Image quality evaluation for a clinical organ-targeted PET camera

Introduction

A newly developed clinical organ-targeted Positron Emission Tomography (PET) system (also known as Radialis PET) is tested with a set of standardized and custom tests previously used to evaluate the performance of Positron Emission Mammography (PEM) systems.

Methods

Imaging characteristics impacting standardized uptake value (SUV) and detectability of small lesions, namely spatial resolution, linearity, uniformity, and recovery coefficients, are evaluated.

Results

In-plane spatial resolution was measured as 2.3 mm ± 0.1 mm, spatial accuracy was 0.1 mm, and uniformity measured with flood field and NEMA NU-4 phantom was 11.7% and 8.3% respectively. Selected clinical images are provided as reference to the imaging capabilities under different clinical conditions such as reduced activity of 2-[fluorine-18]-fluoro-2-deoxy-D-glucose (18F-FDG) and time-delayed acquisitions. SUV measurements were performed for selected clinical acquisitions to demonstrate a capability for quantitative image assessment of different types of cancer including for invasive lobular carcinoma with comparatively low metabolic activity. Quantitative imaging performance assessment with phantoms demonstrates improved contrast recovery and spill-over ratio for this PET technology when compared to other commercial organ-dedicated PET systems with similar spatial resolution. Recovery coefficients were measured to be 0.21 for the 1 mm hot rod and up to 0.89 for the 5 mm hot rod of NEMA NU-4 Image Quality phantom.

Discussion

Demonstrated ability to accurately reconstruct activity in tumors as small as 5 mm suggests that the Radialis PET technology may be well suited for emerging clinical applications such as image guided assessment of response to neoadjuvant systemic treatment (NST) in lesions smaller than 2 cm. Also, our results suggest that, while spatial resolution greatly influences the partial volume effect which degrades contrast recovery, optimized count rate performance and image reconstruction workflow may improve recovery coefficients for systems with comparable spatial resolution. We emphasize that recovery coefficient should be considered as a primary performance metric when a PET system is used for accurate lesion size or radiotracer uptake assessments.

Breast Cancer Detection Using a Low-Dose Positron Emission Digital Mammography System

Purpose To investigate the feasibility of low-dose positron emission mammography (PEM) concurrently to MRI to identify breast cancer and determine its local extent. Materials and Methods In this research ethics board-approved prospective study, participants newly diagnosed with breast cancer with concurrent breast MRI acquisitions were assigned independently of breast density, tumor size, and histopathologic cancer subtype to undergo low-dose PEM with up to 185 MBq of fluorine 18-labeled fluorodeoxyglucose (18F-FDG). Two breast radiologists, unaware of the cancer location, reviewed PEM images taken 1 and 4 hours following 18F-FDG injection. Findings were correlated with histopathologic results. Detection accuracy and participant details were examined using logistic regression and summary statistics, and a comparative analysis assessed the efficacy of PEM and MRI additional lesions detection (ClinicalTrials.gov: NCT03520218). Results Twenty-five female participants (median age, 52 years; range, 32-85 years) comprised the cohort. Twenty-four of 25 (96%) cancers (19 invasive cancers and five in situ diseases) were identified with PEM from 100 sets of bilateral images, showcasing comparable performance even after 3 hours of radiotracer uptake. The median invasive cancer size was 31 mm (range, 10-120). Three additional in situ grade 2 lesions were missed at PEM. While not significant, PEM detected fewer false-positive additional lesions compared with MRI (one of six [16%] vs eight of 13 [62%]; P = .14). Conclusion This study suggests the feasibility of a low-dose PEM system in helping to detect invasive breast cancer. Though large-scale clinical trials are essential to confirm these preliminary results, this study underscores the potential of this low-dose PEM system as a promising imaging tool in breast cancer diagnosis. ClinicalTrials.gov registration no. NCT03520218 Keywords: Positron Emission Digital Mammography, Invasive Breast Cancer, Oncology, MRI Supplemental material is available for this article. © RSNA, 2024 See also commentary by Barreto and Rapelyea in this issue.

