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Perinatal hypoxic ischemic encephalopathy (HIE) results in serious neurological dysfunction and mortality. Clinical trials of multilineage-differentiating stress enduring cells (Muse cells) have commenced in stroke using intravenous delivery of donor-derived Muse cells. Here, we investigated the therapeutic effects of human Muse cells in an HIE model. Seven-day-old rats underwent ligation of the left carotid artery then were exposed to 8% oxygen for 60 min, and 72 hours later intravenously transplanted with 1 × 104 of human-Muse and -non-Muse cells, collected from bone marrow-mesenchymal stem cells as stage-specific embryonic antigen-3 (SSEA-3)+ and −, respectively, or saline (vehicle) without immunosuppression. Human-specific probe revealed Muse cells distributed mainly to the injured brain at 2 and 4 weeks, and expressed neuronal and glial markers until 6 months. In contrast, non-Muse cells lodged in the lung at 2 weeks, but undetectable by 4 weeks. Magnetic resonance spectroscopy and positron emission tomography demonstrated that Muse cells dampened excitotoxic brain glutamatergic metabolites and suppressed microglial activation. Muse cell-treated group exhibited significant improvements in motor and cognitive functions at 4 weeks and 5 months. Intravenously transplanted Muse cells afforded functional benefits in experimental HIE possibly via regulation of glutamate metabolism and reduction of microglial activation.

Positron emission tomography with 15O-labeled gases (15O-PET) is important for in vivo measurement of cerebral oxygen metabolism both in clinical and basic settings. However, there are currently no reports concerning 15O-PET in mice. Here, we developed an 15O-PET method applicable to mice with spontaneous respiration of 15O-gas without a tracheotomy catheter. Sequential 15O-PET was also performed in a mouse model of chronic cerebral hypoperfusion with bilateral common carotid artery stenosis (BCAS) induced by placement of microcoils. 15O-gas with isoflurane was supplied to the nose of mouse with evacuation of excess 15O-gas surrounding the body. 15O-PET was performed on days 3, 7, 14, 21, and 28 after surgery. Cerebral blood flow (CBF), cerebral blood volume, oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO2) were calculated in whole brains. A significant decrease in CBF and compensatory increase in OEF in the BCAS group produced CMRO2 values comparable to that of the sham group at three days post-operation. Although CBF and OEF in the BCAS group gradually recovered over the first 28 days, the CMRO2 showed a gradual decrease to 68% of sham values at 28 days post-operation. In conclusion, we successfully developed a noninvasive 15O-PET method for mice.

We aimed at evaluating the adequacy of the commonly employed compartmental model for quantitation of cerebral metabolic rate of oxygen (CMRO2) using 15O-labeled oxygen (15O2) and positron emission tomography (PET). Sequential PET imaging was carried out on monkeys following slow bolus injection of blood samples containing 15O2–oxyhemoglobin (15O2–Hb), 15O-labeled water (H215O), and C15O-labeled hemoglobin (C15O–Hb) into the internal carotid artery (ICA). Clearance slopes were assessed in the middle cerebral artery territory of the injected hemisphere. The time–activity curves were bi-exponential for both 15O2–Hb and H215O. Single exponential fitting to the early (5 to 40 seconds) and late (80 to 240 seconds) periods after the peak was performed and the 15O2–Hb and H215O results were compared. It was found that a significant difference between the clearance rates of the 15O2–Hb and H215O injections is unlikely, which supports the mathematical model that is widely used to describe the kinetics of 15O2–Hb and H215O in cerebral tissues and is the basis of recent approaches to simultaneously assess CMRO2 and cerebral blood flow in a single PET session. However, it should be noted that more data are necessary to unequivocally confirm the result.

Positron emission tomography ( PET) with 15O tracers provides essential information in patients with cerebral vascular disorders, such as cerebral blood flow ( CBF), oxygen extraction fraction ( OEF), and metabolic rate of oxygen ( CMRO2). However, most of techniques require an additional C15O scan for compensating cerebral blood volume ( CBV). We aimed to establish a technique to calculate all functional images only from a single dynamic PET scan, without losing accuracy or statistical certainties. The technique was an extension of previous dual-tracer autoradiography (DARG) approach, but based on the basis function method (DBFM), thus estimating all functional parametric images from a single session of dynamic scan acquired during the sequential administration of H215O and 15O2. Validity was tested on six monkeys by comparing global OEF by PET with those by arteriovenous blood sampling, and tested feasibility on young healthy subjects. The mean DBFM-derived global OEF was 0.57 ± 0.06 in monkeys, in an agreement with that by the arteriovenous method (0.54 ± 0.06). Image quality was similar and no significant differences were seen from DARG; 3.57% ± 6.44% and 3.84% ± 3.42% for CBF, and −2.79% ± 11.2% and −6.68% ± 10.5% for CMRO2. A simulation study demonstrated similar error propagation between DBFM and DARG. The DBFM method enables accurate assessment of CBF and CMRO2 without additional CBV scan within significantly shortened examination period, in clinical settings.

