X-ray Production/Technology in Radiology Flashcards
What are the principle control variables?
kV
mA
time
What is kV?
refers to the potential between cathode and anode
Tube kV determines peak energy of X ray spectrum (eg 50kVp -> 50keV photon maximum)
What is mA?
refers to the magnitude of electron current passing from cathode to anode
What is X ray photon flux proportional to?
number of electrons striking the anode per second (ie current)
- less than 1 in 100 electrons produce an X ray photon
What is exposure related to?
total number of photons that strike the detector (film or digital sensor)
What is the total number of photons emitted is proportional to?
the tube current and exposure time
What can exposure be characterised by?
mAs (current x time)
What is voltage transformation?
high voltage for high energy electrons
What is voltage rectification?
Anode must be +ve
Diodes (uni-directional current)
Capacitors (smoothing)
High frequency (ripple)
What is tube filtration?
Filtration is a measure of the quality of the emitted X-ray spectrum
Characterised by half value thickness…
…ie. thickness needed to reduce exposure by half
Typically measured in terms of thickness of aluminium
Spectral content can be controlled using filters (Quality)
What is dose area product meter?
a measure of exposure
- ionisation chamber: after filtration, after collimation
DAP reading can be used to infer patient dose
How is a good image produced from an x ray?
Shadowgraph
Focal spot size:Fine focus, Broad focus, Penumbra effects
Differential absorption
Linear region of film response
Photoelectric: Absorption (differential), Dose, Contrast
Compton: Scatter(inelastic > dose), Loss of contrast, Loss of definition
Pair production: Not relevant at diagnostic energies
What is contrast?
defined as the difference in optical density between the regions of interest
C = OD1 – OD2
What is contrast compromised by?
overlying layers of tissue
scatter
For film, what is optical density used for?
to characterise the ‘blackness’ of a film
OD = -log(fraction of light transmitted through the film)
For digital, what does pixal intensity relate to?
exposure
contrast related to differences in pixel intensity
What factors can reduce contrast?
Poor differential absorption in the tissue of interest
Overlying tissues
Scatter
Noise
How can contrast be improvd?
Reduce (‘cone down’) volume of tissue in the beam
Low kV’s are associated with (relatively) low scatter
Scatter reduction grid
Artificial contrast enhancement
How does an X-ray film and cassette work?
AgBr is the active material for image formation.
X-ray film consists of a plastic film coated with an AgBr emulsion.
The emulsion is reduced to Ag particles to form a latent image in response to ionising radiation.
(AgBr Ag+ + Br-)
Ag catalyses further reduction of silver during processing. Fixing hardens gel to preserve image.
Emulsion is on both sides to improve contrast.
However, the film is of low density and atomic number and it is therefore a poor detector of X-rays.
(ie. inefficient; high fluxes needed for adequate images)
Screened film (fluorescent screen) can mitigate this
How does screened film work?
The probability of X-ray detection is greatly increased by sandwiching it between two fluorescent screens.
These are of high density and high atomic number (eg. calcium tungstate).
The presence of an ionising event within the screen releases a shower of visible photons that expose the film.
A reflective layer bounces photons back onto the film if necessary.
The spread of the photon shower at the film is minimised by sandwiching the whole assembly tightly together within a cassette.
The presence of the screen reduces the necessary radiation dose to expose film correctly by a factor of approximately 50.
How does ‘The Grid’ work?
Improves image quality at the expense of dose
The grid is a collimating device that sits above the film/detector.
Consists of many fine lead strips that stop scattered radiation reaching the film/detector.
20-30 strips per mm, approximately 50m thick.
The grid may be static, but some move during exposure to blur out any image of the grid pattern that may appear on the film/detector.
How does fluoroscopy work?
‘Real-time’ X-ray image displayed on video monitor
Intensifier
X-ray fluorescent screen (X-ray to light conversion – CsI)
Photocathode (light to charge conversion)
Electron lens system to transfer image to (small) phosphor screen
Fluorescence phosphor amplif. ~1000
Phosphor image captured by camera
What are the digital techniques of x ray technology?
1) Computed Radiography
Film replaced by a digital plate/cassette (but re-use)
The plate is ‘developed’ to reveal the image (photostimulable phosphor)
2) Digital Radiography
Film replaced by CCD-type chip technology (eg. amorphous silicon)
Energy of X-ray flux directly translated to electronic signals - no ‘developing’ required
- Digital images are amenable to computer manipulation/analysis
What are the principle elements of an X ray (Bremsstrahlung)?
Cathode – thermionic emission
Anode at high potential
High speed electrons
e- impacts on solid anode: High Z, High density
Savage deceleration: (-ve acceleration)
Maxwell – an accelerating charge radiates: Bremsstrahlung – ‘braking radiation’, Continuous spectrum
What is characteristic radiation?
High speed electrons impact on the dense anode.
Inner shell electron ejected from atom (ionisation)
Electron from other shell falls into newly vacant shell…
…with associated release of X-ray radiation
Wavelength is characteristic of energy structure of target material
Energy transfer … kVp
