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.