Comparison of radiation dose between 2D digital stereotactic versus digital breast tomosynthesis-guided breast biopsies

RATIONALE AND OBJECTIVE

Use of digital breast tomosynthesis (DBT) in breast imaging has necessitated DBT-guided biopsy, however, a single DBT acquisition may result in a greater radiation dose than a single DM acquisition. Our objective was to compare the number of images acquired and the resulting radiation dose of DBT versus DM-guided breast biopsies.

METHOD

All biopsies performed on our DM unit from 8/2016 to 1/2017 and on our DM-DBT unit from 8/2017 to 1/2018 were retrospectively reviewed. The number of image acquisitions, average glandular dose (AGD) per acquisition and per procedure were computed and stratified by guidance modality and lesion type.

RESULTS

25 DM-guided biopsies were performed on the DM-only unit, 58 biopsies were performed with DM guidance on the dual unit (DM-DU) and 29 were performed with DBT. The average number of images acquisitions was 10.9 for DM-only unit biopsies, 9.3 images for DM-DU biopsies and 4.3 images for DBT-guided biopsies. Mean procedure AGD for DM-only unit biopsies was 28.77 mGy, versus 22.06 mGy for DM-DU and 10.18 mGy for DBT biopsies. Mean procedure AGD for biopsied calcification-only lesions was 22.3 mGy for DM-DU versus 10.7 mGy for DBT guidance (p < 0.001), with an average of 8.1 images per procedure for DM-DU versus 4.2 for DBT.

CONCLUSION

Fewer image acquisitions were obtained with DBT compared with DM guidance, therefore, the overall dose of DBT-guided procedures was less. The dose reduction obtained with DBT is possible across all lesion types, even for calcification-only lesions.

Update on digital mammography

On January 28, 2000, the U.S. Food and Drug Administration (FDA) approved the first full-field digital mammography unit for clinical use. The approval occurred approximately ten years after a National Cancer Institute (NCI) expert panel determined that, of all emergent technologies, digital mammography held the greatest potential for improving breast cancer detection [1,2]. Currently, four types of digital mammographic systems are under clinical evaluation. This article will review the information from the early clinical trials on digital mammography and will attempt to define the potential impact of digital mammography on the clinical practice of breast imaging.

Linear response theory for detectors consisting of discrete arrays

The optical transfer function (OTF) and the noise power or Wiener spectrum are defined for detectors consisting of a lattice of discrete elements with the assumptions of linear response, Gaussian statistics, and stationarity under the discrete group of translations which leave the lattice fixed. For the idealized classification task of determining the presence or absence of a signal under signal known exactly/background known exactly (SKE/BKE) conditions, the Wiener spectrum, the OTF, along with an analog of the gray-scale transfer characteristic, determine the signal-to-noise ratio (SNR), which quantifies the ability of an ideal observer to perform this task. While this result is similar to the established result for continuous detectors, such as screen-film systems, the theory of discrete lattices of detectors must take into account the fact that the lattice only supports a bounded but (in the limit of a detector of arbitrarily great extent) continuous range of frequencies. Incident signals with higher spatial frequencies appear in the data at lower aliased frequencies, and there are pairs of signals which are not distinguishable by the detector (the SNR vanishes for the task of distinguishing such signals). Further, the SNR will in general change if the signal is spatially displaced by a fraction of the lattice spacing, although this change will be small for objects larger than a single pixel. Some of the trade-offs involved in detectors of this sort, particularly in dealing with signal frequencies above those supported by the lattice, are studied in a simple model.

Image processing algorithms for digital mammography: a pictorial essay

Digital mammography systems allow manipulation of fine differences in image contrast by means of image processing algorithms. Different display algorithms have advantages and disadvantages for the specific tasks required in breast imaging-diagnosis and screening. Manual intensity windowing can produce digital mammograms very similar to standard screen-film mammograms but is limited by its operator dependence. Histogram-based intensity windowing improves the conspicuity of the lesion edge, but there is loss of detail outside the dense parts of the image. Mixture-model intensity windowing enhances the visibility of lesion borders against the fatty background, but the mixed parenchymal densities abutting the lesion may be lost. Contrast-limited adaptive histogram equalization can also provide subtle edge information but might degrade performance in the screening setting by enhancing the visibility of nuisance information. Unsharp masking enhances the sharpness of the borders of mass lesions, but this algorithm may make even an indistinct mass appear more circumscribed. Peripheral equalization displays lesion details well and preserves the peripheral information in the surrounding breast, but there may be flattening of image contrast in the nonperipheral portions of the image. Trex processing allows visualization of both lesion detail and breast edge information but reduces image contrast.

Lossy compression should not be used in certain imaging applications such as chest radiography. Against the proposition

Computational techniques are frequently used to compress image data so that transmission and storage requirements are reduced. If the computational techniques result in no loss in image resolution, the technique is referred to as lossless compression. Greater compression of data may yield some loss in spatial or temporal resolution, and is referred to as lossy compression. In some radiologic examinations [e.g., gastrointestinal (GI) studies], some resolution loss may be tolerable, whereas in others (chest examinations and mammography) it conceivably could result in missed pathology. Without lossy compression, however, data requirements can be overwhelming for transmission, storage and retrieval of images such as chest films. The unanswered question, addressed in this Point/Counterpoint issue, is whether some degree of lossy compression can be tolerated in chest radiography.

