Assessment of myocardial perfusion by magnetic resonance imaging

Development of intravascular contrast agents for MRI using gadolinium chelates

Two MRI contrast agents (CAs) composed of Gd-DO3A conjugated to amino acid building blocks derived from glutamic acid (CA1) and lysine (CA2) have been synthesized by using novel alkyne and propionate linkers, and subsequently characterized. In vitro cell viability assays showed insignificant cytotoxicity of both CAs at low concentrations up to 0.2 mM. The longitudinal relaxivities (r(1) ) of CA1 and CA2 measured at 9.4 T are 6.4 and 5.4 mM(-1)  s(-1) in H(2) O at 25 °C, respectively. Both r(1) values are higher than those of CAs in clinical use: Gd-DTPA (Magnevist, Bayer Schering, Germany) and Gd-DOTA (Dotarem, Guerbet, France). In vivo imaging in Wistar rats demonstrated considerable signal enhancement (∼50 %) in the brain artery by CA2, but lower signal enhancement (∼30 %) by CA1. In contrast to Dotarem, which showed a similar signal enhancement as CA2, the enhancement by CA2 remained high (∼30 %), even at 52 min post-injection. This demonstrates that CA2 has a much longer blood half-life (68.1 min), which could be advantageous for angiography and tissue targeting.

Usage of the T1 effect of an iron oxide contrast agent in an animal model to quantify myocardial blood flow by MRI

BACKGROUND

To present a new method for fully quantitative analysis of myocardial blood flow (MBF) using magnetic resonance imaging. The first pass of an intravascular iron oxide contrast medium can be used to quantify myocardial perfusion. The technique was validated in an animal model using colored microspheres.

MATERIALS AND METHODS

In six pigs, a tracking catheter was positioned in the left anterior descending artery (LAD). Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was performed on a 1.5-T scanner using a hybrid gradient-echo/echoplanar imaging (GRE-EPI) sequence. Regional myocardial blood flow (rMBF) was altered by either inducing vasodilatation with adenosine or creating coronary artery obstruction. The T(1) effect of a superparamagnetic iron oxide-based contrast medium (Resovist) administered at a dose of 8 micromol/kg was used. Upslope, time-to-peak and peak intensity were calculated from the signal intensity-time curves and absolute rMBF using the Kety-Schmidt equation; results were compared to those obtained using colored microspheres.

RESULTS

The mean rMBF calculated by MRI was 1.49 (+/-6.91, quartile width) ml/min/g versus 3.21 (+/-1.61) ml/min/g measured by means of microspheres under resting conditions. rMBF increased to a mean of 6.21 (+/-2.83) ml/min/g versus 4.22 (+/-1.70) ml/min/g under adenosine and was reduced to zero flow in total occlusion. Linear regression showed the best correlation for upslope (R=0.714), time-to-peak (R=0.626) and the Kety-Schmidt equation (R=0.584).

CONCLUSIONS

The T(1) effect of an iron oxide-based contrast medium allows determination of rMBF when using the Kety-Schmidt equation. The results are similar to those obtained with the standard of reference, colored micropheres, but not better than the results of the semiquantitative approach.

Assessment of optical path length in tissue using neodymium and water absorptions for application to near-infrared spectroscopy

Quantitative analysis of blood oxygen saturation using near-IR spectroscopy is made difficult by uncertainties in both the absolute value and the wavelength dependence of the optical path length. We introduce a novel means of assessing the wavelength dependence of path length, exploiting the relative intensities of several absorptions exhibited by an exogenous contrast agent (neodymium). Combined with a previously described method that exploits endogenous water absorptions, the described technique estimates the absolute path length at several wavelengths throughout the visible/near-IR range of interest. Isolated rat hearts (n = 11) are perfused separately with Krebs-Henseleit buffer (KHB) and a KHB solution to which neodymium had been added, and visible/near-IR spectra are acquired using an optical probe made up of emission and collection fibers in concentric rings of diameters 1 and 3 mm, respectively. Relative optical path lengths at 520, 580, 679, 740, 800, 870, and 975 nm are 0.41+/-0.13, 0.49+/-0.21, 0.90+/-0.09, 0.94+/-0.01, 1.00, 0.84+/-0.01, and 0.78+/-0.08, respectively. The absolute path length at 975 nm is estimated to be 3.8+/-0.6 mm, based on the intensity of the water absorptions and the known tissue water concentration. These results are strictly valid only for the experimental geometry applied here.

