Abdominal pathology


Secondary Metastases to Liver

GI cancers commonly metastasise to the liver, as venous blood returning from the bowel filters through the hepatic portal system first before rejoining the general circulation. Depending on the number and location, it may be possible to resect the affected segment(s) of liver.

Main differentials are simple liver cysts, benign lesions such as haemangioma and liver abscesses. To differentiate between them, the HU value may be helpful. Clinical features (such as fever) are also very useful to know.

Appearance: Irregular heterogenous areas of low attenuation.

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Liver Abscess

  • Bacterial: Polymicrobial. e.g E. coli, Klebsiella, Streptococci, Enterococci
  • Fungal: Candida sp.
  • Other: Amoebic (Entamoeba histolytica), Hydatid cysts

Appearance: Bacterial and fungal abscesses usually appear as multiple clustered lesions. Causative features such as biliary obstruction or diverticulitis or appendicitis may also be seen. Amoebic abscess usually appears as a solitary large lesion.

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Gallstones

Ultrasound is the gold standard investigation. CT may miss some stones. Plain X-ray only shows 10% of stones. MRCP is useful for ductal stones. Locations: Gallbladder, Bile ducts, Small bowel (rare).

Appearance: On ultrasound, single or multiple hyperechoic objects in the gallbladder, with acoustic shadowing. On CT, stones may be hyperattenuating (calcified stones), isoattenuating (mixed), or hypoattenuating (cholesterol stones) with regards to bile. Isoattenuating stones can be missed. On MRI, stones are an area of low signal (dark).

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Cholecystitis

Commonly due to gallstone disease.

Biliary colic: Right upper quadrant pain
Cholecystitis: Right upper quadrant pain + Fever
Ascending cholangitis: Right upper quadrant pain + Fever + Jaundice (Charcot’s triad)

Appearance:

  • Distended fluid-filled gallbladder
  • Thick enhancing (bright) gallbladder wall
  • Fluid surrounding gallbladder
  • ‘Fat stranding’ surrounding gallbladder (fluid density due to oedema in fat)

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Pancreatic Cancer

Most commonly arises from the head of pancreas. Very poor prognosis due to late diagnosis.

Appearance: Soft tissue mass which may cause obstruction and dilatation of the bile and pancreatic duct systems (“double-duct sign”).  Mass may invade into surrounding structures (duodenum, vessels).

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Pancreatitis

Release of pancreatic enzymes causes autodigestion of pancreatic tissue, pancreatic ducts, and the surrounding tissues (fat and blood vessels). Complications include peripancreatic fluid collections (if encapsulated, these are called pseudocysts); abscesses; necrosis of pancreatic tissue; thrombosis of splenic/portal veins; and pseudoaneurysms +/- haemorrhage.

Appearance:

  • Enlarged, oedematous pancreas
  • Fuzzy pancreas borders
  • Fluid around pancreas
  • ‘Fat stranding’ in retroperitoneum (fluid density due to oedema in fat)
  • Areas of non-enhancement indicate necrosis

Pancreatitis on CT


Simple Renal Cysts

Extremely common finding, increasing with age. Up to 50% in age >50. Essentially normal. Not premalignant.

Cysts are ‘complex’ (i.e. not simple) if they have septa or contain high attenuation areas (soft tissue/blood/protein). Complex cysts can be associated with malignancy.

Appearance: Well-defined, round, fluid-containing lesions around the periphery of the kidney (“cortical”) or centrally (“parapelvic”). May be solitary or multiple. Cyst size ranges from a few mm to a few cm.

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Renal Cell Carcinoma

Small RCCs often diagnosed incidentally on CT for other reasons, or during investigations for haematuria. Originates from renal tubular epithelium (not transitional cell!). Risk factors are smoking and obesity. Not associated with simple cysts.

CT is the best first-line investigation. MRI may help with staging.

Appearance: Poorly-defined enhancing mass, extension into renal vein/IVC. Enlarged retroperitoneal lymph nodes.
Metastases to bone are classically lytic and expansile.
Metastases to lung are classically ‘cannonball’ (large and round).

Renal cell carcinoma (RCC) on CT


Renal Stones

99% of stones are visible on CT KUB (done without contrast). CT shows the exact location of calcifications to see whether or not they are located within the renal tract. Any associated obstructing effect causing hydroureter or hydronephrosis can be seen. Inflammation around the ureter may also be seen which could indicate a recently passed stone.

‘Follow’ the ureter from where it leaves the renal pelvis, down along the psoas muscle, anterior to the sacroiliac joint, along the pelvic sidewall, until the ischial spine where it turns medially to reach the bladder.

Appearance: Very small (2-10mm) hyperdense object within the renal calyces, renal pelvis, or within the course of the ureters. Possible associated hydronephrosis and hydroureter.

Scroll through the images below. The path of the left ureter is outlined with a yellow arrow. The two renal stones are indicated with red arrows.

