Digital mammography. ROC studies of the effects of pixel size and unsharp-mask filtering on the detection of subtle microcalcifications

We investigated the spatial resolution requirement and the effect of unsharp-mask filtering on the detectability of subtle microcalcifications in digital mammography. Digital images were obtained by digitizing conventional screen-film mammograms with a 0.1 X 0.1 mm2 pixel size, processed with unsharp masking, and then reconstituted on film with a Fuji image processing/simulation system (Fuji Photo Film Co., Tokyo, Japan). Twenty normal cases and 12 cases with subtle microcalcifications were included. Observer performance experiments were conducted to assess the detectability of subtle microcalcifications in the conventional, the unprocessed digital, and the unsharp-masked mammograms. The observer response data were evaluated using receiver operating characteristic (ROC) and LROC (ROC with localization) analyses. Our results indicate that digital mammograms obtained with 0.1 X 0.1 mm2 pixels provide lower detectability than the conventional screen-film mammograms. The detectability of microcalcifications in the digital mammograms is improved by unsharp-mask filtering; the processed mammograms still provide lower accuracy than the conventional mammograms, however, chiefly because of increased false-positive detection rates for the processed images at each subjective confidence level. Viewing unprocessed digital and unsharp-masked images in pairs resulted in approximately the same detectability as that obtained with the unsharp-masked images alone. However, this result may be influenced by the fact that the same limited viewing time was necessarily divided between the two images.     

Evaluation of computer-aided detection of lesions in mammograms obtained with a digital phase-contrast mammography system

A computer-aided detection (CAD) system was evaluated for its ability to detect microcalcifications and masses on images obtained with a digital phase-contrast mammography (PCM) system, a system characterised by the sharp images provided by phase contrast and by the high resolution of 25-μm-pixel mammograms. Fifty abnormal and 50 normal mammograms were collected from about 3,500 mammograms and printed on film for reading on a light box. Seven qualified radiologists participated in an observer study based on receiver operating characteristic (ROC) analysis. The average of the areas under ROC curve (AUC) values for the ROC analysis with and without CAD were 0.927 and 0.897 respectively (P?=?0.015). The AUC values improved from 0.840 to 0.888 for microcalcifications (P?=?0.034) and from 0.947 to 0.962 for masses (P?=?0.025) respectively. The application of CAD to the PCM system is a promising approach for the detection of breast cancer in its early stages.     

Detection of simulated microcalcifications in fixed mammary tissue: An ROC study of the effect of local versus global histogram equalization

PURPOSE: To investigate whether sliding window adaptive histogram equalization (SWAHE) of digital mammograms improves the detection of simulated calcifications, as compared to images normalized by global histogram equalization (GHE). MATERIAL AND METHODS: Direct digital mammograms were obtained from mammary tissue phantoms superimposed with different frames. Each frame was divided into forty squares by a wire mesh, and contained granular calcifications randomly positioned in about 50% of the squares. Three radiologists read the mammograms on a display monitor. They classified their confidence in the presence of microcalcifications in each square on a scale of 1 to 5. Images processed with GHE were first read and used as a reference. In a later session, the same images processed with SWAHE were read. RESULTS: The results were compared using ROC methodology. When the total areas AZ were compared, the results were completely equivocal. When comparing the high-specificity partial ROC area AZ,0.2 below false-positive fraction (FPF) 0.20, two of the three observers performed best with the images processed with SWAHE. The difference was not statistically significant. CONCLUSION: When the reader's confidence threshold in malignancy is set at a high level, increasing the contrast of mammograms with SWAHE may enhance the visibility of microcalcifications without adversely affecting the false-positive rate. When the reader's confidence threshold is set at a low level, the effect of SWAHE is an increase of false positives. Further investigation is needed to confirm the validity of the conclusions.     

