N Manohar

X-ray fluorescence computed tomography (XFCT) imaging of gold nanoparticle-loaded objects using 110 kVp x-rays

A conventional x-ray fluorescence computed tomography (XFCT) technique requires monochromatic synchrotron x-rays to simultaneously determine the spatial distribution and concentration of various elements such as metals in a sample. However, the synchrotron-based XFCT technique appears to be unsuitable for in vivo imaging under a typical laboratory setting. In this study we demonstrated, for the first time to our knowledge, the possibility of performing XFCT imaging of a small animal-sized object containing gold nanoparticles (GNPs) at relatively low concentrations using polychromatic diagnostic energy range x-rays. Specifically, we created a phantom made of polymethyl methacrylate plastic containing two cylindrical columns filled with saline solution at 1 and 2 wt% GNPs, respectively, mimicking tumors/organs within a small animal. XFCT scanning of the phantom was then performed using microfocus 110 kVp x-ray beam and cadmium telluride (CdTe) x-ray detector under a pencil beam geometry after proper filtering of the x-ray beam and collimation of the detector. The reconstructed images clearly identified the locations of the two GNP-filled columns with different contrast levels directly proportional to gold concentration levels. On the other hand, the current pencil-beam implementation of XFCT is not yet practical for routine in vivo imaging tasks with GNPs, especially in terms of scanning time. Nevertheless, with the use of multiple detectors and a limited number of projections, it may still be used to image some objects smaller than the current phantom size. The current investigation suggests several modification strategies of the current XFCT setup, such as the adoption of the quasi-monochromatic cone/fan x-ray beam and XFCT-specific spatial filters or pinhole detector collimators, in order to establish the ultimate feasibility of a bench-top XFCT system for GNP-based preclinical molecular imaging applications.

Quantitative imaging of gold nanoparticle distribution in a tumor-bearing mouse using benchtop x-ray fluorescence computed tomography

X-ray fluorescence computed tomography (XFCT) is a technique that can identify, quantify, and locate elements within objects by detecting x-ray fluorescence (characteristic x-rays) stimulated by an excitation source, typically derived from a synchrotron. However, the use of a synchrotron limits practicality and accessibility of XFCT for routine biomedical imaging applications. Therefore, we have developed the ability to perform XFCT on a benchtop setting with ordinary polychromatic x-ray sources. Here, we report our postmortem study that demonstrates the use of benchtop XFCT to accurately image the distribution of gold nanoparticles (GNPs) injected into a tumor-bearing mouse. The distribution of GNPs as determined by benchtop XFCT was validated using inductively coupled plasma mass spectrometry. This investigation shows drastically enhanced sensitivity and specificity of GNP detection and quantification with benchtop XFCT, up to two orders of magnitude better than conventional x-ray CT. The results also reaffirm the unique capabilities of benchtop XFCT for simultaneous determination of the spatial distribution and concentration of nonradioactive metallic probes, such as GNPs, within the context of small animal imaging. Overall, this investigation identifies a clear path toward in vivo molecular imaging using benchtop XFCT techniques in conjunction with GNPs and other metallic probes.

Experimental demonstration of direct L‐shell x‐ray fluorescence imaging of gold nanoparticles using a benchtop x‐ray source