Abstract P2-09-01: An Emerging Technology for Breast Cancer Detection - Preliminary Data of Breast Cancer Detection using Novel Low Dose Positron Emission Mammography

Purpose: To investigate the feasibility of low-dose Positron Emission Mammography (PEM) to identify breast cancer. Materials and Methods: In an REB-approved ongoing clinical trial, which started in December 2019, all newly diagnosed women with breast cancer who had not undergone neoadjuvant chemotherapy and consented to the study were randomly assigned independently of their mammographic breast density, tumor size, and histopathology cancer subtype to perform PEM using a novel organ-targeted PET system(Radialis PET Imager, Radialis Inc., FDA cleared to image and measure the distribution of injected positron-emitting radiopharmaceuticals) with either 1mCi, 2mCi or 5 mCi of 18F-FDG. The PEM images acquired 1 and 4 hours after 18F-FDG administration were reviewed in consensus by two fellowship-trained breast radiologists blinded to cancer location. PEM imaging features of known malignancies and additional PEM findings were recorded and correlated with histopathology as the ground truth. Results: Our cohort comprised 22 women with a median age of 51 years (range 32-85) with 88 completed bilateral sets of images where 23 cancers (18 invasives, and 5 in situ) were present. The median invasive cancer size on surgical pathology was 28 mm (range: 3-120). Out of 18 invasive cancers, the only PEM images that did not visualize cancer (2 invasive lobular cancers and 2 in-situ cancers) were acquired with the lowest dose of 18F-FDG (1 mCi). A total of 6 (27.3%) subjects received 1mCi 18F-FDG with 5 invasive cancers, 7 (31.8%) of patients received 5 mCi 18F-FDG with 7 invasive cancers, and 9 (40.9%) of patients received 5 mCi 18F-FDG with 6 invasive cancers. A total of 3 false-positive images of benign findings were also present. No additional cancers were identified exclusively by PEM. The PEM performance was similar following an additional 3-hour interval for radiotracer uptake. Conclusion: This preliminary data results show the feasibility of invasive breast cancer detection with a decreased 18F-FDG dose, possibly due to the novel PEM system detectors increasing the sensitivity to radiotracer and the perceived spatial resolution by the visual assessment. Larger-scale clinical trials are required to consolidate our preliminary findings. Clinical Relevance: A novel organ-targeted PET system enables the detection of invasive breast cancer using low-dose PEM, potentially emerging as a promising novel imaging tool.

Citation Format: Vivianne Freitas, Michael L. Waterston, Ken O. Olsen, Oleksandr Bubon, Shayna Parker, Brandon Baldassi, Borys Komarov, Samira Taeb, Alla Reznik. An Emerging Technology for Breast Cancer Detection - Preliminary Data of Breast Cancer Detection using Novel Low Dose Positron Emission Mammography [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-09-01.

Comparative Analysis of Multilayer Lead Oxide-Based X-ray Detector Prototypes

Lead oxide (PbO) photoconductors are proposed as X-ray-to-charge transducers for the next generation of direct conversion digital X-ray detectors. Optimized PbO-based detectors have potential for utilization in high-energy and dynamic applications of medical X-ray imaging. Two polymorphs of PbO have been considered so far for imaging applications: polycrystalline lead oxide (poly-PbO) and amorphous lead oxide (a-PbO). Here, we provide the comparative analysis of two PbO-based single-pixel X-ray detector prototypes: one prototype employs only a layer of a-PbO as the photoconductor while the other has a combination of a-PbO and poly-PbO, forming a photoconductive bilayer structure of the same overall thickness as in the first prototype. We characterize the performance of these prototypes in terms of electron-hole creation energy (W_±) and signal lag-major properties that define a material's suitability for low-dose real-time imaging. The results demonstrate that both X-ray photoconductive structures have an adequate temporal response suitable for real-time X-ray imaging, combined with high intrinsic sensitivity. These results are discussed in the context of structural and morphological properties of PbO to better understand the preparation-fabrication-property relationships of this material.

