Data-driven respiratory gating methods eliminate the requirement for hardware based devices used to respiratory gate PET studies, but few approaches can reliably extend to acquisitions greater than a single axial field of view. A fundamental limitation of many data-driven methods lies in the inherently arbitrary relationship between signal polarity and the physical direction of motion at different axial locations. We have produced a data-driven gating method which exploits continuous bed motion to overcome this issue. Listmode time-of-flight PET data was converted to a time series of spatially filtered histo-projection volumes, and an initial estimate of the respiratory signal was obtained by calculating the time-varying anterior-posterior (AP) displacement. The full acquisition range was then divided into a series of overlapping short axial regions and processed with a data-driven gating method based on spectral analysis, initialized with spectral information from the AP signal. An optimization process was used to combine the axial regions and produce a consistent relationship between the physical direction of motion and the respiratory signal polarity throughout the acquisition range. To produce gated images with axially uniform noise, an adaptive gating methodology was implemented to correct for temporal variations in the respiratory signal characteristics. We analyzed 86 patient acquisitions, and both methods produced similar results between the bladder and the aortic arch, with an average correlation between data-driven and hardware signals of 0.81 (+/- 0.1). Low correlation was frequently found in regions where little or no motion was present. In some cases where low or negative correlation was found, a larger extent of upper lung respiratory motion was identified in images gated with the data-driven signal, suggesting that hardware signals can potentially be less accurate in regions axially distal to the device itself.

PET/MR is an emerging hybrid imaging modality. However, attenuation correction (AC) remains challenging for hybrid PET/MR in generating accurate PET images. Segmentation-based methods on special MR sequences are most widely recommended by vendors. However, their accuracy is usually not high. Individual refinement of available certified attenuation maps may be helpful for further clinical applications. In this study, we proposed a multi-resolution regional learning (MRRL) scheme to utilize the internal consistency of the patient data. The anatomical and AC MR sequences of the same subject were employed to guide the refinement of the provided AC maps. The developed algorithm was tested on 9 patients scanned consecutively with PET/MR and PET/CT (7 [18F]FDG and 2 [18F]FET). The preliminary results showed that MRRL can improve the accuracy of segmented attenuation maps and consequently the accuracy of PET reconstructions.

UNLABELLED:Integrated whole-body PET/MR facilitates the implementation of a broad variety of respiratory motion correction strategies, taking advantage of the strengths of both modalities. The goal of this study was the quantitative evaluation with clinical data of different MR- and PET-data-based motion correction strategies for integrated PET/MR. METHODS:The PET and MR data of 20 patients were simultaneously acquired for 10 min on an integrated PET/MR system after administration of (18)F-FDG or (68)Ga-DOTANOC. Respiratory traces recorded with a bellows were compared against MR self-gating signals and signals extracted from PET raw data with the sensitivity method, by applying principal component analysis (PCA) or Laplacian eigenmaps and by using a novel variation combining the former and either of the latter two. Gated sinograms and MR images were generated accordingly, followed by image registration to derive MR motion models. Corrected PET images were reconstructed by incorporating this information into the reconstruction. An optical flow algorithm was applied for PET-based motion correction. Gating and motion correction were evaluated by quantitative analysis of apparent tracer uptake, lesion volume, displacement, contrast, and signal-to-noise ratio. RESULTS:The correlation between bellows- and MR-based signals was 0.63 ± 0.19, and that between MR and the sensitivity method was 0.52 ± 0.26. Depending on the PET raw-data compression, the average correlation between MR and PCA ranged from 0.25 ± 0.30 to 0.58 ± 0.33, and the range was 0.25 ± 0.30 to 0.42 ± 0.34 if Laplacian eigenmaps were applied. By combining the sensitivity method and PCA or Laplacian eigenmaps, the maximum average correlation to MR could be increased to 0.74 ± 0.21 and 0.70 ± 0.19, respectively. The selection of the best PET-based signal for each patient yielded an average correlation of 0.80 ± 0.13 with MR. Using the best PET-based respiratory signal for gating, mean tracer uptake increased by 17 ± 19% for gating, 13 ± 10% for MR-based motion correction, and 18 ± 15% for PET-based motion correction, compared with the static images. Lesion volumes were 76 ± 31%, 83 ± 18%, and 74 ± 22% of the sizes in the static images for gating, MR-based motion correction, and PET-based motion correction, respectively. CONCLUSION:Respiratory traces extracted from MR and PET data are comparable to those based on external sensors. The proposed PET-driven gating method improved respiratory signals and overall stability. Consistent results from MR- and PET-based correction methods enable more flexible PET/MR scan protocols while achieving higher PET image quality.

