Attenuation Correction Of PET Data Research Articles

Correcting CT misregistration in data-driven gated (DDG) PET with PET self-gating and deformable image registration.

Misregistration between CT and PET data can result in mis-localization and inaccurate quantification of functional uptake in whole body PET/CT imaging. This problem is exacerbated when an abnormal inspiration occurs during the free-breathing helical CT (FB CT) used for attenuation correction of PET data. In data-driven gated (DDG) PET, the data selected for reconstruction is typically derived from the end-expiration (EE) phase of the breathing cycle, making this potential issue worse. The objective of this study is to develop a deformable image registration (DIR)-based respiratory motion model to improve the registration and quantification between misregistered FB CT and PET. Twenty-two whole-body 18 F-FDG PET/CT scans encompassing 48 lesions in misregistered regions were analyzed in this study. End-inspiration (EI) and EE PET data were derived from -10% to 15% and 30% to 80% of the breathing cycle, respectively. DIR was used to estimate a motion model from the EE to EI phase of the PET data. The model was then used to generate PET images at any phase of up to four times the amplitude of motion between EE and EI for correlation with the misregistered FB CT. Once a matched phase of the FB CT was determined, FB CT was deformed to a pseudo CT at the EE phase (DIR CT). DIR CT was compared with the ground truth DDG CT for AC and localization of the DDG PET. Between DDG PET/FB CT and DDG PET/DIR CT, a significant increase in ∆%SUV was observed (p<0.01), with median values elevating from 26.7% to 42.4%. This new method was most effective for lesions ≤3cm proximal to the diaphragm (p<0.001) but showed decreasing efficacy as the distance increased. When FB CT was severely misregistered with DDG PET (>3cm), DDG PET/DIR CT outperformed DDG PET/FB CT alone (p<0.05). Even when patients showed varied breathing patterns during the PET/CT scan, DDG PET/DIR CT still surpassed the efficiency of DDG PET/FB CT (p<0.01). Though DDG PET/DIR CT couldn't match the performance of the DDG PET/CT ground truth (42.4%vs. 53.6%, p<0.01), it reached 84% of its quantification, demonstrating good agreement and a strong overall correlation (regression coefficient of 0.94, p<0.0001). In some cases, anatomical distortion and blurring, and misregistration error were observed in DIR CT, rendering it still unable to correct inaccurate localization near the boundaries of two organs. Based on the motion model derived from gated PET data, DIR CT can significantly improve the quantification and localization of DDG PET. This approach can achieve a performance level of about 84% of the ground truth established by DDG PET/CT. These results show that self-gated PET and DIR CT may offer an alternative clinical solution to DDG PET and FB CT for quantification without the need for additional cine-CT imaging. DIR CT was at times inferior to DDG CT due to some distortion and blurring of anatomy and misregistration error.

List-mode quantitative joint reconstruction of activity and attenuation maps in Time-of-Flight PET

Quantitative PET imaging requires accurate determination of patient-specific attenuation maps, which remains challenging on hybrid PET/MRI systems owing to the lack of a direct relationship between MR image intensity and attenuation coefficients. The aim of the present study is to develop a list-mode based algorithm for accurate and robust attenuation correction of PET data using time-of-flight (TOF) emission information. We analyze and address the challenges of list-mode emission-based maximum-likelihood joint estimation of activity and attenuation (LM-MLAA) in state-of-the-art PET imaging. The proposed method exploits a rapid on-the-fly system matrix calculation algorithm based on elliptic integrals while updating the attenuation map from accumulating list-mode coincidences to achieve accelerated image reconstruction. The scattering compensation is incorporated inside it using an iterative approach in such that the current estimation of attenuation map used on a course grid sampling scattering points to make an estimate of scattering. The performance of the proposed LM-MLAA approach was evaluated on Monte Carlo simulations of a phantom at different time resolutions. The contrast and noise for hot and cold regions on reconstructed images at different time resolutions were analysed. The estimated attenuation map exhibits resilience against noise, effectively eliminates high-frequency cross-talk even in the absence of prior information on attenuation coefficients, and enables discrimination among different anatomical regions in the reconstructed image. The error in the mean estimated attenuation coefficients after 50 iterations was ∼ 2% in water and ∼ -14% in Teflon regions for TOF resolutions corresponding to those of most current commercial PET systems (∼ 500 ps). The proposed LM-MLAA framework can be used for joint reconstruction of activity and attenuation maps from list-mode emission data as standalone or a complementary approach to existing in multimodality imaging such as PET/MRI, where direct measurement attenuation maps is not possible.