PURPOSE To develop a proof-of-principle L-shell x-ray fluorescence (XRF) imaging system that locates and quantifies sparse concentrations of gold nanoparticles (GNPs) using a benchtop polychromatic x-ray source and a silicon (Si)-PIN diode x-ray detector system. METHODS 12-mm-diameter water-filled cylindrical tubes with GNP concentrations of 20, 10, 5, 0.5, 0.05, 0.005, and 0 mg∕cm3 served as calibration phantoms. An imaging phantom was created using the same cylindrical tube but filled with tissue-equivalent gel containing structures mimicking a GNP-loaded blood vessel and approximately 1 cm3 tumor. Phantoms were irradiated by a 3-mm-diameter pencil-beam of 62 kVp x-rays filtered by 1 mm aluminum. Fluorescence∕scatter photons from phantoms were detected at 90° with respect to the beam direction using a Si-PIN detector placed behind a 2.5-mm-diameter lead collimator. The imaging phantom was translated horizontally and vertically in 0.3-mm steps to image a 6 mm×15 mm region of interest (ROI). For each phantom, the net L-shell XRF signal from GNPs was extracted from background, and then corrected for detection efficiency and in-phantom attenuation using a fluorescence-to-scatter normalization algorithm. RESULTS XRF measurements with calibration phantoms provided a calibration curve showing a linear relationship between corrected XRF signal and GNP mass per imaged voxel. Using the calibration curve, the detection limit (at the 95% confidence level) of the current experimental setup was estimated to be a GNP mass of 0.35 μg per imaged voxel (1.73×10(-2) cm3). A 2D XRF map of the ROI was also successfully generated, reasonably matching the known spatial distribution as well as showing the local variation of GNP concentrations. CONCLUSIONS L-shell XRF imaging can be a highly sensitive tool that has the capability of simultaneously imaging the spatial distribution and determining the local concentration of GNPs presented on the order of parts-per-million level within subcentimeter-sized ex vivo samples and superficial tumors during preclinical animal studies.

Quantitative investigation of physical factors contributing to gold nanoparticle-mediated proton dose enhancement.

Some investigators have shown tumor cell killing enhancement in vitro and tumor regression in mice associated with the loading of gold nanoparticles (GNPs) before proton treatments. Several Monte Carlo (MC) investigations have also demonstrated GNP-mediated proton dose enhancement. However, further studies need to be done to quantify the individual physical factors that contribute to the dose enhancement or cell-kill enhancement (or radiosensitization). Thus, the current study investigated the contributions of particle-induced x-ray emission (PIXE), particle-induced gamma-ray emission (PIGE), Auger and secondary electrons, and activation products towards the total dose enhancement. Specifically, GNP-mediated dose enhancement was measured using strips of radiochromic film that were inserted into vials of cylindrical GNPs, i.e. gold nanorods (GNRs), dispersed in a saline solution (0.3 mg of GNRs/g or 0.03% of GNRs by weight), as well as vials containing water only, before proton irradiation. MC simulations were also performed with the tool for particle simulation code using the film measurement setup. Additionally, a high-purity germanium detector system was used to measure the photon spectrum originating from activation products created from the interaction of protons and spherical GNPs present in a saline solution (20 mg of GNPs/g or 2% of GNPs by weight). The dose enhancement due to PIXE/PIGE recorded on the films in the GNR-loaded saline solution was less than the experimental uncertainty of the film dosimetry (<2%). MC simulations showed highly localized dose enhancement (up to a factor 17) in the immediate vicinity (<100 nm) of GNRs, compared with hypothetical water nanorods (WNRs), mostly due to GNR-originated Auger/secondary electrons; however, the average dose enhancement over the entire GNR-loaded vial was found to be minimal (0.1%). The dose enhancement due to the activation products from GNPs was minimal (<0.1%) as well. In conclusion, under the currently investigated conditions that are considered clinically relevant, PIXE, PIGE, and activation products contribute minimally to GNP/GNR-mediated proton dose enhancement, whereas Auger/secondary electrons contribute significantly but only at short distances (<100 nm) from GNPs/GNRs.

Improving x-ray fluorescence signal for benchtop polychromatic cone-beam x-ray fluorescence computed tomography by incident x-ray spectrum optimization: a Monte Carlo study.

