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Spectral Mammography with X-Ray Optics and a Photon-Counting Detector
KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Early detection is vital to successfully treating breast cancer, and mammography screening is the most efficient and wide-spread method to reach this goal. Imaging low-contrast targets, while minimizing the radiation exposure to a large population is, however, a major challenge. Optimizing the image quality per unit radiation dose is therefore essential. In this thesis, two optimization schemes with respect to x-ray photon energy have been investigated: filtering the incident spectrum with refractive x-ray optics (spectral shaping), and utilizing the transmitted spectrum with energy-resolved photon-counting detectors (spectral imaging).

Two types of x-ray lenses were experimentally characterized, and modeled using ray tracing, field propagation, and geometrical optics. Spectral shaping reduced dose approximately 20% compared to an absorption-filtered reference system with the same signal-to-noise ratio, scan time, and spatial resolution. In addition, a focusing pre-object collimator based on the same type of optics reduced divergence of the radiation and improved photon economy by about 50%.

A photon-counting silicon detector was investigated in terms of energy resolution and its feasibility for spectral imaging. Contrast-enhanced tumor imaging with a system based on the detector was characterized and optimized with a model that took anatomical noise into account. Improvement in an ideal-observer detectability index by a factor of 2 to 8 over that obtained by conventional absorption imaging was found for different levels of anatomical noise and breast density. Increased conspicuity was confirmed by experiment. Further, the model was extended to include imaging of unenhanced lesions. Detectability of microcalcifications increased no more than a few percent, whereas the ability to detect large tumors might improve on the order of 50% despite the low attenuation difference between glandular and cancerous tissue. 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.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , x, 56 p.
Series
Trita-FYS, ISSN 0280-316X ; 2009:69
Keyword [en]
mammography; x-ray optics; photon counting; spectral shaping; spectral imaging; collimation; radiation dose; signal-to-noise ratio; quantum noise; anatomical noise; spatial resolution; x-ray flux;
National Category
Engineering and Technology Other Engineering and Technologies Atom and Molecular Physics and Optics Other Engineering and Technologies not elsewhere specified Electrical Engineering, Electronic Engineering, Information Engineering Medical Laboratory and Measurements Technologies Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:kth:diva-11641ISBN: 978-91-7415-516-7 (print)OAI: oai:DiVA.org:kth-11641DiVA: diva2:278600
Public defence
2009-12-18, Kollegiesalen, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100714Available from: 2009-12-04 Created: 2009-11-27 Last updated: 2010-07-15Bibliographically approved
List of papers
1. A Tunable Energy Filter for Medical X-Ray Imaging
Open this publication in new window or tab >>A Tunable Energy Filter for Medical X-Ray Imaging
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2008 (English)In: X-Ray Optics and Instrumentation, ISSN 1687-7632, Vol. 2008Article in journal (Refereed) Published
Abstract [en]

A multiprism lens (MPL) is a refractive X-ray lens, and its chromatic properties can be employed in an energy filtering setup to obtain a narrow tunable X-ray spectrum. We present the first evaluation of such a filter for medical X-ray imaging. The experimental setup yields a 6.6 gain of flux at 20 keV, and we demonstrate tunability by altering the energy spectrum to center also around 17 and 23 keV. All measurements are found to agree well with ray-tracing and a proposed geometrical model. Compared to a model mammography system with absorption filtering, the experimental MPL filter reduces dose 13–25% for 3–7 cm breasts if the spectrum is centered around the optimal energy. Additionally, the resolution is improved 2.5 times for a 5 cm breast. The scan time is increased 3 times but can be reduced with a slightly decreased energy filtering and resolution.

Place, publisher, year, edition, pages
Hindawi, 2008
Keyword
x-ray imaging, mammography, x-ray optics, multiprism lens, energy filtering, radiation dose
National Category
Other Engineering and Technologies not elsewhere specified Medical Laboratory and Measurements Technologies Radiology, Nuclear Medicine and Medical Imaging Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-11633 (URN)10.1155/2008/635024 (DOI)
Note
QC 20100713Available from: 2009-11-27 Created: 2009-11-27 Last updated: 2010-07-15Bibliographically approved
2. An efficient pre-object collimator based on an x-ray lens
Open this publication in new window or tab >>An efficient pre-object collimator based on an x-ray lens
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2009 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 36, no 2, 626-633 p.Article in journal (Refereed) Published
Abstract [en]

