Ultrasound. - Dosimetry. Gail ter Haar. Joint Physics Department, Royal Marsden Hospital: Institute of Cancer Research, Sutton, Surrey UK

Transcription

1 Ultrasound - Dosimetry Gail ter Haar Joint Physics Department, Royal Marsden Hospital: Institute of Cancer Research, Sutton, Surrey UK

2 Measurable in water Exposure Reduced by: tissue acoustic properties environmental factors tissue geometry Dose Tissue target Difficult to measure

3 Exposure Can be measured in water Can be described by I, p, t, f..

4 DOSE

5 Dose A parameter that : 1. Describes the sound field in tissue 2. Tells us about how well we are ablating tissue 3.Allow treatments to be reproduced either on this machine or on another

6 Dose Actual intensity values in tissue depend on: Depth into tissue, tissue type, frequency, non-linearity

7 What is important Clinically?

8 What is important Clinically? Volume of tissue damaged in a given time

9 What is important in Physics terms?

10 What is important to the Physicist?? Volume of tissue damaged Relative to measurable field parameters

11 Potential Dosimetric parameters: 1. I x t 2. Thermal dose

12 Ixt Intensity x time is a measure of total energy useful for purely thermal lesions

13 Calibration Gail ter Haar

14 Oh No!

15 What is calibration? : Quantitative & Qualitative characterisation of the acoustic field

16 Why calibrate? : Clinical Safety: Select treatment output Limit exposure of sensitive sites Clinical efficacy comparison: between patients (repeatability) between different equipment types

17 What do we need to know? Source: Acoustic: Frequency Diameter Focal length Intensity / Power / P - Exposure: Duration Scan speed

18 Beam profiles (calculated): 0cm Focal depth or 15cm from transducer Intensity W/cm^ Radial Distance (cm) Focus

19 Beam profiles (calculated) : 2.0cm deep Intensity (W/cm^2) cm deep Radial Distance (cm)

20 Pressure We measure : Acoustic power We calculate : Intensity There is no way of measuring intensity directly

21 Intensity : I = p 2 /2ρc pressure density speed of sound

22 Describing the beam profile :

23 Intensity Intensity = energy flux (ie energy crossing a unit area in unit time). Intensity will vary in space and time

24 I o Which intensity? : WATER WATER I free field = I o e -αx x WATER TISSUE I in situ = I o e -αx I o I I free field > in situ

25 Intensity is usually calculated either from: acoustic power & beam dimensions or from pressure measurement Isptp IEC defined quantities (originally for diagnostic fields) are based on hydrophone measurements, but lab practice is often different Ispta Isata IEC 61102,61157, 61689

26 Which intensity? : I in situ I SP I SATA I free field

27 Which intensity? : I SAL Beam profile measured at low pressure, power measured at high pressure Accounts for non-linear propagation

28 Intensity : I SAL = I SP Intensity, spatial average measured under linear conditions I SP = 1.56 W/D 2 = 1.8 I SAL Acoustic power Beam width (linear)

29 Which intensity? : Is I SAL or I sp the correct quantity to use? HEAT

30 Or Or should should we we be be using using pressure? pressure? Which intensity? : Is I SAL or I sp the correct quantity to use? HEAT CAVITATION

31 Measurement problems : Highly focused fields Focal region can be <(10 x 1) mm Rapid spatial variation High Intensity/pressure Non linear propagation (water) High local pressures (can damage measurement devices) Cavitation, thermal damage

32 Transducers Frequencies from 0.5 to 4.0 MHz Spatial peak intensity up to 20kWcm -2 Focal lengths from 3 to 15 cm Phased arrays multiple simultaneous foci

33 What do we measure? : Free Field parameters

34 Free field conditions Acoustic absorber lines walls Courtesy of Dr. Coussios

35 Acoustical absorbers Polyurethane based absorbing rubbers; Two types of material produced: - High quality absorber suitable for use as radiation force balance targets; - Low cost single-ply layer for use as tank wall lining. Courtesy of NPL

36 What do we measure? : Acoustic Pressure Acoustic Beam profile & Acoustic radiation force

37 Pressure & beam profiles

38 Acoustic pressure : Measured using a hydrophone The gold standard GEC Marconi 0.5 mm diameter PVDF Membrane Low intensity 100 Wcm cycles, 1 khz

39 Membrane hydrophone with pre-amplifier. Photos and data courtesy of Precision Acoustics Limited.

40 MEMBRANE HYDROPHONE A sheet of unpoled pvdf film (thickness < 30 µm) is stretched over a ring (diameter ~100 mm) with gold / chromium electrodes vacuum deposited on the surfaces. The sheet is only metallised over a small central area, which forms the active element of the device (diameter typically mm).