Zooming method (× 2.0) of digital mammography vs digital magnification view (× 1.8) in full-field digital mammography for the diagnosis of microcalcifications

The purpose of this study was to determine whether the interpretation of microcalcifications assessed on images zoomed (× 2.0) from digital mammograms is at least equivalent to that from digital magnification mammography (× 1.8) with respect to diagnostic accuracy and image quality. Three radiologists with different levels of experience in mammography reviewed each full-field digital mammography reader set for 185 patients with pathologically proven microcalcification clusters, which consisted of digital magnification mammograms (MAGs) with a magnification factor of 1.8 and images zoomed from mammograms (ZOOM) with a zoom factor of 2.0. Each radiologist rated their suspicion of breast cancer in microcalcific lesions using a six-point scale and the image quality and their confidence in the decisions using a five-point scale. Results were analysed according to display methods using areas under the receiver operating characteristic curves (A_z value) for ZOOM and MAGs to interpret microcalcifications, and the Wilcoxon matched pairs signed rank test for image quality and confidence levels. There was no statistically significant difference in the level of suspicion of breast cancer between the ZOOM and MAG groups (A_z = 0.8680 for ZOOM; A_z = 0.8682 for MAG; p = 0.9897). However, MAG images were significantly better than ZOOM images in terms of visual imaging quality (p < 0.001), and the confidence level with MAG was better than with ZOOM (p < 0.001). In conclusion, the performance of radiologists in the diagnosis of microcalcifications using ZOOM was similar to that using MAGs, although image quality and confidence levels were better using MAGs.Magnification mammography produces better spatial resolution and signal-to-noise ratio than does contact mammography. It is well established as a valuable adjunct to contact mammography, especially for the diagnosis of microcalcifications, despite the additional radiation exposure and increased radiation dose because of the shorter distance between the breast and X-ray source during examination [1–4].However, with respect to full-field digital mammography (FFDM), a few studies using zoomed images from contact mammograms have recently been reported and, as a result, a debate has arisen over whether a digital zooming system of FFDM can replace the magnification view of digital mammography [5–7]. Whereas Fischer et al [5] reported that zoomed images of a digital contact mammogram were equivalent to direct magnification of FFDM for the interpretation of microcalcifications, our previous report suggested that magnification mammography yielded better sensitivity and receiver operating characteristic (ROC) analysis than did zoomed images [7]. However, that study compared images zoomed by a factor of 1.3 with images magnified by a factor of 1.8. Therefore, we wondered whether using a zooming factor comparable to a magnification factor of 1.8 would yield the same results.The purpose of this study was to determine whether the diagnostic accuracy and image quality of microcalcification assessments using images twice zoomed from contact mammograms were equivalent to those obtained using digital magnification mammography by a magnification factor of 1.8.     

Direct magnification mammography in combination with computed radiography. First clinical results

Summary Introduction: The combination of direct magnification mammography and computed radiography provides an improvement in spatial resolution of storage phosphor-based digital systems. A clinical study comparing conventional and digital direct magnification mammograms was performed. Methods: 100 survey mammograms in 1.5- or 1.7-fold magnification and 50 4-fold spot magnification views were obtained with a prototype direct magnification mammography system and a storage phosphor-based digital system. An intraindividual comparison of these with previous conventional radiograms of the same patients was carried out. Results: The diagnostic value of digital survey mammograms using the direct magnification technique is comparable to that of conventional radiograms of the breast, especially with regard to the identification of microcalcifications and lesions and the clinical consequences. Spot magnification views performed with this combination of techniques allowed improvement in the evaluation of microcalcifications. In 15 % of cases, diagnostic procedures were adjusted accordingly. Conclusion: The combination of the direct magnification technique with digital storage phosphor radiography systems allows the performance of digital mammography by improving the overall spatial resolution. The diagnostic value of digital direct magnification survey mammograms was comparable to that of conventional mammograms. Digital 4-fold spot magnification views improved visualisation of the morphologic aspects of microcalcifications.      

ROC analysis comparing screen film mammography and digital mammography

PURPOSE: To compare the diagnosis performances of radiologists on screen film versus digital mammography. MATERIAL: and methods: Two sets of 123 mammograms, screen film mammography and storage phosphor digital mammography, are studied comparatively with ROC analysis. RESULTS: Phantom study show that conventional method give better scores for usual tension but the detectability of smaller microcalcification is equivalent. To obtain with digital technic the same conventional score you have to increase the radiation dose. Roc Curves, simulated "detection" mode showed that radiologists performed with higher accuracy with conventional system but this difference is weekly statistically significant. ROC Curves, simulated "diagnostic" mode showed the same results wit no statistically significant difference but when the decision to go to the biopsy is the gold standard, ROC Curves were essentially equivalent for both film screen and digital mammography system. The readers consistently considered the digital mammograms to be less suspicious for cancer findings. The agreement study as proposed by the FDA indicate that probability of a positive digital mammograms given a positive screen film is 75% (threshold value 90%) and the probability of a negative digital mammograms given a negative analog film is 85% (threshold value 85%). CONCLUSION: Analysis of specific discrepancies indicate that spatial resolution is an essential limiting factor for digital method but high resolution phosphor plate are interesting in imaging treated breast, radioluscent lesion, fatty benign tumor, hamartoma, intramammary lymph node, breast with prosthesis.     

