Download The Physics of seeing inside people

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
University 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?
Hi-tech
scanner
Images
(preferably
wacky colours)
come out
Patient
goes in
Medical imaging as seen by “Tomorrow’s World” !
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
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 : Toshiba America Medical Systems
Data
source
: Mayo
Clinic
Visualisation
: Vitrea
2, Vital
Images
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 : FORENAP,
Rouffach, France
Data source : SMIS Ltd
Data source : CSUA, Berkeley
Why use different methods of imaging ?
2. Accessibility and Portability
A sliding scale
Ultrasound
MRI (sometimes!)
MRI (most of the time!)
… and almost everything else.
Why use different methods of imaging ?
3. Repeated exposure and safety
The ALARP Principle - As Low As Reasonably Practiable
• 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.
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:
Ultrasound
MRI
Patient can be accompanied by
friends or relatives.
Patient isolated
Open, comfortable environment
Claustrophobic tunnel
Close contact with radiographer
during scan procedure
Contact only via intercom
Patients can see images of
themselves during the scan.
Patients do not know what
is happening.
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
£20k - £100k
X-ray CT
£500k - £1M
MRI
£2M - £4M
PET
>£5M
Basic Principles of Diagnostic Ultrasound
Reflector
Emitted pulse
c
0
c
d
Transducer
•
•
Lower amplitude
reflected pulse
Based on ultrasound reflection and attenuation coefficients
Position calculated using equation d = ct/2
Use of Ultrasound in Obstetrics
5.5 Weeks
6 Weeks
Data source : Joseph Woo
Use of Ultrasound in Obstetrics
Bi-parietal diameter
Length of femur
Measurements of foetus in utero
Data source : Joseph Woo
Use of Ultrasound in Obstetrics
18 Weeks
19 Weeks
Data source : Joseph Woo
Use of Ultrasound in Obstetrics
Duplex
Duplex of flow in umbilical chord
Measurements of blood flow on the foetus in utero
Data source : Joseph Woo
Use of Ultrasound in Cardiology
Standard real-time B-scan
Duplex scan: colour Doppler superimposed on real-time B-scan
Diagnosis: Severe mitral regurgitation due to flail posterior MV leaflet.
Underlying pathology: Mitral valve prolapse with ruptured chordae tendinae.
Data source : Arizona Society of Echocardiography Image Library
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
Precession
Spin
N
S
•
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.
Spin down
Spin up
Strong
magnetic
field
Emitted photon
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
X-ray
detector
strip
•
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
150 kV
- +
~
Filament
Target
X-ray CT Pictures of the Head and Abdomen
Bone shows up bright
Air is dark
Different densities
of tissue give intermediate results
Basic Principles of Positron Emission Tomography (PET)
g-ray
detector
+
Radioactive nucleus
•
A radioactive isotope is injected and decays,
emitting a b +-particle.
•
Within a short distance, the b +-particle bumps
into an electron and the two annihilate,
producing a pair of g -rays.
•
By detecting and reconstructing where the g rays of come from, we can measure the
location and concentration of radio-isotope.
A Typical PET Scanner Installation
Cyclotron
The PET scanner itself
Radio-chemistry Lab
Image Source: North Carolina Baptist Hospital/Bowman Gray School of Medicine; Children’s Hospital, University of Michigan
Two Typical PET Studies
The highlighted region shows which
part of the brain (the parietal lobe) was
active during a visual stimulation task.
Data source: CVVC, Psychology Dept., Durham Univ,.
“Dead” areas of brain
No glucose metabolism
Data source: Bowman Gray
School of Medicine
FDG Study of Patient with Stroke
FDG “Brain Activation” Study
Principles of Magnetoencephalography (MEG)
•
The patient lies, in a magnetically screened room, next to a large number of very
sensitive magnetic field detectors.
•
Electric currents within the brain create tiny magnetic fields — note the scale on the
graph (fT, I.e., 10-15 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.
Data source: FORENAP, Rouffach, France
Image fusion — combining modalities
Clinical Study: Lung Tumour
CT
Research study: functional imaging
PET
Fused
Data source:
Functional Imaging Laboratory, London
Combined 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 H217O 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.