Combined high-speed NMR imaging of perfusion and microscopic coronary conductance vessels in the isolated rat heart.

Noninvasive characterization of microcirculation at the level of both coronary conductance and resistance vessels is of major importance for the understanding of microvascular adaptive processes in the heart. The objective of this study was to determine simultaneously myocardial perfusion and microvessel diameters in the myocardium by magnetic resonance (MR) imaging within the same heart. A MR imaging method is presented which combines high-resolution perfusion measurement (140 x 140 microm2) by spin labeling with flow-weighted MR microscopy of coronary microvessels (phi > 140 microm). We determined changes in myocardial perfusion and vessel diameters of isolated beating rat hearts (n = 10) at rest and during administration of nitroglycerin (0.5 mg/min). Alterations in perfusion were validated by microsphere measurements. Under the influence of nitroglycerin an increase in perfusion (+2.51 +/- 0.4 ml x min(-1) x g(-1), mean +/- SEM) and vessel diameters (+14.22 +/- 1.92%) could be observed. Endocardial perfusion revealed a modest enhanced susceptibility to nitroglycerin in comparison to epicardial perfusion. Analysis of vessels according to their diameters showed no significant differences. MR imaging allows the noninvasive and simultaneous determination of conducting arteries and smaller resistance vessels in one and the same beating rat heart. Due to an excellent spatial resolution of these methods, transmural characterization of both parameters at rest and during vasodilation is feasible.

Myocardial perfusion imaging using a non-contrast agent MR imaging technique.

INTRODUCTION: A MR imaging (MRI) method has been developed to determine quantitatively myocardial perfusion (P) in the rat heart in vivo. This method has the potential to non-invasively measure cardiac perfusion without the use of a contrast agent by exploiting the endogenous contrast from flowing blood itself. METHOD AND RESULTS: Principle of the technique is the arterial spin labeling of endogenous water protons within the short axis imaging slice. Arterial spin labeling techniques are based on a model that uses inflow effects to relate intrinsic changes in longitudinal relaxation (T1) to tissue perfusion. Perfusion is determined from the difference between a slice selective and a global inversion recovery experiment. Perfusion was determined at rest and during hyperemia induced by intravenous adenosine (3 mg/(kg min)). The MR perfusion values were compared with perfusion data obtained in the same animal using the colored microspheres (MS) technique as the gold standard. The MR perfusion (mean +/- SEM) was 3.3 +/- 0.2 ml/min/g at rest and 4.6 +/- 0.6 ml/min/g during adenosine. Perfusion values obtained by colored MS were 3.4 +/- 0.2 and 4.7 +/- 0.8 ml/min/g at rest and during vasodilation, respectively. Adenosine decreased mean arterial pressure (MAP) from 120 to 65 mmHg which implies a reduction of coronary resistance (CR) to about 50% of baseline. CONCLUSION: Our study shows that quantitative mapping of perfusion may be performed non-invasively by MRI. The MR perfusion data are in excellent correlation with data obtained by the well-established colored MS technique. Determination of perfusion reserve confirms that coronary perfusion is highly dependent on blood pressure due to changes in CR.

Serial magnetic resonance imaging of microvascular remodeling in the infarcted rat heart.

BACKGROUND: Alterations in the coronary circulation are important determinants of myocardial function. Few data are available, however, about microvascular changes in reactive hypertrophy. With MRI, serial determination of myocardial microcirculation after myocardial infarction (MI) is feasible. METHODS AND RESULTS: We quantitatively determined myocardial perfusion and relative intracapillary blood volume using an MRI technique. Infarct size, myocardial mass, and left ventricular volumes were determined with cine MRI. Rats were investigated at 8, 12, and 16 weeks after MI (mean MI size 24.1+/-2.0%) or sham operation. Vasodilation was induced by adenosine. In the infarcted group, maximum perfusion decreased significantly from 8 to 16 weeks (5.6+/-0.3 versus 3.5+/-0.2 mL. g(-1). min(-1), P<0.01) compared with sham animals (5.5+/-0.3 versus 5.0+/-0.2 mL. g(-1). min(-1), P=0.17). Myocardial mass increased significantly (559.1+/-20.8 mg at 8 weeks versus 690.9+/-42.7 mg at 16 weeks, P<0.05) compared with sham-operated animals (516.3+/-41.7 versus 549.2+/-32.3 mg). Basal relative intracapillary blood volume increased significantly to 15.7+/-0.5 vol% at 8 weeks after MI and remained elevated (16.8+/-0.6 vol%) at 16 weeks compared with 12.1+/-0.3 vol% (P<0.01) in sham-operated rats. CONCLUSIONS: Our results indicate that significant microvascular changes occur during cardiac remodeling. Hypoperfusion in the hypertrophied myocardium is related to an increase in vascular capacity, suggesting a compensatory vasodilatory response at the capillary level. These microvascular changes may therefore contribute to the development of heart failure.

Surface plasmon dynamics in silver nanoparticles studied by femtosecond time-resolved photoemission.

