Acquisition count dependence of the specific binding ratio in ¹²³I-FP-CIT SPECT

In the [¹²³I]FP-CIT single-photon emission computed tomography (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 the SBR and to clarify the reliability of SBR values in patients with shorter scan times.

A striatum phantom was used in a phantom study, with the radioactivity concentration adjusted so that the right striatum:left striatum:brain parenchyma ratio was 8:4:1. Changes in SBR values and image quality, expressed as the % coefficient of variation (%CV) and normalized mean squared error (NMSE), with decreasing acquisition counts were evaluated. In the clinical study, 106 patients (73.1 ± 9.6 years) with suspected parkinsonian syndrome underwent [¹²³I]FP-CIT SPECT, and SBR values from normal 30 min acquisitions (_fullSBR) and half-count acquisitions (_halfSBR) were compared. SBR values were calculated using the Tossici-Bolt (SBR_TB) and a fully automatic count-based (SBR_cb) methods.

In the phantom study, image quality decreased with a reduction of acquisition counts. The %CV and NMSE decreased by up to 52.5% and 81.5%, respectively. SBR values decreased slightly as acquisition counts decreased. In the clinical study, the mean values of _halfSBR were lower than those of _fullSBR, and they were significantly different except for SBR_TB without attenuation correction. _halfSBR and _fullSBR values correlated well, with _halfSBR values 1-8% lower than _fullSBR. The accuracy of diagnosis did not decrease even after acquisition counts were reduced by half.

This study demonstrated that SBR values decrease as a function of reduced acquisition counts. Since _halfSBR and _fullSBR showed excellent correlation, it is suggested that _fullSBR can be estimated from _halfSBR using a calibration formula when scan times are reduced.