Questions
2–4 questions per paper
Difficulty
Medium
Importance
High yield for medical imaging and BMRIT board exams
Overview
Computed Tomography (CT) imaging is a diagnostic technique that uses rotating X-ray beams and computer processing to create cross-sectional, 3D images of the body. Mastering this topic is essential for medical and radiologic technology exams as it bridges the gap between basic X-ray physics and advanced diagnostic clinical interpretation.
Principles of CT Imaging
CT imaging relies on the attenuation of X-ray beams as they pass through tissues of varying densities. The system uses a rotating gantry to acquire data from multiple angles, which is subsequently reconstructed using complex algorithms like Filtered Back Projection.
- X-ray source and detectors rotate 360 degrees
- Data acquisition relies on linear attenuation coefficient
- Image reconstruction transforms raw projection data into digital images
- Detectors convert X-ray energy into electrical signals
- Spiral/Helical CT allows continuous patient movement during scanning
Hounsfield Units (HU)
Hounsfield Units represent the relative density of tissues on a CT image on a standardized scale. This scale is vital for identifying tissue types such as bone, fat, air, and soft tissue based on their distinct attenuation properties.
- Air = -1000 HU
- Water = 0 HU
- Dense Bone = +1000 HU
- Formula: HU = 1000 * ((µ_tissue - µ_water) / (µ_water - µ_air))
- Positive values represent dense structures
- Negative values represent air or gas
CT Protocols Basics
CT protocols define the specific scanning parameters required for optimal image quality while minimizing radiation dose. Key parameters include slice thickness, pitch, and windowing settings which are adjusted based on the clinical region of interest.
- Pitch ratio = Table feed per rotation / Slice thickness
- Window Width controls tissue contrast
- Window Level controls image brightness
- Thin slices improve spatial resolution
- ALARA principle governs radiation exposure minimization
Formula Sheet
HU = 1000 * ((µ_tissue - µ_water) / (µ_water - µ_air))
Pitch = Table movement per rotation / Beam width
Exam Tip
Always define Hounsfield Units using the standard reference values for air and water, as this is the most common point-scoring metric in descriptive answers.
Common Mistakes
- Confusing the Hounsfield scale by misremembering air as 0 instead of -1000
- Failing to explain the difference between spatial and contrast resolution in the context of slice thickness
- Assuming radiation dose is fixed rather than being dependent on pitch and tube current
More Revision Notes
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