MEGP vs HEGP collimator for I-131 thyroid scintigraphy

Objectives Thyroid Cancer patients receive scintigraphic images at intervals after treatment to determine presence and extent of residual functioning thyroid tissue or metastases. Many clinics use I-131, but lacking a High Energy Collimator have used a Medium Energy collimator and obtained reasonable quality images. We compare a MEGP collimator to a HEGP collimator for the specific task of I-131 lesion detection.

Methods 1)FWHM and FWTM were measured in air at 10 cm. 2)Penetration Fraction was measured by the method of NEMA Publication NU 1-2007(1), Section 3.3. 3)Sensitivity in air was measured. 4)A phantom was designed and built with hot cylinders of different sizes superimposed on a uniform background. Planar images with a target to background ratio of 4.5 to 1 were acquired for scan durations from 5 seconds to 640 seconds. 5)Repeated acquisitions will be added over a range of scan durations, to estimate the SNR for a Non-prewhitening Matched Filter (SNR NPW) as a function of scan duration.

Results 1)FWHM and FWTM are shown below. Septal penetration of the MEGP is reflected in the large FWTM. 2)At 10 cm, the MEGP penetration fraction was 0.58, and the HEGP penetration fraction was 0.38. 3)The MEGP collimator was 2.3 times more sensitive in total counts, and 1.6 times more sensitive for "properly collimated" counts. 4)Phantom images show that all lesions detectable with the High Energy Collimator are also detectable with the Medium Energy Collimator, for the same combination of lesion size, lesion activity, and scan duration. 5)The scan duration for each collimator will be reported which yields the same SNR NPW, which can be interpreted as the scan duration required to yield the same lesion detectability.

Conclusions For I-131 thyroid imaging, the MEGP collimator has equivalent lesion detectability compared to the HEGP

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