Unit 8 - Medical Physics
Nikki Kelso
Aims of this Session

Production of and uses of thermographic images

Introduce the production of & dangers of using x- rays

Stochastic & Non Stochastic effects

Somatic & Hereditary effects

Uses of Radioisotopes & Nuclear Medicine

Production & uses of Medical Ultrasound

Magnetic Resonance Imaging (MRI)
Thermography
• Infra-red detectors pick
up IR radiation
• Amount of radiation
increases with
temperature therefore
thermography allows you
to visualise variations in
temperature
• computer algorithms used to
interpret data and produce a
usable image
Why is this Useful?


Certain pathologies cause temperature
differentials
Thermography detects these with high sensitivity
& accuracy

Non – invasive

NO Ionising Radiation used
Types of Diagnosis



Sports injuries
Breast cancer
screening
Monitoring of post
operative infection
What we do in Radiology Departments

Plain film radiography

Contrast studies

Computerised Tomography

Radioisotope imaging

Ultrasound

Magnetic resonance imaging

Bone density measurement

Positron emission
tomography
X Rays
•
Discovered in 1895 by
Roentgen
•
“X” Rays because he didn’t
know what they were!
•
An ionising radiation at a
higher level on EM spectrum
•
Higher frequency or
shorter wavelength
X-ray Production
X rays, the risks and dangers.
Ionising Radiation – potentially damaging

Damage is influenced by:
amount of body tissue irradiated
type of body tissue irradiated
dose received
dose rate

Risk minimised using “ALARA” principle

Precautionary Measures

Legislation
Ionising Radiation Regulations 1999
IR(ME)R 2000

In Practice we use
radiation protection
ALARA principle
Staff Protection

Not place themselves in the primary beam

Use of the inverse square law

Use of lead glass panels


Use of lead rubber coats/thyroid shields/lead
glasses
Limit of time spent in fluoroscopy: especially
during pregnancy

QA of the equipment

Dose monitoring
Patient Protection

Correct exposure factors

QA done daily on equipment

Collimation of the primary beam

Correct focus/film distance

Use of appropriate lead rubber protection
where appropriate ie gonads/eyes/thyroid

Appropriate examination

Well trained staff
X Ray Effects

Stochastic – no threshold for damage

Non stochastic – a quantifiable threshold

Effects can take place in somatic cells or be
passed on (hereditary)
Stochastic Effects

Probability of the effect of radiation which can be
either radiation induced cancers or genetic effects.

No safe dose limit

Statistically generated

Lower doses of radiation
Non Stochastic Effects




Also called deterministic effects
There is usually a threshold below which the
effect will not occur
Examples are erythema (skin reddening) or
epilation (hair loss)
Doses are large eg following radiotherapy or as a
result of a radiation accident (Chernobyl)
Damage caused by radiation

SOMATIC caused to the individual

GENETIC passed onto future generations
How are effects measured?
• Sievert is unit of measurement – equivalent to a
deposit of 1 joule of energy per kilogram mass of
tissue
• Relates dose absorbed in tissue to biological damage
caused – “effective” dose
• This will depend on the type of radiation
• Typical background radiation results in an effective
dose of 2.4 mSv/year
Examples of Doses
We’re all exposed to background radiation
Chest
Skull
= few days
= few weeks
Spine/Abdo
= Few months or a year
CT Chest = few years
Additional risk of cancer per exam
1 in 1,000 to 1 in 1,000,000
Risk of cancer 1 in 3
Image production
• Basic form uses
photographic film
• Denser structures attenuate
the x-rays
• When film is exposed to x rays
it turns black
• Image is contrast between two
• Contrast can be manipulated
using exposure factors and
other aids such as contrast
media
Variations in Contrast
Using contrast media
Factors affecting contrast


Transmission – x-ray photons that pass through the
patient unchanged.
Absorbtion – x-ray photons that transfer their energy to
the patient.
Absorbtion is proportional to the degree of attenuation –
thickness, density & atomic number


Scatter – radiation that changes direction or is modified
by decrease in energy as it passes through a body
Attenuation – process that x-rays lose power as it travels
through matter
Plain film radiography
Mobile Radiography



