[en] Developments in instrumentation include the implementation of new detector materials and digital camera technology, resulting in high resolution multi-head systems with count rate capability which enables high flux measurements, e.g. transmission scans or coincidence detection. Also, imaging in nuclear cardiology is greatly enhanced by the availability of computer power which facilitates the implementation of attenuation and scatter correction. (orig.)

[en] Full text of publication follows. Aim: peptide radionuclide receptor therapy is a promising treatment option in metastasized, somatostatin receptor expressing neuroendocrine tumors. A severe side effect of this therapy is radiation induced nephropathy due to renal excretion and reabsorption of the radioactive compound. In this study we investigated the influence of the dynamic scan during the injection and the importance of the following measurement time points to achieve the accumulated activity. Methods: SPECT image data and anterior and posterior planar images were acquired in 7 patients (57±14 years) at 1 h, 24 h, 48 h and 72 h after administration of Lutetium-177-DOTATATE. Furthermore a dynamic planar scan was acquired with 12 frames during the injection. Scatter correction was performed with the triple-energy window method. A co-registered CT was used for attenuation correction in SPECT OSEM reconstruction and in planar images by estimating the patient size. Total activity in kidneys were fitted to a bi-exponential function in dynamic planar images and to a mono-exponential function in SPECT and whole body planar images with the Levenberg-Marquardt-Algorithm. These functions were integrated over time to estimate the organ dose, by applying the kidney specific S value according the MIRD concept. To investigate the effect of the dynamic measurement (dyn-P) the resulting dose was compared to the same measurement without dynamics (stat-P). Furthermore we studied the influence of reducing the number of time points to the dose estimated from planar and SPECT images by omitting one of the time points 1 h, 24 h, 48 h or 72 h after injection. Results: a relative mean deviation of 3.0% was discovered by comparing dyn-P and stat-P. The Spearman Rank Correlation Coefficient (SRCC=0.87) indicated strong correlation between SPECT and planar images when all 4 time points were used. With reduced number of time points a deviation of 7.3% (1 h, 24 h, 48 h), 1.8% (1 h, 24 h, 72 h) and 3.3% (1 h, 48 h, 72 h) in SPECT and 12.9%, 12.1% and 16.2% in stat-P were observed. Conclusions: we found that dispensing with the initial dynamic measurement resulted in a relatively small error in dose calculation from planar images. The small deviation of estimated dose especially when the 72 h measurement was omitted in SPECT processing suggests a further investigation of this approach. This optimization might increase the availability of individualized peptide radionuclide therapy. (authors)

[en] Recent developments of large area avalanche photodiodes (APDs) and fast scintillators with high light yield offer unique advantages for imaging applications. To test possible scintillator-APD combinations, 3.7x3.7x12.0 mm³ LGSO, BGO and LSO crystals were coupled to large area, low capacitance, round and rectangular APDs (5 mm diameter, 3x3 mm² and 5x5 mm², Hamamatsu, Japan). Light output, energy resolution and time resolution were compared. The light output of BGO was about 86% worse and of LGSO about 30% worse compared to LSO (100%). The energy resolution at 511 keV was 13.8±0.5% for LSO and 15.1 ±0.5% for LGSO (FWHM). For BGO, 16.9±0.5% (FWHM) was measured at 662 keV. The coincident time resolution of two opposing single detectors was 2.7±0.2 ns for LSO and 3.9±0.2 ns for LGSO (FWHM). Chemical treatment of crystals showed an improved energy resolution compared to mechanical polishing of more than 1%, providing reduced cost and less processing time. An energy resolution of 10.7±0.5% for LSO could be reached after chemical etching. With the new large area APDs, results similar to PMT readout could be achieved. The scintillation characteristic of LGSO makes this material a promising candidate for APD readout, although the performance was inferior to LSO

