Exhaustive Guide to Photostimulable Phosphor (PSP) Image Capture and Computed Radiography

Introduction to Photostimulable Phosphor Image Capture

Radiographic procedures refer to general X-ray examinations, distinguishing them from other digital modalities such as Computed Tomography (CT), Magnetic Resonance Imaging (MRI), and Ultrasound (US). Photostimulable Phosphor (PSP) plate imaging exists in two primary designs: cassette-based and cassette-less. In cassette-based PSP systems, the cassette is a light-proof container made of durable, lightweight plastic that protects the imaging plate from light and physical handling. The imaging plate replaces traditional radiographic film and is capable of storing an image formed by incident X-ray photon excitation of phosphors.

A simple way to conceptualize this process is that the imaging plate replaces regular X-ray film; when X-rays strike the plate, it "holds onto" or stores the X-ray energy. This stored energy creates a hidden, or latent, image. A laser later reads the plate, turning the stored energy into a digital image on a computer. Once the image is processed, the plate is erased so it can be reused. In one sentence: an imaging plate replaces film and stores X-ray energy until a laser reads it and creates a digital image.

Cassette-less systems using PSP technology operate on the same principles but without a separate cassette. In these systems, the PSP plate is built into the imaging system. During the reading process, the phosphor automatically releases the stored light and converts it into an electrical signal, which is then digitized.

Physical Composition of the PSP Cassette and Imaging Plate

The PSP cassette resembles a conventional screen-film cassette and consists of a durable, lightweight plastic material. It is backed by a thin sheet of aluminum or lead to absorb backscatter X-ray photons. Inside, an antistatic material, usually felt, protects the plate against static electricity buildup, dust collection, and mechanical damage. For image identification, cassette-based systems contain a window with a barcode label or sticker. This allows the technologist to match the image information with patient identifying data from the examination request. For every new examination, the patient identifying barcode and the cassette barcode must be scanned or linked via the computer position or examination menu. In cassette-less systems, the image must be matched with the examination worklist on the computer.

The imaging plate itself is a thin sheet of plastic consisting of several distinct layers. The protective layer is a very thin, tough, clear plastic that protects the phosphor layer. The phosphor, or active layer, is the component that "traps" electrons during exposure to store the latent image; it is usually made of phosphors from the barium fluorohalide family. This layer may also contain a dye that differentially absorbs stimulating light to minimize light spread, similar to dyes in conventional radiographic screens. The reflective layer sends light in a forward direction when released in the cassette reader, though it may be black to reduce the spread of stimulating light and the escape of emitted light, which can cause some detail loss.

Additional layers include the conductive layer, which absorbs and reduces static electricity. Newer plates may feature a color layer located between the active and support layers to absorb stimulating light while reflecting emitted light. The support layer is a semirigid material that provides the imaging sheet with structural strength. Finally, the backing layer is a soft polymer that protects the back of the cassette.

The Physics of Latent Image Formation and Acquisition

In PSP systems, the patient is positioned and X-rayed exactly as in conventional radiography, using a specific combination of exposure factors: Kilovoltage peak ($kVp$), Milliamperage seconds ($mAs$), and Source-to-Image Distance ($SID$). The difference lies in how the exposure is recorded. The remnant beam interacts with electrons in the barium fluorohalide crystals ($BaF X:Eu^{2+}$) within the imaging plate. This interaction gives energy to the electrons in the crystals, trapping them in an area known as the color or phosphor center.

This trapped signal represents the latent image and can remain for hours or even days, though deterioration begins almost immediately. Notably, the trapped signal is never completely lost; a certain amount of exposure remains so that the plate can never be fully erased, though these residual electrons are too few to interfere with subsequent exposures. The term Photostimulated Luminescence (PSL) refers to the emission of light from the phosphor layer after it is stimulated by a relevant light source.

