Medical investigations targeting a quantitative analysis of the position emission tomography (PET) images require the incorporation of additional knowledge about the photon attenuation distribution in the patient. Today, energy range adapted attenuation maps derived from computer tomography (CT) scans are used to effectively compensate for image quality degrading effects, such as attenuation and scatter. Replacing CT by magnetic resonance (MR) is considered as the next evolutionary step in the field of hybrid imaging systems. However, unlike CT, MR does not measure the photon attenuation and thus does not provide an easy access to this valuable information. Hence, many research groups currently investigate different technologies for MR-based attenuation correction (MR-AC). Typically, these approaches are based on techniques such as special acquisition sequences (alone or in combination with subsequent image processing), anatomical atlas registration, or pattern recognition techniques using a data base of MR and corresponding CT images. We propose a generic iterative reconstruction approach to simultaneously estimate the local tracer concentration and the attenuation distribution using the segmented MR image as anatomical reference. Instead of applying predefined attenuation values to specific anatomical regions or tissue types, the gamma attenuation at 511 keV is determined from the PET emission data. In particular, our approach uses a maximum-likelihood estimation for the activity and a gradient-ascent based algorithm for the attenuation distribution. The adverse effects of scattered and accidental gamma coincidences on the quantitative accuracy of PET, as well as artifacts caused by the inherent crosstalk between activity and attenuation estimation are efficiently reduced using enhanced decay event localization provided by time-of-flight PET, accurate correction for accidental coincidences, and a reduced number of unknown attenuation coefficients. First results achieved with measured whole body PET data and reference segmentation from CT showed an absolute mean difference of 0.005 cm⁻¹ (< 20%) in the lungs, 0.0009 cm⁻¹ (< 2%) in case of fat, and 0.0015 cm⁻¹ (< 2%) for muscles and blood. The proposed method indicates a robust and reliable alternative to other MR-AC approaches targeting patient specific quantitative analysis in time-of-flight PET/MR.

A small positron-generating branch in 90-Yttrium ((90)Y) decay enables post-therapy dose assessment in liver cancer radioembolization treatment. The aim of this study was to validate clinical (90)Y positron emission tomography (PET) quantification, focusing on scanner linearity as well as acquisition and reconstruction parameter impact on scanner calibration. Data from three dedicated phantom studies (activity range: 55.2 MBq-2.1 GBq) carried out on a Philips Gemini TF 16 PET/CT scanner were analyzed after reconstruction with up to 361 parameter configurations. For activities above 200 MBq, scanner linearity could be confirmed with relative error margins 4%. An acquisition-time-normalized calibration factor of 1.04 MBq·s/CNTS was determined for the employed scanner. Stable activity convergence was found in hot phantom regions with relative differences in summed image intensities between -3.6% and +2.4%. Absolute differences in background noise artifacts between - 79.9% and + 350% were observed. Quantitative accuracy was dominated by subset size selection in the reconstruction. Using adequate segmentation and optimized acquisition parameters, the average activity recovery error induced by the axial scanner sensitivity profile was reduced to +2.4%±3.4% (mean ± standard deviation). We conclude that post-therapy dose assessment in (90)Y PET can be improved using adapted parameter setups.

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The proposed activity quantification method for 2-D scintigraphies can speed up SPECT/CT-based 3-D dosimetry without losing accuracy.

Pixellated CZT detectors provide a new opportunity to improve the image quality of SPECT detector systems. Their performance has to be evaluated in terms of resolution and efficiency, in a similar way as done earlier for NaI detectors.We have developed an analytical model for spatial resolution and geometric efficiency of collimators specifically for pixellated CZT based detectors. We derive an exact description for static and rotating detector concepts, use NEMA performance criteria for detection efficiency, and adapt measures for spatial resolution of pixellated detectors, based on the sampling of the single pixel response function.Tradeoffs among resolution, efficiency, and signal-to-noise ratio (SNR) have been investigated for different applications. Our analysis shows that the concept of rotating collimators suffers from noise accumulation, except for purely hot spot imaging. We propose multi-pinhole, slat-slit or fan beam-slit collimators in a demagnification mode for optimum efficiency and image quality using pixellated solid-state detectors for SPECT cameras.

• Intravenous contrast medium is essential for many applications of PET/CT • Body surface area adjustment of contrast medium helps standardise contrast enhancement • Underdosing or overdosing of contrast medium will be reduced • PET image quality is not influenced • BSA adjusted contrast medium protocol should be used preferably in combined PET/CT.

• Simultaneous dual-isotope SPECT/CT with (111) In- and (99m) Tc-labelled albumin microspheres is feasible. • Differentiation of two microsphere fractions after transarterial injection is possible. • The origin of an extra-hepatic microsphere deposition can be correlated to the corresponding artery. • This technique could reduce the setup time for selective internal radiation treatment.

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