Establishing minimum performance standards, calibration intervals, and optimal exposure values for a whole breast digital mammography unit

Automatic and robust calibration of optical detector arrays for biomedical diffuse optical spectroscopy

The design and testing of a new, fully automated, calibration approach is described. The process was used to calibrate an image-guided diffuse optical spectroscopy system with 16 photomultiplier tubes (PMTs), but can be extended to any large array of optical detectors and associated imaging geometry. The design goals were accomplished by developing a routine for robust automated calibration of the multi-detector array within 45 minutes. Our process was able to characterize individual detectors to a median norm of the residuals of 0.03 V for amplitude and 4.4 degrees in phase and achieved less than 5% variation between all the detectors at the 95% confidence interval for equivalent measurements. Repeatability of the calibrated data from the imaging system was found to be within 0.05 V for amplitude and 0.2 degrees for phase, and was used to evaluate tissue-simulating phantoms in two separate imaging geometries. Spectroscopic imaging of total hemoglobin concentration was recovered to within 5% of the true value in both cases. Future work will focus on streamlining the technology for use in a clinical setting with expectations of achieving accurate quantification of suspicious lesions in the breast.

Automated analysis of phantom images for the evaluation of long-term reproducibility in digital mammography

The performance of an automatic software package was evaluated with phantom images acquired by a full-field digital mammography unit. After the validation, the software was used, together with a Leeds TORMAS test object, to model the image acquisition process. Process modelling results were used to evaluate the sensitivity of the method in detecting changes of exposure parameters from routine image quality measurements in digital mammography, which is the ultimate purpose of long-term reproducibility tests. Image quality indices measured by the software included the mean pixel value and standard deviation of circular details and surrounding background, contrast-to-noise ratio and relative contrast; detail counts were also collected. The validation procedure demonstrated that the software localizes the phantom details correctly and the difference between automatic and manual measurements was within few grey levels. Quantitative analysis showed sufficient sensitivity to relate fluctuations in exposure parameters (kV(p) or mAs) to variations in image quality indices. In comparison, detail counts were found less sensitive in detecting image quality changes, even when limitations due to observer subjectivity were overcome by automatic analysis. In conclusion, long-term reproducibility tests provided by the Leeds TORMAS phantom with quantitative analysis of multiple IQ indices have been demonstrated to be effective in predicting causes of deviation from standard operating conditions and can be used to monitor stability in full-field digital mammography.

Quality control for digital mammography in the ACRIN DMIST trial: Part I

The Digital Mammography Imaging Screening Trial, conducted by the American College of Radiology Imaging Network, is a clinical trial designed to compare the accuracy of full-field digital mammography (FFDM) versus screen-film mammography in a screening population. Five FFDM systems from four manufacturers (Fischer, Fuji, General Electric, and Lorad) were employed in the study at 35 clinical sites. A core physics team devised and implemented tests to evaluate these systems. A detailed description of physics and quality control tests is presented, including estimates of: mean glandular dose, modulation transfer function (MTF), 2D noise power spectra, and signal-to-noise ratio (SNR). The mean glandular doses for the standard breast ranged from 0.79 to 2.98 mGy, with 1.62 mGy being the average across all units and machine types. For the five systems evaluated, the MTF dropped to 50% at markedly different percentages (22% to 87%) of the Nyquist limit, indicating that factors other than detector element (del) size have an important effect on spatial resolution. Noise power spectra and SNR were measured; however, we found that it was difficult to standardize and compare these between units. For each machine type, the performance as measured by the tests was very consistent, and no predictive benefit was seen for many of the tests during the 2-year period of the trial. It was found that, after verification of proper operation during acceptance testing, if systems failed they generally did so suddenly rather than through gradual deterioration of performance. Because of the relatively short duration of this study further, investigation of the long-term failure characteristics of these systems is advisable.

[The quality of digital mammograms. Development and use of phantoms for optimal safety]

Digital imaging in mammography is becoming more and more accepted using both computed (CR) and direct radiography (DR). These techniques will soon be used in screening programs. Therefore, quality assurance for this technique is indispensable. The relevance of the current regulations, such as EPOC and the German QS-RL was investigated. For the investigation, a breast phantom and appropriate software were developed. Both were tested using digital mammography systems from six manufactures. Quality assurance parameters (such as contrast to noise ratio and contrast resolution) were calculated from these data sets. The results should be considered in future standards for mammography (IEC respectively DIN). In addition, this type of test procedure is time saving and enables a reduction in test devices, i. e. in costs.

How good is the ACR accreditation phantom for assessing image quality in digital mammography?

RATIONALE AND OBJECTIVES

The purpose of this study was to evaluate the American College of Radiology (ACR) accreditation phantom for assessing image quality in digital mammography.

MATERIALS AND METHODS

Digital images were obtained of an ACR accreditation phantom at varying mAs (constant kVp) and varying kVp (constant mAs). The average glandular dose for a breast with 50% glandularity was determined for each technique factor. Images were displayed on a 5 mega-pixel monitor, with the window width and level settings individually optimized for viewing the fibers, specks, and masses in the ACR phantom. Digital images of the ACR phantom were presented in a random manner to eight observers, each of whom indicated the number of objects visible in each image.

RESULTS

Intraobserver variability was greater than interobserver variability for the detection of fibers and specks, but the reverse was true for the detection of masses. As the mAs increased, the number of fibers visible increased from less than one at 5 mAs to all six being visible at 80 mAs. The corresponding number of visible specks increased from 12 to 24, and the number of visible masses increased from 1.25 to about four. Above 26 kVp, object visibility was constant with increasing x-ray tube voltage. Reducing the x-ray tube voltage to 24 kVp, however, reduced the number of visible fibers from six to five, the number of visible specks from 24 to 21.1, and the number of visible masses from four to 3.1. Observer performance was approximately constant for average glandular doses greater than 1.6 mGy, so that the range of lesion detectability in the ACR phantom occurs at doses lower than those normally encountered in clinical practice.

