BME355 Lab Listing
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Gamma Camera


Preparation

Lab Outline

Procedure

Learning more

GOALS

Operate and understand the functioning of the Gamma Camera

Measure and understand the effects on resolution of distance, energy window and attenuation

Understand pinhole collimation

Measure deadtime

Observe the consequences of counting statistics on image quality

Introduction
The Gamma Camera is used to measure the distribution of radioactivity in a subject. The subject ingests, breathes in or is injected with a radioactive compound. These compounds are designed to reflect bodily function, such as circulation, and are often directed to a particular organ. In the heart or brain, such an image could indicate the location of damage due to a heart attack or stroke. In a bone scan, tumors can be visualized. The urinary track, lungs, liver and thyroid, for example, may also be imaged. A single camera, such as we will study, produces a projection of the radioactivity. Cross-sectional images can be produced by imaging at many angles and reconstructing the cross-section from the projections.



Figure 1: Gamma Camera Components

Gamma camera
The Gamma Camera (also called an Anger camera) is an instrument based on scintillation detectors, photomultipliers and a collimator. It is a stationary imaging system sensitive to a large field of view (FOV). First, emitted gamma rays pass through the collimator.

Collimators
The collimator is typically made up of channels of lead that only allow those gamma rays which originated directly below the channel to pass through. In a parallel hole collimator (Figure 2b), the lead strips are parallel to each other and perpendicular to the camera. They ensure that only counts coming from directly below the camera crystal are observed. Counts coming in at an angle are absorbed by the lead. There are also converging (magnifying) and diverging (minifying) collimators (Figure 2c,d). Distance is critical for these collimators.

Another kind of collimator is the pinhole collimator (Figure 2a). The pinhole images like a pinhole camera. The image is reversed and magnified. The magnification factor is set by adjusting the distance of the object to the pinhole. Used for thyroid imaging with 128I because the higher energy photons make standard collimators less practical.



Figure 2: Collimator types

Detection
The gamma ray then hits the scintillator which emits a small amount of visible light. The amount of light is proportional to the energy of the gamma ray. This light is detected by the photomultiplier and is converted to a voltage and amplified. Electronics keep track of the number, location and energy of events and pass that information to the computer to form an image. Thus, the spatial distribution of radioactivity can be measured as well as changes in radioactivity as a function of time.

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