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pubmed: 0161-5505



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6"-18F-Fluoromaltotriose PET Evaluation in Escherichia-Coli-Induced Myositis: is there Uptake Saturation in Control?
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6"-18F-Fluoromaltotriose PET Evaluation in Escherichia-Coli-Induced Myositis: is there Uptake Saturation in Control?

J Nucl Med. 2018 Feb 15;:

Authors: Laffon E, Marthan R

PMID: 29449448 [PubMed - as supplied by publisher]




Tracers for fluorescence-guided surgery: how elongation of the polymethine chain in cyanine dyes alters the pharmacokinetics of a (bimodal) c[RGDyK] tracer.
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Tracers for fluorescence-guided surgery: how elongation of the polymethine chain in cyanine dyes alters the pharmacokinetics of a (bimodal) c[RGDyK] tracer.

J Nucl Med. 2018 Feb 15;:

Authors: Buckle T, van Willigen DM, Spa SJ, Hensbergen AW, van der Wal S, de Korne CM, Welling MM, van der Poel HG, Hardwick JCH, van Leeuwen FWB

Abstract
Objectives: The potential of (receptor-mediated) fluorescence-based image-guided surgery tracers is generally linked to the near-infrared emission profile and good manufacturing production (GMP) availability of fluorescent dyes. Surprisingly, little is known about the critical interaction between the structural composition of the dye and the pharmacokinetics of the tracers. In this study, a bimodal/hybrid tracer design was used to systematically and quantitatively evaluate the influence of elongation of the polymethine chain in a fluorescent cyanine (Cy) dye on the imaging potential of a targeted (RGD-based) tracer. Methods: As model system, the integrin marker ανβ3 was targeted using c[RGDyK] vectors functionalized with a (111In-)DTPA chelate and a fluorescent dye (Cy3-(SO3)Methyl-COOH (λem 580nm), Cy5-(SO3)Methyl-COOH (λem 680nm), or Cy7-(SO3)Methyl-COOH (λem 780nm)). Tracers were analyzed for differences in (photo-) physical properties, serum protein binding, chemical/optical stability and signal penetration through tissue. Receptor affinities (KD) were evaluated using saturation and competition experiments. In vivo biodistribution (SPECT imaging and percentage injected dose per gram of tissue (%ID/g)) was assessed in tumor-bearing mice and complimented with in- and ex vivo fluorescence images obtained using a clinical grade multispectral fluorescence laparoscope. Results: Two carbon-atom-step variations in the polymethine chain of the fluorescent Cy-dyes were shown to significantly influence the chemical and photophysical characteristics e.g. stability, brightness and tissue penetration of the hybrid RGD-tracers. Herein DTPA-Cy5-(SO3)Methyl-COOH-c[RGDyK] systematically outperformed its Cy3- and Cy7- derivatives. Radioactivity-based evaluation of in vivo tracer pharmacokinetics yielded the lowest non-specific uptake and highest tumor-to-background ratio (T/B) for DTPA-Cy5-(SO3)Methyl-COOH-c[RGDyK] (13.2 ± 1.7), with the Cy3- and Cy7- analogs trailing at a respective T/B of 5.7 ± 0.7 and 4.7 ± 0.7. Fluorescence-based assessment of the tumor visibility revealed a similar trend. Conclusion: These findings underline that variations in the polymethine chain lengths of Cy dyes have a profound influence on the photophysical properties, stability and in vivo targeting capabilities of fluorescent imaging tracers. In a direct comparison the intermediate length dye (Cy5) yielded a superior c[RGDyK] -tracer compared to the shorter (Cy3-) and longer (Cy7-) analogs.

PMID: 29449447 [PubMed - as supplied by publisher]




Improving accuracy of simultaneously reconstructed activity and attenuation maps using deep learning.
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Improving accuracy of simultaneously reconstructed activity and attenuation maps using deep learning.

