Physics of Radiography and Radiation Safety 2024 PDF

Physics of radiography and radiation safety Georgina Catlow BVSc MRCVS Learning objectives To describe the generation of x-rays from standard equipment To describe the requirements for safe operation of radiographic equipment To understand the legal requirements for protection of personnel...

Physics of radiography and radiation safety Georgina Catlow BVSc MRCVS Learning objectives To describe the generation of x-rays from standard equipment To describe the requirements for safe operation of radiographic equipment To understand the legal requirements for protection of personnel involved in radiographic exposure To describe the steps to prepare animals for radiographic exposure #universityofsurrey 2 X-ray generation Generator Photons Patient Film #universityofsurrey 3 X-ray generation – the X-ray tube Tungsten target Vacuum Glass envelope Anode Cathode + - Collimators #universityofsurrey 4 Exposure Settings What can you alter to affect the image obtained? KV mA Time (seconds) Film focal distance #universityofsurrey 5 KV KVp = peak voltage across cathode and anode Controls kinetic energy of electrons Increased KVp o More photons o Increased energy o More penetrating #universityofsurrey 6 mA and time mA = tube current to cathode filament Controls number of electrons Increased mA o Increased heat in cathode filament o Increased number electrons o Increased number photons mAs = mA x time (s) o Measure of intensity of beam o Intensity = total number and energy of all x-ray photons #universityofsurrey 7 Film focal distance (FFD) FFD = distance between the focal spot on the anode and the detector under the patient X-ray beam diverges from focal spot Quantity of radiation at any point is proportional to 1/(FFD)2 FFD Santé’s rule kV = (patient width in cm) x2 + (FFD in inches) #universityofsurrey 8 Interaction of X-rays with matter Attenuation Absorption o Removed energy transferred to the patient o Increases with increasing atomic number o Creates contrast between tissues Scatter o Removed energy emitted away from patient o Worse with increasing KV o Hazard to radiographer o Causes loss of contrast due to fogging #universityofsurrey 9 Interaction of X-rays with matter Transmission and interaction with film-screen detector = FILM BLACKENING mAs kVp Adapted from Thrall Veterinary Radiology 5th Ed #universityofsurrey 10 Interaction of X-rays with matter Tissue/Matter Appearance on radiograph Gas Fat Soft tissue/Fluid Bone Metal Contrast = Shade of grey #universityofsurrey 11 Recording and displaying the image Emulsion Produces image Silver bromide crystals Photon + silver bromide crystal = silver atom deposited (Latent image) Latent image Not visible to naked eye Must undergo chemical processing #universityofsurrey 12 Intensifying screens and cassettes Base Advantages Reduced dose Reflective layer Shorter exposure time Less motion blurring Phosphor Less scatter Disadvantage Protective layer Loss some resolution Film #universityofsurrey 13  Digital radiography – Computed Radiography (CR) The cassette contains a storage-phosphor image plate containing The image plate is then photostimulable crystals exposed to intense white light to delete the latent image and allow reuse X-ray energy absorbed and temporarily stored during an exposure to create a latent image Photodiodes capture the light emitted and Cassette put into processer convert to a digital where the image plate is signal removed and scanned by a laser which sets the stored energy as visible light #universityofsurrey 14 Digital radiography – Direct Radiography (DR) Uses flat panel detectors to convert x-rays into electrical charge Can be either direct or indirect converting systems Detectors either sit underneath the x-ray table or on the tabletop and can be used with grids Signal from the detector to the computer can be wired or wireless. Image from BSAVA Manual of Musculoskeletal imaging #universityofsurrey 15 Digital radiography Advantages Disadvantages Greater tolerance to sub-optimal High initial set up cost and ongoing exposure factors maintenance Images can be manipulated Overexposures can be overlooked Images can be shared Interpretation can be limited if computer monitors not of adequate quality Quicker Easier storage No replacement film costs (although initial set up expensive) #universityofsurrey 16 Radiation Safety #universityofsurrey 17 X-ray interaction with tissue Deterministic o Threshold levels for effects exist o Severity proportional to dose received Stochastic o No threshold level of radiation exists o Probability proportional to dose received Hereditary o Stochastic effect which occurs in offspring of exposed #universityofsurrey 18 Principles of radiation protection 1. Radiography should only be undertaken if there is a definite clinical justification for the use of the procedure 2. Any exposure to personnel should be kept to a minimum. (ALARA – As low as reasonably achievable) 3. No legal dose limit should be exceeded #universityofsurrey 19 Principles of radiation protection Radiation protection supervisor Radiation protection adviser Local rules Controlled area o Defined by physical boundary o Walls must be shielded o Warning light and sign #universityofsurrey 20 Limiting occupational exposure #universityofsurrey 21 Limiting occupational exposure 1. Time Rotate staff Record exposure involving staff Exposure chart Electronic timer #universityofsurrey 22 Limiting occupational exposure 2. Distance Stay outside controlled zone I1/I2 = D22 / D12 Use a protective shield Glass should have a lead equivalent of >0.5mm I1 = Intensity at D1 Avoid horizontal beam radiography I2 = Intensity at D2 D = Distance from source #universityofsurrey 23 Limiting occupational exposure Patient Restraint Image courtesy of CardioAcademy Cevalearn #universityofsurrey 24 Limiting occupational exposure 3. Shielding Structural shielding in walls Lead cover to table Personal protective equipment #universityofsurrey 25 What’s wrong!!!! #universityofsurrey 26 Monitoring personal exposure Personal dosimeter One badge per employee Worn on trunk underneath protective clothing Stored outside controlled area Store away from heat and sunlight Changed every 3 months Retain records for 2 years #universityofsurrey 27 Patient preparation Elective vs emergency Food o Withhold for 6-12 hours o Likely will require chemical restraint Clean the coat o Wet or dirty coats will cause artefacts on image What are you imaging and why? o Which projections do you need? o How will the animal be positioned for these? #universityofsurrey 28 Summary X-Ray generation involves the conversion of an electrical current to electrons and then into photons Photons are attenuated differently by tissues giving a variety of shades of grey on the image Radiation is a biological hazard and steps should be taken to avoid occupational exposure #universityofsurrey 29 #universityofsurrey 30 31