Digital Fluoroscopy and Computed Tomography
Gain Reduction and Digital Fluoroscopy
Gain Reduction: (Inlet Diameter)2 / (Output Diameter)2
Further Conversion Factor: Light Output Intensity / Current Input RX. A larger gain implies a smaller image with increased brightness.
Digital Fluoroscopy
Involves removal or subtraction techniques.
- Interrogation Time (TI): The time required for the X-ray tube to be switched on and reach the selected level of kV and mA.
- Extinction Time (TE): The time required for the X-ray tube to be switched off. Digital fluoroscopy uses higher mA than analog, resulting in increased radiation output.
Digital Image Processing
Subtraction techniques:
- Temporal Subtraction: Image subtraction based on time.
- Energy Subtraction: Uses two different X-ray beams (high and low energy) to differentiate tissues, such as iodine in a catheter. High-energy projections highlight iodine more effectively.
- Hybrid Subtraction: A refinement of temporal subtraction, combining both temporal and energy subtraction to highlight iodine, similar to energy subtraction.
Automatic Gain Control and Lightning Protection
A fixed kV is considered standard, with mA being variable. Essential conditions for low light are captured by the monitor (kV for penetration and mA for radiation).
Dose = mA * Exposure Time (seconds)
Collimation = Limiting the X-ray beam to a specific area. Higher kW and lower mA are generally preferred.
Bit depth significantly impacts image resolution and color depth. A 24-bit image uses three 8-bit groups to represent the brightness values for red, green, and blue.
Basic Image Capture System
Contains a lens and a detector. Digital systems can capture and process visual data.
RCD (Charge-Coupled Device) = A matrix composed of hundreds of thousands of microscopic light-sensitive cells.
Classical Tomography
Used in dental panoramic films. It involves an X-ray tube and a detector rotating around a table.
Attenuation (Key Concept in CT)
Forms a 2D image from a 360-degree rotation. X-rays are captured by detectors.
Voxel = The smallest element in a 3D image.
Pixel = A 2D representation of a voxel.
Hounsfield Units
CT uses Hounsfield units, ranging from -1000 (air) to +1000 (bone).
Field of View (FOV)
FOV: The area of exposure. The field of view changes depending on the diameter of vision or exploration, and the area being explored must fit within the CT slice. It is adjusted based on the patient’s anatomy. Slice thickness is determined by the area being visualized.
(e.g., a 512×512 matrix with a 500mm field of view)
Pixel Value Measurement
Calculated by dividing the field of view by the matrix size. A larger field of view, with the same number of pixels, results in a larger pixel size.
Spatial Resolution
Smaller objects will have a larger representation, and larger objects will have a smaller representation.
Depth
Slice thickness, which determines the depth of the voxel. In a 2D image, there are only rows and columns. A collimated beam produces a narrow X-ray, which then fans out. Scattered radiation necessitates higher power for better image quality, especially in larger patients.
Image Quality
Spatial Resolution (High Contrast): Assessed using the Modulation Transfer Function (MTF). Measured in line pairs per millimeter (lp/mm). A value of 0 or 1 is standard, referred to as the limiting resolution.