Additional Resources ❯ Multidetector CT ❯ MDCT: Syllabus: General: The Acute Abdomen: Targeted Evaluation with Helical CT

The Acute Abdomen: Targeted Evaluation with Helical CT

Bruce A. Urban, M.D. and Elliot K. Fishman, M.D.

Introduction

An acute abdomen is defined as any clinical condition characterized by severe abdominal pain which develops over a period of hours. A rapid and accurate diagnosis in patients presenting with an acute abdomen is essential if the morbidity and mortality of diseases processes affecting the abdomen is to be significantly decreased. Clinical assessment is often difficult and inaccurate, primarily due to the overlap of signs and symptoms. Also, laboratory and conventional radiologic studies are many times non-specific. The development of cross-sectional imaging has had a tremendous impact of the diagnosis and treatment of patients with an acute abdomen. Studies using conventional CT have prospectively demonstrated a CT accuracy near 95% in the acute abdomen. Undoubtedly, helical CT has further improved the radiologist's ability to accurately and confidently evaluate the abdomen.

When helical CT for the acute abdomen?

Helical CT has gained widespread acceptance as a reliable and highly accurate modality in evaluation of the patient with an acute abdomen. Helical CT is most indicated in patients presenting with severe abdominal pain that may require surgery or other forms of intervention. It is probably most beneficial in those patients presenting with confusing or conflicting clinical signs and symptoms. Helical CT can provide rapid, cost effective diagnostic information to help guide appropriate clinical management. It is the imaging triage modality of choice, so much so that many hospitals now have helical CT scanners within the physical space of the emergency room. Undoubtedly, the need for conventional radiologic techniques has diminished due to the increasing utility of helical CT scanning for a wide variety of abdominal disorders.

Helical CT technique: OVERVIEW

Helical CT is the optimal technique for imaging patients with acute abdominal pain. Oral and IV contrast enhancement are helpful for diagnosis in the vast majority of cases. Scanning parameters will vary somewhat dependent on the type of scanner available. Single detector CT scanning parameters for routine acquisition in the patient with non-localizing abdominal pain are:

Oral contrast:	750-1000 cc of a 3% Hypaque solution
Intravenous contrast:	110-120 cc non-ionic contrast (Omnipaque-350) at 2 cc/sec
Acquisitions:	Single phase
Scan delay:	40 - 50 seconds
Area scanned:	Diaphragm to symphysis pubis
Slice thickness:	5 mm
Pitch:	1.6 (speed of 8 mm / rotation)
Reconstruction interval:	5 mm abdomen, 8 mm pelvis

When a multidetector scanner is used the 2.5 mm collimators are selected and a slice thickness of 5mm is routinely used.

Often, the patient's clinical signs and symptoms focus a differential diagnosis to a particular anatomic region and/or specific diagnosis. In these patients, it is important to appropriately target the CT exam for the individual clinical presentation. Scanning parameters will vary depending on the working clinical diagnosis. Failure to target the helical CT scan can greatly reduce the ability to accurately and confidently detect disease.

This exhibit provides an overview of the utility of helical CT in the setting of the patient with acute abdominal pain. Variations from the above routine protocol are addressed for each clinical presentation.

BILIARY SYSTEM

Acute cholecystitis

Indications:
- Equivocal or complicated US or NM
- Severe RUQ pain, elevated WBC, fever
Protocol variations:
- None
Helical CT findings:
- Gallbladder wall thickening > 3 mm
- Distended gallbladder lumen
- Gallbladder wall enhancement
- Pericholecystic fluid or haziness
- Air in wall or lumen
Helical CT advantages:
- Depicts complications, including abscesses
Diagnostic pitfalls:
Rarely have false negative CT exam
Ill patients may have baseline distension

Choledocolithiasis

Indications:
- Severe RUQ pain, biliary colic
Protocol variations:
- Slice thickness: 3 mm in upper abdomen
- Incrementation: 3 mm
Helical CT findings:
- Biliary duct dilatation in absence of mass
- High density nidus within CBD
- Gallbladder stones
Helical CT advantages:
- Sensitivity of 88%; accuracy of 94%
Diagnostic pitfalls:
- Subtle cholesterol stones can be missed
- Findings mimicked by small tumors