There are two types of PSP readers: point scan and line scan. Point scan readers utilize an optical stage, a scanning laser beam, translation mechanics, a light pickup guide, a photomultiplier, a signal transformer/amplifier, and an Analog-to-Digital Converter (ADC). At any point in time, only a single laser point radiates the imaging plate, reading it one tiny spot at a time. Line scan readers are based on the simultaneous stimulation of the imaging plate one line at a time. This system requires a scanning module with several linear laser units and optical light collection lenses. In line scan systems, the acquisition of the PSL signal occurs with a Charged-Coupled Device (CCD) linear array photo detector.

Reader Mechanics and Digitization

The laser, which stands for Light Amplification by Stimulated Emission of Radiation, creates and amplifies a narrow, intense beam of coherent light. In the reader, the laser scans the imaging plate to release stored electrons. There are two scan directions: the fast scan direction, which is the movement of the laser across the imaging plate, and the slow scan (or translation/sub-scan) direction, which is the movement of the plate through the reader. The translation speed must be precisely coordinated with the scan direction to maintain consistent spacing.

During the reading process, the plate is scanned with a helium-neon laser beam or a solid-state laser diode ($633\,nm$ for helium-neon or $670\,nm$ to $690\,nm$ for solid-state). The beam is approximately $100\,\mu m$ wide. As the laser scans the plate in a raster pattern, it provides energy (approximately $2\,eV$) to the trapped electrons, allowing them to escape the active layer. As they relax into lower energy levels, they emit visible blue light with an energy of $3\,eV$. A photodetector detects this light and converts it into an electrical signal.

To ensure consistent output, the laser beam is "shaped" by special optics to keep the size, shape, and speed independent of the beam position. If the beam angle causes an elliptical shape that is not corrected, the spatial resolution would differ at the edges compared to the middle. Mirrored deflectors move the beam rapidly back and forth while light collection optics direct the energy to an optical filter and then to the photodetector. Typical throughput is approximately $50$ cassettes per hour.

Digitization involves assigning a numerical value to each light photon. The electrical signal is sampled and digitized to represent specific locations within the image matrix. A matrix is a group of squares called pixels (picture elements). The number of pixels determines the image resolution; more pixels lead to greater resolution for a fixed field of view. Each pixel contains bits of information, and the number of bits per pixel defines the bit depth (shades of gray). The number of gray tones a pixel can produce is calculated as $2^{n}$, where $n$ is the bit depth. For example, a bit depth of $8$ produces $2^{8} = 256$ shades of gray. Some PSP systems utilize bit depths of $10$ or $12$, allowing for even more shades of gray.

Spatial Resolution and Image Erasure

Spatial resolution is the amount of detail present in an image. In PSP, this is determined by phosphor layer thickness and pixel size. Thinner phosphor layers and smaller pixel diameters result in higher resolution. While film/screen radiography can reach $10\,lp/mm$, general PSP imaging yields approximately $2.55$ to $5\,lp/mm$. Despite this lower numerical resolution, digital imaging often appears to have more detail because the higher bit depth allows for the visualization of more tissue densities and varying shades of gray (e.g., fat pads on a lateral elbow are easier to see on digital images).

After reading, the imaging plate must be erased to prevent the buildup of background signal. While the reading process returns most electrons to a lower energy state, some remain trapped. Readers have an erasure mode where the plate is flooded with very bright white light to remove any residual electrons. If the last time of erasure is unknown, or if the plate has not been used for some time, it should be erased before use. It is recommended that plates be run through an erase cycle at least once a week to remove background radiation and scatter.

Technical Factors and Exposure Selection

Technical factors such as $kVp$, $mAs$, and distance are chosen similarly to conventional radiography. $kVp$ is selected for penetration and tissue type, typically ranging from $45$ to $120\,kVp$. Values outside this range are not recommended as they may produce inconsistent excitation of phosphors. The K-edge of phosphor imaging plates ranges from $30$ to $50\,keV$, making $60$ to $110\,kVp$ the optimum range for exposure. A major difference in digital systems is that image contrast is no longer solely dependent on $kVp$; instead, it is determined by computer processing. Digital receptors have a much higher dynamic recording range, producing a wide variety of gray values.

