SPECT is a rapidly changing field, and the past several years have produced new developments in both hardware technology and image-processing algorithms. At the component level there have been improvements in scintillators and photon transducers as well as a greater availability of semiconductor technology. These devices permit the fabrication of smaller and more compact systems that can be customized for particular applications. New clinical devices include high-count sensitivity cardiac SPECT systems that do not use conventional collimation and the introduction of diagnostic-quality hybrid SPECT/CT systems. While there has been steady progress with reconstruction algorithms, exciting new processing algorithms have become commercially available that promise to provide substantial reductions in SPECT acquisition time without sacrificing diagnostic quality. Preclinical small-animal SPECT systems have become a major focus in nuclear medicine. These systems have pushed the limits of SPECT into the submillimeter range, making them valuable molecular imaging tools capable of providing information unavailable from other modalities. From the beginning of radionuclide imaging, there has been a dedicated effort to produce tomographic images of the internal distribution of radiopharmaceuticals. Although the early development of SPECT will not be discussed in this article, an excellent review of the key investigators and their ground-breaking work can be found in a recent article by Jaszczak (1). The present article will focus on recent developments in SPECT that cover approximately the past 3 y. I will begin by briefly reviewing the fundamental physical assumptions underlying SPECTand follow that with a discussion of the advances in detection instrumentation. Clinical and research devices designed for patient studies will be reviewed next. Small-animal SPECT systems will also be described. These sections will be followed by a review of reconstruction algorithms and correction methods. An outstanding in-depth source of information on all of these topics for the interested reader can be found in the book Emission Tomography: The Fundamentals of PET and SPECT (2).Conventional nuclear medicine images compress the 3-dimensional (3D) distributions of radiotracers into a 2-dimensional image. As a result, the contrast between areas of interest and the surrounding territory is often significantly reduced. This reduction limits the diagnostic information that is available in the study. In addition, the exact location of an abnormality can be difficult to determine. Tomographic images remove these difficulties but at the price of longer acquisition times, poorer spatial resolution, and the susceptibility to artifacts. Recent advances in SPECT instrumentation and processing have made marked improvements in each of these areas. SPECT FUNDAMENTALSThe fundamental objective of any tomographic imaging is to determine the internal distribution of an object solely from external measurements. This can be accomplished only if the following re...

Arrays of nanowires (NWs) are currently being established as vehicles for molecule delivery and electrical- and fluorescence-based platforms in the development of biosensors. It is conceivable that NW-based biosensors can be optimized through increased understanding of how the nanotopography influences the interfaced biological material. Using state-of-the-art homogenous NW arrays allow for a systematic investigation of how the broad range of NW densities used by the community influences cells. Here it is demonstrated that indium arsenide NW arrays provide a cell-promoting surface, which affects both cell division and focal adhesion up-regulation. Furthermore, a systematic variation in NW spacing affects both the detailed cell morphology and adhesion properties, where the latter can be predicted based on changes in free-energy states using the proposed theoretical model. As the NW density influences cellular parameters, such as cell size and adhesion tightness, it will be important to take NW density into consideration in the continued development of NW-based platforms for cellular applications, such as molecule delivery and electrical measurements.

