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VCE Physics
Unit 1
Topic 3
Medical Physics
Unit Outline
• This unit covers the following topics
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applications of radioisotopes to medical diagnosis and treatment;
the operation of optical fibres as endoscopes and other applications
for diagnosis and treatment;
the use of laser treatments considering a laser as an intense energy
source;
processes of medical imaging using two or more of ultrasound, Xrays, CT and PET
make simple interpretations of images of the human body produced
by ultrasound, X-rays, CT, PET and MRI.
Chapter 1
Nuclear Medicine:
Radioisotopes
1.0 Nuclear Medicine –
A Little History
Medical uses of radioactive elements had its
beginnings in the work of the Curies, Pierre
(1859 – 1906) and his wife Marie (1867 – 1934).
Together they discovered two highly radioactive
elements Polonium (400 times more radioactive
than Uranium) and Radium (900 times more
radioactive).
The first recorded medical use of a radioactive substance
occurred in France in 1901 when radium was used as a cancer
treatment.
The first recorded radium use in Australia was by a Melbourne
dermatologist in 1903.
The first diagnostic use of a radioisotope was in 1924
when a decay product of Radium was injected into the
bloodstream and its movement through the body was
recorded with a geiger counter.
1.1 Radioisotopes
Radioisotopes (sometimes called radionuclides) are
unstable atoms which, in searching for stability, emit either
energy (in the form of gamma rays), or matter (in the form
of neutrons, alpha or beta particles).
Radioisotopes can be naturally occuring, eg Carbon-14 (14C) or man made, eg
Cobalt-60 (60Co).
Man made radionuclides are manufactured in either a Cyclotron (a particle
accelerator) or in a nuclear reactor which, in Australia’s case, is located at
Lucas Heights, just south of Sydney.
Radioisotopes are used in many areas:
In Agriculture - to investigate plant growth and fertiliser take up.
In Industry - to check important welds in pipes etc, and to
measure metal thickness.
In Archaeology - to carbon date ancient objects.
In Sewage Disposal - to trace water flows.
In Medicine – to detect and treat disease.
1.2 Medical Radioisotopes
When used in Medicine, radioisotopes
fall into one of two groups:
1. Diagnostic Radionuclides
2. Therapeutic Radionuclides
Radioisotope
Half Life
A representative list of
medical radioisotopes
is shown in the table
Uses
Sodium – 24 (24Na)
15 Hours
Study of general biological
processes
Iron – 59 (59Fe)
46.3 Days
Diagnosis of Blood Disease
Technetium - 99m (99mTc)
6 Hours
Diagnosis of various
diseases
Cobalt - 60 (60Co)
5.3 Years
Treatment of Cancer
Strontium - 90 (90Sr)
27.7 Years
Treatment of Tumors
Iodine – 131 (131I)
2.6 Minutes
Treatment of thyroid cancers
1.3 Diagnostic Radioisotopes
To be useful as a diagnostic tool, a radioisotope must meet
certain criteria. It must:
(a) have a short half life, ideally about the same as the time
required to perform the diagnosis.
(b) not emit alpha or beta radiation, because they would be
trapped inside the patient and could not be detected
externally.
(c) emit gamma radiation which is energetic enough to allow
its exact source to be identified.
(d) be energetic enough to provide useful clinical information
but not so energetic as to be dangerous to the patient.
From a field of more than 2300 radioisotopes, only a
handful come close to satisfying the criteria for use
as diagnostic tools.
Of these, the reactor produced Technetium – 99m,
is by far the best, being used in more that 80% of all
nuclear diagnostic tests performed.
Note: the m in the symbol 99mTc means this is the
“metastable” form of Tc, which radiates gamma
rays and low energy electrons.
1.4 Technetium 99m
The radioisotope most widely used in medicine is
technetium-99m.
It is an isotope of the reactor-produced element
technetium and it has almost ideal characteristics for a
nuclear medicine scan.
These are:
a. It has a half-life of six hours which is long enough to
examine metabolic processes yet short enough to
minimise the radiation dose to the patient.
b. Technetium-99m decays by an "isomeric" process
which emits gamma rays and low energy electrons.
