Radiology Physics. Just take a deep breath. Books to Consider. Why worry about physics? The Game Plan. 1 st Period

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1 Radiology Physics Just take a deep breath OR: I DIDN T SIGN UP TO LEARN THIS STUFF Chris Ober, DVM, PhD, DACVR 7 February 2011 Why worry about physics? Know what the system can give you Know what the system CAN T give you Recognize errors and know how to correct them Understand the importance of radiation safety It s on the test, so I might as well teach it Books to Consider Thrall. Textbook of Veterinary Diagnostic Radiology. 4 th or 5 th ed or Morgan & Silverman. Techniques of Veterinary Radiography. 4 th ed *Bushberg, Seibert, et al. The Essential Physics of Medical Imaging. 2 nd ed * Masochists only The Game Plan 1 st Period X-rays Generation of X-rays Interaction of X-rays with matter Accessory equipment X-rays Generation of X-rays Interaction of X-rays with matter Accessory equipment

2 What are X-rays? Type of electromagnetic radiation Can act as wave or particle (photon) No mass, no charge Travel at speed of light What are X-rays? Shorter wavelength than visible light Higher energy than visible light High energy makes them Ionizing Radiation Ionizing Radiation 2 nd Period Radiation that is capable of generating ions Can cause disruption of molecular bonds Thus important in radiation safety and radiation therapy Note: X-rays and gamma rays are different only in source X: outside nucleus, interaction of highspeed particles Gamma: within nucleus due to spontaneous decay X-rays Generation of X-rays Interaction of X-rays with matter Accessory equipment What We Need The X-ray Tube Interaction of high-speed charged particles Particles: electron source High speed: method of accelerating electrons Interaction: target for electrons to crash into

3 The X-ray Tube The X-ray Tube Cathode Negatively charged Filament made of tungsten Set into focusing cup Thermionic Emission Current thru filament Heat generated Electrons released (boiled off) in cloud around filament Cathode Number of electrons released is directly proportional to: Current across filament (milliamperes = ma) Exposure time (seconds = s) mas = ma x s 10 mas = 600 ma x 1/60 s 10 mas = 100 ma x 1/10 s Cathode Number of X-rays produced is directly proportional to electron number & mas Operator selects mas (or ma and s separately) Time selector (s) ma selector Cathode Cathode Number of X-rays produced is directly proportional to mas Operator selects mas (or ma and s separately) ma selector Focal Spot size determined by filament size & focusing cup Most machines have 2 filaments Small focal spot: greater detail (spatial resolution) Large focal spot: routine work (higher output capability) Time (s) or mas selector

4 Cathode Cathode Larger penumbra = more unsharpness The X-ray Tube Anode Anode Positively charged Target made of tungsten (mostly) Focal Spot Small area where interaction with electrons occurs Actual site of X-ray production Stationary Anode Fixed target at end of X-ray tube Lightweight with fewer moving parts good for portables Limited heat dissipation means limited X-ray production Rotating Anode High speed rotating disc interactions spread over larger area Good heat dissipation means higher X-ray output Stationary Anode Rotating Anode

5 Anode Focal Spot Target surface angled relative to the path of the electron beam Larger actual focal spot (interaction area) for better heat dissipation Smaller effective focal spot for better image detail The X-ray Tube Envelope Pyrex container holding cathode & anode Vacuum inside Housing Lead shielding enclosing envelope Window to let X-rays out in the desired direction High potential difference (voltage) applied to X-ray tube Generally kv Electrons (-) pulled toward anode (+) Value set for kvp determines electron energy photon energy X-ray Production X-ray Production Electrons collide with target, interacting with tungsten atoms X-rays are produced X-ray Generation X-ray Production Electrons interacting with target produce X-rays in 2 possible ways Characteristic radiation: Photons of specific energies / wavelengths Bremsstrahlung: Photons of broad energy range (photons of many wavelengths) Electrons with more kinetic energy will produce X-rays with greater energy Electron energy determined by potential across tube kilovolt peak (kvp) Operator selects kvp Maximum X-ray energy will be equal to kvp, though average X-ray energy is only about 1/3-1/2 kvp

