A theoretical comparison of attenuation measurement techniques from backscattered ultrasound echoes

Abstract

Accurate characterization of tissue pathologies using ultrasonic attenuation is strongly dependent on the accuracy of the algorithm that is used to obtain the attenuation coefficient estimates. In this paper, computer simulations were used to compare the accuracy and the precision of the three methods that are commonly used to estimate the local ultrasonic attenuation within a region of interest (ROI) in tissue; namely, the spectral log difference method, the spectral difference method, and the hybrid method. The effects of the inhomgeneities within the ROI on the accuracy of the three algorithms were studied, and the optimal ROI size (the number of independent echoes laterally and the number of pulse lengths axially) was quantified for each method. The three algorithms were tested for when the ROI was homogeneous, the ROI had variations in scatterer number density, and the ROI had variations in effective scatterer size. The results showed that when the ROI was homogeneous, the spectral difference method had the highest accuracy and precision followed by the spectral log difference method and the hybrid method, respectively. Also, when the scatterer number density varied, the spectral difference method completely failed, while the log difference method and hybrid method still gave good results. Lastly, when the scatterer size varied, all of the methods failed.

Using speckle statistics to improve attenuation estimates for cervical assessment

“…Our previous studies simulated the role of ROI size in calculation of attenuation and found that the Spectral Log difference algorithm is less dependent on the homogeneity assumption for the tissue compared to the Spectral difference algorithm, but the Spectral Difference algorithm was more precise when the tissue was homogeneous [39]. In addition, our simulations showed that if the ROI size is small, the attenuation estimates obtained will have higher variance [39]. However, the homogeneity assumption might be better approximated by smaller ROI sizes.…”

“…We can rewrite equation The size of the ROI was varied in both the axial and lateral direction. Our previous studies have shown that approximately 15 pulse lengths in the axial direction and 15 pulse lengths in the axial direction and 15 echoes in the lateral direction gave a standard deviation on the order of 25% in the attenuation estimates for simulations, phantom experiments, and in the rat cervix when using the spectral log difference algorithm, while the spectral difference algorithm had a standard deviation of less than 13% for the same ROI size [9,37,39].The pulse length was calculated by taking the auto correlation coefficient of time samples from RF echo lines of the phantom data. Samples with absolute correlation coefficient greater than 0.2 were considered correlated and determined the time pulse length.…”

“…Spectral difference is more sensitive to changes in backscatter resulting from tissue heterogeneity [39]. Whereas the spectral log difference algorithm is less sensitive to changes in backscatter, but it requires computing the changes in the backscattered power spectrum over a larger region for the same variance in the attenuation estimate (poorer attenuation-map resolution).…”

“…Our previous studies had shown that approximately 15 pulse lengths in the axial direction and 15 echoes in the lateral direction gave a standard deviation on the order of 25% in the attenuation estimates when the tissue was homogeneous. Therefore, we varied our region sizes about these values for our study [11,37,39].…”

“…The phantom had scattering targets of average scatter diameter of 35µm with a number density of 160mm -3 . A calibrated reference phantom is required to calculate the attenuation of the cervix so as to cancel the effects of diffraction term and machine transmit/receive characteristics [39]. The date of delivery was also noted for each subject by following up with the patient after the completion of the scans.…”