Digital mammography: observer performance study of the effects of pixel size on the characterization of malignant and benign microcalcifications

RATIONALE AND OBJECTIVES: The authors performed this study to evaluate the effects of pixel size on the characterization of mammographic microcalcifications by radiologists. MATERIALS AND METHODS: Two-view mammograms of 112 microcalcification clusters were digitized with a laser scanner at a pixel size of 35 microm. Images with pixel sizes of 70, 105, and 140 microm were derived from the 35-microm-pixel size images by averaging neighboring pixels. The malignancy or benignity of the microcalcifications had been determined with findings at biopsy or 2-year follow-up. Region-of-interest images containing the microcalcifications were printed with a laser imager. Seven radiologists participated in a receiver operating characteristic (ROC) study to estimate the likelihood of malignancy. The classification accuracy was quantified with the area under the ROC curve (Az). The statistical significance of the differences in the Az values for different pixel sizes was estimated with the Dorfman-Berbaum-Metz method and the Student paired t test. The variance components were analyzed with a bootstrap method. RESULTS: The higher-resolution images did not result in better classification; the average Az with a pixel size of 35 microm was lower than that with pixel sizes of 70 and 105 microm. The differences in Az between different pixel sizes did not achieve statistical significance. CONCLUSION: Pixel sizes in the range studied do not have a strong effect on radiologists' accuracy in the characterization of microcalcifications. The low specificity of the image features of microcalcifications and the large interobserver and intraobserver variabilities may have prevented small advantages in image resolution from being observed.     

Detection of masses and calcifications by soft-copy reading: comparison of two postprocessing algorithms for full-field digital mammography

Purpose  The purpose of this study was to determine the effects of a commercially available postprocessing algorithm on the detection of masses and microcalcifications of breast cancer by soft-copy reading. Materials and methods  The study included 64 digital mammograms with 16 histologically proven abnormal findings (eight masses and eight microcalcifications) and 48 normal breasts. Two image-processing algorithms were applied to the digital images, which were acquired using General Electric units. The commercially available advanced and standard postprocessed digital mammograms were evaluated in a localization receiver operating characteristic (ROC) curve experiment involving seven mammography radiographers. Results  The mean area under the ROC curve was 0.921 ± 0.022 for the commercially available advanced postprocessed digital mammograms session and 0.904 ± 0.026 for the standard postprocessed digital mammograms session (P = 0.1953). Observer agreement among the readers was better for the advanced postprocessed digital mammograms than for the standard postprocessed digital mammograms. Conclusion  During soft-copy reading, the interpretation accuracy might be influenced by the postprocessing algorithm.     

Spectral phase based medical image processing

RATIONALE AND OBJECTIVES: To exploit the spectral phase characteristics of digital or digitized mammograms for early detection of microcalcifications, shape, and sizes of suspected lesions and to demonstrate its use for training radiologists to discriminate signal features in different spatially varying backgrounds. MATERIALS AND METHODS: We propose two algorithms: in the phase-only image (POI) reconstruction algorithm the spectral phase of the digital mammogram is extracted from its Fourier spectrum. This is coupled with unit magnitude and inverse Fourier transformed to reconstruct the POI thus enhancing the features of interest such as microcalcifications, shape, and sizes of suspected lesions. In the algorithm for image reconstruction from a priori phase-only information, spectral phase is used to extract signal features of the digital mammogram and then this is combined with spectral magnitude that is extracted and averaged over an ensemble of unrelated digital mammograms. RESULTS: The results for several digital phantoms and mammograms show that POI reconstructs only high spatial frequencies related to the features such as microcalcifications, shape, and size of masses like cysts and tumors. The results on image reconstruction from a priori phase-only information demonstrate the changes in the visibility of signal features when buried in a wide variety of real world mammogram backgrounds with different densities. CONCLUSION: The POI can aid radiologists in early detection of microcalcifications, lesions, and other masses of interest in digital mammograms. This reconstruction method is self-adaptive to changes in the background. The image reconstruction from a priori phase-only information can help the radiologist as a training tool in his decision-making process. Preliminary experiments indicate the potential of the techniques for early diagnosis of breast cancer. Clinical studies on these algorithm procedures are in progress for application as a diagnostic CAD tool in digital mammography. These methods can in general be applied to other medical images such as CT and MRI images.     