Ultra-low dose whole-body CT for attenuation correction in a dual tracer PET/CT protocol for multiple myeloma.

To investigate within phantoms the minimum CT dose allowed for accurate attenuation correction of PET data and to quantify the effective dose reduction when a CT for this purpose is incorporated in the clinical setting. The NEMA image quality phantom was scanned within a large parallelepiped container. Twenty-one different CT images were acquired to correct attenuation of PET raw data. Radiation dose and image quality were evaluated. Thirty-one patients with proven multiple myeloma who underwent a dual tracer PET/CT scan were retrospectively reviewed. 18F-fluorodeoxyglucose PET/CT included a diagnostic whole-body low dose CT (WBLDCT: 120kV-80mAs) and 11C-Methionine PET/CT included a whole-body ultra-low dose CT (WBULDCT) for attenuation correction (100kV-40mAs). Effective dose and image quality were analysed. Only the two lowest radiation dose conditions (80kV-20mAs and 80kV-10mAs) produced artifacts in CT images that degraded corrected PET images. For all the other conditions (CTDIvol≥0.43mGy), PET contrast recovery coefficients varied less than±1.2%. Patients received a median dose of 6.4mSv from diagnostic CT and 2.1mSv from the attenuation correction CT. Despite the worse image quality of this CT, 94.8% of bone lesions were identifiable. Phantom experiments showed that an ultra-low dose CT can be implemented in PET/CT procedures without any noticeable degradation in the attenuation corrected PET scan. The replacement of the standard CT for this ultra-low dose CT in clinical PET/CT scans involves a significant radiation dose reduction.

An Investigation of the Required MR Bone Attenuation Correction for Quantitative Whole-Body PET/MR Imaging Using Clinical NaF PET/CT Studies

Tissue-classification-based attenuation correction strategies have been previously proposed to correct for bone attenuation in PET/MR imaging and simulated using computed tomography. However, the complication of voxel averaging uniquely associated with bone has not been considered explicitly in the past. This study investigated the effect of voxel averaging between bone and soft tissue in attenuation images and determined how accurately bone must be detected in MR images in order to perform acceptable attenuation correction of PET data by using CT-simulated attenuation correction. We found out that treating bone as soft tissue caused a mean quantification difference of -9.9% ± 5.5% in all 119 bone lesions. There were no significant differences between lesions in the pelvis and the vertebrae. The nominal difference in lesions in the ribs was significantly lower, likely due to the spatial misregistration between the emission and attenuation images. Interestingly, a non-monotonic relationship between the bone imaging ability and the absolute PET quantification accuracy was observed, with the minimal quantification difference achieved at a BVF around 40% for skull lesions (2.6% ± 2.1%), and 30% for non-skull lesions (1.4% ± 1.1%) and all lesions (1.5% ± 1.3%). This study established that a bone classification sensitivity of approximately 30% BVF is required in order for MR-based attenuation correction methods to achieve optimal quantification in whole-body PET/MR studies. For this purpose, higher bone imaging ability of MR may not be necessary.

Attenuation correction for hybrid MR/PET scanners: a comparison study.

Attenuation correction of PET data acquired in hybrid MR/PET scanners is still a challenge. Different methods have been adopted by several groups to obtain reliable attenuation maps (mu-maps). In this study we compare three methods: MGH, UCL, Neural-Network. The MGH method is based on an MR/CT template obtained with the SPM8 software. The UCL method uses a database of MR/CT pairs. Both generate mu-maps from MP-RAGE images. The feed-forward neural-network from Juelich (NN-Juelich) requires two UTE images; it generates segmented mu-maps. Data from eight subjects (S1-S8) measured in the Siemens 3T MR-BrainPET scanner were used. Corresponding CT images were acquired. The resulting mu-maps were compared against the CT-based mu-maps for each subject and method. Overlapped voxels and Dice similarity coefficients, D, for bone, soft-tissue and air regions, and relative differences images were calculated. The true positive (TP) recognized voxels for the whole head were 79.9% (NN-Juelich, S7) to 92.1% (UCL method, S1). D values of the bone were D=0.65 (NN-Juelich, S1) to D=0.87 (UCL method, S1). For S8 the MHG method failed (TP=76.4%; D=0.46 for bone). D values shared a common tendency in all subjects and methods to recognize soft-tissue as bone. The relative difference images showed a variation of -10.9% - +10.1%; for S8 and MHG method the values were -24.5% and +14.2%. A preliminary comparison of three methods for generation of mu-maps for MR/PET scanners is presented. The continuous methods (MGH, UCL) seem to generate reliable mu-maps, whilst the binary method seems to need further improvement. Future work will include more subjects, the reconstruction of corresponding PET data and their comparison.

