1. THE DIGITAL
RADIOGRAPHY SYSTEM
Digital radiography is performed by a system consisting of
the following functional Components:
·
A digital image receptor
·
A digital image processing unit
·
An image management system
·
Image and data storage devices
·
Interface to a patient information system
·
A communications network
·
A display device with viewer operated controls
In this and other modules, each of these components
will be considered and detail. At this time we will briefly introduce the
various components.
2. The Digital
Receptor
The digital receptor is the device that intercepts the
x-ray beam after it has passed through the patients body and produces an image
in digital form, that is, a matrix of pixels, each with a numerical value.
This replaces the cassette
containing intensifying screens and film that is used in non-digital,
film-screen radiography.
As we will soon see, there are
several different types of digital radiography receptors.
3. The Image
Management System
Image
management is a function performed by the computer system associated with the
digital radiography process.
These functions consist of controlling the movement of the
images among the other components and associating other data and information
with the images.
Some of these functions might be
performed by the computer component of a specific digital radiography device or
by a more extensive Digital Image Management System (DIMS) that serves many
imaging devices within a facility. Note: it is not unusual for the DIMS
to be referred to by an older, and somewhat less appropriate name, PACS
(Picture Archiving and Communications System).
4. Patient
Information System
The Patient Information System,
perhaps known as the Radiology Information System (RIS), is an adjunct to the
basic digital radiography system. Through the interface, information such
as patient ID, scheduling, actual procedures performed, etc is transferred.
5. Imaging
Processing
One of the major advantages of digital radiography is the
ability to process the images after they are recorded.
Various forms of digital
processing can be used to change the characteristics of the digital images.
For digital radiographs the
ability to change and optimize the contrast is of great value.
It is also possible to use digital
processing to enhance visibility of detail in some radiographs.
The various processing methods
are explored in much more detail in another module.
6. Digital Image Storage
Digital radiographs, and other
digital medical images, are stored as digital data. Advantages (compared to
images recorded on film) include:
· Rapid
storage and retrieval
· Less
physical storage space required
· Ability
to copy and duplicate without loss of image quality.
The digital image storage methods and process is
explored in more detail in another module.
7. Communications
Network
Another advantage of digital
images is the ability to transfer them from one location to another very
rapidly.
This can be:
· Within
the imaging facility to the storage and display devices
· To other
locations (Teleradiology)
· Anywhere
in the world (by means of the internet)
The total network available for
transferring digital images is made up of a variety of integrated systems as
will be described in another module.
8. Digital Image Display and Display Control
Compared
to radiographs recorded and displayed on film, i.e. "softcopy", there
are advantages of "softcopy" displays.
One major advantage is the
ability of the viewer to adjust and optimize image characteristics such as
contrast.
Other advantages include the
ability to zoom, compare multiple images, and perform a variety of analytical
functions while viewing the images.
9. The Direct
Digital Radiographic Receptor
We can think of the direct digital radiographic receptor
as "a digital x-ray camera".
The receptor is in the form of a
matrix of many individual pixel elements. They are based on a combination
of several different technologies, but all have this common characteristic:
when the pixel area is exposed by the x-ray beam (after passing through the
patient's body), the x-ray photons are absorbed and the energy produces an
electrical signal. This signal is a form of analog data that is then converted
into a digital number and stored as one pixel in the image.
10. Stimulable
Phosphor Radiographic Receptor
We can think of the stimualible
phosphor receptor as being like a conventional radiographic intensifying screen
in that it absorbs the x-ray photons and and then produces light.
The difference is that there is a delay between the
x-ray exposure and the production of the light. This is how it works:
·
First, a receptor (cassette) containing only a stimualible
phosphor screen is exposed to record an image. At this stage the image
recorded by the screen is an invisible latent image.
·
The next step is to process the receptor through
the reader and processing unit. In this unit the screen is scanned by a
very small laser beam. When the laser beam strikes a spot on the screen
it causes light to be produced (the stimulation process). The light that is
produced is proportional to the x-ray exposure to that specific spot. The
result is that an image in the form of light is produced on the surface of the
stimualible phosphor screen.
·
A light detector measures the light and sends the
data on to produce a digitized image.
11.
Image Formation
As the surface of the
stimualible phosphor screen is scanned by the laser beam, the analog data
representing the brightness of the light at each point is converted into
digital values for each pixel and stored in the computer memory as a digital
image.
12.
Digital Receptor Dynamic Range
One of
the significant characteristics of most digital radiographic receptors is that
they have a wide dynamic range.
