Evaluation of semi-quantitative method for quantification of dopamine transporter in human PET study with ¹⁸F-FE-PE2I

Effect of scan-time shortening on the 11C-PHNO binding potential to dopamine D3 receptor in humans and test-retest reliability

OBJECTIVE

¹¹C-PHNO is a PET radioligand most specific to dopamine D₃ receptor (D₃R). The long scan duration of 120 min used in quantification of ¹¹C-PHNO binding to D₃R in previous studies is challenging to subjects. The main objective of this study was to investigate the effects of shorter scan times on the binding of ¹¹C-PHNO to D₃R and test-retest reliability using the latest digital whole-body PET system.

METHODS

Two 120-min ¹¹C-PHNO brain scans were performed in 7 healthy subjects using a digital whole-body PET/CT. The binding potential relative to non-displaceable tracer in the tissue (BP_ND) of D₃R-rich regions: the pallidum, ventral striatum (VST), substantia nigra (SN) and hypothalamus, were quantified using the simplified reference tissue model. The bias, correlation, and test-retest reliability of BP_ND, which includes the test-retest variability (TRV) and intraclass correlation coefficient (ICC), were evaluated and compared between scans of shorter durations (40-110 min post-injection) and the original 120-min scan acquisitions.

RESULTS

Progressively, shorter scan durations were associated with underestimation of BP_ND, slightly decreased correlation with 120-min derived BP_ND, and decrease in test-retest reliability. The BP_ND values of the pallidum, VST and SN from the shortened 90-min scans showed excellent correlation with those derived from the 120-min scans (determination coefficients > 0.98), and the bias within 5%. The test-retest reliability of BP_ND in these regions derived from 90-min scan (TRV of 3% in the VST and pallidum, 7% in the SN and the ICC exceeded 0.88) was comparable to those obtained in previous 120-min studies using brain-dedicated PET scanners. In the hypothalamus, the BP_ND values obtained from scan-time less than 110 min showed bias larger than 5% and the TRV more than 9%.

CONCLUSION

The scan-time shortening causes bias and decreasing test-retest reliability of ¹¹C-PHNO BP_ND. However, in the whole-body PET system, 90-min scan duration was sufficient for estimating the ¹¹C-PHNO BP_ND in the D₃R-rich striatum and SN with small bias and at the test-retest reliability comparable to those derived from 120-min scans using the brain-dedicated PET systems.

Small-animal PET study for noninvasive quantification of transmembrane AMPA receptor regulatory protein -8 (TARP -8) in the brain

Transmembrane AMPA receptor regulatory protein γ-8 (TARP γ-8) mediates various AMPA receptor functions. Recently, [¹¹C]TARP-2105 was developed as a PET ligand for TARP γ-8 imaging. We performed a full kinetic analysis of [¹¹C]TARP-2105 using PET with [¹¹C]TARP-2105 for the first time. The distribution volume (V_T), which is a macro parameter consisting of the K₁-k₄ rate constants in the two-tissue compartment model analysis, exhibited the following rank order: hippocampus (1.4 ± 0.3) > amygdala (1.0 ± 0.2) > frontal cortex (0.9 ± 0.2) > striatum (0.8 ± 0.2) ≫ cerebellum (0.5 ± 0.1) ≈ thalamus (0.5 ± 0.1) > pons (0.4 ± 0.1 mL/cm³). These heterogenous V_T values corresponded with the order of biological distribution of TARP γ-8 in the brain. To validate the reference tissue model, the binding potential (BP_ND) of [¹¹C]TARP-2105 for TARP γ-8 was estimated using general methods (SRTM, MRTM0, Logan reference model, and ratio method). These BP_NDs based on reference models indicated excellent correlation (R² > 0.9) to the indirect BP_NDs based on 2TCM with moderate reproducibility (%variability ≈ 10). PET with [¹¹C]TARP-2105 enabled noninvasive BP_ND estimation and visual mapping of TARP γ-8 in the living rat brain.

