Gamma Camera

Gamma Camera

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Gamma cameras are amazing machines that help doctors see what's happening inside your body. They use a special imaging agent and a detector that's super sensitive to detect gamma photons. Gamma photons are like tiny particles of energy that are emitted from the imaging agent in your body. The gamma camera then converts the photons to lower energy, so they don't just pass through the machine without being detected. This is really important for techniques like PET, which require extreme accuracy. The detector in a gamma camera is so sensitive that it can even record individual photon arrivals, which is pretty cool! That's why gamma cameras are used in procedures like Single Photon Emission Computerised Tomography (SPECT) and Positron Emission Tomography (PET). So, the next time you need a medical imaging test, you'll know what's happening behind the scenes!

The gamma camera

The gamma camera is a complex machine that's used to detect gamma photons emitted from medical tracers in the body. Unlike a regular camera, which measures the colour and intensity of visible light across an image, the gamma camera needs to be super sensitive to detect individual gamma photon arrivals. This allows it to measure the intensity of gamma radiation, which shows how the body is processing the radiopharmaceutical in the tissue. This kind of scan is called scintigraphy and is really helpful for diagnosing how a compound is functioning in the body. Gamma cameras are used in both handheld scanners and larger scintigraphy machines, making them really versatile tools for doctors and medical professionals.

The key components of a gamma camera, not displayed to scale
The key components of a gamma camera, not displayed to scale

The gamma camera is a fascinating piece of technology that doctors use to detect gamma photons emitted from medical tracers in the body. Here's how it works! First, the medical tracer is processed by the body and concentrates in certain areas. Gamma photons are then emitted from the tracer in all directions, with intensity proportional to the concentration of radiopharmaceutical in that area. Next, the photons that head towards the gamma camera meet the collimator, which only allows parallel photons to pass through. This is important because the camera only produces an image of the area below it. After passing through the collimator, the photons arrive at the scintillator layer, which absorbs high-energy gamma photons and emits thousands of lower-energy visible light photons. The visible light photons then pass through a light guide into the PhotoMultiplier Tubes (PMTs), which convert them into an electric pulse proportional to their intensity. The PMTs are arranged in a hexagonal grid, and their electrical pulse output is connected to a computer. This computer uses software to calculate the photon impact positions on the scintillator layer, creating an accurate representation of the medical tracer concentrations in the body.

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.

A photomultiplier tube (PMT)
A photomultiplier tube (PMT)

The process of photon detection in a gamma camera is truly fascinating. When a photon strikes a photocathode, it's absorbed and only a 'photoelectron' is ejected. The photocathode is typically made of alkali-metal films such as potassium bromide (KBr), caesium iodide (ScI), and rubidium telluride (RbTe). The photoelectron is then accelerated towards the first dynode, which is held at a +100V potential. This accelerates the electron to a high speed, and upon collision with the first dynode, on average, produces four secondary electrons. These are then accelerated to the second dynode, which is held at a higher potential, and 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. For example, in a 9-dynode tube, one incident photon would result in the generation of 262,144 electrons at the anode. This collection of electrons at the anode flows through a resistor to produce a voltage pulse signal, indicating the detection of a photon.


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.Technetium-99m (99mTc) is a metastable nuclear isomer of technetium-99, symbolized as 99mTc, that is used in tens of millions of medical diagnostic procedures annually, making it the most commonly used medical radioisotope in the world. 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.

Radionuclide imaging techniques use a gamma camera as a sensitive detector to view the intensity of gamma radiation emitted from the tissue beneath the camera. The collimator ensures only gamma photons travelling parallel to the camera axis are detected. This is important for accuracy as it allows the emission location of the photon to be known. The scintillator and light guide convert a single gamma photon into thousands of visible light photons that pass into the PMTs. The most common scintillator material is sodium iodide. Photomultiplier tubes (PMTs) amplify the signal from visible light photons into an electrical pulse, which can be recorded by a computer. The location of the photon impact on the scintillator can then be calculated using the signals from the array of PMTs. Photon impact locations are processed to produce a representation of radiopharmaceutical concentrations in the tissue beneath the camera.

Overall, Tc-99m radiopharmaceuticals are used in a variety of medical diagnostic procedures, including gamma camera scans, and are the most commonly used medical radioisotope in the world. The versatile chemistry of technetium-99m is a major advantage in radiopharmaceutical development.

Gamma Camera

What is gamma camera used for?

The gamma camera is a device that is used in scintigraphy scans in order to detect gamma photons emitted from a medical tracer in the patient’s body. By measuring the intensity of radiation, the concentrations of medical tracer can be visualised, allowing diagnosis of the body processes and functions.

How does a gamma camera work step by step?

The function of the gamma camera is to convert individual photon arrivals to an electrical pulse that a computer can detect.The first step in a gamma photon’s journey through the camera is to pass through the collimator, which only allows through photons that are travelling parallel to the camera axis.These gamma photons are then converted into thousands of visible light photons by the scintillator layer. The visible light photons produced by the scintillator after gamma photon impacts pass into an array of photomultiplier tubes (PMTs), which amplify and convert photon arrivals into electrical pulses.These pulses are recorded by a computer, which produces a visualisation of the concentrations of medical tracer in the patient’s tissue beneath the camera.

What is the advantage of gamma camera?

Nuclear medicine-based imaging using a gamma camera has several advantages over traditional anatomical scan techniques like CT or MRI:Allows for imaging of physiological function rather than simply anatomy.Smaller and lighter equipment.The greater penetrating power of radiation allows imaging of different body parts.More complex techniques like PET can produce highly accurate 3D images of anatomy and function.Can produce real-time images.

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