CR IRs can be portable or fixed in a table or upright x-ray unit. The CR IR includes a cassette that houses the imaging plate (IP) (Figure 6-1). The radiation exiting the patient interacts with the IP, where the photon intensities are absorbed by the phosphor. Although some of the absorbed energy is released as visible light (luminescence), a sufficient amount of energy is stored in the phosphor to produce a latent image. Luminescence is the emission of light when stimulated by radiation.

FIGURE 6-1 Typical CR cassette showing the imaging plate housed within.

The IP primarily consists of a support layer, phosphor layer, and protective layer (Figure 6-2). The phosphor layer is composed of barium fluorohalide crystals doped with europium, referred to as the photostimulable phosphor (PSP). This type of phosphor emits visible light when stimulated by a high-intensity laser beam, a phenomenon termed photostimulable luminescence.

FIGURE 6-2 Cross-section of CR imaging plate.

CR imaging requires a two-step process for image acquisition: image capture in the IP and image readout. The latent image is formed in the PSP when the exit x-ray intensities are absorbed by the phosphor and the europium atoms become ionized by the photoelectric effect. The absorbed energy excites the electrons, and they are elevated to a higher energy state where they become stored or trapped (Figure 6-3). The number and distribution of these trapped electrons are proportional to the tissue’s differential x-ray absorption and form the latent image. These electrons remain in this higher energy state until released during the laser beam scanning of the readout stage. The acquired image data (released energy) are extracted from the digital receptor, converted to digital data, and computer processed for image display. Exposed IPs should be processed within a relatively short amount of time (within 1 hour) because the latent image dissipates over time.

FIGURE 6-3 Trapped electrons following CR exposure.

Important Relationship

Computed Radiography Digital Image Receptors

The CR latent image is acquired in the PSP layer of the IP. Most energy from the exit radiation intensities is stored in the PSP for extraction in the reader unit.

The exposed IP is placed in or sent to a reader unit that converts the analog data into digital data for computer processing (Figure 6-4). Reader units are available in single-plate or multiplate configurations. The major components of a typical reader unit are a drive mechanism to move the IP through the scanning process; an optical system, which includes the laser, beam-shaping optics, collecting optics, and optical filters; a photodetector, such as a photomultiplier tube (PMT); and the analog-to-digital converter (ADC). Manufacturers differ in the CR reader mechanics. Some devices move the IP, and some move the optical components. There are three important stages in digitizing the latent image: scanning, sampling, and quantization.

FIGURE 6-4 The exposed CR imaging plate is placed in a reader unit to release the stored image, convert the analog image to a digital image, and send the data to a computer monitor, laser printer for a hard copy or both. The reader unit also erases the exposed imaging plate in preparation for the next exposure.

The purpose of scanning is to convert the latent image into an electrical signal (voltage) that can be subsequently digitized and displayed as a manifest digital image. Once in the reader unit, the IP is removed from the cassette and scanned with a helium-neon laser beam or a solid-state laser diode to release the stored energy as visible light (Figure 6-5). Absorption of the laser beam energy releases the trapped electrons, and they return to a lower energy state. During this process, the excess energy is emitted as visible light (photostimulable luminescence). The scanning of the plate results in a continuous pattern of light intensities being sent to the PMT, whose output is directed to the ADC for sampling and quantization.

FIGURE 6-5 A neon-helium laser beam scans the exposed CR imaging plate to release the stored energy as visible light. The photomultiplier tube collects, amplifies, and converts the light to an electrical signal. The analog-to-digital converter converts the analog data to digital data.

A PMT collects, amplifies, and converts the visible light to an electrical signal proportional to the range of energies stored in the IP. The signal output from the PMT is digitized by an ADC in order to produce the digital image. To digitize the analog signal from the PMT, it must first be sampled. An important performance characteristic of an ADC is the sampling frequency, which determines how often the analog signal is reproduced in its discrete digitized form. Increasing the sampling frequency of the analog signal increases the pixel density of the digital data and improves the spatial resolution of the digital image (Figure 6-6). The closer the samples are to each other (increased sampling frequency), the smaller the sampling pitch, or distance between the sampling points (Figure 6-7). Increased sampling frequency decreases the sampling pitch and results in smaller-sized pixels. The distance between the midpoint of one pixel to the midpoint of an adjacent pixel describes the pixel pitch. Spatial resolution is improved with an increased number of smaller pixels resulting in a more faithful digital representation of the acquired analog image.

FIGURE 6-6 CR sampling and its effect on the digital data. A represents the sampling points of the analog waveform. B shows the improper digital waveform that results from low sampling frequency.

Important Relationship

Sampling Frequency and Spatial Resolution

Increasing the sampling frequency results in a smaller sampling and pixel pitch, which improves the spatial resolution of the digital image. Decreasing the sampling frequency results in a larger sampling and pixel pitch and decreased spatial resolution.

