[en] The development of two different low-cost scanners for positron emission tomography (PET) based on 3D acquisition are presented. The first scanner consists of two rotating scintillation cameras, and produces quantitative images, which have shown to be clinically useful. The second one is a system with two opposed sets of detectors, based on the limited angle tomography principle, dedicated for mammographic studies. The development of low-cost PET scanners can increase the clinical impact of PET, which is an expensive modality, only available at a few centres world-wide and mainly used as a research tool. A 3D reconstruction method was developed that utilizes all the available data. The size of the data-sets is considerably reduced, using the single-slice rebinning approximation. The 3D reconstruction is divided into 1D axial deconvolution and 2D transaxial reconstruction, which makes it relatively fast. This method was developed for the rotating scanner, but was also implemented for multi-ring scanners with and without inter plane septa. An iterative 3D reconstruction method was developed for the limited angle scanner, based on the new concept of 'mobile pixels', which reduces the finite pixel errors and leads to an improved signal to noise ratio. 100 refs

[en] By collecting the radon (Rn-222) emanating from a sample enclosed in an accumulation can, the mean exhalation rate (Bq s^-1) can easily be deduced by extracting gas from the can and analysing it for radon. This so-called closed-can method has been used in various forms since the beginning of this century. The radon exhalation rate from a sample free in air and in equilibrium with its surroundings (i.e. the radon concentration gradient inside the porous sample is constant in time) is referred to as the free exhalation rate of the sample. The natural aim of most exhalation measurement techniques is to determine this free exhalation rate. Unfortunately, the radon accumulating variant of the closed-can technique fails in this (except for samples that are very thick compared with the diffusion length), and what is really remarkable is that this failure can pass unnoticed, even under carefully controlled laboratory conditions. The objective of this paper is to illustrate, by means of theoretical calculations using diffusion theory and Fick's law, why it is so difficult to experimentally determine the free exhalation rate with closed-can methods. The temporal variation of the radon exhalation rate and the corresponding radon gas concentration in the sample enclosure (outer volume) for 'one-dimensional' samples will be presented. The conclusions drawn are, in principle, also valid for more realistic sample geometries, as long as the dimensions of the sample relative to the diffusion length are kept the same

[en] The accident at the nuclear power plant of Chernobyl on April 26, 1986 led to radioactive contamination of many countries including Sweden. The population was exposed to released radionuclides, both by inhalation and from contaminated food. We have studied the content of gamma-emitting radioisotopes in the thyroid glands of a normal population from southern Sweden using measurements of samples taken at autopsy. The first samples are from a person who died on April 27, 1986. This report contains results for ¹³¹I and ¹³³I. The time-activity curve for ¹³¹I shows an immediate uptake with a maximum 18-26 days after the accident. No measurable levels were observed after 93 days. We have found that the increase in dose equivalent to the thyroid for the population of southern Sweden due to the released ¹³¹I and ¹³³I will be less than 0.1 mSv. This may lead to an increase in the incidence of thyroid cancer of 0.1% during a period of 25 yr

[en] In this paper, the authors have developed a new method for three-dimensional image reconstruction in PET, which preserves the simplicity of the two-dimensional methods. It was primarily developed for our constructed PET system consisting of two rotating scintillation cameras with a restricted axial field-of-view. In principle, however, it can also be used for multi-ring cameras with the interplane septa removed. The basic principle of the method is to organize the projection data so that 2D reconstruction can be used. The final image can be obtained using any available 2D reconstruction technique. The method consists of a combination of analytical and algebraical algorithms, including an iterative process with two-dimensional reconstruction of transaxial slices and deconvolution of a position dependent axial spread function. With this method the authors avoid the complex and time-consuming operation of 3D backprojection, but can still utilize all the detected events and take advantage of the three-dimensional information

[en] Investigations on the effect of normal healthy ageing on the muscarinic system have shown conflicting results. Also, in vivo determination of muscarinic receptor binding has been hampered by a lack of subtype selective ligands and differences in methods used for quantification of receptor densities. Recent in vitro and in vivo work with the muscarinic antagonist (R,R)-I-QNB indicates this ligand has selectivity for m₁ and m₄ muscarinic receptor subtypes. Therefore, we used (R,R)[¹²³I]-I-QNB and single photon emission tomography to study brain m₁ and m₄ muscarinic receptors in 25 healthy female subjects (11 younger subjects, age range 26-32 years and 14 older subjects, age range 57-82 years). Our aims were to ascertain the viability of tracer administration and imaging within the same day, and to evaluate whether normalization to whole brain, compared to normalization to cerebellum, could alter the clinical interpretation of results. Images were analyzed using the simplified reference tissue model and by two ratio methods: normalization to whole brain and normalization to cerebellum. Significant correlations were observed between kinetic analysis and normalization to cerebellum, but not to whole brain. Both the kinetic analysis and normalization to cerebellum showed age-related reductions in muscarinic binding in frontal, orbitofrontal, and parietal regions. Normalization to whole brain, however, failed to detect age-related changes in any region. Here we show that, for this radiotracer, normalizing to a region of negligible specific binding (cerebellum) significantly improves sensitivity when compared to global normalization

