Measurement of axial vertebral rotation using three-dimensional ultrasound images

Background

Axial vertebral rotation (AVR) is one of the important parameters to evaluate the
severity and predict the progression of scoliosis. However, the AVR measurements on
radiographs may underestimate its actual value. This pilot study investigated a new
three-dimensional (3D) ultrasound method to measure AVR.

Methods

Three cadaveric vertebrae T7, L1, and L3 were scanned with a 3D medical ultrasound
system. Nine sets of ultrasound data, the vertebral rotation from 0 to 40° with 5°
increments, were recorded from each vertebra. An in-house program was developed to
reconstruct and measure the 3D vertebral images. The rotation of each reconstructed
vertebra was determined by the angle between the line going through the centres of
either laminae (L-L) or transverse processes (TP-TP) and a reference vertical plane.
Three raters measured the rotation in 3 sessions, in which they used the mouse pointer
to select the L-L or TP-TP according to their knowledge of vertebral anatomy. The
program detected the 3D coordinates of these points and calculated the AVR.

The intra-class correlation coefficients (ICCs) were used to calculate the intra-reliability
and inter-reliability. The mean absolute difference (MAD±SD) and the range of difference
(RD) between the actual values and the average measurements of each rater were computed
to evaluate the accuracy of methods.

Results

When rotation was greater than 30° for both L1 and L3, all raters found it difficult
to determine one of the lamina areas due to the lack of ultrasound information in
an area behind the spinous process. Therefore, the corresponding measurements were
excluded. The ICC values of the intra-reliability (L-L, TP-TP) for the three raters
were (0.987, 0.991), (0.989, 0.998) and (0.997, 1.000), respectively; meanwhile, the
inter-reliability were 0.991 for (L-L) and 0.992 for (TP-TP). All ICC values were
greater than 0.98 indicating both methods were highly reliable. The MAD±SD values
(L-L, TP-TP) for the three raters were (1.5±0.3°, 1.2±0.2°), (1.6±0.3°, 1.3±0.3°),
and (1.7±0.5°, 0.9±0.2°), respectively. The RD (L-L, TP-TP) were (0-4.5°, 0-3.5°),
(0-5.1°, 0-4.3°), and (0-5.1°, 0-2.8°) for the three raters, respectively.

Conclusions

The (L-L) and (TP-TP) methods could be used to measure AVR reliability from the 3D
ultrasound images.