Showing papers by "Habib Zaidi published in 2011"

Design and performance evaluation of a whole-body Ingenuity TF PET-MRI system

Abstract: The Ingenuity TF PET-MRI is a newly released whole-body hybrid PET-MR imaging system with a Philips time-of-flight GEMINI TF PET and Achieva 3T X-series MRI system. Compared to PET-CT, modifications to the positron emission tomography (PET) gantry were made to avoid mutual system interference and deliver uncompromising performance which is equivalent to the standalone systems. The PET gantry was redesigned to introduce magnetic shielding for the photomultiplier tubes (PMTs). Stringent electromagnetic noise requirements of the MR system necessitated the removal of PET gantry electronics to be housed in the PET-MR equipment room. We report the standard NEMA measurements for the PET scanner. PET imaging and performance measurements were done at Geneva University Hospital as described in the NEMA Standards NU 2-2007 manual. The scatter fraction (SF) and noise equivalent count rate (NECR) measurements with the NEMA cylinder (20 cm diameter) were repeated for two larger cylinders (27 cm and 35 cm diameter), which better represent average and heavy patients. A NEMA/IEC torso phantom was used for overall assessment of image quality. The transverse and axial resolution near the center was 4.7 mm. Timing and energy resolution of the PET-MR system were measured to be 525 ps and 12%, respectively. The results were comparable to PET-CT systems demonstrating that the effect of design modifications required on the PET system to remove the harmful effect of the magnetic field on the PMTs was negligible. The absolute sensitivity of this scanner was 7.0 cps kBq(-1), whereas SF was 26%. NECR measurements performed with cylinders having three different diameters, and image quality measurements performed with IEC phantom yielded excellent results. The Ingenuity TF PET-MRI represents the first commercial whole-body hybrid PET-MRI system. The performance of the PET subsystem was comparable to the GEMINI TF PET-CT system using phantom and patient studies. It is conceived that advantages of hybrid PET-MRI will become more evident in the near future.

An outlook on future design of hybrid PET/MRI systems

Abstract: Early diagnosis and therapy increasingly operate at the cellular, molecular, or even at the genetic level. As diagnostic techniques transition from the systems to the molecular level, the role of multimodality molecular imaging becomes increasingly important. Positron emission tomography (PET) and magnetic resonance imaging (MRI) are powerful techniques for in vivo molecular imaging. The inability of PET to provide anatomical information is a major limitation of standalone PET systems. Combining PET and CT proved to be clinically relevant and successfully reduced this limitation by providing the anatomical information required for localization of metabolic abnormalities. However, this technology still lacks the excellent soft-tissue contrast provided by MRI. Standalone MRI systems reveal structure and function but cannot provide insight into the physiology and/or the pathology at the molecular level. The combination of PET and MRI, enabling truly simultaneous acquisition, bridges the gap between molecular and systems diagnosis. MRI and PET offer richly complementary functionality and sensitivity; fusion into a combined system offering simultaneous acquisition will capitalize the strengths of each, providing a hybrid technology that is greatly superior to the sum of its parts. A combined PET/MRI system provides both the anatomical and structural description of MRI simultaneously with the quantitative capabilities of PET. In addition, such a system would allow exploiting the power of MR spectroscopy (MRS) to measure the regional biochemical content and to assess the metabolic status or the presence of neoplasia and other diseases in specific tissue areas. This paper briefly summarizes state-of-the-art developments and latest advances in dedicated hybrid PET/MRI instrumentation. Future prospects and potential clinical applications of this technology will also be discussed.

NEMA NU-04-based performance characteristics of the LabPET-8™ small animal PET scanner.

Abstract: The objective of this study is to characterize the performance of the preclinical avalanche photodiode (APD)-based LabPET-8™ subsystem of the fully integrated trimodality PET/SPECT/CT Triumph™ scanner using the National Electrical Manufacturers Association (NEMA) NU 04-2008 protocol. The characterized performance parameters include the spatial resolution, sensitivity, scatter fraction, counts rate performance and image-quality characteristics. The PET system is fully digital using APD-based detector modules with highly integrated electronics. The detector assembly consists of phoswich pairs of Lu(1.9)Y(0.1)SiO(5) (LYSO) and Lu(0.4)Gd(1.6)SiO(5) (LGSO) crystals with dimensions of 2 × 2 × 14 mm(3) having 7.5 cm axial and 10 cm transverse field of view (FOV). The spatial resolution and sensitivity were measured using a small (22)Na point source at different positions in the scanner's FOV. The scatter fraction and count rate characteristics were measured using mouse- and rat-sized phantoms fitted with an (18)F line source. The overall imaging capabilities of the scanner were assessed using the NEMA image-quality phantom and laboratory animal studies. The NEMA-based radial and tangential spatial resolution ranged from 1.7 mm at the center of the FOV to 2.59 mm at a radial offset of 2.5 cm and from 1.85 mm at the center of the FOV to 1.76 mm at a radial offset of 2.5 cm, respectively. Iterative reconstruction improved the spatial resolution to 0.84 mm at the center of the FOV. The total absolute system sensitivity is 12.74% for an energy window of 250-650 keV. For the mouse-sized phantom, the peak noise equivalent count rate (NECR) is 183 kcps at 2.07 MBq cc(-1), whereas the peak true count rate is 320 kcps at 2.5 MBq cc(-1) with a scatter fraction of 19%. The rat-sized phantom had a scatter fraction of 31%, with a peak NECR of 67 kcps at 0.23 MBq cc(-1) and a peak true count rate of 186 kcps at 0.27 MBq cc(-1). The average activity concentration and percentage standard deviation were 126.97 kBq ml(-1) and 7%, respectively. The performance of the LabPET-8™ scanner was characterized based on the NEMA NU 04-2008 standards. The all in all performance demonstrates that the LabPET-8™ system is able to produce high-quality and highly contrasted images in a reasonable time, and as such it is well suited for preclinical molecular imaging-based research.

