Marie-Laure Camborde

The second generation HRRT - a multi-centre scanner performance investigation

The high resolution research tomograph (HRRT) is one of the most complex existing positron emission tomographs: it is the only human size scanner capable of decoding the depth of the /spl gamma/-ray interaction in the crystal, using a lutetium LSO/LYSO phoswich detector arrangement. In this study we determined basic scanner hardware characteristics, such as scanner data acquisition stability, and their variability across eleven centres. In addition a subset of the NEMA NU-2001 standards measurements was performed. We found (i) significant variability in the DOI decoding results between centres, (ii) a trend toward an increasing number of detected true coincident events as a function of elapsed time from scanner calibration likely due to a shifting energy spectrum, (iii) a count-rate dependent layer identification, (iv) scatter fraction ranging from /spl sim/ 42% to 54% where the variability was partly related to the shifting of the energy spectrum, (v) sensitivity ranging from /spl sim/5.5% to 6.5% across centres, (vi) resolution of /spl sim/(2.5 mm)/sup 3/, fairly consistent across centres, (vii) image quality which is very comparable to other scanners.

In vivo measurement of density and affinity of the monoamine vesicular transporter in a unilateral 6-hydroxydopamine rat model of PD

This is the first in vivo determination of the vesicular monoamine transporter (VMAT2) density (Bmax) and ligand–transporter affinity (Kdapp) in six unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats using micro-positron emission tomography (PET) imaging with [11C]-(+)-α-dihydrotetrabenazine (DTBZ). A multiple ligand concentration transporter assay (MLCTA) was used to determine a Bmax value of 178 ± 32 pmol/mL and a Kdapp of 47.7 ± 9.3 pmol/mL for the non-lesioned side and 30.52 ± 5.84 and 43.4 ± 15.52 pmol/mL for the lesioned side, respectively. While Bmax was significantly different between the two sides, no significant difference was observed for the Kdapp. In addition to demonstrating the feasibility of in vivo Scatchard analysis in rats, these data confirm the expectation that a 6-OHDA lesion does not affect the affinity; a much simpler binding potential (BP) measure can thus be used as a marker of lesion severity (LS) in this rat model of Parkinsons disease. A transporter occupancy curve demonstrated negligible transporter occupancy (∼1%) at a specific activity (SA) of 1100 nCi/pmol (assuming an injected dose of 100 μCi/100 g), while 10% occupancy was estimated at 100 nCi/pmol. An indirect measurement indicated that the degree of occupancy as a function of SA is independent of LS. Finally, BP measurement reproducibility was assessed and found to be 11% ± 7% for the healthy and 8% ± 12% for the lesioned side. Quantitative PET results can thus be obtained even for severely lesioned animals with the striatum on one side not clearly visible provided accurate image analysis methods are used.

A central positron source to perform the timing alignment of detectors in a PET scanner

Accurate timing alignment and stability are important to maximize the true counts and minimize the random counts in positron emission tomography. Its importance increases in time-of-flight (TOF) scanners. We propose using a central positron emitting source enclosed in a detector which detects the excess energy of the positron before it annihilates as a timing reference. All crystals can be time-aligned with respect to this central source. We evaluated 10 /spl mu/Ci /sup 22/Na and /sup 68/Ge sources embedded in cylinders of plastic scintillator coupled to a fast PMT. Light flashes produced after the parent isotope emits positrons are detected, and the anode signals from the PMT are the reference time for each positron decay. The time delay before the gamma ray is detected by the scanners conventional gamma ray detectors is the time offset to be applied to that crystal. Since all detectors are almost the same distance from the central source, TOF errors are minimized. Preliminary results show a mean signal amplitude of >0.5 V from /sup 22/Na at 1000-V PMT bias, a timing FWHM of 850 ps with respect to a small LSO crystal. This suggests it could be useful to align both conventional and TOF PET scanners.

Monte Carlo modelling of singles-mode transmission data for small animal PET scanners

The attenuation corrections factors (ACFs), which are necessary for quantitatively accurate PET imaging, can be obtained using singles-mode transmission scanning. However, contamination from scatter is a largely unresolved problem for these data. We present an extension of the Monte Carlo simulation tool, GATE, for singles-mode transmission data and its validation using experimental data from the microPET R4 and Focus 120 scanners. We first validated our simulated PET scanner for coincidence-mode data where we found that experimental resolution and scatter fractions (SFs) agreed well for simulations that included positron interactions and scatter in the source material. After modifying GATE to model singles-mode data, we compared simulated and experimental ACFs and SFs for three different sized water cylinders using 57Co (122 keV photon emitter) and 68Ge (positron emitter) transmission sources. We also propose a simple correction for a large background contamination we identified in the 68Ge singles-mode data due to intrinsic 176Lu radioactivity present in the detector crystals. For simulation data, the SFs agreed to within 1.5% and 2.5% of experimental values for background-corrected 68Ge and 57Co transmission data, respectively. This new simulation tool accurately models the photon interactions and data acquisition for singles-mode transmission scans.

Effect of normalization method on image uniformity and binding potential estimates on microPET

This study investigates different detector normalization procedures for a small animal scanner, specifically the Concorde microPET/sup /spl reg// R4. The procedures were compared in terms of: (i) image uniformity, (ii) performance as a function of count rate, and (iii) impact on the estimate of the binding potential (BP) in brain rat studies. Image uniformity studies were performed on two cylindrical phantoms of different size filled with an aqueous concentration of /sup 11/C (38 kBq/mL and 615 kBq/mL). BP was estimated with the Logan graphical approach on 12 /sup 11/C-Methylphenidate and 9 /sup 11/C-Dihydrotetrabenazine rat studies processed with all the normalization procedures. We found that: 1) the combination, the geometry normalized combination, the component-point, and the geometry normalized component procedures significantly improve radial image uniformity compared to the direct-point and direct-cylinder procedures, 2) the geometry normalized combination procedure seems to provide the best radial and axial uniformity, 3) a mismatch between the count rates at which the normalization and the emission scans are acquired degrades the axial uniformity by 47% to 98% whereas this effect was not observed for radial uniformity, and 4) the difference in BP values obtained from data corrected with different normalization procedures is as high as 15% for normal striatum and 75% for lesion striatum.

