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Frontiers in Physics
Copyright © 2020 Siekkinen, Teuho, Smith, Fenwick, Kirjavainen, Koskensalo, Saraste and Teräs. ...doi:10.3389/fphy.2020.00148 doaj:244f551789234c618bd9cee64df3d3c3 fatcat:mwxwxs2gynhhxcq7v27hrhyy7i
Magnetic resonance imaging-only radiotherapy treatment planning (MRI-only RTP) and positron emission tomography (PET)–MRI imaging require generation of synthetic computed tomography (sCT) images from MRI images. In this study, initial dosimetric evaluation was performed for a previously developed MRI-based attenuation correction (MRAC) method for use in MRI-only RTP of the brain. MRAC-based sCT images were retrospectively generated from Dixon MR images of 20 patients who had previously receiveddoi:10.3390/diagnostics10050299 pmid:32422950 fatcat:bc5uok6zyvg5tayup7xqhtcgxe
more »... external beam radiation therapy (EBRT). Bone segmentation performance and Dice similarity coefficient of the sCT conversion method were evaluated for bone volumes on CT images. Dose calculation accuracy was assessed by recalculating the CT-based EBRT plans using the sCT images as the base attenuation data. Dose comparison was done for the sCT- and CT-based EBRT plans in planning target volume (PTV) and organs at risk (OAR). Parametric dose comparison showed mean relative differences of <0.4% for PTV and <1.0% for OARs. Mean gamma index pass rates of 95.7% with the 2%/2 mm agreement criterion and 96.5% with the 1%/1 mm agreement criterion were determined for glioma and metastasis patients, respectively. Based on the results, MRI-only RTP using sCT images generated from MRAC images can be a feasible alternative for radiotherapy of the brain.
Publication in the conference proceedings of EUSIPCO, Poznan, Poland, 2007doi:10.5281/zenodo.40431 fatcat:umhmfon4izb4dgkcpuapipxkre
The increasing use of PET for assessing cerebral blood flow, oxygen metabolism, and blood volume in critically ill patients has created a need for reliable technical solutions for delivering (15)O-tracer gases to mechanically ventilated subjects. Our objective was to create such a solution. We designed a ventilator add-on unit that enables complex functional brain studies using labeled oxygen and carbon monoxide gases as tracers. The unit manages both steady-state and bolus inhalations, and thepmid:15175404 fatcat:5cjylupfhjcizlpdj2l37chhma
more »... latter can be manually initiated using a remote trigger. All parts conducting breathing gases can be sterilized. The unit can be operated during both spontaneous pressure support breathing and volume-controlled ventilation. It supports the standard safety features and alarms of the ventilator and includes an overflow valve in the bolus reservoir. The count rate curves obtained using the new unit were similar to those from the standard bag-inhalation method. The unit we describe offers an economical and easily operated solution for providing uninterrupted ventilator treatment while performing PET brain studies, and the provided treatment meets intensive care criteria.
This study was performed to evaluate the feasibility of preclinical imaging in a clinical PET/MR system. Preliminary sequences were evaluated for establishing preclinical protocols for rat brain and rabbit knee. Rats were placed in a stereotactic holder, allowing a 30 minute scan time before re-administration of anesthesia. In-house developed warm-water heating system was used to maintain the body temperature at 37.5°C, monitored using an MR-compatible rectal probe. Brain imaging was performeddoi:10.1186/2197-7364-1-s1-a88 pmid:26501680 pmcid:PMC4546025 fatcat:rjkjyuycorarvaf7yd6v32h2c4
more »... ith a dedicated 4 channel phased array receive coil (RAPID Biomedical GmbH, Germany). High resolution coronal images were acquired using conventional T1-SE (0.30x0.30x1.2mm) and T2-TSE (0.23x0.23x0.7mm) with a total scan time of 30 min. PET/MR imaging was performed on two white rabbits. The rabbits were imaged in a custom wooden holder. PET/MR protocol had a total duration of 45 minutes. No external heating was used. MR protocol consisted of anatomical T1, T2 and PDW of the knees, using a SENSE Flex-S coil. MR attenuation correction (MRAC) was acquired with 3D T1-FFE using three-class segmentation. A dynamic 30 minute PET acquisition was started on injection of 33.8MBq of Ga-68. Animal coils enabled high resolution images to be acquired in reasonable acquisition time with regards to animal handling and anesthesia. T1 and T2 images provided good differentiation of anatomy in the rat brain with high contrast. T1, T2 and PDW images of the rabbit knee had high resolution and differentiation of anatomical structures. MRAC was able to distinguish the knees and the body contour. Image fusion of PET and MR was able to localize the infection, which was confirmed by a physician. Pre-clinical imaging with the Ingenuity TF was deemed feasible, although PET imaging is limited by the resolution of the scanner. The preliminary sequences were successfully implemented for future studies on the Ingenuity TF.
