CT Imaging Fundamentals

Computed Tomography (CT) Basics

• Conventional radiographs show a 3D object as a 2D image, causing overlapping tissues.
• Computed Tomography (CT) overcomes this limitation by scanning thin body sections with a rotating X-ray beam, producing cross-sectional images.
• CT's unique physics allows differentiation between tissues with similar densities, unlike conventional radiography.

Advantages of CT over Conventional Radiography

• Eliminates superimposed structures.
• Enables differentiation of small density differences in anatomical structures and abnormalities.
• Provides superior image quality.

Terminology

• Tomography comes from the Greek word "tomo," meaning to cut, section, or layer.
• In CT, a sophisticated computer method produces cross-sectional slices of the human body.
• Older scanning systems are often referred to as computerized axial tomography (CAT) scan.

CT Image Terminology

• The preliminary images produced are sometimes called "topograms," "scouts," or "scanograms", depending on the vendor.
• "Continuous acquisition scanning," "spiral," "helical," or "isotropic" scanning refer to methods for continuous image acquisition.

CT Image Quality

• Spatial resolution describes the ability to distinctly locate small objects.
• Low-contrast resolution refers to a system's ability to differentiate objects with similar densities.
• Temporal resolution refers to the speed of data acquisition. Faster acquisition reduces artifacts from motion, like heart imaging.

CT Slice Thickness

• Each CT slice represents a specific plane in the body.
• The slice thickness is determined by the Z-axis.
• Limiting the X-ray beam to the chosen slice thickness reduces scatter radiation and overlapping structures.
• Collimating (using small shutters) controls the X-ray beam path.

Pixels and Voxels

• CT slice data is organized into elements: pixel (picture element)
• A matrix (grid) of pixels (X and Y axes) creates the image. Adding the Z-axis creates a voxel (volume element)
• A common CT matrix size is 512x512, with a total of 512 x 512 pixels.
• Each pixel contains information on the scanned area.

Beam Attenuation

• An X-ray beam consists of photons, which can pass through, be redirected (scattered), or absorbed by a structure.
• The amount of reduction in the X-ray beam due to interaction with the body is called attenuation.
• The degree of attenuation depends on the X-ray beam strength, structure characteristics, and the structure’s path.
• In CT, the X-ray beam passes through the body and the detected photons are used to create an image.
• The number of photons that either pass through the body or are absorbed/scattered determines the shades of gray in the image.

Density and Attenuation

• Density is the degree of concentration of an object's mass per unit volume.
• High-attenuation structures have a strong effect on the X-ray beam, showing up bright in the image (e.g., bone).
• Low-attenuation structures have less effect on the X-ray beam, showing up dark in the image (e.g., air).
• Linear attenuation coefficient (μ) quantifies the amount of attenuation per unit thickness.

Hounsfield Units (HU)

• In CT, measurements are expressed in Hounsfield units, named after Godfrey Hounsfield.
• These units are also called CT numbers or density values.
• HU quantify the degree to which a structure attenuates an X-ray beam.
• Water is assigned 0 HU, bone 1000 HU, and air -1000 HU
• Helps in identifying characteristics of different tissues and structures.

Artifacts

• Artifacts are objects on the image that are not present in the actual scanned area.
• Beam-hardening artifacts occur with preferential absorption of low-energy photons, resulting in dark streaks or a cupping effect, mainly seen with dense structures.
• These artifacts reduce image quality.

Contrast Agents

• Used to create temporary artificial density differences in the body.
• Increase the difference in densities between different structures, enabling better visibility of structures.
• Positive contrast agents (such as barium sulfate and iodine) have higher densities than the surrounding tissues, producing white areas on the images.
• Low-density contrast agents are used to highlight lower density structures, reducing their appearance to dark areas.

Polychromatic X-ray Beams

• CT and conventional radiography use polychromatic x-ray energy (photons with varying energies).
• Low-energy photons are absorbed more effectively than high-energy photons.
• This difference affects the image, potentially leading to artifacts.

Filtering the X-ray Beam

• Filtering the beam with materials like Teflon, Aluminum can improve image quality by reducing the range of X-ray energies and creating a more homogenous beam, eliminating artifacts.
• Reducing softer (lower energy) photons also reduces radiation dose to the patient.