CH 21 Computed Radiography (CR) Imaging

• Computed Radiography (CR) is a form of digital radiography involving a photostimulable storage phosphor imaging plate (PSP or IP) often used inside a cassette
• CR cassettes are similar to traditional film-screen ones, suitable for Bucky and mobile exams
• CR systems involve image acquisition, image processing, and image display

CR Imaging Plates

• CR imaging plates are rigid sheets with multiple layers designed to record and enhance image transmission from ionizing radiation
• The layers include a protective layer (insulates from handling trauma), a phosphor layer (holds the photostimulable phosphor), a support layer (base for other layers), a conductor layer (grounds the plate), and a light-shielding layer (prevents light erasure)
• Common phosphors include barium fluorohalide bromides and iodides with europium activators (BaFBr:Eu and BaFI:Eu), as well as BaSrFBr:Eu, RbBr:T1, RbCl, and CsBr:Eu

Latent Image Production

• Incident x-rays produce a latent image within the imaging plate's photostimulable fluorohalides
• Photoelectric interactions excite low-energy electrons creating holes called F-centers in the crystal phosphor
• Millions of F-centers accumulate in the crystal phosphor's atomic band gap forming the latent image
• Europium electron holes become the latent image, losing about 25% of its energy in 8 hours
• Processing cassettes shortly after exposure is important

Reading CR Data

• The imaging plate is scanned by a helium-neon laser beam inside an image reader device (IRD)
• The laser beam causes phosphors to emit the stored latent image as light photons
• Photosensitive receptors detect this light, converting it to an electrical signal, then to a digital value
• After reading, the plate is erased to remove any latent image vestiges
• A finely focused red laser beam (670–690 nm) frees trapped electrons, is known as photostimulated luminescence (PSL).
• Releasing blue-purple light photons (415 nm) are proportional to radiation absorbed
• Scanning occurs in a raster pattern
• Data collection starts at the upper left corner and proceeds line by line
• Some use a dual-sided PSP with a screen on both sides in order to get more information
• Plate throughput ranges from 30 to over 200 plates/hour depending on the processor type
• Pixel pitch depends on the sampling frequency, which is the line spacing correlated to a finely focused laser beam

Changing Analog Signals

• Analog-to-digital conversion (ADC) is needed to change analog signals into components for digital manipulation
• The analog signal from the PM tube is converted by the ADC into limited discrete values with sampling rates at the Nyquist frequency for better representation

Pixel Bit Depth

• Pixel bit depth quantifies voltage values, digitizing them determined by a computer chip
• Bit depth determines gray levels
• 10-bit (1,024), 12-bit (4,096), and 16-bit (65,536) ADCs determine respective gray levels
• Pixel size, matrix, and bit depth impact a computer file's size

Image Processing

• Image processing is a two-step operation: pre-processing and post-processing.
• Pre-processing prepares raw image data, and post-processing manipulates pre-processed information

Pre-processing

• Exposure field recognition recognizes the clinically useful area and removing signals from outside the collimated field
• The system must determine the orientation of the part and number of projections
• Exposure data recognition (EDR) or segmentation is used
• Data clipping is used to remove irrelevant data exposure
• Values of interest (VOI) determines the information

Histogram Analysis Error

• Histogram analysis error can occur with incorrect body part settings, leading to improper image display
• Most CR systems use algorithms to detect the edges of exposure
• Incorrect identification of these edges results in the inclusion of off-focus radiation and scatter in the histogram
• Excess exposure from poor collimation can cause a error in VOI location, resulting in rescaling errors leading to a poor image
• Significant anatomical density changes in atypical locations can also cause a error

Look-Up Table Adjustments

• Changes can be made in brightness or contrast
• Some systems restrict these which are used as calibrations
• Skilled users can enhance visualization of difficult-to-see pathologies

Scanning Detection Pattern

• CR programs the reader for scanning detection patterns, automatic, semi-automatic, and fixed for user control
• Automatic adjusts the latitude and sensitivities for an image and placing multiple images on a plate
• Semi-automatic sets the latitude and correct kVp technique only and is used for navicular/odontoid projections
• Semi allows the user to select and divide into nine areas
• Fixed has same latitude and sensitivity as a film with screen

Histogram Equalization

• In some environmental settings, histograms of the data from the image receptor are adjusted to enhance a particular element, chest CR stacking bone enhanced images, for example
• CR stores raw data which corrects menu anatomical and re-processes quality lost

Post-Processing

• Spatial location and frequency domain approaches are used in post-processing
• Both edge enhancement and image stitching are an example

Enhancement Techniques

• Amplifying image noise is important to see that the techniques are amplified
• High-pass filtering is useful and radiologists can clearly see the changes and patterns
• It is possible to stitch together multple images relying on the registration that will over lap

Image Display

• The last step is image display
• Digital images displayed are from soft copy or on a computer
• The characteristics and quality can effect digital imagery

CR Selection Factors

• Systems must be selected to what degree the exposure and image
• Exposure techniques and AEC are needed to minimize repeat shots
• Even though the KVP cannot effect and cannot measure contrast
• Radiographers should understand that MAs subotomial numbers create noise
• Some CR can prevent the use above 80 KVP that does not produce fog
• More than film screen can decrease that same exposure

Advantages of Digital

• Digital imaging offers a wide range and is responsible
• The response of CR is liner
• Radiation has to get a little bit over film and screens so IR has to go through the cassette

Grid Use

• Grid is unethical
• They violate the ALARA
• High frequency and all use that grid

Positioning Reminders

• It is proper technique
• Long term oversaturation