Klaus A. Hamacher

Possible axonal regrowth in late recovery from the minimally conscious state

We used diffusion tensor imaging (DTI) to study 2 patients with traumatic brain injury. The first patient recovered reliable expressive language after 19 years in a minimally conscious state (MCS); the second had remained in MCS for 6 years. Comparison of white matter integrity in the patients and 20 normal subjects using histograms of apparent diffusion constants and diffusion anisotropy identified widespread altered diffusivity and decreased anisotropy in the damaged white matter. These findings remained unchanged over an 18-month interval between 2 studies in the first patient. In addition, in this patient, we identified large, bilateral regions of posterior white matter with significantly increased anisotropy that reduced over 18 months. In contrast, notable increases in anisotropy within the midline cerebellar white matter in the second study correlated with marked clinical improvements in motor functions. This finding was further correlated with an increase in resting metabolism measured by PET in this subregion. Aberrant white matter structures were evident in the second patients DTI images but were not clinically correlated. We propose that axonal regrowth may underlie these findings and provide a biological mechanism for late recovery. Our results are discussed in the context of recent experimental studies that support this inference.

Validation of GATE Monte Carlo simulations of the GE Advance/Discovery LS PET scanners

The recently developed GATE (GEANT4 application for tomographic emission) Monte Carlo package, designed to simulate positron emission tomography (PET) and single photon emission computed tomography (SPECT) scanners, provides the ability to model and account for the effects of photon noncollinearity, off-axis detector penetration, detector size and response, positron range, photon scatter, and patient motion on the resolution and quality of PET images. The objective of this study is to validate a model within GATE of the General Electric (GE) Advance/Discovery Light Speed (LS) PET scanner. Our three-dimensional PET simulation model of the scanner consists of 12 096 detectors grouped into blocks, which are grouped into modules as per the vendors specifications. The GATE results are compared to experimental data obtained in accordance with the National Electrical Manufactures Association/Society of Nuclear Medicine (NEMA/SNM), NEMA NU 2-1994, and NEMA NU 2-2001 protocols. The respective phantoms are also accurately modeled thus allowing us to simulate the sensitivity, scatter fraction, count rate performance, and spatial resolution. In-house software was developed to produce and analyze sinograms from the simulated data. With our model of the GE Advance/Discovery LS PET scanner, the ratio of the sensitivities with sources radially offset 0 and 10 cm from the scanners main axis are reproduced to within 1% of measurements. Similarly, the simulated scatter fraction for the NEMA NU 2-2001 phantom agrees to within less than 3% of measured values (the measured scatter fractions are 44.8% and 40.9 +/- 1.4% and the simulated scatter fraction is 43.5 +/- 0.3%). The simulated count rate curves were made to match the experimental curves by using deadtimes as fit parameters. This resulted in deadtime values of 625 and 332 ns at the Block and Coincidence levels, respectively. The experimental peak true count rate of 139.0 kcps and the peak activity concentration of 21.5 kBq/cc were matched by the simulated results to within 0.5% and 0.1% respectively. The simulated count rate curves also resulted in a peak NECR of 35.2 kcps at 10.8 kBq/cc compared to 37.6 kcps at 10.0 kBq/cc from averaged experimental values. The spatial resolution of the simulated scanner matched the experimental results to within 0.2 mm.

TH‐D‐330A‐01: Annual Gamma Camera QC: Techniques And Phantoms To Simplify The Implementation Of The NEMA NU‐1 Procedures For Tests Recommended By The ACR

The objective of this work is to present techniques to perform the NEMA tests that are suggested by the ACR for annual gamma camera QC. Since the recommendation contains nine tests, the goal is to devise simple and efficient procedures to reduce the required time without compromising the quality. Also included is a comparison of test equipment conforming to NEMA specifications to equipment made in‐house. The time to complete the annual QC is markedly reduced by the availability of specifically designed phantoms. They provide a reliable means to compare the cameras and to establish a performance baseline. For two tests, Multi Window Spatial Registration and SPECT Resolution with scatter, the NEMA prescribed equipment is compared with readily available devices. The comparison of the commercial MWSP device and the one produced in‐house showed that the average maximum difference of the center of mass for the three Ga‐67 peaks measured at various points across the crystal face is 0.35 mm vs. 0.33 mm for the commercial and the in‐house device, respectively. For the SPECT resolution using three capillary tubes filled with Tc‐99m, a NEMA phantom is compared to a modified Jaszczak phantom. The FWHM is 4.78 pxl vs. 4.79 pxl (radial) and 4.144 pxl vs. 4.167 pxl (azimuthal) for NEMA and Jaszczak phantom, respectively. Further topics include sensitivity, count rate performance, comparison of fit methods, and monitor/printer QC.

SU‐EE‐A4‐03: Validation of GATE Monte Carlo Simulations of the Noise Equivalent Count Rate and Image Quailty for the GE Discovery LS PET Scanner

Purpose: The needs of radiation therapytreatment planning impose higher demands on PET/CT imaging accuracy. The recently developed GATE (Geant4 Application for Tomographic Emission) Monte Carlo package, provides the possibility to model accurately the factors contributing to decreased PET resolution and image degradation. The purpose of this study is to test GATEs ability to predict time curves and image quality (IQ) for the GE Discovery LS/Advance PET scanner. Method and Materials: Our 3D PET simulation model of the GE Discovery LS scanner and phantoms follows both the vendors and NEMAs specifications and was previously validated for the PET scatter fraction and sensitivity tests. Simulations with this model were performed for the count rate and IQ NEMA‐2001 tests as a function of activity concentration. The Software for Tomographic Image Reconstruction (STIR) package was used to reconstruct the simulated data which was then compared to experiment. Results: Our simulations correctly predict the shape and magnitude of the true, scatter, random, and NEC rates. The simulated peak true and NEC rates are both within 3 kBq/cc of the measured data. Scatter and random rejection in the Monte Carlo data dramatically improved the agreement between measured and simulated contrast ratios. The cold sphere contrast ratio is within 9% of the measured data when the scatter and random coincidences were rejected. Larger discrepancies for the hot sphere contrast are currently observed and investigated. Conclusion:Monte Carlo simulation of PETimages and the corresponding NEC rates can aid in improving PETimage quality. The ability to model the features of PET scanners accurately makes GATE a potentially useful tool in improving PETs performance, which is necessary for its effective use in radiation treatment planning This work is sponsored by Program Project Grant CA059017‐12 from the National Cancer Institute and by an internal departmental grant.