Our objective was to compare the quality and diagnostic performance of 18F-fluoride PET/MR imaging with that of 18F-fluoride PET/CT imaging in patients with foot pain of unclear cause. Methods: Twenty-two patients (9 men, 13 women; mean age, 48 ± 18 y; range, 20–78 y) were prospectively included in this study and underwent a single-injection dual-imaging protocol with 18F-fluoride PET/CT and PET/MR. At a minimum, the PET/MR protocol included T1-weighted spin echo and proton-density fat-saturated sequences in 2 planes each with simultaneous acquisition of PET over 20 min. PET/CT included a native isotropic (0.6 mm) diagnostic CT scan (80 kV, 165 mAs) and a subsequent PET scan (2 min per bed position). By consensus, 2 masked interpreters randomly assessed both PET datasets for image quality (3-point scale) and for the presence of focal lesions with increased 18F-fluoride uptake (maximum of 4 lesions). For each dataset (PET/CT vs. PET/MR), the diagnoses were defined using both PET and a morphologic dataset. Standardized uptake values (SUVs) from the 2 devices were compared using linear correlation and Bland–Altman plots. Moreover, we estimated the potential for dose reduction for PET/MR compared with PET/CT considering the longer acquisition time of PET/MR analyzing count rate statistics. Results: Image quality was rated diagnostic for both PET datasets. However, with a mean rating of 3.0/3 for PET/MR and 2.3/3 for PET/CT, image quality was significantly superior for PET/MR (P < 0.0001). The sensitivity of the PET datasets in PET/MR and PET/CT was equivalent, with the same 42 lesions showing focal 18F-fluoride uptake. In PET/MR, the mean SUVmean was 10.4 (range, 2.0–67.7) and the mean SUVmax was 15.6 (range, 2.9–94.1). In PET/CT, the corresponding mean SUVmean of PET/CT was 10.2 (range, 1.8–55.6) and the mean SUVmax was 16.3 (range, 2.5–117.5), resulting in a high linear correlation coefficient (r = 0.96, P < 0.0001, for SUVmean and for SUVmax). A final consensus interpretation revealed the most frequent main diagnoses to be osteoarthritis, stress fracture, and bone marrow edema. PET/CT was more precise in visualizing osteoarthritis, whereas PET/MR was more specific in nondegenerative pathologies because of the higher soft-tissue and bone marrow contrast. The longer acquisition time of MR compared with CT would potentially allow 18F-fluoride dose reduction using hybrid 18F-fluoride PET/MR imaging of at least 50% according to the counting rate analysis. Conclusion: In patients with foot pain of unclear cause, 18F-fluoride PET/MR is technically feasible and is more robust in terms of image quality and SUV quantification than 18F-fluoride PET/CT. In most patients, 18F-fluoride PET/MR provided more diagnostic information at a higher diagnostic certainty than did PET/CT. Thus, PET/MR combines the high sensitivity of 18F-fluoride PET to pinpoint areas with the dominant disease activity and the specificity of MR imaging for the final diagnosis with the potential for a substantial dose reduction compared with PET/CT.