Effect of soft-copy display supported by CAD on mammography screening performance

Diagnostic performance and reading speed for conventional mammography film reading is compared to reading digitized mammograms on a dedicated workstation. A series of mammograms judged negative at screening and corresponding priors were collected. Half were diagnosed as cancer at the next screening, or earlier for interval cancers. The others were normal. Original films were read by fifteen experienced screening radiologists. The readers annotated potential abnormalities and estimated their likelihood of malignancy. More than 1 year later, five radiologists reread a subset of 271 cases (88 cancer cases having visible signs in retrospect and 183 normals) on a mammography workstation after film digitization. Markers from a computer-aided detection (CAD) system for microcalcifications were available to the readers. Performance was evaluated by comparison of A_z-scores based on ROC and multiple-Reader multiple-case (MRMC) analysis, and localized receiver operating characteristic (LROC) analysis for the 271 cases. Reading speed was also determined. No significant difference in diagnostic performance was observed between conventional and soft-copy reading. Average A_z-scores were 0.83 and 0.84 respectively. Soft-copy reading was only slightly slower than conventional reading. Using a mammography workstation including CAD for detection of microcalcifications, soft-copy reading is possible without loss of quality or efficiency.     

Characterization of microcalcification: can digital monitor zooming replace magnification mammography in full-field digital mammography?

The aim of this study was to compare the diagnostic accuracy and image quality of microcalcifications in zoomed digital contact mammography with digital magnification mammography. Three radiologists with different levels of experience in mammography reviewed 120 microcalcification clusters in 111 patients with a full-field digital mammography system relying on digital magnification mammogram (MAG) images and zoomed images from contact mammography (ZOOM) using commercially available zooming systems on monitors. Each radiologist estimated the probability of malignancy and rated the image quality and confidence rate. Performance was evaluated by sensitivity, specificity, positive predictive value, negative predictive value, and receiver operating characteristic (ROC) analysis. All three radiologists rated MAG images higher than ZOOM images for sensitivity with statistical significance (average value, 92% vs. 87%, P < 0.05) and performance by ROC analysis improved with MAG imaging. The confidence rate for diagnosis decision and the assessment of lesion characteristics were also better in MAG images than in ZOOM images with statistical significance (P < 0.0001). Digital magnification mammography can enhance diagnostic performance when characterizing microcalcifications. Images zoomed from digital contact mammography cannot serve as an alternative to direct magnification digital mammography. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007-313-E00363).     

A comparison of digitized storage phosphors and conventional mammography in the detection of malignant microcalcifications

The detectability of malignant tumor-derived microcalcifications with conventional mammography was compared to that with digital images (2000 X 2510 pixels by 10 bits) derived from a storage phosphor-based digital radiography system capable of 5 line pair/mm resolution at identical exposure factors (30 kVp, 250 mAs, 65 cm film-focus distance). Microcalcifications (50-800 microns in diameter) were randomly superimposed on a preserved human breast specimen. ROC analysis based on 480 observations made by four readers indicated that the ability to detect the calcifications with digital images (ROC area = 0.871 +/- 0.066) was equivalent to conventional mammography (ROC area = 0.866 +/- 0.075) despite lower spatial resolution. With digital mammography, 62% of all clusters were correctly localized, but only 23.6% of the individual calcifications were counted. With conventional mammography 61% of all clusters were correctly localized, but significantly more of the individual calcifications (31.5%) were counted.     

