HUVECs are commercially available and can be expanded to a large number in vitro, therefore providing a convenient EC source for this proof-of-principle study to examine whether or not SL-MRI is able to detect temporal changes of regional MBF in response to EC engraftment in the infarcted heart. Our results suggest that EC transplantation induces a strong neovascularization response in the infarcted region detectable at 2-weeks post injection, leading to a substantial increase of regional blood flow and capillary density accompanied by incorporation of grafted ECs into capillaries in the infarcted and border regions (). The localized responses, although strong, appear to be short-termed and transit to a prominent increase of MBF in the remote territory (). This finding is intriguing because it is generally expected that only infarcted and border zones would benefit from EC engraftment. However, there is compelling evidence that the remote region is affected during unfavorable post-MI remodeling 28
and stem cells may partially rescue/stabilize that region 29
. Consistently, an increase in MBF was accompanied by a recovery of wall motion (Ecc
) in the remote region.
SL-MRI (also called arterial spin labeling) has achieved a great success in measuring blood flow in the brain 30
. In conventional brain SL, the RF inversion pulse is typically introduced at an upstream location proximal to the tissue of interest. For MBF measurement, however, on-slice
SL, namely, flow-sensitive alternating inversion recovery (FAIR)31
technique, can minimize the magnetization transfer artifact and the underestimation of flow when feeding arteries have tortuous paths. SL-based MBF in small rodents is based on T1 mapping and the arterial transit time (ATT) is ignored. This approach might be justified by the fact that small rodents have much higher MBF (3–5 mL/min/g 11,20,32
) than humans (0.7–1 mL/min/g 33
) and are studied at higher field strength than clinical scanners, leading to prolonged blood T1. Therefore, the ATT of un-inverted blood spins is much shorter than blood T1, which allows a measurable inflow effect in slice-selective T1 values 12
. ATT of human heart was estimated at 1.5T recently34
. While further study is necessary to evaluate the effect of ATT on MBF quantification in small rodents, our validation study suggests the T1 mapping approach is in excellent agreement with standard microsphere method.
While the goals of this study have been fulfilled, several limitations should be discussed. First, compared to the resting MBF, coronary flow reserve, CFR, might be more representative of coronary microvascular function than the resting MBF 35
. CFR measurement can be implemented via pharmacological stimulation in SL-MRI protocol for future studies. Second, single slice MBF was obtained in the current protocol due to relatively lengthy acquisition time (≥ 25 min), which could lead to concerns that MBF may vary during acquisition. Inversion recovery based T1 mapping, while being the most robust method, is inherently time-consuming. Non-Cartesian k-space trajectory such as spiral 16
or radial 36
imaging techniques can reduce acquisition time and resist respiratory motion. Third, due to inflammatory reactions induced by acute MI and/or the use of human cells in a rat model (albeit immune-compromised), injection of ECs during MI surgery might have a negative impact on EC survival. Therefore, better EC engraftment is expected if they are injected 7 days post-MI as we have shown with embryonic stem cell derived cardiomyocytes.19
Finally, the high post-surgery mortality rate and the exclusion of 7 rats from the Aim-2 analysis because of unqualified infarct size raise the possibility of selection bias in addition to impacting statistical power.
In summary, our results indicate excellent agreement of MBF in free-breathing rats measured by SL-MRI and the standard color microsphere technique. Non-invasive SL-MRI allows serial assessments of regional MBF in response to EC treatment. The presented method offers a promising framework to quantify MBF as a specific and sensitive index to evaluate EC-mediated therapy for the infarcted heart.