Operate and understand the functioning of the Gamma Camera
Measure and understand the effects on resolution of distance, energy window
Understand pinhole collimation
Observe the consequences of counting statistics on image quality
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
Figure 1: Gamma Camera Components
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.
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
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.