Quantitative accuracy of positron emission tomography (PET) is decreased by the partial volume effect (PVE). The PVE correction (PVC) methods proposed by Alfano et al., Rousset et al., Müller-Gärtner et al. and Meltzer et al. were evaluated in the present study to obtain guidelines for selecting among them. For accuracy evaluation, the Hoffman brain phantom was scanned with three PETs of differing spatial resolution in order to measure the effect of PVC on radioactivity distribution.doi:10.4103/0971-6203.35723 pmid:21157530 pmcid:PMC3000501 fatcat:u7xy7dwlirbzraqrl7osmnuqhm
more »... data consisting of duplicate dynamic emission recordings of the dopamine D2-receptor ligand [ 11 C] raclopride obtained in eight healthy control subjects were used to test the correction effect in different regions of interest. The PVC method proposed by Alfano et al. gave the best quantification accuracy in the brain gray matter region. When the effect of PVC on reliability was tested with human data, the method of Meltzer et al. proved to be the most reliable. The method by Alfano et al. may be better for group comparison studies and the method by Meltzer et al. for intra-subject drug-effect studies. Keywords: Brain imaging, partial volume effects, positron emission tomography Positron emission tomography (PET) is a medical imaging method that records the simultaneous emission of gamma rays from the point of positron-electron annihilation in living tissue. Neuroreceptors and cerebrometabolic pathways can be targeted with specific tracers which bear positron-emitting isotopes in their chemical structure. Unfortunately, despite the good quantitative capabilities of PETs, the PET imaging technique suffers from relatively poor spatial resolution, which impairs the spatial localization of the radioactivity signals and also results in the so-called partial volume effects (PVE). As a result of PVE, the quantitative accuracy of PET images is reduced. The PVE smoothes PET images so that some of the radioactivity from regions of higher concentration is mis-attributed to adjacent regions of lower activity. In brain imaging with tracers of high radioactivity in brain gray matter (GM), PVE yields lower radio activities than their real values for GM structures and higher radioactivity values for the brain white matter (WM). As a result of this, the radioactivity concentration values are differently variated in different regions of interest (ROI) of the brain. The main correlating element for PVE is the size of the image resolution, which is mainly determined by the crystal size, the ring diameter of the tomograph and the travel distance of the positron. The travel distance before annihilation degrades the resolution, and it correlates with the kinetic energy of the positron of the used tracer. Other contributing factors include the size and shape of the radioactivity source and the signal-to-noise ratio of the PET image. In addition to the physical causes of PVE, also the computational methods that are used in order to reconstruct the PET image can induce PVE in the resulting image. These include histogramming and reconstruction from coincidence raw data to sinogram data, sinogram data reconstruction to PET images and image post-filtering after reconstruction of the PET images. Various methods for partial volume effect correction (PVC) have been proposed. The method reported by Meltzer et al. corrects the radioactivity values of the conjunction of the WM and GM regions. The methods of Müller-Gärtner and Alfano et al. process the WM and GM regions separately and use an estimate of the true WM mean radioactivity when correcting the radioactivity values of the GM region. Modifications of these two methods have also been proposed. Rousset et al. have proposed a method for correcting the mean activity values of ROIs. Most methods mentioned above have been further refined in subsequent years,[6,7] and the robustness of the PVC methods against errors in their processing steps such as MRI-PET co-registration and MRI segmentation has also been considered in several studies.[8-11] The method of Meltzer et al. has been reported to be robust to errors in preprocessing and in homogeneities in the PET image. It is notable that the methods amplify the existing noise.  An increase of variance in the time-activity curves has been reported when using the method by Rousset et al.  If the noise is amplified too much, the usefulness of recovering the regional mean radioactivity values decreases. Therefore, both the effect of PVC on noise and on the accuracy of the mean values of ROIs should be evaluated. Further, when dispersion in image voxels is high, it is difficult to conclude whether small differences in images or regions originate from real differences in the studied objects, or if they are caused by the data-collecting and reconstruction processes.  