Department of Engineering Science. Robin Cleveland

Transcription

1 Department of Engineering Science Robin Cleveland 1

2 Engineering Science What is Engineering? What is Engineering Science at Oxford? Biomedical Ultrasound Applications 2

3 Scientists discover Engineers create T. von Karman MOTION STRUCTURES ENERGY DESIGN CREATE 3

4 Motion Aerodynamics: Trains, planes and automobiles, motorbikes, rockets, sails Hydrodynamics:Boats, submarines Fluids: Oil and gas pipelines Artificial hearts DNA chips 4

5 Production and Manufacturing Automation Materials processing Manufacturing process 5

6 Energy Engines: Combustion, turbines, electric Power generation: thermal, nuclear, gas turbine, wind HVAC: Heating, ventilation, air-conditioning 6

7 Structures and Materials Structures: Wings, bridges, bones Micro-devices Nano-devices Materials: Steel, concrete, titanium, Kevlar, plastics, carbon fibre composites 7

8 What Professional Engineers Do Not Do (For A Living) Repair televisions Plumbing Building work Install telephones Operate lathes Tinker with engines

9 Selection Process Lots of applicants from many countries - need to differentiate attainment from potential Applications read by tutors from two colleges - looking for academic ability and interest engineering - minimum requirement is predicted grades = offer Applicants sit Physics Aptitude Test - compare students with different qualifications All information is used to select candidates

10 Admissions - Interviews Interviews at two Colleges: first choice (might get reallocated) second college (allocated by computer) Interviews ~30-40 minutes Both on the same day Accommodation provided if necessary Probably 14 th, 15 th & 16 th December 2015

11 Academic Requirements Maths and A Level (or equivalent) SUITABLE 3 rd A LEVELS Chemistry Computing Design & Technology Further Maths Economics History/English CURRENT STANDARD OFFERS A Level : A*A*A (A*s in Maths, Physics or FM) Adv. Highers: AAA/AAB IB: 40 points (HL 776 including HL 7 in Maths & Physics) ~20 Open Offers college determined in August

12 Years 1 & 2 Provide A common, broad foundation in the fundamentals of engineering analysis and design in all the major engineering disciplines

13 ~10 Hours/Week of Lectures Mechanical Engineering Thermodynamics & Fluid Mechanics Civil & Structural Engineering Materials Electrical & Electronic Engineering Control & Information Engineering Mathematical Methods Business/Management/Economics Design & Engineering Applications

14 Specialisations in 3 rd and 4 th years Students have a free choice of options, each of which is associated with one or two of the following specialisations: Biomedical Engineering Chemical Engineering Civil Engineering Electrical Engineering Information Engineering Mechanical Engineering

15 Applications in Biomedical Ultrasound Ultrasound Imaging High-intensity focused ultrasound (HIFU) Shock wave lithotripsy 15

16 Biomedical Ultrasound, Biotherapy and Biopharmaceutical Laboratory PIs: Constantin Coussios, Eleanor Stride, Robert Carlisle and Robin Cleveland 16

17 Diagnostic ultrasound Imaging contrast in mechanical properties Imaging anatomic features Measuring blood flow 17

18 Ultrasound Ø Ultrasound is defined as sound of a frequency higher than the upper limit of the human hearing range (f>20 khz) Ø In biomedical applications megahertz ultrasound is used as it is able to penetrate through the soft tissue of the body as a wave to clinically relevant depths. With some important caveats: bone and air! Ø The motion is described by the wave equation. Therefore many phenomena associated with light also apply to sound.

19 Ultrasound Physics 1 D Wave Equation 2 p x 2 1 c p t 2 = 0 p: pressure (Pa) c 0 : sound speed (m/s) D Alembert s Solution Right traveling p(t, x) = f (t x / c 0 ) + g(t + x / c 0 ) Plane progressive wave p(t, x) = f (t x / c 0 ) Left traveling Intensity I = p2 RMS 0 c 0 Power/Area (W/m2) ρ 0 : density (kg/m3) 19

20 Speed of Sound, Frequency and Wavelength Ø In many cases sound consists of harmonic waves (sinusoids). Ø The waveform is characterised by one of these properties: f frequency, T period or λ wavelength : λ = ct = c f Ø For air c = 340 m/s, if f =1 khz, T=1 ms, λ=0.3 m Ø For water/tissue c = 1500 m/s if f=1 MHz, T=1 µs, λ=1.5 mm

21 Reflection and Transmission Normal Incidence Plane interface Pressure coefficients R = Z 2 Z 1 Z 2 + Z 1 T = 2Z 2 Z 2 + Z 1 Incident p=f(t-x/c 0 ) Reflected R f(t+x/c 0 ) Z 1= ρ 1 c 1 Z 2= ρ 2 c 2 Transmitted T f(t-x/c 0 ) Z is the specific acoustic impedance Z=p/u equivalent to V/I in electrical circuits 21

22 Sound Speed and Impedance Material Velocity (mm/µs) Impedance(MRayl) Water Blood Liver Kidney Muscle Fat Soft tissue Dense bone Air

23 Impedance Mismatch Specular reflection in the body: Pressure reflection: Energy reflection: Pulse-echo imaging is based on the use of reflected echoes to locate impedance mismatch Soft-tissue: Tissue - bone: Tissue - air: R<0.01 R=0.61 R=

