Recent developments in time-of-flight PET

While the first time-of-flight (TOF)-positron emission tomography (PET) systems were already built in the early 1980s, limited clinical studies were acquired on these scanners. PET was still a research tool, and the available TOF-PET systems were experimental. Due to a combination of low stopping power and limited spatial resolution (caused by limited light output of the scintillators), these systems could not compete with bismuth germanate (BGO)-based PET scanners. Developments on TOF system were limited for about a decade but started again around 2000. The combination of fast photomultipliers, scintillators with high density, modern electronics, and faster computing power for image reconstruction have made it possible to introduce this principle in clinical TOF-PET systems. This paper reviews recent developments in system design, image reconstruction, corrections, and the potential in new applications for TOF-PET. After explaining the basic principles of time-of-flight, the difficulties in detector technology and electronics to obtain a good and stable timing resolution are shortly explained. The available clinical systems and prototypes under development are described in detail. The development of this type of PET scanner also requires modified image reconstruction with accurate modeling and correction methods. The additional dimension introduced by the time difference motivates a shift from sinogram- to listmode-based reconstruction. This reconstruction is however rather slow and therefore rebinning techniques specific for TOF data have been proposed. The main motivation for TOF-PET remains the large potential for image quality improvement and more accurate quantification for a given number of counts. The gain is related to the ratio of object size and spatial extent of the TOF kernel and is therefore particularly relevant for heavy patients, where image quality degrades significantly due to increased attenuation (low counts) and high scatter fractions. The original calculations for the gain were based on analytical methods. Recent publications for iterative reconstruction have shown that it is difficult to quantify TOF gain into one factor. The gain depends on the measured distribution, the location within the object, and the count rate. In a clinical situation, the gain can be used to either increase the standardized uptake value (SUV) or reduce the image acquisition time or administered dose. The localized nature of the TOF kernel makes it possible to utilize local tomography reconstruction or to separate emission from transmission data. The introduction of TOF also improves the joint estimation of transmission and emission images from emission data only. TOF is also interesting for new applications of PET-like isotopes with low branching ratio for positron fraction. The local nature also reduces the need for fine angular sampling, which makes TOF interesting for limited angle situations like breast PET and online dose imaging in proton or hadron therapy. The aim of this review is to introduce the reader in an educational way into the topic of TOF-PET and to give an overview of the benefits and new opportunities in using this additional information

GE Signa integrated PET/MR: NEMA PET performance measurements

We evaluated the PET performance of the GE SIGNA integrated PET/MR (SGINA) and adjusted potential post-filtering settings to harmonise image quality and quantitative accuracy between different PET-systems at our institution. METHODS: The NEMA NU 2-2007 protocol was followed for studying the PET performance of the SIGNA. The following measurements were performed: spatial resolution; scatter fraction, sensitivity; accuracy of correction for count losses and randoms. Furthermore, a NEMA image quality phantom was measured with and without simultaneous MR acquisition. The reconstructed spatial resolution (SR) and mean recovery coefficients (RCmean) in a TOF-OSEM reconstruction (VPFX, 2 subsets 28 iterations, no post-filtering) were determined using the method of [1]. Moreover, we analysed the effect of different post-filters to obtain a SR that is comparable to a Siemens Hirez PET/CT (HIREZ) at our institution. RESULTS: An transaxial SR of 4.1 and 5.1 mm in full width at half maximum (FWHM) was measured at 1 and 10 cm off-center. In the axial direction values were 6.1 and 6.9 mm, respectively. The system sensitivity was 21.8 cps/kBq along the center and 21.2 cps/kBq and at 10 cm off-center. The scatter fraction was 43.35%, and the peak noise-equivalent count rate was 216 kcps at 18.6 kBq/mL. The maximum absolute value of the relative count rate error due to dead-time losses and randoms was 2.92%. The average lung residual error was 1.6% for PET only and 2.1% for PET with simultaneous MR. The reconstructed SR of the TOF-OSEM reconstruction was 4.4 mm resulting in RCmeans of 0.87, 0.85, 0.80, 0.75, 0.70, 0.62 for the six spheres in the NEMA phantom. A Gaussian post-filtering with 5.4mm was necessary to match the SR of the SIGNA with the one of the HIREZ (7mm). CONCLUSION: The SIGNA offers a very promising PET performance and reconstructed SR which is crucial for lesion detectability and reduced partial volume effects. REFERENCES: [1] Hofheinz et al., “Effects of cold sphere walls in PET phantom measurements on the volume reproducing threshold.”, Med Phys 40:082503, 201