A multiprism lens (MPL) is a refractive x-ray lens with one-dimensional focusing properties. If used as a pre-object collimator in a scanning system for medical x-ray imaging, it reduces the divergence of the radiation and improves on photon economy compared to a slit collimator. Potential advantages include shorter acquisition times, a reduced tube loading, or improved resolution. We present the first images acquired with a MPL in a prototype for a scanning mammography system. The lens showed a gain of flux of 1.32 compared to a slit collimator at equal resolution, or a gain in resolution of 1.31–1.44 at equal flux. We expect the gain of flux in a clinical setup with an optimized MPL and a custom-made absorption filter to reach 1.67, or 1.45–1.54 gain in resolution.

Keyword
x-ray imaging, mammography, x-ray optics, multiprism lens, collimation, x-ray flux, acquisition time, resolution
National Category
Other Engineering and Technologies not elsewhere specified Atom and Molecular Physics and Optics Radiology, Nuclear Medicine and Medical Imaging Medical Laboratory and Measurements Technologies
Identifiers
urn:nbn:se:kth:diva-11634 (URN)10.1118/1.3062926 (DOI)000262852600037 ()2-s2.0-59249085647 (Scopus ID)
Note
QC 20100713Available from: 2009-11-27 Created: 2009-11-27 Last updated: 2017-12-12Bibliographically approved
3. A low-absorption x-ray energy filter for small-scale applications
Open this publication in new window or tab >>A low-absorption x-ray energy filter for small-scale applications
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2009 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 17, no 14, 11388-11398 p.Article in journal (Refereed) Published
Abstract [en]

We present an experimental and theoretical evaluation of an x-ray energy filter based on the chromatic properties of a prism-array lens (PAL). It is intended for small-scale applications such as medical imaging. The PAL approximates a Fresnel lens and allows for high efficiency compared to filters based on ordinary refractive lenses, however at the cost of a lower energy resolution. Geometrical optics was found to provide a good approximation for the performance of a flawless lens, but a field-propagation model was used for quantitative predictions. The model predicted a 0.29 ΔE/E energy resolution and an intensity gain of 6.5 for a silicon PAL at 23.5 keV. Measurements with an x-ray tube showed good agreement with the model in energy resolution and peak energy, but a blurred focal line contributed to a 29% gain reduction. We believe the blurring to be caused mainly by lens imperfections, in particular at the periphery of the lens.

Keyword
X-ray optics, Diffractive lenses, Refractive lenses, Filters, Medical x-ray imaging, Radiation, Wave propagation
National Category
Atom and Molecular Physics and Optics Radiology, Nuclear Medicine and Medical Imaging Other Engineering and Technologies not elsewhere specified Medical Laboratory and Measurements Technologies
Identifiers
urn:nbn:se:kth:diva-11635 (URN)10.1364/OE.17.011388 (DOI)000267761200018 ()2-s2.0-67650544809 (Scopus ID)
Note
QC 20100713Available from: 2009-11-27 Created: 2009-11-27 Last updated: 2017-12-12Bibliographically approved
4. Energy filtering with x-ray lenses: Optimization for photon-counting mammography
Open this publication in new window or tab >>Energy filtering with x-ray lenses: Optimization for photon-counting mammography
2010 (English)In: Radiation Protection Dosimetry, ISSN 0144-8420, E-ISSN 1742-3406, Vol. 139, 339-342 p.Article in journal (Refereed) Published
Abstract [en]

Chromatic properties of the multi-prism and prism-array x-ray lenses (MPL and PAL) can potentially be utilized for efficient energy filtering and dose reduction in mammography. The line-shaped foci of the lenses are optimal for coupling to photon-counting silicon strip detectors in a scanning system. A theoretical model was developed and used to investigate the benefit of two lenses compared to an absorption-filtered reference system. The dose reduction of the MPL filter was 15% compared to the reference system at matching scan time, and the spatial resolution was higher. The dose of the PAL-filtered system was found to be 20% lower than for the reference system at equal scan time and resolution, and only 20% higher than for a monochromatic beam. An investigation of some practical issues remains, including the feasibility of brilliant-enough x-ray sources and manufacturing of a polymer PAL.

