controlling scatter

Question 1

what happens to x ray photons when they are directed at a patient

Answer 1

absorbed
scattered
transmitted

Question 2

scatter distribution

Answer 2

• the average angle of scattering decreases as the incident photon energy increases
• remember that higher tube voltages ( kVps ) result in higher energy photons
• higher energy photons tend to continue travelling in a forward direction ( i.e. along their original course )
• This makes it more likely that high energy scatter will interact with the IRD
• scattering angle - depends on energy lost, relatively small energy losses produce significant scattering angles

Question 3

what is the liklihood of compton scatter

Answer 3

• compton scatter increases with density of the attenuating medium ( tissues of patient )
• compton scatter doesn’t vary much between elements ( only hydrogen markedly different - down to electron density )
• compton scatter decreases with photon energy ( i.e. lower energy photons are absorbed rather than scattered )

Question 4

effects of scatter

Answer 4

• increases patient radiation dose
• contributes to staff radiation dose
• reduces image contrast, thus reducing the overall quality of the image

Question 5

factors influencing scatter

Answer 5

• electron density has an effect on the probability of compton scatter, so the no. electrons in the tissues and the physical thickness of the anatomical area
• energy ( quality ) of the beam at energies of 100 keV
• NB higher beam energies result in more energentic forward travelling, scattered photons

Question 6

how can we reduce scatter at source

Answer 6

• collimation
• reduction of tube voltage kVp
• reduces tissue density
• remove dense objects from area of interest
• protecting the image recording device with grid or bucky or an air gasp is secondary to minimising the amount of scatter at source

Question 7

how does tissue displacement effect scatter

Answer 7

• tissue thickness contributes to scatter formation ( thicker the tissues / anatmoical and the more electrons present )
• more scatter generated from abdo / pelvic areas compared to wrist or tibia and fibula
• this technique also allows the operator to select a lower kVp, thus further reducing scatter issues

Question 8

how does the removal of dense objects affect scatter

Answer 8

• dense objects in, or adjecent to the area of interest increase scatter formation
• back scatter from lateral spine projections ( lead rubber sheet )

Question 9

overview of buckys and grids

Answer 9

• designed to transmit the useful image forming beam while attenuating any scatter
• a bucky is essentially a moving grid
• not very selective - will always attenuate some of the useful image forming beam while transmitting some of the scatter
• beam intensity is reduced
• exposure factors have to be increased to compensate for a loss in beam intensity
• both devices result in increased radiation dose

Question 10

secondary radiation grids

Answer 10

• composed of parrarell strips / slats of lead
• radiolucent interspaces support lead slats while allowing useful beam to be transmitted
• attenuate obliquely travelling scattered photons

Question 11

disadvantages of grids

Answer 11

• never 100% efficient at distinguishing between scatter and useful image forming photons
• they reduce intensity of the beam, operator needs to compensate for the drop in photons by increasing the exposure factors
• usually a grid increases patient dose but reduces amount of scatter reaching IRD
• increased exposure factors leads to increase in patient dose

Question 12

parallel grids

Answer 12

• carbon fibre interspaces with lead or tungsten slats
• no FFD requirements
• permit some angulation
• ideal for use where re positioning patient may be difficult

Question 13

focussed grids

Answer 13

• still has parallel lines
• slats are angled as you move away from the midline
• avoids grid cut off at the margins, trasnmits more of a useful image forming photons
• requires definitive FFD
• will tolerate some angulation along the long axis of the grid lines but not across them
• less flexible and requires more precision from operator

Question 14

cross hatched grids

Answer 14

• only used in instances where there is a lot of scatter generated
• 2 parallel grids turned 90 degreed to one another and superimposed
• do not permit any angulation of the beam thus no flexibility

Question 15

comparing grids

Answer 15

• the greater the amount of lead/tungsten in a grid, the more scatter it will attenuate
• NB will also attenuate more image forming beam as well ( reduce beam intensity )

Question 16

grid ratio

Answer 16

• relationship between height of the lead slats and the gap between them
• higher grid ratios attenuate more scatter and require higher exposure factors
• higher grid ratios demand higher precision
• grids with high ratios have more lead/ attenuating slats

Question 17

grid cut off

Answer 17

• reduced image density due to attenuation of the image forming beam
• result of poor centring, FFD selection of angling across the grid lines
• low grid ratios offer more opportunity for angling along the grid lines
• high grid ratios are less tolerant to operator errors and adaption of radiographic technique

Question 18

bucky device

Answer 18

• moving grid incorporated into the x ray table
• it oscillates back and forth over the IRD, attenuating scatter
• because it is moving the grid lines are blurred out

Question 19

air gap technique

Answer 19

• used for CXRs, horizontal beam lateral hips and lateral cervical spines
• relies on increasing OFD to 30cm which is essentially an increase in FFD
• as scatter leaves patient, it travels more obliquely and is reduced significantly after travelling 10-20cm
• intensity of useful beam remains relatively unchanged, but the scatter intensity has been reduced significantly
• improves contrast, and in comparison to using a grid, offers reduced radiation dose to patient
• however and air gap technique does require a higher kVp and a larger source to image distance