The impact of automatic tube current modulation related settings of a modern GE CT scanner on image quality and patient dose; details do matter.

PURPOSE: To investigate the operation principles of the automatic tube current modulation (ATCM) of a modern GE healthcare CT scanner, and the impact of related settings on image quality and patient dose. MATERIAL & METHODS: A dedicated phantom (Mercury 4.0) was scanned using two of the most frequently used clinical scanning protocols (chest and abdomen-pelvis). The preset protocol settings were used as starting points (reference conditions). Scan direction, scan mode (helical vs. axial), total beam width, tube potential (kVp), and ATCM settings were then modified individually to understand their impact on radiation dose and image quality. Regarding the ATCM settings, the SmartmA minimum and maximum mA limits, and the noise index (NI) values were varied. As surrogates of patient dose, the CTDIvol and DLP values of each scan were used. As surrogates of image quality were used the image noise and the detectability index (d') of five different materials (air, solid water, polystyrene, iodine, and bone) embedded in the Mercury phantom calculated with the ImQuest software. RESULTS: The scanning direction did not have any effect on ATCM curves, unlike what has been observed in CT scanners from other manufacturers. Total beam width does matter, however, the SmartmA limit settings and kVp selection had the greatest impact on image quality and dose. It was seen that improper minimum mA limit settings practically invalidated the ATCM operation. In contrast, when full modulation was allowed without restrictions, noise standard deviation, and detectability index became much more consistent across the wide range of phantom diameters. For lower kVp settings an impressive dose reduction was observed that requires further investigation. CONCLUSION: SmartmA is a tool that if not properly used may increase the patient doses considerably. Therefore, its settings should be carefully adjusted for each preset different clinical protocol.

Automatic image quality evaluation in digital radiography using for-processing and for-presentation images.

PURPOSE: To investigate the impact of digital image post-processing algorithms on various image quality (IQ) metrics of radiographic images under different exposure conditions. METHODS: A custom-made phantom constructed according to the instructions given in the IAEA Human Health Series No.39 publication was used, along with the respective software that automatically calculates various IQ metrics. Images with various exposure parameters were acquired with a digital radiography unit, which for each acquisition produces two images: one for-processing (raw) and one for-presentation (clinical). Various examination protocols were used, which incorporate diverse post-processing algorithms. The IQ metrics' values (IQ-scores) obtained were analyzed to investigate the effects of increasing incident air kerma (IAK) on the image receptor, tube potential (kVp), additional filtration, and examination protocol on image quality, and the differences between image type (raw or clinical). RESULTS: The IQ-scores were consistent for repeated identical exposures for both raw and clinical images. The effect that changes in exposure parameters and examination protocol had on IQ-scores were different depending on the IQ metric and image type. The expected positive effect that increasing IAK and decreasing tube potential should have on IQ was clearly exhibited in two IQ metrics only, the signal difference-to-noise-ratio (SDNR) and the detectability index (d'), for both image types. No effect of additional filtration on any of the IQ metrics was detected on images of either type. An interesting finding of the study was that for all different image acquisition selections the d' scores were larger in raw images, whereas the other IQ metrics were larger in clinical images for most of the cases. CONCLUSIONS: Since IQ-scores of raw and their respective clinical images may be largely different, the same type of image should be consistently used for monitoring IQ constancy and when results from different X-ray systems are compared.

Technical specifications of dose management systems: An international atomic energy agency survey.

PURPOSE: Dose management systems (DMS) have been introduced in radiological services to facilitate patient radiation dose management and optimization in medical imaging. The purpose of this study was to gather as much information as possible on the technical characteristics of DMS currently available, regarding features that may be considered essential for simply ensuring regulatory compliance or desirable to fully utilize the potential role of DMS in optimization of many aspects of radiological examinations. METHODS: A technical survey was carried out and all DMS developers currently available (both commercial and open source) were contacted and were asked to participate. An extensive questionnaire was prepared and uploaded in the IAEA International Research Integration System (IRIS) online platform which was used for data collection process. Most of the questions (93%) required a "Yes/No" answer, to facilitate an objective analysis of the survey results. Some free text questions and comments' slots were also included, to allow participants to give additional information and clarifications where necessary. Depending on the answer, they were considered either as "Yes" or "No." RESULTS: Given the way that the questions were posed, every positive response indicated that a feature was offered. Thus, the percentage of positive responses was used as a measure of adherence. The percentages of positive answers per section (and sub-section) are presented in graphs and limitations of this type of analysis are discussed in detail. CONCLUSIONS: The results of this survey clearly exhibit that large differences exist between the various DMS developers. Consequently, potential end users of a DMS should carefully determine which of the features available are essential for their needs, prioritize desirable features, but also consider their infrastructure, the level of support required and the budget available before selecting a DMS.

