The time window of MRI of murine atherosclerotic plaques after administration of CB2 receptor targeted micelles: inter-scan variability and relation between plaque signal intensity increase and gadolinium content of inversion recovery prepared versus non-prepared fast spin echo.

Single fast spin echo scans covering limited time frames are mostly used for contrast-enhanced MRI of atherosclerotic plaque biomarkers. Knowledge on inter-scan variability of the normalized enhancement ratio of plaque (NER(plaque)) and relation between NER(plaque) and gadolinium content for inversion-recovery fast spin echo is limited. Study aims were: evaluation of (1) timing of MRI after intravenous injection of cannabinoid-2 receptor (CB2-R) (expressed by human and mouse plaque macrophages) targeted micelles; (2) inter-scan variability of inversion-recovery fast spin echo and fast spin echo; (3) relation between NER(plaque) and gadolinium content for inversion-recovery fast spin echo and fast spin echo. Inversion-recovery fast spin echo/fast spin echo imaging was performed before and every 15 min up to 48 h after injection of CB2-R targeted or control micelles using several groups of mice measured in an interleaved fashion. NER(plaque) (determined on inversion-recovery fast spin echo images) remained high (∼2) until 48 h after injection of CB2-R targeted micelles, whereas NER(plaque) decreased after 36 h in the control group. The inter-scan variability and relation between NER(plaque) and gadolinium (assessed with inductively coupled plasma- mass spectrometry) were compared between inversion-recovery fast spin echo and fast spin echo. Inter-scan variability was higher for inversion-recovery fast spin echo than for fast spin echo. Although gadolinium and NER(plaque) correlated well for both techniques, the NER of plaque was higher for inversion-recovery fast spin echo than for fast spin echo. In mice injected with CB2-R targeted micelles, NER(plaque) can be best evaluated at 36-48 h post-injection. Because NER(plaque) was higher for inversion-recovery fast spin echo than for fast spin echo, but with high inter-scan variability, repeated inversion-recovery fast spin echo imaging and averaging of the obtained NER(plaque) values is recommended.

Brain death-related energetic failure of the donor heart becomes apparent only during storage and reperfusion: an ex vivo phosphorus-31 magnetic resonance spectroscopy study on the feline heart.

BACKGROUND AND OBJECTIVE: Recently, we have shown, by using localized in vivo phosphorus-31 magnetic resonance spectroscopy (31P MRS) of the anterior left ventricular wall, that brain death (BD) is not associated with reduced myocardial energy status. In this study, we applied ex vivo 31P MRS of the entire heart to study the effects of BD on the energy status of the feline donor heart following explantation. METHODS: We used cats (6 BD and 6 controls [C]) in a 26-hour protocol. After 2 hours of preparation, we induced BD by filling an intracranial balloon at t = 0 hour. At t = 6 hours, the hearts were arrested with St. Thomas' Hospital cardioplegic solution, explanted, and stored in the same solution at 4 degrees C in a 4.7 Tesla magnet for 17 hours. Subsequently, the hearts were reperfused in the Langendorff mode at 38 degrees C for 1 hour. The first 5-minute 31P MRS spectrum was obtained 1 hour after crossclamping the aorta; we obtained subsequent spectra every hour during storage and every 5 minutes during reperfusion. At the end, the hearts were dried and weighed. Phosphocreatine (PCr), gamma-adenosine triphosphate (gamma-ATP), inorganic phosphate (Pi), and phosphomonoesters (PME), were expressed per g dry heart weight. The intracellular pH (pH(i)) and the PCr/ATP ratio were calculated. RESULTS: During storage, we identified a significant but similar decrease of pH(i), PCr/ATP ratio, and PCr in both groups. During reperfusion, pH(i) and PCr/ATP ratio recovered similarly in both groups, whereas the recovery of PCr in the BD group was significantly lower (p < 0.05). The Pi and PME increased in both groups during storage but to a lesser extent in the BD group (p < 0.05). This difference disappeared during reperfusion. The gamma-ATP was already significantly lower in the BD group at the onset of storage, and this remained so throughout storage and reperfusion (p < 0.05 vs C). Contractile capacity was lost in all hearts, except for 1 heart in the BD group. CONCLUSION: Brain death-related failure of the energetic integrity of the feline donor heart becomes apparent only when using 31P MRS during ischemic preservation and subsequent reperfusion.

