Radiographic Image Quality Quiz

Radiographic Image Quality

• Radiographic image quality is defined as a radiograph that accurately reproduces tissue structures and is considered high-quality.
• Key characteristics of radiographic image quality include spatial resolution, contrast resolution, noise, and artifacts.

Spatial Resolution

• Spatial resolution refers to the ability to image small structures with high subject contrast, such as a bone-soft tissue interface.

Contrast Resolution

• Contrast resolution is the ability to differentiate anatomical structures with similar subject contrast, such as liver-spleen or gray matter-white matter.

Radiographic Noise

• Radiographic noise is the random variation in optical density (OD) of an image.
• Screen-film radiographic noise has four components: film graininess, structure mottle, quantum mottle, and scatter radiation.
  • Film graininess is the distribution of silver halide grain size and spacing in the emulsion.
  • Structure mottle refers to the phosphor in the intensifying screen.
  • Quantum mottle results from the random nature of x-ray interactions with the image receptor.
  • Scatter radiation is scattered x-rays.

Film Factors

• Sensitometry describes the relationship between the intensity of exposure to the film and its blackness after processing.
• The characteristic curve demonstrates the connection between optical density and radiation exposure. It has three key parts: toe, straight-line portion, and shoulder.

Geometric Factors

• Magnification is the enlargement of an object in an image. Minimizing magnification is important in most medical images.
• Magnification is minimized by using a large source-to-image distance (SID) and a small object-to-image distance (OID).
• Distortion occurs because of object thickness, position, shape and focal-spot blur.

Subject Factors

• Subject contrast depends on patient thickness, tissue mass density, and effective atomic number.

  • A thicker body part attenuates more x-rays than a thin part.
  • Tissues can have equal thickness but different densities.
  • High subject contrast is seen when effective atomic numbers of adjacent tissues differ significantly. Variations affect the amount of x-rays absorbed in each tissue.

• Object shape influences subject contrast

  • A structure with a shape that closely aligns with the exposure beam creates the highest possible subject contrast.

• Kilovolt peak (kVp) is crucial in influencing subject contrast.

  • Low kVp creates high subject contrast (short grayscale), while high kVp leads to low subject contrast (long grayscale).

Motion Blur

• Motion blur occurs when the patient or x-ray tube moves during exposure. Minimizing motion blur involves using short exposure times, immobilizing patients, and utilizing larger SID and smaller OID values.
• Voluntary movement of limbs can be minimized through immobilization techniques.
• Involuntary movement of the heart and lungs is best controlled using short exposure times.

Tools for Improved Radiographic Image Quality

• Patient Positioning: The body part under examination should be close to the image receptor, with its axis parallel to the receptor plane. The central x-ray beam should hit the center of the body part to optimize image quality. Effective immobilization of the patient is critical.
• Image Receptors: Standard screen-film image receptors are commonly used in radiology.
• Technique Selection: Optimum technique factors (kVp, mAs, exposure time) are chosen for the examination

Scatter Radiation

• Image-forming x-rays include those that pass through the patient without interacting and those that are Compton scattered within the patient.
• As scatter radiation increases, image contrast decreases, and the image becomes dull
• Factors affecting scatter radiation intensity are kVp, field size, and patient thickness.
  • Higher kVp leads to more x-rays and higher scatter radiation intensity.
  • A larger field size results in a greater amount of scatter radiation.
  • Thicker body parts produce more scatter radiation due to more interactions within the tissue.

Control of Scatter Radiation

• Beam-restricting devices include aperture diaphragms, cones or cylinders, and variable-aperture collimators.
  • Aperture diaphragms are simple lead-lined metal plates attached to the x-ray tube head.
• Cones and cylinders are modifications of aperture diaphragms, expanding the beam to a required size.
• Variable-aperture collimators use collimator blades with motorized leaves to restrict the X-ray beam.
• Positive beam limiting (PBL) is mandated by the FDA for consistent beam restriction to the size of the image receptor.

Radiographic Grids

• Radiographic grids are structures utilized to reduce scatter radiation and improve the quality of radiographic images.
  • They consist of radiopaque and radiolucent alternating strips.
• Grids have three main dimensions: the grid strip thickness, interspace material width, and height.
  • Grid ratio (h/D) is the most critical dimension influencing scatter reduction.
• Grid frequency is the number of grid strips per centimeter.
• Grids improve contrast by absorbing scatter radiation.
  • High-ratio grids are superior at lowering scatter compared to low-ratio grids

Radiographic Grids Performance

• Contrast improvement factor (k) gauges the contrast improvement achieved by using a grid.
• Bucky factor (B) measures the reduction in patient dose with the use of a grid.

Radiographic Grid Types

• Parallel Grids: Lead strips are parallel to each other.
• Crossed Grids: Lead strips run parallel to both the long and short axes.
• Focused Grids: Lead strips are curved to better redirect the scattered radiation towards the image receptor.
• Moving Grids: Improve image quality by minimizing image blur introduced by movement.
  • Reciprocal motion, Oscillating motions

Radiographic Grids Problems

• Off-level Grid: The central ray is not perpendicular to the grid.
• Off-center Grid: The central ray does not align with the grid center.
• Off-focus Grid: The x-ray beam is not properly aligned with the focal axis of the grid.
• Upside-down Grid: The grid is positioned in the wrong direction, thus reversing the image.

Radiographic Grids Selection

• Selecting an appropriate grid for an imaging procedure relies on high-quality imaging and reducing the amount of patient radiation dose.

Computed Radiography (CR)

• CR is a digital radiography technique that uses phosphor plates (PSP).
• Advantages of CR include its workflow and flexibility. It reduces manual handling when compared to screen-film radiography.
• The critical element in CR is the Storage Phosphor Imaging Plate (IP). PSPs capture and record the image data.
• CR uses a laser to release light energy and transform absorbed x-ray energy into a visible light signal.

Digital Radiography

• Digital radiography (DR) refers to techniques that convert X-rays into electronic signals for image display.
• Capture elements, coupling elements, and collection elements translate x-rays into displayable electron signals. Each element has different properties affecting the image quality. Examples of capturing elements are amorphous silicon (a-Se), cesium iodide (CsI), and barium fluorohalide for CR. Coupling elements transfer the signal from the capturing element. Collection elements display the signal in a final format to the user. Typical collection elements are a CCD or flat-panel detector.

CCD Technology

• CCDs (Charge-Coupled Devices) are silicon-based semiconductor elements.
• They have high sensitivity and dynamic range.

CsI/CCD Technology

• CsI transfers X-ray input to a light signal, which is further transferred to a CCD.

CsI/a-Si Technology

• CsI/a-Si is an indirect DR approach.
• CsI converts x-rays to light.
• Amorphous Silicon (a-Si) converts light to an electronic signal.
• Fill factor is the proportion of pixels that actually detect x-rays.

Amorphous Selenium (a-Se) Technology

• a-Se is directly used to detect x-rays.
• The x-rays create electron-hole pairs in the a-Se material.
• The a-Se acts as both a capture and coupling element.