Investigating dopaminergic neurotransmission with 123I-FP-CIT SPECT: comparability of modern SPECT systems

With (123)I-labeled N-omega-fluoropropyl-2-beta-carbomethoxy-3-beta-(4-iodophenyl)nortropane (FP-CIT) SPECT increasingly gaining access into routine patient care, the comparability of the results of different SPECT systems in the quantification of receptor binding is important for accurate clinical decision making and the translation of imaging results between institutions (e.g., as part of multicenter therapy trials).

In a series of studies using phantoms (containing target cylinders of 2- and 2.8-cm diameter) and (123)I-FP-CIT patient studies (n = 49), we compared target-to-background (T/BG) and (123)I-FP-CIT striatal uptake ratios recovered by a conventional triple-head SPECT system and a dedicated high-resolution brain SPECT system. All patient studies were acquired on both SPECT systems successively (<15-min interscan gap) using a single-injection protocol (group A [n = 20] was first scanned on the triple-head SPECT system, and group B [n = 29] was first scanned on the dedicated brain SPECT system).

In phantom studies, the T/BG ratios recovered by both systems correlated strongly with the true T/BG ratios (R(2) > 0.96), with the linear regression slopes being 0.86-1.17 and 0.41-0.52 (less steep for smaller target size and lower T/BG ratios) for the dedicated brain SPECT and the triple-head SPECT system, respectively. Although both systems yielded markedly different results, they showed a high linear correlation with each other (R(2) > 0.95, no significant effect from target size). In (123)I-FP-CIT patient studies, a similar linear intersystem correlation was found (R(2) [A/B] = 0.79/0.80, 0.52/0.68, and 0.83/0.85 for the uptake ratios of the striatum, caudate, and putamen, respectively, to the occipital reference region).

A linear transformation of striatal uptake ratios between different SPECT systems appears to be achievable for ligands such as (123)I-FP-CIT. An evaluation is needed of whether the present method can do this with sufficient accuracy for clinical purposes or whether methodologic adaptations are necessary. Proper study timing has to be ensured.