Since there is no high energy beta emission the
radiation dose to the patient is low.
c. The low energy gamma rays it emits easily escape
the human body and are accurately detected by a
gamma camera. Once again the radiation dose to the
patient is minimised.
d. The chemistry of technetium is so versatile it can
form tracers by being incorporated into a range of
biologically-active substances to ensure that it
concentrates in the tissue or organ of interest.
1.5 Technetium Delivery
Technetium generators are popularly known as
“technetium cows” because they can be “milked” of
technetium as needed.
The generator consists of a lead pot enclosing a glass
tube containing the radioisotope, is supplied to
hospitals from the nuclear reactor where the isotopes
are made.
It contains molybdenum-99, with a half-life of 66 hours,
which progressively decays to technetium-99m.
The Tc-99m is washed out of the lead pot by saline
solution when it is required.
The generator is exhausted after approximately two
weeks and returned for recharging.
1.6 The Gamma Camera
Once produced, 99mTc is linked to chemical compounds which
permit specific physiological processes to be scrutinised.
It can be given by injection, inhalation or orally.
The gamma ray photons are detected by
a gamma camera which can view
organs from many different angles.
The camera builds up an image from the
points from which radiation is emitted.
This image is enhanced by a computer
and viewed by a physician on a monitor
for indications of abnormal conditions.
Gamma Camera
1.7 Diagnosis
Positioning of the radiation source within the body
is the fundamental difference between nuclear
medicine imaging and other imaging techniques
such as x-rays.
Gamma imaging provides a view of the position and
concentration of the radioisotope within the body.
Organ malfunction can be indicated if the isotope is either
partially taken up in the organ (cold spot), or taken up in
excess (hot spot).
A series of images are taken over a period of time that
show unusual patterns or rates of isotope movement could
indicate malfunction in the organ.
A distinct advantage of nuclear imaging over x-ray
techniques is that both bone and soft tissue can be
imaged very successfully.
This has led to its common use in developed countries
where the probability of anyone having such a test is
about one in two and rising.
1.8 Therapeutic Radioisotopes
Rapidly dividing cells are particularly sensitive to damage by
radiation.
For this reason, some cancerous growths can be controlled or
eliminated by irradiating the area containing the growth.
External irradiation can be carried out using a gamma
beam from a radioactive cobalt-60 source, though in
developed countries the much more versatile linear
accelerators are now being utilised as a high-energy x-ray
source (gamma and x-rays are much the same).
Internal radiotherapy is by administering or planting a small
radiation source, usually a gamma or beta emitter, in the
target area.
Iodine-131 is commonly used to treat thyroid cancer, probably
the most successful kind of cancer treatment.
Iridium-192 implants are used especially in the head and
breast.
They are produced in wire form and are introduced through a catheter to the
target area. After administering the correct dose, the implant wire is removed
to shielded storage. This procedure gives less overall radiation to the body, is
more localised to the target tumour and is cost effective.
1.9 A Cure for Anything !
At present, approximately 35 radioisotopes
are commonly used in the detection and
treatment of illness or disease. Those used
for treatment include:
Radioisotope
Cobalt – 60:
Dysprosium-165:
Iodine-125:
Iodine-131:
Phosphorus-32:
Rhenium-188:
Samarium-153:
Boron – 10:
Use
External cancer radiation
Treatment, arthritis
Treatment, cancer of prostate, brain
Treatment, cancer of thryoid
Treatment, excess red blood cells
Treatment, coronary artery disease
Treatment, breast, prostate cancers
Treatment, brain tumours
Chapter 2
Optical Instruments:
Endoscopes
2.0 Endoscopes
The name endoscope is derived from two Greek
words: endom (within) and skopein (view).
The endoscope is an optical instrument used for
viewing internal organs through natural openings
(ear, throat, rectum, etc.) or through a small
incision in the skin.
There are 2 basic types of endoscopes: Rigid and Flexible
Various rigid & flexible
endoscopes
In rigid endoscopes the image is conveyed by a
relay of lenses.
The classical rigid endoscopes have a number of
periscopic and field lenses in order to convey the
image from distal end to the eyepiece.
Generally, a flexible endoscope is referred to as a
fibrescope.
