Charles C. Watson

ECAT ART — a continuously rotating PET camera: Performance characteristics, initial clinical studies, and installation considerations in a nuclear medicine department

Advances in fully three-dimensional (3D) image reconstruction techniques have permitted the development of a commercial, rotating, partial ring, fully 3D positron emission tomographic (PET) scanner, the ECAT ART. The system has less than one-half the number of bismuth germanate detectors compared with a full ring scanner with the equivalent field of view, resulting in reduced capital cost. The performance characteristics, implications for installation in a nuclear medicine department, and clinical utility of the scanner are presented in this report. The sensitivity (20 cm diameter×20 cm long cylindrical phantom, no scatter correction) is 11400 cps·kBq−1·ml−1. This compares with 5800 and 40500 cps·kBq−1·ml−1 in 2D and 3D respectively for the equivalent full ring scanner (ECAT EXACT). With an energy window of 350–650 keV the maximum noise equivalent count (NEC) rate was 27 kcps at a radioactivity concentration of ~15 kBq·ml−1 in the cylinder. Spatial resolution is ~6 mm full width at half maximum on axis degrading to just under 8 mm at a distance of 20 cm off axis. Installation and use within the nuclear medicine department does not appreciably increase background levels of radiation on gamma cameras in adjacent rooms and the dose rate to an operator in the same room is 2 µSv·h−1 for a typical fluorine-18 fluorodeoxyglucose (18F-FDG) study with an initial injected activity of 370 MBq. The scanner has been used for clinical imaging with18F-FDG for neurological and oncological applications. Its novel use for imaging iron-52 transferrin for localising erythropoietic activity demonstrates its sensitivity and resolution advantages over a conventional dual-headed gamma camera. The ECAT ART provides a viable alternative to conventional full ring PET scanners without compromising the performance required for clinical PET imaging.

Extension of single scatter simulation to scatter correction of time-of-flight PET

We extend the single scatter simulation (SSS) algorithm for scatter correction of three dimensional positron emission tomography to include the case in which the scattered annihilation radiation is discriminated according to its differential time-of-flight (TOF). Good agreement between computed and measured TOF dependent scatter is observed in phantom and human data. Significant differences between TOF dependent and non-TOF dependent scatter are found. Because TOF scoring removes the degeneracy in ray sum calculations, TOF SSS takes about seven times longer than non-TOF SSS to compute the scatter sinogram itself. However, when the overhead due to image computation and iteration is factored in, the clinical performance is only a factor of three slower.

Completion of a Truncated Attenuation Image From the Attenuated PET Emission Data

Positron emission tomographs (PET) are currently almost exclusively designed as hybrid systems. The current standard is the PET/CT combination, while prototype PET/MRI systems are being studied by several research groups. One problem in these systems is that the transaxial field of view of the second system is smaller than that of the PET camera. The problem is limited for PET/CT, it is more pronounced in PET/MRI. Because this second system provides the image for attenuation correction, the smaller field of view causes truncation of the attenuation map. In this paper, we propose a maximum-a-posteriori algorithm for estimating the missing part of the attenuation map from the PET emission data.

Performance Characteristics of a New LSO PET/CT Scanner With Extended Axial Field-of-View and PSF Reconstruction

A new combined lutetium oxyorthosilicate (LSO) PET/CT scanner with an extended axial field-of-view (FOV) of 21.8 cm has been developed (Biograph TruePoint PET/CT with TrueV; Siemens Molecular Imaging) and introduced into clinical practice. The scanner includes the recently announced point spread function (PSF) reconstruction algorithm. The PET components incorporate four rings of 48 detector blocks, 5.4 cm times 5.4 cm in cross-section. Each block comprises a 13 times 13 matrix of 4 times 4 times 20 mm3 elements. Data are acquired with a 4.5 ns coincidence time window and an energy window of 425-650 keV. The physical performance of the new scanner has been evaluated according to the recently revised National Electrical Manufacturers Association (NEMA) NU 2-2007 standard and the results have been compared with a previous PET/CT design that incorporates three rings of block detectors with an axial coverage of 16.2 cm (Biograph TruePoint PET/CT; Siemens Molecular Imaging). In addition to the phantom measurements, patient Noise Equivalent Count Rates (NECRs) have been estimated for a range of patients with different body weights (42-154 kg). The average spatial resolution is the same for both scanners: 4.4 mm (FWHM) and 5.0 mm (FWHM) at 1 cm and 10 cm respectively from the center of the transverse FOV. The scatter fractions of the Biograph TruePoint and Biograph TruePoint TrueV are comparable at 32%. Compared to the three ring design, the system sensitivity and peak NECR with smoothed randoms correction (1R) increase by 82% and 73%, respectively. The increase in sensitivity from the extended axial coverage of the Biograph TruePoint PET/CT with TrueV should allow a decrease in either scan time or injected dose without compromising diagnostic image quality. The contrast improvement with the PSF reconstruction potentially offers enhanced detectability for small lesions.

