Hongyoon Choi

Noninvasive imaging of radiolabeled exosome-mimetic nanovesicle using (99m)Tc-HMPAO.

Exosomes known as nano-sized extracellular vesicles attracted recent interests due to their potential usefulness in drug delivery. Amid remarkable advances in biomedical applications of exosomes, it is crucial to understand in vivo distribution and behavior of exosomes. Here, we developed a simple method for radiolabeling of macrophage-derived exosome-mimetic nanovesicles (ENVs) with 99mTc-HMPAO under physiologic conditions and monitored in vivo distribution of 99mTc-HMPAO-ENVs using SPECT/CT in living mice. ENVs were produced from the mouse RAW264.7 macrophage cell line and labeled with 99mTc-HMPAO for 1 hr incubation, followed by removal of free 99mTc-HMPAO. SPECT/CT images were serially acquired after intravenous injection to BALB/c mouse. When ENVs were labeled with 99mTc-HMPAO, the radiochemical purity of 99mTc-HMPAO-ENVs was higher than 90% and the expression of exosome specific protein (CD63) did not change in 99mTc-HMPAO-ENVs. 99mTc-HMPAO-ENVs showed high serum stability (90%) which was similar to that in phosphate buffered saline until 5 hr. SPECT/CT images of the mice injected with 99mTc-HMPAO-ENVs exhibited higher uptake in liver and no uptake in brain, whereas mice injected with 99mTc-HMPAO showed high brain uptake until 5 hr. Our noninvasive imaging of radiolabeled-ENVs promises better understanding of the in vivo behavior of exosomes for upcoming biomedical application.

Metabolic and metastatic characteristics of ALK-rearranged lung adenocarcinoma on FDG PET/CT

INTRODUCTION ALK rearrangement in lung cancer has been identified as a novel molecular target in lung adenocarcinoma. In this study, we evaluated metabolic and metastatic features of lung adenocarcinoma by using FDG PET/CT, with regard to specific genotypes of ALK and EGFR mutation. METHODS Patients with lung adenocarcinoma initially diagnosed and examined with FDG PET/CT and molecular genotyping with biopsy specimen, from September 2009 to September 2011, were selected retrospectively. ALK fluorescence in situ hybridization and EGFR mutations were tested. Maximum standardized uptake value (SUVmax) and metastatic characteristics on FDG PET/CT were analyzed with regard to ALK and EGFR status. RESULTS Of the 331 lung adenocarcinoma patients, 18 were ALK positive (ALK(+)), 156 were EGFR mutation positive (EGFR(+)), and 157 were wild type (WT) for both ALK and EGFR mutation. The ALK(+) tumor showed significantly higher SUVmax and more common metastasis to lymph nodes and distant organs than those of other genotypes in overall patients (P<0.01). In a subgroup analysis of advanced stage (stage IIIb and IV), ALK(+) lung cancer showed significantly higher SUVmax (P<0.05) than EGFR(+) tumors. In another subgroup analysis of size matched groups, ALK(+) tumors showed significant difference in SUVmax, lymph node and distant metastasis (P<0.01), particularly in the moderate-sized tumors (1.5-3cm). CONCLUSION ALK-rearranged lung adenocarcinoma represents higher glucose metabolism and more rapid metastasis to lymph nodes or distant sites compared with those with EGFR mutation and wild type, which suggests more aggressive features of ALK rearrangement.

Illuminating the physiology of extracellular vesicles

Extracellular vesicles play a crucial role in intercellular communication by transmitting biological materials from donor cells to recipient cells. They have pathophysiologic roles in cancer metastasis, neurodegenerative diseases, and inflammation. Extracellular vesicles also show promise as emerging therapeutics, with understanding of their physiology including targeting, distribution, and clearance therefore becoming an important issue. Here, we review recent advances in methods for tracking and imaging extracellular vesicles in vivo and critically discuss their systemic distribution, targeting, and kinetics based on up-to-date evidence in the literature.

