Our purpose was to evaluate the performance of the Biograph Vision Quadra PET/CT system. This new system is based on the Biograph Vision 600, using the same silicon photomultiplier-based detectors ...with 3.2 × 3.2 × 20 mm lutetium-oxoorthosilicate crystals. The 32 detector rings of the Quadra provide a 4-fold larger axial field of view (AFOV) of 106 cm, enabling imaging of major organs in 1 bed position.
The physical performance of the scanner was evaluated according to the National Electrical Manufacturers Association NU 2-2018 standard, with additional experiments to characterize energy resolution. Image quality was assessed with foreground-to-background ratios of 4:1 and 8:1. Additionally, a clinical
F-FDG PET study was reconstructed with varying frame durations. In all experiments, data were acquired using the maximum ring distance of 322 crystals (MRD 322), whereas image reconstructions could be performed with a maximum ring distance of only 85 crystals (MRD 85).
The spatial resolution at full width at half maximum in the radial, tangential, and axial directions was 3.3, 3.4, and 3.8 mm, respectively. The sensitivity was 83 cps/kBq for MRD 85 and 176 cps/kBq for MRD 322. The noise-equivalent count rates (NECRs) at peak were 1,613 kcps for MRD 85 and 2,956 kcps for MRD 322, both at 27.49 kBq/mL. The respective scatter fractions at peak NECR equaled 36% and 37%. The time-of-flight resolution at peak NECR was 228 ps for MRD 85 and 230 ps for MRD 322. Image contrast recovery ranged from 69.6% to 86.9% for 4:1 contrast ratios and from 77.7% to 92.6% for 8:1 contrast ratios reconstructed using point-spread function time of flight with 8 iterations and 5 subsets. Thirty-second frames provided readable lesion detectability and acceptable noise levels in clinical images.
The Biograph Vision Quadra PET/CT device has spatial and time resolution similar to those of the Biograph Vision 600 but exhibits improved sensitivity and NECR because of its extended AFOV. The reported spatial resolution, time resolution, and sensitivity make it a competitive new device in the class of PET scanners with an extended AFOV.
The main inhibitory neurotransmitter in the mammalian brain, GABA, mediates multiple forms of inhibitory signals, such as fast and slow inhibitory postsynaptic currents and tonic inhibition, by ...activating a diverse family of ionotropic GABA(A) receptors (GABA(A)Rs). Here, we studied whether distinct GABA(A)R subtypes mediate these various forms of inhibition using as approach mice carrying a point mutation in the alpha-subunit rendering individual GABA(A)R subtypes insensitive to diazepam without altering their GABA sensitivity and expression of receptors. Whole cell patch-clamp recordings were performed in hippocampal pyramidal cells from single, double, and triple mutant mice. Comparing diazepam effects in knock-in and wild-type mice allowed determining the contribution of alpha1, alpha2, alpha3, and alpha5 subunits containing GABA(A)Rs to phasic and tonic forms of inhibition. Fast phasic currents were mediated by synaptic alpha2-GABA(A)Rs on the soma and by synaptic alpha1-GABA(A)Rs on the dendrites. No contribution of alpha3- or alpha5-GABA(A)Rs was detectable. Slow phasic currents were produced by both synaptic and perisynaptic GABA(A)Rs, judged by their strong sensitivity to blockade of GABA reuptake. In the CA1 area, but not in the subiculum, perisynaptic alpha5-GABA(A)Rs contributed to slow phasic currents. In the CA1 area, the diazepam-sensitive component of tonic inhibition also involved activation of alpha5-GABA(A)Rs and slow phasic and tonic signals shared overlapping pools of receptors. These results show that the major forms of inhibitory neurotransmission in hippocampal pyramidal cells are mediated by distinct GABA(A)Rs subtypes.
