Spatial Resolution Phantom Research Articles

A phantom for the quantitative determination and improvement of the spatial resolution in slice-selective 2D-FT magnetic resonance micro-imaging and -microscopy based on Deep X-ray Lithography (DXRL)

Introduction: The most important assessed quality-control (QC) criteria for improvements in high-resolution imaging are represented by the contrast-to-noise-ratio and spatial resolution. Ultra-High-Field (UHF) Magnetic-Resonance-scanners (B ≥ 7 T) for medical research allowed for the improvement in spatial resolution up to the microimaging and nominal microscopy range [pixel-size: ps < (100 μm)2], even in-vivo on humans just recently. Preclinical MRI- and dedicated MR-microscopy (MRM) scanners already allow for microimaging and MRM (1-256 μm) but lack a sensible spatial resolution phantom for QC and performance improvements in hardware, pulse-sequencing and MRprotocols. In most scientific MRI articles, the spatial resolution is characterized by the ps, though this measurement parameter only limits the actual resolution.Methods: Here the Modulation-Transfer-Function (MTF) is used as evaluation concept for the determination of the spatial resolution in MRM using simple intensity profiles. The resolution limit is defined using a critical modulation-level. In approaching visual impressions on spatial resolution an additional criterion derived from the Modulation-depth-to-Noise-Ratio (MNR) is proposed. A practical method for assessment based on a concrete phantom design and its realization is shown.Results: The phantom design consists of several sets of fine grids, specifically featuring high structural anisotropy for optimum SNR and CNR, with different spatial periods ranging from a1 = 256 μm down to a8 = 2 μm, not only for a quick visual qualitative check, but also for quantification of resolution using the MTF for two different spatial encodings in two orthogonal in-plane directions. The challenging demands on the manufacturing technology especially with regard to the aspect-ratio are approached using Deep-X-Ray-Lithography (DXRL) relying on the high brilliance of Synchroton-radiation. Smallest grid plates with width of 4 μm corresponding to 125 line pairs/mm at a plate depth of 100 μm were achieved.Discussion: MR-microscopic images, originating from a microscopy insert on a human UHF-MR-scanner, were used for demonstration of the evaluation process with two independent resolution-criteria. The developed prototype offers unique possibilities for quantitative resolution QC on UHF human and preclinical MR-scanners. Such a resolution-phantom might be very important for the improvement of MR-pulse-sequences, MR-protocols and even hardware. In principle the phantom can also be used for other microscopic imaging-modalities as for instance μCT and Optical-Coherence-Tomography (OCT).

Metal artifact correction in photon-counting detector computed tomography: metal trace replacement using high-energy data.

Metal artifacts have been an outstanding issue in computed tomography (CT) since its first uses in the clinic and continue to interfere. Metal artifact reduction (MAR) methods continue to be proposed and photon-counting detectors (PCDs) have recently been the subject of research toward this purpose. PCDs offer the ability to distinguish the energy of incident x-rays and sort them in a set number of energy bins. High-energy data captured using PCDs have been shown to reduce metal artifacts in reconstructions due to reduced beam hardening. High-energy reconstructions using PCD-CT have their drawbacks, such as reduced image contrast and increased noise. Here, we demonstrate a MAR algorithm, trace replacement MAR (TRMAR), in which the data corrupted by metal artifacts in full energy spectrum projections are corrected using the high-energy data captured during the same scan. The resulting reconstructions offer similar MAR to that seen in high-energy reconstructions, but with improved image quality. Experimental data were collected using a bench-top PCD-CT system with a cadmium zinc telluride PCD. Simulations were performed to determine the optimal high-energy threshold and to test TRMAR in simulations using the XCAT phantom and a biological sample. For experiments a 100-mm diameter cylindrical phantom containing vials of water, two screws, various densities of Ca(ClO4 )2 , and a spatial resolution phantom was imaged with and without the screws. The screws were segmented in the initial reconstruction and forward projected to identify them in the sinogram space in order to perform TRMAR. The resulting reconstructions were compared to the control and to reconstructions corrected using normalized metal artifact reduction (NMAR). Additionally, a beef short rib was imaged with and without metal to provide a more realistic phantom. XCAT simulations showed a reduction in the streak artifact from -978HU in uncorrected images to -10HU with TRMAR. The magnitude of the metal artifact in uncorrected images of the 100-mm phantom was -442 HU, compared to the desired -81HU with no metal. TRMAR reduced the magnitude of the artifact to -142 HU, with NMAR reducing the magnitude to -96 HU. Relative image noise was reduced from 176% in the high-energy image to 56% using TRMAR. Density quantification was better with NMAR, with the Ca(ClO4 )2 vial affected most by metal artifacts showing 0.8% error compared to 2.1% with TRMAR. Small features were preserved to a greater extent with TRMAR, with the limiting spatial frequency at 20% of the MTF fully maintained at 1.31 lp/mm, while with NMAR it was reduced to 1.22 lp/mm. Images of the beef short rib showed better delineation of the shape of the metal using TRMAR. NMAR offers slightly better performance compared to TRMAR in streak reduction and image quality metrics. However, TRMAR is less susceptible to metal segmentation errors and can closely approximate the reduction in the streak metal artifact seen in NMAR at 1/3 the computation time. With the recent introduction of PCD-CT into the clinic, TRMAR offers notable potential for fast, effective MAR.