Validation of the geometric equivalent field concept in total scatter factor calculations, for half-, quarter- and off-isocenter asymmetric square fields.

OBJECTIVE: Monitor unit (MU) verification for any symmetric or asymmetric field is performed using a total scatter factor (Scp ), that is calculated based on the geometric equivalent square field (GESF) concept. In this study, we measured the Scp of various asymmetric square fields (ASFs ) and their respective GESFs. METHODS: Square half-fields (SHFs ), square quarter-fields (SQFs ) and square off-isocenter fields (SOFs ), with sizes ranging from 3×3 cm2 to 20×20 cm2 were created, by varying the collimator jaws of two Varian iX Linacs (6/18 and 6/23 MV). A semi-flex ion chamber was used to measure Scp at a depth of 10 cm within a water phantom, at the effective field center (EFC) of all ASFs , and at the isocenter (IC) of their respective GESFs. The later Scp values were corrected by the off-axis ratio [OAR(r)] of the 40×40 cm2 field size, where r is the distance between EFC and IC. RESULTS: The results show that the Scp (EFC) is independent of the type of the ASF (SHF, SQF, or SOF) and no significant difference exists between the 18 and 23 MV beams. Compared with the Scp (IC), the Scp (EFC) increased with increasing r, by up to 2% and 4% for 18/23 and 6 MV, respectively. CONCLUSIONS: The GESF concept provides acceptable accuracy (< 2%) for the calculation of Scp of the ASFs used in most clinical situations (except from SOF with EFC at large r), and thus can be used in MU verification calculations.

Patient exposure during videofluoroscopic swallowing studies performed by speech-language pathologists.

Videofluoroscopic swallowing studies (VFSSs) are fluoroscopic examinations performed by speech and language pathologists (SLPs), for the evaluation of the oral and pharyngeal phases of swallowing, in patients who are diagnosed with symptoms like dysphagia and speech impairment. The study was focused on the evaluation of the patient doses from VFSS performed at Hamad Medical Corporation hospitals. Data on the patient exposure and examination parameters were extracted from the Radiation Dose Monitoring system, statistically analysed and compared with literature. For adult patients, the mean (median) values for fluoroscopy time and kerma-air product were 2.8 (2.7) min and 181 (144) cGycm2, respectively. For children, the respective mean (median) values were 2.6 (2.4) min and 15.3 (9.2) cGycm2. The results of the study indicate that the VFSS are performed by well-trained health professionals, and as a result, image quality sufficient for a confident diagnosis is obtained at relatively low dose levels.

Digital radiography image quality evaluation using various phantoms and software.

PURPOSE: To investigate the effect of the exposure parameters on image quality (IQ) metrics of phantom images, obtained automatically using software or from visual evaluation. METHODS: Three commercial phantoms and a homemade phantom constructed according to the instructions given in the IAEA Human Health Series No. 39 publication were used, along with the respective software that estimate automatically various IQ metrics. Images with various exposure parameters were acquired in a digital radiography (DR) unit. For the commercial phantoms, visual evaluations were also performed. The IQ scores obtained were analyzed to investigate the effects of increasing incident air kerma (IAK), tube potential (kVp), additional filtration, and acquisition protocol on IQ. RESULTS: The effects of the exposure parameters on the IQ metrics, determined with the commercial and the IAEA phantoms, were not the same. For example, clear trends of improvement of IQ scores with increased IAK and reduction of most IQ scores with increased kVp were observed mostly with the IAEA phantom, but not with the commercial phantoms (for both automatic and visual scoring methods). For all phantoms, the maximum variations in IQ scores observed for repeated identical exposures were almost always below 10% with automatic evaluation whereas, for visual evaluation, reached 17%. CONCLUSIONS: Failure to detect some expected trends with the complex commercial phantoms may be attributed to the fact that IQ in DR is more strongly affected by the post-processing procedures, which may mask the effect of other parameters on IQ, something that was not observed with the simple IAEA phantom.