Dark Current Modeling for a Polyimide-Amorphous Lead Oxide-Based Direct Conversion X-ray Detector

The reduction of the dark current (DC) to a tolerable level in amorphous selenium (a-Se) X-ray photoconductors was one of the key factors that led to the successful commercialization of a-Se-based direct conversion flat panel X-ray imagers (FPXIs) and their widespread clinical use. Here, we discuss the origin of DC in another X-ray photoconductive structure that utilizes amorphous lead oxide (a-PbO) as an X-ray-to-charge transducer and polyimide (PI) as a blocking layer. The transient DC in a PI/a-PbO detector is measured at different applied electric fields (5-20 V/μm). The experimental results are used to develop a theoretical model describing the electric field-dependent transient behavior of DC. The results of the DC kinetics modeling show that the DC, shortly after the bias application, is primarily controlled by the injection of holes from the positively biased electrode and gradually decays with time to a steady-state value. DC decays by the overarching mechanism of an electric field redistribution, caused by the accumulation of trapped holes in deep localized states within the bulk of PI. Thermal generation and subsequent multiple-trapping (MT) controlled transport of holes within the a-PbO layer governs the steady-state value at all the applied fields investigated here, except for the largest applied field of 20 V/μm. This suggests that a thicker layer of PI would be more optimal to suppress DC in the PI/a-PbO detector presented here. The model can be used to find an approximate optimal thickness of PI for future iterations of PI/a-PbO detectors without the need for time and labor-intensive experimental trial and error. In addition, we show that accounting for the field-induced charge carrier release from traps, enhanced by charge hopping transitions between the traps, yields an excellent fit between the experimental and simulated results, thus, clarifying the dynamic process of reaching a steady-state occupancy level of the deep localized states in the PI. Practically, the electric field redistribution causes the internal field to increase in magnitude in the a-PbO layer, thus improving charge collection efficiency and temporal performance over time, as confirmed by experimental results. The electric field redistribution can be implemented as a warm-up time for a-PbO-based detectors.

The X-ray Sensitivity of an Amorphous Lead Oxide Photoconductor

The photoconductor layer is an important component of direct conversion flat panel X-ray imagers (FPXI); thus, it should be carefully selected to meet the requirements for the X-ray imaging detector, and its properties should be clearly understood to develop the most optimal detector design. Currently, amorphous selenium (a-Se) is the only photoconductor utilized in commercial direct conversion FPXIs for low-energy mammographic imaging, but it is not practically feasible for higher-energy diagnostic imaging. Amorphous lead oxide (a-PbO) photoconductor is considered as a replacement to a-Se in radiography, fluoroscopy, and tomosynthesis applications. In this work, we investigated the X-ray sensitivity of a-PbO, one of the most important parameters for X-ray photoconductors, and examined the underlying mechanisms responsible for charge generation and recombination. The X-ray sensitivity in terms of electron–hole pair creation energy, W_±, was measured in a range of electric fields, X-ray energies, and exposure levels. W_± decreases with the electric field and X-ray energy, saturating at 18–31 eV/ehp, depending on the energy of X-rays, but increases with the exposure rate. The peculiar dependencies of W_± on these parameters lead to a conclusion that, at electric fields relevant to detector operation (~10 V/μm), the columnar recombination and the bulk recombination mechanisms interplay in the a-PbO photoconductor.

Bilayer lead oxide X-ray photoconductor for lag-free operation

Polycrystalline Lead Oxide (poly-PbO) was considered one of the most promising photoconductors for the direct conversion X-ray medical imaging detectors due to its previous success in optical imaging, i.e., as an optical target in so-called Plumbicon video pick-up tubes. However, a signal lag which accompanies X-ray excitation, makes poly-PbO inapplicable as an X-ray-to-charge transducer in real-time X-ray imaging. In contrast, the recently synthesized Amorphous Lead Oxide (a-PbO) photoconductor is essentially lag-free. Here, we report on our approach to a PbO detector where a thin layer of a-PbO is combined with a thick layer of poly-PbO for lag-free operation. In the presented a-PbO/poly-PbO bilayer structure, the poly-PbO layer serves as an X-ray-to-charge transducer while the a-PbO acts as a lag prevention layer. The hole mobility in the a-PbO/poly-PbO bilayer structure was measured by photo-Charge Extraction by Linearly Increasing Voltage technique at different temperatures and electric fields to investigate charge transport properties. It was found that the hole mobility is similar to that in a-Se—currently the only commercially viable photoconductor for the direct conversion X-ray detectors. Evaluation of the X-ray temporal performance demonstrated complete suppression of signal lag, allowing operation of the a-PbO/poly-PbO detector in real-time imaging.

