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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?
Medical imaging as seen on TV !
Hi-tech scanner
Patientgoes in
Images (preferably wacky colours)come out
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
Data source : Toshiba America Medical SystemsVisualisation : Vitrea 2, Vital Images
Data source : Mayo Clinic
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.
• X-ray CT measures the X-ray attenuation coefficient of the body at each point. True 3-D images.
• Ultrasound maps the reflection and attenuation of sound.
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Data source : SMIS Ltd
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.
• MEG maps directly the magnetic fields generated by currents flowing in the brain.
Data source : CSUA, Berkeley
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
• Electromagnetism
• Statistical Mechanics
• Classical and Quantum Mechanics
• Nuclear Physics
• 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
Image source : GE Medical Systems, VA Imaging Centre, University of Florida
• What is the tunnel into which the patient slides?
I
n turns / unit length
B = 0 n I
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
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• Nuclear Physics
Many nuclei have an intrinsic spin
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
• Magnetism We exert a couple on the nuclear spin by placing
a magnetic field across the sample.
B
What happens in a scan?Magnetic fields and resonance
m
PrecessionSpin
If the particle were not spinning, it would align like a compass, but, because of the spin, it precesses instead.
• 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
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
• Resonant frequency is related to the magnetic field.
f B
What was happens in a scan?Magnetic field gradients
x
BHeadFeet
• If we vary the magnetic field across the sample, then the frequency of emitted radio waves varies.
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
• A computer to process the data
What happens in a scan?Equipment being used
• A large (normally superconducting) magnet
Image source : GE Medical Systems
• An aerial to transmit and receive radiofrequency signals
• A “gradient set” to create a varying magnetic field
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
• Basic Level 2 Physics tells us how to create our magnetic field gradient.
Maxwell Pair
Saddle Coil / Golay Pair
What happens in a scan?The MRI gradient set
• It also explains why the gradients make a knocking noise.
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
a Our NMR signal is made up of the sum of lots of different frequencies, corresponding to different spatial positions.
What happens in a scan?Processing the data
c Fourier transforms, from Level 2 Maths, can unscramble the data and make an image.
a
b
c
FT
b Acquiring multiple signals gives us a 2-D dataset.
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)
• We can inject a chemical called a contrast agent into the bloodstream.
• 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.
-0,05
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,45
0 20 40 60 80 100 120 140 160 180
Time (s)
Gd
Co
nce
ntr
atio
n
• As it reaches the brain, the signal intensity rises.
Data source: James D’Arcy and David Collins, Institute of Cancer Research, Sutton
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
• MR images can be sensitised to the rate of diffusion of water molecules.
Data source: Geoff Parker, Institute of Neurology, London
Image contrast in the brain (2): neural fibre tracking
• 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.
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
Data source: Functional Imaging Laboratory, LondonCombined PET / MRI study
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!!
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 ...
Data source : SMIS
Lumbar spine Knee sports injuries
Data source : SMIS
Liver Lung
Liver motion during normal breathing
Data source : ICR, Sutton
The Physics of Seeing Inside People — Presentation by Simon J Doran, Dept of Physics, University of Surrey
A virtual tour round the human colon ...
Computer-generated “flythrough” model
Original MR images
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
MRI in cardiology• High resolution
anatomical imaging
• Imaging of coronary arteries
• “Movies” — data acquired in < 20 s
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
• It is money — the human brain is a very valuable thing — which has led to the incredible developments that we see today.
• It is study of basic Physics (electromagnetism, nuclear physics, mechanics) which has discovered the principles.
• The different methods tell us different things.
• There are many different ways of imaging the human body.