Photostimulable Phosphor Image Capture

Chapter 4: Photostimulable Phosphor Image Capture

Objectives

• Basic Construction: Understand the construction of a photostimulable phosphor (PSP) cassette and imaging plate.

• Layer Purposes: Describe the purpose of each layer in the imaging plate.

• Photostimulation Process: Explain how photostimulation occurs in the imaging plate.

• Reading and Erasing Process: Understand how to read and erase the imaging plate.

• Conventional vs. PSP Systems: Compare conventional radiographic screen/film with PSP systems.

• Body Part Examination: Discuss the importance of matching body parts with examination menus.

• Technical Factors: Discuss selection of technical factors for density, contrast, and penetration.

• Imaging Plate and Grid Selection: Describe the imaging plate and grid selection process.

• Preprocessing Importance: Discuss the importance of preprocessing collimation and image marking.

• Exposure Indicators: Compare exposure indicators from major manufacturers and vendors.

Key Terms

• Artifacts: Any undesired changes or errors in the image.

• Backing Layer: Layer that provides support and helps absorb backscatter radiation.

• Barcode Label: Used for identifying and correlating the imaging plate with patient data.

• Bit Depth: The number of bits used for each pixel; determines the number of gray levels available.

• Cassette: A light-proof container for the imaging plate.

• Color Layer: A layer that absorbs stimulating light while reflecting emitted light in newer imaging plates.

• Collimation: The technique for limiting the size of the x-ray beam to reduce scatter radiation.

• Conductive Layer: Layer that reduces static electricity in the imaging plate.

• Fast Scan Direction: The direction of movement of the laser beam during scanning.

• Grid Frequency: The number of grid lines per unit distance; impacts image quality.

• Grid Ratio: The ratio of the height of the lead strips to the distance between them; affects how much scatter is absorbed.

• Imaging Plate: The layer that contains the photostimulable phosphor and captures the image.

• Kilovoltage Peak (kVp): The maximum voltage applied; determines the penetration power of x-rays.

• Laser: The light source used for reading the imaging plate.

• Milliamperage Seconds (mAs): A measure of x-ray exposure time and intensity.

• Moiré: An artifact that appears when grid lines interfere with image acquisition.

• Phosphor Layer: The active layer of the imaging plate where the image is created.

• Photodetector: Device that counts the light released from the phosphor layer.

• Photostimulable Phosphor: Phosphor material that stores the image and is stimulated by light to release it.

• Protective Layer: The outer layer that protects the phosphor.

• Reflective Layer: Layer that directs emitted light towards the photodetector.

• Quantum Mottle: Noise in the image due to insufficient exposure.

• Quantum Noise: Variability in signal output; often caused by low photon levels.

• Slow Scan Direction: Direction of the imaging plate movement through the reader.

• Support Layer: Provides structural integrity to the imaging plate.

Photostimulable Storage Phosphor Equipment

• Cassette-Based PSP Systems: Different from conventional radiography; the cassette serves as a light-proof container that protects the imaging plate from light and handling.
    - The imaging plate replaces traditional film and can store images formed by x-ray excitation of phosphors.

• Cassetteless Systems: Operate without a cassette, utilizing a similar process. During reading, the phosphor releases stored light which is converted into an electrical signal and digitized.

Cassette Specifications

• Resembles a film/screen cassette

• Made of durable, lightweight plastic

• Backed with aluminum or lead to absorb backscatter x-rays

• Includes antistatic materials to protect against static build-up, dust collection, and mechanical damage.

Imaging Plate Construction

1. Protective Layer: Thin, tough plastic that protects the phosphor layer.

2. Phosphor Layer (Active Layer): Contains photostimulable phosphors (often barium fluorohalide) that trap electrons during exposure.

3. Reflective Layer: Sends light emitted from excited phosphors in a forward direction; may be black to reduce stimulating light spread.

4. Conductive Layer: Reduces static electricity and minimizes image artifacts.

5. Color Layer: Absorbs stimulating light and reflects emitted light, improving image quality in newer plates.

6. Support Layer: Provides strength and stability to the imaging plate.

7. Backing Layer: Protects the rear of the cassette from damage.

Barcode Identification

• Cassette Identification: Each PSP system includes a barcode for linking patient data with imaging plates. Technologists must scan this barcode; in cassetteless systems, matching the image with a computer worklist is necessary.

