Engineering Workshop presentation

Department of
Engineering
Science
Robin Cleveland
1
Engineering Science
What is Engineering?
What is Engineering Science at Oxford?
Biomedical Ultrasound Applications
2
“Scientists discover …
Engineers create”
T. von Karman
MOTION
ENERGY
STRUCTURES
DESIGN
CREATE
3
Motion
Aerodynamics: Trains, planes and automobiles,
motorbikes, rockets, sails
Hydrodynamics:Boats, submarines
Fluids: Oil and gas pipelines
Artificial hearts
DNA chips
4
Production and Manufacturing
Automation
Materials processing
Manufacturing process
5
Energy
Engines: Combustion, turbines, electric
Power generation:
thermal, nuclear,
gas turbine, wind
HVAC: Heating, ventilation,
air-conditioning
6
Structures and Materials
Structures: Wings, bridges, bones
Micro-devices
Nano-devices
Materials: Steel, concrete, titanium,
Kevlar, plastics, carbon fibre composites
7
WHAT PROFESSIONAL ENGINEERS
DO NOT DO (FOR A LIVING)
•  Repair televisions
•  Plumbing
•  Building work
•  Install telephones
•  Operate lathes
•  Tinker with engines
Engineering Science at Oxford
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
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 15th, 16th & 17th December 2014
ACADEMIC REQUIREMENTS
•  Maths and Physics @ A Level (or equivalent)
SUITABLE 3rd A LEVELS
•  Chemistry
•  Computing
•  Design & Technology
•  Further Maths
•  Economics
•  History/English
CURRENT STANDARD OFFERS
A Level :
A*AA (A* in one of 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
YEARS 1 & 2 PROVIDE
•  A common, broad foundation in the
fundamentals of engineering
analysis and design in all the major
engineering disciplines
~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
SPECIALISATIONS in 3rd and 4th year
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
Biomedical Ultrasound
Ultrasound Imaging
Shock wave lithotripsy
High-intensity focused ultrasound (HIFU)
15
Biomedical Ultrasound, Biotherapy
and Biopharmaceutical Laboratory
PIs: Constantin Coussios, Eleanor Stride, Robert Carlisle and Robin Cleveland
16
Diagnostic ultrasound
● Imaging contrast in mechanical properties
● Imaging anatomic features
● Measuring blood flow
http://www.medical.philips.com/main/products/ultrasound/image_library/
17
Ultrasound
Ø 
Ultrasound is defined as sound of a frequency higher than the
upper limit of the human hearing range (f>20 kHz)
Ø 
Therefore, all ultrasound is sound and the same physical
principles that describe sound propagation are fully applicable
to ultrasound
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.
Ultrasound Physics – Wave Equation
2
2
∂ p 1∂ p
− 2 2 =0
2
∂ x c0 ∂ t
p: pressure (Pa)
c0: sound speed (m/s)
Right
traveling
p(t, x) = f (t − x / c0 ) + g(t + x / c0 )
Left
traveling
Plane progressive wave
p(t, x) = f (t − x / c0 )
Intensity
p2RM S
I=
⇢0 c 0
Power/Area (W/m2)
ρ0: density (kg/m3)
19
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 :
c
λ = cT =
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
€
Reflection and Transmission – Normal Incidence
•  Plane interface
Pressure coefficients
Z2 − Z1
R=
Z2 + Z1
2Z2
T=
Z2 + Z1
Z1=ρ1c1
Z2=ρ2c2
Incident
p=f(t-x/c0)
Reflected
R•f(t+x/c0)
Transmitted
T•f(t-x/c0)
•  Z is the specific acoustic impedance
•  Z=p/u equivalent to V/I in electrical circuits
21
Sound Speed and Impedance
Material
Velocity (mm/µs) Impedance(MRayl)
Water
1.48
1.48
Blood
1.57
1.61
Liver
1.55
1.65
Kidney
1.56
1.62
Muscle
1.58
1.70
Fat
1.45
1.40
Soft tissue
1.54
1.63
Dense bone
4.10
7.8
Air
0.33
0.0004
22
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=-0.9995
23
Impedance Mismatch
Soft-tissue:
R<0.01
Tissue - bone:
R~0.6
Tissue - air:
R=-0.9995
24
Oblique Incidence
Specular Reflection
sin θ i = sin θ r
Snell’s Law
Z1
y Z2
θR
θT
θI
x
sin θ i sin θ t
=
c1
c2
Rayleigh Reflection Coefficient
Z 2 cosθ t − Z1 / cosθ i
R=
Z 2 cosθ t + Z1 / cosθ i
( c)
cosθ t = 1− c2
1
2
sin 2 θ i
Total Internal Reflection
Ø 
For c1<c2 , there exists a critical angle θc such that, when θi > θc,
cosθ t is imaginary. This critical angle is given by:
sin θ C ≡
Ø 
Ø 
c1
c2
; θ C is the "critical angle"
Beyond the critical angle the magnitude of the reflection coefficient is
€unity. All the incident energy is reflected.