Tissue-mimicking phantoms for performance evaluation of photoacoustic microscopy systems.

Phantom-based performance test methods are critically needed to support development and clinical translation of emerging photoacoustic microscopy (PAM) devices. While phantoms have been recently developed for macroscopic photoacoustic imaging systems, there is an unmet need for well-characterized tissue-mimicking materials (TMMs) and phantoms suitable for evaluating PAM systems. Our objective was to develop and characterize a suitable dermis-mimicking TMM based on polyacrylamide hydrogels and demonstrate its utility for constructing image quality phantoms. TMM formulations were optically characterized over 400-1100 nm using integrating sphere spectrophotometry and acoustically characterized using a pulse through-transmission method over 8-24 MHz with highly confident extrapolation throughout the usable band of the PAM system. This TMM was used to construct a spatial resolution phantom containing gold nanoparticle point targets and a penetration depth phantom containing slanted tungsten filaments and blood-filled tubes. These phantoms were used to characterize performance of a custom-built PAM system. The TMM was found to be broadly tunable and specific formulations were identified to mimic human dermis at an optical wavelength of 570 nm and acoustic frequencies of 10-50 MHz. Imaging results showed that tungsten filaments yielded 1.1-4.2 times greater apparent maximum imaging depth than blood-filled tubes, which may overestimate real-world performance for vascular imaging applications. Nanoparticles were detectable only to depths of 120-200 µm, which may be due to the relatively weaker absorption of single nanoparticles vs. larger targets containing high concentration of hemoglobin. The developed TMMs and phantoms are useful tools to support PAM device characterization and optimization, streamline regulatory decision-making, and accelerate clinical translation.

Flat-panel imager energy-dependent proton radiography for a proton pencil-beam scanning system

In proton-based radiotherapy, proton radiography could allow for direct measurement of the water-equivalent path length (WEPL) in tissue, which can then be used to determine relative stopping power (RSP). Additionally, proton radiographs allow for imaging in the beam’s-eye-view. In this work, a proton radiography technique using a flat-panel imager and a pencil-beam scanning (PBS) system is demonstrated in phantom. Proton PBS plans were delivered on a Varian ProBeam system to a flat-panel imager. Each proton plan consisted of energy layers separated by 4.8 MeV, and a field size of 25 cm × 25 cm. All measured data is binned into a layer-by-layer delivery in post processing. To build a calibration curve correlating detector response to WEPL, the plans were delivered to slabs of solid water of increasing thickness. Pixel-by-pixel detector response in the time/energy domain is then fit as a function of WEPL. Tissue equivalent phantoms are imaged for evaluation of WEPL accuracy. A spatial resolution phantom and a head phantom are also imaged. For all experiments, the detector was run with an effective pixel size of 0.4 mm × 0.4 mm. The proposed method reconstructed RSP with mean errors of 2.65%, −0.14%, and 0.61% for lung, soft tissue, and bone, respectively. In a 40 mm thick spatial resolution phantom, a 2 mm deep pinhole with 1 mm diameter can be seen. The accuracy and spatial resolution of the method show that it could be implemented for patient position verification. Further development could lead to patient-specific verification of RSP to be used for treatment guidance.

