ALLPCB
Overview
Nuclear medicine imaging devices detect and display the in vivo distribution of radiopharmaceuticals (commonly called isotope drugs). Nuclear medicine imaging visualizes organs or lesions based on differences in radioactivity concentration between normal and diseased tissues. Compared with CT and MRI, nuclear medicine imaging examinations (ECT) can detect and diagnose certain diseases earlier. Nuclear medicine imaging is a functional imaging modality that uses radiopharmaceuticals.
1. Classification and Characteristics of Nuclear Medicine Imaging Equipment
Gamma Camera
Gamma camera components:
(1) Scintillation probe: includes the collimator, scintillation detector, and photomultiplier tubes.
(2) Electronic circuits: includes front-end amplifiers, single-pulse height analyzers, and correction circuits.
(3) Display devices: oscilloscope, camera, etc.
(4) Auxiliary equipment for the gamma camera.
Characteristics:
(1) Continuous imaging allows dynamic studies by tracking and recording the passage of radiopharmaceuticals through an organ, showing both morphology and function.
(2) Short examination time and operational simplicity make it suitable for pediatric and critically ill patients.
(3) Rapid imaging facilitates multi-position and multi-site observation.
(4) Image processing can produce diagnostic data and quantitative parameters.
Single Photon Emission Computed Tomography (SPECT)
Imaging principle
SPECT uses gamma cameras that rotate around the region of clinical interest to collect and count gamma photons emitted at different angles. Image reconstruction methods similar to those used in X-CT are applied to obtain the distribution of radiopharmaceutical concentration within a slice of the body, producing multi-plane tomographic images or 3D reconstructions.
Current SPECT systems measure photon energies in the range of 50-600 keV, with spatial resolution of about 6-11 mm.
Differences from X-CT
(1) Images are relatively coarse and have lower spatial resolution.
(2) SPECT is an emission-type tomographic method.
Positron Emission Tomography (PET)
1. PET uses positron-emitting radionuclides. These radionuclides are often elements that are basic constituents of human tissues and can label various biologically important compounds and their metabolic analogs without altering their biological activity. They participate in physiological and biochemical processes. Because these radionuclides generally have short half-lives, relatively large doses can be administered during examinations to improve image contrast and spatial resolution. Consequently, PET images reflect physiological, biochemical, pathological, and functional information of tissues.
2. Since positrons have short ranges in matter and only exist momentarily, they cannot penetrate thick organs or tissues. Therefore, positron detection is performed by measuring the gamma photons produced by annihilation.
Limitations:
Clinical deployment of PET is constrained by two factors: (1) Positron-emitting radionuclides have short half-lives and are typically produced by cyclotrons, so PET facilities need access to medical cyclotron production near the PET unit. (2) Facilities need rapid radiochemistry laboratories and equipment to prepare short-lived radiolabeled radiopharmaceuticals.
2. Nuclear Medicine Imaging Process and Basic Requirements
(1) A radionuclide is labeled to a pharmaceutical to form a radiopharmaceutical, which is then introduced into the body. When taken up by organs and tissues, it becomes an internal radiation source.
(2) External gamma detection devices detect the gamma rays emitted during radioactive decay, enabling construction of an image representing the in vivo distribution density of the radionuclide.
Because radiopharmaceuticals participate in normal metabolic processes, nuclear medicine images not only reflect organ and tissue morphology but also provide functional, physiological, and biochemical information.
3. Fundamental Characteristics of Nuclear Medicine Imaging
(1) Nuclear medicine imaging is based on differences in radioactivity concentration between regions inside and outside an organ or between parts of an organ, producing static and dynamic images. These images show the location, morphology, and size of tissues, organs, and lesions, and they can reveal very small local functional changes and differences within an organ.
(2) Nuclear medicine supports multiple dynamic imaging modes. Because organs uptake, absorb, and excrete radiopharmaceuticals, blood flow and functional status of organs and lesions can be displayed dynamically and quantitatively, providing parameters that reflect perfusion, metabolism, and receptor-related information.
(3) Some radionuclides preferentially accumulate in specific organs or lesions, giving high specificity to radionuclide imaging. This can reveal different tumor types, various neural receptors, inflammation, metastases, and other findings that are difficult to obtain from morphological imaging alone.
4. Summary
Nuclear medicine is a developing discipline with distinct advantages. Imaging equipment has evolved alongside the field, and imaging devices are integral throughout nuclear medicine practice. From single-function measurement instruments to complex integrated scanners and modern imaging systems, advances in instrumentation have driven progress in nuclear medicine.