The physical properties of yttrium-90 (90Y) allow for imaging with positron emission tomography/computed tomography (PET/CT). The increased sensitivity of long axial field-of-view (LAFOV) PET/CT ...scanners possibly allows to overcome the small branching ratio for positron production from 90Y decays and to improve for the post-treatment dosimetry of 90Y of selective internal radiation therapy.
For the challenging case of an image quality body phantom, we compare a full Monte Carlo (MC) dose calculation with the results from the two commercial software packages Simplicit90Y and Hermes. The voxel dosimetry module of Hermes relies on the 90Y images taken with a LAFOV PET/CT, while the MC and Simplicit90Y dose calculations are image independent.
The resulting doses from the MC calculation and Simplicit90Y agree well within the error margins. The image-based dose calculation with Hermes, however, consistently underestimates the dose. This is due to the mismatch of the activity distribution in the PET images and the size of the volume of interest. We found that only for the smallest phantom sphere there is a statistically significant dependence of the Hermes dose on the image reconstruction parameters and scan time.
Our study shows that Simplicit90Y’s local deposition model can provide a reliable dose estimate. On the other hand, the image based dose calculation suffers from the suboptimal reconstruction of the 90Y distribution in small structures.
•Comparison of three dose calculation methods in a challenging imaging situation.•Evaluation of 90Y dosimetry based on long axial field-of-view PET/CT images.•Underestimation of the dose by image based algorithm.
Background
Our aim was to determine sets of reconstruction parameters for the Biograph Vision Quadra (Siemens Healthineers) PET/CT system that result in quantitative images compliant with the ...European Association of Nuclear Medicine Research Ltd. (EARL) criteria. Using the Biograph Vision 600 (Siemens Healthineers) PET/CT technology but extending the axial field of view to 106 cm, gives the Vision Quadra currently an around fivefold higher sensitivity over the Vision 600 with otherwise comparable spatial resolution. Therefore, we also investigated how the number of incident positron decays—i.e., exposure—affects EARL compliance. This will allow estimating a minimal acquisition time or a minimal applied dose in clinical scans while retaining data comparability.
Methods
We measured activity recovery curves on a NEMA IEC body phantom filled with an aqueous
18
F solution and a sphere to background ratio of 10–1 according to the latest EARL guidelines. Reconstructing 3570 image sets with varying OSEM PSF iterations, post-reconstruction Gaussian filter full width at half maximum (FWHM), and varying exposure from 59 kDecays/ml (= 3 s frame duration) to 59.2 MDecays/ml (= 1 h), allowed us to determine sets of parameters to achieve compliance with the current EARL 1 and EARL 2 standards. Recovery coefficients (RCs) were calculated for the metrics RC
max
, RC
mean
, and RC
peak
, and the respective recovery curves were analyzed for monotonicity. The background’s coefficient of variation (COV) was also calculated.
Results
Using 6 iterations, 5 subsets and 7.8 mm Gauss filtering resulted in optimal EARL1 compliance and recovery curve monotonicity in all analyzed frames, except in the 3 s frames. Most robust EARL2 compliance and monotonicity were achieved with 2 iterations, 5 subsets, and 3.6 mm Gauss FWHM in frames with durations between 30 s and 10 min. RC
peak
only impeded EARL2 compliance in the 10 s and 3 s frames.
Conclusions
While EARL1 compliance was robust over most exposure ranges, EARL2 compliance required exposures between 1.2 MDecays/ml to 11.5 MDecays/ml. The Biograph Vision Quadra’s high sensitivity makes frames as short as 10 s feasible for comparable quantitative images. Lowering EARL2 RC
max
limits closer to unity would possibly even permit shorter frames.
Avoiding measurement variability from
F phantom preparation by using
Ge/
Ga phantoms for the determination of
F recovery curves (RC) in clinical quality assurance measurements and for PET/CT site ...qualification in multicentre clinical trials.