A Probe of Valence and Conduction Band Electronic Structure of Lead Oxide Films for Photodetectors

We investigate how the electronic structure of amorphous lead oxide (a-PbO) films deposited on ITO substrate is changed after annealing at various temperatures. Both experimental soft X-ray spectroscopic and density functional theory (DFT) based computational techniques are used to explore the electronic structure of this material. X-ray emission, resonant X-ray inelastic scattering, and X-ray absorption spectroscopic techniques are employed to directly probe the valence and conduction bands. We discover that the films are very stable and remain amorphous when exposed to temperatures below 300 °C. An amorphous-to-polycrystalline (α-PbO phase) transformation occurs during annealing at 400 °C. At 500 °C, an alpha to beta phase change is observed. These structural modifications are accompanied by the band gap value changing from 1.4±0.2 eV to 2.0±0.2 eV upon annealing at 400 °C and to 2.6±0.2 eV upon annealing at 500 °C. A difference between surface and bulk structural properties is found for all samples annealed at 500 °C and above; these samples also exhibit an unexpected suppression of O : 2p density of states (DOS) near the bottom of the conduction band, whereas additional electronic states appear well within the valence band. This study provides a significant step forward to understanding the electronic properties of two polymorphic forms of PbO needed for optimization of this material for use in X-ray sensors.

Characterization of polycrystalline lead oxide for application in direct conversion X-ray detectors

While polycrystalline lead oxide (poly-PbO) is known to be one of the most promising photoconductors for utilization in X-ray detectors, its major performance parameters such as charge yield and mobility-lifetime product (μτ) are still not well established and require further investigation. Combining the conventional X-ray induced photocurrent and pulse height spectroscopy techniques we examine the X-ray photogeneration and recombination processes in poly-PbO. The measurements indicate that the amount of energy required to release a single electron hole pair W_± (inverse of charge yield) strongly depends on applied electric field and at 10 V/μm reaches ~20 eV/ehp. Fitting the measured pulse height spectra with the Hecht formula provided μτ for holes and electrons to be 4.1 × 10⁻⁸ cm²/V and 10⁻⁹ cm²/V, respectively. Obtained μτ values combined with recently reported mobility values of charge carriers in PbO suggest a new direction towards improvement of PbO technology by incorporation of Frisch grid or X-ray transistor architectures.

Charge transport mechanism in lead oxide revealed by CELIV technique

Although polycrystalline lead oxide (PbO) belongs to the most promising photoconductors for optoelectronic and large area detectors applications, the charge transport mechanism in this material still remains unclear. Combining the conventional time-of-flight and the photo-generated charge extraction by linear increasing voltage (photo-CELIV) techniques, we investigate the transport of holes which are shown to be the faster carriers in poly-PbO. Experimentally measured temperature and electric field dependences of the hole mobility suggest a highly dispersive transport. In order to analyze the transport features quantitatively, the theory of the photo-CELIV is extended to account for the dispersive nature of charge transport. While in other materials with dispersive transport the amount of dispersion usually depends on temperature, this is not the case in poly-PbO, which evidences that dispersive transport is caused by the spatial inhomogeneity of the material and not by the energy disorder.

An Amorphous Selenium Based Positron Emission Mammography Camera with Avalanche Gain

Early diagnosis of breast cancer is crucial for effective treatment, and the need exists for greater detection ability and specificity than possible by screening x-ray mammography (currently the primary imaging technique for the detection of breast lesions). Positron Emission Tomography (PET) using the radiotracer 2-[fluorine-18]-fluoro-2-deoxy-D-glucose (FDG) offers a noninvasive, highly sensitive method for the diagnosis of breast cancer. Images from PET contain unique metabolic information that is not available from anatomical imaging techniques. We propose a Positron Emission Mammography (PEM) imaging system that maintains the established high specificity of FDG PET while providing improved collection efficiency for the radiotracer signal and the potential for images with better spatial resolution. This PEM system will enable detection of lesions that are considerably smaller than those that can be visualized using whole body PET imaging.

The compact dual-head PEM camera will be based on an amorphous selenium (a-Se) avalanche photodetector and the scintillator lutetium oxyorthosilicate (LSO). The camera promises high collection efficiency by combining the fast scintillation light decay and high light yield of LSO with the excellent quantum efficiency, large avalanche gain, and rapid response time of a-Se. We have measured the gain and readout time of an 8 μm a-Se layer and demonstrated the feasibility of the proposed PEM camera.

