Imaging Acute Abdomen Part 1: General Concepts
Rathachai Kaewlai, M.D.
www.RiTradiology.com Updated: May 2009
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Overview
Role, indications and limitations of each imaging modality: radiography, US, CT, MR imaging, scintigraphy
Appropriateness criteria
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Acute Abdomen: A Clinical Challenge
“Severe abdominal pain develops over a period of hours”
Common chief complaints:
In USA, stomach and abdominal pain ranked first in patient presentation to emergency departments
Difficult diagnosis:
Broad differentials
Nonspecific history and clinical examination
Nonspecific lab tests
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Acute Abdomen: A Clinical Challenge
Require all resources to reach accurate diagnosis, timely management and proper disposition
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Conventional RadiographyOften the first imaging evaluation
“Acute abdominal series”
Upright chest to evaluate for pneumonia, subdiaphragmatic pneumoperitoneum
Upright and supine abdomen
Decubitus view of abdomen if upright radiograph not possible
To detect small pneumoperitoneum
The patient must be in decubitus position for several minutes before radiograph taken to allow relocation of pneumoperitoneum to perihepatic space
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Conventional Radiography
Helpful for the detection of:
Pneumoperitoneum
Bowel obstruction
Pneumonia mimicking abdominal pain
Suspected emphysematous pyelonephritis or emphysematous cholecystitis on ultrasound
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Large pneumoperitoneum: supine chest radiograph in a 70-year-old man shows a large amount of pneumoperitoneum under the dome of the
diaphragm bilaterally. The patient had perforated stomach following biopsy.
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diaphragm
liver
diaphragm
Small bowel obstruction: Supine and upright abdominal radiographs show disproportionate dilatation of small bowel (SB) with a relatively small amount
of colonic gas (C). There are air-fluid levels (arrows) with different height in the same small bowel loops. Small bowel obstruction due to adhesion
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Pitfalls/Limitations
Poor sensitivity to detect several causes of acute abdomen including appendicitis, cholecystitis and diverticulitis
Poor sensitivity to detect small pneumoperitoneum and free fluid
Low interobserver agreement on the diagnosis of bowel obstruction (particularly with low-grade small bowel obstruction)
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Conventional Radiography
Small pneumoperitoneum not detected on chest radiograph: Axial CT image of the upper abdomen shows small dots of extraluminal air in the
omentum (long arrow) and gastrohepatic ligament (short arrows) in a 54-year-old man who had perforated gastric ulcer.
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free air
“Pseudo” small bowel obstruction on radiography: Supine abdominal radiograph shows multiple loops of dilated small bowel (SB) with paucity of colonic gas. Coronal CT image of the abdomen performed on the same day
does not show evidence of bowel obstruction.
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SB
SB
Ultrasound
Right upper quadrant (RUQ) ultrasound
Renal ultrasound
Abdominal ultrasound
Limited ultrasound
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RUQ Ultrasound
Evaluation of biliary tree (i.e. liver, intrahepatic biliary duct, common bile duct and gallbladder), pancreas, right kidney
Indications
Right upper quadrant pain attributed to hepatobiliary tract
Imaging of choice to evaluate acute cholecystitis
Intra/extrahepatic biliary duct dilatation
Right hydronephrosis, calculi
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Acute cholecystitis: Sagittal ultrasound image of a 63-year-old man presenting with right upper quadrant pain shows an impacted gallstone in
the gallbladder neck and a positive sonographic’s Murphy sign. Surgically and pathologically proven acute cholecystitis.
gallstone
Biliary ductal dilatation: (A) Transverse grey-scale ultrasound image of the liver shows a “double-duct” sign (between arrows). They represent a dilated intrahepatic duct and a portal vein branch. In a normal subject, a portal vein is the only structures in portal triads visualized in the periphery of the liver. (B) The color Doppler image of the same patient shows a dilated common bile duct anterior to the main portal vein. Obstructive biliary system due to
pancreatic head cancer.
