Download VCE Physics

Drouin Secondary College
VCE Physics
Topic: Medical Physics
VCE Physics
Unit 1
Topic 3
Medical Physics
Page- 1 -
Drouin Secondary College
1.0
Nuclear Medicine
VCE Physics
–
Topic: Medical Physics
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 _______________ (400 times
more radioactive than Uranium) and _____________ (900 times more radioactive).
The first recorded medical use of a radioactive substance occurred in France in 1901
when radium was used as a _____________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 _____________________ and its movement through the body was
recorded with a geiger counter.
1.1
Radioisotopes
Radioisotopes (sometimes called _______________________) are unstable atoms which,
in searching for stability, emit either energy (in the form of ___________ rays), or matter
(in the form of neutrons, ___________ or _________ 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 _______________ in pipes etc, and to measure metal
thickness.
In Archaeology - to carbon date ancient objects.
Page- 2 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
In Sewage Disposal - to trace water flows.
In Medicine – to detect and _____________ disease.
1.2
Medical Radioisotopes
When used in Medicine, radioisotopes fall into one of two groups:
1. _________________Radionuclides
2. _________________ Radionuclides
A representative list of medical radioisotopes is shown in the table
Radioisotope
Half Life
Uses
Sodium – 24 (24Na)
15 Hours
Study of general
Iron – 59 (59Fe)
46.3 Days
biological
processes
Diagnosis of Blood
Technetium - 99m (99mTc)
6 Hours
Cobalt - 60 (60Co)
5.3 Years
Disease
Diagnosis of various
diseases
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 _________half life, ideally about the same as the time required to
perform the diagnosis.
(b) not emit ________ or __________ radiation, because they would be trapped
inside the patient and could not be detected externally.
(c) emit ______________ 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.
Page- 3 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
From a field of more than ___________ 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 __________ 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 _______ 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 ___________ 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.
Page- 4 -
Drouin Secondary College
1.5
VCE Physics
Topic: Medical Physics
Technetium Delivery
Technetium generators are popularly known as “technetium ________” because they can
be “____________” of technetium as needed.
The generator consists of a lead pot enclosing a glass tube containing the radioisotope,
is supplied to hospitals from the ______________ ____________ where the isotopes are
made.
It contains molybdenum-99, with a half-life of _______ 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 __________ 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 point where the radiation is emitted.
This image is enhanced by a computer and viewed by a physician on a monitor for
indications of abnormal conditions.
1.7
Diagnosis
Positioning of the radiation source within the _______ is the fundamental difference
between nuclear medicine imaging and other imaging techniques such as x-rays.
Page- 5 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
Gamma imaging provides a view of the position and ______________________ of the
radioisotope within the body.
Organ malfunction can be indicated if the isotope is either partially taken up in the organ
(_________ spot), or taken up in excess (_________ 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 _________
and ________ 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 _______________.
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 ____________
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 ___________ cancer, probably the most successful
kind of cancer treatment.
Iridium-192 implants are used especially in the ___________ 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.
Page- 6 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
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 _______ radioisotopes are commonly used in the detection
and treatment of illness or disease. Those used for treatment include:
Radioisotope
Use
Cobalt – 60:
External cancer radiation
Dysprosium-165:
Iodine-125:
Iodine-131:
Phosphorus-32:
Rhenium-188:
Samarium-153:
Boron – 10:
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
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: _________ and _____________
In rigid endoscopes the image is conveyed by a relay of _____________________.
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 ____________________.
In flexible endoscopes, a bundle of precisely aligned flexible __________ ___________ is
used.
Page- 7 -
Drouin Secondary College
2.1
VCE Physics
Topic: Medical Physics
Endoscopes – Some History
The concept of endoscopy originated in the early __________ 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 _____________
_____________ 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
_______________ 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 ________dense to a ________ 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
Page- 8 -
Drouin Secondary College
θ θ
1
Some Reflected Light
Topic: Medical Physics
n1 > n2
Lost
Light
1
θ 1 < θC
VCE Physics
θ
θ
θ
C
2
2
Critical
Angle
All Light Reflected
θ2 > θC
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 flexible, thin, cylindrical and made of transparent materials
such as _____________ and plastic.
The most abundant and widespread material used to make optical fibre is glass and most
often this is an oxide glass based on silica (SiO2) with some 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 “_________ ____________ 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 __________ 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 ____________INDEX FIBRE (GRIN).
Page- 9 -
Drouin Secondary College
VCE Physics
Cross Section
Fibre
Type
Topic: Medical Physics
Refractive Index Profile
RI
Step
Index
n2
n1
n1
r
n2
r
RI
Graded
Index
n1
n2
n1
r
n2
r
2.4
Step vs Graded Fibres
In SI fibre, the light rays zigzag between the core/cladding on each side of the fibre axis.
