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The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
The Physics of Seeing Inside People
An introduction to the science of Medical Imaging
or
“MRI for beginners”
Dr. S. J. Doran
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Summary of today’s lecture
• What is Medical Imaging?
• How does MRI work?
• What can we do with the images?
• How does all this relate to what a typical
Physics undergraduate might be doing for the
three years at University?
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
What is Medical Imaging?
Hi-tech
scanner
Images
(preferably
wacky colours)
come out
Patient
goes in
Medical imaging as seen on TV !
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
What is Medical Imaging?
• The application of basic Physics to see inside the human
body
• Not one subject but many — lots of different techniques
• Each one measures a different physical property of the
sample.
Ultrasound
Composite MRI + PET
X-ray CT
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Why use different methods of imaging ?
Different methods reveal different features
• Plane-film X-ray maps the total
attenuation of X-rays along a path
through the body, giving a projection
image. Good for bone structure in
accidents.
Data source : Mayo Clinic
• X-ray CT measures the X-ray attenuation
coefficient of the body at each point. True
3-D images.
Data source :
Visualisation :
• Ultrasound maps the reflection and
attenuation of sound.
Toshiba America Medical Systems
Vitrea 2, Vital Images
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Why use different methods of imaging ?
Different methods reveal different features (cont.)
• MRI maps the distribution and
“environment” of water
molecules in the body.
• PET maps the distribution of
radioactively labelled
compounds.
Data source : SMIS Ltd
• MEG maps directly the magnetic
Data source : CSUA, Berkeley
fields generated by currents
flowing in the brain.
Data source: FORENAP,
Rouffach, France
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
The Physics of MRI
• Nuclear Physics
• Classical and Quantum Mechanics
• Statistical Mechanics
• Electromagnetism
• Mathematics, Signal and Image
Processing
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Magnet for Whole-Body Imager
•
What is the tunnel into which the patient slides?
I
B = m0 n I
n turns / unit length
Image source : GE Medical Systems, VA Imaging Centre, University of Florida
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
What happens in a scan?
Fundamentals of MRI
• Nuclear Physics

Many nuclei have an intrinsic spin

A spinning charged particle has a
magnetic moment.
• Mechanics

Newton’s Law says that a torque will cause the
dipole to precess in a direction perpendicular to
the torque and the spin vector.
Spin
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
What happens in a scan?
Magnetic fields and resonance
• Magnetism

We exert a couple on the nuclear spin by placing
a magnetic field across the sample.

If the particle were not spinning, it would align
like a compass, but, because of the spin, it
precesses instead.
B
m
Precession
• Resonance

An electromagnetic field that oscillates at
exactly the same frequency as the nucleus will
be absorbed.

Under certain conditions, the nucleus will also
emit a quantum of RF energy
Spin
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
What was happens in a scan?
Magnetic field gradients
• Resonant frequency is related to the magnetic field.
f  B
• If we vary the magnetic field across the sample,
then the frequency of emitted radio waves varies.
B
Feet
Head
x
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
What happens in a scan?
Equipment being used
• A large (normally
superconducting) magnet
• An aerial to transmit and
receive radiofrequency
signals
• A “gradient set” to create a
varying magnetic field
• A computer to process the data
Image source : GE Medical Systems
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
What happens in a scan?
The MRI gradient set
• Basic Level 2 Physics tells
us how to create our
magnetic field gradient.
• It also explains why the
Maxwell Pair
gradients make a
knocking noise.
Saddle Coil / Golay Pair
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
What happens in a scan?
Processing the data
c
a
a Our NMR signal is made up of
the sum of lots of different
frequencies, corresponding to
different spatial positions.
b
b Acquiring multiple signals
gives us a 2-D dataset.
c Fourier transforms, from Level
2 Maths, can unscramble the
data and make an image.
FT
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
The Human Brain as seen by MRI
Data sources : Left - The Whole-brain Atlas, K. A. Johnson and J. A. Becker, Harvard; Right SMIS UK Ltd.
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Image contrast in the brain (1)
0,45
• We can inject a chemical
• As it reaches the brain, the
signal intensity rises.
• This allows us to calculate the
concentration of the agent.
• We can use this to find out
various parameters of the
blood supply to the brain
tissue.
Data source: James D’Arcy and David Collins,
Institute of Cancer Research, Sutton
0,35
Gd Concentration
called a contrast agent into
the bloodstream.
0,4
0,3
0,25
0,2
0,15
0,1
0,05
0
-0,05
0
20
40
60
80
100
Time (s)
120
140
160
180
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Image contrast in the brain (2): neural fibre tracking
• MR images can be sensitised
to the rate of diffusion of
water molecules.
• Water diffuses faster along
nerve fibres than
perpendicular to them.
• This allows us to map the
local direction of a fibre and
create a map of the fibres.
• Finally, we can overlay them
on a computer model of the
head.
Data source: Geoff Parker, Institute of Neurology, London
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Image contrast in the brain (3): Functional imaging
• We acquire one image of the
brain in a “resting” state.
• We follow this by a
corresponding image where
the brain is active.
• Any differences between
these two images correspond
to places where the brain is
working.
• We can see you think!!
Data source: Functional Imaging Laboratory, London
Combined PET / MRI study
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
MRI can image much more than just the brain ...
Liver motion during
normal breathing
Liver
Lung
Data source : ICR, Sutton
Lumbar spine
Data source : SMIS
Knee sports injuries
Data source : SMIS
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
A virtual tour round the human colon ...
Original MR images
Computer-generated
“flythrough” model
Data source : Dr David Lomas, Dept. of Radiology, Addenbrokes Hospital, Cambridge
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
More about image contrast … MR angiography
• The contrast in MR images can be made
sensitive either to “flowing” material, or to
an externally administered contrast agent.
• In both cases, one can obtain images of
the body’s blood vessels with exquisite
detail.
Prof. Arlart, Katharinen Hosp., Stuttgart, Germany Data source, via GE publicity material
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
• High resolution
MRI in cardiology
anatomical imaging
• “Movies” — data acquired in < 20 s
• Imaging of
coronary arteries
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
And finally … the state of the art in cardiac imaging
Data source: GE Medical Systems
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Conclusion
• There are many different ways of imaging the
human body.
• The different methods tell us different things.
• It is study of basic Physics (electromagnetism,
nuclear physics, mechanics) which has
discovered the principles.
• It is money — the human brain is a very valuable
thing — which has led to the incredible
developments that we see today.