Comparative Assessment of Energy-Mapping Approaches in CT-Based Attenuation Correction for PET

Abstract: Introduction Reliable quantification in positron emission tomography (PET) requires accurate attenuation correction of emission data, which in turn entails accurate determination of the attenuation map (µ-map) of the object under study. One of the main steps involved in CT-based attenuation correction (CTAC) is energy-mapping, or the conversion of linear attenuation coefficients (µ) calculated at the effective CT energy to those corresponding to 511 keV.

Scattered radiation emission imaging: principles and applications

Abstract: Imaging processes built on the Compton scattering effect have been under continuing investigation since it was first suggested in the 50s. However, despite many innovative contributions, there are still formidable theoretical and technical challenges to overcome. In this paper, we review the state-of-the-art principles of the so-called scattered radiation emission imaging. Basically, it consists of using the cleverly collected scattered radiation from a radiating object to reconstruct its inner structure. Image formation is based on the mathematical concept of compounded conical projection. It entails a Radon transform defined on circular cone surfaces in order to express the scattered radiation flux density on a detecting pixel. We discuss in particular invertible cases of such conical Radon transforms which form a mathematical basis for image reconstruction methods. Numerical simulations performed in two and three space dimensions speak in favor of the viability of this imaging principle and its potential applications in various fields.

Is metal artefact reduction mandatory in cardiac PET/CT imaging in the presence of pacemaker and implantable cardioverter defibrillator leads?

Abstract: Purpose Cardiac PET/CT imaging is often performed in patients with pacemakers and implantable cardioverter defibrillator (ICD) leads. However, metallic implants usually produce artefacts on CT images which might propagate to CT-based attenuation-corrected (CTAC) PET images. The impact of metal artefact reduction (MAR) for CTAC of cardiac PET/CT images in the presence of pacemaker, ICD and ECG leads was investigated using both qualitative and quantitative analysis in phantom and clinical studies.

Reduction of artefacts caused by hip implants in CT-based attenuation-corrected PET images using 2-D interpolation of a virtual sinogram on an irregular grid

Abstract: Purpose Metallic prosthetic replacements, such as hip or knee implants, are known to cause strong streaking artefacts in CT images. These artefacts likely induce over- or underestimation of the activity concentration near the metallic implants when applying CT-based attenuation correction of positron emission tomography (PET) images. Since this degrades the diagnostic quality of the images, metal artefact reduction (MAR) prior to attenuation correction is required.

Quantitative Techniques in PET-CT Imaging

Abstract: The appearance of hybrid PET/CT scanners has made quantitative whole body scanning of radioactive tracers feasible. This paper deals with the novel concepts for assessing global organ function and disease activity based on combined functional (PET) and structural (CT or MR) imaging techniques, their advantages over current quantitative techniques and their potential clinical applications in the management of various diseases. First the complicated kinetic modeling and methods for calculation of the standardized uptake value (SUV) that have been utilized in the practice of clinical PET are briefly described. Subsequently we discuss the quantitative concepts in PET-CT imaging that have been developed in recent years: (a) SUV analysis in the dual-time point and delayed PET imaging, (b) partial volume correction of SUV for small lesions (c) assessment of global metabolic activity in the whole organ or of diseased sites and (d) the novel image segmentation techniques with FDG-PET and newer tracers to precisely define the diseased or intended normal tissue which is of great value for image guided radiation therapy.

