Abdominal Imaging

Abdominal imaging is usually taught as a catalogue of signs, which is the wrong order. The sign only matters once the correct test has been performed, and the commonest clinical error is choosing the wrong modality rather than misreading the right one.

Two questions decide the choice.

What tissue property distinguishes the thing you are looking for? Ultrasound distinguishes fluid from solid and detects stones by their acoustic shadow. CT distinguishes tissues by density and is unmatched for gas, fat, calcium and blood. MRI distinguishes tissues by proton behaviour, which is why it is superior for the biliary tree, for characterising liver lesions and for soft tissue.

What is the cost of the information? Radiation, contrast nephropathy, time, availability and expense are all real, and in a young patient or in pregnancy the radiation cost dominates the decision.

A third idea prevents most of the diagnostic errors. Free gas, free fluid and dilated bowel are the three findings that change management immediately, and they should be looked for deliberately on every acute abdominal film rather than noticed incidentally.

1. Choosing the Modality

Ultrasound is operator-dependent and defeated by gas and by body habitus, which is its principal limitation and the reason a normal scan in a difficult patient does not exclude disease.

Non-contrast CT is used for renal stones because contrast obscures them, since a stone and contrast in the collecting system are both dense.

2. The Acute Abdominal Radiograph

The plain film has a shrinking role, but three findings still matter.

Free gas under the diaphragm on an erect chest radiograph indicates a perforated viscus. It requires an erect film with the patient upright for several minutes, and it is absent in a substantial proportion of perforations, so a normal film does not exclude one.

Rigler sign, gas outlining both sides of the bowel wall, indicates a large pneumoperitoneum on a supine film.

Dilated bowel distinguishes obstruction by pattern. Small bowel lies centrally with valvulae conniventes crossing the full width, and is dilated above 3 cm. Large bowel lies peripherally with haustra that do not cross the full width, and is dilated above 6 cm, or 9 cm at the caecum.

The 3-6-9 rule is the practical version, and caecal dilatation beyond 9 cm indicates imminent perforation.

Volvulus produces characteristic shapes. Sigmoid volvulus gives a coffee bean sign arising from the pelvis and pointing to the right upper quadrant. Caecal volvulus points to the left upper quadrant.

Two further plain film signs are worth carrying. Thumbprinting, indentation of the bowel lumen by thickened oedematous mucosal folds, indicates colitis of any cause including ischaemic colitis. And the absence of gas throughout the abdomen, a gasless abdomen, occurs in high obstruction with persistent vomiting and in severe fluid-filled bowel.

3. The Liver Lesion

Characterising a focal liver lesion is a matter of watching enhancement over time, which is why single-phase imaging is inadequate.

Arterial enhancement with portal venous washout is essentially diagnostic of hepatocellular carcinoma in a cirrhotic liver, and it is one of the few situations in medicine where imaging alone establishes a malignant diagnosis without biopsy.

The reason is vascular. Normal liver receives most of its blood from the portal vein, while hepatocellular carcinoma recruits an arterial supply, so it enhances early and then loses contrast as the surrounding liver enhances late.

4. The Acute Abdomen on CT

Certain patterns recur and are worth recognising as units.

Appendicitis: a dilated, non-compressible appendix above 6 mm with wall thickening, periappendiceal fat stranding and sometimes an appendicolith.

Diverticulitis: sigmoid diverticula with wall thickening and pericolic fat stranding, and the presence of an abscess or free gas changes management.

Acute pancreatitis: an enlarged gland with peripancreatic fat stranding. CT is not indicated in the first days unless the diagnosis is uncertain or the patient deteriorates, because necrosis takes 72 hours or more to become apparent and early scanning gives false reassurance.

Bowel ischaemia: the findings are frequently subtle and the diagnosis is often late. Look for bowel wall thickening or paradoxical thinning, absent mural enhancement, pneumatosis intestinalis, portal venous gas and mesenteric vessel occlusion.

Absent mural enhancement is the most specific sign of infarcted bowel, and it is easily overlooked because it is an absence rather than a presence.

5. Ultrasound in Practice

Ultrasound answers more abdominal questions in India than any other modality, because it is available, portable and free of radiation, so it is worth understanding what it can and cannot do.

How the image is formed

The probe emits pulses and listens for echoes returning from tissue interfaces. The greater the difference in acoustic impedance between two tissues, the stronger the echo, which is why fat and fluid interfaces are conspicuous.

Three consequences follow directly and explain almost every ultrasound sign.

Fluid transmits sound almost perfectly, so it appears black and the tissue behind it appears unusually bright. That is posterior acoustic enhancement, and it identifies a structure as cystic rather than solid.

Stone and bone reflect almost all the sound, so nothing returns from behind them and a black band extends deep to the structure. That is acoustic shadowing, and it is what identifies a gallstone.

Gas scatters sound chaotically, which is why bowel gas ruins the image and why the pancreas and retroperitoneum are so often obscured.

