A method of computing the beam profiles for multi-element multiple-beam radiotherapy treatment is presented. This is applicable to the treatment planning of conformal radiotherapy using an isocentric rotation technique. The method begins with the treatment dose prescription and calculates the beam profiles rather than the reverse 'conventional' planning technique. The method is the iterative optimisation method of simulated annealing. It is shown using published 'difficult' clinical treatment planning problems that the high-dose region can be very precisely tailored to the tumour volume even when this has a re-entrant (concave) periphery. Simultaneously the dose can be constrained in other sensitive regions. The mathematics of the method is explained together with this implementation. The analogy of this optimisation with certain reconstruction problems in medical imaging is drawn and by way of experiment dose distributions are presented in image form and beam profiles as sinograms.

http://imrt1.persiangig.com/Articles/Optimizing/Optimization%20Algorithms%20in%20IMRT/SA%20(Stimulated%20Anealing)/1.Optimisation%20of%20CRT%20dose%20distributions%20by%20SA.pdf

Optimisation of conformal radiotherapy dose distributions by simulated annealing.

Positron emission tomography (PET) is a tool for metabolic imaging that has been utilized since the earliest days of nuclear medicine. A key component of such imaging systems is the detector modules—an area of research and development with a long, rich history. Development of detectors for PET has often seen the migration of technologies, originally developed for high energy physics experiments, into prototype PET detectors. Of the many areas explored, some detector designs go on to be incorporated into prototype scanner systems and a few of these may go on to be seen in commercial scanners. There has been a steady, often very diverse development of prototype detectors, and the pace has accelerated with the increased use of PET in clinical studies (currently driven by PET/CT scanners) and the rapid proliferation of pre-clinical PET scanners for academic and commercial research applications. Most of these efforts are focused on scintillator-based detectors, although various alternatives continue to be considered. For example, wire chambers have been investigated many times over the years and more recently various solid-state devices have appeared in PET detector designs for very high spatial resolution applications. But even with scintillators, there have been a wide variety of designs and solutions investigated as developers search for solutions that offer very high spatial resolution, fast timing, high sensitivity and are yet cost effective. In this review, we will explore some of the recent developments in the quest for better PET detector technology.

https://iopscience.iop.org/article/10.1088/0031-9155/53/17/R01/pdf

Recent developments in PET detector technology.

FastSPECT II is a recently commissioned 16-camera small-animal SPECT imager built with modular scintillation cameras and list-mode data-acquisition electronics. The instrument is housed in a lead-shielded enclosure and has exchangeable aperture assemblies and adjustable camera positions for selection of magnification, pinhole size, and field of view. The calibration of individual cameras and measurement of an overall system imaging matrix (1 mm/sup 3/ voxels) are supported via a five-axis motion-control system. Details of the system integration and results of characterization and performance measurements are presented along with first tomographic images. The dynamic imaging capabilities of the instrument are explored and discussed.

/pdf/fastspect-ii-a-second-generation-high-resolution-dynamic-9sg04lhurx.pdf

FastSPECT II: a second-generation high-resolution dynamic SPECT imager

The development of radiation detectors capable of delivering spatial information about gamma-ray interactions was one of the key enabling technologies for nuclear medicine imaging and, eventually, single-photon emission computed tomography (SPECT). The continuous sodium iodide scintillator crystal coupled to an array of photomultiplier tubes, almost universally referred to as the Anger Camera after its inventor, has long been the dominant SPECT detector system. Nevertheless, many alternative materials and configurations have been investigated over the years. Technological advances as well as the emerging importance of specialized applications, such as cardiac and preclinical imaging, have spurred innovation such that alternatives to the Anger Camera are now part of commercial imaging systems. Increased computing power has made it practical to apply advanced signal processing and estimation schemes to make better use of the information contained in the detector signals. In this review we discuss the key performance properties of SPECT detectors and survey developments in both scintillator and semiconductor detectors and their readouts with an eye toward some of the practical issues at least in part responsible for the continuing prevalence of the Anger Camera in the clinic.

/pdf/spect-detectors-the-anger-camera-and-beyond-1crz00c7wv.pdf

SPECT detectors: the Anger Camera and beyond.

In any gamma-ray detector, each event produces electrical signals on one or more circuit elements. From these signals, we may wish to determine the presence of an interaction; whether multiple interactions occurred; the spatial coordinates in two or three dimensions of at least the primary interaction; or the total energy deposited in that interaction. We may also want to compute listmode probabilities for tomographic reconstruction. Maximum-likelihood methods provide a rigorous and in some senses optimal approach to extracting this information, and the associated Fisher information matrix provides a way of quantifying and optimizing the information conveyed by the detector. This paper will review the principles of likelihood methods as applied to gamma-ray detectors and illustrate their power with recent results from the Center for Gamma-ray Imaging.

