Dose assessment following an overexposure of a worker at a Swiss nuclear power plant.

AIM: The aim of this work was to assess the doses received by a diver exposed to a radiation source during maintenance work in the fuel transfer pool at a Swiss nuclear power plant, and to define whether the statutory limit was breached or not. METHOD: Onsite measurements were carried out and different scenarios were simulated using the MicroShield Software and the MCNPX Monte Carlo radiation transport code to estimate the activity of the irradiating object as well as the doses to the limbs and the effective dose delivered to the operator. RESULTS: The activity of the object was estimated to 1.8 TBq. From the various dose estimations, a conservative value of 7.5 Sv was proposed for the equivalent dose to the skin on the hands and an effective dose of 28 mSv. CONCLUSION: The use of different experimental and calculation methods allowed us to accurately estimate the activity of the object and the dose delivered to the diver, useful information for making a decision on the most appropriate scheme of follow up for the patient.

Exposure of the Swiss population to computed tomography.

BACKGROUND: The frequency of CT procedures has registered a significant increase over the last decade, which led at the international level to an increasing concern on the radiological risk associated with the use of CT especially in paediatrics. This work aimed at investigating the use of computed tomography in Switzerland, following the evolution of CT frequency and dose data over a decade and comparing it to data reported in other countries. METHODS: The frequency and dose data related to CT are obtained by means of a nationwide survey. National frequencies were established by projecting the collected data, using the ratio of the number of CT units belonging to the respondents to the total number of CT units in the country. The effective doses per examination were collected during an auditing campaign. RESULTS: In 2008 about 0.8 Million CT procedures (~100 CT examinations/1000 population) were performed in the country, leading to a collective effective dose of more than 6000 man.Sv (0.8 mSv/caput). In a decade the frequency of CT examinations averaged over the population and the associated average effective dose per caput increased by a factor of 2.2 and 2.9 respectively. CONCLUSIONS: Although the contribution of CT to the total medical X-rays is 6% in terms of the frequency, it represents 68% in terms of the collective effective dose. These results are comparable to those reported in a number of countries in Europe and America with similar health level.

Model-based iterative reconstruction in pediatric chest CT: assessment of image quality in a prospective study of children with cystic fibrosis.

BACKGROUND: The potential effects of ionizing radiation are of particular concern in children. The model-based iterative reconstruction VEO(TM) is a technique commercialized to improve image quality and reduce noise compared with the filtered back-projection (FBP) method. OBJECTIVE: To evaluate the potential of VEO(TM) on diagnostic image quality and dose reduction in pediatric chest CT examinations. MATERIALS AND METHODS: Twenty children (mean 11.4 years) with cystic fibrosis underwent either a standard CT or a moderately reduced-dose CT plus a minimum-dose CT performed at 100 kVp. Reduced-dose CT examinations consisted of two consecutive acquisitions: one moderately reduced-dose CT with increased noise index (NI = 70) and one minimum-dose CT at CTDIvol 0.14 mGy. Standard CTs were reconstructed using the FBP method while low-dose CTs were reconstructed using FBP and VEO. Two senior radiologists evaluated diagnostic image quality independently by scoring anatomical structures using a four-point scale (1 = excellent, 2 = clear, 3 = diminished, 4 = non-diagnostic). Standard deviation (SD) and signal-to-noise ratio (SNR) were also computed. RESULTS: At moderately reduced doses, VEO images had significantly lower SD (P < 0.001) and higher SNR (P < 0.05) in comparison to filtered back-projection images. Further improvements were obtained at minimum-dose CT. The best diagnostic image quality was obtained with VEO at minimum-dose CT for the small structures (subpleural vessels and lung fissures) (P < 0.001). The potential for dose reduction was dependent on the diagnostic task because of the modification of the image texture produced by this reconstruction. CONCLUSIONS: At minimum-dose CT, VEO enables important dose reduction depending on the clinical indication and makes visible certain small structures that were not perceptible with filtered back-projection.

Usefulness of specific calibration coefficients for gamma-emitting sources measured by radionuclide calibrators in nuclear medicine.

