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Questions and Answers
What primarily causes attenuation in CT imaging?
What primarily causes attenuation in CT imaging?
Which factor has the dominant role in CT contrast?
Which factor has the dominant role in CT contrast?
At approximately what photon energy does Compton interactions account for around 91% in muscle?
At approximately what photon energy does Compton interactions account for around 91% in muscle?
How does the photoelectric effect correlate with energy in diagnostic imaging?
How does the photoelectric effect correlate with energy in diagnostic imaging?
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What is the effective atomic number of bone as indicated in the content?
What is the effective atomic number of bone as indicated in the content?
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What does Beer’s Law describe in the context of X-ray beams?
What does Beer’s Law describe in the context of X-ray beams?
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What is the significance of the attenuation coefficient μ?
What is the significance of the attenuation coefficient μ?
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Which factors necessitate corrections to the attenuation coefficient μ in real-world applications?
Which factors necessitate corrections to the attenuation coefficient μ in real-world applications?
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What phenomena can result from the use of polychromatic radiation in X-ray production?
What phenomena can result from the use of polychromatic radiation in X-ray production?
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Which of the following statements about characteristic X-rays is true?
Which of the following statements about characteristic X-rays is true?
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Study Notes
Computed Tomography (CT)
- CT is an imaging technique that produces two-dimensional cross-sectional images (slices) from three-dimensional body structures.
- The creation of these images is based on the mathematical principle developed by Radon in 1917.
- Images are created using X-ray beams that are attenuated by the patient.
- Attenuation is a reduction in the intensity of the X-ray beam due to interaction with the body's structures.
- The amount of attenuation varies depending on the density and atomic number of different tissues.
- CT uses linear attenuation coefficients (μ) to characterize attenuation. μ is the sum of individual linear attenuation coefficients for each interaction type.
- μ depends on the density, atomic number, and energy of the photons.
- CT contrast arises mainly from physical properties affecting Compton scatter. These include physical density and electron density (approximately constant for tissues other than lung).
- The photoelectric effect also affects contrast, primarily in tissues with high atomic numbers (like bone).
- CT images are created by calculating μ through a ray from the source to the detector.
- The raw data is corrected for sensor variations, inhomogeneities, and other factors.
- Logarithmic transformation is used to convert attenuation to μ.
Image Formation/Reconstruction
- CT image formation starts with obtaining raw data through many ray lines projected from the source to the detector.
- The process assumes parallel ray data.
- Modern CT uses fan-beam geometry that needs re-binning to parallel-beam format
- Reconstructing the data from projections to a CT image is done using several methods, including back projection and filtered back projection.
- Iterative methods are also used.
- Iterative methods refine the image by comparing the calculated data and measured data and updating the guess.
CT Image Characteristics
- Resulting images are axial slices.
- Each pixel in a CT image is a voxel, with depth corresponding to reconstructed slice thickness.
- Images are typically 512x512 pixel matrices.
- Matrix size affects spatial resolution and noise levels.
CT Numbers (Hounsfield Units)
- CT numbers represent the linear attenuation coefficient scaled to water.
- Values are given in Hounsfield Units (HU).
- Different tissues have unique CT numbers: water = 0, air = -1000, bone = ~1000.
Multiplanar Reconstruction (MPR)
- MPR generates sagittal and coronal images from axial data.
- Thinning the slices improves resolution.
3D Reconstruction
- Slice data can be rendered to form 3D surfaces.
- Multiple projections from a single point to create a 3D model.
- Based on calculations like maximum intensity projection (MIP).
Data Processing
- Data is the total attenuation along a ray line (average μ).
- The data needs to be converted to the initial distribution.
Reconstruction Algorithms
- Back Projection: Simplest method; each projection is 'smeared' across the image.
- Filtered Back Projection: Improves image quality by filtering out noise. Methods such as Fourier transforms are used.
- Iterative Methods: Refine the image using comparisons and updates.
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Description
Explore the fundamentals of Computed Tomography (CT), an advanced imaging technique that generates cross-sectional images of the body through X-ray beams. Learn about the principles of attenuation, linear attenuation coefficients, and the impact of tissue density and atomic number on the imaging process.