Manufacturers of CR equipment vary in the method of sampling IPs of different sizes. Some manufacturers fix the sampling frequency to maintain a fixed spatial resolution, whereas others vary the sampling frequency to maintain a fixed matrix size. If the spatial resolution is fixed, the image matrix size is simply proportional to the IP size. A larger IP has a larger matrix to maintain spatial resolution (Figure 6-8). If the matrix size is fixed, changing the size of the IP would affect the spatial resolution of the digital image. For example, with a fixed matrix size system, changing from a 14 × 17 inch (35 × 43 cm) to a 10 × 12 inch (25 × 30 cm) IP size, for the same field of view (FOV), would result in improved spatial resolution (Figure 6-9). Spatial resolution is improved because in order to maintain the same matrix size and number of pixels, the pixels must be smaller in size. It is recommended to use the smallest IP size reasonable for the anatomic area of interest.

FIGURE 6-8 Fixed sampling frequency. A fixed sampling frequency will maintain a fixed spatial resolution. A larger IP size will have a larger matrix to maintain the same pixel size. Note: Pixel size not to scale and used for illustration only.

Important Relationship

Imaging Plate Size and Matrix Size

For a fixed matrix size CR system, using a smaller IP for a given field of view (FOV) results in improved spatial resolution of the digital image. Increasing the size of the IP for a given FOV results in decreased spatial resolution.

Another important ADC performance characteristic is degree of quantization or pixel bit depth, which controls the number of gray shades or contrast resolution of the image. During the process of quantization, each pixel, representing a brightness value, is assigned a numeric value. Quantization reflects the precision with which each sampled point is recorded. Chapter 3 discussed the characteristics of a digital image in terms of matrix size, pixel, and pixel bit depth. As previously discussed, the pixel size and pitch determine the spatial resolution, and the pixel bit depth determines the system’s ability to display a range of shades of gray to represent the anatomic tissues. Pixel bit depth is fixed by the choice of ADC, and CR systems manufactured with a greater pixel bit depth (i.e., 14-bit [2¹⁴ can display 16,384 shades of gray]) improve the contrast resolution of the digital image.

Before the IP is returned to service, the plate is exposed to an intense white light to release any residual energy that could affect future exposures. PSPs can be reused and are estimated to have a life of 10,000 readings before they need to be replaced. Advancements in the PSP material, laser beam technology, and dual-sided IP scanning will continue to improve the process of CR image acquisition. (Box 6-1 describes quality control methods for evaluating CR equipment.)

BOX 6-1 Quality Control Check: Computed Radiography

• Manufacturers of CR equipment have developed quality control activities specific to their equipment; however, there are basic universal procedures.

• The sensitivity of individual IPs should be checked routinely to ensure the consistency of optical densities. Expose selected IPs to varying amounts of x-ray intensities, print the images, and measure the optical density to verify it does not vary more than ±0.20.

• Linearity of the system can be evaluated by proportionally increasing and decreasing the radiation exposure to the IP and validating that the exposure indicator responds accordingly and within ±20%.

• Reproducibility of optical densities within an individual IP and uniformity among multiple IPs should be within ±0.25 optical density.

• Spatial resolution can be monitored by imaging a line-pair resolution test tool; the resolution should be within 10% of the resolution specified by the manufacturer.

• The laser beam performance can be evaluated by imaging an opaque straight-edge object and visually checking for any jitter along the edges of the object.

• Contrast resolution can be measured by imaging a phantom or step wedge device and measuring density differences. Optical density differences should remain stable over time.

• The thoroughness of the IP erasure function can be evaluated by performing a secondary erasure on the IP and checking for any residual optical density (ghosting).

Direct Digital Radiography

DR IRs have a self-scanning readout mechanism that employs an array of x-ray detectors that receive the exit radiation and convert the varying x-ray intensities into proportional electronic signals for digitization. In contrast to CR, which requires a two-step image acquisition process, DR imaging combines image capture and image readout. As a result, DR images are available almost instantly after exposure. However, DR receptors are more fragile and much more expensive than CR IRs. Several types of electronic detectors are available for DR.

Flat Panel Detectors

Flat panel detectors are solid-state IRs employing a large area active matrix array of electronic components ranging in size from 43 × 35 cm to 43 × 43 cm (17 × 14 inches to 17 × 17 inches). Flat panel detectors are constructed with layers in order to receive the x-ray photons and convert them to electrical charges for storage and readout (Figure 6-10). Signal storage, signal readout, and digitizing electronics are integrated into the flat panel device. The first layer is composed of the x-ray converter, the second layer houses the thin-film transistor (TFT) array, and the third layer is a glass substrate. The TFT array is divided into square detector elements (DEL), each having a capacitor to store electrical charges and a switching transistor for readout. Electrical charges are read out separately from each detector element. The electronic signal is then sent to the ADC for digitization.