From the use of exposure index in quality control testing to the use of exposure index for quality control of clinical images.

Aim: The exposure index (EI) is used in routine quality control (QC) tests performed in the radiographic equipment installed in our hospitals. This study aimed at investigating the factors affecting the calculation of EI in QC and clinical images, and the implementation of target EI (EIT) and deviation index (DI) in clinical practice. Methods: The EI is 100 times the incident air kerma (IAK) in µGy on the image receptor, using the RQA-5 X-ray beam quality. Conformance to this relationship was investigated in QC images and clinical images acquired using anthropomorphic phantom body parts and different examination protocols, tube potential settings and radiation field sizes. Furthermore, a survey on EIT and DI data from clinical images was performed. Results: Though automatic exposure control (AEC) systems have been adjusted for an IAK of 2.5 µGy, for most anthropomorphic phantom images the EIs were far from 250, depending on the manufacturer, the anatomy imaged, and the examination protocol. Regarding the survey results, DI calculation was feasible in only 38 % of the systems, since for the rest EIT values have not been set. However, the rationale based on which EIT have been selected is unclear. Some systems use only one while others many different EIT values. Conclusion: Before using EI for quality control of clinical images image all receptors and AEC systems should be properly calibrated. Then, the methodology of selecting appropriate EIT should be refined, since the EI calculation may vary, depending on the manufacturer, the anatomy imaged, and the examination protocol.

Image Quality Comparison between Digital Breast Tomosynthesis Images and 2D Mammographic Images Using the CDMAM Test Object.

To evaluate the image quality (IQ) of synthesized two-dimensional (s2D) and tomographic layer (TL) mammographic images in comparison to the 2D digital mammographic images produced with a new digital breast tomosynthesis (DBT) system. Methods: The CDMAM test object was used for IQ evaluation of actual 2D images, s2D and TL images, acquired using all available acquisition modes. Evaluation was performed automatically using the commercial software that accompanied CDMAM. Results: The IQ scores of the TLs with the in-focus CDMAM were comparable, although usually inferior to those of 2D images acquired with the same acquisition mode, and better than the respective s2D images. The IQ results of TLs satisfied the EUREF limits applicable to 2D images, whereas for s2D images this was not the case. The use of high-dose mode (H-mode), instead of normal-dose mode (N-mode), increased the image quality of both TL and s2D images, especially when the standard mode (ST) was used. Although the high-resolution (HR) mode produced TL images of similar or better image quality compared to ST mode, HR s2D images were clearly inferior to ST s2D images. Conclusions: s2D images present inferior image quality compared to 2D and TL images. The HR mode produces TL images and s2D images with half the pixel size and requires a 25% increase in average glandular dose (AGD). Despite that, IQ evaluation results with CDMAM are in favor of HR resolution mode only for TL images and mainly for smaller-sized details.

Evaluation of automatic tube current modulation of CT scanners using a dedicated and the CTDI dosimetry phantoms.

PURPOSE: To investigate the operation principles of the automatic tube current modulation (ATCM) of a CT scanner, using a dedicated phantom and the CT dosimetry index (CTDI) phantom. MATERIAL AND METHODS: The Mercury 4.0 phantom and three different configurations of the CTDI dosimetry phantom were employed. A frequently used clinical scanning protocol was employed as a basis for the acquisitions performed with all phantoms, using both scanning directions. Additional acquisitions with different pitch and examination protocols were performed with Mercury phantom, to further explore their effect on ATCM and the resulting image quality. Different software named DICOM Info Extractor, ImageJ, and imQuest, were used to derive CTDIvol and table position, image noise, and water equivalent diameter (WED) of each phantom CT image, respectively. ImQuest was also used to derive the detectability index (d') of five different materials (air, solid water, polystyrene, iodine, and bone) embedded in the Mercury phantom. RESULTS: It was exhibited with all four phantoms that the scanning direction greatly affects the modulation curves. The fitting of the dose modulations curves suggested that for each table position what determines the CTDIvol value is the WED values of the phantom structures laying ahead towards the scanning direction, for a length equal to the effective width of the X-ray beam. Furthermore, it was also exhibited that ATCM does not fully compensate for larger thicknesses, since images of larger WED phantom sections present more noise (larger SD) in all four phantoms and in Mercury 4.0 phantom smaller detectability (d'). CONCLUSION: Mercury 4.0 is a dedicated phantom for a complete and in-depth evaluation of the ATCM operation and the resulting image quality. However, in its absence, different CTDI configurations can be used as an alternative to investigate and comprehend some basic operation principles of the CT scanners' ATCM systems.

