Daniel Gagnon

A new modular and scalable detector for a Time-of-Flight PET scanner

We describe a detector design for a new low-cost Time-of-Flight PET scanner. Each independent detector module is two-side buttable in the transaxial direction. To maximize the optical collection efficiency, the design uses two different-sized circular PMT types with concave photocathodes and each type is placed at a different distance from the scintillator array. The detector consists of multiple axial segments. Each axial segment is read-out by four PMTs arranged in a rectangular pattern, and neighboring segments share two PMTs in order to reduce channel count and improve optical collection efficiency. To reduce deadtime and pile-up effects, each axial segment is essentially optically isolated from its neighbors. The detector length is scalable in the axial direction by changing the number of segments. In addition to the detector design, we present timing resolution measurements as a function of singles rate.

Timing calibration for time-of-flight PET using positron-emitting isotopes and annihilation targets

Adding time-of-flight (TOF) technology has been proved to improve the image quality in PET. In order for TOF information to significantly reduce the statistical noise in reconstructed PET images, good timing resolution is needed across the scanner field of view (FOV). This work proposes an accurate, robust and practical crystal-based timing calibration method using FDG positron-emitting sources together with a spatially separated annihilation target. We calibrated a prototype Toshiba TOF PET scanner using this method and then assessed its timing resolution at different locations in the scanner FOV.

Count-level dependent image domain PSF kernel width selection for fully 3D PET image reconstruction

We present an image-domain point spread function (PSF) modeling approach for resolution recovery where spatially varying PSF kernel widths are adjusted based on data quality. This approach attempts to maximize contrast recovery while minimizing edge artifacts (ringing) associated with PSF modeling. We choose broader PSF kernels for noisier datasets where the extent of ringing is comparable to noise standard deviation levels and therefore result in minimal visible ringing artifacts. Similarly, we choose narrower, under-modeled PSF kernels for high count datasets to avoid edge artifacts associated with broader kernels which would have been visible at low image noise levels. We quantify ringing by measuring the difference between the highest overshoot and lowest undershoot levels around the mean background activity level. We define “visible ringing” by the difference between ringing and twice the background standard deviation and use it as our primary metric for measuring artifact levels. We show through simulations that broader PSF kernels can be used for noisier datasets with minimal visible ringing for improved contrast recovery. We use these results to determine the broadest PSF kernel widths to be used at each count level to achieve the highest level of contrast recovery with minimal visible ringing.

Comparison of two projectors with image-based resolution models in a fully 3D list-mode PET reconstruction

In this paper, an image-based and spatially variant resolution model (i.e. PSF model) was derived from reconstructions of a series of point sources, and used to recover the resolution loss in PET imaging. Since this PSF model was not only system dependent, but also reconstruction algorithm dependent, two reconstruction projectors were compared: one was the single-ray-tracing (1-ray) and another was the Area-Simulating-Volume (ASV). An IEC phantom and a female patient were reconstructed using a fully 3D time-of-flight list-mode ordered-subsets expectation-maximization algorithm with different projector-PSF combinations: 1-ray, 1-ray-PSF, ASV and ASV-PSF. Physical corrections such as attenuation, scatter, random, detector normalization, decay and dead time were also included in these reconstructions. For both phantom and patient datasets, projectors with resolution models demonstrate superior quantitation performance compared to those without resolution models in terms of resolution, contrast and noise. Results also show 1-ray-PSF has very comparable image qualities to ASV-PSF, though the latter calculates more accurate geometric probabilities in the system matrix. Therefore, a simple projector with an image-based PSF model might be suggested for the future reconstruction development, especially in reconstruction with a GPU acceleration.

PET random reduction with a FOV-dependent coincidence window and tangential TOF-mask

Time-of-flight (TOF) positron emission tomography (PET) has achieved a 500-600 ps coincidence timing resolution for clinical whole-body scanners. TOF PET has been used to narrow the coincidence window to 4-6 ns for whole-body imaging and thus reduced random coincidence rate and improved the noise-equivalent-count (NEC) rate. In this paper, we revisit the random reduction by deriving an explicit coincidence window equation, and propose a tangential TOF-mask to tailor the coincidence window to conform to the FOV without any patient morphological information. Experimental line source data at the edge of various FOVs are used to validate the coincidence window equation and TOF-mask. A new signal-to-noise ratio (SNR) formula via NEC is given with this TOF mask. Multiple FOVs of NEMA NU-2 experimental count-rate data are used to demonstrate the relative gain in NEC with this TOF mask.

A method for measuring Time-of-Flight resolution of Positron Emission Tomography scanner

Time-of-Flight Positron Emission Tomography (TOF-PET) image reconstruction could significantly improve the overall image quality and robustness to inconsistent data corrections. The timing resolution of each PET scanner, which is directly related to the SNR gain in the reconstructed image, plays a key role in the system performance and image quality improvement. However, there is no consensus on how to measure the TOF resolution in the literature. In this paper, we propose a new method for measuring TOF resolution directly from a histogram of raw list-mode data without any calibration or reconstruction. The evaluation results from both GATE simulation and real scanner show that the proposed method could yield an accurate estimation of the TOF resolution of the system. Furthermore, the proposed method also provides the measurements of the intrinsic transverse and axial spatial resolutions without any calibration or reconstruction.

Impacts of reduction of CT radiation dose on PET in PET/CT imaging

We studied the impacts of CT dose reduction on PET quantitation with combination of currently available techniques by phantom and human studies on a Toshiba PET/CT prototype scanner with TOF capability. For the phantom studies, an anthropomorphic torso phantom and an IEC phantom were scanned using both diagnostic quality CT protocols and dose-reduced CT protocols. To achieve reduced-dose CT scans, we used tube current reduction, spectral shaping, helical pitch optimization and AIDR 3DTM (Adaptive Iterative Dose Reduction). Then, both the high and low dose acquired CT images were resampled and used to generate attenuation maps for PET. The CT images, attenuation maps and reconstructed PET images were compared between high and low dose CT protocols. ROI-based analysis for each tissue type was also performed to see the impact of the dose reduction on PET quantitative accuracy among different dose protocols. Finally, the selected optimal low dose CT protocol was used for attenuation correction in whole-body PET/CT imaging on twelve patients. The PET data were reconstructed with fully 3D list-mode TOF ordered-subset expectation maximization algorithm with physical corrections of attenuation, scatter, random, detector normalization, decay, duration and dead time. The reconstructed PET images show that in PET/CT imaging, the PET diagnostic quality could be remained unchanged, while CT dose could be dramatically reduced compared to the diagnostic protocol, when CT is not used for diagnostic purpose. There is big room for CT dose reduction as long as the bias and noise in CT could be controlled in an acceptable range without introducing artifacts through CT-based PET attenuation correction. This study demonstrates that CT could be tailored for the intended purpose in PET/CT imaging to dramatically reduce patient radiation dose, while still maintaining high quality PET images.