This is a condition of total internal reflection.
Attenuation: Absorption + Scattering
Transmit
Receive
Absorption: conversion to heat
Scattering: energy re-directed out of direction of propagation
Plane wave
p(x) = p0 e
(f )x
Attenuation coefficient α (f) Np/m
27
Attenuation
In tissue attenuation increases linearly with frequency
↵(f ) = ↵1 f
and then report α1 measured at 1 MHz and extrapolate
Typical attenuation in soft tissue:
Kidney
Fat
Muscle
Skin
“Average”
3.7 Np/m/MHz
7.2 Np/m/MHz
15 Np/m/MHz
38 Np/m/MHz
5.8 Np/m/MHz
28
Pulse-Echo Imaging
A- Mode Display
Electronic
System
1
Echo 3 Arrives
at Transducer
A
3
‘time’ translated into ‘distance’
via d = ct/2
B
C
3
2
D
29
From Goldberg & Kimmelmann (1988)
Early Developments
Howery's B-29 Ultrasonic Tomographic
System
Annotated "Image" of the Neck
Adapted from Szabo (2004)
Milestone Imaging Systems
Radar
Equipment!
J. Reid
J.J. Wild
Early Mechanically Scanned System
Early Phased Array System
"HP 70020A"
Axial resolution – two targets
Transducer
Pulses well separated
0
10
20
30
40
Pulses begin to overlap
0
10
20
30
40
Targets non resolved
0
10
20
32
30
40
Axial resolution - improves with higher frequency
2 targets are resolved if: difference in echo time > pulse duration
Pulse duration ~ one period
Axial resolution inversely proportional to frequency
33
Attenuation affects imaging depth
Freq (MHz)
λ (mm)
Att. coeff. (Np/m)
Imaging depth (cm)
2.0
0.75
11.6
20
3.5
0.45
20.3
11
5
0.30
29
8
7.5
0.20
43.5
6
10
0.15
58
4
Wavelength:
Frequency
↔ Attenuation
↔ Imaging depth
Imaging depth is usually on the order of 400 wavelengths (echo 1%).
34
One Dimensional Array
http://www.ndk.com/en/sensor/ultrasonic/basic02.html
€
Focussed Arrays
Arrays can be focused by adding time delays that
simulate a curved wavefront.
