[(18)F]FDG uptake reflects the immediate tumor activity of bone metastases, whereas the radiographic morphology changes vary greatly with time among patients.

A recently introduced method (dual-photon X-ray absorptiometry, DEXA) capable of measuring skeletal density in man (at present in the spine and hips, but ultimately for the whole body) has been evaluated in terms of its ability to perform long-term assessment of bone density changes. The method, which uses X rays rather than gamma rays as its photon source, represents a significant improvement over present systems both in image quality and precision (reproducibility) of results, which is better than 1% in vivo. Scanning time is approximately halved compared with present techniques and the radiation dose is reduced by 25%. First data on long-term drift of results and effects of changes in patient composition (i.e. thickness and fat content) are given and show the new method to be superior to present radionuclide systems. It is likely that this new method will become the standard for bone density measurements.

We discuss recent trends in collimator design and technology, with emphasis on theoretical and practical issues of importance for single photon emission tomography (SPET). The well-known imaging performance parameters of parallel-hole collimators are compared with those of fan-beam collimators, which have enjoyed considerable success in recent years, particularly for brain SPET. We review a simplistic approach to the collimator optimization problem, as well as more sophisticated "task-dependent" treatments and important considerations for SPET collimator design. Practical guidance is offered for understanding trade-offs that must be considered for clinical imaging. Finally, selective comparisons among different SPET systems and collimators are presented for illustrative purposes.

3-Iodo-6-methoxybenzamide (123I-IBZM), a new Dopamine D2 receptor ligand, was used in conjunction with SME 810 brain tomography to study six subjects (one normal volunteer, four schizophrenics and one DAT patient). Initial Dynamic SPET was followed by multislice SPET. High-resolution images of the D2 receptor distribution in the basal ganglia were obtained. The specific binding in D2 receptors of the basal ganglia is highest from 2-4 h p.i. Patients on anti-psychotic drugs showed significantly lower specific binding. Dopamine D2 brain receptor availability in man may now be studied with SPET. Continuous data acquisition with single slice tomography is particularly important in the study of this type of radiotracers.

Multiplexing is a way of increasing the sensitivity in a multi-slit-slat SPECT system by allowing the overlap of projections from neighboring apertures. The fundamental objective of multiplexing is to increase the signal-to-noise ratio for a given system resolution. Multiplexing may therefore lead to an improved tradeoff between resolution and sensitivity. Overlapped projections, however, introduce ambiguities in the data which can lead to non-uniqueness of solution for the inverse problem, deterioration in the quality of reconstruction and ultimately loss of resolution. Therefore, it is not straightforward to evaluate the advantage of the extra sensitivity gained by multiplexing, without first devising a method to overcome the image artifacts caused due to this overlapping of projection data. In this paper we investigate the effect of multiplexing on the reconstructed image quality and we determine whether reconstruction of multiplexed data could be improved by the addition of non-multiplexed data. For this purpose we have done simulations based on three digital phantoms. We compared the reconstructed images both qualitatively and quantitatively for different degrees of multiplexing and different fractions of non-multiplexed data. Our results indicate that the recovery coefficient (and therefore spatial resolution) can be maintained with a high degree of multiplexing leading to a significant increase in the SNR (up to 25%) due to a reduced noise level. This gain in the SNR corresponds to a 75% increase in counts or sensitivity which can be utilized to reduce acquisition time, patient dose or/and improve image quality.