PURPOSE: In nuclear medicine, the activity of a radionuclide is measured with a radionuclide calibrator that often has a calibration coefficient independent of the container type and filling. METHODS: To determine the effect of the container on the accuracy of measuring the activity injected into a patient, The authors simulated a commercial radionuclide calibrator and 18 container types most typically used in clinical practice. The instrument sensitivity was computed for various container thicknesses and filling levels. Monoenergetic photons and electrons as well as seven common radionuclides were considered. RESULTS: The quality of the simulation with gamma-emitting sources was validated by an agreement with measurements better than 4% in five selected radionuclides. The results show that the measured activity can vary by more than a factor of 2 depending on the type of container. The filling level and the thickness of the container wall only have a marginal effect for radionuclides of high energy but could induce differences up to 4%. CONCLUSIONS: The authors conclude that radionuclide calibrators should be tailored to the uncertainty required by clinical applications. For most clinical cases, and at least for the low-energy gamma and x-ray emitters, measurements should be performed with calibration coefficients specific to the container type.

Paediatric cardiac CT examinations: impact of the iterative reconstruction method ASIR on image quality--preliminary findings.

BACKGROUND: Radiation dose exposure is of particular concern in children due to the possible harmful effects of ionizing radiation. The adaptive statistical iterative reconstruction (ASIR) method is a promising new technique that reduces image noise and produces better overall image quality compared with routine-dose contrast-enhanced methods. OBJECTIVE: To assess the benefits of ASIR on the diagnostic image quality in paediatric cardiac CT examinations. MATERIALS AND METHODS: Four paediatric radiologists based at two major hospitals evaluated ten low-dose paediatric cardiac examinations (80 kVp, CTDI(vol) 4.8-7.9 mGy, DLP 37.1-178.9 mGy·cm). The average age of the cohort studied was 2.6 years (range 1 day to 7 years). Acquisitions were performed on a 64-MDCT scanner. All images were reconstructed at various ASIR percentages (0-100%). For each examination, radiologists scored 19 anatomical structures using the relative visual grading analysis method. To estimate the potential for dose reduction, acquisitions were also performed on a Catphan phantom and a paediatric phantom. RESULTS: The best image quality for all clinical images was obtained with 20% and 40% ASIR (p < 0.001) whereas with ASIR above 50%, image quality significantly decreased (p < 0.001). With 100% ASIR, a strong noise-free appearance of the structures reduced image conspicuity. A potential for dose reduction of about 36% is predicted for a 2- to 3-year-old child when using 40% ASIR rather than the standard filtered back-projection method. CONCLUSION: Reconstruction including 20% to 40% ASIR slightly improved the conspicuity of various paediatric cardiac structures in newborns and children with respect to conventional reconstruction (filtered back-projection) alone.

On the interplay effect for moving targets treated with the CyberKnife static tracking system.

PURPOSE: To assess the interplay effect amplitude between different planned MU distributions and respiratory patterns in the CyberKnife system when treating moving targets with static tracking technique. METHODS: Small- and Large-Respiratory Motions (SRM and LRM) differing in amplitude and frequency were simulated in a semi-anthropomorphic dynamic thorax phantom. The interplay effect was evaluated for both respiration motions in terms of GTV coverage and conformity for three plans designed with an increasing range of MU per beam (small, medium and large). Each plan was delivered three times changing the initial beam-on phase to assess the inter-fraction variation. Dose distributions were measured using radiochromic films placed in the GTV axial and sagittal planes. RESULTS: Generally, SRM plans gave higher GTV coverage and were less dependent on beam-on phases than LRM plans. For SRM (LRM) plans, the GTV coverage ranged from 95.2% to 99.7% (85.9% to 99.8%). Maximum GTV coverage was found for large MU plans in SRM and for small MU plans in LRM. Minimum GTV coverage was found for medium MU plans for both SRM and LRM. For SRM plans, dose conformity decreased with increasing MU range while the variation was reduced for LRM plans. Large MU plans reduced the inter-fraction variation for SRM and LRM. CONCLUSIONS: We confirmed the interplay effect between target motion and beam irradiation time for CyberKnife static tracking. Plans with large MU per beam improved the GTV coverage for small motion amplitude and the inter-fraction dose variation for large motion amplitude.

A delivery quality assurance tool based on the actual leaf open times in tomotherapy.

