• Principal Investigator: Prof. TG Leighton• Co-investigators: Dr A Coleman, Dr G Ball• Researcher: AR Jamaluddin, F Fedele

Sponsor: Engineering and Physical Sciences Research Council

Collaboration:

Institute of Sound and Vibration ResearchUniversity of Southampton

Dept. Medical Physics and Bioengineering, Guy’s and St Thomas’ Health Trust

Development of an in-vivo acoustic diagnostic for lithotripter-induced shock-tissue interaction

INTRODUCTION

Lithotripsy is the favoured technique for the destruction of kidney and gall stones. Thousands of shock waves, emitted one per second, are focused

onto the stones and break them up. We are devising an instrument which can be placed onto the patient’s abdomen. It listens to the echoes as the shock hits the stone and from that tells us:

• Whether the stone is in the target area (if not, healthy tissue is shocked and the stone remains intact)

• The moment the stone has broken, so that treatment can cease (which avoids prolonged exposure, allowing more patients to be treated and saving very costly wear and tear on the shock wave source)

Lithotripsy

←←

Lithotripsy is a clinical process by which kidney stones are broken up by passing high amplitude ultrasonic shock waves through the human body.

In the early devices, patients were lowered into a water bath. The water provided good acoustic coupling to assist the shock to pass into the body.

The shock was generated by a focused source in the base of the bath (red arrow).

The patient is carefully positioned to make sure the acoustic focus, where the shock wave has highest amplitude, is located at the kidney stone (green arrow).

One of the issues we are researching is how to monitor effect that the shock wave is having on stone. Is the shock wave hitting the stone? Has the stone broken up (so that we can stop the treatment)?

Lithotripsy (continued)

In later devices, the shock wave source is encapsulated so that the patient no longer needs to be immersed in a water bath.

Several hundred shocks are passed through the patient at roughly one second intervals.

In collaboration with Guys and St Thomas’ Health Trust (GSTHT), we are investigating whether the sound that can be recorded when the shock wave hits or misses the stone can tell us this information.

This is a picture of a device developed at GSTHT; it is a remote hydrophone which can be placed onto the body, and the readout on the screen tells us a lot about the effect the shock is having on the tissue.

Time/ s0 4 8

60

0MPa

Lithotripterpulse

Pres

sure

rad

iate

d by

bub

ble

0 100 200Time/ s

Bub

ble

radi

us

T im e/ s

Lithotripter pulse

Bubble radius

This typical output from the sensor (volts relating to acoustic pressure linearly) shows a number of peaks.

Time-frequency representation of the same data shows that three of these peaks (i, ii, iii) contain significant broadband energy. What does this tell us about the processes going on at the lithotripter focus?

Well, the lithotripter pulse lasts for only a few microseconds…

…but the bubble activity lasts much longer. The model shows that the 60 MPa compressive part of the lithotripter pulse (red) causes the bubble radius (yellow) to decrease: the bubble collapses, but then rebounds. It then remains expanded for some 200 ms, before undergoing a series of collapses on rebounds. The amplitude and interval between the rebounds decreases.Each time the bubble rebounds it emits a pressure wave, and these are the source of the peaks (i, ii, iii) measured in the data. From these peaks we can monitor the cavitational activity and hence the damage to the stone. These models assume the bubble remain spherical, but we are now predicting the pressures generated when the bubble collapses in a more realistic jetting fashion.

The CFD simulation below shows the half-space in which an initially spherical gas bubble in a compressible liquid is subjected to a lithotripter shock wave.

After the lithotripter pulse has passed (left to right) over the bubble, it collapses. When the liquid jet has passed through the bubble, it impacts the remote bubble wall and generates a blast wave. The movie shows:

• Schlieren images of• 60 micron radius spherical bubble of• air (ideal gas equation) in water

(Tait equation of state), under a• lithotripter shock wave• The Free Lagrange code can

incorporate solid target with real material properties

1.2 mm x 0.6 mm

Axis of symmetry

Bubble wall

The CFD simulation below shows the half-space in which an initially spherical gas bubble in a compressible liquid is subjected to a lithotripter shock wave.

After the lithotripter pulse has passed (left to right) over the bubble, it collapses. When the liquid jet has passed through the bubble, it impacts the remote bubble wall and generates a blast wave. The movie shows:

• Schlieren images of• 60 micron radius spherical bubble of• air (ideal gas equation) in water

(Tait equation of state), under a• lithotripter shock wave• The Free Lagrange code can

incorporate solid target with real material properties, as shown on the next overhead:

Frame size=1.2 mm x 0.6 mm

Axis of symmetry

Total stress in Al (x-dir)

Total stress in Al (y-dir)

Pressure in water and gas

Pressure in water and gas

Contour=1 MPa (t=1 us)

Principal Investigator: TG Leighton (ISVR, University of Southampton)Co-investigators: AJ Coleman (Guy’s and St Thomas’ Trust), GJ Ball (AWE)Students: AR Jamaluddin, CK Turangan, F. FedeleSponsor: EPSRCGrant: “Development of an in-vivo acoustic diagnostic for lithotripter-induced shock-tissue interaction “

The Free Lagrange code, which was used in the previous slide to simulate the collapse of a bubble far any solids, can also incorporate a solid target with real material properties, and be used to calculate the pressure in the water and gas, and stress in the solid (Aluminium in the example below). This can be used to assess the likelihood of a cavitation collapse causing damage to the solid. It is hoped to apply this to kidney and gall stone subjected to lithotripter pulses.