Christina Söderman

Image quality dependency on system configuration and tube voltage in chest tomosynthesis—A visual grading study using an anthropomorphic chest phantom

PURPOSE To investigate the potential benefit of increasing the dose per projection image in chest tomosynthesis, performed at the current standard dose level, by reducing the angular range covered or the projection image density and to evaluate the influence of the tube voltage on the image quality. METHODS An anthropomorphic chest phantom was imaged using nine different projection image configurations and ten different tube voltages with the GE VolumeRAD tomosynthesis system. The resulting image sets were representative of being acquired at the same total effective dose. This was achieved partly by applying a simulated dose reduction to the projection images due to restrictions concerning the tube load settings on the VolumeRAD system. Four observers were included in a visual grading study where the reconstructed tomosynthesis section images were rated according to a set of image quality criteria. Image quality was evaluated relative to the default configuration and default tube voltage on the VolumeRAD system. RESULTS Overall, the image quality decreased with decreasing projection image density. Regarding angular range covered by the projection images, the image quality increased with decreasing angular range for two of the criteria, whereas for a criterion related to the depth resolution in the section images the reduced angular ranges resulted in inferior image quality as compared to the default configuration. The image quality showed little dependence on the tube voltage. CONCLUSIONS At the standard dose level of the VolumeRAD system, the potential benefits from increasing the dose per projection do not fully compensate for the negative effects resulting from a reduction in the number of projection images. Consequently, the default configuration consisting of 60 projection images acquired over 30° is a good alternative. The tube voltage used in tomosynthesis does not have a large impact on the image quality.

A simple method to retrospectively estimate patient dose-area product for chest tomosynthesis examinations performed using VolumeRAD

PURPOSE The purpose of the present work was to develop and validate a method of retrospectively estimating the dose-area product (DAP) of a chest tomosynthesis examination performed using the VolumeRAD system (GE Healthcare, Chalfont St. Giles, UK) from digital imaging and communications in medicine (DICOM) data available in the scout image. METHODS DICOM data were retrieved for 20 patients undergoing chest tomosynthesis using VolumeRAD. Using information about how the exposure parameters for the tomosynthesis examination are determined by the scout image, a correction factor for the adjustment in field size with projection angle was determined. The correction factor was used to estimate the DAP for 20 additional chest tomosynthesis examinations from DICOM data available in the scout images, which was compared with the actual DAP registered for the projection radiographs acquired during the tomosynthesis examination. RESULTS A field size correction factor of 0.935 was determined. Applying the developed method using this factor, the average difference between the estimated DAP and the actual DAP was 0.2%, with a standard deviation of 0.8%. However, the difference was not normally distributed and the maximum error was only 1.0%. The validity and reliability of the presented method were thus very high. CONCLUSIONS A method to estimate the DAP of a chest tomosynthesis examination performed using the VolumeRAD system from DICOM data in the scout image was developed and validated. As the scout image normally is the only image connected to the tomosynthesis examination stored in the picture archiving and communication system (PACS) containing dose data, the method may be of value for retrospectively estimating patient dose in clinical use of chest tomosynthesis.

Evaluation of Accuracy and Precision of Manual Size Measurements in Chest Tomosynthesis using Simulated Pulmonary Nodules

RATIONALE AND OBJECTIVES To investigate the accuracy and precision of pulmonary nodule size measurements on chest tomosynthesis images. MATERIALS AND METHODS Artificial ellipsoid-shaped nodules with known sizes were inserted in clinical chest tomosynthesis images. The volume of the nodules corresponded to that of a sphere with a diameter of 4.0, 8.0, or 12.0 mm. Four thoracic radiologists were given the task to determine the longest diameter of the nodules. All nodules were measured twice. Measurement accuracy in terms of the mean measurement error was determined. Intraobserver and interobserver variabilities, as well as variability because of differences between nodules and their locations, were used as measures of precision. RESULTS The mean measurement error ranged from -0.3 to 0.1 mm for the nodule size groups and observers. Of the smallest nodules, the observers found 7-17 of total 50 nodules nonmeasurable. The intraobserver and interobserver variabilities were of similar magnitude, indicating relatively small differences between the observers. The internodule variability was in general larger, indicating that the different characteristics of the nodules and their location are sources of variability. CONCLUSIONS The results suggest a high accuracy and precision for manual measurements of the nodules in chest tomosynthesis images. However, small nodules (<5.0 mm) may be difficult to measure at all because of poor visibility.

