Test–retest variability and reliability of123I-IBZM SPECT measurement of striatal dopamine D2 receptor availability in healthy volunteers and influence of iterative reconstruction algorithms

A brain imaging study of dopamine receptor D2 availability in cannabis dependent users after recovery from cannabis-induced psychosis

There is increased risk of psychosis associated with cannabis use disorder and the interaction of THC with dopamine neurotransmission is complex. It is important to investigate the recovery from cannabis-induced psychosis and its effects on the brain's dopamine neurotransmission. This study was to evaluate dopamine receptor D2 availability in the striatum (caudate/putamen) in recently abstinent cannabis dependent users after recovery from psychosis in comparison with abstinent MDMA "ecstasy" abusers and healthy control participants. Participants were eight abstinent ex cannabis-dependent users who were treated for cannabis-induced psychosis with anti-psychotic medication and psychosocial support for 4 months in an inpatient treatment center for drug users. They were compared with nine abstinent ex MDMA "ecstasy" abusers who received medication and psycho-social treatment for 4 months at the same treatment facility and eight healthy control participants. All participants were scanned with bolus and constant infusion of [123I] Iodobenzamide (IBZM) in Single Photon Computed Tomography (SPECT). Cannabis abstinent users who were treated for cannabis-induced psychotic episodes showed no difference in dopamine D2 receptor availability in the caudate compared with abstinent MDMA "ecstasy" abusers and healthy control participants. This finding indicates minimal effects of cannabis-induced psychosis on dopamine reward mechanisms. There is evidence for reduced D2 receptor availability measures in the right putamen (uncorrected) which may indicate a residual effect of anti-psychotic medication.

Cross-camera comparison of ROI-based semi-quantitative ¹²³I-IBZM SPECT data in healthy volunteers using an anthropomorphic phantom for calibration

BACKGROUND

In (123)I-Iolopride (IBZM) SPECT reference values may diverge between camera systems. If multicenter pooling of normal material databases is needed, differences in measured semi-quantitative data due to equipment performance and reconstruction parameters have to be investigated in each instance to determine the comparability.

PURPOSE

To explore the differences in (123)I-IBZM measured uptake ratios between two different gamma cameras in healthy controls, the intra-rater reproducibility of the image evaluation method and the possibility to equalize uptake ratios by calibration through an anthropomorphic phantom.

MATERIAL AND METHODS

Differences in ROI-based semi-quantitative data from two different gamma camera systems, the three-headed brain dedicated Neurocam and the two-headed multipurpose hybrid system Infinia Hawkeye, were studied using image data from a group of healthy volunteers and an anthropomorphic brain-phantom scanned with both cameras. Several reconstruction methods and corrections were applied. To test the reliability of the ROI method, the intra-observer reproducibility was determined for the ROI method in this study.

RESULTS

The ROI method had a high reliability. Differences in mean measured uptake (123)I-IBZM ratios in healthy controls varied between 2.9% and 6.5% depending on reconstruction and correction for attenuation and scatter. After calibration, the differences decreased. There were no statistically significant differences between corrected ratios from the two camera systems in the study when images were reconstructed with attenuation correction.

CONCLUSION

The conformity of uptake ratios in attenuation corrected (123)I-IBZM images derived from the two different cameras was improved by using an anthropomorphic phantom for calibration.

Schizotypy trait and striatal dopamine receptors in healthy volunteers

Individuals with schizotypal features exhibit cognitive, perceptual and social deficits that are similar to but less prominent than those seen in patients with schizophrenia. Dopaminergic hyperactivity in the striatum has been related to the positive symptoms of schizophrenia, and brain-imaging studies of dopamine uptake in the striatum are thought to be linked to the pathophysiological mechanisms underlying schizophrenia. The aim of this study was to investigate whether the increased availability of striatal dopamine (DA) D(2/3) receptors is related to elevated levels of schizotypal features in healthy individuals. The Schizotypal Personality Questionnaire (SPQ) was administered to 55 healthy volunteers. The availability of their striatal DA D(2/3) receptors was analysed using [(123)I] iodobenzamide single photon emission computed tomography (SPECT). Although the SPQ total scores showed no correlation with the availability of total (left and right) striatal DA D(2) receptors, the SPQ disorganised subscale scores were positively correlated with the availability of right striatal DA D(2/3) receptors. Our findings demonstrated that the availability of striatal DA D(2/3) receptors may be associated with schizotypal features in healthy volunteers.

