Uptake of MnCl2and mangafodipir trisodium in the myocardium: A magnetic resonance imaging study in pigs

Manganese-Enhanced T1 Mapping in the Myocardium of Normal and Infarcted Hearts

Background

Manganese-enhanced MRI (MEMRI) has the potential to identify viable myocardium and quantify calcium influx and handling. Two distinct manganese contrast media have been developed for clinical application, mangafodipir and EVP1001-1, employing different strategies to mitigate against adverse effects resulting from calcium-channel agonism. Mangafodipir delivers manganese ions as a chelate, and EVP1001-1 coadministers calcium gluconate. Using myocardial T1 mapping, we aimed to explore chelated and nonchelated manganese contrast agents, their mechanism of myocardial uptake, and their application to infarcted hearts.

Methods

T1 mapping was performed in healthy adult male Sprague-Dawley rats using a 7T MRI scanner before and after nonchelated (EVP1001-1 or MnCl2 (22 μmol/kg)) or chelated (mangafodipir (22-44 μmol/kg)) manganese-based contrast media in the presence of calcium channel blockade (diltiazem (100-200 μmol/kg/min)) or sodium chloride (0.9%). A second cohort of rats underwent surgery to induce anterior myocardial infarction by permanent coronary artery ligation or sham surgery. Infarcted rats were imaged with standard gadolinium delayed enhancement MRI (DEMRI) with inversion recovery techniques (DEMRI inversion recovery) as well as DEMRI T1 mapping. A subsequent MEMRI scan was performed 48 h later using either nonchelated or chelated manganese and T1 mapping. Finally, animals were culled at 12 weeks, and infarct size was quantified histologically with Masson's trichrome (MTC).

Results

Both manganese agents induced concentration-dependent shortening of myocardial T1 values. This was greatest with nonchelated manganese, and could be inhibited by 30-43% with calcium-channel blockade. Manganese imaging successfully delineated the area of myocardial infarction. Indeed, irrespective of the manganese agent, there was good agreement between infarct size on MEMRI T1 mapping and histology (bias 1.4%, 95% CI -14.8 to 17.1 P>0.05). In contrast, DEMRI inversion recovery overestimated infarct size (bias 11.4%, 95% CI -9.1 to 31.8 P=0.002), as did DEMRI T1 mapping (bias 8.2%, 95% CI -10.7 to 27.2 P=0.008). Increased manganese uptake was also observed in the remote myocardium, with remote myocardial ∆T1 inversely correlating with left ventricular ejection fraction after myocardial infarction (r=-0.61, P=0.022).

Conclusions

MEMRI causes concentration and calcium channel-dependent myocardial T1 shortening. MEMRI with T1 mapping provides an accurate assessment of infarct size and can also identify changes in calcium handling in the remote myocardium. This technique has potential applications for the assessment of myocardial viability, remodelling, and regeneration.

Monitoring dynamic alterations in calcium homeostasis by T (1)-weighted and T (1)-mapping cardiac manganese-enhanced MRI in a murine myocardial infarction model

Manganese has been used as a T(1)-weighted MRI contrast agent in a variety of applications. Because manganese ions (Mn(2+)) enter viable myocardial cells via voltage-gated Ca(2+) channels, manganese-enhanced MRI is sensitive to the viability and inotropic state of the heart. In spite of the established importance of Ca(2+) regulation in the heart both before and after myocardial injury, monitoring strategies to assess Ca(2+) homeostasis in affected cardiac tissues are limited. This study implements a T(1)-mapping method to obtain quantitative information both dynamically and over a range of MnCl(2) infusion doses. To optimize the current Mn(2+) infusion protocols, we performed both dose-dependent and temporal washout studies. A non-linear relationship between infused MnCl(2) solution dose and increase in left ventricular wall relaxation rate (DeltaR(1)) was observed. Control mice also exhibited significant Mn(2+) clearance over time, with a decrease in DeltaR(1) of approximately 50% occurring in just 2.5 h. The complicated efflux time dependence possibly suggests multiple efflux mechanisms. With the use of the measured relationship between infused Mn(2+) dose, DeltaR(1), and inductively coupled plasma mass spectrometry data analysis provided a means of estimating the absolute heart Mn concentration in vivo. We show that this technique has the sensitivity to observe or monitor potential alterations in Ca(2+) handling in vivo because of the physiological remodeling after myocardial infarction. Left ventricular free wall DeltaR(1) values were significantly lower (P = 0.005) in the adjacent zone, surrounding the injured myocardial tissue, than in healthy tissue. This inferred reduction in Mn concentration can be used to estimate potentially salvageable myocardium in vivo for future treatment or evaluation of disease progression.

Contrast enhancement of manganese-hydroxypropyl-tetraacetic acid, an MR contrast agent with potential for detecting differences in myocardial blood flow

PURPOSE

To determine whether the contrast agent MnHPTA has potential for detecting differences in myocardial blood flow.

MATERIALS AND METHODS

R1 in the myocardium was calculated from MR signal intensity measurements in 18 pigs after intravenous injection of 5, 15, or 25 micromol MnHPTA/kg body weight. Measurements were made in each animal after administration at rest and during dobutamine-induced stress.

RESULTS

A difference of approximately 0.1 sec-1 in the R1 increase between rest and stress still remained 31 minutes after administration of 25 micromol MnHPTA/kg body weight. When two consecutive MnHPTA injections were performed, the second injection induced a lower R1 increase than the corresponding first injection.

CONCLUSION

MnHPTA at a dose of 25 micromol/kg body weight (b.w.) has the potential to detect perfusion differences in myocardium. When two consecutive injections of MnHPTA were administered, the R1 change after the second injection was affected by the earlier administration. Therefore, a protocol including more than one administration is not ideal for this contrast agent.

Dobutamine-induced stress affects intracellular uptake of manganese: a quantitative magnetic resonance imaging study in pigs

PURPOSE

To determine whether there are differences in increase in longitudinal relaxation rate (delta R1) in the myocardium between bolus administration of Mn2+ ions during rest and during dobutamine-induced stress and, additionally, to determine whether there are differences in delta R1 between bolus injection and infusion of Mn2+ ions during dobutamine-induced stress.

MATERIALS AND METHODS

Pigs were divided into three groups with six pigs in each group. All animals received 15 micromol MnCl2/kg of body weight (b.w.) intravenously either as a bolus injection (groups 1 and 2) or as an infusion over one minute (group 3). The animals in groups 2 and 3 were subjected to dobutamine stress before injection of MnCl2, while those in group 1 were not given dobutamine. T1 was quantified in the myocardium and left ventricular blood pool before contrast injection and repeatedly during a one-hour postinjection period.

RESULTS

A significant difference in delta R1 between the groups with and the group without dobutamine stress was noted in the myocardium up to 45 minutes after contrast agent injection. No such significant difference was found between pigs that received the contrast agent as a bolus injection compared to infusion.

CONCLUSION

Dobutamine stress increases uptake of manganese ions in the myocardium. This increase was independent of whether the contrast agent was administered as a bolus injection or as an infusion.