I. Basic Principles of Digital Imaging Flashcards
Digital Image Characteristics
Picture elements – pixels
The pixel is a two-dimensional representation of a corresponding tissue volume.
Numeric information contained in each pixel is a CT number or Hounsfield unit (HU).
Digital Image Characteristics
Spatial resolution
The ability to distinguish small structures within the image.
Factors affecting spatial resolution: Pixel size, matrix size, and characteristics of the imaging receptor.
Digital Image Characteristics
Pixel size
Determines the in-plane resolution.
Formula: Pixel size = Field of view / Matrix size
Digital Image Characteristics
Matrix size
The grid of rows and columns of pixels that forms the digital image.
Larger matrix sizes improve detail but increase data requirements.
Digital Receptors
Amorphous selenium/thin film transistor (TFT) arrays
Directly converts X-ray energy into an electrical signal without intermediate steps.
A-Se is a direct DR process where x-rays are converted directly into an electronic signal. The a-Se acts as both the capture and coupling element.
Digital Receptors
Cesium iodide/amorphous silicon TFT arrays
CsI/a-Si is an indirect DR process where x-rays are first converted into light, then into an electronic signal via a silicon pixel array.
Digital Receptors
Charged-coupled device (CCD) and complementary metal oxide semiconductor (CMOS) systems
CCD: Light-sensitive device converting X-rays into digital images.Solid-state device that converts visible light photons to electrons. CCDs are highly light-sensitive, have a wide dynamic range, and small physical size. They detect light from scintillators like CsI and convert it into an electrical signal.
CMOS: Similar to CCD but smaller and faster, widely used in imaging systems.
Digital Receptors
Photostimulable phosphor (PSP) plates
a. Cassette-based systems:
PSP plates are inserted into cassettes and require processing to produce an image. PSP plates store x-ray energy and require a laser scanner for image extraction.
Digital Receptors
Photostimulable phosphor (PSP) plates
b. Cassette-less systems:
Fixed PSP plates directly send data to processing systems without physical movement.
Detective quantum efficiency (DQE)
Measures a detector’s ability to convert X-ray input into a quality image with minimal noise.
Spatial resolution
Sampling frequency – pixel pitch
Sampling frequency – pixel pitch: Smaller pixel pitch improves resolution.
Spatial resolution
Receptor size
Receptor size: Larger receptors capture more area but may reduce resolution.
Spatial resolution
Light spread – phosphor layer thickness
Light spread – phosphor layer thickness: Thicker layers reduce sharpness by spreading light.
Spatial resolution
TFT detector element size (DEL)
TFT detector element size (DEL): Smaller elements increase image clarity.
Dynamic range of the detector
Range of exposure levels the detector can capture, from darkest to brightest.
Latitude – allowable error for optimal image acquisition
Film-screen systems: Narrow latitude.
Digital systems: Wide latitude, allowing for greater exposure tolerance.