Assessment of the attenuation of metal-infused filaments for 3D printing a gamma camera calibration phantom

The capability to produce customizable 3D printed imaging phantoms out of a growing number of materials has contributed to the increased use of such phantoms in clinical practice and research. Many of these materials have not been characterized at clinically relevant parameters for nuclear medicine imaging. In this work, we measured the attenuation of several 3D printing metal-infused filament materials with the potential for blocking 122 keV gamma photons using a scintillation detector. Understanding the ability of each material to attenuate gamma rays would allow modelling of varying levels of attenuation required for different body tissues at this energy. In addition, it allows for the determination of the thickness of the material needed to attenuate 122 keV photons, in order to build high-contrast spatial resolution phantoms. To achieve this goal, we performed attenuation experiments for three different 3D printing materials in this work (Tungsten infused Acrylonitrile Butadiene Styrene (ABS), Iron infused Polylactic Acid (PLA), and Stainless Steel (SS) infused PLA), by observing gamma transmission through blocks made of each material as a function of thickness. A Cobalt-57 (Co-57; 122 keV) source was chosen because of its relevance to nuclear medicine. Tungsten infused ABS showed the highest measured attenuation coefficient out of the three candidates at this energy (1.452 cm2/g). We further designed and 3D printed an imaging phantom to aid in characterizing the spatial resolution of novel gamma imaging systems at the above energy. The phantom design contains multiple line pairs (lp) located in four quadrants ranging from 1.2 mm – 8 mm. The 3D printed phantom was imaged by a molecular breast tomosynthesis (MBT) gamma camera. Assessment of the intrinsic spatial resolution of the MBT camera showed that our 3D printed phantom might be a viable option for routine spatial resolution quality control (QC) of gamma cameras used in clinical practice.

Design and performance evaluation of spheroid geometry for brain PET scanner using Monte Carlo modeling

Introduction: There has been a curiosity about the spheroid geometry for PET scanners developments since several years ago, therefore in this study, we are aiming to evaluate the performance of this geometry and compare its performance with cylindrical geometry using Monte Carlo simulation. Methods: We simulated a spheroid geometry with a radius of 199 mm, patient bore with of radius of 175 mm, which is compatible with brain size. In second design, cylindrical geometry was simulated with transaxial FOV and ring radius of 175 mm as well. Photon detection efficiency (PDE), NEMA line source sensitivity, spatial resolution and Derenzo phantom image quality were analyzed. Results: We obtained PDE about 21.7% versus 23.8% in 250-750 keV and 19.5% versus 21.3% in 410-613 keV for point source in center of FOV for spheroid and cylindrical PET respectively. The results of NEMA sensitivity measurements indicate 3.29 kcps/MBq versus 3.64 kcps/MBq for spheroid and cylindrical designs. The spatial resolution (FWHM) calculations using MLEM reconstruction algorithm show around 1.6 mm for transvers and axial resolution for point source placed in center of FOV for both scanners. Also we found for spheroid and cylindrical designs 4.8 and 2.7 mm versus 4 and 3.6 mm as transvers and axial mean resolution for off-center point sources. Conclusion: Performance evaluation study indicates that the spheroid geometry delivers better axial resolution whereas cylindrical design can still provide higher sensitivity and transvers spatial resolution than the spheroid geometry PET with same scanner bore size.

Performance evaluation of noise reduction algorithm with median filter using improved thresholding method in pixelated semiconductor gamma camera system: A numerical simulation study

Abstract To improve the noise characteristics, software-based noise reduction algorithms are widely used in cadmium zinc telluride (CZT) pixelated semiconductor gamma camera system. The purpose of this study was to develop an improved median filtering algorithm using a thresholding method for noise reduction in a CZT pixelated semiconductor gamma camera system. The gamma camera system simulated is a CZT pixelated semiconductor detector with a pixel-matched parallel-hole collimator and the spatial resolution phatnom was designed with the Geant4 Application for Tomography Emission (GATE). In addition, a noise reduction algorithm with a median filter using an improved thresholding method is developed and we applied our proposed algorithm to an acquired spatial resolution phantom image. According to the results, the proposed median filter improved the noise characteristics compared to a conventional median filter. In particular, the average for normalized noise power spectrum, contrast to noise ratio, and coefficient of variation results using the proposed median filter were 10, 1.11, and 1.19 times better than results using conventional median filter, respectively. In conclusion, our results show that the proposed median filter using improved the thresholding method results in high imaging performance when applied in a CZT semiconductor gamma camera system.