Positron emission tomography (PET) with [11C]raclopride has been used to investigate the density ( Bmax) and affinity ( Kd) of dopamine D2 receptors related to several neurological and psychiatric disorders. However, in assessing the Bmax and Kd, multiple PET scans are necessary under variable specific activities of administered [11C]raclopride, resulting in a long study period and unexpected physiological variations. In this paper, we have developed a method of multiple-injection graphical analysis (MI-GA) that provides the Bmax and Kd values from a single PET scan with three sequential injections of [11C]raclopride, and we validated the proposed method by performing numerous simulations and PET studies on monkeys. In the simulations, the three-injection protocol was designed according to prior knowledge of the receptor kinetics, and the errors of Bmax and Kd estimated by MI-GA were analyzed. Simulations showed that our method could support the calculation of Bmax and Kd, despite a slight overestimation compared with the true magnitudes. In monkey studies, we could calculate the Bmax and Kd of diseased or normal striatum in a 150 mins scan with the three-injection protocol of [11C]raclopride. Estimated Bmax and Kd values of D2 receptors in normal or partially dopamine-depleted striatum were comparable to the previously reported values.

Udgivelsesdato: 2007-Sep An international group of experts in pharmacokinetic modeling recommends a consensus nomenclature to describe in vivo molecular imaging of reversibly binding radioligands.

One of the most limiting factors for the accurate quantification of physiologic parameters with positron emission tomography (PET) is the partial volume effect (PVE). To assess the magnitude of this contribution to the measurement of regional cerebral blood flow (rCBF), the authors have formulated four kinetic models each including a parameter defining the perfusable tissue fraction (PTF). The four kinetic models used were 2 one-tissue compartment models with (Model A) and without (Model B) a vascular term and 2 two-tissue compartment models with fixed (Model C) or variable (Model D) white matter flow. Furthermore, rCBF based on the autoradiographic method was measured. The goals of the study were to determine the following in normal humans: (1) the optimal model, (2) the optimal length of fit, (3) the model parameters and their reproducibility, and (4) the effects of data acquisition (2D or 3D). Furthermore, the authors wanted to measure the activation response in the occipital gray matter compartment, and in doing so test the stability of the PTF, during perturbations of rCBF induced by visual stimulation. Eight dynamic PET scans were acquired per subject (n = 8), each for a duration of 6 minutes after IV bolus injection of H215O. Four of these scans were performed using 2D and four using 3D acquisition. Visual stimulation was presented in four scans, and four scans were during rest. Model C was found optimal based on Akaike's Information Criteria (AIC) and had the smallest coefficient of variance after a 6-minute length of fit. Using this model the average PVE corrected rCBF during rest in gray matter was 1.07 mL·min−1·g−1 (0.11 SD), with an average coefficient of variance of 6%. Acquisition mode did not affect the estimated parameters, with the exception of a significant increase in the white matter rCBF using the autoradiographic method (2D: 0.17 mL·min−1·g−1 (0.02 SD); 3D: 0.21 mL·min−1·g−1 (0.02 SD)). At a 6-minute fit the average gray matter CBF using Models C and D were increased by 100% to 150% compared with Models A and B and the autoradiographic method. There were no significant changes in the perfusable tissue fraction by the activation induced rCBF increases. The largest activation response was found using Model C (median = 39.1%). The current study clearly demonstrates the importance of PVE correction in the quantitation of rCBF in normal humans. The potential use of this method is to cost-effectively deliver PVE corrected measures of rCBF and tissue volumes without reference to imaging modalities other than PET.

To investigate the effect of the level of baseline cerebral blood flow (CBF) on local CBF augmentation by activation, we have used positron emission tomography to measure regional CBF (rCBF) in 12 normal volunteers with and without photic stimulation during hypocapnia, normocapnia, and hypercapnia. The increase in rCBF in the primary visual cortex by photic stimulation was 10.8 ± 3.1, 18.6 ± 9.3, and 19.5 ± 6.1 ml 100 ml−1 min−1 in hypo-, normo-, and hypercapnia, respectively. The increase was significantly smaller in hypocapnia than in normocapnia ( p < 0.005). The fractional CBF increase caused by the photic stimulation was the same in all breathing conditions. This result indicates that the magnitude of the CBF increase induced by neuronal activity correlates proportionally with the level of baseline CBF.