Simultaneous trimodal MR-PET-EEG imaging for the investigation of resting state networks in humans.

Glucose is the principal source of energy for the brain and its relationship to neuronal activity are poorly understood. The human brain uses 80% of its energy for ongoing neural activity that occurs in isolation from any particular stimulus. A promising tool for the investigation of glucose metabolism and its relationship to neuronal activity is simultaneous trimodal MR-PET-EEG data imaging. We here demonstrate the first in vivo human trimodal data at 3T. In one session MR, FDG-PET and EEG data were recorded simultaneously at a 3T hybrid MR-BrainPET scanner (Siemens, Germany) equipped with a 32 channel MR-compatible EEG system (Brain Products, Germany) in 11 healthy volunteers (11 males, mean age: 25.2 years SD: 1.2). MR and EEG data acquisition MP-RAGE (TR = 2250 ms, TE= 3.03 ms, 176 sagittal slices. 1 mm, GRAPPA factor 2. MR-based attenuation correction of PET data via UTE: flip angle=15. Two different echo times TE1=0.07 and TE2=2.46 ms, TR=200 ms. EPI sequence (TR: 2.2 s, TE: 30 ms, FOV: 200 mm, 165 volumes, The subjects were requested to close their eyes and relax EEG data were recorded using a 32-channel MR compatible EEG system. App. 200 MBq/μmol FDG were injected, data were acquired in list mode and iteratively reconstructed with all necessary corrections into 153 slices with 256 x 256 voxels sized 1.25 mm3. The trimodal approach, recording PET data, MR data and EEG data simultaneously was successful. The high neuronal activity of the structures within the default mode network occurs on the basis of a high glucose consumption rate within the default node network. The activity of the default mode is not tied to a special EEG frequency band.

Effects of ferumoxytol on quantitative PET measurements in simultaneous PET/MR whole-body imaging: a pilot study in a baboon model.

BackgroundSimultaneous PET/MR imaging depends on MR-derived attenuation maps (mu-maps) for accurate attenuation correction of PET data. Currently, these maps are derived from gradient-echo-based MR sequences, which are sensitive to susceptibility changes. Iron oxide magnetic nanoparticles have been used in the measurement of blood volume, tumor microvasculature, tumor-associated macrophages, and characterizing lymph nodes. Our aim in this study was to assess whether the susceptibility effects associated with iron oxide nanoparticles can potentially affect measured 18F-FDG PET standardized uptake values (SUV) through effects on MR-derived attenuation maps.MethodsThe study protocol was approved by the Institutional Animal Care and Use Committee. Using a Siemens Biograph mMR PET/MR scanner, we evaluated the effects of increasing concentrations of ferumoxytol and ferumoxytol aggregates on MR-derived mu-maps using an agarose phantom. In addition, we performed a baboon experiment evaluating the effects of a single i.v. ferumoxytol dose (10 mg/kg) on the liver, spleen, and pancreas 18F-FDG SUV at baseline (ferumoxytol-naïve), within the first hour and at 1, 3, 5, and 11 weeks.ResultsPhantom experiments showed mu-map artifacts starting at ferumoxytol aggregate concentrations of 10 to 20 mg/kg. The in vivo baboon data demonstrated a 53% decrease of observed 18F-FDG SUV compared to baseline within the first hour in the liver, persisting at least 11 weeks.ConclusionsA single ferumoxytol dose can affect measured SUV for at least 3 months, which should be taken into account when administrating ferumoxytol in patients needing sequential PET/MR scans.Advances in knowledge1. Ferumoxytol aggregates, but not ferumoxytol alone, produce significant artifacts in MR-derived attenuation correction maps at approximate clinical dose levels of 10 mg/kg.2. When performing simultaneous whole-body 18F-FDG PET/MR, a single dose of ferumoxytol can result in observed SUV decreases up to 53%, depending on the amount of ferumoxytol aggregates in the studied tissue.Implications for patient careAdministration of a single, clinically relevant, dose of ferumoxytol can potentially result in changes in observed SUV for a prolonged period of time in the setting of simultaneous PET/MR. These potential changes should be considered in particular when administering ferumoxytol to patients with expected future PET/MR studies, as ferumoxytol-induced SUV changes might interfere with therapy assessment.Electronic supplementary materialThe online version of this article (doi:10.1186/s40658-015-0109-0) contains supplementary material, which is available to authorized users.

A hybrid method for generation of attenuation map for MR-based attenuation correction of PET data in prostate PET/MR imaging.

