Dopamine Transporter Imaging Using ^99mTc-TRODAT-1 SPECT in Parkinson’s Disease

Background

Current advances in molecular imaging techniques, such as single-photon emission computerized tomography (SPECT) and positron emission tomography (PET), have allowed the functional assessment of the nigrostriatal pathway using specific dopamine radiotracers. Studies on PD have revealed that the degeneration of dopaminergic neurons in the substantia nigra leads to a decrease in the density of presynaptic dopaminergic nerve terminals and dopamine transporters (DAT) in the striatum. Striatal DAT imaging disclosed by SPECT and PET has been considered an in vivo biomarker of dopaminergic neuron loss in the substantia nigra, and may help to determine the diagnosis and progression of PD [1–4].

Although the decrease in striatal DAT density has been extensively described in North American, European, and Asian PD patients [5–8], few reports concerning DAT imaging are available on the rest of the world population.

The objective of this study was to examine the diagnostic utility of SPECT with 99mTc-TRODAT-1 in Brazilian PD patients and healthy controls by means of a cross-sectional study design and receiver operating characteristic (ROC) analysis. Diagnostic accuracies of DAT density in the striatum, caudate nucleus, and putamen, as well as DAT density measurements provided by 2 independent readers, were compared.

Material and Methods

BRAIN SPECT STUDIES:

SPECT assessments using 99mTc-TRODAT-1, a radiotracer with high selectivity and specificity for the DAT, were performed between January 2008 and December 2009. Antiparkinsonian drugs were withdrawn for at least 12 h before the examinations, although these drugs have not been demonstrated to influence brain DAT density. Subjects were scanned using a double-headed gamma camera equipped with fan beam collimators (Infinia™ Hawkeye® SPECT/CT, GE Healthcare), 4 h following intravenous injection of 814 MBq (±74) of 99mTc-TRODAT-1. The TRODAT-1 kits, produced by the Institute of Nuclear Energy Research (Taiwan, ROC), were labeled and controlled before administration, and the radiochemical purity control was verified by solvent extraction method. Images were transferred to a Xeleris workstation (GE Medical Systems) and reconstructed taking into account the anteroposterior commissural line by using a standard back-projection technique with a Butterworth low-pass filter (cut-off frequency 0.45). Photon attenuation correction was performed using Chang’s first-order correction method [10].

DAT density was calculated using binding potential from regions of interest (ROI), bilaterally drawn in the striatum, caudate nucleus, putamen, and occipital lobe, by 2 independent readers (P.B.N and I.R.B), blind to subject identity, diagnosis, and group. DAT binding potential was obtained according to the equation: (specific binding – nonspecific binding)/nonspecific binding. Data from occipital lobes were considered as a reference for nonspecific DAT binding.

STATISTICAL ANALYSIS:

Descriptive statistical data are given as median values with lower and upper quartiles (25^th and 75^th percentiles, respectively). Concerning the DAT binding potentials in the striatum, caudate nucleus, and putamen, the lowest of bilateral measures of each participant was selected for analysis. Data provided by observer 1 were randomly chosen and were considered for descriptive data.

Data distribution was assessed using the Kolmogorov-Smirnov test. For intergroup comparisons, the Mann-Whitney U test was used. Chi-squared analysis with Yates’ correction was used when appropriate. The cut-off values for the differentiation between PD and healthy subjects were established by receiver operating characteristic (ROC) curves. Correlations were performed using nonlinear regression analysis. To verify the interobserver reproducibility of the ^99mTc-TRODAT-1 SPECT results, DAT binding potentials from both studied groups, provided by the readers 1 and 2, were compared, using the Wilcoxon signed rank test. Differences were considered significant for p values <0.05.

Results

ROC CURVE ANALYSIS:

Regarding striatal DAT density, the cut-off value of 0.90 for indicating PD was associated with a sensitivity of 100% and a specificity of 89% (area under the curve: 0.998; p<0.05). Striatal ^99mTc-TRODAT-1 binding <0.90 (defined as presynaptic dopaminergic deficit according to the current study) was found in all PD patients (100%), and in 1 (11%) of 9 healthy subjects (p<0.05). The decrease in striatal ^99mTc-TRODAT-1 binding was bilateral in all PD patients.

Concerning caudate nucleus DAT density, the cut-off value of 1.40 for indicating PD was associated with a sensitivity of 100% and a specificity of 56% (area under the curve: 0.994; p<0.05). Caudate nucleus ^99mTc-TRODAT-1 binding <1.40, considered as presynaptic dopaminergic deficit in this study, was found in all PD patients (100%), and in 4 (44%) of 9 controls (p<0.05).

Considering putaminal DAT density, the cut-off value of 0.76 for indicating PD was associated with a sensitivity of 85% and a specificity of 89% (area under the curve: 0.983; p<0.05). Putaminal 99mTc-TRODAT-1 binding <0.76 (defined as presynaptic dopaminergic deficit in the present study) was found in 17 PD patients (85%), and in 1 (11%) of 9 control individuals (p<0.05). Tables 3 and 4 summarize ROC curve analysis data.

