Matthias J. Schmand

Simultaneous MR/PET Imaging of the Human Brain: Feasibility Study

The purpose of this study was to apply a magnetic resonance (MR) imaging-compatible positron emission tomographic (PET) detector technology for simultaneous MR/PET imaging of the human brain and skull base. The PET detector ring consists of lutetium oxyorthosilicate (LSO) scintillation crystals in combination with avalanche photodiodes (APDs) mounted in a clinical 3-T MR imager with use of the birdcage transmit/receive head coil. Following phantom studies, two patients were simultaneously examined by using fluorine 18 fluorodeoxyglucose (FDG) PET and MR imaging and spectroscopy. MR/PET data enabled accurate coregistration of morphologic and multifunctional information. Simultaneous MR/PET imaging is feasible in humans, opening up new possibilities for the emerging field of molecular imaging.

The ECAT HRRT: performance and first clinical application of the new high resolution research tomograph

The ECAT HRRT is a three-dimensional (3-D) only dedicated brain tomograph employing the new scintillator lutetium-oxy-orthosilicate (LSO) and using depth of interaction (DOI) information to achieve uniform isotropic resolution across a 20-cm diameter volume. With its unique technological innovations it represents the prototype of a new generation of high-resolution brain tomographs. The physical performance with respect to count rate, live time, scatter, sensitivity, and resolution was evaluated with phantom studies and measurements with a point source. The HRRTs imaging performance was tested with phantoms and fluorodeoxyglucose (FDG) scans performed in animal and human brains. We find that due to the significantly improved resolution and the large solid angle covered by the panel detectors, several issues that have been adequately solved for older generation scanners demand new attention for the HRRT, like acquiring and handling large amounts of data effectively, strategies for optimal reconstruction, shielding, and correction of random coincidences.

Toward Implementing an MRI-Based PET Attenuation-Correction Method for Neurologic Studies on the MR-PET Brain Prototype

Several factors have to be considered for implementing an accurate attenuation-correction (AC) method in a combined MR-PET scanner. In this work, some of these challenges were investigated, and an AC method based entirely on the MRI data obtained with a single dedicated sequence was developed and used for neurologic studies performed with the MR-PET human brain scanner prototype. Methods: The focus was on the problem of bone–air segmentation, selection of the linear attenuation coefficient for bone, and positioning of the radiofrequency coil. The impact of these factors on PET data quantification was studied in simulations and experimental measurements performed on the combined MR-PET scanner. A novel dual-echo ultrashort echo time (DUTE) MRI sequence was proposed for head imaging. Simultaneous MR-PET data were acquired, and the PET images reconstructed using the proposed DUTE MRI–based AC method were compared with the PET images that had been reconstructed using a CT-based AC method. Results: Our data suggest that incorrectly accounting for the bone tissue attenuation can lead to large underestimations (>20%) of the radiotracer concentration in the cortex. Assigning a linear attenuation coefficient of 0.143 or 0.151 cm−1 to bone tissue appears to give the best trade-off between bias and variability in the resulting images. Not identifying the internal air cavities introduces large overestimations (>20%) in adjacent structures. On the basis of these results, the segmented CT AC method was established as the silver standard for the segmented MRI-based AC method. For an integrated MR-PET scanner, in particular, ignoring the radiofrequency coil attenuation can cause large underestimations (i.e., ≤50%) in the reconstructed images. Furthermore, the coil location in the PET field of view has to be accurately known. High-quality bone–air segmentation can be performed using the DUTE data. The PET images obtained using the DUTE MRI– and CT-based AC methods compare favorably in most of the brain structures. Conclusion: A DUTE MRI–based AC method considering all these factors was implemented. Preliminary results suggest that this method could potentially be as accurate as the segmented CT method and could be used for quantitative neurologic MR-PET studies.

