Principles of PET/MR Imaging

Disselhorst, Jonathan A.; Bezrukov, Ilja; Kolb, Armin; Parl, C.; Pichler, Bernd J.

doi:10.2967/jnumed.113.129098

Cited by 77 publications

(65 citation statements)

References 81 publications

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“…PET/ MR imaging is a powerful technology that is now expanding worldwide (1); integrated PET/MR imaging systems are already commercially available for clinical applications (2,3). Special attention is thus being paid to the ability of simultaneous PET/MR imaging to provide quantitative information and artifact-free images.…”

mentioning

confidence: 99%

Shine-Through in PET/MR Imaging: Effects of the Magnetic Field on Positron Range and Subsequent Image Artifacts

Kolb

Sauter

Eriksson³

et al. 2015

J Nucl Med

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Simultaneous PET/MR imaging is an emerging hybrid modality for clinical and preclinical imaging. The static magnetic field of the MR imaging device affects the trajectory of the positrons emitted by the PET radioisotopes. This effect translates into an improvement of the spatial resolution in transaxial images. However, because of the elongation of the positron range distribution along the magnetic field, the axial resolution worsens and shine-through artifacts may appear. These artifacts can lead to misinterpretation and overstaging. The aim of this work was to study the relevance of this effect. Methods: Measurements were performed in a 3-tesla PET/ MR scanner. A 1-cm 2 piece of paper, soaked with a radioisotope and placed in air, was scanned, and the magnitude of the shinethrough was quantified from the PET images for various radioisotopes. Additionally, PET/MR and PET/CT images of the lungs and the larynx with trachea of a deceased swine were obtained after injecting a mixture of NiSO 4 and 68 Ga to simulate hot tumor lesions. Results: For the radioactive paper, shine-through artifacts appeared in the location of the acrylic glass backplane, located 3 cm from the source in the axial direction. The ratio between the activity of the shine-through and the activity reconstructed in the original location ranged from 0.9 ( 18 F) to 5.7 ( 68 Ga). For the larynx-withtrachea images, the magnitude of the artifacts depended on the organ orientation with respect to the magnetic field. The shine-through activity could reach 46% of the reconstructed activity (larynx lesion). The lesion within the trachea produced 2 artifacts, symmetrically aligned with the magnetic field and characterized by artifact-to-lesion volumeof-interest ratios ranging from 21% to 30%. Conclusion: In simultaneous PET/MR imaging, the effect of the magnetic field on positrons may cause severe artifacts in the PET image when the lesions are close to air cavities and high-energy radioisotopes are used. For accurate staging and interpretation, this effect needs to be recognized and adequate compensation techniques should be developed. PET/ MR imaging is a powerful technology that is now expanding worldwide (1); integrated PET/MR imaging systems are already commercially available for clinical applications (2,3). Special attention is thus being paid to the ability of simultaneous PET/MR imaging to provide quantitative information and artifact-free images. A particular phenomenon of this integrated technology is the effect of the static magnetic field on positrons. Because of the Lorentz force, positrons follow helical paths along the magnetic field lines. Consequently, the positron range distribution narrows in the plane perpendicular to the magnetic field, thus improving the transaxial resolution (4-9). On the other hand, little attention has been devoted to the behavior of positrons in the axial direction. The positron range distribution elongates along the magnetic field, and the axial resolution subsequently degrades (9-11). The positron range can rea...

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mentioning

confidence: 99%

Shine-Through in PET/MR Imaging: Effects of the Magnetic Field on Positron Range and Subsequent Image Artifacts

Kolb

Sauter

Eriksson³

et al. 2015

J Nucl Med

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“…Accurate MRI-guided attenuation correction in PET/MRI has been a major challenge since the introduction of this clinical hybrid imaging technology (Disselhorst et al, 2014). Since then, a number of strategies have been proposed to derive patient-specific attenuation maps either from MR images and PET emission data or co-registered MR/CT datasets.…”

Section: Discussionmentioning

confidence: 99%

Quantitative analysis of MRI-guided attenuation correction techniques in time-of-flight brain PET/MRI