What that means is that the
receptors respond to x-ray exposure and produce digital data over a wide range
of x-ray exposure values as illustrated here.
There are distinct advantages of
this characteristic as we will see later.
We can appreciate this characteristic by comparing
it to that of film in the next step.
13. Film Latitude (Dynamic
Range)
Radiographic film has a somewhat limited dynamic range
which is generally referred to as the film latitude.
The latitude (or dynamic range)
is the range of receptor exposures over which an image and contrast will be formed.
The relationship between receptor exposure and the resulting film density is
usually described by the film characteristic (or H & D) curve as we see
here.
The latitude (or dynamic range)
is associated with that part of the curve where there is some slope and
contrast will be formed.
In the region of the toe of the curve, there is no
significant contrast formed, and this corresponds to under-exposed areas within
an image.
In the region of the shoulder of the curve there is
no significant contrast formed and this corresponds to areas of overexposure.
This somewhat limited latitude or dynamic range is
a characteristic of film because of the way images are formed with the silver
halide crystals.
Digital receptors do not have this limitation.
14. The Exposure Histogram
Before we proceed with exploring the characteristics of
digital receptors, let's develop the concept of the exposure histogram as we
see here.
X-ray images and image contrast
are formed as the x-ray beam passes through the body and experiences different
levels of attenuation through the various anatomical regions.
In the example of the chest, the
low-density lung areas produce a relatively high exposure to the receptor and
dark areas in the image. The more dense areas ,like the spine and below the
diaphragm, produce relatively low exposure to the receptor and light areas in
the image.
The histogram, as we see here, shows the amount of
image area (in a digital image this is the number of pixels) that receives the
different levels of exposure that forms the image.
At this time our primary interest is in the range
of exposures (width of the histogram) that reaches the receptor.
15. Imaging with
Film
One of the challenges in doing
film radiography is to get the range of exposures produce by the body (as
described by the exposure histogram) fitted into the latitude or dynamic range
of the film.
If the exposure falls outside of the latitude, there will
be little or no image contrast formed.
There are generally two
conditions that contribute to receptor exposure outside of a film's latitude:
·
One is just an error in setting the correct
exposure
·
The other is that some body regions, such as the
chest, produce a relatively wide range of exposure (histogram) that exceeds the
latitude of the film.
Using a film with a wide latitude, as is usually
done for chest imaging, can reduce this problem but the tradeoff is that a film
with a wide latitude generally produces less contrast than a so-called contrast
film.
16.
The Advantage of a Wide Dynamic Range
Here we
see one of the advantages of a digital receptor that has a wide dynamic range.
Even when there is a wide
exposure range coming from the body (wide histogram) and exposures at different
levels (because of exposure errors), we see that they still fit within the wide
dynamic range of the digital receptor.
This means that good image
contrast can be formed over a wide range of exposures.
17.
Digital Image Contrast
In a
digital image contrast is represented by the different pixel values.
A typical digital radiographic
receptor has a linear relationship between exposure and the resulting pixel
value as shown here. We have also seen that this relationship extends
over a relatively wide range of exposures to produce the wide dynamic range.
This can be contrasted with the non-linear (curved)
relationship between exposure and density, or image brightness, for film.
As we have just seen, film also has a very limited latitude or "working
range" of exposures.
18. Optimum Exposure in Digital Radiography
The wide dynamic range and linear response of the typical
digital receptor is like a "two-edged sword".
The advantage is that a wide range of exposures, and
exposure errors, will still produce good image contrast. That is, the
loss of contrast with exposure error is not a limiting factor as it is with
film.
So, what is the problem?
It is that while images can be produced throughout the range (as far as
contrast is concerned) there are two potential problems as we see here.
Even though images with good
contrast can be produced with relatively low exposures, they will have a high
level of quantum noise. We recall from other modules that the level of
image (quantum) noise depends on the exposure to the receptor. When a low
exposure is used, the result can be excessive image noise.
The other problem is that excessively high and
unnecessary exposures can be used to form images. While these images will
have good quality (low noise) there will be unnecessary exposure to the
patient. This problem does not exist with film radiography because the
increased exposure will result in a visibly overexposed film.
In general, for a radiographic procedure there is
an optimum exposure that produces a good balance between image noise and
patient exposure. The challenge to the technologist is to make sure that
the technique factors are set to produce this optimum exposure.
19. Monitoring Exposure Levels
As we have just observed, one of the challenges with
digital radiography is knowing when the receptor is correctly and optimally
exposed. It is not like film radiography where under and over exposures are
obvious.