The Role of CT-Based Attenuation Correction and Collimator Blurring Correction in Striatal Spect Quantification

Purpose. Striatal single photon emission computed tomography (SPECT) imaging of the dopaminergic system is becoming increasingly used for clinical and research studies. The question about the value of nonuniform attenuation correction has become more relevant with the increasing availability of hybrid SPECT-CT scanners. In this study, the value of nonuniform attenuation correction and correction for collimator blurring were determined using both phantom data and patient data. Methods. SPECT imaging was performed using 7 anthropomorphic phantom measurements, and 14 patient studies using [I-123]-FP-CIT (DATSCAN). SPECT reconstruction was performed using uniform and nonuniform attenuation correction and collimator blurring corrections. Recovery values (phantom data) or average-specific uptake ratios (patient data) for the different reconstructions were compared at similar noise levels. Results. For the phantom data, improved recovery was found with nonuniform attenuation correction and collimator blurring corrections, with further improvement when performed together. However, for patient data the highest average specific uptake ratio was obtained using collimator blurring correction without nonuniform attenuation correction, probably due to subtle SPECT-CT misregistration. Conclusions. This study suggests that an optimal brain SPECT reconstruction (in terms of the lowest bias) in patients would include a correction for collimator blurring and uniform attenuation correction.

Striatal and extrastriatal microPET imaging of D2/D3 dopamine receptors in rat brain with [¹⁸F]fallypride and [¹⁸F]desmethoxyfallypride

In this study, we compared two different D(2/3) receptor ligands, [¹⁸F]fallypride and [¹⁸F]desmethoxyfallypride ([¹⁸F]DMFP) with respect to the duration of the scan, visualization of extrastriatal receptors, and binding potentials (BP(ND) ) in the rat brain. In addition, we studied the feasibility of using these tracers following a period of awake tracer uptake, during which the animal may perform a behavioral task. Male Sprague-Dawley rats were imaged with [¹⁸F]fallypride and with [¹⁸F]DMFP in four different studies using microPET. All scans were performed under isoflurane anesthesia. The first (test) and second (retest) study were 150-min baseline scans. No retest scans were performed with [¹⁸F]DMFP. A third study was a 60-min awake uptake of radiotracer followed by a 90-min scan. A fourth study was a 150-min competition scan with haloperidol (0.2 mg/kg) administered via tail vein at 90-min post-[¹⁸F]fallypride injection and 60-min post-[¹⁸F]DMFP. For the test-retest studies, BP(ND) was measured using both Logan noninvasive (LNI) method and the interval ratios (ITR) method. Cerebellum was used as a reference region. For the third study, the binding was measured only with the ITR method, and the results were compared to the baseline results. Studies showed that the average transient equilibrium time in the dorsal striatum (DSTR) was at 90 min for [¹⁸F]fallypride and 30 min for [¹⁸F]DMFP. The average BP(ND) for [¹⁸F]fallypride was 14.4 in DSTR, 6.8 in ventral striatum (VSTR), 1.3 in substantia nigra/ventral tegmental area (SN/VTA), 1.4 in colliculi (COL), and 1.5 in central gray area. In the case of [¹⁸F]DMFP, the average BP(ND) values were 2.2 in DSTR, 2.7 in VSTR, and 0.8 in SN/VTA. The haloperidol blockade showed detectable decrease in binding of both tracers in striatal regions with a faster displacement of [¹⁸F]DMFP. No significant changes in BP(ND) of [¹⁸F]fallypride due to the initial awake state of the animal were found, whereas BP(ND) of [¹⁸F]DMFP was significantly higher in the awake state compared to baseline. We were able to demonstrate that dynamic PET using MicroPET Inveon allows quantification of both striatal and extrastriatal [¹⁸F]fallypride binding in rats in vivo. Quantification of the striatal regions could be achieved with [¹⁸F]DMFP.

Pre- and postsynaptic dopamine SPECT in the early phase of idiopathic parkinsonism: a population-based study

PURPOSE

The aim of this study was to assess the diagnostic contribution of pre- and postsynaptic dopamine SPECT in drug-naïve patients with early idiopathic parkinsonism and to investigate possible differences between idiopathic Parkinson's disease (PD) and atypical parkinsonian syndromes (APS) and possible differences in motor subtypes of parkinsonism.