In flexible endoscopes, a bundle of precisely
aligned flexible optical fibres is used.
2.1 Endoscopes – Some History
The concept of endoscopy originated in
the early 19th century.
Philip Bonzini, an Italian doctor, is
credited with the first use of a rigid
endoscope in humans in the early
1800’s
In 1930, German medical student, Heinrich
Lamm was the first person to assemble a
bundle of optical fibres to carry an image.
Lamm's goal was to look inside
inaccessible parts of the body.
During his experiments, he reported
transmitting the image of a light bulb.
However the image was of poor quality.
The first endoscope made of optical fibres (fibrescope)
was used for viewing the stomach and esophagus at
the University of Michigan School of Medicine in 1957.
Since then, there has been rapid progress in
endoscope development.
2.2 Principles of Optical Fibres
Total internal reflection (TIR) is the most important phenomenon for the guiding of
light in optical fibres.
With TIR light can be completely reflected at the optical fibre surface without any
reflective coating.
TIR can only occur for light travelling from a more dense to a less dense medium.
Thus, in the diagram, the refractive index of the actual optical fibre n1 is greater
than that of the cladding n2 .
For TIR to occur the angle of incidence (θ) must be greater than the critical
angle
n1 > n2
Lost Light
θ1 θ1
Some Reflected Light
θ1< θC
θC
Critical Angle
θ2 θ2
n2 Cladding
n1 Optical Fibre
All Light Reflected
θ2> θC
n2 Cladding
For light with with θ < θC , much of the light is refracted out of the optical fibre
For light with θ = θC , all light is refracted so it just grazes the surface of the fibre
For light with θ > θC , light is totally internally reflected and will continue to do
so whenever it strikes the fibre’s surface.
2.3 Optical Fibre Construction
Usually optical fibres are
Cross Section
Refractive Index Profile
Fibre Type
flexible, thin, cylindrical
and made of transparent
RI
materials such as glass
Step
r n1
n2
n1
and plastic.
n2
Index
r
The most abundant and
widespread material used
to make optical fibre is
RI
n1
glass and most often this
Graded
r
is an oxide glass based on
n2
n1
Index
n2
silica (SiO2) with some
r
additives.
The required properties for an optical fibre are:
optical quality, mechanical strength, and flexibility.
For these reasons, plastic optical fibres have been made with polymethylmethacrylate (PMMA). They have a “tighter turning circle” than glass fibres.
In general, optical fibres have a cylindrical core and are surrounded by a cladding.
If both Refractive Indexes, (n1) and (n2) are uniform across their cross sections,
the fibre is called a STEP INDEX FIBRE (SI) .
If (n1) varies with the core radius (i.e., (n1) gradually decreases from the centre of
the core to n2 at the outer radius), it is a GRADED INDEX FIBRE (GRIN).
2.4 Step vs Graded Fibres
Cladding
Core
Fibre
Accptance
Cone
Step Index Fibre
In GRIN fibre, the
gradient in the
refractive index
gradually bends the
rays back toward the
axis.
In SI fibre, the light
rays zigzag between
the core/cladding on
each side of the fibre
axis.
The Fibre Acceptance
Cone represents the
range of angles for
which the incidence
angles are greater
than the critical angle
Core
Fibre
Accptance
Cone
Graded Index Fibre
2.5 Fibre Bundles
It is impossible for a single fibre to transmit an image.
An individual fibre can transmit only a spot of a certain color and intensity.
To transmit an image, a large number of single fibres must be aligned and
fused together.
This means assembly of optical fibres in which the fibres are ordered in
exactly the same way at both ends of the bundle to create an image.
This type of fibre bundle is called a Coherent Bundle
(a) is a low power endoscope
(b) is a high power endoscope
Incoherent Bundles are
groups of fibres which are
not ordered at both ends.
They are used as light pipes
to bring light from an
external source down the
endoscope to illuminate the
area under view.
Object seen by
endoscope
Image projected to
eyepiece
Object seen by
endoscope
Image projected to
eyepiece
2.6 Endoscope Construction
Fibre Optic Endoscopes
Endoscopic
have a number of
“Pictures”
basic components:
1. A Coherent Bundle for
bringing the image to
the eyepiece (or video
monitor).
