Microvascular obstruction and hemorrhage influence T1 and T2 relaxation parameters in the detection of edema in acute myocardial infarction
Nilesh R. Ghugre1,2,3, Venkat Ramanan1, Jing Yang1, Idan Roifman3, Mohammad I Zia3, Bradley H Strauss3, Kim A Connelly4, and Graham A Wright1,2,3

1Physical Sciences Platform, Sunnybrook Research Institute, Toronto, ON, Canada, 2Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada, 3Schulich Heart Research Program, Sunnybrook Health Sciences Centre, Toronto, ON, Canada, 4Cardiology, St. Michaels Hospital, Toronto, ON, Canada

Synopsis

T1 and T2 relaxation parameters have been instrumental in quantifying both edema and hemorrhage in acute myocardial infarction (AMI). Unlike T2, the combined effect of edema and hemorrhage on T1 has not been well described. Our study assessed and compared the impact of microvascular obstruction (MVO) and hemorrhage on the sensitivity of T1 and T2 in the quantification of edema. Our study indicated that the capacity of T1 to detect edema is affected by the counteracting influence from hemorrhage. Furthermore, T2 may be more sensitive to edema than T1 in AMI, particularly in the presence of MVO and hemorrhage.

Background

Ischemia-reperfusion injury (IRI) is an undesirable sequela following successful reperfusion in acute myocardial infarction (AMI). The lethal form of IRI can result in aggravated edema, microvascular obstruction (MVO) and injury and myocardial hemorrhage (1). Hemorrhage in association with MVO is also considered to be independent predictor of adverse outcomes (2). In this respect, T1 and T2 relaxation parameters have been instrumental in quantitatively characterising both edema and hemorrhage in AMI (3–5). It is well known that T2 is affected by counteracting influences from edema and hemorrhage, when simultaneously present in the tissue, resulting in an underestimation of edematous content. However this confounding effect has been neglected when evaluating the performance of T1 mapping techniques; this could be an important consideration when evaluating clinical outcomes.

Purpose

The objective of our study was to access and compare the impact of MVO and hemorrhage on the sensitivity of T1 and T2 relaxation parameters in the quantification of infarct zone edema following AMI.

Methods

18 patients with ST-segment elevation myocardial infarction (STEMI) underwent a MRI exam on a 1.5T scanner (GE MR450w) at 48 hrs post percutaneous-coronary-intervention (PCI). T2 was quantified using a previously validated cardiac-gated free-breathing T2-prepared spiral imaging sequence: TE=2.9-184ms, 12 spiral interleaves, 4096 points (16.4 ms duration) (6). T1 was quantified using a Modified Look-Locker Inversion recovery (MOLLI) sequence acquired with two inversion schedules in a 3-5 pattern with 3 pausing heart-beats resulting in 8 images in 11 heart-beats. T2* was utilized as a reference for the identification of hemorrhage; the sequence was a multi-echo gradient-echo acquisition with 8 echos, TE=1.4-12.7ms and TR=14.6 ms. Finally, gadolinium-DTPA (Magnevist, 0.2 mmol/kg) was administered to identify regions of MVO and infarction with early- and late gadolinium enhancement (EGE, LGE) imaging initiated at 1 min and 8 min post injection, respectively. Patients were divided into two groups: a) with MVO (MVO+) and b) without MVO (MVO-). Relaxation parameters were then measured in infarcted, core MVO and remote myocardial territories based on contrast-enhanced images.

Results

MVO was identified on EGE and LGE images in 8 of the 18 patients (44%). In the MVO+ group, low T2* values in the MVO region (26.1±6.3 vs. 34.7±3.2 ms remote) confirmed the presence of hemorrhage (see Fig. 1); the MVO- group was non-hemorrhagic. T2 values were significantly higher in the infarcted tissue in both MVO+ (55.4±6.9 vs. 37.6±1.6 ms remote) and MVO- (50.7±5.2 vs. 37.6±1.5 ms remote) groups, indicative of edematous development. As expected, T2 in the core MVO region was significantly depressed compared to that in the infarct zone (44.9±6.3 ms, p=0.0004) due to interaction with hemorrhage; this value was still higher than remote tissue (p<0.007) given the simultaneous influence from edema (see Fig 2). Interestingly, T1 also demonstrated an identical behaviour. T1 in the infarct zone was driven up by edema in both MVO+ (1156.4±50.8 vs. 996.8±33.7 ms remote) and MVO- (1112.2±80.6 vs. 979.6±35.8 ms remote) groups. However, T1 in the core MVO region was significantly lower than the infarct value (1088.1±71.2, p=0.002) as a result of hemorrhagic byproducts while still being higher than that in remote myocardium due to an effect from edema (see Fig. 3). Figure 4 demonstrates the effect of hemorrhage on T1 maps in a representative patient with MVO. Overall in the MVO+ group, T2 values were 48% and 20% higher while T1 values were 16% and 9% higher in the infarct and MVO regions, respectively, when compared with remote values; this highlights the influence of hemorrhage in the presence of edema.

Conclusion

T1 and T2 relaxation parameters are sensitive to both edema and hemorrhage in clinical AMI. Similar to T2, the capacity of T1 to detect edema is also affected by the counteracting influence from hemorrhage. Our findings suggest that T2 may be more sensitive to edema than T1 in AMI, particularly in the presence of MVO and hemorrhage. The effects of the underlying pathophysiology need to be carefully taken under consideration when interpreting relaxation parameters with respect to their role in predicting clinical outcomes and therapeutic response in AMI.

Acknowledgements

We would like to acknowledge funding support from the Ontario Research Fund, the Canadian Institutes of Health Research and GE Healthcare.

References

1. Yellon DM, Hausenloy DJ, N Engl J Med 2007;357:1121–1135.

2. Ganame J, et al., Eur Heart J 2009;30:1440–1449.

3. Ugander M, et al., JACC Cardiovasc Imaging 2012;5:596–603.

4. Pedersen SF, et al., J. Cardiovasc. Magn. Reson. 2012;14:59.

5. Ghugre NR, et al., Magn. Reson. Med. 2011;66:1129–1141.

6. Zia MI, et al., Circ Cardiovasc Imaging 2012;5:566–572.

Figures

Figure 1: Plot demonstrates T2* in the infarct, MVO and remote myocardial regions in the MVO+ and MVO- groups. Low T2* values in the MVO region is indicative of myocardial hemorrhage.

Figure 2: Plot demonstrates T2 in the infarct, MVO and remote myocardial regions in the MVO+ and MVO- groups. Elevated T2 values in the infarct zone indicate development of edema while low T2 values in the MVO region is indicative of myocardial hemorrhage.

Figure 3: Plot demonstrates T1 in the infarct, MVO and remote myocardial regions in the MVO+ and MVO- groups. Elevated T1 values in the infarct zone indicate development of edema while low T1 values in the MVO region is indicative of myocardial hemorrhage. This demonstrates that T1 is also affected by hemorrhage in the presence of edema, very similar to T2.

Figure 4: Images from a representative patient from the MVO+ group demonstrating the effect of MVO and hemorrhage on T1 maps. (a) Arrows on EGE image show a hypointense core indicative of MVO. (b) With in the region of MVO, low T2* values indicated the presence of hemorrhage (color overlay). (c) T1 map demonstrates regions of elevated T1 representing edema (black arrows) while low T1 values (red arrows) within the region of MVO are suggestive of hemorrhage, that spatially agreed with T2*.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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