Validation of dynamic [18F]FE-PE2I PET for estimation of relative regional cerebral blood flow: a comparison with [15O]H2O PET

BACKGROUND

Dopamine transporter (DAT) imaging is used in the diagnostic work-up in suspected parkinsonian syndromes and dementia with Lewy bodies but cannot differentiate between these syndromes, and an extra brain imaging examination of the regional cerebral blood flow (rCBF) or glucose metabolism is often needed for differential diagnosis. The requirement of two different imaging examinations is resource-consuming and inconvenient for the patients. Therefore, imaging of both cortical blood flow and DAT imaging with the same radiotracer would be more convenient and cost-effective. The aim of this study was to test whether relative regional cerebral blood flow (rCBF_R) can be measured with the DAT-specific positron emission tomography (PET) tracer [¹⁸F]FE-PE2I (FE-PE2I), by validation with cerebral perfusion measured with [¹⁵O]H₂O PET (H₂O).

METHODS

The rCBF_R was quantified by kinetic modeling for FE-PE2I (R1) and H₂O (F). The R1 was calculated using the simplified reference tissue model, and F was calculated with a modified Koopman double-integration method. The linear relationship and intraclass correlation (ICC) between R1 and F were tested in image data derived from 29 patients with recent onset parkinsonism and 30 healthy controls.

RESULTS

There was a strong linear correlation across all subjects between R1 and F in the frontal, parietal, temporal, cingulate and occipital cortex as well as in the striatum (r  ≥  0.731-0.905, p  < 0.001) with a good-to-excellent ICC, ranging from 0.727 to 0.943 (p < 0.001).

CONCLUSIONS

Our results suggest that FE-PE2I may be used as a proxy for cerebral perfusion, thus potentially serving as a radiotracer for assessment of both DAT availability and rCBF_R in one single dynamic scan. This could be valuable in the differential diagnosis of parkinsonian syndromes.

TRIAL REGISTRATION

EUDRA-CT 2015-003045-26. Registered 23 October 2015 https://www.clinicaltrialsregister.eu/ctr-search/search?query=2015-003045-26.

Fully Automated GMP-Compliant Synthesis of [18F]FE-PE2I

In the struggle to understand and accurately diagnose Parkinson's disease, radiopharmaceuticals and medical imaging techniques have played a major role. By being able to image and quantify the dopamine transporter density, noninvasive diagnostic imaging has become the gold standard. In the shift from the first generation of SPECT tracers, the fluorine-18-labeled tracer [18F]FE-PE2I has emerged as the agent of choice for many physicians. However, implementing suitable synthesis for the production of [18F]FE-PE2I has proved more challenging than expected. Through a thorough analysis of the relevant factors affecting the final radiochemical yield, we were able to implement high-yielding fully automated GMP-compliant synthesis of [18F]FE-PE2I on a Synthera®+ platform. By reaching RCYs up to 62%, it allowed us to isolate 25 GBq of the formulated product, and an optimized formulation resulted in the shelf life of 6 h, satisfying the increased demand for this radiopharmaceutical.

Dopamine transporter imaging in neurodegenerative movement disorders: PET vs. SPECT

Purpose

The dopamine transporter (DAT) serves as biomarker for parkinsonian syndromes. DAT can be measured in vivo with single-photon emission computed tomography (SPECT) and positron emission tomography (PET). DAT-SPECT is the current clinical molecular imaging standard. However, PET has advantages over SPECT measurements, and PET radioligands with the necessary properties for clinical applications are on the rise. Therefore, it is time to review the role of DAT imaging with SPECT compared to PET.

Methods

PubMed and Web of Science were searched for relevant literature of the previous 10 years. Four topics for comparison were used: diagnostic accuracy, quantitative accuracy, logistics, and flexibility.

Results

There are a few studies directly comparing DAT-PET and DAT-SPECT. PET and SPECT both perform well in discriminating neurodegenerative from non-neurodegenerative parkinsonism. Clinical DAT-PET imaging seems feasible only recently, thanks to simplified DAT assessments and better availability of PET radioligands and systems. The higher resolution of PET makes more comprehensive assessments of disease progression in the basal ganglia possible. Additionally, it has the possibility of multimodal target assessment.

