Beam quality optimization in mammography traditionally considers detection of a target obscured by quantum
noise on a homogenous background. It can be argued that this scheme does not correspond well to the
clinical imaging task because real mammographic images contain a complex superposition of anatomical structures,
resulting in anatomical noise that may dominate over quantum noise. Using a newly developed spectral
mammography system, we measured the correlation and magnitude of the anatomical noise in a set of mammograms.
The results from these measurements were used as input to an observer-model optimization that included
quantum noise as well as anatomical noise. We found that, within this framework, the detectability of tumors
and microcalcifications behaved very differently with respect to beam quality and dose. The results for small
microcalcifications were similar to what traditional optimization methods would yield, which is to be expected
since quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however,
low-frequency anatomical noise was the limiting factor. Because anatomical structure has similar energy dependence
as tumor contrast, optimal x-ray energy was significantly higher and the useful energy region wider than
traditional methods suggest. Measurements on a tissue phantom confirmed these theoretical results. Furthermore,
since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially
without sacrificing tumor detectability. Exposure settings used clinically are therefore not necessarily optimal
for this imaging task. The impact of these findings on the mammographic imaging task as a whole is, however,
at this stage unclear.
Breast tomosynthesis is an imaging modality that recently became available for breast examination. For conventional
projection mammography quality control procedures are well described. For breast tomosynthesis, on the other hand,
such procedures have not yet been established. In this paper we propose a simple method and phantom for daily quality
control (DQC). With DQC image quality problems arising after acceptance of the system should be detected. Therefore,
the DQC procedure needs to monitor the stability of the most critical components of the system over time. For breast
tomosynthesis we assume that the most critical items are the image receptor, X-ray tube and the tomosynthesis motion.
In the proposed procedure the image receptor homogeneity and system stability are evaluated using an image of a
homogeneous block of PMMA. The z-resolution is assumed to be dependent on the tomosynthesis motion. To monitor
this motion the nominal z-resolution using the slice sensitive profile is measured. Shading artefacts that arise due to
objects with high attenuation are also typical for tomosynthesis systems. Analysing those artefacts may provide
additional information about the tomosynthesis motion. The proposed DQC procedure has been evaluated on two
different breast tomosynthesis systems: A multi slit scanning system and a system using a stationary a-Se detector.
Preliminary results indicate that the proposed method is useful for DQC, although some minor changes to the phantoms
are advised. To verify that this method detects image quality problems sufficiently, more experience with different DBT
systems, over longer periods of time are needed.
In digital mammography noise characteristics are measured in quality control procedures. In the European Guidelines a
method of measurement to investigate noise in digital mammography systems was proposed to evaluate the presence of
additional noise beside quantum noise. However this method of noise analysis does not discriminate sufficiently between
systems with and without additional noise. Therefore a different noise analysis is proposed. In this analysis the noise of a
digital system is subdivided into three components: electronic, quantum and structured noise and the noise dose
dependency of these components is studied. The usefulness of this analysis in both the frequency and spatial domain is
investigated on a number of DR and CR systems.
The results show that large differences between digital mammography systems exists. Some systems do have a large
range in detector dose for which quantum noise is the largest noise component. For one system however, electronic and
structured noise are more dominant. In addition to the differences between systems smaller differences in noise
characteristics exist between different target-filter combinations on a particular system. These differences might be
attributed to the limited flatfield calibration, the heel effect and difference in sensitivity. The noise analysis in both the
frequency and spatial domain give useful information about the noise characteristics of systems. The analysis in the
spatial domain is relatively easy to perform and to interpret. This analysis might be suitable for QC purposes. The
analysis in the frequency domain does give additional information and might be used for thorough investigations.
The purpose of this study was to evaluate physical characteristics of a clinical prototype for full-field digital mammography (FFDM) with an amorphous selenium (a-Se) flat-panel detector (FPD) and to compare these results with the currently available systems for FFDM. The effective FPD area consists of 2816 x 2016 pixel matrix having a pixel pitch of 85μm. This yields to theoretical maximum spatial frequency of ~5.9lp/mm. The basic performance of Instrumentarium clinical prototype direct mammography system has been assessed on the basis of measured response curve, the modulation transfer function (MTF), the noise power spectrum (NPS), the noise equivalent quanta (NEQ) and the detective quantum efficiency (DQE) in the clinical setting. The system shows a linear response curve over a dynamic range from 0.4 mR to 57 mR. The presampling MTF was found to be approximately 0.91, 0.72 and 0.50 at 2, 4 and 5.9 (Nyquist frequency) lp/mm. The NEQ is linearly related to the exposure starting from about 8 mR above which value the system can be considered noise quantum limited. The DQE, evaluated in clinical conditions (28kVp Mo-Mo spectrum hardened by 4cm of PMMA) is at close to zero spatial frequency approximately 69% at 46.4 mR and 61% at 8.3 mR. Below 8 mR the DQE(0) falls to 54.4%, 46% and 32.5% at 5.2, 3.0 and 1.4 mR respectively due to structured and electronic noise. The results of quantitative analysis demonstrate a high MTF as we expected due to direct conversion technology and a high DQE over the exposure range from 8 mR to 50 mR. The NEQ shows that the system can be considered noise quantum limited above 8 mR suggesting the exposure level that should be set in the clinical practice to ensure an adequate image quality.
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