Ga-68 Prostate Specific Membrane Antigen also known as PSMA is currently used in prostate cancer PET imaging. The resulting images show high uptakes in kidney and bladder which could produce a photopenic artifact (halo) and potentially mask tumor lesions or bone metastasis at the level of kidney or bladder. The measured contrasts between these organs and background could be as high as 200:1 and 50:1 for kidney and bladder respectively. The correct quantification in these areas requires precise scatter correction which needs to account for the effect of prompt gamma. Ga-68 has a prompt gamma at 1077 keV with a branching ratio of 3.2%. An unscattered prompt gamma ray of 1077 keV in the object has a small probability to be detected. An object scattered prompt gamma has a higher detection probability. When the contrast is low, more accurate quantification can be achieved. On the contrary, when the contrast is very high, halo artifact can be observed around high uptake organs. The purpose of this work is to evaluate the effect of Ga-68 prompt gamma in clinical PSMA studies. The halo artifact around kidney and bladder is strongly reduced by applying a Prompt Gamma Correction. Selected studies were performed on a Siemens mCT and acquired by Technische Universität München, Germany.

Accurate localization and uptake-quantification of lesions in the chest and abdomen using PET imaging is challenged by respiratory motion occurring during the exam. This work describes how a stack-of-stars MRI acquisition on integrated PET/MRI systems can be used to derive a high-resolution motion model, how many respiratory phases need to be differentiated, how much MRI scan time is required, and how the model is employed for motion-corrected PET reconstruction. MRI self-gating is applied to perform respiratory gating of the MRI data and simultaneously acquired PET raw data. After gated PET reconstruction, the MRI motion model is used to fuse the individual gates into a single, motion-compensated volume with high signal-to-noise ratio (SNR). The proposed method is evaluated in vivo for 15 clinical patients. The gating requires 5-7 bins to capture the motion to an average accuracy of 2 mm. With 5 bins, the motion-modeling scan can be shortened to 3-4 min. The motion-compensated reconstructions show significantly higher accuracy in lesion quantification in terms of standardized uptake value (SUV) and different measures of lesion contrast compared to ungated PET reconstruction. Furthermore, unlike gated reconstructions, the motion-compensated reconstruction does not lead to SNR loss. (C) 2014 Elsevier B.V. All rights reserved.

Technologic specifications of recently introduced integrated PET/MR instrumentation, such as MR-based attenuation correction, may particularly affect brain imaging procedures. To evaluate the qualitative performance of PET/MR in clinical neuroimaging, we systematically compared results obtained with integrated PET/MR with conventional PET/CT in the same patients examined for assessment of cognitive impairment. Methods: Thirty patients underwent a single-injection (F-18-FDG), dual-imaging protocol including PET/CT and integrated PET/MR imaging in randomized order. Attenuation and scatter correction were performed using low-dose CT for the PET/CT and segmented Dixon MR imaging data for the PET/MR. Differences between PET/MR and PET/CT were assessed via region-of-interest (ROI) based and voxel-based statistical group comparison. Analyses involved attenuation-corrected (AC) and non attenuation-corrected (NAC) data. Individual PET/MR and PET/CT datasets were compared versus a predefined independent control population, using 3-dimensional stereotactic surface projections. Results: Generally, lower measured PET signal values were obtained throughout the brain in ROI-based quantification of the PET signal for PET/MR as compared with PET/CT in AC and NAC data, independently of the scan order. After elimination of global effects, voxel-based and ROI-based group comparison still revealed significantly lower relative tracer signal in PET/MR images in frontoparietal portions of the neocortex but significantly higher relative signal in subcortical and basal regions of the brain than the corresponding PET/CT images of the AC data. In the corresponding NAC images, the discrepancies in frontoparietal portions of the neocortex were diminished, but the subcortical overestimation of tracer intensity by PET/MR persisted. Conclusion: Considerable region-dependent differences were observed between brain imaging data acquired on the PET/MR, compared with corresponding PET/CT images, in patients evaluated for neurodegenerative disorders. These findings may only in part be explained by inconsistencies in the attenuation-correction procedures. The observed differences may interfere with semiquantitative evaluation and with individual qualitative clinical assessment and they need to be considered, for example, for clinical trials. Improved attenuation-correction algorithms and a PET/MR-specific healthy control database are recommended for reliable and consistent application of PET/MR for clinical neuroimaging.

