L Goldman

Principles of CT: Radiation Dose and Image Quality

This article discusses CT radiation dose, the measurement of CT dose, and CT image quality. The most commonly used dose descriptor is CT dose index, which represents the dose to a location (e.g., depth) in a scanned volume from a complete series of slices. A weighted average of the CT dose index measured at the center and periphery of dose phantoms provides a convenient single-number estimate of patient dose for a procedure, and this value (or a related indicator that includes the scanned length) is often displayed on the operators console. CT image quality, as in most imaging, is described in terms of contrast, spatial resolution, image noise, and artifacts. A strength of CT is its ability to visualize structures of low contrast in a subject, a task that is limited primarily by noise and is therefore closely associated with radiation dose: The higher the dose contributing to the image, the less apparent is image noise and the easier it is to perceive low-contrast structures. Spatial resolution is ultimately limited by sampling, but both image noise and resolution are strongly affected by the reconstruction filter. As a result, diagnostically acceptable image quality at acceptable doses of radiation requires appropriately designed clinical protocols, including appropriate kilovolt peaks, amperages, slice thicknesses, and reconstruction filters.

Principles of CT and CT Technology

This article provides a review of the basic principles of CT within the context of the evolution of CT. Modern CT technology can be understood as a natural progression of improvements and innovations in response to both engineering problems and clinical requirements. Detailed discussions of multislice CT, CT image quality evaluation, and radiation doses in CT will be presented in upcoming articles in this series.

Principles of CT: Multislice CT

This article describes the principles and evolution of multislice CT (MSCT), including conceptual differences associated with slice definition, cone beam effects, helical pitch, and helical scan technique. MSCT radiation dosimetry is described, and dose issues associated with MSCT—and with CT in general—as well as techniques for reducing patient radiation dose are discussed. Factors associated with the large volume of data associated with MSCT examinations are presented.

An exposure indicator for digital radiography

Digital radiographic imaging systems, such as those using photostimulable storage phosphor, amorphous selenium, amorphous silicon, CCD, and MOSFET technology, can produce adequate image quality over a much broader range of exposure levels than that of screen/film imaging systems. In screen/film imaging, the final image brightness and contrast are indicative of over- and underexposure. In digital imaging, brightness and contrast are often determined entirely by digital postprocessing of the acquired image data. Overexposure and underexposures are not readily recognizable. As a result, patient dose has a tendency to gradually increase over time after a department converts from screen/film-based imaging to digital radiographic imaging. The purpose of this report is to recommend a standard indicator which reflects the radiation exposure that is incident on a detector after every exposure event and that reflects the noise levels present in the image data. The intent is to facilitate the production of consistent, high quality digital radiographic images at acceptable patient doses. This should be based not on image optical density or brightness but on feedback regarding the detector exposure provided and actively monitored by the imaging system. A standard beam calibration condition is recommended that is based on RQA5 but uses filtration materials that are commonly available and simple to use. Recommendations on clinical implementation of the indices to control image quality and patient dose are derived from historical tolerance limits and presented as guidelines.

An exposure indicator for digital radiography: AAPM Task Group 116 (Executive Summary)

Digital radiographic imaging systems, such as those using photostimulable storage phosphor, amorphous selenium, amorphous silicon, CCD, and MOSFET technology, can produce adequate image quality over a much broader range of exposure levels than that of screen/film imaging systems. In screen/film imaging, the final image brightness and contrast are indicative of over- and underexposure. In digital imaging, brightness and contrast are often determined entirely by digital postprocessing of the acquired image data. Overexposure and underexposures are not readily recognizable. As a result, patient dose has a tendency to gradually increase over time after a department converts from screen/film-based imaging to digital radiographic imaging. The purpose of this report is to recommend a standard indicator which reflects the radiation exposure that is incident on a detector after every exposure event and that reflects the noise levels present in the image data. The intent is to facilitate the production of consistent, high quality digital radiographic images at acceptable patient doses. This should be based not on image optical density or brightness but on feedback regarding the detector exposure provided and actively monitored by the imaging system. A standard beam calibration condition is recommended that is based on RQA5 but uses filtration materials that are commonly available and simple to use. Recommendations on clinical implementation of the indices to control image quality and patient dose are derived from historical tolerance limits and presented as guidelines.

Ongoing quality control in digital radiography: Report of AAPM Imaging Physics Committee Task Group 151.