Development and implementation of a quality control protocol for B-mode ultrasound equipment.

PURPOSE: Quality assurance (QA) of ultrasound (US) equipment is currently required in only a few countries around the world. In Greece, no national or other norms exist for regulating the use of US equipment. However, to obtain accreditation for the radiology department of a Greek hospital, the establishment and implementation of a quality control (QC) protocol and a QA programme for US equipment was required. MATERIALS AND METHODS: A literature review regarding US QC/QA procedures was performed. The information collected was used as a guide to create a QC/QA protocol and to obtain an appropriate US QC phantom. Drafting and testing of the initial protocol lasted 6 months. Its final version was implemented for 18 months in two US systems and five US transducers. RESULTS: The QC tests included in the protocol evaluate mechanical and electrical safety, image display, uniformity, penetration depth, distance accuracy, greyscale display, anechoic object imaging, geometric distortion, and axial/lateral resolution. The only QC test that failed was the test for uniformity since intense non-uniformities were observed that led to the replacement of two linear transducers. CONCLUSION: US imaging is considered safe and, where appropriate, is preferred over imaging modalities that use ionizing radiation. However, the lack of QC/QA implies that US image quality is not routinely monitored. Therefore, the possibility of malfunctions that may go undetected and lead to wrong diagnosis cannot be excluded. A QC/QΑ programme can contribute to the elimination of such errors and ensure that performance is maintained over time.

Radiation dose monitoring in computed tomography: Status, options and limitations.

In the last few years there has been an increasing interest on radiation dose to patients undergoing various diagnostic or therapeutic procedures with the use of ionizing radiation. Especially for CT examinations and interventional procedures, where it is known that patient doses are much higher than conventional radiography, new norms have been published that require to have appropriate radiation dose indices registered in the patient medical record. Because of these demands, dose monitoring has been recommended and adopted into many clinical practices as a routine procedure for every patient and every examination. Dedicated dose monitoring systems (DMS) that facilitate data collection and processing, statistical comparisons, reporting and management of radiation dose related information have been devised and are being used worldwide. In this review paper, a brief flashback to the reasons that necessitated dose monitoring in radiology will be first presented. Furthermore, since the focus of this manuscript is on CT, the CT dosimetry principles and metrics will be summarized. The limitations of these metrics will be also discussed, so that DMS users are aware of the semantics of the parameters shown in the DMS reports. The operation of DMS systems will be outlined to make users aware of functions, limitations, and available options of DMS systems. Furthermore, the usefulness of DMS systems as an optimization tool will be presented and discussed. Finally, information about the DMS solutions available in the market and relevant links will be presented.

An Automated Method for Quality Control in MRI Systems: Methods and Considerations.

OBJECTIVE: The purpose of this study was to develop an automated method for performing quality control (QC) tests in magnetic resonance imaging (MRI) systems, investigate the effect of different definitions of QC parameters and its sensitivity with respect to variations in regions of interest (ROI) positioning, and validate the reliability of the automated method by comparison with results from manual evaluations. MATERIALS AND METHODS: Magnetic Resonance imaging MRI used for acceptance and routine QC tests from five MRI systems were selected. All QC tests were performed using the American College of Radiology (ACR) MRI accreditation phantom. The only selection criterion was that in the same QC test, images from two identical sequential sequences should be available. The study was focused on four QC parameters: percent signal ghosting (PSG), percent image uniformity (PIU), signal-to-noise ratio (SNR), and SNR uniformity (SNRU), whose values are calculated using the mean signal and the standard deviation of ROIs defined within the phantom image or in the background. The variability of manual ROIs placement was emulated by the software using random variables that follow appropriate normal distributions. RESULTS: Twenty-one paired sequences were employed. The automated test results for PIU were in good agreement with manual results. However, the PSG values were found to vary depending on the selection of ROIs with respect to the phantom. The values of SNR and SNRU also vary significantly, depending on the combination of the two out of the four standard rectangular ROIs. Furthermore, the methodology used for SNR and SNRU calculation also had significant effect on the results. CONCLUSIONS: The automated method standardizes the position of ROIs with respect to the ACR phantom image and allows for reproducible QC results.