Maximum-Likelihood Methods for Processing Signals From Gamma-Ray Detectors

A modular gamma ray camera is described that gives useful image information over its entire crystal face. The lack of dead area on the periphery of the camera is made possible by a unique application of digital electronics and optimal position estimation using maximum likelihood (ML) estimates. The ML estimates are calculated directly from photomultiplier tube responses and stored in a lookup table, so the restriction of calculating the position estimates in separate circuitry is removed. Each module is designed to be optically and electronically independent, so that many modules can be combined in a large system. Results from a prototypical module, which has an active crystal area of 10 cm X 10 cm, are presented.

A full-field modular gamma camera.

Digital implementation of position estimation for the modular scintillation camera is presented. The modular camera system, introduced at the 1983 IEEE Nuclear Science Symposium, comprises an array of small, mechanically and electronically independent, all-digital scintillation cameras. This paper addresses how to best determine the event position given the PMT signals. Two Bayesian estimators are investigated, namely maximum-likelihood (ML) and minimum-meansquare-error (MS) estimators. Both estimators performed similarly. The more important aspect of the estimation problem was found to be the shape of the detector responses as a function of position.

Digital Position Estimation for the Modular Scintillation Camera

Modular scintillation cameras are gamma cameras with relatively small crystal faces, a small number of photomultiplier tubes (PMTs), and independent processing electronics. Our prototypical module has a 10 cm square crystal face, four PMTs, and digital processing electronics. Scintillation event information is transferred to images by mapping digitized PMT response combinations to optimal position estimates of event locations. In our prototype, a look-up table is used to perform this mapping. To encode scintillation event information more effectively, we use nonlinear compression of each of the PMT signals. Also introduced are logarithmic matched filtering and likelihood windowing, two processing techniques that result from exploitations of the Poisson model of the distribution of photopeak events. Logarithmic matched filtering is a method of obtaining estimates of mean detector response functions having greater accuracy than that indicated by the digitization of the PMT responses. Likelihood windowing is the utilization of a likelihood threshold, rather than the familiar energy window, as a discriminant of photopeak and scatter events. We have implemented each of the above on our prototypical module. Performance characteristics of this module include energy resolution of 10% full width at half maximum (FWHM) at 140 keV and spatial resolution of better than 4mm FWHM over 90% of the crystal.

Modular Scintillation Cameras: A Progress Report

The design of a parallel processor containing 60 32-bit processors with on-chip floating point hardware, 60 Mbytes of memory, hardware random number generation, and a high bandwidth, re- configurable interprocessor communications system is discussed. The system is especially well- suited to run Monte Carlo-based algorithms, such as simulated annealing, for optimization and estimation problems in nuclear medicine and other areas. Introduction In image reconstruction problems in nuclear medicine, as well as in optimization and estimation problems in many other areas, one wants to find a "best" estimate of some quantity. Often, it is either impossible or impractical to handle these problems with a straightforward deterministic algorithm. In such cases, there are a number of Monte Carlo-based methods which provide techniques for obtaining satisfactory solutions. Examples include simulated annealing1"4, genetic algorithms5, and others. Although these techniques are very powerful, they typically require very large amounts of computation for their effective use, and this has prevented them from being implemented on a widespread basis for many applications.A very attractive and cost-effective approach to providing the computational resources needed to execute these optimization methods rapidly is to use a combination of special-purpose hardware and microprocessor-based parallel processing. In this paper we present the design of a parallel processor that we have developed using such an approach.

TRIMM: A High Speed Parallel Processor For Optimization And Estimation Problems

This paper describes a small, digital, modular gamma camera that can be used individually as a small organ, planar imaging system or can be combined to create modular single-photon emission computed tomography (SPECT) systems that contain no moving parts. We describe two systems currently under design: a cardiac imager capable of multi-slice, two-dimensional SPECT, and a full three-dimensional brain imager. Finally, we present a two-dimensional tomographic image generated by a prototypical modular camera SPECT system.

A. L. Landesman

Papers

A full-field modular gamma camera.

Digital Position Estimation for the Modular Scintillation Camera

Modular Scintillation Cameras: A Progress Report

TRIMM: A High Speed Parallel Processor For Optimization And Estimation Problems

The Design and Implementation of Modular SPECT Imaging Systems