Recently introduced PET/MRI scanners present significant advantages in comparison with PET/CT. However, the lack of accurate method for generation of µmap from MR images for implementation of MRAC is hampering further development. In this study, we present a new method including short echo-time pulse sequence to detect bone signal along with a robust and automatic image segmentation method base on FCM, active contouring and shape analysis to provide a three-classes µmap. The proposed imaging protocol implemented on a 1.5T Avanto scanner. The acquisition parameters were 1.11 msec and 20 msec for TE and TR, respectively, with FA=20. The image-processing protocol includes five major steps: (I) intensity-inhomogeneity correction using non parametric method, which is an essential step for removing bias field; (II) separation of bone and air from other areas using active contouring based on gradient vector method; (III) FCM in order to segment the image into two clusters, one cluster is bone and air and another cluster is soft tissue; (IV) separation of bone and air areas using shape analysis; (V) generation of µmap. The accuracy of the proposed segmentation method was validated using comparison against manual segmentation. The corresponding segmentation images and generated µmap prove the validity of algorithm. Quantitative analysis on accuracy, sensitivity and specificity for 15 segmented images in comparison with manual segmentation performed by an expert radiologist. The proposed strategy shows that the three tissue class µmap can be successfully generated from MR images in pelvis region using STE pulse sequence following by the image processing steps. The method can be a potential alternative to UTE-based attenuation correction. The algorithm is still under development and will be validated in more details based on comparison with the µmaps generate from CT images. This study has been developed particularly for improving the accuracy of MRAC in prostate imaging.

Towards using scattered PET emission data for reconstruction of attenuation map in PET/MRI.

Despite promising advances, attenuation correction of PET data remains challenging in PET/MRI. Known approaches to estimating attenuation maps from PET emission data take into account only true coincidences and fail to completely determine the attenuation map without prior information. Even TOF PET emission data determines the attenuation sinogram up to an unknown constant only. We propose to extract information from scattered PET coincidences to fully determine the attenuation map. Scatter-to-attenuation reconstruction has been approximated by a proof-of-concept scatter-to-attenuation back-projection. The latter is based on determining possible scattering locations from energy measurements (via Compton scattering angles) and summing contributions from scattered coincidences in image voxels. Contributions are weighted based on geometrical considerations and various corrections derived from a scatter-measurement equation similar to the single-scatter-simulation formula; this equation represents a weighted Radon transform on surfaces in curvilinear coordinates. The back-projection technique was evaluated with data from GATE simulations of various activity and attenuation distributions in a clinical scanner model. Finally, the potential of the back-projection operator for determination of a unique attenuation map has been evaluated in an iterative scheme for small-scale problems. Scatter-to-attenuation back-projection of coincidences with scattered photon energies in the range of 248–478 keV are shown for general cases as well as for highly degenerate cases with perfect spherical symmetry of attenuation and activity distributions. The parameter-space trajectory followed by the iterative scheme shows that the proposed scatter-to-attenuation back-projection can be used to estimate true attenuation values despite an initially unknown scaling factor. Scattered coincidences provide information that may complement existing reconstruction approaches. The proposed scatter-to-attenuation back-projection may constitute a missing piece for resolving ambiguities in the simultaneous reconstruction of activity and attenuation. Lacking an analytical reconstruction procedure, iterative techniques need to be developed to fully integrate information from true and scattered coincidences.

MO‐G‐17A‐03: MR‐Based Cortical Bone Segmentation for PET Attenuation Correction with a Non‐UTE 3D Fast GRE Sequence

Purpose:To determine the feasibility of identifying cortical bone on MR images with a short‐TE 3D fast‐GRE sequence for attenuation correction of PET data in PET/MR.Methods:A water‐fat‐bone phantom was constructed with two pieces of beef shank. MR scans were performed on a 3T MR scanner (GE Discovery™ MR750). A 3D GRE sequence was first employed to measure the level of residual signal in cortical bone (TE1/TE2/TE3=2.2/4.4/6.6ms, TR=20ms, flip angle=25°). For cortical bone segmentation, a 3D fast‐GRE sequence (TE/TR=0.7/1.9ms, acquisition voxel size=2.5×2.5×3mm3) was implemented along with a 3D Dixon sequence (TE1/TE2/TR=1.2/2.3/4.0ms, acquisition voxel size=1.25×1.25×3mm3) for water/fat imaging. Flip angle (10°), acquisition bandwidth (250kHz), FOV (480×480×144mm3) and reconstructed voxel size (0.94×0.94×1.5mm3) were kept the same for both sequences. Soft tissue and fat tissue were first segmented on the reconstructed water/fat image. A tissue mask was created by combining the segmented water/fat masks, which was then applied on the fast‐GRE image (MRFGRE). A second mask was created to remove the Gibbs artifacts present in regions in close vicinity to the phantom. MRFGRE data was smoothed with a 3D anisotropic diffusion filter for noise reduction, after which cortical bone and air was separated using a threshold determined from the histogram.Results:There is signal in the cortical bone region in the 3D GRE images, indicating the possibility of separating cortical bone and air based on signal intensity from short‐TE MR image. The acquisition time for the 3D fast‐GRE sequence was 17s, which can be reduced to less than 10s with parallel imaging. The attenuation image created from water‐fat‐bone segmentation is visually similar compared to reference CT.Conclusion:Cortical bone and air can be separated based on intensity in MR image with a short‐TE 3D fast‐GRE sequence. Further research is required to optimize the strategy to reduce Gibbs artifacts.