6 X-Ray Generation kvp X-ray Generation Only 1% of electrons energy X-rays Wide range of X-ray energies are produced (the kvp is the PEAK energy) Very low energy photons can t get out of tube Moderately low energy photons can get out, but are diagnostically worthless, so filters are used to absorb them just outside the window Energy not used in primary X-ray beam is converted to heat in tube X-ray Generation Heat can kill the tube Oil housing helps dissipate heat Rotating anode helps dissipate heat Two-step exposure Warm up high-output tubes Overheating will Burn out filament Pit anode Cause metal deposits on envelope Cost $$$ X-ray technique Technique = combination of kvp and mas used to make a radiograph Values determine overall blackness as well as overall contrast of image We ll get to this in more detail in a couple of lectures X-rays Generation of X-rays Interaction of X-rays with matter Accessory equipment 3 rd Period Interaction with Matter Photons have 3 choices: Pass through (Transmission) Deposit all of their energy (Absorption) Be deflected (Scatter)

7 Interaction with Matter Photons have 3 choices: Pass through (Transmission) Deposit all of their energy (Absorption) Be deflected (Scatter) Interaction with Matter Photons have 3 choices: Pass through (Transmission) Deposit all of their energy (Absorption) Be deflected (Scatter) Photoelectric effect Compton scatter Interaction with Matter The Noble Step Wedge Determinants of Transmission High energy photon more transmission Dense material less transmission High atomic number (Z) material less transmission Thicker material less transmission Transmitted photons result in image formation Transmitted photons turn film black High Energy Low Energy Atomic Number and Density Basis of 5 basic radiopacities (remember those?) Air Fat Fluid / Soft Tissue Bone / Mineral Metal

8 Atomic Number and Density Gas Low Z High Z Mineral ST Fat Metal Thin vs. Thick Material Thick Thin Interaction with Matter Absorption of X-ray photons = radiation exposure (important for radiation safety) Absorption generation of small amount of heat Differential photon absorption/transmission of various structures is what produces the diagnostic image Scatter Radiation Degrades image (photons don t convey info, as we don t know where they came from) Adds to personnel exposure Unavoidable

9 Scatter Radiation Minimizing Scatter Make radiographed part thinner Collimate Use (antiscatter) grid Use air gap technique Lots of it Not very much of it Making Part Thinner Making Part Thinner Less material for photons to pass through = fewer opportunities for ricochet Commonly used in mammography Radiolucent paddle used to displace other structures e.g. squish abdomen, get intestine out of way Must decrease mas to account for decreased thickness Making Part Thinner Air Gap Technique Space between patient and film Scatter photons more likely to miss the film Extra point try vs. 50-yd. field goal Rare in vet med Distance between tube and patient must also be large

10 Air Gap Technique Space between patient and film Scatter photons more likely to miss the film Extra point try vs. 50-yd. field goal Rare in vet med Distance between tube and patient must also be large X-rays Generation of X-rays Interaction of X-rays with matter Accessory equipment The OT X-rays are emitted from target in all directions Lead housing blocks most photons Remainder go thru window, but even this small beam is wider than is needed for most purposes Use a collimator Restrict X-ray beam to area of interest Original collimators were lead cones We still use the phrase cone down when referring to collimation Now sets of lead shutters variable aperture much more versatile Required by OSHA for radiation safety Improves image quality by reducing scatter radiation

11 Cone Collimator Fixed Aperture Lead Shutters Variable Aperture Used to minimize scatter Fewer photons passing through material at the edges means less opportunity to deflect into the area of interest Must walk fine line Only include what you need in the field, but make sure the study is complete Always collimate at least a little bit No Collimation + No Patient ID Label = Grids Plates with thin lead strips alternating with radiolucent material Placed between patient and film Decreases scatter reaching film DOES NOT decrease scatter reaching staff Generally located in table or Bucky tray, but can have loose varieties Grid absorbs many of the scatter photons, but also some primary photons Exposure technique must be increased

12 Grids Focused grids most common Lead strips angled to match X-ray beam divergence Distance from tube to grid must match grid s focal distance Grids can be unfocused (parallel strips) Grids Grids generally not used if part thickness <10cm Not enough scatter produced to be worth it Grid Ratio = Height of strips divided by distance between them Grids High grid ratio Remove more scatter photons Also remove more primary photons Thus require higher exposure technique Low grid ratio Less elimination of scatter Exposure technique doesn t have to be increased as much Common grid ratio is 8:1 or 10:1

13 Trouble with Grids Need to increase exposure technique Often 3-5x higher than without grid Grid lines visible on image Distracting Use Potter-Bucky mechanism (tray in table) to oscillate grid during exposure and blur lines DR systems may have software-based grid line suppression Trouble with Grids Damaged grid strips will be visible on image Poor alignment of grid and X-ray beam will cause various types of grid cutoff artifact Grid Artifacts What Have We Learned? Upside-Down Focused Grid Off-Level Focused Grid You don t have to be Einstein to understand how X-rays are produced Scatter is not our friend The collimator and grid, however, ARE our friends