How do you interpret a CT scan?

Interpreting CT Scans: A Systematic Approach

CT scan interpretation should follow a structured, systematic approach to ensure all abnormalities are identified and correctly characterized, prioritizing findings that impact patient mortality and morbidity.

General Principles of CT Interpretation

Systematic Approach to CT Review

1. Initial Overview

   • Begin with a general survey of the entire scan
   • Note image quality, artifacts, and technical adequacy
   • Identify obvious major abnormalities

2. Structured Review by Region

   • Use a consistent pattern (e.g., "4As, 3Bs, 2Cs, and 1D" approach) 1
   • Review each anatomic region systematically
   • Compare symmetrical structures

3. Window Settings

   • Adjust window settings appropriate for the tissue being evaluated:
     • Lung window: For pulmonary parenchyma
     • Soft tissue window: For abdominal organs, muscles
     • Bone window: For skeletal structures
     • Brain window: For intracranial contents

4. Multiplanar Reconstruction

   • Review axial, coronal, and sagittal planes when available
   • Use 3D reconstructions for complex anatomy

Specific Approaches by Body Region

Head CT Interpretation

1. Brain Parenchyma

   • Assess for hemorrhage, infarction, mass effect
   • Look for midline shift or herniation
   • Evaluate ventricular size and symmetry
   • In stroke, use ASPECTS score to quantify early ischemic changes (score ≥6 indicates small-to-moderate ischemic core) 2

2. Bone Windows

   • Examine skull for fractures
   • Check sinuses and mastoid air cells

Chest CT Interpretation

1. Lung Parenchyma

   • Assess for consolidation, ground-glass opacities, nodules
   • Look for interstitial patterns and air trapping
   • In suspected hypersensitivity pneumonitis, look for the "three-density sign" (areas of decreased attenuation from air-trapping, normal lung, and ground-glass opacification) which has 93% specificity when present in three or more lobes 2

2. Mediastinum

   • Evaluate cardiac size and contour
   • Assess great vessels and lymph nodes
   • Measure main pulmonary artery diameter (>29.5mm suggests pulmonary hypertension) 2

3. Coronary Arteries

   • Report coronary artery calcification (CAC) using visual assessment:
     • None, mild, moderate, or heavy/severe 2
   • Heavy/severe CAC correlates with increased coronary heart disease risk and warrants further evaluation 2

Abdominal CT Interpretation

1. Solid Organs

   • Liver: Size, contour, density, focal lesions
   • Spleen: Size and density
   • Pancreas: Contour, density, ductal dilation
   • Kidneys: Size, enhancement, hydronephrosis

2. Hollow Organs

   • Stomach and bowel: Wall thickness, obstruction, pneumatosis
   • In suspected duodenal trauma, look for periduodenal fluid/hematoma, extraluminal air 2

3. Vascular Structures

   • Aorta and major branches: Aneurysm, dissection
   • Portal and hepatic veins: Thrombosis, patency

4. Abdominal Wall

   • Evaluate for hernias and their contents
   • Assess surrounding soft tissues 3

CT Colonography Specific Approach

1. Primary Search

   • Use both 2D and 3D search methods for optimal polyp detection
   • 2D search: Evaluate enlarged images sequentially from rectum to cecum
   • 3D search: Review endoluminal surface of the colon 2

2. Lesion Characterization

   • Assess morphology by correlating 2D and 3D images
   • Determine lesion density by visual interrogation
   • Judge lesion mobility by comparing supine and prone images
   • Measure polyps on 2D images with lung window settings 2

Special Considerations

Trauma Imaging

1. Blunt Abdominal Trauma

   • CT has high sensitivity (97%) and specificity (95%) for detecting significant intra-abdominal injuries 2
   • Liver and spleen injuries are relatively easy to evaluate
   • Bowel, pancreas, and diaphragm injuries are more difficult to detect 2
   • Consider repeat CT scan after 12-48 hours in doubtful cases of pancreatic-duodenal lesions 2

2. Radiation Dose Considerations

   • Use appropriate dose reduction techniques when possible
   • For CT colonography, lower dose settings are appropriate (100 mAs) 2
   • For [18F]FDG PET/CT, consider radiation exposure from both PET (1.9x10-2 mSv/MBq) and CT components 2

Common Pitfalls to Avoid

1. Satisfaction of Search

   • After finding one abnormality, continue systematic review to avoid missing additional findings

2. Perceptual Errors

   • Small lesions adjacent to normal structures may be overlooked
   • Use multiplanar reconstructions to confirm findings

3. Interpretation Errors

   • Reader experience affects accuracy, particularly for subtle findings
   • Consider consultation with subspecialty radiologists for complex cases
   • Multidisciplinary discussion may increase diagnostic confidence in uncertain cases 2

4. Technical Limitations

   • Metal artifacts can obscure adjacent structures
   • Use metal artifact reduction techniques when available 2
   • Review both attenuation-corrected and non-attenuation-corrected images when metal is present 2

When to Consider Advanced Imaging

1. MRI

   • Consider when soft tissue characterization is needed
   • Useful for evaluating suspected prostate cancer in high-risk patients 2
   • Provides additional information through advanced techniques (perfusion/diffusion, spectroscopy) 2

2. PET/CT

   • Consider for infection/inflammation assessment
   • Evaluate for abnormal sites of increased uptake
   • Correlate findings with morphologic data from CT component 2

By following this systematic approach to CT interpretation, clinicians can maximize diagnostic accuracy while minimizing the risk of missing significant findings that could impact patient outcomes.