EFFECTIVE DOSE TO PATIENTS FROM THORACIC SPINE EXAMINATIONS WITH TOMOSYNTHESIS

The purposes of the present work were to calculate the average effective dose to patients from lateral tomosynthesis examinations of the thoracic spine, compare the results with the corresponding conventional examination and to determine a conversion factor between dose-area product (DAP) and effective dose for the tomosynthesis examination. Thoracic spine examinations from 17 patients were included in the study. The registered DAP and information about the field size for each projection radiograph were, together with patient height and mass, used to calculate the effective dose for each projection radiograph. The total effective doses for the tomosynthesis examinations were obtained by adding the effective doses from the 60 projection radiographs included in the examination. The mean effective dose was 0.47 mSv (range 0.24-0.81 mSv) for the tomosynthesis examinations and 0.20 mSv (range 0.07-0.29 mSv) for the corresponding conventional examinations (anteroposterior + left lateral projection). For the tomosynthesis examinations, a conversion factor between total DAP and effective dose of 0.092 mSv Gycm(-2) was obtained.

RETROSPECTIVE ESTIMATION OF PATIENT DOSE-AREA PRODUCT IN THORACIC SPINE TOMOSYNTHESIS PERFORMED USING VOLUMERAD.

The aim of this study was to evaluate the use of a recently developed method of retrospectively estimating the patient dose-area product (DAP) of a chest tomosynthesis examination, performed using VolumeRAD, in thoracic spine tomosynthesis and to determine the necessary field-size correction factor. Digital imaging and communications in medicine (DICOM) data for the projection radiographs acquired during a thoracic spine tomosynthesis examination were retrieved directly from the modality for 17 patients. Using the previously developed method, an estimated DAP for the tomosynthesis examination was determined from DICOM data in the scout image. By comparing the estimated DAP with the actual DAP registered for the projection radiographs, a field-size correction factor was determined. The field-size correction factor for thoracic spine tomosynthesis was determined to 0.92. Applying this factor to the DAP estimated retrospectively, the maximum difference between the estimated DAP and the actual DAP was <3 %. In conclusion, the previously developed method of retrospectively estimating the DAP in chest tomosynthesis can be applied to thoracic spine tomosynthesis.

EFFECT OF RADIATION DOSE LEVEL ON ACCURACY AND PRECISION OF MANUAL SIZE MEASUREMENTS IN CHEST TOMOSYNTHESIS EVALUATED USING SIMULATED PULMONARY NODULES

The aim of the present study was to investigate the dependency of the accuracy and precision of nodule diameter measurements on the radiation dose level in chest tomosynthesis. Artificial ellipsoid-shaped nodules with known dimensions were inserted in clinical chest tomosynthesis images. Noise was added to the images in order to simulate radiation dose levels corresponding to effective doses for a standard-sized patient of 0.06 and 0.04 mSv. These levels were compared with the original dose level, corresponding to an effective dose of 0.12 mSv for a standard-sized patient. Four thoracic radiologists measured the longest diameter of the nodules. The study was restricted to nodules located in high-dose areas of the tomosynthesis projection radiographs. A significant decrease of the measurement accuracy and intraobserver variability was seen for the lowest dose level for a subset of the observers. No significant effect of dose level on the interobserver variability was found. The number of non-measurable small nodules (≤5 mm) was higher for the two lowest dose levels compared with the original dose level. In conclusion, for pulmonary nodules at positions in the lung corresponding to locations in high-dose areas of the projection radiographs, using a radiation dose level resulting in an effective dose of 0.06 mSv to a standard-sized patient may be possible in chest tomosynthesis without affecting the accuracy and precision of nodule diameter measurements to any large extent. However, an increasing number of non-measurable small nodules (≤5 mm) with decreasing radiation dose may raise some concerns regarding an applied general dose reduction for chest tomosynthesis examinations in the clinical praxis.

