Motion effect on the measurement of stiffness on ultrasound elastography: A moving liver fibrosis phantom study

Abstract

Purpose The recommended method for measurement of tissue elasticity using ultrasound elastography is the breath-hold technique. However, for infants and young children, and even sometimes for adults, sustaining a breath-hold is not easy. To utilize the advantages of ultrasound elastography in these populations, it is essential to know how motion affects the measurement of elasticity. Therefore, the purpose of this study was to evaluate the effect of motion on tissue elasticity measurement on ultrasound elastography using moving liver fibrosis phantoms.

Materials and Methods Ultrasound elastography was performed with both convex and linear transducers of supersonic shear wave imaging (SSI) and acoustic radiation force impulse imaging (ARFI). Liver elasticity phantoms of custom-made stiffnesses of 3.0 and 16.9 kPa were used, and the Orbital Shaker was used to make regular circular and horizontal motions. The elasticities were measured at depths of 2, 3, 4, and 5 cm with measurements repeated 20 times for each depth. Motion velocities were 30 and 60 rpm with the phantoms in an upright position on the shaker to replicate the respiratory rates of children. To simulate different directional motion, we laid the phantoms on their side on the table of shaker at 30 rpm. The Wilcoxon signed-rank test was used to compare values between moving and static status. The Mann-Whitney U and Kruskal-Wallis tests were used to compare the differences between moving and static status between all depths. The number of measurement failures was counted during the acquisitions.

Results In SSI, the convex transducer was less affected by motion at 30 rpm with the 3 kPa phantom. At 30 rpm, SSI showed a tendency for elasticity values to increase during movement than in static status, regardless of the direction of the motion. Otherwise, most values from SSI were different between moving and static status in the higher velocity motion (60 rpm), or in the higher stiffness phantom (16.9 kPa). In ARFI, when using the convex transducer, measurements made at 4 cm depth were less affected by motion. With the linear transducer, measurements made at 2 cm depth were less affected by motion when using the 3 kPa phantom. There were frequent measurement failures using ARFI, especially in moving status.

Conclusion Motion affected the measurement of elasticity differently according to machine, phantom stiffness, acquisition depth, transducer, and velocity and direction of the motion. The convex transducer of SSI was more stable for the slowly moving (represents normal respiratory rates, except for neonates), low stiffness phantom (represents normal liver). However, using SSI, the values in moving status were significantly different in the higher velocity motion or in the stiffer phantom (such as in liver fibrosis), even though values could be easily obtained without measurement failures. With ARFI, measurements made at specific depths were shown to be more stable during motion according to which transducer was applied. However, there were frequent measurement failures using ARFI, especially in moving status. This study supported that the different setting was needed according to the purpose of examination and condition of the patients for measurement of elasticity.

목적 탄성 초음파를 사용하여 간의 탄성도를 측정함에 있어서 가장 권유되는 방법은 숨을 참은 상태에서 탄성도를 측정하는 것이다. 하지만 소아 환자에 있어서 이러한 방법은 실제로 적용이 불가능한 경우가 많으며, 나이가 많거나 질병이 있는 성인의 경우에도 호흡을 참는 방법이 어려운 경우가 많아 탄성 초음파의 적용에 제한이 되고 있다. 그러므로 본 연구에서는 동적인 간 섬유화 모형을 이용하여 탄성 초음파를 이용한 간의 탄성도 측정 시 움직임이 주는 영향을 알아보고자 하였다.

재료 및 방법 탄성 초음파는 supersonic shear wave imaging (SSI)과 acoustic radiation force impulse imaging (ARFI)를 사용하였고, 각각에서 곡면 및 선형 탐촉자를 사용하였다. 간 섬유화 모형은 3.0과 16.9 kPa의 탄성도를 가진 모형을 사용하였고 규칙적인 원형 및 수평면의 움직임을 구현하기 위하여 Orbital Shaker를 사용하였다. 탄성도는 각 모형의 2, 3, 4, 5cm 깊이에서 움직이는 상태 및 정적인 상태 각각 20회씩 측정하였다. 먼저 소아의 정상적인 호흡 수를 구현하기 위하여 shaker 위에 모형을 세운 채로 각각 30, 60 rpm의 속도로 움직임을 주어 탄성도를 측정하였고, 다른 방향의 움직임을 구현하기 위하여 shaker 위에 모형을 눕힌 채로 30 rpm...