Radiologists' preferences for digital mammographic display. The International Digital Mammography Development Group

PURPOSE: To determine the preferences of radiologists among eight different image processing algorithms applied to digital mammograms obtained for screening and diagnostic imaging tasks. MATERIALS AND METHODS: Twenty-eight images representing histologically proved masses or calcifications were obtained by using three clinically available digital mammographic units. Images were processed and printed on film by using manual intensity windowing, histogram-based intensity windowing, mixture model intensity windowing, peripheral equalization, multiscale image contrast amplification (MUSICA), contrast-limited adaptive histogram equalization, Trex processing, and unsharp masking. Twelve radiologists compared the processed digital images with screen-film mammograms obtained in the same patient for breast cancer screening and breast lesion diagnosis. RESULTS: For the screening task, screen-film mammograms were preferred to all digital presentations, but the acceptability of images processed with Trex and MUSICA algorithms were not significantly different. All printed digital images were preferred to screen-film radiographs in the diagnosis of masses; mammograms processed with unsharp masking were significantly preferred. For the diagnosis of calcifications, no processed digital mammogram was preferred to screen-film mammograms. CONCLUSION: When digital mammograms were preferred to screen-film mammograms, radiologists selected different digital processing algorithms for each of three mammographic reading tasks and for different lesion types. Soft-copy display will eventually allow radiologists to select among these options more easily.     

Computer-aided detection of microcalcifications in mammograms. Methodology and preliminary clinical study

The authors investigated the feasibility of using computer methods for automated detection of clustered microcalcifications on clinical mammograms. A new difference-image approach using a matched filter/box-rim filter combination effectively removed the structured background from the image. A locally adaptive gray-level thresholding technique was then used for extraction of the signals from the resulting difference image. Signal-extraction criteria based on the size, contrast, number, and clustering properties of microcalcifications were next imposed on the detected signals to distinguish true signals from noise or artifacts. The detection accuracy of the computer scheme was evaluated by means of a free response receiver operating characteristic (FROC) analysis. It was found that, for simulated subtle microcalcifications superimposed on normal mammograms, the difference-image approach with a matched filter/box-rim filter combination could yield a true-positive cluster detection rate of 80% at a false-positive detection rate of one cluster per image. In a study of 20 clinical images containing moderately subtle microcalcifications, the automated computer scheme obtained an 82% true-positive cluster detection rate at a false-positive detection rate of one cluster per image. These results indicate that the automated method has the potential to aid radiologists in screening mammograms for clustered microcalcifications.     

Evaluation of a wavelet-based computer-assisted detection system for identifying microcalcifications in digital full-field mammography

PURPOSE: To evaluate a new wavelet-based computer-assisted detection (CAD) system for detecting and enhancing microcalcifications. MATERIAL AND METHODS: A total of 280 mammograms acquired by full-field digital mammography (Senographe 2000D; G.E. Medical Systems Milwaukee, Wisc., USA) were analyzed with and without a new wavelet-based CAD system for detecting and enhancing microcalcifications. The mammograms comprised roughly equal numbers of cases from each of the BIRADS (Breast Imaging, Reporting and Data System, according to the American College of Roentgenology) categories 1-5. Histologic confirmation was available for all of the 180 cases assigned BIRADS categories 3-5. Four readers interpreted all 280 images for suspicious microcalcifications using a scale of 1-5. The readers alternately assessed 5 images with and 5 without CAD. In a second reading immediately following the first, the readers had to reassess the 280 mammograms. The images that had already been interpreted without CAD were now presented with CAD and vice versa. The images were interpreted as soft copies on a diagnostic mammography workstation (Image Diagnost GmbH, Neufahrn/Munich, Germany). All images interpreted with CAD were presented with enhancement of microcalcifications by wavelet algorithms and prompting of microcalcifications. ROC (receiver operating characteristic) analyses were performed, and image interpretation time with and without CAD was measured. RESULTS: The overall time for interpretation required by all 4 readers together was 483 min with CAD compared to 580 min without CAD. ROC analysis revealed no significant advantage of CAD for the individual readers. Readers 3 (0.811/0.817) and 4 (0.799/0.843) had a slightly improved AUC (area under the curve) with CAD. Readers 1 and 2 had a slightly lower AUC with CAD (0.832 versus 0.861 and 0.818 versus 0.849). CONCLUSION: The CAD system significantly (P<0.05, t test) speeded up image interpretation with respect to the identification of microcalcifications, while the diagnostic quality remained almost identical under the study conditions.     

