The PSP cassette looks like the conventional screen-film cassette. It consists of a durable, lightweight plastic material (Figure 4-1). The cassette is backed by a thin sheet of aluminum or lead that absorbs backscatter x-ray photons (Figure 4-2). In addition to holding the PSP plate, the cassette contains an antistatic material (usually felt) that protects against static electricity buildup, dust collection, and mechanical damage to the plate (Figure 4-3).

With PSP systems, no chemical processor or darkroom is necessary. Instead, following exposure, the cassette is fed into a reader (Figure 4-7) that removes the imaging plate and scans it with a laser to release the stored electrons. When referring to the PSP interaction within the reader, a technologist often will note two scan directions. Fast scan direction is the movement of the laser across the imaging plate (also known as “scan”) and slow scan direction is the movement of the imaging plate through the reader (also known as “translation” or “sub-scan direction”).

The Laser.

A laser, or light amplification of stimulated emission of radiation, is a device that creates and amplifies a narrow, intense beam of coherent light (Figure 4-8). The atoms or molecules of a crystal such as ruby or garnet or of a gas, liquid, or other substance are excited so that more of them are at high energy levels rather than low energy levels. Surfaces at both ends of the laser container reflect energy back and forth as atoms bombard each other, stimulating the lower energy atoms to emit secondary photons in the same frequency as the bombarding atoms. When the energy builds sufficiently, the atoms discharge simultaneously as a burst of coherent light; it is coherent because all of the photons are traveling in the same direction at the same frequency. The laser requires a constant power source to prevent output fluctuations. The laser beam passes through beam-shaping optics to an optical mirror that directs the laser beam to the surface of the imaging plate (Figure 4-9).

Using the Laser to Read the Imaging Plate.

During the reading process, the imaging plate is scanned with a helium laser beam or, in more recent systems, solid-state laser diodes. This beam, about 100 micrometers (µm) wide with a wavelength of 633 nanometers (nm) (or 670 to 690 nm for solid state), scans the plate with red light in a raster pattern and gives energy to the trapped electrons. The red laser light is emitted at approximately 2 electron volts eV, which is necessary to energize the trapped electrons. This extra energy allows the trapped electrons to escape the active layer where they emit visible blue light at an energy of 3 eV as they relax into lower energy levels (Figure 4-10). As the imaging plate moves through or remains stationary in the reader, the laser scans across the imaging plate multiple times. The plate movement through the scanner is known as translation because it moves in a parallel manner at a certain rate through the reader. This scan process produces lines of light intensity information that are detected by a photodetector. The photodetector amplifies the light and sends it to an ADC. The translation speed of the plate must be coordinated with the scan direction of the laser, or the spacing of the scan lines will be affected.

The action of moving the laser beam across the imaging plate is much like holding a flashlight at the same height and moving it back and forth across a wall. The more angled the beam is, the more elliptical the shape of the beam. The same thing happens with the reader laser beam as it scans. This means that if this change in the beam shape were ignored, the output of the screen would differ from the middle to the edges, resulting in differing spatial resolution and inconsistent output signals, depending on the position and angle of the laser beam. To correct this, the beam is “shaped” by special optics that keep the beam size, shape, and speed largely independent of the beam position. A beam deflector moves the laser beam rapidly back and forth across the imaging plate to stimulate the phosphors. Mirrors are used to ensure that the beam is positioned consistently. Because the type of phosphor material in the imaging plate has an effect on the amount of energy required, the laser and the imaging plate should be designed to work together. The light collection optics direct the released phosphor energy to an optical filter and then to the photodetector (Figure 4-11).

Although there will be variances among manufacturers, the typical throughput is 50 cassettes per hour. Some manufacturers claim that a rate of up to 150 cassettes per hour is possible, but based on average hospital department work flow, 50 cassettes per hour is a more realistic expectation.

Digitizing the Signal.