PURPOSE To develop an accurate and comprehensive Monte Carlo (MC) model of an experimental benchtop polychromatic cone-beam x-ray fluorescence computed tomography (XFCT) setup and apply this MC model to optimize incident x-ray spectrum for improving production/detection of x-ray fluorescence photons from gold nanoparticles (GNPs). METHODS A detailed MC model, based on an experimental XFCT system, was created using the Monte Carlo N-Particle (MCNP) transport code. The model was validated by comparing MC results including x-ray fluorescence (XRF) and scatter photon spectra with measured data obtained under identical conditions using 105 kVp cone-beam x-rays filtered by either 1 mm of lead (Pb) or 0.9 mm of tin (Sn). After validation, the model was used to investigate the effects of additional filtration of the incident beam with Pb and Sn. Supplementary incident x-ray spectra, representing heavier filtration (Pb: 2 and 3 mm; Sn: 1, 2, and 3 mm) were computationally generated and used with the model to obtain XRF/scatter spectra. Quasimonochromatic incident x-ray spectra (81, 85, 90, 95, and 100 keV with 10 keV full width at half maximum) were also investigated to determine the ideal energy for distinguishing gold XRF signal from the scatter background. Fluorescence signal-to-dose ratio (FSDR) and fluorescence-normalized scan time (FNST) were used as metrics to assess results. RESULTS Calculated XRF/scatter spectra for 1-mm Pb and 0.9-mm Sn filters matched (r ≥ 0.996) experimental measurements. Calculated spectra representing additional filtration for both filter materials showed that the spectral hardening improved the FSDR at the expense of requiring a much longer FNST. In general, using Sn instead of Pb, at a given filter thickness, allowed an increase of up to 20% in FSDR, more prominent gold XRF peaks, and up to an order of magnitude decrease in FNST. Simulations using quasimonochromatic spectra suggested that increasing source x-ray energy, in the investigated range of 81-100 keV, increased the FSDR up to a factor of 20, compared to 1 mm Pb, and further facilitated separation of gold XRF peaks from the scatter background. CONCLUSIONS A detailed MC model of an experimental benchtop XFCT system has been developed and validated. In exemplary calculations to illustrate the usefulness of this model, it was shown that potential use of quasimonochromatic spectra or judicious choice of filter material/thickness to tailor the spectrum of a polychromatic x-ray source can significantly improve the performance of benchtop XFCT, while considering trade-offs between FSDR and FNST. As demonstrated, the current MC model is a reliable and powerful computational tool that can greatly expedite the further development of a benchtop XFCT system for routine preclinical molecular imaging with GNPs and other metal probes.

Design of an Yb-169 source optimized for gold nanoparticle-aided radiation therapy

PURPOSE To find an optimum design of a new high-dose rate ytterbium (Yb)-169 brachytherapy source that would maximize the dose enhancement during gold nanoparticle-aided radiation therapy (GNRT), while meeting practical constraints for manufacturing a clinically relevant brachytherapy source. METHODS Four different Yb-169 source designs were considered in this investigation. The first three source models had a single encapsulation made of one of the following materials: aluminum, titanium, and stainless steel. The last source model adopted a dual encapsulation design with an inner aluminum capsule surrounding the Yb-core and an outer titanium capsule. Monte Carlo (MC) simulations using the Monte Carlo N-Particle code version 5 (MCNP5) were conducted initially to investigate the spectral changes caused by these four source designs and the associated variations in macroscopic dose enhancement across the tumor loaded with gold nanoparticles (GNPs) at 0.7% by weight. Subsequent MC simulations were performed using the EGSnrc and norec codes to determine the secondary electron spectra and microscopic dose enhancement as a result of irradiating the GNP-loaded tumor with the mcnp-calculated source spectra. RESULTS Effects of the source filter design were apparent in the current MC results. The intensity-weighted average energy of the Yb-169 source varied from 108.9 to 122.9 keV, as the source encapsulation material changed from aluminum to stainless steel. Accordingly, the macroscopic dose enhancement calculated at 1 cm away from the source changed from 51.0% to 45.3%. The sources encapsulated by titanium and aluminum/titanium combination showed similar levels of dose enhancement, 49.3% at 1 cm, and average energies of 113.0 and 112.3 keV, respectively. While the secondary electron spectra due to the investigated source designs appeared to look similar in general, some differences were noted especially in the low energy region (<50 keV) of the spectra suggesting the dependence of the photoelectron yield on the atomic number of source filter material, consistent with the macroscopic dose enhancement results. A similar trend was also shown in the so-called microscopic dose enhancement factor, for example, resulting in the maximum values of 138 and 119 for the titanium- and the stainless steel-encapsulated Yb-169 sources, respectively. CONCLUSIONS The current results consistently show that the dose enhancement achievable from the Yb-169 source is closely related with the atomic number (Z) of source encapsulation material. While the observed range of improvement in the dose enhancement may be considered moderate after factoring all uncertainties in the MC results, the current study provides a reasonable support for the encapsulation of the Yb-core with lower-Z materials than stainless steel, for GNRT applications. Overall, the titanium capsule design can be favored over the aluminum or dual aluminum/titanium capsule designs, due to its superior structural integrity and improved safety during manufacturing and clinical use.

