Single-Photon Emission Computed Tomography/Computed Tomography: Basic Instrumentation and Innovations

https://doi.org/10.1053/j.semnuclmed.2006.05.005Get rights and content

Correlation of the anatomical and functional information presented by single-photon emission computed tomography (SPECT) and computed tomography (CT) can aid in the decision-making process by enabling better localization and definition of organs and lesions and improving the precision of surgical biopsies. Technical developments over the past 20 years have led to the development of better software techniques for image fusion and, more recently, to the development of modern SPECT/CT systems. While image fusion techniques have been in clinical use for many years, the first commercial SPECT/CT system was only developed in 1999. Following the commercial success of PET/CT systems that employed multidetector CT (MDCT) scanners, there has been renewed interest in the development of comparable SPECT/CT systems. This has resulted in the development of a range of SPECT/CT devices varying from a simple CT add-on to a conventional SPECT system that can provide low-dose CT images to a full MDCT scanner integrated with a SPECT system. The advantages of combining SPECT with CT are numerous and are primarily due to the anatomic referencing and the attenuation correction capabilities of CT. Depending on system design, there are varying technical issues surrounding the different SPECT/CT devices, ranging from cost, radiation dose, planning, and siting requirements to system-specific issues such as table sag and CT artifacts due to patient motion. Motion artifacts should be less prevalent with the faster acquisition times of modern scanners, but are still problematic in the thorax and have not yet been fully resolved as they pertain to the use of CT data for cardiac attenuation correction. As this technology matures, we can expect to see a range of SPECT/CT devices available on the market that range from low-dose 1-4 slice inexpensive CT upgrades of conventional SPECT systems, to SPECT systems incorporating 64 or 128 slices CT scanners. The cost of the high-end CT scanners will exceed the cost of the SPECT scanner and hence the justification for such devices will be heavily dependent on clear demonstration of their value in clinical practice.

Section snippets

Software Approach to Image Fusion

Image fusion usually is performed between an anatomical imaging technique such as CT or MRI and a functional imaging technique such as positron emission tomography (PET) or SPECT. Before the introduction of dedicated PET/CT or SPECT/CT systems, considerable work had been done on the development of software algorithms for the coregistration of anatomical and functional images. It is worth briefly reviewing what has been accomplished with these techniques, particularly as we go forward and look

Development of SPECT/CT Devices

Much of the early work on the development of a combined SPECT/CT unit was performed at University of California, San Francisco, by Dr Hasegawa and colleagues. Their initial work was focused on the development of a system that could perform simultaneous CT and SPECT studies. Figure 2 shows a schematic diagram of their first system, which used an array of high-purity germanium detectors to simultaneously detect 40 to 100 keV x-rays from an external source and 140 keV gamma rays from an internally

Technical Aspects of SPECT/CT Imaging

The advantages of combining SPECT with CT are numerous and are primarily due to the anatomic referencing and the attenuation correction capabilities of CT. Whether the CT component that is used in the combined imaging approach should be a conventional MDCT scanner or the more compact, low current CT add-on used on the GE Hawkeye system is currently a matter of debate.

Sources of Error

There are several sources of error in the application of SPECT/CT, depending on the system configuration. These errors include misregistration, truncation, scatter, and beam hardening artifacts. A major issue for CT type systems is misregistration between the emission and transmission data, resulting in incorrect matching of the attenuation map to the emission data.48 This may occur for a number of reasons, including sagging of the emission table, respiratory and cardiac motion, and patient

Anatomic Referencing

Coregistration of anatomy and function is less dependent on the fidelity of the CT image than the attenuation correction algorithm. However, the accurate coregistration of the SPECT and CT data are just as important as with attenuation correction, and many of the pitfalls discussed previously, vis-à-vis table sagging, patient motion, and respiratory and cardiac motion, all apply equally to image fusion. Most vendors now include a calibration procedure to insure that, in the absence of

Planning/Siting Requirements for SPECT/CT

The space required for a SPECT/CT system depends on the type of system being installed. The GE Hawkeye or GE Hawkeye-4 only requires the same room size as conventional SPECT systems. Minimum room size for these types of units is typically 14′ × 16′. Because of the x-ray tube on these systems, some lead shielding of the room is required. The exposure rate from these systems is approximately 20 times less than that from a conventional MDCT scanner. Hence lead shielding in the walls is usually

Future Applications and Advances for SPECT/CT Technology

In addition to attenuation correction and co-registration, other possible applications for this emerging technology include patient dosimetry and radiotherapy. The development of more-sophisticated co-registration applications should permit estimation of organ or tumor volume from the anatomical data rather than the emission data. Traditional calculations of organ and tumor size from emission data are problematic, particularly for small tumors in which the limited spatial resolution of SPECT

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