Transcript Medical Physics Revision File

Medical Physics
Medical Physics
X-rays
Diagnostic methods in medicine
Ultrasound
Medical Physics
Describe the nature of x-rays
1. Electromagnetic waves
2. Travel at speed of light / 3x108 m/s (in a vacuum)
3. Travel in a vacuum
4. Can cause ionisation
5. Wavelength about 10-10m
6. X-rays are high energy photons
Medical Physics
Describe in simple terms how x-rays are
produced
1. Electrons are accelerated through high voltage
2. High speed electrons hit metal
3. 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 !
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)
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
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
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.
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
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 .
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 Xrays.
Medical Physics
X-rays
Diagnostic methods in medicine
Ultrasound
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
Medical Physics
Describe the main components of a gamma
camera
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 .
Medical Physics
Describe the principles of positron emission
tomography (PET)
Positron emitting
substance is
injected into the
patient.
Annihilation of
electron and
positron
Annihilation of
positron and electron
produces two gamma
photons
The gamma photons travel
in opposite directions
The patient is surrounded
by a ring of gamma
detectors
The arrival times of the
photons ( in diametrically
opposite directions) are
used to pinpoint areas of
increased activity.
A 3D image is created
using the detector
signals with the aid of a
computer
A PET Scan looks at the
function of the brain
Medical Physics
Describe the main
components of an MRI
scanner
Strong electromagnet,
Radio frequency transmitting coils,
Radio frequency receiving coils, gradient coils and a
computer.
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
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
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
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
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
Medical Physics
X-rays
Diagnostic methods in medicine
Ultrasound
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.
Medical Physics
Describe the principles of ultrasound scanning
1. Pulses of ultrasound (sent into the body)
2. Wave / ultrasound / pulse / signal is reflected (at
boundary of tissue)
3. Time of delay used to determine depth / thickness
4. The fraction of reflected signal is used to identify the
tissue
5. Small wavelength used which means finer detail can be
seen / greater resolution
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
Medical Physics
Describe the difference between A-scan and Bscan
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