Advances in scatter correction for 3D PET/CT

We report on several significant improvements to the implementation of image-based scatter correction for 3D PET and PET/CT. Among these advances are: a new algorithm to scale the estimated scatter sinogram to the measured data, thereby largely compensating for external scatter; the ability to handle CT image truncation during this scaling; the option to iterate the scatter calculation for improved accuracy; the use of ordered subset estimation maximization (OSEM) reconstruction for the estimated emission images from which the scatter contributions are simulated; reporting of data quality parameters such as scatter and randoms fractions, and noise equivalent count rate (NECR), for each patient bed position; and extensive quality control output. Scatter correction (2 iterations, OSEM) typically requires 15-45 sec per bed. Very good agreement between the estimated scatter and measured emission data for several typical clinical scans is reported for CPS Pico-3D and HiRez LSO PET/CT systems. The data characteristics extracted during scatter correction are useful for patient specific count rate modeling and scan optimization

Evaluation of clinical PET count rate performance

We propose a methodology for analyzing clinical PET count rate performance that involves matching net trues (prompts - delayeds) and randoms (delayeds) count rate responses measured on a reference phantom to the actual patient data. We employ these estimated response curves to compute a performance metric closely related to the noise equivalent count rate, as a function of the total singles event rate in the system. Front this we can determine the peak performance value relative to the measured performance for any individual acquisition. This maximum performance value is largely independent of the magnitude of the activity present in the patient at the time of the acquisition, but depends mainly on the emitter and attenuator distributions. These peak count rates can be used to derive frame durations for equivalent noise, and correlated with patient weight. We can also determine lite singles rate, and thereby activity, necessary to achieve maximum performance, and based on the known activity in the patient, predict optimal injected doses.

Count rate dependence of local signal-to-noise ratio in positron emission tomography

The count rate dependence of the signal-to-noise ratio (SNR) in positron emission tomography (PET) data is frequently characterized by a global quantity such as the noise equivalent count rate (NECR). However, it has not been clear to what extent global optimization of data variance (e.g., through varying injected dose or uptake period) also optimizes SNR in local regions of interest. In this paper, we present a new methodology for predicting the dependence of local image mean and variance on count rate that can be applied when an actual measurement at only a single count rate is available, as is frequently the case in human clinical PET scanning. We define local count rates and SNR based on the methods of Huesman and Alpert et al. We demonstrate that local trues and randoms count rates can be accurately modeled from global phantom count rate curves. Using these results, we compare local and global SNR count rate responses in phantom and human examples. Except in the vicinity of the bladder, we find only small differences between the local and global response curves, suggesting that global optimization of SNR or NECR is likely generally adequate for optimizing most clinical scanning protocols, at least with respect to injected dose.

Physical Performance and Clinical Workflow of a new LSO HI-REZ PET/CT Scanner

A new combined LSO PET/CT scanner with an extended axial field-of-view of 21.6 cm has been developed (Biograph TruePoint TrueV PET/CT, Siemens Molecular Imaging) and introduced into clinical practice. The scanner incorporates four rings of 48 detector blocks, 5.4 cm times 5.4 cm in cross-section; each block comprises a 13 times 13 matrix of 4 times 4 times 20 mm3 elements. The scanner is operated with a coincidence time window of 4.5 ns and an energy window of 425-650 keV. The physical performance of the new scanner has been evaluated according to the NEMA NU 2-2001 protocol and the results have been compared with a previous design that incorporated three rings of detectors with an axial coverage of 16.2 cm (Biograph TruePoint PET/CT, Siemens Molecular Imaging). The spatial resolution is the same for both scanners: 4.4 mm and 5 mm at 1 cm and 10 cm respectively from the center of the transverse field-of-view. The scatter fraction is 34%, similar to that of the three ring design. However, compared to the three ring scanner, the system sensitivity and peak noise equivalent count rate (NECR) increase by 82% and 73% respectively. In addition to the phantom measurements, patient NECRs have been estimated for a range of patients with different body sizes. The increase in sensitivity and extended axial coverage facilitates a decrease in either scan time or injected dose without compromising diagnostic image quality.

A method for calibrating the CT-based attenuation correction of PET in human tissue

The use of x-ray computed tomography (CT)-based attenuation correction for positron emission tomography (PET) in PET/CT systems requires the transformation of CT Hounsfield units (HU) to linear attenuation coefficients at 511 keV (LAC/sub 511/). This cannot be done perfectly from a single peak kilovolt (kVp) CT scan due to variability in Compton and photoelectric composition and, thus, an approximate transformation must be employed. One difficulty in this lies in accurately determining the linear attenuation coefficients (LAC) in actual human tissue. Typically, phantoms consisting of synthetic materials thought to be approximate human tissue equivalents are employed instead. A potentially more accurate approach would be to use dual kVp CT scans to estimate LAC/sub 511/ in actual human tissue and then base the single kVp transformation on these data. This approach would also permit an assessment of the dispersion of actual tissue values about the two-component trend lines typically used for the single kVp transformation. In this paper, we develop and assess this methodology.

Design and performance of collimated coincidence point sources for simultaneous transmission measurements in 3-D PET

The authors have implemented a simultaneous emission-transmission measurement for three-dimensional positron emission tomography (3-D PET) using a collimated coincidence point source design employing a fast, dedicated, reference detector close to the transmission source. This design reduces the effects of randoms, scatter, dead time, and sensitivity loss on the emission data compared to previous implementations. It also greatly reduces the effect of emission contamination of the transmission data compared to the use of rod sources. Here, the authors present performance characterizations of this measurement technique on both the Siemens/CTI ECAT ART and PET/SPECT tomographs. The main effect of the transmission sources on the emission measurement is an increased randoms rate, which lends to a 10-25% reduction in NECR at specific activities >2 kBq/mL in a 21-cm-diameter phantom on the PET/SPECT. Emission contamination effects on the transmission measurement are estimated to be less than 1% for up to 20 kBq/mL in a 21-cm phantom on the PET/SPECT. Both the emission and transmission NECR are dominated by the effects of randoms. Considering the effects of both emission and transmission noise on the final corrected image, it appears that 3-6 kBq/mL of emitter concentration is an optimal imaging range for simultaneous acquisitions. The authors present the first images of a normal volunteer using this system on a Siemens/CTI PET/SPECT tomograph.