Exposure Variables and Image Characteristics

Question 1

what is attenuation?

Answer 1

A measure of how easily a material can be penetrated by an x-ray beam

It quantifies how much the beam is “attenuated” (weakened) by the material it is passing through

Attenuated x-ray photons are those that are absorbed, transmitted with a lower energy or scattered

Question 2

why is attenuation important

Answer 2

if all different types of tissues in our bodies stopped x-rays in the same way we would not have a distinct image but just a uniform grey image on our screen.

Question 3

Attenuation Summary

Answer 3

Can be referred to as the total reduction in the number of x-ray photons passing through tissue

Combination of Photoelectric Effect and Compton Scatter

As the atomic number (Z) and tissue mass density increase:
There is an increase in both Compton scatter and photoelectric effect
Greater attenuation of the x-ray photons

As the x-ray photon energy increases:
There is a decrease in photoelectric effect and Compton scatter becomes the predominant cause of attenuation
There is a reduction in the total attenuation so more photons are transmitted through the tissue

Question 4

controllable variables
- what factor affect image quality and patient dose.

Answer 4

variables which affect image quality and patient dose.
- control x-ray photon energies.
- control the amount of x-ray photons available
- control the length of time x-ray photons are available
- control the overall exposure - AEC
- control the number of scattered photons reaching the target
- control the area of the patient being exposed
- control the distance between the source of x-ray photons and image receptor (SID)
- control the thickness of the area being exposed?

Question 5

X-ray Photon Energies

Answer 5

• Homogenous or heterogenous x-ray beam?
  The beams produced by x-ray tubes are photons of a wide range of energies
• The lower-energy photons are attenuated more than the higher-energy photons, leaving overall a larger number high energy photons = beam hardening
• The resulting beam is of a higher average energy and can penetrate tissue easier
• Inherent filtration and additional filtration help to create a homogenous x-ray beam

Question 6

Filtration

Answer 6

Results in less lower energy photons

Increased:
Average energy of photons
Decreased:
Total number of photons

Question 7

X-ray Photon Energies - kV

Answer 7

The voltage used for an exposure indicates the power/intensity of the x-ray photons produced
The higher the kV, the greater the penetrative power of the x-ray beam
The kV selected determines the _contrast________ within an image

The amount of x-ray contrast produced is determined by the object characteristics:
atomic number
tissue density
tissue thickness

Question 8

Contrast

Answer 8

For an object or anatomical structure to be visible in an x-ray image, it must have physical contrast in relationship to the tissue or other material in which it is embedded

The difference in x-ray penetration between different tissues represents the contrast in the image

In other words, image contrast is how easily we can tell different structures apart

Question 9

Amount of X-Ray Photons - mAs

Answer 9

The __current_____ and ___exposure time __________ used indicate the amount of x-ray photons generated
The higher the mAs, the more x-ray photons produced
The mAs selected determines the (radiographic) density within an image

Radiographic density is the amount of blackness (or lack of it) on an image

resulting gray scale image on xray
air - fat - soft tissue - bone - metal

Question 10

Density

Answer 10

Radiographic density is directly proportional to the quantity of photons used to create any part of the image

the more photons hitting the detector the darker the image

Question 11

Grey Scale Image

Answer 11

An x-ray image is made of different levels of grey or a combination of varying radiographic densities

Radiolucent:
an area of the image where a larger amount of the xray beam passed through unobstructed to reach the detector - blacker on the x-ray image

Radiopaque:
an area of the patient that is absorbed or scattered a large amount of the incident x-ray beam prior to it reaching the detector - the tissues in question block the x-rays from reaching the detector - whiter on the x-ray image

Question 12

Exposure Time

Answer 12

The longer the exposure, the more chance that the patient will move, which creates motion blur in the resulting image

When is it useful to have a shorter exposure time?

Question 13

controllable variable

Answer 13

variables which affect image quality and patient dose;
- control the x-rays photon energies
- control the amount of xrays photons available
- control the length of time xray photons are available.

Question 14

Exposure Time - When is useful to have a longer exposure time?

Answer 14

the longer the exposure the more chance that the patient will move which creates motion blur in the resulting image.

when is it useful to have a shorter exposure time?

• paediatric imaging
• patients in pain who cannot keep still
• confused patients
• voluntary/ involuntary patient movement (chest, abdomen)

when is useful to have a longer exposure time?

a long exposure time e.g 2 seconds results in blurring of the bowel gas, ribs and diaphragm

• the spinal bony anatomy remains sharp
• patient is asked to keep still but remain breathing during the exposure

Question 15

control variables - control the overall exposure - AEC

Answer 15

Automatic exposure control is a device incorporated into radiographic imaging systems.

