Unit 8 Medical Physics

Paul Kane

Radiographer

Aims for this Session

• Understand the production of and uses for thermographic images

• Understand X-Ray production • Understand how X-Rays are used to

produce images• Understand the dangers of X-Rays• Evaluate the use of both modalities• Understand Radiation, its uses and

dangers

The Electromagnetic Spectrum

Thermography

• Infrared detectors pick up IR radiation

• Amount of radiation varies with temperature

• Computer algorithms used to interpret data and produce a usable image

Why is this useful?

• Certain pathologies will cause temperature differentials

• Thermography detects these with high sensitivity and accuracy

• Non – Invasive

• No ionising radiation used

What sort of diagnoses?

• Sports Injuries

• Ca Breast screening

• Monitoring of post operative infection

X-Rays

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

XRAY TUBE

X-Ray Production

Rotating Anode Tube

XRAY TUBE

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

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)

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

Precautionary Measures

• Legislation

1. Ionising Radiation Regulations

2. IR(ME)R 2000

• In practice we use

1. Radiation Protection

2. ALARA principle

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

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 that which does not interest you

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?

Radiation

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 Bequerels – 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 modelling

Uses for Radioisotopes

• Nuclear Medicine • Branch of imaging

science which uses unsealed radioactive sources

• Gamma sources are those 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 aborbs isotope• Gamma emission is detected by a gamma

camera• Image is digitally produced

Gamma Camera

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

• “Milking a cow”

Production Cont’d

• Used uranium fuel has a content of molybdenum99

• Easily extracted

• Technetium99 is daughter product

• A few micrograms of molybdenum99 will produce enough technetium99 to image approx 10,000 patients

Precautions

• Unsealed source• Main protection for

staff is time, distance and shielding

• Patients only need worry about the period immediately around scan

Time Distance Shielding

Detectors

• Scintillation Counters – uses materials which fluoresce when irradiated

• Geiger Counters – uses a gas which becomes a conductor if irradiated

• Film Badges – uses photographic film

Film Badge

• Piece of wrapped photographic film

• Film holder - a plastic holder containing various metal and plastic filters

• Tin, Cadmium, Lead Indium, plastic of differing densities.

Experimenting with Radiation

• Any experiments must be properly regulated and kept safe – radiation brings other considerations

• Strict international regulations

• Adequate protection measures must be in place

• QA vital

• Participants monitored

QUESTIONS?