Derivation of attenuation map for attenuation correction of PET data in the presence of nanoparticulate contrast agents using spectral CT imaging

Uptake value in quantitative PET imaging is biased due to the presence of CT contrast agents when using CT-based attenuation correction. Our aim was to examine spectral CT imaging to suppress inaccuracy of 511keV attenuation map in the presence of multiple nanoparticulate contrast agents. Using a simulation study we examined an image-based K-edge ratio method, in which two images acquired from energy windows located above and below the K-edge energy are divided by one another, to identify the exact location of all contrast agents. Multiple computerized phantom studies were conducted using a variety of NP contrast agents with different concentrations. The performance of the proposed methodology was compared to conventional single-kVp and dual-kVp methods using wide range of contrast agents with varying concentrations. The results demonstrate that both single-kVp and dual-kVp energy mapping approaches produce inaccurate attenuation maps at 511keV in the presence of multiple simultaneous contrast agents. In contrast, the proposed method is capable of handling multiple simultaneous contrast agents, thus allowing suppression of 511keV attenuation map inaccuracy. Attenuation map produced by spectral CT clearly outperforms conventional single-kVp and dual-kVp approaches in the generation of accurate attenuation maps in the presence of multiple contrast agents.

Anatomic Evaluation of 3-Dimensional Ultrashort-Echo-Time Bone Maps for PET/MR Attenuation Correction

Ultrashort-echo-time (UTE) sequences have been proposed in the past for MR-based attenuation correction of PET data, because of their ability to image cortical bone. In the present work we assessed the limitations of dual-echo UTE imaging for bone segmentation in head and neck imaging. Sequentially acquired MR and PET/CT clinical data were used for this purpose. Twenty patients referred for a clinical oncology examination were scanned using a trimodality setup. Among the MR sequences, a dual-echo UTE acquisition of the head was acquired and used to create tissue R2 maps. The different undesired structures present in these maps were identified by an experienced radiologist. Global and local measurements of the overlap between R2-based and CT-based bone masks were computed. UTE R2 maps displayed a nonfunctional relation with CT data. The obtained bone masks showed acceptable overlap with the corresponding CT data, in the case of the skull itself (e.g., 47% mismatch for the parietal region), with decreased performance in the base of the skull and in the neck (e.g., 78% for the maxillary region). Unwanted structures were detected, both anatomic (e.g., sternocleidomastoid, temporal, and masseter muscles) and artifactual (e.g., dental implants and air-tissue interfaces). It is indeed possible to estimate the anatomic location of bone tissue using UTE sequences. However, using pure parametric maps for attenuation correction may lead to bias close to certain anatomic structures and areas of high magnetic field inhomogeneity. More sophisticated approaches are necessary to compensate for these effects.

MRI-guided attenuation correction in whole-body PET/MR: assessment of the effect of bone attenuation

Hybrid PET/MRI presents many advantages in comparison with its counterpart PET/CT in terms of improved soft-tissue contrast, decrease in radiation exposure, and truly simultaneous and multi-parametric imaging capabilities. However, the lack of well-established methodology for MR-based attenuation correction is hampering further development and wider acceptance of this technology. We assess the impact of ignoring bone attenuation and using different tissue classes for generation of the attenuation map on the accuracy of attenuation correction of PET data. This work was performed using simulation studies based on the XCAT phantom and clinical input data. For the latter, PET and CT images of patients were used as input for the analytic simulation model using realistic activity distributions where CT-based attenuation correction was utilized as reference for comparison. For both phantom and clinical studies, the reference attenuation map was classified into various numbers of tissue classes to produce three (air, soft tissue and lung), four (air, lungs, soft tissue and cortical bones) and five (air, lungs, soft tissue, cortical bones and spongeous bones) class attenuation maps. The phantom studies demonstrated that ignoring bone increases the relative error by up to 6.8% in the body and up to 31.0% for bony regions. Likewise, the simulated clinical studies showed that the mean relative error reached 15% for lesions located in the body and 30.7% for lesions located in bones, when neglecting bones. These results demonstrate an underestimation of about 30% of tracer uptake when neglecting bone, which in turn imposes substantial loss of quantitative accuracy for PET images produced by hybrid PET/MRI systems. Considering bones in the attenuation map will considerably improve the accuracy of MR-guided attenuation correction in hybrid PET/MR to enable quantitative PET imaging on hybrid PET/MR technologies.