When we talk about digitizing a signal, such as the light signal from a photodetector, we are talking about assigning a numerical value to each light photon. As humans, we experience the world analogically. We see the world as infinitely smooth gradients of shape and color. Analog refers to a device or system that represents changing values as continuously variable physical quantities. A typical analog device is a watch: the hands move continuously around the face and are capable of indicating every possible time of day. In contrast, a digital clock is capable of representing only a finite number of times (e.g., every tenth of a second). The scanning process results in the conversion of the light emitted from the storage phosphor into an electrical signal. The electrical signal is sampled and digitized to represent a specific location within the image matrix and displays as a specific brightness. A matrix is the group of squares that make up the image information.

Each square is called a pixel or picture element. The typical number of pixels in a matrix ranges from about 512 × 512 to 1024 × 1024 for CT but can be as large as 2500 × 2500 for radiography. The more pixels there are, the greater the image resolution for a fixed field of view. The image is digitized both by position (spatial location) and by intensity (gray level). Each pixel contains bits of information, and the number of bits per pixel that define the shade of each pixel is known as bit depth. If a pixel has a bit depth of 8, for example, then the number of gray tones that pixel can produce is 2 to the power of the bit depth, or 2⁸, or 256 shades of gray. Therefore the number of photons detected within the pixel will determine the amount of gray level or bit depth within that pixel. Some PSP systems have bit depths of 10 or 12, resulting in more shades of gray. Each pixel can have a gray level between 1 (2⁰) and 4096 (2²⁰). The gray level will be a factor in determining the quality of the image.

Spatial Resolution.

The amount of detail present in any image is known as its spatial resolution. Just as the crystal size and thickness of the phosphor layer determine resolution in film/screen radiography, phosphor layer thickness and pixel size help determine resolution in PSP. The thinner the phosphor layer, the higher the resolution. In film/screen radiography, resolution at its best is limited to approximately 10 line pairs (lp) per millimeter (mm). In general projection radiography PSP imaging, resolution is approximately 2.55 to 5 lp/mm, resulting in less detail. Resolution detail is also affected by the laser beam spot size, translation speed, sampling frequency, and the laser beam sweep in point beam readers. However, because the bit depth, or the number of available shades of gray that can be displayed, is much higher, the difference in resolution is more difficult to discern. More tissue densities on the digital radiograph are seen, giving the appearance of more detail. For example, the fat pads on a lateral elbow are difficult to discern on a film image (Figure 4-12A). Fat pads are a very important sign for the radiologists to see in pediatric elbow fractures. In the digital image, the fat pads are easily seen (Figure 4-12B). This is because of the ability to display more shades of gray, thereby making it possible to visualize more tissues of varying densities; this does not, however, mean that the digital image contains additional detail.

Erasing the Image.

The process of reading the image returns most but not all of the electrons to a lower energy state, effectively removing the image from the plate. However, imaging plates are extremely sensitive to scatter radiation and should be erased to prevent a buildup of background signal. At least once a week the plates should be run under an erase cycle to remove background radiation and scatter. PSP readers have an erasure mode that allows the surface of the imaging plate to be scanned without recoding the generated signal. Systems automatically erase the plate by flooding it with light to remove any electrons still trapped after the initial plate reading (Figure 4-13). Cassettes should be erased before using if the last time of erasure is unknown.

Preprocessing, Processing, and Forwarding the Image.

Once the imaging plate has been read, the signal is sent to the computer where it is preprocessed. The data then go to a monitor where the technologist can review the image, manipulate it if necessary (postprocessing), and send it to the quality control (QC) station and ultimately to the picture archiving and communication system (PACS).