2. An Incoherent Bundle
for taking an external
light source down the
A Ball Bearing
endoscope to
lodged in the
illuminate the viewing
oesophagus
area.
3. Optional tubes or
channels for the
passage of air, water,
as well as remote
control implements
such as biopsy
forceps or cytology
Stomach Ulcer
brushes.
A piece of dried
pork crackling stuck
in oesophagus
A coin in the
stomach
2.7 Endoscope Man
An incredible number of
endoscopes have been
developed for both diagnosis
and treatment.
Some of the more common are
shown on “Endoscope Man”
Ultra thin endoscope for
investigating blood vessels
ARTHROSCOPE
Famous in “Aussie Rules” for
investigating knee injuries
Typical Rigid Endoscope
Most commonly used
endoscope in general
surgery.
Chapter 3
Lasers &
Laser Treatments
3.0 Laser Basics
"Laser" is an acronym for Light Amplification by
Stimulated Emission of Radiation.
Although there are many types of lasers, all have
certain common features.
In explaining laser operation, the common ruby laser
will be used as an example.
In a laser, the lasing medium (the
ruby crystal) is so called
Ruby Crystal
“pumped” to get the electrons of
the ruby atoms into an excited
Ruby Atoms
state.
Flash Tube
These excited electrons then
release their excess energy
Mirror
as photons of red light.
Ruby Atoms
Partially silvered mirror
These red photons
rush back and forth
finally exiting the
tube as a coherent
beam
Typically, very intense
flashes of light from a flash
tube (or from an electrical
discharge pump) enter the
lasing medium and create a
large collection of excitedstate atoms (atoms with
higher- energy electrons).
3.1 Laser Types
Since their development in 1960, lasers used in
medicine and surgery have evolved, and while
medical lasers have never become the "magic ray"
that some had hoped, they have become powerful
and indispensable tools in clinical practice.
There are many medical laser systems available
today, but they all use the principal of selective
photothermolysis which means getting the right
amount of the right wavelength of laser energy to the
right tissue to damage or destroy only that tissue,
and nothing else.
Letters etched on a human
hair using an Eximer Laser
Note: YAG = YttriumAluminium- Garnett
KTP = potassium-titanylphosphate
Ruby
Laser
Some of the many medical and surgical lasers in use.
3.1 Laser Types & Treatments
Laser
CO2
Wavelength
(nm)
10,600
Use
Surgery (used as a “scalpel”)
Er: YAG
2940
“Shaving” of skin to remove wrinkles
Ho: YAG
2070
Shaving bones (eg, arthroscopes), kidney stone remov
Nd: YAG
1064
Blue/black ink tattoo removal; hair removal
Diode
800 to 900
Hair removal; dental surgery
Alexandrite
755
Blue/black ink tattoo removal; hair removal
Ruby
694
Treatment and removal of moles, freckles, birthmarks
Pulsed Dye
KTP
577 to 585
532
Argon
488 to 514
Eximer
193
Treatment of port wine birthmarks and spider veins
Cutting tissue, red/yellow tattoo ink removal
Retinal and ear surgery, removal of birthmarks
Laser eye correction
Tattoo removal using
Nd:YAG laser
Chapter 4
Ultrasound
4.0 Ultrasound Basics
Definition of Ultrasound
Sound consists of travelling pressure waves
Speed of sound waves in human tissue: ~ 1500 ms-1.
Frequency range: between 2 MHz and 10 MHz
Ultrasound is produced using piezo-electric transducers,
crystals which change shape under the action of an electric
field.
Quartz is the most commonly known piezo-electric material.
The disk is placed between 2
electrodes and applying a voltage
causes the crystal to vibrate.
Better performing piezo –
electric materials (such
as BARIUM TITANATE or
LEAD ZIRCONATE) is
formed into disks.
The crystal will vibrate at the same frequency
as the supply voltage, producing sound waves
with frequencies between 2 and 10 MHz
Variable frequency
A.C Voltage:
V = VoCos 2π ft
The crystal’s vibrations set up
Ultrasonic sound waves in the
medium around the crystal
4.1 Echo Location
Ultrasound or ultrasonography is a medical imaging technique that uses high
frequency sound waves and their echoes.