PURPOSE: To validate a delivery quality assurance (DQA) protocol for tomotherapy based on the measurement of the leaf open times (LOTs). In addition, to show the correlation between the mean relative LOT discrepancy and the dose deviation in the planning target volume (PTV). MATERIALS AND METHODS: We used a LOT measurement algorithm presented in a previous work on our two tomotherapy treatment units (TOMO1 and TOMO2). We generated TomoPhant plans with intentional random LOT discrepancies following Gaussian distributions of -6%, -4%, -2%, 2%, 4%, and 6%. We irradiated them on the Cheese Phantom with two ion chambers and collected the raw data on both our treatment units. Using the raw data, we measured the actual LOTs and verified that the induced discrepancies were highlightable. Then, we calculated the actual dose using Accuray's standalone dose calculator and verified that the calculated dose agreed with the ion chamber measurement. We randomly chose 60 clinical treatment plans, delivered them in air, and collected the raw detector data. We measured the actual LOTs from the raw data and calculated the corresponding dose distributions using Accuray's standalone dose calculator. We assessed the Pearson coefficient correlation of the deviation between expected and actual dose in the PTV (a) with the mean relative LOT discrepancy and (b) with the γ-index pass rate for different tolerances. RESULTS: The mean relative discrepancy between actual (measured by the algorithm) and expected LOTs on the modified TomoPhant plans was 1.10 ± 0.05% on TOMO1 and 0.02 ± 0.03% on TOMO2, respectively. The agreement between measured and calculated dose was 0.2 ± 0.3% on TOMO1 and 0.1 ± 0.3% on TOMO2, respectively. On clinical plans, the means of the relative LOT discrepancies ranged from -3.0 % to 1.4%. The dose deviation in the PTVs ranged from -1.6% to 2.4%. The Pearson coefficient correlation between the mean relative LOT discrepancy and the dose deviation in the PTV was 0.76 (P ≈ 10-15 ) on TOMO1 and 0.65 (P ≈ 10-10 ) on TOMO2, respectively. There was no correlation between the γ-index pass rate and the dose deviation in the PTV. CONCLUSION: The method made it possible to measure and to correctly highlight the LOT discrepancies on the TomoPhant plans. The dose subsequently calculated was accurate. On clinical plans, the mean LOT discrepancy correlated with the dose deviation in the PTV. This makes the mean LOT discrepancy a handy indicator of the plan quality.

Measurement of the useful field of view for single slices of different imaging modalities and targets.

Purpose: With three-dimensional (3-D) images displayed as stacks of 2-D images, radiologists rely more heavily on vision away from their fixation point to visually process information, guide eye movements, and detect abnormalities. Thus the ability to detect targets away from the fixation point, commonly characterized as the useful field of view (UFOV), becomes critical for these 3-D imaging modalities. We investigate how the UFOV, defined as the eccentricity, in which detection performance degrades to a given probability, varies across imaging modalities and targets. Approach: We measure the detectability of different targets at various distances from gaze locations for single slices of liver computed tomography (CT), 2-D digital mammograms (DM), and single slices of digital breast tomosynthesis (DBT) cases. Observers with varying expertise were instructed to maintain their gaze at a point while a short display of the image was flashed and an eye tracker verified observer's steady fixation. Display times were 200 and 1000 ms for CT images and 500 ms for DM and DBT images. Results: We find variations in the UFOV from 9 to 12 deg for liver CT to as small as 2.5 to 5 deg for calcification clusters in breast images (DM and DBT). We compare our results to those reported in the literature for lung nodules and discuss the differences across methods used to measure the UFOV, their dependence on case selection/task difficulty, viewing conditions, and observer expertise. We propose a complementary measure defined in terms of performance degradation relative to the peak foveal performance (relative-UFOV) to circumvent UFOV's variations with case selection/task difficulty. Conclusion: Our results highlight the variations in the UFOV across imaging modalities, target types, observer expertise, and measurement methods and suggest an additional relative-UFOV measure to more thoroughly characterize the detection performance away from point of fixation.

Search of low-contrast liver lesions in abdominal CT: the importance of scrolling behavior.