Different digital radiography
systems might not do it the same way, but a typical approach is for the
equipment to calculate and display on the image the exposure information.
This is displayed as an
"S" number. The displayed value generally indicates the
calculated SPEED of the receptor that would match the actual exposure used.
A low exposure would result in a high calculated S
number (like S=1000) and a high exposure would produce low S numbers (like
S=50).
The staff should determine what is the appropriate
range of S values to be used and then monitor the values to insure the
exposures are optimum.
The optimum S numbers might be different for
different digital radiographic systems and also depend on the specific clinical
procedure.
20.
Digital Radiography Quality Characteristics
Like all medical images, digital
radiographs have the five specific quality characteristics as we see here .
We will now see how three of these, contrast,
detail, and noise are effected by the characteristics and operation of the
digital system.
21. Digital Radiograph Contrast Characteristics
The contrast sensitivity of a
digital radiographic procedure and the image contrast depend on several
factors.
Two of these, the x-ray beam spectrum and the effects of
scattered radiation are similar to film radiography.
What is different, and generally
an advantage, with digital radiography is the ability to adjust and optimize the
contrast after the image has been recorded.
This usually occurs through the
digital processing of the image and then the adjustment of the window when the
image is being viewed.
The details of image processing and windowing are
explored in another module.
22. Digital Radiographic Detail
As in all medical images, visibility of detail is reduced
and limited by the blurring that occurs at different stages of the imaging
process as we see here.
What
is common to both digital and film radiography are three sources of blurring:
·
The focal spot (depends on size and object location)
·
Motion (if it is present)
·
The receptor (generally because of light spreading
within the fluorescent or phosphor screen)
What is specific to digital
radiography is that additional blurring is introduced by dividing the image
into pixels. Each pixel is actually a blur. As we have already observed
in other modules, the size of a pixel (amount of blurring) is the ratio of the
field of view (image size relative to the anatomy) and the matrix size.
Pixel
size is a factor that must be considered because it limits detail in the
images.
There
is at least one form of digital image processing that can be used to increase
visibility of detail and it will be described in another module.
23. Noise in Digital Radiographs
The most predominant source of noise in digital
radiography is generally the quantum noise associated with the random
distribution of the x-ray photons received by the image receptor.
As we have just observed, the
level of noise depends on the amount of receptor exposure used to produce an
image. With digital radiography it can be adjusted over a rather wide
range because of the wide dynamic range of the typical digital receptor.
The noise is controlled by using
the appropriate exposure factors.
24.
Summary and Conclusion
The digital radiography system
consist of a variety of functional components interacting to provide all of the
advantages of digital radiography.
There are specific features of the digital
radiography process that affect the characteristics and quality of the
images. This must be considered and adjusted to obtain optimum image
quality.
Learning Objectives
1. Identify
and briefly describe the functions of a total digital radiography system.
2. Describe
the general advantages of digital radiography compared to film/screen
radiography.
3. Describe
the general function of the receptor within a total digital radiography system.
4. Describe
the general function of the image management component within a total digital
radiography system.
5. Describe
the general function of the image processing component within a total digital
radiography system.
6. Describe
the general function of the memory and storage component within a total digital
radiography system.
7. Describe
the general function of the display and display control component within a
total digital radiography system.
8. Describe
the general function of the communications network within a total digital
radiography system.
9. Describe
the general function of the patient information system with respect to a
digital radiography system.
10. Describe
and explain the general function of a direct digital radiography receptor.
11. Describe
and explain the imaging process using a stimualible phosphor digital
radiography receptor.
12. Compare
the advantages and disadvantages of direct and stimualible phosphor receptor
systems.
13. Illustrate
and describe the concept of dynamic range for a radiography receptor.
14. Sketch
and explain the factors that limit the dynamic range (latitude) of radiographic
film.
15. Sketch
and compare the dynamic range (latitude) of film and digital radiography
receptors.
16. Describe
some advantages and possible disadvantages of the wide dynamic range of digital
radiography receptors.
17. Identify
and briefly describe the factors in a digital radiography system that have an
effect on contrast sensitivity and image contrast.
18. Identify
and briefly describe the factors in a digital radiography system that have an
effect on blurring and visibility of detail.
19. Identify
and describe the principle factor that affects image noise in digital
radiography.
20. Describe
the process that a radiographer should follow to insure a proper balance
between patient exposure and image noise in digital radiography.
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