Volume 33, Issue 6, June 2006
Index of content:
- Imaging Scientific Session: Room 330 D
- Advances in Radiographic Imaging
WE‐E‐330D‐01: The Production of Ultrafast Bright K‐Alpha X‐Rays From Laser Produced Plasmas for Medical Imaging33(2006); http://dx.doi.org/10.1118/1.2241791View Description Hide Description
Purpose: To show the potential of improving image quality with a cleaner, brighter, quasi‐monochromatic X‐ray micro‐source via laser produced plasmas(LPP).Method and Materials: First generation targets consisting of ten micron thick gold formed into free‐standing pyramids have been built. PIC (Particle‐In‐Cell) simulations have been performed in order to validate this target geometry. Preliminary experiments with a Ti‐Sapphire CPA laser have been achieved with these targets. Second generation parabolic cone targets with an optimal angle for electron transport have also been built. This new nano‐fabricated target could optimize X‐ray source characteristics. Results:PIC (Particle‐In‐Cell) simulations show that conical targets optically guide laser light resulting in a higher density of hot electrons at the apex These simulations show a possible ten times augmentation in hot electron density and a three times increase in electron temperature with a conical verses flat target. This increase in collimated suprathermal electrons boosts total photon yield as well as possibly enhancing line emission verses the bremsstrahlung continuum. Preliminary experiments demonstrate a three‐fold higher X‐ray yield and a two‐fold reduction in focal spot with the pyramidal verses the flat target. Furthermore, the geometry of the conical targets not only reduces focal spot size to a few microns and pulse duration to a couple picoseconds, but allows the particles to escape the target perpendicular to the surface resulting in a particle‐free, ultra‐short X‐ray micro‐beam. Conclusion: Comparing LPP X‐ray source parameters to that of a standard X‐ray tube shows substantial improvements in focal spot size, photon flux, spectral range and emission duration. Focusing on target design can provide a cleaner, brighter, quasi‐monochromatic X‐ray source that could improve image quality in any medicaldiagnostic regime. Such advancements show promising applications in mammography and angiography. Conflict of Interest: Research sponsored by DOE/NNSA under University of Nevada Reno grant #DE‐FC52‐01NV14050.
33(2006); http://dx.doi.org/10.1118/1.2241792View Description Hide Description
Purpose: Coherent scatterimaging has been developed to elucidate the chemical composition of calcifications. Breast calcifications can be divided into two broad categories. Type I are calcium oxalate dehydrate, while Type II are calcium hydroxyapatite. Type II calcifications are known to be associated with carcinoma. It is generally accepted that the exclusive finding of type I calcifications is indicative of benign lesions. Method and Materials: An imaging system has been built that utilizes a molybdenum target x‐ray tube with niobium filtration to isolate the molybdenum Kαcharacteristic radiation. The system is designed to interrogate calcifications in the field with a pencil‐beam of radiation. The transmitted beam is attenuated, and the scattered beam is recorded on an x‐ray image intensifier optically‐coupled to a CCD camera. The system is typically operated at 36 kVp and 25–100 mAs. Results: Reagent grade calcium oxalate and calcium hydroxyapatite were made into blocks of thickness 42–510 mg/cm2. Pinhole sizes varying from 0.3–2.0 mm in diameter were tested to observe the effects of irradiation area on the resolution of the diffraction patterns. All pinhole sizes produced distinctive spectra, but a pinhole size of 0.75 mm appears to be near optimal, as there is sufficient angular resolution and photon fluence to produce distinguishable diffraction patterns. Scatteringmaterials (simulating glandular tissue and fat) were placed upstream and downstream of the calcific material to probe the influence of the surrounding tissue on diffraction. Material thicknesses >1 cm dramatically degraded the measured diffraction patterns. Analysis of diffraction patterns show that calcifications are readily discernable based upon their scatteringcharacteristics. We are able to match these diffraction patterns to calculated theoretical patterns when the later is convolved with a Gaussian‐based filter. Conclusion: We have demonstrated in proof‐of‐principle that we can discern different types of calcifications using coherent scatterimaging.
