Author :  Mr. Ng Choong Kheng
Last Updated : 23 June 2003 

HIFU URObot

Introduction 

Prostate cancer is most common among men between the ages of 60 and 80. It accounts for 29% of all cancers diagnosed in males in the United States. This means that approximately 200,000 men in the United States alone were diagnosed with prostate cancer in the year 2000. In Singapore, prostate cancer is the sixth commonest cancer, after lung, stomach, liver, colorectal and breast cancer.

Presently, there are several therapeutic procedures available to treat prostate cancer. The more commonly practiced procedures include radical prostatectomy, brachytherapy, cyrotherapy and electropolation. With the recent technological advances in medicine, yet another type of therapeutic procedure is becoming acceptable as a form of treatment for prostate cancer. Non-invasive surgery is an alternative surgical procedure that requires no physical damage to healthy tissue. If the same therapeutic results can be obtained by non-invasive surgery, it would clearly be the procedure of choice as compared to invasive and minimal invasive surgeries. Microwave and external beam radiation therapies are examples of non-invasive surgery. Although they provide therapy to the tissue of interest without the use of intrusive tools, these methods also result in damages to intervening healthy tissue. High Intensity Focused Ultrasound (HIFU) can be considered a possible candidate as a form of therapeutic non-invasive surgery. HIFU focuses acoustic energy at a selected depth within the body, thus producing a region of high energy density. This causes the rise in the temperature that destroys specific cancerous tissue without damaging the overlying and intervening tissue.

Proposed Protocol 

Figure 1.   Proposed surgical planner flow.

The flow-chart of the proposed Surgical Planner to use HIFU to treat prostate cancer is shown in Figure 1. Pre-operatively, slices of MRS prostate images of the patient are stored into the imaging processing/path planning module in the computer. In the intra-operative treatment, the patient lies in the supine position on the operating table with both legs lifted up. A specially designed robot houses the Transrectal Ultrasound (TRUS) probe and the HIFU transducer. The robot first manipulates the TRUS probe to scan the prostate to obtain a series of 2D transverse ultrasound images. These images are captured at defined intervals as the probe is traversed linearly in and out of the rectum. A 3D model of the prostate can be developed using these ultrasound images. This model is then mapped with a similar model obtained from the earlier MRS images. With knowledge of the cancerous sites in the prostate, the surgeon then defines the target position within the prostate where he wants the HIFU transducer to focus on.

After the target has been defined, the computer commands the robot to manipulate the HIFU transducer such that its focus point coincides with the defined target position. A water bag (with degassed water) is placed between the perineal wall and the HIFU transducer to enable the effective transmission of the HIFU beam. The HIFU beam is then fired at the target for a few seconds. Following this, the water bag is taken away and the HIFU transducer moves back to its home position. The cycle is repeated for every target point defined by the surgeon.

Objectives 

The objectives of the project are as follows:

Localization of prostate cancer with MRS guidance 

From the MRS images, the locations of the prostate cancer can be obtained pre-operatively. By stacking the images in series (Figure 2 b), a computer generated volume of the prostate can be obtained showing the locations of the cancer (Figure 2 c). This volume can be match with the intra-operative transrectal ultrasound volume of the prostate obtained from the same patient. The resulting matched volume (Figure 2 d) shows the positions of the prostate cancer with respect to the ultrasound probe. This information can then be used to position the HIFU transducer.

Therefore, the images guidance provides more accuracy and requires less skill from the practitioner. With the pre-operative MRS introduced, the prostate surface and cancer locations can be identified quickly in the intra-operative ultrasound images.   

Figure 2.   Corresponding Ultrasound and MRS images.  

Development of a HIFU robot with TRUS 

In our laboratory, a robotic system (Figure 3 ) was specially designed to position a HIFU transducer to treat prostate cancer via the transperineal route. The robot is made up of 3 modules; Firing Positioner, Imaging and Manipulator modules. The Firing Positioner module comprises of the HIFU transducer and its manipulator. The aim of this module is to accurately position the HIFU transducer to fire at the specific cancerous location in the prostate gland. The Imaging module carries the TRUS probe for localization purposes. The Manipulator module positions the Firing Positioner and Imaging modules with respect to the patient.

Two dimensional ultrasound images will first be captured by means of a TRUS probe. Knowing the relative positions of these images with respect to the probe, a three dimensional model of the prostate can be produced. With this model, the surgeon can manually pinpoint the areas of interest in the prostate. Recent advances in imaging technology seem to give the possibilities of using MRS to determine the locations of cancerous tumours in the prostate. Having defined the area of interest in the prostate, the robot is activated to automatically position the focal point of the HIFU beam at the particular point in the prostate.

Figure 3.   Proposed HIFU robot.  

 Non-invasive temperature rise estimation

Besides real-time imaging, an ideal robotic system should also provide the surgeon with updated temperature profile estimates, so that information of the region being treated as well as the delivered treatment can be readily monitored. Recent results from a number of research groups have suggested the use of MRI, impedance tomography, microwave radiometry, and backscattered ultrasound for non-invasive temperature estimation. Our group is especially interested in using ultrasound to estimate temperature.

The major advantages of using ultrasound are:

Among the ultrasound techniques, different approaches have been suggested to estimate temperature changes:

Our group plans to investigate the possibility of using these techniques to monitor the rise in temperature cause by the HIFU transducer. In doing so, we plan to develop a close loop temperature monitoring system to better control the treatment of prostate cancer using HIFU.  

Publications related to HIFU.

We would be glad if you could sign our guest book.

For more information, please contact the principal investigator:

A/P Ng Wan Sing
School of Mechanical & Aerospace Engineering
Nanyang Technological University
Nanyang Avenue, Singapore 639798
Fax:(65) 6791 1859