Margarita Chevalier

Patient dose in digital mammography.

In the present investigation, we analyze the dose of 5034 patients (20,137 images) who underwent mammographic examinations with a full-field digital mammography system. Also, we evaluate the system calibration by analyzing the exposure factors as a function of breast thickness. The information relevant to this study has been extracted from the image DICOM header and stored in a database during a 3-year period (March 2001-October 2003). Patient data included age, breast thickness, kVp, mAs, target/filter combination, and nominal dose values. Entrance surface air kerma (ESAK) without backscatter was calculated from the tube output as measured for each voltage used under clinical conditions and from the tube loading (mAs) included in the DICOM header. Mean values for the patient age and compressed breast thickness were 56 years (SD: 11) and 52 mm (SD: 13), respectively. The majority of the images was acquired using the STD (for standard) automatic mode (98%). The most frequent target/filter combination automatically selected for breast smaller than 35 mm was Mo/Mo (75%); for intermediate thicknesses between 35 and 65 mm, the combinations were Mo/Rh (54%) and Rh/Rh (38.5%); Rh/Rh was the combination selected for 91% of the cases for breasts thicker than 65 mm. A wide kVp range was observed for each target/filter combination. The most frequent values were 28 kVp for Mo/Mo, 29 kVp for Mo/Rh, and 29 and 30 kV for Rh/Rh. Exposure times ranged from 0.2 to 4.2 s with a mean value of 1.1 s. Average glandular doses (AGD) per exposure were calculated by multiplying the ESAK values by the conversion factors tabulated by Dance for women in the age groups 50 to 64 and 40 to 49. This approach is based on the dependence of breast glandularity on breast thickness and age. The total mean average glandular dose (AGD(T)) was calculated by summing the values associated with the pre-exposure and with the main exposure. Mean AGD(T) per exposure was 1.88 mGy (CI 0.01) and the mean AGD(T) per examination was 3.8 mGy, with 4 images per examination on average. The mean dose for cranio-caudal view (CC) images was 1.8 mGy, which is lower than that for medio-lateral oblique (MLO) view because the thickness for CC images was on average 10% lower than that for MLO images. Mean AGD(T) for the oldest group of women (1.90) was 3% higher than the AGD(T) for the younger group (1.85) due to the larger compressed breast thickness of women in the older group (10% on average). Differences between the corresponding AGD(T) values of each age group were lowest for breast thicknesses in the range 40-60 mm, being slightly higher for the women in the older group.

Evaluation of the technical performance of three different commercial digital breast tomosynthesis systems in the clinical environment

The aim of this work was to research and evaluate the performance of three different digital breast tomosynthesis (DBT) systems in the clinical environment (Siemens Mammomat Inspiration, Hologic Selenia Dimensions, and Fujifilm Amulet Innovality). The characterization included the study of the detector, the automatic exposure control, and the resolution of DBT projections and reconstructed planes. The modulation transfer function (MTF) of the DBT projections was measured with a 1mm thick steel edge, showing a strong anisotropy (30-40% lower MTF0.5 frequencies in the tube travel direction). The in-plane MTF0.5, measured with a 25μm tungsten wire, ranges from 1.3 to 1.8lp/mm in the tube-travel direction and between 2.4 and 3.7lp/mm in the chest wall-nipple. In the latter direction, the MTF peak shift is more emphasized for large angular range systems (2.0 versus 1.0lp/mm). In-depth resolution of the planes, via the full width at half maximum (FWHM) from the point spread function of a 25μm tungsten wire, is not only influenced by angular range and yields 1.3-4.6mm among systems. The artifact spread function from 1mm diameter tungsten beads depends mainly on angular range, yielding two tendencies whether large (FWHM is 4.5mm) or small (FWHM is 10mm) angular range is used. DBT delivers per scan a mean glandular dose between 1.4 and 2.7mGy for a 45mm thick polymethyl methacrylate (PMMA) block. In conclusion, we have identified and analysed specific metrics that can be used for quality assurance of DBT systems.