Purpose: Visual search using volumetric images is becoming the standard in medical imaging. However, we do not fully understand how eye movement strategies mediate diagnostic performance. A recent study on computed tomography (CT) images showed that the search strategies of radiologists could be classified based on saccade amplitudes and cross-quadrant eye movements [eye movement index (EMI)] into two categories: drillers and scanners. Approach: We investigate how the number of times a radiologist scrolls in a given direction during analysis of the images (number of courses) could add a supplementary variable to use to characterize search strategies. We used a set of 15 normal liver CT images in which we inserted 1 to 5 hypodense metastases of two different signal contrast amplitudes. Twenty radiologists were asked to search for the metastases while their eye-gaze was recorded by an eye-tracker device (EyeLink1000, SR Research Ltd., Mississauga, Ontario, Canada). Results: We found that categorizing radiologists based on the number of courses (rather than EMI) could better predict differences in decision times, percentage of image covered, and search error rates. Radiologists with a larger number of courses covered more volume in more time, found more metastases, and made fewer search errors than those with a lower number of courses. Our results suggest that the traditional definition of drillers and scanners could be expanded to include scrolling behavior. Drillers could be defined as scrolling back and forth through the image stack, each time exploring a different area on each image (low EMI and high number of courses). Scanners could be defined as scrolling progressively through the stack of images and focusing on different areas within each image slice (high EMI and low number of courses). Conclusions: Together, our results further enhance the understanding of how radiologists investigate three-dimensional volumes and may improve how to teach effective reading strategies to radiology residents.

Dose indicator for CyberKnife image-guided radiation therapy.

PURPOSE: The purpose of this study was to calculate dose distributions from CyberKnife image-guided radiation therapy (IGRT) for brain, H&N, lung, and pelvis treatment regions and use them to extract the corresponding effective dose and estimate-related risk. METHODS: We developed a CyberKnife IGRT kV beam model in a standard treatment planning system and validated it against measurements in heterogeneous phantoms. Five brain, five head and neck, five thorax, and 10 (five male and five female) pelvis patient computed tomographies (CTs) were contoured. The dose distribution resulting from different CyberKnife IGRT protocols was calculated. From them, the effective dose was calculated according to ICRP publication Nr 103, using the average dose to contoured organs. The corresponding risk factors were calculated. Entrance surface dose (ESD) was also calculated and compared with existing data. RESULTS: The maximum effective dose produced by CyberKnife IGRT protocols was 0.8 mSv (brain), 1.9 mSv (H&N), 20.2 (pelvis), and 42.4 mSv (thorax) per fraction for a risk estimate of 0.004% (brain), 0.01% (H&N), 0.1% (pelvis), and 0.2% (thorax). Calculated ESD were compatible with existing data. CONCLUSIONS: Dose calculation models for CyberKnife IGRT kV beams were implemented in a clinical treatment planning system and validated in water and heterogeneous phantoms. We determined the effective dose and the related risk estimate resulting from CyberKnife IGRT protocols for brain, head and neck, thorax, and pelvis cases. The effective doses calculated for CyberKnife IGRT protocols were similar to those obtained for cone beam CT protocols on conventional C-arm linear accelerators, except for extreme irradiation conditions for thorax cases (140 kV X-ray tube tension).

An absolute dose determination of helical tomotherapy accelerator, TomoTherapy High-Art II.

PURPOSE: A helical tomotherapy accelerator presents a dosimetric challenge because, to this day, there is no internationally accepted protocol for the determination of the absolute dose. Because of this reality, we investigated the different alternatives for characterizing and measuring the absolute dose of such an accelerator. We tested several dosimetric techniques with various metrological traceabilities as well as using a number of phantoms in static and helical modes. METHODS: Firstly, the relationship between the reading of ionization chambers and the absorbed dose is dependent on the beam quality value of the photon beam. For high energy photons, the beam quality is specified by the tissue phantom ratio (TPR20,10) and it is therefore necessary to know the TPR20,10 to calculate the dose delivered by a given accelerator. This parameter is obtained through the ratio of the absorbed dose at 20 and 10 cm depths in water and was measured in the particular conditions of the tomotherapy accelerator. Afterward, measurements were performed using the ionization chamber (model A1SL) delivered as a reference instrument by the vendor. This chamber is traceable in absorbed dose to water in a Co-60 beam to a water calorimeter of the American metrology institute (NIST). Similarly, in Switzerland, each radiotherapy department is directly traceable to the Swiss metrology institute (METAS) in absorbed dose to water based on a water calorimeter. For our research, this traceability was obtained by using an ionization chamber traceable to METAS (model NE 2611A), which is the secondary standard of our institute. Furthermore, in order to have another fully independent measurement method, we determined the dose using alanine dosimeters provided by and traceable to the British metrology institute (NPL); they are calibrated in absorbed dose to water using a graphite calorimeter. And finally, we wanted to take into account the type of chamber routinely used in clinical practice and therefore measured the dose using a Farmer-type instrument (model NE 2571) as well. RESULTS: We found the tomotherapy TPR20,10 value to be around 0.629, which is close to a 4 MV conventional linear accelerator value. During static irradiation, the secondary standard and the alanine dosimeters were compatible within 0.5%. The A1SL relative deviation to the secondary standard was 1.2% and the NE2571 relative deviation to the secondary standard was -1.7%. The measurement in dynamic helical mode found the different dosimeters compatible within 1.4% and the alanine dosimeters and the secondary standard were even found under 0.2%. CONCLUSIONS: We found that the different methods are all within uncertainties as well as globally coherent, and the specific limitations of the various dosimeters are discussed in order to help the medical physicist design an independent reference system. We demonstrated that, taking into account the particular reference conditions, one can use an ionization chamber calibrated for conventional linear accelerators to assert the absolute dose delivered by a tomotherapy accelerator.

