Exhaustive Guide to Photostimulable Phosphor (PSP) Image Capture

Introduction to Photostimulable Phosphor Image Capture and Systems

Radiographic procedures refer to general X-ray procedures and are distinct from other digital modalities such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US). Digital imaging began with its first phase, commonly known as CR (Computed Radiography). In CR, the cassette is the primary tool used to house and protect the photostimulable phosphor (PSP) plate. This differs from the traditional film-screen method where the cassette contained an actual piece of film. Photostimulable phosphor (PSP) plate systems are categorized into two types: cassette-based and cassette-less designs. Cassette-based PSP systems differ from conventional radiography in that the cassette is simply a light-proof container that protects an imaging plate from light and handling. The imaging plate replaces the regular X-ray film and is capable of storing an image formed by incident X-ray photon excitation of phosphors. In cassette-less systems, the PSP technology does the same thing, but the PSP plate is built into the imaging system so that the plate is read automatically without manual transport.

Physical Composition and Design of the PSP Cassette and Plate

The PSP cassette looks like a conventional screen-film cassette and consists of a durable, lightweight plastic material. It is backed by a thin sheet of aluminum or lead that absorbs backscatter X-ray photons to prevent image degradation. It contains an antistatic material, usually felt, to protect against static electricity build-up, dust collection, and mechanical damage to the plate. The cassette will often include orientation markers, such as an arrow or a green marker, indicating the correct direction for insertion into the reader. It may also include size identifiers, such as $14 \times 17$, and centering lines to help the technologist align the anatomy with the center of the plate.

Inside the cassette is the imaging plate (IP), which 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 layer, also known as the active layer, contains the photostimulable phosphors which trap electrons during exposure to store the latent image. These phosphors are typically from the barium fluorohalide family ($BaF X:Eu^{2+}$). This layer may also contain a dye that differentially absorbs stimulating light to minimize light spread, functioning similarly to the dye added to 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. The conductive layer is designed to absorb and reduce static electricity that might interfere with the imaging process. Newer plates may contain a color layer between the active and support layers, which absorbs stimulating light but reflects emitted light. The support layer is a semi-rigid material that gives the imaging sheet its strength. Finally, the backing layer is a soft polymer that protects the back of the cassette.

The Physics of the Latent Image and Photostimulated Luminescence (PSL)

In PSP systems, the patient is X-rayed exactly the same way as in conventional radiography, using a combination of exposure factors including kilovoltage peak ($kVp$), milliamperage seconds ($mAs$), and source-to-image distance ($SID$). The difference is how the exposure is recorded. In PSP, the remnant beam interacts with electrons in the barium fluorohalide crystals. This interaction stimulates, or gives energy to, electrons in the crystals, trapping them in an area known as the color center or phosphor center (also called F-centers). Approximately $50\%$ of these electrons get trapped, representing the stored latent image. While the trapped signal begins to deteriorate almost immediately, it can remain for hours or even days. The trapped signal is never completely lost; a certain amount of exposure remains trapped so that the imaging plate can never be fully erased, though the residual electrons are so few that they do not interfere with subsequent exposures.

Photostimulated luminescence (PSL) refers to the emission of light from the phosphor layer after stimulation by a relevant light source. When the reader scans the plate with a red laser, the trapped electrons are released and move back into the valence band. As they relax into lower energy levels, they emit visible blue light with an energy of approximately $2\,eV$. The red laser light itself is about $2\,eV$, which is necessary to energize the electrons and allow them to escape the active layer. This emitted blue light is what is captured and converted into the digital image.

PSP Reader Mechanics and Scanning Processes

There are two primary types of PSP readers: point scan and line scan. Point scan readers use 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. With line scan readers, the acquisition of the PSL signal occurs with a charge-coupled device (CCD) linear array photodetector. A scanning module in a line scan system contains several linear laser units and optical light collection lenses, requiring a lens array to focus each laser beam to a corresponding point on the CCD array.

During the reading process, two scan directions are noted. The fast scan direction is the movement of the laser across the imaging plate, while the slow scan direction (also known as translation or sub-scan direction) is the movement of the imaging plate through the reader. The translation speed of the plate must be coordinated with the scan direction of the laser to avoid spacing issues. As the laser scans, it produces lines of light intensity information that are detected by the photodetector. The laser itself is a device that creates and amplifies a narrow, intense beam of coherent light (Light Amplification by Stimulated Emission of Radiation). In the laser, atoms of a crystal (such as ruby or garnet), liquid, or gas are excited to high energy levels. Atoms discharge simultaneously as a burst of coherent light because all photons travel in the same direction and frequency. PSP readers typically use a helium-neon laser beam or solid-state laser diodes with a beam about $100\,\mu m$ wide and a wavelength of $633\,nm$ (or $670\,nm$ to $690\,nm$ for solid-state).

Digitization and Image Structure

Digitizing the signal involves assigning a numerical value to each light photon. The scanning process results in the conversion of the light emitted from the storage phosphor into an electrical signal. This analog electrical signal is sampled and digitized by the ADC to represent a specific location within the image matrix. A matrix is a group of squares that make up the image information, and each square is called a pixel (picture element). The image is digitized by position (spatial location) and intensity (gray level). Each pixel contains bits of information; the number of bits per pixel defines the shade of each pixel, which is known as bit depth. 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 $2^8$ or $256$ shades of gray. Some systems have bit depths of $10$ or $12$, providing even more shades of gray. The more photons detected within the pixel, the higher the gray level. Ultimately, the ADC assigns numeric values to brightness levels, which the computer stores as binary code.

