Unit
- CT imaging overview: Computed tomography produces cross-sectional images by rotating an x-ray tube around the patient and measuring attenuation from multiple angles. Detector arrays capture the transmitted radiation, and reconstruction algorithms convert raw data into pixel-based slices. This process eliminates superimposition of structures seen in conventional radiography.
- Historical development: The first clinical CT scanner developed by Hounsfield in the 1970s used a narrow beam and a single detector, requiring long scan times. Subsequent generations introduced fan beams, multiple detectors, helical acquisition, and multi-slice systems, dramatically reducing scan times and improving resolution.
- Attenuation and Hounsfield units: Tissue attenuation coefficients are mapped to a standardized scale where water is 0 HU and air is -1000 HU. Different tissues occupy characteristic ranges, enabling differentiation of bone, soft tissue, fat, and fluid. Window width and level settings enhance visualization of specific structures.
- Image acquisition parameters: Key parameters include tube voltage (kVp), tube current (mA), rotation time, slice thickness, and pitch. These factors directly affect image noise, contrast, radiation dose, and spatial resolution. Balancing these parameters requires understanding their interdependencies.
- Multi-slice CT technology: Modern scanners use multiple detector rows (16 to 640 slices) to acquire several slices per rotation. This allows isotropic voxels, faster coverage, and improved three-dimensional reconstructions. Detector configuration and slice collimation are critical for achieving desired spatial resolution.
- Contrast resolution and noise: Contrast resolution in CT is determined by the ability to distinguish tissues with small attenuation differences. Noise, arising from quantum fluctuations and electronic sources, degrades contrast. Increasing dose reduces quantum noise, but must be balanced against patient exposure.
- Spatial resolution and reconstruction kernel: Spatial resolution describes the ability to visualize small objects. It depends on focal spot size, detector element size, and reconstruction kernel. Sharp kernels enhance edge detail but increase noise, while smooth kernels reduce noise at the cost of detail.
- Helical and axial scanning: In helical scanning, the table moves continuously during rotation, enabling fast volumetric acquisition. Axial (step-and-shoot) scanning acquires one slice per rotation and is used for certain applications like cardiac CT. Pitch (table movement per rotation divided by collimation) affects dose and coverage.
- Dose management and ALARA principles: Radiation dose in CT is a critical concern, managed through techniques such as tube current modulation, automatic exposure control, and appropriate use of pitch. The ALARA (As Low As Reasonably Achievable) principle guides protocol optimization. Dose metrics including CTDIvol and DLP are monitored to ensure patient safety while maintaining diagnostic image quality.
- Iterative reconstruction techniques: Unlike traditional filtered back projection, iterative reconstruction uses mathematical models to reduce noise and artifacts while lowering radiation dose. These algorithms iteratively refine the image by comparing forward projections of estimated images to actual measured data. Clinical implementation requires careful calibration to avoid altering texture or introducing bias.
- Artifact reduction and quality assurance: Common CT artifacts include beam hardening, partial volume, motion, and ring artifacts. Quality assurance programs involve daily, weekly, and monthly tests using phantoms to monitor CT number accuracy, noise, spatial resolution, and slice positioning. Artifact reduction techniques include patient coaching, appropriate scan parameters, and post-processing correction algorithms.
Scope of Practice
Role: CT Technology Manager
Within scope:
- Selecting appropriate imaging parameters for clinical protocols
- Overseeing calibration and quality control of CT scanners
- Training technologists on proper acquisition techniques
- Evaluating new CT technology for purchase decisions
Outside scope:
- Interpreting CT images for diagnostic purposes
- Performing patient positioning or direct scanner operation during clinical exams
- Modifying reconstruction algorithms at the physics level
- Recommending specific patient treatments based on imaging findings
Quick Revision
- CT creates cross-sectional images using rotating x-ray tube and detector arrays.
- Hounsfield units quantify tissue attenuation relative to water and air.
- Multi-slice technology enables faster, high-resolution volumetric scanning.
- Image quality is determined by contrast, noise, spatial resolution, and acquisition parameters.
- Helical scanning provides continuous coverage; axial scanning is step-and-shoot.