Unit 8Medical Physics

Walter Hall

School of Health Sciences

University of Ulster

Topics for this session

• Fibre Optics and Lasers in medicine

• Light in therapy

• Ultrasound in therapy

• Radioisotopes in therapy

• Choosing modalities

Fibre Optics

Fibre Optics Explained

• Glass fibres around 120 micrometres dia

• Can carry light up to 50km without need for repeaters or boosting

• Commonly known applications are in data and voice communications

Why the light “Stays in”

• When light meets an interface between 2 materials it will be transmitted or refracted

• Fibre optics manipulate this phenomenon

Refractive Index and TIR

ZigZag – NOT STRAIGHT!

Lower Refractive Index

Medical Application

• Fibre optic tube• Light source• Lens• Can be used to

explore natural body cavities

• OR small artificial ones

Medical Application

Advantages

• Can examine a variety of structures without requiring surgery

• Minor apertures can expand role further

• Can carry out surgical procedures via specialised variants of endoscope

• Incision is small – post-operative recovery is much quicker

Light in Therapy

• Fibre optics can convey special light such as LASERs

• Light Amplification by Stimulated Emission of Radiation

• LASER is useful as surgical tool

How It Works

• Pump energy – optical or electrical

• Excite Atoms

• Create population inversion – more excited than non-excited atoms

• Stimulate emission

Properties

• Directional

• Monochromatic

• Coherent – all the photons are in step with each other

Types

• Helium – Neon

• Argon Ion

• Neodymium – YAG

• CO2

• Solid State – GaAlAs

High Power Laser Applications• Eye Surgery –

– corrective surgery– retinal welding

• Cosmetic - Hair Removal– Procedures on skin – birthmark removal

• General Surgery – tumour removal

• Neurosurgery

• Oncology – activation of specialised drugs

Surgical

• Cutting

• Cauterisation

• Opening Blocked Arteries

• Retinal Re-attachment

• Corrective Eye Surgery

• Photo-stimulation

Other Light Applications

• SAD (Seasonal Affective Disorder)

• Skin Disorders

• Delayed Sleep Phase Syndrome

Low Level Laser Therapy• Physiotherapy

• Cells are stimulated by pulses of light, usually from a solid-state laser

• Pain Relief

• Increases Blood Flow

• Stimulates more rapid healing• Wounds• Strained Muscles• Leg Ulcers

Ultrasound in Therapy

• Useful with injuries to soft tissue and joints

• Can aid the bodies own repair mechanisms – heat and increased circulation

• Relieve pain and increase mobility

• Sports medicine makes great use of it

Specialised Techniques

• Lithotripsy• Derived from the Greek for “Breaking

Stones”

• Break down of areas of calcification in areas like kidneys and gall bladder

• TENS• Transcutaneous Electrical Nerve Stimulation

Why use Ultrasound?

• Readily available

• Proven technology

• Painless

• No need for surgery

• Safe and relatively cheap

• Not always effective

• Some procedures may require repetition

Radioisotopes in Therapy

• Nuclear medicine utilises unsealed radioactive sources often attaching to a pharmaceutical to encourage uptake by specific tissue

• Therapeutic applications also use sealed sources

Radiations

helium nucleus – penetrates only a few cm in air – stopped by a sheet of paper

energetic electrons – penetrate up to a metre in air – stopped by a few mm of Al

high energy electromagnetic rays – penetrate many metres in air - only stopped by several cm of lead

Example Application

• Iodine131

• Half life 8 days emitter

• Used to test thyroid function

• Used to treat thyroid cancer or thyrotoxicosis

Example Application

• Iridium 192

• Half life 74 days emitter

• Used to treat breast cancer

• Used as a wire which is inserted for a time and then removed

Example Application

• Caesium 137

• Half life 30 years emitter

• Used to treat reproductive organ tumours

• Up to 200 others are used– Lutetium 177 - and – Yttrium 90

Example Application

• Cobalt 60

• Half life 5.27 years emitter

• General radiation therapy

• Teletherapy units

• Largely replaced by linear accelerators which produce high energy X-rays

Half Life

• Overall half life is a combination of – radioactive half-life– biological half-life

– Radioisotope decays– Radioisotope is excreted

Ionisation

Ionisation In Tissue One million ion pairs per mm 10,000 ion pairs per mm 1,000 ion pairs per mm

• Water is ionised

• H+ and OH- attack cell walls and chromosomes

• Kills or modifies cells

Afterloading

• Catheters introduced to treatment area

• Therapist controls insertion of sources from behind shielding

Teletherapy

• Useful for a wide variety of radiotherapy applications

Attributes of Therapeutic Isotopes

• In a physical form which can be manipulated• Can be beta emitters• Co60 is a gamma emitter

Problems

• Radiation protection issues

• Dosimetry changes over time

• Finite life span of isotopes

• Side effects

Side Effects

• Hair loss – may be permanent

• Fatigue

• Inducing other cancers

• Damage to epithelial surfaces

• Inflamation and swelling

• Infertility

Which Modality?

• What is available

• What does the clinician recommend

• What can the patient tolerate

• Is there a safer/cheaper alternative?

• Can potential risks be justified?

QUESTIONS?