Abdominal CT and X-Rays
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þ General recommendations for contrast administration in abdominal CT^s; 
 - Renal colic -> no oral/IV contrast 
 - Appendicitis -> oral + IV contrast if conventional CT; otherwise, 
   no contrast if helical or MDCT; give IV contrast if thin or pediatric 
   patient (where intraperitoneal fat may be lacking) 
 - SBO-> IV contrast only (but can be non-contrast) AAA and leakage -> no contrast 
   (though you may need IV contrast to visualize if there is a concurrent dissection) 
 - Pancreatitis, hepatobiliary disease, high grade obstruction -> 
   IV contrast 
 - Diverticular disease, intra-abdominal abscess, partial SBO, mesenteric ischemia, 
   bowel perforation -> oral and IV contrast 

þ Measurement of kidney stone can predict passage - 
  1 mm diameter: 87% - spontaneous passage 
  2-4 mm: 76%  
  5-7 mm: 60% 
  7-9 mm: 48% 
  >9 mm: 25% 

þ Small bowel obstruction (SBO) 
 - 80% c/w 20% for LBO
 - Abdominal plain films have low sensitivities (49%) for detecting bowel obstructions. 
 - Can be "negative" if 
  + Early or incomplete SBO 
  + Intermittent or low-grade SBO 
  + Obstruction is proximal 
  + Obstructed bowel is filled with fluid only. Signs: 

þ Signs of SBO on plain film: 
 - air-fluid levels (AFL) 
 - 'string of pearls" on upright film (from the air being trapped under the 
   valvulae conniventes of an obstructed loop of bowel) 
 - dilated loops proximally and collapsed bowel distally; differential or dynamic AFL 
   (the step-ladder appearance - AFL at different heights - suggests mechanical 
   obstruction, over adynamic ileus. The greater the height differential, the more likely 
   it is a mechanical obstruction.) 

þ Abdominal CTs 
 - are much more sensitive (75-84%) and specific than (90%) plain films in SBO detection. 
 - They can also provide useful information about the etiology of obstruction 
   (adhesions, mass, abscess) and localize the site of obstruction. 
 - Oral contrast is not needed because the fluid naturally collecting from the obstruction 
   provides enough contrast, and the patient has slow transit times and is usually 
   vomiting. 
 - IV contrast can be useful to highlight any abnormal bowel wall or evaluate for 
   ischemia. 
   
þ How to read CT for SBO: 
 - scroll with cine function from the rectum proximally to "clear" colon and confirm 
   that the dilated bowel is not colon. Locate the terminal ileum - if collapsed. 
   suspect SBO; if dilated, excludes SBO. 

þ CT findings of SBO: 
 - Dilated loops of bowel (>3 cm) proximal to the point of obstruction 
 - Collapsed distal small bowel 
 - Air-fluid levels 
 - Point of obstruction (tapering at the transition point): beak appearance 
 - Bowel wall may appear enhanced 
 - Free fluid may be present 
 - Obstructing lesion (if no lesions are seen, then adhesions are presumed) - 
 - "Fecalization" of small bowel contents (fluid and gas bubbles in the small bowel that 
   simulates appearance of feces). It is seen at the zone of transition, just proximal to 
   obstruction. 
 - Bowel wall ischemia 
  + thickening of the bowel wall > 3 mm 
  + target sign (two concentric layers of enhancement of the thickened bowel wall) 
  + mesenteric edema and hemorrhage 
  + intramural gas 
  + ascites 
  + free intraperitoneal air 

þ Closed loop obstruction 
 - Bowel is obstructed at two points 
 - High risk for ischemia 
 - Needs surgery 
 - Caused by adhesions or hemias (external and internal) 

þ CT signs of closed loop bowel obstruction: 
 - Dilated loops of bowel and mesenteric vessels converging to a central point 
 - Multiple bowel loops clustered in one spot 
 - c-shaped or u-shaped loop of dilated bowel 
 - 2 adjacent collapsed, triangular shaped loops of bowel 
 - whirl sign - twisted mesenteric vessels at point of volvulus 

þ Large bowel obstruction (LBO)
 - Cecum can become very distended - diameter > 9-10cm. 
 - Obstruction is usually on the L side of the colon. 
 - If the ileocecal valve is incompetent, there will be air-fluid levels and dilated 
   small bowel loops. 
 - In colonic obstructions caused by tumor, the most common site of perforation is near 
   the tumor. 
 - In colonic obstructions from other causes are, the most common site of perforation 
   is the cecum. 
 - Can be diagnosed on abdominal plain films alone but the sensitivity is low. 
   CT findings rarely alter management since these patients will have to be 
   explored and operatively decompressed. 
 - Can be seen with noncontrast study. Rectal contrast may better demonstrate a mass or 
   evidence of metastatic disease. IV contrast can discern if there is bowel wall 
   ischemia. 
 - Unlike SBO, LBOs almost always require operative intervention. 

þ Is it a phlebolith or a renal stone?   
 - Comet-tail sign: can see the vein in which there is a phlebolith, 
   looks like a comet tail
 - Rim sign: see a rim around the stone that is the edematous ureter
 - These two signs are specific but not sensitive