MI 237 - Unit 1

A

transmission

B

absorption

C

scatter

Compton Scattering

• energy is >20kV

• ionization of outer shell

• scattered photon - yes

• free electron - yes

• dose is minimal - significant to patient and most significant to radiographer

  • probaility : as kVp increases, probability decreases, but it is more common at higher kVps than photoelectric

what is happening in this image

compton scattering

what does scatter do to an image

• adds unwanted noise

• decreases image contrast levels

3 factors that impact scatter production

• part thickness

• field size

• kVp

part thickness

increasing patient size or body part increases scatter

• more matter more scatter

field size

increasing field size exposed increases scatter

• more matter = more scatter

kVp

increasing kVp decreases the photoelectric and compton interactions that happen

• there are more compton interactionsthat happen over photoelectric

• the scattered photon has higher remaining energy when higher kVp was used

increasing kVp = increases scatter

2 factors to help scatter control

beam-restricting devices

radiographic grids

beam restricting devices

AFFECTS SCATTER PRODUCTION

• decreases the x-ray beam field size and the amount of tissue irradiated, reducing the amount of scatter produced in the patient

radiographic grids

AFFECTS SCATTER CLEAN UP AFTER PRODUCTION

• used to improve radiographic image quality by absorbing scatter radiation that exits the patient, reducing the amount of scatter reaching the image receptor

beam restriction purpose

• limits patient exposure

• reduces scatter production

increasing beam restriction

decreases patient dose

decreases scatter produced in the patient

decreases scatter reaching the IR

increases radiographic contrast

types of beam restricting devices

• aperture diaphragm

• cones and cylinders

• collimators

• automatic collimators (PBLs)

• lead masks

aperture diaphragm definition

a flat piece of lead (diaphragm) that has a hole (aperture) in it and is placed directly below the x-ray tube window

what is this

aperture diaphragm

cones and cylinders definition

an aperture diaphragm with an extended flange attached to it

• almost always made to produce a circular projected field

what is A

cylinder

what is B

cone

collimators

located immediately below the tube window

2 sets of lead shutters

first (entrance shutter)

limits the field size similar to aperture diaphragm

second (adjustable shutters)

longitudinal and lateral “leaves” or “blades”

limits off-focus radiation (extrafocal radiation)

x-rays created anywhere outside of focal spot

x-rays not aligned with regular beam

adds noise to the image

collimators are equipped with

a white light source and a mirror to project a light field onto the patient which indicated exposure area

an x-ray field measurement guide in case of light failure

a plastic template with crosshairs to indicate centering

automatic collimators are also known as

positive beam-limiting (PBL) device

what do PBLs do

automatically limits the size and shape of primary beam to the size and shape of the IR

lead blockers/masks

similar to aperture diaphragm

• limited to that shape and size

• cut out to shape of body part

who invented the first grid

Gustav Bucky

Gustav Bucky designed a

cross hatched grid

Hollis-Potter design is a

grid with linear strips only, thinner strips, Potter-Bucky diaphragm moved during grid exposure to blur grid lines

radigraphic grids are

very thin lead strips that absorb scatter from the patient before it can reach the IR

• it is placed between patient and IR

typrical grids use

part thickness > 10cm

kVp above 60

strips of the grid are made of

lead

interspace material of grids are made of

aluminum or plastic

grid dimensions include

height (H)

thickness lead strip (T)

distance between strips (D)

grid ratio is

height of grid/distance between lead strips

grid construction is describe by

grid ratio and grid frequency

grid ratio ranges from

5:1 to 16:1

as the ratio increases so does the efficiency of scatter clean up and the image quality/contrast

grid frequency is

the number of lead lines per unit length (cm, mm, inches)

ranges from 60-110 lines/inch

most common grid frequency

85-103 LPI

linear grid pattern

lead lines run in only one direction

• most popular

• allows angulation of central ray along the length of lead strips because only absorbs scatter in one direction

crosses grid pattern

lead lines run at right angles to one another

• most effective at absorbing scatter

• difficult to use

  • must center perfectly and cannot angle central ray

non-focused/parallel grid

lead lines run parallel to one another

focused grid

lead lines are angled to match the angle of divergence of the primary beam

• most common

• allows more transmitted photons to reach the IR

• can only be used at specific SID’s so that the divergence of beam matches lead strips

convergent point/convergent line

determines the focal distance of a focused grid

focal distance (grid radius)

distance between the grid and the convergent line or point

focal range

the recommended range of source to image receptor distances (SIDs) that can be used with a focused grid

multi-focus grids

grids with a wide range of focal distances

• lower grid ratios

• shorter height strips

• decreased strip angles

• tech doesn’t have to change out

• not as precise scatter clean up as focused grid at one distance

stationary , nonmoving grids

• grid cap

  • contains a permanently mounted grid and allows the IR to slide in behind it

• wafer (slip on) grid

  • matches the size of the cassette and is used by placing it on top of the IR

• grid cassette

  • IR that has a grid permanently mounted to its front surface

moving or reciprocating grids (oscillating)

• potter-bucky diaphragm

  • located directly below the radiographic tabletop and just above the tray that holds the IR

• grid motion controlled electrically by the x-ray exposure switch

• grid moves slightly back and forth in a lateral direction over the IR during the entire exposure

grid conversion formula

• used to maintain receptor exposure with changing grid ratio

mAs1/mAs2 = GCF2/GCF1 OR mAs2 = (mAs1 x GCF2)/GCF1

grid cut off

a decrease in the number of transmitted photons that reach IR because of some misalignment of the grid

5 types of errors that can result in grid cut off

• off-level

• off-angle

• off-center

• off-focus

• upside-down

grid lines may also occur

off level error

can affect focused and parallel grids

grid not flat

central ray not perpendicular to grid strips

loss of exposure across entire image

off angle error

affects focused and parallel grids

beam is angled across/against the lead strips

loss of exposure across entire image

off center error

only affects focused grid

occurs when the central of the x-ray beam is not aligned from side to side with the center of a focused grid

loss of exposure across entire image

off focus error

only affects focused grids

occurs when using an SID (greater than or less than) outside of the recommended focal range

loss of exposure at the periphery of the image (outer edge only)

upside down error

only affects focused grids

focused grid is placed upside down on the IR, resulting in the grid lines going opposite the angle of divergence of the x-ray beam

significant loss of exposure on outer edges only

grid lines - reciprocating grid

grid lines can be visible if

• reciprocating grid fails to move

• exposure time is too short for grid to move

grid lines - stationary grids

grid lines can become visible if useful beam is absorbed by lead strips

grid usage

for parts 10 cm or larger

60 kVp or higher

grid selection involves consideration

• contrast improvement

• patient dose

• likelihood of grid cutoff

virtual grids

use complex algorithms to analyze and correct images

*still need a real grid for some larger body partx

virtual grids benefits

• can allow for lower mAs to be used as no grid is used = lower pt dose

• less repeats

• digital post processing allows for real time corrections

virtual grid challenges

• high costs

• large datasets needed for machine learning

• integration complexity

air gap technique

provides another way for limiting the scatter reaching the IR

based on increased OID

acts like a grid

large OID

allows time for scattered photons to diverge before they reach the IR

less scatter = less grays

increased contrast

air gap disadvantages

reduces quantity/receptor exposure

creates size distortion