Bio-energetic response of the heart to dopamine following brain death-related reduced myocardial workload: a phosphorus-31 magnetic resonance spectroscopy study in the cat.

OBJECTIVE: Long-term exposure of the donor heart to high dosages of dopamine in the treatment of brain death-related hemodynamic deterioration has been shown to reduce myocardial phosphocreatine (PCr) and adenosine triphosphate (ATP) in myocardial biopsy specimens and may preclude heart donation for transplantation. Short-term exposure to the acute catecholamine release during the onset of brain death has shown an unchanged PCr/ATP ratio using in vivo phosphorus-31 magnetic resonance spectroscopy (31P MRS). In this study 31P MRS was used to evaluate in vivo myocardial energy metabolism during long-term dopamine treatment. METHODS: Twelve cats were studied in a 4.7 Tesla magnet for 360 minutes. At t = 0 minutes, brain death was induced (n = 6). At 210 minutes, when myocardial workload in the brain-death group was reduced significantly, dopamine was infused (n = 12) at 5 microg/kg/min and its dose was consecutively doubled every 30 minutes and was withheld during the last 30 minutes of the experiment. Phosphorus-31 magnetic resonance spectra were obtained from the left ventricular wall during 5-minute time frames, and PCr/ATP ratios were calculated. The hearts were histologically examined. RESULTS: Although significant changes in myocardial workload were observed after the induction of brain death and during support and withdrawal of dopamine in both groups, the initial PCr/ATP ratio of 2.00+/-0.12 and the contents of PCr and ATP did not vary significantly. Histologically identified sub-endocardial hemorrhage was observed in 3 of 6 of the brain-dead animals and in 1 of 6 of the control animals. CONCLUSIONS: High dosages of dopamine in the treatment of brain death-related reduced myocardial workload do not alter PCr/ATP ratios and the contents of PCr and ATP of the potential donor heart despite histologic damage.

Characterization and in vitro and in vivo testing of CB2-receptor- and NGAL-targeted paramagnetic micelles for molecular MRI of vulnerable atherosclerotic plaque.

PURPOSE: Atherosclerotic plaque macrophages express the peripheral cannabinoid receptor (CB2-R) and promote fibrous cap degradation by secretion of neutrophil gelatinase-associated lipocalin 2 (NGAL). In this study, we report the preparation, characterization, and in vitro and in vivo testing of double-labeled (MR and fluorescent) CB2-R- and NGAL-targeted micelles. PROCEDURES/RESULTS: Specific CB2-R agonists or antibodies directed to 24p3 (mouse homolog of NGAL) were incorporated into di-oleoyl-polyethylene glycol-phosphatidylethanolamine 1000 (DOPE-PEG1000) micelles or di-stearoyl-polyethylene glycol-phosphatidylethanolamine 2000 (DSPE-PEG2000) micelles. The hydrodynamic diameter, determined by dynamic light scattering, was 16.5 and 19.0 nm for CB2-R-targeted DOPE-PEG1000 and DSPE-PEG2000 micelles, respectively, and 23.0 nm for Ab-conjugated DSPE-PEG2000 micelles. In vitro and in vivo MRI and fluorescence microscopy showed specific binding of CB2-R-targeted and 24p3-targeted micelles to in vitro systems and to aortic plaque in apoE(-/-)/eNOS(-/-) mice, respectively. CONCLUSIONS: CB2-R- and NGAL-targeted micelles show promise as tools for in vivo characterization of vulnerable plaque.