24 Oblique Incidence Specular Reflection y Z 1 Z 2 Snell s Law sinθ i = sinθ r sinθ i c 1 = sinθ t c 2 θ R θ I θ T x Rayleigh Reflection Coefficient R = Z 2 Z 2 cosθ Z / cosθ t 1 i cosθ t + Z 1 / cosθ i cosθ t = 1 ( c )2 2 sin 2 θ c1 i

25 Total Internal Reflection Ø For c 1 <c 2, there exists a critical angle θ c such that, when θ i > θ c, cosθ t is imaginary. This critical angle is given by: sinθ C c 1 c 2 ; θ C is the "critical angle" Ø Beyond the critical angle the magnitude of the reflection coefficient is unity. All the incident energy is reflected. Ø This is a condition of total internal reflection.

26 Attenuation: Absorption + Scattering Transmit Receive Absorption: conversion to heat Scattering: energy re-directed out of direction of propagation Plane wave p(x) =p 0 e (f)x Attenuation coefficient α (f) Np/m 26

27 Attenuation In tissue attenuation increases linearly with frequency (f) = 1 f and then report α 1 measured at 1 MHz and extrapolate Typical attenuation in soft tissue: Kidney 3.7 Np/m/MHz Fat 7.2 Np/m/MHz Muscle 15 Np/m/MHz Skin 38 Np/m/MHz Average 5.8 Np/m/MHz 27

28 Pulse-Echo Imaging A- Mode Display Electronic System Echo 3 Arrives at Transducer 3 A B C D time translated into distance via d = ct/2 28

29 An Ultrasound Image is made out of lots of one dimensional lines Number of frames/second? Typical image: 128 lines Depth 20 cm Velocity 1540 m/s Time for 1 line = 260 µs Time for 128 lines, ie 1 image 34 ms Frame rate = 29 frame/s 29

30 Axial resolution two targets Transducer Pulses well separated Pulses begin to overlap Targets non resolved

31 Axial resolution frequency dependence 2 targets are resolved if: difference in echo time > pulse duration Pulse duration ~ one period Axial resolution inversely proportional to frequency 31

32 Attenuation affects imaging depth Freq (MHz) λ (mm) Att. coeff. (Np/m) Imaging depth (cm) Wavelength: Frequency Attenuation Imaging depth Imaging depth is usually on the order of 400 wavelengths (echo 1%). 32

33 High Intensity Focused Ultrasound Ultrasound Source Liver Skin Tumor Frequency ~ 1 MHz Pressure 10 MPa ~ 100 atm Duration ~ 10s 33

34 Lesion Beef Liver US beam direction 34

35 Focal Volume Approximations D F L -6dB l -6dB Cigar shaped with -6dB dimensions: Example F : focal length D: transducer diameter f = 1 MHz D=6.4 cm F=6.3 cm l -6dB = 1.5 mm L -6dB = 10 mm 35

36 Heat Deposition by Ultrasound n By conservation of energy, intensity lost due to absorption must go into heat. For a plane harmonic wave q s = di dz = 2 α f ( ) I where I is the acoustic intensity, α is the attenuation coefficient, which is a function of frequency. Qs has units W/m3. If all heat changes the local temperature then the change in energy: E = mc V Where cv is the specific heat of the tissue Neglected: conduction and convection (perfusion by blood). T

37 Temperature Rise by Ultrasound E t = Vc V T t = q s c V q s =2 ˆp2 2 c T t = ˆp2 2 c v c T t For tissue c V =4000 J/kg/K At 1 MHz and 1MPa =0.94 K/s

38 Applications of HIFU Opthamology FDA approval 1985 Cancer Liver, kidney, prostate, breast, brain, skin Non Cancer Uterine fibroids, liver surgery, BPH, Trauma Care Acoustic hemostasis through vessel occlusion Transcutaneous Intraoperative HAIFU JC-Tumor Therapy System 38

39 Clinical HIFU at Churchill Hospital Liver cancer trial Superior /Dome 2 days Pre-HIFU 1 day post-hifu Plane of surgical resection R L Plane of histological slice Inferior / Free edge 39

40 Kidney Stones Stones form in collecting system of kidney Stones have layered structure; 100 µm crystalline (calcium oxylate) and 15 µm glue 1995: 10% of males and 4% of female have one episode by 70 years 2005: 13% of males and 7% of female have one episode 40 by 70 years

41 Electrohydraulic Shock Wave Lithotripsy Coupling liquid Kidney stone p+=42 MPa T+=1µs Semi-ellipsoidal reflector P-=-12 MPa + - Spark source Day surgery Typically with mild sedation SWs at 1-2 Hz (30-90 mn) Some discomfort - pain in 10% of patients Some soreness at shock wave entry site Hematuria for 1-2 days 41

42 Ellipsoidal Reflector Ray theory KZK equation f d

43 Storz Modulith SLX F2 Electromagnetic Lithotripter 43

44 Stone Disintegration 44 44

45 Conclusions: the future of biomedical ultrasound Ultrasound is cheap, portable, presents no risk to the user and little or no risk to the patient. Ultrasound imaging has become widely accepted in the clinical arena providing real-time structural information but can t penetrate bone or air. Ultrasound therapy is emerging for soft-tissue ablation. Shock wave lithotripsy is first line treatment for kidney stones.