Intro
Despite the years since its development, computed tomography (CT) remains an invaluable diagnostic tool. It allows for detailed imaging of internal structures, providing accessibility and safety due to the absence of a magnetic field and associated risks. However, because it uses ionizing radiation, there is a continuous need to optimize patient radiological protection to minimize the risks associated with radiation exposure.
Dose Optimization in Computed Tomography
The ALARA principle (As Low As Reasonably Achievable) is fundamental in optimizing radiological protection. It involves organizing medical procedures to keep radiation doses as low as possible while still achieving the necessary diagnostic image quality. According to this principle, any diagnostic procedure involving ionizing radiation should only be performed when necessary, and the radiation dose must be optimized. The essence of this principle has been incorporated into Polish legislation.
While the legislator publishes diagnostic reference levels in the form of the weighted computed tomography dose index (CTDIw) for a single tube rotation or single layer in spiral technique, as well as dose-length product (DLP) for a single study phase, and defines the image parameters that should be achieved, each facility is responsible for developing its examination parameters.
It is often the case that protocols were created many years ago, with the main priority of those responsible being image quality, based on the assumption that a higher dose yields a better image. Dose optimization in computed tomography involves adjusting scanning parameters and utilizing advanced technologies that reduce radiation dose without compromising diagnostic image quality. Key parameters affecting computed tomography radiation dose include the X-ray tube's voltage and current, pitch, and detector configuration. Lowering the X-ray tube voltage (kV) can significantly reduce radiation dose; however, it is essential to note that excessively low kV values may increase image noise. On the other hand, using a voltage below 120 kV can significantly enhance the visibility of the contrast agent.
The X-ray tube current (mAs) is another key parameter, and reducing it is one of the most effective ways to lower radiation dose. Modern computed tomography systems allow for automatic mAs adjustment in the Z, X, and Y axes, adapting to the patient’s body shape. An example of such a system is the “smart mA” function in GE devices. This adjustment is further controlled by the quality factor for AEC (“Noise Index” in GE systems), which is also a parameter that can be optimized.
Pitch, or the distance the patient table moves during one gantry rotation divided by the beam width in the Z-axis, also affects the radiation dose. Increasing the pitch value shortens scan time and reduces radiation dose; however, it may increase the likelihood of artifacts appearing.
Beam collimation and detector configuration are also subject to optimization. Reducing collimation of the X-ray tube along the X-axis limits the field of view (FoV), while along the Z-axis, it increases the number of rotations required by the tube-detector assembly to image the patient. However, both of these adjustments reduce scattered radiation reaching the detectors, which positively impacts image quality.
Modern Technologies Supporting Dose Optimization
For many years, medical equipment manufacturers have introduced additional features that facilitate both dose optimization and image quality enhancement. The most important include:
- Angular and Longitudinal Tube Current Modulation: Allows real-time adjustments of radiation dose based on the thickness and density of the tissue being examined.
- Organ-based Tube Current Modulation: Reduces the radiation dose directed at sensitive organs, such as the eyes or thyroid, by lowering mAs when the tube is positioned at specific angles during scanning.
- Advanced Image Reconstruction: Iterative reconstruction (IR) techniques or the use of artificial intelligence in reconstruction enable high-quality imaging at significantly lower radiation doses compared to traditional filtered back projection (FBP) methods.
Research and Tests Confirming Optimization Effectiveness
With such a large number of parameters, creating an optimal protocol is a complex process. Before applying it in clinical practice, the effectiveness of the optimization must be verified. Using an unverified protocol may reduce the quality of the images obtained, leading to decreased effectiveness in disease detection.
One of the options proposed in the literature is testing on cadavers. Unfortunately, not every imaging facility can conduct this type of testing. Furthermore, it is not feasible to conduct comparative protocol tests between radiology departments.
Therefore, the primary tool for analyzing new protocols in study optimization is phantoms, such as ProCT Mk II from Diagnomatic. These allow for precise examination of image parameters, including uniformity and noise, modulation transfer function (MTF), and low-contrast detection.
Proper optimization requires multiple scan repetitions, gradually adjusting a single parameter. This allows for precise fine-tuning of settings to achieve the best results. Diagnomatic Pro-Control software, with its capability for rapid analysis of numerous tests, is a powerful tool in the effort to reduce patient radiation dose.
After completing the optimization and achieving appropriate image parameters, it is essential to consider the feedback from the team of radiologists. If the team determines that the quality of the images does not meet the required diagnostic standards, the implemented changes should be withdrawn, and the optimization process should be restarted. This time, the process should be supplemented with additional information and guidance obtained from consultations with the medical team.
Summary
The dose optimization process in computed tomography is complex and involves comprehensive editing of numerous scanning protocol parameters. Tests on phantoms and dedicated software play a key role in evaluating the effectiveness of implemented changes, together ensuring high imaging quality and patient safety.
Art. 33d of the Act of 29 November 2000 - Atomic Law
REGULATION OF THE MINISTER OF HEALTH of 6 December 2022 on diagnostic reference levels
ANNOUNCEMENT OF THE MINISTER OF HEALTH of 10 November 2015 on the announcement of a list of reference radiological procedures in the field of radiology - imaging diagnostics and interventional radiology
https://www.iaea.org/resources/rpop/resources/online-training-in-radiation-protection
https://www.jmirs.org/article/S1939-8654(20)30296-4/fulltext
Lell MM, May MS, Brand M, Eller A, Buder T, Hofmann E, et al. Imaging the Parasinus Region with a Third-Generation Dual-Source CT and the Effect of Tin Filtration on Image Quality and Radiation Dose. AJNR Am J Neuroradiol 2015; 36: 1225–30.
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