Iterative reconstruction with boundary detection for carbon ion computed tomography

In heavy ion radiation therapy, improving the accuracy in range prediction of the ions inside the patient’s body has become essential. Accurate localization of the Bragg peak provides greater conformity of the tumor while sparing healthy tissues. We investigated the use of carbon ions directly for computed tomography (carbon CT) to create the relative stopping power map of a patient’s body. The Geant4 toolkit was used to perform a Monte Carlo simulation of the carbon ion trajectories, to study their lateral and angular deflections and the most likely paths, using a water phantom. Geant4 was used to create carbonCT projections of a contrast and spatial resolution phantom, with a cone beam of 430 MeV/u carbon ions. The contrast phantom consisted of cranial bone, lung material, and PMMA inserts while the spatial resolution phantom contained bone and lung material inserts with line pair (lp) densities ranging from 1.67 lp cm−1 through 5 lp cm−1. First, the positions of each carbon ion on the rear and front trackers were used for an approximate reconstruction of the phantom. The phantom boundary was extracted from this approximate reconstruction, by using the position as well as angle information from the four tracking detectors, resulting in the entry and exit locations of the individual ions on the phantom surface. Subsequent reconstruction was performed by the iterative algebraic reconstruction technique coupled with total variation minimization (ART-TV) assuming straight line trajectories for the ions inside the phantom. The influence of number of projections was studied with reconstruction from five different sets of projections: 15, 30, 45, 60 and 90. Additionally, the effect of number of ions on the image quality was investigated by reducing the number of ions/projection while keeping the total number of projections at 60. An estimation of carbon ion range using the carbonCT image resulted in improved range prediction compared to the range calculated using a calibration curve.

Filtered back-projection reconstruction for attenuation proton CT along most likely paths

This work investigates the attenuation of a proton beam to reconstruct the map of the linear attenuation coefficient of a material which is mainly caused by the inelastic interactions of protons with matter. Attenuation proton computed tomography (pCT) suffers from a poor spatial resolution due to multiple Coulomb scattering (MCS) of protons in matter, similarly to the conventional energy-loss pCT. We therefore adapted a recent filtered back-projection algorithm along the most likely path (MLP) of protons for energy-loss pCT (Rit et al ) to attenuation pCT assuming a pCT scanner that can track the position and the direction of protons before and after the scanned object. Monte Carlo simulations of pCT acquisitions of density and spatial resolution phantoms were performed to characterize the new algorithm using Geant4 (via Gate). Attenuation pCT assumes an energy-independent inelastic cross-section, and the impact of the energy dependence of the inelastic cross-section below 100 MeV showed a capping artifact when the residual energy was below 100 MeV behind the object. The statistical limitation has been determined analytically and it was found that the noise in attenuation pCT images is 411 times and 278 times higher than the noise in energy-loss pCT images for the same imaging dose at 200 MeV and 300 MeV, respectively. Comparison of the spatial resolution of attenuation pCT images with a conventional straight-line path binning showed that incorporating the MLP estimates during reconstruction improves the spatial resolution of attenuation pCT. Moreover, regardless of the significant noise in attenuation pCT images, the spatial resolution of attenuation pCT was better than that of conventional energy-loss pCT in some studied situations thanks to the interplay of MCS and attenuation known as the West–Sherwood effect.

Evaluation of the OSC‐TV iterative reconstruction algorithm for cone‐beam optical CT

