Attenuation Correction Of Positron Emission Tomography Data Research Articles

Ferumoxytol Does Not Impact Standardized Uptake Values on PET/MR Scans.

Tumor response assessments on positron emission tomography (PET)/magnetic resonance imaging (MRI) scans require correct quantification of radiotracer uptake in tumors and normal organs. Historically, MRI scans have been enhanced with gadolinium (Gd)-based contrast agents, which are now controversial due to brain deposition. Recently, ferumoxytol nanoparticles have been identified as an alternative to Gd-based contrast agents because they provide strong tissue enhancement on MR images but are not deposited in the brain. However, it is not known if the strong T1- and T2-contrast obtained with iron oxide nanoparticles such as ferumoxytol could affect MR-based attenuation correction of PET data. The purpose of our study was to investigate if ferumoxytol administration prior to a 2-deoxy-2-[18F]fluoro-D-glucose [18F]FDG PET/MR scan would change standardized uptake values (SUV) of normal organs. Thirty pediatric patients (6-18years) with malignant tumors underwent [18F]FDG-PET/MR scans (dose 3MBq/kg). Fifteen patients received an intravenous ferumoxytol injection (5mgFe/kg) prior to the [18F]FDG-PET/MR scans (group 1). Fifteen additional age- and sex-matched patients received unenhanced [18F]FDG-PET/MR scans (group 2). For attenuation correction of PET data, we used a Dixon-based gradient echo sequence (TR 4.2ms, TE 1.1, 2.3ms, FA 5), which accounted for soft tissue, lung, fat, and background air. We used a mixed linear effects model to compare the tissue MRI enhancement, quantified as the signal-to-noise ratio (SNR), as well as tissue radiotracer signal, quantified as SUVmean and SUVmax, between group 1 and group 2. Alpha was assumed at 0.05. The MRI enhancement of the blood and solid extra-cerebral organs, quantified as SNR, was significantly higher on ferumoxytol-enhanced MRI scans compared to unenhanced scans (p < 0.001). However, SUVmean and SUVmax values, corrected based on the patients' body weight or body surface area, were not significantly different between the two groups (p > 0.05). Ferumoxytol administration prior to a [18F]FDG PET/MR scan did not change standardized uptake values (SUV) of solid extra-cerebral organs. This is important, because it allows injection of ferumoxytol contrast prior to a PET/MRI procedure and, thereby, significantly accelerates image acquisition times.

PET/MRI

Magnet resonance imaging (MRI) is an excellent anatomical reference method for the combination with positron emission tomography (PET). But MRI does not produce data, which can be directly used for attenuation correction of PET data, potentially compromising quantitative accuracy of PET. Hybrid-positron emission tomography/computed tomography (PET/CT) is an established standard diagnostic tool, particularly for staging and restaging in oncology. Attenuation correction of PET data is performed with a µMAP derived from low-dose-CT, considered as a robust method. Using standardized MRI-sequences, tissue classes are segmented and attenuation maps are obtained, based on empirical density values. In addition, new reconstruction algorithms and the possibility to acquire PET and MRI simultaneously with MRI-based motion correction are available. These advances have improved image quality and quantitative accuracy of the PET-data in PET/MRI. In numerous oncological studies PET/CT and PET/MR were rated as equal in their diagnostic performance. The combination of functional-metabolic PET and multiparametric MRI with excellent soft tissue contrast complement each other with regard to their diagnostic information in numerous tumor entities. The standard diagnostic workup for lung cancer is currently still based on PET/CT. In numerous tumor entities, the combination of PET/MRI can provide additional diagnostic information.

Single STE-MR Acquisition in MR-Based Attenuation Correction of Brain PET Imaging Employing a Fully Automated and Reproducible Level-Set Segmentation Approach.

The aim of this study is to introduce a fully automatic and reproducible short echo-time (STE) magnetic resonance imaging (MRI) segmentation approach for MR-based attenuation correction of positron emission tomography (PET) data in head region. Single STE-MR imaging was followed by generating attenuation correction maps (μ-maps) through exploiting an automated clustering-based level-set segmentation approach to classify head images into three regions of cortical bone, air, and soft tissue. Quantitative assessment was performed by comparing the STE-derived region classes with the corresponding regions extracted from X-ray computed tomography (CT) images. The proposed segmentation method returned accuracy and specificity values of over 90% for cortical bone, air, and soft tissue regions. The MR- and CT-derived μ-maps were compared by quantitative histogram analysis. The results suggest that the proposed automated segmentation approach can reliably discriminate bony structures from the proximal air and soft tissue in single STE-MR images, which is suitable for generating MR-based μ-maps for attenuation correction of PET data.

