Operating principle and basic physics
Modern computed tomography scanners are based on the principles established by Sir Godfrey Hounsfield. A rotating X-ray tube emits a narrow fan-shaped beam of radiation (fan beam) that is strictly collimated to the target. As the rays pass through the patient's body, attenuation occurs. The degree of this attenuation directly depends on the density of specific tissues and their atomic number.
After passing through body structures, the X-rays are captured by a detector array located on the gantry directly opposite the source. The rotation of the tube around the patient makes it possible to collect a dataset from multiple different angles. The raw signals (raw data) are then processed by the computer system into a grayscale anatomical map consisting of three-dimensional voxels.
Evolution of rotation technologies
Historically, the X-ray tubes of early scanners (such as the EMI Mark I, which imaged only the brain) required a significant amount of time for positioning and scanning. Initially, a single scan took about an hour, and reconstruction took days.
A key breakthrough occurred in the 1980s with the introduction of slip-ring technology. This engineering solution enabled continuous rotation of the X-ray tube without cable twisting, which instantly reduced acquisition time for a single axial slice to one second and paved the way for helical scanning.
Modern multislice CT (MSCT) systems are characterized by rapid tube rotation — less than 0.3 seconds per revolution. Scanners are divided into two main types based on the number of sources:
- Single-tube: image reconstruction requires a gantry rotation of at least 180 degrees.
- Dual-source: equipped with two independent tube/detector systems. A rotation of only 90 degrees is sufficient for data acquisition, providing an ultra-short acquisition time ranging from 80 to 200 milliseconds.
Radiation control and current modulation
Automatic tube current modulation (ATCM) is a crucial tool for optimizing radiation dose. The system adjusts the tube current in real time to maintain a preset level of image noise. However, the method has certain limitations:
- The tube current automatically increases when X-rays pass through dense anatomical regions (shoulder joints, lung bases, diaphragm, pelvis).
- The effectiveness of angular modulation in young children is controversial because their cross-sectional body shape is more uniformly "round".
- The use of bismuth shielding is discouraged, as the system may interpret it as dense tissue, which falsely increases tube current and radiation exposure.
To protect radiosensitive organs (breasts, thyroid gland), modern algorithms reduce X-ray generation power as the tube passes over these zones in the anteroposterior projection. Furthermore, the issue of over-ranging in helical scanning is addressed by using dynamic collimators that physically block the radiation beam at the start and end of the spiral.
Tube operating parameters in pediatric practice
X-ray tube settings are strictly regulated by protocols, especially in pediatrics. Differences depend on the scan region and the clinical task:
- Routine abdominal CT: helical mode (helical) is used. Tube voltage is 100 kV, reference value is 120 mAs. Tube rotation time is 0.5 s, collimation is 0.6 mm (for patients weighing up to 15 kg).
- Routine head CT: sequential mode (sequential) is applied. Tube voltage is increased to 120 kV, reference current is 300 mAs. Tube rotation time is 1 s.
- Hydrocephalus evaluation: also sequential mode, voltage 120 kV, but reference current is reduced to 200 mAs. Rotation time is 1 s.
- Low-dose 3D head CT (maxillofacial region): helical mode with a tube voltage of 100 kV and a reference current of 300 mAs. Tube rotation time is 1 s.
Portable radiographic units, unlike stationary systems, have strict limitations on maximum tube current and voltage. This precludes the use of high-voltage techniques and requires longer exposure times, frequently resulting in motion artifacts and unacceptable image noise due to underexposure.
Class and exam questions
What is the essence of the "flying focal spot" technology?
This is a technology ('flying focal spot') in which the X-ray focal spot on the tube anode is purposefully deflected. This allows for doubling the number of collected projections along the z-axis without physically increasing the number of detector rows.
Why is a 90-degree rotation sufficient for dual-source systems?
Dual-source systems operate with two independent X-ray tubes and two detector arrays positioned at an angle to each other. Due to simultaneous data collection from different angles, a complete dataset for a slice is formed twice as fast (in a quarter of a rotation).
What limitations does the X-ray tube face in portable units?
Mobile X-ray units cannot provide the high voltage and current levels of stationary systems. Consequently, exposure times must be increased, which raises the risk of image blurring (motion artifacts) and results in high noise levels.
What else to study on this topic
- Design features of CT detectors (scintillators and photodiodes)
- Reconstruction algorithms: high and low spatial resolution
- Operating principle of dental cone-beam computed tomography (CBCT)
- Dual-energy subtraction radiography
- Immobilization and management strategies for pediatric patients during CT
Master the entire topic in the "Diagnostic Radiology" course
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Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.