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The Link Between Radiation Optimization and Quality
RADIATION SENSIBILITIES RICHARD L. MORIN, PHD, DONALD P. FRUSH, MD
The Link Between Radiation Optimization and Quality William F. Sensakovic, PhD, David R. Warden IV, MD, Laura W. Bancroft, MD The application of radiation to a patient for medical procedures should be governed, in part, by the principle of optimization of protection [1]. This principle answers the question: When performing an imaging procedure, are we keeping dose as low as reasonably achievable, taking into account economic and societal factors? This ensures that the benefitto-risk ratio is maximized when a procedure is performed. The risk associated with medical imaging is typically thought of as detriment caused by radiation. This detriment takes the form of possible cancer induction and, occasionally, tissue effects such as erythema, epilation, and necrosis. Given these risks, it is natural to want to minimize radiation dose. However, this is a critical mistake because it ignores the primary aim of medical imaging: diagnosis. The correct diagnosis is crucially dependent on sufficient image quality to allow diagnostic interpretation. Radiation dose and image quality are two sides of the same coin, and decreasing (or worse, minimizing) dose will adversely affect image quality. Image quality is dependent on the photon energy and amount of emitted radiation (quantity) absorbed in the image receptor. As the quantity of radiation emitted by the source decreases, fewer photons are sent toward the patient causing fewer interactions in patient tissues (lower patient dose) and fewer interactions
850
in the image receptor. Each photon that passes through the patient and is absorbed in the receptor adds information to the image; thus too few photons result in insufficient information in the image. This is typically visualized as an increase in mottle (noise) and a corresponding decrease in signal-to-noise ratio (SNR) (ie, detectability). A rule of thumb is that quadrupling the quantity of photons hitting the receptor halves noise and doubles SNR, so large changes in dose are often required to make an appreciable impact on image quality. Energy affects image quality and dose in a very different (and more complex) way from quantity. In projection imaging (fluoroscopy, radiography, and mammography), as energy increases, radiation becomes more penetrating. Increased penetration allows more of the emitted radiation to travel through patient tissue and interact in the receptor, thus improving mottle and SNR. If we instead decide to decrease quantity as we increase energy, then we can produce an image with similar mottle that uses less radiation (thus indirectly lowering patient dose). A rule of thumb is that for every 15% increase in energy, the quantity can be decreased by 50% while roughly maintaining constant receptor dose. As energy increases, the amount of scattered radiation from the patient also increases, which decreases
contrast. Additionally, as energy increases, the amount of radiation blocked by different tissues becomes similar, which also reduces contrast. Luckily, the decrease in contrast with increasing energy is relatively small (except for imaging contrast media) and in projection imaging can often be offset by image processing for moderate energy increases (w10 kV). Optimization occurs through improvements in technique and technology. What is considered “sufficient” for image quality is task specific, and technique optimization focuses on identifying tasks using excess radiation and altering technique (eg, energy and quantity) to decrease dose and maintain sufficient image quality. For example, an abdominal CT scan used for localization during PET can be set to use a much lower dose (and have much higher mottle) than a routine diagnostic scan. Technique optimization should ideally involve the expertise of radiologists, physicists, and technologists. The radiologist provides feedback throughout the optimization process to ensure that sufficient task-specific image quality is maintained. The physicist optimizes the protocols, keeping sufficient image quality while eliminating excess dose. The technologist ensures that the specified protocol is feasible in the clinical workflow and executed correctly. Diagnostic reference levels [2], the ACR Dose Index Registry
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ª 2017 American College of Radiology http://dx.doi.org/10.1016/j.jacr.2017.03.020
[3], and white papers from groups such as Image Wisely [4], Image Gently [5], and the American Association of Physicists in Medicine can be used to identify possible protocols for optimization and to guide the optimization process. However, these are not absolute limits on dose and should not substitute for image quality assessment made during the optimization process. Optimization through improved technology can also dramatically affect both dose and image quality. Newer receptors can capture a higher percentage of radiation, allowing decreased dose. Automatic exposure control in projection imaging, as well as voltage selection and current modulation in CT, analyze the patient and attempt to automatically generate an image with a predefined amount of mottle (or SNR). When set up and used correctly, these
technologies make small alterations to energy and/or quantity to reduce excess radiation. Modern equipment with intelligent, vendor-specific postprocessing or reconstructions can enhance the image to improve mottle, resolution, and image contrast and even reduce artifacts. Unfortunately, these algorithms can also be unpredictable at very low and high receptor doses and can cause perceptually worse image quality and even create image artifacts that hinder diagnosis. The primary responsibility of every imaging professional is to ensure sufficient image quality for diagnosis using only the necessary amount of radiation. In the modern era of imaging, a missed diagnosis or misdiagnosis due to poor image quality is often a greater concern than the possible radiation-induced detriment caused by a diagnostic scan (assuming reasonable protocols are used). In
trying to do the best for our patients, it is crucial that the goal of optimization not inadvertently become a race to the bottom resulting in nondiagnostic image quality. Optimization must consider both image quality and patient radiation dose: optimization is not minimization.