Its function is to automatically terminate exposure when a preset amount of radiation reaching the image receptor has been detected

Helps to provide a consistent image density/signal-to-noise ratio, regardless of patient factors such as size and density

Assist radiographers to achieve an optimum exposure and reduce ‘dose creep’ that can occur with inadvertent overexposure

Question 16

AEC Device

Answer 16

The AEC works by measuring the amount of ionisation that occurs withan ionisation chamber while radiation is passing through it

Most diagnostic systems use a combination of three chambers

A selection of the chambers can be activated over the region of interestusing the control panel

Question 17

AEC - Ionisation

Answer 17

Ionisation chambers measure exposure by detecting the liberated electron charge when x-ray photons ionise the gas within the chamber

The chambers need a high positive voltage applied at the collecting anode to attract the liberated electrons

The electron charge is collected and used to determine the radiographic exposure

The exposure is stopped when sufficient amount of x-ray photons have reached the image detector (based on the kV and mAs used)

This ensures that a reproducible optimum image density is achieved independent of the object density

AEC usage

Question 18

general rules

Answer 18

kV and mAs can be used to offset each other

If the kV is increased by 10, the mAs should be halved to achieve the same exposure
If kV is decreased by 10, the mAs should be doubled

For every 4cm increase in tissue thickness, the mAs needs to be doubled to maintain the same image density

The current (mA) and time (s) are generally considered together (mA x s) but can be considered individually if necessary, ie. for shorter exposures

Question 19

Overall Exposure

Answer 19

The overall x-ray image is determined by a combination of the kV and mAs set for the exposure

_overexposure___________ and _____underexposure_______ are used to describe suboptimal images where too much/too little kV/mAs have been used respectively

Exposure factors are decided based on several factors such as:
Area being imaged
Tissue thickness/density
Clinical indications/pathology

Key points:
kV, contrast, mAs, density cannot be considered in isolation
Goal is not to have high/low contrast but optimum contrast resolution

Question 20

Digital Imaging

Answer 20

Digital imaging systems are designed to optimise the image contrast and density irrespective of the exposure factors used!
Brightness and contrast algorithms post-process the appearance of an image to reduce the effect of over/underexposure

Loss of link between exposure and visual appearance of the x-ray image

Risk of overexposure and higher dose to the patient

Question 21

Exposure Index

Answer 21

Exposure Index (EI) is the measure of the amount of exposure received by the image receptor (IR)

It provides useful feedback to the radiographer about the accuracy of the exposure utilised

Vendor specific but there is an international standard for EI

Question 22

Scatter Radiation

Answer 22

X-ray photons engaging in Compton interactions produce scattered radiation
Some of this scattered radiation leaves the body in the same general direction as the primary beam and exposes the image receptor

Scatter increases with:
Field size
Patient thickness
Higher kV

Scatter radiation degrades ___image contrast ______________

Question 23

controllable variables

Answer 23

variables which affect image quality and patient dose
- control the xrays photon energies
- control the amount of xray photons available
- control the number of scattered photons reacching the target + area of the patient being exposed.

Question 24

Controlling Scatter - Collimation

Answer 24

Amount of scattered radiation is generally proportional to the total mass of tissue contained within the primary x-ray beam
Determined by the thickness of the patient and the area/field size being exposed
Controlling the area exposed/thickness = controlling scatter radiation

Collimation:
By limiting the size of the primary beam, the area/field size exposed is smaller so less amount of tissue exposed
Less chance of scatter radiation to reach the image receptor
Improved image contrast resolution and lower patient dose

Question 25

Controlling Scatter - Grids

Answer 25

Another way of controlling scatter radiation is to reduce the amount of scatter radiation reaching the image receptor

Named after Dr. Gustave Bucky who constructed the first grid in 1913

Grids are placed between the patient and the image receptor to reduce the scattered radiation reaching it and thus improve image contrast

Grid – a grid made of parallel strips of high attenuating material such as lead with an interspace filled with low attenuating material such as carbon fibre or organic spacer

Question 26

Grids

Answer 26

The ideal grid would absorb all scattered radiation and allow all primary x-rays to penetrate to the receptor
Unfortunately, there is no ideal grid, because they all absorb some primary radiation and allow some scattered radiation to pass through to some extent

A grid’s efficiency is described by the grid ratio:
The ratio of the height of the lead strips to the distance between two strips (interspace)

Grid Types:
Stationary
Parallel
Focused
Crossed
Moving

Question 27

Grid Issues

Answer 27

Grid Lines
Since the grid is physically located between the patient and the receptor, there is always a possibility that it will interfere with the formation of the image
Depends on the thickness of the strips and the interspaces
Overcome by using a moving/oscillating grid

Grid Cut Off
Misalignment of the x-ray tube focal spot with respect to the focal point of the grid

Question 28

Grid Usage

Answer 28

Generally used where the anatomy is >10 cm
Skull
Chest
Abdomen
Pelvis/Hips
Spine (except cervical)

A higher kV is required for thicker body section = larger amount of scatter radiation
The exposure will need to be increased to compensate for the x-ray photons absorbed by the grid
Trade off between contrast improvement and patient dose

Question 29

Controlling Scatter - Air Gap

Answer 29

The quantity of scattered radiation in an x-ray beam reaching the image receptor can also be reduced by increasing the distance between the patient’s body and receptor surface

This separation is known as an air gap

Scattered radiation leaving a patient’s body is more divergent than the primary x-ray beam
Therefore, scattered radiation spreads out of the primary beam area

Question 30

Controlling the SID

Answer 30

The source image receptor distance known as the SID is the distance of the tube from the image receptor

The x-ray beam is diverging from the source

An SID of 1m is used for most x-ray projections
PA chest x-rays are usually done at 180cm

Increasing the SID:
Reduces magnification (M=SID/SOD)
Reduces patient dose (inverse square law)

Question 31

Controlling the Thickness

Answer 31

A reduction in the thickness of the area being imaged results in:
Reduced scatter radiation
Less exposure
Improved image quality

It is often not possible to reduce the thickness for the general projectional radiography
Some modified projections result in an inherently thicker area to be exposed