Ultrasonic Heating Detects Lipiodol Deposition within Liver Tumors after Transarterial Embolization: An In Vivo Approach

Abstract

Computed tomography (CT) is the standard method to evaluate Lipiodol deposition after transarterial embolization (TAE) for a long period. However, iodine but not Lipiodol can be observed on CT. A minimally invasive other method to detect Lipiodol has been needed to evaluate accurate evaluation after procedure. The purpose of this study was to evaluate the efficacy of using the rate of change in sound velocity caused by ultrasonic heating to reflect Lipiodol accumulation after TAE in a rat liver tumor model. We analyzed the association of this developed technique with CT images and histological findings. Eight rats bearing N1S1 cells were prepared. After confirmation of tumor development in a rat liver, Lipiodol was injected via the hepatic artery. Seven days after TAE, CT scan and sound velocity changes caused by ultrasonic heating were measured, and then the rats were sacrificed. An ultrasonic pulse-echo method was used to measure the sound velocity. The temperature coefficient of the sound velocity in each treated tumor was evaluated and compared with the mean CT value and the histological Lipiodol accumulation ratio. Pearson's correlation coefficients were calculated to assess the correlation between the measured values. The correlation coefficient (r) of the mean CT value and histological Lipiodol accumulation ratio was 0.835 (p = 0.010), which was considered statistically significant. Also, those of the temperature coefficient of the sound velocity and the histological Lipiodol accumulation ratio were statistically significant (r = 0.804; p = 0.016). To our knowledge, this is the first study that reported the efficacy of ultrasonic heating to detect Lipiodol accumulation in rat liver tumors after TAE. Our results suggest that the rate of change in sound velocity caused by ultrasonic heating can be used to evaluate Lipiodol accumulation in liver tumors after TAE, and thus could represent an alternative to CT in this application. This new innovative technique is easy to treat and less invasive in terms of avoiding radiation compared with CT.

Keywords: Lipiodol; TACE; TAE; liver tumor; temperature coefficient of the sound velocity; ultrasonic heating.

Figure 1

Schematic illustration of the integrated probe of coaxial transducers for heating and measurement.

Figure 2

Sound pressure distribution formed by the transducer for heating.

Figure 3

Experimental setup for ultrasonic heating and ultrasonic measurement system.

Figure 4

Process flow for the calculation of the rate of change in the sound velocity in the specimen.

Figure 5

Example of a received waveform from a specimen.

Figure 6

(a) Example of the time difference of echoes between before and after heating; (b) example of echoes before and after heating from the ROI.

Figure 7

(a) One of the slices close to the center of the tumor; (b) extracted tumor lesion from (a); (c) Lipiodol accumulation ratio calculated by the analysis software.