Gamma camera | Components & Function l Visual explanation

The gamma camera is an imaging device used to image gamma-emitting radioisotopes. Just like an X-ray, the gamma camera will yield a two-dimensional projection of a three-dimensional object. However, 3D images can also be produced using SPECT, an imaging technique that consists of taking images as the camera rotates around the patient.

This allows us to create frontal, transversal and sagittal cuts. The camera accumulates counts of gamma photons, which are detected by crystals in the camera. ​​ The gamma camera consists of a collimator, a crystal plane, and an array of photomultiplier tubes connected to a computer system.

The collimator is the first device to encounter the emitted photons from the patient's body and many have compared its function to that of a photographic lens. As photons are emitted in all directions, without the presence of a collimator an image cannot be obtained, as it would be indistinct. The collimator is a plate, usually of lead, full of holes separated from each other by septa and these holes control which photons will reach the detector.

Only photons travelling parallel to the collimator holes will reach the crystal while the majority will be absorbed by the septa and therefore will not contribute to the image. This greatly limits the sensitivity of the camera system. High-resolution collimators have small or long holes and thicker septa and therefore have low sensitivity.

Large amounts of radiation must be present to provide enough exposure for the camera system to detect sufficient scintillation dots to form an image. Collimators with high sensitivity have larger holes but a low resolution because more angled photons are able to reach the crystal. The two basic types of collimators are pinhole and multihole collimators.

With pinhole collimation, Radiation must pass through the pinhole to be imaged, and the image is always inverted on the scintillation crystal. Because little of the radiation coming from the object of interest is allowed to pass through the pinhole over a given time period, the pinhole collimator has very poor sensitivity. The poor sensitivity of a pinhole collimator makes placement near the organ of interest critical.

Pinhole collimators are routinely used for very high-resolution images of small organs, such as the thyroid. The holes in a multihole collimator may be aligned in a parallel, diverging, or converging manner. The most commonly used type of collimator is the parallel-hole collimator.

Once photons reach the crystal, they are absorbed and this absorbed energy is emitted as flashes of light, a process called scintillation. The brightness of light is proportional to the energy absorbed by the crystal. The thickness of the crystal results in a trade-off between absorption and spatial resolution.

Thicker crystals absorb a larger proportion of gamma rays but allow for greater scatter and therefore have a poorer spatial resolution. These flashes of light are detected by an array of photomultiplier tubes, which act both as a converter and an amplifier. They convert the extremely weak light into a measurable electrical signal that is ultimately processed to produce an image.

A PMT is a vacuum provided of a thin layer of a photoemissive material. This is where photons are converted into electrons through the photoelectric effect. These electrons are accelerated toward an amplification system consisting of several electrodes called dynodes.

When an accelerated electron strikes the surface of a dynode, the energy deposited causes the re-emission of several electrons. These electrons are then accelerated to the next dynode, producing an avalanche multiplication process, which ends with the collection of the electrical charge on a collector electrode or anode. The electrical signals are then analyzed by a computer to construct an image.