Electronic collimation of radiographic images: does it comprise an overexposure risk?

OBJECTIVE: To investigate whether electronic collimation software, which is available in all digital X-ray systems, may comprise an overexposure risk. METHODS: In the context of surveys on Diagnostic Reference Levels carried out in two radiographic facilities, along with data on exposure factors, the radiographic field sizes were also recorded. In one facility (Unit A), a wireless flat panel detector is used with a conventional X-ray unit, while in the other, a fully digital system is installed (Unit B). The electronically collimated image sizes were compared with the original radiation field sizes. The differences between these two systems concerning the field sizes and the mode of electronic collimation utilization were investigated. RESULTS: In Unit A, manual electronic collimation was extensively used and cases where the radiation field size was up to three times larger than that electronically collimated, were identified. On the contrary, in Unit B radiation fields were smaller and electronic collimation was automatic. CONCLUSION: When electronic collimation is used in manual mode instead of proper pre-exposure collimation, then it does comprise an overexposure risk. The risk is larger in radiographic units where the field size is not automatically selected according to the examination protocol and no interlocks against oversized collimation settings exist. Advances in knowledge: When radiologists review masked images to make the diagnosis, possible suboptimal X-ray field collimation practices may go unnoticed for long. Therefore, radiologists and medical physicists should periodically survey the original images to determine the actual radiation field sizes used for each radiographic examination type.

Utilizing a simple CT dosimetry phantom for the comprehension of the operational characteristics of CT AEC systems.

PURPOSE: To investigate the utility of the nested polymethylacrylate (PMMA) phantom (which is available in many CT facilities for CTDI measurements), as a tool for the presentation and comparison of the ways that two different CT automatic exposure control (AEC) systems respond to a phantom when various scan parameters and AEC protocols are modified. METHODS: By offsetting the two phantom's components (the head phantom and the body ring) half-way along their longitudinal axis, a phantom with three sections of different x-ray attenuation was created. Scan projection radiographs (SPRs) and helical scans of the three-section phantom were performed on a Toshiba Aquilion 64 and a Philips Brilliance 64 CT scanners, with different scan parameter selections [scan direction, pitch factor, slice thickness, and reconstruction interval (ST/RI), AEC protocol, and tube potential used for the SPRs]. The dose length product (DLP) values of each scan were recorded and the tube current (mA) values of the reconstructed CT images were plotted against the respective Z-axis positions on the phantom. Furthermore, measurements of the noise levels at the center of each phantom section were performed to assess the impact of mA modulation on image quality. RESULTS: The mA modulation patterns of the two CT scanners were very dissimilar. The mA variations were more pronounced for Aquilion 64, where changes in any of the aforementioned scan parameters affected both the mA modulations curves and DLP values. However, the noise levels were affected only by changes in pitch, ST/RI, and AEC protocol selections. For Brilliance 64, changes in pitch affected the mA modulation curves but not the DLP values, whereas only AEC protocol and SPR tube potential selection variations affected both the mA modulation curves and DLP values. The noise levels increased for smaller ST/RI, larger weight category AEC protocol, and larger SPR tube potential selection. CONCLUSIONS: The nested PMMA dosimetry phantom can be effectively utilized for the comprehension of CT AEC systems performance and the way that different scan conditions affect the mA modulation patterns, DLP values, and image noise. However, in depth analysis of the reasons why these two systems exhibited such different behaviors in response to the same phantom requires further investigation which is beyond the scope of this study.

Comparative performance evaluation of a flat detector and an image intensifier angiographic system both used for interventional cardiology procedures in adult and pediatric patients.