The high-yield findings

Gallstones: mobile, echogenic foci with posterior acoustic shadowing. Acute cholecystitis adds wall thickening above 3 mm, pericholecystic fluid and a sonographic Murphy sign, meaning maximal tenderness under the probe over the gallbladder.

Biliary obstruction: dilated intrahepatic ducts running alongside portal vein branches, producing the parallel channel sign, with a common bile duct above about 6 mm in a patient with a gallbladder.

Hydronephrosis is graded by the degree of pelvicalyceal dilatation, and its absence does not exclude obstruction in the first hours before the system has distended.

Free fluid collects in dependent spaces, which is why the FAST examination looks at Morison pouch, the splenorenal recess, the pelvis and the pericardium.

Doppler adds flow information without contrast, which is why it is used to assess portal vein patency and direction, to detect testicular and ovarian torsion, and to distinguish a vascular from an avascular structure at the bedside.

6. Contrast: Benefit and Risk

Iodinated contrast improves detection of vascular structures, inflammation and tumour, and many CT questions cannot be answered without it.

Two risks dominate.

Contrast-induced nephropathy is a rise in creatinine after intravascular contrast. Its incidence has been revised downward substantially in recent literature, and much of what was attributed to contrast reflects the underlying illness, but the risk is not zero in advanced renal impairment.

Contrast reactions range from mild urticaria to anaphylaxis, and previous reaction to iodinated contrast is by far the strongest risk factor. Shellfish allergy is not, despite the persistent belief, since the reaction is not to iodine content.

Metformin is withheld around contrast administration in patients with significant renal impairment, not because contrast interacts with metformin but because contrast-induced renal impairment could allow metformin to accumulate and cause lactic acidosis.

Gadolinium for MRI carries a different concern, namely nephrogenic systemic fibrosis in severe renal impairment, which has become rare with modern agents but governs practice.

7. Radiation and the Pregnant Patient

The dose ordering is worth carrying, because it drives modality choice in young patients.

Ultrasound and MRI involve no ionising radiation at all. A chest radiograph delivers a very small dose, comparable to a few days of natural background exposure. An abdominal or pelvic CT delivers a dose several hundred times greater than a chest radiograph, and interventional and nuclear medicine studies vary widely depending on the procedure.

The principle is ALARA, as low as reasonably achievable, and it operates through justification, meaning the study should change management, and optimisation, meaning the lowest dose that answers the question.

In pregnancy the priorities are clear. Ultrasound is first-line. MRI is used without gadolinium where ultrasound is inconclusive, since gadolinium crosses the placenta. CT is not absolutely contraindicated, and a necessary CT should not be withheld from a pregnant woman with a serious diagnosis in question, because the risk of missing appendicitis or pulmonary embolism exceeds the fetal risk from a single properly justified study.

8. Contrast Studies and Interventional Work

Fluoroscopic studies

Barium studies have narrowed in use but retain characteristic appearances that are examined.

Barium swallow shows the bird beak of achalasia, the corkscrew oesophagus of diffuse spasm, and the shouldered irregular stricture of carcinoma against the smooth tapering stricture of a benign cause.

Barium meal and follow-through demonstrate the string sign of a narrowed terminal ileum in Crohn disease, alongside skip lesions and fistulae.

Barium enema shows the apple core lesion of colonic carcinoma and the lead pipe colon of chronic ulcerative colitis, in which haustra are lost.

Barium is contraindicated where perforation is suspected, because barium in the peritoneum causes a severe chemical peritonitis. A water-soluble contrast agent is used instead, and the same principle applies before surgery.

Interventional radiology

Several procedures now replace operations and appear in management questions.

Percutaneous drainage of an abscess or collection avoids laparotomy in many patients, and its success depends on a safe window and on drainable rather than solid contents.

Transarterial chemoembolisation treats hepatocellular carcinoma by exploiting the same arterial supply that makes the tumour enhance early, delivering chemotherapy and occluding the feeding vessel while sparing portal-fed liver.

Percutaneous transhepatic biliary drainage relieves obstruction when endoscopic access fails, and uterine artery embolisation treats fibroids and controls postpartum haemorrhage.

Embolisation for gastrointestinal bleeding is used when endoscopy fails to control it, with the same logic of reaching the lesion through the vessel that supplies it.

9. Worked Examples

Example 1. A 32-year-old woman at 20 weeks of pregnancy has right iliac fossa pain. Ultrasound is inconclusive because of bowel gas. What next?

MRI without gadolinium. In pregnancy, ultrasound is the first-line investigation, and where it fails to answer the question, MRI provides excellent soft tissue detail without ionising radiation and has become the standard next step for suspected appendicitis in pregnancy.

Gadolinium is avoided because it crosses the placenta and is excreted into amniotic fluid, where its residence time and dissociation behaviour are not fully characterised.

If MRI is unavailable, a CT should not be refused on principle. The fetal risk from a single justified abdominal CT is small in absolute terms, whereas a missed and perforated appendicitis threatens both mother and fetus, so the comparison is between two risks rather than between risk and safety.