The present work evaluates an iterative reconstruction approach, namely, the ordered subsets convex (OSC) algorithm with regularization via total variation (TV) minimization in the field of cone-beam optical computed tomography (optical CT). One of the uses of optical CT is gel-based 3D dosimetry for radiation therapy, where it is employed to map dose distributions in radiosensitive gels. Model-based iterative reconstruction may improve optical CT image quality and contribute to a wider use of optical CT in clinical gel dosimetry. This algorithm was evaluated using experimental data acquired by a cone-beam optical CT system, as well as complementary numerical simulations. A fast GPU implementation of OSC-TV was used to achieve reconstruction times comparable to those of conventional filtered backprojection. Images obtained via OSC-TV were compared with the corresponding filtered backprojections. Spatial resolution and uniformity phantoms were scanned and respective reconstructions were subject to evaluation of the modulation transfer function, image uniformity, and accuracy. The artifacts due to refraction and total signal loss from opaque objects were also studied. The cone-beam optical CT data reconstructions showed that OSC-TV outperforms filtered backprojection in terms of image quality, thanks to a model-based simulation of the photon attenuation process. It was shown to significantly improve the image spatial resolution and reduce image noise. The accuracy of the estimation of linear attenuation coefficients remained similar to that obtained via filtered backprojection. Certain image artifacts due to opaque objects were reduced. Nevertheless, the common artifact due to the gel container walls could not be eliminated. The use of iterative reconstruction improves cone-beam optical CT image quality in many ways. The comparisons between OSC-TV and filtered backprojection presented in this paper demonstrate that OSC-TV can potentially improve the rendering of spatial features and reduce cone-beam optical CT artifacts.

Enhanced spatial resolution in fluorescence molecular tomography using restarted L1-regularized nonlinear conjugate gradient algorithm.

Owing to the high degree of scattering of light through tissues, the ill-posedness of fluorescence molecular tomography (FMT) inverse problem causes relatively low spatial resolution in the reconstruction results. Unlike L2 regularization, L1 regularization can preserve the details and reduce the noise effectively. Reconstruction is obtained through a restarted L1 regularization-based nonlinear conjugate gradient (re-L1-NCG) algorithm, which has been proven to be able to increase the computational speed with low memory consumption. The algorithm consists of inner and outer iterations. In the inner iteration, L1-NCG is used to obtain the L1-regularized results. In the outer iteration, the restarted strategy is used to increase the convergence speed of L1-NCG. To demonstrate the performance of re-L1-NCG in terms of spatial resolution, simulation and physical phantom studies with fluorescent targets located with different edge-to-edge distances were carried out. The reconstruction results show that the re-L1-NCG algorithm has the ability to resolve targets with an edge-to-edge distance of 0.1 cm at a depth of 1.5 cm, which is a significant improvement for FMT.

Optimisation of an MDCT abdominal protocol: Image quality assessment of standard vs. iterative reconstructions

This work aims to construct a method to objectively evaluate CT image quality when new clinical protocol performances must be compared with a standard reference. We compare iterative reconstruction in the image space with filtered back projection reconstruction and accurately quantify the dose reduction.The comparison strategy accounts for both physical and clinical image qualities that are evaluated using a standard metric. The quasi-ideal observer metric is also explored to verify its reportedly high correlation with perceived image quality.Water or spatial resolution phantom images are used to characterise the physical image quality using the classic metrics in the Fourier domain by calculating the modulation transfer functions and noise power spectra (NPS).The clinical-image quality is evaluated with a 4-alternative forced-choice test. The human observers are asked to detect a positive image that contains a simulated lesion in a background image. Then, the same positive images are characterised with the quasi-ideal observer metric, which calculates the non-prewhitening matched filter signal-to-noise ratio (SNRNPWMF).Iterative reconstruction strongly reduces the image noise, but the NPS are slightly shifted to lower frequencies, which gives the images a coarse graininess. Compared with the reference FBP protocol for abdomen exams, the highest dose reduction is 40% if the standard metric is used and 30% if the SNRNPWMF metric is used. The detectability test results achieve a better correlation with SNRNPWMF than with the standard metric.The identified Fourier metric is a useful descriptor of human quality perception and can be used for future protocol optimisation.

Application of Response Surface Methodology to systematically optimize image quality in computer tomography: A case study using fresh chestnuts (Castanea spp.)