We developed a new technique of weighted integration for the measurement of local cerebral blood flow (LCBF) and the blood-tissue partition coefficient ip) using dynamic positron emission tomography (PET) and H152O. The weighted integration in the new technique is carried out on the equation of the first time integration of the Kety–Schmidt differential equation. Practically, serially accumulated images with sequentially prolonged accumulation times are weighted by two arbitrary functions. The weighting functions do not have to be differentiated because of the exclusion of the differential term in the starting equation. Consequently, the method does not require data at the end of the scan. The technique was applied to H152O dynamic PET performed on four normal subjects, and was verified to provide a better signal-to-noise ratio than the previously developed integrated projection (IP) technique. Computer simulations were carried out to investigate the effects of statistical noise, tissue heterogeneity, and time delay and dispersion in arterial input function. The simulation showed that the new technique provided about a 1.4 times lower statistical error in both LCBF and p at 50 ml 100 g−1 min−1 compared to the IP technique, and it should be noted that the new technique was less sensitive to the shape of the weighting functions. The new technique provides a new strategy with respect to the statistical error for estimation of LCBF and p.

CBF/CMRO2 may be quantified using PET with 15O-tracers, but the conventional three-step technique (3S) requires a long study period, attributed to need for separate acquisition for three radioactive tracers for CBF (H215O or C15O2), CMRO2 (15O2), and CBV (C15O). Simultaneous determination of CBF, CMRO2 and CBV from a single dynamic 15O2 scan is known to suffer from statistical uncertainty. We recently developed a rapid technique that allows sequential administration of two of three tracers (for CBF and CMRO2) in a single PET scan, and generates quantitative CBF/CMRO2 images with a dual-tracer integration method (DTI). This study asked whether this technique is feasible in normal human subjects and how accurate quantitative values it provides in comparison with the 3S and non-linear fitting method (NLM). Eight normal subjects (24.2+-2.6 y.o) participated in this study. The protocol of the study was approved by the local ethical committee. All subjects received three emission scans, i.e., a 4-min scan after short bolus administration of C15O, a 9.5-min scan with dual administration of C15O2 followed by 15O2, and a 9-min scan with 15O2 followed by H215O. With correction for blood volume using the first C15O scan data, CBF/CMRO2 images were generated by 3S using the first part of the second (CO2) and the third scan (O2), whereas those by DTI method were calculated using both part of PET data in each of the second or the third scan. The calculation used a mathematical formula derived from a single-tissue compartment model including a correction for blood volume and an assumed partition coefficient for water. Regional quantitative values of CBF/CMRO2 in the DTI was compared with those by 3S and NLM. We also compared by voxel-by-voxel CBF or CMRO2 images between the second and the third DTI scan to test the regional difference. Global CBF or CMRO2 values for the DTI did not differ from those by the 3S, and between the second and third DTI scans. The cortical regional values of the DTI neither differed significantly from those of 3S and NLM. In contrast, CMRO2 values of the white matter region in both 3S and DTI differed significantly from NLM, as was predicted from a simulation study for the error in the assumed partition coefficient. Voxel-based comparison of CBF images between the second and the third DTI scan showed significant difference at brain regions around nasal cavity, and at the somatosensory areas that may come from the difference in methodology (inhalation or venous-injection of CBF tracer) and physiology (sensation of intravenous injection), respectively. These were also seen when compared CMRO2 images, but at a lower height threshold. These results suggest that this DTI technique generates CBF/CMRO2 images with a considerably shortened scan period, which are comparable to those by 3S, in terms of quantitative accuracy and image quality. The technique may be of use for clinical studies in patients particularly with acute stroke, and also for investigation of metabolic coupling during neuronal activation or pharmacological challenges in human.

Delay time of arterial input function measured by an external radiation detector must be estimated correctly in order to accurately measure rCBF or CMRO2 with O-15 tracer and PET by autoradiographic method. Conventionally, in order to estimate the delay time, the measured arterial input curve is compared with tissue time activity curve (TTAC) from reconstructed image and the delay time is determined by the fitting procedure. Instead of getting TTAC from the reconstructed image (image based method), Shidahara et al1 proposed a method to get TTAC from the sinogram data (sinogram based method), which enables more rapid estimation of the delay time. CO2 or O2 gas is usually supplied by a mask and a subject inhales the radioactive gas through the mask. During PET acquisition, radioactive gas in mask and nasal cavity contributes large artifact on the sinogram data and it is difficult to estimate delay time from the sinogram data. We proposed a new method to estimate the delay time using the sinogram data and the attenuation map (attenuation weighted sinogram method). In the present method, the attenuation map was used to eliminate the effect of the gas outside the brain region from the sinogram data. For the validation of the present method, PET data with CO2 (n=10) were analyzed. Three methods, namely the image base method, the sinogram base method and the attenuation weighted sinogram method were used to estimate the delay time. The estimated delay times and calculated rCBF images by three methods were compared. Due to the radioactivity outside of the brain, the sinogram method significantly overestimated the delay time and thus underestimated the rCBF value compared with the image base method. On the other hand, there were good agreements between the delay times estimated by the attenuation weighted sinogram method and the image base method. The present method can eliminate the effect of the radioactivity outside of the brain on the sinogram data and estimate the delay time accurately and fast enough for clinical use.

