A multiparameter optimization of digital mammography

Thickness of Molybdenum Filter and Squared Contrast-to-Noise Ratio per Dose for Digital Mammography

OBJECTIVE

The objective of our study was to test whether the lesion-tissue contrast-to-noise ratio (CNR) at a given dose level can be improved by increasing the thickness of the molybdenum (Mo) filter currently used in digital mammography.

MATERIALS AND METHODS

We studied how the CNR between breast and a 5-mm simulated infiltrating ductal carcinoma (IDC) embedded in a 5-cm-thick breast changes with Mo filter thickness. We performed phantom imaging experiments by modifying the filter wheel of a Senographe 2000D unit with Mo filters that ranged from 15 to 90 microm in thickness. A 5-cm-thick 50% glandular-50% adipose breast phantom with a 5-mm insert simulating IDC was used as the phantom for all the cases. The CNRs between the breast phantom and the IDC insert were measured, and average glandular doses were calculated using a filtration-dependent X-ray spectra model and a breast dosimetry model based on a validated Monte Carlo simulation.

RESULTS

The lesion-tissue CNR at a given dose level increases with increasing Mo filter thickness from 15 to 90 microm. The measured squared CNR per dose increased by 8%, 14%, 17%, and 17% for 45-, 60-, 75-, and 90-microm Mo filters, respectively, compared with the standard 30-microm Mo filter. Meanwhile, the exposure times were increased by 35% (45 microm), 71% (60 microm), 177% (75 microm), and 229% (90 microm).

CONCLUSION

Increasing Mo filter thickness from 30 to 60 microm can increase lesion-tissue squared CNR per dose by 14% with a tolerable increase in the duration of exposure.

A conceptual optimisation strategy for radiography in a digital environment

Using a completely digital environment for the entire imaging process leads to new possibilities for optimisation of radiography since many restrictions of screen/film systems, such as the small dynamic range and the lack of possibilities for image processing, do not apply any longer. However, at the same time these new possibilities lead to a more complicated optimisation process, since more freedom is given to alter parameters. This paper focuses on describing an optimisation strategy that concentrates on taking advantage of the conceptual differences between digital systems and screen/film systems. The strategy can be summarised as: (a) always include the anatomical background during the optimisation, (b) perform all comparisons at a constant effective dose and (c) separate the image display stage from the image collection stage. A three-step process is proposed where the optimal setting of the technique parameters is determined at first, followed by an optimisation of the image processing. In the final step the optimal dose level-given the optimal settings of the image collection and image display stages-is determined.

Effect of area x-ray beam equalization on image quality and dose in digital mammography

In mammography, thick or dense breast regions persistently suffer from reduced contrast-to-noise ratio (CNR) because of degraded contrast from large scatter intensities and relatively high noise. Area x-ray beam equalization can improve image quality by increasing the x-ray exposure to under-penetrated regions without increasing the exposure to other breast regions. Optimal equalization parameters with respect to image quality and patient dose were determined through computer simulations and validated with experimental observations on a step phantom and an anthropomorphic breast phantom. Three parameters important in equalization digital mammography were considered: attenuator material (Z = 13-92), beam energy (22-34 kVp) and equalization level. A Mo/Mo digital mammography system was used for image acquisition. A prototype 16 x 16 piston driven equalization system was used for preparing patient-specific equalization masks. Simulation studies showed that a molybdenum attenuator and an equalization level of 20 were optimal for improving contrast, CNR and figure of merit (FOM = CNR2/dose). Experimental measurements using these parameters showed significant improvements in contrast, CNR and FOM. Moreover, equalized images of a breast phantom showed improved image quality. These results indicate that area beam equalization can improve image quality in digital mammography.

Optimization of technique factors for a silicon diode array full-field digital mammography system and comparison to screen-film mammography with matched average glandular dose