Abnormal metabolic connectivity in the pilocarpine-induced epilepsy rat model: A multiscale network analysis based on persistent homology

Temporal lobe epilepsy is associated with dysfunctional brain networks. Here we investigated metabolic connectivity in the pilocarpine-induced epilepsy rat model and applied a new multiscale framework to the analysis of metabolic networks of small-animal brains. [(18)F]fluorodeoxyglucose PET was acquired in pilocarpine-induced chronic epilepsy rats and controls to yield interregional metabolic correlation by inter-subject manner. When interregional correlation of epilepsy rats and controls was compared directly, the epilepsy rats showed reduced connectivity involving the left amygdala and left entorhinal cortex. When regional graph properties were calculated to characterize abnormal nodes in the epileptic brain network, the epilepsy rats showed reduced nodal and local efficiencies in the left amygdala. Then, a new multiscale framework, persistent brain network homology, was used to examine metabolic connectivity with a threshold-free approach and the difference between two networks was analyzed using single linkage distances (SLDs) of all pairwise nodes. We found a tendency for longer SLDs between the left insula/left amygdala and bilateral cortical/subcortical structures in the epilepsy rats. Persistent brain network homology analysis as well as interregional correlation study implied the abnormal left limbic-paralimbic-neocortical network in the pilocarpine-induced epilepsy rat models. In conclusion, we found a globally disrupted network in the epileptic brain in rats, particularly in the limbic and paralimbic structures by direct comparison, graph properties and multiscale network analysis. These results demonstrate that the multiscale and threshold-free network analysis can be used to find the network abnormality in small-animal brains as a preclinical research.

Imaging of Integrin αvβ3 Expression Using 68Ga-RGD Positron Emission Tomography in Pediatric Cerebral Infarct:

Enhanced expression of integrin αvβ3 is commonly used as a biomarker for angiogenesis, which is one of the key pathophysiologic processes in cerebral infarct. Integrin αvβ3 can be imaged with arginine-glycine-aspartic acid (RGD) peptide agents. In this study, characteristics of positron emission tomography (PET) using a 68Ga-labeled RGD were investigated in pediatric cerebral infarct. Pediatric patients with moyamoya disease underwent 68Ga-RGD PET in a research protocol for neovascularization evaluation. In these patients, 17 cerebral infarct lesions of 10 patients were included in the analysis. On 68Ga-RGD PET, the infarct lesion to contralateral brain ratio (LCR) of the infarct lesion was measured and analyzed with regard to postinfarct time interval (PTI) and perfusion single-photon emission computed tomography (SPECT) findings. An increase in 68Ga-RGD uptake was observed in cerebral infarct, particularly in recent lesions. The LCR was significantly higher in the recent than in the chronic lesions, and a significant correlation existed between the LCR and PTI. Additionally, the LCR was significantly higher in the lesions with hyperperfusion on SPECT. This study, as the first human study using an RGD agent for in vivo cerebral infarct imaging, demonstrated that 68Ga-RGD PET has a potential for molecular imaging of integrin αvβ3 expression in cerebral infarct as a biomarker of angiogenesis.

In vivo imaging of mGluR5 changes during epileptogenesis using [11C]ABP688 PET in pilocarpine-induced epilepsy rat model.

