-sectional imaging, is what could answer the question.

Distended abdomen, vomiting, and central loops with lines crossing the full width. Small bowel obstruction. Examine the hernial orifices and ask about previous surgery.

A fall on an outstretched hand with snuffbox tenderness and a normal radiograph. Treat as a scaphoid fracture and arrange follow-up imaging. The normal film does not exclude it.

A young woman needs abdominal imaging for possible appendicitis. Ask about pregnancy, and consider whether ultrasound can answer the question first.

Commonly confused

A normal film and no disease. The film excludes what it is capable of showing, when it was taken.

Supine and erect films. They answer different questions; free gas needs the patient upright.

Small and large bowel. Central with full-width lines, against peripheral with partial lines.

Caution about radiation and avoidance of imaging. Caution means choosing well and shielding; it does not mean withholding a test the patient needs urgently.

Rapid revision

Free gas rises — an erect film shows it, a supine film may not.

Small bowel: central, lines across the full width. Large bowel: peripheral, lines part way.

Examine the hernial orifices in bowel obstruction.

Scaphoid tenderness with a normal film — treat as fractured.

Early osteomyelitis is invisible on plain film.

Ask about pregnancy; prefer ultrasound in children — but do not delay urgent imaging.

Syllabus points

Gas outside the bowel indicating perforation
Free gas rising, and why an erect film is needed to demonstrate it
Why a normal supine film does not exclude perforation
Small bowel obstruction — central loops, lines crossing the full width, gasless colon beyond
Large bowel obstruction — peripheral loops, fewer and larger, lines crossing part way
The film suggesting the level while the history suggests the cause
Examining the hernial orifices in a patient with obstruction
Fractures invisible on the first film, with the scaphoid as the classic example
Treating snuffbox tenderness as a fracture despite a normal radiograph
Early osteomyelitis showing no plain-film change until mineral is lost
Believing the patient when the clinical picture and the film disagree
Asking every woman of reproductive age about pregnancy before abdominal imaging
Children's greater radiation sensitivity and the preference for ultrasound
Why an urgent indication still outweighs radiation risk
Choosing imaging capable of answering the question actually being asked
A normal image excluding only what that image can show, at that time

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