High-sensitivity imaging and quantification of intratumoral distributions of gold nanoparticles using a benchtop x-ray fluorescence imaging system.

A high-sensitivity benchtop x-ray fluorescence (XRF) imaging system, based on a high-power x-ray source and silicon drift detector, has been developed. This system allows gold L-shell XRF-based quantitative imaging of gold nanoparticles (GNPs) at concentrations as low as 0.007 mg/cm3 (7 ppm) in biological tissues/water. Its capability for biomedical applications was demonstrated by imaging the GNP distribution within a small (∼12×11×2 mm3) ex vivo sample (extracted from a murine tumor after intravenous GNP administration). The results suggest direct translatability for routine preclinical ex vivo imaging tasks involving GNPs, as well as the possibility for in vivo imaging of small/superficial animal tumors.

Dosimetric study on learning-based cone-beam CT correction in adaptive radiation therapy.

INTRODUCTION: Cone-beam CT (CBCT) image quality is important for its quantitative analysis in adaptive radiation therapy. However, due to severe artifacts, the CBCTs are primarily used for verifying patient setup only so far. We have developed a learning-based image quality improvement method which could provide CBCTs with image quality comparable to planning CTs (pCTs). The accuracy of dose calculations based on these CBCTs is unknown. In this study, we aim to investigate the dosimetric accuracy of our corrected CBCT (CCBCT) in brain stereotactic radiosurgery (SRS) and pelvic radiotherapy. MATERIALS AND METHODS: We retrospectively investigated a total of 32 treatment plans from 22 patients, each of whom with both original treatment pCTs and CBCTs acquired during treatment setup. The CCBCT and original CBCT (OCBCT) were registered to the pCT for generating CCBCT-based and OCBCT-based treatment plans. The original pCT-based plans served as ground truth. Clinically-relevant dose volume histogram (DVH) metrics were extracted from the ground truth, OCBCT-based and CCBCT-based plans for comparison. Gamma analysis was also performed to compare the absorbed dose distributions between the pCT-based and OCBCT/CCBCT-based plans of each patient. RESULTS: CCBCTs demonstrated better image contrast and more accurate HU ranges when compared side-by-side with OCBCTs. For pelvic radiotherapy plans, the mean dose error in DVH metrics for planning target volume (PTV), bladder and rectum was significantly reduced, from 1% to 0.3%, after CBCT correction. The gamma analysis showed the average pass rate increased from 94.5% before correction to 99.0% after correction. For brain SRS treatment plans, both original and corrected CBCT images were accurate enough for dose calculation, though CCBCT featured higher image quality. CONCLUSION: CCBCTs can provide a level of dose accuracy comparable to traditional pCTs for brain and prostate radiotherapy planning and the correction method proposed here can be useful in CBCT-guided adaptive radiotherapy.

Evaluation of dose point kernel rescaling methods for nanoscale dose estimation around gold nanoparticles using Geant4 Monte Carlo simulations.

The absence of proper nanoscale experimental techniques to investigate the dose-enhancing properties of gold nanoparticles (GNPs) interacting with radiation has prompted the development of various Monte Carlo (MC)-based nanodosimetry techniques that generally require considerable computational knowledge, time and specific tools/platforms. Thus, this study investigated a hybrid computational framework, based on the electron dose point kernel (DPK) method, by combining Geant4 MC simulations with an analytical approach. This hybrid framework was applied to estimate the dose distributions around GNPs due to the secondary electrons emitted from GNPs irradiated by various photon sources. Specifically, the equivalent path length approximation was used to rescale the homogeneous DPKs for heterogeneous GNPs embedded in water/tissue. Compared with Geant4 simulations, the hybrid framework halved calculation time while utilizing fewer computer resources, and also resulted in mean discrepancies less than 20 and 5% for Yb-169 and 6 MV photon irradiation, respectively. Its appropriateness and computational efficiency in handling more complex cases were also demonstrated using an example derived from a transmission electron microscopy image of a cancer cell containing internalized GNPs. Overall, the currently proposed hybrid computational framework can be a practical alternative to full-fledged MC simulations, benefiting a wide range of GNP- and radiation-related applications.

MRI-based treatment planning for brain stereotactic radiosurgery: Dosimetric validation of a learning-based pseudo-CT generation method.

Magnetic resonance imaging (MRI)-only radiotherapy treatment planning is attractive since MRI provides superior soft tissue contrast without ionizing radiation compared with computed tomography (CT). However, it requires the generation of pseudo CT from MRI images for patient setup and dose calculation. Our machine-learning-based method to generate pseudo CT images has been shown to provide pseudo CT images with excellent image quality, while its dose calculation accuracy remains an open question. In this study, we aim to investigate the accuracy of dose calculation in brain frameless stereotactic radiosurgery (SRS) using pseudo CT images which are generated from MRI images using the machine learning-based method developed by our group. We retrospectively investigated a total of 19 treatment plans from 14 patients, each of whom has CT simulation and MRI images acquired during pretreatment. The dose distributions of the same treatment plans were calculated on original CT simulation images as ground truth, as well as on pseudo CT images generated from MRI images. Clinically-relevant DVH metrics and gamma analysis were extracted from both ground truth and pseudo CT results for comparison and evaluation. The side-by-side comparisons on image quality and dose distributions demonstrated very good agreement of image contrast and calculated dose between pseudo CT and original CT. The average differences in Dose-volume histogram (DVH) metrics for Planning target volume (PTVs) were less than 0.6%, and no differences in those for organs at risk at a significance level of 0.05. The average pass rate of gamma analysis was 99%. These quantitative results strongly indicate that the pseudo CT images created from MRI images using our proposed machine learning method are accurate enough to replace current CT simulation images for dose calculation in brain SRS treatment. This study also demonstrates the great potential for MRI to completely replace CT scans in the process of simulation and treatment planning.