Quality control (QC) in medical imaging is an ongoing process and not just a series of infrequent evaluations of medical imaging equipment. The QC process involves designing and implementing a QC program, collecting and analyzing data, investigating results that are outside the acceptance levels for the QC program, and taking corrective action to bring these results back to an acceptable level. The QC process involves key personnel in the imaging department, including the radiologist, radiologic technologist, and the qualified medical physicist (QMP). The QMP performs detailed equipment evaluations and helps with oversight of the QC program, the radiologic technologist is responsible for the day-to-day operation of the QC program. The continued need for ongoing QC in digital radiography has been highlighted in the scientific literature. The charge of this task group was to recommend consistency tests designed to be performed by a medical physicist or a radiologic technologist under the direction of a medical physicist to identify problems with an imaging system that need further evaluation by a medical physicist, including a fault tree to define actions that need to be taken when certain fault conditions are identified. The focus of this final report is the ongoing QC process, including rejected image analysis, exposure analysis, and artifact identification. These QC tasks are vital for the optimal operation of a department performing digital radiography.

TU-FG-209-08: Distribution of the Deviation Index (DI) in Digital Radiography Practices Across the United States

PURPOSE To characterize the distribution of the deviation index (DI) in digital radiography practices across the United States. METHODS DI data was obtained from 10 collaborating institutions in the United States between 2012 and 2015. Each institution complied with the requirements of the Institutional Review Board at their site. DI data from radiographs of the body parts chest, abdomen, pelvis and extremity were analyzed for anteroposterior, posteroanterior, lateral, and decubitus views. The DI data was analyzed both in aggregate and stratified by exposure control method, image receptor technology, patient age, and participating site for each body part and view. The number of exposures with DI falling within previously published control limits for DI and descriptive statistics were calculated. RESULTS DI data from 505,930 radiographic exposures was analyzed. The number of exposures with DI falling within published control limits for DI varied from 10 to 20% for adult patients and 10 to 23% for pediatric patients for different body parts and views. Mean DI values averaged over other parameters for radiographs of the abdomen, chest, pelvis, and extremities ranged from 0.3 to 1.0, -0.6 to 0.5, 0.8, and -0.9 to 0.5 for the different adult views and ranged from -1.6 to -0.1, -0.3 to 0.5, -0.1, -0.2 to 1.4 for the different pediatric views, respectively (DI data was solicited only for anteroposterior view of pelvis). Standard deviation values of DI from individual sites ranged from 1.3 to 3.6 and 1.3 to 3.0 for the different adult and pediatric views, respectively. Also of interest was that target exposure indicators varied by up to a factor of 6 between sites for certain body parts and views. CONCLUSION Previously published DI control limits do not reflect the state of clinical practice in digital radiography. Mean DI and target exposure indicators are targets for quality improvement efforts in radiography.

SU‐E‐I‐101: Initial Implementation and Evaluation of AAPM TG‐150 Draft Image Receptor Non‐Uniformity Testing Recommendations

PURPOSE To implement in software the procedures described in AAPM Task Group 150s draft recommendations for image receptor performance testing, and to evaluate the effectiveness and practicality of these procedures. METHODS Images of flat fields were acquired using digital x-ray image receptors at 6 cooperating institutions. Four flat field images obtained with each detector spanned a range of input detector air kerma. Software based on AAPM TG150s draft report processed the test images and generated results. Image receptor response and several measures of non-uniformity were evaluated. Images were divided into 10 mm square regions, after eliminating 10 mm borders. For each region, signal (mean), noise (standard deviation) and SNR were calculated. Characteristic signal, noise and SNR were calculated based on average values from all regions. Local non-uniformity for signal (SLN), noise (NLN) and SNR (SNRLN) were expressed as the maximum ratio of the absolute difference between each regions value and its 4 nearest neighbors, to the respective characteristic value. Global non-uniformity (SGN, NGN, SNRGN) were expressed similarly but differences between maximum and minimum values obtained from the regions were used (without comparison to local neighbors). RESULTS TG150 tests discriminated between good and poorly performing detectors. Improper detector calibration was detectable, with noise non-uniformity proving to be a more sensitive measure than signal or SNR non-uniformity. Detector rotation relative to calibration conditions produced a greater change in signal non-uniformity than the other measures. Image receptor structured noise was characterized by an increase in noise non-uniformity with incident air kerma. CONCLUSIONS AAPM TG150s proposed approach to image receptor testing was implemented and evaluated. The approach appears to be an effective and practical one for routine quality assurance testing of digital radiographic image receptors.