An attempt to estimate out-of-plane lung nodule elongation in tomosynthesis images

In chest tomosynthesis (TS) the most commonly used reconstruction methods are based on Filtered Back Projection (FBP) algorithms. Due to the limited angular range of x-ray projections, FBP reconstructed data is typically associated with a low spatial resolution in the out-of-plane dimension. Lung nodule measures that depend on depth information such as 3D shape and volume are therefore difficult to estimate. In this paper the relation between features from FBP reconstructed lung nodules and the true out-of-plane nodule elongation is investigated and a method for estimating the out-of-plane nodule elongation is proposed. In order to study these relations a number of steps that include simulation of spheroidal-shaped nodules, insertion into synthetic data volumes, construction of TS-projections and FBP-reconstruction were performed. In addition, the same procedure was used to simulate nodules and insert them into clinical chest TS projection data. The reconstructed nodule data was then investigated with respect to in-plane diameter, out-of-plane elongation, and attenuation coefficient. It was found that the voxel value in each nodule increased linearly with nodule elongation, for nodules with a constant attenuation coefficient. Similarly, the voxel value increased linearly with in-plane diameter. These observations indicate the possibility to predict the nodule elongation from the reconstructed voxel intensity values. Such a method would represent a quantitative approach to chest tomosynthesis that may be useful in future work on volume and growth rate estimation of lung nodules.

Application of a computed tomography based cystic fibrosis scoring system to chest tomosynthesis

In the monitoring of progression of lung disease in patients with cystic fibrosis (CF), recurrent computed tomography (CT) examinations are often used. The relatively new imaging technique chest tomosynthesis (CTS) may be an interesting alternative in the follow-up of these patients due to its visualization of the chest in slices at radiation doses and costs significantly lower than is the case with CT. A first step towards introducing CTS imaging in the diagnostics of CF patients is to establish a scoring system appropriate for evaluating the severity of CF pulmonary disease based on findings in CTS images. Previously, several such CF scoring systems based on CT imaging have been published. The purpose of the present study was to develop a CF scoring system for CTS, by starting from an existing scoring system dedicated for CT images and making modifications regarded necessary to make it appropriate for use with CTS images. In order to determine any necessary changes, three thoracic radiologists independently used a scoring system dedicated for CT on both CT and CTS images from CF patients. The results of the scoring were jointly evaluated by all the observers, which lead to suggestions for changes to the scoring system. Suggested modifications include excluding the scoring of air trapping and doing the scoring of the findings in quadrants of the image instead of in each lung lobe.

Evaluation of a corrected implementation of a method of simulating pulmonary nodules in chest tomosynthesis

Background A method of simulating pulmonary nodules in tomosynthesis images has previously been developed and evaluated. An unknown feature of a rounding function included in the computer code was later found to introduce an artifact, affecting simulated nodules in low-signal regions of the images. The computer code has now been corrected. Purpose To perform a thorough evaluation of the corrected nodule-simulation method, comparing the detection rate and visual appearance of artificial nodules with those of real nodules in an observer performance experiment. Material and Methods A cohort of 64 patients with a total of 129 pulmonary nodules was used in the study. Artificial nodules, each matching a corresponding real nodule by size, attenuation, and anatomical location, were generated and simulated into the tomosynthesis images of the different patients. The detection rate and visual appearance of artificial nodules generated using both the corrected and uncorrected computer code were compared to those of real nodules. The results were evaluated using modified receiver operating characteristic (ROC) analyses. Results The difference in detection rate between artificial and real nodules slightly increased using the corrected computer code (uncorrected code: area under the curve [AUC], 0.47; 95% CI, 0.43–0.51; corrected code: AUC, 0.42; 95% CI, 0.38–0.46). The visual appearance was however substantially improved using the corrected computer code (uncorrected code: AUC, 0.70; 95% CI, 0.63–0.76; corrected code: AUC, 0.49; 95% CI, 0.29–0.65). Conclusion The computer code including a correct rounding function generates simulated nodules that are more visually realistic than simulated nodules generated using the uncorrected computer code, but have a slightly different detection rate compared to real nodules.

INFLUENCE OF THE IN-PLANE ARTEFACT IN CHEST TOMOSYNTHESIS ON PULMONARY NODULE SIZE MEASUREMENTS.

The aim of the present study was to investigate how the in-plane artefact present in the scan direction around structures in tomosynthesis images should be managed when measuring the size of nodules in chest tomosynthesis images in order to achieve acceptable measurement accuracy. Data from measurements, performed by radiologists, of the longest diameter of artificial nodules inserted in chest tomosynthesis images were used. The association between the measurement error and the direction of the longest nodule diameter, relative to the scan direction, was evaluated using the Kendall rank correlation coefficient. All of the radiologists had chosen to not include the artefact in the measurements. Significant association between measurement error and the direction of the longest diameter was found for nodules larger than 12 mm, which indicates that, for these nodules, there is a risk of underestimating the nodule size if the in-plane artefact is omitted from manual diameter measurements.