150 Objectives This study introduces a new motion correction technique, which combines cardiac ECG gating and bioimpedance-based respiratory gating in cardiac PET. Methods Cardiac PET/CT studies were conducted with 12 patients in one bed position (10min, listmode). ECG and transthoracic bioimpedance were measured simultaneously using standard ECG locations (II limb lead). The bioimpedance measurement was used to monitor electrical conductivity variation in thorax due to changes in respiratory depth and thorax shape and allowed the delineation of direct respiratory motion [1-3]. Attenuation corrected static (ST), end-expiratory (RE), end-diastolic (CA) and dual-gated (end-expiratory and -diastolic, DUAL) images were reconstructed (3D-OSEM, 3i21s, no filter). To evaluate the effect of gating, images were analyzed for ventricle volume and lateral wall thickness (FWHM) basal to papillary muscles. Results Respiratory and cardiac gating were successful in all patients. In ventricle volume analysis (7 patients) the median difference between ST vs. RE, ST vs. CA, ST vs. DUAL and CA vs. DUAL images were 3ml, 24ml, 30ml and 1ml, respectively. Respiratory gating resulted in small wall thickness reduction (median 0.5mm, NS, Wilcoxon) compared to ST images. Dual-gating reduced wall thickness significantly (5.2mm, p=0.002). Wall thickness reduction from CA to DUAL was small (0.4mm, NS), but reached 4.6mm in one patient. Conclusions This study shows the feasibility of a new dual-gating method for cardiac PET. The technique is easily adoptable and enables simultaneous cardiac and respiratory gating using only 4 electrodes. In addition, the volume analysis of this study implies that the advantage of dual-gating over cardiac gating may be limited in measurements of global cardiac parameters. Based on relevant thinning of wall thickness in single patients in this study, dual-gating may offer increased signals in imaging of certain molecular targets, such as in myocardial inflammations.

Published online: December 5, 2013. 2014;55:191-197. J Nucl Med. Denys J. Loeffelbein, Ernst J. Rummeny, Sibylle I. Ziegler, Markus Schwaiger and Ambros J. Beer Matthias Eiber, Toshiki Takei, Michael Souvatzoglou, Marius E. Mayerhoefer, Sebastian Fürst, Florian C. Gaertner, for Evaluation of Malignant Bone Lesions F-FDG PET/MR in Comparison to PET/CT 18 Performance of Whole-Body Integrated http://jnm.snmjournals.org/content/55/2/191 This article and updated information are available at: http://jnm.snmjournals.org/site/subscriptions/online.xhtml Information about subscriptions to JNM can be found at: http://jnm.snmjournals.org/site/misc/permission.xhtml Information about reproducing figures, tables, or other portions of this article can be found online at:

Fully integrated PET/MR imaging holds great promise as a novel hybrid imaging modality in oncology and might offer advantages to PET/CT in many instances, especially because of the superior soft-tissue contrast of MR imaging, compared with CT. However, lung metastases are a frequent finding in oncologic patients, and for imaging of the lung CT is still the modality of choice. Thus, we prospectively evaluated differences in quality, detection rate, size, and radiotracer uptake of pulmonary lesions in 18F-FDG PET/CT and PET/MR imaging. Methods: Institutional review board approval and informed consent were obtained. Forty patients (23 men, 17 women; mean age ± SD, 53.2 ± 13.1 y) underwent a single-injection dual-imaging protocol with 18F-FDG PET/CT and PET/MR imaging. Pulse sequences for the lung included T1-weighted VIBE (volumetric interpolated breath-hold examination) Dixon for attenuation correction and contrast-enhanced VIBE pulse sequences. All patients underwent a diagnostic CT of the chest in deep inspiration, which also served as a standard of reference. Two masked readers assessed in consensus all images randomly concerning quality, detection, standardized uptake value (SUV), and size of pulmonary nodules. Correlations were performed using linear correlation. Results: Overall, 47 pulmonary lesions (mean size ± SD, 10.0 ± 11.4 mm; range, 2–60 mm) in 25 of 40 patients were detected. The PET datasets of PET/MR imaging and PET/CT revealed 22 of 47 pulmonary lesions with focal 18F-FDG uptake. SUVs of lung lesions in PET/MR imaging and PET/CT correlated significantly (R = 0.9; P = 0.0001) and showed no significant difference (mean SUV PET/MR imaging, 6.3; PET/CT, 5.1; P = 0.388). There was a significantly lower image quality comparing Dixon and VIBE sequence with CT whereas PET from PET/CT and PET from PET/MR imaging showed the same results (2.8). Dixon images detected 15 of 47 lung lesions whereas VIBE images detected 32 of 47 lesions, respectively. The detection rates for small lung lesions less than 1 cm in diameter (n = 33) of MR imaging was significantly lower, with a detection rate of 9 of 33 for the Dixon sequence and 15 of 33 for the VIBE sequence (P < 0.0001 for VIBE and Dixon sequence). There was a high correlation of pulmonary lesion size of CT versus VIBE (R = 0.97). Conclusion: PET image quality and detection rate of 18F-FDG–positive lung lesions in PET/MR imaging is equivalent to PET/CT despite differences in attenuation-correction techniques. Additionally, a high linear correlation coefficient in the SUVs for the PET images from PET/CT and PET/MR imaging was found. The detection rate of lung lesions can be significantly improved by adding a diagnostic contrast-enhanced VIBE sequence to the PET/MR imaging protocol. However, the detection rate of small lung lesions is still inferior, compared with PET/CT with diagnostic CT of the chest.

We describe a new MR-based attenuation correction (MRAC) method for neurological studies performed using integrated PET/MR scanners. The method, combining the advantages of image segmentation and atlas-based approaches to generate a high-resolution template, is based on the widely available SPM8 software and provides robust and accurate linear attenuation coefficients (LACs) for head while requiring minimal user interaction. Atlas generation: 3T MR and CT images from 15 glioblastoma subjects were used to generate the high-resolution atlas. MR images were segmented into 6 tissue classes: GM, WM, CSF, soft tissue, bone and air)[1]. Tissue classes were then coregistered using an iterative diffeomorphic image registration algorithm [2] to form the template. Atlas validation: The template was validated on 16 subjects. SyN [3] and IRTK [4], considered state-of-the-art for non-rigid image registration[5], were used for comparison. Final attenuation maps were created from the warped CT atlas following [6]. PET images were then reconstructed using the proposed methods as well as the manufacturer’s built-in method (dual-echo Dixon-VIBE sequence) [7] and compared to the gold standard CT-based attenuation correction (CTAC). The qualitative and quantitative analysis of the attenuation maps revealed that the SPM8-based method produces very robust results (Figure ​(Figure1).1). In terms of the PET data quantification, we observed improvements of > 70% compared to the VIBE-based method (Table ​(Table11 and Figure ​Figure2).2). When compared to SyN-based image registration, the SPM8 approach showed improved global results on the brain area (Figures ​(Figures11 and ​and22). Figure 1 Comparison of LACs from a validation subject for our proposed method (A), the SyN method (B) and the manufacturer’s built-in Dixon method (C) to the gold standard CTAC (D). Image differences with respect to the gold standard CTAC of our method ... Table 1 Summary of voxel- and ROI-based results between our method (atlas) and the current manufacturer’s method (Dixon) Figure 2 PET images from a validation subject reconstructed with our proposed method (A), with the SyN method (B) and with the manufacturer’s built-in Dixon method (C), compared with the gold standard CTAC (D). Relative changes (in % with respect to gold ... We presented a new MRAC technique for brain images acquired on simultaneous PET/MR scanners. The new approach relies on segmentation- and atlas-based features to provide robust and more accurate LACs than using state-of-art non-rigid image registration while avoiding sophisticated user input or interaction.

