Diagnostic Imaging for Veterinary Technicians: Chapter 7 - Digital Imaging

Computed Radiography (CR): A digital imaging method using phosphor plates.
Detective Quantum Efficiency (DQE): The efficiency of an imaging system defined as the percentage of the X-ray energy that strikes the detector.
Digital Radiography (DR): A form of radiography that generates images digitally without the need for traditional film.
Dose Creep: The tendency to increase the exposure factors to improve image quality, which can lead to higher patient radiation doses.
Grayscale Bit Depth: The number of shades of gray that can be represented in a digital image.
Image Noise: Random variations in the pixel values of a digital image that degrade image quality.
Photostimulable Storage Phosphor Imaging Plate (PSP IP): A device used in CR that captures and holds the latent image until processed.
Pixel: The smallest unit of a digital image.
Voxel: A three-dimensional pixel representing a volume of tissue in an image.

Digital radiography is progressively replacing traditional film-based radiography due to several advantages:
- Increased Safety:
- Minimal repeat exposures are necessary due to software corrections for exposure issues.
- Elimination of hazardous chemicals used in film processing enhances safety for both personnel and patients.

Two main types of digital systems:
- Computerized Radiography (CR): Uses phosphor plates in a cassette to record images.
- Digital Radiography (DR): Sometimes called direct digital imaging (DDI), has built-in detectors, eliminating the need for cassettes and separate imaging units.
Differences:
- Both systems utilize standard X-ray machines; however, they differ in how the latent image is produced and processed for display.

The quality of digital images is determined by:
- Resolution: Sharpness of the image, inversely related to pixel size.
- Image Noise: Unwanted random variations can decrease clarity and contrast.
- Pixel Size and Number: Higher pixel counts lead to sharper images.
- Field of View: The observable area captured in the image.
Matrix: Digital images are captured in a matrix of rows and columns, where pixels represent the smallest elements of the image and correspond to a three-dimensional area called a voxel.

Each pixel can display a variety of shades of gray, determined by the grayscale bit depth.
- Ranges typically from 8 to 32 bits.
- A 12-bit grayscale bit depth yields 4096 shades of gray (calculated as $2^{12}$ ).
- The resolution of an image is directly influenced by pixel size; larger matrices lead to more pixels, enhancing clarity.

CR systems share similarities with standard film-based systems but utilize cassettes containing special photostimulable phosphors:
- Working Mechanism:
- Electrons in the IP get excited by X-rays, creating a latent image.
- During processing, a laser retrieves the high-energy electrons, releasing light proportional to the radiation exposure, which is then converted into an electronic signal and digitized.
Components of Imaging Plates:
- Layers:
- Protective layer: Shields the plate.
- Phosphor layer: Contains stimulable phosphors.
- Conductor layer: Reduces electrostatic interference.
- Light-shielding layer: Protects against light damage.
Note: Imaging plates are sensitive to scatter radiation and may experience “ghosting” if not properly erased.

DR includes image receptors integrated into X-ray tables, eliminating the need for separate cassettes:
- Efficiency:
- DQE indicates system sensitivity, defined as the percentage of X-ray energy captured. Typical DQE values are around 80%, with 20% created by software to enhance the final image.

Direct Detector Systems:
- Utilize thin-film-transistor (TFT) technology and amorphous selenium which generates a digital image directly by releasing electrons in response to X-ray exposure.
Indirect Detector Systems:
- Include TFT and Charged-Coupled Devices (CCD): both systems use scintillators that generate light when struck by X-rays, which is then converted to electrical energy.

Software applications are essential in translating signals into viewable images:
- Includes operations such as:
- Signal conversion.
- Image optimization.
- Artifact reduction.
Histogram Generation:
- A histogram depicts signal intensity across pixels; systems analyze this data to standardize image brightness, optimizing based on predetermined factors for different body parts:
- Look-Up Table: An internal standard for adjusting image contrast.
- Windowing and Leveling:
  - Controls density range and brightness respectively.
  - The ideal display is on medical-grade LCD monitors, although certain consumer-grade models can suffice for routine work.

Encompasses all technology, procedures, and processes for managing digital images:
- DICOM: Digital Imaging and Communications in Medicine, a universal format facilitating integration into electronic medical records with identifying information.

Artifacts can arise from various issues affecting image quality:
- Incomplete erasure leads to ghost images; may require multiple erasure cycles.
- Light spots may occur from dust on the imaging plate; often removable.
- Fogging due to background and scatter radiation affects image clarity. Digital plates are increasingly susceptible to radiation compared to film.
- Linear white lines can be traced back to foreign materials affecting image reception. Regular cleaning of the imaging system is essential to prevent accumulation.

Digital systems are classified as either computerized radiography (CR) or digital radiography (DR), with significant enhancements in safety and quality due to reduced exposure incidents and the avoidance of hazardous chemicals.
Image quality is intrinsically tied to resolution, noise, pixel size, and field of view, with noise decreasing contrast and overall clarity.
CR systems utilize photostimulable phosphor imaging plates, while DR systems include direct and indirect detector options, differing mainly in their imaging mechanisms. PACS serves a vital role in managing digital image data using the DICOM standard.