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“double duct” dilated CBD
A B
RUQ Ultrasound: Limitations (1)
Recent meal (within 4-6 hours) will contract gallbladder, therefore:
Limiting evaluation for gallstones
May lead to ‘false-positive’ thickening of gallbladder wall
Recent morphine will contract gallbladder and mask the presence of sonographic Murphy’s sign
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RUQ Ultrasound: Limitations (2)
Limited evaluation in patients with
Obesity (poor ultrasound beam penetration)
Fatty liver (obscuring liver pathology)
Significant bowel gas (obscuring pancreas)
Low sensitivity to detect CBD stones (CBD often cannot be visualized in its entirety)
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Severe fatty liver: Transverse ultrasound image of the liver shows marked attenuation of the liver echo due to the presence of fatty change. Internal
structures of the liver (i.e. hepatic veins, portal veins, bile ducts) cannot be visualized.
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gallbladder
liver
Common bile duct stone not detected on ultrasound: An ultrasound image of the right upper quadrant shows a dilated common bile duct (CBD), and
intrahepatic duct (not shown) in a 76-year-old man with acute pain and mild jaundice. Follow-up ERCP shows multiple CBD stones obstructing the CBD.
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CBD stones
dilated CBD
Renal Ultrasound
Evaluation of kidneys and bladder
Acute indications:
Hydronephrosis
Renal infection (pyelonephritis is not an imaging diagnosis although US can occasionally suggest the diagnosis)
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Hydronephrosis due to obstructed upper ureteric stone: Sagittal ultrasound image of the right kidney shows dilated renal collecting system
and proximal ureter in a 57-year-old man presenting with acute renal failure. He had bilateral hydronephrosis due to obstructing ureteric stones.
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hydronephrosis
hydroureter
Abdominal Ultrasound
Evaluation of hepatobiliary tract, both kidneys, spleen, +/- aorta and IVC
Acute indications:
Patients contraindicated or unable to undergo CT or MR imaging
Pregnant patients with trauma
Pediatric patients with abdominal pain
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Limited Ultrasound
Ultrasound performed at specific anatomic location(s) according to clinical suspicion
Free fluid in trauma patients (FAST)
Suspected appendicitis
Suspected intussusception in pediatric patients
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Diagram showing the areas included in FAST (focused abdominal sonography for trauma). These four areas are 1) perihepatic and hepato-
renal space, 2) perisplenic, 3) pelvis, and 4) pericardium.
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1 2
3
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Acute appendicitis: (A) Transverse ultrasound image of the right lower quadrant, using a “graded compression” technique, shows a dilated fluid-
filled tubular structure, which is non-compressible. (B) Color Doppler image shows hyperemia of the inflamed appendix.
Surgically- and pathologically-proven acute appendicitis.
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non-compressible appendix hyperemia of appendix
A B
Ileocolic intussusception: (A) Transverse ultrasound image of the right lower quadrant of a 6-month-old boy shows a mass containing several
concentric rings of hyperechogenicity. (B) Longitudinal scan of the “mass” shows a “pseudo-kidney” sign of intussusception. Hyperechoic region
inside the mass represents intussuscepted mesenteric fat.
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A B
mass
massintussuscepted
omental fat
Computed Tomography (CT)
Evaluation of the whole abdomen and pelvis is required
Options:
Without oral or IV contrast (urinary tract stone, retroperitoneal hematoma)
With oral and without IV contrast (cannot receive IV contrast)
With IV and without oral contrast (mesenteric ischemia, high-grade small bowel obstruction)
With both oral and IV contrast (most indications)
With rectal contrast (appendicitis, colonic pathology i.e. penetrating trauma)
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Computed Tomography (CT)
Indications
Contraindications
Inappropriate use
History of severe contrast reaction (CECT*)
Renal insufficiency (CECT)
Concerns
Use of iodinated contrast medium: nephrotoxicity, adverse reactions
Radiation exposure
28*CECT = contrast-enhanced CT
Value of CT in Acute Abdomen
Changes leading diagnosis
Changes were shown to be as high as 1/3 of all cases in prospective investigations1,2
Increases physician’s diagnostic certainty
CT doubled diagnostic certainty of ED physicians, particularly in the elderly
Changes patient management plan
CT influenced disposition in up to 60% of cases1,2
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1. Nagurney JT, Brown DF, Chang Y, et al. J Emerg Med. 2003;25:363-371.