$mAs$ selection is based on the number of photons needed for the body part. If too few photons reach the plate, the image will be grainy, a condition known as quantum mottle or quantum noise. PSP systems often use Automatic Exposure Controls (AECs), which must be recalibrated when switching from film/screen to digital to ensure the desired exposure indicators. Proper part selection from the workstation menu is critical. If a technician selects a "skull" algorithm for a "knee" exposure, the computer will interpret the data incorrectly, potentially leading to an image that is too dark, too light, or grainy. Selecting the wrong menu may also lead to patient overexposure.

Equipment and Grid Selection

There are two types of imaging plates: standard and high resolution. High-resolution plates have a thinner phosphor layer, which reduces lateral light spread and results in greater image sharpness. These are typically used for extremities and mammography where fine detail is necessary. If the matrix remains constant, reducing the field of view ($FOV$) decreases pixel size, thereby increasing spatial resolution.

Grids are used to absorb scatter radiation, which is particularly important as PSP plates are highly sensitive to scatter. Grid selection involves frequency, ratio, and focus. Grid frequency refers to the number of lead strips per centimeter or inch; typical frequencies are between $80$ and $152\,lines/inch$. High-frequency grids require more X-rays (higher patient dose) but produce finer grid lines. If the grid frequency is close to the laser scanning frequency, a wavy artifact known as a Moire pattern can occur. Moving grids (buckies) eliminate this by blurring the grid lines.

The grid ratio is the relationship between the height of the lead strips and the space between them. Higher ratios absorb more scatter but require more precise positioning. A $6:1$ ratio is appropriate for mobile radiography, while a $12:1$ ratio is standard for departmental grids. Focused grids use angled lead strips to match the divergence of the X-ray beam and must be used at specific distances and centered precisely.

Collimation, Shuttering, and Artifacts

Collimation is the physical reduction of the X-ray beam area using lead shutters, which reduces the volume of tissue irradiated and the resulting scatter, thereby increasing contrast resolution. After exposure, technologists may use a software tool called shuttering to add a black background around the collimated edges. Shuttering is for visual comfort and does not replace physical collimation nor does it reduce patient dose.

Artifacts are undesirable densities on an image. There are four categories: imaging plate artifacts, plate reader artifacts, image processing artifacts, and printer artifacts. Imaging plate artifacts include cracks or scratches from use, which appear as areas of radiolucency (white). Static electricity from low humidity can cause hair to cling to the plate. Backscatter from the back of the cassette can cause dark line artifacts if the lead coating is worn. Plate reader artifacts, such as white lines parallel to the direction of plate travel, are caused by dirt on the light guide. Printer artifacts are often fine white lines caused by debris on the laser printer mirror.

Operator errors also contribute to artifacts. Insufficient collimation can cause improper exposure indicator calculations. If a cassette is placed backward (back toward the source), hardware from the back of the cassette will be imaged. Insufficient erasure after overexposure can lead to residual "ghosting" of the previous image. Finally, improper orientation of a stationary grid (grid lines not perpendicular to the laser scan) will cause a Moire effect.

Questions & Discussion

Q: Where is the latent image stored?A: In the PSP imaging plate as trapped energy within the phosphor layer centers.

Q: What does the laser release the stored energy as?A: It is released as light, specifically Blue Photostimulated Luminescence (PSL).

Q: What device converts the light signal to a digital image?A: The Analog-to-Digital Converter (ADC).

Q: What is the benefit of smaller pixels?A: Smaller pixels provide better spatial resolution and increased image detail.

Q: What is the benefit of greater bit depth?A: Greater bit depth allows for better contrast resolution by providing more available shades of gray.

Q: Are lead markers required for legal reasons?A: Yes. Physical lead markers placed at the time of exposure identify the technologist and the patient's side, providing legal credibility and allowing for technologist testimony in court cases. Electronic markers should not replace them.