Mammography & Fluoro Flashcards
Tube Voltage???
25 - 35kvP
Focal spot size (Normal vs Mag)
Normal 0.3mm
Mag 0.1mm
Molybdenum
Characteristic x rays
Rhodium
- 17.9 and 19.5 keV
Atomic Number = 42
Molybdenum filter is 0.3mm
- 20.2 and 22.7keV
Atomic number = 45
Exposure times??
Can be up to 4 seconds in dense breasts
Compression
- 110 - 200N
Magnification Mammography
Breast is moved 15-30 cm closer to the x-ray source, creates air gap also
Focus to film distance remains the same!!!
Magnification = FFD/FOD
(focus to film distance/focus to object distance)
GRID IS NOT USED IN MAGNIFICATION MAMMOGRAPHY
Source to film distance??
65cm
What is tube output proportional to in Mammography?
Proportional to kV3
Molybdenum Filter thickness?
0.03mm
Image intensifier chamber material
Made of:
- Glass, ceramic or aluminium
Surrounded by metal housing preventing light from reaching tube. Also shields from magnetic field
NO LEAD INSIDE TUBE
Input Phosphor - Amount of light photons produced at input phosphor from 1 x-ray photon??
1 x-ray photon = 3000 light photons in BLUE part of spectrum
GREEN light produced at output
Input window material ?
Aluminium or Titanium
Which phosphor is thicker?
Input phosphor
Scatter at output window - how to minimise?
Thick glass at output window
Does efficieny of input screen affect gain?
NO
Gain is only dependent on flux gain and minification gain
Iris
When brightness decreases, e.g in magnification mode - iris aperture can open to let more light in. This saves an increase in dose
In magnification - there is a change in the voltage of the intermediate electrodes
TV Camera
Electron beam deposits electrons onto the surface
- each light photon from the output screen that is absorbed into the CCD gives rise to ONE electron-hole pair
Automatic brightness control
DOES NOT USE 3 ionisation chambers - that is DR
- Measures brightness at the centre of the output screen and adjusts mA and kV accordingly
- Uses isowatt curves
- Curve A: increases mA and kV
- Curve B: holds kV and increases mA (higher dsoe but good for contrast studies)
- Curve C: Low dose - high kV
Contrast study and DSA
mA increases when radiological thickness (not the kV as would be expected)
Pulsed Fluoro
Pulse rate is between 2 and 20ms
Uses grid controlled tube
Dose of a single exposure
0.1 - 5µGy
Spatial resolution of Digital mammography vs Film?
Digital = 7lp/mm
Film = 15lp/mm
Typical dose to each breast?
1.5 - 3mGy per breast
What is kV of mammography unit?
24 - 35kV