Cladding
Core
Step Index Fibre
In GRIN fibre, the gradient in the refractive index gradually bends the rays back toward the axis.
Core
Graded Index Fibre
Page- 10 -
Drouin Secondary College
2.5
VCE Physics
Topic: Medical Physics
Fibre Bundles
It is impossible for a single fibre to transmit an image.
An individual fibre can transmit only a spot of a certain ___________and _______________.
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 _______________ Bundle
_____________________ 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
Object seen by
endoscope
2.6
Image projected to
eyepiece
Image projected to
eyepiece
Endoscope Construction
Fibre Optic Endoscopes have a number of basic components:
1. A Coherent Bundle for bringing the _____________ to the eyepiece (or video monitor).
Page- 11 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
2. An Incoherent Bundle for taking an external __________ source down the endoscope to
illuminate the viewing area.
3. Optional tubes or channels for the passage of air, water, as well as remote control
implements such as biopsy forceps or cytology brushes.
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”
ARTHROSCOPE
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 ______________ laser will be used as an example.
In a laser, the lasing medium (the ruby crystal) is so called “_______________” to get the
electrons of the ruby atoms into an excited state.
Typically, very intense flashes of ______________ from a flash tube (or from an electrical
discharge pump) enter the lasing medium and create a large collection of excited-state atoms
(atoms with higher- energy electrons).
Page- 12 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
High energy photons from the flash tube excite electrons
The electrons then release their energy as red light.
Flash Tube
Ruby Crystal
Ruby Atoms
3.1
Mirror
Ruby Atoms
Laser Types
silvered
Since their development in 1960, lasers used in medicinePartially
and surgery
have evolved, and while
mirror
medical lasers have never become the "magic _____" that some had hoped, they have become
powerful and indispensible 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 ___________ amount of the __________ wavelength
of laser energy to the __________ tissue to damage or destroy only that tissue, and nothing else.
Human Hair etched by Eximer
Laser
Some of the many medical and surgical lasers in use.
Page- 13 -
Drouin Secondary College
3.1
Laser
CO2
VCE Physics
Topic: Medical Physics
Laser Types & Treatments
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 removal
Nd: YAG
1064
Blue/black ink tattoo removal; hair removal
Diode
800 to 900
Alexandrit
e
Ruby
755
Blue/black ink tattoo removal; hair removal
694
Treatment and removal of moles, freckles, birthmarks
Pulsed
Dye
KTP
577 to 585
Argon
488 to 514
Eximer
193
4.0
Hair removal; dental surgery
532
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
Ultrasound Basics
Definition of Ultrasound
Sound consists of travelling pressure waves
Speed of sound waves in human tissue: ~ 1500 ms-1.
Frequency range: between ____ MHz and ____MHz
Ultrasound is produced using piezo-electric transducers, crystals which change shape under the
action of an _______________ field.
______________ is the most commonly known piezo-electric material.
Better performing piezo – electric material (such as BARIUM TITANATE or LEAD ZIRCONATE) is
formed into disks.
The disk is placed between 2 electrodes and applying a voltage causes the crystal to vibrate.
The crystal’s vibrations set up Ultrasonic sound waves in the medium around the crystal
The crystal will vibrate at the same frequency as the supply voltage, producing sound waves with
frequencies between 2 and 10 MHz
Page- 14 -
Drouin Secondary College
4.1
VCE Physics
Topic: Medical Physics
Echo Location
Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound
waves and their ____________.
The technique is similar to the echolocation used by bats, whales and dolphins, as well as
________________ used by submarines.
The ultrasound signals generated as previously described leave the handpiece and are reflected
back from various ______________ and_______________.
These reflected waves strike the handpiece causing the piezo electric crystal to contract and
expand.
This change in ______________ 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
skullbone.
4.3
Sound Intensity Profile
Beam Properties:
Page- 15 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
Longitudinal Waves - the wave in which the particle motion is parallel to the direction of the wave
travel.
A series of ___________________________ waves make up the ultrasound beam.
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.
Focal Zone - the region where the beam diameter is most concentrated giving the greatest degree
of focus.
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 __________ Zone.
4.4
An Ultrasound Examination
In ultrasound examination, the following events happen:
1. High-frequency sound pulses are transmitted into your body using a _______________.
2. The waves travel into your body and hit a ____________________ between tissues (e.g.
between fluid and soft tissue, soft tissue and bone).
3. Some of the sound waves get reflected back to the probe, while some travel on further
until they reach another boundary and get reflected.
4. The reflected waves are picked up by the probe and relayed to the machine.
5. 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 a second).
Page- 16 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
6. The machine displays the distances and 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 _______-dimensional 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.
4.6
Doppler Ultrasound
Page- 17 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
Doppler ultrasound is based upon the Doppler Effect.
When the object reflecting the ultrasound waves is moving, it changes the ________________ of
the ___________, 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 _____________ 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.