The objective of this study is to describe a method that will systematically and efficiently obtain a model of computerized tomographic (CT) scanning factor levels, which returns optimized high quality CT images. Chestnut (Castanea spp.) two-dimensional CT images were used to describe this optimization procedure, considered to be a critical step in the development of a fast, nondestructive technique, capable of assessing fresh internal quality attributes and components of chestnuts, and other agricultural commodities. Response Surface Methodology (RSM), using a three-factor, three-level Box–Behnken statistical design and digital image processing, were used to optimize the factors affecting image quality, which include X-ray voltage, current, and slice thickness. Response variables representing image quality were digitally and automatically inferred from fresh chestnut image Signal to Noise Ratio, Teflon® cylinder reference volume accuracy, quality assurance (QA) High Contrast Spatial Resolution phantom, and QA Low Contrast Detectability phantom. Second-order RSM prediction models for each response variable reflected a combined maximized CT image quality at a voltage, current, and slice thickness equal to 120kV, 170mA, and 2.5mm respectively. The experiment yielded optimal chestnut CT images that can accurately reflect internal decay of fresh chestnuts with an overall accuracy rate equal to 96%, taking as reference data the of Subjective Quality Rating of five trained chestnut experts.

Photon counting spectral CT versus conventional CT: comparative evaluation for breast imaging application

Spectral CT systems with photon counting detectors have more advantages compared to conventional CT systems. However, clinical applications have been hampered for a long time due to the high demands of clinical systems and limitations of spectroscopic x-ray detectors. Photon counting detector technology has gained considerable improvements in the past decade, and spectral CT has become a hot topic. Several experimental spectral CT systems are under investigation. The purpose of this work was to perform the first direct, side-by-side comparison of existing spectral CT technology with a mature clinical CT system based on a conventional energy integrating detector. We have built an experimental spectral CT system whose main parameters are similar to the parameters of a clinical CT system. The system uses a spectroscopic cadmium zinc telluride (CZT) detector. The detector includes two rows of CZT pixels with 256 pixels in each row. The pixel size is 1 × 1 mm2, and the maximum count rate is 2 Mcounts/pixel/s. The spectral CT system has a magnification factor of 1.62 and the source to detector and source to image distances of 85 and 53 cm, respectively. The above parameters are similar to those of the clinical CT system, Siemens Sensation 16, used for comparison. The two systems were compared by imaging spatial resolution and contrast resolution phantoms made from acrylic cylinders with 14 cm diameters. The resolution phantom included Al wires with 0.3, 0.6, and 1 mm diameters, and 0.25 g cc−1 CaCO3 contrast. The contrast phantom included contrast elements with 1.7, 5, and 15 mg cc−1 iodine, and 1.1, 3.3, and 10 mg cc−1 gadolinium. The phantoms were imaged with the two systems using 120 kVp tube voltage and 470 mR total skin exposure. The spectral CT showed CT numbers, image noise, and spatial and contrast resolutions to be similar within 10% compared to the Siemens 16 system, and provided an average of 10% higher CNR. However, the spectral CT system had a major advantage in that the iodine, gadolinium, and CaCO3 contrasts were decomposed by dual-energy and K-edge subtraction methods using energy selective CT data acquired in a single CT scan and fixed tube voltage. It is concluded that photon counting spectral CT technology is close to feasibility for routine clinical applications. Furthermore, it is ready for some clinical applications such as dedicated breast CT which has relatively lower demands on photon counting detectors.

Low-dose megavoltage cone-beam CT imaging using thick, segmented scintillators

Megavoltage, cone-beam computed tomography (MV CBCT) employing an electronic portal imaging device (EPID) is a highly promising technique for providing soft-tissue visualization in image-guided radiotherapy. However, current EPIDs based on active matrix flat-panel imagers (AMFPIs), which are regarded as the gold standard for portal imaging and referred to as conventional MV AMFPIs, require high radiation doses to achieve this goal due to poor x-ray detection efficiency (∼2% at 6 MV). To overcome this limitation, the incorporation of thick, segmented, crystalline scintillators, as a replacement for the phosphor screens used in these AMFPIs, has been shown to significantly improve the detective quantum efficiency (DQE) performance, leading to improved image quality for projection imaging at low dose. Toward the realization of practical AMFPIs capable of low dose, soft-tissue visualization using MV CBCT imaging, two prototype AMFPIs incorporating segmented scintillators with ∼11 mm thick CsI:Tl and Bi4Ge3O12 (BGO) crystals were evaluated. Each scintillator consists of 120 × 60 crystalline elements separated by reflective septal walls, with an element-to-element pitch of 1.016 mm. The prototypes were evaluated using a bench-top CBCT system, allowing the acquisition of 180 projection, 360° tomographic scans with a 6 MV radiotherapy photon beam. Reconstructed images of a spatial resolution phantom, as well as of a water-equivalent phantom, embedded with tissue equivalent objects having electron densities (relative to water) varying from ∼0.28 to ∼1.70, were obtained down to one beam pulse per projection image, corresponding to a scan dose of ∼4 cGy–-a dose similar to that required for a single portal image obtained from a conventional MV AMFPI. By virtue of their significantly improved DQE, the prototypes provided low contrast visualization, allowing clear delineation of an object with an electron density difference of ∼2.76%. Results of contrast, noise and contrast-to-noise ratio are presented as a function of dose and compared to those from a conventional MV AMFPI.