SPLEEN

Splenic infarct

Indications:
- Severe LUQ pain or back pain
Protocol variations:
- Acquisitions: optional delayed images
Helical CT findings:
- Wedge-shaped lesion extending to surface
Helical CT advantages:
- Depicts surrounding splenic vasculature
- Evaluates for adjacent pancreatic masses
Diagnostic pitfalls:
- Can mimic splenic abscess or tumor
- Confusing early enhancement of spleen

PANCREAS

Acute pancreatitis

Indications:
- Mid abdominal pain, elevated amylase
Protocol variations:
- Oral contrast: consider water only
- IV contrast: increase rate to 3 cc/sec
- Acquisitions: consider dual-phase through upper abdomen at 30 sec and 70 sec
- Slice thickness: 3 mm for arterial phase
Helical CT findings:
- Glandular enlargement, irregular contour
- Focal hypodense regions of necrosis/edema
- Increase in density of peripancreatic fat.
- Peripancreatic fluid collections
Helical CT advantages:
- Clinical severity correlates with CT picture
- Scan often predicts clinical outcome
Diagnostic pitfalls:
- Up to 1/3 of patients will show a normal
- pancreas early in course of disease

GENITOURINARY TRACT

Acute pyelonephritis

Indications:
- Flank pain, hematuria, WBC in urine
Protocol variations:
- Acquisitions: consider dual phase imaging at 30 seconds and at 90 seconds; consider delayed images at 4 minutes
Helical CT findings:
- Striated or wedge-shaped perfusion defects
- Loss of corticomedullary differentiation
- Renal enlargement, perinephric stranding
Helical CT advantages:
- Detecting subtle cases of pyelonephritis
Diagnostic pitfalls:

Focal pyelonephritis can mimic renal mass Renal infarct

Indications:
- Flank pain, hematuria
Protocol variations:
- Acquisitions: consider dual-phase imaging at 30 seconds and at 90 seconds
Helical CT findings:
- Wedge-shaped lesion extending to surface
- Diffuse absence of perfusion
- Preserved capsular enhancement
Helical CT advantages:
- Also evaluates abdominal aorta for dissections and aneurysms
Diagnostic pitfalls:
- Can be mimicked by focal pyelonephritis

Ureteral stones

Indications:
- Flank pain, hematuria
Protocol variations:
- Oral contrast: none
- Intravenous contrast: none
- Slice thickness: 3 mm
- Reconstruction interval: 3 mm
Helical CT findings:
- Radiodense calculus or ureter
- Ureteral edema or stranding
- Hydronephrosis
Helical CT advantages:
- Nearly 100% of all calculi detected
- Smaller calculi are detected
- Essentially eliminates need for IVP
- Rapid diagnosis even if obstructed
Diagnostic pitfalls:
- Phleboliths can mimic renal calculi

Tubo-ovarian abscess

Indications:
- Pelvic pain, fever, elevated WBC
- Confusing or complicated US findings
Protocol variations:
- Slice thickness: 5 mm in pelvis
Helical CT findings:
- Complex cystic adnexal mass
- Dilated, enhancing fallopian tube
Helical CT advantages:
- Can easily differentiate from appendicitis
Diagnostic pitfalls:
- Distinguishing from other adnexal masses

GASTROINTESTINAL TRACT

Appendicitis

Indications:
- RLQ pain, elevated WBC, fever
Protocol variations:
- Contrast: similar diagnostic capabilities with oral, IV, and / or rectal contrast
- Slice thickness: 5 mm through RLQ
- Reconstruction interval: 5 mm
Helical CT findings:
- Distended appendiceal lumen
- Thickened, enhancing appendiceal wall
- Periappendiceal stranding
Helical CT advantages:
- Rapid test, especially non-contrast
- Greater than 95% accuracy
Diagnostic pitfalls:
- Mistake terminal ileum for appendix