RCs were obtained from PET/CT measurements of seven differently sized phantom spheres filled either with
F or with
Ga. RCs for the respective other isotope were then determined by two different methods: In the first method, images were convolved with positron range transconvolution functions derived from positron annihilation distributions found in literature. This method generated recasted images matching images using the respective other isotope. In the second method, the PET/CT system's isotope independent (intrinsic) point spread function was determined from said phantom measurements and convolved with numerical representations simulating hot spheres filled with the respective other isotope. These simulations included the isotope specific positron annihilation distributions. Recovered activity concentrations were compared between recasted images, simulated images, and the originally acquired images.
F and
Ga recovery was successfully determined from image acquisitions of the respective opposite isotope as well as from the simulations.
Ga RCs derived from
F data had a normalized root-mean-square deviation (NRMSD) from real
Ga measurements of 0.019% when using the first method and of 0.008% when using the second method.
F RCs derived from
Ga data had a NRMSD from real
F measurements of 0.036% when using the first method and of 0.038% when using the second method.
Applying the principles of transconvolution,
F RCs can be recalculated from
Ga phantom measurements with excellent accuracy. The maximal additionally introduced error was below 0.4% of the error currently accepted for RCs in the site qualification of multicentre clinical trials by the EARL program of the European Association of Nuclear Medicine (EANM). Therefore, our methods legitimately allow for the use of long-lived solid state
Ge/
Ga phantoms instead of manually prepared
F phantoms to characterize comparability of
F measurements across different imaging sites or of longitudinal
F measurements at a single PET/CT system.
Background
For multicenter clinical studies, PET/CT and SPECT/CT scanners need to be validated to ensure comparability between various scanner types and brands. This validation is usually performed ...using hollow phantoms filled with radioactive liquids. In recent years, 3D printing technology has gained increasing popularity for manufacturing of phantoms, as it is cost-efficient and allows preparation of phantoms of almost any shape. So far, however, direct 3D printing with radioactive building materials has not yet been reported. The aim of this work was to develop a procedure for preparation of
99m
Tc-containing building materials and demonstrate successful application of this material for 3D printing of several test objects.
Method
The desired activity of a
99m
Tcpertechnetate solution eluted from a
99
Mo/
99m
Tc-generator was added to the liquid 3D building material, followed by a minute amount of trioctylphosphine. The resulting two-phase mixture was thoroughly mixed. Following separation of the phases and chemical removal of traces of water, the radioactive building material was diluted with the required volume of non-radioactive building material and directly used for 3D printing.
Results
Using our optimized extraction protocol with trioctylphosphine as complex-forming phase transfer agent, technetium-99m was efficiently transferred from the aqueous
99
Mo/
99m
Tc-generator eluate into the organic liquid resin monomer. The observed radioactivity concentration ratio between the organic phase and the water phase was > 2000:1. The radioactivity was homogeneously distributed in the liquid resin monomer. We did not note differences in the 3D printing behavior of the radiolabeled and the unlabeled organic liquid resin monomers. Radio-TLC and SPECT studies showed homogenous 2D and 3D distribution of radioactivity throughout the printed phantoms. The radioactivity was stably bound in the resin, apart from a small amount of surface-extractable radioactivity under harsh conditions (ethanol at 50 °C).
Conclusions
3D printing of radioactive phantoms using
99m
Tc-containing building materials is feasible. Compared to the classical fillable phantoms, 3D printing with radioactive building materials allows manufacturing of phantoms without cold walls and in almost any shape. Related procedures with longer-lived radionuclides will enable production of phantoms for scanner validation and quality control.