The PET method is typically used in group comparison studies and intra-subject drug effect studies. The application of a PVC method is sensible if it is capable of revealing the true differences between the groups and between the scans regardless of the added noise. In the present study, testing with a phantom object was done using three different tomographs (GE Advance, GE DSTE and Siemens HRRT) to reveal differences in the noise amplification between the PVC methods proposed by Alfano et al., Rousset et al., Müller-Gärtner et al. and Meltzer et al. In total, eight methods were evaluated, as three different WM radioactivity estimation methods proposed by Alfano et al., Rousset et al. and Müller-Gärtner et al. were combined with the methods by Alfano et al. and Müller-Gärtner et al. In addition, the binding potential values with eight patients scanned on two separate occasions with the dopamine D2 receptor ligand [ 11 C ] raclopride were analyzed with four of the methods in order to reveal the reproducibility and reliability characteristics of the evaluated PVC methods. Real differences between scans are assumed to be minimal in this test-retest material. Materials and Methods Human PET studies lack explicit information on the true radioactivity levels and their distributions. Other means are therefore needed to evaluate the correction methods. Computer simulation is one way to model the imaging process, but this is not easy due to the analog radioactivity signal and due to the complicated behavior of the PET tomograph. While 3D computer simulations allow the definition of arbitrary radioactivity sources with known variable noise levels as input, they may lack the realism of the real scanning situation. They can also require huge amounts of computational resources. Alternatively, phantom objects consisting of fluid-Evaluation of partial volume effect correction methods for brain positron emission tomography... http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3000501/?report=printable spillover from the GM. Another way of doing the estimation is to use the method of Rousset et al., reviewed above (referred to as R-WME). Third, in the method proposed by Alfano et al.  (referred to as A-WME), an image calculated from convoluted mask images with the formula (X GM I PSF )/((X GM PSF )+(X WM I PSF )) is used. In the locations where the values of the image are high, the PET radioactivity values contain little of the true radioactivity from the WM region. Conversely, low values in the image indicate that PET radioactivity values in these locations consist mainly of the true radioactivity from the WM region. In A-WME, the PET radioactivity values are projected against the convoluted mask image. A regression line is then fitted to characterize PET radioactivity values and the probabilities that the radioactivity value originates from the WM region. Finally, the WM estimation is made from an interception of this line. PVC methods applied in this work are from the software developed in the 'PVE Out' project (
Dual cardiac and respiratory gating is a well-known technique for motion compensation in nuclear medicine imaging. In this study, we present a new data fusion framework for dual cardiac and respiratory gating based on multidimensional microelectromechanical (MEMS) motion sensors. Our approach aims at robust estimation of the chest vibrations, that is, high-frequency precordial vibrations and low-frequency respiratory movements for prospective gating in positron emission tomography (PET),doi:10.3390/s19194137 pmid:31554282 pmcid:PMC6811750 fatcat:777bcl2n3fbu7g54ey4cum2dza
more »... d tomography (CT), and radiotherapy. Our sensing modality in the context of this paper is a single dual sensor unit, including accelerometer and gyroscope sensors to measure chest movements in three different orientations. Since accelerometer- and gyroscope-derived respiration signals represent the inclination of the chest, they are similar in morphology and have the same units. Therefore, we use principal component analysis (PCA) to combine them into a single signal. In contrast to this, the accelerometer- and gyroscope-derived cardiac signals correspond to the translational and rotational motions of the chest, and have different waveform characteristics and units. To combine these signals, we use independent component analysis (ICA) in order to obtain the underlying cardiac motion. From this cardiac motion signal, we obtain the systolic and diastolic phases of cardiac cycles by using an adaptive multi-scale peak detector and a short-time autocorrelation function. Three groups of subjects, including healthy controls (n = 7), healthy volunteers (n = 12), and patients with a history of coronary artery disease (n = 19) were studied to establish a quantitative framework for assessing the performance of the presented work in prospective imaging applications. The results of this investigation showed a fairly strong positive correlation (average r = 0.73 to 0.87) between the MEMS-derived (including corresponding PCA fusion) respiration curves and the reference optical camera and respiration belt sensors. Additionally, the mean time offset of MEMS-driven triggers from camera-driven triggers was 0.23 to 0.3 ± 0.15 to 0.17 s. For each cardiac cycle, the feature of the MEMS signals indicating a systolic time interval was identified, and its relation to the total cardiac cycle length was also reported. The findings of this study suggest that the combination of chest angular velocity and accelerations using ICA and PCA can help to develop a robust dual cardiac and respiratory gating solution using only MEMS sensors. Therefore, the methods presented in this paper should help improve predictions of the cardiac and respiratory quiescent phases, particularly with the clinical patients. This study lays the groundwork for future research into clinical PET/CT imaging based on dual inertial sensors.