Phoenix, the first Mars Scout mission, capitalizes on the large NASA investments in the Mars Polar Lander and the Mars Surveyor 2001 missions. On 4 August 2007, Phoenix was launched to Mars from Cape Canaveral, Florida, on a Delta 2 launch vehicle. The heritage derived from the canceled 2001 lander with a science payload inherited from MPL and 2001 instruments gives significant advantages. To manage, build, and test the spacecraft and its instruments, a partnership has been forged between the Jet Propulsion Laboratory, the University of Arizona (home institution of principal investigator P. H. Smith), and Lockheed Martin in Denver; instrument and scientific contributions from Canada and Europe have augmented the mission. The science mission focuses on providing the ground truth for the 2002 Odyssey discovery of massive ice deposits hidden under surface soils in the circumpolar regions. The science objectives, the instrument suite, and the measurements needed to meet the objectives are briefly described here with reference made to more complete instrument papers included in this special section. The choice of a landing site in the vicinity of 68°N and 233°E balances scientific value and landing safety. Phoenix will land on 25 May 2008 during a complex entry, descent, and landing sequence using pulsed thrusters as the final braking strategy. After a safe landing, twin fan‐like solar panels are unfurled and provide the energy needed for the mission. Throughout the 90‐sol primary mission, activities are planned on a tactical basis by the science team; their requests are passed to an uplink team of sequencing engineers for translation to spacecraft commands. Commands are transmitted each Martian morning through the Deep Space Network by way of a Mars orbiter to the spacecraft. Data are returned at the end of the Martian day by the same path. Satisfying the mission's goals requires digging and providing samples of interesting layers to three on‐deck instruments. By verifying that massive water ice is found near the surface and determining the history of the icy soil by studying the mineralogical, chemical, and microscopic properties of the soil grains, Phoenix will address questions concerning the effects of climate change in the northern plains. A conclusion that unfrozen water has modified the soil naturally leads to speculation as to the biological potential of the soil, another scientific objective of the mission.

The method of superposition of configurations is applied to the singlet sigma states of HeH+ which correlate to the separated atom states of principal quantum number less than or equal to 3. The calculations are carried out for internuclear separations in the ranges 0(.1)34.5, 35.0(.5)50.0 bohr. Energy eigenvalues and the expectation values of the electron coordinate along the internuclear line are discussed in this paper. Dipole transition matrix elements and radial coupling matrix elements are presented in a companion paper. The quality of the calculations is exhibited by comparisons of the length and velocity forms of the dipole matrix elements, by the Hellmann-Feynman theorem, and by the approach to atomic properties at large internuclear separations. Supplementary calculations of higher quality wavefunctions for the lowest states are also reported. These are used to help estimate the extent to which the results of the main calculations have converged toward the exact values. Comparisons with other work are given. It is shown that for internuclear separations greater than about 3 bohr a one-electron electron model potential approach is quite successful for the excited states. At smaller internuclear separations, and as the united atom limit is approached, current model potential methods cannot be trusted. In this region the model potential, being based on the separated atoms, cannot describe the actual changing charge distributions of the core electron. This work demonstrates the feasibility, for simple systems, of the ab initio structure calculations required for the quantitative description of low-energy atom-atom collisions.

Configuration interaction calculations of the dipole, gradient, and radial coupling matrix elements among the lowest 10-12 l~ states of HeH+ are reported for internuclear separations in the range o :$R:$50 a.u. The forms (n la/aR I m) and (Em -En )-1 (n la VlaR I m) of the radial coupling element are compared. The results are compared with those of other investigators. The accurate evaluation of these matrix elements in the present work demonstrates the feasibility of a quantitative ab initio treatment of low-energy collisions between light atoms.

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Two types of wireless position sensitive X -ray proportional counters are under development at the Danish Space Research Institute. One detector -the microstrip proportional counter -employs very narrowly spaced conducting strips deposited on an isolating substrate instead of wires. The gas gain is uniform over the entire detector area and the achievable energy resolution is close to or better than the best achievable for single wire proportional counters. The position readout is made using a wedge and strip electrode mounted on the back -side of the insulating glass substrate. The second detector employs a uniform electric field between two parallel electrodes in order to achieve amplification. Parallel electrodes have better energy resolution, better timing resolution and are easier to construct and more durable than multiwire chambers. The parallel gap is formed between an etched Ni mesh and a segmented anode in the form of a wedge and strip electrode. The X -ray photon energy is derived from the mesh electrode, whereas the position information is taken from the anode. Submilimeter position resolutions have been achieved.In both detectors the energy signal has a very fast risetime and background rejection based on pulse shape analysis is, therefore, very efficient. The background rejection efficiencies achieved and the optimum rejection method for spaceborne detectors will be discussed.