Depending on the type of system being used, the technologist will choose the body part imaged either prior to exposure of the image receptor or after exposure. If the examination room has a PSP housed in the detector in the table or a wall stand, the patient worklist will most likely be in the room’s workstation, which means the technologist may choose the appropriate body part automatically. Always check to make sure the appropriate part has been selected. When using a cassette-based system, the selection of the body part is usually done after exposure and it is imperative that cassettes are kept apart so that the technologist knows which cassette goes with which body part. For example, if a skull examination is to be performed, the technologist would choose “skull” from the workstation menu (Figure 4-14). Selecting the proper body part and position is important for the proper conversion to take place. Image recognition is accomplished through complex mathematical computer algorithms, and if the improper part and/or position is designated, the image may be processed incorrectly and fail to display properly. For example, if a knee examination is to be performed and the examination selected is for skull, the computer will interpret the exposure for the skull, resulting in improper image display (Figure 4-15). The resultant image might appear too dark or too light and it might appear grainy or like it was underexposed. It is not acceptable to select a body part or position other than that being performed simply because it provides a better image. If the proper examination/part selection results in a suboptimal image, then service personnel should be notified of the problem to correct it as soon as possible. Improper menu selections may lead to overexposure of the patient and/or repeated exposure.

When exposing a patient, the larger the volume of tissue being irradiated, the more scatter will be produced. Whereas the use of a grid absorbs the scatter that exits the patient and affects latent image formation, properly used collimation reduces the area of irradiation and the volume of tissue in which scatter can be created. Collimation is the reduction of the area of beam that reaches the patient through the use of two pairs of lead shutters encased in a housing attached to the x-ray tube. Collimation results in increased contrast resolution as a result of the reduction of scatter. Through postexposure image manipulation known as shuttering, a black background can be added around the original collimation edges, virtually eliminating the distracting white or clear areas (Figure 4-19). However, this technique is not a replacement for proper preexposure collimation. It is an image aesthetic only and does not change the amount or angles of scatter. There is no substitute for appropriate collimation because collimation reduces patient dose.

As the imaging plate ages, it becomes prone to cracks from the action of removing and replacing the imaging plate within the reader. Cracks in the imaging plate appear as areas of radiolucency on the image (Figure 4-20). The imaging plate must be replaced when cracks occur in clinically useful areas. Adhesive tape used to secure lead markers to a surface can leave residue behind (Figure 4-21). If static exists because of low humidity, hair can cling to the imaging plate, creating another type of image plate artifact (Figure 4-22).

Backscatter created by x-ray photons transmitted through the back of the cassette can cause dark line artifacts (Figure 4-23). Areas of the lead coating on the cassette that are worn or cracked allow scatter to image these weak areas. Proper collimation and regular cassette inspection help to eliminate this problem.

The intermittent appearance of extraneous line patterns can be caused by problems in the plate reader’s electronics (Figure 4-24). Reader electronics may have to be replaced to remedy this problem.

White lines that are parallel to the direction of plate travel are caused by dirt, dust, or scratches on the light guide. Service personnel will need to clean or replace the light guide.

A rare but possible artifact can occur when multiple imaging plates are loaded into a single cassette. In this instance, usually only one of the plates will be extracted, which leaves the other plate to be exposed multiple times. The result is similar to a conventional film/screen double-exposed cassette (Figure 4-25).

Insufficient erasure after an overexposure may result in residual image information being left in the imaging plate before the next exposure. This may also occur if the erasure lamp is in need of repair. The results will vary depending on how much residual image is left and where it is located.

The orientation of a stationary grid relative to the direction of the laser scan is critical to reduce the likelihood of the moiré artifact (Figure 4-26). The grid lines must be perpendicular to the laser scan direction. Moving grids generally do not demonstrate the moiré artifact because the grid lines are blurred out. Vendors have identified the specific grid frequency for stationary grids that will prevent the moiré artifact with their equipment.

Insufficient collimation can result in an improper calculation of the exposure indicator. This may result in a misrepresentation of the displayed image (Figure 4-27).

If the cassette is exposed with the back of a cassette toward the source, the result will be artifacts from any hinges or other hardware present on the back of the cassettes. Care should be taken to expose only the tube side of the cassette (Figure 4-28).

Underexposure produces quantum mottle, and overexposure reduces contrast. The proper selection of technical factors is critical for both patient dose, image quality, and to ensure the appropriate production of light from the imaging plate (Figure 4-29).