Enhanced detection efficiency of direct conversion X-ray detector using polyimide as hole-blocking layer

In this article we demonstrate the performance of a direct conversion amorphous selenium (a-Se) X-ray detector using biphenyldisnhydride/1,4 phenylenediamine (BPDA/PPD) polyimide (PI) as a hole-blocking layer. The use of a PI layer with a-Se allows detector operation at high electric fields (≥10 V/μm) while maintaining low dark current, without deterioration of transient performance. The hole mobility of the PI/a-Se device is measured by the time-of-flight method at different electric fields to investigate the effect of the PI layer on detector performance. It was found that hole mobility as high as 0.75 cm²/Vs is achievable by increasing the electric field in the PI/a-Se device structure. Avalanche multiplication is also shown to be achievable when using PI as a blocking layer. Increasing the electric field within a-Se reduces the X-ray ionization energy, increases hole mobility, and improves the dynamic range and sensitivity of the detector.

Origin of n- and p-type conductivity in undoped α-PbO: role of defects

First-principles calculations have been applied to study the crystallographic defects in α-PbO in order to understand an origin of n- and p-type conductivity in otherwise undoped α-PbO. It was found that deposition in an oxygen-deficient environment defined in our simulations by the Pb-rich/O-poor limit stimulates a formation of O vacancies and Pb interstitials both characterized by quite low formation energies ∼1.0 eV. The O vacancy, being occupied by two electrons, shifts the balance of electrons and holes between these two defects to an excess of electrons (four electrons against two holes) that causes n-type doping. For the Pb-poor/O-rich limit, an excess of oxygen triggers the formation of the O interstitials characterized by such a low formation energy that a spontaneous appearance of this defect is predicted. It is shown that the concentration of O interstitials is able to reach an extreme magnitude equal to the number of possible defect sites (∼10(22) cm(-3)). The localized state formed by the O interstitial is occupied by two holes and because there are no other defects in reasonable concentration to balance the hole redundancy, p-type doping is induced.

Lead monoxide α-PbO: electronic properties and point defect formation

The electronic properties of polycrystalline lead oxide consisting of a network of single-crystalline α-PbO platelets and the formation of native point defects in the α-PbO crystal lattice are studied using first-principles calculations. The results suggest that the polycrystalline nature of α-PbO causes the formation of lattice defects (i.e., oxygen and lead vacancies) in such a high concentration that defect related conductivity becomes the dominant mechanism of charge transport. The neutral O vacancy forms a defect state at 1.03 eV above the valence band which can act as a deep trap for electrons, while the Pb vacancy forms a shallow trap for holes located just 0.1 eV above the valence band. The ionization of O vacancies can account for the experimentally found dark current decay in ITO/PbO/Au structures.

Electroded avalanche amorphous selenium (a-Se) photosensor

Although avalanche amorphous selenium (a-Se) is a very promising photoconductor for a variety of imaging applications, it is currently restricted to applications with electron beam readout in vacuum pick-up tube called a High-gain Avalanche Rushing Photoconductor (HARP). The electron beam readout is compatible with high definition television (HDTV) applications, but for use in solid-state medical imaging devices it should be replaced by an electronic readout with a two-dimensional array of metal pixel electrodes. However, due to the high electric field required for avalanche multiplication, it is a technological challenge to avoid possible dielectric breakdown at the edges, where electric field experiences local enhancement. It has been shown recently that this problem can be overcome by the use of a Resistive Interface Layer (RIL) deposited between a-Se and the metal electrode, however, at that time, at a sacrifice in transport properties.Here we show that optimization of RIL deposition technique allows for electroded avalanche a-Se with transport properties and time performance previously not achievable with any other a-Se structures. We have demonstrated this by detailed analysis of transport properties performed by Time-of-Flight (TOF) technique. Our results showed that a stable gain of 200 is reached at 104 V/μm for a 15-μm thick a-Se layer, which is the maximum theoretical gain for this thickness. We conclude that RIL is an enabling technology for practical implementation of solid-state avalanche a-Se image sensors.

Amorphous and polycrystalline photoconductors for direct conversion flat panel x-ray image sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI₂ and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.

