Horst Halling

Comparison of LuYAP, LSO, and BGO as scintillators for high resolution PET detectors

For high resolution positron emission tomographs based on scintillation detectors a fast, dense, and bright scintillator is required. A sample of a new scintillator, Lu/sub 0.8/Y/sub 0.2/AlO/sub 3/:Ce (LuYAP) with a density of 7.7 g/cm/sup 3/ and a scintillation decay time of 20 and 160 ns is compared with LSO and BGO crystals of the same size to estimate the potential of the crystal for high resolution PET detectors. Special attention was paid to use an application specific measurement setup with respect to high resolution PET. The light yield of the crystals using polished crystals as well as PTFE tape and BaSO/sub 4/ as reflector material and the temperature dependence of the light yield are measured using crystals of 2 /spl times/ 2 /spl times/ 10 mm/sup 3/. At room temperature, the light yields of BGO : LuYAP : LSO are 1 : 1.5 : 4.5 with energy resolutions of 27.4% (BGO), 20% (LuYAP) and 15% (LSO), respectively. LuYAP shows an increase in light yield with increasing temperature whereas both BGO and LSO show a decrease.

Coincidence detection by digital processing of free-running sampled pulses

Coincident events in two scintillator crystals coupled to photomultipliers (PMT) are detected by processing just the digital data of the recorded pulses. For this purpose the signals from both PMTs are continuously sampled by freerunning ADCs at a sampling rate of 40 MHz: For each sampled pulse the starting time is determined by processing the pulse data. Even a fairly simple interpolating algorithm results in a FWHM of about 2 ns: r 2002 Elsevier Science B.V. All rights reserved.

Pulse recording by free-running sampling

Pulses from a position-sensitive photomultiplier (PS-PMT) are recorded by free running ADCs at a sampling rate of 40 MHz. A four-channel acquisition-board has been developed which is equipped with four 12 bit-ADCs connected to one FPGA (field programmable gate array). The FPGA manages data acquisition and the transfer to the host computer. It can also work as a digital trigger, so a separate hardware-trigger can be omitted. The method of free running sampling provides a maximum of information, besides the pulse charge and amplitude also pulse shape and starting time are contained in the sampled data. These informations are crucial for many tasks such as distinguishing between different scintillator materials, determination of radiation type, pile-up recovery, coincidence detection or time-of-flight applications. The absence of an analog integrator allows coping with very high count rates. Since this method is going to be employed in positron emission tomography (PET), the position of an event is another important information. The simultaneous readout of four channels allows localization by means of center-of-gravity weighting. First results from a test setup with LSO-scintillators coupled to the PS-PMT are presented.

Measurement of rubidium and xenon absolute polarization at high temperatures as a means of improved production of hyperpolarized 129Xe

We report on a rubidium–xenon (Rb–Xe) polarization unit for the continuous production of large quantities of hyperpolarized 129Xe. The unit includes two diagnostic systems which enable the absolute measurement of both the 85Rb and 129Xe polarization in situ and at high temperatures. The Rb diagnostic system allows the measurement of one‐ or two‐dimensional images of the absolute Rb polarization and thus enables the experimental study of light penetration into the optical pumping cell. The equilibrium Xe polarization measured in the optical pumping cell and in the freezing unit is typically ∼20%, under optimal flowing conditions, and this is much lower than the volume‐averaged rubidium polarization. Copyright

Recent results of the TierPET scanner

At the Forschungszentrum Julich a high resolution positron emission tomograph (TierPET) for imaging small laboratory animals, especially rats, has been constructed. The scanner is based on arrays of YAP crystals. As a special feature the detector distances can be varied continuously from 16 to 58 cm. Due to the variable detector distances the performance of the scanner has to be evaluated in various configurations. Special attention was paid to dedicated data acquisition protocols and adequate applications with regard to the system sensitivity.

