E. Fredenberg, E. Roessl, T. Koehler, U. van Stevendaal, I. Schulze-Wenck, N. Wieberneit, M. Stampanoni, Z. Wang, R. Kubik-Huch, N. Hauser, M. Lundqvist, M. Danielsson, M. Åslund
Phase-contrast imaging is an emerging technology that may increase the signal-difference-to-noise ratio in medical
imaging. One of the most promising phase-contrast techniques is Talbot interferometry, which, combined with
energy-sensitive photon-counting detectors, enables spectral differential phase-contrast mammography. We have
evaluated a realistic system based on this technique by cascaded-systems analysis and with a task-dependent
ideal-observer detectability index as a figure-of-merit. Beam-propagation simulations were used for validation
and illustration of the analytical framework. Differential phase contrast improved detectability compared to
absorption contrast, in particular for fine tumor structures. This result was supported by images of human
mastectomy samples that were acquired with a conventional detector. The optimal incident energy was higher
in differential phase contrast than in absorption contrast when disregarding the setup design energy. Further,
optimal weighting of the transmitted spectrum was found to have a weaker energy dependence than for absorption
contrast. Taking the design energy into account yielded a superimposed maximum on both detectability as a
function of incident energy, and on optimal weighting. Spectral material decomposition was not facilitated by
phase contrast, but phase information may be used instead of spectral information.
We have designed a mammography system that for the first time combines photon-counting spectral imaging with
tomosynthesis. The present study is a comprehensive physical evaluation of the system; tomosynthesis, spectral
imaging, and the combination of both are compared using an ideal-observer model that takes anatomical noise
into account. Predictions of signal and noise transfer through the system are verified by contrast measurements
on a tissue phantom and 3D measurements of MTF and NPS. Clinical images acquired with the system are
discussed in view of the model predictions.
Spectral imaging is a method in medical x-ray imaging to extract information about the object constituents by the material-specific energy dependence of x-ray attenuation. Contrast-enhanced spectral imaging has been
thoroughly investigated, but unenhanced imaging may be more useful because it comes as a bonus to the
conventional non-energy-resolved absorption image at screening; there is no additional radiation dose and no need
for contrast medium. We have used a previously developed theoretical framework and system model that include
quantum and anatomical noise to characterize the performance of a photon-counting spectral mammography
system with two energy bins for unenhanced imaging. The theoretical framework was validated with synthesized
images. Optimal combination of the energy-resolved images for detecting large unenhanced tumors corresponded
closely, but not exactly, to minimization of the anatomical noise, which is commonly referred to as energy
subtraction. In that case, an ideal-observer detectability index could be improved close to 50% compared to absorption imaging. Optimization with respect to the signal-to-quantum-noise ratio, commonly referred to as energy weighting, deteriorated detectability. For small microcalcifications or tumors on uniform backgrounds, however, energy subtraction was suboptimal whereas energy weighting provided a minute improvement. The performance was largely independent of beam quality, detector energy resolution, and bin count fraction. It is clear that inclusion of anatomical noise and imaging task in spectral optimization may yield completely different results than an analysis based solely on quantum noise.
We present the first evaluation of a recently developed silicon-strip detector for photon-counting dual-energy
breast tomosynthesis. The detector is well suited for tomosynthesis with high dose efficiency and intrinsic scatter
rejection. A method was developed for measuring the spatial resolution of a system based on the detector in terms
of the three-dimensional modulation transfer function (MTF). The measurements agreed well with theoretical
expectations, and it was seen that depth resolution was won at the cost of a slightly decreased lateral resolution.
This may be a justifiable trade-off as clinical images acquired with the system indicate improved conspicuity of
breast lesions. The photon-counting detector enables dual-energy subtraction imaging with electronic spectrumsplitting.
This improved the detectability of iodine in phantom measurements, and the detector was found to be
stable over typical clinical acquisition times. A model of the energy resolution showed that further improvements
are within reach by optimization of the detector.
Dual-energy subtraction imaging (DES) is a method to improve the detectability of contrast agents over a lumpy
background. Two images, acquired at x-ray energies above and below an absorption edge of the agent material,
are logarithmically subtracted, resulting in suppression of the signal from the tissue background and a relative
enhancement of the signal from the agent. Although promising, DES is still not widely used in clinical practice.
One reason may be the need for two distinctly separated x-ray spectra that are still close to the absorption edge,
realized through dual exposures which may introduce motion unsharpness.
In this study, electronic spectrum-splitting with a silicon-strip detector is theoretically and experimentally
investigated for a mammography model with iodinated contrast agent. Comparisons are made to absorption
imaging and a near-ideal detector using a signal-to-noise ratio that includes both statistical and structural noise.
Similar to previous studies, heavy absorption filtration was needed to narrow the spectra at the expense of a
large reduction in x-ray flux. Therefore, potential improvements using a chromatic multi-prism x-ray lens (MPL)
for filtering were evaluated theoretically. The MPL offers a narrow tunable spectrum, and we show that the
image quality can be improved compared to conventional filtering methods.