WE‐G‐211‐07: Quasi‐Monochromatization of 110 KVp X‐Rays for Bench‐Top X‐Ray Fluorescence Computed Tomography (XFCT) Imaging of Gold Nanoparticle‐Loaded Objects

Purpose: To assess the impact of adopting a quasi‐monochromatic x‐ray source for bench‐top x‐ray fluorescencecomputed tomography (XFCT) imaging of goldnanoparticle‐loaded objects. Methods: A Monte Carlo(MC) model of an experimental XFCT system was created using the MCNP5 code. This model was used to simulate irradiation of a small animal‐sized plastic phantom containing water columns loaded with goldnanoparticles (GNPs) at various concentrations (2% or less by weight) with a Pb‐filtered polychromatic x‐ray source. MC results were verified with measurements to validate the model. Subsequently, MC simulations were repeated with monochromatic x‐ray sources (85, 90, and 95 keV) to determine the most ideal x‐ray energy to discriminate between gold K‐shell fluorescence peaks and the Compton scatter background. As a practical alternative to monochromatic x‐rays, a quasi‐monochromatic x‐ray spectrum was experimentally created from a polychromatic x‐ray spectrum (110 kVp) by using a highly oriented pyrolitic graphite (HOPG) crystal. This spectrum was used for further MC simulations to assess the effect of a quasi‐ monochromatic x‐ray spectrum on the detection of gold K—shell fluorescence x‐rays. Results: Monochromatic x‐ray energy needed to exceed 95 keV in order for the gold K‐shell fluorescence peaks (67 and 69 keV) to be discernible over the Compton scatter background at the detector position (90 degrees with respect to the beam axis). At the same GNP concentration, there was at least a three‐fold increase in the goldfluorescence peak‐to‐scatter background ratio from using the HOPG‐ generated quasi‐monochromatic source spectrum, compared to that from using the Pb‐filtered polychromatic source spectrum. Conclusions: Since a monochromatic synchrotron x‐ray beam, although ideal, is not readily available for a bench‐top XFCT system, quasi‐monochromatization of a polychromatic x‐ray source spectrum to peak at certain energies appears to help dramatically improve the detection efficiency of K‐shell fluorescence x‐rays from GNPs.

TH‐A‐213CD‐03: Polychromatic Cone‐Beam X‐Ray Fluorescence Computed Tomography of Gold Nanoparticle‐Loaded Objects

Purpose: To determine the spatial distribution and amount of goldnanoparticles (GNPs) within small‐animal‐sized objects using polychromatic cone‐beam x‐ray fluorescencecomputed tomography (XFCT) under realistic constraints on x‐ray dose, scan time, and image resolution. Methods: 6‐mm‐diameter cylindrical tubes containing saline solution and 0.5–2.0 wt. % of GNPs were inserted into a cylindrical polymethyl methacrylate (PMMA) phantom, 3 cm in diameter and 5 cm in height. The phantom was irradiated by a cone‐beam of polychromatic 105 kVp x‐rays filtered by 0.9 mm of tin. Energy‐sensitive cadmium telluride detectors behind a 2.5 mm diameter lead pinhole collimator collected the spectrum of emitted gold K‐shell fluorescence and Compton scattered photons at an angle of 90° relative to the beam central axis as the phantom was rotated to a series of 60 projection angles in 6° steps. Sinograms of goldfluorescencephoton signals were constructed by extracting the goldfluorescence peak height from the Compton background, and the image of GNP location and concentration was reconstructed using a maximum likelihood iterative reconstruction algorithm. Results: Using the measured sinograms, XFCT images of GNP‐loaded objects were successfully reconstructed, accurately determining both the GNP location and concentration. The x‐ray dose delivered during the XFCT scanning was measured using the AAPM TG‐61 protocol, and it was determined that, using an array detector, it would be possible to acquire these images in one hour with a tissue dose of approximately 20 cGy. Conclusions: With a few easily achievable modifications, the current benchtop setup would be capable of producing useful XFCT images of small animals injected with GNPs with the total scanning time and a tissue dose comparable to other modalities currently available for routine pre‐clinical in‐vivo imaging such as micro‐CT. Supported in part by NIH/NCI grant 1R01CA155446. Supported in part by NIH/NCI grant 1R01CA155446

