  Osteopenia is indicated as a common phenomenon in patients who have
scoliosis. Quantitative ultrasound (QUS) has been used to assess skeletal
status for decades, and recently ultrasound imaging using reflection signals
from vertebra were as well applied to measure spinal curvatures on children
with scoliosis. The objectives of this study are to develop a new method which
can robustly extract a parameter from ultrasound spinal data for estimating
bone quality of scoliotic patients and to investigate the potential for the
parameter on predicting curve progression. The frequency amplitude index (FAI)
was calculated based on the spectrum of the original radio frequency (RF)
signals reflected from the tissue-vertebra interface. The correlation between
FAI and reflection coefficient was validated using decalcified bovine bone
samples in vitro, and the FAIs of scoliotic subjects were investigated in vivo
referring to BMI, Cobb angles and curve progression status. The results showed
that the intra-rater measures were highly reliable between different trials
(ICC=0.997). The FAI value was strongly correlated to the reflection
coefficient of bone tissue ($R^{2}=0.824$), and the lower FAI indicated the
higher risk of curve progression for the non-mild cases. This preliminary study
reported that the FAI method can provide a feasible and promising approach to
assess bone quality and monitor curve progression of the patients who have AIS.
