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Conventional radiographs depict three-dimensional objects as three-dimensional images.
Conventional radiographs depict three-dimensional objects as three-dimensional images.
False
CT scanning produces images by rotating a narrow X-ray beam around the body.
CT scanning produces images by rotating a narrow X-ray beam around the body.
True
CT has the ability to distinguish between two tissues with identical densities.
CT has the ability to distinguish between two tissues with identical densities.
False
The term tomography originates from a Greek word meaning to cut.
The term tomography originates from a Greek word meaning to cut.
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The acronym CAT scan stands for computerized axial tomography.
The acronym CAT scan stands for computerized axial tomography.
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A topogram, scout, and scanogram are terms used to describe the same preliminary image produced by a CT scanner.
A topogram, scout, and scanogram are terms used to describe the same preliminary image produced by a CT scanner.
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Spatial resolution in CT refers to the ability to define small objects distinctly.
Spatial resolution in CT refers to the ability to define small objects distinctly.
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Temporal resolution in CT imaging concerns the quality of the images produced.
Temporal resolution in CT imaging concerns the quality of the images produced.
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The thickness of the CT slice is determined by the Y-axis.
The thickness of the CT slice is determined by the Y-axis.
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Collimators are used to adjust the X-ray beam's opening based on the operator's selection.
Collimators are used to adjust the X-ray beam's opening based on the operator's selection.
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A voxel is defined as a two-dimensional square in CT imaging.
A voxel is defined as a two-dimensional square in CT imaging.
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In CT imaging, the most common matrix size is 512 x 256.
In CT imaging, the most common matrix size is 512 x 256.
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Beam attenuation refers to the absorption, scattering, or transmission of X-ray photons.
Beam attenuation refers to the absorption, scattering, or transmission of X-ray photons.
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The total number of pixels in a matrix is found by adding the number of rows and columns.
The total number of pixels in a matrix is found by adding the number of rows and columns.
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The information used to create CT images is derived from scanning and recorded by the detectors.
The information used to create CT images is derived from scanning and recorded by the detectors.
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Scatter radiation enhances the clarity of the images captured in CT scans.
Scatter radiation enhances the clarity of the images captured in CT scans.
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X-ray photons that pass through objects unimpeded are represented by a white area on the image.
X-ray photons that pass through objects unimpeded are represented by a white area on the image.
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An object that absorbs a large amount of the X-ray beam is referred to as having low attenuation.
An object that absorbs a large amount of the X-ray beam is referred to as having low attenuation.
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The density of an object is defined as the volume of a substance per unit mass.
The density of an object is defined as the volume of a substance per unit mass.
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The linear attenuation coefficient expresses how much of the X-ray beam is absorbed per unit thickness of the absorber.
The linear attenuation coefficient expresses how much of the X-ray beam is absorbed per unit thickness of the absorber.
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The number of interacting photons increases with the thickness and density of the object.
The number of interacting photons increases with the thickness and density of the object.
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In general, the attenuation coefficient increases with increasing photon energy.
In general, the attenuation coefficient increases with increasing photon energy.
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Dense elements with a high atomic number provide fewer opportunities for photon interaction than less dense elements.
Dense elements with a high atomic number provide fewer opportunities for photon interaction than less dense elements.
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For a 125-kVp x-ray beam, the linear attenuation coefficient for water is approximately 0.18 cm.
For a 125-kVp x-ray beam, the linear attenuation coefficient for water is approximately 0.18 cm.
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Low-energy X-ray photons are less readily attenuated by the patient than high-energy X-ray photons.
Low-energy X-ray photons are less readily attenuated by the patient than high-energy X-ray photons.
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Artifacts in CT imaging are always beneficial for image quality.
Artifacts in CT imaging are always beneficial for image quality.
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Beam-hardening artifacts are caused by the preferential absorption of low-energy photons.
Beam-hardening artifacts are caused by the preferential absorption of low-energy photons.
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Filtering the X-ray beam can improve CT images by making the beam more homogeneous.