SU‐GG‐J‐119: Induction of Plasmonic Heating Inside Breast Tumor Phantom Using Gold Nanorods and Near‐Infrared Laser

Purpose: To demonstrate and quantify goldnanorods (GNRs)‐induced plasmonicheating inside breast tumor phantom during near‐infrared (NIR) laser illumination.Method and Materials: GNRs were fabricated using published procedures. Two types of breast gel phantoms were fabricated. The first phantom was nearly water‐equivalent as it was fabricated using 1.5% agar gel (98.5% water). The second phantom, on the other hand, was fabricated using a more turbid intralipid‐based gel (2% agar, 2% intralipid, and 96% water). Both phantoms contained an identical GNR‐filled cavity with 4 mm inner diameter. The cavity was filled with approximately 0.1 weight percent GNRs and located at the center of a quadrant of hemispherical phantom at 2.5 cm depth measured from the base of the phantom. Both phantoms were illuminated with an 808 nm NIR laser at 1 W power under the identical geometry. Thermocouples were used to measure the temperature changes within the cavity and surrounding medium. Results: There was significant difference between the two gel phantoms in terms of achievable temperature rise within a GNR‐filled cavity. For a 60 second illumination, the cavity inside an agar‐based gel phantom was heated up to 15°C above the background temperature, while there was a temperature change of only about 1.5°C within the cavity inside an intralipid‐based gel phantom. Conclusion: The current results suggest tumors inside breast tissue with high water content may be easily heated up to a few degrees above the surrounding body temperature within a short time interval (e.g., on the order of 10 seconds) without much difficulty. According to the current results, however, it might be difficult to achieve such a temperature change for tumors inside breast tissue similar to intralipid‐based gel in terms of its optical property. This work has been supported by the US Department of Defense, Breast Cancer Research Program, Concept Award, W81XWH‐08‐1‐0686.

MO-FG-BRA-02: Modulation of Clinical Orthovoltage X-Ray Spectrum Further Enhances Radiosensitization of Cancer Cells Targeted with Gold Nanoparticles

Purpose: To assess the potential to amplify radiosensitization of cancer cells targeted with gold nanoparticles by augmenting selective spectral components of X-ray beam. Methods: Human prostate cancer cells were treated for 24h with gold nanorods conjugated to goserelin acetate or pegylated, systematically washed and irradiated with 250 kVp X-rays (25mA, 0.25mm Cu- filter, 8x8cm2 field size, 50cm SSD) with or without an additional 0.25 mm Erbium (Er) filter. As demonstrated in a companion Monte Carlo study, Er-filter acted as an external target to feed Erbium K-shell X-ray fluorescence photons (∼50 keV) into the 250 kVp beam. After irradiation, we performed measurements of clonogenic viability with doses between 0 -6Gy, irreparable DNA damage assay to measure double-strand breaks via γH2AX-foci staining, and production of stable reactive oxygen species (ROS). Results: The clonogenic assay for the group treated with conjugated nanoparticles showed radiosensitization enhancement factor (REF), calculated at the 10% survival fraction aisle, of (1.62±0.07) vs. (1.23±0.04) with/without the Er-filter in the 250 kVp beam, respectively. The group treated with pegylated nanoparticles, albeit retained in modest amounts within the cells, also showed statistically significant REF (1.13±0.09) when the Erbium filter was added to the beam. No significant radiosensitization was observed for other groups. Measurements of ROS levels showed increments of (1.9±0.2) vs. (1.4±0.1) for combined treatment with targeted nanoparticles and Er-filtered beam. γH2AX-foci showed 50% increase for the same treatment combination, confirming the enhanced radiosensitization in a consistent fashion. Conclusion: Our study demonstrates the feasibility of enhancing radiosensitization of cancer cells by combining actively targeted gold nanoparticles and modulating the X-ray spectrum in the desired energy range. The established technique will not only help develop strategies to maximize nanoparticle-mediated radiosensitization but also offer a convenient way to acquire unprecedented insights into the role of photon energy for the observed radiosensitization effects. Supported by DOD/PCRP grant W81XWH-12-1-0198