Comparative study of dose values and image quality in mammography in the area of Madrid

Dose values and image quality in mammography resulting from a study carried out in the area of Madrid during 1990-1991 are presented in a two-part sequence. In this paper are reported the values of air kerma and mean glandular dose per film estimated at 14 centres by means of the Leeds TOR(MAX) mammographic phantom and from patient samples. The results obtained from patient measurements allow us to deduce that the TOR(MAX) phantom assembly used (4.5 cm thick) is equivalent to an average 5 cm thick compressed breast from the population in the area of Madrid. Approximately 75% of the air kerma values resulting from phantom estimations are below 6.6 and 12.2 mGy for X-ray units with and without grid, respectively. The mean glandular doses per film for 75% of the units with and without grid are below 1.5 and 1.7 mGy, respectively. The results showed that the higher values of both air kerma and mean glandular dose obtained for the units without grid result from the use of slow speed recording systems. On the other hand, the quality control programme carried out shows important faults in the performance of the X-ray units which have an important impact on both dose and image quality. Finally, the third quartile values of the air kerma and mean glandular dose per film (10.4 and 1.7 mGy, respectively), deduced by considering all the units (with and without grid), are proposed to be the reference values of the mammographic examinations during the aforementioned period. In a second paper, the image quality is analysed by using the same Leeds TOR(MAX) phantom. Results for each test object included in the phantom are discussed as a function of the X-ray unit characteristics. Subsequently, the phantom images obtained with each X-ray system are scored and the resulting values are compared with the corresponding dose values, so that the dose-image quality relationship for each unit is obtained.

A CDMAM Image Phantom Software Improvement for Human Observer Assessment

A software tool is presented to improve the features of CDMAM image phantom by University Hospital Nijmegen. This software tool ensures that the 4-alternative forced choice method of CDMAM is actually kept, even when is being scored by highly expertise observers familiar on the test object pattern. For digital images, the developed software tool automatically changes the image position of the four corners. It can be selected a fixed rotation angle or a random one, so making impossible that any observer is able to remember the exact corner position of the target disc inside any cell. Two alternative successful algorithms have been tested. ROC curve analysis obtained by 36 observers shows that both original and computer-modified images are indistinguishable. The ROC area was 0.507±0.024 for first algorithm and 0.522±0.026 for the second one, indicating that there was no statistical difference between real and computer-modified images for both of them.

Edge image quality assessment: a new formulation for degraded edge imaging

We discuss a formalism to characterize degraded edge images formed in a diffraction limited system (with circular pupil of unit radius) under conditions of incoherent illumination. We introduce a novel definition of degraded edges and consider this approach to model a basic optical mechanism involved in the perception of visual depth and edge detection. We introduce a degradation parameter to quantize the degree of edge blur. We present a generalization of such a procedure by assuming the Heaviside function to be a systematic generator of degraded edges. We reproduce experimentally the predictions made by the formalism proposed herein.

Diagnostic value of the stand-alone synthetic image in digital breast tomosynthesis examinations

ObjectiveTo demonstrate the non-inferiority of synthetic image (SI) mammography versus full-field digital mammography (FFDM) in breast tomosynthesis (DBT) examinations.MethodsAn observational, retrospective, single-centre, multireader blinded study was performed, using 2384 images to directly compare SI and FFDM based on Breast Imaging Reporting and Data System (BIRADS) categorisation and visibility of radiological findings. Readers had no access to digital breast tomosynthesis slices. Multiple reader, multiple case (MRMC) receiver operating characteristic (ROC) methodology was used to compare the diagnostic performance of SI and FFDM images. The kappa statistic was used to estimate the inter-reader and intra-reader reliability.ResultsThe area under the ROC curves (AUC) reveals the non-inferiority of SI versus FFDM based on BIRADS categorisation [difference between AUC (ΔAUC), -0.014] and lesion visibility (ΔAUC, -0.001) but the differences were not statistically significant (p=0.282 for BIRADS; p=0.961 for lesion visibility). On average, 77.4% of malignant lesions were detected with SI versus 76.5% with FFDM. Sensitivity and specificity of SI are superior to FFDM for malignant lesions scored as BIRADS 5 and breasts categorised as BIRADS 1.ConclusionsSI is not inferior to FFDM when DBT slices are not available during image reading. SI can replace FFDM, reducing the dose by 45%.Key Points• Stand-alone SI demonstrated performance not inferior for lesion visibility as compared to FFDM.• Stand-alone SI demonstrated performance not inferior for lesion BIRADS categorisation as compared to FFDM.• Synthetic images provide important dose savings in breast tomosynthesis examinations.