Generation of a Four-Class Attenuation Map for MRI-Based Attenuation Correction of PET Data in the Head Area Using a Novel Combination of STE/Dixon-MRI and FCM Clustering.

The aim of this study is to generate a four-class magnetic resonance imaging (MRI)-based attenuation map (μ-map) for attenuation correction of positron emission tomography (PET) data of the head area using a novel combination of short echo time (STE)/Dixon-MRI and a dedicated image segmentation method. MR images of the head area were acquired using STE and two-point Dixon sequences. μ-maps were derived from MRI images based on a fuzzy C-means (FCM) clustering method along with morphologic operations. Quantitative assessment was performed to evaluate generated MRI-based μ-maps compared to X-ray computed tomography (CT)-based μ-maps. The voxel-by-voxel comparison of MR-based and CT-based segmentation results yielded an average of more than 95% for accuracy and specificity in the cortical bone, soft tissue, and air region. MRI-based μ-maps show a high correlation with those derived from CT scans (R (2) > 0.95). Results indicate that STE/Dixon-MRI data in combination with FCM-based segmentation yields precise MR-based μ-maps for PET attenuation correction in hybrid PET/MRI systems.

Integrated Whole-Body PET/MR Hybrid Imaging

Integrated whole-body positron emission tomography (PET)/magnetic resonance (MR) scanners have recently been introduced and potentially offer new possibilities in hybrid imaging of oncologic patients. Integration of PET in a whole-body MR system requires new PET detector technology and new approaches to attenuation correction of PET data based on MR imaging. The aim of this study was to evaluate the clinical performance and image quality parameters of integrated whole-body PET/MR hybrid imaging in intraindividual comparison with PET/CT in oncologic patients. Eighty patients underwent a single-injection, dual-imaging protocol including whole-body PET/computed tomography (CT) and subsequent whole-body PET/MR hybrid imaging. Positron emission tomography/computed tomography was performed after adequate resting time (73 ± 13 minutes post injectionem of 227 ± 52.7 MBq Fluor-18-Fluordesoxyglucose, 3 minutes of acquisition time for each of 7 bed positions), followed by PET/MR (172 ± 33 minutes post injectionem, 10 minutes acquisition time for each of 4 bed positions). Positron emission tomographic data for both modalities were reconstructed iteratively. Two observers evaluated the following parameters: qualitative correlation of tracer-avid lesions in PET/CT versus PET/MR and PET image quality of PET/CT versus PET/MR. Magnetic resonance image quality of standard sequences (T1-weighted, T2-weighted), performance of the Dixon sequence for MR-based attenuation correction in comparison with corresponding T1-weighted images, artifacts in PET/MR data, and spatial coregistration of PET and MR data were evaluated by another observer. In 70 of the 80 patients, both image data sets were complete. In these patients, 192 tracer-avid lesions were identified on PET/CT; 195, on PET/MR. A total of 187 lesions were identified concordantly by both modalities, and this corresponds to an agreement rate of 97.4%. The overall PET image quality was rated good to excellent for PET from PET/CT (12/28, excellent, 42.9%; 16/28, good, 57.1%; 0/28, poor, 0.0%) and slightly superior compared with PET from PET/MR, which was rated good (3/28, excellent, 10.7%; 20/28, good, 71.4%; 5/28, poor, 17.9%) in a subset of 28 patients. The overall image quality of the MR image data sets in all 70 of the 80 patients was rated excellent (260/280, excellent, 92.8%; 15/280, good, 5.4%; 5/280, poor, 1.8%). The Dixon sequence and conversion to μ-maps for MR-based attenuation correction provided robust tissue segmentation in all 280 bed positions of the acquired PET/MR data. No artifacts such as elevated noise and radiofrequency disturbances related to hardware cross talk between the PET and MR components in the hybrid system could be detected in the MR images. No major spatial mismatches between PET and MR data were detected. Integrated PET/MR hybrid imaging is feasible in a clinical setting with similar detection rates as those of PET/CT. Attenuation correction can be performed sufficiently with Dixon sequences, although bone is disregarded. The administration of specific radiotracers and dedicated imaging sequences will foster this hybrid imaging modality in various indications.