We present an approach for head MR-based attenuation correction (AC) based on the Statistical Parametric Mapping 8 (SPM8) software, which combines segmentation- and atlas-based features to provide a robust technique to generate attenuation maps (mu maps) from MR data in integrated PET/MR scanners. Methods: Coregistered anatomic MR and CT images of 15 glioblastoma subjects were used to generate the templates. The MR images from these subjects were first segmented into 6 tissue classes (gray matter, white matter, cerebrospinal fluid, bone, soft tissue, and air), which were then nonrigidly coregistered using a diffeomorphic approach. A similar procedure was used to coregister the anatomic MR data for a new subject to the template. Finally, the CT-like images obtained by applying the inverse transformations were converted to linear attenuation coefficients to be used for AC of PET data. The method was validated on 16 new subjects with brain tumors (n = 12) or mild cognitive impairment (n = 4) who underwent CT and PET/MR scans. The p maps and corresponding reconstructed PET images were compared with those obtained using the gold standard CT-based approach and the Dixon-based method available on the Biograph mMR scanner. Relative change (RC) images were generated in each case, and voxel- and regionof-interest based analyses were performed. Results: The leave-one-out cross-validation analysis of the data from the 15 atlas-generation subjects showed small errors in brain linear attenuation coefficients (RC, 1.38% +/- 4.52%) compared with the gold standard. Similar results (RC, 1.86% +/- 4.06%) were obtained from the analysis of the atlas-validation datasets. The voxel- and region-of-interest based analysis of the corresponding reconstructed PET images revealed quantification errors of 3.87% +/- 5.0% and 2.74% +/- 2.28%, respectively. The Dixon-based method performed substantially worse (the mean RC values were 13.0% +/- 10.25% and 9.38% +/- 4.97%, respectively). Areas closer to the skull showed the largest improvement. Conclusion: We have presented an SPM8-based approach for deriving the head p map from MR data to be used for PET AC in integrated PET/MR scanners. Its implementation is straightforward and requires only the morphologic data acquired with a single MR sequence. The method is accurate and robust, combining the strengths of both segmentation- and atlas-based approaches while minimizing their drawbacks.

Because of its higher soft-tissue contrast, whole-body integrated PET/MR offers potential advantages over PET/CT for evaluation of bone lesions. However, unlike PET/CT, PET/MR ignores the contribution of cortical bone in the attenuation map. Thus, the aims of this study were to evaluate the diagnostic performance of whole-body integrated 18F-FDG PET/MR specifically for bone lesions and to analyze differences in standardized uptake value (SUV) quantification between PET/MR and PET/CT. Methods: One hundred nineteen patients with 18F-FDG–avid primary malignancies underwent a single-injection, dual-imaging protocol using 18F-FDG on a PET/CT scanner and a subsequent PET/MR scan with a T1-weighted volumetric interpolated breath-hold examination (VIBE) Dixon sequence for attenuation correction and an unenhanced coronal T1-weighted turbo spin-echo (TSE) sequence for bone analysis. Three sets of images (CT with PET [from PET/CT; set A], T1-weighted VIBE Dixon with PET [set B], and T1-weighted TSE with PET [both from PET/MR; set C]) were analyzed. Two readers rated every lesion using a 4-point scale for lesion conspicuity on PET, a 4-point scale for anatomic allocation of PET-positive lesions, and a 5-point scale for the nature of every lesion based on its appearance on morphologic imaging and uptake on PET. For all lesions and for representative regions of normal bone, SUV analysis was performed for PET/MR and PET/CT. Results: In total, 98 bone lesions were identified in 33 of 119 patients, and 630 regions of normal bone were analyzed. Visual lesion conspicuity on PET was comparable for PET/CT (mean rating, 2.82 ± 0.45) and PET/MR (2.75 ± 0.51; P = 0.3095). Anatomic delineation and allocation of suggestive lesions was significantly superior with T1-weighted TSE MRI (mean rating, 2.84 ± 0.42) compared with CT (2.57 ± 0.54, P = 0.0001) or T1-weighted VIBE Dixon MRI (2.57 ± 0.54, P = 0.0002). No significant difference in correct classification of malignant bone lesions was found among sets A (85/90), B (84/90), and C (86/90). For bone lesions and regions of normal bone, a highly significant correlation existed between the mean SUVs for PET/MR and PET/CT (R = 0.950 and 0.917, respectively, each P < 0.001). However, substantially lower mean SUVs were found for PET/MR than for PET/CT both for bone lesions (12.4% ± 15.5%) and for regions of normal bone (30.1% ± 27.5%). Conclusion: Compared with PET/CT, fully integrated whole-body 18F-FDG PET/MR is technically and clinically robust for evaluation of bone lesions despite differences in attenuation correction. PET/MR, including diagnostic T1-weighted TSE sequences, was superior to PET/CT for anatomic delineation and allocation of bone lesions. This finding might be of clinical relevance in selected cases—for example, primary bone tumors, early bone marrow infiltration, and tumors with low uptake on PET. Thus, a diagnostic T1-weighted TSE sequence is recommended as a routine protocol for oncologic PET/MR.