A solid-state amorphous selenium avalanche technology for low photon flux imaging applications

PURPOSE

The feasibility of a practical solid-state technology for low photon flux imaging applications was investigated. The technology is based on an amorphous selenium photoreceptor with a voltage-controlled avalanche multiplication gain. If this photoreceptor can provide sufficient internal gain, it will be useful for an extensive range of diagnostic imaging systems.

METHODS

The avalanche photoreceptor under investigation is referred to as HARP-DRL. This is a novel concept in which a high-gain avalanche rushing photoconductor (HARP) is integrated with a distributed resistance layer (DRL) and sandwiched between two electrodes. The avalanche gain and leakage current characteristics of this photoreceptor were measured.

RESULTS

HARP-DRL has been found to sustain very high electric field strengths without electrical breakdown. It has shown avalanche multiplication gains as high as 10(4) and a very low leakage current (< or = 20 pA/mm2).

CONCLUSIONS

This is the first experimental demonstration of a solid-state amorphous photoreceptor which provides sufficient internal avalanche gain for photon counting and photon starved imaging applications.

Design and feasibility of active matrix flat panel detector using avalanche amorphous selenium for protein crystallography

Protein crystallography is the most important technique for resolving the three-dimensional atomic structure of protein by measuring the intensity of its x-ray diffraction pattern. This work proposes a large area flat panel detector for protein crystallography based on direct conversion x-ray detection technique using avalanche amorphous selenium (a-Se) as the high gain photoconductor, and active matrix readout using amorphous silicon (a-Si:H) thin film transistors. The detector employs avalanche multiplication phenomenon of a-Se to make the detector sensitive to each incident x ray. The advantages of the proposed detector over the existing imaging plate and charge coupled device detectors are large area, high dynamic range coupled to single x-ray detection capability, fast readout, high spatial resolution, and inexpensive manufacturing process. The optimal detector design parameters (such as detector size, pixel size, and thickness of a-Se layer), and operating parameters (such as electric field across the a-Se layer) are determined based on the requirements for protein crystallography application. The performance of the detector is evaluated in terms of readout time (<1 s), dynamic range (approximately 10(5)), and sensitivity (approximately 1 x-ray photon), thus validating the detector's efficacy for protein crystallography.

Online gamma-camera imaging of 103Pd seeds (OGIPS) for permanent breast seed implantation

Permanent brachytherapy seed implantation is being investigated as a mode of accelerated partial breast irradiation for early stage breast cancer patients. Currently, the seeds are poorly visualized during the procedure making it difficult to perform a real-time correction of the implantation if required. The objective was to determine if a customized gamma-camera can accurately localize the seeds during implantation. Monte Carlo simulations of a CZT based gamma-camera were used to assess whether images of suitable quality could be derived by detecting the 21 keV photons emitted from 74 MBq (103)Pd brachytherapy seeds. A hexagonal parallel hole collimator with a hole length of 38 mm, hole diameter of 1.2 mm and 0.2 mm septa, was modeled. The design of the gamma-camera was evaluated on a realistic model of the breast and three layers of the seed distribution (55 seeds) based on a pre-implantation CT treatment plan. The Monte Carlo simulations showed that the gamma-camera was able to localize the seeds with a maximum error of 2.0 mm, using only two views and 20 s of imaging. A gamma-camera can potentially be used as an intra-procedural image guidance system for quality assurance for permanent breast seed implantation.

Mathematical model of manganese ion catalyzed microwave deactivation of Enterococcus faecalis, Staphylococcus aureus and Escherichia coli

Enterococcus faecalis, Staphylococcus aureus and Escherichia coli survival was investigated using microwave irradiation (power 130 W) both in a water control and in the presence of a 1 microM manganese ion solution. Measured survival dependencies had "bell" shape form with maximum bacterial viability between 1-2 min of microwave heating. Additional heating revealed bacteria survival decreasing up to 3 min of microwave heating when viability became insignificantly small. The total deactivation time of bacteria in the presence of manganese ions was significantly smaller then that of bacteria irradiated in the microwave without manganese ions present (4-5 min). One possible explanation for the rapid reduction of bacterial survival during microwave irradiation in the presence of manganese ions is that increasing manganese ion penetration into bacteria along with microwave irradiation related to an increase of kinetic energy of ions, and damaging of bacteria by metal ions. The proposed mathematical model for microwave heating took into account "growth" and "death" factors of bacteria. It assumes that rates of bacterial growth and decay are linear functions of water temperature, and rate of bacterial decay that relates with metal concentration into water is also linear, which influenced the differential equation for the dependence between number of survival bacteria and temperature water. By using proportionality between the time of microwave heating and water temperature we derived the differential equation, between bacterial viability and time of microwave irradiation which was used as mathematical model for microwave heating in the presence of metal ions. This model had forms of second-degree polynomial functions. We received good relationships (with coefficient of correlation 0.92-0.99) between proposed mathematical model and experimental data for all bacterial deactivation.

