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Purpose Today, attenuation correction (AC) of positron emission tomography/magnetic resonance (PET/MR) hardware components is performed by using an established method from PET/CT hybrid imaging. As shown in previous studies, the e...
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Purpose Today, attenuation correction (AC) of positron emission tomography/magnetic resonance (PET/MR) hardware components is performed by using an established method from PET/CT hybrid imaging. As shown in previous studies, the established mathematical conversion from computed tomography (CT) to PET attenuation coefficients may, however, lead to incorrect results in PET quantification when applied to AC of hardware components in PET/MR. The purpose of this study is to systematically investigate the attenuating properties of various materials and electronic components frequently used in the context of PET/MR hybrid imaging. The study, thus, aims at improving hardware component attenuation correction in PET/MR. Materials and methods Overall, 38 different material samples were collected; a modular phantom was used to for CT, PET, and PET/MR scanning of all samples. Computed tomography‐scans were acquired with a tube voltage of 140?kVp to determine Hounsfield Units (HU). PET transmission scans were performed with 511?keV to determine linear attenuation coefficients (LAC) of all materials. The attenuation coefficients were plotted to obtain a HU to LAC correlation graph, which was then compared to two established conversions from literature. Hardware attenuation maps of the different materials were created and applied to PET data reconstruction following a phantom validation experiment. From these measurements, PET difference maps were calculated to validate and compare all three conversion methods. Results For each material, the HU and corresponding LAC could be determined and a bi‐linear HU to LAC conversion graph was derived. The corresponding equation was y = 1.64 ? 10 - 5 × HU + 1000 + 8.3 ? 10 - 2 . While the two established conversions lead to a mean quantification PET bias of 4.69%?±?0.27% and ?2.84%?±?0.72% in a phantom experiment, PET difference measurements revealed only 0.5?% bias in PET quantification when applying the new conversion resulting from this study. Conclusions An optimized method for the conversion of CT to PET attenuation coefficients has been derived by systematic measurement of 38 different materials. In contrast to established methods, the new conversion also considers highly attenuating materials, thus improving attenuation correction of hardware components in PET/MR hybrid imaging.
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Generally, the Chang method depends on counts for the attenuation correction (AC) method in brain perfusion single-photon emission computed tomography (SPECT), because the head outlined for a uniform attenuation coefficient map is...
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Generally, the Chang method depends on counts for the attenuation correction (AC) method in brain perfusion single-photon emission computed tomography (SPECT), because the head outlined for a uniform attenuation coefficient map is set to the sinogram of the projection data by the threshold (Sinogram Threshold Chang method). Magnetic resonance imaging (MRI) is a routine examination in our hospital. Patients who underwent N-isopropyl-p-[123I] iodoamphetamine (123I-IMP) SPECT are undergoing MRI. Therefore, we thought it help AC accuracy to set an accurate head outline by using the image. We jointly made “Software for an attenuation coefficient map using MRI” for trial purposes. This paper investigated whether the AC method using MRI promotes the accuracy of brain perfusion SPECT in some clinical samples. With AC methods using gamma ray transmission computed tomography (TCT) or X-ray CT (CT) also being taken into account, the AC method using MRI was compared with the Sinogram Threshold Chang method. As a result, count dependency was excluded by an accurate head outline setting that used MRI, and the AC method using MRI approached the effect of the AC method using TCT and CT more than the Sinogram Threshold Chang method. Therefore, it is suggested that the AC method using MRI is useful for the accuracy of brain perfusion SPECT.
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Abstract Purpose The aim of this study was to compare and evaluate three different bilinear conversion curves for attenuation correction (AC) of a 16‐channel radiofrequency (RF) coil in positron emission tomography/magnetic reson...