J Nucl Med. 2018 Feb 15;:

Authors: Hwang D, Kim KY, Kang SK, Seo S, Paeng JC, Lee DS, Lee JS

Abstract
Simultaneous reconstruction of activity and attenuation using the maximum likelihood reconstruction of activity and attenuation (MLAA) augmented by time-of-flight (TOF) information is a promising method for positron emission tomography (PET) attenuation correction. However, it still suffers from several problems, including crosstalk artifacts, slow convergence speed, and noisy attenuation maps (μ-maps). In this work, we developed deep convolutional neural networks (CNNs) to overcome these MLAA limitations, and we verified their feasibility using a clinical brain PET data set. Methods: We applied the proposed method to one of the most challenging PET cases for simultaneous image reconstruction (18F-FP-CIT PET scans with highly specific binding to striatum of the brain). Three different CNN architectures (convolutional autoencoder (CAE), U-net, hybrid of CAE and U-net) were designed and trained to learn x-ray computed tomography (CT) derived μ-map (μ-CT) from the MLAA-generated activity distribution and μ-map (μ-MLAA). PET/CT data of 40 patients with suspected Parkinson's disease were employed for five-fold cross-validation. For the training of CNNs, 800,000 transverse PET slices and CTs augmented from 32 patient data sets were used. The similarity to μ-CT of the CNN-generated μ-maps (μ-CAE, μ-Unet, and μ-Hybrid) and μ-MLAA was compared using Dice similarity coefficients. In addition, we compared the activity concentration of specific (striatum) and non-specific binding regions (cerebellum and occipital cortex) and the binding ratios in the striatum in the PET activity images reconstructed using those μ-maps. Results: The CNNs generated less noisy and more uniform μ-maps than original μ-MLAA. Moreover, the air cavities and bones were better resolved in the proposed CNN outputs. In addition, the proposed deep learning approach was useful for mitigating the crosstalk problem in the MLAA reconstruction. The hybrid network of CAE and U-net yielded the most similar μ-maps to μ-CT (Dice similarity coefficient in the whole head = 0.79 in the bone and 0.72 in air cavities), resulting in only approximately 5% errors in activity and biding ratio quantification. Conclusion: The proposed deep learning approach is promising for accurate attenuation correction of activity distribution in TOF PET systems.

PMID: 29449446 [PubMed - as supplied by publisher]




Evaluation of penalized likelihood estimation reconstruction on a digital time-of-flight PET/CT scanner for 18F-FDG whole-body examinations.
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Evaluation of penalized likelihood estimation reconstruction on a digital time-of-flight PET/CT scanner for 18F-FDG whole-body examinations.

J Nucl Med. 2018 Feb 15;:

Authors: Lindström E, Sundin A, Trampal C, Lindsjö L, Ilan E, Danfors T, Antoni G, Sörensen J, Lubberink M

Abstract
Resolution and quantitative accuracy of positron emission tomography (PET) are highly influenced by the reconstruction method. Penalized likelihood estimation algorithms allow for fully convergent iterative reconstruction, generating a higher image contrast while limiting noise compared to ordered subsets expectation maximization (OSEM). In this study, block-sequential regularized expectation maximization (BSREM) was compared to time-of-flight OSEM (TOF-OSEM). Various strengths of noise penalization factor β were tested along with scan durations and transaxial field of views (FOVs) with the aim to evaluate the performance and clinical use of BSREM for 18F-FDG-PET-computed tomography (CT), both in quantitative terms and in a qualitative visual evaluation. Methods: Eleven clinical whole-body 18F-FDG-PET/CT examinations acquired on a digital TOF PET/CT scanner were included. The data were reconstructed using BSREM with point spread function (PSF) recovery and β 133, 267, 400 and 533, and TOF-OSEM with PSF, for various acquisition times/bed position (bp) and FOVs. Noise, signal-to-noise ratio (SNR), signal-to-background ratio (SBR), and standardized uptake values (SUVs) were analysed. A blinded visual image quality evaluation, rating several aspects, performed by two nuclear medicine physicians complemented the analysis. Results: The lowest levels of noise were reached with the highest β resulting in the highest SNR, which in turn resulted in the lowest SBR. Noise equivalence to TOF-OSEM was found with β 400 but produced a significant increase of SUVmax (11%), SNR (22%) and SBR (12%) compared to TOF-OSEM. BSREM with β 533 at decreased acquisition (2 min/bp) was comparable to TOF-OSEM at full acquisition duration (3 min/bp). Reconstructed FOV had an impact on BSREM outcome measures, SNR increased while SBR decreased when shifting FOV from 70 to 50 cm. The visual image quality evaluation resulted in similar scores for reconstructions although β 400 obtained the highest mean while β 267 was ranked best in terms of overall image quality, contrast, sharpness and tumour detectability. Conclusion: Comparison of penalized reconstruction to TOF-OSEM resulted in an increase of tumour SUVmax as well as improved SNR and SBR at a matched level of noise. BSREM reconstruction allowed for a shorter image acquisition time compared to TOF-OSEM with a resulting equal image quality outcome.

PMID: 29449445 [PubMed - as supplied by publisher]