We aimed to print a solid-state germanium-68 phantom for positron emission tomography (PET) with excellent control of the activity concentration in the phantom material, with safe handling ...characteristics, and without inactive walls. To this end, we developed a novel method for incorporating germanium-68 into hydrophobic stereo-lithography monomers. The spontaneous phase transfer process provides complexation and induces phase transfer of aqueous ionic germanium-68 into the hydrophobic monomers, ensures homogenous isotope distribution in the rigid polymer matrix, and prevents leaching in aqueous environments. With this method, we additively manufactured radioactive spheres following the International Electrotechnical Commission design with the addition of a small sphere of 7.7-mm diameter. Extended leaching testing demonstrated near perfect source tightness. The same phantom can be repeatedly used in the quality management of multicenter clinical trials, without preparative errors. Long-lived solid-state phantoms produced with this method will provide consistent and reproducible validation of total body PET/CT systems. PET images with these phantoms acquired within a decaying gallium-68 background at different signal-to-background ratios showed homogenous activity distribution and no fill-related artefacts. The varying background allowed the assessment of trial-specific acquisition protocols that match a given clinical question much better than today’s predefined standard protocol.
Purpose:
A PET/CT system’s imaging capabilities are best described by its point spread function (PSF) in the spatial domain or equivalently by its modulation transfer function (MTF) in the spatial ...frequency domain. Knowing PSFs or MTFs is a prerequisite for many numerical methods attempting to improve resolution and to reduce the partial volume effect. In PET/CT, the observed PSF is a convolution of the system’s intrinsic imaging capabilities including image reconstruction (PSF0) and the positron range function (PRF) of the imaged β
+ emitting isotope. A PRF describes the non-Gaussian distribution of β
+ annihilation events around a hypothetical point source. The main aim was to introduce a new method for determining a PET/CT system’s intrinsic MTF (MTF0) from phantom measurements of hot spheres independently of the β
+ emitting isotope used for image acquisition. Secondary aim was to examine non-Gaussian and nonlinear MTFs of a modern iterative reconstruction algorithm.
Methods:
PET/CT images of seven phantom spheres with volumes ranging from 0.25 to 16 ml and filled either with 18F or with 68Ga were acquired and reconstructed using filtered back projection (FBP). MTFs were modeled with linear splines. The spline fit iteratively minimized the mean squared error between the acquired PET/CT image and a convolution of the thereof derived PSF with a numerical representation of the imaged hot phantom sphere. For determining MTF0, the numerical sphere representations were convolved with a PRF, simulating a fill with either 18F or 68Ga. The MTFs determined by this so-called MTF fit method were compared with MTFs derived from point source measurements and also compared with MTFs derived with a previously published PSF fit method. The MTF fit method was additionally applied to images reconstructed by a vendor iterative algorithm with PSF recovery (Siemens TrueX).
Results:
The MTF fit method was able to determine 18F and 68Ga dependent MTFs and MTF0 from FBP reconstructed images. Root-mean-square deviation between fit determined MTFs and point source determined MTFs ranged from 0.023 to 0.039. MTFs from Siemens TrueX reconstructions varied with size of the imaged sphere.
Conclusions:
MTF0 can be determined regardless of the imaged isotope, when using existing PRF models for the MTF fit method presented. The method proves that modern iterative PET/CT reconstruction algorithms have nonlinear imaging properties. This behaviour is not accessible by point source measurements. MTFs resulting from these clinically applied algorithms need to be estimated from objects of similar geometry to those intended for clinical imaging.
Purpose
To determine the thyroid clearance effective half‐life T with a common handheld electronic dosimeter (ED) in patients undergoing radioiodine treatment for hyperthyroidism.
Methods
Dose rates ...from 12 inpatients were measured daily with an ED and with a clinical uptake counter. The ED was attached to the patient with two different setups, one using a cervical collar and another employing a neck strap. Estimation of T was performed by linear regression analysis of the log of both the ED and the uptake counter measurements versus time. The latter provided the reference data.
Results
Based on repeated neck strap dose rate measurements, individual Ts were determined with clinically required accuracy. The mean difference from the reference method equaled to −0.09 ± 0.35 days.
Conclusions
Determination of individual T is feasible with a common handheld ED using the simple and easy to instruct neck strap measurement setup. This simple method complements stationary uptake counter measurements and thus may improve the accuracy of radioiodine treatment planning by adding an individual T for dose calculation.
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