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Abstract Purpose The aim of this study was to compare and evaluate three different bilinear conversion curves for attenuation correction (AC) of a 16‐channel radiofrequency (RF) coil in positron emission tomography/magnetic resonance (PET/MR) breast cancer imaging. Methods The quantitative impact of three different bilinear conversions of computed tomography (CT) data for the AC of a 16‐channel RF breast coil was systematically evaluated in phantom measurements and on n = 20 PET/MR patients with breast cancer. PET data were reconstructed four times: (1) no coil AC (C‐NAC) serving as a reference, (2) established bilinear conversion by Carney et?al., (3) bilinear conversion by Paulus et?al., and (4) bilinear conversion by Oehmigen et?al. Relative differences in PET data were calculated. Results Independent of the choice of bilinear conversion, significant gains in PET signal, compared to C‐NAC, were measurable in all phantom and patient measurements. Mean relative differences of ca. 10% in SUVmean (i.e., standardized uptake value; maximal relative differences up to 30%) due to the integration of the coil AC were calculated, compared to C‐NAC in phantom and patient measurements. Relative difference images depict that the quantitative impact of coil AC is highest in regions close to the RF coil when compared to no AC data. Bilinear conversion by Carney et?al. shows a slightly overcorrection (2.9%), whereas the conversion by Paulus et?al. provides a slight undercorrection of the PET images (?1.6%) in comparison to the no‐coil measurement. The bilinear conversion proposed by Oehmigen et?al. provides the most appropriate AC for the breast coil in this phantom experiment (?0.2%). A total of 23 congruent lesions could be detected in all patients. All lesions could be detected in all reconstructions. Conclusions For the best possible PET image quality and accurate PET quantification in breast PET/MRI, the AC of MR hardware components is important. The bilinear conversion proposed by Oehmigen et?al. provides the most appropriate AC for the breast coil in this study.
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Previously, we proposed using an interpolated average CT (IACT) method for attenuation correction (AC) in positron emission tomography (PET), which is a good, low-dose approximation of cine average CT (CACT) to reduce misalignment...
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Previously, we proposed using an interpolated average CT (IACT) method for attenuation correction (AC) in positron emission tomography (PET), which is a good, low-dose approximation of cine average CT (CACT) to reduce misalignments and improve quantification in PET/CT. This study aims to evaluate the performance of IACT for different motion amplitudes. We used the digital four-dimensional (4-D) extended cardiac-torso phantom (XCAT) to simulate maximum of 2, 3, and 4 cm respiratory motions. The respiratory cycle was divided into 13 phases, with average activity and attenuation maps to represent $^{18}$ F-fluorodeoxyglucose ($^{18}$F-FDG) distributions with average respiratory motions and CACT, respectively. The end-inspiration, end-expiration, and midrespiratory phases of the XCAT attenuation maps represented three different helical CTs (i.e., HCT-1, HCT-5, and HCT-8). The IACTs were generated using: 1) 2 extreme + 11 interpolated phases (IACT$_{rm 2o}$ ); 2) 2 phases right after the extreme phases + 11 interpolated phases (IACT $_{rm 2s}$); 3) 4 original + 9 interpolated phases (IACT$_{rm 4o}$). A spherical lesion with a target-to-background ratio (TBR) of 4:1 and a diameter of 25 mm was placed in the base of right lung. The noise-free and noisy sinograms with attenuation modeling were generated and reconstructed with different noise-free and noisy AC maps (CACT, HCTs, and IACTs) by Software for Tomographic Image Reconstruction, respectively, using ordered subset expectation maximization(OS-EM) with up to 300 updates. Normalized mean-square error, mutual information (MI), TBR, image profile, and noi- e-contrast tradeoff were analyzed. The PET reconstructed images with AC using CACT showed least difference as compared to the original phantom, followed by IACT $_{rm 4o}$, IACT$_{rm 2o}$, IACT$_{rm 2s}$, HCT-5, HCT-8, and HCT-1. Significant artifacts were observed in the reconstructed images using HCTs for AC. The MI differences between IACT $_{rm 2o}$ and IACT$_{rm 4o}$ /CACT were <0.41% and <2.17%, respectively. With a slight misplacement of the two extreme phases, IACT$_{rm 2s}$ was still comparable to IACT $_{rm 2o}$ with MI difference of <2.23%. The IACT is a robust and accurate low-dose alternate to CACT.