REFERENCES 1. International Commission on Radiological Protection. The 2007 recommendations of the International Commission on Radiological Protection. ICRP Publication No. 103. Ann ICRP 2007;37:2-4. 2. Geise RA, Monticciolo DL, Timins JK, et al. ACR–AAPM practice parameter for diagnostic reference levels and achievable doses in medical x-ray imaging. Resolution 47. Rev. 2013. Reston, Virginia: American College of Radiology. 3. Morin RL, Coombs LP, Chatfield MB. ACR Dose Index Registry. J Am Coll Radiol 2011;8:288-91. 4. American College of Radiology. Image Wisely. Available at: http://www.imagewisely. org. Accessed March 1, 2017. 5. The Image Gently Alliance. Image Gently. Available at: http://www.imagegently.org. Accessed March 1, 2017.
William F. Sensakovic, PhD, David R. Warden IV, MD, and Laura W. Bancroft, MD, are from Florida Hospital, Orlando, Florida. The authors have no conflicts of interest related to the material discussed in this article. William F. Sensakovic, PhD: Imaging Administration, Florida Hospital, 601 E. Rollins Street, Orlando, FL 32803; e-mail: [email protected].
Journal of the American College of Radiology Sensakovic, Warden, Bancroft n Radiation Sensibilities
851
The Link Between Radiation Optimization and Quality William F. Sensakovic, PhD, David R. Warden IV, MD, Laura W. Bancroft, MD The application of radiation to a patient for medical procedures should be governed, in part, by the principle of optimization of protection [1]. This principle answers the question: When performing an imaging procedure, are we keeping dose as low as reasonably achievable, taking into account economic and societal factors? This ensures that the benefitto-risk ratio is maximized when a procedure is performed. The risk associated with medical imaging is typically thought of as detriment caused by radiation. This detriment takes the form of possible cancer induction and, occasionally, tissue effects such as erythema, epilation, and necrosis. Given these risks, it is natural to want to minimize radiation dose. However, this is a critical mistake because it ignores the primary aim of medical imaging: diagnosis. The correct diagnosis is crucially dependent on sufficient image quality to allow diagnostic interpretation. Radiation dose and image quality are two sides of the same coin, and decreasing (or worse, minimizing) dose will adversely affect image quality. Image quality is dependent on the photon energy and amount of emitted radiation (quantity) absorbed in the image receptor. As the quantity of radiation emitted by the source decreases, fewer photons are sent toward the patient causing fewer interactions in patient tissues (lower patient dose) and fewer interactions
850
in the image receptor. Each photon that passes through the patient and is absorbed in the receptor adds information to the image; thus too few photons result in insufficient information in the image. This is typically visualized as an increase in mottle (noise) and a corresponding decrease in signal-to-noise ratio (SNR) (ie, detectability). A rule of thumb is that quadrupling the quantity of photons hitting the receptor halves noise and doubles SNR, so large changes in dose are often required to make an appreciable impact on image quality. Energy affects image quality and dose in a very different (and more complex) way from quantity. In projection imaging (fluoroscopy, radiography, and mammography), as energy increases, radiation becomes more penetrating. Increased penetration allows more of the emitted radiation to travel through patient tissue and interact in the receptor, thus improving mottle and SNR. If we instead decide to decrease quantity as we increase energy, then we can produce an image with similar mottle that uses less radiation (thus indirectly lowering patient dose). A rule of thumb is that for every 15% increase in energy, the quantity can be decreased by 50% while roughly maintaining constant receptor dose. As energy increases, the amount of scattered radiation from the patient also increases, which decreases
