CT Basics and Terminology

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.