[en] The PETRRA positron camera consists of two 60 cmx40 cm annihilation photon detectors mounted on a rotating gantry. Each detector contains large BaF₂ scintillators interfaced to large area multiwire proportional chambers filled with a photo-sensitive vapour (tetrakis-(dimethylamino)-ethylene). The spatial resolution of the camera has been measured as 6.5±1.0 mm FWHM throughout the sensitive field-of-view (FoV), the timing resolution is between 7 and 10 ns FWHM and the detection efficiency for annihilation photons is ∼30% per detector. The count-rates obtained, from a 20 cm diameter by 11 cm long water filled phantom containing 90 MBq of ¹⁸F, were ∼1.25x10⁶ singles and ∼1.1x10⁵ cps raw coincidences, limited only by the read-out system dead-time of ∼4 μs. The count-rate performance, sensitivity and large FoV make the camera ideal for whole-body imaging in oncology

[en] A 3D reconstruction method originally developed by us for 3D PET scanners has been implemented for data obtained with a 2D scanner. The scanner consists of 4 detector rings and gives 7 simultaneous planes including cross-planes. The axial sampling is improved by moving the patient couch. This gives a smaller plane separation, but there will be cross-talk between the planes. The axial response profile for each direct plane is fairly independent of the transaxial position, but it varies dramatically for the cross-planes. Our 3D reconstruction method is based on 1D deconvolution of a position dependent axial spread function and 2D reconstruction of transaxial planes. Evaluations have been made with Monte Carlo simulations and phantom studies. 3D reconstruction gives an improved axial resolution at the cost of an increased noise level. (orig.)

[en] A three-dimensional (3D) image reconstruction method, which was originally developed for a positron emission tomography (PET) system consisting of two rotating scintillation cameras, has now been implemented for a multi-ring PET scanner with retractable septa. The method is called 'single-slice rebinning with axial deconvolution' (SSAD), and can be described as follows. The projection data are sorted into transaxial 2D sinograms. Correction for the axial blurring is made by deconvolution in the sinograms. To obtain the axial spread functions, which depend on the activity distribution, 2D reconstruction is first made using a limited axial acceptance angle. The final 3D image is obtained by 2D reconstruction of transaxial planes. The method is simple but not approximate, has a modest memory requirement, and can be combined with different 2D techniques. Evaluations by Monte Carlo simulations and phantom studies have been made. (Author)

[en] A fast accurate iterative reconstruction (FAIR) method suitable for low-statistics positron volume imaging has been developed. The method, based on the expectation maximization-maximum likelihood (EM-ML) technique, operates on list-mode data rather than histogrammed projection data and can, in just one pass through the data, generate images with the same characteristics as several ML iterations. Use of list-mode data preserves maximum sampling accuracy and implicitly ignores lines of response (LORs) in which no counts were recorded. The method is particularly suited to systems where sampling accuracy can be lost by histogramming events into coarse LOR bins, and also to sparse data situations such as fast whole-body and dynamic imaging where sampling accuracy may be compromised by storage requirements and where reconstruction time can be wasted by including LORs with no counts. The technique can be accelerated by operating on subsets of list-mode data which also allows scope for simultaneous data acquisition and iterative reconstruction. The method is compared with a standard implementation of the EM-ML technique and is shown to offer improved resolution, contrast and noise properties as a direct result of using improved spatial sampling, limited only by hardware specifications. (author)

[en] We present three novel multi-slit-slat (MSS) system designs which allow for the acquisition of data with variable multiplexing in order to optimize the use of a high intrinsic resolution detector for clinical brain SPECT. In this paper we first study the relationship between the geometric parameters of a MSS collimator system and the resulting resolution and sensitivity for an on-axis point at the centre of the field-of-view (FOV), assuming a continuous cylindrical detector model. The model predicts that for optimal system sensitivity and resolution, the ratio of the detector radius to slit collimator radius should be 1.3-1.5, as any further increase in this ratio results in significant deterioration in both system resolution and sensitivity. The analytical results were used to fix the geometric parameters for the three novel MSS system designs. Comparison of the three designs, asymmetric rotating collimator (ARC), asymmetric rotating detector (ARD) and symmetric rotating collimator (SRC) with variable slit spacing, suggests that the SRC system performs better in terms of the system sensitivity (5.1 x 10^-4) for the same average resolution (6.0 mm) in comparison to designs based on an ARC (3.7 x 10^-4) and ARD (4.2 x 10^-4).