In air and in vivo measurement of the leaf open time in tomotherapy using the on-board detector pulse-by-pulse data.

PURPOSE: We developed an algorithm to measure the leaf open times (LOT) from the on-board detector (OBD) pulse-by-pulse data in tomotherapy. We assessed the feasibility of measuring the LOTs in dynamic jaw mode and validated the algorithm on machine QA and clinical data. Knowledge of the actual LOTs is a basis toward calculating the delivered dose and performing efficient phantom-less delivery quality assurance (DQA) controls of the multileaf collimator (MLC). In tomotherapy, the quality of the delivered dose depends on the correct performance of the MLC, hence on the accuracy of the LOTs. MATERIALS AND METHODS: In the detector signal, the period of time during which a leaf is open corresponds to a high intensity region. The algorithm described here locally normalizes the detector signal and measures the FWHM of the high intensity regions. The Daily QA module of the TomoTherapy Quality Assurance (TQA) tool measures LOT errors. The Daily QA detector data were collected during 9 days on two tomotherapy units. The errors yielded by the method were compared to these reported by the Daily QA module. In addition, clinical data were acquired on the two units (25 plans in total), in air without attenuation material in the beam path and in vivo during a treatment fraction. The study included plans with fields of all existing sizes (1.05, 2.51, 5.05 cm). The collimator jaws were in dynamic mode (TomoEDGETM ). The feasibility of measuring the LOTs was assessed with respect to the jaw aperture. RESULTS: The mean discrepancy between LOTs measured by the algorithm and those measured by TQA was of 0 ms, with a standard deviation of 0.3 ms. The LOT measured by the method had thus an uncertainty of 1 ms with a confidence level of 99%. In 5.05 cm dynamic jaw procedures, the detector is in the beam umbra at the beginning and at the end of the delivery. In such procedures, the algorithm could not measure the LOTs at jaw apertures between 7 and maximum 12.4 mm. Otherwise, no measurement error due to the jaw movement was observed. No LOT measurement difference between air and in vivo data was observed either. CONCLUSION: The method we propose is reliable. It can equivalently measure the LOTs from data acquired in air or in vivo. It handles fully the static procedures and the 2.51 cm dynamic procedures. It handles partially the 5.05 cm dynamic procedures. The limitation was evaluated with respect to the jaw aperture.

Channelized Hotelling observer correlation with human observers for low-contrast detection in liver CT images.

Task-based image quality procedures in CT that substitute a human observer with a model observer usually use single-slice images with uniform backgrounds from homogeneous phantoms. However, anatomical structures and inhomogeneities in organs generate noise that can affect the detection performance of human observers. The purpose of this work was to assess the impact of background type, uniform or liver, and the viewing modality, single- or multislice, on the detection performance of human and model observers. We collected abdominal CT scans from patients and homogeneous phantom scans in which we digitally inserted low-contrast signals that mimicked a liver lesion. We ran a rating experiment with the two background conditions with three signal sizes and three human observers presenting images in two reading modalities: single- and multislice. In addition, channelized Hotelling observers (CHO) for single- and multislice detection were implemented and evaluated according to their degree of correlation with the human observer performance. For human observers, there was a small but significant improvement in performance with multislice compared to the single-slice viewing mode. Our data did not reveal a significant difference between uniform and anatomical backgrounds. Model observers demonstrated a good correlation with human observers for both viewing modalities. Human observers have very similar performances in both multi- and single-slice viewing mode. It is therefore preferable to use single-slice CHO as this model is computationally more tractable than multislice CHO. However, using images from a homogeneous phantom can result in overestimating image quality as CHO performance tends to be higher in uniform than anatomical backgrounds, while human observers have similar detection performances.

