X-ray lecture 3

Something to do with the way the spectrum changes as it passes through matter.

X-rays generated by the tube have a wide range of energies

Lower energy photons are attenuated more rapidly than higher energy photons. (duh)

• As beam penetrates tissue and bone..
• - Shape of spectrum changes
• -Effective energy of beam (E_ff) increases which leads to
• -reduced differences in attenuation coeffs (between bone and soft tissue)
• Reduced image contrast

• Inherent x-ray tube filtration eg
• X-ray window
• 1-2mm glass/metal
• Si (z=14), Al(z=14)
• Cut off below 15keV

• Added filtration
• Use it (usually Al) to attenuate low energy photons that add to dose but not image

Absorbers (including the patient) 'hardens' the beam.

Thickness of a material that reduces beam intensity by half

• Assumptions
• Narrow beam geometry
• No scattering

• Used as an approximation for diagnostic beams
• Usually expressed in mm Al

• -Quantifies the ability of the beam to penetrate tissue
• -indirect measurement of the energies (quality) of an xray beam
• -normally used to characterise polyenergetic beams

• Tissue ranges between 2.5-3cm
• Mammography is 1cm

• 10% for chest
• 1% for skull
• 0.5% for abdominal

HVL increases, but it is not the same thing as real HVL(?)

X-rays leaving the anode tangential to the anode surface are reduced in intensity when arriving at the detector

Reduction is due to self absorption of photons by the anode, caused by microscopic roughness of anode surface.

• Beam on the anode side has
• lower intensity
• higher effective enrgy

• x-ray tube output is the dose per unit mAs 
• Proportional to Z_target and kVp^2

Units are mGy/mAs (milli in both cases) at 1m for a set kVp

Can be calculated if dose is known at a different distance using inverse square law

kVp^2

Noise, spatial resolution, contrast, scatter

Even fr uniform X-ray exposure, adjacent areas have differing photon counts, following a poisson distribution

For N photons reaching the detector, the noise is: Noise ∝ √N

Its another word for relative noise, with the formula

Relative noise ∝ (√N/N)*100

In diagnostic radiology this comes to N>10⁵ for every mm² so thats a relative noies value of 0.3%

Abiligy to distinguish smal objects placed very close together. It has units of line pairs per mm (lp/mm)

• A line pair is a pair of dark and light lines (opaque and radiolucent)
• So 1lp means you can see a 0.5mm wide bar with 2 0.25 lines either side of it

• X lp/mm shows that X line pairs can be resolved in 1mm. 
• Typical limiting resolution: 6lp/mm

• Subject contrast,
• Object contrast
• Radiographic contrast

The difference in attenuation characteristics of adjacent materials

The difference in x-ray intensity transmitted through different adjacent materials

The difference in density (i.e greyness between areas in the radiograph)

I₁ = I₀e^-μ₁b

I₂ = I₀e^{-μ₁(b-c)-μ₂c}

• It is highest at lower kVp
• 

• Increasing Xray beam intensity (increasing mAs).
• It compensates for low penetration at low kVp
• It increases patient dose

• You get a higher transmission/penetration
• You get a decreased contrast

Low kVp

high kVp, high mAs

• High kVp
• Low mAs

The minimum contrast difference that can be distinguished

• Depends on: 
• Signal to noise ratio
• Size of object of interest

• Use test objects
• 

Secondary photon interactions mainly produced by compton interactions

• Increases with kV (Due to reduction in compton scattering, but more photons are scattered forward).
• Increases with thickness of material
• Increases with size of field of view

• It increases image noise
• Reduces image contrast

• where P = no. of primary (unscattered) photons
• S = no. of scattered photons
• R = S/P

• Air gap technique
• Anti-scatter grids
• Reciprocating grids

• Made of narrow lead bars that only allow photons travelling in a specific range of angles.
• The scattered photons at high angles will be absorbed by the lead strips.

• Ratio of strip height to grid gap between bars.
• 
• Increasing grid ratio:
• Improves image contrast
• Needs more careful alignment (susceptible to artefacts)
• Needs higher expocure

They move during the exposure, spreading the image of the grid lines to render them invisible

You cause artefacts

• They are adjustable parallel opposed lead shutters
• They define the x-ray beam size and shape emerging from the X-ray window
• It can reduce the total patient mass irradiated and thus reduce patient dose and exposure.

By using a light source and mirror to define the X-ray field.

• Its the penumbra that results from xrays arriving at slightly different locations in the focal spot since the focal spot is actually an area
• 

• Increased magnification
• Increased focal spot size

Small focal spot

• Exposure is terminated when the right radiation amount reaches the detectors
• A thin radiation detector is placed in front of the detector and behind the grid
• AEC systems are almost radiolucent
• Level of exposure is set to provide optimal image quality