Analogic versus digital mammographic examination a radiological study of mammary microcalcifications on 52 surgical samples

PURPOSE: Digital mammography is known to have lower spatial resolution compared to conventional analogic mammography. The aim of this study was to evaluate whether this physical feature could compromise the perception of microcalcifications in radiological findings. MATERIALS AND METHODS: Fifty-two surgical samples of non-palpable breast lesions with microcalcifications were imaged using both techniques. The images were examined by four different radiologists. Data processing was limited to comparing the number of microcalcifications found on the conventional and digital images, in both standard and magnified modality. The cases were classified into 3 groups according to the number of calcifications demonstrated in the surgical sample: less than 10, 10 to 30, and more than 30. The differences in the count of microcalcifications with the two acquisition modalities were evaluated with the Kappa test. In order to compare the differences we synthesised by percentage those cases exhibiting a larger or lesser number of calcifications. RESULTS: The Kappa test was 0.546 in standard analogic vs standard digital, 0.582 in magnified analogic vs magnified digital, 0.828 in standard analogic vs magnified analogic and 0.492 in standard digital vs magnified digital. The most significant results were observed on comparing the magnifications produced with the two modalities: in 25% of cases, digital magnification detected more calcifications than did traditional magnification. The number of cases where standard digital images allowed the detection of more calcifications than standard analogic images was significant, although less important (17.8%). CONCLUSIONS: The study was able to provide data that confirm the overall equivalence of the two techniques, as far as subtle mammographic findings (such as microcalcifications) are concerned. In particular, as applied to the series we examined, there is a cautious advantage in favour of the digital technique. More clinical studies, on larger series, will be necessary for a further and more thorough comparison of the two techniques, so that the results might be consistently useful in clinical practice.     

A comparison of dual-energy digital radiography and screen-film imaging in the detection of subtle interstitial pulmonary disease

An ROC study is described which compares the performance of three types of images--conventional screen-film, single-energy digital and dual energy bone cancelled (soft tissue) digital--in detecting subtle interstitial pulmonary disease. Marginally detectable nodular and reticulonodular patterns (12 different patterns of each) were superimposed over the lungs of a frozen human chest phantom to simulate the clinical situation. The digital images were formatted on film at full size (ie, 35 cm X 43 cm). A total of 156 images (52 of each type, of which 28 were normal and 24 had simulated pathology) were used in the study and read by five experienced chest radiologists. Using a paired t-test, the areas under the individual ROC curves were compared for three combinations of images--single-energy digital and conventional, soft tissue digital and conventional, and soft tissue and single-energy digital. No statistically significant difference was observed between the conventional and single-energy digital images. The readers performed better with both conventional and single-energy digital images than with the soft tissue digital images at statistically significant levels (P = 0.05 for conventional vs. soft tissue digital and P = 0.02 for single-energy digital vs. soft tissue digital). The results suggest that there is no advantage in employing dual-energy soft tissue images to assist in diagnosing interstitial disease in the clinical setting. They also suggest that spatial resolution requirements are less demanding in digital chest systems that obtain scatter-free images than in digital systems utilizing conventional scatter control techniques.     

Observer variability in screen-film mammography versus full-field digital mammography with soft-copy reading

Full-field digital mammography (FFDM) with soft-copy reading is more complex than screen-film mammography (SFM) with hard-copy reading. The aim of this study was to compare inter- and intraobserver variability in SFM versus FFDM of paired mammograms from a breast cancer screening program. Six radiologists interpreted mammograms of 232 cases obtained with both techniques, including 46 cancers, 88 benign lesions, and 98 normals. Image interpretation included BI-RADS categories. A case consisted of standard two-view mammograms of one breast. Images were scored in two sessions separated by 5 weeks. Observer variability was substantial for SFM as well as for FFDM, but overall there was no significant difference between the observer variability at SFM and FFDM. Mean kappa values were lower, indicating less agreement, for microcalcifications compared with masses. The lower observer agreement for microcalcifications, and especially the low intraobserver concordance between the two imaging techniques for three readers, was noticeable. The level of observer agreement might be an indicator of radiologist performance and could confound studies designed to separate diagnostic differences between the two imaging techniques. The results of our study confirm the need for proper training for radiologists starting FFDM with soft-copy reading in breast cancer screening. Presented at ECR, Wien 2006.     