METHODS

A group of 128 newly diagnosed idiopathic parkinsonian patients and 48 healthy controls was studied. Presynaptic baseline SPECT with (123)I-FP-CIT was performed in all patients and in 120 patients also a baseline postsynaptic SPECT with (123)I-IBZM. Clinical diagnoses were reassessed after 12 months.

RESULTS

Presynaptic uptake in the putamen and caudate was significantly reduced in patients compared to controls. Presynaptic uptake ratios were not different between PD patients and patients with APS, and postsynaptic uptake in APS was not significantly reduced compared to PD or controls. In half of the APS patients both pre- and postsynaptic uptake ratios were reduced on the same side in the striatum. Impaired motor performance was associated with decreased presynaptic uptake in the putamen in PD. The postural instability and gait difficulty (PIGD) subtype of PD had lower presynaptic uptake ratios than patients with tremor-dominated (TD) symptoms.

CONCLUSION

Not only presynaptic putamen uptake ratios, but also caudate ratios were reduced in a majority of the patients in our study. At baseline scan, i.e. in an early stage of the disease, the accuracy of excluding APS in the whole study population was 85% using a combination of pre- and postsynaptic SPECT. Already at baseline, lower presynaptic SPECT ratios were seen in PD with PIGD at onset compared to those with TD subtype.

Genetically determined measures of striatal D2 signaling predict prefrontal activity during working memory performance

BACKGROUND

Variation of the gene coding for D2 receptors (DRD2) has been associated with risk for schizophrenia and with working memory deficits. A functional intronic SNP (rs1076560) predicts relative expression of the two D2 receptors isoforms, D2S (mainly pre-synaptic) and D2L (mainly post-synaptic). However, the effect of functional genetic variation of DRD2 on striatal dopamine D2 signaling and on its correlation with prefrontal activity during working memory in humans is not known.

METHODS

Thirty-seven healthy subjects were genotyped for rs1076560 (G>T) and underwent SPECT with [123I]IBZM (which binds primarily to post-synaptic D2 receptors) and with [123I]FP-CIT (which binds to pre-synaptic dopamine transporters, whose activity and density is also regulated by pre-synaptic D2 receptors), as well as BOLD fMRI during N-Back working memory.

RESULTS

Subjects carrying the T allele (previously associated with reduced D2S expression) had striatal reductions of [123I]IBZM and of [123I]FP-CIT binding. DRD2 genotype also differentially predicted the correlation between striatal dopamine D2 signaling (as identified with factor analysis of the two radiotracers) and activity of the prefrontal cortex during working memory as measured with BOLD fMRI, which was positive in GG subjects and negative in GT.

CONCLUSIONS

Our results demonstrate that this functional SNP within DRD2 predicts striatal binding of the two radiotracers to dopamine transporters and D2 receptors as well as the correlation between striatal D2 signaling with prefrontal cortex activity during performance of a working memory task. These data are consistent with the possibility that the balance of excitatory/inhibitory modulation of striatal neurons may also affect striatal outputs in relationship with prefrontal activity during working memory performance within the cortico-striatal-thalamic-cortical pathway.

Within-subject comparison of striatal D2 receptor occupancy measurements using [123I]IBZM SPECT and [11C]Raclopride PET

Antipsychotic-induced D2 receptor occupancy values tend to be lower when measured with [(123)I]IBZM SPECT than with [(11)C]Raclopride PET. To clarify this issue, D2 receptor occupancy was measured in the same subjects using both techniques. Twenty patients with schizophrenia on monotherapy with risperidone (n=7; 3-9 mg/d), olanzapine (n=5; 5-20 mg/d) or clozapine (n=8; 150-450 mg/d) at stable doses, and ten healthy volunteers (HV) underwent both a [(123)I]IBZM SPECT and a [(11)C]Raclopride PET examinations in random order on different days within a week. Patients with schizophrenia were scanned at a fixed interval after last dose administration. Quantification of receptor availability was performed using the most conventional methods from the literature: the tissue ratio derived specific uptake ratios (SUR) were used for SPECT, and simplified reference tissue model (SRTM) derived binding potentials (BP(ND)) for PET. Analysis was performed using both occipital cortex and cerebellum as reference regions for both modalities. Striatal D2 receptor occupancy was measured as the percentage reduction of [(123)I]IBZM SUR or [(11)C]Raclopride BP(ND) compared to the population average measured in HV using the same modality. Occupancy values measured by SPECT were lower than those measured with PET, by 12.4% and 13.8% when occipital cortex and cerebellum were used as reference regions. This difference should be taken in consideration when interpreting reported antipsychotic striatal D2 receptor occupancy values from the literature.

