A-LEVEL PHYSICS 15/09/ :49 Medical Physics

A-LEVEL PHYSICS 15/09/ :49 X-rays Basic concept X-rays can expose photographic film X-rays can pass through soft tissue X-rays can not pass through bone Put a body over some photographic film and then “shine” X-rays on them Develop the film Photograph of the inside of the patient

A-LEVEL PHYSICS 15/09/ :49 Major applications Radiography –Diagnostic imagery Radiotherapy –Destroying dangerous cells

A-LEVEL PHYSICS 15/09/ :49 Properties of X-rays Can not be reflected Can not be refracted Can not be focused

A-LEVEL PHYSICS 15/09/ :49 Rotating-anode X-ray tube

A-LEVEL PHYSICS 15/09/ :49 Attenuation –Intensity of X-rays reduced –Passage through matter Linear attenuation coefficient

A-LEVEL PHYSICS 15/09/ :49 Half-value thickness HVT The thickness of material needed to reduce intensity to 50% of its initial value x II0I0 0 X I

A-LEVEL PHYSICS 15/09/ :49 Half-thickness value X1/2 Half the original intensity Cancel I 0 Take natural logs Make x the subject

A-LEVEL PHYSICS 15/09/ :49 Measuring  0 X Log I

A-LEVEL PHYSICS 15/09/ :49 X-ray spectrum Wavelength [x10exp11] Intensity [arbitary units] Continuous radiation Bremsstrahlung Line spectrum min

A-LEVEL PHYSICS 15/09/ :49 Continuous radiation Bremsstrahlung Energy of the emitted X-ray is equal to the energy lost by the electron –Some…why there is a range –All…why there is a minimum wavelength

A-LEVEL PHYSICS 15/09/ :49 Line spectrum High energy incident electron Removes DEEP-LYING orbiting electron Outer electron falls into the space Emits X-ray photon –Much more energy than visible light –Light photons use higher orbital transitions

A-LEVEL PHYSICS 15/09/ :49 Line spectrum

A-LEVEL PHYSICS 15/09/ :49 Line spectrum

A-LEVEL PHYSICS 15/09/ :49 Line spectrum

A-LEVEL PHYSICS 15/09/ :49 Continuous spectrum

A-LEVEL PHYSICS 15/09/ :49 X-ray spectrum Photon energy [keV] Intensity [arbitary units] Range of energies corresponding to frequency of X-ray Maximum value corresponding to min

A-LEVEL PHYSICS 15/09/ :49 Quality & Intensity: Different supply voltage Photon energy [keV] Intensity [arbitary units] kV 50kV More energy available to create X-rays from each incident electron

A-LEVEL PHYSICS 15/09/ :49 Quality & Intensity: Different filement current Photon energy [keV] Intensity [arbitary units] Top 20mA Lower 10mA Greater number of electrons striking the anode so more X-rays produced

A-LEVEL PHYSICS 15/09/ :49 Interaction with matter Simple scattering Photoelectric absorption Compton scattering Pair production

A-LEVEL PHYSICS 15/09/ :49 Simple scattering Low energy Can’t remove an electron Incident x-ray is deflected No loss of energy Material scatter X-ray Without absorbing energy

A-LEVEL PHYSICS 15/09/ :49 Photoelectric absorption Similar to the production of line spectra Incident high energy electron replaced by X-ray photon X-ray loses ALL energy as an inner electron is ejected –Photoelectron…ionizes other atoms Lower energy photon released as an electron falls into the vacant orbit

A-LEVEL PHYSICS 15/09/ :49 Photoelectric absorption

A-LEVEL PHYSICS 15/09/ :49 Compton effect

A-LEVEL PHYSICS 15/09/ :49 Pair production High energy X-ray photons –1.022MeV [rest mass of electron + positron] Nucleus interaction –X-ray vanishes –Electron-positron pair created Electron ionizes atoms Positron annihilated by electron –Two photons created

A-LEVEL PHYSICS 15/09/ :49 Filtration Limit X-rays to the “useful” energies –Remove unwanted low energy X-rays –Heterogeneous Range of values Absorbing filter material –Photoelectric absorption –All energy of incident X-ray absorbed –Low energy photon emitted

A-LEVEL PHYSICS 15/09/ :49 Effect of filtering Photon energy [keV] Intensity [arbitary units]

A-LEVEL PHYSICS 15/09/ :49 Filtration HVT increases Higher proportion of high-energy X-ray photons

A-LEVEL PHYSICS 15/09/ :49 Beam size and alignment 4 adjustable lead shields

A-LEVEL PHYSICS 15/09/ :49 Beam size and alignment Mirror reflects light that is transparent to X-rays

A-LEVEL PHYSICS 15/09/ :49 X-ray image quality Focal spot Angled target –Enlarged focus

A-LEVEL PHYSICS 15/09/ :49 X-ray image quality

A-LEVEL PHYSICS 15/09/ :49 Scattering To produce an image only use X-rays that have passed directly through the body Grid –Vertical lead strips 5mm deep 0.5mm apart 0.05mm thick –Moves from side to side prevent grid from casting its own shadow

A-LEVEL PHYSICS 15/09/ :49 Contrast media How to take an X-ray of the stomach or other organ? –Barium meal –High Z [atomic number] –Absorb X-rays –So get a better contrast on the image

A-LEVEL PHYSICS 15/09/ :49 Intensifying screens Intensifying cassette –Reduces patient’s exposure time –White plastic –Fluorescent crystals Absorb X-rays Give out light Film much more sensitive to light Metal prevents back scattering

A-LEVEL PHYSICS 15/09/ :49 Fluoroscopy Real time images X-ray to light Nice idea but… –X-ray dose too high –Patient dies! Image intensifier

A-LEVEL PHYSICS 15/09/ :49 Computer Tomography Need a powerful computer Compares signals from tiny detectors X-ray source rotated around the patient –pules of X-rays produced Complex mathematical algorithm used to determine the attenuation at each point in the body Excellent resolution Able to be used on soft tissue

A-LEVEL PHYSICS 15/09/ :49 Computer Tomography The NET for… –CT scanning –Principles –Images Encarta for… –Uses –Treatment

A-LEVEL PHYSICS 15/09/ :49 Attenuation…again Mass attenuation co-efficient Consider water –ice –water –steam All have different densities But ALL have the same MAC