The effects of gray scale image processing on digital mammography interpretation performance

RATIONALE AND OBJECTIVES: To determine the effects of three image-processing algorithms on diagnostic accuracy of digital mammography in comparison with conventional screen-film mammography. MATERIALS AND METHODS: A total of 201 cases consisting of nonprocessed soft copy versions of the digital mammograms acquired from GE, Fischer, and Trex digital mammography systems (1997-1999) and conventional screen-film mammograms of the same patients were interpreted by nine radiologists. The raw digital data were processed with each of three different image-processing algorithms creating three presentations-manufacturer's default (applied and laser printed to film by each of the manufacturers), MUSICA, and PLAHE-were presented in soft copy display. There were three radiologists per presentation. RESULTS: Area under the receiver operating characteristic curve for GE digital mass cases was worse than screen-film for all digital presentations. The area under the receiver operating characteristic for Trex digital mass cases was better, but only with images processed with the manufacturer's default algorithm. Sensitivity for GE digital mass cases was worse than screen film for all digital presentations. Specificity for Fischer digital calcifications cases was worse than screen film for images processed in default and PLAHE algorithms. Specificity for Trex digital calcifications cases was worse than screen film for images processed with MUSICA. CONCLUSION: Specific image-processing algorithms may be necessary for optimal presentation for interpretation based on machine and lesion type.     

Application of a computer-aided detection (CAD) system to digitalized mammograms for identifying microcalcifications]

PURPOSE: It is estimated that during mammographic screening programs radiologists fail to detect approximately 25% of breast cancers visible on retrospective review; this percentage rises to 50% if minimal signs are considered. Independent double reading is now strongly recommended as it allows to reduce the rate of false negative examinations by 5-15%. Recent technological progress has allowed to develop a number of computer-aided detection (CAD) systems. The aim of CAD is to help radiologists interpret lesions by serving as a second reader. In this study the authors developed and applied a CAD system to measure its ability to microcalcifications detect and compare its performance with that of a human observer. MATERIAL AND METHODS: The study was performed as part of the CALMA (computer-aided library for mammography) project of the Pisa section of the National Institute of Nuclear Physics. The aim of this project is to set up a large database of digital mammograms and to develop a CAD system. Our study series consisted of 802 mammograms - corresponding to 213 patients - digitalized between March and June 2000. We performed traditional mammography and then digitalized the mammograms using a CCD linear scanner (pixel size of 85 x 85 microm2, 12 bits). The images were evaluated by two radiologists with similar experience and then by the CAD system. This CAD system searches for microcalcifications by using ad hoc algorithms and an artificial neural network (Sanger type). RESULTS: The number of clusters in our database was 141 corresponding to 140 images; 692 images were non pathological. The CAD system identified a variable number of clusters depending on the threshold values. The threshold value is a number over which the probability of finding a lesion is highest. With thresholds of 0.13 and 0.14 the CAD system identified 140/141 clusters (99.3%); with a threshold of 0.15 it identified 139/141 clusters (98.6%); with a threshold of 0.16, 137/141 (97.2%); with a threshold of 0.18, 133/141 (94.3%); with thresholds of 0.18 and 0.20, 130/141 (92.2%). With threshold values of 0.13, 0.14, 0.15, 0.16 and 0.17 the system's sensitivity was greater than 82%, whereas with values of 0.18 and 0.20 it was greater than 80%. The number of false positive region of interest (ROI) / image was greater with low threshold values: in particular, thresholds of 0.13 and 0.14 yielded 16 false positives /image, thresholds of 0.15 and 0.16 yielded 9 and 7 false positives/image, and both 0.18 and 0.20 only 5/image. DISCUSSION: ROC curve shows how the use of high threshold values determined a very high specificity despite very low sensitivity rates. Conversely, low threshold values allowed to have a high sensitivity and a very low specificity. The best performance of our CAD system was obtained with threshold values at 0.15 and 0.16. In fact these thresholds resulted in a high sensitivity (greater than 82%) with an acceptable number of false positives/image, 9 and 7/image, respectively. It is not yet known how radiologists can deal with large numbers of false positives in screening programmes but in our opinion the most important feature of a good CAD system is a high sensitivity. CONCLUSIONS: In the near future the use of CAD systems will be widespread and easier to apply to everyday practice above all in centers where digital mammography is performed. Mammograms could be directly shown to radiologists after the CAD system has selected the ROI and analysed the images. Thanks to its high sensitivity and despite its low specificity CAD represents a concrete aid for radiologists.