X-ray Production/Technology in Radiology

Question 1

What are the principle control variables?

Answer 1

kV
mA
time

Question 2

What is kV?

Answer 2

refers to the potential between cathode and anode

Tube kV determines peak energy of X ray spectrum (eg 50kVp -> 50keV photon maximum)

Question 3

What is mA?

Answer 3

refers to the magnitude of electron current passing from cathode to anode

Question 4

What is X ray photon flux proportional to?

Answer 4

number of electrons striking the anode per second (ie current)
- less than 1 in 100 electrons produce an X ray photon

Question 5

What is exposure related to?

Answer 5

total number of photons that strike the detector (film or digital sensor)

Question 6

What is the total number of photons emitted is proportional to?

Answer 6

the tube current and exposure time

Question 7

What can exposure be characterised by?

Answer 7

mAs (current x time)

Question 8

What is voltage transformation?

Answer 8

high voltage for high energy electrons

Question 9

What is voltage rectification?

Answer 9

Anode must be +ve
Diodes (uni-directional current)
Capacitors (smoothing)
High frequency (ripple)

Question 10

What is tube filtration?

Answer 10

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)

Question 11

What is dose area product meter?

Answer 11

a measure of exposure
- ionisation chamber: after filtration, after collimation
DAP reading can be used to infer patient dose

Question 12

How is a good image produced from an x ray?

Answer 12

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

Question 13

What is contrast?

Answer 13

defined as the difference in optical density between the regions of interest
C = OD1 – OD2

Question 14

What is contrast compromised by?

Answer 14

overlying layers of tissue

scatter

Question 15

For film, what is optical density used for?

Answer 15

to characterise the ‘blackness’ of a film

OD = -log(fraction of light transmitted through the film)

Question 16

For digital, what does pixal intensity relate to?

Answer 16

exposure

contrast related to differences in pixel intensity

Question 17

What factors can reduce contrast?

Answer 17

Poor differential absorption in the tissue of interest
Overlying tissues
Scatter
Noise

Question 18

How can contrast be improvd?

Answer 18

Reduce (‘cone down’) volume of tissue in the beam
Low kV’s are associated with (relatively) low scatter
Scatter reduction grid
Artificial contrast enhancement

Question 19

How does an X-ray film and cassette work?

Answer 19

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

Question 20

How does screened film work?

Answer 20

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.

Question 21

How does ‘The Grid’ work?

Answer 21

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 50m 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.

Question 22

How does fluoroscopy work?

Answer 22

‘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

Question 23

What are the digital techniques of x ray technology?

Answer 23

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

Question 24

What are the principle elements of an X ray (Bremsstrahlung)?

Answer 24

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

Question 25

What is characteristic radiation?

Answer 25

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