Projection radiography Flashcards
How is traditional Xray film constructed?
Conventional film is layered he active component is an emulsion of radiation-sensitive crystals coated onto a transparent base material. Most film used in radiography has an emulsion layer on each side of the base - double-sided film - so that it can be used with two intensifying screens simultaneously
What is the emulsion in traditional film made from?
The emulsion is the active component in which the image is formed and consists of many small, irregularly-shaped silver halide crystals (grains) suspended in gelatin. The gelatin supports, separates, and protects the crystals. The typical emulsion is approximately 10 μm thick.
The silver halide typically used in medical imaging films is silver bromide.
How are traditional xray film images produced?
An image on film is created by converting silver ions into metallic silver, which causes each processed grain to become black. This requires two steps
First, the film is exposed to radiation, typically light from an intensifying screen, which activates the emulsion material but produces no visible change. The exposure creates a latent image - an invisible image that is present in the emulsion after exposure and before development
Second, the exposed film is processed in a series of chemical solutions that convert the invisible latent image into an image of developed silver halide crystals that is visible as different shades of grey. The darkness or density of the film increases as the exposure is increased and can be measured in terms of optical density
What happens on an atomic level in traditional film to turn it black?
Absorption of light photons by the bromide ions frees the extra electrons which move to and get trapped by the sensitive spot, causing it to become negatively charged. A positive silver ion is attracted to the spot and is neutralised, depositing an atom of black metallic silver. The process is repeated several times (depending on the number of photons that reach the grain) to give several metallic silver atoms – forming the latent image. The developer solution supplies electrons that migrate into the sensitised grains of silver bromide and converts the other silver ions into black metallic silver:
What are the 2 steps in traditional film after exposure?
Development
Fixing - Fixing is that part of the photographic process in which the unexposed and unreduced silver halide is removed to render the image stable in white light.
What in the process will effect image quality in traditional film process?
The strength and composition of the processing chemicals
The length of time that the film spends in the chemicals
The temperature of the chemicals
What is optical density?
The darkness of the film increases as the exposure is increased and this is measured as optical density. Optical density (D) measures film blackening, in terms of the transmission of light.
What is the traditional curve of optical density?
Low exposure (toe): It takes several silver atoms to make a crystal developable. At low exposures, is unlikely that crystals will have enough silver atoms, so small increases in exposure produce hardly any change in density (the toe of the curve), resulting in a white image with no contrast
Linear region: At higher exposures, where each crystal is likely to have some silver atoms produced, the probability of producing a developable crystal increases with exposure. This is the useful part of the characteristic curve and it is desirable to expose film within this region. This region has the highest contrast
High exposure (shoulder): At high exposures, nearly all the crystals can be developed, so the optical density reaches a maximum (the shoulder of the curve), resulting in a dark image with no contrast
Why does the characteristic optical density curve never reach zero?
Base plus fog
This is the density produced on an unexposed part of a developed film. This is partly due to the density of the film base itself, which is not completely transparent, and partly from the blackening caused by the emulsion.
WHat does film speed refer to?
One of the most important characteristics of film is its sensitivity, often referred to as film speed. The sensitivity of a particular film determines the amount of exposure required to produce an image. A film with a high sensitivity requires less exposure than a film with a lower sensitivity
What are the types of contrast on traditional film?
Radiation contrast - Also known as subject contrast this is the ratio of radiation intensities transmitted by different tissues or structures. (Change in x in the trad curve)
Radiographic contrast - When an image is produced on a film, the radiographic contrast depends on the subject contrast, and on any amplification introduced by the image production process. It is measured as the difference (not the ratio) between optical densities produced in areas of the film corresponding to different tissues. (change in y on the trad curve)
Gamma - The ratio between radiographic contrast and radiation contrast is called the film gamma (the slope of the curve)
what is the latitude of the plain film characteristic curve?
To obtain an image with an adequate range of contrasts, all the film densities must lie in the linear part of the characteristic curve. The latitude is a measure of how much room there is for variation in the exposure.
The film latitude measures the range of exposures that will result in a density within the useful range of film densities (0.25-2). The exposure latitude measures the range of exposure factors (kilovoltage (kV) and milliampere-second (mAs)) that will produce a radiograph with all the densities in the appropriate range
Film screen systems with a wide latitude have poor contrast (small gamma) and vice versa
How do you think the film speed and contrast are affected by the use of small or large grain sizes in the emulsion?
Large, uniform grain size results in:
Fast emulsion
Large gamma
Large grains also reduce the spatial resolution, though film unsharpness is rarely a limiting factor.
