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IMAGING WITH FILM The greatest challenge
of film radiography is to get the range of exposure to fit into the dynamic range of the film.
If the exposure falls outside the latitude, there will be little or no image contrast formed.
IMAGING WITH FILM There are two
conditions that contribute to receptor exposure outside the film latitude: Error in setting the
correct exposure factors. Some regions of the
body have wide dynamic of exposure that exceeds the latitude of the film.
Using a wider latitude film can correct this but results in lower image contrast.
THE ADVANTAGE OF A WIDE DYNAMIC RANGE
Here we can see the advantages of a digital receptor that has a wide dynamic range.
Even when there is a wide range of exposure coming from the body (wide histogram) and exposure at different levels (exposure errors) they still fit within the dynamic range.
DIGITAL IMAGE CONTRAST
In a digital image, contrast is represented by different pixel values.
A typical digital receptor will have a linear relationship between exposure and resulting pixel value.
DIGITAL IMAGE CONTRAST
We have seen that this relationship extends over a relatively wide range of exposures.
This is contrasted with the non-linear relationship seen with film.
OPTIMUM EXPOSURE IN DIGITAL RADIOGRAPHY
The wide dynamic range and linear response of digital receptors is like a double edged sword.
The advantage is a wide range of exposures and exposure errors will still produce an image with good contrast.
OPTIMUM EXPOSURE IN DIGITAL RADIOGRAPHY
So, what is the problem? There are two that we can see here.
Even though good contrast with low exposure is obtained. Due to the low exposure, we have high noise.
OPTIMUM EXPOSURE IN DIGITAL RADIOGRAPHY
With film the image would be light.
The other problem is that exceptionally good images are obtained with high exposure ( very low noise).
With film the image would be dark or over exposed
OPTIMUM EXPOSURE IN DIGITAL RADIOGRAPHY
In general, the challenge is to make sure the exposure factors are set for optimum image quality.
MONITORING EXPOSURE LEVELS
One of the challenges is to know when the digital image is properly exposed since it is unlike film where under and over exposure is obvious.
Each manufacturer of digital receptors will provide an approach to calculate the image exposure information.
MONITORING EXPOSURE LEVELS
Some will display the “S” number. This displayed value generally indicates the speed of the receptor that would match the actual exposure.
MONITORING EXPOSURE LEVELS
A low exposure would result in a high calculated S number (like S=1000) and a high exposure would produce a low number (like s=50).
This is opposite of screen-film speed.
MONITORING EXPOSURE LEVELS
The operator should determine the appropriate range of values for optimum exposure and monitor the values.
The method varies by manufacturer and clinical procedure.
DIGITAL EXPOSURE
Proper exposure Over exposure
DIGITAL RADIOGRAPHY IMAGE QUALITY
Like all x-ray images, there are 5 specific quality characteristics. Spatial detail Detail Contrast Noise Artifacts
We will now see how 3 of these, contrast, detail and noise are effected by the operation of digital radiography.
DIGITAL RADIOGRAPH CONTRAST
Contrast sensitivity of digital procedures and image contrast depend upon several factors.
Two are:X-ray beam
spectrumScattered radiation Similar to film.
DIGITAL RADIOGRAPH CONTRAST
What is different is the ability to adjust and optimize contrast after the image is recorded.
This is done through the digital processing of the image and then the adjustment of the window when the image is being viewed.
DIGITAL RADIOGRAPHIC DETAIL
Visibility of detail is reduced and limited by the blurring that occurs at different stages of the imaging process.
Some are common to both film and digital radiography.
COMMON SOURCES OF BLURRING
Common sources of blurring;The focal spot
(depends upon size and object location.)
Motion if presentThe receptor (generally
due to light spreading within the fluorescent or phosphor screen)
DIGITAL SOURCES OF BLURRING
Additional blurring is the result of dividing the image into pixels.
The size of a pixel (amount of blurring) is the ratio of the image size (image size relative to the anatomy) and the matrix size.
DIGITAL SOURCES OF BLURRING Pixel size is another
factor that must be considered because it limits the detail of the image.
1024 x 1280 on a 18cm x 24 cm image would be sharper than on a 35 cm x 43 cm image.
Generally, digital images lack the spatial detail of film images due to the digital processing.
NOISE IN DIGITAL RADIOGRAPHS
The most predominate source of noise in digital imaging is quantum noise associated with the random distribution of the x-ray photons received by the image receptor.
NOISE IN DIGITAL RADIOGRAPHS
The level of noise depends upon the amount of receptor exposure used to produce the image.
With digital imaging, it can be adjusted over a wide range due to the wide dynamic range.
NOISE IN DIGITAL RADIOGRAPHS
Noise is controlled by using the appropriate exposure factors.
DIGITAL IMAGE NOISE
Underexposed Proper exposure
