Optimizing injected dose in clinical PET by accurately modeling the counting-rate response functions specific to individual patient scans.

UNLABELLED: To optimize the injected dose of radiopharmaceutical in PET, one needs to know its relationship to some metric of data quality for individual patient scans, such as noise-equivalent counting rate (NECR). In this paper, we show how one may accurately model the clinical NECR response corresponding to specific patient scans much as if a counting-rate test had been performed on each patient. We apply this technique to patient data and show how it can lead to improved clinical scanning protocols. METHODS: True and random coincidence rates expressed as functions of an appropriate measurable system parameter such as the detector single-event rate have functional forms that are largely independent of the object being scanned. Thus, reference true and random response functions may be scaled directly to the specific counting rates measured on a clinical scan, thereby yielding a curve of NECR versus injected dose. We have applied this technique to 2 groups of 163 clinical (18)F-FDG scans each. One of the groups was obtained on a lutetium oxyorthosilicate PET/CT scanner with conventional front-end electronics, and the other was obtained on a lutetium oxyorthosilicate PET/CT scanner with a new digital data processing system (Pico-3D). RESULTS: At 90%-95% of maximum signal-to-noise ratio (SNR), the mean optimal dose for a 60-min uptake period ranged from 366 to 717 MBq depending on the electronics and randoms processing method. There was only a slight (1 MBq/kg) dependence of optimal dose on patient weight but a larger dependence on position in the body. Pico-3D electronics improved optimal data SNR by 35% for a 70-kg person, but in both cases NECR fell rapidly with increasing weight (1.4%/kg). For an equivalent data SNR, a 120-kg person would have to be scanned 2.3 times longer than a 60-kg person. Over this range of weight, the mean scatter fraction increased by 12% whereas the ratio of mean randoms to trues increased by 48%. CONCLUSION: The methodology developed here allows one to directly estimate the optimal dose to inject for specific clinical scans and permits a detailed analysis of the sources of noise in PET data and of their variation with parameters such as patient weight.

NEMA NU 2 performance tests for scanners with intrinsic radioactivity.

Performance tests on lutetium oxyorthosilicate (LSO)-based PET scanners cannot be conducted strictly according to the National Electrical Manufacturers Association (NEMA) NU 2 standards because of the presence of intrinsic radioactivity within the LSO crystal scintillator material. This background radiation gives rise mainly to random coincidence events but also to a small number of true coincidences, which cannot be eliminated from measurements on such scanners and must therefore be corrected for in the data analysis. The current NU 2 standards do not take account of these backgrounds and hence can lead to erroneous results on LSO-based machines. Nevertheless, the intent of the standards can be met with appropriate modifications to the acquisition and processing procedures. In this paper, we propose certain changes to the NEMA specifications to accommodate this class of scanners. These changes affect mainly the estimation of sensitivity, scatter, randoms, and count losses. Using these modified procedures, the NU 2 performance of LSO-based systems can accurately be measured.

PET performance measurements for an LSO-based combined PET/CT scanner using the National Electrical Manufacturers Association NU 2-2001 standard.

UNLABELLED: Results of performance measurements for a lutetium oxyorthosilicate (LSO)-based PET/CT scanner using new National Electrical Manufacturers Association (NEMA) NU 2-2001 standards are reported. METHODS: Performance measurements following the NU 2-2001 standards were performed on an LSO-based PET/CT scanner. In addition, issues associated with the application of the NEMA standard to LSO-based tomographs in the presence of intrinsic radiation are discussed. RESULTS: We report on some difficulties experienced in following the suggested NEMA measurement techniques and describe alternative approaches. Measurements with the new standard (as compared with NU-1994) incorporate the effects of activity outside the scanner and facilitate measurements of the entire axial field of view. Realistic clinical conditions are also simulated in image quality measurements of a torso phantom. CONCLUSION: We find that, with appropriate modifications, NU 2-2001 can be successfully applied to LSO-based scanners.

The dynamics of physics in PET.

From a technical perspective, there are fundamentally two forces driving the evolution of instrumentation in positron emission tomography (PET) and nuclear medicine generally: clinical needs and technical innovation. This essay considers some of the dynamics of these forces as they act on physics-related developments in PET and suggests that progress will be greatest if these differing motivations are kept in balance as the field evolves.

Physics and applications of positron beams in an integrated PET/MR.