Digital Breast Tomosynthesis: Image Quality and Dose Saving of the Synthesized Image

In this paper, the impact on image quality and dose reduction related to the use of a synthesized 2D image in digital breast tomosynthesis examinations is analyzed. 2D and 3D images of the TORMAM phantom were acquired at clinical conditions. Syntesized 2D images (C-View) were also obtained. Seven observers scored the detectability and visibility of microcalcification (MC) clusters in both types of images. Low contrast objects were studied measuring contrast-to-noise ratio (CNR) and applying a non-prewhitening matched filter (NPW) model observer. Glandular doses were estimated from a sample of 50 patients. The detectability and visibility of the microcalcification clusters were higher in C-View than in 2D images (50% and 100%, respectively). CNR values were higher for C-View for all contrasts. The NPW got slightly higher detectability values for the lowest contrast details in C-View. We have estimated a dose reduction of 43% by replacing the conventional 2D by the C-View image.

A phantom using titanium and Landolt rings for image quality evaluation in mammography

A phantom for image quality evaluation of digital mammography systems is presented and compared to the most widely used phantoms in Europe and the US. The phantom contains objects for subjective detection of Landolt rings (four-alternative, forced-choice task) and for objective calculation of signal-difference-to-noise ratios (SDNR), both in a titanium background within a 12-step wedge. Evaluating phantom images corresponding to exposures between 15 and 160 mAs (average glandular dose between 0.2 and 2 mGy), the resulting scores were compared to the scores obtained following the European EPQC and American College of Radiology (ACR) protocols. Scores of the Landolt test equal to 19 and 8.5 and SDNR equal to 20 and 11 were found to be equivalent to the acceptable limiting values suggested by EPQC and ACR. In addition, the Landolt and SDNR tests were shown to take into account the anatomical variations in thickness and tissue density within the breast. The simplified evaluation method presented was shown to be a sensitive, efficient and reliable alternative for image quality evaluation of mammography systems.

Use of the cross-correlation component of the multiscale structural similarity metric (R* metric) for the evaluation of medical images

PURPOSE The aim of the present work is to analyze the potential of the cross-correlation component of the multiscale structural similarity metric (R*) to predict human performance in detail detection tasks closely related with diagnostic x-ray images. To check the effectiveness of R*, the authors have initially applied this metric to a contrast detail detection task. METHODS Threshold contrast visibility using the R* metric was determined for two sets of images of a contrast-detail phantom (CDMAM). Results from R* and human observers were compared as far as the contrast threshold was concerned. A comparison between the R* metric and two algorithms currently used to evaluate CDMAM images was also performed. RESULTS Similar trends for the CDMAM detection task of human observers and R* were found in this study. Threshold contrast visibility values using R* are statistically indistinguishable from those obtained by human observers (F-test statistics: p > 0.05). CONCLUSIONS These results using R* show that it could be used to mimic human observers for certain tasks, such as the determination of contrast detail curves in the presence of uniform random noise backgrounds. The R* metric could also outperform other metrics and algorithms currently used to evaluate CDMAM images and can automate this evaluation task.

Automatic scoring of CDMAM using a model of the recognition threshold of the human visual system: R*

A software tool is presented for the automatic evaluation of the CDMAM phantom images that are currently used for the quality assessment of the image quality in mammography. This software tool is based on the use of the cross-correlation component of the index MS-SSIM*, R*, oriented to the recognition threshold of different image representations. Given an image sequence, whose images begin as unrecognizable and are gradually refined to include more information, the recognition threshold corresponds to first the image in the sequence in which an observer indentifies the content. We have validated our software tool by comparing our readouts with those obtained by a total of 4 expert observers in the evaluation of 8 CDMAM images. The correlations obtained between both readout sets are better than 0.99 and the range of useful correlation comprises diameters from 0.16 mm to 2.0 mm of the gold disks inside the CDMAM.