Technical Note: A benchtop cone-beam x-ray fluorescence computed tomography (XFCT) system with a high-power x-ray source and transmission CT imaging capability.

PURPOSE: This report describes upgrades and performance characterization of an experimental benchtop cone-beam x-ray fluorescence computed tomography (XFCT) system capable of determining the spatial distribution and concentration of metal probes such as gold nanoparticles (GNPs). Specifically, a high-power (~3 kW) industrial x-ray source and transmission CT capability were deployed in the same platform under the cone-beam geometry. METHODS: All components of the system are described in detail, including the x-ray source, imaging stage, cadmium-telluride detector for XFCT, and flat-panel detector for transmission CT imaging. The general data acquisition scheme for XFCT and transmission CT is also explicated. The detection limit of the system was determined using calibration samples containing water and GNPs at various concentrations. Samples were then embedded in a small-animal-sized phantom and imaged with XFCT and CT. The reconstructed XFCT and CT images were compared and analyzed using the contrast-to-noise ratio for each GNP-containing region of interest. Also, measurements of the incident beam spectra used for XFCT and CT imaging were made and the corresponding x-ray dose rates were estimated, along with the imaging dose. RESULTS: The present configuration produced a GNP detection limit of 0.03 wt. % with the delivery of an effective dose of 1.87 cGy per projection. XFCT scan of an animal-sized phantom containing low concentrations (down to 0.03 wt. %) of GNP-loaded inserts can be performed within an hour. CONCLUSIONS: The high performance of the system combined with the ability to perform transmission CT in tandem with XFCT suggests that the currently developed benchtop cone-beam XFCT/CT system, in conjunction with GNPs, can be used for routine multimodal preclinical imaging tasks with less stringent dose constraints such as ex vivo imaging. With further effort to minimize XFCT imaging dose as discussed in this report, it may also be used for in vivo imaging.

Radiosensitization of Prostate Cancers In Vitro and In Vivo to Erbium-filtered Orthovoltage X-rays Using Actively Targeted Gold Nanoparticles.

Theoretical investigations suggest that gold nanoparticle (GNP)-mediated radiation dose enhancement and radiosensitization can be maximized when photons interact with gold, predominantly via photoelectric absorption. This makes ytterbium (Yb)-169, which emits photons with an average energy of 93 keV (just above the K-edge of gold), an ideal radioisotope for such purposes. This investigation tests the feasibility of tumor-specific prostate brachytherapy achievable with Yb-169 and actively targeted GNPs, using an external beam surrogate of Yb-169 created from an exotic filter material - erbium (Er) and a standard copper-filtered 250 kVp beam. The current in vitro study shows that treatment of prostate cancer cells with goserelin-conjugated gold nanorods (gGNRs) promotes gonadotropin releasing hormone receptor-mediated internalization and enhances radiosensitivity to both Er-filtered and standard 250 kVp beams, 14 and 10%, respectively. While the degree of GNP-mediated radiosensitization as seen from the in vitro study may be considered moderate, the current in vivo study shows that gGNR treatment plus Er-filtered x-ray irradiation is considerably more effective than radiation treatment alone (p < 0.0005), resulting in a striking reduction in tumor volume (50% smaller) 2 months following treatment. Overall, the current results provide strong evidence for the feasibility of tumor-specific prostate brachytherapy with Yb-169 and gGNRs.

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.

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.

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.

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.

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.

Experimental demonstration of benchtop x-ray fluorescence computed tomography (XFCT) of gold nanoparticle-loaded objects using lead- and tin-filtered polychromatic cone-beams.

This report presents the first experimental demonstration, to our knowledge, of benchtop polychromatic cone-beam x-ray fluorescence computed tomography (XFCT) for a simultaneous determination of the spatial distribution and amount of gold nanoparticles (GNPs) within small-animal-sized objects. The current benchtop experimental setup successfully produced XFCT images accurately showing the regions containing small amount of GNPs (on the order of 0.1 mg) within a 3 cm diameter plastic phantom. In particular, the performance of the current XFCT setup was improved remarkably (e.g., at least a factor of 3 reduction in XFCT scan time) using a tin-filtered polychromatic beam in comparison with a lead-filtered beam. The results of this study strongly suggest that the current benchtop XFCT configuration can be made practical with a few modifications such as the deployment of array detectors, while meeting realistic constraints on x-ray dose, scan time and image resolution for routine pre-clinical in vivo imaging with GNPs.

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.