Correlation between 99mTc-TRODAT-1 SPECT and 18F-FDOPA PET in patients with Parkinson’s disease: a pilot study

Objective To determine whether technetium-99m-labeled tropane derivative single-photon emission computed tomography (99mTc-TRODAT-1 SPECT) provides results comparable to those of the less widely available, less accessible tool fluorine-18-labeled fluorodopa positron-emission tomography (18F-FDOPA PET) in the setting of a movement disorders clinic. Materials and Methods In this prospective pilot study, eight subjects with a clinical diagnosis of Parkinson’s disease were randomly selected from among patients under treatment at a movement disorders clinic and submitted to 99mTc-TRODAT-1 SPECT and 18F-FDOPA PET. The results were read by two experienced observers, and a semiquantitative analysis was performed. Results The visual and semiquantitative analyses were concordant for all studies, showing that radiotracer uptake in the contralateral striatum on the most affected side was lower when 99mTc-TRODAT-1 SPECT was employed. The semiquantitative analysis demonstrated a significant correlation between 18F-FDOPA PET and 99mTc-TRODAT-1 SPECT (r = 0.73; p < 0.01). Conclusion It appears that 99mTc-TRODAT-1 SPECT is a valid option for the study of dopaminergic function in a clinical setting.


INTRODUCTION
The main neurochemical hallmark of Parkinson's disease (PD) is insufficient dopamine production by the substantia nigra pars compacta. Although PD is still diagnosed on the basis of clinical findings (1) , it is difficult to make the early differential diagnosis between true PD and other, nondegenerative forms of parkinsonism, such as those that are drug-induced, vascular, or even psychogenic, in which for some time now, with well-established indications and diagnostic yields (3,4) . Although PET provides higher spatial resolution than does SPECT and is better for quantification purposes, SPECT is more affordable and more readily available in clinical practice as well as for epidemiological studies (5,6) .
The use of DAT-SPECT with 123 I-ioflupane (DaTscan) has been approved by the US Food and Drug Administration (FDA) and European Medicines Agency (EMEA). In contrast, technetium-99m-labeled tropane derivative SPECT ( 99m Tc-TRODAT-1 SPECT) has not been approved for use in the United States or Europe, despite the fact that 99m Tc-TRODAT-1 has an effective half-life for imaging of 6.02 h and that 99m Tc-TRODAT-1 SPECT is extensively used in a number of other countries (7)(8)(9) . In addition, 99m Tc-TRODAT-1 is a biomarker of the DAT, which is found on the presynaptic membrane of dopaminergic projections and plays a role in extracellular dopamine regulation (10) . Several small studies have shown 99m Tc-TRODAT-1 SPECT to be a more affordable, accessible, kit-based imaging alternative that can effectively distinguish between normal and abnormal subjects (11,12) .
The aim of this study was to investigate the correlation between 99m Tc-TRODAT-1 SPECT and 18 F-FDOPA PET.

MATERIALS AND METHODS Subjects
Eight patients with a clinical diagnosis of PD, according to current criteria (1) , were recruited prospectively and randomly from among patients under treatment at a movement disorders clinic. To reduce the risk of misdiagnosis, we recruited only patients who had been diagnosed at least 2 years prior and had shown a sustained response to dopamine replacement therapy.
All of the patients underwent clinical examination by an experienced neurologist and scored according to the Hoehn and Yahr (H&Y) scale (13) and the Movement Disorder Society-sponsored revision of the Unified Parkinson's Disease Rating Scale (MDS-UPDRS) part III (14) in the off state. Each subject underwent a 99m Tc-TRODAT-1 SPECT and an 18 F-FDOPA PET, in random order.
The study protocol was approved by the local research ethics committee. All participating patients gave written informed consent.