Contrast-detail experiments were performed to optimize technique factors for the detection of low-contrast lesions using a silicon diode array full-field digital mammography (FFDM) system under the conditions of a matched average glandular dose (AGD) for different techniques. Optimization was performed for compressed breast thickness from 2 to 8 cm. FFDM results were compared to screen-film mammography (SFM) at each breast thickness. Four contrast-detail (CD) images were acquired on a SFM unit with optimal techniques at 2, 4, 6, and 8 cm breast thicknesses. The AGD for each breast thickness was calculated based on half-value layer (HVL) and entrance exposure measurements on the SFM unit. A computer algorithm was developed and used to determine FFDM beam current (mAs) that matched AGD between FFDM and SFM at each thickness, while varying target, filter, and peak kilovoltage (kVp) across the full range available for the FFDM unit. CD images were then acquired on FFDM for kVp values from 23-35 for a molybdenum-molybdenum (Mo-Mo), 23-40 for a molybdenum-rhodium (Mo-Rh), and 25-49 for a rhodium-rhodium (Rh-Rh) target-filter under the constraint of matching the AGD from screen-film for each breast thickness (2, 4, 6, and 8 cm). CD images were scored independently for SFM and each FFDM technique by six readers. CD scores were analyzed to assess trends as a function of target-filter and kVp and were compared to SFM at each breast thickness. For 2 cm thick breasts, optimal FFDM CD scores occurred at the lowest possible kVp setting for each target-filter, with significant decreases in FFDM CD scores as kVp was increased under the constraint of matched AGD. For 2 cm breasts, optimal FFDM CD scores were not significantly different from SFM CD scores. For 4-8 cm breasts, optimum FFDM CD scores were superior to SFM CD scores. For 4 cm breasts, FFDM CD scores decreased as kVp increased for each target-filter combination. For 6 cm breasts, CD scores decreased slightly as kVp increased for Mo-Mo, but did not change significantly as a function of kVp for either Mo-Rh or Rh-Rh. For 8 cm breasts, Rh/Rh FFDM CD scores were superior to other target-filter combinations and increased significantly as kVp increased. These results indicate that low-contrast lesion detection was optimized for FFDM by using a softer x-ray beam for thin breasts and a harder x-ray beam for thick breasts, when AGD was kept constant for a given breast thickness. Under this constraint, optimum low-contrast lesion detection with FFDM was superior to that for SFM for all but the thinnest breasts.

The effect of the antiscatter grid on full-field digital mammography phantom images

Computer Analysis of Mammography Phantom Images (CAMPI) is a method for making quantitative measurements of image quality. This article reports on a recent application of this method to a prototype full-field digital mammography (FFDM) machine. Images of a modified ACR phantom were acquired on the General Electric Diagnostic Molybdenum Rhodium (GE-DMR) FFDM machine at a number of x-ray techniques, both with and without the scatter reduction grid. The techniques were chosen so that one had sets of grid and non-grid images with matched doses (200 mrads) and matched gray-scale values (1500). A third set was acquired at constant 26 kVp and varying mAs for both grid conditions. Analyses of the images yielded signal-to-noise-ratio (SNR), contrast and noise corresponding to each target object, and a non-uniformity measure. The results showed that under conditions of equal gray-scale value the grid images were markedly superior, albeit at higher doses than the non-grid images. Under constant dose conditions, the non-grid images were slightly superior in SNR (7%) but markedly less uniform (60%). Overall, the grid images had substantially greater contrast and superior image uniformity. These conclusions applied to the whole kVp range studied for the Mo-Mo target filter combination and 4 cm of breast equivalent material of average composition. These results suggest that use of the non-grid technique in digital mammography with the GE-DMR-FFDM unit, is presently not warranted. With improved uniformity correction procedure, this conclusion would change and one should be able to realize a 14% reduction in patient dose at the same SNR by using a non-grid technique.

Scattered radiation in scanning slot mammography

Monte Carlo simulations were used to quantify the amount of scattered radiation a scanning slot detector geometry designed for use in digital mammography. Ratios of the scatter to primary (S/P) x-ray photon energy absorbed in the detector were obtained for a Lucite phantom, and were investigated as a function of photon energy, phantom thickness, and slot detector width. Over a Lucite phantom thickness range of 2-6 cm, the S/P ratios range from about 0.10 to 0.17 for a 4 mm wide slot detector at the x-ray photon energies used in mammography. These ratios increased by a factor of approximately 1.8 when the slot width was increased to 10 mm. In general, 20 keV photons gave S/P ratios similar to those of a 30 kVp x-ray spectrum (Mo target + 30 microns Mo filtration). The use of a 3 cm air gap reduced the S/P ratios by a factor of between 2.5 and 3.4, depending on the phantom thickness. For a constant primary energy fluence, coherent scatter was reduced as photon energy increased, whereas Compton scatter increased with increasing photon energy. With no air gap, the contributions of coherent and Compton scatter were found to be equal at 25 keV, whereas the introduction of a 3 cm air gap resulted in equal contributions for the two scatter processes at 36 keV. A 10 mm wide slot detector consisting of a 36.7 mg/cm2 thick Gd2O2S:Tb phosphor screen was compared to an ideal detector absorbing all incident primary/scatter photons. Average differences in the S/P ratios for these two detectors were 7% with no air gap and approximately 4% with a 3 cm air gap. The results obtained in this study will assist in the design of an optimal slot detector for use in digital mammography.