The impact of NaI(Tl) crystal hydration on gamma camera spectral response and image uniformity

NaI(Tl) scintillators used in most gamma cameras are hermetically sealed to prevent the absorption of water molecules from the surrounding environment. If this seal is compromised, crystal hydration, a localized defect resulting in non-uniform attenuation of scintillation photons, may occur.

The purpose of this study was to evaluate the effects of crystal hydration across multiple radionuclides, characterizing the impact on spectral response and image uniformity.

Hydration was assessed using on-peak and off-peak imaging for several radionuclides. The progression of hydration was assessed by calculating the contrast-to-noise ratio in hydrated regions on monthly uniformity maps. Spectra were captured for both hydrated and non-hydrated regions of the detector to determine photopeak energy and energy resolution.

The visual appearance of the hydration effect in off-peak images was greatest for ¹³³Xe. The effect was more substantial in the 10% low off-peak image than in the 10% high off-peak image. Hydration was not observed in on-peak images for either ¹³³Xe or ^99mTc and was only barely visible for ¹³¹I. CNR measurements show a slow but notable progression of hydration in uniformity maps over time. The measured photopeak in hydrated regions was lower than that of non-hydrated regions. Hydration also resulted in a degradation in energy resolution with the effect being more significant at lower energies.

Hydrated regions of the detector demonstrated an energy-dependent degradation in energy resolution which corresponded to an energy dependence in the detectability of hydration in off-peak images. Monthly updates to the uniformity correction maps were able to adequately account for hydration. When hydration has been identified, routine evaluation of its impact on uniformity maps may provide a convenient method of tracking progression.