Spatial resolution is the ability of a gamma camera to differentiate two close radioactive sources. It takes "full width at half maximum" (FWHM) of the radioactivity count rate from a point source or a line source. FWHM in this case is defined as possible positions of the source when the count rate is half of its maximum. Therefore, the smaller the FWHM, the higher the spatial resolution 1.
The intrinsic resolution of the gamma camera (excluding the collimator) is the best possible spatial resolution achievable 1. Adding other components such as a collimator or imaging hardware only serves to degrade the spatial resolution of the system 2. The system resolution (Rs) of the gamma camera can be described as follows 1:
Rs = Ri2 + Rc2
where
Ri = intrinsic resolution
Rc = collimator resolution
Therefore, the system resolution of the gamma camera is always worse than the spatial resolution of any single component inside it. The collimator resolution is the dominant contributor to the system resolution. A typical modern gamma camera has a spatial resolution of 5 to 7 mm 1.
The spatial resolution of a gamma camera depends on the following 1:
Intrinsic resolution
The higher the gamma photon energy, the higher the number of scintillation photons reaching a photomultiplier tube (PM tube). Thus, there are fewer variations of the position of the source 1.
The thicker the scintillation crystal, the worse the spatial resolution because of variations in depths that the photons are generated. Besides, the probability of the Compton effect within the crystal increases with thickness. Therefore, scintillation crystals are typically thin, measuring 6 to 13 mm thick 1.
A good connection between the scintillation crystal and the photomultiplier tube allows maximum light collection. Hexagonal or square-shaped PM tubes cover a greater area of the crystal instead of circular ones because there is less wasted space between the PM tubes 1.
A larger number of PM tubes allows better sampling of light, as long as the minimum number of scintillation photons collected per tube is achieved.
More efficient PM tube releases more photoelectrons per photon detected, thus improving spatial resolution.
Filtering of lower energy signals away from the position calculations enables the elimination of noisy signals, thus improving spatial resolution 1.
Linearity (or spatial distortion) correction improves spatial resolution 1.
The intrinsic resolution of gamma camera is 2.5 to 4 mm of FWHM 1.
Collimator resolution
The spatial resolution of the collimator (Rc) is given below:
Rc = d (1 + (b/h))
where
d = hole diameter
b = distance from the source to the collimator
h = length of the collimator hole
Thus, the greater the length of the collimator hole, the higher the spatial resolution because gamma photons will be confined within the collimator hole instead of spreading out during travel 1.
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