Spinal phantom comparability study of Cobb angle measurement of scoliosis using digital radiographic imaging

Abstract

Background: Computed radiography (CR), digital radiography (DR) and biplanar radiography (EOS™ imaging system) are common imaging tools for radiographic evaluation of adolescent idiopathic scoliosis (AIS). The effect of imaging methods in relation to later-on Cobb angle measurements on radiographs is not yet quantified. The study aimed to examine the compatibility between CR, DR and EOS for scoliotic quantification by evaluating the reliability, agreement of different imaging methods, and assessing the prediction performance for EOS measurement from that of CR and DR.

Method: A flexible spine phantom was used to simulate 32 different scoliotic curves ranging from 10° to 60°. Each curvature was imaged using DR, CR and EOS systems accordingly. Each of the six observers independently measured Cobb angle twice on each image at a two-week interval. Intraclass correlation coefficient (model 2 and 3), Bland-Altman plot and linear regression analysis were completed to evaluate the reliability, agreement, and the prediction of Cobb angle measurement, respectively.

Results: Reliability analysis showed excellent intra-observer reliability (Intraclass correlation coefficient >0.9) for each observer and good inter-observer reliability (Intraclass correlation coefficient = 0.84 for EOS; 0.739 for CR; 0.877 for DR) for each method. Bland-Altman plots demonstrated good agreement between imaging methods without fixed or proportional bias. Excellent coefficient of determination was achieved, with 0.980 for CR versus EOS measurements, and 0.973 for DR versus EOS measurements.

Conclusions: Radiographs produced by all of the three methods can provide reliable and accurate Cobb angle measurements for scoliosis assessments. None of the methods systemically underestimates or overestimates the Cobb angle measurement. Additionally, all of the evaluated methods are satisfactory in obtaining images for Cobb angle measurement in AIS. However, the 3D post-processing techniques offered by EOS should also be taken into consideration as it takes a vital role in treatment and monitoring of 3D deformity in the case of scoliosis.

The translational potential of this article: In view of the limited availability of biplanar radiography (EOS™ imaging system), computed radiography and digital radiography are demonstrated to be reliable alternatives in scoliosis monitoring as evident in the reliability, agreement and prediction of Cobb angle measurement.

Keywords: Cobb angle; Comparison study; Imaging evaluation; Scoliosis; Spine phantom.

Figure 1

Illustration of the spine phantom. The spine phantom was manipulated with a right-sided thoracic scoliosis fixed in an immobilizer frame.

Figure 2

Experimental set-up and the corresponding radiographs for different imaging methods. The phantom was placed on the chair in true anteroposterior/posteroanterior position with consistent centring and collimation (A) in DR setting, (B) in CR setting, and (C) in EOS setting. CR = computed radiography; DR = digital radiography; EOS^™ imaging system.

Figure 3

Interobserver reliability of Cobb angle measurements within imaging methods. Two-way random ICC model 2 with absolute agreement for single measure was used. For all estimates, test value = 0.9. ICC, intraclass correlation coefficient; CR = computed radiography; DR = digital radiography; EOS™ imaging system.

Figure 4

Bland-Altman plots of observer 4 between the Cobb angle measurements of different imaging methods. (A) CR versus DR. (B) CR versus EOS. (C) DR versus EOS. CR = computed radiography; DR = digital radiography; EOS™ imaging system.

Figure 5

Regression variation plot of observer 4 between Cobb angle measured using CR/DR and EOS. Estimated Cobb angle value in EOS is predicted with known value of Cobb angle in CR/DR measurement. CR = computed radiography; DR = digital radiography; EOS™ imaging system.