Conclusion

DAT-SPECT is established for differentiating degenerative from non-degenerative parkinsonism. For further differentiation within neurodegenerative Parkinsonian syndromes, DAT-PET has essential benefits. Nowadays, because of wider availability of PET systems and radioligand production centers, and the possibility to use simplified quantification methods, DAT-PET imaging is feasible for clinical use. Therefore, DAT-PET needs to be considered for a more active role in the clinic to take a step forward to a more comprehensive understanding and assessment of Parkinson’s disease.

Quantification and discriminative power of 18F-FE-PE2I PET in patients with Parkinson's disease

RATIONALE

Dopamine transporter (DAT) imaging is an important adjunct in the diagnostic workup of patients with Parkinsonism. ¹⁸F-FE-PE2I is a suitable PET radioligand for DAT quantification and imaging with good pharmacokinetics. The aim of this study was to determine a clinical optimal simplified reference tissue-based image acquisition protocol and to compare the discriminatory value and effect size for ¹⁸F-FE-PE2I to that for ¹²³I-FP-CIT scan currently used in clinical practice.

METHODS

Nine patients with early Parkinson's disease (PD, 64.3 ± 6.8 years, 3M), who had previously undergone a ¹²³I-FP-CIT scan as part of their diagnostic workup, and 34 healthy volunteers (HV, 47.7 ± 16.8 years, 13M) underwent a 60-min dynamic ¹⁸F-FE-PE2I PET-MR scan on a GE Signa 3T PET-MR. Based on dynamic data and MR-based VOI delineation, BP_ND, semi-quantitative uptake ratio and SUVR_[t1-t2] images were calculated using either occipital cortex or cerebellum as reference region. For start-and-end time of the SUVR interval, three time frames [t₁-t₂] were investigated: [15-40] min, [40-60] min, and [50-60] min postinjection. Data for putamen (PUT) and caudate nucleus-putamen ratio (CPR) were compared in terms of quantification bias versus BP_ND and discriminative power.

RESULTS

Using occipital cortex as reference region resulted in smaller bias of SUVR with respect to BP_ND + 1 and higher correlation between SUVR and BP_ND + 1 compared with using cerebellum, irrespective of SUVR [t₁-t₂] interval. Smallest bias was observed with the [15-40]-min time window, in accordance with previous literature. The correlation between BP_ND + 1 and SUVR was slightly better for the late time windows. Discriminant analysis between PD and HV using both PUT and CPR SUVRs showed an accuracy of ≥ 90%, for both reference regions and all studied time windows. Semi-quantitative ¹²³I-FP-CIT and ¹⁸F-FE-PE2I values and relative decrease in the striatum for patients were highly correlated, with a higher effect size for ¹⁸F-FE-PE2I for PUT and CPR SUVR.

CONCLUSION

¹⁸F-FE-PE2I is a suitable radioligand for in vivo DAT imaging with high discriminative power between early PD and healthy controls. Whereas a [15-40]-min window has lowest bias with respect to BP_ND, a [50-60]-min time window at pseudoequilibrium can be advocated in terms of clinical feasibility with optimal discriminative power. The occipital cortex may be slightly preferable as reference region because of the higher time stability, stronger correlation of SUVR with BP_ND + 1, and lower bias. Moreover, the data suggest that the diagnostic accuracy of a 10-min static ¹⁸F-FE-PE2I scan is non-inferior compared with ¹²³I-FP-CIT scan used in standard clinical practice.