Imaging analysis
Patients were required to discontinue all dopaminergic medication overnight prior to both studies. For the 99m Tc-TRODAT-1 SPECT study, other potential test-altering medications, such as certain antidepressants, were also discontinued.
For the 18 F-FDOPA PET examination, patients were premedicated with 200 mg of carbidopa (to inhibit peripheral L-dopa metabolism) 60 min before the administration of 370 MBq of 18 F-FDOPA (ANMAT certificate no. 58050; Tecnonuclear SA, Buenos Aires, Argentina). At 120 min after tracer administration, images were acquired in a 64-slice PET scanner (Discovery 690; GE Healthcare, Waukesha, WI, USA). Patients were awake, in the supine position, with their head immobilized. Images were acquired with a static (30 min) protocol and were reconstructed using the ordered-subset expectationmaximization algorithm (2 iterations; 24 subsets). For the 99m Tc-TRODAT-1 SPECT examination, patients were injected with approximately 20 mCi of isotope DOPA-TEC (ANMAT certificate no. 57706; Tecnonuclear SA) and images were acquired 240 min later.
Visual and semiquantitative analysis was performed for 99m Tc-TRODAT-1 SPECT and 18 F-FDOPA PET. Images were reconstructed by back-projection with a Butterworth filter at a cutoff frequency of 0.45, and the Chang method was employed for attenuation correction (15) .
For each pair of images from an individual patient, spatial co-registration was applied. The 18 F-FDOPA PET images were spatially normalized to a reference 18 F-FDOPA template (16) . Calculated transformation was applied to 18 F-FDOPA PET and 99m Tc-TRODAT-1 SPECT images. Regions of interest (ROIs) for the striatum (caudate nucleus and putamen) were delineated in the 18 F-FDOPA template and applied automatically to the 18 F-FDOPA PET and 99m Tc-TRODAT-1 SPECT images. The ROIs in the occipital cortex were delineated and applied in a similar fashion to serve as reference areas. Specific uptake ratios (SURs) were calculated for each ROI and divided by the mean count in the occipital area, the SUR for each region thus being obtained. Asymmetry indices were calculated for the caudate nucleus and putamen through subtraction of the SUR on each side, divided by the mean values for both. The readers were blinded to the clinical data as well to previous individual PET or SPECT study results.

Statistical analysis
The SURs obtained for the ROIs on 99m Tc-TRODAT-1 SPECT were correlated with those obtained on 18 F-FDOPA PET. To compensate for the signal effects due to individual variation in specific uptake incorporation, we calculated the mean signal for each ROI pair (ipsilateral and contralateral to the most affected side). To evaluate the consistency between the two acquisition methods in terms of the difference between the ipsilateral and contralateral signals, linear regression was performed and Pearson's correlation coefficient was calculated.

RESULTS
The sample included three women and five men. The mean age was 58.3 ± 14.3 years (range, 35.8-74.9 years), and the mean disease duration was 34.3 ± 14.1 months (range, 15.1-57.1 months). One patient was classified as H&Y stage 1, whereas six were classified as H&Y stage 2 and one was classified as H&Y stage 3. The mean MDS-UPDRS part III (motor examination) score was 31.5 ± 14.2 (range, 10.0-53.0). All patients had asymmetric symptoms; one (subject 5) presented mild dyskinesias and motor fluctuations ( Table 1). The mean time elapsed between the 18 F-FDOPA PET and 99m Tc-TRODAT-1 SPECT examinations (or vice versa) was 3.6 ± 2.8 months (range, 0.4-8.3 months), and no treatment modifications were made during the interim.
For each patient, the visual comparison between the two methods was performed by two experienced observers and was concordant across all studies. Greater striatal signal loss was observed contralateral to the more affected side ( Figure 1). As expected, the caudate nucleus and putamen were both better defined spatially and more readily identifiable on 18 F-FDOPA PET images than on 99m Tc-TRODAT-1 SPECT images.   Semiquantitative analysis of the caudate nucleus and putamen ROIs showed greater loss of dopaminergic activity in the striatum on the contralateral side, and the degree of loss was comparable between the two techniques; except in subject 5, in whom, on 99m Tc-TRODAT-1 SPECT, the ipsilateral caudate nucleus was more affected, although the degree of asymmetry was only 3.4%, which was not significant ( Table 2).
Taking all of the studies together, the semiquantitative analysis showed a statistically significant correlation between the two methods (r = 0.73; p < 0.01). The linear regression of predetermined ROIs (for the caudate nucleus and putamen, on the most affected and contralateral sides) is shown in Figure 2. The box plots in Figure 3 show that the differences between the ipsilateral and contralateral sides were greater on 99m Tc-TRODAT-1 SPECT images than on 18 F-FDOPA PET images. The results of the visual and semiquantitative analyses were concordant across all studies, as well as being consistent with the clinical findings.