[en] To fully utilize positron emission tomography (PET) as a non-invasive tool for tissue characterization, dedicated instrumentation is being developed which is specially suited for imaging mice and rats. Semiconductor detectors, such as avalanche photodiodes (APDs), may offer an alternative to photomultiplier tubes for the readout of scintillation crystals. Since the scintillation characteristics of lutetium oxyorthosilicate (LSO) are well matched to APDs, the combination of LSO and APDs seems favourable, and the goal of this study was to build a positron tomograph with LSO-APD modules to prove the feasibility of such an approach. A prototype PET scanner based on APD readout of small, individual LSO crystals was developed for tracer studies in mice and rats. The tomograph consists of two sectors (86 mm distance), each comprising three LSO-APD modules, which can be rotated for the acquisition of complete projections. In each module, small LSO crystals (3.7 x 3.7 x 12 mm³) are individually coupled to one channel within matrices containing 2 x 8 square APDs (2.6 x 2.6 mm² sensitive area per channel). The list-mode data are reconstructed with a penalized weighted least squares algorithm which includes the spatially dependent line spread function of the tomograph. Basic performance parameters were measured with phantoms and first experiments with rats and mice were conducted to introduce this methodology for biomedical imaging. The reconstructed field of view covers 68 mm, which is 80% of the total detector diameter. Image resolution was shown to be 2.4 mm within the whole reconstructed field of view. Using a lower energy threshold of 450 keV, the system sensitivity was 350 Hz/MBq for a line source in air in the centre of the field of view. In a water-filled cylinder of 4.6 cm diameter, the scatter fraction at the centre of the field of view was 16% (450 keV threshold). The count rate was linear up to 700 coincidence counts per second. In vivo studies of anaesthetized rats and mice showed the feasibility of in vivo imaging using this PET scanner. The first LSO-APD prototype tomograph has been successfully introduced for in vivo animal imaging. APD arrays in combination with LSO crystals offer new design possibilities for positron tomographs with finely granulated detector channels. (orig.)

[en] Lutetium oxyorthosilicate (LSO) scintillation crystals coupled to avalanche photodiodes (APDs) have been proven to be an interesting detector module for high resolution positron emission tomography (PET). Compact detector modules were built to test their performance in a prototype tomograph. Each detector module consisted of sixteen 3.7 x 3.7 x 12.0 mm³ LSO crystals, which are one-to-one coupled to a two dimensional APD array (2 x 8 diodes, 3 mm OE, 4 mm spacing). The signals were read out by a low noise, fast, charge sensitive preamplifier, a 50 ns shaping amplifier and CAMAC electronic modules. Two detector modules were mounted on a rotatable gantry, to simulate a detector ring. Long term runs over 110 hours have shown that the detector remains stable over a long period of time. The energy resolution (FWHM) was 13.4% for 511 keV gamma rays and the intrinsic spatial resolution was 2.2 mm (FWHM). The gain dependence of the APD on temperature had been determined as 3.5%/K. The time resolution was 3.2 ± 0.1 ns (FWHM). Phantom measurements showed the suitability of the new LSO/APD detector for high resolution animal PET

[en] To assess how ASIR (adaptive statistical iterative reconstruction) contributes to dose reduction and affects image quality of non-contrast cranial computed tomography (cCT). Non-contrast emergency CT scans of the head acquired in 177 patients were evaluated. The scans were acquired and processed using four different protocols: Group A (control): 120 kV, FBP (filtered back projection) n=71; group B1: 120 kV, scan and reconstruction performed with 20 % ASIR (blending of 20 % ASIR and 80 % FBP), n=86; group B2: raw data from group B1 reconstructed using a blending of 40 % ASIR and 60 % FBP, n=74; group C1: 120 kV, scan and reconstruction performed with 30 % ASIR, n=20; group C2: raw data from group C1 reconstructed using a blending of 50 % ASIR and 50 % FBP, n=20. The effective dose was calculated. Image quality was assessed quantitatively and qualitatively. Compared to group A, groups B1/2 and C1/2 showed a significantly reduced effective dose of 40.4 % and 73.3 % (p < 0.0001), respectively. Group B1 and group C1/2 also showed significantly reduced quantitative and qualitative image quality parameters. In group B2, quantitative measures were comparable to group A, and qualitative scores were lower compared to group A but higher compared to group B1. Diagnostic confidence grading showed groups B1/2 to be adequate for everyday clinical practice. Group C2 was considered acceptable for follow-up imaging of severe acute events such as bleeding or subacute stroke. Conclusion: Use of ASIR makes it possible to reduce radiation significantly while maintaining adequate image quality in non-contrast head CT, which may be particularly useful for younger patients in an emergency setting and in follow-up.

No abstract available

[en] Modern SPECT instrumentation allows attenuation correction in cardiac scans by using transmission measurements. Simultaneous transmission and emission measurements are feasible with different geometries in multihead SPECT systems and are already commercially available. The selection of the transmission nuclide affects the quality of data, depending on the emission nuclide used. Iterative reconstruction methods have to be implemented to get accurate results in the case of heterogeneous attenuation distributions. Methodological aspects concerning simultaneous scatter and attenuation correction have to be studied in future. The clinical significance of routine attenuation correction measurements for myocardial perfusion has to be shown in a large patient population. (orig.)