Novel inverse planning optimization algorithm for robotic radiosurgery: First clinical implementation and dosimetric evaluation.

PURPOSE: A novel optimization algorithm (VOLO™) for robotic radiosurgery in the Precision™ treatment planning system was evaluated for different SRS/SBRT treatments and compared with the previous Sequential Optimization (SO) algorithm. MATERIALS AND METHODS: Fifty cases of brain, spine, prostate and lung tumors previously optimized with SO, were re-planned with VOLO™ algorithm keeping the same prescription, collimator type and size, optimization shells, and blocking structures. The dosimetric comparison involved target coverage, conformity (CI), gradient (GI) and homogeneity indexes, specific indicators of dose to OARs and number of nodes, beams, MU and delivery time. For brain only, plans were IRIS- and MLC-based (10 each). The remaining 30 plans were all IRIS-based. RESULTS: VOLO™ optimization was significantly superior for target coverage for prostate and spine, CI for brain, and for brain and urethra dose sparing. SO gave significantly better results for GI for prostate. VOLO™ showed a significantly steeper dose fall-off for brain MLC-based, while for prostate and spine SO was superior. For IRIS-based plans, VOLO™ significantly reduced the nodes (36%), beams (14%), and MU (31%). This led to an average reduction of delivery time of 20% (from 8% for brain to 30% for prostate). For MLC-based plans, VOLO™ significantly increased the nodes and beams (42%) keeping the same number of MU. The averaged delivery time increased by 18%. CONCLUSIONS: With respect to SO, VOLO™ optimization algorithm provided better results in terms of delivery time for IRIS-based and of quality of dose distribution for MLC-based plans, respectively.

Mammographic texture synthesis: second-generation clustered lumpy backgrounds using a genetic algorithm.

Synthetic yet realistic images are valuable for many applications in visual sciences and medical imaging. Typically, investigators develop algorithms and adjust their parameters to generate images that are visually similar to real images. In this study, we used a genetic algorithm and an objective, statistical similarity measure to optimize a particular texture generation algorithm, the clustered lumpy backgrounds (CLB) technique, and synthesize images mimicking real mammograms textures. We combined this approach with psychophysical experiments involving the judgment of radiologists, who were asked to qualify the visual realism of the images. Both objective and psychophysical approaches show that the optimized versions are significantly more realistic than the previous CLB model. Anatomical structures are well reproduced, and arbitrary large databases of mammographic texture with visual and statistical realism can be generated. Potential applications include detection experiments, where large amounts of statistically traceable yet realistic images are needed.

Inter-laboratory comparison of channelized hotelling observer computation.

PURPOSE: The task-based assessment of image quality using model observers is increasingly used for the assessment of different imaging modalities. However, the performance computation of model observers needs standardization as well as a well-established trust in its implementation methodology and uncertainty estimation. The purpose of this work was to determine the degree of equivalence of the channelized Hotelling observer performance and uncertainty estimation using an intercomparison exercise. MATERIALS AND METHODS: Image samples to estimate model observer performance for detection tasks were generated from two-dimensional CT image slices of a uniform water phantom. A common set of images was sent to participating laboratories to perform and document the following tasks: (a) estimate the detectability index of a well-defined CHO and its uncertainty in three conditions involving different sized targets all at the same dose, and (b) apply this CHO to an image set where ground truth was unknown to participants (lower image dose). In addition, and on an optional basis, we asked the participating laboratories to (c) estimate the performance of real human observers from a psychophysical experiment of their choice. Each of the 13 participating laboratories was confidentially assigned a participant number and image sets could be downloaded through a secure server. Results were distributed with each participant recognizable by its number and then each laboratory was able to modify their results with justification as model observer calculation are not yet a routine and potentially error prone. RESULTS: Detectability index increased with signal size for all participants and was very consistent for 6 mm sized target while showing higher variability for 8 and 10 mm sized target. There was one order of magnitude between the lowest and the largest uncertainty estimation. CONCLUSIONS: This intercomparison helped define the state of the art of model observer performance computation and with thirteen participants, reflects openness and trust within the medical imaging community. The performance of a CHO with explicitly defined channels and a relatively large number of test images was consistently estimated by all participants. In contrast, the paper demonstrates that there is no agreement on estimating the variance of detectability in the training and testing setting.

Semiautomatic mammographic parenchymal patterns classification using multiple statistical features.