The technique is similar to the echolocation used by bats, whales and dolphins,
as well as SONAR used by submarines.
The ultrasound signals
generated as previously
described leave the handpiece
and are reflected back from
various tissues and bones.
These reflected waves strike the handpiece causing
the piezo electric crystal to contract and expand.
This change in shape causes a voltage to be
generated which is then processed into a “picture”.
4.2 Ultrasonic Speeds
When ultrasonic waves are applied to various body tissues they travel at
varying speeds from a low of 1450 ms-1 through fat to a high of 4080 ms-1
through skull bone.
Ultrasound image of
yolk sac and fetus at 6
week gestation.
4.3 Sound Intensity Profile
Field Zones
Near Field - the region of a sound
beam in which the beam diameter
decreases as the distance from the
transducer increases. This zone is
called the Fresnel (Fra-nel, the s is
silent) zone.
Beam Properties:
Focal Zone - the region where the
beam diameter is most concentrated
giving the greatest degree of focus.
Longitudinal Waves - the wave in which
the particle motion is parallel to the
direction of the wave travel.
A series of longitudinal waves make up
the ultrasound beam.
Far Field - the region where the
beam diameter increases as the
distance from the transducer
increases. This zone is called the
Fraunhoffer zone
The best ultrasound images are produced with the transducer operating in the
Focal Zone.
4.4 An Ultrasound Examination
1.
2.
3.
4.
5.
6.
In ultrasound examination, the following
events happen:
Below is an Ultrasound image of a
High-frequency sound pulses are
growing fetus (approximately 12
transmitted into your body using a probe.
weeks old) inside a mother's
The waves travel into your body and hit a
uterus.
boundary between tissues (e.g. between
This is a side view of the baby,
fluid and soft tissue, soft tissue and bone). showing:
Some of the sound waves get reflected
back to the probe, while some travel on
further until they reach another boundary
and get reflected.
The reflected waves are picked up by the
probe and relayed to the machine.
The machine calculates the distance from
the probe to the tissue or organ
(boundaries) using the speed of sound in
tissue and the time of the each echo's
return (usually on the order of millionths of
Legs
a second).
Neck Head
The machine displays the distances and
Torso
intensities of the echoes on the screen,
forming a two dimensional image.
4.5 Ultrasound in 3D
In the past few years,
ultrasound machines
capable of threedimensional imaging have
been developed.
In these machines, several
two-dimensional images
are acquired by moving
the probes across the
body surface or rotating
inserted probes.
The two-dimensional
scans are then combined
by specialized computer
software to form 3-D
images.
3-D ultrasound images Photo courtesy Philips Research
The same computer technology is used to
produce the famous “dancing babies”
images
4.6 Doppler Ultrasound
Doppler ultrasound is based upon the Doppler Effect.
When the object reflecting the ultrasound waves is moving, it
changes the frequency of the echoes, creating a higher
frequency if it is moving toward the probe and a lower
frequency if it is moving away from the probe.
How much the frequency is changed
depends upon how fast the object is
moving.
Doppler ultrasound measures the
change in frequency of the echoes to
calculate how fast an object is
moving.
Doppler ultrasound has been used
mostly to measure the rate of blood
flow through the heart and major
arteries.
Chapter 5
X Rays
5.0 X rays
Wilhelm Conrad
Roentgen (1845-1923)
Roentgen found that, if the discharge tube is enclosed in a
sealed, thick black carton to exclude all light, and if he worked
in a dark room, a paper plate covered on one side with the
compound barium platinocyanide placed in the path of the
rays became fluorescent (gave out a greenish light) even when
it was as far as two metres from the discharge tube
Following this discovery, he
asked his wife to hold her hand in
In 1895 Röntgen was studying
the path of rays between the tube
what happened when an electric
and a photographic plate.
current was passed through a gas
He observed, after developing the
of extremely low pressure in
plate, an image of his wife's hand
apparatus called Crooke’s Tubes.
which showed the shadows
The
thrown by the bones of her hand
“first X ray” and that of a ring she was
wearing.
Because the nature of the new rays was then
unknown, he gave them the name X-rays.
Later it was shown that they are of the same
electromagnetic nature as light, but differ from it
only in the higher frequency of their vibration.
