What are the key components and implications of cervical spine measurements on imaging?

Cervical Spine Measurements on Imaging

The key cervical spine measurements vary by imaging modality and clinical context, with CT being the gold standard for osseous measurements (fractures, alignment), MRI superior for soft tissue assessment (cord, ligaments, discs), and conventional radiography useful for basic alignment and dynamic instability evaluation.

Imaging Modality Selection Based on Clinical Context

CT Cervical Spine

CT is the gold standard for identifying cervical spine fractures and provides essential osseous measurements 1. Key measurements include:

• Vertebral body dimensions: Anteroposterior depth increases from C3 (16.56 ± 2.21 mm) to C7 (19.32 ± 2.30 mm), with greater AP depth at inferior endplates at C5 (20.75 ± 2.87 mm) and C6 (20.56 ± 2.31 mm) 2
• Fracture detection and characterization: CT detects significantly more fractures than plain radiographs, identifying approximately three times as many injuries 1
• Cervicothoracic and craniocervical junction evaluation: Areas traditionally poorly visualized on plain films where up to 60% of cervical injuries may occur 1

MRI Cervical Spine

MRI is the gold standard for soft tissue injuries and provides critical measurements for ligamentous stability and neural compromise 1. Essential measurements include:

• Spinal cord compression and signal changes: MRI demonstrates 100% sensitivity for cervical paraspinal muscles, intervertebral disc, and interspinous ligament injuries, though specificity is modest (64-77%) 1
• Discoligamentous complex integrity: Critical for assessing mechanical instability, with MRI detecting 25% of soft tissue injuries not visible on CT or plain films 1
• Subarachnoid space reduction: Flexion MRI views show 1.17-fold more brainstem compression and 1.06-fold more reduction in subarachnoid space at atlanto-axial level 1
• Epidural hematoma and cord contusion: Essential for surgical planning in unstable cervical spine 1

Conventional Radiography

Plain radiographs provide basic alignment measurements and dynamic instability assessment, though sensitivity is limited (73-90% for injuries) 1. Key measurements include:

• Atlanto-dental interval (ADI): CR shows greater ADI in flexion than MRI (p<0.001), with significant differences between neutral and flexion/extension positions (p<0.0001) 1
• Vertebral height: Measured with 3.9% relative error using computer-aided protocols 3
• Disc height: Lowest at posterior disc space (2.45-2.95 mm from C2-3 to C7-T1) compared to anterior disc space (3.55-4.34 mm) 2
• Posteroanterior displacement: Measured with 2.8% error of mean vertebral depth 3
• Dens-atlas gap: Measured with <1.8% error of C2 depth 3
• Prevertebral soft tissue: >6 mm at C3 and >22 mm at lower levels suggests ligamentous injury with high specificity but low sensitivity 1

Specific Measurements by Anatomic Region

Atlanto-Axial and Atlanto-Occipital Region

CR and CT are superior to MRI for atlanto-axial and atlanto-occipital lesions, while MRI is superior for odontoid lesions 1:

• Atlanto-axial subluxation (AAS): More detail seen with MRI using flexion/extension views, with anterior AAS seen more in flexion on CR than MRI (p<0.005) 1
• Odontoid erosions: MRI detects 67.5% compared to CR (12.5%) and CT (41%), with lateral views showing 1.57-fold more erosions than open-mouth views 1
• C0-C2 angle: Shows significant differences across all spinal alignment types and serves as compensatory mechanism in cervical spondylosis 4

Subaxial Cervical Spine (C3-C7)

Measurements focus on alignment, disc space, and vertebral body dimensions 2, 3:

• Cervical lordosis angles: Global cervical angles (between posterior walls of C2 and C7) and practical cervical angles (sum of C3-C7 endplate angles) correlate with thoracic kyphosis, C0-C2 angle, and T1 slope 4
• Disc space height: Posterior disc height consistently lower (2.45-2.93 mm) than anterior (3.55-4.34 mm) across all levels 2
• Mid-axis of disc space: Located approximately 3 mm above anterior midpoint of annulus fibrosus, requiring 5° cephalad angulation to reach posterior disc space 2

Comparative Imaging Performance

Erosion and Bone Lesion Detection

CT outperforms plain radiography, with MRI showing intermediate sensitivity 1:

• MRI vs CT: Sensitivity 0.61-0.68, specificity 0.92-0.96 for hand/wrist erosions (detection rate 0.71-fold) 1
• CR vs CT: Sensitivity 0.14-0.54, specificity 0.92-1.0 (detection rate 0.34-fold) 1
• CR vs MRI: Sensitivity 0.0-0.55, specificity 0.5-1.0 (detection rate 0.38-fold) 1

Cervical Spine Involvement Detection Rates

MRI demonstrates highest detection rate for overall cervical spine involvement 1:

• CR: 47.5% detection rate 1
• MRI: 70% detection rate 1
• CT: 28.2% detection rate (best for lateral AAS) 1

Clinical Pitfalls and Caveats

Plain Radiography Limitations

Up to 49% of lateral cervical films fail to visualize the cervicothoracic junction, and 10-20% of missed injuries result from misinterpretation of suboptimal radiographs 1. The sensitivity of lateral cervical film alone is only 73-90%, missing approximately 15% of injuries 1.

MRI Specificity Concerns

While MRI has 100% sensitivity for soft tissue injuries, specificity is only 64-77% for clinically significant injuries 1. Care must be taken in interpretation, as 28% show effusion and 62% show pannus on MRI despite normal plain radiographs in asymptomatic subjects 1.

CT Limitations for Soft Tissue

CT alone is insufficient for excluding significant soft-tissue pathology in patients with neurologic symptoms, with less than 1% of patients showing cervical spine instability on MRI not appreciated on CT 1.

Fusion Assessment

CT is more accurate than plain radiography for assessing fusion status, with plain films showing 83-96% fusion rates compared to CT showing only 50-70% at corresponding time points 1. Two-dimensional reformatted CT images are essential for accurate fusion assessment 1.