Ch 3 The xray beam

Producing Xrays (how it takes place)

• requires a rapidly moving stream of electrons that are suddenly decelerated or stopped

  • negative electrode (cathode) is heated, and electrons are emitted

  • electrons are attracted to the anode

Bremsstrahlung interactions

• break down radiation

• 85% of xray beam

• Greater than 70 kVp 100%

charactesitic interactions

• projectile electron energy at least 69.5 keV

• inner shell electron ionized

• 15% of xray beam

• 100% when it’s less than 70kVp

Brems vs Characteristic

Most xray interactions produced in diagnostic radiology result from bremsstrahlung interactions. Characteristic occurs at 69.5

cathode (preparing xray tube)

• filament current heats up the filament

• this heat boils the electrons off the filament (thermionic emission)

• these electrons gather in a cloud around the filament (space charge)

• the negatively charged focusing cup keeps the electron cloud focused together

• the # of electrons in the space charge is limited (space charge effect)

anode (preparing xray tube)

the rotating target begins to turn rapidly, quickly reaching top speed

cathode (making xray exposure)

• high negative charge strongly repels electrons

• these electrons stream away from the cathode and toward the anode (tube current)

anode (making an xray exposure)

• high negative charge strongly attracts electron sin the tube current

• therse electron strike the anode

• xrays and heat are produced

kinetic energy convertion

as electrons strike anode target, more than 99% of their kinetic energy is converted to heat, whereas less than 1% of their energy is converted to xrays

tube current

• electrons flow from cathode to anode (only in 1 direction)

• measured in milliamperes

electrical current

• Approximately 3-5 A

• opperates at approximately 10 V

• current flowing through the filament depends on the mA set at control panel

kilovoltage (kVp)

• determines speed of electrons in tube current (direct relationship, NOT proportional)

• higher electron speed—→ higher quantity and energy of primary beam

• increase electron speed will inclease xray beam penetrability

  • changing kVp: affects energy or quality of xrays produced but it is NOT a proportional relationship

milliampere

• higher mA—> more electrons moving in tube current from cathode to anode

• mA is directly proportional to quantity of xrays produced

Quiality vs quantity

quantity→ mA

quality→kV (also secondary quantity)

exposure time

• longer exposure time→more electrons moving in tube (cathode → anode)

• more electrons in tube current→ more xrays produced

increasing time will increase total # of xrays produced

(exposure time and xray quantity are directly proportional)

effects of changing prime exposure factors

prime exp factors: kilovoltage, milliampere, & exposure time

voltage ripple

• describe voltage waveforms in terms of how much the voltage varies during xray production

kVp settings

• higher kVp → electrons move faster from cathode to anode

• higher energy of xray photons→ greater penetrability

actual focal spot (AFS)

area on anode target exposed to tube current electrons

effective focal spot (EFS)

• projected focal spot size as measured directly under the anode target

• Smaller EFS → Higher Resolution (Sharper image, better detail).

• Larger EFS → Lower Resolution (More blurring, less detail).

beam intensity variations

• greater intensity (# of xrays ) on cathode side

• lower intensity toward anode side

how anode heel effect works and intensity differences

xrays are more intense on the cathode side of the tube, their intensity decreases toward the anode side

• varies 45%

beam filtration/ compensating filters purpose

• process of removing low energy (soft) xray photons from the beam before they reach the pt. Improves quality and reduces pt dose

beam filtration/ compensating filters types

• added→ added filtration anywhere below the port of the xray tube. (Aluminum is the primary material used for this purpose)

  • Aluminum absorbs more lower energy photons, while higher photons can penetrate aluminum and exit

• Inherent→ filtration that is permanently in path of xray beam.

  • 3 components (envelope of tube, oil surrounding tube, & window in the tube housing

• total→ sum of added and inherent filtration

extending tube life

• Warm Up the Tube according to manufacture’s specifications

• avoid excessive heat unit generation

• don’t hold down rotor button w/o making exposure

• use lower tube currents w longer exp times

• don’t move tube while it’s energized

• if rotor makes noise, stop using tube until it has been inspected