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.