Accuracy of CT-based attenuation correction in PET/CT bone imaging

We evaluate the accuracy of scaling CT images for attenuation correction of PET data measured for bone. While the standard tri-linear approach has been well tested for soft tissues, the impact of CT-based attenuation correction on the accuracy of tracer uptake in bone has not been reported in detail. We measured the accuracy of attenuation coefficients of bovine femur segments and patient data using a tri-linear method applied to CT images obtained at different kVp settings. Attenuation values at 511 keV obtained with a 68Ga/68Ge transmission scan were used as a reference standard. The impact of inaccurate attenuation images on PET standardized uptake values (SUVs) was then evaluated using simulated emission images and emission images from five patients with elevated levels of FDG uptake in bone at disease sites. The CT-based linear attenuation images of the bovine femur segments underestimated the true values by 2.9 ± 0.3% for cancellous bone regardless of kVp. For compact bone the underestimation ranged from 1.3% at 140 kVp to 14.1% at 80 kVp. In the patient scans at 140 kVp the underestimation was approximately 2% averaged over all bony regions. The sensitivity analysis indicated that errors in PET SUVs in bone are approximately proportional to errors in the estimated attenuation coefficients for the same regions. The variability in SUV bias also increased approximately linearly with the error in linear attenuation coefficients. These results suggest that bias in bone uptake SUVs of PET tracers ranges from 2.4% to 5.9% when using CT scans at 140 and 120 kVp for attenuation correction. Lower kVp scans have the potential for considerably more error in dense bone. This bias is present in any PET tracer with bone uptake but may be clinically insignificant for many imaging tasks. However, errors from CT-based attenuation correction methods should be carefully evaluated if quantitation of tracer uptake in bone is important.

Reduction of dental filling metallic artifacts in CT‐based attenuation correction of PET data using weighted virtual sinograms optimized by a genetic algorithm

The presence of metallic dental fillings is prevalent in head and neck PET/CT imaging and generates bright and dark streaking artifacts in reconstructed CT images. The resulting artifacts would propagate to the corresponding PET images following CT-based attenuation correction (CTAC). This would cause over- and/or underestimation of tracer uptake in corresponding regions thus leading to inaccurate quantification of tracer uptake. The purpose of this study is to improve our recently proposed metal artifact reduction (MAR) approach and to assess its performance in a clinical setting. The proposed MAR algorithm is performed in the virtual sinogram space to overcome the challenges associated with manipulating raw CT data. The corresponding bins of the virtual sinogram affected by metallic objects are obtained by forward projection of segmented metallic objects in the original CT image. These bins are then substituted by weighted values of three estimates: the affected bins in the original sinogram, the bins in the corrected sinogram using spline interpolation, and the sinogram bins in the neighboring column of the sinogram matrix. The optimized weighting factors (alpha, beta, and gamma) were estimated using a genetic algorithm (GA). The optimized combination of weighting coefficients was obtained using the GA applied to 24 clinical CT data sets. The proposed MAR method was then applied to 12 clinical head and neck PET/CT data sets containing dental artifacts. Analysis of the results was performed using Bland and Altman plots and a method allowing analysis in the absence of gold standard called regression without truth (RWT). The proposed method was also compared to an image-based MAR method. Optimization of the weighting coefficients using the GA resulted in an optimum combination of parameters of alpha=0.26, beta=0.67, and gamma=0.07. According to Bland and Altman plots generated for both CT and PET images of the clinical data, the proposed MAR algorithm is efficient for reduction of streak artifacts in CT images and such reduce the over- and/or underestimation o tracer uptake. The RWT method also confirmed the effectiveness of the proposed MAR method. The obtained figures of merit revealed that attenuation corrected PET data corrected using CTAC after applying the MAR algorithm are more similar to the assumed gold standard. Comparison with the knowledge-based method revealed that the proposed method mainly corrects the artifactual regions without modifying the unaffected regions. The knowledge-based method globally modifies the images including those that do not include metallic artifacts. The proposed MAR algorithm improves the quality and quantitative accuracy of clinical head and neck PET/CT images and could be easily integrated in clinical setting.