In PET/MR systems having the PET component within the uniform magnetic field interior to the MR, positron beams can be injected into the PET field of view (FOV) from unshielded emission sources external to it, as a consequence of the action of the Lorentz force on the transverse components of the positron's velocity. Such beams may be as small as a few millimeters in diameter, but extend 50 cm or more axially without appreciable divergence. Larger beams form 'phantoms' of annihilations in air that can be easily imaged, and that are essentially free of γ-ray attenuation and scatter effects, providing a unique tool for characterizing PET systems and reconstruction algorithms. Thin targets intersecting these beams can produce intense annihilation sources having the thickness of a sheet of paper, which are very useful for high resolution measurements, and difficult to achieve with conventional sources. Targeted beams can provide other point, line and surface sources for various applications, all without the need to have radioactivity within the FOV. In this paper we discuss the physical characteristics of positron beams in air and present examples of their applications.

Sensitivity analysis for computer model projections of hurricane losses.

Projecting losses associated with hurricanes is a complex and difficult undertaking that is fraught with uncertainties. Hurricane Charley, which struck southwest Florida on August 13, 2004, illustrates the uncertainty of forecasting damages from these storms. Due to shifts in the track and the rapid intensification of the storm, real-time estimates grew from 2 billion dollars to 3 billion dollars in losses late on the 12th to a peak of 50 billion dollars for a brief time as the storm appeared to be headed for the Tampa Bay area. The storm struck the resort areas of Charlotte Harbor and moved across the densely populated central part of the state, with early poststorm estimates in the 28 dollars to 31 billion dollars range, and final estimates converging at 15 billion dollars as the actual intensity at landfall became apparent. The Florida Commission on Hurricane Loss Projection Methodology (FCHLPM) has a great appreciation for the role of computer models in projecting losses from hurricanes. The FCHLPM contracts with a professional team to perform onsite (confidential) audits of computer models developed by several different companies in the United States that seek to have their models approved for use in insurance rate filings in Florida. The team's members represent the fields of actuarial science, computer science, meteorology, statistics, and wind and structural engineering. An important part of the auditing process requires uncertainty and sensitivity analyses to be performed with the applicant's proprietary model. To influence future such analyses, an uncertainty and sensitivity analysis has been completed for loss projections arising from use of a sophisticated computer model based on the Holland wind field. Sensitivity analyses presented in this article utilize standardized regression coefficients to quantify the contribution of the computer input variables to the magnitude of the wind speed.

Uncertainty analysis for computer model projections of hurricane losses.

Projecting losses associated with hurricanes is a complex and difficult undertaking that is wrought with uncertainties. Hurricane Charley, which struck southwest Florida on August 13, 2004, illustrates the uncertainty of forecasting damages from these storms. Due to shifts in the track and the rapid intensification of the storm, real-time estimates grew from 2 to 3 billion dollars in losses late on August 12 to a peak of 50 billion dollars for a brief time as the storm appeared to be headed for the Tampa Bay area. The storm hit the resort areas of Charlotte Harbor near Punta Gorda and then went on to Orlando in the central part of the state, with early poststorm estimates converging on a damage estimate in the 28 to 31 billion dollars range. Comparable damage to central Florida had not been seen since Hurricane Donna in 1960. The Florida Commission on Hurricane Loss Projection Methodology (FCHLPM) has recognized the role of computer models in projecting losses from hurricanes. The FCHLPM established a professional team to perform onsite (confidential) audits of computer models developed by several different companies in the United States that seek to have their models approved for use in insurance rate filings in Florida. The team's members represent the fields of actuarial science, computer science, meteorology, statistics, and wind and structural engineering. An important part of the auditing process requires uncertainty and sensitivity analyses to be performed with the applicant's proprietary model. To influence future such analyses, an uncertainty and sensitivity analysis has been completed for loss projections arising from use of a Holland B parameter hurricane wind field model. Uncertainty analysis quantifies the expected percentage reduction in the uncertainty of wind speed and loss that is attributable to each of the input variables.

The ECAT ART Scanner for Positron Emission Tomography. 1. Improvements in Performance Characteristics.

The widespread use of positron emission tomography (PET) has been to some extent limited by the cost and complexity of PET instrumentation. Recognition of the wider applicability of clinical PET imaging is reflected in the ECAT ART design, a low cost PET scanner targeted for clinical applications, particularly in oncology. The ART comprises two asymmetrically opposed arrays of BGO block detectors. Each array consists of 88 (transaxial) by 24 (axial) crystals, and the arrays rotate continuously at 30 rpm to acquire a full 3D projection data set. Sensitivity and count rate limitations are key performance parameters for any imaging device. This paper reports on improved performance characteristics of the ART, achieved by operating the scanner with a decreased block integration time, reduced coincidence time window, and collimated singles transmission sources. Compared to the standard ART configuration, these modifications result in both improved count rate performance and higher quality transmission scans.