Computed Tomography Notes

Conventional radiographs project a 3D object onto a 2D image, causing superimposition of tissues, which is a major limitation.

Tomography originates from the Greek word "tomos," meaning to cut, section, or layer.
CT uses computerized methods to acquire and transform data.
Components for CT imaging were available 20 years before Hounsfield's demonstration in 1970.
Godfrey Hounsfield (EMI, Ltd.) and Alan Cormack (Tufts University) are key figures in CT development.
Cormack developed the mathematics for CT image reconstruction and shared the 1979 Nobel Prize in physics with Hounsfield.

Spatial Resolution: Ability to distinctly define small objects.
Low-Contrast Resolution: Ability to differentiate objects with similar densities.
Temporal Resolution: Speed of data acquisition, important for reducing motion artifacts (e.g., in cardiac imaging).

CT uses a computer to process x-ray beam information from an area of anatomy to create cross-sectional images.
Each CT slice represents a specific plane in the patient’s body.
Z-axis: Represents the thickness of the CT slice.

Pixel: A two-dimensional square element (picture element) defined by width (X) and height (Y).
A CT image is a composite of thousands of pixels.
Voxel: A three-dimensional cube element (volume element) when the Z-axis (slice thickness) is considered.

Illustration of Pixel/Voxel relationship

Beam Attenuation: The degree to which an x-ray beam is reduced by an object, dependent on:
- Density
- Thickness
- Atomic number
Linear Attenuation Coefficient: The amount of the x-ray beam that is scattered or absorbed per unit thickness of the absorber.

Example: For a 125-kVp x-ray beam, the linear attenuation coefficient for water is approximately $0.18 \, cm^{-1}$ . This means about 18% of the photons are either absorbed or scattered when the x-ray beam passes through 1 cm of water.

Hounsfield units quantify the degree of x-ray beam attenuation.
- Bone: $+1,000$ HU
- Distilled Water: $0$ HU
- Air: $-1,000$ HU
1 HU equals a 0.1% difference between the linear attenuation coefficient of the tissue compared to water.

CT and conventional radiography use polychromatic x-ray beams (photons with varying energies).

Beam Hardening: Artifact that appears as dark streaks or vague areas of decreased density (cupping artifacts).

Volume averaging occurs when different tissue attenuation values are averaged to produce one less accurate pixel reading.
Thicker CT slices increase the likelihood of missing small objects.
Example: If 10-mm slices are used and a pathologic tissue measures 2 mm, the 8 mm of normal tissue is averaged in, potentially making the pathologic tissue less apparent.
Thinner slices of the same area may be useful if an area scanned produces images that are suspicious for a mass, but not definitive

Why use thicker cuts in some scanning protocols?

Radiation Dose: Scanning procedures aim to balance image quality and radiation dose.
Large Scan Area: Scanning a large area with thin slices produces a huge number of slices.

Larger pixel sizes increase the chance that a pixel will contain tissues of different densities.
Example: A pixel containing equal parts calcium (600 HU) and lung tissue (-600 HU) will have a resulting density of 0 HU, which does not accurately reflect either tissue.

Raw Data: Data that have not been sectioned into pixels; Hounsfield unit values have not been assigned.
Image Reconstruction: The process of using raw data to create an image.
Image Data: Data with assigned Hounsfield unit values used to create an image.
Prospective Reconstruction: The reconstruction that is automatically produced during scanning.
Retrospective Reconstruction: Using the same raw data later to generate new images.

Step-and-Shoot Scanning (1980s):
- X-ray tube rotated 360° around the patient to acquire data for a single slice.
- The motion of the x-ray tube was halted while the patient was advanced on the CT table to the location appropriate to collect data for the next slice.
- The process was repeated until the desired area was covered.
- Also known as axial scanning, conventional scanning, or serial scanning.
Helical (Spiral) Scanning (1990s):
- Continuous acquisition scanning mode.
- Development of systems that eliminated cables allowed for continuous gantry rotation.
- Uninterrupted data acquisition traces a helical path around the patient.
Multidetector Row CT Scanning:
- The first helical scanners emitted x-rays that were detected by a single row of detectors, yielding one slice per gantry rotation.
- In 1992, scanners were introduced that contained two rows of detectors, capturing data for two slices per gantry rotation.
- Further improvements equipped scanners with multiple rows of detectors, allowing data for many slices to be acquired with each gantry rotation.

Illustration of imaging planes: Axial, Coronal, Sagittal.

Includes anatomical terms and direction indicators like superior, inferior, anterior, posterior, medial, lateral, proximal, and distal.

CT scan process:
- Data Acquisition (GET DATA)
- Image Reconstruction (USE DATA)
- Image Display (DISPLAY DATA)