CONCLUSION

The current design of the ACR phantom is unsatisfactory for assessing image quality in digital mammography.

ROC curve analysis of lesion detectability on phantoms: comparison of digital spot mammography with conventional spot mammography

Although conventional screen--film mammography has excellent spatial resolution and is commonly used as a screening tool, certain inherent limitations prevent its further improvement. New digital mammography techniques, despite lower spatial resolution than screen--film mammography, may overcome these limitations. This study compared lesion detectability between charge coupled device-based digital spot mammography and conventional spot mammography. A total of 100 sets of images of specially designed breast phantoms was acquired, with variable background achieved by overlapping several layers of grapefruit fibre on a 4 cm thick lucite slab, using both modalities. 75 sets were "normal" images and 25 sets were images with simulated lesions. Four radiologists assessed the images according to a five-point confidence scale. The results were used to construct receiver operating characteristic curves. No statistical difference was observed between the two sets of curves for individual radiologists as well as pooled data. The lower spatial resolution of digital mammography was compensated for by its higher contrast sensitivity relative to conventional spot mammography.

Comparison of full-field digital mammography with screen-film mammography for cancer detection: results of 4,945 paired examinations

PURPOSE

To prospectively compare full-field digital mammography (FFDM) with screen-film mammography (SFM) for cancer detection in a screening population.

MATERIALS AND METHODS

At two institutions, 4,945 FFDM examinations were performed in women aged 40 years and older presenting for SFM. Two views of each breast were acquired with each modality. SFM and FFDM images were interpreted independently. Findings detected with either SFM or FFDM were evaluated with additional imaging and, if warranted, biopsy.

RESULTS

Patients in the study underwent 152 biopsies, which resulted in the diagnosis of 35 breast cancers. Twenty-two cancers were detected with SFM and 21 with FFDM. Four were interval cancers that became palpable within 1 year of screening and were considered false-negative findings with both modalities. The difference in cancer detection rate was not significant. FFDM had a significantly lower recall rate (11.5%; 568 of 4,945) than SFM (13.8%; 685 of 4,945) (P <.001, McNemar chi(2) model; P <.03, generalized estimating equations model). The positive biopsy rate for findings detected with FFDM (30%; 21 of 69) was higher than that for findings detected with SFM (19%; 22 of 114), but this difference was not significant.

CONCLUSION

No difference in cancer detection rate has yet been observed between FFDM and SFM. FFDM has so far led to fewer recalls than SFM.

Breast imaging using an amorphous silicon-based full-field digital mammographic system: stability of a clinical prototype

An amorphous silicon-based full-breast imager for digital mammography was evaluated for detector stability over a period of 1 year. This imager uses a structured CsI:TI scintillator coupled to an amorphous silicon layer with a 100-micron pixel pitch and read out by special purpose electronics. The stability of the system was characterized using the following quantifiable metrics: conversion factor (mean number of electrons generated per incident x-ray), presampling modulation transfer function (MTF), detector linearity and sensitivity, detector signal-to-noise ratio (SNR), and American College of Radiology (ACR) accreditation phantom scores. Qualitative metrics such as flat field uniformity, geometric distortion, and Society of Motion Picture and Television Engineers (SMPTE) test pattern image quality were also used to study the stability of the system. Observations made over this 1-year period indicated that the maximum variation from the average of the measurements were less than 0.5% for conversion factor, 3% for presampling MTF over all spatial frequencies, 5% for signal response, linearity and sensitivity, 12% for SNR over seven locations for all 3 target-filter combinations, and 0% for ACR accreditation phantom scores. ACR mammographic accreditation phantom images indicated the ability to resolve 5 fibers, 4 speck groups, and 5 masses at a mean glandular dose of 1.23 mGy. The SMPTE pattern image quality test for the display monitors used for image viewing indicated ability to discern all contrast steps and ability to distinguish line-pair images at the center and corners of the image. No bleeding effects were observed in the image. Flat field uniformity for all 3 target-filter combinations displayed no artifacts such as gridlines, bad detector rows or columns, horizontal or vertical streaks, or bad pixels. Wire mesh screen images indicated uniform resolution and no geometric distortion.

A comparison of digital and screen-film mammography using quality control phantoms

AIM

To compare the performance of a direct digital mammography system with normal-view and magnified-view conventional screen-film methods using quality control phantoms.

MATERIALS AND METHODS

Using a Siemens Mammomat((R))3000 and an Opdima((R))digital spot imaging and biopsy attachment, film and direct digital images of two phantoms [DuPont and TOR (MAM)] were obtained under normal operating conditions. These were assessed by three groups of observers with differing expertise - radiologists, radiographers and medical physicists. Each observer was asked to compare the direct digital image with films taken in standard view and magnified view, providing scores for object visibility and confidence. For the digital images, observers were allowed to vary the image presentation parameters.

RESULTS

Both phantoms showed that overall the direct digital view and the magnified view film performed significantly better (P < 0.05) than standard view film. For certain small or low contrast objects the differences became very highly significant (P < 0.001).

CONCLUSION

Only the TOR (MAM) phantom showed any significant difference between digital and magnified modalities, with magnified views performing better for fine, faint filaments and digital acquisition better for low contrast objects. Almost no difference existed between the three observer groups. Undrill, P. E. (2000). Clinical Radiology53, 782-790.