Gordon L. Brownell

Estimation of the Local Statistical Noise in Emission Computed Tomography

A simple modification of the filtered backprojection algorithm is presented for the computation of the local statistical noise in emission computed tomography. The technique is general in that any distribution of radioactivity may be accommodated. When applied to positron emission tomography, it is shown that the effects of photon absorption, random coincidences, radioactive decay, and detector nonuniformity may be included. Calculations have shown the effects of resolution, object size, and photon absorption on the statistical noise of disk-shaped emitters. Comparison of calculation and experiment show close agreement both in magnitude and spatial variation. Measurements of the noise level in tomograms of the brain obtained during continuous inhalation of 150-CO2 demonstrate that estimates of radioactivity concentration with a precision of a few percent are readily attainable.

Boron neutron capture therapy for the treatment of cerebral gliomas. I: Theoretical evaluation of the efficacy of various neutron beams

The technique of boron neutron capture therapy in the treatment of cerebral gliomas depends upon the selective loading of the tumor with a 10B-enriched compound and subsequent irradiation of the brain with low-energy neutrons. The charged particles produced in the 10B (n,alpha) 7Li reaction have ranges in tissue of less than 10 mum so that the dose distribution closely follows the 10B distribution even to the cellular level. The effectiveness of this therapy procedure is dependent not only on the 10B compound but on the spectral characteristics of the neutron source as well. Hence, an optimization of these characteristics will increase the chances of therapeutic success. Transport calculations using a neutral particle transport code have been made to determine the dose-depth distributions within a simple head phantom for five different incident neutron beams. Comparison of these beams to determine their relative therapeutic efficacy was made by the use of a maximum useable depth criterion. In particular, with presently available compounds, the MIT reactor (MITR) therapy beam (a) is not inferior to a pure thermal neutron beam, (b) would be marginally improved if its gamma-ray contamination were eliminated, (c) is superior to a partially 10B-filtered MITR beam, and (d) produces a maximum useable depth which is strongly dependent upon the tumor-to-blood ratio of 10B concentrations and weakly dependent upon the absolute 10B concentration in tumor. A pure epithermal neutron beam with a mean energy of 37 eV is shown to have close to the optimal characteristics for boron neutron capture therapy. Futhermore, these optimal characteristics can be approximated by a judiciously D2O moderated and 10B-filtered 252Cf neutron source. This tailored 252Cf source would have at least a 1.5 cm greater maximum useable depth than the MITR therapy beam for realistic 10B concentrations. However, at least one gram of 252Cf would be needed to make this a practical therapy source. If the moderated 252Cf source is not 10B filtered, the resultant neutron beam has characteristics similar to those of the MITR beam with no gamma-ray contamination. For usch a beam, 100 mg of 252Cf would produce a flux of 2.4 X 10(8) neutrons/(cm2 sec), which is an intensity suitable for therapy applications.

Dopamine fiber detection by [11c]-cft and Pet in a primate model of parkinsonism

MONKEYS were treated on two regimens of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) injections to achieve dopamine fiber degeneration of differing severities. A rapid treatment regimen produced a severe parkinsonian syndrome, whereas an intermittent regimen did not cause locomotor symptoms to appear up to 25 weeks. High resolution PET scanning of dopamine nerve terminals revealed that the specific binding of the dopamine transporter [11C]-WIN 35,428 ([11C]-CFT) was diminished by 94% (caudate nucleus) and by 93% (putamen) in the symptomatic monkey. Decreases of 65 and 67% were detected in these regions in the non-symptomatic monkey. Post-mortem immunocytochemical evaluation of presumed dopamine fibers by tyrosine-hydroxylase showed similar reductions in the symptomatic animal.

NOMENCLATURE FOR TRACER KINETICS.

Zusammenfassung This report, written by the Task Group on Tracer Kinetics of the ICRU, is concerned with the nomenclature, concepts and procedures used in medical tracer studies. It aims at achieving a certain measure of uniformity in their usage. To that effect, it reviews the basic concepts of tracer kinetics and proposes a set of definitions and a unified system of symbols. Dieser Bericht, der von der Arbeitsgruppe fur Tracerkinetik der ICRU verfasst wurde, beschaftigt sich mit der Nomenklatur, den Konzepten und den Methoden, die in medizinischen Traceruntersuchungen verwendet werden. Sein Ziel ist es, eine grossere Vereinheitlichung zu erreichen. Zu diesem Zweck wird eine Ubersicht uber die grundlegenden Kozepte der Tracerkinetik gegeben und eine Reihe von Definitionen und ein einheitliches System von Symbolen vorgeschlagen.