Qualitative and Quantitative Assessment of Metal Artifacts Arising from Implantable Cardiac Pacing Devices in Oncological PET/CT Studies: A Phantom Study

Abstract: We evaluate the magnitude of metallic artifacts caused by various implantable cardiac pacing devices (without leads) on both attenuation maps (μ-maps) and positron emission tomography (PET) images using experimental phantom studies. We also assess the efficacy of a metal artifact reduction (MAR) algorithm along with the severity of artifacts in the presence of misalignment between μ-maps and PET images. Four pacing devices including two pacemakers (pacemakers 1 and 2) and two cardiac resynchronization therapy (CRT) devices of pacemaker (CRT-P) and defibrillator (CRT-D) type were placed in three phantoms including a cylindrical Ge-68 phantom, a water-bath phantom and an anthropomorphic heart/thorax phantom. The μ-maps were derived from computed tomography (CT) images reconstructed using the standard method supplied by the manufacturer and those reconstructed using the MAR algorithm. In addition, the standard reconstructed CT images of the last two phantoms were manually misaligned by 10 mm along the patient’s axis to simulate misalignment between CT and PET images. The least and severest artifacts produced on both μ-maps and PET images of the Ge-68 phantom were induced by CRT-P and pacemaker 1 devices, respectively. In the water-bath phantom, CRT-P induced 17.5% over- and 9.2% underestimation of tracer uptake whereas pacemaker 1 induced 69.6% over- and 65.7% underestimation. In the heart/thorax phantom representing a pacemaker-bearing patient, pacemaker 1 induced 41.8% increase and 36.6% decrease in tracer uptake and attenuation coefficients on average in regions corresponding to bright and dark streak artifacts, respectively. Statistical analysis revealed that the MAR algorithm was successful in reducing bright streak artifacts, yet unsuccessful for dark ones. In the heart/thorax phantom, the MAR algorithm reduced the overestimations to 4.4% and the underestimations to 35.5% on average. Misalignment between μ-maps and PET images increased the peak of pseudo-uptake by approximately 20%. This study demonstrated that, depending on their elemental composition, different implantable cardiac pacing devices result in varying magnitudes of metal artifacts and thus pseudo-uptake on PET images. The MAR algorithm was not successful in compensating for underestimations which calls for a more efficient algorithm. The results showed that misalignments between PET and CT images render metal-related pseudo-uptake more severe.

⁹⁰Y Time-of-flight PET/MR on a hybrid scanner following liver radioembolisation (SIRT).

Abstract: Several studies have reported on the use of high-resolution Y PET/CT scans in the evaluation of the biodistribution of Ylabelled glass or resin based microspheres after radioembolisation of malignant liver lesions [1–4]. PET/MR is an emerging technology providing particularly high soft tissue contrast and no inherent radiation exposure. A 67-year-old man with multifocal unresectable hepatocellular carcinoma was referred for selective internal radiation therapy (SIRT). The pretreatment management consisted of angio-CT of the liver, embolisation of the right gastric and gastroduodenal arteries followed by Tc-MAA SPECT/CT. Two weeks later, 4.3 GBq of Y-labelled TheraSpheres® (MDS Nordion, Kanata, Canada) were administered in a single bolus into the right liver lobe. Subsequently, the patient

Accurate Monte Carlo modeling and performance assessment of the X-PET subsystem of the FLEX triumph preclinical PET/CT scanner.

Abstract: Purpose: X-PET™ is a commercial small animalPETscanner incorporating several innovative designs to achieve improved performance It is employed as a PET subsystem in the FLEX Triumph™ preclinical PET/CT scanner, the first commercial small animal PET/CT scanner worldwide The authors report on a novel Monte Carlo(MC)model designed for the evaluation of performance parameters of the X-PET™ Methods: The Geant4 Application for Tomographic Emission (GATE) MC code was used as a simulation tool The authors implemented more accurate modeling of the geometry of detector blocks and associated electronic chains, including dead-time and time-independent parameters, compared to previously presented MCmodels of the X-PET™scanner Validation of the MCmodel involved comparison between simulated and measured performance parameters of the X-PET™, including spatial resolution, sensitivity, and noise equivalent count rate (NECR) Thereafter, various simulations were performed to assess scanner performance parameters according to NEMA NU 4-2008 standards with the aim to present a reliable Monte Carlo platform for small animalPETscanner design optimization Results: The average differences between simulated and measured results were 112%, 333%, and 91% for spatial resolution, sensitivity, and NECR, respectively The average system absolute sensitivity was 27% Furthermore, the peak true count rate, peak NECR, and scatter fraction were 2050 kcps, 1520 kcps, and 47%, respectively, for a mouse phantom and 1017 kcps, 469 kcps, and 182%, respectively, for a rat phantom Spatial resolution was also measured in ten different positions at two axial locations The radial, tangential, and axial FWHM ranged from 131 to 196 mm, 117 to 211 mm, and 177 to 244 mm, respectively, as the radial position varied from 0 to 25 mm at the centre of the axial field-of-view Conclusions: The developed MC simulation platform provides a reliable tool for performance evaluation of small animalPETscanners and has the potential to be used in other applications such as detector design optimization, correction of image degrading factors such as randoms, scatter, intercrystal scatter, parallax error, and partial volume effect