Filtering the X-ray beam can improve CT images by making the beam more homogeneous.
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Filtering low-energy photons in X-ray imaging increases the radiation dose to the patient.
Filtering low-energy photons in X-ray imaging increases the radiation dose to the patient.
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Bone attenuates fewer photons than lung tissue when the kVp is kept constant.
Bone attenuates fewer photons than lung tissue when the kVp is kept constant.
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Soft tissues have a linear attenuation coefficient that is roughly proportional to physical density.
Soft tissues have a linear attenuation coefficient that is roughly proportional to physical density.
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Air appears as white areas on a CT image due to its high density.
Air appears as white areas on a CT image due to its high density.
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Contrast agents can create a temporary artificial density difference between objects in a CT image.
Contrast agents can create a temporary artificial density difference between objects in a CT image.
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Metals have the lowest capacity for beam attenuation in comparison to soft tissues.
Metals have the lowest capacity for beam attenuation in comparison to soft tissues.
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Positive contrast agents, such as those containing barium sulfate, have a higher density than the structures they fill.
Positive contrast agents, such as those containing barium sulfate, have a higher density than the structures they fill.
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The representation of surgical clips on a CT image is typically shown as black areas.
The representation of surgical clips on a CT image is typically shown as black areas.
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A contrast agent permanently changes the physical properties of the structure containing it.
A contrast agent permanently changes the physical properties of the structure containing it.
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Hounsfield units can have both positive and negative values based on the object's beam attenuation compared to water.
Hounsfield units can have both positive and negative values based on the object's beam attenuation compared to water.
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A measurement of 4 Hounsfield units suggests an object is likely filled with pure water.
A measurement of 4 Hounsfield units suggests an object is likely filled with pure water.
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The Hounsfield unit is not directly related to the linear attenuation coefficient of materials.
The Hounsfield unit is not directly related to the linear attenuation coefficient of materials.
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Distilled water is assigned a Hounsfield unit value of 1000.
Distilled water is assigned a Hounsfield unit value of 1000.
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Polychromatic X-ray beams consist of photons that all have the same energy level.
Polychromatic X-ray beams consist of photons that all have the same energy level.
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Hounsfield units are named after Godfrey Hounsfield, who contributed to the development of CT technology.
Hounsfield units are named after Godfrey Hounsfield, who contributed to the development of CT technology.
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Factors such as poor equipment calibration do not affect Hounsfield unit measurements.
Factors such as poor equipment calibration do not affect Hounsfield unit measurements.
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In a CT scan, the density reading of an unknown structure can help approximate its composition.
In a CT scan, the density reading of an unknown structure can help approximate its composition.
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Study Notes
CT Basics
- Conventional radiographs display 3D objects as 2D images, causing tissue overlap. This is a limitation.
- Computed Tomography (CT) solves this by scanning thin body sections with a narrow X-ray beam rotating around the body. This creates cross-sectional images.
- Another limitation of conventional radiographs is the inability to differentiate tissues with similar densities.
- CT's unique physics allows for the differentiation between tissues of similar densities.
- Advantages of CT include eliminating superimposed structures, the ability to identify minor density differences in anatomy and abnormalities, alongside superior image quality.
Terminology
- Tomography originates from the Greek word "tomo," meaning to cut, section, or layer.
- In CT, a sophisticated computerised method crafts cross-sectional slices of the human body.
- Older scanning systems were called computerized axial tomography, hence the acronym CAT scan.
Image Terminology
- Preliminary images produced by CT scanners are often called "topograms," "scouts," or "scanograms" depending on the manufacturer.
- Continuous acquisition scanning – a common method – is also sometimes known as "spiral" or "helical" scans.
- Some, are referred to as “isotropic” scans.
CT Image Quality
- CT image quality is assessed based on several factors.
- Spatial resolution: A system's ability to precisely locate small objects in an image.
- Low-contrast resolution: A system's ability to distinguish objects with similar densities.
- Temporal resolution: The speed at which data is acquired, which is crucial for minimizing motion artifacts, particularly vital when imaging areas that move, like the heart.