Performance results of a new DOI detector block for a high resolution PET-LSO research tomograph HRRT

To improve the spatial resolution and uniformity in modern high resolution brain PET systems over the entire field of view (FOV), it is necessary to archive the depth of interaction (DOI) information and correct for spatial resolution degradation. In this work the authors present the performance results of a high resolution LSO/GSO phoswich block detector with DOI capability. This detector design will be used in the new CTI High Resolution Research Tomograph, ECAT HRRT. The two crystal layer (19/spl times/19/spl times/7.5 mm/sup 3/) and a light guide are stacked on each other and mounted on a (2/spl times/2) PMT set, so that the corners of the phoswich are positioned over the PMT centers. The crystal phoswich is cut into a 8/spl times/8 matrix of discrete crystals. The separation of the LSO and the GSO layer by pulse shape discrimination allows discrete DOI information to be obtained. The high light output and the light guide design results in an accurate identification of the 128 single crystals per block. Flood source measurements document a very good homogeneity of events, energy centroid stability and energy resolution (14-20% FWHM) per single crystal. An intrinsic resolution of /spl sim/1.3 mm and the DOI feasibility is extracted by coincidence measurements with a single GSO crystal.

MRI-Assisted PET Motion Correction for Neurologic Studies in an Integrated MR-PET Scanner

Head motion is difficult to avoid in long PET studies, degrading the image quality and offsetting the benefit of using a high-resolution scanner. As a potential solution in an integrated MR-PET scanner, the simultaneously acquired MRI data can be used for motion tracking. In this work, a novel algorithm for data processing and rigid-body motion correction (MC) for the MRI-compatible BrainPET prototype scanner is described, and proof-of-principle phantom and human studies are presented. Methods: To account for motion, the PET prompt and random coincidences and sensitivity data for postnormalization were processed in the line-of-response (LOR) space according to the MRI-derived motion estimates. The processing time on the standard BrainPET workstation is approximately 16 s for each motion estimate. After rebinning in the sinogram space, the motion corrected data were summed, and the PET volume was reconstructed using the attenuation and scatter sinograms in the reference position. The accuracy of the MC algorithm was first tested using a Hoffman phantom. Next, human volunteer studies were performed, and motion estimates were obtained using 2 high-temporal-resolution MRI-based motion-tracking techniques. Results: After accounting for the misalignment between the 2 scanners, perfectly coregistered MRI and PET volumes were reproducibly obtained. The MRI output gates inserted into the PET list-mode allow the temporal correlation of the 2 datasets within 0.2 ms. The Hoffman phantom volume reconstructed by processing the PET data in the LOR space was similar to the one obtained by processing the data using the standard methods and applying the MC in the image space, demonstrating the quantitative accuracy of the procedure. In human volunteer studies, motion estimates were obtained from echo planar imaging and cloverleaf navigator sequences every 3 s and 20 ms, respectively. Motion-deblurred PET images, with excellent delineation of specific brain structures, were obtained using these 2 MRI-based estimates. Conclusion: An MRI-based MC algorithm was implemented for an integrated MR-PET scanner. High-temporal-resolution MRI-derived motion estimates (obtained while simultaneously acquiring anatomic or functional MRI data) can be used for PET MC. An MRI-based MC method has the potential to improve PET image quality, increasing its reliability, reproducibility, and quantitative accuracy, and to benefit many neurologic applications.

Design of a small animal PET imaging system with 1 microliter volume resolution

The design of a new scanner for use in small animal PET imaging is described. The goal is to achieve 1 mm FWHM resolution in each of three orthogonal directions throughout a volume suitable for whole body mouse imaging, roughly 40 mm diameter /spl times/ 80 mm long. Simultaneously, the design should achieve a sensitivity of greater than 5% of all decays from a point source located at the center of the scanner. The scanner uses 12, plane detector banks mounted in a 160 mm diameter ring on a rotating gantry. Each detector bank consists of a 48 /spl times/ 108 array of 20 mm long LSO crystals with an array pitch of 0.87 mm. Each bank uses two Hamamatsu H8500 large-area, multi-anode photomultiplier tubes for fluorescence detection. The detector banks are divided into two sets with the respective lines of response offset by one quarter of the array pitch to give increased sampling density. Tests using a prototype crystal array demonstrate that individual crystals can be resolved. Simulations have been performed to evaluate the performance expected in the complete scanner. With F-18 point sources, the FWHM resolutions in the radial, tangential, and axial directions are less than 1 mm for source positions throughout the desired field of view (FOV). Simultaneously, the detector sensitivity is greater than 7% of all decays for a point source located at the center of the FOV. Results are also presented for simulations using different PET isotopes to investigate the effect of positron range, and for a phantom containing hot spots added to a uniform background to evaluate the scanner performance for an extended object.