CT-Based Attenuation Correction on the FLEX Triumph Preclinical PET/CT Scanner

Abstract: Positron Emission Tomography (PET) has emerged as a valuable molecular imaging modality for quantitative measurement of biochemical processes in vivo in the clinical and preclinical imaging domains. However, PET imaging suffers from various physical degrading factors including photon attenuation, which can be corrected using CT-based attenuation correction (CTAC) on combined PET/CT scanners. The attenuation map is calculated by converting CT numbers derived from low-energy polyenergetic x-ray spectra to linear attenuation coefficients at 511 keV. Generation of accurate attenuation maps is crucial for reliable attenuation correction of PET data and hence is a prerequisite for accurate quantification of biological processes. In this study, we implemented the CTAC procedure on the FLEX Triumph™ preclinical PET/CT scanner and evaluated tube voltage dependence for different kVps (40, 50, 60, 70, and 80). The quantitative impact of both bilinear and quadratic based energy-mapping methods on linear attenuation coefficients, attenuation maps and corrected PET images was assessed at different CT tube voltages. Attenuation maps were calculated from CT images of a cylindrical polyethylene phantom containing different concentrations of K2HPO4 in water. Correlation coefficients and best regression fit equations were calculated for both methods. Phantom and rodent PET/CT images were used to assess improvements in image quality and quantitative accuracy. It was observed that the slopes of the bilinear calibration curves for CT numbers greater than 0 HU increase with increasing tube voltage. In addition, higher correlation coefficients were obtained for the quadratic compared to the bilinear energy-mapping method. Tube voltage of 70 kVp produced the smallest relative error and higher correlation coefficient compared to other tube voltages. For low concentrations of K2HPO4, the mean relative difference (in %) between theoretical and calculated attenuation coefficients when using bilinear and quadratic energy-mapping methods are 1.39 ± 1.9 and 1.33 ± 1.8, respectively. They are 2.78 ± 1.3 and 2.5 ± 1.3, respectively, for high concentrations of K2HPO4 . As expected, higher activity concentrations were obtained for PET after attenuation correction. The increased PET signal for mouse tissues ranged between 21 and 31% for bilinear energy-mapping and between 21.8 and 35% for quadratic energy-mapping, whereas these varied from 40 to 51% and from 41 to 56%, respectively, for rat tissues. For biological tissues having a high atomic number such as bone, the quadratic energy-mapping method produced slightly improved results compared to the bilinear energy-mapping method. Phantom and rodent PET studies were successfully corrected for photon attenuation using the developed CTAC procedure.

NEMA NU-04-based performance characteristics of the LabPET-8™ small animal PET scanner

Abstract: The objective of this study is to characterize the performance of the preclinical avalanche photodiode (APD)-based LabPET-8™ subsystem of the fully integrated trimodality PET/SPECT/CT Triumph™ scanner using the NEMA NU 04 - 2008 protocol The characterized performance parameters include the spatial resolution, sensitivity, scatter fraction, counts rate performance, and image quality characteristics The PET system is fully digital using APD-based detector modules with highly integrated electronics The detector assembly consists of phoswich pairs of LYSO and LGSO crystals with dimensions of 2×2×14 mm3 having 75 cm axial and 10 cm transverse field-of-view (FOV) The spatial resolution and sensitivity were measured using a small 22Na point source at different positions in the scanner's FOV The scatter fraction and count rate characteristics were measured using a mouse- and rat-sized phantoms fitted with 18F line source The overall imaging capabilities of the scanner were assessed using the NEMA image quality phantom and laboratory animal studies The NEMA-based radial and tangential spatial resolution ranged from 17 mm at the center of the FOV to 259 mm at a radial offset of 25 cm and from 185 mm at the center of the FOV to 176 mm at a radial offset of 25 cm, respectively Iterative reconstruction improved the spatial resolution to 084 mm at the center of the FOV The total absolute system sensitivity is 1274% for an energy window of 250–650 keV For the mouse-sized phantom, the peak noise equivalent count rate (NECR) is 183 kcps at 207 MBq/cc whereas the peak true count rate is 320 kcps at 25 MBq/cc with a scatter fraction of 19% The rat-sized phantom had a scatter fraction of 31%, with a peak NECR of 67 kcps at 023 MBq/cc and a peak true count rate of 186 kcps at 027 MBq/cc The average activity concentration and percentage standard deviation (%STD) are 12697 kBq/ml and 7%, respectively The performance of the LabPET-8™ scanner was characterized based on the NEMA NU 04 - 2008 standards The all in all performance demonstrates that the LabPET-8™ system is able to produce high quality and highly contrasted images in a reasonable time, and as such it is well suited for preclinical molecular imaging-based research

