1. Medical Physics

2. Medical Physics
X-rays
Diagnostic methods in medicine
Ultrasound

3. Medical Physics Describe the nature of x-rays Electromagnetic waves
Travel at speed of light / 3x108 m/s (in a vacuum)
Travel in a vacuum
Can cause ionisation
Wavelength about 10-10m
X-rays are high energy photons

4. Medical Physics Describe in simple terms how x-rays are produced
Electrons are accelerated through high voltage
High speed electrons hit metal
Kinetic energy of electrons ‘produces’ X-ray photons
Remember if an electron is accelerated through say 90,000V it will have 90,000 eV of energy !

5. Medical Physics
Describe how x-rays interact with matter in the photoelectric effect
Incoming X ray photon has enough energy to eject an electron, the remainder of its energy appearing as the kinetic energy of the electron.( Atom is ionised)

6. Medical Physics
Describe how x-rays interact with matter in the Compton Effect
X-ray photon collides with an orbital electron
The scattered photon has a longer wavelength / lower frequency / lower energy AND
The electron is ejected from the atom at high speed

7. Medical Physics
Describe how x-rays interact with matter in pair production
Incoming photon disappear and produces electron-positron pair.
Be aware of E =mc2 type calculations

8. Medical Physics
Describe the operation of a computerised axial tomography (CAT) scanner
X Ray beam passes through the patient at different angles
X Ray tube rotates around the patient
A thin fan shaped beam is used.
Images of slices through the patient are produced with the help of computer software
X ray detectors are moved along the patient for the next slice.

9. Medical Physics Describe the advantages
of a CAT scan compared with an x-ray image
Differences:
Simple X-ray is one directional & produces single image
Computer processes data / image constructed from many slices
Advantages: X-ray image is 2D / CT scan produces 3D image
Greater detail / definition / contrast with CT scan / ‘soft tissues can be seen’
Image can be rotated

10. Medical Physics
Describe how image intensifiers can improve the quality of X ray images
Lower exposure or fewer X-rays needed if image intensifier used
Intensifier used as X-ray would pass through film
Intensifier converts X-ray photon to visible light photons using a phosphor
These photons are used to create many electrons .
Electrons are accelerated and focussed and used to produce more visible light photons .

11. Medical Physics
Explain how soft tissues like the intestines can be imaged using barium meals
In order to make soft tissue more visible, contrast media, such as barium, are used.( High Z value)
The patient swallows a liquid rich in barium as it will readily absorb X-rays.
The barium meal coats the wall of the tract enabling its outline to be seen in X-rays.

12. Medical Physics
X-rays
Diagnostic methods in medicine
Ultrasound

13. Medical Physics
Describe the use of medical tracers like technetium-99m to diagnose the function of organs.
Radioactive substance that is ingested / injected (into patient)
Technetium(-99) / Iodine(-131) absorbed by organ
Tracer administered will be giving off radiation so the path can be followed.
It must not interfere with any functions of the body.
It must emit detectable radiation so that the image of the organs can be observed

14. Medical Physics
Describe the main components of a gamma camera

15. Medical Physics
Collimator – gamma ray photons travel along the axis of lead tubes travel to the scintillator
Having thin / long / narrow lead tubes makes the image sharper / less blurred
Scintillator – gamma ray photon produces many or thousands of photons of visible light
Photomultiplier – An electrical pulse or electrons are produced from the light photons
Computer – signals from photomultiplier tubes are used to produce an image .

16. Medical Physics
Describe the principles of positron emission tomography (PET)
Positron emitting substance is injected into the patient.
The gamma photons travel in opposite directions
A 3D image is created using the detector signals with the aid of a computer
The patient is surrounded by a ring of gamma detectors
Annihilation of electron and positron
Annihilation of positron and electron produces two gamma photons
The arrival times of the photons ( in diametrically opposite directions) are used to pinpoint areas of increased activity.
A PET Scan looks at the function of the brain

17. Medical Physics Strong electromagnet,
Describe the main
components of an MRI
scanner
Strong electromagnet,
Radio frequency transmitting coils,
Radio frequency receiving coils, gradient coils and a computer.

18. Medical Physics Outline the principle of MRI
Protons or nuclei have spin & behave like tiny magnets
Protons or nuclei precess about the magnetic field provided by the strong electromagnet
Transmitting coils provide pulses of radio waves of frequency equal to the Larmor frequency
The protons or nuclei absorb energy or radio waves & resonate and flip into a higher energy state

19. Medical Physics When protons or nuclei flip back to a lower energy
state they emit photons of radio waves
The relaxation time of the protons / nuclei depends
on the surrounding tissues
The radio waves are picked up by the receiving coils
The gradient coils alter the magnetic flux density
through the body

20. Medical Physics The Larmor frequency of the protons / nuclei varies
throughout the body
The computer processes all the signals from the
receiving coils and generates the image
An MRI Scan looks at the structure of the brain

21. Medical Physics Describe the advantages & disadvantages of MRI
Advantages: uses non ionising radiation
better soft tissue contrast
Generates data to produce a 3D image
Information can be displayed on a screen as a section in any direction
Disadvantages: heating effect of metal objects eg
effect on cardiac pacemakers .
Takes a long time to perform MRI scan

22. Medical Physics
Describe the need for non invasive techniques in diagnosis
No entry into body / no cutting / incision of patient / no surgery.
Lower risk of infection / less trauma

23. Medical Physics
X-rays
Diagnostic methods in medicine
Ultrasound

24. Medical Physics Describe the piezoelectric effect
The application of a p.d. across a material / crystal causes an expansion / contraction / vibration.
An alternating voltage will cause the crystal to vibrate.

25. Medical Physics Describe the principles of ultrasound scanning
Pulses of ultrasound (sent into the body)
Wave / ultrasound / pulse / signal is reflected (at boundary of tissue)
Time of delay used to determine depth / thickness
The fraction of reflected signal is used to identify the tissue
Small wavelength used which means finer detail can be seen / greater resolution

26. Medical Physics
Explain qualitatively how the Doppler effect can be used to determine the speed of blood
Doppler effect uses ultrasound waves.
Sound waves are reflected by the moving blood cells.
Changes in frequency or wavelength enables the speed of blood flow or rate of flow of blood to be found . Δf α v

27. Medical Physics Describe the difference between A-scan and B-scan
A-scan In one direction only / range or distance or depth finding
B-scan Uses a number of sensors or a sensor in different positions or angles to build up a 2D/3D image
Is made up of many A scans