Introduction Metabotropic glutamate receptor 5 (mGluR5) that regulates glutamatergic neurotransmission contributes to pathophysiology of epilepsy. In this study, we monitored the changes of mGluR5 in vivo using [11C]ABP688 PET during the epileptogenesis in a pilocarpine-induced epilepsy rat model. Methods In vivo mGluR5 images were acquired using [11C]ABP688 microPET/CT in pilocarpine-induced chronic epilepsy rat models and controls. We also acquired microPET/CT at acute, subacute as well as chronic periods after status epilepticus. Non-displaceable binding potential (BPND) of [11C]ABP688 was calculated using simplified reference tissue model in a voxel-based manner. mGluR5 BPND of the rat brains of epilepsy models and controls were compared. Results Status epilepticus developed after pilocarpine administration and was followed by recurrent spontaneous seizures for more than 3 weeks. In chronic epilepsy rat model, BPND in hippocampus and amygdala was reduced on a voxel-based analysis. Temporal changes of mGluR5 BPND was also found. In acute period after status epilepticus, mGluR5 BPND was reduced in the whole brain. BPND of caudate-putamen was restored in subacute period, while BPND of the rest of the brain was still lower. In chronic period, global BPND was normalized except in hippocampus and amygdala. Conclusions In vivo imaging of mGluR5 using [11C]ABP688 microPET/CT could successfully reveal the regional changes of mGluR5 binding potential of the rat brain in a pilocarpine-induced epilepsy model. The temporal and spatial changes in mGluR5 availability suggest [11C]ABP688 PET imaging in epilepsy provide abnormal glutamatergic network during epileptogenesis.

Autoclustering of Non-small Cell Lung Carcinoma Subtypes on (18)F-FDG PET Using Texture Analysis: A Preliminary Result.

PurposeTexture analysis on 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) scan is a relatively new imaging analysis tool to evaluate metabolic heterogeneity. We analyzed the difference in textural characteristics between non-small cell lung carcinoma (NSCLC) subtypes, namely adenocarcinoma (ADC) and squamous cell carcinoma (SqCC).MethodsDiagnostic 18F-FDG PET/computed tomography (CT) scans of 30y patients (median age, 67; range, 42-88) with NSCLC (17 ADC and 13 SqCC) were retrospectively analyzed. Regions of interest were manually determined on selected transverse image containing the highest SUV value in tumors. Texture parameters were extracted by histogram-based algorithms, absolute gradient-based algorithms, run-length matrix-based algorithms, co-occurrence matrix-based algorithms, and autoregressive model-based algorithms. Twenty-four out of hundreds of texture features were selected by three algorithms: Fisher coefficient, minimization of both classification error probability and average correlation, and mutual information. Automated clustering of tumors was based on the most discriminating feature calculated by linear discriminant analysis (LDA). Each tumor subtype was determined by histopathologic examination after biopsy and surgery.ResultsFifteen texture features had significant different values between ADC and SqCC. LDA with 24 automate-selected texture features accurately clustered between ADC and SqCC with 0.90 linear separability. There was no high correlation between SUVmax and texture parameters (|r| ≤ 0.62).ConclusionEach subtype of NSCLC tumor has different metabolic heterogeneity. The results of this study support the potential of textural parameters on FDG PET as an imaging biomarker.

Segmentation-Based MR Attenuation Correction Including Bones Also Affects Quantitation in Brain Study: An Initial Result of 18F-FP-CIT PET/MR for Patients with Parkinsonism

Attenuation correction (AC) with an ultrashort echo time (UTE) sequence has recently been used in combination with segmentation for cortical bone identification for brain PET/MR studies. The purpose of this study was to evaluate the quantification of 18F-fluoropropyl-carbomethoxyiodophenylnortropane (18F-FP-CIT) binding in brain PET/MR, particularly focusing on effects of UTE-based AC including bone segmentation. Methods: Sixteen patients with initially suspected parkinsonism were prospectively enrolled. An emission scan was acquired 110 min after 18F-FP-CIT injection on a dedicated PET/MR scanner, immediately followed by another emission scan using a PET/CT scanner 120 min after the injection. A UTE-based attenuation map was used to classify the voxels into 3 tissues: bone, soft tissue, and air. All PET images were spatially normalized, and a specific-to-nonspecific dopamine transporter (DAT) binding ratio (BR) was calculated using statistical probabilistic anatomic mapping. The level of agreement was assessed with intraclass correlation coefficients (ICCs). Voxelwise comparison between PET images acquired from PET/MR and PET/CT was performed. We compared non–attenuation-corrected images to analyze UTE-based AC effects on DAT quantification. Results: BR in the putamen obtained from PET/MR and PET/CT showed low interequipment variability, whereas BR in the caudate nucleus showed significant variability (ICC = 0.967 and 0.682 for putamen and caudate nucleus, respectively). BR in the caudate nucleus was significantly underestimated by PET/MR, compared with PET/CT (mean difference of BR = 0.66, P < 0.0001). Voxelwise analysis revealed that PET/MR showed significantly low BR in the periventricular regions, which was caused by a misclassification of the ventricle as air on the attenuation map. We also compared non-AC images, revealing low interequipment variability even in the caudate nucleus (ICC = 0.937 and 0.832 for putamen and caudate nucleus, respectively). Conclusion: Our data demonstrate spatial bias of the DAT BR on 18F-FP-CIT PET/MR. Voxelwise analysis and comparison to non-AC images identified the misclassification of ventricle as air to be the cause of bias. To obtain reliable quantification for brain PET/MR studies including 18F-FP-CIT PET, alternative and more reliable segmentation strategies are required.