Multiphoton photoelectron spectroscopy reveals the multiple excitation of the surface plasmon in silver nanoparticles on graphite. Resonant excitation of the surface plasmon with 400 nm femtosecond radiation allows one to distinguish between photoemission from the nanoparticles and the substrate. Two different previously unobserved decay channels of the collective excitation have been identified, namely, decay into one or several single-particle excitations.

Myocardial perfusion and intracapillary blood volume in rats at rest and with coronary dilatation: MR imaging in vivo with use of a spin-labeling technique.

PURPOSE: To validate a magnetic resonance (MR) imaging technique that is not first pass and that reveals perfusion and regional blood volume (RBV) in the intact rat. MATERIALS AND METHODS: Measurement of perfusion was based on the perfusion-sensitive T1 relaxation after magnetic spin labeling of water protons. RBV was determined from steady-state measurements of T1 before and after administration of an intravascular contrast agent. The colored microsphere technique was used as a reference method for perfusion measurement. RBV and perfusion maps were obtained with the rats at rest and during administration of 3 mg of adenosine phosphate per kilogram of body weight per minute. RESULTS: At MR imaging, perfusion during resting conditions was 3.5 mL/g/min +/- 0.1 (SEM), and RBV was 11.6% +/- 0.6 (SEM). Adenosine phosphate significantly increased perfusion to 4.5 mL/g/min +/- 0.3 (SEM) and decreased mean arterial pressure from 120 mm Hg to 65 mm Hg, which implies a reduction of coronary resistance to 40% of baseline. RBV increased consistently to 23.8% +/- 0.6 (SEM). CONCLUSION: The study results show that quantitative mapping of perfusion and RBV may be performed noninvasively by means of MR imaging in the intact animal. The presented method allows determination of vasodilative and perfusion reserve, which reflects the in vivo regulation of coronary microcirculation for a given stimulus.

Perfusion-corrected mapping of cardiac regional blood volume in rats in vivo.

Measurement of regional blood volume (RBV) in the myocardium in vivo is important for the assessment of tissue viability and function. The method in this work is based on the acquisition of a T(1) map before and after intravascular contrast agent application. It is known that this method is influenced by perfusion that causes an overestimation of RBV values. In order to solve this problem, the new method is proposed which acquires T(1) maps with slice selective inversion pulses. Due to blood flow nonexcited spins enter the detection slice, which leads to an acceleration of the relaxation time. A model that divides tissue into two compartments is adapted to slice selective inversion in order to derive a simple expression for perfusion-corrected RBV. The aim of the study is to demonstrate the feasibility and accuracy of this technique for quantification of RBV in rat myocardium in vivo. RBV maps were obtained for five rats, and the reproducibility was determined by repeating the experiment several times. A mean RBV value of 12.8 +/- 0.7% (v/v) over all animals was obtained in the myocardium. The results were compared with RBV maps obtained with perfusion-sensitive RBV imaging in the same five rats and with first-pass RBV studies. In order to demonstrate the strength of the new method the vasodilator adenosine was administered and alterations in microcirculation were imaged. Magn Reson Med 42:500-506, 1999.

In vivo quantitative mapping of cardiac perfusion in rats using a noninvasive MR spin-labeling method.

Measurement of myocardial perfusion is important for the functional assessment of heart in vivo. Our approach is based on the modification of the longitudinal relaxation time T1 induced by magnetic spin labeling of endogenous water protons. Labeling is performed by selectively inverting the magnetization within the detection slice, and longitudinal relaxation is measured using a fast gradient echo MRI technique. As a result of blood flow, nonexcited spins enter the detection slice, which leads to an acceleration of the relaxation rate. Incorporating this phenomenon in a mathematical model that describes tissue as two compartments yields a simple expression that allows the quantification of perfusion from a slice-selective and a global inversion recovery experiment. This model takes into account the difference between T1 in blood and T1 in tissue. Our purpose was to evaluate the feasibility and reproducibility of this technique to map quantitatively myocardial perfusion in vivo in rats. Quantitative maps of myocardial blood flow were obtained from nine rats, and the reproducibility of the technique was evaluated by repeating the whole perfusion experiment four times. Evaluation of regions of interest within the myocardium yielded a mean perfusion value of 3.6 +/- .5 ml x min(-1) x g(-1) over all animals, which is in good agreement with previously reported literature values.

Quantitative regional blood volume studies in rat myocardium in vivo.