Example 2. A 60-year-old cirrhotic patient has a 3 cm liver lesion that enhances brightly in the arterial phase and becomes hypodense to liver in the portal venous phase. What is it, and why does it behave this way?

Hepatocellular carcinoma. In a cirrhotic liver, arterial hyperenhancement followed by portal venous washout is considered diagnostic, and this is one of the few circumstances where imaging alone establishes a malignant diagnosis without biopsy.

The behaviour follows the blood supply. Normal hepatic parenchyma receives roughly three-quarters of its blood from the portal vein, so it enhances relatively late. Hepatocellular carcinoma undergoes neoangiogenesis and derives its supply almost entirely from the hepatic artery, so it enhances intensely and early. As portal venous contrast then reaches the background liver, the tumour, which has no comparable portal inflow and rapid washout through its arterialised bed, becomes relatively hypodense.

A haemangioma would behave quite differently, with peripheral nodular enhancement filling in centripetally and retaining contrast on delayed images.

Example 3. A patient with abdominal distension has a supine film showing gas outlining both the inner and outer surfaces of the bowel wall. What is the sign and what does it mean?

Rigler sign, indicating a large pneumoperitoneum.

Normally only the inner surface of the bowel wall is outlined by luminal gas, so the wall itself is not visible as a distinct line. When free gas accumulates in the peritoneal cavity, it outlines the serosal surface as well, and the bowel wall becomes visible as a line with gas on both sides.

Its importance is that it is visible on a supine film. The classic sign of pneumoperitoneum, free gas under the diaphragm, requires an erect chest radiograph with the patient upright for several minutes, which is often impossible in an unwell or immobile patient.

The finding indicates a perforated viscus and is a surgical emergency. CT is more sensitive than any radiograph and will additionally localise the perforation, but Rigler sign on a film already taken should prompt immediate action rather than further imaging first.

Example 4. A patient with acute pancreatitis is scanned with CT on the day of admission, which shows only mild peripancreatic stranding. The team is reassured. Comment.

The reassurance is unfounded because the scan was performed too early.

Pancreatic necrosis takes at least 72 hours to become radiologically apparent, since it depends on demonstrating absent parenchymal enhancement, and that requires the necrotic process to be established. An early scan therefore underestimates severity systematically and can give false reassurance in a patient who will subsequently deteriorate.

Early CT is not routinely indicated in acute pancreatitis at all. Its legitimate early uses are diagnostic uncertainty, where an alternative surgical diagnosis such as perforation must be excluded, and clinical deterioration.

Severity is assessed clinically and biochemically in the first days, using organ failure and scoring systems, and CT is reserved for around 72 to 96 hours or later, or earlier if the patient worsens.

Example 5. A 45-year-old with flank pain and haematuria is booked for a contrast-enhanced CT to look for a renal stone. Comment.

The scan should be non-contrast.

Renal stones are dense on CT regardless of their composition, including those that are radiolucent on plain radiography such as uric acid stones. Non-contrast CT therefore detects almost all of them with very high sensitivity, and it is quick, requires no preparation and carries no contrast risk.

Giving intravenous contrast is actively counterproductive for this question, because excreted contrast in the collecting system is itself dense and can obscure a stone lying within it, so a stone can be missed on the very study intended to find it.

Contrast is added only when a different question is being asked, such as characterising a renal mass or assessing the vascular anatomy, and in those situations a non-contrast phase is usually acquired first.

Summary

Choosing the wrong modality is a commoner error than misreading the right one.

Ultrasound separates fluid from solid and shows stones by acoustic shadowing.

CT distinguishes gas, fat, calcium and blood by density.

MRI is superior for the biliary tree, liver characterisation and soft tissue.

Ultrasound is operator-dependent and defeated by gas and body habitus.

Renal stone CT is performed without contrast, because contrast obscures stones.

Free gas, free fluid and dilated bowel change management immediately.

An erect chest film misses a substantial proportion of perforations.

Rigler sign shows pneumoperitoneum on a supine film.

Small bowel is central with valvulae crossing fully; large bowel is peripheral with haustra that do not.

The 3-6-9 rule: 3 cm small bowel, 6 cm colon, 9 cm caecum.

Sigmoid volvulus points to the right upper quadrant; caecal volvulus to the left.

Arterial enhancement with portal venous washout in cirrhosis is diagnostic of hepatocellular carcinoma.

Haemangioma fills in centripetally and retains contrast.

Appendicitis on CT: appendix above 6 mm, non-compressible, with fat stranding.

CT in pancreatitis is deferred to 72 hours or more unless the patient deteriorates.

Absent mural enhancement is the most specific sign of infarcted bowel.

Metformin is withheld around contrast because of the lactic acidosis risk if renal function falls.

Gadolinium is avoided in severe renal impairment and in pregnancy.

Ultrasound first in pregnancy, then MRI without gadolinium, and a justified CT is not withheld.