5.1 X Ray Production
An x-ray machine, like
that used in a doctor's
or a dentist's office, is
really very simple.
X-rays are just like any other kind
of electromagnetic radiation.
They are produced in parcels of
Individual Photon
energy called photons, just like
light.
There are two different atomic
processes that can produce xray photons.
Inside the machine is an x- 1. The first is called
Bremsstrahlung, which is a
ray tube.
An electron gun inside the fancy German name meaning
"braking radiation."
tube shoots high energy
electrons at a target made
of heavy atoms, such as
2. The other is called K-shell
tungsten.
emission. They can both occur
X-rays come out because of
in heavy atoms like tungsten.
atomic processes induced
by the energetic electrons
shot at the target.
5.2 Types of X Rays
1. Bremsstrahlung. This form of X
radiation occurs when the velocity
of electrons fired towards the
tungsten nucleus changes.
2. K Shell. The K-shell is the lowest energy
state of an atom.
The incoming electron can give the K shell
electron enough energy to knock it out of its
energy state.
This electron slows down after
swinging around the nucleus of a Then, a tungsten electron of higher energy
tungsten atom and loses energy by (from an outer shell) can fall into the K-shell.
The energy lost by the falling electron
radiating x-rays.
In this process, a lot of photons of shows up in an emitted x-ray photon.
Meanwhile, higher energy electrons fall into
different wavelengths are
produced, but none of the photons the vacated energy state in the outer shell,
has more energy than the electron and so on.
had to begin with.
K-shell emission produces higher-intensity
After emitting the spectrum of xx-rays than Bremsstrahlung, and the x-ray
ray radiation the original electron
photon comes out at a single wavelength.
is slowed down or stopped.
5.3 X Ray Diagnostics
X rays are most
commonly used
for investigation
of the skeleton,
the diagnosis of
broken bones
and the display
of the effects of
trauma on the
body.
Steel spikes
in wrist
Shotgun Pellets
Shattered Femur
Broken Femur
Chapter 6
CT Scans
6.0 CT Scans
One of the first
dedicated head CT
scanners, in 1974
CT or Computerised Tomography, also
know as CAT or Computerised Axial
Tomography Scans use an X-ray
source coupled with an X-ray detector
on the opposite side of the body, which
are rotated together to give a crosssectional picture of the body at one
level or cut.
CT scans are of greatest value for
showing physical changes in tissue,
although small tumours may be
missed if absorption properties are
like those of normal tissue.
COMPUTED AXIAL TOMOGRAPHY
CAT Scan of the
Pelvic Region
• Images the body using X-rays.
• Initial research: 1960s
• Applied research: 1970s-80s
• X-rays are sent through the body at
various angles, resulting in cross-sectional
images.
6.1 CT - History
Tomography is from the Greek
word "tomos" meaning "slice"
or "section" and graphia
meaning "describing".
CT was invented in 1972 by
British engineer Godfrey
Hounsfield of EMI
Laboratories, England, and
independently by South
African born physicist Allan
Cormack of Tufts University,
Massachusetts.
CT image of a normal brain using
a state-of-the-art CT system and a
512 x 512 matrix image.
The first clinical CT scanners were installed
between 1974 and 1976. The original systems
were dedicated to head imaging only, but "whole
body" systems with larger patient openings
became available in 1976.
CT became widely available by about 1980.
There are now 30,000 installed worldwide.
The first CT scanner took several hours to
acquire the raw data for a single scan or "slice"
and took days to reconstruct a single image
from this raw data.
The latest multi-slice CT systems can collect up
to 4 slices of data in about 350 ms and
reconstruct a 512 x 512-matrix image from
Original CT image from scanner
millions of data points in less than a second. An
circa 1975. This image is a coarse
entire chest (forty 8 mm slices) can be scanned
128 x 128 matrix, showing a slice of in five to ten seconds using the most advanced
the brain
multi-slice CT system.
Chapter 7
PET Scans
7.1 PET Scans
Positron Emission
Tomography, or PET,
scanning is an
imaging technique
that uses radioactive
positrons (positively
charged particles) to
detect subtle changes
in the body's
metabolism and
chemical activities.
