Digital Imaging Systems Thanks to the work of Dr. Perry Sprawls of Emory University and the Sprawls Educational Foundation, this material is available on-line.
The Digital Radiography System • Digital radiography is performed with a system of the following functional components: • A digital image receptor • A digital image processing unit • An image management system
The Digital Radiography System • A communications Interface to a patient information system • network • A display for viewer operated controls. • At this time we will briefly introduce the various components.
Image Receptor • The image receptor intercepts the x-ray beam after it passes through the patient’s body. • It produces an image in digital form, that is a matrix of pixels, each with a numerical value.
Image Receptor • It replaces the cassette with intensifying screens and film. • It can be the entire receptor that looks like a Bucky as in Direct Digital. • It can be a cassette the contains a Stimulable Phosphor as in Computed Radiography.
Image Management System • Image management is a function of a computer associated with the digital radiography process. • Functions consist of: • Movement of the images among the other components • Associating other data and information with the images.
Image Management System • Functions may be performed by the computer of the specific radiography device or by a more extensive Digital Image Management System (DIMS) that serves many imaging devices within the facility. • Note that sometimes the DIMS is called by it’s old name PACS (Picture Archiving and Communication System.
Patient Information System • The Patient Information System , sometime the RIS (Radiology Information System) is an adjunct to the basic digital system. • Through the system information such as the patient I.D., billing and scheduling is transferred.
Image Processor • One of the major advantages of digital imaging is the ability to process the image after they are recorded. • Various forms of digital processing can be used the change the image.
Image Processor • The ability to change or optimize the density and contrast of the image is of great value. • It is also possible to enhance visibility of detail in some radiographs with magnification or inversion.
Digital Image Storage • Digital radiographs and other information are stored as digital data. • Advantages (compared to film) include: • Rapid storage and retrieval • Less physical space required • Ability to copy and duplicate without loss of image quality.
Communications Network • Another advantage of digital images is the ability to transfer them from one location to another rapidly. • This can be: • Within the facility to storage and display devices • To other locations around the world via the internet.
Display Devices • Digital images displayed on a monitor are referred to as softcopy. • One major advantage is the ability of the viewer to adjust and optimize the image characteristics such as contrast and density.
Display Devices • Other advantages include: • Zoom • Compare multiple images • Perform analytical functions such as measure distances and angles accurately.
Direct Digital Image Receptor • With direct digital imaging, the phosphor is built into the Wall Bucky or Table Bucky. • Almost as soon as the image is produced, it can be viewed. • Great through-put but less flexible than CR.
Direct Digital Image Receptor • The direct digital receptor is basically a digital x-ray camera. • The pixel area is exposed by the x-rays exiting the patient. • The photons are absorbed and the energy produces an electrical signal.
Direct Digital Image Receptor • The electrical signal is a form of analog data that is converted into a digital number and stored as one pixel in the image.
Computed Radiography Receptor • CR uses a Stimualible Phosphor Receptor inside a cassette. • It can be used with existing radiographic systems. • The x-ray exposure produces an invisible latent image.
Computed Radiography Receptor • The difference between intensifying screens and computed radiography is that there is a delay between exposure and the production of the light. • Here is how it works:
Computed Radiography Receptor • First a receptor containing the phosphor plate is exposed to record an image. • At this point it is an invisible latent image. • The next step is the cassette is placed into reader and processor unit. • The plate is scanned with a very small laser beam.
Computed Radiography Receptor • The laser beam stimulates the plate causing light to be produced. • The light that is produced is proportional to the x-ray exposure to that specific spot. • The result is an image formed by the light produced on surface of the plate or screen.
Reading Phase • A light detector measures the light and sends the data on to produce a digitized image. • 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 to digital values for each pixel.
Reading Phase • The digital data is stored as a digital image. • The process takes about 50 seconds compared to two minutes or more with film.
Image Receptor • BaSrFBrl:Eu phosphors are used to produce the image. • Its luminescence spectrum is at about 390 nm in lattices of the BaFBr-type. • The top of the spectrum is shifted slightly to longer wavelengths due to the incorporation of iodide. • The stimulation spectrum is much broader then pure BaFBr. The shift is the result of partial replacement of Ba by SR and by the iodide. • The red shift of the stimulation spectrum assures maximum stimulability at 633 nm, the wavelength of the stimulating laser.
Agfa CR 35 Computed Radiography Unit • This is the digital radiographic processor used in the clinic. • We will go into detail on how to use it in 9th Quarter when you start taking radiographs on student patients.
Digital Receptor Dynamic Range • A wide dynamic range of exposure is a characteristic of many digital radiography systems. • This means that the receptor respond to x-ray exposure and produce digital data over a wide range of exposure.
Film Latitude or Dynamic Range • Most film systems have a very limited dynamic range of exposure. • Latitude is the range of exposure that forms an image. • Latitude is associated with the slope part of the H & D curve.
Film Latitude or Dynamic Range • The region of the toe of the curve has no significant contrast and it corresponds to the underexposed areas of the image. • The shoulder region also has no significant contrast and corresponds to over exposure.
Film Latitude or Dynamic Range • The limited latitude is due to the way the image is formed with the silver halide crystals. • Digital receptors do not have this limitation.
The Exposure Histogram • Before we go deeper into characteristics of digital receptors, lets develop the concept of the exposure histogram. • The x-ray image and contrast are formed as the beam passes through the body and experiences different levels of attenuation through the anatomical regions.
The Exposure Histogram • In the chest, the low-density lung areas produce a relatively high exposure to the receptor and dark areas of the image. • The more dense areas like the spine and below the diaphragm produce relatively low exposure and light areas of the image.
The Exposure Histogram • The histogram as we see it here, shows the amount of image area that receives the different levels of exposure that forms the image. • At this time our primary interest is in the range of exposures.
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.
ACR Digital Radiography Guidelines • The American College of Radiology also provides a chart to estimate what exposure change is needed to achieve optimum exposure. • With the LGM system to go from 1.80 to 2.20, the mAs is doubled or kVp increased 50% or increase kVp by 15%.
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.