Diverticulitis

Indications:
- LLQ pain, elevated WBC, fever
Protocol variations:
- Contrast: consider rectal contrast
- Slice thickness: 5 mm
- Reconstruction interval: 5 mm
Helical CT findings:
- Thickened colon wall
- Inflammatory stranding in pericolonic fat
Helical CT advantages:
- Greater than 90% sensitivity for diagnosis
- Demonstrates perforations and abscesses
Diagnostic pitfalls:
- Differentiating diverticulitis from cancer

Ischemic Bowel

Indications:
- Abdominal pain, elevated lactate
Protocol variations:
- Acquisitions: consider arterial phase to assess mesenteric vascular patency
Helical CT findings:
- Thickened bowel wall
- Pneumatosis, portal venous air if severe
Helical CT advantages:
- Depicts etiology: arteriosclerosis,thrombosis, mass
Diagnostic pitfalls:
- "Negative" exam does not exclude diagnosis
- Findings often non-specific finding

Gastrointestinal perforation

Indications:
- Severe abdominal pain, pneumoperitoneum
Protocol variations:
- Acquisitions: view on "lung windows"
Helical CT findings:
- Extraluminal air and fluid
Helical CT advantages:
- Most sensitive modality to depict free air
- Often depicts underlying cause
Diagnostic pitfalls:
- Location of air does not necessarily correlate with site of perforation

Small bowel obstruction

Indications:
- Abdominal pain, nausea, vomiting
Protocol variations:
- Oral contrast: can do without as fluid filled loops provide "natural" contrast
Helical CT findings:
- Dilated small bowel loops proximal to transition point
- Other findings depend on cause of SBO
Helical CT advantages:
- Accurate for site, cause of obstruction
- Differentiates between hernia, mass, adhesions, intussusception, volvulus, etc.
- Rapid test, usually obviates need for SBS
- Depicts ischemia and perforation
Diagnostic pitfalls:
- Rare difficulty: obstruction from ileus
- Detecting low-grade obstruction

Peptic ulcer disease

Indications:
- Epigastric pain, nausea, vomiting
Protocol variations:
- Oral contrast: consider water only
Helical CT findings:
- Gastric / duodenal wall thickening
- Stranding or fluid in adjacent fat
Helical CT advantages:
- Depicts complications of perforation and pancreatitis
Diagnostic pitfalls
- Findings often non-specific
- "Negative" exam does not exclude diagnosis

VASCULAR SYSTEM

Aortic dissection

Indications:
- Tearing abdominal pain, hypertension
Protocol variations:
- Oral contrast: water only
- Intravenous contrast: 150 cc @ 3 cc/sec
- Scan delay: 30 seconds
- Slice thickness: 3 mm
Helical CT findings:
- Intimal flap, true and false aortic lumens
Helical CT advantages:
- Rapid, accurate diagnosis
- 3D images provide "angiographic" display
Diagnostic pitfalls:
- Intimal flap can be "missed"on conventional window due to bright contrast: view on wide window setting

Aortic aneurysm rupture

Indications:
- Acute abdominal pain, hypotension
Protocol variations:
- Oral contrast: water only
- Intravenous contrast: 150 cc @ 3 cc/sec
- Scan delay: 30 seconds
- Slice thickness: 3 mm
Helical CT findings:
- Retroperitoneal hematoma
- IV contrast extravasation from aneurysm
Helical CT advantages:
- Rapid, accurate diagnosis
- 3D images provide "angiographic" display

Hemorrhage

Indications:
- Acute abdominal pain
- GI bleeding
- Falling hematocrit, hypotension
Protocol variations:
- Acquisitions: begin scan earlier at 30 sec
- IV contrast: increase rate to 3 -4 cc / sec
Helical CT findings:
- Active contrast extravasation
- Hematoma
Helical CT advantages:
- Localizes site of bleed, directs embolization
Diagnostic pitfalls:
- Small or intermittent bleeds can be missed

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