Keyword
x-ray imaging, mammography, x-ray optics, multiprism lens, prism-array lens, energy filtering, radiation dose, spatial resolution, exposure time
National Category
Other Engineering and Technologies not elsewhere specified Atom and Molecular Physics and Optics Radiology, Nuclear Medicine and Medical Imaging Medical Laboratory and Measurements Technologies
Identifiers
urn:nbn:se:kth:diva-11640 (URN)10.1093/rpd/ncq049 (DOI)000277738200064 ()2-s2.0-77953329327 (Scopus ID)
Note
QC 20100714. Uppdaterad från submitted till published (20100714)Available from: 2009-11-27 Created: 2009-11-27 Last updated: 2017-12-12Bibliographically approved
5. Energy resolution of a photon-counting silicon strip detector
Open this publication in new window or tab >>Energy resolution of a photon-counting silicon strip detector
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2010 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, 156-162 p.Article in journal (Refereed) Published
Abstract [en]

A photon-counting silicon strip detector with two energy thresholds was investigated for spectral X-ray imaging in a mammography system. Preliminary studies already indicate clinical benefit of the detector, and the purpose of the present study is optimization with respect to energy resolution. Factors relevant for the energy response were measured, simulated, or gathered from previous studies, and used as input parameters to a cascaded detector model. Threshold scans over several X-ray spectra were used to calibrate threshold levels to energy, and to validate the model. The energy resolution of the detector assembly was assessed to range over ΔE/E=0.12–0.26 in the mammography region. Electronic noise dominated the peak broadening, followed by charge sharing between adjacent detector strips, and a channel-to-channel threshold spread. The energy resolution may be improved substantially if these effects are reduced to a minimum. Anti-coincidence logic mitigated double counting from charge sharing, but erased the energy resolution of all detected events, and optimization of the logic is desirable. Pile-up was found to be of minor importance at typical mammography rates.

Keyword
Spectral X-ray imaging, Mammography; Silicon strip detector, Photon counting, Energy resolution, Cascaded detector model
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Medical Laboratory and Measurements Technologies Other Engineering and Technologies not elsewhere specified Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:kth:diva-11636 (URN)10.1016/j.nima.2009.10.152 (DOI)000274772800023 ()2-s2.0-77949279240 (Scopus ID)
Note
QC 20100714Available from: 2009-11-27 Created: 2009-11-27 Last updated: 2017-12-12Bibliographically approved
6. Contrast-enhanced spectral mammography with a photon-counting detector
Open this publication in new window or tab >>Contrast-enhanced spectral mammography with a photon-counting detector
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2010 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 37, no 5, 2017-2029 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: 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. In particular, the detectability of a contrast agent can be improved over a lumpy background. We have investigated a photon-counting spectral imaging system with two energy bins for contrast-enhanced mammography. System optimization and the potential benefit compared to conventional non-energy-resolved imaging was studied.

Methods: A framework for system characterization was set up that included quantum and anatomical noise, and a theoretical model of the system was benchmarked to phantom measurements.

Results: It was found that optimal combination of the energy-resolved images corresponded approximately to minimization of the anatomical noise, and an ideal-observer detectability index could be improved more than a factor of two compared to absorption imaging in the phantom study. In the clinical case, an improvement close to 80% was predicted for an average glandularity breast, and a factor of eight for dense breast tissue. Another 70% was found to be within reach for an optimized system.

Conclusions: Contrast-enhanced spectral mammography is feasible and beneficial with the current system, and there is room for additional improvements.

Keyword
spectral imaging, mammography, contrast agent, photon counting, dual-energy subtraction, energy weighting, anatomical noise
National Category
Other Engineering and Technologies not elsewhere specified Computer Vision and Robotics (Autonomous Systems) Radiology, Nuclear Medicine and Medical Imaging Medical Laboratory and Measurements Technologies
Identifiers
urn:nbn:se:kth:diva-11637 (URN)10.1118/1.3371689 (DOI)000277242800009 ()2-s2.0-77952404690 (Scopus ID)
Funder
EU, European Research Council, LSHCCT2007-037642
Note
QC 20100714 ändrad från submitted till published 20110103Available from: 2009-11-27 Created: 2009-11-27 Last updated: 2017-12-12Bibliographically approved

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