C. What Life Scientists Should Know about Imaging Modalities
3. Nuclear Imaging: Physical Principles and Instrumentation: Part 2
Moderators: Roger Fulton and Gary Luker
Presentation 1: Principles of PET and SPECT
Medical Physics, Westmead Hospital, Sydney, NSW, Australia; Brain and Mind Research Institute, University of Sydney, Sydney, NSW, Australia
- Be able to explain the tracer principle and its importance in PET and SPECT molecular imaging.
- Be able to explain to your peers how radiation emitted from the body is detected externally using SPECT and PET instrumentation.
- Be able to explain the key principles in forming a reconstructed image of the tracer distribution in the body.
This presentation explores the principles and methods that underpin two key molecular imaging techniques based on the radioactive tracer principle: single photon emission computed tomography (SPECT) and positron emission tomography (PET). Topics covered include the radioactive tracer principle, radioisotope production and decay, radiation transport in tissue, radiation detection, PET and SPECT instrumentation and tomographic image reconstruction. On completion of this lecture, students will have a basic understanding of the imaging chain as it relates to PET and SPECT, starting with the emission of radiation in the body, leading to its external detection and, finally, a reconstructed image of the radioactive tracer distribution in the body. The factors affecting the accuracy and noise properties of molecular images will be briefly explored. Students will also have an appreciation of how to use these imaging technologies to exploit the properties of the radioactive tracer principle and make estimates of important physiological parameters.
Disclosure of author financial interest or relationships: R. Fulton, None.
Presentation 2: SPECT and PET Detector Technologies
Dennis R. Schaart
Applied Sciences, 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 (preclinical PET and SPECT in particular). 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.
- van Eijk, CWE. Inorganic scintillators in medical imaging. Phys Med Biol. 47:R85-R106, 2002
- Lewellen, TK. Recent developments in PET Detector technology. Phys Med Biol. 53:R287-R317, 2008
Presentation 3: PET and SPECT Based Hybrid Imaging Systems
Radiology, University of Washington, Seattle, Washington, USA
- 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.
A theme in medical imaging is that two images combined together can often provide more than twice the information as one. The two images can be a functional (e.g., PET or SPECT) and an anatomic image (e.g., CT or MRI); images from two different pharmaceuticals or contrast agents; or images before and after therapy. In this session we will focus on nuclear medicine based hybrid imaging systems (e.g., PET/CT and SPECT/CT). We will begin with an overview of the rationale for hybrid imaging both for clinical and pre-clinical applications. We will describe the synergistic benefits of these systems. We will then discuss the different types of nuclear medicine hybrid instrumentation. The main focus will be on PET/CT and SPECT/CT as they have the largest user base. However, we will also cover PET/MRI, SPECT/MRI, and other types of hybrid imaging systems. We will cover the challenges (i.e., imaging artifacts) associated with hybrid imaging systems and also the benefits of having multiple sets of complementary data. Finally, we will go over the special data processing requirements that are necessary and available for the various imaging combinations.
Disclosure of author financial interest or relationships: R. Miyaoka, Philips Healthcare, Grant/research support; GE Healthcare, Grant/research support.