41 HYDROPHONE ARRAY (NPL / PRECISION ACOUSTICS) 25 elements, deposited on a pvdf membrane. The minimum element diameter is 0.2 mm with a minimum pitch of 0.3 mm. The overall span of the array is 12 mm.

42 HYDROPHONE ARRAY All 25 channels are available simultaneously. Direct and quick measurement of beam profile (avoids scanning the ultrasonic field with the hydrophone). Now available commercially. However, it is an expensive device.

43 Needle Hydrophones

44 Needle hydrophones Precision Acoustics Courtesy of Dr. Coussios

45 Primary standard for hydrophone calibration Optical interferometer displacement of pellicle (foil) within field measured using a Michelson interferometer; Displacement simply related to the acoustic pressure in the transducer far-field; Hydrophone to be calibrated is placed at the same position in the field and its output voltage determined; Used to calibrate secondary standard hydrophones with uncertainties of ±3% to ±5% (200 khz to 20 MHz); Approaching 20 years old. Courtesy of NPL

46 New primary standard laser interferometer Courtesy of NPL

47 What do we measure? : FWHM (6dB) widths Full width half pressure (6dB) beam width

48 Beam profiles (transverse) : Transducer 20010A3, 06/08/2003 Transverse radial plot, varying x Pressure (mv) FWHM Mean of 3 plots Distance from Max (mm)

49 Beam profiles (axial) : Transducer 20010A2, 17th Sept 2003 Axial plot, varying z pressure (mv) FWHM Mean of 3 plots distance from maximum (mm)

50 Acoustic power

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53 Determination of power Acoustic power can be measured in two main ways: Planar scanning using hydrophones Performed close to output face of scanner head Intensity integrated over beam area Radiation force methods Often useful to perform both, to obtain independent check of results

54 POWER FROM INTENSITY MEASUREMENTS Spatially integrate the temporal average intensity (measured with the hydrophone) over the entire 2-D beam area. Power = I ta da whole beam This is a (time-consuming) alternative to measuring the power using a power balance, but may be the only option for diagnostic ultrasound fields.

55 Radiation force - principles Travelling acoustic wave has associated momentum Net transfer of energy from transducer into medium A target placed in the path of the beam experiences a radiation force, which is proportional to the power contained in the beam Requires target large enough to intercept whole beam

56 Radiation force balances If the target is connected to an appropriate force-measuring device (e.g. a weighing balance), then the power output of the device may be determined very easily Sensitive balance needed Power content of 1 W produces a change in weight of 69 mg Typical scanners will thus produce changes in the range 7-70 mg

57 Absorbing: TARGETS h = 1 Difficult to produce target that absorbs 100% of sound and target heats up, changing its buoyancy and hence weight. Reflecting: θ θ θ h = 2cos 2 θ Reflected ultrasound is absorbed by absorbing materials lining the tank, which can heat up and cause convection currents in the tank.

58 Radiation force : Measured using a force balance Degassed water Reflecting target Counter-balance Pre-focus

59 Radiation force : Measured using a force balance Balanced Forces Pivot point pivots Counter balancing weight Target Sound beam target

60 Radiation force : Measured using a force balance Balanced Forces l F rad = m g l l o Pivot point mg Counter balancing weight l o Target F rad Sound beam

61 Isp (Wcm-2) Isp data from to 01/05/ /11/00 05/12/00 11/12/00 04/01/01 18/01/01 22/01/01 31/01/01 22/02/01 16/03/01 20/03/01 27/03/01 04/04/01 10/04/01 23/04/01 01/05/01 22/05/01 05/06/ db

62 Radiation force : Measured using a force balance Target may be absorbing or reflecting Geometry may be vertical or horizontal

63 POWER MEASUREMENT: POWER BALANCE Microbalance Absorber Target Transducer Control and compensation Produces a signal that is proportional to the force required to keep the target stationary once acoustic power is applied. P = cf h h depends on geometry and size of target. Typical weight-equivalent force from diagnostic beam is just 70µg.

64 Shotton Balance Reflecting target

65 Primary standard radiation force balance Sartorius M25-D microbalance; Reflecting and absorbing targets; Power range: 1 mw to 1W; Frequency range: 1 to 20 MHz; Typical uncertainties ±2.5% to ±6%; Therapy-level balance also available, up to 20W. Courtesy of NPL

66 NPL Power Membrane Uses pyro-electric effect Adam Shaw, Institute of Physics Publishing Journal of Physics: Conference Series 1 (2004)

67 Network analyser QA and fault diagnosis first indicator!

68 Network analysis : Electrical properties in water Log Magnitude Real impedance Imaginary impedance SWR Smith Chart

69 Smith Chart

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