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We investigated PET image quantification when using a uniform attenuation coefficient (mu) for attenuation correction (AC) of anthropomorphic density phantoms derived from high-resolution breast CT scans. A breast PET system was m...
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We investigated PET image quantification when using a uniform attenuation coefficient (mu) for attenuation correction (AC) of anthropomorphic density phantoms derived from high-resolution breast CT scans. A breast PET system was modeled with perfect data corrections except for AC. Using uniform mu for AC resulted in quantitative errors roughly proportional to the difference between mu used in AC (mu AC) and local mu, yielding approximately +/- 5% bias, corresponding to the variation of mu for 511-keV photons in breast tissue. Global bias was lowest when uniform mu AC was equal to the phantom mean mu (mu mean). Local bias in 10-mm spheres increased as the sphere mu deviated from mu mean, but remained only 2%-3% when the mu sphere was 6.5% higher than mu mean. Bias varied linearly with and was roughly proportional to local mu mismatch. Minimizing local bias, e.g., in a small sphere, required the use of a uniform mu value between the local mu and the mu mean. Thus, biases from using uniform-mu AC are low when local mu sphere is close to mu mean. As the mu sphere increasingly differs from the phantom mu mean, bias increases, and the optimal uniform mu is less predictable, having a value between mu sphere and the phantom mu mean.
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To evaluate the influence of Gadolinium contrast agent on image segmentation in magnetic resonance (MR)-based attenuation correction (AC) with four-segment dual-echo time Dixon-sequences in whole-body [18F]-fluorodeoxyglucose posi...
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To evaluate the influence of Gadolinium contrast agent on image segmentation in magnetic resonance (MR)-based attenuation correction (AC) with four-segment dual-echo time Dixon-sequences in whole-body [18F]-fluorodeoxyglucose positron emission tomography (PET)/MR imaging, and to analyze the consecutive effect on standardized uptake value (SUV).
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Attenuation correction (AC) is essential for the generation of artifact-free and quantitatively accurate positron emission tomography (PET) images. PET AC based on computed tomography (CT) frequently results in artifacts in attenu...
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Attenuation correction (AC) is essential for the generation of artifact-free and quantitatively accurate positron emission tomography (PET) images. PET AC based on computed tomography (CT) frequently results in artifacts in attenuation-corrected PET images, and these artifacts mainly originate from CT artifacts and PET-CT mismatches. The AC in PET combined with a magnetic resonance imaging (MRI) scanner (PET/MRI) is more complex than PET/CT, given that MR images do not provide direct information on high-energy photon attenuation. Deep-learning (DL)-based methods for the improvement of PET AC have received significant research attention as alternatives to conventional AC methods. Many DL studies were focused on the transformation of MR images into synthetic pseudo-CT or attenuation maps. Alternative approaches that are not dependent on the anatomical images (CT or MRI) can overcome the limitations related to current CT- and MRI-based ACs and allow for more accurate PET quantification in stand-alone PET scanners for the realization of low radiation doses. In this article, a review is presented on the limitations of the PET AC in current dual-modality PET/CT and PET/MRI scanners, in addition to the current status and progress of DL-based approaches, for the realization of improved performance of PET AC.
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Attenuation correction (AC) is important for the quantitative merits of positron emission tomography (PET). However, attenuation coefficients cannot be derived from magnetic resonance (MR) images directly for PET/MR systems. In th...
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Attenuation correction (AC) is important for the quantitative merits of positron emission tomography (PET). However, attenuation coefficients cannot be derived from magnetic resonance (MR) images directly for PET/MR systems. In this work, we aimed to derive continuous AC maps from Dixon MR images without the requirement of MR and computed tomography (CT) image registration. To achieve this, a 3-D generative adversarial network with both discriminative and cycle-consistency loss (Cycle-GAN) was developed. The modified 3-D U-net was employed as the structure of the generative networks to generate the pseudo-CT/MR images. The 3-D patch-based discriminative networks were used to distinguish the generated pseudo-CT/MR images from the true CT/MR images. To evaluate its performance, datasets from 32 patients were used in the experiment. The Dixon segmentation and atlas methods provided by the vendor and the convolutional neural network (CNN) method which utilized registered MR and CT images were employed as the reference methods. Dice coefficients of the pseudo-CT images and the regional quantification in the reconstructed PET images were compared. Results show that the Cycle-GAN framework can generate better AC compared to the Dixon segmentation and atlas methods, and shows comparable performance compared to the CNN method.