SU-E-I-103: Detecting Anomalous Pixels and Correlated Artifacts in Digital Detectors from Flat-Field Images

PURPOSE Anomalous pixels may be defined as those pixels whose exposure response relationship is deviant from the typical, expected or calibrated response. A group of anomalous pixels may Result in visible correlated artifacts. Here we demonstrate an approach to identify anomalous pixels and correlated artifacts using flat-field images. METHODS Using manufacturer specific calibration geometry, sets of four flat-field images per detector were obtained with varying input air kerma values (0.5 to 160 μGy) from 9 digital detectors at 6 institutions. Images obtained before and after calibration, with both proper and improper gain maps and structured artifacts were additionally acquired with some detectors. Image analysis methodology under consideration by AAPM Task Group 150 was used.After eliminating 10mm borders, images were divided into square regions (100mm2 ). Anomalous pixels were identified as pixels within each region with valuesabove or below ±3 standard deviations (SD) relative to the mean value of the region. If these pixels were identified in all four images comprising a set, then they were reported as anomalous. Line artifacts were identified as rows and columns with cumulative profile values that were above or below ±3 SD with respect to the mean value of neighboring profiles in the set of four flat-field mages. Results were verified with visual inspection of the images. RESULTS For four sets of images, the algorithm did not identify any anomalous pixels, and none were spotted on visible inspection as well, while for five sets of images the identified anomalous pixels matched visual inspection results. Anomalous pixel detection failed in regions with an unusually large number of defects and structured noise, since those regions exhibited relatively large SD. Line artifacts consistent with visual analysis were identified correctly when present. CONCLUSIONS A practical approach to identify anomalous pixels and correlated artifacts from flat-field images is demonstrated.

MO‐A‐L100F‐01: Patient Dose in Digital Radiography: Exposure Indexes and Optimization Procedures

RadiationDoses in DR: Part 1 — Status report on AAPM TG116: A Recommended Standard Detector Exposure Index in Digital Radiography. This presentation will briefly cover image detector exposure indices in digital radiography. The best way to understand what these indices tell us about image quality is to understand how they are related to image noise in digital radiography and the appropriateness of the technique factors used to make an exposure and create an image. The phenomenon of “Exposure creep” will be reviewed and an update will be provided on the AAPMs and the International Electrotechnical Commissions efforts to improve the tools available to cope with it. Educational Objectives: 1. Understand how over‐ and under‐ exposures are manifested in digital radiography. 2. Recognize the advantages and disadvantages of wide exposure latitude in digital radiography and ho it may impact patient dose. 3. Be able to explain the root cause of “exposure creep” and identify clinical methods to reduce it. 4. Understand current detector exposure indices in use today, how they are related to image quality, and how they are defined by various manufacturers. 5. Recognize the problem caused by having multiple index definitions in clinical use. 6. Be familiar with the exposure indices being proposed by the AAPM (TG116) and the IEC (WG43). 7. Understand why it is important to allow the exposure index to change with corrections applied to an incorrectly identified VOI by the technologist after the image is acquired. 8. Be familiar with the concept of a “Relative” exposure index or “Deviation Index”. 9. Understand the importance of maintaining exposure index logs and tracking them over time. 10. Understand the importance of establishing clear rules for repeats for the technologists and what the AAPM (TG116) will recommend for them. 11. RadiationDoses in DR: Part 2 — Towards Optimizing CR/DR Techniques. Unlike their film/screen forerunners, CR/DR systems provide radiology with the potential of optimizing receptor exposures—and therefore patient exposures—on an exam‐specific basis. A necessary step towards this goal is assessing and monitoring receptor exposure levels currently being used. Issues associated with this task, including the current lack CR/DR exposure index standards, imprecise and/or incorrect CR/DR calibration by service personnel will be reviewed. Practical problems encountered in measuring and comparing receptor exposures among different systems will be discussed, along with suggestions for medical physicists working this area. Educational Objectives: Upon completion of this presentation, the attendee will: 1. Understand that optimum doses for CR and DR may be customized to specific examinations and clinical operations. 2. Understand the importance of monitoring current practice to the development of optimum radiographic techniques. 3. Understand the importance of exposure indices in stabilizing clinical techniques. 4. Recognize some problems encountered in current clinical practice. 5. Understand some of the issues related to measuring and comparing receptor exposures on different systems. 6. Understand some ideas being developed to optimize CR/DR techniques.