Sparsity constrained sinogram inpainting for metal artifact reduction in x-ray computed tomography

Abstract: In this paper, we proposed a new projection completion metal artifact reduction (MAR) algorithm in x-ray computed tomography (CT) using a sparsity based sinogram inpainting (interpolation) technique We developed the MAR algorithm on a Bayesian framework in which a wavelet-based generalized Gaussian (lp) prior was applied and then the inpainting problem was formulated as a constrained optimization problem For the optimization, we derived a projected gradient descent algorithm using a majorization-minimization technique The gradient step was performed by a soft thresholding operator for an l1 prior, and a hard thresholding with a decaying threshold for an l0 prior We utilized a tight frame of translation-invariant wavelets implemented by undecimated discrete wavelet transform As in the clinical setting there is no ground truth CT image to objectively evaluate the performance of a proposed MAR algorithm, we also introduced a novel approach to simulate metal artifacts in a real CT dataset The results showed that the proposed MAR algorithm using hard thresholding efficiently recovers and inpaints the sinogram projections corrupted by metallic implants

Metal artifact reduction in CT-based attenuation correction of PET using sobolev sinogram restoration

Abstract: In patients bearing metallic implants, CT images reconstructed by the filtered back-projection algorithm usually suffer substantially from streaking metal artifacts. The CT-based attenuation correction of PET using such images can lead to pseudo-uptakes and thus equivocal findings. In this paper, we introduce a new metal artifact reduction (MAR) algorithm based on Bayesian iterative restoration techniques applied in the sinogram space. We proposed a Sobolev prior in the maximum a posteriori (MAP) estimation of the projections corrupted by metallic implants. The Sobolev prior was invoked in order to impose the a priori knowledge that a CT sinogram is a smooth dataset in which it is highly probable that neighboring projections have similar photon counts. We compared the proposed prior with a total variation (TV) one which imposes piece-wise smoothness on the sinograms being restored. We also compared it with a smoothed TV (TVs) prior which ranks between the Sobolev and TV ones. We formulated the MAP estimation as a convex constrained optimization problem and solved it for the Sobolev and TVs priors by a projected gradient descent algorithm and for the TV/s priors by a sophisticated primal-dual projected gradient algorithm. The results of artifact simulations in a real CT image showed that the Sobolev-based MAR algorithm outperforms its TV and TVs-based counterparts in terms of convergence rate and is comparable with the TVs in projection recovery. It was demonstrated that the proposed MAR algorithm has high applicability in fast and efficient CT-based attenuation correction of PET data.

Novel detector design for reducing intercell x-ray cross-talk in the variable resolution x-ray CT scanner: A Monte Carlo study

Abstract: Purpose: The variable resolution x-ray (VRX) CT scanner provides substantial improvement in the spatial resolution by matching the scanner's field of view (FOV) to the size of the object being imaged. Intercell x-ray cross-talk is one of the most important factors limiting the spatial resolution of the VRX detector. In this work, a new cell arrangement in the VRX detector is suggested to decrease the intercell x-ray cross-talk. The idea is to orient the detector cells toward the opening end of the detector. Methods: Monte Carlo simulations were used for performance assessment of the oriented cell detector design. Previously published design parameters and simulation results of x-ray cross-talk for the VRX detector were used for model validation using the GATE Monte Carlo package. In the first step, the intercell x-ray cross-talk of the actual VRX detector model was calculated as a function of the FOV. The obtained results indicated an optimum cell orientation angle of 28 degrees to minimize the x-ray cross-talk in the VRX detector. Thereafter, the intercell x-ray cross-talk in the oriented cell detector was modeled and quantified. Results: The intercell x-ray cross-talk in the actual detector model was considerably high, reaching up to 12% at FOVs from 24 to 38 cm. The x-ray cross-talk in the oriented cell detector was less than 5% for all possible FOVs, except 40 cm (maximum FOV). The oriented cell detector could provide considerable decrease in the intercell x-ray cross-talk for the VRX detector, thus leading to significant improvement in the spatial resolution and reduction in the spatial resolution nonuniformity across the detector length. Conclusions: The proposed oriented cell detector is the first dedicated detector design for the VRX CT scanners. Application of this concept to multislice and flat-panel VRX detectors would also result in higher spatial resolution. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3555035]

Impact of using different tissue classes on the accuracy of MR-based attenuation correction in PET-MRI