Mobile unit can be
moved to patients
bedside, A&E dept or
theatre
Can be mains or battery
powered
Can produce images as
good as purpose built
units.
Digital imaging

Images stored on
computer

No films

Image manipulation

Multiple viewing

Storage

Volume

Physical principles
remain the same
But because its Windows
based
“C” arm for angiography
Ultrasound

Ultrasound uses sound
waves to produce images

Becoming highly skilled

Increasingly specialised

Images are very dependent
on the ultrasonographers
skill
Ultrasound images
Ultrasound images
Computerised Tomography
CT explained
Tomography
tomos – slice
graphia – describing

Where digital geometry processing is used to
generate a three-dimensional image of the
internals of an object from a large series of two
dimensional x-ray images taken around a single
axis of rotation.

CT in practice





Data is obtained digitally
Algorithms allow
manipulation of data
Windowing is process of
using a variety of Hounsfield
Units
Setting a top and bottom of
range allows various tissue
types to be imaged
Can “get rid” of what you
are not interested in
Magnetic Resonance Imaging




The latest imaging tool
Images are similar in appearance to CT but
produced without radiation
Technology utilises radio waves and a huge
magnet to produce images
The magnet must be kept cool to allow
superconductivity. It has to be cooled with liquid
helium to -270 degrees.
MR scanner
MR Precautions


Not everybody can have an
MRI scan
Metal implants eg cardiac
pacemakers, aneurysm clips

Tattoos

Metallic foreign bodies

Pregnant women

claustrophobics
MRI Images
CT versus MR



Principles of data
collection are the
same
MR is Non Ionising
Better at imaging
softer tissue
Which Modality to use

What are you attempting to image?

What level of information do you wish to obtain?

How do you wish to manipulate it?

What protection measures need to be considered?
Radioisotope Imaging

What is an isotope?
Nuclei of atoms consist of protons and neutrons.
The number of protons is called the atomic number
The number of protons and neutrons is called the
mass number
All the atoms of one element with the same atomic
number but different mass number are called
isotopes
Radioisotopes

Isotopes behave chemically the same
some of the radioisotopes will be radioactive ie emit
radiation
By attaching these radioactive isotopes to certain
pharmaceuticals we can use the emitted radiation
to produce images

Most commonly used isotope is Technetium99m
because it decays by gamma emission
What is Radioactivity?

Certain elements have isotopes which are unstable

The unstable atoms emit particles or energy

The particles or energy are radiation

The process is unpredictable

It is measured in Becquerels – 1 Bq is one
“decay” event per second
Radiation Types



Alpha – helium
nuclei stopped by
paper
Beta – electron, can
be stopped by light
metal
Gamma – EM
photon, requires
dense material to
absorb
Half Life




The time taken for half of the atoms of a given
sample to decay
Stays the same for a given isotope regardless of
the actual quantity
Expressed as a unit of time
Can be validated using experimentation and
computer modeling
Uses for Isotopes



Nuclear Medicine
Branch of imaging
science which uses
unsealed radioactive
sources
Gamma sources are
isotope of choice
How does it work?

Radioactive isotopes are labelled with
pharmaceuticals

Now known as radiopharmaceuticals

Introduced into the body

Pharmaceuticals influence tissue type which
absorbs isotope

Gamma emission is detected by a gamma camera

Image is digitally produced
Gamma Camera



Detects individual Gamma
photons
Builds up an image over a
period of several minutes
Useful to show biological
(metabolic) processes eg
infections/secondary boney
cancer deposits
Why do we use Nuclear Medicine?

Radiopharmaceuticals do not cause much harm in
proportion to benefit derived

Body will excrete material

Radioactivity is short lived – matter of hours

Can be used to image anatomy and physiology

Can be integrated with other modalities (PET)
Production

Most useful isotopes are not natural

Must be produced by reactors

Side product of used nuclear fuel

Used uranium fuel has a content of
molybdenum99

Easily extracted

Technetium99 is a daughter product

A few micrograms of molybdenum99
will produce enough technetium 99 to
image 10,000 patients
Radioisotope/NM Images
Positron Emission Tomography
latest radiology tool to image patients
Cyclotron – particle
accelerator
3-30 MeV
Positron Emission Tomography
PET
QUESTIONS?