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Abstract Purpose Aim of this study was to evaluate the use of computer‐aided design (CAD) models for attenuation correction (AC) of hardware components in positron emission tomography/magnetic resonance (PET/MR) imaging. Methods ...
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Abstract Purpose Aim of this study was to evaluate the use of computer‐aided design (CAD) models for attenuation correction (AC) of hardware components in positron emission tomography/magnetic resonance (PET/MR) imaging. Methods The technical feasibility and quantitative impact of CAD‐AC compared to computer tomography (CT)‐based AC (reference) was investigated on a modular phantom consisting of 19 different material samples (plastics and metals arranged around a cylindrical emission phantom) typically used in phantoms, patient tables, and radiofrequency (RF) coils in PET/MR. The clinical applicability of the CAD‐AC method was then evaluated on a 16‐channel RF breast coil in a PET/MR patient study. The RF breast coil in this study was specifically designed PET compatible. Using this RF breast coil, the impact on clinical PET/MR breast imaging was systematically evaluated in breast phantom measurements and, furthermore, in n?=?10 PET/MR patients with breast cancer. PET data were reconstructed three times: (1) no AC (NAC), (2) established CT‐AC, and (3) CAD‐AC. For both phantom measurements, a scan without attenuating hardware components (material probes or RF breast coil) was acquired serving as reference. Relative differences in PET data were calculated for all experiments. Results In all phantom and patient measurements, significant gains in PET signal compared to NAC data were measurable with CT and CAD‐AC. In initial phantom experiments, mean relative differences of –0.2% for CT‐AC and 0.2% for CAD‐AC were calculated compared to reference measurements without the material probes. The application to a RF breast coil depicts that CAD‐AC results in significant gains compared to NAC data (10%) and a slight underestimation in PET signal of –1.3% in comparison to the no‐coil reference measurement. In the patient study, a total of 15 congruent lesions in all 10 patients with a mean relative difference of 14% (CT and CAD‐AC) in standardized uptake value compared to NAC data could be detected. Conclusions To ensure best possible PET image quality and accurate PET quantification in PET/MR imaging, the AC of hardware components such as phantoms and RF coils is important. In initial phantom experiments and in clinical application to an RF breast coil, it was found that CAD‐based AC results in significant gains in PET signal compared to NAC data and provides comparably good results to the established method of CT‐based AC.
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Purpose of Review In the years 2011-2014, three different whole-body PET/MRI hybrid systems from different vendors have been introduced to the market. While this rather new hybrid imaging modality is primarily used in clinical onc...
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Purpose of Review In the years 2011-2014, three different whole-body PET/MRI hybrid systems from different vendors have been introduced to the market. While this rather new hybrid imaging modality is primarily used in clinical oncolog-ic and neurologic hybrid imaging, cardiovascular applications are now gaining broader utilization. Cardiovascular hybrid imaging with PET/MRI is especially challenging because the independent data acquisition with PET and MRI is to be synchronized to breathing and cardiac motion to fully make use of the diagnostic potential inherent to this new hybrid method. Recent Findings New methodological developments and refinements in attenuation, truncation, and motion correction for PET/MRI further improve overall image quality and PET quantification in cardiovascular imaging applications. The rather complex hybrid imaging workflow integrating motion correction techniques is further streamlined to facilitate usability and to increase the diagnostic application spectrum. Summary Instrumentation and methodology over the recent years have matured to a level that PET/MRI today is a powerful addition to the palette of hybrid imaging modalities in cardiovascular applications.
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