and keywords Abstract Dual gating is a method of dividing the data of a cardiac PET scan into smaller bins according to the respiratory motion and the ECG of the patient. It reduces the undesirable motion artefacts in images but produces several images for interpretation and decreases the quality of single images. By using motion correction techniques, the motion artefacts in the dual gated images can be corrected and the images can be combined into a single motion-free image with gooddoi:10.1097/mnm.0000000000000539 pmid:27258990 fatcat:2au4brxd7veqpn4pc5y5qzol74
more »... s. The aim of the present study is to develop and evaluate motion correction methods for cardiac PET studies. We have developed and compared two different methods: CT-PET-based and CTonly methods. The methods were implemented and tested with a cardiac phantom and three patient datasets. In both methods, anatomical information of CT images is used to create models for the cardiac motion. In the patient study the CT-only-method reduced motion (measured as the centre of mass of the myocardium) on average 43%, increased contrast-to-noise ratio on average 6.0% and reduced the target-size on average 10%. Slightly better figures (51%, 6.9% and 28%) were obtained with the CT-PET-based method. Even better results were obtained in the phantom study for both the CT-only-method (57%, 68% and 43%) and the CT-PETbased method (61%, 74% and 52). We conclude that using anatomical information of CT for motion correction of cardiac PET images, both respiratory and pulsatile motions can be corrected with good accuracy. 3
Laaksonen MS, Kalliokoski KK, Luotolahti M, Kemppainen J, Teräs M, Kyröläinen H, Nuutila P, Knuuti J. Myocardial perfusion during exercise in endurance-trained and untrained humans. ...doi:10.1152/ajpregu.00771.2006 pmid:17522118 fatcat:v5oxs2zcffe5legp2awrrxknz4
We investigated the effects of mobile phone radiation on cerebral glucose metabolism using high-resolution positron emission tomography (PET) with the 18 F-deoxyglucose (FDG) tracer. A long half-life (109 minutes) of the 18 F isotope allowed a long, natural exposure condition outside the PET scanner. Thirteen young right-handed male subjects were exposed to a pulse-modulated 902.4 MHz Global System for Mobile Communications signal for 33 minutes, while performing a simple visual vigilance task.doi:10.1038/jcbfm.2011.128 pmid:21915135 pmcid:PMC3323189 fatcat:nw44k5jthbgihblqgprwq7q2me
more »... Temperature was also measured in the head region (forehead, eyes, cheeks, ear canals) during exposure. 18 F-deoxyglucose PET images acquired after the exposure showed that relative cerebral metabolic rate of glucose was significantly reduced in the temporoparietal junction and anterior temporal lobe of the right hemisphere ipsilateral to the exposure. Temperature rise was also observed on the exposed side of the head, but the magnitude was very small. The exposure did not affect task performance (reaction time, error rate). Our results show that short-term mobile phone exposure can locally suppress brain energy metabolism in humans.
We studied whether TOF reduces error propagation from attenuation correction to PET image reconstruction in PET/MR neuroimaging, by using imperfect attenuation maps in a clinical PET/MR system with 525 ps timing resolution. Ten subjects who had undergone 18F-FDG PET neuroimaging were included. Attenuation maps using a single value (0.100 cm−1) with and without air, and a 3-class attenuation map with soft tissue (0.096 cm−1), air and bone (0.151 cm−1) were used. CT-based attenuation correctiondoi:10.3390/app12094605 fatcat:ud7fjveczbhsdlxumpjr4omrb4
more »... s used as a reference. Volume-of-interest (VOI) analysis was conducted. Mean bias and standard deviation across the brain was studied. Regional correlations and concordance were evaluated. Statistical testing was conducted. Average bias and standard deviation were slightly reduced in the majority (23–26 out of 35) of the VOI with TOF. Bias was reduced near the cortex, nasal sinuses, and in the mid-brain with TOF. Bland–Altman and regression analysis showed small improvements with TOF. However, the overall effect of TOF to quantitative accuracy was small (3% at maximum) and significant only for two attenuation maps out of three at 525 ps timing resolution. In conclusion, TOF might reduce the quantitative errors due to attenuation correction in PET/MR neuroimaging, but this effect needs to be further investigated on systems with better timing resolution.
Teräs provided helpful discussions and contributions for the gyrocardiography implications from the clinical points of view. M. Pänkäälä and T. ...doi:10.1038/s41598-017-07248-y pmid:28754888 pmcid:PMC5533710 fatcat:ehaa6hi6qrggxcl2r6afar3qam
Frontiers in Physics
In this review, we will summarize the past and current state-of-the-art developments in attenuation and scatter correction approaches for hybrid positron emission tomography (PET) and magnetic resonance (MR) imaging. The current status of the methodological advances for producing accurate attenuation and scatter corrections on PET/MR systems are described, in addition to emerging clinical and research applications. Future prospects and potential applications that benefit from accurate datadoi:10.3389/fphy.2019.00243 fatcat:msnyzua32nbr5ihtvseqy6ey4a
more »... ctions to improve the quantitative accuracy and clinical applicability of PET/MR are also discussed. Novel clinical and research applications where improved attenuation and scatter correction methods are beneficial are highlighted.