The tracer appearance time relative to the radial artery–sampling site has been evaluated in six brain locations in five human subjects using dynamic positron emission tomography (PET) following the bolus injection of H215O. There was a maximum difference of ± 2 s from the average in each location. T o globally adjust the timing difference between the measured arterial curve and the PET scan, a correction method was developed based on a nonlinear least-squares fitting procedure. This new technique determined the global time delay with an accuracy of ± 0.5 s. On the other hand, the linear backward extrapolation method resulted in a systematic error of 4 s.

A system for CBF measurement using an H215O autoradiographic method and positron emission tomography (PET) has been designed and installed as a clinical tool. Following an intravenous injection of H215O, a radioactivity accumulation in the brain tissue for 60 s and a continuous record of radioactivity in arterial blood were measured by a high counting speed PET device and a beta-ray detector, respectively, and CBF was calculated by a table-lookup procedure. First, this method was compared with the C15O2 inhalation steady-state method on 17 cerebrovascular disease patients and four normal subjects. The two values for CBF agreed with each other when H215O autoradiographic method was applied by correction for the dispersion in the measured arterial radioactivity–time curve. However, without the correction, the CBF by the H215O autoradiographic method revealed substantial overestimation by 30.6 ± 17.5%. A reduced gray/white ratio of CBF was also observed in the H215O autoradiographic method. Second, simulation was performed in order to determine optimal accumulation time by PET scan; the result was that errors due to dispersion and time mismatch became critical as the accumulation time was shortened to

In order to investigate the validity of the single compartment model in measuring CBF with the use of 15O-labeled water (H215O), dynamic positron emission tomography (PET) was performed following bolus injection of H215O. Careful attention was paid to accuracy in the measurement system (especially for the input function). In the region of the putamen, which includes the smallest mixture of gray and white matters in addition to the smallest contamination of cerebrospinal fluid (CSF) spaces, the partition coefficient obtained was 0.88 ± 0.06 (ml/g). The discrepancy from the prediction estimated from the brain/blood water content ratio was only 7%. This finding suggests that there is no more complicated model than the usual single compartment one to describe the physiological behaviour of 15O water. On the other hand, in the other cortical regions, the discrepancy was larger (e.g., about 12% for the insular cortex and 26% for the frontal cortex) than in the region of the putamen, and a significant fit–interval dependence was observed in the calculated parameters. These observations suggest a significant effect of tissue heterogeneity and/or contamination with nonperfusable spaces in actual clinical PET data.

The oxygen extraction fraction (OEF) at maximally vasodilated tissue in patients with chronic cerebrovascular disease was evaluated using positron emission tomography. The vascular responsiveness to changes in PaCO2 was measured by the H215O autoradiographic method. It was correlated with the resting-state OEF, as estimated using the 15O steady-state method. The subjects comprised 15 patients with unilateral or bilateral occlusion and stenosis of the internal carotid artery or middle cerebral artery or moyamoya disease. In hypercapnia, the scattergram between the OEF and the vascular responsiveness to changes in PaCO2 revealed a significant negative correlation in 11 of 19 studies on these patients, and the OEF at the zero cross point of the regression line with a vascular responsiveness of 0 was 0.53 ± 0.08 (n = 11). This OEF in the resting state corresponds to exhaustion of the capacity for vasodilation. The vasodilatory capacity is discussed in relation to the lower limit of autoregulation.

The effect of the inaccuracy of the input function on CBF measured by the H152O autoradiographic method was investigated. In H152O autoradiography the measured input function usually includes a larger dispersion than the true input function, as well as the absolute time axis having been already lost. The time constant of the external dispersion that occurred in our continuous sampling system was evaluated as 10–12 s when the dispersion function was approximated by a monoexponential function. The internal dispersion occurring in arterial lines in a human body was evaluated as 4–6 s. Such dispersion, indispensable in a patient study, was found to produce large errors in calculating CBF, e.g., 5(10) s of the dispersion caused + 15(33) and + 10(20)% systematic overestimations for the 40- and 60-s accumulation time, respectively. An analytical correction employing an inverse Laplace transform was applied to clinical CBF studies, and the results were compared with those from the C15O2steady-state inhalation method. Correction by 10 s in time constant, corresponding to the external dispersion, reduced the overestimation significantly from 70–100% to ∼20%. Further correction by 5 s, corresponding to the internal dispersion, resulted in a negligible difference (less than a few percent) from the steady-state method.