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Everything you need to know about Computed Tomography (CT) & CT Scanning


Colon: Non Neoplastic Disease: The Acute Abdomen: Targeted Evaluation With Helical CT

 

Bruce A. Urban, MD, Elliot K. Fishman, MD

 


 

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 wide-spread 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. 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) @ 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

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 @ 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 @ 30 seconds and @ 90 seconds; consider delayed images @ 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 @ 30 seconds and @ 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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