Introduction to Computed Tomography

Course Information: Medical Radiation Sciences 1 (MRAD 1000) at Loyalist College

Attenuation: The reduction of the intensity of the X-ray beam as it passes through matter, influenced by the mass density and effective atomic number of the tissues.

Superimposition: To place one thing over another, typically allowing both items to remain observable.
Artifact: An imperfection seen on an image that is not actually present, resulting from the imaging modality or the superimposition of items.
Algorithm: A set of predefined instructions that a computer follows to process data and perform calculations.
Z-Axis: The long axis of the patient in the coordinate system used for imaging.
Interpolation: A method of estimating unknown values from known data points.

Explore the general principles of CT.
List and describe the various generations of computed tomography (CT) imaging systems.
Describe and relate CT imaging system components to their functions.
Explain how attenuation data is captured and processed into image data.

Clinical Procedures:
- General Radiography: Respiratory - High
- General Radiography: Skeletal - High
- General Radiography: Digestive - Medium
- General Radiography: Urinary - Medium
- Computed Tomography - Medium
- Bone Mineral Densitometry - Low
- Fluoroscopy - Low
- Interventional Radiography - Low
- Mammography - Low

Understanding of CT imaging and the necessity for dual training in X-Ray and CT in many hospital settings.

The process involves directing an X-ray beam at the patient.
Detectors measure attenuated radiation, which is then transmitted to a computer.
A computer reconstructs the image and displays it on a monitor using various algorithms.

Image Quality: CT creates a transverse (axial) image perpendicular to the body’s long axis.
Coronal and sagittal images can be derived from the axial dataset.
As the source-detector assembly rotates, X-rays are attenuated according to mass density and effective atomic number.
Radiation intensity detected varies based on this attenuation, leading to an intensity profile, or projection.
This process of collecting multiple projections occurs rapidly, storing digital data for further processing.

Features:
- A finely collimated X-ray beam with a single detector assembly.
- Required 180 translations with 1-degree rotation steps to complete an image, taking nearly 5 minutes.

Characteristics:
- Translate-and-rotate type with multiple detectors and fan-shaped X-ray beams (5 to 30 detectors).
- Advantage: faster imaging due to fewer translations required for equivalent data collection compared to first-generation CT.
- Time limitation of about 20 seconds due to mechanical motion inertia.
- Utilizes a “bow-tie” filter to compensate for radiation intensity variations across the fan beam.

Design:
- The X-ray source and detector array rotate around the patient, producing images in under 100 ms.
- Employs a curvilinear detector array providing constant source-to-detector path length and superior image quality.
- Disadvantages include the potential for ring artifacts which can be corrected with advanced algorithms.

Structure:
- The X-ray source rotates around a stationary circular array of detectors (up to 4000 detectors).
- This design eliminates ring artifacts observed in third-generation CT systems.

Operating Console: Controls the technique factors including kVp, mA, slice thickness, patient positioning, and image reconstruction.
Computer: Requires high computing capacity (~250,000 simultaneous equations) for fast image reconstruction, often using an array processor.
Gantry: Houses essential components like the X-ray tube, detector array, and high-voltage generator, and facilitates image production.

X-Ray Tube: Must handle high thermal demands with anodes rated for at least 50,000 exposures and cooling capabilities.
Detector Array: Made of high-efficiency scintillation solid-state detectors enhancing image quality and reducing radiation doses.
Collimation: Incorporated pre-patient and pre-detector collimators to limit the beam exposure and improve image contrast.
High Voltage Generator: Supplies high-frequency power to the multislice systems, mounted directly on the gantry for compactness.
Patient Couch: Designed from low atomic number material, motor-driven for precise positioning.
Slip-Ring Technology: Allows continuous rotation of the gantry without interruption, enabling helical CT scanning.

Reconstruction Algorithm: Uses attenuation data from detectors to construct images systematically.
Back Projection: Compiles attenuation profiles into an image, may introduce artifacts that filtering must correct.
Filtered Back Projection (FBP): Enhances sharpness by applying filters to raw data during reconstruction.
Iterative Reconstruction: Compares initial images to real-time data for continual refinement, reducing noise and necessary radiation doses up to 50% compared to FBP.
Interpolation Algorithms: Allow for image reconstruction along the z-axis and prevent blurring in reformatted images.

Imaging primarily occurs in the axial plane, but reconstructed images can exist in multiple orientations. Modern scanners primarily utilize third-generation technology due to advancements in processing and imaging techniques.

Bushong, S. C. (2021). Radiologic Science for Technologists: Physics, Biology, and protection. Elsevier