Spatial and Contrast Resolution

Spatial resolution is the amount of detail present in an image, defined as the ability to see or distinguish between two small objects. In PSP systems, this is determined by phosphor layer thickness and pixel size. The thinner the phosphor layer, the higher the resolution. While film/screen radiography resolution is limited to approximately $10\,lp/mm$, general PSP imaging resolution is approximately $2.5\,lp/mm$ to $5\,lp/mm$. Spatial resolution is also affected by laser beam spot size (smaller diameter equals higher resolution), translation speed (slower speed allows more exposure detection), and sampling frequency (higher frequency equals more exposure detected). Sampling frequency refers to how often the reader measures light; for a $10\,cm$ length, sampling every $1\,mm$ yields $100$ samples, while every $0.1\,mm$ yields $1,000$ samples. Higher sampling frequencies result in better resolution.

Contrast resolution is the ability to display many shades of gray. Because the bit depth in digital systems is much higher than in film, the difference in resolution is often difficult to discern. For example, fat pads on a lateral elbow are hard to see on film but easily seen on digital images because more tissue densities are visualized through varying shades of gray. Signal-to-Noise Ratio (SNR) is also critical. Utilizing an analogy, the signal is like a voice at a concert, and noise (quantum mottle or graininess) is the crowd noise. If the signal (voice) is much higher than the noise (crowd), the image quality is better.

Technical Factors and Patient Dose

Kilovoltage peak ($kVp$) selection, milliamperage seconds ($mAs$), and distance are chosen similarly to film/screen radiography. $kVp$ is chosen for penetration and varies from $45\,kVp$ to $120\,kVp$; values outside this range are inconsistent and not recommended. The K-edge of phosphor imaging plates ranges from $30\,keV$ to $50\,keV$, making an exposure range of $60\,kVp$ to $110\,kVp$ optimum. Digital image contrast is no longer solely dependent on $kVp$ because contrast is determined by computer processing. $mAs$ selection is based on the number of photons needed for the part. If there are too few photons, the result is quantum mottle or noise, making the image look grainy.

When using PSP systems, automatic exposure controls (AECs) should be recalibrated. Proper source-to-image distance ($SID$) follows the inverse square law, where intensity changes by a factor of $4$ if the distance is doubled or halved. Always follow the ALARA principle (As Low As Reasonably Achievable). Grids should be used when part thickness exceeds $4\,inches$ or when using $70\,kVp$ or higher. While grids improve contrast by absorbing scatter, they increase patient dose because higher technical factors are required. Grid frequency ($80\,lines/inch$ to $192\,lines/inch$) and grid ratio ($6:1$ for mobile, $12:1$ for departmental) are key selection factors. High-frequency grids require more photons and therefore a higher patient dose. If the grid frequency is too close to the laser scanning frequency, a Moire pattern (wavy artifact) may occur. Moving grids (buckets) eliminate this by blurring the grid lines.

Image Management and Maintenance

Proper patient identification is essential. Cassette-based systems use barcodes to match the image information with the patient identifying barcode on the examination request. For each new exam, both markers must be scanned. In cassette-less systems, the image is matched via an examination worklist on the computer. Selecting the correct body part and position from the workstation menu is imperative for proper mathematical algorithm application (histogram analysis). Improper selection can lead to images appearing too dark, too light, or grainy, and may lead to patient overexposure.

Imaging plates are extremely sensitive to scatter radiation and should be erased at least once a week to prevent the buildup of background signal. If the time of last erasure is unknown, the cassette should be erased before use. The reader automatically erases the plate by flooding it with bright white fluorescent light to remove any electrons still trapped after the initial reading. Because stored energy dissipates over time, plates should be read as quickly as possible to avoid losing information. For maintenance, technologists should review images at the QC station, perform post-processing (adjusting window levels, rotating, flipping, or annotating), and then send them to the Picture Archiving and Communication System (PACS).

Artifacts in PSP Systems

Artifacts are undesirable densities on the processed image. There are four main types. Imaging plate artifacts include cracks caused by the aging of the plate, which appear as areas of radiolucency (white). Static from low humidity can cause hair to cling to the plate. Backscatter from photons transmitted through the back of the cassette can cause dark line artifacts, especially if the lead coating is worn. Plate reader artifacts include horizontal white lines from dirt or dust on the light guide, or intermittent line patterns from reader electronics malfunctions. Printer artifacts consist of fine white lines from debris on the laser printer mirror.

Operator errors also cause significant artifacts. Insufficient collimation can result in improper exposure indicator calculation or misrepresentation of the image. Insufficient erasure after overexposure can cause "ghosting" or phantom images. If a cassette is exposed with the back toward the source, hardware from the back of the cassette will appear. Improper technical factors like underexposure produce quantum mottle, while overexposure reduces contrast. Grid orientation also matters; grid lines must be perpendicular to the laser scan direction to prevent the Moire artifact. Additionally, bulky clothing or graphics/metallic paint on T-shirts can create artifacts mistaken for pathology.

Questions & Discussion

Q: Where is the latent image stored? A: In the PSP (Photostimulable Phosphor) imaging plate as trapped energy.

Q: What does the laser release the stored energy as? A: Light (Blue PSL).

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

Q: What is the benefit of smaller pixels? A: Better spatial resolution and detail.

Q: What is the benefit of greater bit depth? A: Better contrast resolution (more shades of gray).

Q: Are lead markers required for legal reasons? A: Yes. Images without physical lead markers may not be admissible in court proceedings.