INTEROBSERVER DATA COMPARISON:

Taking into account the DAT density obtained from all participants, there was no significant difference between measures provided by readers 1 and 2 for the right and left striatum or the putamen. In contrast, a significant difference was found between data measured by readers 1 and 2 regarding the caudate nuclei (Table 5).

Discussion

This investigation reports data on the diagnostic accuracy and interobserver reproducibility of SPECT using 99mTc-TRODAT-1 for evaluating the DAT densities in the striatum, caudate nucleus, and putamen in PD patients and healthy subjects. Our results proved a significant reduction in the 99mTc-TRODAT-1 binding in the aforementioned basal ganglia structures in PD patients compared with data from control individuals, which are in line with findings of prior studies [5–7,11,12]. Analysis of ROC curves disclosed that the striatal 99mTc-TRODAT-1 bindings presented the highest diagnostic accuracy compared to those estimates from the caudate nucleus and putamen. For the diagnosis of PD, striatal 99mTc-TRODAT-1 binding cut-off value of 0.90 was associated with a sensitivity of 100% and a specificity of 89%. Moreover, there was no significant difference between striatal 99mTc-TRODAT-1 binding measures provided by different readers, contrary to 99mTc-TRODAT-1 binding estimates in the caudate nucleus.

It is worth noting that the diagnostic accuracy of striatal 99mTc-TRODAT-1 binding, reported in the present study, refers to PD patients versus healthy subjects, and not to PD patients versus those with atypical Parkinsonian syndrome. SPECT or PET using DAT radioligands are relevant methods to identify conditions associated with loss of dopaminergic nigrostriatal nerve terminals, such as PD, multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and dementia with Lewy bodies (DLB); this means that DAT imaging can distinguish patients with and without impairment of the nigrostriatal dopaminergic system. Therefore, in cases of parkinsonism and tremor disorders, normal striatal DAT density highlights the possibility of conditions, such as essential tremor (ET), psychogenic, vascular and drug-induced parkinsonism, as well as dystonic tremor [1,3,13]. To differentiate PD from ET, the sensitivity of SPECT using DAT tracers has been reported to be around 90% [14,15]. In a North American multicenter assessment of DAT imaging, SPECT with 123I-β-CIT was demonstrated to reliably and effectively distinguish between individuals with Parkinson’s syndrome (PD and PSP) and without Parkinson’s syndrome (healthy controls and ET) [6]. According to the European Association of Nuclear Medicine guidelines [4], 123I-FP-CIT imaging helps discriminate ET from parkinsonian syndromes related to PD, MSA, and PSP, but DAT imaging is unable to discriminate between the latter 3 conditions. Interestingly, the use of 99mTc-TRODAT-1 for the evaluation of clinically unclear parkinsonian syndromes has been recently reported, with an accuracy of 80%, a sensitivity of 100%, and a specificity of 70% when it was compared to the definitive clinical diagnosis established about 2 years later [16].

The commonly used SPECT tracers are 123I-FP-CIT, 123I-β-CIT, and 99mTc-TRODAT-1; the latter was selected for this study because 99mTc provides suitable energy and half-life for imaging, in addition to its wide availability and lower cost, in contrast to the limited availability and higher cost of 123I [8,12]. Although PET has advantages over SPECT in image resolution and capacity of quantification, the results of 99mTc-TRODAT-1 SPECT were demonstrated to be comparable with the findings of 18F-FDOPA PET [11]. Also, PET is more expensive, less widely available, more time-consuming, and more manpower-dependent. The combination of all these factors can make the use of 99mTc-TRODAT-1 SPECT more feasible in some countries.

The main limitations of the present study are: 1) small sample, particularly of the control group (9 subjects), because it is not easy to convince healthy people to undergo radiation unnecessarily, and 2) lack of a criterion standard in vivo examination for definitive diagnosis of PD; the accuracy of clinical diagnosis of PD is only approximately 90% [9].

Recently, transcranial sonography has become a promising tool for the evaluation of movement disorders [17–26]. Future studies must address the utility of striatal DAT density in combination with substantia nigra hyperechogenicity, in addition to a number of non-motor signs of PD, such as depression, olfactory dysfunction, neuropsychological deficits (visuospatial processing and sequential planning), idiopathic rapid-eye-movement (REM) sleep behavior disorder, and pain [27]. At present, there is strong evidence that decreased striatal dopamine transporters uptake and SN hyperechogenicity are risk markers of PD in patients with idiopathic REM sleep behavior disorder [28].

Conclusions

Striatal DAT imaging with 99mTc-TRODAT-1 SPECT can be considered a biomarker for differentiating PD patients from healthy individuals, with good interobserver reproducibility. Further studies comparing PD with atypical Parkinson’s syndrome are needed.