• A chronic progressive transmural granulomatous inflammatory bowel disease • There are typically discontinuous (‘skip’) lesions with asymmetrical bowel wall involvement • It can affect any part of the GI tract – however it almost always affects the terminal ileum (in 95% of cases) • Inflammatory polyps (pseudopolyps): small, discrete round filling defects • Thickened valvulae conniventes: they can also be distorted, blunted or flattened (they are due to hyperplasia of the lymphoid tissue which causes an obstructive lymphoedema) • Fistulae formation: this can involve adjacent loops of ileum, caecum or sigmoid colon • Bowel wall sacculation: this is secondary to fibrosis within healing eccentric ulcers – NECT: the stratified appearance is due to a thickened muscularis mucosae and submucosal fatty infiltration – CECT: the stratified appearance is due to acute inflammation with submucosal oedema and enhancement of the mucosa and muscularis propria • Although any part of the GI tract can be involved, the ileum is the commonest involved site (esp. the terminal ileum and ileocaecal junction) • Discrete transverse and circumferential ulcers, mucosal fold thickening and strictures are the main radiological features • Ulcerative, hypertrophic or fibrotic forms are described: • Fleischner sign: a thickened patulous ileocaecal valve seen in conjunction with a narrowed terminal ileum • Stierlin’s sign: this is due to rapid emptying of contrast through a gaping incompetent ileocaecal valve and into a conically contracted caecum • A submucosal mesenchymal (non-epithelial) tumour appearing to arise from the muscularis propria • The commonest gastrointestinal mesenchymal tumour (< 1% of all GI tract tumours) • Location: stomach (40–70%) > small intestine (20–50%) > large intestine and rectum (5%) • 90% of tumours will express KIT (CD 117) which is a tyrosine kinase growth factor receptor • Increased prevalence of GIST with NF-1 • Unfavourable prognostic signs: tumour size >5cm • Carney’s triad: a genetic syndrome of young women
Small bowel
CROHN’S DISEASE (CD)
CROHN’S DISEASE
DEFINITION
RADIOLOGICAL FEATURES
Barium studies
Aphthoid ulcers: characteristic superficial ulcers that do not penetrate the muscularis mucosa 
they appear as small collections of barium with surrounding radiolucent oedematous margins 
en face they appear as a dense amorphous barium pool with a surrounding black halo
Longitudinal ulcers: these run along the ileal mesenteric border
these are not a frequent finding
Thickened bowel wall segments will displace adjacent barium-filled loops
‘Skip lesions’: discontinuous involvement of the bowel wall
These may be short, long, single or multiple (the latter is virtually diagnostic of CD) 
solitary strictures are common and may be accompanied by proximal (prestenotic) dilatation
‘String sign’: tubular narrowing of the intestinal lumen secondary to oedema and spasm (± scarring)
Other sites: urinary bladder 
perianal region (leading to a ‘watering can’ perineum) 
occasionally the skin and vagina
it can also be seen with ischaemic strictures or scleroderma (with wide ‘square’-shaped diverticulae)
CT
The transmural disease leads to greater wall thickening than seen with UC 
mild reactive adenopathy (<1cm) can be present
‘Dirty fat’: transmural inflammation of the small bowel usually involves the adjacent mesentery
‘Target’ or ‘halo’ sign: a homogeneous or stratified appearance is seen on both NECT and CECT:
‘Creeping fat’: fat accumulates on the serosal surfaces
INFECTIONS/INFESTATIONS OF THE SMALL BOWEL
TUBERCULOSIS (TB)
Radiological features
there are often multiple bowel lesions
Barium follow through (ileocaecal tuberculosis)
Ulcerative: there are discrete ulcers with a ‘shaggy’ edge – these tend to be large and circumferential
GASTROINTESTINAL STROMAL TUMOURS (GISTs) AND CARCINOID TUMOURS
GASTROINTESTINAL STROMAL TUMOURS (GISTs)
Definition
GISTs may arise from the interstitial cells of Cajal (these serve a gastric pacemaker function)
leiomyomas and leiomyosarcomas are rare at these sites
Pearls
this differentiates a GIST from other gastrointestinal mesenchymal tumours (e.g. leiomyoma or leiomyosarcoma)
infiltration into adjacent organs 
metastases 
a high mitotic and proliferation index
CARCINOID TUMOURS OF THE GASTROINTESTINAL TRACT
Clinical presentation
they may lead to abscess formation, sinuses or fistulae
it is the most common cause of bowel loop separation in CD
these may contain gas – this is usually due to enteric or cutaneous fistulae and sinus tracts rather than due to a gas-producing bacteria
ulceration 
cobblestoning 
a ‘target’ appearance 
fibrofatty proliferation 
vascular engorgement 
adenopathy 
luminal narrowing and stenosis
spondylitis 
symmetrical sacroilitis 
renal stones 
complications of steroid treatment (e.g. avascular necrosis) 
primary sclerosing cholangitis
it used to be secondary to pulmonary disease but is now more likely to be of primary bovine origin (from drinking unpasteurized milk)
liver 
spleen 
lymph nodes 
peritoneum 
female genital tract
fever 
weight loss 
diarrhoea 
intestinal obstruction 
bowel perforation (rare)
it may be difficult to distinguish from lymphoma
there may be enlarged rim enhancing low-density mesenteric nodes (due to caseous liquefaction)
acute inflammation of the terminal ileum is often indistinguishable clinically from an acute appendicitis
tortuous, thickened mucosal folds with small discrete nodular filling defects of lymphoid hyperplasia 
there can be mural thickening
the appendix is the most commonly affected site
previous surgery 
neoplasms 
diabetes 
steroids 
poor dental hygiene
it is contracted through contaminated drinking water
small well-defined nodular lymphoid hyperplasia in patients who also have dysgammaglobulinaemia
thickened or absent valvulae conniventes within the duodenum and proximal jejunum 
severe cases may show a rigid ‘pipestem’ stenosis with irregular narrowing
it may be indistinguishable from Crohn’s disease (although the mucosa remains intact)
it is very commonly seen in the tropics
there can be associated oesophagitis, gastritis or colitis
thickened small bowel folds
abdominal pain 
abdominal mass 
ileus 
GI bleeding 
weight loss
the border of the smooth mucosal surface forms right or obtuse angles with the adjacent mucosa
coarse mottled calcifications (25%) 
occasionally pedunculated (and may obstruct the pylorus or duodenum/act as a lead point for an intussusception) 
hypo- or hypervascular
T2WI: low to intermediate SI 
T1WI + Gad: variable heterogeneous enhancement
it is multiple in ⅓ of cases
it can present with abdominal pain, obstruction, or an abdominal mass 
GI haemorrhage is very rare