Many pathophysiological processes in the myocardium are in close relation to changes of the regional blood volume and regional myocardial blood flow or perfusion. Only few methods exist to obtain quantitative values for these parameters. Quantitative regional blood volume (RBV) studies in rat myocardium are presented using snapshot fast low angle shot (FLASH) inversion recovery T1 measurements with two different blood pool contrast agents, gadolinium diethylenetriaminopentaacetic acid (Gd-DTPA) albumin and Gd-DTPA polylysine. In contrast to previous attempts, each snapshot FLASH image acquisition was ECG-triggered under breathhold conditions. To measure relaxation times shorter than a heart cycle, each T1 sequence was repeated two times with different delays between inversion pulse and first image acquisition. The experiments were performed on a Bruker Biospec 70/21 using a homogeneous transmitter coil and a circularly polarized surface receiver coil, a special ECG trigger unit, and a respirator that is controlled by the pulse program. Based on a fast exchange model RBVm maps were calculated from the relaxation time maps for different concentrations of the two blood pool contrast agents. A significant dependence of the RBVm values on blood T1 was found. This is in accordance with a model that has been developed recently relating the dependence of RBVm on T1 of blood to perfusion. For Gd-DTPA albumin, the application of the model to the experimental data yields realistic values for RBV and perfusion. The values, which are in accordance with literature data, were obtained at highest contrast agent concentrations i.e., lowest relaxation times of blood (ca. 200 ms).

Magnetic resonance microimaging for noninvasive quantification of myocardial function and mass in the mouse.

The purpose of this work was to develop high-resolution cardiac magnetic resonance imaging techniques for the in vivo mouse model for quantification of myocardial function and mass. Eight male mice were investigated on a 7-Tesla MRI scanner. High-quality images in multiple short axis slices (in-plane resolution 117 microm2, slice thickness 1 mm) were acquired with an ECG-gated cine sequence. Left ventricular end-diastolic and end-systolic volumes and mass were calculated from segmented slice volumes. There was precise agreement of left ventricular mass determined ex vivo and by MRI. Intraobserver (5%) and interobserver (5%) variability of in vivo MR measurements were low.

Study of microcirculation by coloured microspheres and NMR-microscopy in isolated rat heart: effect of ischaemia, endothelin-1 and endothelin-1 antagonist BQ 610.

Although the investigation of coronary microcirculation is of great importance, available methods have severe restrictions. They do not allow the study of vasodynamics of resistance vessels and microscopic conductance vessels simultaneously in the isolated beating rat heart. We now demonstrate that the combined measurement of perfusion which reflects the state of resistance vessels and cross-sections of microscopic conductance vessels is feasible in the model of the isolated constant flow perfused rat heart. Perfusion measurement was based on injection of coloured microspheres. Cross-sections of microscopic conductance vessels (diameter >140 micron) were determined by NMR-microscopy by flow weighted imaging. Both methods were established recently by our group. The combined measurement was applied to hearts which were subjected to ischaemia and reperfusion (group 1: n=5, 15 min ischaemia/group 2: n=7, 30 min ischaemia/measurements before ischaemia and 15/30 min after reperfusion), 200 pmol endothelin-1 bolus application (group 3: n=6/measurements before and 5 min after drug application), continuous infusion of the endothelin-1 antagonist BQ 610 (group 4: n=6/measurements before and 20 min after onset of infusion), and 200 pmol endothelin-1 application superimposed on 20 min of continuous BQ 610 infusion (group 5: n=7/combined measurement before BQ 610 infusion and 5 min after endothelin-1 application). In group 1, 15 min reperfusion restored the pre-ischaemic perfusion state, whereas conductance vessels were dilated (80.8+/-2.6%), after 30 min reperfusion pre-ischaemic conditions were also restored for conductance vessels. In group 2, a redistribution of perfusion from left ventricular endocardium to the right ventricular wall was observed. Post-ischaemic rhythm disturbances made NMR-imaging in this group impossible. In group 3, a shift of perfusion from the left ventricular myocardium to the right ventricular wall was observed. Similarly, the cross-section of left ventricular conductance vessels decreased (-32.6+/-2.1%), whereas size of right ventricular vessels increased. In group 4, BQ 610 had no effect on perfusion nor on vessel size and antagonized the effect of endothelin-1 on perfusion and vessel size in group 5.

In vivo colored microspheres in the isolated rat heart for use in NMR.

Myocardial perfusion measurement with colored microspheres may become an alternative for radioactive microsphere techniques. We use and validate a spectrophotometric method that has been previously established for large animals in the isolated perfused rat heart. The perfusion system was adapted for use in a NMR microscope. Hearts were perfused with constant coronary flow that was adjusted to a coronary perfusion pressure of 100 mmHg. Homogeneous coronary inflow of microspheres was represented by equal distribution of microspheres of two different colors after simultaneous injection. Mean regional myocardial blood flow was 17.76 +/- 5.01 ml/min/g, mean wet heart weight was 1.13 +/- 0.34 g and mean global flow was 20.06 +/- 0.60 ml/min. Heart rate was 296 +/- 8.9 beats/min and left ventricular pressure was similar 5 min before (149.1 +/- 14.27 mmHg) and after (147.1 +/- 13.49 mmHg) microsphere injection. Microspheres of four colors that were injected sequentially, at various coronary flows, demonstrated linearity and reproducibility of the technique. A cumulative use of less than 90 000 microspheres showed no effect on hemodynamics especially on left ventricular pressure.