Image Acquisition Process

• When radiographing, the patient is positioned using appropriate techniques with correct alignment to the image receptor.

• Exposure involves using proper kVp, mAs, and distance; the difference lies in how that exposure is recorded.

Interaction with Phosphor Crystals

• In PSP, the remnant x-ray beam interacts with trapped electrons in the barium fluorohalide crystals, creating a well-defined captured image.

• The energy trapped in the phosphors may last for hours or days, although some signal deterioration happens over time, meaning complete erasure may not occur.

Types of PSP Readers

1. Point Scan Reader: Uses a single laser point to read the image and includes components like scanning laser beams and photomultipliers.

2. Line Scan Reader: Simultaneously stimulates one line of the imaging plate with a charge-coupled device (CCD) photodetector.

Scanning Process

• Fast Scan Direction: Movement of the laser across the imaging plate (known as scan).

• Slow Scan Direction: Movement of the imaging plate through the reader (known as translation).

Laser Dynamics

• A laser amplifies a narrow beam of coherent light.

• Excitation of atoms within a substance (e.g., ruby) emits light through stimulated emission.

• Requires consistent power to maintain output stability; directed towards the imaging plate through optics.

Reading the Imaging Plate

• The scanning incorporates red laser light to energize trapped electrons, resulting in emitted blue light upon their release. The process is executed in a zigzag pattern to cover the imaging plate effectively.

• The system aims to keep the laser size consistent to prevent resolution discrepancies from edge effects.

Throughput Consideration

• Typical throughput is about 50 cassettes per hour, although some manufacturers claim rates as high as 150 cassettes per hour under ideal conditions.

Digitizing the Signal

• Analog vs. Digital: Analog devices represent continuous variables, while digital systems assign numeric values to light photons. The scanning results in the conversion of emitted light into electrical signals.

• Image Matrix: Composed of pixels (e.g., 512×512 to 2500×2500) affecting resolution; more pixels equate to greater detail.

• Bit Depth Significance: Determines how many shades of gray a pixel can produce, impacting overall image quality. For instance, a pixel with a bit depth of 8 can produce $2^8 = 256$ shades of gray.

Spatial Resolution

• Determination Factors: The thickness of the phosphor layer and pixel size directly impact spatial resolution, with thinner layers yielding higher resolution (film resolution vs. PSP resolution).

• Resolution Limits: PSP systems have resolutions from 2.55 to 5 line pairs per millimeter (lp/mm) in comparison to 10 lp/mm in analog systems. Factors such as laser beam size and speed during scanning can influence results.

Erasing the Image

• Reading Process: Removes majority trapped electrons, clearing the plate for new exposures. It is crucial to erase plates regularly to eliminate latent images and background radiation carryover.

• Erasure Mode: Involves scanning the plate with bright light to clear it effectively.

Image Processing

• After reading, the imaging plate sends data to a computer, followed by preprocessing before being reviewed and adjusted. Technologists must ensure that collimation and patient identifiers match to avoid processing errors.

Technical Factors Selection

• kVp: Chosen based on penetration requirements and patient body type, usually ranging from 45 to 120 kVp; higher kVp allows for lower mAs settings depending on the anatomy examined.

• mAs: Directly correlates with exposure needed for adequate phosphor stimulation to ensure clear imaging.

Grid and Imaging Plate Selection

• Grid Selection: More crucial in PSP; grids must match frequency and ratio to minimize artifacts like the Moiré pattern caused by grid lines parallel to laser scans.

• Imaging Plate: The choice between standard and high-resolution imaging plates impacts image sharpness and is critical for techniques like mammography.

Collimation Importance

• Proper collimation optimizes exposure, minimizes scatter, and improves image contrast, promoting better patient safety.

• Post-Exposure Shuttering: Manipulation technique to enhance aesthetics but should not replace pre-exposure collimation.

Artifacts and Error Management

• Common Artifacts: Include those from imaging plate wear, static electricity, adhesive residue, and backscatter issues.

• Operator Errors: Proper operation is critical; many errors arise from insufficient collimation or exposure misalignment, which can produce artifacts or degraded images.

Summary

• PSP Cassette System: Features specially designed cassettes made of robust plastic, layered imaging plates, with barcodes for patient data correlations.

• Image Formation: Involves barium fluorohalide crystals from which light energy is released and digitized, eventually resulting in high-resolution images that are sent for quality control and long-term storage (PACS).