Abstract: Diagnosis , staging and treatment of disease depends on the morphological and functional information obtained from multimodality molecular imaging systems. The combination of functional and morphological information is now routinely performed to overcome the limitations of each individual modality. Attenuation of photons in the object under study is one of the main limitations of quantitative PET imaging. Attenuation correction plays a pivotal role in PET imaging. However, the availability of CT data on hybrid PET/CT scanners made it possible to build an accurate attenuation map. One of the well-known methods for generation of the attenuation map on PE/MRI systems is MR-based attenuation correction (MRAC) where image segmentation is used to classify MRI into several classes corresponding to different attenuation factors. In this study we investigate the effect of using different numbers of classes for the generation of attenuation maps on the accuracy of attenuation correction of PET data. The study was carried out using simulations of the XCAT phantom and 10 clinical studies. For the later, CT and PET images of 10 patients were used with CT-based attenuation correction assumed as reference. MRI was classified into different classes to produce two, three and four-class attenuation maps using the ITK library. The relative error showed that the lower number of classes will increase the global error over 8%. The elimination of bony structures from the attenuation map will cause a local error over 3%. In clinical studies, SUV mean and SUV max were calculated for each AC method. The results seem to indicate an underestimation of 11% because of neglecting bone.

B-spline based free form deformation thoracic non-rigid registration of CT and PET images

Abstract: Accurate attenuation correction of emission data is mandatory for quantitative analysis of PET images. One of the main concerns in CT-based attenuation correction(CTAC) of PET data in multimodality ...

[11C]acetate PET/CT Visualizes Skeletal Muscle Exercise Participation, Impaired Function, and Recovery after Hip Arthroplasty; First Results

Abstract: Purpose Based on skeletal muscle acetate physiology we aimed studying muscle function after hip arthroplasty with [11C]acetate PET.

Assessment of scatter for the micro-CT subsystem of the trimodality FLEX Triumph preclinical scanner.

Abstract: Purpose: This work aims at assessing, through experimental measurements and Monte Carlo calculations, the scatter to primary ratio (SPR) for the micro-CT subsystem of the FLEX Triumph (TM) preclinical PET-CT scanner to improve its quantitative capabilities. Methods: Experimental measurements were carried out using the single blocker method, where five cylindrical blockers with diameters ranging between 3 and 11.65 mm were used to assess the SPR without the blocker through interpolation. Because of the vertical layout of the imaging device, the blocker was placed over rat-sized and mouse-sized phantoms and central and peripheral SPR values were obtained by rotating the source and detector. The influence of beam energy (30, 50, and 80 kVp), geometrical magnification (1.3 and 2.0) and phantom diameter (25 and 50 mm) and density (polyethylene and water) were investigated. Monte Carlo (MC) simulations using the MCNP4C code were also performed and compared to experimental results to validate their accuracy. Results: The highest difference was found in the extreme peripheral region of the small phantom, while the maximum difference at the center of the phantom is about 6%, indicating that MC simulations can reproduce well the experimental results, at least in the region inside the phantom. The maximal SPR (0.562) was obtained for the large phantom at 30 kVp and a magnification of 1.3. The full SPR profile was calculated using MC simulations and used to express its dependency on beam energy (quadratic), air gap (asymptotic), and phantom diameter (quadratic). Conclusions: The obtained results are in good agreement with theoretical predictions. MC simulations were valuable for the evaluation of the influence of various acquisition parameters on the SPR estimates.(C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3598438]

Breathing adapted radiotherapy: a 4D gating software for lung cancer

Abstract: Purpose: Physiological respiratory motion of tumors growing in the lung can be corrected with respiratory gating when treated with radiotherapy (RT). The optimal respiratory phase for beam-on may be assessed with a respiratory phase optimizer (RPO), a 4D image processing software developed with this purpose. Methods and Materials: Fourteen patients with lung cancer were included in the study. Every patient underwent a 4D-CT providing ten datasets of ten phases of the respiratory cycle (0-100% of the cycle). We defined two morphological parameters for comparison of 4D-CT images in different respiratory phases: tumor-volume to lungvolume ratio and tumor-to-spinal cord distance. The RPO automatized the calculations (200 per patient) of these parameters for each phase of the respiratory cycle allowing to determine the optimal interval for RT. Results: Lower lobe lung tumors not attached to the diaphragm presented with the largest motion with breathing. Maximum inspiration was considered the optimal phase for treatment in 4 patients (28.6%). In 7 patients (50%), however, the RPO showed a most favorable volumetric and spatial configuration in phases other than maximum inspiration. In 2 cases (14.4%) the RPO showed no benefit from gating. This tool was not conclusive in only one case. Conclusions: The RPO software presented in this study can help to determine the optimal respiratory phase for gated RT based on a few simple morphological parameters. Easy to apply in daily routine, it may be a useful tool for selecting patients who might benefit from breathing adapted RT.

