"Abdominal Imaging" — Practice

Question 1

The investigation of choice for suspected renal colic is

Accepted Answer

B

Question 2

Gas outlining both the inner and outer surfaces of the bowel wall on a supine abdominal film is

Accepted Answer

B

Question 3

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

Accepted Answer

MRI without gadolinium. In pregnancy ultrasound is first-line, 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, 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.

Question 4

A patient with acute pancreatitis has CT on the day of admission showing only mild peripancreatic stranding, and the team is reassured. Comment.

Accepted Answer

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.

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.

Question 5

Explain why a gallstone casts a shadow while a simple cyst brightens the tissue behind it.

Accepted Answer

Both are consequences of how much sound energy reaches the tissue beyond the structure.

A gallstone has an acoustic impedance very different from that of bile and soft tissue, so almost all the incident sound is reflected at its surface and very little is transmitted onward. Nothing returns from behind it, and the machine displays that region as a black band, which is acoustic shadowing.

A simple cyst contains fluid, which transmits sound with minimal attenuation. More energy therefore reaches the tissue behind the cyst than reaches tissue at the same depth elsewhere, so those echoes are stronger and the region appears brighter. This is posterior acoustic enhancement.

Both are diagnostic findings rather than artefacts: shadowing identifies a strongly reflective structure such as a stone or calcification, and enhancement confirms that a lesion is truly cystic rather than solid.

Question 6

A 60-year-old cirrhotic has a 3 cm liver lesion enhancing brightly in the arterial phase and becoming hypodense to liver in the portal venous phase. What is it and why does it behave this way?

Accepted Answer

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. The same arterial dependence is exploited therapeutically in transarterial chemoembolisation.

Question 7

A supine film shows dilated peripheral loops with incomplete folds, a caecum measuring 10 cm and a large gas-filled loop arising from the pelvis pointing to the right upper quadrant. Interpret and act.

Accepted Answer

This is sigmoid volvulus causing large bowel obstruction with a caecum at the threshold of perforation.

The peripheral position and incomplete haustral folds identify large bowel rather than small bowel, where valvulae conniventes cross the full lumen. The large gas-filled loop arising from the pelvis and pointing to the right upper quadrant is the coffee bean appearance of sigmoid volvulus, which points away from its origin.

A caecal diameter of 10 cm exceeds the 9 cm threshold and indicates imminent perforation. By Laplace's law the caecum has the largest diameter and therefore the highest wall tension, which is why it perforates first in a closed-loop or distal obstruction.

Management is urgent rather than elective. Resuscitation, decompression by flexible sigmoidoscopy with flatus tube placement where the bowel is viable, and surgery where there is peritonism, failed decompression or evidence of ischaemia. CT confirms the diagnosis and assesses viability if the picture is uncertain, but it must not delay treatment of a caecum at 10 cm.

Question 8

A diabetic on metformin with an eGFR of 25 needs a contrast CT. A colleague says metformin reacts with contrast. Correct this and outline management.

Accepted Answer

The stated mechanism is wrong, though the precaution is right.

Contrast does not interact with metformin pharmacologically. The concern is sequential: iodinated contrast can cause a decline in renal function in susceptible patients, and metformin is renally cleared, so a fall in glomerular filtration allows metformin to accumulate and predisposes to lactic acidosis, which carries high mortality.

The practical consequences follow from that reasoning. The precaution applies to patients with significant renal impairment, such as this one, rather than to every diabetic receiving contrast. Metformin is withheld at the time of the study and for a period afterwards, with renal function rechecked before restarting.

Alongside this, the study itself should be justified, alternatives without iodinated contrast considered, adequate hydration ensured, and the lowest necessary contrast volume used. Contrast-induced nephropathy incidence has been revised downward substantially in recent literature, but the risk is not zero at this level of function.

Question 9

A patient with suspected anastomotic leak after bowel surgery is booked for a barium study. Comment and state what should be done.

Accepted Answer

Barium is contraindicated where a leak or perforation is suspected, and a water-soluble contrast study should be performed instead.

Barium sulphate is inert within the bowel lumen, which is what makes it an excellent mucosal contrast agent, but it is not absorbed and not cleared if it escapes. In the peritoneal cavity it provokes a severe granulomatous chemical peritonitis with adhesion formation and high morbidity, and it cannot be removed adequately at operation.

Water-soluble iodinated contrast is used instead. It is absorbed if it leaks, causing far less peritoneal reaction, and it still demonstrates the leak adequately. Its own caution is that aspiration of hyperosmolar water-soluble contrast can cause pulmonary oedema, so it is avoided where aspiration is likely.

The same principle applies before planned surgery, since residual barium obscures subsequent imaging and complicates the operative field. In practice, CT with oral and intravenous contrast is often the better study, since it demonstrates the leak, any collection and the surrounding anatomy together.

Practice — "Abdominal Imaging"

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