WE‐SAM‐201B‐01: PET/CT Technology Update, Quality Assurance and Applications

In the past few years, positron emission tomography/computed tomography (PET/CT) imaging has increasingly been used for the diagnosis, staging, and restaging of malignant diseases. The success of this emerging modality has primarily been due to its ability to combine the advantages of both PET and CT imaging while minimizing their separate weaknesses. One of the main advantages of PET/CT imaging is its ability to generate functional images depicting the biodistribution of radioactive compounds that are correlated with anatomical landmarks thereby increasing the physicians' confidence in image interpretation and improving patient management.The aim of this lecture is to provide an overview of the basic physics principles of PET/CT imaging as well as the advantages and drawbacks of using CT for attenuation correction of PET data. In addition, the lecture will cover the latest in design specifics of commercially available PET/CT scanners from different manufacturers as well as review PET/CT quality control and assurance techniques.Learning Objectives:1. To learn the basic physics principles of PET/CT imaging2. To understand the advantages and drawbacks of using CT for attenuation correction of PET images.3. Become familiar with design specifics of commercially available PET/CT scanners.4. Learn quality control and assurance techniques for PET/CT imaging

A virtual sinogram method to reduce dental metallic implant artefacts in computed tomography-based attenuation correction for PET

Attenuation correction of PET data requires accurate determination of the attenuation map (mumap), which represents the spatial distribution of linear attenuation coefficients of different tissues at 511 keV. The presence of high-density metallic dental filling material in head and neck X-ray computed tomography (CT) scanning is known to generate streak artefacts in the resulting CT images and thus in the corresponding mumaps generated using CT-based attenuation correction. Consequently, an under/overestimation of activity concentration occurs in corresponding regions of the corrected PET images. The purpose of this study is to develop a simple yet practical approach for reduction of metallic dental implant artefacts in the generated mumaps. Currently available sinogram-based metal artefact reduction (MAR) algorithms operate directly on the raw sinograms. These usually consist of huge files stored in proprietary format not easily disclosed by the manufacturers and thus are not straightforward to read and manipulate. The proposed method uses the concept of virtual sinograms produced by forward projection of CT images in Dicom format for MAR. The projection data affected by metallic objects are detected in the sinogram space through segmentation of metallic objects in the CT image followed by forward projection of the metal-only image. Thereafter, the affected sinogram bins are replaced by interpolated values from adjacent projections using the spline interpolation technique. The algorithm was assessed using a polyethylene phantom containing materials simulating different tissues and a dedicated jaw phantom scanned before and after the insertion of metallic objects, where the corrected and noncorrected mumaps were compared with the artefact-free mumap. In addition, the Jaszczak and standard germanium phantoms including four metallic inserts were scanned on a PET/CT scanner to evaluate the impact of the MAR procedure on PET data through the comparison of uncorrected and corrected PET images to the actual activity concentrations in the phantoms. The proposed algorithm was also applied to head and neck CT images of 10 patients with metallic dental implants. The MAR method proved to be practical in a clinical setting and reduced substantially the visible metal induced artefacts. The mean relative error in regions close to metallic objects is reduced by approximately 90%. The statistical analysis of the Jaszczak and solid Ge-68 phantoms PET images did not reveal statistically significant differences between the corrected and artefact-free images (P>0.05). Moreover, the evaluation of clinical studies did not reveal statistically significant differences between the attenuation coefficients of the corrected mumaps and the expected theoretical values. The proposed MAR method provides a simple and convenient approach allowing correction for the presence of metal artefacts caused by dental implants without the need to manipulate the complex raw CT data. Further evaluation using a larger clinical PET/CT database is under way to evaluate the potential of the technique in a clinical setting.

WE‐SAM‐304A‐01: PET/CT Technology Update, Quality Assurance and Applications

In the past few years, positron emission tomography/computed tomography (PET/CT) imaging has increasingly been used for the diagnosis, staging, and restaging of malignant diseases. The success of this emerging modality has primarily been due to its ability to combine the advantages of both PET and CT imaging while minimizing their separate weaknesses. One of the main advantages of PET/CT imaging is its ability to generate functional images depicting the biodistribution of radioactive compounds that are correlated with anatomical landmarks thereby increasing the physicians' confidence in image interpretation and improving patient management.The aim of this lecture is to provide an overview of the basic physics principles of PET/CT imaging as well as the advantages and drawbacks of using CT for attenuation correction of PET data. In addition, the lecture will cover the latest in design specifics of commercially available PET/CT scanners from different manufacturers as well as review PET/CT quality control and assurance techniques.Learning Objectives:1. To learn the basic physics principles of PET/CT imaging2. To understand the advantages and drawbacks of using CT for attenuation correction of PET images.3. Become familiar with design specifics of commercially available PET/CT scanners.4. Learn quality control and assurance techniques for PET/CT imaging

