Radiation dose reduction in chest CT—Review of available options
Introduction
X-ray computed tomography (CT) has become an indispensable diagnostic tool, providing patients’ detailed anatomical information. The number of CT examinations is increasing. During the last two decades, the number of CT examinations has been steadily growing by roughly 10% per year to reach about 67 million in 2006 [1]. Consequently, the possible detrimental effects caused by ionizing radiation from diagnostic imaging have become a concern [2].
The rapid increase in CT scan use has raised an argument on the possible overutilization of CT examinations or inappropriate use of CT examinations, which might be responsible for deaths from malignant tumors.
There are varieties of measures that we can adopt to improve the safety of CT examinations. We need to understand these methods and use them wisely to achieve dose reduction.
Indeed, no single method works perfectly for all diagnostic needs. Users of diagnostic imaging should be aware of the advantages and disadvantages of these techniques so that they can use recent technical developments with minimal loss of diagnostic quality. Therefore, the purpose of this article is to review the gamut of methods that can contribute to the observation of the ALARA (As Low As Reasonably Achievable) principle of diagnostic radiation exposure. The first part of this article addresses the problem of growing radiation exposure related with medical imaging. Next, principles and methods of dose reduction in CT are presented. Then, the evidence for the feasibility of radiation dose reduction is reviewed.
Section snippets
Current status of growing radiation exposure
Diagnostic imaging has become a significant source of radiation exposure and has become the largest source in some areas of the world [1]. The U.S. National Council on Radiation Protection and Measurements (NCRP) reported an almost 10-fold increase in the frequency of radiological examinations between 1950 and 2006 in the United States [3]. As a result, the U.S. per-capita annual effective dose from medical procedures reached 3.01 mSv in 2006, with CT examinations making the largest contribution
Causes of the CT radiation dose growth
There are reasons for the increase in the number of CT examinations that needs attention. Because the clinical decision as to whether or not to request a CT examination depends on the risk-benefit ratio, educating physicians about CT radiation doses and their potential risk should help reduce low-yield CT examinations. Studies have shown that physicians are not knowledgeable about the radiation doses related to CT examinations. Lee et al. demonstrated that all patients, 73 percent of emergency
Principles of CT radiation dose reduction
Given that many factors can influence the radiation dose either directly or indirectly, there are various measures to achieve radiation dose reduction safety.
First, it should be also noted that radiation dose to the patient can be lowered by carefully following proper techniques. Correct patient positioning [9] and appropriate scan length decrease the radiation exposure to the patients without any cost to image quality [10]. Shielding of the radiosensitive organs for reduced radiation exposure
Reduction of current-time product
Reduction of current-time product is the simplest method to reduce radiation dose to the patients. In principle, the radiation dose is proportional to the tube current if all other parameters are the same. Most often, tube current is modified to lower current-time product.
Other factors, including gantry rotation time and helical pitch, also influence the current-time product. High-pitch scanning lowers the radiation dose. On single source scanner, pitch can be raised up to 1.5 without gap of
Automatic tube current modulation
Using constant tube current throughout the CT scan is not efficient because of the heterogeneity of attenuation values in parts of the human body. If tube current is fixed, it needs to be set to the level where the worst image quality in the scan range still remains acceptable. Consequently, some parts may be excessively exposed to X-rays.
