C. Lois

Effect of MR contrast agents on quantitative accuracy of PET in combined whole-body PET/MR imaging

PurposeClinical PET/MR acquisition protocols entail the use of MR contrast agents (MRCA) that could potentially affect PET quantification following MR-based attenuation correction (AC). We assessed the effect of oral and intravenous (IV) MRCA on PET quantification in PET/MR imaging.MethodsWe employed two MRCA: Lumirem® (oral) and Gadovist® (IV). First, we determined their reference PET attenuation values using a PET transmission scan (ECAT-EXACT HR+, Siemens) and a CT scan (PET/CT Biograph 16 HI-REZ, Siemens). Second, we evaluated the attenuation of PET signals in the presence of MRCA. Phantoms were filled with clinically relevant concentrations of MRCA in a background of water and 18F-fluoride, and imaged using a PET/CT scanner (Biograph 16 HI-REZ, Siemens) and a PET/MR scanner (Biograph mMR, Siemens). Third, we investigated the effect of clinically relevant volumes of MRCA on MR-based AC using human pilot data: a patient study employing Gadovist® (IV) and a volunteer study employing two different oral MRCA (Lumirem® and pineapple juice). MR-based attenuation maps were calculated following Dixon-based fat–water segmentation and an external atlas-based and pattern recognition (AT&PR) algorithm.ResultsIV and oral MRCA in clinically relevant concentrations were found to have PET attenuation values similar to those of water. The phantom experiments showed that under clinical conditions IV and oral MRCA did not yield additional attenuation of PET emission signals. Patient scans showed that PET attenuation maps are not biased after the administration of IV MRCA but may be biased, however, after ingestion of iron oxide-based oral MRCA when segmentation-based AC algorithms are used. Alternative AC algorithms, such as AT&PR, or alternative oral contrast agents, such as pineapple juice, can yield unbiased attenuation maps.ConclusionIn clinical PET/MR scenarios MRCA are not expected to lead to markedly increased attenuation of the PET emission signals. MR-based attenuation maps may be biased by oral iron oxide-based MRCA unless advanced AC algorithms are used.

A feasibility study on the use of arrays of discrete SiPMs for MR compatible LYSO readout using Monte Carlo simulation

This paper shows a feasibility study on the use of discrete SiPMs to read out monolithic scintillator-based detectors for use in molecular imaging applications. Monte Carlo simulations are carried out in order to evaluate the basic performance of 8 × 8 arrays of discrete SiPMs with different PDE values and compare it to conventional 64 channel multi-anode PMT (MA-PMT) readout. A detailed optical transport model was incorporated into the Monte Carlo simulation and a detector module based on a monolithic scintillator crystal of 50 × 50 × 4 mm3 coupled to a MA-PMT was built for experimental validation. The effect of the SiPM dynamic range was also investigated by including a model of the saturation effects into the optical transport simulation. The results show that a detector module based on an array of 8 × 8 discrete SiPM devices (3 × 3 mm2 and PDE > 32%) is feasible as a replacement of a 64 channel MA-PMT in order to read out large monolithic crystals for MR-compatible gamma cameras.

Comparison of NEMA NU 4-2008 vs NEMA NU 2-2001 for the performance evaluation of the microPET R4 system

Positron Emission Tomography (PET) techniques have gained an important role in the diagnosis of human diseases as well as for pharmaceutical and biomedical research. In order to best optimize their capabilities, evaluation of their performance has to be achieved through standardized methodology from specific standards. The aim of this work was to assess the performance evaluation of the microPET R4 system for imaging rodents according to the new NEMA NU 4-2008 standards and compare it against its previous evaluation according the adapted clinical NEMA NU 2-2001. All measurements were made for an energy window of 350-650 keV. Spatial resolution of 2.12 mm FWHM in the radial direction, 2.66 mm FWHM in the tangential direction and 2.23 mm FWHM in the axial direction was measured in the centre of the axial FOV. Scatter fraction for mouse and rat phantoms were found to be 9.5% and 22% respectively. The maximum noise-equivalent count rate (NECR) was 110 kcps at 121.3 MBq for the rat phantom 287 kcp at 129.8 MBq for the mouse phantom. The absolute sensitivity of the scanner was found to be 1.66% at the centre of the field of view.

