C. What Life Scientists Should Know about Imaging Modalities
3. Nuclear Imaging: Physical Principles and Instrumentation: Part 3
Moderators: Steven Meikle and Robert Miyaoka
Presentation 1: Principles of SPECT and PET
Radiology, University of Washington, Seattle, WA, USA
- To learn about the tracer principle and its importance in molecular imaging
- To understand how radiation emitted from the body is detected externally using tomographic systems
- To appreciate the key principles in forming a reconstructed image of the tracer distribution in the body
Positron emission tomography (PET) and single photon computed tomogprahy (SPECT) are nuclear medicine based imaging techniques that provide functional or biochemical information about the subject being studied. PET and SPECT are molecular imaging techniques that rely upon the tracer principle for studying human physiology and disease processes. The tracer principle will be described and how it relates to PET and SPECT imaging. Criteria for what makes a radioisotope a good candidate for human imaging will be described as well as the most common radioisotopes for PET and SPECT procedures.
Positrons are positively charged beta particles and after they are ejected from the nucleus during radioactive decay. The basic principles of positron emission and the physics behind coincidence imaging and event localization will be described. For SPECT imaging only a single gamma is produced so other methods are used to help localize where the radioactive decay occurred. The most common practice is to place a collimator in front of the SPECT detector to only allow photons traveling perpendicular to the face of the detector to be detected. Just like PET, the use of a collimator fixes a detected photon from having to have originated along a specific line of response. In addition to these principles, the methods for spatial localization within the detector system will be described.
Both PET and SPECT data are usually stored in structures called sinograms prior to image formation or image reconstruction. Analytic and iterative reconstruction methods are used to produce three-dimensional images of the activity distribution within the patient. The most common analytic image reconstruction method is filtered back projection. It is still used quite a bit for SPECT imaging; however, iterative image reconstruction techniques are now the norm for PET imaging. The basic methodologies for both filtered back projection and iterative image reconstruction will be explained. With the advent of SPECT/CT systems and a trend toward quantitative SPECT imaging, iterative reconstruction is becoming more commonplace for SPECT image reconstruction, also.
In wrapping up the presentation, an overview of clinical PET and SPECT systems and organ specific systems will be provided.
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- Muehllehner, G and Karp, JS. Positron emission tomography. Phys Med Biol. 51:R117-R137, 2006
- Bushberg JT, Seibert JA, Leidholdt Jr. EM, Boone JM. The Essential Physics of Medical Imaging, Third Edition, Publisher, Lippincott Williams and Wilkins, 2011.
Presentation 2: Detector Technologies
Dennis R. Schaart
Radiation Science & Technology, Delft University of Technology, Delft, Netherlands
- To obtain a basic understanding of the physical principles of radiation detection
- To learn the essential principles of inorganic scintillators and their most important properties
- To acquire basic knowledge of photosensors and readout electronics relevant to radiation detection
This lecture provides a basic introduction into the physical principles of the detection of gamma radiation in nuclear medicine imaging (in particular PET, SPECT, and hybrid imaging modalities such as PET/MRI). The emphasis lies on scintillation detectors, which are the most commonly applied type of detector. The interaction of the gamma quanta with the scintillation material forms the basis for the signal formation. Therefore, a basic understanding of the different types of interaction that may occur is required to understand the operation and performance of a scintillation detector. Furthermore, the physics underlying the conversion of the energy of a gamma quantum into scintillation photons will be briefly discussed. The next step in the detection chain is the conversion of the relatively weak light signal emitted by the scintillator into an electronic signal by means of a photosensor. An important aspect of detector design is the optimization of the crystal-photosensor geometry in order to achieve a good balance between multiple and often conflicting requirements on the detector performance at reasonable costs. The classical and still most widely used photosensor is the photomultiplier tube (PMT). For a long time, the relatively large internal gain and low noise of these devices made them the first, if not only, choice for the detection of very small amounts of light, down to the single photon level. However, advances in semiconductor technology have recently given birth to several new types of low-level light sensor, some of which have distinct advantages compared to PMTs for certain applications. Examples include avalanche photodiodes (APDs) and silicon photomultipliers (SiPMs). These solid state devices enable new crystal-sensor geometries, as well as new combinations of imaging modalities such as PET and MRI in highly integrated multimodality systems. For optimum detector performance, the use of dedicated front-end electronics, adapted to the specific properties of the scintillator-photosensor combination, is paramount. The readout electronics and data acquisition (DAQ) architecture furthermore need to be tailored to the imaging modality and application, as the corresponding requirements may vary greatly. The overall detector performance, expressed in terms of parameters such as spatial resolution, energy resolution, timing resolution, and detection efficiency, is the result of the design choices made with respect to each of the above components making up the detection chain (scintillation material, photosensor, detector geometry, electronics). Since the performance of the detectors impose ultimate limits on the image quality achievable with any scanner, it is not surprising that many research groups work on new and better detectors. Some of these recent developments will be briefly highlighted in this lecture.
