The Physics of Seeing Inside People A brief overview of medical imaging using MRI, PET, Ultrasound, X-ray CT and MEG Dr. S. J. Doran Department of Physics.

The Physics of Seeing Inside People

A brief overview of medical imaging using MRI, PET, Ultrasound, X-ray CT and MEG

Dr. S. J. Doran

Department of PhysicsUniversity of Surrey

Structure of the Talk

• What is Medical Imaging?

• Why use different methods of imaging?

• Basic principles of imaging Ultrasound

MRI

X-ray computed tomography (CT)

PET

MEG

What is Medical Imaging?

Medical imaging as seen by “Tomorrow’s World” !

Hi-tech scanner

Patientgoes in

Images (preferably wacky colours)come out

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

Data source : Toshiba America Medical SystemsVisualisation :Vitrea 2, Vital ImagesData source : Mayo Clinic

Why use different methods of imaging ?

1. 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.

Data source : SMIS Ltd

Why use different methods of imaging ?

1. 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

Why use different methods of imaging ?

2. Accessibility and Portability

Ultrasound MRI (sometimes!)

A sliding scale

MRI (most of the time!)… and almost everything else.

Why use different methods of imaging ?

3. Repeated exposure and safety

• No medical imaging scan will be done unless the prescribing doctor is sure that it will not harm you.

• Nevertheless, human biology is a complicated thing that none of us completely understands, so we try to keep all risks to an absolute minimum.

• Different modalities of imaging use different types of radiation (e.g., MRI uses radio waves, PET uses gamma rays), which have different characteristics. At all times, we should pick the least invasive method.

The ALARP Principle - As Low As Reasonably Practiable

Why use different methods of imaging ?

4. Patient acceptability

• Different types of scan entail different degrees of patient discomfort. Some might require an injection of a contrast agent. The scanning environment is also important:

Patient can be accompanied by Patient isolatedfriends or relatives.

Open, comfortable environment Claustrophobic tunnel

Close contact with radiographer Contact only via intercomduring scan procedure

Patients can see images of Patients do not know whatthemselves during the scan. is happening.

Ultrasound MRI

Why use different methods of imaging ?

5. Cost Effectiveness

• A good guiding principle in many walks of life is …

Always pick the simplest solution for your problem.

• In many cases the cheapest solution is the best.You do not need to give every pregnant mother in the country an MRI scan.

• The capital cost of installation varies widely:

Ultrasound X-ray CT MRI PET

£20k - £100k £500k - £1M £2M - £4M >£5M

Basic Principles of Diagnostic Ultrasound

Lower amplitudereflected pulse

0

ReflectorEmitted pulse

Transducer

• Based on ultrasound reflection and attenuation coefficients

• Position calculated using equation d = ct/2

c

cd

Use of Ultrasound in Obstetrics

Data source : Joseph Woo

5.5 Weeks 6 Weeks

Use of Ultrasound in Obstetrics

Data source : Joseph Woo

Bi-parietal diameter Length of femur

Measurements of foetus in utero

Use of Ultrasound in Obstetrics

Data source : Joseph Woo

18 Weeks 19 Weeks

Use of Ultrasound in Obstetrics

Data source : Joseph Woo

Duplex Duplex of flow in umbilical chord

Measurements of blood flow on the foetus in utero

Use of Ultrasound in Cardiology

Data source : Arizona Society of Echocardiography Image Library

Standard real-time B-scan Duplex scan: colour Doppler super-imposed on real-time B-scan

Diagnosis: Severe mitral regurgitation due to flail posterior MV leaflet. Underlying pathology: Mitral valve prolapse with ruptured chordae tendinae.

Duplex Doppler Flow Data

“Signed” velocity “Doppler Power”

Flow pattern at the bifurcation of the carotid artery

Data source : St. Paul’s Hospital, BC, Canada

Basic Principles of MRI

Strong magnetic

field

PrecessionSpin

S

N

Emitted photon

Spin down

Spin up

• In a magnetic field, spinning nuclei precess at the Larmor frequency.

• The spin-up and spin-down nuclei have different energies.

• Transitions between levels lead to emission of photons, which we can detect.

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.

Basic Principles of X-ray CT

• A standard tube produces X-rays with an energy of approximately 150 keV.

• A fan-shaped beam passes through the patient and is detected to form a projection.

• The source and detector are rotated to obtain a larger number of different projections.

• Images are reconstructed using a technique called back projection and give a measure of the X-ray attenuation coefficient at every point.

X-ray source

X-ray detector

strip

~

- +150 kV Target

Filament

Bone shows up bright

Air is dark

Different densities of tissue give inter-mediate results

X-ray CT Pictures of the Head and Abdomen

Basic Principles of Positron Emission Tomography (PET)

-ray detector

+-

Radioactive nucleus

• A radioactive isotope is injected and decays, emitting a +-particle.

• Within a short distance, the +-particle bumps into an electron and the two annihilate, producing a pair of -rays.

• By detecting and reconstructing where the -rays of come from, we can measure the location and concentration of radio-isotope.

A Typical PET Scanner Installation

The PET scanner itself

Cyclotron

Radio-chemistry Lab

Image Source: North Carolina Baptist Hospital/Bowman Gray School of Medicine; Children’s Hospital, University of Michigan

Two Typical PET Studies

FDG Study of Patient with Stroke

“Dead” areas of brainNo glucose metabolism

Data source: Bowman Gray School of Medicine

FDG “Brain Activation” Study

The highlighted region shows whichpart of the brain (the parietal lobe) was active during a visual stimulation task.

Data source: CVVC, Psychology Dept., Durham Univ,.

Principles of Magnetoencephalography (MEG)

• The patient lies, in a magnetically screened room, next to a large number of very sensitive magnetic field detectors.

Data source: FORENAP, Rouffach, France

• Electric currents within the brain create tiny magnetic fields — note the scale on the graph (fT, I.e., 1015 times the magnetic field in an MRI scanner).

• The complete set of measurements allows us to “solve” the “inverse problem” and find out what sort of current distribution created the magnetic fields.

• We can then plot the result on, for example, an “anatomical” MRI scan.

Image fusion — combining modalities

Clinical Study: Lung Tumour

CT PET

Fused

Research study: functional imaging

Data source: Functional Imaging Laboratory, LondonCombined PET / MRI study

Typical uses of the different types of imaging (1)

• X-ray imaging Plane-film: bony trauma e.g., car crash Fluoroscopy (real-time projection imaging, e.g., intra-operative) CT: head scans involving fracture, bleeding

scanning of other organs via addition of contrast agents

• Ultrasound Obstetrics Flow and cardiac imaging Applications requiring high portability of equipment

• MRI Method of choice for most soft tissue imaging Cancer, stroke, multiple sclerosis, degenerative diseases Functional brain mapping

Typical uses of the different types of imaging (2)

• PET Functional brain imaging using H2

17O or radio-labelled glucose (fluoro-deoxy glucose FDG).

Metabolic studies Receptor studies

• MEG Epilepsy — location of seizure focus Brain function

• + other techniques not discussed in detail SPECT — Single Photon Emission Computed Tomography Optical imaging Optical tomography

Conclusion

• There are many different ways of imaging the human body.

• The different methods tell us different things.

• It is study of basic Physics (acoustics, magnetism, nuclear and particle physics) 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.