Analysis of signal propagation in optically coupled detectors for digital mammography: II. Lens and fibre optics

An x-ray detector for digital x-ray mammography is under investigation, which consists of a phosphor screen coupled by a demagnifying fibre-optic taper to a time-delay integration mode, charge-coupled device (CCD) image array. The signal propagation through such a detector depends on the intensity and angular emission of light from the phosphor screen, the angular acceptance and transmission of light through the optics, and the spectral sensitivity of the CCD to the fluorescent light. The production of light by the phosphor screen was considered in a previous paper. Here, the issues related to the optics are examined. For phosphor screens coupled by lenses with limiting acceptance angles of less than 30 degrees, it was calculated that the coupling efficiency would be 10% greater than would be estimated under the assumption of a Lambertian source. These increases occur because a phosphor screen typically produces light which is more forward directed than a Lambertian source. Similar increases in efficiency are observed when a phosphor screen is coupled to a fibre-optic faceplate or taper. For fibre optics, exact estimation of the optical coupling efficiency requires knowledge of the angular-dependent transmission efficiency of the fibres.

Comparison of receiver operating characteristic curves on the basis of optimal operating points

RATIONALE AND OBJECTIVES

We developed a method of comparing receiver operating characteristic (ROC) curves on the basis of the utilities associated with their optimal operating points (OOPs).

METHODS

OOPs were computed for paired ROC curves on the basis of isocost lines in ROC space with slopes ranging from 0.1 to 3.0. For each pair of OOPs corresponding to a single isocost slope, the difference in costs and the variance of this difference was computed. A sensitivity analysis was thus obtained for the difference between the two curves over a range of isocost slopes. Three published data sets were evaluated using this technique, as well as by comparisons of areas under the curves and of true-positive fractions at fixed false-positive fractions.

RESULTS

The OOPs of paired ROC curves often occur at different false-positive fractions. Comparisons of ROC curves on the basis of OOPs may provide results that differ from comparisons of curves at a fixed false-positive fraction.

CONCLUSION

ROC curves may be compared on the basis of utilities associated with their OOPs. This comparison of the optimal performance of two diagnostic tests may differ from conventional statistical comparisons.

Analysis of signal propagation in optically coupled detectors for digital mammography: I. Phosphor screens

The angular emission of light from turbid phosphor screens has been measured and modelled. As a first approximation, turbid phosphor screens have traditionally been modelled as Lambertian sources; however, Giakoumakis et al have previously shown that light emission from turbid phosphor screens is in fact more forward peaked. In this article, we extend the theory of Giakoumakis to include turbid phosphor screens that incorporate a transparent overcoat. The refractive index of the optical coupling medium in contact with the overcoat is shown to have a direct effect on both light output and angular emission from the screen. It was found that simple laws of refraction adequately describe this phenomenon. To model the angular emission of light from such phosphor screens, a term was included to describe the refraction from the overcoat into the adjacent coupling medium. The data obtained are required to calculate the propagation of the signal in a digital mammography detector in which a phosphor screen is optically coupled to a charge-coupled device (CCD) image array.

Analysis of the spatial-frequency-dependent DQE of optically coupled digital mammography detectors

The effect of optical coupling efficiency on the spatial-frequency-dependent propagation of signal and noise is considered for x-ray image detectors for digital mammography in which a phosphor screen is optically coupled to a charge-coupled device (CCD) image array. For experimental purposes, optical coupling between a Gd2O2S:Tb phosphor screen and a CCD image array was provided by relay lenses. Neutral density filters were inserted between the lenses to vary the optical coupling efficiency without altering the inherent spatial resolution. The total coupling efficiency, defined as the number of electrons (e-) recorded in the CCD per x-ray interaction in the phosphor, was calculated in each case. The modulation transfer function, and the contributions to the total noise power spectrum (NPS) of x-ray quantum noise, secondary quantum noise, and inherent detector noise were measured as a function of coupling efficiency. These data were used to calculate the spatial-frequency-dependent detective quantum efficiency [DQE(f)]. The NPS due to x-ray quantum noise had a significant spatial-frequency dependence for coupling efficiencies of more than 9 e- per x-ray interaction, but little spatial-frequency dependence for coupling efficiencies of less than 2 e- per x-ray interaction. These results indicate that to preserve high spatial-frequency values of DQE(f), and to ensure that images are x-ray quantum-noise limited at high spatial frequencies, a coupling efficiency on the order of 10 e- per x-ray interaction is required.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic range requirements in digital mammography

The dynamic range and the number of gray levels, gamma s, required for digital mammography has been evaluated using an energy transport model. The effects of molybdenum (Mo) and tungsten (W) target spectra and the energy-dependent attenuation by elemental filters, breast tissue, and a phosphor screen were included in the model. For detectors with ideal optical coupling and no inherent detector noise, 3,100 gray levels are discernable (requiring 12 bits per pixel), assuming a 40 kVp, W target spectrum (1.0 mm A1 filtration), a mean glandular dose to a 5 cm thick breast of 0.6 mGy, and an ideal observer with a 5 mm diam viewing aperture. The effects of inherent detector noise and realistic coupling efficiency on gamma s were also examined. For the 40 kVp, W spectrum, a detector with total coupling efficiency of 16 electrons (e-) per x-ray interaction and a dynamic range of 3000 (maximum carrier signal of 1.93 x 10(5) e-/pixel and inherent detector noise of 64 e- pixel) would decrease the number of gray levels that could be resolved by only 2% compared to a detector with ideal coupling and no inherent noise. A detector with a total coupling efficiency of 2.0 electrons per x-ray interaction and a dynamic range of 240 (maximum carrier signal 2.41 x 10(4) e-/pixel and inherent detector noise of 100 e-/pixel) would reduce the number of gray levels by 26% for the 40 kVp spectrum. On the basis of dynamic range, W spectra are preferable for digital mammography, since Mo spectra yielding the same signal-to-noise ratio require a detector with dynamic range twice as large, and with a 30% greater saturation signal. When no scatter rejection method is used, scattered radiation over a 254 cm2 imaging field reduces the number of discernable gray levels by 23% for a 5 cm thick breast and 34% for an 8 cm thick breast.