Variable grain size results in:
Small gamma
A film with a double emulsion:
A. Will have greater density for the same exposure than a single emulsion film
B. Will have improved resolution compared with a single emulsion film
C. Will have increased gamma compared with a single emulsion film
D. Will take longer to develop than a single emulsion film
E. Will have a total optical density of the sum of the individual emulsion densities
A. Correct. Both emulsions will be blackened, and their optical densities will add.
B. Incorrect. Resolution will be poorer because of crossover and parallax effects.
C. Correct. Imagine two areas of the film with densities of 0.7 and 0.8 - a difference of 0.1. When the densities of two superimposed emulsions are added they become 1.4 and 1.6 - a difference of 0.2.
D. Incorrect. The two emulsions develop together as the film passes through the processor: there will be no need for extra processing time, though more chemical replenishment may be needed.
E. Correct. Optical densities are superimposed in double-sided film.
It should be noted that this answer assumes that the emulsions on a double-sided film are the same as that on the single-sided one. Single emulsion films may be made with thicker emulsions, reducing the differences between the two types.
A film with a wide exposure latitude:
A. Will have a large gamma
B. Will be faster than one with smaller latitude
C. Will produce a satisfactory image for a wider range of exposure factors than one with smaller latitude
D. Requires automatic exposure control to produce a good image
E. Is particularly suited to high kV techniques
A. Incorrect. Latitude and gamma always oppose – a film with large gamma shows a large change in density for a small change in exposure.
B. Incorrect. Speed is independent of latitude – you can have a fast film with great or small latitude.
C. Correct. This is almost the definition of latitude.
D. Incorrect. With a wide latitude film, you are more likely to get a correct exposure, so while you might well use automatic exposure control (AEC), it is certainly not more important than with a high contrast film.
E. Incorrect. Using a high kV will reduce the radiation contrast, so that it is not necessary to have such a wide latitude film.
Base plus fog:
A. Is the optical density of the film base plus the unexposed emulsion
B. Is the optical density of an unprocessed film
C. Is increased at high kV
D. Is increased by storing film for long periods
E. Should be about 0.25
A. Correct. This is the definition of base plus fog.
B. Incorrect. Base plus fog is the density of unexposed, processed film.
C. Incorrect. Base plus fog is the density of unexposed film so the kV used for a subsequent exposure is irrelevant. However, it is true that film stored in a room where high kV procedures are taking place is at risk of being accidentally fogged by scattered radiation.
D. Correct. Fog tends to increase with the age of the film.
E. Incorrect. Base plus fog should not produce a density greater than 0.2 – so 0.25 is too high.
Radiographic contrast:
A. Is decreased at higher tube voltage (kV)
B. Is independent of the film exposure
C. Is improved at optical densities greater than 2
D. Is the reciprocal of gamma
E. Between two parts of an image is the ratio of their optical densities
A. Correct. Contrast is reduced at higher kV.
B. Incorrect. Contrast varies with the exposure and is described by the characteristic curve. Contrast is greatest in the linear portion of this curve.
C. Incorrect. Densities greater than 2 are towards the shoulder of the characteristic curve where the contrast is smaller.
D. Incorrect. Radiographic contrast is proportional to gamma.
E. Incorrect. The contrast is the difference between densities, not the ratio.
The quality of any image can be expressed using what six descriptors?
Contrast
Resolution
Noise
Magnification
Distortion
Artefacts
Define contrast.
Contrast is the difference in signal between different parts of an image.
What is subject contrast?
Subject contrast is the ratio of the radiation intensities in different parts of an image.
In film screen systems the optical density in the image varies with the log of the exposure. Many digital imaging systems also use a log scale to represent exposure.
if you ignore scatter does overlying tissue change the subject contrast?
ignoring scatter then no as the ratio transmitted will still be the same.
How does scatter effect contrast?
scatter reduces contrast as the scattered photons do not travel in their original directions, so they strike the image receptor in the wrong place to contribute useful information and apply a ‘fog’ over the whole image thus reducing the ratio between areas.
How does gamma effect image contrast in film screen systems?
Using a system with a larger gamma will result in a larger difference in optical densities for the same ratio of radiation intensities.
define noise.
The random variation in the signal is called the noise
What is the noise roughly equal to?
the square root of the actual number of photons
What is SNR equal to?
SNR = N/√N
SNR = √N
SNR will increase with the number of photons - in other words with dose.
Is it possible to reduce noise with amplification?
