Gamma Camera Ilker Ozsahin Oct 3 2017

Questions Which radiations can be stopped in which medium or material and at what thickness? Why is this important? What is the mostly used scintillation crystal, and their properties, e.g. NaI(Tl) Thallium-doped Sodium Iodide and its density, atomic number, etc.? Spatial Resolution & Sensitivity Formula? Compton Kinematics? Characteristics of Scintillation Detectors: Energy Resolution, Decay Time, Efficiency, Effective Z, Density, Photon Yield, Relative Light Output, Attenuation Coefficient?

Anger Camera

Gamma Camera Invented by Hal Anger, the gamma camera is usually based on the use of a single large area (e.g. 50 cm x 40 cm of NaI(Tl)) phosphor coupled to up to a hundred PMTs. The camera can detect gamma rays emitted by a radiotracer distributed in the body. The lead collimator placed in front of the scintillation counter selects the direction of the gamma rays entering the device and allows an image of the biodistribution of the tracer to be made.

Components The gamma camera, or Anger camera, is the traditional workhorse of nuclear medicine imaging and its components are illustrated here. Gamma camera systems are comprised of four basic elements: the collimator, which defines the lines of response (LORs); the radiation detector, which counts incident gamma photons; the computer system, which uses data from the detector to create 2‑D histogram images of the number of counted photons; and the gantry system, which supports and moves the gamma camera and patient.

Components

Crystal

Bulk Crystal

Crystal?

Purpose The overall function of the system is to provide a projection image of the radioactivity distribution in the patient by forming an image of gamma rays exiting the body

Collimators Gamma rays from radioactive source within the body emitted in all directions, so high degree of collimation needed Operating principle: absorptive collimation Only gamma rays travelling in certain directions go through and reach the detector, the others are absorbed in the septa where most radiation is absorbed Made usually from tungsten or lead to provide high absorption probability

Collimators The drawing on the left demonstrates the image of two point sources that would result without the collimator. It provides very little information about the origin of the photons and, thus, no information about the activity distribution in the patient. The drawing on the right illustrates the role of the collimator and how it defines lines of response (LOR) through the patient.

Collimators As mentioned above, the collimator defines LORs. The collimator accomplishes this by preventing photons emitted along directions that do not lie along the LOR from reaching the detector. Thus, collimators consist of a set of holes in a dense material with a high atomic number, typically lead. The holes are parallel to the LORs. Ideally, each point in the object would contribute to only one LOR. This requires the use of collimator holes that are very long and narrow. However, such holes would allow very few photons to pass through the collimator and be detected. Conversely, increasing the diameter or decreasing the length of the holes results in a much larger range of incident angles passing through the collimator.

Collimator Size

Collimator Size Illustration of the concept of spatial resolution and how collimator hole length and diameter affect spatial resolution. The lines from the point source through the collimator indicate the furthest apart that two sources could be and still have photons detected at the same point on the image plane (assumed to be the back of the collimator). Thus, sources closer together than this would not be fully resolved (though they might be partially resolved). From this, we see that the resolution decreases as a function of distance. It should also be noted that the resolution improves proportionally with a reduction in the width of the collimator holes and improves (though not proportionally) with the hole length.

Multi Hole Collimators Examples of the three major hole shapes used in multi-hole collimators. They are from left to right: round, hexagonal and square holes. In all cases, black indicates septa and white open space. The diameter is d and the septal thickness is s.

Multi Hole Collimators Multi-hole collimators typically have many thousands of holes. The uniformity of the image critically depends on the holes having uniform sizes and spacing. As a result, high quality fabrication is essential. The septa must be made of a high density, high Z material in order to stop the incident γ rays. Lead is the material of choice for most multi-hole collimators due to its relatively low cost, low melting temperature and malleability

Collimator Holes

Types of Collimator Parallel holes are the most commonly used collimators. The LORs are parallel, and there is, thus, a one to one relationship between the size of the object and image. In converging hole collimators, the LORs converge to a point (focal point) or line (focal line) in front of the collimator, and there is magnification of the image. These two collimator types are referred to as cone-beam and fan-beam collimators, respectively. These are useful for imaging small objects, such as the heart or brain

Types of Collimator In diverging hole collimators, the LORs converge to a point or line behind the collimator. This results in minification of the image. Diverging hole collimators are useful for imaging large objects on a small camera. However, they result in a poor resolution versus noise trade-off, and are, thus, infrequently used. Pinhole collimators use a single hole to define the LORs. In terms of geometry, they are similar to cone-beam collimators but with the focal point between the image plane and the object being imaged. As a result of this, the image is inverted compared to the object. In addition, the object can be either minified or magnified depending on whether the distance from the image plane to the focal point is less than or greater than the distance from the pinhole to the object plane.

Advantages/Disadvantages 3D image Organ Specific Exposing to radioactivity Further discussion will be made after learning about SPECT and PET system

Making Slides!!! No copy&paste, instead, read, understand, write with your own words. During the presentation, make sure if the audience is listening or reading? Always have references at the end

PMT-Photocathode-Dynode-Anode

PMT and PreAmp

Scintillation Crystal

Scintillation Process Scintillation is a general term referring to the process of giving off light, it is used both literally and figuratively. When a gamma ray hits the crystal, it ejects an outer shell electron. When an excited electron in the scintillator returns to its ground state, it can release a photon in the UV or visible light range.