Advantages using pulse shape discrimination to assign the depth of interaction information (DOI) from a multi-layer phoswich detector

Recently new high resolution brain PET and PET/SPECT tomographs have been discussed using phoswich detectors. The detector design is based on two layers of scintillation crystals in the z-axis with different physical properties, such as light decay time or light yield. This makes it feasible to assign the depth of interaction (DOI) information in order to be able to correct for spatial degradation in PET or to operate the detector in PET or SPECT mode. The pixel DOI detectors are cost effectively arranged in a block design. In this work the authors discuss the possibility of separating events from the two different layers in the time domain using pulse shape discrimination or in the energy domain using pulse height discrimination. Furthermore, the authors have investigated the feasibility of interdetector scatter suppression using pulse shape discrimination. The measurements have been done using a LSO/LSO high resolution PET detector, a LSO and GSO crystal in coincidence and a NaI(TI)/LSO combined PET/SPECT detector. The investigations show, that the pulse shape discrimination technique has a significant higher identification probability compared to the pulse height discrimination and does make a interdetector scatter detection feasible.

SiPM performance in PET applications: An experimental and theoretical analysis

Silicon Photomultipliers (SiPMs) are increasingly being studied for their use in clinical and pre-clinical PET applications. Many groups have evaluated the performance of Multi-Pixel Photon Counters (MPPCs) from Hamamatsu Photonics. When coupled to PET scintillator crystals, these devices have shown promising results in terms of energy and timing resolution. The purpose of this paper is to analyze the main factors that determine the performance of SiPM based PET detectors and to provide guidelines for further optimization towards the performance levels of state-of-the-art PMT detectors. We present a statistical signal analysis that links the energy and time resolution to fundamental device characteristics, such as photon detection efficiency, cell density, secondary avalanche probability and dark rate. The trade-offs and impact of these device parameters on the overall detector performance is analyzed and discussed.

Scintillation properties of LSO:Ce boules

We investigated the scintillation characteristics of 1880 LSO crystals that were cut from 76 crystal boules. We measured the light output and energy resolution of all 1880 crystals, and also measured the decay times of 1169 of the crystals. We observed trends in light output and energy resolution that were not previously evident from studies of small numbers of crystals, and in addition, a correlation between light output and decay time became evident for the first time. The results may be interpreted in terms of the properties of the two Ce scintillation centers in LSO, the effect of quenching centers, and the effects of Ce segregation during crystal growth.

Performance evaluation of a new LSO high resolution research tomograph-HRRT

In order to improve the capability for investigating the living human brain using positron emission tomography with regard to blood flow, metabolism and receptor characteristics for small structures such as cortical sublayers and nuclei, the spatial resolution has to be improved relative to what is available today. A spatial resolution of 2 mm or less in all three dimensions may be necessary to reach these research goals. In order to meet this goal a new next generation high resolution 3D-only LSO brain PET has been developed at CTI. The HRRT (High Resolution Research Tomograph) is not only the first LSO PET for human studies it is also the first PET with full DOI capability over an extended FOV. The HRRT has been delivered to the MPI Cologne in February 1999 and is presently being optimized in terms of energy discrimination, crystal positioning, coincidence timing and PSD to achieve optimal system performance with respect to resolution, count rate efficiency and scatter. The panel detector setup with its new setup challenges is still under development and investigations. First evaluation measurements are presently obtained. The measurements promise an excellent high resolution performance with a high count rate capability of the HRRT. The high random rates and the SF fraction measurements underline the necessity for a short coincidence time window and an improved energy resolution for such an in the images reveal a not yet optimized system setup and reconstruction parameters. The authors believe the results demonstrate the high capability of the new brain tomograph and justify the excitement for the first LSO tomograph with DOI capability for human brain investigations and the new scintillator LSO, superior for coincidence timing and energy resolution.