Investigation of time-of-flight benefits in an LYSO-based PET/CT scanner: A Monte Carlo study using GATE

a b s t r a c t The advent of fast scintillators yielding great light yield and/or stopping power, along with advances in photomultiplier tubes and electronics, have rekindled interest in time-of-flight (TOF) PET. Because the potential performance improvements offered by TOF PET are substantial, efforts to improve PET timing should prove very fruitful. In this study, we performed Monte Carlo simulations to explore what gains in PET performance could be achieved if the coincidence resolving time (CRT) in the LYSO-based PET component of Discovery RX PET/CT scanner were improved. For this purpose, the GATE Monte Carlo package was utilized, providing the ability to model and characterize various physical phenomena in PET imaging. For the present investigation, count rate performance and signal to noise ratio (SNR) values in different activity concentrations were simulated for different coincidence timing windows of 4, 5.85, 6, 6.5, 8, 10 and 12 ns and with different CRTs of 100-900 ps FWHM involving 50 ps FWHM increments using the NEMA scatter phantom. Strong evidence supporting robustness of the simulations was found as observed in the good agreement between measured and simulated data for the cases of estimating axial sensitivity, axial and transaxial detection position, gamma non-collinearity angle distribution and positron annihilation distance. In the non-TOF context, the results show that the random event rate can be reduced by using narrower coincidence timing window widths, demonstrating considerable enhancements in the peak noise equivalent count rate (NECR) performance. The peak NECR had increased by $ 50% when utilizing the coincidence window width of 4 ns. At the same time, utilization of TOF information resulted in improved NECR and SNR with the dramatic reduction of random coincidences as a function of CRT. For example, with CRT of 500 ps FWHM, a factor of 2.3 reduction in random rates, factor of 1.5 increase in NECR and factor of 2.1 improvement in SNR is achievable. The results of this study show that in addition to the high sensitivity of Discovery RX PET/CT scanner, the implementation of TOF with proper CRT can efficiently improve the image quality in this scanner. Having successfully simulated the DRX scanner and utilization of TOF information, our research goal is to use the Monte Carlo simulation technique to arrive at powerful, accurate and feasible reconstruction algorithms

A novel energy mapping approach for CT-based attenuation correction in PET

Purpose: Dual-energy CT (DECT) is arguably the most accurate energy mapping technique in CT-based attenuation correction (CTAC) implemented on hybrid PET/CT systems. However, this approach is not attractive for clinical use owing to increased patient dose. The authors propose a novel energy mapping approach referred to as virtual DECT (VDECT) taking advantage of the DECT formulation but using CT data acquired at a single energy (kV(P)). For this purpose, the CT image acquired at one energy is used to generate the CT image at a second energy using calculated kV(P) conversion curves derived from phantom studies. Methods: The attenuation map (mu-map) at 511 keV was generated for the XCAT phantom and clinical studies using the bilinear, DECT, and VDECT techniques. The generated mu-maps at 511 keV are compared to the reference derived from the XCAT phantom serving as ground truth. PET data generated from a predefined activity map for the XCAT phantom were then corrected for attenuation using mu-maps generated using the different energy mapping approaches. In addition, the generated mu-maps using the above described methods for a cylindrical polyethylene phantom containing different concentrations of K2HPO4 in water were compared to actual attenuation coefficients. Likewise, CT images of five clinical whole-body studies were used to generate mu-maps using the various energy-mapping approaches were compared with mu-maps acquired at 511 keV using Ge-68/Ga-68 rod sources for the clinical studies. Results: The results of phantom studies demonstrate that the proposed method is more accurate than the bilinear technique. All three mu-maps yielded almost similar results for soft and lung tissues whereas for bone tissues, the DECT and the VDECT methods produced a much smaller mean relative difference (3.0% and 2.8%, respectively) than the bilinear approach (11.8%). Likewise, the comparison of PET images corrected for attenuation using the various methods showed that the proposed method provides better accuracy (6.5%) than the bilinear method (13.4%). Clinical studies further demonstrated that, compared to the bilinear method, the VDECT approach has better agreement for bony structures with the DECT technique (1.5% versus 8.9%) and transmission scanning (8.8% versus 17.7%). Conclusions: It was concluded that the proposed method outperforms the bilinear method especially in bony structures. Further evaluation using a large clinical PET/CT database is underway to evaluate the potential of the technique in a clinical setting. (C) 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.3694108

A novel energy mapping approach for CT-based attenuation correction in PET.

Dual-energy CT (DECT) is arguably the most accurate energy mapping technique in CT-based attenuation correction (CTAC) implemented on hybrid PET/CT systems. However, this approach is not attractive for clinical use owing to increased patient dose. The authors propose a novel energy mapping approach referred to as virtual DECT (VDECT) taking advantage of the DECT formulation but using CT data acquired at a single energy (kV(P)). For this purpose, the CT image acquired at one energy is used to generate the CT image at a second energy using calculated kV(P) conversion curves derived from phantom studies