The multi-prism lens (MPL) is a refractive x-ray lens consisting of two rows of prisms facing each other at an
angle. Rays entering the lens at the periphery will encounter a larger number of prisms than will central ones,
hence experiencing a greater refraction. The focusing effect of the MPL can be used to gather radiation from a
large aperture onto a smaller detector, and accordingly to make better use of the available x-ray flux in medical
x-ray imaging. Potential advantages of a better photon economy include shorter acquisition times, a reduced
tube loading, or an improved resolution. Since the focusing effect is one-dimensional it matches the design of
scanning systems.
In this study we present the first images acquired with an MPL instead of the pre-breast slit collimator in a
scanning mammography system. According to the measurements, the MPL is able to increase the flux 32% at
equal resolution compared to the slit collimator, or to improve the resolution 2.4 mm-1 at equal flux. If used
with a custom-made absorption filter in a clinical set-up, the gain of flux of the MPL is expected to be at least
45%, and the corresponding improvement in resolution to be 3 mm-1.
Measurements and simulations of the signal-difference-to-noise ratio (SDNR) and average glandular dose (AGD) have been performed on a photon counting full-field digital mammography system to determine the optimal operating conditions. Several beam qualities were experimentally evaluated by using different combinations of tube voltage, added filters and thickness of BR12 with a tungsten target
x-ray tube. The SDNR and AGD were also calculated theoretically for an extended number of operating conditions and a more accurate breast
model. As figure of merit for each operating condition, a spectral quantum efficiency (SQE) was calculated as the polychromatic SDNR squared over the optimal monochromatic SDNR squared at the same AGD. The theoretical model agreed within ±4% relative the measured SDNR throughout the evaluated breast thickness (30-70 mm) and tube voltage range (26-38 kV). The optimization was performed with a constant
SDNR-rate as compared to using a fixed filter thickness. The optimal combinations of tube voltage-filter material were: 32 kV-Ag, 34
kV-Cd, 36 kV-Sn for a breast thickness of 30, 50 and 70 mm respectively. These K-edge filter materials increased the SQE by less than 4% compared to the optimal Al filtration.
KEYWORDS: Sensors, Breast, Mammography, Signal detection, X-rays, Image quality, Tissues, Digital mammography, Signal to noise ratio, Atomic force microscopy
In mammography, there is an optimal photon energy and current time product that produce the required image quality at the minimal dose. The task of an automatic exposure control (AEC), in full field digital mammography (FFDM) is to minimize the dose by using optimized exposure settings. Each point in a mammogram has different radiological thickness. A conventional AEC samples the thickness in some regions to set the current time product and possibly also the beam quality. We define an ideal AEC as one that optimizes the beam quality and exposure in each point to produce a constant contrast-to-noise ratio (CNR) of structures of interest throughout the image. This paper presents the results from a theoretical evaluation of an AEC proposed for a scanning photon-counting FFDM system. The geometry enables the AEC to use information from the leading detector line to adjust the scan velocity during the scan. Thus, the irradiation can be better optimized in the scanning-direction as compared to a conventional AEC. The scan time is further reduced by increased velocity over sections that contain no tissue. The results are quantified in terms of reduction of entrance dose and scan time. The presented AEC is compared to an ideal AEC, a conventional AEC and is also benchmarked against an ideal regulator. The effect of the detector width is evaluated. Compared to a conventional AEC, both evaluated on a set of 266 mammograms, the ideal AEC would reduce the entrance dose by 39% on average while the proposed AEC for scanning systems reduces the entrance dose by 10-20% and scan-time by 25-32% on average, depending on detector width.
KEYWORDS: Sensors, Breast, Photons, Monte Carlo methods, Digital mammography, Signal detection, Scatter measurement, X-rays, Lead, Computer simulations
Measurements and Monte Carlo simulations were used to investigate
the scatter properties of a scanned multi-slit digital mammography system. Scatter to primary ratio (S/P) in the center of the image field was calculated for different thickness of breast equivalent material and different tube potentials. The simulated model also varied the angular acceptance, the number of slits and the distance between the slits of a dedicated scatter rejection device.
In addition to the expected scatter from the breast equivalent material, scatter within the detector contributes to the S/P-ratio. The main part of the scatter is identified as coming from this process. Measured total S/P-ratios below 3% are reported for breast range 3-8 cm. The scatter-DQE is used as figure-of-merit for comparison to other imaging geometries and scatter rejection schemes.
Sectra Microdose is the first single photon counting mammography detector. An edge-on crystalline silicon detector is connected to application specific integrated circuits that individually process each photon. The detector is scanned across the breast and the rejection of scattered radiation exceeds 97% without the use of a Bucky. Processing of each x-rays individually enables an optimization of the information transfer from the x-rays to the image in a way previously not possible. Combined with an almost absence of noise from scattered radiation and from electronics we foresee a possibility to reduce the radiation dose and/or increase the image quality. We will discuss fundamental features of the new direct photon counting technique in terms of dose efficiency and present preliminary measurements for a prototype on physical parameters such as Noise Power Spectra (NPS), MTF and DQE.
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