Dual time point method for the quantification of irreversible tracer kinetics: A reference tissue approach applied to [18F]-FDOPA brain PET

The Patlak graphical analysis (PGA_REF) for quantification of irreversible tracer binding with a reference tissue model was approximated by a dual time point imaging approach (DTP_REF). The DTP_REF was applied to 18 [¹⁸F]-FDOPA brain scans using the occipital cortex as reference region (DTP_OCC) and compared to both PGA_OCC and striatal-to-occipital ratios (SOR). Pearson correlation analysis and Bland-Altman plots showed an excellent correlation and good agreement between DTP_OCC and PGA_OCC, while correlations between SOR and PGA_OCC were consistently lower. Linear discriminant analysis (LDA) demonstrated a similar performance for all methods in differentiating patients with Parkinson's disease (PD) from healthy controls (HC). Specifically for [¹⁸F]-FDOPA brain imaging, these findings validate DTP_OCC as an approximation for PGA_OCC, providing the same quantitative information while reducing the acquisition time to two short static scans. For PD patients, this approach can greatly improve patient comfort while reducing motion artifacts and increasing image quality. In general, DTP_REF can improve the clinical applicability of tracers with irreversible binding characteristics when a reference tissue is available.

Introduction of nuclear medicine research in Japan

There were many interesting presentations of unique studies at the Annual Meeting of the Japanese Society of Nuclear Medicine, although there were fewer attendees from Europe than expected. These presentations included research on diseases that are more frequent in Japan and Asia than in Europe, synthesis of original radiopharmaceuticals, and development of imaging devices and methods with novel ideas especially by Japanese manufacturers. In this review, we introduce recent nuclear medicine research conducted in Japan in the five categories of Oncology, Neurology, Cardiology, Radiopharmaceuticals and Technology. It is our hope that this article will encourage the participation of researchers from all over the world, in particular from Europe, in scientific meetings on nuclear medicine held in Japan.

Optimal Acquisition Time Window and Simplified Quantification of Dopamine Transporter Availability Using 18F-FE-PE2I in Healthy Controls and Parkinson Disease Patients

¹⁸F-(E)-N-(3-iodoprop-2-enyl)-2β-carbofluoroethoxy-3β-(4'methylphenyl)nortropane (¹⁸F-FE-PE2I) is a newly developed dopamine transporter (DAT) PET radioligand. Full quantification methods rely on dynamic acquisition of ¹⁸F-FE-PE2I, but in a clinical setting a simplified protocol is preferable. The aims of this study were to identify the optimal acquisition time window for ¹⁸F-FE-PE2I and to validate the specific binding ratio (SBR) as a simplified quantification method.

METHODS

Ten Parkinson disease (PD) patients and 10 controls were included. Ninety-three-min dynamic PET measurements with ¹⁸F-FE-PE2I were conducted using the high-resolution research tomograph (HRRT). The dynamic measurement was also smoothed to the resolution of a clinical PET system (HR). Regions of interest for the caudate, putamen, ventral striatum, substantia nigra (SN), and cerebellum were manually drawn on coregistered MR images. The outcome measure was the SBR, and the gold standard was the binding potential obtained with wavelet-aided parametric imaging (WAPI BP_ND). The cerebellum was used as a reference region. In a preliminary analysis, SBR was computed for 8 time windows (SBR_dyn). Linear regression analysis and Bland-Altman plots were used to select the optimal acquisition time window. An average image from the selected time window was created, from which new SBR values (SBR calculated on the average image on the HRRT and SBR calculated on the average image on the simulated HR images) were calculated and compared with WAPI BP_ND The effect size was calculated.

RESULTS

SBR_dyn values for the time window between 16.5 and 42 min correlated best with WAPI BP_ND (r² = 0.98, P < 0.001). Significant correlations (P < 0.001) were observed between SBR_HR and WAPI-BP_ND (r² = 0.95 in controls and 0.97 in PD patients). In the striatum, SBR_HR values were 37% lower than BP_ND in controls, 29% in PD patients, whereas in the SN the underestimation was 22% in controls and 15% in PD patients. Similar effect sizes for BP_ND and SBR_HR were found in the caudate (0.6), putamen (1.7 and 1.4), ventral striatum (0.7), and SN (0.5 and 0.4).

CONCLUSION

A single ¹⁸F-FE-PE2I acquisition between 16.5 and 42 min provides the best outcome measure for simplified DAT quantification. Despite underestimation of the BP_ND, the SBR can be used in a clinical setting as a valid quantification method for DAT using ¹⁸F-FE-PE2I, because it provides differentiation similar to BP_ND between controls and PD patients.