SU-GG-I-135: Average CT in Colorectal Cancer PET Studies

Purpose: To assess the effect of average CT (ACT) used for attenuation correction of PET data on SUVmax and gross tumor volume (GTV) in liver lesions of colorectal cancer patients. Method and Materials: We studied 34 colorectal cancer patients with metastasis in the liver. A routine PET/CT scan was immediately followed by a cine CT scan of the liver region for ACT. Patients were free‐breathing during the helical CT (HCT) and ACT acquisitions. Both HCT and ACT were used for attenuation correction of the PET data. 59 liver lesions were included in the study and SUVmax for each lesion was measured on PETHCT and PETACT. Both PETdata sets were imported to Pinnacle3 7.6 treatment planning system. GTVs for PETimages were generated based on the SUV thresholds, normally used for assessing tumor response to therapy. Percentage volume change, centroid shift and concordance index were defined for each lesion. Results: 67% of the studies showed respiratory artifacts in PETHCT while 15% demonstrated a difference in SUVmax above 20%. The effect of misregistration between HCT and PET data was more evident for smaller lesions with the most pronounced case showing a 29% increase in SUVmax for PETACT, 246% increase in GTV, 2mm shift in GTV centroid location and concordance index value of 0.34. We analyzed correlation between the variations and the lesion size with a non‐paired t‐test at 0.05 p‐value level. The means of concordance indexes in two distributions of the lesions normalized to their size (less and greater than 5cm3) were shown to be significantly different (p=0.007). Conclusion: ACT is effective in improving the registration between the CT and PET data in PET/CT, which is important when using PETimages for treatment planning. Misregistration is more pronounced for small lesions and should be considered in the generation of the GTVs.

Comparing 511 keV Attenuation Maps Obtained from Different Energy Mapping Methods for CT Based Attenuation Correction of PET Data

Introduction: The advent of dual-modality PET/CT scanners has revolutionized clinical oncology by improving lesion localization and facilitating treatment planning for radiotherapy. In addition, the use of CT images for CT-based attenuation correction (CTAC) decreases the overall scanning time and creates a noise-free attenuation map (6map). CTAC methods include scaling, segmentation, hybrid scaling/segmentation, bilinear and dual energy methods. All CTAC methods require the transformation of CT Hounsfield units (HU) to linear attenuation coefficients (LAC) at 511 keV. The aim of this study is to compare the results of implementing different methods of energy mapping in PET/CT scanners. Materials and Methods: This study was conducted in 2 phases, the first phase in a phantom and the second one on patient data. To perform the first phase, a cylindrical phantom with different concentrations of K2HPO4 inserts was CT scanned and energy mapping methods were implemented on it. For performing the second phase, different energy mapping methods were implemented on several clinical studies and compared to the transmission (TX) image derived using Ga-68 radionuclide source acquired on the GE Discovery LS PET/CT scanner. Results: An ROI analysis was performed on different positions of the resultant 6maps and the average 6value of each ROI was compared to the reference value. The results of the 6maps obtained for 511 keV compared to the theoretical values showed that in the phantom for low concentrations of K 2 HPO 4 all these methods produce 511 keV attenuation maps with small relative difference compared to gold standard. The relative difference for scaling, segmentation, hybrid, bilinear and dual energy methods was 4.92, 3.21, 4.43, 2.24 and 2.29%, respectively. Although for high concentration of K 2 HPO 4 the three methods; hybrid scaling/segmentation, bilinear and dual energy produced the lowest relative difference of 10.91, 10.88 and 5%, respectively. For patients it was found that for soft tissues all the mentioned energy mapping methods produce acceptable attenuation map at 511 keV. The relative difference of scaling, segmentation, hybrid, and bilinear methods compared to TX method was 6.95, 4.51, 7, and 6.45% respectively. For bony tissues, the quantitative analysis showed that scaling and segmentation method produce high relative difference of 26 and 23.2%, respectively and the relative difference of hybrid and bilinear in comparison to TX method was 10.7 and 20%, respectively. Discussion and Conclusion: Based on the result obtained from these two studies it can be concluded that for soft tissues all energy mapping methods yield acceptable results while for bony tissues all the mentioned methods except the scaling and segmentation yield acceptable results.