2. Rosen MP, Sands DZ, Longmaid HE, et al. AJR Am J Roentgenol. 2000;174:1391-1396.
CT - Intravenous Contrast
Often required in acute abdomen imaging
Iodinated contrast medium enhances visibility of vascular structures and organs
Characters
Water-based
Non-ionic (mostly used at present) vs. ionic
Less osmolality - decreases adverse reactions and side effects
More hydrophilic - less tendency to cross cell membranes
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CT - IV Contrast Reactions
Can range from minimal (e.g. hives) to anaphylactoid reactions; mostly idiosyncratic (unpredictable, not dose-dependent)
Acute or delayed
Delayed reaction = 1 hour to 7 days after injection; usually mild
Incidence1
Mild reactions up to 3% (LOCM), 15% (HOCM)
Severe reactions 0.04% (LOCM), 0.22% (HOCM)
Fatal reactions exceedingly rare in both (1:170,000)
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LOCM = low-osmolar contrast medium; HOCM = high-osmolar contrast medium1. Morcos SK, Thomsen HS. Eur Radiol 2001;11:1267-1275.
CT - IV Contrast Reactions
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Predisposing Factors1 x Risk
History of asthma or bronchospasm 6-10
Previous reaction to iodinated contrast medium 5
History of allergy of atopy 3
Dehydration, cardiac disease, hematologic/metabolic conditions, very young or old age, use of medications such as b-blockers, IL-2, aspirin, NSAIDs
N/A
1. Morcos SK, Thomsen HS. Eur Radiol 2001;11:1267-1275.
CT - Premedication
If the risk exists - the patient should be pre-medicated.
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Regimen 1 Regimen 2
Medication Prednisolone Methylprednisolone
Route oral IV
Dose 50 mg 125 mg
Schedule 13, 7, and 1 hour prior to CT 6 and 1 hour prior to CT
Diphenhydramine 50 mg oral or IV 1 hour prior to CT50 mg oral or IV 1 hour prior to CT
CT - IV Contrast Nephrotoxicity
“Increase in serum creatinine by more than 25% or 44 umol/l occurring within 3 days following IV contrast administration and in the absence of alternative etiology.”
Reduces renal perfusion and injured renal tubular cells
Manifestations
Reduced GFR, proteinuria, oliguria
Persistent nephrogram on conventional radiography or CT
Usually self-limiting and resolve within 1-2 weeks but it can increase risk of severe non-renal complications and prolong hospital stay
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CT - IV Contrast Nephrotoxicity
Incidence
0-10% in normal population (normal renal function)
12-27% in patients with pre-existing renal impairment
Predisposing factors
Patient factors: Pre-existing renal impairment, particularly diabetic nephropathy, dehydration, congestive heart failure, concurrent nephrotoxic medications, e.g. NSAIDs
Large dose of IV contrast medium, injection in renal arteries
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CT - IV Contrast Nephrotoxicity
Prevention
Adequate hydration
Use low- or iso-osmolar contrast media
Stop administration of nephrotoxic medications for at least 24 hours prior to contrast administration
Consider alternative imaging methods
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Contrast-induced nephropathy: Coronal CT image of the abdomen without IV contrast in a 76-year-old man, status post cardiac catheterization
24 hours ago, shows persistent renal nephrograms.
CT - IV Contrast and Metformin
Patients with pre-existing renal impairment and are on Metformin are at risk of developing Metformin-associated lactic acidosis (MALA).
The use of IV contrast in this patient subset could lead to contrast-induced nephropathy that in turn worsens MALA
The American College of Radiology recommends checking the renal function and patient’s comorbidities for lactic acidosis before determining if IV contrast could be given
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CT - IV Contrast and Metformin
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CT - IV Contrast: IV Access
Peripheral IV should be used.
Most PICC lines CANNOT be used for IV contrast administration
Not designed to allow rapid injection
Risk of line disruption
‘Power PICC’ (as shown in picture on the right) can be used.
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Image credit: http://home.caregroup.org/centralLineTraining/
CT - Radiation Exposure
CT accounts for 5% of radiologic examinations but contributes 34% of collective radiation dose, worldwide1
Risk of radiation exposures
Deterministic effect: cell death; threshold level specified when effects would occur; rarely seen with diagnostic x-ray and CT
Stochastic effect: cancer, genetic effects; “linear, non-threshold” model generally believed; seen with diagnostic x-ray and CT
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1. United Nations Scientific Committee on the Effects of Atomic Radiation. 2000 report to the
General Assembly, Annex D: medical radiation exposures New York, NY: United Nations, 2000.
