Lecture Notes: Image Terms and Characteristics

Basic Terms

• Matrix: A grid structure of rows and columns forming an image.
  • Made of pixels (picture elements) with number values determining brightness.
• Pixel: Smallest element of an image.
  • Has a gray value range, stored in bytes.

Spatial Resolution

• Spatial Resolution: Related to pixel size; smaller pixels = higher resolution.
  • Larger matrix with smaller pixels offers better resolution.

Field of View

• Field of View: Size of area being imaged.
  • Collimation: Adjusting field of view to enhance image quality.

Pixels and Voxels

• Voxel: 3D equivalent of a pixel; computer processes images in binary (0 or 1).
  • Bits and Bytes: 8 bits = 1 byte, with 256 possible gray values.

Image Properties

• Pixel Bit Depth: Number of bits per pixel, affecting tonal range.
  • Greater bit depth = more tonal variation.
• Pixel Pitch: Distance between centers of adjacent pixels; smaller pitch = better resolution.
• Pixel Density: Number of pixels per unit area.

Sampling and Frequency

• Sampling Frequency: Number of pixels sampled per mm, related to image scanning.
• Nyquist Frequency: Relationship between sampling frequency and spatial resolution.

Analog to Digital Conversion

• ADC (Analog to Digital Converter): Converts x-ray signals to digital images.

Dynamic Range and Exposure Latitude

• Dynamic Range: Range of x-ray intensities the detector can differentiate.
  • Wider dynamic range allows for technique errors while maintaining image quality.

Image Noise

• Image Noise: Often from scatter radiation; primarily from the patient.
  • Controlled through technique and collimation.
  • Quantum Noise: Due to insufficient exposure (mass).

Signal-to-Noise Ratio (SNR)

• SNR/CNR: Ratio of signal reaching the receptor versus noise level.

Histogram

• Histogram: Graphical display of pixel intensities, crucial for selecting correct settings.

Spatial vs Contrast Resolution

• Spatial Resolution: Detail and clarity, ability to distinguish borders.
• Contrast Resolution: Density differences in neighboring regions.
  • Controlled by lookup tables in digital imaging.

Modulator Transfer Function (MTF)

• MTF: Ability of a system to represent spatial frequencies.
  • Scaled from 0 to 1; closer to 1 indicates better resolution.

Look-Up Tables and Window Levels

• Lookup Table: Expected data values, control image appearance.
• Window Level: Controls image brightness.
• Window Width: Controls image contrast and grayscale.
  • Wider width = more gray levels (low contrast), narrower width = high contrast.

Key Comparisons

• Penguins (high contrast, black and white) vs. Elephants (low contrast, more gray).
  • Visual metaphor for understanding contrast differences.