PET Scanner
Human Brain Performing
A PET scan provides a
color-coded image of a
body organ in function
rather than its structure.
During a PET scan, a
positron-producing
radioisotope called a
tracer is either injected
into a vein or inhaled as
a gas.
This tracer is typically a chemical that is normally
found in the body (carbon, nitrogen, oxygen) that has
been altered to allow it to emit positrons.
Once the tracer enters the body, it travels through the
bloodstream to a specific target organ, such as the
brain or heart.
There the tracer emits positrons, which collide with electrons (negatively
charged particles), producing gamma rays (similar to X-rays).
These gamma rays are detected by a ringed-shaped PET scanner and
analyzed by a computer to form an image of the target organ's metabolism or
other functions.
Chapter 8
MRI Scans
8.0 MRI - Basic Operation
Typical MRI Scanner
MRI (Magnetic Resonance Imaging) started out as
a tomographic imaging (CT) technique, that is, it
produced an image of a thin slice through the
human body.
MRI has advanced beyond a tomographic imaging
technique to a volume imaging technique.
The human body is primarily
fat and water.
Both fat and water have many
hydrogen atoms which make
the human body roughly 63%
hydrogen atoms.
MRI takes advantage of the fact
that the nuclei of certain atoms,
hydrogen and phosphorous, in
particular, behave like a
magnet.
When the field is turned-off, the
nuclei against the field spin and
release a characteristic radiofrequency photon emission.
In the absence of an external
magnetic field, these
hydrogen atoms are not
lined up in any particular
direction.
When those atoms are placed in a
strong magnetic field, their nuclei
align the axis of spin either with
or against the direction of the
field.
These emissions are
collected and fed into a
computer which
produces the MRI image.
8.1 MRI Scans
MRI scanners are good at looking at the
non-bony parts or "soft tissues" of the
body.
In particular, the brain, spinal cord and
nerves are seen much more clearly with
MRI than with regular x-rays and CAT
scans.
Also, muscles, ligaments and
Knee MRI
Colour Enhanced
tendons are seen quite well so
that MRI scans are commonly
used to look at knees and
shoulders following injuries.
An advantage of MRI is the radio waves used are a
trillion times less energetic (and potentially less
damaging) than X rays.
Neck
Brain
Kidneys
The Magnetic Fields used by
MRI’s are about 1 million times
stronger than the Earth’s field.
So beware funny things can
happen when these machines
are switched on !
A disadvantage of MRI is it’s
higher cost compared to a
regular x-ray or CAT scan.
Chapter 9
Image Interpretation
9.1 Image Interpretation:
X Rays
Bullet lodged
in shoulder
Coin lodged in
child’s oesophageus
Needle in
child’s foot
Broken Ulna Bone
in forearm
9.2 Image Interpretation:
CT Scans
Brain Scan
Cuts due
to MVA
Liver Scan
Stroke – Bleeding
into brain
Brain Scan
Calf (Lower Leg) Scan
Sub Dural Haematoma –
Bleeding inside skull due
to head injury from MVA
DVT (Deep Vein Thrombosis)
Economy Class Syndrome
9.3 Image Interpretation:
MRI
Brain Scan
Brain Tumor
Ruptured Cruciate
Ligaments
Knee Scan
Heart Scan
Movie
Spinal Scan
Breast Scan
Ruptured Disc
Actual
Colour
Enhanced
Breast
Cancer
9.4 Image Interpretation:
PET
Scans
Whole Body
Brain scan – growing child
Scan
Brain Scan
These days, scans
are “colour coded”
making them much
easier to “read”.
Dark Spots
are cancers
Scans comparing
brain activity
during various
activities with the
same brain in its
resting state.
Shows the remarkable increase in
brain activity during the 1st year of life
Resting
Brain
9.5 Image Interpretation:
Ultrasound
Twins shown in colour
enhanced scan
3-D scan of fetus
with a Clubfoot
Information sources:
•Uranium Information Centre (UIC)
•ANSTO
•www.sciencephoto.com/htm
•www.nlm.nih.gov/
•www.spine-inc.com/
•www.epub.org
•www.cancer-therapy-options.com
Ollie Leitl 2003