contrast. Additionally, as energy increases, the amount of radiation blocked by different tissues becomes similar, which also reduces contrast. Luckily, the decrease in contrast with increasing energy is relatively small (except for imaging contrast media) and in projection imaging can often be offset by image processing for moderate energy increases (w10 kV). Optimization occurs through improvements in technique and technology. What is considered “sufficient” for image quality is task specific, and technique optimization focuses on identifying tasks using excess radiation and altering technique (eg, energy and quantity) to decrease dose and maintain sufficient image quality. For example, an abdominal CT scan used for localization during PET can be set to use a much lower dose (and have much higher mottle) than a routine diagnostic scan. Technique optimization should ideally involve the expertise of radiologists, physicists, and technologists. The radiologist provides feedback throughout the optimization process to ensure that sufficient task-specific image quality is maintained. The physicist optimizes the protocols, keeping sufficient image quality while eliminating excess dose. The technologist ensures that the specified protocol is feasible in the clinical workflow and executed correctly. Diagnostic reference levels [2], the ACR Dose Index Registry
1546-1440/17/$36.00
n
ª 2017 American College of Radiology http://dx.doi.org/10.1016/j.jacr.2017.03.020
[3], and white papers from groups such as Image Wisely [4], Image Gently [5], and the American Association of Physicists in Medicine can be used to identify possible protocols for optimization and to guide the optimization process. However, these are not absolute limits on dose and should not substitute for image quality assessment made during the optimization process. Optimization through improved technology can also dramatically affect both dose and image quality. Newer receptors can capture a higher percentage of radiation, allowing decreased dose. Automatic exposure control in projection imaging, as well as voltage selection and current modulation in CT, analyze the patient and attempt to automatically generate an image with a predefined amount of mottle (or SNR). When set up and used correctly, these
technologies make small alterations to energy and/or quantity to reduce excess radiation. Modern equipment with intelligent, vendor-specific postprocessing or reconstructions can enhance the image to improve mottle, resolution, and image contrast and even reduce artifacts. Unfortunately, these algorithms can also be unpredictable at very low and high receptor doses and can cause perceptually worse image quality and even create image artifacts that hinder diagnosis. The primary responsibility of every imaging professional is to ensure sufficient image quality for diagnosis using only the necessary amount of radiation. In the modern era of imaging, a missed diagnosis or misdiagnosis due to poor image quality is often a greater concern than the possible radiation-induced detriment caused by a diagnostic scan (assuming reasonable protocols are used). In
trying to do the best for our patients, it is crucial that the goal of optimization not inadvertently become a race to the bottom resulting in nondiagnostic image quality. Optimization must consider both image quality and patient radiation dose: optimization is not minimization.
REFERENCES 1. International Commission on Radiological Protection. The 2007 recommendations of the International Commission on Radiological Protection. ICRP Publication No. 103. Ann ICRP 2007;37:2-4. 2. Geise RA, Monticciolo DL, Timins JK, et al. ACR–AAPM practice parameter for diagnostic reference levels and achievable doses in medical x-ray imaging. Resolution 47. Rev. 2013. Reston, Virginia: American College of Radiology. 3. Morin RL, Coombs LP, Chatfield MB. ACR Dose Index Registry. J Am Coll Radiol 2011;8:288-91. 4. American College of Radiology. Image Wisely. Available at: http://www.imagewisely. org. Accessed March 1, 2017. 5. The Image Gently Alliance. Image Gently. Available at: http://www.imagegently.org. Accessed March 1, 2017.
William F. Sensakovic, PhD, David R. Warden IV, MD, and Laura W. Bancroft, MD, are from Florida Hospital, Orlando, Florida. The authors have no conflicts of interest related to the material discussed in this article. William F. Sensakovic, PhD: Imaging Administration, Florida Hospital, 601 E. Rollins Street, Orlando, FL 32803; e-mail: [email protected].
Journal of the American College of Radiology Sensakovic, Warden, Bancroft n Radiation Sensibilities
851