Mathematical models for conventional and microwave thermal deactivation of Enterococcus faecalis, Staphylococcus aureus and Escherichia coli

Temperature dependencies of survival fecal coliforms such as Enterococcus faecalis, Staphylococcus aureus and Escherichia coli in water were investigated between 25-65 degrees C. Measured dependencies had "bell" shaped form with maximum bacterial viability at 35-45 degrees C. The rates of growth and decay of bacterial viability depend on specific forms of bacteria. At temperatures of 60-65 degrees C the number of viable bacteria decreased in one hundred times in comparison with the maximum value. Similar "bell" shape forms were found for dependencies between bacterial viability and time of microwave (dielectric) heating of water. The dependencies had maximum value at 1-2 min of microwave heating. Then, the number of viable bacteria decreased, and at 4-5 min of microwave heating, became insignificantly small. The proposed mathematical models for conventional and microwave heating took into account "growth" and "death" factors of bacteria, and had forms of second degree polynomial functions. The results showed good relationships (with coefficient correlation 0.84-0.99) between the proposed mathematical models and experimental data for both conventional and microwave heating.

In-situ surfactant/surfactant-nutrient mix-enhanced bioremediation of NAPL (fuel)-contaminated sandy soil aquifers

SCOPE AND BACKGROUND

Contamination of soils, aquifers and groundwater by nonaqueous phase liquid (NAPL) pollutants constitutes a major environmental issue of concern, worldwide. The residual (biodegradation-resistant) hydrophobic fuel hydrocarbons entrapped in the soil porous matrix, possess a particular bioremediation challenge due to their becoming virtually immobile, nor desorbable, or water dispersible. Consequently, they are not available as substrates to the micro-organism-based biodegradation.

MATERIALS AND METHODS

Our research involves the development of economically feasible, surfactant/surfactant-nutrient mix (SSNM)-enhanced bioremediation methodologies for sustainable, in situ bioremediation of fuel-contaminated aquifers. This requires, methodologically, (a) the optimization, via in vitro 'flow' (columns) lab experiments and screening processes, of an effective mixture for the intended SSNM-enhanced bioremediation; and (b) the study of the combined effect of the optimized SSNM on the solubilization/mobilization and biodegradation of NAPL (fuel) in in vitro site/aquifer-simulated bioremediation.

RESULTS AND DISCUSSION

The essence of our findings: (1) kerosene's maximum enhanced mobilization - f = 3.6, compared with that of deionized water, was achieved with an SSNM having the composition of linear alkylbenzene sulfonate (LABS): coco-amphodiacetate (containing N): surfactant-nutrient X (containing both N and P) = 0.15: 0.15: 0.05 g/L, respectively; (2) 62-64% of the initial amount of kerosene in the initially saturated soil matrix, 'packed' in a column, has been eluted from it during approximately 30 days, compared with 68% of kerosene biodegradation in 'vessel' settings, in 21 days.

CONCLUSIONS

(1) The indigenous microorganisms present in th vadose zones of fuel-contaminated sandy soil aquifers are potentially capable of unassisted removal of approximately 80% of the initially contained fuel (kerosene), during a period of about 42 days; (2) the major effects of the SSNM addition are (a) enhanced mobilization of the bulky NAPL; and (b) enhanced desorbtion/ solubilization/dispersion of the entrapped NAPL which, in turn, facilitate their enhanced biodegradation.

RECOMMENDATIONS AND PERSPECTIVE

Our findings suggest that pre-optimized, biodegradable SSNM is essential for surfactants-based bioremediation of NAPL-contaminated aquifers, in order to make this in-situ methodology both technologically and economically feasible.