Clough-Tocher interpolation of virtual sinogram in a Delaunay triangulated grid for metal artifact reduction of PET/CT images

Abstract: Metallic implants, such as hip implants, are known to induce streaking artifacts in CT images which can cause over/underestimation of the activity uptake in CT-based attenuation corrected PET images Hence, metal artifact reduction (MAR) of CT images is essential in order to obtain accurate quantification of PET data The proposed MAR technique replaces the projection bins of the virtual sinogram affected by metallic implants using a 2D cubic interpolation scheme Since removing the affected projection bins renders the sinogram grid irregular, a Delaunay triangulated gridding together with Clough-Tocher cubic interpolation, which is compatible with this irregular grid, is used to substitute the values of the affected bins A cylindrical phantom filled with uniform activity concentration incorporating metallic inserts and 30 clinical PET-CT studies containing hip prostheses were used to assess the performance of the proposed approach The resulting images were compared to those obtained using the built-in MAR algorithm on a Siemens mCT64 PET/CT scanner Phantom and clinical studies showed that the proposed algorithm performed considerably better than Siemens's method in the regions corresponding to dark streaking artifacts (underestimated regions), whereas it performed equally well compared to Siemens's method in the other regions In the underestimated regions, the proposed method increased the uptake value up to 45%, whereas the Siemens's method kept almost the same uptake as the uncorrected PET images In the overestimated regions both methods decreased the uptake by ∼45% The phantom experiment also revealed that the proposed approach is in better agreement with the actual activity concentration compared to both the uncorrected and corrected images using Siemens's method It can be concluded that the proposed method allows more accurate attenuation correction of PET data thus preventing misinterpretation of activity uptake in regions adjacent to metallic objects

Sparsity Constrained Sinogram Inpainting for Metal Artifact Reduction in X-ray Computed

Abstract: In this paper, we proposed a new projection completion metal artifact reduction (MAR) algorithm in x-ray computed tomography (CT) using a sparsity based sinogram inpainting (interpolation) technique. We developed the MAR algorithm on a Bayesian framework in which a wavelet-based generalized Gaussian (A p ) prior was applied and then the inpainting problem was formulated as a constrained optimization problem. For the optimization, we derived a projected gradient descent algorithm using a majorization-minimization technique. The gradient step was performed by a soft thresholding operator for an A 1 prior, and a hard thresholding with a decaying threshold for an A 0 prior. We utilized a tight frame of translation-invariant wavelets implemented by undecimated discrete wavelet transform. As in the clinical setting there is no ground truth CT image to objectively evaluate the performance of a proposed MAR algorithm, we also introduced a novel approach to simulate metal artifacts in a real CT dataset. The results showed that the proposed MAR algorithm using hard thresholding efficiently recovers and inpaints the sinogram projections corrupted by metallic implants.

A novel energy mapping approach in CT-based attenuation correction of PET data using multi-energy CT imaging

Abstract: A major source of potential pitfalls in CT-based attenuation correction (CTAC) of PET data is the use of integral mode of CT detectors in the presence of polychromatic x-rays resulting in limited information for determination of exact tissue content The wide range of bone mineral contents and densities in the human body makes it difficult to map the CT number to Linear Attenuation Coefficient (LAC) at the PET energy when merely using one or two scaling factors In this study we proposed an alternative approach in order to use energy sensitive CT imaging techniques as opposed to integrating CT imaging The multi-energy strategy would promise significant improvements in tissue determination and leads to accurate energy mapping results in CTAC, which can be especially critical and useful in the presence of varying bone tissues In order to accurately validate our method a novel bone model based on cortical and marrow mixtures is proposed Furthermore, a two-step energy mapping algorithm is implemented For validation, tomographic projections of phantom in five energy bin were acquired and reconstructed The proposed energy mapping technique was used to estimate the LAC of different bone tissues at 511 keV The results had 11% error at maximum compared to true values To test the precision, the effect of 10% variation in effective energy was investigated In different bone tissues, maximum errors induced by the pricewise linear and hybrid methods were 80% and 146%, respectively; whereas in the proposed multi-energy method, errors was 16%, at maximum

Characterization of scattered radiation profile in volumetric 64 slice CT scanner: Monte Carlo study using GATE

Abstract: It is commonly understood that scattered radiation in X-ray computed tomography (CT) reduces the CT number and degrades the quality of reconstructed images. This effect is more pronounced in multi detector CT scanners with extended detector aperture mostly using cone-beam configurations, which are much less immune to scatter than fan-beam and single-slice CT scanners. To perform accurate scatter correction, it is essential to characterize scattered radiation in Volumetric CT. As characterization of scattered radiation behavior using experimental measurement is a difficult and time consuming approach, Monte Carlo simulation can be an ideal method. In this study we used Geant4-based simulation package, GATE, to model x-ray photon interactions in the phantom and detector. The Monte Carlo simulation was validated through comparison with experimental measurement data. Thereafter, the effect of different parameters such as tube voltage and phantom material on the scatter profile and Scatter to Primary Ratio (SPR) was calculated. We also compared the simulated SPR curves with experimental data which was measured with array blocker method. The experimental technique assumed to be the gold standard technique. The comparison between simulation and experimental data in SPR showed error less than 5 %. The results indicate that the GATE Monte Carlo code is a useful tool for investigation of scattered radiation characterization in CT scanners. Moreover, there is a possibility of take advantage of GATE for simulation of PET and CT scanners in order to simultaneously asses the contribution of scattered radiation in PET/CT scanners.