[en] Improving system efficiency without jeopardizing spatial resolution is one of the main problems of small animal PET scanners. In pursuit of this goal, the future LSO-APD-PET prototype MADPET-II will combine highly granulated detector modules with a dual layer structure. The individual readout of the LSO crystals allows separately handling multiple signals related to those photons scattering between different crystal units (inter-crystal scatter, ICS). The contribution of ICS events can significantly increase the system efficiency. Such coincidences are not characterized by a unique LOR. However, in order to minimize resolution degradation, it would be desirable to identify the primary path of the ICS events. Since ICS is geometry dependent, this work was aimed at investigating the effects of ICS in the performance of the dual layer prototype. Different recovery algorithms to select the primary crystal were implemented and developed, and applied to Monte Carlo simulated data. Some of these algorithms were based on the properties of Compton kinematics. For a centred point source and a 100 keV lower energy threshold, the absolute system efficiency was found to increase by 35% when including ICS events: from 1.8% without ICS events to 2.8% with ICS. Similarly, for a threshold of 200 keV, the contribution of ICS coincidences still represented ∼20% of the total detected coincidences, leading to an absolute system efficiency of almost 2%. The mispositioning introduced by processing ICS coincidences only led to a moderate broadening of the axial line spread function (LSF), especially at the tails of the profile (FWTM). This effect was also noticeable in the transaxial plane. In presence of scattering media (water-filled cylinder), the resolution degradation was dominated by the contribution of object scatter. The reconstructed images from a simulated homogeneous cylinder filled with activity with a non-active rod at its centre were employed to estimate the impact of ICS on the image quality. In general, the use of ICS coincidences increased the signal-to-noise ratio (SNR) but worsened contrast. The effects of ICS on resolution could be reduced by employing a new identification scheme based on the maximum signal and the Compton kinematics. This method yielded the highest identification rate for the correct photon trajectory, even for a finite energy resolution of 15% (511 keV). This technique also increased the SNR by 17% to 30% and preserved the image contrast. In conclusion, by combining individual crystal readout, a low energy threshold and an appropriate recovery scheme, the processing of ICS coincidences significantly increases the system efficiency without any substantial deterioration of the image quality

[en] Expression of the translocator protein (TSPO) is upregulated in activated macrophages/microglia and is considered to be a marker of neuroinflammation. We investigated the novel TSPO ligand [¹⁸F]GE-180 in patients with relapsing-remitting multiple sclerosis (RRMS) to determine the feasibility of [¹⁸F]GE-180 PET imaging in RRMS patients and to assess its ability to detect active inflammatory lesions in comparison with the current gold standard, contrast-enhanced magnetic resonance imaging (MRI). Nineteen RRMS patients were prospectively included in this study. All patients underwent TSPO genotyping and were classified as high-affinity, medium-affinity or low-affinity binders (HAB/MAB/LAB). PET scans were performed after administration of 189 ± 12 MBq [¹⁸F]GE-180, and 60-90 min summation images were used for visual analysis and assessment of standardized uptake values (SUV). The frontal nonaffected cortex served as a pseudoreference region (PRR) for evaluation of SUV ratios (SUVR). PET data were correlated with MRI signal abnormalities, i.e. T2 hyperintensity or contrast enhancement (CE). When available, previous MRI data were used to follow the temporal evolution of individual lesions. Focal lesions were identified as hot spots by visual inspection. Such lesions were detected in 17 of the 19 patients and overall 89 [¹⁸F]GE-180-positive lesions were found. TSPO genotyping revealed 11 patients with HAB status, 5 with MAB status and 3 with LAB status. There were no associations between underlying binding status (HAB, MAB and LAB) and the signal intensity in either lesions (SUVR 1.87 ± 0.43, 1.95 ± 0.48 and 1.86 ± 0.80, respectively; p = 0.280) or the PRR (SUV 0.36 ± 0.03, 0.40 ± 0.06 and 0.37 ± 0.03, respectively; p = 0.990). Of the 89 [¹⁸F]GE-180-positive lesions, 70 showed CE on MRI, while the remainder presented as T2 lesions without CE. SUVR were significantly higher in lesions with CE than in those without (2.00 ± 0.53 vs. 1.60 ± 0.15; p = 0.001). Notably, of 19 [¹⁸F]GE-180-positive lesions without CE, 8 previously showed CE, indicating that [¹⁸F]GE-180 imaging may be able to detect lesional activity that is sustained beyond the blood-brain barrier breakdown. [¹⁸F]GE-180 PET can detect areas of focal macrophage/microglia activation in patients with RRMS in lesions with and without CE on MRI. Therefore, [¹⁸F]GE-180 PET imaging is a sensitive and quantitative approach to the detection of active MS lesions. It may provide information beyond contrast-enhanced MRI and is readily applicable to all patients. [¹⁸F]GE-180 PET imaging is therefore a promising new tool for the assessment of focal inflammatory activity in MS. (orig.)