τn
[
=
2
r − (x r − x n ) + z r2
c
]+ t
r
= distance from origin to
focal point
xn
= distance from origin to
center of nth element (np)
to
= constant delay added
to avoid negative delays
o
Focal Volume Approximations
Cigar shaped with -6dB dimensions:
L-6dB
D
l-6dB
F
F : focal length
D: transducer diameter
Example
f = 3.5 MHz
l-6dB = 1 mm
D=3.0 cm
L-6dB = 16 mm
F=7.0 cm
37
Doppler: Concepts
•  Doppler Equation:
–  f is frequency transmitted
–  v is velocity of blood
–  c is sound of speed in medium
38
Doppler Shift
•  f = 2- 10 MHz
•  v=0-5 m/s
fD=0 - 15 kHz
fDis maximised when blood flow is parallel with
ultrasound beam
fD=0 when blood flow is perpendicular to
ultrasound beam
39
Lithotripsy
Coupling
liquid
Semi-ellipsoidal
reflector
Spark source
+
40
p+=40 MPa
map13aug:si187z20xp0.6
30
Pressure (MPa)
• Extracorporeal shock waves
focused onto the stone
• 75% of cases in the US
• Day surgery
• Typically with mild sedation
• 1000-4000 SWs at 1-2 Hz
(30-90 minutes)
• Some discomfort - pain in
10% of patients
• Some soreness at shock wave
entry site
• Hematuria for 1-2 days
• Retreatment required in 30 to
50% of cases
Kidney
stone
20
T+=0.8 µs
10
0
rt=30 ns
-10
p-=-10 MPa
0
1
40
2
Time (µs)
3
4
Storz Modulith SLX–F2
Electromagnetic Lithotripter
41
Stone Disintegration
42
42
Ellipsoidal Reflector
Ray theory
f
d
KZK equation
High Intensity Focused Ultrasound
Ultrasound
Source
Skin
Liver
Tumor
§  Frequency ~ 1 MHz
§  Pressure 10 MPa ~ 100 atm
§  Duration ~ 10s
44
Not physical therapy
§  Frequency ~ 1 MHz
§  Pressure < 1 atm
§  Duration ~ 30s
45
Focal Region for a Therapy Transducer
Normalized Pressure
1
0.8
0.6
0.4
0.2
0
-6
Example
f = 1 MHz
D=6.4 cm
F=6.3 cm
l-6dB = 1.5 mm
L-6dB = 10 mm
-2
0
2
4
6
20
30
Radial Distance (mm)
1
Normalized Pressure
Sonic Concepts Model H101
Focal length: 63 mm
Aperture: 64 mm
Driven at 1 MHz in water
-4
0.8
0.6
0.4
0.2
0
-30
-20
-10
0
10
Axial Distance (mm)
Heat Deposition by Ultrasound
n 
By conservation of energy, intensity lost due to absorption must
go into heat. For a plane harmonic wave
dI
dz
= 2 α( f ) I
qs = −
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 = mcV
T
Where cv is the specific heat of the tissue
Neglected: conduction and convection (perfusion by blood).
Temperature Rise by Ultrasound
E
= ⇢ V cV
t
T
qs
=
t
⇢cV
T
t
For tissue
cV=4000 J/kg/K
Lesion
Beef Liver
US beam
direction
49
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’
50
Clinical HIFU at Churchill Hospital
Liver cancer trial
Superior /Dome
2 days Pre-HIFU
1 day post-HIFU
Plane of surgical resection
R
L
Inferior / Free edge
Plane of histological slice
51
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.
•  Shock wave lithotripsy is first line treatment for kidney
stones.
•  Ultrasound therapy is emerging for soft-tissue ablation.
53
An Engineering Science degree will allow you to create, by
design:
Lego
Boats
Running shoes
Cars
Trucks
Artificial Limbs
Power plants
Aircraft
Planes
Motorcycles
Pneumatic Drills
Baseballs
Surgical Tools
Golf clubs
Heart Valves
Robots
Breweries
Pumps Spacecraft Bicycles
Buildings
Pipe lines
Submarines
Trains
Jet Engines
Ultrasound Scanners Satellites
Drug delivery
Rockets
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Freq (MHz)
λ (mm)
Att. coeff. (Np/m)
Imaging depth (cm)
2.0
0.75
11.6
20
3.5
0.45
20.3
11
5
0.30
29
8
7.5
0.20
43.5
6
10
0.15
58
4

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