Investigation of ultrasonic soft tissue-bone reflection coefficients correlating with curve severity in children with adolescent idiopathic scoliosis

Abstract

Adolescent idiopathic scoliosis (AIS) is a three-dimensional curvature of spine. Children with AIS and low bone quality have higher chance to get curve progression leading to bigger spinal curvature. In addition, bone quality affects acoustic impedance of bone, thus influencing the reflection coefficient of ultrasound signal from the soft tissue-bone interface. This study aimed to estimate the bone quality of AIS patients based on the reflection coefficients to determine the correlation of the bone quality with curve severity. A simple bone model was used to develop an equation to calculate the reflection coefficient value. Experiments were conducted on five different phantoms. Acrylic was used to design a vertebral shape to study the effect of surface roughness and inclination, including: smooth flat surface (SFS), smooth curved surface (SCS), rough curved surface (RCS), and the rough curved inclined surface (RCIS). A clinical study with 37 AIS patients were recruited. The estimated reflection coefficient values of plate phantoms agreed well with the predicted values and the maximum error was 6.7%. The reflection coefficients measured from the acrylic-water interface for the SFS, SCS, RCS, RCIS (3° and 5°) were 0.37, 0.33, 0.28, (0.23 and 0.12), respectively. The surface roughness and inclination increased the reflection loss. From the clinical data, the average reflection coefficients for children with AIS were 0.11 and 0.07 for the mild curve group and the moderate curve group, respectively. A moderate linear correlation was found between the reflection coefficients and curve severity ($r^2$ = 0.3). Patients with lower bone quality have observed to have larger spinal curvature.

Keywords: Adolescent idiopathic scoliosis; bone quality; correlation; curve severity; ultrasound reflection coefficient.

Figure 1.

(a) A simple bone model with a transducer on top of soft tissue which covers the cortical bone and (b) the primary and multiple reflections within the tissue mimicking layer used to estimate $A_0$ and ${\alpha }_t$ .

Figure 2.

(a) Experimental setup to measure the ultrasound properties of Blue Phantom and an acrylic plate and (b) experimental setup to measure the reflection from a rough curved surface. The surface is also tilted about 5° around the X axis (in YZ plane).

Figure 3.

(a) The ultrasound scan of children with AIS in standing position and (b) a B-mode image of a vertebra. A cadaver vertebra is shown in the inset.

Figure 4.

(a) Envelopes of the recorded echo and two reverberations within the BP. The amplitude of the second reverberation is small and the zoomed signal is shown in the inset. (b) The linear regression line of the three data points. (c) The simulated amplitude ratio with change of soft-tissue thickness for three ${\alpha }_s$ -values. (d) Comparison between the measured and predicted reflection coefficients. Error bars denote the standard deviations (see Table 2).

Figure 5.

The ultrasonographs (Left) and the corresponding RF data (Right) of the following target: (a) acrylic phantom with the SFS, (b) acrylic phantom with the RCIS, (c) lumbar vertebra phantom, and (d) lumbar vertebra of a subject.

Figure 6.

The correlation between the $R_{\mathrm{sb}}$ and Cobb angle.