Respiratory motion may degrade image quality in cardiac PET imaging. Since cardiac PET studies often involve cardiac gating by ECG, a separate respiratory monitoring system is required increasing the logistic complexity of the examination, in case respiratory gating is also needed. Thus, we investigated the simultaneous acquisition of both respiratory and cardiac gating signals using II limb lead mimicking electrode configuration during cardiac PET scans of 11 patients. In addition to conventional static and ECG-gated images, bioimpedance technique was utilized to generate respiratory- and dual-gated images. The ability of the bioimpedance technique to monitor intrathoracic respiratory motion was assessed estimating cardiac displacement between end-inspiration and -expiration. The relevance of dual gating was evaluated in left ventricular volume and myocardial wall thickness measurements. An average 7.6  ±  3.3 mm respiratory motion was observed in the study population. Dual gating showed a small but significant increase (4 ml, p = 0.042) in left ventricular myocardial volume compared to plain cardiac gating. In addition, a thinner myocardial wall was observed in dual-gated images (9.3  ±  1.3 mm) compared to cardiac-gated images (11.3  ±  1.3 mm, p = 0.003). This study shows the feasibility of bioimpedance measurements for dual gating in a clinical setting. The method enables simultaneous acquisition of respiratory and cardiac gating signals using a single device with standard ECG electrodes.

The Motion in PET studies degrades image quality and introduces bias and partial volume artifacts, which are critical considerations for high resolution scanners. There are two kinds of motion, such as rigid (e.g. brain) and nonrigid (e.g. respiratory and cardiac). Elastic motion correction is needed for nonrigid-motion artifacts. There are three basic steps in this approach, acquisition of a gating signal, extraction of elastic motion, and reconstruction. First, the gating signal is acquired by hardware, such as EKG, Belt, RPM, and MR, or from the analysis PET list-mode data. This is the most important step because, if the data are not properly gated, it is not possible to extract accurate motion vectors. Second, motion information for each gated signal can be extracted from CT for PET/CT or MRI for MR/PET. The motion information can also be extracted from the PET data themselves, and optical flow methods have been shown to be very robust in this approach. Third, image reconstruction with motion correction is commonly performed through summing gated images in a common reference frame. However, the combination of processed data with poor statistics generally results in high image noise and bias in the final image. A better approach is to incorporate motion information into the reconstruction process itself. Motion-correction reconstruction has been shown to produce less noise and bias in the image domain than conventional summing methods. The ideal method for motion correction in emission data should produce quantitatively accurate images which retain noise properties of conventional images, all while introducing no additional subject dose or inconvenience.