33(2006); http://dx.doi.org/10.1118/1.2241793View Description Hide Description
Purpose: This study is aimed at investigating the feasibility of high‐resolution contrast enhanced digital mammography (CEDM). Method and Materials: Recent studies report certain promising aspects of contrastmammography [Jong et al., Radiology 228, 842–50, 2003] for identifying subtle lesions that might not be detectable by conventional mammography. In this study we investigate certain physical aspects of high‐resolution CEDM. The objective was to study the feasibility of high resolution and low dose CEDM with acceptable contrast characteristics. We used a prototype imager [Vedantham et al., Med Phys 31, 1462–72, 2004] that consists of a 2 × 2, CCD array. The imager was operated in a 78 μm mode by pixel binning. Computational studies with a 49 kVp, W spectrum with 0.6 mm Cu added filtration (1st HVL: 1.9 mm of Al) indicated dose levels in the range of 0.1–0.5 mGy for a 5 cm thick, 50% glandular breast for the entire mammography exam. Theoretical modeling was performed using the parallel cascaded approach described by Cunningham and Yao [Proc. SPIE 3336, 220–30, 1998, Med Phys 28, 2020–38, 2001] for various physical conditions. In addition, experimental evaluation of the physical characteristics of the imager was conducted. Results: The resolution characteristics at 10% MTF was ∼7.8 and ∼4.2 cycles/mm and the DQE(0) estimate was ∼0..4 and ∼0.65 for 150 and 450 μm thick CsI:Tl scintillators respectively. Model results for pixel size range of 39–156 μm and CsI:Tl thickness range of 150–300 μm indicate that a 250–300 μm thick CsI scintillator with an imager pixel size of 78 μm could potentially offer a reasonable trade‐off between spatial resolution and DQE(f) characteristics. Conclusion: The results suggest that high‐resolution CEDM appears to be feasible at dose levels substantially lower then digital mammography. This research was supported in part by: NIH‐NIBIB Grant RO1‐EB002123 and the Georgia Cancer Coalition.
WE‐E‐330D‐04: High‐Performance Dual‐Energy Imaging with a Flat‐Panel Detector: Answering the Challenge of Dual‐KVp Flood‐Field Correction33(2006); http://dx.doi.org/10.1118/1.2241794View Description Hide Description
Purpose: Flood‐Field correction is a critical step in achieving high image quality in digital radiography (DR) and dual‐energy (DE) imaging. The optimal Dark‐Flood correction scheme suggests collection of Flood‐Fields at the same technique as the Projection data. In practical applications calibration data are often collected at the start of the day, and Flood‐Fields not collected for all techniques. The problem of proper Flood‐Field correction is compounded in DE imaging where two projections at different energies need to be considered. The purpose of this study is to quantitatively examine the effects of various Flood‐Field correction schemes on DE imaging performance. Method and Materials: In DE imaging two Projections are collected: a low‐energy image (e.g., 60–90 kVp) and a high‐energy image (e.g., 120–150 kVp). Five Flood‐Field correction schemes were considered: optimal correction (Flood‐Field at the same kVp as the Projection) and four sub‐optimal cases (variations wherein the Flood‐Field kVp is different from that of the Projection). Imaging performance was evaluated in terms of the uniformity, noise‐power spectrum (NPS), and detective quantum efficiency (DQE) in Projection and DE image data. Phantom images were used to assess the contrast‐to‐noise ratio and perceived image quality of DE images processed under each correction scheme. Results: The results reveal a systematic degradation in the performance of the corrections as energy separation between the Projections and the Flood‐Field increases. Sub‐optimal correction schemes degraded imaging performance significantly: image uniformity degraded by a factor of 5–10; soft‐tissue contrast degraded by ∼13%; low‐frequency NPS was significantly increased; and DQE was degraded by >10% at low‐ and mid‐frequencies. Conclusion: The choice of Flood‐Field correction scheme has significant impact on DE imaging performance. This study provides valuable guidance in the implementation of a high‐performance calibration scheme for DE imaging. Deployment in a pre‐clinical DE chest imaging system at our institution is underway.
WE‐E‐330D‐05: Investigation of Imaging Performance and Acquisition Technique for a New Dual‐Energy Chest Imaging System33(2006); http://dx.doi.org/10.1118/1.2241795View Description Hide Description
Purpose: A novel, high‐performance, cardiac‐gated dual‐energy (DE) chest system is under development in our lab. This paper investigates the influence of key image acquisition technique parameters (viz., selection of kVp, filtration, and dose) on DE imaging performance. Method and Materials: Experiments were conducted on a DE imaging bench with a custom‐built phantom containing simulated lung nodules of varying contrast. Performance was quantified in terms of nodule contrast‐noise ratio (CNRDE) in DE ‘tissue‐only’ images. Low‐ and high‐kVp were varied from 60–90 kVp and 120–150 kVp, respectively. Differential added filtration in low‐ and high‐kVp projections was analyzed in terms of soft‐tissue CNRDE both theoretically across the entire Periodic Table (Z=1−92) and experimentally for specific material types (Al, Ce, Cu, and Ag). Allocation of dose (defined A=ESDlow/ESDhigh) between low‐ and high‐energy projections was analyzed at various levels of total entrance surface dose, ESD, over a broad range of allocation. Results: The results provide valuable guidance of technique selection for high‐performance DE imaging. Optimal performance was achieved at a technique of [60/130] kVp, increasing soft‐tissue CNRDE by 32% compared to [90/120] kVp. Differential added filtration [0.2 mm Ce / 0.6 mm Ag] increased soft‐tissue CNRDE by 21% compared to the undifferentiated case ([1 mm Al / 1 mm Al]). Dose allocation was found to have significant influence on performance, with CNRDE increasing by more than ∼30% for A<1 compared to higher A>3 (with optima suggested in the range A∼0.3–0.5). Conclusion: Knowledgeable selection of kVp pairs, differential added filtration, and dose allocation provide significant increase in the soft‐tissue CNR of DE images compared to conventional or sub‐optimal techniques. Quantitative theoretical and experimental evaluation demonstrates the importance of optimized acquisition techniques for high‐performance DE imaging and guides the implementation of a novel DE imaging system under development for pre‐clinical imaging trials.
