[en] Attenuation correction in PET is the primary prerequisite for quantification of the radiotracer's signal. Absolute quantification is the key to improve diagnostic performance, to enable comparisons between follow-up examinations and to perform pharmacokinetic modeling. A large fraction of the 511 keV annihilation photons from the positron emitters are scattered by the patient's body. Thus, they are discarded or do not even reach the PET detectors, while others are identified at the wrong location after being scattered. To account for these effects and thus generate quantitative PET images showing the actual activity distribution, it is necessary to determine an attenuation map with the appropriate attenuation coefficients for 511 keV photons at each voxel. In hybrid PET/CT systems, this is achieved using the information about the tissue electron density provided by the CT and adjusting it for the difference in photon energy. In PET/MR systems, there is no mechanism to directly measure the attenuation coefficients of the tissue. Determining the attenuation map in PET/MR is an important challenge involving two problems: the determination of the patient's attenuation map and the determination of the attenuation introduced by additional hardware components. We describe the approaches investigated to deal with these problems and, based on the experience with a fully integrated PET/MR system, we finally discuss potential solutions and limitations in a close to routine setting. (orig.)

[en] Attenuation correction (AC) in brain PET/MR has recently emerged as one of the challenging tasks in the PET/MR field. It has been shown that to ignore the attenuation produced by bone can lead to errors ranging from 5-30% in regions close to bone structures. Since the information provided by the MR signal is not directly related to tissue attenuation, alternative methods have to be developed. Signal from bone tissue is difficult to measure given its short transverse relaxation time (T2). Ultrashort-echo time (UTE) pulse sequences were developed to measure signal from tissues with short T2. A combination of two consecutive UTE echoes has been used in several works to measure signal from bone tissue. The first echo is able to measure signal from bone tissue in addition to soft tissue, while the second echo contains most of the soft tissue contained in the first echo but not bone. In this work we extract the attenuation information from the difference between the logarithm of two images obtained after applying two consecutive UTE pulse sequences using the mMR scanner (Siemens Healthcare). Subsequently, image processing techniques are applied to reduce the noise and extract air cavities within the head. The resulting image is converted to linear attenuation coefficients, generating what is known as µ-map, to be used during reconstruction. For comparison purposes PET/CT scans of the same patients were acquired prior to the PET/MR scan. Additional µ-maps obtained for comparison were extracted from a Dixon sequence (used in clinical routine) and an additional µ-map calculated by the scanner based on UTE pulse sequences. Preliminary quantitative results measured in the cerebellum, using the value obtained with CT-based AC as reference, show differences of 34% without AC, 13% using the Dixon-based and UTE-based provided by the scanner, and 0.8% with the AC strategy presented here.

[en] This review will focus on the clinical potential of PET/CT for the characterization of cardiovascular diseases. We describe the technical challenges of combining instrumentation with very different imaging performance and discuss the clinical applications in the field of cardiology.

[en] Non-invasive imaging in the form of single-photon emission-computed tomography (SPECT), positron-emission tomography (PET), computed tomography (CT), echocardiography or magnetic resonance imaging (MRI) is a very useful tool for cardiovascular research as it allows assessment of biological processes in vivo. Nuclear imaging with SPECT and PET offers the advantage of high sensitivity, the potential for serial imaging, and reliable quantification. Currently a wide range of established as well as innovative agents is available and can be imaged with dedicated preclinical and clinical SPECT and PET imaging systems. These scanners can be equipped with CT and MRI components to form hybrid imaging systems. This review provides an outline on SPECT and PET as capable tools for translational research in cardiology as part of a work flow similar to the one used in clinical imaging illustrating the concept “from bench to bedside”.

[en] Positron emission tomography (PET) is the gold standard for non-invasive assessment of myocardial viability and allows accurate detection of coronary artery disease by assessment of myocardial perfusion. Magnetic resonance imaging (MRI) provides high resolution anatomical images that allow accurate evaluation of ventricular structure and function together with detection of myocardial infarction. Potential hybrid PET/MR tomography may potentially facilitate the combination of information from these imaging modalities in cardiology. Furthermore, the combination of anatomical MRI images with the high sensitivity of PET for detecting molecular targets may extent the application of these modalities to the characterization of atherosclerotic plaques and to the evaluation of angiogenetic or stem cell therapies, for example. This article reviews studies using MRI and PET in parallel to compare their performance in cardiac applications together with the potential benefits and applications provided by hybrid PET/MRI systems. (orig.)

[en] Respiratory motion of lung lesions is a limiting factor of quantification of positron emission tomography (PET) data. As some important applications of PET such as therapy monitoring and radiation therapy treatment planning require precise quantification, it is necessary to correct PET data for motion artefacts. The method is based on list-mode data. First, the motion of the lesion was detected by a centre of mass approach. In the second step, data were sorted corresponding to the breathing state. A volume of interest (VOI) around the lesion was defined manually, and the motion of the lesion in this VOI was measured with reference to the end-expiration image. Then, all voxels in the VOI were shifted according to the measured lesion motion. After optimisation of parameters and verification of the method using a computer-controlled motion phantom, it was applied to nine patients with solitary lesions of the lung. Fifty percent difference in measured lesion volume and 26% in mean activity concentration were found comparing PET data before and after applying the correction algorithm when simulating a motion amplitude of 28 mm in phantom studies. For patients, maximum changes of 27% in volume and 13% in mean standardised uptake values (SUV) were found. As respiratory motion is affecting quantification of PET images, correction algorithms are essential for applications that require precise quantification. We described a method which improves the quantification of moving lesions by a local motion correction using list-mode data without increasing acquisition time or reduced signal-to-noise ratio of the images. (orig.)

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[en] After a decade of PET/MR, the case of attenuation correction (AC) remains open. The initial four-compartment (air, water, fat, soft tissue) Dixon-based AC scheme has since been expanded with several features, the latest being MR field-of-view extension and a bone atlas. As this potentially changes quantification, we evaluated the impact of these features in PET AC in prostate cancer patients. Two hundred prostate cancer patients were examined with either ${}^{18}$F- or ${}^{68}$Ga-prostate-specific membrane antigen (PSMA) PET/MR. Qualitative and quantitative analysis (SUV${}_{mean}$, SUV${}_{max}$, correlation, and statistical significance) was performed on images reconstructed using different AC schemes: Dixon, Dixon+MLAA, Dixon+HUGE, and Dixon+HUGE+bones for ${}^{18}$F-PSMA data; Dixon and Dixon+bones for ${}^{68}$Ga-PSMA data. Uptakes were compared using linear regression against standard Dixon. High correlation and no visually perceivable differences between all evaluated methods (r > 0.996) were found. The mean relative difference in lesion uptake of ${}^{18}$F-PSMA and ${}^{68}$Ga-PSMA remained, respectively, within 4% and 3% in soft tissue, and within 10% and 9% in bones for all evaluated methods. Bone registration errors were detected, causing mean uptake change of 5% in affected lesions. Based on these results and the encountered bone atlas registration inaccuracy, we deduce that including bones and extending the MR field-of-view did not introduce clinically significant differences in PSMA diagnostic accuracy and tracer uptake quantification in prostate cancer pelvic lesions, facilitating the analysis of serial studies respectively. However, in the absence of ground truth data, we advise against atlas-based methods when comparing serial scans for bone lesions.

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No abstract available