Renal stones on a CT KUB

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Hydronephrosis

Caused by a distal obstruction, e.g. stones, cancers, prostatic hypertrophy, pregnancy, congenital, large blood clot, retroperitoneal fibrosis; or backflow e.g. vesicoureteric reflux. The affected kidney frequently has impaired function. Further radiological tests to assess function include delayed phase contrast CT, intravenous pyelogram, or nuclear medicine tests.

Appearance: Enlarged renal calyces and renal pelvis. Look for visible cause of obstruction.

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CT with contrast - hydronephrosis and hydroureter of left kidney
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Small Bowel Obstruction

Causes of small bowel obstruction include adhesions, herniae and gallstone ileus. The ‘transition point’ is the point at which proximal bowel is dilated, and distal bowel is collapsed. This usually indicates site of obstruction.

Small bowel ileus occurs due to metabolic derangements or post-operatively, with dilatation but no transition point.

Appearance: Dilated small bowel loops proximal to a transition point; Multiple air-fluid levels; Possible ischaemic bowel (a complication).

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Large Bowel Obstruction

Common causes of large bowel obstruction are cancers, diverticulosis and volvulus. An obstruction of the large bowel causes dilatation of proximal large bowel. However, the small bowel may or may not be dilated as well, depending on whether the ileocaecal valve is competent. It is competent in ~70% of people.  A competent valve traps contents in the large bowel, so the small bowel is not dilated. However, this causes the large bowel to dilate much more and much quicker, putting it at a higher risk of perforation.

Appearance: Dilated large bowel loops proximal to a transition point; Multiple air-fluid levels; Dilated small bowel loops (if valve incompetent).

The 3-6-9 rule:

Dilated Small bowel: > 3 cm

Dilated Large bowel: > 6 cm

Dilated Caecum: > 9 cm

 

Large bowel obstruction on CT


Small Bowel Inflammation

Most common cause is Crohn's disease, which affects patchy areas along the entire gut, most commonly the terminal ileum. Common complications are perforation (free air outside the bowel), abscesses, fistulae to other structures and obstructing strictures causing bowel dilatation.

Appearance: “Target sign” involving the small bowel (on CT).

  • Enhancement (bright) of serosa and mucosa due to more blood flow
  • Low attenuation (dark) wall: oedema

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Large Bowel Inflammation

Ulcerative colitis affects a continuous section of large bowel, most commonly the rectum. Infective causes, such as Clostridium difficile infection, are also common. If the entire large bowel is affected, it is called a pancolitis. Both may cause toxic megacolon (seen as marked colonic dilatation).

Appearance: “Target sign” involving the large bowel (on CT)

  • Enhancement (bright) of serosa and mucosa due to more blood flow
  • Low attenuation (dark), thickened wall due to wall oedema
  • Ascites also seen with C Difficile colitis

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Appendicitis

Appendicitis can present at any age. In children, mesenteric adenitis is a common and harmless differential that should be considered. In older people, an underlying caecal carcinoma needs to be considered. The diagnosis can be made on US although the appendix is frequently not visible due to being obscured by overlying bowel gas.

Appearance:

  • Dilated, fluid-filled appendix
  • ‘Fat stranding’ around appendix
  • Possible associated soft tissue mass (lymphoid tissue, cancer); or hyperdense faecolith at base of appendix

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Diverticulitis

Diverticulitis typically occurs in those over 40, on a background of diverticulosis. It may be referred to as 'right-sided appendicitis' as it can present very similarly (classically periumbilical pain initially, later localising to the left iliac fossa).

Terminology

  • Diverticulum: Single small outpouching of the bowel
  • Diverticula: Plural of diverticulum (i.e. more than one diverticulum)
  • Diverticulosis: Multiple diverticula, however no evidence of diverticulitis
  • Diverticulitis: Inflammation or infection of a diverticulum
  • Diverticular disease: When diverticula are associated with symptoms (e.g. abdominal pain)

 

Appearance: Diverticula are multiple small outpouchings, particularly affecting the sigmoid colon. If these are inflamed (diverticulitis), there will be fat stranding, with possible localised perforation or collections.

Diverticulosis on CT

Diverticulosis on CT

 

Diverticulitis on CT

Diverticulosis on CT


Ischaemic Bowel

Bowel ischaemia may be caused by arterial insufficiency (thrombosis or embolism), venous obstruction, or as a result of bowel obstruction.

Appearance: Variety of appearances including hypo- or hyper-density of the bowel wall, bowel distension and congestion of the mesentery. Arterial filling defect.