WE‐E‐330D‐06: Conceptual Examination of Conformal, Transparent, Indirect Detection, Active Matrix Mammographic Imagers33(2006); http://dx.doi.org/10.1118/1.2241796View Description Hide Description
Purpose: The recent development of techniques to create high‐quality amorphous silicon (a‐Si:H) at relatively low deposition temperatures enables the creation of active matrix arrays of thin‐film transistors(TFTs) on very thin, flexible plastic sheets, rather than on thick, rigid glass substrates. In this presentation, an examination of the potential advantages and theoretical performance of indirect detectionmammographicimagers based upon such arrays will be reported. Methods and Materials: Prototype active matrix arrays of thin‐film transistors based on low‐temperature a‐Si:H and deposited on plastic substrates have demonstrated TFT performance essentially equivalent to that of devices produced with conventional a‐Si:H. In addition, other prototype imagers have demonstrated that the incorporation of continuous photodiode structures can provide improved signal gain compared to arrays with discrete photodiodes at small pixel pitches. Techniques based on Monte Carlo simulations and cascaded systems analysis, parameterized by empirical information obtained from these early prototypes, have been employed to explore the performance of optimized imaging array designs operated under mammographicimaging conditions. Results: The excellent performance of transistors and photodiodes fabricated from low temperature a‐Si:H, coupled with the incorporation of continuous photodiode structures, provides good signal and noise characteristics, even for sub‐100 μm pitch designs. In addition, the flexibility and x‐ray transparency of a thin plastic substrate allows for the possibility of conforming the shape of the detector to an arc and to position the scinitillator (and opposing active matrix array circuits) on the opposite side of the substrate relative to the x‐ray source — leading to improvements in spatial resolution and DQE. Conclusion: Early investigations of the potential performance of indirect detection active matrix mammographicimagers based on low‐temperature a‐Si:H and fabricated on plastic substrates suggest that significant advantages would accrue from the development and implementation of such devices.
WE‐E‐330D‐07: Empirical Studies of Polycrystalline Silicon‐Based Flat‐Panel Imagers Incorporating Pixel‐Amplifiers33(2006); http://dx.doi.org/10.1118/1.2241797View Description Hide Description
Purpose: To investigate the potential of achieving significant improvements in the DQE performance of active‐matrix flat‐panel imagers at low fluoroscopic exposures and high spatial frequencies through incorporation of novel pixel architectures based on polycrystallinesilicon thin‐film transistors(TFTs). Methods and Materials: Detailed empirical studies have recently been performed on the signal and noise characteristics of a series of arrays incorporating poly‐Si TFTs. These indirect detection designs involved three pixel architectures employing either a single TFT switch, a single‐stage amplifier, or a dual‐stage amplifier — along with a continuous photodiode structure. Determinations of MTF, NPS, and DQE, as well as of individual pixel properties (sensitivity, linearity, trapping, noise) were performed under fluoroscopic and radiographic conditions. Circuit simulations were also performed to explore the potential performance of these and other hypothetical array designs. Results: The studies indicate that the high mobilities of poly‐Si lead to potential frame rates of at least an order of magnitude greater than those of conventional arrays with a‐Si:H TFTs. In addition, the single‐ and dual‐stage pixel‐amplifier arrays demonstrate signal gain (∼×10 and ∼×25, respectively) very close to design expectations. Furthermore, empirical data taken from these early prototypes demonstrate a small, but non‐negligible enhancement in signal‐to‐noise performance compared to that of similar arrays using conventional designs, as a result of pixel amplification and the use of repeated, non‐destructive readout. Analysis based on these empirical results and circuit simulations indicates that, with circuit design optimization and improved TFT quality, further significant enhancement of performance should be possible. Conclusion: These results indicate that substantial improvements in DQE performance are possible through incorporation of poly‐Si circuits in flat panel pixel designs. Factors limiting the performance of present designs will be described and future steps in the development of this technology will be discussed. This work is supported by NIH grant R01 EB000558.