Gamma Rays and Detection

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In the electromagnetic energy spectrum, the highest energy photons (shortest wavelength, highest frequency) are gamma rays. Gamma rays arise out of nuclear events during radioactive decay. For in vivo imaging purposes, the best gamma rays are of low energy, in the range of 100-511 keV. Gamma rays in this energy range can be efficiently stopped and therefore measured by external detectors. Approximately 80-90% of nuclear medicine imaging is accomplished using radioactive 99mTc, which emits a 140-keV gamma ray during its radioactive decay. 99mTc has a 6-h half-life and is continuously available from regional nuclear pharmacies. It is the decay product of "Mo (half-life = 66 h) and is eluted daily from the "Mo/"mTc generator system and therefore available at very high specific activity and low cost. 99mTc is chelated (complexed) with various compounds that have different biological characteristics.

A typical mobile Anger gamma camera for planar imaging is shown in Fig. 1A. Gamma camera imaging requires the use of a collimator, a solidly constructed gamma-ray attenuator (usually made from lead) that is placed between the subject and the gamma-ray detector. There are various types of collimators, some more specific for low-energy gamma rays, while other are specific for higher ranges of gamma-ray energies. A pinhole collimator and parallel-hole collimator is shown in Figs. IB and ID, respectively. The pinhole collimator has a small round hole at the end (inset, Fig. 1C) that allows projection of the gamma rays onto the detector crystal, thus forming an image like a pinhole camera. In contrast, the parallel-hole collimator allows passage

Figure 1 Gamma camera and collimators. (A) Anger 420/550 mobile radioisotope gamma camera (Technicare, Solon, OH); (B) gamma camera detector head with the pinhole collimator; (C) close-up of the pinhole; (D) gamma camera detector head with the high resolution parallel-hole collimator. The gamma camera has one detector head; the collimators are changed for the particular application.

Figure 1 Gamma camera and collimators. (A) Anger 420/550 mobile radioisotope gamma camera (Technicare, Solon, OH); (B) gamma camera detector head with the pinhole collimator; (C) close-up of the pinhole; (D) gamma camera detector head with the high resolution parallel-hole collimator. The gamma camera has one detector head; the collimators are changed for the particular application.

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Detector Crystal

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Pinhole Collimator

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Figure 2 Diagram showing cross-section of the detector head with a mouse in position for imaging. (A) Imaging with a parallel hole collimator; (B) imaging with a pinhole collimator; (C) data acquisition computer (NumaStation, Amherst, NH) showing collected images. A gamma ray is depicted in A and B passing through the collimators and interacting with the detector crystal (l ), leading to production of light that is detected by the photomultiplier tubes.

of gamma rays that are perpendicular to the plane of the collimator. Figure 2 presents a diagram illustrating a cross section of the gamma camera equipped with either a high-resolution parallel-hole collimator or a pinhole collimator. The mice are in position for imaging and were previously injected with a compound called methylene diphosphonate labeled with "mTc (99mTc-MDP). This compound localized in areas of bone with high osteoblastic activity by 4 h after intravenous injection. The gamma rays emitted from the animal are stopped by the detector crystal (Fig. 2, "1") and visible light photons are emitted. These photons are captured by the photomultiplier tubes adjacent to the crystal and converted to a voltage pulse. The X, Y location of the interaction event is recorded, as well as the magnitude of the voltage pulse (Z, pulse height), which is proportional to the energy of the gamma ray that was stopped. The gamma camera in this example has an intrinsic spatial resolution of 3 mm; therefore, individual vertebra of the mouse were not detected separately. However, uptake in the spine and knee joints is clearly visualized.

Additional examples of "mTc complexes for human imaging include 99mTc-MAG3 (mercaptoacetyltriglycine) for imaging renal function and

"mTc-HMPAO (hexamethylpropyleneamineoxime) for imaging blood flow in the brain. Peptides and antibodies radiolabeled with "mTc are also approved for human imaging applications. Most often, the 99mTc is attached to protein with a bifunctional chelater. With this system one part of the chelator binds 99mTc in stable conformation, while a second part is used for attachment of the complex to the protein. Besides 99mTc, other radionuclides that are used for imaging include 67Ga, mIn, 123I, and 131I (see Table I). These radionuclides have different gamma-ray emissions; simultaneous imaging with 99mTc is possible. Multi-gamma-ray imaging is one feature that differentiates gamma camera imaging from PET. The latter is limited to detection of positron annihilation events, and therefore radionuclides lacking positron emission are not detected. The image presented in Fig. 2 is a planar image that represents a two-dimensional distribution of the 99mTc-MDP at 4 h after intravenous injection. Single photon emission computed tomography (SPECT) is also possible with specialized gamma cameras that are routinely available in nuclear medicine departments. SPECT is accomplished by collecting multiple images (or projections) at various angles around the subject; the gamma camera usually moves. A tomographic image of the distribution of the radioactivity is produced following reconstruction of these projections. The typical spatial

Table I

Radionuclides Commonly Used in Imaging

Table I

Radionuclides Commonly Used in Imaging

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