PURPOSE: To compare two angiography systems of different image capture technology, one with flat detector (FD) and one with image intensifier (II), in terms of entrance surface air kerma (ESAK) rate, detector dose (DD) rate and image quality (IQ), in interventional cardiology procedures concerning both adult and pediatric patients. MATERIALS AND METHODS: In order to determine ESAK and DD rates, a digital dosimeter and polymethylmethacrylate (PMMA) plates were used. For the evaluation of IQ, two contrast objects (the Leeds TOR 18FG and a 5 mm-thick Aluminum plate) were used and two figures of merit were defined in fluoroscopy and cine acquisition modes. Measurements of ESAK, DD rates and IQ were made for various fields of view, pulse and frame acquisition rates. RESULTS: For the particular setup used in this study was noted that ESAK values in the II system were generally larger than the respective values in the FD system (on average 70% for fluoro mode, 5 times for cine mode). When halving the fluoroscopy pulse rate, reduction in ESAK was not proportional, in fluoroscopy mode. Image quality evaluations indicated that II performs better in terms of low contrast sensitivity (LCS) and signal-to-noise ratio (SNR) than the FD system which performs better regarding high contrast resolution (HCR). However, when considering image quality in relation to ESAK the FD system performs better than the II system (with the exception of low thicknesses and zooms for high pulse rates in the fluoroscopy mode). CONCLUSIONS: The FD system, generally, provides a better image quality-dose relation than the II system although II unit provides better LCS and SNR. This means that with the right adjustments to both systems, FD unit is able to provide same image quality with lower dose. However, newer technology does not automatically imply better image quality and further investigation is necessary for deriving safe conclusions for units which utilize different capture technology.

A comprehensive method for calculating patient effective dose and other dosimetric quantities from CT DICOM images.

OBJECTIVE: The purpose of this article is to present a method for the calculation of effective dose using the DICOM header information of CT images. MATERIALS AND METHODS: Using specialized software, the DICOM data were automatically extracted into a spreadsheet containing embedded functions for calculating effective dose. These data were used to calculate the dose-length product (DLP) fraction that corresponds to each image, and the respective effective dose was obtained by multiplying the image DLP by a conversion coefficient that was automatically selected depending on the CT scanner, the tube potential, and the anatomic position to which each image corresponded. The total effective dose was calculated as the sum of effective doses of all images plus the contribution of overscan. The conversion coefficient tables were derived using dosimetry calculator software for both the International Commission on Radiological Protection (ICRP) 60 and ICRP 103 organ-weighting schemes. This method was applied for 90 chest, abdomen-pelvis, and chest-abdomen-pelvis examinations performed in three different MDCT scanners. RESULTS: The DLP values calculated with this method were in good agreement with those calculated by the CT scanners' software. The effective dose values calculated using the ICRP 103 conversion coefficient compared with those calculated using the ICRP 60 conversion coefficient were roughly equal for the chest-abdomen-pelvis examinations, smaller for the abdomen-pelvis examinations, and larger for the chest examinations. The applicability of this method for estimating organ doses was also explored. CONCLUSION: With this method, all patient dose-related quantities, such as the DLP, effective dose, and individual organ doses, can be calculated.

CT dosimetry considerations in non typical conditions: the effect of scan field of view and table height selection.

PURPOSE: To experimentally investigate the effect of the scan field of view (SFOV) selection and table height settings on the Computed Tomography Dose Index (CTDI) and the implications concerning patient effective and skin dose. METHODS: Air-kerma length product (AKLP) measurements were carried out in a helical CT scanner using a pencil type dosimeter positioned in air and inside the holes of a head and a body phantom, using all available SFOV selections and different table height settings. Furthermore, using radiotherapy verification films placed on the CT table surface, the entrance surface air kerma (ESAK) profiles were derived with different SFOV and table height selections, both with and without a phantom on top of the films. RESULTS: The AKLP is strongly dependent on the SFOV selection and the table height settings. Different SFOV selections correspond to the selection of different bowtie filters that shape the X-ray beam intensity, resulting in different ESAK values at the isocenter and at the other points within the scanning plane. With the off-center positioning the calculated CTDI values within the center and the periphery of the phantom change also, as a result of the different intensity and width of the X-ray beam to which are exposed to. CONCLUSIONS: The existing protocols for calculating effective dose are limited to only two patient anatomy-SFOV combinations and cannot account for off-center positioning. Therefore, more work will be required to estimate the effective and skin dose for non-standard SFOV-patient anatomy combinations and off-center patient positioning.