Evaluation of Spatial Resolution for Heavy Ion CT System Based on the Measurement of Residual Range Distribution With HIMAC

We report experimental results from a heavy ion CT system based on the measurement of residual range distribution using an X-ray intensifying screen and a charged coupled device (CCD) camera system. This technique was first investigated by Zygmanski (2000) for proton beams, and they reported that the spatial resolution was significantly degraded by multiple Coulomb scattering (MCS) effects in the irradiated medium. Experiments were done on the spatial resolution phantom by using helium and carbon beams accelerated up to 120 MeV/u and 230 MeV/u by the Heavy Ion Medical Accelerator in Chiba (HIMAC), installed in the National Institute of Radiological Sciences (NIRS) in Japan, using a high performance intensified CCD (ICCD) camera. We show that the MCS blurring effect can be significantly reduced in the reconstructed image by using a carbon beam with this technique. Our results suggest that heavier particles such as carbon would be more useful if this technique is envisioned as a clinical tool to obtain data that would aid proton and/or heavy ion treatment planning.

The effect of β + energy on performance of a small animal PET camera

The effective spatial resolution of a positron emission tomography (PET) scanner is determined in part by the initial energy of the positron, which is a function of the radionuclide. For F-18 ( E max=0.633 MeV) the mean positron range in water is small (0.6 mm). However, many other useful positron-emitting nuclides have higher energies, for example Ga-68 ( E max=1.899 MeV, mean range 2.9 mm) has one of the highest. The performance of a non-rotating, 16 module high density avalanche chamber (quad-HIDAC) small animal PET scanner was measured using both F-18 and Ga-68 to represent the extremes of high and low positron energy. The count rate performance—scatter fraction and noise-equivalent count rate (NEC)—were measured for both isotopes. Data were also collected for a spatial resolution phantom with rectangular arrays of holes of diameter 2.0, 1.5, 1.0 and 0.5 mm with the centres separated by 4.0, 3.0, 2.0 and 1.0 mm respectively. The NEC, measured for both 70 and 200 cm 3 cylindrical phantoms, was approximately linear up to 30 MBq, but shows a rapid drop-off above this value. The spatial resolution phantom showed that 1 mm objects are just resolved with F-18, but none of the targets are resolved for Ga-68. In conclusion, spatial resolution is dominated by the choice of isotope down to 1 mm, with sensitivity and count-rate data being largely independent of positron range.

SU‐FF‐I‐03: Computation‐Efficient Cone Beam Image Reconstruction for Image‐ Guided Radiation Therapy Applications Using 3D Weighted Filtered Backprojection (CB‐FBP) Algorithm

Purpose: To extend the 3D weighted cone beam filtered backprojection (CB‐FBP) algorithm for diagnostic CT imaging to image‐guided radiation therapy (IGRT) applications. Method and Materials: 3D isotropic spatial resolution is one of the most attracting features of state‐of‐the‐art volumetric CT for diagnostics imaging. However, in IGRT treatment planning, the CT image slice thickness is usually larger than what is determined by detector row width (namely thin image). A straightforward way to generate thicker image is the combination of weighted adjacent thin images in image domain, which is computationally expensive because each thin image has to go through a computation expensive 3D backprojection. Another way is to carry out cross‐row filtering in projection domain, which may cause shading/glaring artifacts and uneven slice thickness as isotropic 3D geometry is distorted. To optimize both image quality (IQ) and computational efficiency, a virtual reconstruction plane (RP) based algorithm is proposed and implemented. By using the 3D weighted CB‐FBP algorithm, a thick image is still a weighted combination of adjacent thin images, but the combination is implemented in projection domain using virtual RPs. To maintain the IQ of thick image, the 3D weighted CB‐FBP algorithm is applied by tracking re‐sampled projection data. The tracking process is to improve computational efficiency further by making use of the projection data corresponding to involved virtual RPs only, while the re‐sampling process is to improve IQ by increasing the in‐plane sampling‐rate in virtual RPs. Results: By using a helical body phantom, spatial resolution phantom and 20 cm water phantom, the performance of the proposed algorithm, such as suppression of artifacts, uniformity of slice thickness and noise characteristics, are experimentally evaluated and verified. Conclusion: The experimental evaluation shows the proposed algorithm is indeed an optimized image reconstruction solution for IGRT applications in terms of both image quality and computational efficiency.

