How do you interpret a computed tomography (CT) scan film?

How to Interpret a CT Scan Film

Interpreting CT scans requires systematic evaluation of image quality, anatomical completeness, and methodical assessment of all visualized structures using appropriate window settings, with recognition that even experienced clinicians miss significant findings in 2-9% of cases.

Essential Technical Principles

Understanding CT Image Formation

• CT images consist of a grid of pixels, each representing x-ray attenuation by tissues in the tomographic slice 1
• Gray scale manipulation allows optimal visualization of different tissue types within the same slice 1
• The absence of structure superimposition is CT's primary advantage over conventional radiography 1

Critical Image Quality Assessment

• High-resolution imaging (1.5-2 mm collimation) is essential for detecting subtle fractures and axially-oriented injuries that may be missed on thicker slices 2
• Modern helical CT with thin-section reconstructions provides superior diagnostic accuracy compared to older scanning protocols 2
• Identify common artifacts that can mimic or obscure pathology: aliasing, ring artifacts, beam hardening, metal artifacts, motion, partial volume averaging, and streaking 1

Systematic Approach to CT Interpretation

Verify Anatomical Completeness First

• Ensure all relevant anatomy is included in the scan - incomplete imaging is a major source of missed injuries 3
• For head CT: confirm visualization of the entire brain including inferior cerebellum, brainstem, and skull base 3
• For cervical spine: the craniocervical and cervicothoracic junctions frequently conceal injuries and require complete visualization 2
• 25% of plain films are technically inadequate at these critical junctions 2

Window Settings and Tissue Evaluation

• Use multiple window settings to evaluate different tissue types optimally 1
• Bone windows: essential for detecting fractures, particularly in the skull, facial bones, and spine 2
• Soft tissue windows: evaluate for hemorrhage, edema, masses, and soft tissue injuries 2
• Brain windows: assess for intracranial hemorrhage, mass effect, midline shift, and parenchymal abnormalities 3

Specific Anatomical Regions

Head CT Interpretation

• Fractures are the most commonly missed finding (18 of 62 false-negatives in one study) 4
• Chronic ischemic foci are frequently overlooked (12 of 62 false-negatives) 4
• Evaluate systematically: skull, brain parenchyma, ventricles, cisterns, gray-white differentiation, and extra-axial spaces 4
• Emergency physicians have 35 false-negatives and 53 false-positives per 544 scans compared to radiologists 5

Cervical Spine CT Interpretation

• CT detects more fractures while plain films detect more malalignment - the modalities are complementary 2
• Three-view plain films combined with directed CT detects >99% of cervical spine injuries 2
• Non-directed entire cervical spine CT detects an additional 8-14% of injuries missed by directed scanning 2
• Transverse process fractures account for 13.2% of cervical fractures, with 78% extending into the intervertebral foramen 2

Soft Tissue and Vascular Structures

• CT with contrast is recommended for neck masses at increased risk for malignancy 6
• CT advantages include visualization of both soft tissue and bone with brief scan time 6
• Evaluate for incidental findings: pneumothorax, mandible fractures, skull base fractures (7-9% detection rate) 2

Common Pitfalls and How to Avoid Them

Interpretation Errors

• Do not assume non-visualized areas are normal - incomplete scans must be completed 3
• Suboptimal and anatomically incomplete imaging combined with misinterpretation account for the largest number of missed injuries 2
• Resident accuracy improves significantly with training level (91% overall agreement with neuroradiologists, but only 2% significant disagreement rate) 4
• Senior surgical residents achieve 81.3% overall accuracy, with maxillofacial CT being the weakest category at 50% 7

Technical Limitations

• CT alone can miss ligamentous injuries - MRI is superior for soft tissue and ligamentous pathology 2
• Plain films reveal only 23% of abnormalities in suspected cervical injury, while MRI reveals 79% 2
• However, MRI detects many findings of uncertain clinical significance - only 5.7% required surgical intervention in one series 2

Clinical Decision-Making

• The combination of plain films and directed CT provides false-negative rates <0.1% for cervical spine clearance 2
• Entire cervical spine CT adds only 11-12 minutes to scan time and provides superior injury detection 2
• Radiation dose for cervical CT is only 0.5 mSv (3% of abdominal CT dose) 2

Training and Quality Improvement

Skill Development

• Level of training significantly affects interpretation accuracy (p=0.032) 4
• Upper-level residents demonstrate higher agreement rates with attending radiologists 4
• Focus training on fracture identification and recognition of chronic ischemic changes 4

Verification and Consultation

• Emergency physicians demonstrate higher sensitivity but lower specificity compared to residents 5
• All interpretations should be verified by radiologists when clinical decisions depend on imaging findings 5, 4
• Continuing medical education is necessary for emergency physicians to maintain interpretation skills 5