RATIONALE AND OBJECTIVES: Our project was to investigate a complete methodology for the semiautomatic assessment of digital mammograms according to their density, an indicator known to be correlated to breast cancer risk. The BI-RADS four-grade density scale is usually employed by radiologists for reporting breast density, but it allows for a certain degree of subjective input, and an objective qualification of density has therefore often been reported hard to assess. The goal of this study was to design an objective technique for determining breast BI-RADS density. MATERIALS AND METHODS: The proposed semiautomatic method makes use of complementary pattern recognition techniques to describe manually selected regions of interest (ROIs) in the breast with 36 statistical features. Three different classifiers based on a linear discriminant analysis or Bayesian theories were designed and tested on a database consisting of 1408 ROIs from 88 patients, using a leave-one-ROI-out technique. Classifications in optimal feature subspaces with lower dimensionality and reduction to a two-class problem were studied as well. RESULTS: Comparison with a reference established by the classifications of three radiologists shows excellent performance of the classifiers, even though extremely dense breasts continue to remain more difficult to classify accurately. For the two best classifiers, the exact agreement percentages are 76% and above, and weighted kappa values are 0.78 and 0.83. Furthermore, classification in lower dimensional spaces and two-class problems give excellent results. CONCLUSION: The proposed semiautomatic classifiers method provides an objective and reproducible method for characterizing breast density, especially for the two-class case. It represents a simple and valuable tool that could be used in screening programs, training, education, or for optimizing image processing in diagnostic tasks.

Primary activity measurements with 4pigamma NaI(Tl) counting and Monte Carlo calculated efficiencies.

The radioactive concentrations of (18)F, (88)Y and (152)Eu solutions and the activity of (222)Rn gas ampoules are measured using a primary method involving 4pigamma NaI(Tl) integral counting with a well-type NaI(Tl) detector and efficiencies computed by Monte Carlo simulations. The simulations use the GEANT code coupled with a routine (sch2for), which generates randomly the decay paths and emissions depending on the decay scheme parameters. The resulting radioactive concentrations of (88)Y, (152)Eu and (18)F are found to agree with those measured with other primary measurement methods, such as 4pi (beta, e, X)-gamma coincidence counting or liquid scintillation counting. Results of the determination of the activity of (222)Rn gas ampoules by this method also match the results of an absolute standardisation technique in which radon is condensed onto a cold surface and its alpha-emissions are detected through an accurately specified solid angle.

Assessment of low contrast detection in CT using model observers: Developing a clinically-relevant tool for characterising adaptive statistical and model-based iterative reconstruction.

PURPOSE: This study aims to assess CT image quality in a way that would meet specific requirements of clinical practice. Physics metrics like Fourier transform derived metrics were traditionally employed for that. However, assessment methods through a detection task have also developed quite extensively lately, and we chose here to rely on this modality for image quality assessment. Our goal was to develop a tool adapted for a fast and reliable CT image quality assessment in order to pave the way for new CT benchmarking techniques in a clinical context. Additionally, we also used this method to estimate the benefits brought by some IR algorithms. MATERIALS AND METHODS: A modified QRM chest phantom containing spheres of 5 and 8mm at contrast levels of 10 and 20HU at 120kVp was used. Images of the phantom were acquired at CTDIvol of 0.8, 3.6, 8.2 and 14.5mGy, before being reconstructed using FBP, ASIR 40 and MBIR on a GE HD 750 CT scanner. They were then assessed by eight human observers undergoing a 4-AFC test. After that, these data were compared with the results obtained from two different model observers (NPWE and CHO with DDoG channels). The study investigated the effects of the acquisition conditions as well as reconstruction methods. RESULTS: NPWE and CHO models both gave coherent results and approximated human observer results well. Moreover, the reconstruction technique used to retrieve the images had a clear impact on the PC values. Both models suggest that switching from FBP to ASIR 40 and particularly to MBIR produces an increase of the low contrast detection, provided a minimum level of exposure is reached. CONCLUSION: Our work shows that both CHO with DDoG channels and NPWE models both approximate the trend of humans performing a detection task. Both models also suggest that the use of MBIR goes along with an increase of the PCs, indicating that further dose reduction is still possible when using those techniques. Eventually, the CHO model associated to the protocol we described in this study happened to be an efficient way to assess CT images in a clinical environment. In the future, this simple method could represent a sound basis to benchmark clinical practice and CT devices.