Exposure Variables and Image Characteristics Flashcards
what is attenuation?
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
why is attenuation important
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.
Attenuation Summary
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
controllable variables
- what factor affect image quality and patient dose.
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?
X-ray Photon Energies
- 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
Filtration
Results in less lower energy photons
Increased:
Average energy of photons
Decreased:
Total number of photons
X-ray Photon Energies - kV
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
Contrast
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
Amount of X-Ray Photons - mAs
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
Density
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
Grey Scale Image
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
Exposure Time
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?
controllable variable
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.
Exposure Time - When is useful to have a longer exposure time?
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
control variables - control the overall exposure - AEC
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
AEC Device
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
AEC - Ionisation
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
general rules
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
Overall Exposure
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
Digital Imaging
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
Exposure Index
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
Scatter Radiation
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 ______________
controllable variables
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.
Controlling Scatter - Collimation
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
Controlling Scatter - Grids
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
Grids
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
Grid Issues
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
Grid Usage
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
Controlling Scatter - Air Gap
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
Controlling the SID
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)
Controlling the Thickness
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