X-ray imaging with amorphous selenium: Pulse height measurements of avalanche gain fluctuations

Avalanche multiplication in amorphous selenium (a-Se) can provide a large, adjustable gain for active matrix flat panel imagers (AMFPI), enabling quantum noise limited x-ray imaging during both radiography and fluoroscopy. In the case of direct conversion AMFPI, the multiplication factor for each x ray is a function of its depth of interaction, and the resulting variations in gain can reduce the detective quantum efficiency (DQE) of the system. An experimental method was developed to measure gain fluctuations by analyzing images of individual x rays that were obtained using a video camera with an a-Se target operated in avalanche mode. Pulse height spectra (PHS) of the charge produced per x ray were recorded for monoenergetic 30.9, 49.4, and 73.8 keV x-ray sources. The rapid initial decay and long tail of each PHS can be explained by a model in which positive charge dominates the initiation of avalanche. The Swank information factor quantifies the effect of gain fluctuation on DQE and was calculated from the PHS. The information factor was found to be 0.5 for a 25 microm a-Se layer with a maximum gain of approximately 300. Changing the energy of the incident x ray influenced the range of the primary photoelectron and noticeably affected the tail of the experimental PHS, but did not significantly change the avalanche Swank factor.

Indirect flat-panel detector with avalanche gain: Fundamental feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat panel imager)

An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The final electronic image is read out using an active matrix array of thin film transistors (TFT). We call the proposed detector SHARP-AMFPI (scintillator HARP active matrix flat panel imager). The advantage of the SHARP-AMFPI is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. The purpose of this paper is to investigate the important design considerations for SHARP-AMFPI such as avalanche gain, which depends on both the thickness d(Se) and the applied electric field E(Se) of the HARP layer. To determine the optimal design parameter and operational conditions for HARP, we measured the E(Se) dependence of both avalanche gain and optical quantum efficiency of an 8 microm HARP layer. The results were used in a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and nonavalanche modes as a function of E(Se): (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; (4) gain nonuniformity resulting from thickness nonuniformity; and (5) effects of direct x-ray interaction in HARP. Our results showed that a HARP layer thickness of 8 microm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.

A newly developed intra-operative gamma camera: performance characteristics in a laboratory phantom study

PURPOSE

Radioguided surgery depends on the intra-operative detection of radiolabelled tissues. This is currently accomplished with hand tools capable of providing a tone signal, depending on the proximity and direction of a radioactive source in relation to the probe. The advantages of visual images of radiolabelled tissues are well recognised, but satisfactory means of acquiring such images intra-operatively are not yet available. The goal of this study was to examine the performance of a newly developed intra-operative gamma camera, compact enough to be a hand tool and capable of yielding a visual image of the source field.

METHODS

The study was performed in the laboratory with a phantom consisting of a water bath and small hollow spheres (1-2 cm in internal diameter) filled with 99mTc (1-5 microCi/cc), placed in different configurations within the bath. For comparison, studies were also performed using a standard intra-operative gamma probe, and others using a standard single-head high-resolution gamma camera.

RESULTS

Compared with the gamma probe, the intra-operative camera was found to possess a superior ability to distinguish small, deep and weakly localised radioactivity sources from background. By acquiring images from different angles, it allowed a 3D understanding of multiple radioactive sources. It detected "cold" defects within a "hot" radiolabelled sphere. It discriminated a weak source located near a much "hotter" radioactivity source, similar to discrimination with the standard gamma camera, and discerned localised sources against a background of radioactivity.

CONCLUSION

It is anticipated that the high imaging potential of the camera tested in this study will offer clinical advantages.

Negative adaptation to physical training in senile mice

Three age groups (6, 22 and 27 months) of CWI mice (female, outbred strain) were subjected to a physical training program for up to 5 weeks. During the training period the body weight remained constant in all groups. Hind leg muscle mass increased in response to the exercise in 6 and 22 month old mice but decreased in the senescent (27 months) animals. Physical exercise also increased the key enzyme creatine kinase in the 6 months group and to a lesser degree in the 22 months group. By contrast, senile animals showed a progressive loss of the enzyme throughout the training period. Similar changes - though less-pronounced - were seen with regard to soluble protein and to total DNA. The data indicate that old animals (22 months) are still able to adapt adequately to physical training while senile mice (27 months) respond paradoxically by a negative adaptation to a physical challenge.