Acquisition Count Dependence of Specific Binding Ratio in 123I-FP-CIT SPECT

Objectives: In the [¹²³I]FP-CIT SPECT examination, the specific binding ratio (SBR) calculated from the ratio of the striatal specific to extra-striatal background non-specific binding in the brain is now commonly used as a quantitative index of parkinsonian syndrome. The purpose of this study was to examine the influence of count reduction on quantitative SBR, as well as to clarify the reliability of SBR in patients with shorter scan time.

Methods: In the phantom study, a striatal phantom was used, and the ratio of the radioactivity concentration was adjusted so that the right striatum: left striatum: brain parenchyma (BG) = 8:4:1. A Changes in SBR and image quality, i.e. % coefficient of variation (% CV) and normalized mean squared error (NMSE), according to decrease in acquisition counts were evaluated.In the clinical study, 106 consecutive patients (73.1 ± 9.6 years) who suspected of parkinsonian syndrome were assigned. They underwent [¹²³I]FP-CIT SPECT and SBRs from 30 min common acquisition (SBR_full) and those from at half a count (SBR_half) were compared. The SBR was calculated using two methods of Tossici-Bolt’s and fully automatic count-based one.

Results: In the phantom study, the image quality was decreased according to reduction of the acquisition counts. The % CV and NMSE decreased up to 52.5% and 81.5%, respectively. The SBR tended to decrease slightly as reduction of the acquisition count. In the clinical study, the mean values of SBR_half was smaller than those of SBR_full, and they were significantly different except for the SBR by the Tossici-Bolt’s method without attenuation correction. SBR_half and SBR_full correlated well although SBR_half was 2-9% smaller than SBR_full. The accuracy of diagnosis did not decrease even if the acquisition count was reduced by half.

Conclusions: The SBR tended to decrease as reduction of the acquisition count, but it was not a significant decrease. Since SBR_half and SBR_full showed a strong correlation, SBR_full will be estimated from SBR_half using a calibration formula even when the scan time is short.

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