The impact of overscan on patient dose with first generation multislice CT scanners.

Helical scanning requires the irradiation of larger lengths than those planned. This is referred to as overscan and results to an increase of patient dose. Its impact on patient dose was investigated for three first generation multislice CT scanners; a six-, a quad- and a dual-slice. The amount of overscan was determined using the scanners' dose-length product (DLP) indications and films positioned on the CT table. With the preset protocol for the chest examination selected in all CT scanners, the overscan length calculated from the DLP indications was 6.3, 3.5 and 2cm respectively, whereas the corresponding figures derived from the films were 6.6, 4.8 and 2.5 to 3.2cm. For a 30cm scan length, the respective contributions of overscan to the DLP values were 17, 10 and 6%, whereas for a scan length of 20cm the respective values increased to 24, 15 and 9%. For the smallest scan lengths allowed in helical mode, the respective contributions reached 53, 88 and 67% because for the six-slice scanner the smallest scan length was limited to twice the collimation, whereas in the quad and dual scanners no limitation existed. For small scan lengths the presence of overscan cancels out any dose reduction offered by helical scanning with pitch factor values larger than one and therefore the axial mode should be preferred, when this is not prohibited by the diagnostic task in question.

Patient dose considerations in computed tomography examinations.

Ionizing radiation is extensively used in medicine and its contribution to both diagnosis and therapy is undisputable. However, the use of ionizing radiation also involves a certain risk since it may cause damage to tissues and organs and trigger carcinogenesis. Computed tomography (CT) is currently one of the major contributors to the collective population radiation dose both because it is a relatively high dose examination and an increasing number of people are subjected to CT examinations many times during their lifetime. The evolution of CT scanner technology has greatly increased the clinical applications of CT and its availability throughout the world and made it a routine rather than a specialized examination. With the modern multislice CT scanners, fast volume scanning of the whole human body within less than 1 min is now feasible. Two dimensional images of superb quality can be reconstructed in every possible plane with respect to the patient axis (e.g. axial, sagital and coronal). Furthermore, three-dimensional images of all anatomic structures and organs can be produced with only minimal additional effort (e.g. skeleton, tracheobronchial tree, gastrointestinal system and cardiovascular system). All these applications, which are diagnostically valuable, also involve a significant radiation risk. Therefore, all medical professionals involved with CT, either as referring or examining medical doctors must be aware of the risks involved before they decide to prescribe or perform CT examinations. Ultimately, the final decision concerning justification for a prescribed CT examination lies upon the radiologist. In this paper, we summarize the basic information concerning the detrimental effects of ionizing radiation, as well as the CT dosimetry background. Furthermore, after a brief summary of the evolution of CT scanning, the current CT scanner technology and its special features with respect to patient doses are given in detail. Some numerical data is also given in order to comprehend the magnitude of the potential radiation risk involved in comparison with risk from exposure to natural background radiation levels.

Comparison of measured and calculated skin doses in CT-guided interventional procedures.

OBJECTIVE: The objective of our study was to compare theoretic estimations of the dose to the patient's skin during CT-guided interventional procedures with measurements performed using radiation therapy verification films. MATERIALS AND METHODS: In each of the 12 interventions studied, a Kodak EDR2 film was positioned under the patient's anatomic area of concern. After processing, each film was scanned with a medical-grade scanner to produce a digital image from which the gray-scale profiles were obtained using the appropriate software. From these data and respective data from a series of calibration films, the entrance skin dose (ESD) profiles were derived. These ESD profiles were compared with the ESD profiles produced using a theoretic model and its revised version, which utilizes the DICOM data of each slice (i.e., tube potential, tube loading, slice thickness, slice location, pitch, and table height) and air-kerma output measurements from the CT tube. RESULTS: In general, the ESD profiles calculated using the revised theoretic method were in better agreement with the profiles derived from the verification films than the ESD profiles derived from the original theoretic method. The deviations from the peak skin doses (PSDs) derived from the digital film images were within -3% and 27% of the PSDs derived from the verification films. The respective deviations of the ESD profiles calculated with the original theoretic method were quite larger. CONCLUSION: The theoretic model provides a useful tool for estimating skin doses during CT-guided interventions with a reasonable level of accuracy. With further refinement and a little automation this method could be implemented for daily use.