MRI-Based Attenuation Correction for PET/MRI Using Multiphase Level-Set Method

Inaccuracy in MR image–based attenuation correction (MR-AC) leads to errors in quantification and the misinterpretation of lesions in brain PET/MRI studies. To resolve this problem, we proposed an improved ultrashort echo time MR-AC method that was based on a multiphase level-set algorithm with main magnetic field (B0) inhomogeneity correction. We also assessed the feasibility of this level-set–based MR-AC method (MR-AClevel), compared with CT-AC and MR-AC provided by the manufacturer of the PET/MRI scanner (MR-ACmMR). Methods: Ten healthy volunteers and 20 Parkinson disease patients underwent 18F-FDG and 18F-fluorinated-N-3-fluoropropyl-2-β-carboxymethoxy-3-β-(4-iodophenyl)nortropane (18F-FP-CIT) PET scans, respectively, using both PET/MRI and PET/CT scanners. The level-set–based segmentation algorithm automatically delimited air, bone, and soft tissue from the ultrashort echo time MR images. For the comparison, MR-AC maps were coregistered to reference CT. PET sinogram data obtained from PET/CT studies were then reconstructed using the CT-AC, MR-ACmMR, and MR-AClevel maps. The accuracies of SUV, SUVr (SUV and its ratio to the cerebellum), and specific–to–nonspecific binding ratios obtained using MR-AClevel and MR-ACmMR were compared with CT-AC using region-of-interest– and voxel-based analyses. Results: There was remarkable improvement in the segmentation of air cavities and bones and the quantitative accuracy of PET measurement using the level set. Although the striatal and cerebellar activities in 18F-FP-CIT PET and frontal activity in 18F-FDG PET were significantly underestimated by the MR-ACmMR, the MR-AClevel provided PET images almost equivalent to the CT-AC images. PET quantification error was reduced by a factor of 3 using MR-AClevel (SUV error < 10% in MR-AClevel and < 30% in MR-ACmMR [version VB18P], and < 5% in MR-AClevel and < 15% in MR-ACmMR [VB20P]). Conclusion: The results of this study indicate that our new multiphase level-set–based MR-AC method improves the quantitative accuracy of brain PET in PET/MRI studies.

18F-FDG PET/CT in anti-LGI1 encephalitis: initial and follow-up findings.

Anti-LGI1 encephalitis is a subgroup of autoimmune encephalitis, characterized by memory impairments, seizures, and behavioral problems. The diagnosis can be made by clinical manifestation with a help of serum autoantibody test. There was lack of imaging studies to evaluate and monitor the disease activity by anatomical and functional information. Here, we report serial F-FDG PET/CT findings in a patient with anti-LGI1 encephalitis. Intense F-FDG uptake was initially noted in bilateral limbic system at active disease status, and then decreased and eventually normalized according to the clinical improvement after treatment. F-FDG PET/CT can be used to evaluate treatment response of encephalitis.