CT - Radiation Exposure
Effective radiation dose of one abdominal-pelvic CT scan equals to1
10 mSv, comparable to 3 years of natural background radiation
100 chest radiographs
“Estimated risk of cancer death for those undergoing CT is 12.5/10,000 population for each pass of the CT scan through the abdomen”2
Any efforts to reduce radiation dose from CT should be done.
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1 = http://www.radiologyinfo.org/en/safety/index.cfm?pg=sfty_xray#3
2 = Gray JE. Safety (risk) of diagnostic radiology exposures. In: Janower ML, Linton OW, eds. Radiation risk: a primer. Reston, Va:
American College of Radiology, 1996; 15-17.
MR Imaging
Advantages over CT
High contrast resolution (good for imaging of pelvis, hepatobiliary tract and pancreas)
No ionizing radiation
Can be performed in pregnancy
Total exam time usually <30 minutes. No contrast needed in most cases
Limitations
Contraindications for MR: pacemaker, claustrophobia, etc.
Critically ill patients require MR-compatible life support equipments
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MR Imaging
Scientific evidence for MRI in acute abdomen still is not extensive
Clinical applications
Suspected acute appendicitis (particularly during pregnancy, and in children). Note that gadolinium-based contrast agent cannot be used in pregnant women.
Good results shown for MRI in sigmoid diverticulitis, common bile duct stone, acute cholecystitis, pancreatitis
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Acute appendicitis: Axial STIR MR image of the pelvis in a young pregnant woman shows an enlarged appendix with high signal intensity of the wall
and small periappendiceal fluid.
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appendix
Scintigraphy
Major drawback is limited availability in acute setting; requires efforts to gather a team off-hours; and limited resolution
Clinical applications
Acute cholecystitis: hepatobiliary scintigraphy1
Higher accuracy and specificity than ultrasound
Reserved for patients whom diagnosis is unclear after ultrasound
Acute pulmonary embolism: ventilation-perfusion (V/Q) scan
Considered V/Q scan in patients with a normal chest radiograph suspected of having PE when there is a contraindication to CT scan (renal impairment, severe contrast reaction)
461. Strasberg SM. New Eng J Med 2008;358:2804-2811.
Acute cholecystitis: Anterior (A) and right lateral (B) images of a HIDA scan performed at 4 hours after radiotracer injection show no excretion into the
gallbladder. Image credit: MedPixTM
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Acute pulmonary embolism: 55-year-old man. Perfusion lung scan in right posterior oblique view shows multisegmental defects which do not match the findings seen on a ventilation scan obtained earllier (V/Q mismatch).
Image credits: Radiographics 2003;23:1521-1539
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Appropriateness Criteria1
Clinical VariantMost
Appropriate2nd Most
Appropriate
Non-localizing pain, fever, no recent operation CT with contrast X-ray, US, CT without contrast
Non-localizing pain, pregnant, fever US MRI without contrast
RUQ pain, fever, elevated WBC, positive Murphy sign US X-ray, CT
RUQ pain, suspected acalculous cholecystitis Scintigraphy X-ray, CT
RUQ pain, no fever, normal WBC US CT
RUQ pain, no fever, normal WBC, US shows only gallstones Scintigraphy CT
RLQ pain, fever, elevated WBC, adults, typical appendicitis CT with contrast CT without contrast
RLQ pain, fever, elevated WBC, adults and adolescents, atypical presentation CT with contrast X-ray, US, CT without contrast
RLQ pain, fever, elevated WBC, pregnant US MRI without contrast
RLQ pain, fever, elevated WBC, atypical presentation in children (<14 years) US CT with contrast
LLQ pain, typical diverticulitis, old age CT with contrast CT without contrast
LLQ pain, acute, severe CT with contrast CT without contrast
LLQ pain, woman of childbearing age US CT with contrast
LLQ pain, obese patient CT with contrast X-ray, US, CT without contrast1 = Adapted from the American College of Radiology Appropriateness Criteria. Available at URL: http://www.acr.org/
SecondaryMainMenuCategories/quality_safety/app_criteria/pdf/ExpertPanelonGastrointestinalImaging.aspx
Conclusions
Imaging plays an increasingly important role in diagnosis of etiology of acute abdomen
CT is widely used in several acute abdominal indications; along with ultrasound and MR imaging
Limitations of each imaging method and appropriateness criteria should be considered before selecting an imaging test for a particular patient
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