5.0
X rays
In 1895 Wilhelm Röntgen was studying what happened when an electric _____________
was passed through a gas of extremely low
pressure in apparatus called Crooke’s Tubes.
Roentgen found that, if the discharge (Crooke’s)
tube is enclosed in a sealed, thick black carton to
exclude all light, and if he worked in a __________ 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 _____________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 the path of rays between
the tube and a photographic plate.
He observed, after developing the plate, an image of his wife's hand which showed the
shadows thrown by the __________ of her hand and that of a __________ she was
wearing.
Page- 18 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
Because the nature of the new rays was then unknown, he gave them the name
________.
Later it was shown that they are of the same electromagnetic nature as light, but differ
from it only in the higher __________________ 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.
Inside the machine is an X-ray tube.
An electron ______ inside the tube shoots high energy
electrons at a target made of heavy _________, such as
tungsten.
X-rays come out because of atomic processes induced by
the energetic electrons shot at the target.
X-rays are just like any other kind of _________________ radiation.
They are produced in parcels of energy called ________________, just like light.
There are two different atomic processes that can produce x-ray photons.
1. The first is called Bremsstrahlung, which is a fancy German name meaning "braking
radiation."
2. The other is called K-shell emission. They can both occur in heavy atoms like
tungsten.
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.
This electron __________ down after swinging around the nucleus of a tungsten atom
and _________ energy by radiating x-rays.
Page- 19 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
In this process, a lot of X ray photons of _____________ wavelengths are produced, but
none of the photons has more energy than the electron had to begin with.
After emitting the spectrum of x-ray radiation the original electron is slowed down or
stopped.
2. K Shell. The K-shell is the _____________ energy state of an atom.
The incoming electron can give the K shell electron enough
energy to knock it out of its energy state.
Then, a tungsten electron of higher energy (from an outer shell)
can fall into the K-shell.
The energy _____________ by the falling electron shows up in an emitted x-ray photon.
Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell,
and so on.
K-shell emission produces higher-intensity x-rays than Bremsstrahlung, and the x-ray
photon comes out at a ________________ wavelength.
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.
Page- 20
-
Drouin Secondary College
6.0
VCE Physics
Topic: Medical Physics
CT Scans
CT or Computerised ___________________, also know as CAT or Computerised
__________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 cross-sectional
picture of the body at one __________ or cut.
CT scans are of greatest value for showing physical changes in ____________, although
small tumors 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.
Original CT image from
scanner circa 1975. This
image is a coarse 128 x 128
matrix, showing a slice of the
brain
Page- 21 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
The first clinical CT scanners were installed between 1974 and 1976. The original
systems were dedicated to __________ 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 ___________ to acquire the raw data for a single scan
or "slice" and took ____________ 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 millions of data points in less than a
_____________. An entire chest (forty 8 mm slices) can be scanned in five to ten seconds
using the most advanced multi-slice CT system.
CT image of a normal
brain using a state-of-theart CT system and a 512 x
512 matrix image.
7.1
PET Scans
Positron Emission Tomography, or PET, scanning is an imaging technique that uses
_________________ positrons (positively charged particles) to detect subtle changes in
the body's metabolism and chemical activities.
A PET scan provides a color-coded image of a body organ in ______________ rather than
its structure.
Page- 22 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
During a PET scan, a positron-producing radioisotope called a __________ 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 ________________ rays (similar to Xrays).
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.
8.0
MRI - Basic Operation
MRI (Magnetic Resonance Imaging) started out as a tomographic imaging (CT) technique,
that is, it produced an image of a _______ __________ through the human body.
MRI has advanced beyond a tomographic imaging technique to a _______________
imaging technique.
The human body is primarily fat and water.
Both fat and water have many hydrogen atoms which make the human body roughly
_______% hydrogen atoms.
MRI takes advantage of the fact that the nuclei of certain atoms, hydrogen and
phosphorous, in particular, behave like a _____________.
Page- 23 -
Drouin Secondary College
VCE Physics
Topic: Medical Physics
In the absence of an external _______________ field, the hydrogen atoms are not lined up
in any particular direction.
When those atoms are placed in a strong magnetic field, their
______________ align the axis of spin either with or against the
direction of the field.
When the field is turned-off, the nuclei ___________ the field spin and
release a characteristic radio- frequency photon emission.
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 ___________, spinal cord and ____________ are seen much more
clearly with MRI than with regular x-rays and CAT scans.
Also, muscles, ligaments and tendons are seen quite well so that MRI scans are
commonly used to look at knees and shoulders following injuries.
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 !
An advantage of MRI is the radio waves used are a trillion times less energetic (and
potentially less damaging) than X rays.
A disadvantage of MRI is it’s higher cost compared to a regular X-ray or CAT scan.
Page- 24
-