CT is the most useful imaging modality in acute abdomen.
The vast majority of intra-abdominal pathologies can be seen on CT.
However, this involves a radiation dose, so care should be taken in young patients.

Abdominal X-rays are useful in diagnosing bowel obstruction, and erect chest x-rays can indicate perforation of viscus.

Ultrasound is most useful to detect gallstones, cholecystitis, and hydronephrosis. It can also detect pathology of the pancreas and appendicitis, although these areas are frequently poorly visualised due to overlying bowel gas. As it does not involve radiation, it is frequently used first line in children and young adults.

Pain from retroperitoneal structures (pancreas, kidneys, duodenum) tends to radiate to the back.

Suspected GI cancers are most commonly investigated with endoscopy. If a patient is unfit for colonoscopy, a CT ‘virtual’ colonoscopy can be done, with a 3D reconstruction of the intraluminal view. The disadvantage is that biopsies cannot be done.

Contrast studies (swallow/meal/follow-through/enema) are also frequently performed with suspected obstructing lesions. Barium is usually used, but if there is possible perforation, Gastrografin (water soluble) is used instead. Barium is highly irritant and will cause inflammation and fibrosis if it leaks out. A tumour in the wall will cause a ‘filling defect’ where it displaces the contrast. A circumferential wall lesion is seen as an ‘apple core lesion’.

Once cancer is confirmed, a staging CT (i.e chest/abdo/pelvis with contrast) is performed to inform treatment options and prognosis.

The best modality to detect renal stones, particularly in first presentation of renal colic, is CT KUB (kidneys ureters bladder).

X-ray KUB may not detect small stones. Also, phleboliths (calcification within veins) are frequently found in the pelvis and are difficult to differentiate from renal stones on X-ray. CT KUB can show whether or not these are located within the renal tract.

Painful jaundice is investigated with USS to look for obstructing stones in the gallbladder and bile ducts. If bile duct dilatation is seen, but stones are not seen, the next step is MRCP (magnetic resonance cholangiopancreatography) which is better at detecting ductal stones.

Painless jaundice is more concerning and would warrant an urgent CT with contrast for pancreatic cancer.

Gynaecological disorders are usually imaged with ultrasound as first line. This avoids radiation exposure, and the structures are viewed better than on CT. Transvaginal ultrasound is generally better than transabdominal ultrasound. Although it is more invasive, it provides better image quality, and is not affected by patient habitus.

Testicular disorders are also generally imaged with ultrasound, which can easily differentiate between a cystic lesion and a solid lesion.

CT as well as ultrasound can be used by interventional radiologists to perform guided biopsies. The choice between CT and ultrasound is generally made by considering the proximity of the lesion of interest to the body surface, and whether there are any structures in the way. Preferred spots to take biopsies are immobile, peripheral structures such as lymph nodes, peritoneal lesions, and omental ‘cake’.

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