Ultrasonic Nakagami Visualization of HIFU-Induced Thermal Lesions

Meng-Lin Li1, Dai-Wei Li1, Hao-Li Liu2, Ming-Shi Lin2 1Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan

2Department of Electrical Engineering, Chang Gung University, Taoyuan, Taiwan

Abstract—Using high intensity focused ultrasound (HIFU) to locally perform thermal ablation has shown its potential in auxiliary excision surgery, especially in thermal ablation of tumor. However, it is challenging for clinical physicians to locate the region being under HIFU thermal ablation and determine the applied ablation level only by conventional ultrasound B-mode images, causing dangers in therapy. In this study, we propose an ultrasonic Nakagami imaging technique to monitor HIFU thermal ablation in real time. It has been demonstrated that ultrasonic Nakagami imaging is capable of characterizing tissues with different scatterer concentration and distribution. Because in histological sections with and without HIFU thermal ablation, there are differences in scatterer concentration and distribution, we hypothesize that Nakagami parameters of tissues being HIFU thermal ablated will change, accordingly, thus potentially enabling Nakagami visualization of HIFU-induced thermal lesions. Ex vivo thermal ablation experiments of porcine livers were conducted to demonstrate our idea. Experiment results showed that Nakagami images could reveal the HIFU-induced thermal lesion, which was difficult to be located in conventional B-mode images because of no appearance of bubbles. Moreover, in the cases with apparent bubble formation, both of B-mode images and Nakagami images could locate the position of the thermal ablation area, while Nakagami images owned higher contrast. In summary, we experimentally demonstrated the feasibility of monitoring HIFU thermal ablation by Nakagami imaging. The contrast of the ablated regions between the Nakagam images before and after thermal ablation outpeform that of B-mode images. Because the complexity of the Nakagami imaging algorithm is low, it can be easily integrated as part of post processing in current array systems – indicating that real-time visualization of HIFU thermal lesion with Nakagami imaging is possible.

Keywords- High-intensity focused ultrasound, thermal ablation, Nakagami imaging

I. INTRODUCTION Non-invasive therapy has been highly regarded recently, and

particularly, high-intensity focused ultrasound (HIFU) has been a novel tool in local thermal ablation and has great potential in developing as a supplementing surgery tool for lesion resection, especially for thermal ablation of tumor[1][2]. During the HIFU therapy, in order to detect the degree and area of thermal ablation

so as not to injure neighboring normal tissue, a system offering real-time detection of thermal ablation such as ultrasound imaging systems is necessary. Ultrasound-guided HIFU has been used to treat the breast tumors, the uterine fibroids in human body as well as a variety of tumors. However, clinical practitioners may still have difficulty in determining thermal ablation area and extent by using conventional B-mode ultrasound image only and the risk during medication may be thus increased. Particularly, in case of no apparent bubble formation during thermal ablation process, the thermal ablation position would be difficult to be distinguished with B-mode images. To solve such problems, ultrasonic temperature imaging techniques and ultrasonic elastography have been proposed to monitor HIFU thermal ablation and have been demonstrated their ability to detection thermal ablation area. high-intensity focused ultrasound therapy was demonstrated to have the ability to detect thermal ablation [3-5]; however, the computational complexity of the adopted algorithms are high.

In this study, we propose an ultrasonic Nakagami imaging technique to monitor HIFU thermal ablation in real time. It has been demonstrated that ultrasonic Nakagami imaging is capable of characterizing tissues with different scatterer concentration and distribution [6][7]. Because in histological sections with and without HIFU thermal ablation, there are differences in scatterer concentration and distribution, we hypothesize that Nakagami parameters of tissues being HIFU thermal ablated will change, accordingly, thus potentially enabling Nakagami visualization of HIFU-induced thermal lesions.

II. MATERIALS AND METHODS

A. Nakagami probability density function Speckle statistics of ultrasonic back-scattered envelopecan be described by the following Nakagami probability density function (PDF)[6].

2 122( ) exp( ) ( )

( )

m m

m

m r mf r r U rm

−

= −Γ Ω Ω , (1)

whereΓ (． ) and U(． ) are gamma function and unit step function, respectively. Define E(．) is statistical mean and R is

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the back-scattered envelope. The scaling parameter Ω and Nakagami parameter m can be calculated using the following formula.

2 2

2 2 2

[ ( )][ ( )]

E RmE R E R

=− , (2)

2=E(R )Ω . (3) Nakagami parameter m is a shape parameter determining statistical distribution of the ultrasonic back-scattered envelope. the statistical distribution of the envelope data would vary from pre-Rayleigh distribution to Rayleigh distribution; as m is greater than 1, the statistical distribution would lean to post-Rayleigh distribution. Therefore, different m value would correspond to different conditions for scatterer distribution, and Nakagami distribution model is suitable for describing the statistical model for ultrasound backscattering because of its simple mathematical operation and various characteristics.

B. Nakagami imaging method Nakagami PDF has been reported by Tsui et.al for application

in ultrasound parametric imaging, where images are formed with m parameter of the Nakagami PDF as pixels in images. In the imaging algorithm, a moving window size is defined first, and then the envelope data inside the moving window is collected to calculate the local statistics distribution of back-scattered signal envelope. As illustrated in Fig. 1, the window size is firstly defined for the coordinates. Local statistical distribution is then calculated using the envelope data contained in the window for obtaining the m parameter of Nakagami PDF for the window area. The m value is then filled for the coordinate in the center of the window. The window then moves in an interval of one pixel and other procedures are repeated so as to obtain corresponding m values for each pixel in the image and form the Nakagami image.

