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.

https://jnm.snmjournals.org/content/jnumed/48/4/661.full.pdf

Recent Advances in SPECT Imaging

Today the majority of clinical molecular imaging procedures are carried out with single-photon emitters and gamma cameras, in planar mode and single-photon emission computed tomography (SPECT) mode. Thanks to the development of advanced multi-pinhole collimation technologies, SPECT imaging of small experimental animals is rapidly gaining in popularity. Whereas resolutions in routine clinical SPECT are typically larger than 1 cm (corresponding to >1,000 μl), it has recently proved possible to obtain spatial resolutions of about 0.35 mm (≈0.04 μl) in the mouse. Meanwhile, SPECT systems that promise an even better performance are under construction. The new systems are able to monitor functions in even smaller structures of the mouse than was possible with dedicated small animal positron emission tomography (≈1 mm resolution, corresponding to 1 μl). This paper provides a brief history of image formation with pinholes and explains the principles of pinhole imaging and pinhole tomography and the basics of modern image reconstruction methods required for such systems. Some recently introduced ultra-high-resolution small animal SPECT instruments are discussed and new avenues for improving system performance are explored. This may lead to many completely new biomedical applications. We also demonstrate that clinical SPECT systems with focussing pinhole gamma cameras will be able to produce images with a resolution that may become superior to that of PET for major clinical applications. A design study of a cardiac pinhole SPECT system indicates that the heart can be imaged an order of magnitude faster or with much more detail than is possible with currently used parallel-hole SPECT (e.g. 3–4 mm instead of ≈8 mm system resolution).

The pinhole: gateway to ultra-high-resolution three-dimensional radionuclide imaging.

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

This article describes a range of results, from passive magnet design to optimal feedback control of a distributed ferrofluid. Representing magnetic forces as the gradient of a magnetic energy allowed design and demonstration, in animal experiments, of a simple open-loop two-magnet system to inject nanoparticles into the inner ear. Then a petri-dish test bed was used to develop and test optimal closed-loop manipulation of a single ferrofluid droplet.The demonstrated algorithms exploited the quadratic dependence of the magnetic forces on control inputs while accounting for magnet time delays and spatial discontinuities in the optimal control. All dimensional parameters were reduced to three essential nondimensional numbers, and ferrofluid behavior was mapped across the entire feasible drug delivery parameter space.

Towards Control of Magnetic Fluids in Patients: Directing Therapeutic Nanoparticles to Disease Locations

Charge-coupled devices (CCDs) coupled to scintillation crystals can be used for high resolution imaging with x-rays and gamma-rays. When the CCD images can be read out fast enough, the energy and interaction position of individual gamma quanta can be estimated by real-time image analysis of scintillation light flashes ('photon counting mode'). We tested a set-up in which an electron-multiplying CCD was coupled to a 1 mm thick columnar CsI crystal by means of a fibre-optic taper. We found that, compared to light integration, photon counting improves the intrinsic spatial resolution by a factor of about 3 to 6. Applying our set-up to Tc-99m and I-125 imaging, we were able to obtain intrinsic resolutions below 60 μm (full width at half maximum). Counting losses due to overlapping of light flashes are negligible for event rates typical for biomedical radio-nuclide imaging and do strongly depend on energy window settings. Energy resolution was estimated to be approximately 35 keV FWHM for a 1:1 taper. We conclude that CCD-based gamma cameras have great potential for applications such as in vivo imaging of gamma emitters.

http://iopscience.iop.org/article/10.1088/0031-9155/50/12/N01/pdf

Gerralt A de Vree

Papers

Photon-counting versus an integrating CCD-based gamma camera: important consequences for spatial resolution.