TU‐C‐ValA‐01: Image Fusion and Deformation Using a Genetic Algorithm

Purpose: To develop an image fusion application utilizing genetic algorithms, segment the fused images using treatment‐planning contours, perform a slice‐by‐slice sub‐fusion of the segmented images in order to measure deformation, and then utilize the measured deformation for adaptive therapy on a helical tomotherapy treatment delivery. Methods and Materials: A reference CT image of a density and spatial resolution phantom was obtained using a MVCT imaging. A secondary MVCT fusion image was obtained with the phantom offset by a known amount with plugs removed or rotated. An image fusion algorithm was created using genetic programming to perform image registration of the MVCT images. Contours of the plugs were used to extract sub‐images that were separately registered and deformed using the genetic algorithm. Adaptive therapy was achieved thorough a treatment delivery sinogram deformation algorithm. The sinogram deformation algorithm was tested using a geometric test case that consisted of a dose triangle with a 5.2‐cm base located inside a dose circle with of 3.14‐cm radius. The test dose pattern was moved by known amounts by deforming the treatment delivery sinogram. Results: The initial genetic fusion of the reference and secondary MVCT images was achieved in approximately 15 generations. The time required to perform the genetic fusion was typically 10 to 15 seconds. The images were fused to within 0.7‐mm of the correct position. At the end of the initial fusion, the genetic algorithm correctly identified one of the plugs as missing in the secondary MVCT dataset. The genetic algorithm correctly segmented the second resolution plug in a sub‐image and deformed it to within 1‐degree and 0.7‐mm of the correct position. The delivery deformation tests moved the dose to within 5‐mm of the desired position. Conclusions: A genetic algorithm has been developed for performing image fusion and simple deformation of defined regions of interest.

SU‐DD‐A3‐04: Quantitative Evaluation of Cone Beam Digital Tomosynthesis (CBDT) for Image‐Guided Radiation Therapy

Purpose: Cone beam digital tomosynthesis (CBDT) is a new imaging technique proposed by us recently as a rapid approach for creating cross sectional images of a patient in the radiotherapy treatment room. Similar to the cone beam computed tomography (CBCT) approach, the CBDT uses an X‐ray source and an X‐ray detector on a Linac to acquire projection data by rotation around the patient. Unlike CBCT, CBDT utilizes partial scans, typically in the range of 20–60 degrees of gantry arc. The purposes of this work are (1) to evaluate quantitatively the image quality of CBDT in terms of signal‐to‐noise ratio and spatial resolution; (2) to demonstrate that we can use CBDT to image soft tissue targets, e.g., the prostate. Method and Materials: An experimental CBDT system has been built on a Linac with a recently developed flat‐panel detector. A number of phantoms including a spatial resolution phantom, two contrast‐detail phantoms and a custom‐made anthropomorphic pelvic phantom (CIRS Inc.) were used in the evaluation. CBDT phantom images have been generated and analyzed for different degrees of gantry arc and compared to those from CBCT. Results: Quantitative results on signal‐to‐noise ratio and spatial resolution have been obtained for different degrees of gantry arc. Compared to CBCT, CBDT has a worse spatial resolution in the direction perpendicular to the planes of reconstruction but a better resolution in the parallel direction. The image quality of CBDT is acceptable in the planes most relevant to the treatment. It has been shown that the prostate is visible on CBDT reconstructed images of the pelvic phantom. Conclusion: This work indicates that the CBDT approach can be a viable and rapid tomographical imaging technique for treatment verification in radiotherapy. Conflict of Interest: This work was partially supported by Siemens Medical Solutions, Inc.