Single Photon Emission Computerised Tomography (SPECT) and Positron Emission Tomography (PET) allow body functions to be visualised by detecting gamma photons emitted from a radionuclide imaging agent in the body. However, detecting gamma photons is not straightforward. The photons must first be converted to lower energy to prevent them from simply passing through the imaging hardware, and the detector must be extremely sensitive. That’s to say, it must be able to record individual photon arrivals with great accuracy to enable techniques like PET. The type of detector designed to achieve this is known as a gamma camera.
The gamma camera
The gamma camera consists of a series of different components, which isolate, amplify, detect, and display the intensities of gamma photons emitted from medical tracers in the tissue beneath the camera. Its requirements are quite different from a photography camera. While a typical camera needs to measure the wavelength (colour) and intensity of visible light across the image, it receives many thousands of photons per pixel to gather this information. The gamma camera is not required to measure the wavelength of photons but must be extremely sensitive and capable of detecting individual gamma photon arrivals.
Measuring the intensity of gamma radiation shows the concentration of radiopharmaceutical in the tissue, which indicates how the body is processing the compound and allows its function to be diagnosed. This type of scan is known as scintigraphy. Gamma cameras have applications as both a handheld scanner and the detector component of larger scintigraphy machines.
Figure 1. The key components of a gamma camera, not displayed to scale.
To understand how the gamma camera works, we will follow each stage of a gamma photon’s journey from the radiopharmaceutical in the body to displaying an image on the computer.
- The radiopharmaceutical or medical tracer is processed by the body and concentrates in certain locations, depending on how the body transports the compound. Gamma photons are emitted from the tracer in all directions, with emission intensity being proportional to the concentration of radiopharmaceutical in that area.
- Photons that travel towards the gamma camera first meet the collimator. The function of this component is to allow only those photons to pass that are travelling parallel to the camera axis. This is necessary because the camera only produces an image of the region directly below it. If off-axis photons were allowed to pass through, there would be no way to determine where they originated, which would reduce the accuracy of the image. The collimator consists of a honeycomb-shaped grid of thin lead tubes. This means photons that travel along the camera axis will pass through the tubes, while an off-axis photon will hit the side of a tube and be absorbed.
- After passing through the collimator, the photons arrive at the scintillator layer. This is a component that absorbs a single high-energy gamma photon and emits thousands of lower-energy visible light photons. The probability of a gamma photon interacting with the scintillator to produce this effect is about 1 in 10, meaning 90% of gamma photons go undetected by the camera. There are several materials the scintillator can be made from, with the most common being sodium iodide.
- The visible light photons emitted by the scintillator pass through a light guide into the PhotoMultiplier Tubes (PMTs). The function of these tubes is to convert the visible light photons into an electric pulse proportional to their intensity. The detail of how this is achieved is covered later. The PMTs are arranged in a hexagonal grid, with the electrical pulse output of each being connected to a computer. Software processes the arrivals of electrical pulses to calculate photon impact positions on the scintillator layer. These impact positions are then used to produce a high-quality representation of the medical tracer concentrations within the patient’s body.
A key difference between x-ray imaging techniques and a gamma camera image is that while x-rays are used to see the
body anatomy, the gamma camera is used to view the
body function and processes.
Photomultiplier tubes
The PMTs are a critical part of the gamma camera responsible for converting photons indicating a gamma photon collision into an electrical signal that can be processed by a computer. The key requirement for the PMTs is to amplify single photon arrival signals so that they can be reliably detected.
Figure 2. A photomultiplier tube (PMT).
A photon strikes a photocathode, which absorbs the photon and ejects only a ‘photoelectron’. Common photocathode materials are alkali-metal films, including potassium bromide (KBr), caesium iodide (ScI), and rubidium telluride (RbTe).
The electron is accelerated towards the first dynode (electrode), which is held at +100V potential. This accelerates the electron to a high speed, and the collision with the first dynode on average produces four secondary electrons. These are then accelerated to the second dynode, held at a higher potential, which, upon impact, produces another four secondary electrons. This process is repeated at each successive dynode, with the number of electrons multiplying by four each time.
We can see that for the 9-dynode tube shown in Figure 2, one incident photon would result in the generation of 262,144 electrons at the anode.
\(4^9 = 262144\)
The collection of electrons at the anode flows through a resistor to produce a voltage pulse signal, which indicates the detection of a photon.
Radiopharmaceuticals
The gamma camera allows for the diagnosis of patients by observing how the body processes radiopharmaceutical medical tracer compounds. These are radioisotopes combined with another molecule, such as glucose, which the body transports. Gamma-emitting sources are ideal for this application as this type of radiation is less ionising than alpha or beta, and the high-energy photons can pass through the body to be detected externally. It is also important to select an isotope with a relatively short half-life, as this ensures the source is highly active, meaning less time is required, and that the substance decays quickly after the procedure, reducing the duration of exposure for the patient.
A commonly used radioisotope is Technetium-99m, which emits a gamma photon with a half-life of six hours and can be used to image many major organs in the body. This isotope is produced by the natural decay of molybdenum-99. The Mo-99 isotope has a half-life of 67 hours and decays by beta-minus emission to form a Tc-99m nucleus.
\(^{99}_{42}Mo \xrightarrow{67h} ^{99m}_{43}Tc + ^{0}_{-1}e + \bar{V_e} \xrightarrow{6.0h} ^{99}_{43} Tc + \gamma \xrightarrow {210,000 yrs} ^{99}_{44} Ru + ^{0}_{-1}e+ \bar{V_e}\)
The ‘m’ in Tc-99m indicates a ‘metastable’ nucleus, which stays in a higher-energy state than the stable nucleus for longer than expected. The Tc-99m loses this energy by emission of a gamma photon with an energy of exactly 140keV and a half-life of 6 hours. In its stable state of Tc-99, the isotope has a half-life of 210,000 years.
A Tc-99m based medical tracer is NaTcO4, which is an inorganic compound made by chemically combining TC-99m with sodium and oxygen. This compound is transported to the brain when injected into the body, allowing a gamma camera to be used to observe how a patient’s body brings compounds to the brain.
Some other types of scans that can be performed with Tc-99m radiopharmaceuticals are summarised below.
Radiopharmaceutical | Scan application |
Sodium pertechnetate\((NaTcO_4)\) | Brain (primarily thyroid), salivary glands, urinary bladder |
Technetium-99m methyl diphosphonate\((Tc -99m \space MDP)\) | Bone metastasis, cancer |
Technetium tetrofosmin\((C_{36}H_{80}O_{10}P_{4}Tc)\) | Heart |
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