CT Slice Thickness
- Each CT slice represents a specific plane within the patient's body.
- The thickness of this plane, known as the Z-axis, defines the slice thickness.
- Limiting the X-ray beam to this specific volume decreases scatter radiation and tissue overlap.
- This is accomplished by mechanical components called collimators, which adjust the beam based on the operator's selection.
Pixel and Voxel
- CT slice data is further divided into elements.
- Width (X) and height (Y) define the pixel (picture element).
- A pixel is a two-dimensional square.
- Combining numerous pixels constructs the CT image which appears on the monitor.
- Adding the Z-axis creates a cube called a voxel (volume element).
- A CT matrix is composed of rows and columns of pixels.
- The standard matrix size is 512 rows by 512 columns, making a total of 512 x 512 pixels.
- Each pixel contains data acquired during the scanning process.
Beam Attenuation
- An X-ray beam comprises photon bundles, which can either pass through, be redirected (scattered), or be absorbed by a structure.
- The degree to which an X-ray beam is decreased is known as attenuation.
- Attenuation depends on the photon energy, structural characteristics of the body, and the photon's path length.
- In conventional radiology, the X-ray beam exposes photographic film.
- In CT, the X-ray beam is passed through the body, and the recorded data used to create the image.
Image Density
- By convention, X-ray photons that pass unimpeded appear as black areas (low attenuation).
- Those completely absorbed appear white (high attenuation).
- Intermediate attenuation levels show up as different shades of gray.
- Density is the mass of a substance per unit volume.
- Dense materials, with high atomic numbers, absorb more X-rays compared to less dense materials.
Beam Attenuation (cont.)
- To understand how an object's physical properties affect beam attenuation, consider a single X-ray photon passing through.
- More atoms in the path (a thicker, denser object) increases the likelihood of photon interaction.
- The amount of beam attenuation per unit thickness is shown by the linear attenuation coefficient, denoted by μ.
- The linear attenuation coefficient for water is approximately 0.18 cm⁻¹ (meaning about 18% of photons are attenuated per centimeter).
Attenuation Coefficient Factors
- The attenuation coefficient in general, decreases with increased photon energy and increases with increased atomic number and density.
- With constant kVp, a denser material such as bone, will have a higher attenuation coefficient than a less dense tissue such as lung. This means bone absorbs or scatters more photons than lung tissue, and thus showing up brighter/lighter shades of gray.
Hounsfield Units
- CT quantifies beam attenuation using Hounsfield Units (HU), named after Godfrey Hounsfield.
- HU values are also known as CT numbers or density values.
- Distilled water has a HU value of 0.
- Dense bone is assigned a value of 1000
- Air is assigned -1000
- Naturally occurring tissues have HU values within the range of -1000 to 1000.
- A 1 HU difference corresponds to a 0.1% difference in linear attenuation coefficient compared to water.
Creating a CT Value
- CT values, such as 4 HU, can be used to estimate the composition of an unknown structure.
- A low attenuation (dark) area in an image, might represent fluid, like a cyst.
Polychromatic X-ray Beams
- CT X-ray beams are polychromatic, meaning comprised of photons with various energies.
- Low-energy photons are more readily attenuated than high-energy photons.
- This difference in attenuation is not captured by the detectors.
Artifacts
- Artifacts appear on CT scans, but are not present in the original body part being scanned.
- Beam-hardening artifacts, for example arise due to preferential absorption of low-energy photons within a dense structure.
- Such artifacts appear as streaks or areas of decreased density.
X-ray Beam Filtering
- Filtering an X-ray beam using materials such as Teflon or aluminum reduces the range of X-ray energies.
- This creates a more homogeneous beam, which minimizes artifacts and reduces radiation dose.
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Description
Explore the fundamentals of Computed Tomography (CT) and its advantages over conventional radiography. Understand key terminology and the process by which CT generates detailed cross-sectional images of the human body. This quiz is perfect for those studying medical imaging techniques and their applications.