2016

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Purpose: In quantitative PET/MR imaging, attenuation correction (AC) of PET data is markedly challenged by the need of deriving accurate attenuation maps from MR images. A number of strategies have been developed for MRI-guided attenuation correction with different degrees of success. In this work, we compare the quantitative performance of three generic AC methods, including standard 3-class MR segmentation-based, advanced atlasregistration-based and emission-based approaches in the context of brain time-of-flight (TOF) PET/MRI. Materials and methods: Fourteen patients referred for diagnostic MRI and 18 F-FDG PET/CT brain scans were included in this comparative study. For each study, PET images were reconstructed using four different attenuation maps derived from CT-based AC (CTAC) serving as reference, standard 3-class MR-segmentation, atlas-registration and emission-based AC methods. To generate 3-class attenuation maps, T1-weighted MRI images were segmented into background air, fat and soft-tissue classes followed by assignment of constant linear attenuation coefficients of 0, 0.0864 and 0.0975 cm −1 to each class, respectively. A robust atlas-registration based AC method was developed for pseudo-CT generation using local weighted fusion of atlases based on their morphological similarity to target MR images. Our recently proposed MRI-guided maximum likelihood reconstruction of activity and attenuation (MLAA) algorithm was employed to estimate the attenuation map from TOF emission data. The performance of the different AC algorithms in terms of prediction of bones and quantification of PET tracer uptake was objectively evaluated with respect to reference CTAC maps and CTAC-PET images.Results: Qualitative evaluation showed that the MLAA-AC method could sparsely estimate bones and accurately differentiate them from air cavities. It was found that the atlas-AC method can accurately predict bones with variable errors in defining air cavities. Quantitative assessment of bone extraction accuracy based on Dice similarity coefficient (DSC) showed that MLAA-AC and atlas-AC resulted in DSC mean values of 0.79 and 0.92, respectively, in all patients. The MLAA-AC and atlas-AC methods predicted mean linear attenuation coefficients of 0.107 and 0.134 cm . The evaluation of the relative change in tracer uptake within 32 distinct regions of the brain with respect to CTAC PET images showed that the 3-class MRAC, MLAA-AC and atlas-AC methods resulted in quantification errors of −16.2 ± 3.6%, −13.3 ± 3.3% and 1.0 ± 3.4%, respectively. Linear regression and Bland-Altman concordance plots showed that both 3-class MRAC and MLAA-AC methods result in a significant systematic bias in PET tracer uptake, while the atlas-AC method results in a negligible bias. Conclusion: The standard 3-class MRAC method significantly underestimated cerebral PET tracer uptake. While current state-of-the-art MLAA-AC methods look promising, they were unable to noticeably reduce quantification errors in the context of brain imaging. Conversely, the propose...

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“…Segmentation methods have the following main limits: 1) segmentation errors and consequently classification errors may occur; for example, fine structures such as nasal cavities and cerebrospinal fluid (CSF) are sometimes misclassified [7] and lack of identification of the air cavities may introduce overestimations in adjacent brain areas [16]. This may be due to errors occurring during the execution of the mathematical function underlying the segmentation process and errors are favoured by limits in the available MR sequences [17]).…”

Section: ) Segmentation-based Methodsmentioning

confidence: 99%

“…These methods may work well, especially for the distinction of brain parenchyma, but they are often less accurate in regions with high anatomical variability [7]. Moreover, because of the anatomic adaptation during the registration process, these methods produce patient specific attenuation correction maps [18].…”

Section: ) Methods Based On Atlas or Templatementioning

confidence: 99%

“…The first PET/MR devices were built [4][5] even before the introduction of PET/CT [6]. Despite more than two decades of extensive research on PET/MR, only a small number of centres are currently equipped with these devices [7]. Several technical difficulties make the combination of these two modalities challenging (see for review [3,7]) and PET/MR also presents some disadvantages.…”

Section: Introductionmentioning

confidence: 99%

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MR-Based Attenuation Correction in Brain PET/MR Studies: A Short Review

Giovannini¹,

Giovacchini²,

Nicolosi³

et al. 2017

J Diagn Imaging Ther

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In the previous few years we have witnessed to an increased diffusion of positron emission tomography/magnetic resonance (PET/MR) tomographs and equally an increasing number of clinical studies with these hybrid devices in both the neurological and psychiatric fields. Although PET/MR contains many features that facilitate its application in brain imaging, accurate quantification is still hindered by difficulties in developing MR-based attenuation correction methods. In this paper, we have reviewed the three main methods currently used for attenuation correction in PET/MR: namely segmentation-based methods including, atlas and template. In addition to procedures based on the combination of PET emission data and MR anatomical information (or reconstruction-based methods). Many research centers are actively working to refine available methods and substantial improvements are expected in future years. Clinical studies using PET/MR focused mainly on neurooncological and neurodegenerative disorders. Simultaneous PET/MR was shown to provide very promising scientific results and to be logistically more convenient for patients. More studies are expected in the near future, as the availability of PET/MR and the clinical use of new tracers for neurodegenerative disorders will further increase.

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Principles of PET/MR Imaging

Cited by 77 publications

References 81 publications

Shine-Through in PET/MR Imaging: Effects of the Magnetic Field on Positron Range and Subsequent Image Artifacts

Shine-Through in PET/MR Imaging: Effects of the Magnetic Field on Positron Range and Subsequent Image Artifacts

Quantitative analysis of MRI-guided attenuation correction techniques in time-of-flight brain PET/MRI

MR-Based Attenuation Correction in Brain PET/MR Studies: A Short Review

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