Presentation 3: SPECT and PET Based Hybrid Imaging Systems
Jae Sung Lee
Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Republic of Korea
- To become acquainted with a range of hybrid imaging methods, their rationale and principles.
- To develop an appreciation of the applications of clinical and preclinical hybrid imaging.
- To gain an understanding of technical challenges facing designers of hybrid imaging systems, and recent technological innovations.
- To be familiar with potential sources of artifacts in hybrid imaging and how they may be counteracted.
Nuclear medicine imaging methods that use radionuclides, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT), offer highly sensitive and quantitative tools for detection and localization of the biochemical and functional abnormalities associated with various diseases. In addition, molecular imaging techniques based on radionuclide imaging are the most sensitive methods that are readily translatable to clinical use. However, the major drawbacks of stand-alone PET and SPECT systems are their relatively poor spatial resolutions and low signal-to-noise ratios. The absence of background anatomical information in stand-alone PET and SPECT images of highly target-specific radiotracers sometimes make it difficult to interpret the distributions of these tracers. The introduction of dual-modality PET/CT and SPECT/CT systems in the late 1990s, in which PET and SPECT are combined with X-ray CT in a clinical setting, is regarded as a revolutionary advance in modern diagnostic imaging. In these systems, the PET or SPECT images are acquired sequentially with the CT images using a single device and without moving the patient from the bed, eliminating differences in patient positioning and minimizing the misalignments caused by internal organ motion. The anatomical information provided by the CT images enhances the user’s confidence in the PET and SPECT findings. Additionally, the attenuation map derived from the X-ray CT for the gamma-rays emitted from the radionuclides offers useful ways to correct for the attenuation and scatter artifacts in PET and SPECT with minimal addition to the scan time and the image noise. The concept of simultaneous acquisition of PET and MR images was also suggested in the early days of dual-modality systems development, and the development of PET/MR scanners started in the 1990s. However, progress in the development process was relatively slow and the realization of clinical PET/MR scanners was greatly delayed because of technical difficulties when operating PET and MR scanners in close proximity combined with a lack of industrial interest and concern over the high cost of the combined device. The great success of nuclear medicine imaging modalities when combined with CT has, however, revived interest in the combination of PET and MR scanners. The technical advances made over the long development period to minimize the mutual interference between the PET and MR data acquisition processes have led to combined clinical PET/MR scanners with sequential and simultaneous imaging strategies in recent years. The major advantages of PET/MRI include a smaller radiation burden than PET/CT, better soft tissue contrast when using MRI rather than CT, and possible simultaneous acquisition of images. In this lecture, the basic principles, fields of application, and recent advances will be reviewed for each hybrid imaging device.
- Townsend DW, et al. Multimodality imaging of structure and function. Phys Med Biol. 53:R1-R39, 2008
- Seo Y, Mari C, Hasegawa BH. Technological development and advances in single-photon emission computed tomography/computed tomography. Semin Nucl Med 2008;38(3):177-198
- Zaidi H, Alavi A. Current trends in PET and combined (PET/CT and PET/MR) systems design. PET Clinics 2007;2(2):109-123
- Cherry SR. Multimodality imaging: beyond PET/CT and SPECT/CT. Semin Nucl Med 2009;39(5):348-353
- Rahmim A, Zaidi H. PET versus SPECT: strengths, limitations and challenges. Nucl Med Commun 2008;29(3):193-207
- Lee JS, Kim JH. Recent advances in hybrid molecular imaging systems. Semin Musculoskel R. 2014;18:103–122