Recent Advances in Hybrid Imaging for Radiation Therapy Planning: The Cutting Edge.

Abstract: In the past decade, there have been many contributions demonstrating the advantages of combining morphologic and molecular imaging, and with commercial PET/CT scanners emerging in the same time frame, PET/CT has already had a significant impact on patient management. Ultimately, molecular imaging-guided radiotherapy holds the promise of improved definition of tumor target volumes. Yet, despite considerable progress to date, challenges remain if the potential of PET/CT-guided radiotherapy treatment planning is to be fully exploited in clinics.

Evaluation of scatter fraction and count rate performance of two small-animal PET scanners using dedicated phantoms

Abstract: Positron Emission Tomography (PET) image quality deteriorates as the object size increases owing to increased detection of scattered and random events. The characterization of the scatter component in small animal PET imaging has received little attention owing to the small scatter fraction when imaging rodents. The purpose of this study is first to design and fabricate a dedicated cone-shaped phantom which can be used for measurement of object size dependent SF and noise equivalent count (NEC) rates and second, to evaluate those parameters for two small animal PET scanners, namely the X-PET™ and LabPET™-8 as function of radial offset, object size and lower energy threshold. Both scanners were modeled as realistically as possible using GATE Monte Carlo simulation platform. The simulation models were validated against experimental measurements in terms of sensitivity, SF and noise equivalent count rate (NECR). The dedicated phantom was designed and fabricated in-house using high-density polyethylene. The optimized dimensions of the cone-shaped phantom are 150 mm (length), 20 mm (minimum diameter), 70 mm (maximum diameter) and taper angle of 9°.The relative difference between simulated and experimental results for the LabPET™-8 scanner varied between 0.66% and 10% except for few results where it was below 16%. Depending on the radial offset from the axial centre for a central field of view (3–6 cm diameter), the SF for the cone-shaped phantom varied from 26.3 to 18.2% (X-PET™); 34.4 to 26.9% (LabPET™-8), 18.6 to 13.1% (X-PET™); 19.1 to 17.0% (LabPET™-8); 10.1 to 7.6% (X-PET™) and 9.1 to 7.3% (LabPET™-8) for lower energy thresholds of 250, 350 and 425 keV, respectively. The SF increases as the radial offset decreases, lower energy threshold (LET) decreases and object size increases. The SF values are higher for the LabPET™-8 compared to the X-PET™. The NECR increases as the radial offset increases and object size decreases. Maximum NECR was obtained at a LET of 350 keV for LabPET™-8 whereas 250 keV for X-PET™. High correlation coefficients (R2) for SF and NECR were observed between the cone-shaped phantom and an equivalent volume cylindrical (EVC) phantom for the three considered axial fields-of-view. A single cone-shaped phantom enables the assessment of effects of three factors, namely radial offset, energy threshold and object size on small animal PET imaging characteristics like SF and NECR. Hence, a cone-shaped phantom may be more suited for evaluation of object size-dependent SF and NECR instead of using various discrete phantoms of different size.

Atlas-guided automated analysis of small-animal PET studies

Abstract: This work aims to develop a methodology for automated atlas-guided analysis of small animal PET data through deformable registration to an anthropomorphic mouse atlas A non-rigid registration technique is used to put into correspondence relevant anatomic regions of rodents to the predefined atlas The technique consists in registering CT images from actual mouse PET/CT data to corresponding CT images of the Digimouse atlas, thus providing a pre-segmented anatomical model consisting of 21 anatomical regions suitable for automated analysis Image registration is performed using the Elastix package, which is a modular toolbox based on the ITK library allowing the implementation of various image registration algorithms Once the optimal parameters were derived, these were applied to all data sets The accuracy of image registration was assessed by segmenting mice CT images into 7 regions: brain, lungs, heart, kidneys, bladder, skeleton and rest of the body This was realized previous to image registration in a semiautomated way using the ITK-Snap toolbox Each segmented mouse was transformed using the output transformation parameters obtained during CT image registration The resulting segmentation was compared with the original Digimouse atlas to quantify image registration accuracy using established figures of merit such as Dice coefficient and Hausdorff distance showing fair to excellent agreement and a mean registration mismatch distance of about 6 mm Pre-registration was applied to some PET images which were slightly misaligned with the corresponding CT images PET images were then transformed using the same method used earlier The results demonstrate good quantification accuracy in most regions, especially the brain As expected, relatively large deviations were obtained for the bladder It can be concluded that the proposed automated technique is reliable and suitable for fast quantification of preclinical PET data in large serial studies