It is not possible to reduce noise by amplifying a signal. You would also amplify the noise so the percentage of noise remains the same
What is quantum mottle?
The variation in optical density, or image brightness, that is caused by the variation in the number of
x-ray photons detected, is called quantum mottle.
Does noise effect high or lose dose images more with low contrast?
Low dose
Noise reduces the ability to distinguish small levels of contrast.
Does changing the noise always improve image quality?
Reducing the noise in an image does not necessarily improve visibility, especially that of high contrast structures.
In order to reduce noise we must increase the number of photons used to form each element of the image. Listed here are a number of ways that this might be done.
A. Increasing the dose used to produce the image, so that more photons reach each part of the image
B. Make the image receptor from a material with a greater attenuation coefficient, so that more of the photons are captured
C. Make the image receptor thicker, so more of the photons are captured
D. Use an image receptor that gives a higher signal per absorbed x-ray photon
E. Use a narrower window when displaying the image, so that the contrast is more easily visualised
F. Use an image receptor that has larger detector elements
A. True. Signal to noise ratio depends on the square root of the number of photons forming each part of the image. Signal to noise ratio is increased at the expense of greater patient dose.
B. True. More photons are used, and this will increase the signal to noise ratio. There is no dose penalty.
C. True. More photons are used as above. However, in phosphor-based receptors the greater thickness will allow light to spread out more, increasing the unsharpness.
D. False. The noise will be amplified by the same amount as the signal and there will be no improvement in signal to noise ratio.
E. False. The narrower window will produce greater image contrast. However, changes in image brightness arising from noise will also increase and there will be no improvement in signal to noise ratio.
F. True. The number of photons per detector element will be higher, increasing the signal to noise ratio. However, the spatial resolution will decrease.
What are the main types of unsharpness?
Image receptor unsharpness
Geometric unsharpness
Movement unsharpness
Edge unsharpness
How can unsharpness be introduced by the intensifying screens?
When an x-ray is absorbed by an intensifying screen, the light produced can spread out before it reaches the film.
How does intensifying screen depth effect introduced unsharpness?
A thicker screen, which would make use of more of the x-rays, would allow more spreading of the light.
How might unsharpness be introduced in digital images without intensifying screens?
the detector element associated with a given pixel may lie on the border between a light and a dark area, so that the pixel will receive a signal that is an average of the two values.
What does geometric unsharpness introduce to edges in the image?
a penumbra - its size is a measure of the unsharpness.
What would happen to the size of the penumbra if the object were moved further away from the image receptor, increasing its magnification?
If the object is moved further away, the size of the penumbra is increased.
How large would the penumbra be if the object were placed in direct contact with the image receptor?
With the object in direct contact, there would be no penumbra.
what is motion blurring most often caused by?
movements such as breathing
Why does edge unsharpness occur?
An object might have a sharp edge, but have tapering thickness, so that the attenuation gradually decreases towards the edge. This will produce a gradual change in signal value in the image
What is resolution a measure of?
how far two objects must be apart before they can be seen as separate details in the image
How can resolution be measured?
It can be measured in line pairs per millimetre (lp/mm) where a line pair can be regarded as a black line and a white line. These line pairs/mm measure spatial frequency.
It can also be measured in units of distance (e.g. mm), the distance indicating how far two objects must be separated in order to be seen as separate. If two objects can be seen as separate, they are said to be resolved.
Does unsharpness effect resolution?
Unsharpness affects the resolution of an imaging system.
Each part of an imaging system will have some unsharpness associated with it. For example a film may be capable of showing 20 lp/mm, but intensifying screen unsharpness may reduce the overall resolution to 10 lp/mm, and geometric unsharpness will reduce this further. Even after the sources of unsharpness in the system have been accounted for, movement unsharpness may reduce the resolution in the final image even more.
How can you measure unsharpness mathematically?
by looking at how spread out the image of a sharp object becomes as a line spread function.
How is the ability of a system to reproduce spatial frequencies measured?
modulation transfer function (MTF)
What happens if there is a spatial frequency higher than the system capability?
The consequence is that the system is only able to use part of its range of greyscale or optical density. Although the pattern is seen at the correct spatial frequency, the contrast is much reduced.
If the spatial frequency of the pattern were increased further, the contrast in the image would be reduced further, until it became a uniform grey.
At some stage, the contrast would be too small for the pattern to be discernible.
If there were only small subject contrast in the object, the pattern would become indiscernible much earlier.
Resolution is ultimately limited by contrast. At low contrast, resolution is poorer.
