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 beamNot place themselves in the primary beam Use of the inverse square lawUse of the inverse square law Use of lead glass panelsUse of lead glass panels Use of lead rubber coats/thyroid shields/lead Use of lead rubber coats/thyroid shields/lead

glassesglasses Limit of time spent in fluoroscopy: especially Limit of time spent in fluoroscopy: especially

during pregnancy during pregnancy QA of the equipmentQA of the equipment Dose monitoringDose monitoring

Patient Protection

Correct exposure factorsCorrect exposure factors QA done daily on equipmentQA done daily on equipment Collimation of the primary Collimation of the primary beam Correct focus/film distanceCorrect focus/film distance Use of appropriate lead rubber protection Use of appropriate lead rubber protection

where appropriate ie gonads/eyes/thyroidwhere appropriate ie gonads/eyes/thyroid Appropriate examinationAppropriate examination Well trained staffWell 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 radiationWe’re all exposed to background radiation

Chest Chest = few days= few days

Skull Skull = few weeks= few weeks

Spine/AbdoSpine/Abdo = Few months or a year= Few months or a year

CT ChestCT Chest = few years= few years

Additional risk of cancer per examAdditional risk of cancer per exam

1 in 1,000 to 1 in 1,000,0001 in 1,000 to 1 in 1,000,000

Risk of cancer 1 in 3Risk 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 Tomographylatest radiology tool to image patients

C yclo tron – partic le acce lera tor

3 -30 M eV

C yclo tron – partic le acce lera tor

3 -30 M eV

Positron Emission TomographyPET

QUESTIONS?