Characterization of low energy Lu background on continuous LYSO blocks

The presence of the naturally occurring isotope 176Lu gives rise to background count rates in LSO/LYSO crystals to be used in PET and SPECT systems. The aim of this work is to measure 176Lu background count rates and its spatial distributions in a continuous LYSO block. Our results show that the low energy component of 176Lu background is related to partial energy deposition mainly produced near the crystal edge showing a hot-perimeter artifact. Furthermore, 176Lu background rate is observed to be x10 times lower for a 100-180 keV than for a 411-611 keV energy window. Our findings indicate that when using monolithic LYSO crystal blocks, 176Lu background is not expected to significantly deteriorate the performance of single photon detection systems.

Analytical Study of the Effect of the System Geometry on Photon Sensitivity and Depth of Interaction of Positron Emission Mammography

Positron emission mammography (PEM) cameras are novel-dedicated PET systems optimized to image the breast. For these cameras it is essential to achieve an optimum trade-off between sensitivity and spatial resolution and therefore the main challenge for the novel cameras is to improve the sensitivity without degrading the spatial resolution. We carry out an analytical study of the effect of the different detector geometries on the photon sensitivity and the angle of incidence of the detected photons which is related to the DOI effect and therefore to the intrinsic spatial resolution. To this end, dual head detectors were compared to box and different polygon-detector configurations. Our results showed that higher sensitivity and uniformity were found for box and polygon-detector configurations compared to dual-head cameras. Thus, the optimal configuration in terms of sensitivity is a PEM scanner based on a polygon of twelve (dodecagon) or more detectors. We have shown that this configuration is clearly superior to dual-head detectors and slightly higher than box, octagon, and hexagon detectors. Nevertheless, DOI effects are increased for this configuration compared to dual head and box scanners and therefore an accurate compensation for this effect is required.

Design simulations of a LSO crystal block detector module for dual PET/SPECT systems

Monte Carlo simulations were carried out in order to guide the choice of crystal block material and thickness for a small FOV PET/SPECT camera. A continuous crystal block of 50mm x 50mm was simulated considering several thickness (8 mm, 16 mm and 32 mm) and two scintillation crystal types(LSO:Ce and NaI:Tl). Detection module parameters like spatial resolution at centre and border of FOV, efficiency and energy resolution were evaluated. Despite there is a common opinion that LSO is a less valuable material for SPECT detection, our findings demonstrate that LSO crystal blocks have interesting design features for the development of an hybrid SPECT and PET system. Our results show that by adequate corrections of non-linearity of a thick crystal block response, a high detection efficiency can be preserved (nearly 80% for PET and 99.9% for SPECT) without compromising high spatial resolution (about 1.9 mm for PET and 1.45 mm for SPECT).

The silicon tracker of the LHCb experiment

LHCb is one of the experiments of the Large Hadron Collider at CERN, dedicated to B-physics and CP-violation measurements. To fully exploit the physics potential, a good tracking performance with high efficiency in a high particle density environment close to the beam pipe is required. Silicon strip detectors with large read-out pitch and long strips will be used for the LHCb inner tracker after the magnet and the trigger tracker station in front of the magnet. We report here about the design of the silicon tracker, test beam results and the electrical tests foreseen during module production.

Monte Carlo optimization of SiPM readout configurations for continuous LYSO blocks

Monte Carlo simulations were carried out in order to evaluate the performance of readout solutions for monolithic LYSO blocks, considering several 2D array configurations of discrete SiPMs. Results were compared to conventional multianode PMT readout. Our findings show that a detector module based on an array of 8×8 discrete SiPM may lead to better energy resolution and comparable spatial resolution. Our work shows the feasibility of using discrete SiPMs as readout for monolithic scintillator-based detector modules.

LHCb Silicon Tracker Performance Studies

LHCb is one of the experiments for the Large Hadron Collider at CERN and is dedicated to B-physics and CP-violation measurements. To exploit its physics potential good tracking performance with high efficiency in a high particle density environment close to the beam pipe is required. Silicon strip detectors with large read-out pitch and long strips will be used for the LHCb Inner Tracker behind the magnet and the Trigger Tracker station in front of the magnet. We report here about the design of the Silicon Tracker, test beam results and the electrical tests foreseen during production

The Silicon Tracker of the LHCb Experiment

LHCb is one of the experiments of the Large Hadron Collider at CERN, dedicated to B-physics and CP-violation measurements. To fully exploit the physics potential, a good tracking performance with high efficiency in a high particle density environment close to the beam pipe is required. Silicon strip detectors with large read-out pitch and long strips will be used for the LHCb inner tracker after the magnet and the trigger tracker station in front of the magnet. We report here about the design of the silicon tracker, test beam results and the electrical tests foreseen during module production.