Ischaemic bowel on CT


Infarcted Bowel

Infarcted and gangrenous tissues will start to produce gas. Therefore, severely infarcted or necrotic bowel may have gas within its walls. The portal venous system drains the bowel walls. As a result, the portal vein fills with gas that drains into the liver. The liver shows a widespread branching pattern of portal venous gas, which is a premorbid sign (this is different from pneumobilia, which looks similar but the air is actually within the intrahepatic biliary ducts, seen when there is a fistula between bowel and gallbladder)

Appearance: Air within bowel walls; Air in hepatic portal venous system.

Bowel infarction on CT


Traumatic Injury

Trauma can be either blunt force or sharp penetrating trauma. Common mechanisms include high speed road traffic accidents, falls from height and assault. Both blunt and penetrating trauma types can cause solid organ, bowel and mesenteric injuries. In high energy trauma, a whole body (head/neck/chest/abdo/pelvis) trauma series CT with two phases of contrast enhancement is usually done.  Injuries to the liver, kidneys and spleen are associated with rib injuries. There may be a laceration with associated haematoma, with extension to involve the vessels at the hilum of the organ. There may also be active bleeding which would require emergent surgery or interventional radiology input to embolise the bleeding vessel.

Appearance: Disruption in continuity of the organ cortex, associated soft tissue injury, haematoma, contrast extravasation indicating active bleeding.

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Gas in Peritoneum

Causes of gas/air within intra- or retroperitoneal spaces:

  • Perforation - of hollow viscus (usually duodenum or sigmoid)
  • Recent laparoscopic surgery - from CO2 insufflation

Appearance: Free air within the peritoneum.  Location of the air depends on the segment of perforated bowel i.e. intra- or retro-peritoneal. Air can extend into other spaces e.g. pneumomediastinum. Perforation of the rectum - air in mesorectum (mesentery surrounding rectum).

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Gas in Bladder

Causes of gas within the urinary bladder:

  • Catheterisation - recent or current
  • Fistula - between bladder and bowel

A fistula would allow infecting organisms to easily colonise the bladder leading to frequent UTIs and pyelonephritis.

Appearance: Gas within the bladder, forming an air-fluid level with the urine inside the bladder.

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Abdominal Aortic Aneurysm

Focal dilatation of the abdominal aorta. Two-thirds of patients with ruptured AAAs die before reaching hospital. Of those who make it to emergency surgery, half die. Ultrasound used for diagnosis and screening. CT used for suspected leak, but this must not delay emergency surgery.

Screening for AAA is offered to all men when they turn 65. It is a quick ultrasound scan to measure the abdominal aorta, done in the community.

  • If the test is negative (< 3 cm), there will be no further recall scans, unlike other screening programmes
  • If the test is positive (> 3 cm), further action depends on the diameter of the aneurysm: 3.0 - 4.4 cm = repeat in 1 year; 4.5 - 5.4 cm = repeat in 3 months; > 5.5 cm = immediate referral to vascular surgeons to consider elective repair

 

Appearance: Aneurysm wall usually calcified. If ruptured, free blood (higher density than fluid e.g. in stomach/GB) with possible contrast leak. Mural thrombus common, seen as circumferential low density area.

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Gynaecology

The preferred modality for initial diagnosis of a suspected gynaecological pathology is ultrasound (transabdominal or transvaginal).

Uterine fibroids can be very large and cause pain if they degenerate. Endometriosis may present as ovarian masses, partially cystic, or as deposits anywhere within the abdominal cavity.

Gynaecologic malignancies are staged with MRI & CT. Ovarian cancer tends to present late, frequently with spread to the peritoneum and omentum by diagnosis.

A ‘Krukenberg tumour’ refers to a metastatic deposit in the ovary which originated from a primary elsewhere within the peritoneal cavity.

Appearance: Uterine fibroids can calcify and are seen as rounded masses with calcific rims. MRI can show the blood within endometriotic deposits on ovaries and the rectovaginal space.

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Peritoneal and Omental Disease

Soft tissue nodularity and thickening of the omentum and peritoneum can be seen in either metastatic disease (usually from gastric, pancreatic, colonic or ovarian carcinomas) or from infections such as tuberculosis. There may also be nodularity on the serosal surface of the bowel, and in metastatic disease in particular, the bowel may become encased in widespread malignant tissue. Ultrasound or CT guidance can be used to biopsy the omentum to identify the primary source.

Appearance: Nodular thickening of omentum deep to abdominal wall (“cake”) and peritoneal nodularity. Necrotic lymph nodes are a feature of TB.

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Ascites

Ascites is seen in chronic liver disease, heart failure and abdominopelvic malignancies. It presents as abdominal distension.

It can be seen easily on ultrasound and CT.
Ultrasound guided paracentesis/drainage can be carried out. A sample of ascitic fluid can be obtained for laboratory testing.

Appearance: Depending on the quantity, it is usually seen as fluid around the liver and spleen, surrounding the bowel, and in the pelvic cavity. It travels to dependent locations, i.e. the left and right paracolic gutters (folds of peritoneum) posteriorly.

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