Ultrasonically Generated Bubble Clouds for Kidney Stone Erosion

1. Introduction

This research involves optimising the destruction of kidney stones and gallstones using an ultrasonically excited cavitation-bubble cloud. This research is part of a larger proposal with the ultimate aim of developing radically new techniques to cure gallstones without the sided-effects associated with lithotripsy, and to cure kidney stones without the need for surgery. This has the potential for huge benefits for patient welfare, and also for significant cost savings in the NHS (and worldwide).

1.1 Anatomy and Physiology

Kidney stones or "renal calculi" are collections of crystals that form in urine when the compositions of certain compounds are in high enough concentration. Just like the way sugar or salt crystals form in a concentrated solution, kidney stones may form in a similar manner.

1.2 Statistics

About 3 in 20 men and 1 in 20 women in the UK develop a kidney stone at some stage in their life. They can occur at any age, but most commonly occur between the ages of 20 and 40. About half of people who develop a kidney stone will have at least one recurrence at some stage in the future.

1.3 High Intensity Focused Ultrasound (HIFU)

HIFU is characterised by its ability to noninvasively penetrate deep into the human body and precisely deliver energy to a specific site.HIFU therapy occurs in the deepseated focal region of the acoustic field where the acoustic intensity reaches the order of 1000–10,000 W/cm²which is sufficiently high so that cavitation generally happens. In this instance, it is recognised that the collapse of cavitation bubbles contributes to stone comminution. The target is selectively destroyed while surrounding areas are spared, all from outside the body. Unlike radiation and radio surgery, HIFU has no apparent cumulative effect on intervening tissue and leads to better patient outcomes, shorter hospital stays, and lower per-patient costs.

Dual-frequency HIFU.

The Servo system.

HIFU transducer

2. Methods

2.1 Initial chalk experiments

The experimental set-up allows us to first of all visualise and control the ultrasonic field produced by a 2.7 MHz, 1000V-driven HIFU transducer (HIFU radius 60mm, focal length 150mm, focus area radius 1.5mm, focus area length 25mm). Stone crushing tests are also performed, which involves focussing this ultrasonic beam from the HIFU transducer onto a stone model, in this case a piece of chalk, and successfully eroding one side with a precision of 1mm.

 

Chalk experiments

2.2 Biological Models

To understand the effect of therapeutic ultrasound, the changes in tissue caused by the thermal and/or mechanical effects of ultrasound have to be analysed. This effect is investigated using appropriate tissue-mimicking materials. The latter should exhibit similar acoustic properties to the tissue to which they are representing. Granulated agar has been used as a solidifying agent, although further experiments still need to be done in order to produce different grades of tissue densities that would mimic real-life human soft tissue, including skin, fat and muscle.

Different grades of agar tissue-mimicking material

2.3 Computer Simulations

In order to optimise the HIFU transducers to our needs, it has been deemed more beneficial to design and manufacture our own using PZFlex, rather than use commercial devices. PZFlex is the medical industry’s premier tool for modelling ultrasound from transducers, and can model pulsed and continuous-wave operation, nonlinear propagation, frequency-dependent attenuation, and thermal effects over time. It is used to visualise the beam path through tissue layers and to predict, monitor, and control the probe’s motion, tracking, and heat-generation. Forecasting the beam’s absolute location and its effects ensures more precise positioning of the focal point and better control over exposure times, increasing patient safety. Some typical simulation results are shown below.

 

 

 

PZFlex simulation of a HIFU wave pattern