DISCUSSION
In our sample of patients with mild to moderate PD, randomly selected from among patients under treatment at a movement disorders clinic, we found that 99m Tc-TRO-DAT-1 SPECT and 18 F-FDOPA PET correlated well in relation to the visual and semiquantitative analyses. The 18 F-FDOPA PET technique has become a well-established imaging modality for the study of such activity, providing good spatial resolution and allowing reliable quantification of dopaminergic activity, ultimately identifying the different parts of the striatum and quantifying levels of activity, which makes it very useful for research purposes. The 18 F-FDOPA PET and 99m Tc-TRODAT-1 SPECT techniques differ with respect to the binding sites. Because 18 F-FDOPA measures aromatic L-amino acid decarboxylase activity and 99m Tc-TRODAT-1 measures DAT availability, each is susceptible to different compensatory mechanisms. In 99m Tc-TRODAT-1 SPECT, compensation in early-stage PD consists in downregulation of presynaptic DATs in order to maintain high dopamine concentrations within the synapse. That increases the diagnostic sensitivity of this technique during the early stages of the disease.
To our knowledge, there has been only one study demonstrating that 99m Tc-TRODAT-1 SPECT has adequate sensitivity and specificity for the diagnosis of PD (7) . One previous study compared 99m Tc-TRODAT-1 SPECT and 18 F-FDOPA PET in patients with PD in Taiwan (17) . However, there have been no such studies employing 18 F-FDOPA PET as the gold-standard for evaluating dopaminergic function and comparing it with 99m Tc-TRODAT-1 SPECT in patients with PD in Latin America.
In the present study, the semiquantitative analysis showed that the variation within ROIs was greater for 99m Tc-TRODAT-1 SPECT than for 18 F-FDOPA PET, likely because of the lower spatial resolution of SPECT, together with the nonspecific uptake of 99m Tc-TRODAT-1 and the higher counts obtained with PET, which would result in significant variations in quantification. In one instance (the caudate nucleus in subject 5), there was not even agreement between the quantitative and qualitative analyses. Therefore, we believe quantitative analysis in Radiol Bras. 2021 Jul/Ago;54(4):232-237 99m Tc-TRODAT-1 SPECT should not be interpreted separately from a visual analysis by an experienced observer.
Our study has some limitations. Pathology specimens were not available to confirm PD in the individuals recruited, and the diagnosis was therefore established on the basis of the clinical findings alone, in accordance with currently accepted criteria (1) . In addition, because this was a pilot study, the number of subjects was limited. Nevertheless, the statistical power attained for the correlation analysis was acceptable.
Comparative studies have shown that SPECT techniques involving the use of tracers such as iodine-123-ß-CIT can be good alternatives to PET (18,19) . In 2011, the FDA approved the use of the DaTscan technique for the study of dopaminergic function, and the technique is now widely used. In contrast, the use of 99m Tc-TRODAT-1 SPECT has yet to be approved by the FDA or the EMEA. Nevertheless, in many countries, where 99m Tc-labeled tracers are more accessible and less expensive, 99m Tc-TRODAT-1 SPECT has become a widely available alternative in clinical practice.

CONCLUSION
Given the accessibility and lower cost of 99m Tc-TRO-DAT-1 SPECT, together with the fact that it appears to correlate well with 18 F-FDOPA PET, we believe that it should be considered a viable option for the imaging of patients with PD in daily practice.