Preliminary studies of a micro-CT for a combined small animal PET/CT scanner

We are developing an X-ray computed tomography (CT) system which will be combined with a high resolution animal PET system. This permits acquisition of both molecular and anatomical images in a single machine. In particular the CT will also be utilized for the quantification of the animal PET data by providing accurate data for attenuation correction. A first prototype has been built using a commercially available plane silicon diode detector. A cone-beam reconstruction provides the images using the Feldkamp algorithm. First measurements with this system have been performed on a mouse. It could be shown that the CT setup fulfils all demands for a high quality image of the skeleton of the mouse. It is also suited for soft tissue measurements. To improve contrast and resolution and to acquire the X-ray energy further development of the system, especially the use of semiconductor detectors and iterative reconstruction algorithms are planned.

Development of a multi-pinhole detector for high-sensitivity SPECT imaging

We report on the development of a multi-pinhole detector for high-sensitivity SPECT imaging. The work is based on a high resolution SPECT system which already is in successful operation since a couple of months. The goal of the present work is to increase the sensitivity of the current detector by a factor of 30 to 40 and to optimize it for I-123 tracer development and evaluation. Besides improvements in the detector and aperture physics a multi-pinhole arrangement is envisaged. A series of Monte-Carlo simulations and preliminary phantom studies have shown the feasibility of this approach although a number of careful optimizations concerning the layout of the aperture geometry and the iterative reconstruction algorithm are still required. Results of the simulations and tests with simulated phantom data will be presented and the quality assessment procedures will be discussed.

Homogenization of the MultiChannel PM gain by inserting light attenuating masks

MultiChannel Photomultipliers (PM), like the R7600-00-M64 or R5900-00-M64 from Hamamatsu, are often chosen as photodetectors in high-resolution positron emission tomography (PET). A major problem of this PM is the nonuniform channel gain. In order to solve this problem, light attenuating masks were created. The aim of the masks is a homogenization of the output of all 64 channels using different hole sizes at the channel positions. The hole area, which is individually defined for the different channels, is inversely proportional to the channel gain. The measurements by inserting light attenuating masks improved a homogenization to a ratio of 1:1.2.

Molecular gastrin receptor localisation in mice using high-resolution SPET-MRI image fusion

The images demonstrate the biodistribution of 99mTcDemogastrin (99mTc-minigastrin) studied with a high-resolution SPET camera in a mouse bearing an AR4-2J subcutaneous tumour (rat exocrine pancreatic cancer expressing gastrin receptors). SPET sections were matched and fused with high-resolution (300 μm isotropic resolution) MR images of the same animal. Uptake of the 99mTc-Demogastrin can be seen in the renal cortex (A), the receptor-positive tumour (B), the stomach (organ with the highest physiological receptor expression (C)) and, to a very small extent, in the liver. The uptake in the tumour correlates with the MR signal of the double echo in the steady state (DESS) sequence. Intermediate MR signal areas, indicating vital tumour tissue, show high gastrin uptake. Areas of higher MR signal intensity, indicating oedema and/or necrosis, show lower 99mTc-Demogastrin uptake.

High temperature superconductor dc-SQUID microscope with a soft magnetic flux guide

A scanning SQUID microscope based on high-temperature superconductor (HTS) dc-SQUIDs was developed. An extremely soft magnetic amorphous foil was used to guide the flux from room temperature samples to the liquid-nitrogen-cooled SQUID sensor and back. The flux guide passes through the pick-up loop of the HTS SQUID, providing an improved coupling of magnetic flux of the object to the SQUID. The device measures the z component (direction perpendicular to the sample surface) of the stray field of the sample, which is rastered with submicron precision in the x–y direction by a motorized computer-controlled scanning stage. A lateral resolution better than 10 µm, with a field resolution of about 0.6 nT Hz−1/2 was achieved for the determination of the position of the current carrying thin wires. The presence of the soft magnetic foil did not significantly increase the flux noise of the SQUID.