Collimators in Cardiac Imaging

Basic Review of Types and Purposes

Purpose of Collimators in Nuclear Medicine Collimators are used in nuclear imaging to

define the space and location of an energy source

Due to the nature of radioactive emission, without a collimator to define a sources’ shape, images would be undefined, appear blurry, and flood the crystal (as we see in a daily intrinsic flood)

Collimators are made of material that can absorb radioactive emissions and have septa which only allow gamma rays entering at a certain angle to be counted in the image

Purpose of Collimators in Nuclear Medicine Collimators can vary in the:

Material they are made of High density/high Z number Most commonly lead

Thickness of the collimator (septa length) Diameter of the septa Thickness of the septa (between holes)

These values define both the resolution and the sensitivity of the system

Resolution As the diameter of the septa increases, the resolution becomes worse

As the length of the septa increases, the resolution becomes better

The distance of the source from the collimator will also effect resolution (as distance increases, resolution becomes worse)

Sensitivity The efficiency of the collimator

Number of photons that pass through the collimator vs. number emitted

As the diameter of the septa increases, the sensitivity becomes better

As the thickness of the septa increases, the sensitivity becomes worse

The distance of the source from the collimator has a minimal effect on the sensitivity of the system

Collimator Characteristics There is a trade-off between characteristics

enhancing resolution and those enhancing sensitivity when deciding what collimator to use

As resolution increases, sensitivity decreases

Parallel Hole Collimators Most widely used Used in Cardiac SPECT Several types (low energy with acquisitions

using Tc99m) General Purpose (LEGP) High Resolution (LEHR) Ultra High Resolution (LEUHR) High Sensitivity (rare) (LEHS)

Collimator Characteristics

• Above are the collimator specifications for the maiCAM180 system: • by comparing the system resolution and system sensitivity

between LEUHR, LEHR, and LEGP, the tradeoff can easily be evaluated

• only the system resolution has been evaluated at different distances from the collimator because only the resolution is noticeably affected by it

• lower resolution value = smaller object that can be resolved• higher sensitivity value = more counts acquired

Collimator Characteristics High Resolution collimators are most

commonly used in cardiac applications The maiCAM180 uses LEHR collimators

The maiCAM180 is not intended for general nuclear applications, therefore, the increased sensitivity of the general purpose collimators is not as beneficial; the increased resolution of the LEHR collimators is most beneficial when scanning myocardial perfusion images