Medical Physics

Medical Physics X-rays Diagnostic methods in medicine Ultrasound

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

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 !

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 X-rays.

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. 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

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.

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 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

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 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