Part II Instrumentation - The Gamma Camera

Each camera has a set of PM tubes
The arrangement of tubes covers the entire surface of the crystal
There maybe as little as 19 (old system) or as many as 91 or more tubes (current technology)
Depending on where the gamma ray scintillates with the crystal, light will be given off and shared between the PMTs

Diagram shows one gamma ray scintillating between four PM tubes - light is shared equally between the four PMTs. The positioning circuits then define its location via the light shared
Diagram also shows gamma ray scintillation occurring in the center of one PM tube. Here the majority of light is received by one tube, which the positioning circuits identify
As light energy is converted to electrons x/y coordinates are identified
After the scintillation has be accepted by the PHA the data is stored and then recalled for display on a CRT screen
Usually there are 100,000 plus counts recorded, giving definition of the are being imaged
Consider a similar situation with a regular camera. The greater the aperture opening the more light enters
Other devices also record data and may include: formatter (that has film), and monitor or p-scope (for immediate visual display)

All PM tubes vary slightly in its sensitivity to pick up the scintillation
Z pulse corrects for this variation
To make this work the sensitivity of all the PM tubes in the imaging field must be equal
Failure to correctly adjust the PMTs would cause an abnormal collection of data and a loss of uniformity. If you collect data from a uniform source of activity then a uniform flood is seen below (left image) However, if the PMTs' Z pulse is off you might see abnormal distribution of activity (right image)
When the Z pulse is being used the system with either remove or add counts from each PM tube in order to get an even distribution of counts across the entire crystal

Subtracting counts increases the time of collecting imaging, but is considered the "true" counts
Adding counts decreases imaging time, but it adds counts that are not really there
In order to determine how many counts should be subtracted or added a uniformity matrix flood is collect which ranges between 30 to 120 million or more counts
Data is stored and Z pulse can be determined for each PM tube

Here are two examples of a uniform flood. Houston, "What seems to be the problem."

On the surface of crystal a collimator is placed to "focus" the incoming gamma
A collimator is made up of septa which are composed of lead
Incoming gamma that fails within the "field of view" (umbra) interacts with the crystal at the appropriate keV (see diagram)
If an incoming gamma falls outside its "field of view," the gamma will encounter a lead septa

Septa will absorb all the energy of the gamma ray, or
Septa will slow down the gamma ray, reducing the keV resulting in pulse height that falls below the LLD
Counts that fall below the LLD will be rejected

Septa that are short in length allow for greater amount of gamma rays to be detected because the umbra is larger (see LEHS). Sensitivity improves, but at a cost to resolution) - Known as High Sensitivity (HS)
Septa that are longer in length allow less gamma rays are detected (smaller umbra). This improves resolution (reduce counts) - known as High Resolution (HR)
In general, the relationship between HS and HR is inversely proportional - you cannot have HS and HR collimator
However, a collimator known as General All Purpose (GAP) scarifies both sensitivity and resolution
Closer look to septa - http://www.nuclearfields.com/collimators-nuclear-medicine.htm

Thinner septa with higher energy gammas cause cause septal penetration of the gammas, it crosses over and interact with the wrong area of the crystal and are recorded. This is referred to as cross talk
Therefore, septa thickness must increase as the energy gamma increases, in order to prevent cross talk from occurring
Low Energy (LE) septa have a range of 100 - 200 keV
Medium Energy (ME) septa have a range of 200 - 300 keV
High Energy (HE) septa have a range of 300 or greater keV
Hence, collimators are classified as LE, ME, or HE

The closer the camera head is to the patient the better the resolution
The further away from the detector is from the patient the greater the loss of resolution. Note how the red dot on the left is seen in the right location whereas the dot on the right gets recorded on the wrong location
This is referred to as scatter radiation

As the distances increases, the angle of the umbra increases
Draw a straight line from the septa, and compare the angle of the umbra. Gamma rays that are within the umbra are recorded
The initial location of the emitting gamma and the distance between the source of the gamma ray and the detector become very important in attaining image quality
Therefore, when imaging the patient make sure the camera head is as close as possible - This improves resolution!