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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