Generation of attenuation map for MR-based attenuation correction of PET data in the head area employing 3D short echo time MR imaging

Attenuation correction is a crucial step to get accurate quantification of Positron Emission Tomography (PET) data. An attenuation map to provide attenuation coefficients at 511keV can be generated using Magnetic Resonance Images (MRI). One of the main steps involved in MR-based attenuation correction (MRAC) of PET data is to separate bone from air. Low signal intensity of bone in conventional MRI makes it difficult to separate bone from air in the head area, while their attenuation coefficients are very different. In literature, several groups proposed ultrashort echo-time (UTE) sequences to differentiate bone from air [4,5,7], because these sequences are capable of imaging tissues with short T2⁎ relaxation time, such as cortical bone; however, they are difficult to use, expensive and time-consuming. Employing short echo-time (STE) MRI in combination with long echo-time (LTE) MRI, and along with high performance image processing algorithms is a good substitute for UTE-based PET attenuation correction; they are widely available, easy to use, inexpensive and much faster than UTE pulse sequences. In this work, we propose the use of STE sequences along with LTE ones, as well as a dedicated image processing method to differentiate bone from air cavities in the head area by creating contrast between the tissues. Attenuation coefficients at 511kev, relying on literature [5], will then be assigned to the voxels. Acquisition was performed on a clinical 3T Tim Trio scanner (Siemens Medical Solution, Erlangen, Germany), employing a dual echo sequence. To achieve an optimized protocol with the best result for discrimination of bone and air, two types of acquisitions were performed, with and without fat suppression; the acquisition parameters were as follows: TE=1.21/5ms, TR=5/17, FA=30, and TE=1.12/3.16ms, TR=5/5, FA=12 for non-fat-suppressed and fat-suppressed protocol, respectively. Contrast enhancement and tissue segmentation were applied as processing steps, to successfully classify voxels into bone, air and soft tissue classes, yielding accuracy, sensitivity and specificity of 88%, 77% and 94%, for non-fat suppressed acquisition, respectively. This method could potentially be as an efficient method for generation of attenuation map in 511keV for MR-based attenuation correction of PET data in clinical PET/MR applications with mixed air and bone signals.

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

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.

Coronary calcium score scan-based attenuation correction in cardiovascular PET imaging

Cardiac positron emission tomography (PET)/CT imaging is a noninvasive procedure allowing the assessment of coronary artery disease (CAD). CT-based attenuation correction of PET data is essential for accurate quantitative analysis in PET/CT imaging. Coronary artery calcium scoring CT (CaScCT) is used as a noninvasive tool for the diagnosis of atherosclerosis in patients with medium risk for CAD. In addition to the CaScCT examination, current cardiac rest/stress NH3 or ¹⁸F-fluorodeoxyglucose viability PET/CT protocols incorporate a correlated low-dose CT scan for attenuation correction purposes (ACCT). As a result, the patient receives a non-negligible radiation dose. The aim of this study is to evaluate the possibility of using CaScCT images for AC of myocardial rest/stress/viability PET data with the aim of reducing patient dose. Since in cardiac PET/CT protocols, the CaScCT examination is usually reconstructed using a small field-of-view, the CaScCT data were reconstructed again with extended field-of-view (ExCaScCT) and used for AC of the corresponding PET data. The feasibility study was performed using 10 patients including four NH3 perfusion and six ¹⁸F-fluorodeoxyglucose viability examinations acquired on the Biograph TP 64 PET/CT scanner. The assessment of PET images corrected using both ACCT and ExCaScCT images was carried out through qualitative assessment performed by an expert nuclear medicine specialist in addition to the regression analysis and the Bland-Altman plots, and 20-segment myocardial bull's eye view analysis. Despite the good agreement between PET images corrected using ACCT and ExCaScCT images as expressed by the correlation coefficient and slope of the regression line in viability (0.949 ± 0.041 and 0.994 ± 0.124) and stress perfusion examinations (0.944 ± 0.008 and 0.968 ± 0.055), the rest perfusion examinations had weak correlation (0.454 ± 0.203 and 0.757 ± 0.193). This is attributed to the fact that the CaScCT scan is performed immediately after the stress/viability ACCT in our protocol that leads to a small misalignment between the CaScCT and stress/viability ACCT images, whereas there is a large misalignment between the CaScCT and rest ACCT images. The bull's eye view analysis showed that the difference between the uptake values was larger in the inferior wall because of diaphragm motion. Our preliminary results seem to suggest that the calcium score study could be used for attenuation correction of cardiac PET images, thus allowing the elimination of ACCT in viability and stress perfusion studies and as such reduce patient dose.