Impact of the spatial normalization template and realignment procedure on the SPM analysis of [11C]Raclopride PET studies

Background/Objectives: In this paper, we address the issue of how the preprocessing steps of a typical neuroreceptor PET study may influence the outcome of a statistical parametric mapping (SPM) analysis examining intercondition differences. Two preprocessing steps and their possible interaction were investigated: 1) the realignment of dynamic images to correct for patients motion and 2) the effect of the template in the spatial normalization step required by SPM. Methods : The impact of different realignment methods was tested by performing the SPM analysis on a six patient-two conditions /sup 11/C-raclopride (RAC) study where the dynamic images of each subject were re-aligned with four methods. Once realigned, the images were spatially normalized to the standard cerebral blood flow (CBF) template, available in SPM, and to a RAC template. The SPM pattern showing the intercondition differences obtained in each case was compared with the expected one. The accuracy of the four realignment methods used was further tested using a set of motion-free RAC studies. Results/Conclusion: We found that: 1) different realignment methods may create different intercondition difference patterns; 2) such differences in the results may be due to a failure in accuracy of some realignment methods; and 3) the use of a template that closely matches the spatial distribution of the tracer under investigation can minimize these problems.

Stereotactic body radiotherapy for unresectable hepatocellular carcinoma: A population based analysis of tumors up to 15 cm.

322 Background: Stereotactic body radiotherapy (SBRT) has an emerging role for patients with hepatocellular (HCC). The purpose of this study is to describe the efficacy of HCC SBRT at a population level without any tumor size restriction. Methods: A retrospective study of the first 49 HCC patients treated with SBRT between March 2011-July 2015 at the British Columbia Cancer Agency was conducted. All patients were either ineligible for or failed standard local therapies (partial hepatectomy, radiofrequency ablation, percutaneous ethanol injection, transarterial chemoembolization, or radioembolization) and discussed in a multidisciplinary rounds setting. Local control (LC), progression free survival (PFS) (defined as freedom from failure elsewhere in the liver) and overall survival (OS) were analyzed at 1 and 2 years. Changes to Child’s Pugh (CP) score at 3 months post-SBRT were also analyzed. Results: Median follow-up was 14 months with 35 patients (71%) alive at last follow-up. Fifty-two separate HCC lesi...

TH-C-19A-03: Characterization of the Dose Per Pulse Dependence of Various Detectors Used in Quality Assurance of FFF Treatment Plans

PURPOSE To present the dose per pulse dependence of various QA devices under Flattening Filter Free (FFF) conditions. METHODS Air and liquid filled ion chamber arrays, diode arrays, radiochromic film and optically stimulated luminescence detectors were investigated. All detectors were irradiated under similar conditions of varying dose per pulse on a TrueBeam linac. Dose per pulse was controlled by varying SSD from 70 to 160 cm providing a range from ˜0.5 to ˜3 mGy per pulse. MU rates of up to 2400 MU/min for 10X FFF and 1400 MU/min for the 6X FFF beam were used. Beam pulses were counted using the Profiler™ diode array and pulse timing was confirmed by examining linac node files. Delivered doses were calculated with the Eclipse™ treatment planning system. RESULTS The detectors show a range of behaviors depending on the detector type, as expected. Diode arrays show up to 4% change in sensitivity (sensitivity increases with increasing dose per pulse) over the range tested. Air and liquid ion chambers arrays show a change in sensitivity of up to 3% (air) and 6% (liquid) (sensitivity decreases with increasing dose per pulse) while film and OSLD do not demonstrate a dependence on dose per pulse. CONCLUSION Dependence of detector response on dose per pulse varies considerably depending on detector design. Interplay between dose per pulse and MU rate also exists for some detectors. Due diligence is required to characterize detector response prior to implementation of a QA protocol for FFF treatment delivery. During VMAT delivery, the MU rate may also vary dramatically within a treatment fraction. We intend to further investigate the implications of this for VMAT FFF patient specific quality assurance. T Karan and F Viel have received partial funding through the Varian Research program.

Accelerated microPET Transmission Imaging

The Siemens microPET family of scanners provides for acquiring a measured attenuation correction using a 57Co point transmission source that spirals around the field of view (FOV). The standard transmission source mechanism advances the source 0.5 mm for every spiral about the FOV. The transmission scan measurement requires approximately 10 minutes, so that often in the interest of time it is not done. The aim of this work was to reduce the transmission scan time on the microPET without compromising the accuracy of the measured attenuation map. The transmission source mechanisms of a microPET R4 and Focus 120 were modified by replacing the 0.5 mm pitch source driving rod with rods of pitches 1.0 mm and 1.5875 mm, thus reducing the transmission scanning times by 50% and 68% respectively. The reductions in transmission imaging acquisition times drop to 40% and 56% for the two rods when the time required to configure the system to low energy singles mode is considered. Transmission data were acquired of uniform cylinders and of a euthanized rat. The reconstructed attenuation maps showed no apparent artefacts introduced by the increase in source pitch. Image noise increased according to that expected from Poisson statistics. The effects of increased image noise had little effect on the attenuation map when segmentation methods were used. We conclude that accurate measured attenuation maps can be produced in as little as 1/3 of the normal transmission scan time by modifying the transmission source mechanism to have a more aggressive source pitch.