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RA 220: RADIATION PHYSICS

RA 220: RADIATION PHYSICS. X-RAY TUBE CONSTRUCTION AND FUNCTION. X-RAY TUBE. TUBE HOUSING. Shield from leakage radiation 100 mr/hr at 1 meter from tube Window/port Electrical and thermal insulation. ENCLOSURE. PYREX GLASS – Why? METAL – Advantages? CCONTAINS A VACUUM - Why?.

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RA 220: RADIATION PHYSICS

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  1. RA 220:RADIATION PHYSICS X-RAY TUBE CONSTRUCTION AND FUNCTION

  2. X-RAY TUBE

  3. TUBE HOUSING • Shield from leakage radiation • 100 mr/hr at 1 meter from tube • Window/port • Electrical and thermal insulation

  4. ENCLOSURE • PYREX GLASS – Why? • METAL – Advantages? • CCONTAINS A VACUUM - Why?

  5. CATHODE ASSEMBLY • FILAMENT • FUNCTION? • COIL OF WIRE • THORIATED TUNGSTEN – Why? • DUAL FOCUS TUBES • TWO FILAMENTS • LARGE AND SMALL – Why?

  6. CATHODE ASSEMBLY • FOCUSING CUP • FUNCTION? • NEGATIVELY CHARGED METAL – Why? • ACTS AS EXPOSURE SWITCH IN GRID CONTROLLED TUBES

  7. THERMIONIC EMISSION • DEFINITION: • FILAMENT HEATED TO THE POINT AT WHICH ELECTRONS ARE BURNED OFF • SPACE CHARGE EFFECT • ELECTRON CLOUD AROUND FILAMENT • SATURATION CURRENT (VOLTAGE) • VOLTAGE LEVEL WHERE ALL AVAILABLE ELECTRONS ARE SENT TO ANODE • NO INCREASE IN MA AS VOLTAGE INCREASES

  8. ANODE ASSEMBLY • FUNCTION? • PROVIDES TARGET FOR ELECTRONS • PRODUCES THE X-RAY BEAM • CONDUCTS ELECTRONS THROUGH THE CIRCUIT • THERMAL DISSIPATOR

  9. ANODE ASSEMBLY • STATIONARY ANODE • Tungsten target • Imbedded in copper • ROTATING ANODE • Molybdenum disk • Tungsten-rhenium coating • Rotates during exposure

  10. TARGET MATERIAL • TUNGSTEN • HIGH ATOMIC NUMBER • Importance? • HIGH MELTING POINT • Importance? • HIGH THERMAL CONDUCTIVITY • Importance?

  11. ROTATING ANODE • ANODE ROTATES DURING EXPOSURE • 34,000 RPM - 10,000 RPM (High Speed) • Driven by an induction motor • ELECTRON BEAM INTERACTS WITH A LARGE AREA OF THE TARGET • THE FOCAL SPOT BECOMES THE FOCAL TRACK • WHAT IS THE VALUE OF THIS?

  12. TARGET • FOCAL SPOT: • Area of the target from which x-rays are actually emitted • Size is based on the size of the chosen filament • As small as possible – Why?

  13. Angling the anode face (target) produces an EFFECTIVE focal spot that is smaller than the ACTUAL focal spot Allows for higher heat ratings ACTUAL FOCAL SPOT Area on the anode actually struck by electrons EFFECTIVE FOCAL SPOT Focal spot as “seen” by the film Projected focal spot LINE FOCUS PRINCIPLE

  14. LINE FOCUS PRINCIPLE

  15. HEEL EFFECT • VARIATION IN BEAM INTENSITY ALONG THE LONGITUDINAL AXIS • CATHODE INTENSITY GREATER THAN ANODE INTENSITY • DUE TO ABSORPTION OF BEAM BY ANODE • INCREASES AS ANODE ANGLE DECREASES

  16. HEEL EFFECT

  17. OFF-FOCUS RADIATION • RADIATION PRODUCED FROM INTERACTION OF ELECTRONS WITH AREAS OF THE TARGET OTHER THAN THE FOCAL SPOT • INCREASES PATIENT DOSE • CREATES IMAGES OUTSIDE THE INTENDED AREA

  18. TUBE FAILURE - CAUSES • VAPORIZING OF TUNGSTEN FROM FILAMENT • THINNING OF FILAMENT – BREAKING • ARCING • ANODE SURFACE DAMAGE • PITTING • SURFACE MELTS • CRACKING • BEARING DAMAGE • CRACKING OF GLASS ENCLOSURE

  19. TUBE DAMAGE - REDUCTION • MINIMIZE FILAMENT PREP TIME • USE LOWER MA • FOLLOW TUBE RATING CHARTS • LIMIT OPERATION TO 80% OF MAXIMUM EXPOSURE RATING • DO NOT MAKE HIGH MA EXPOSURES ON A COLD TUBE • LIMIT ROTOR START-STOP OPERATIONS

  20. TUBE RATING CHARTS • RADIOGRAPHIC TUBE RATING CHARTS • TUBE COOLING CURVES • ANODE COOLING CURVES

  21. TUBE RATING CHARTS • Indicate safe vs unsafe exposures • Reconcile KV, MA and TIME • UNSAFE = above the line • SAFE = below the line

  22. COOLING CURVES • Demonstrates ability of an anode to dissipate stored heat

  23. HEAT UNITS • USED TO MEASURE AMOUNT OF HEAT ON THE ANODE DUE TO A SINGLE EXPOSURE • CALCULATION: • MA X TIME X KV - SINGLE PHASE • MA X TIME X KV X 1.35 – 3 PHASE, 6 PULSE • MA X TIME X KV X 1.41 – 3 PHASE, 12 PULSE, HIGH FREQUENCY

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