Objective Inflammation is an important contributor to atherosclerosis progression. A glucose analogue 18 F-fluorodeoxyglucose ([ 18 F]FDG) has been used to detect atherosclerotic inflammation. However, it is not known to what extent [ 18 F]FDG is taken up in different stages of atherosclerosis. We aimed to study the uptake of [ 18 F]FDG to various stages of coronary plaques in a pig model. Methods First, diabetes was caused by streptozotocin injections (50 mg/kg for 3 days) in farm pigs (n =doi:10.1371/journal.pone.0131332 pmid:26120829 pmcid:PMC4487365 fatcat:yj6n4f4fnnh3rjhdgxcmnhz24e
more »... . After 6 months on high-fat diet, pigs underwent dual-gated cardiac PET/CT to measure [ 18 F]FDG uptake in coronary arteries. Coronary segments (n = 33) were harvested for ex vivo measurement of radioactivity and autoradiography (ARG). Results Intimal thickening was observed in 16 segments and atheroma type plaques in 10 segments. Compared with the normal vessel wall, ARG showed 1.7±0.7 times higher [ 18 F]FDG accumulation in the intimal thickening and 4.1±2.3 times higher in the atheromas (P = 0.004 and P = 0.003, respectively). Ex vivo mean vessel-to-blood ratio was higher in segments with atheroma than those without atherosclerosis (2.6±1.2 vs. 1.3±0.7, P = 0.04). In vivo PET imaging showed the highest target-to-background ratio (TBR) of 2.7. However, maximum TBR was not significantly different in segments without atherosclerosis (1.1±0.5) and either intimal thickening (1.2±0.4, P = 1.0) or atheroma (1.6±0.6, P = 0.4). We found increased uptake of [ 18 F]FDG in coronary atherosclerotic lesions in a pig model. However, uptake in these early stage lesions was not detectable with in vivo PET imaging. Further studies are needed to clarify whether visible [ 18 F]FDG uptake in coronary arteries represents more advanced, highly inflamed plaques. [ 18 F]FDG Accumulation in Early Coronary Atherosclerotic Lesions PLOS ONE |
In PET, corrections for photon scatter and attenuation are essential for visual and quantitative consistency. MR attenuation correction (MRAC) is generally conducted by image segmentation and assignment of discrete attenuation coefficients, which offer limited accuracy compared with CT attenuation correction. Potential inaccuracies in MRAC may affect scatter correction, because the attenuation image (m-map) is used in single scatter simulation (SSS) to calculate the scatter estimate. Wedoi:10.2967/jnumed.117.190231 pmid:28336781 fatcat:ncd43qerivedffbvvvhkfpx2ee
more »... the impact of MRAC to scatter correction using 2 scatter-correction techniques and 3 m-maps for MRAC. Methods: The tail-fitted SSS (TF-SSS) and a Monte Carlo-based single scatter simulation (MC-SSS) algorithm implementations on the Philips Ingenuity TF PET/MR were used with 1 CT-based and 2 MR-based m-maps. Data from 7 subjects were used in the clinical evaluation, and a phantom study using an anatomic brain phantom was conducted. Scatter-correction sinograms were evaluated for each scatter correction method and m-map. Absolute image quantification was investigated with the phantom data. Quantitative assessment of PET images was performed by volume-of-interest and ratio image analysis. Results: MRAC did not result in large differences in scatter algorithm performance, especially with TF-SSS. Scatter sinograms and scatter fractions did not reveal large differences regardless of the m-map used. TF-SSS showed slightly higher absolute quantification. The differences in volume-of-interest analysis between TF-SSS and MC-SSS were 3% at maximum in the phantom and 4% in the patient study. Both algorithms showed excellent correlation with each other with no visual differences between PET images. MC-SSS showed a slight dependency on the m-map used, with a difference of 2% on average and 4% at maximum when a m-map without bone was used. Conclusion: The effect of different MR-based m-maps on the performance of scatter correction was minimal in non-time-of-flight 18 F-FDG PET/MR brain imaging. The SSS algorithm was not affected significantly by MRAC. The performance of the MC-SSS algorithm is comparable but not superior to TF-SSS, warranting further investigations of algorithm optimization and performance with different radiotracers and time-of-flight imaging.
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