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| Fig. 1: Here we can see an example of Tc-99m being administered to a patient through a shielded syringe. (Source: Wikimedia Commons) |
Nuclear medicine maps physiological function within a living patient by using radiopharmaceuticals, which are molecules engineered to localize in a target organ and are labeled with a radionuclide whose emitted radiation is measured by an external detector. Technetium-99m (Tc- 99m) is one of the most important radionuclides in this field, accounting for 80% of the nearly 30 million nuclear medicine procedures performed annually. Fig. 1 shows Tc-99m being administered to a patient. [1]
There are three unique properties that make Tc-99m uniquely suited for diagnostic imaging: its 140.5 keV gamma-ray energy, a 6.01-hour half-life, and a decay mode that produces no alpha or beta particles. For imaging to work, a gamma ray emitted inside the body must escape through several centimeters of tissue without being absorbed, then deposit its energy into an external detector. This creates a fundamental problem: low-energy photons are absorbed by tissue before they escape, while high-energy photons pass through the detector without stopping. Since Tc-99m has a 140.5 keV gamma, its photon which orginates 10 cm deep has a roughly 22% chance of escaping, and standard NaI(Tl) scintillation crystals capture about 90% of the photons that do arrive. Image formation also requires knowing the direction each photon came from, and with 140.5 keV, lead collimators with narrow parallel channels that admit only near-perpendicular photons are highly effective, blocking off-angle photons without excessive absorption. [2,3]
The half-life and decay mode of Tc-99m are also critical factors in its use for medical imaging, as most radioisotopes either decay too quickly to complete the imaging workflow or persist long enough to accumulate a significant radiation dose. With a half-life of 6.01 hours, Tc- 99ms half-life is long enough to encompass the full clinical process within one to two half-lives, yet short enough that 94% of the injected activity has decayed by the following morning. Tc-99m decays by isomeric transition, emitting only the diagnostic gamma ray with no alpha or beta particles. Those particles cause most biological damage in nuclear medicine because they deposit their energy directly in tissue over microns to millimeters, whereas penetrating gamma rays escape the body and are captured by the camera. [2,3]
One challenge with Tc-99ms 6-hour half-life is getting the radioisotopes to hospitals before they decay. For Tc-99m, current systems exploit the relationship between Tc-99m and Mo-99, which has a 65.94-hour half-life, and can be shipped to hospitals, where it continuously generates Tc-99m. Mo-99 is produced in research reactors by fission of U-235, appearing as a fission fragment with 6.1% yield; 87.5% of its decays populate the Tc-99m isomeric state. [4] Because the Mo- 99 and Tc-99m differ in chemical properties, flushing the column with saline selectively elutes Tc-99m as pertechnetate while Mo-99 remains bound. After each elution, Tc-99m regrows, peaking at about 67.7% of Mo-99 activity roughly 23 hours later; hospitals elute once daily and recover approximately 95% of the available Tc-99m. [5] A generator arriving by overnight courier delivers sterile pertechnetate on demand for up to two weeks before Mo-99 activity falls too low to be useful.
Once eluted, the pertechnetate is combined with a targeting molecule and injected. Forming an image then requires knowing the direction of each detected photon, since the decay emits gamma rays in all directions. A lead collimator physically blocks any photon not traveling perpendicular to the detector face. When an accepted photon strikes the crystal, it produces a flash of visible light detected by an array of photomultiplier tubes, and the interaction position is computed as a weighted average of signals across the array. A pulse-height analyzer is set to a 20% energy window around 140.5 keV, which rejects Compton-scattered photons that would otherwise blur the image. In single-photon emission computed tomography (SPECT), one or two camera heads rotate around the patient, acquiring 60-128 projections that are reconstructed into a three-dimensional map of tracer concentration using iterative algorithms. [6,7]
Because pertechnetate can be attached to a wide range of targeting molecules, Tc- 99m can image almost any organ's physiology; the most common application is myocardial perfusion imaging, accounting for roughly half of all Tc-99m procedures in the United States. [8] In a myocardial perfusion imaging, a standard one-day stress protocol administers 24-36 mCi (888-1,332 MBq). [9] Human biodistribution measurements show only 1.2% of injected activity localizes in the myocardium at rest and 1.5% during exercise stress, placing approximately 11-14 MBq in the heart; the remaining 98.5-98.8% distributes to the liver, kidneys, and skeletal muscle. [10] Imaging is therefore delayed by 15-60 minutes after injection to allow the hepatic and subdiaphragmatic activity to clear the field of view. [9] Of the roughly 10-12 million 140.5 keV photons emitted per second from those 11-14 MBq, chest-wall attenuation, which has a narrow-beam linear attenuation coefficient 0.150 -1 at 140 keV, transmits approximately 17% through a mean cardiac depth of 12 cm, yielding on the order of 2 x 106; unscattered photons per second exiting the chest. [11] The parallel-hole collimator then admits only near-perpendicular photons, and a conventional dual-head Anger camera records a myocardial count rate of approximately 47,000 counts per minute during stress acquisition, which is roughly 10,000 times lower than the emission rate at the heart. [12,13] This four-order-of-magnitude reduction sets the acquisition time at 12-16 minutes and limits reconstructed spatial resolution to approximately 15 mm FWHM.
One of the primary questions around these procedures: the injected dose must be justified by its diagnostic value. A standard myocardial perfusion study delivers approximately 9-12 mSv; a bone scan delivers roughly 4-5 mSv. [14,15] The average American receives 3.1 mSv annually from background radiation, and cardiac CT angiography delivers 5-15 mSv. The same physical properties that make Tc-99m effective also ensure the dose is manageable, with the 6-hour half-life removing 94% of the injected activity within 24 hours, and the absence of particulate emission means essentially all energy deposited in the body comes from the same penetrating gamma rays the camera captures, rather than from locally absorbed radiation that contributes dose without diagnostic benefit.
Tc-99m dominates nuclear medicine because of three properties: its 140.5 keV gamma ray, its 6-hour half-life, and it does not produce alpha or beta particles. The Mo-99/Tc-99m generator system then addresses the logistical challenge of delivering a short-lived isotope to hospitals worldwide, without requiring an on-site reactor. The result is a technology that can map conditions from outside the body, providing physiological information that no anatomical imaging modality can supply.
© Anirudh Mazumder. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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