A Multi-Crystal Positron Camera

A multicrystal positron camera designed for use with radionuclides produced with a medical cyclotron is described. The camera uses coincidence techniques for collimation by detecting the positron annihilation radiation. Typical radionuclides are 11C, 13N, 15O with half lives of 20, 10, and 2 minutes, respectively. The principal object of the design is to achieve a data rate capacity to 5 × 104 coincidence events per second, as well as high sensitivity and resolution. This will allow time sequential images of fractional second duration. Each of the cameras two identical detectors consist of a planar array of 127 small NaI(Tl) crystals viewed by an array of 72 phototubes. Timing signals are derived from the phototubes and are subsequently used to identify 2549 crystal pairs. The coincidence data is stored in a magnetic-core memory and used for a local data display; alternatively data may be transferred under program control to a PDP-9 computer. The sensitive field area is 27 cm × 30 cm and the spatial resolution is less than 1 cm at the midplane. The camera is currently being used for a number of clinical applications.

Gamma-ray Dosimetry of Internal Emitters. Monte Carlo Calculations of Absorbed Dose from Point Sources

Monte Carlo calculations of the absorbed dose delivered by point source γ emitters to tissue equivalent phantoms have been performed and the results tabulated in tables of A.F.; i.e. that fraction of the emitted energy absorbed by the phantom. The use of these tables makes it possible to calculate γ dose-rate in a manner analogous to that used for β-ray dosimetry. where Σγ is a constant for a particular isotope, C the average isotopic concentration and (A.F.) M,Eγ the A.F. for a particular phantom mass and γ energy. The effects of changing phantom mass, shape and source configuration have been considered and the results compared to analytical treatments of the same problem performed by others. Of some clinical interest are the following conclusions: (1) Tables of absorbed fraction afford an accurate and convenient method of estimating the total body γ dose; (2) The absorbed fraction is relatively independent of body height, shape, and the position of radioactivity within the patient; (3) A restricted dist...

Theory of radioisotope scanning

Abstract The visualization of radioisotope distributions is an increasingly important application of radioisotopes. A general relation is derived, valid for any detecting system, yielding the optimal radioisotope content as a function of resolution, detector efficiency, time duration of scan, and dimensions of the object being scanned. Various collimating systems are considered and isoefficiency curves calculated. The simplest collimator consists of a cylindrical aperture in a lead shield. The use of a tapered aperture improves the sensitivity somewhat for a given resolution. A focusing detector may be constructed by means of a series of tapered apertures aimed at a focal spot. Finally, the detection of annihilation radiation resulting from positron detection is considered. The resolution and efficiency of these collimating systems are compared. Numerous factors modify the general design considerations. The effect of penetration of lead by the γ-rays is considered as well as the effect of finite source sizes and the attenuation of radiation in the object being scanned. Background radiation arises from many sources and results in an effective limit to the amount of radioisotope which can be employed. Examples of single-channel and coincidence scanning techniques are given.

Cylindrical PET detector design

A cylindrically shaped high-resolution PET (positron-emission-tomography) detector that uses cross-plane coincidence events is being developed. A 2-D analog coded position-sensitive detector is used. It consists of a hexagonal array of photomultiplier tubes and a rectangular array of crystals, eight elements per tube. The optics has been designed to maximize the light collection and to provide uniform spatial resolution. The detector will be 60 cm in diameter by 11.5 cm wide, the crystals are 3 mm*5.7 mm*30 mm. The design, associated electronics, and results of measurements on a sector of the detector are presented. >

Accelerated image reconstruction for a cylindrical positron tomograph using Fourier domain methods

The authors present a method to reconstruct projection images through assembly in the Fourier domain, rather than by backprojection. By bypassing the rate-limiting backprojection step of filtered backprojection, the proposed algorithm operates more quickly than previous methods. Fourier domain operations are also used to form forward-projected views of the imaged object; portions of these views are required to operate a full three-dimensional reconstruction using the cross-plane projection planes. Inaccuracies in images recovery and artifacts in the image are reduced by oversampling in the Fourier domain; this is easily accomplished by zero-padding the projection data prior to entering the Fourier domain. These Fourier domain algorithms form the basis for the reconstruction algorithm in PCR-II, a volumetric positron imager under construction at Massachusetts General Hospital. >

A method for labeling cells for positron emission tomography (PET) studies.

Positron emission tomography (PET) may provide an ideal means of monitoring the delivery of cells to normal and pathological tissue owing to its high resolution, specificity and three dimensional imaging capabilities. In order to implement such a technique, it is important to develop a labeling method for cells which provides a rapid and stable incorporation of the positron emitter without altering the viability or functional activity of the cells to be studied. Two approaches have been explored to achieve this end: metabolic incorporation of 2-[18F]fluorodeoxyglucose (2-[18F]FDG) or protein labeling with [11C]methyl iodide (MI). IL-2 activated mouse natural killer lymphocytes were expanded in culture for 7 days and used for these studies. Uptake of 2-[18F]FDG by the natural killer cells was found to occur rapidly and to level off after 30 min. The amount of cell-associated label was found to decay at a steady rate immediately following the procedure, with approximately 21% loss of label after 1 h. In contrast, labeling of cells with MI provided a stable association for 60 min which did not alter the viability or the cytotoxic activity of the cells. Injection of the labeled cells into a normal mouse followed by a full body scanning procedure demonstrated the accumulation of the cells in the lungs which corresponded to those seen by microscopy. These findings suggest that labeling lymphocyte populations with MI may provide a rapid and practical means to quantify systemic cell distribution by PET.