Photomultiplier Tube Technology
Historically, gamma camera technology dates back to the late 1950s when Hal Anger invented the first clinically successful scintillation camera. Earlier, Benedict Cassen’s invention of the rectilinear scanner capable of performing static studies using radioactive tracers brought about the emergence of nuclear imaging. However, it was Anger’s initial scintillation camera design based on seven photomultiplier tubes (PMTs) coupled to a Nal scintillation crystal that revolutionized nuclear imaging and established itself as a core medical imaging modality. The application of PMT technology to accurately determine the position of where scintillation occurred (and thus the photon’s origin) enabled clinicians to capture functional images of entire organs with faster acquisition times and higher resolution. Although manufacturers have achieved significant improvements in image resolution and uniformity since that time, the majority of today’s commercial gamma camera systems still utilize the same underlying detection technology used in traditional Anger cameras. Detection occurs through a two-step process; first, a NaI(TI) (thallium-doped sodium iodide) scintillation crystal captures and transforms a released γ-photon into a light photon. A photomultiplier tube mounted onto the scintillation crystal then transforms the light photon into an amplified electric signal proportional to the energy of the original scintillation light.
State-of-the-art PMT technology now equips photomultiplier tubes with individual analog-to-digital converters (ADC) that can carry out individual digitalization and signal processing including sampling, integration, and event positioning. Earlier systems composed of analog electrical components offered less system stability and greater noise, resulting in lower quality images than digital systems. Manufacturers realized that the earlier the digitalization of an electrical signal was performed, the more one could minimize signal and image degradation. In addition, manufacturers now construct PMTs in various geometries (rectangular, circular, and hexagonal). Consequently, cameras now contain larger and higher-efficient fields-of-view (FOV) that minimize dead space and increase image resolution. Finally, significant improvements in attenuation correction and image reconstruction algorithms have resulted in clearer and more uniform images.
However, the demand for cameras with larger FOVs by end-users (especially in nuclear cardiology) has also resulted in manufacturers increasing the number of PMTs within its detectors. End-users are attracted to cameras with larger FOVs for their ability to minimize image artifacts and improve workflow. However, to create larger FOVs, manufacturers must increase the number of PMTs, resulting in significant increases in system weight, bulkiness, and cost. These larger footprints for PMT-based systems have placed severe limitations on the mobility and installation options for gamma cameras.
Solid-State Detector Technology
The development of solid-state detectors in nuclear imaging provides an affordable and attractive solution for the next generation of SPECT cameras. Historically, earlier generation solid-state detectors severely underperformed PMT-based cameras which resulted in limited clinical adoption and stagnant sales. However, advancements in solid-state technology electronics and scintillation-detector materials have significantly improved solid-state based SPECT detector performance such that it equals or exceeds PMT-based cameras in all major performance categories, including energy resolution, intrinsic spatial footprint size, and power requirements. In addition, solid-state based electronics offer higher sensitivity and a low signal-to-noise ratio (SNR), which translates to images with higher clarity, that are produced with shorter acquisition times.
Most important is the fact that SPECT cameras based on cadmium zinc telluride (CZT) and cesium iodide thallium (CsI(TI)) crystals can convert and digitalize gamma radiation in a single step, eliminating the need for bulky PMT technology. The advances made allow manufacturers to offer systems with smaller footprints and increased mobility as both weight and size dimensions are drastically reduced. According to Dr. Jack Juni of CardiArc, despite the fact that virtually all cardiologists refer patients for SPECT imaging to diagnose CAD development, less than 10 percent of them possess imaging equipment. With size being the most important limiting factor, solid-state manufacturers emphasize that their systems have the advantage of being small enough to fit into the majority of physician offices, with system footprints as small as 7 X 8 feet*, requiring minimal to no room modifications. With prices similar to traditional Anger-based cameras, preference for dedicated cardiac cameras should shift to solid-state digital detectors. Growing adoption of these cameras by freestanding imaging centers and physician offices could significantly increase procedure volumes, driving demand for additional installed base units.
Historically, 15 years after Anger made his scintillation camera commercially available, a clinical survey showed that greater than half of its respondents still used the rectilinear scanner despite Anger’s camera offering superior image quality and faster acquisition times. Similarly, solid-state detector technology has been approved by the FDA since 1997, but high cost and sporadic ability of CZT crystals have limited the adoption of the technology by a majority of companies. In addition, long replacement cycles and substantial cuts in reimbursement may have had a stronger adverse impact on technology adoption than previously expected. One strong driver for solid-state technology could be its ability to be coupled with other imaging modalities, such as MRI, to create novel hybrid systems capable of producing both functional and anatomical high-resolution images. In addition, solid-state crystals have been demonstrated to contain the best low-energy resolution and are sensitive to a wider range of energies, thus potentially allowing dual radionucleotide imaging of similar energies with high accuracy. Despite the promise of this new technology, major manufacturers have been slow to adopt it. While some have created prototype SPECT cameras with CZT digital detectors, none are currently prepared to offer solid-state digital detectors on the commercial market. One restraint could be the high manufacturing costs and sporadic availability of CZT crystals. Despite the fact that CZT crystals offer the best energy resolution, some manufacturers have chosen to work with CsI crystals that offer comparable sensitivity and image resolution at a lower cost. Although the future for solid-state digital detectors remains uncertain, it looks very promising, as manufacturers strive to meet the growing demand for smaller and more versatile dedicated cardiac cameras for nuclear cardiology.
*This applies to currently available commercial solid-state detectors. At least one manufacturer (CardiArc) is taking pre-orders for a gamma camera that would offer a 6' X 7' footprint.