Pharmacokinetics and time-course of D(2) receptor occupancy induced by atypical antipsychotics in stabilized schizophrenic patients

The (123)I-IBZM SPECT measured D(2) receptor occupancy (D(2)RO) in chronically dosed, stabilized schizophrenic patients and its relationship with antipsychotic (AP) pharmacokinetics (PK) over time is still unclear. The aims of this study were: 1) To define the relationship between striatal D(2) receptor occupancy (D( 2)RO) and plasma concentration (C(P)) in stabilized schizophrenic patients on clinically relevant doses using (123)I-IBZM SPECT; 2) To investigate the time course of AP-induced D(2)RO and corresponding C(P). Forty-six schizophrenic patients on their clinically required doses of risperidone, olanzapine, clozapine or quetiapine were included. D( 2)RO and C(P) were measured over time following a sparse-sampling experimental design, and individual PK and D(2)RO-time profiles were estimated using a population approach. Observed striatal D(2)RO and C(P) ranges were 28-75% and 9.4-60.5 ng/mL for risperidone, 22-84% and 8.6-89.5 ng/mL for olanzapine, 5-53% and 41.6-818.2 ng/mL for clozapine and 0-64% and 37.9-719.6 ng/mL for quetiapine. A PK-D(2)RO relationship was found for the four APs. D(2)RO pattern over time was stable for risperidone, olanzapine and clozapine but fluctuating for quetiapine. Stabilized schizophrenic patients show a wide range of both D(2)RO and C(P) at clinically effective doses of the four AP, suggesting that clinical response to these AP may be maintained with D(2)RO below 65%. D(2)RO patterns over time differ between AP. These results should be considered for accurate interpretation of D(2)RO measurements, proper design of studies and optimization of drug regimens for patients on AP treatment.

Sequential and simultaneous dual-isotope brain SPECT: comparison with PET for estimation and discrimination tasks in early Parkinson disease

Parkinson disease (PD) is the second most frequently occurring cerebral degenerative disease, after Alzheimer disease. Treatments are available, but their efficacy is diminished unless they are administered in the early stages. Therefore, early identification of PD is crucial. In addition to providing perfectly registered studies, simultaneous 99mTc/123I imaging makes possible the assessment of pre- and postsynaptic neurotransmission functions under identical physiological conditions, while doubling the number of counts for the same total imaging time. These advantages are limited, however, by cross talk between the two radionuclides due to the close emission energies of 99mTc (140 keV) and 123I (159 keV). PET, on the other hand, provides good temporal and spatial resolution and sensitivity but usually requires the use of a single radionuclide. In the present work, the authors compared brain PET with sequential and simultaneous dual-isotope SPECT for the task of estimating striatal activity concentration and striatal size for a normal brain and two stages of early PD. Realistic Monte Carlo simulations of a time-of-flight PET scanner and gamma cameras were performed while modeling all interactions in the brain, collimator (gamma camera) and crystal (detector block in PET), as well as population biological variability of pre- and postsynaptic uptake. For SPECT imaging, we considered two values of system energy resolution and scanners with two and three camera heads. The authors used the Cramer-Rao bound, as a surrogate for the best theoretical performance, to optimize the SPECT acquisition energy windows and objectively compare PET and SPECT. The authors determined the discrimination performance between 500 simulated subjects in every disease stage as measured by the area under the ROC curve (AUC). The discrimination accuracy between a normal subject and a subject in the prodromal disease stage was AUC = 0.924 with PET, compared to 0.863 and 0.831 with simultaneous and sequential SPECT, respectively. The significant improvement in performance obtained with simultaneous dual-isotope SPECT compared to sequential imaging (p = 0.019) was due primarily to the increased number of counts detected and resulted in comparable performance when performing simultaneous SPECT on a two-head camera with 9.2% energy resolution to that obtained with sequential SPECT on a three-head camera with 6.2% energy resolution.