Dynamic tube current adjustment during an examination becomes possible by the use of automatic tube current modulation systems [16]. These systems are
Adaptive dose shield
For multidetector CT scanning, a half gantry rotation beyond the longitudinal scan range is necessary at both ends of the scan range to collect complete set of projection data for the scan range. Whole data collected in the outermost half rotations at both sides are not used to produce images and some data are therefore ‘wasted’. Adaptive dose shield, which is developed to avoid the unnecessary radiation exposure, is realized with asymmetric control of collimator, blocking nonproductive X-ray
Mode of scanning
Change in the mode of scanning can lead to radiation dose reduction. Cardiac CT used to be realized retrospective reconstruction of continuously collected raw data with very low pitch. Parts of the collected data are not usable because there is significant cardiac motion at the time of data acquisition. Recently, wide longitudinal coverage of the multidetector scanner made it practical to initiate the scan on ECG-trigger to catch the diastolic phase, which decreases the radiation dose for
Use of lower tube potential
Lower tube potential (peak kilovoltage, kVp) has been applied for radiation dose reduction. The choice of peak kilovoltage is not as flexible as tube current, usually limited to several fixed tube voltage peaks. Typically selectable options are 80, 100, 120, or 140 kVp. Switching to lower kilovoltage will lead to significant reduction of photon fluence and hence a considerable increase in the image noise. Radiation dose is proportional to peak kilovoltage to the power of 2.5 [22]. For example,
Shielding of radiosensitive organs
For patient safety, selective protection of radiosensitive organs such as the breasts, thyroid, and eye lens is a reasonable approach for X-ray imaging. Regarding chest CT, mammary glands are the most important organ at risk. They are among the organs with the highest tissue weighting factor (0.12) according to the latest International Commission on Radiological Protection (ICRP) recommendations [31]. Moreover, breast tissue is particularly radiosensitive in younger patients. Protective devices
Image processing and image filters
Image filters are software applications designed to improve image quality. These filters are applied after data collection on the scanner. For low dose images, noise reduction filters and artifact removal filters can be applied for better image quality. Mitigation of image quality deterioration caused by radiation dose reduction may contribute to radiation dose reduction. Imaging filters can work on the projection data (raw data filter) or on the reconstructed images (spatial domain filter).
Reconstruction methods
Methods of CT image reconstruction have been improved with the sophistication of filtered back projection methods. Recently, the iterative reconstruction method, a standard method for emission tomography, has been introduced for CT to decrease image noise [41].
Iterative reconstruction involves calculation of projection data from reconstructed images, taking into account the effect of the scanner's physical properties. The calculated projection data is compared with measured projection data to
Reduction of sampling density
Using recent multidetector CT scanners, collecting data for the whole lung is easily performed with a single breath hold. A scan of the whole chest without interslice gaps is the standard scanning procedure. However, lower sampling density can be applicable to some disease conditions, leading to lower radiation exposure to patients. Although the volumetric data acquisition allows us to evaluate the extent of the disease more precisely [48], monitoring the progression of a known disease
Evidence for application of low dose computed tomography techniques
Regardless of the method used to achieve radiation dose reduction, the effects of dose reduction should be critically assessed to make sure that the quality of the examination is not affected. For that purpose, images obtained with a radiation dose reduction technique should be compared with images obtained according to a generally accepted examination protocol. Quantitative indexes of image quality such as standard deviation of the CT values do not provide information about the feasibility of
Effects on computer-assisted analysis of radiation dose reduction
Recently, computer analysis of volumetric CT data is more widely applied for various conditions as a marker of disease severity [79]. Ideally, the image quality should be as fine as possible for the best results, as noise in CT images can affect the analysis results. Therefore, the effect of radiation dose reduction on the various types of computer analysis results should be elucidated to make computer analysis reliable.
Using simulated low dose images Ley-Zaporozhan et al. demonstrated that
Summary
Radiation dose related to computed tomography and radiation dose reduction measures were reviewed.
CT examinations are the largest and fastest growing source of ionizing radiation exposure to patients and, therefore, dose reduction is an urgent issue. Avoiding low-yield examinations should be part of the CT dose reduction effort. Technological innovations have made it possible to reduce the radiation dose to patients. Dose-sparing technology, including automatic tube current modulation, should
Conflict of interest statement
None.
Acknowledgments
The authors would like to express appreciation for Dr. Pei-Jan Lin, Dr. Shiva Gautam, Dr. Sebastian Ley, Dr. Julia Ley-Zaporozhan, Dr. Wolfram Stiller, and Dr. Mizuki Nishino whose comments and suggestions were of inestimable value for this review article.
The authors are indebted to Koji Koizumi, Kimiko Sameshima, and Kyoji Higashimura for technical support and permission to use images.
I would also like to thank Prof. Kaori Togashi and Prof. Kazuo Sugimura who gave me invaluable comments and
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