Download Ultrasound

The Physics of Seeing Inside People
A brief overview of medical imaging using MRI, PET, Ultrasound
and X-ray CT
Dr. S. J. Doran
Department of Physics
University of Surrey
Structure of the Talk
• Why use different methods of imaging ?

To look at different things

Accessibility - who can undergo a scan?

Repeated exposure and safety

Cost effectiveness
• Basic principles of imaging

Ultrasound

MRI

X-ray computed tomography (CT)

PET
Why use different methods of imaging ?
1. Different methods reveal different features
• X-ray CT maps the attenuation X-rays
Good for mapping bone structure
• Ultrasound maps the reflection and
attenuation of sound
• MRI maps the distribution and
“environment” of water molecules in the
body
• PET maps the distribution of radioactively
labelled compounds
Why use different methods of imaging ?
2. Accessibility and Portability
MRI
A sliding scale
Ultrasound
Why use different methods of imaging ?
3. Repeated exposure and safety
The ALARA Principle - As Low As Reasonably Achievable
• 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. 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 and 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
Magnet for Whole-Body Imager
Data source : VA Imaging Centre, University of Florida
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.
MRI can image much more than just the brain ...
Knee
Knee (close up, different patient)
Data source : SMIS
MRI can image much more than just the brain ...
Chest region
Arm, wrist and hand
Lumbar spine
Data source : UoS, SMIS
Magnetic Resonance Angiography
Neck
Small part of head
MRI can also be used to visualise patterns of blood vessels. In these pictures, the image has been
sensitised to movement, in this case the movement of blood inside veins and arteries.
Data source : The Whole-brain Atlas, K. A. Johnson and J. A. Becker, Harvard
Functional Imaging
Functional MRI (fMRI) is the “hottest” topic in MRI for
many years. If we acquire one image of the brain in a
resting state, followed by another with the brain
“stimulated” in some way, we can compare the two.
Any regions which are different correspond to areas of
brain activation.
The image is a 3-D picture of a human brain, which
has been “sliced” in the computer to reveal the interior
at a particular level in the brain (white and grey).
Superimposed on this are coloured regions, which
correspond to the activated areas of the brain.
Data source : UW Madison MRI Center
Basic Principles of X-ray CT
X-ray
detector
•
A standard tube produces X-rays with
an energy of approximately 150 keV.
•
The X-ray focal spot scans across and
around the patient.
•
The technique measures the X-ray
attenuation coefficient of the different
tissues in the body.
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
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
Image Fusion — Combining Modalities
Clinical Study - Lung Tumour
CT
Research Study MRI + PET
PET
Fused
Location of Dopamine Receptors
Data source: Bowman Gray School of Medicine
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.