The moving window size determines the spatial resolution of Nakagami imaging and estimation accuracy of Nakagami m parameter. Higher resolution can be achieved by using small window size, whereas the included envelope information is too deficient to provide accurate estimation of Nakagami m parameter in each pixel. Higher accuracy of m parameter can be achieved with large window size; however, large window will hamper the spatial resolution. The window in the previously proposed studies [7] is square-shaped, and has a fixed side length of 3 fold pulse length. In this study, based on axial resolution and lateral resolution of the used ultrasound imaging system, the window size is obtained with multiples of axial resolution (6dB) and lateral resolution (6dB) instead of square window. Simulations were performed to determine the window size. It is shown that multiples =4 may achieve both image resolution and m precision. Therefore, the length and width of the window are equal to 4-fold axial resolution and 4-fold lateral resolution, respectively for estimating local Nakagami m parameter, and the

actual length and width of the window for computing Nakagami imaging is 1.786 and 3.4576 mm based on the system resolution for following experimental analysis.

Fig. 1. Flow chart of the Nakagami imaging method

III. EXPERIMENTAL RESULTS Ex vivo thermal ablation experiments of porcine livers were

conducted to demonstrate our idea. A 1.5-MHz HIFU transducer was employed to perform the thermal ablation, and ultrasound envelope data during ablation were collected with a clinical ultrasound imaging system. The clinical ultrasound array imaging system comprised a 128-element linear array transducer with frequency ranging from 5MHz to12MHz. Experimental settings were with center frequency of 5MHz, sampling frequency of 20MHz, imaging depth of 5cm, and data size of 1300*256 points. A tank was filled with 30℃ degassed water and a fresh porcine liver was pre-heated in the tank till 30 ℃ and then placed on a platform. The porcine liver was then thermally ablated in an indirect manner by HIFU. The heating time was set to 1.8 second and then stopped for 1.2 second for capturing envelope data and B-mode image for further analysis. Each cycle was 3 second and there were 20 cycles (60 seconds) for the total heating time. The pocine liver was then cooled for 120 second, i.e. 40 cycles, and envelope data and B-mode image was captured in each cycle. The total experimental time was 180 seconds, i.e. 60 cycles. Below two cases with and without apparent bubble formation during HIFU thermal ablation were presented.

A. Without apparent bubble formation Figs. 2(a) and 2(b) illustrate the B-mode images before and

after 25w thermal ablation for 60 seconds, and it is hard to distinguish the thermal ablation position due to no apparent bubble formation. Figs. 2(c) and 2(d) are Nakagami m images before and after 25w thermal ablation for 60 seconds, where the red box illustrates the area with obvious m-value changes. It is shown that the Nakagami m image may assist in thermal ablation identification as the B-mode image may not show when there is

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no apparent bubble formation. Fig. 2(e) illustrates the subtraction result of Nakagami m images before and after thermal ablation, and the area shown with Nakagami parameter variation is fairly identical in size and position in comparison to the thermally ablated and denatured area shown in the taken picture of the ablated porcine liver (Fig. 2(f)). It is shown that in this case, B-mode images can not locate the position of the thermal ablation area without appearance of bubbles, while Nakagami m images can.

. Fig. 2. Images when there is no apparent bubble formation during HIFU thermal ablation. (a) and (b): B-mode images before and after HIFU thermal ablation, respectively. (c) and (d): Nakagami m images before and after HIFU thermal ablation, respectively. (e): Differential Nakgami m image ((d) – (c)). (f) picture of the ablated porcine liver.

B. With apparent bubble formation Referring to Figs. 3(a) and 3(b), there is apparent bubble formation at the thermally ablated area of porcine liver in the B-mode images; therefore, the thermally-ablated area may be detected roughly but not precisely. Instead, Figs. 3(c) and 3(d) show Nakagami m images before and after 40w thermal ablation for 60 seconds, where the thermal ablation area can be distinguished. Fig. 3(e) illustrates the subtraction result of Nakagami m images before and after thermal ablation, and the area shown with Nakagami parameter variation is fairly identical in size and position in comparison to the thermally ablated and denatured area shown in the taken picture of the ablated porcine liver (Fig. 3(f)). It is shown that in this case, Nakagami m images could locate the position and size of the thermal ablation areas more precisely than B-mode images do.

IV. CONCLUSIONS In summary, we experimentally demonstrated the feasibility

of monitoring HIFU thermal ablation by Nakagami imaging. Nakagami imaging may assist in thermal ablation identification as the B-mode images may not show when there is no apparent bubble formation and also precisely show the thermally ablated and denatured area with better contrast when compared to B-mode images. Furthermore, in case of apparent bubble formation,

Nakagami imaging may also precisely show the thermally ablated and denatured area with better contrast and definition when compared to B-mode images. The contrast of the ablated regions between the Nakagam images before and after thermal ablation outpeforms that of B-mode images. Because the complexity of the Nakagami imaging algorithm is low, it can be easily integrated as part of post processing in current array systems – indicating that real-time visualization of HIFU thermal lesion with Nakagami imaging is possible.

Fig. 3. Images when there is apparent bubble formation during HIFU thermal ablation. (a) and (b): B-mode images before and after HIFU thermal ablation, respectively. (c) and (d): Nakagami m images before and after HIFU thermal ablation, respectively. (e): Differential Nakgami m image ((d) – (c)). (f) picture of the ablated porcine liver.

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