INSTRUMENTATION ELECTRICAL REQUIREMENTS AND EFFICIENCY Effective voltage High

INSTRUMENTATION

ELECTRICAL REQUIREMENTS AND EFFICIENCY § Effective voltage § High frequency inverter technology (over 10, 000 Hz) § Constant potential output (<13% ripple) § k. Vp should be set according to machine’s ripple content.

Advantages of High. Frequency/Constant Potential Units § Ability to use lower k. Vp settings § Greater efficiency which results in shorter § § § exposure times Less patient motion. Lower x-ray dose to the patient. Tube will last longer because output is higher. Less interference from reciprocity law failure. More compact

KV SELECTION § Compton § Photoelectric

Technical Factors Defined § k. Vp – peak kilovolts; measure of the highest energy applied to electrons during the exposure. § m. As- The number of electrons accelerated to the target during a given exposure. § ke. V – ( kilo-electron volts) The energy received by the electrons.

PHOTOELECTRIC EFFECT § Depends on atomic number of substance § Molybdenum (atomic #42) produces § § § characteristic peaks at 17. 4 -19. 6 ke. V. Glandular tissue (atomic #’s 7, 8) Fatty tissue (atomic # 6) Calcifications (atomic #’s 15, 20) Low k. V beam essential to maximize photoelectric interactions. 25 -27 k. V optimal

COMPTON § AVOID COMPTON § Deals with electron densities § No contrast differentiation between fat and glandular tissue with electron density. § Accounts for most interactions within tissues above 28 k. V

Target Material/Filter § Molybdenum – 17 -20 ke. V 25 k. Vp § Rhodium – 20 -23 ke. V 30 k. Vp § Tungsten – 15 -25 ke. V 22 -26 k. Vp

RECIPROCITY LAW FAILURE § § § Exposure must be <1 sec to avoid this Grids make it difficult to avoid Invites client motion Increases dosage Capture of excessive scattered radiation on the film § Instead of making exposures beyond the RLF of the film, increase density of film by raising k. Vp setting.

GRIDS § Design § Ratio § Effect on Dose § Effect on Images

COMPRESSION § Decreased motion § Decreased geometric unsharpness § Increased contrast § Separates superimposed tissues § Decreased dose § Increased uniform film density

COMPRESSION PADDLES § Made of thin Plexiglas § Straight chest wall edge § Sufficient height and angle of chest wall § Squared rather than rounded § Remains parallel when compressed § Vertical alignment between chest wall edge and image receptor § Final compression manually controlled

MAGNIFICATION § No grid: causes increase in tube loading, dose and motion due to prolonged exposure time. § Disadvantages of mag: § Increased dose § Decreased sharpness § FSS decreased to improve sharpness: §. 2 mm with 1. 5 x mag §. 1 mm with 2 x mag

MAGNIFICATION CONT…. § Magnification factors range from 1. 4 x to 2 x § 1. 5 x mag can see 13 line pairs/mm § 2 x only sees 7 line pairs

X-RAY TUBE § HVL – amount of AL that when added to the beam, will decrease intensity by 1/2. An indirect measurement of the x-ray beam and expressed in mm-AL. § Includes: § Beryllium window § Moly filter § Mirror § Compression paddle

X-RAY TUBE § § Heel effect Electron stream driven by ___ Amount of electrons controlled by ___ When angle of anode is increased: § can withstand larger stream of electrons (greater heat) § results in smaller effective focal spot which provides better resolution § more pronounced heel effect (reduction in intensity of beam from chest to nipple)

X-RAY TUBE § Focal Spot Projection § Reference Axis

AUTOMATIC EXPOSURE CONTROL § Radiation that is transmitted through an object is converted into an electronic signal, which terminates the exposure when the predetermined level of radiation has been reached.

AUTOMATIC EXPOSURE CONTROL § Single Pickup Phototimers § Confined to the center of the chest wall edge § Can move ~3 inches from chest § Difficult to determine adequate placement § Smart Phototimers § A series of photocells § Averages signals of each cell

Smart Phototimers § Track from one k. Vp setting to another § Compensate for differences in breast thickness § Obtain an adequate density for all varying breast § § § densities Compensate for reciprocity law failure of the film Provide a k. Vp and m. As readout Reproduce accurate densities on each film

SINGLE EMULSION FILM § More silver halide and gelatin per emulsion layer than double emulsion § Longer immersion time in developer due to thicker emulsion § More susceptible to artifacts § Anti-halation layer § Base + fog no more than. 16 -. 20

H&D CURVE § High contrast films have a steeper average gradient § Fast films respond to lower exposures creating higher optical density with less light exposure.

DOUBLE EMUSION FILM § Contrast is less but 60% faster § Less susceptible to processor problems and artifacts

CASSETTES/SCREENS § Fluorescent layers either calcium tungstate (blue) or rare earth phosphor (green orthochromatic film) § An increase in screen speed will not cause as much of an increase in quantum mottle (noise) as film would. § The thicker the screen, the greater the blur, but the faster the screen becomes, thus the lower the dose.

PROCESSING § Extended processed films remain in developer 47 seconds (as opposed to 23 sec in standard). § Diffusion of developer through the thicker emulsion requires: § Longer immersion in developer § Increase in developer temp § Movement of film through rollers to give developer better access to silver halide

EXTENDED PROCESSING § Increased contrast § Increased film speed § Decreased dose § Increased noise

STANDARD PROCESSING § Decreased noise § Increased dose