Radiography Chapter 6 Flashcards

CCD Overview

• A CCD (Charge Coupled Device) is the oldest indirect conversion system for digital radiography, used to obtain digital images.
• X-ray photons interact with a scintillation material, generating signals captured by the CCD, which stores charges in a patterned array.

CCD Construction

• Comprised of photosensitive receptors and electronics embedded in a silicon chip substrate.
• Contains components like a polysilicon layer, silicon dioxide insulator, and silicon substrate for charge storage.

Detector Elements and Functionality

• A Del (Detector Element) features three electrodes, forming an electrical potential well that holds electrons, functioning like a bucket brigade to collect charges.
• The blooming effect occurs when Del's overflow; this is controlled using built-in overflow drains.

Scintillator Properties

• The design of the scintillator influences the absorption of photons, light production, and the resultant color or wavelength of emitted light.
• CSI (Cesium Iodide) detectors possess structured needles that focus light, minimizing spread and allowing for thicker scintillators without loss of spatial resolution.

Image Size Reduction Considerations

• Important components for reducing image size include scintillator properties, light collection efficiency, and noise levels.
• Gadolinium is dubbed unstructured due to turbid phosphors causing light spread, thereby reducing detector efficiency.

Efficiency Metrics

• CCD efficiency relies on maximizing light directed to the chip while minimizing noise generation.
• Quantum efficiency of a CCD measures electron production related to incident light from the scintillator, representing the effectiveness of signal generation.

Performance Influencers

• Geometric distortions, including light scattering and lens imperfections, can negatively affect optical performance.
• The quantum efficiency is directly linked to Detective Quantum Efficiency (DQE), which is a ratio of output signal to noise, making it critical for performance evaluation.

Noise Types in CCDs

• Common noise types include statistical noise (quantum mottle), dark or current noise, and amplification noise which affect image quality.
• Statistical noise arises from insufficient light photons emitted by the scintillator.

Applications of CCD Technology

• CCDs are employed in various medical imaging modalities such as digital fluoroscopy, breast biopsy, digital mammography, and general radiography.
• In digital fluoro, CCDs replace older television pickup tubes and offer improved dynamics and compactness without susceptibility to scanning errors.

Advantages and Disadvantages of CCD

• Benefits of CCD technology include simplicity, ease of repair, and cost-effectiveness compared to TFTs (Thin Film Transistors).
• Limitations include demagnification issues that can impair DQE.

CMOS Technology

• CMOS (Complementary Metal Oxide Semiconductor) was developed by NASA, utilizing scintillators with individual pixel amplifiers and built-in ACDs for enhanced efficiency and reduced space requirements.