Realignment of myocardial first-pass MR perfusion images using an automatic detection of the heart-lung interface

Magnetic resonance first-pass imaging of a bolus of contrast agent is well adapted to distinguish normal and hypoperfused areas of the myocardium. In most cases, the signal intensity-time curves in user defined regions of interest (ROI) are studied. As image acquisition is ECG-gated, the images are acquired at the same moment in the cardiac cycle, and the basic shape of the heart is similar from one view to the next. However, superficial respiratory motion can displace the heart in the short-axis plane. The aim of this study is to correct for the respiratory motion of the heart by performing an automatic realignment of the myocardial ROI based on a method tracking the movement of the lung-myocardium interface. Visual and quantitative analyses performed on 120 curves show a very good concordance between two automatic methods and the manual one.

Detection of myocardial cell damage in isolated rat hearts with near-infrared spectroscopy

One hallmark of cell death resulting from prolonged ischemia is cell membrane disruption. We apply optical spectroscopy to gauge membrane disruption in isolated rat hearts by monitoring (1) the washout of myoglobin (Mb) and (2) the accumulation of an exogenous contrast agent in permeabilized cells. The contrast agent, a neodymium (Nd) chelate, has several absorptions in the visible and near-IR, and when present in the perfusate, it cannot penetrate cellular membranes. When membrane integrity is disrupted, however, it is expected to accumulate within the intracellular space; moreover, cellular Mb is expected to wash out. To test this hypothesis, rat hearts (n=12) are perfused with Krebs-Henseleit buffer (KHB), followed by perfusion with KHB in which a 5 mM Nd-DTPA solution is present. Membrane damage is then induced by infusion of digitonin into the Nd-KHB perfusate to provide a digitonin concentration of 2.5, 5, or 10 microg/mL. After 30 min of infusion, Mb levels fall to 46+/-14% of baseline levels and Nd-DTPA rises to 161+/-19% of predigitonin levels. No apparent dependence of total membrane disruption on digitonin concentration over the concentration range studied is found, although higher concentrations do lead to more rapid membrane disruption.

Regional variations in myocardial tissue oxygenation mapped by near-infrared spectroscopic imaging

Near-infrared fibre-optic single point spectroscopy has been widely exploited to provide information regarding blood volume and oxygenation in vivo, but it does not provide any information on regional differences in perfusion. We have combined the chemical sensitivity of spectroscopy with the spatial sensitivity of imaging to generate maps of regional cardiac oxygenation. Spectroscopic images were acquired for isolated, arrested, blood-perfused porcine hearts (n=4) over the wavelength range 650 and 1050 nm. Spectroscopic images were acquired during normal perfusion, regional ischemia (occlusion of left anterior descending artery) global ischemia, and reperfusion. Hemoglobin (Hb) and myoglobin (Mb) content and oxygenation were determined by reconstructing the tissue spectra measured at each pixel as weighted sums of water, oxy- and deoxy-Hb (and -Mb) absorptivity spectra. The spectroscopic images acquired during regional ischemia clearly revealed increased deoxy-(Hb+Mb) levels and decreased oxy-(Hb+Mb) levels in the ischaemic regions relative to the normally-perfused regions. Global ischemia produced a dramatic decrease in oxy-(Hb+Mb) levels and a moderate increase in deoxy-(Hb+Mb). These images confirm that blood oxygenation can be mapped in cardiac tissue by near-infrared spectroscopic imaging.

[Methods for coronary functional assessment]

Functional evaluation of coronary vasomotion encompasses the assessment of dynamic changes in coronary lumen, vessel wall, blood flow, intracoronary pressure and myocardial perfusion in response to specific pharmacologic stimuli. These parameters are obtained to characterize mechanisms of physiologic regulation and to evaluate pathophysiologic processes and potential therapeutic strategies, especially with regard to the development of coronary atherosclerosis. To this end, a variety of direct (invasive) and indirect (non-invasive) diagnostic tools are employed. Among the invasive methods are registration of intracoronary Doppler flow, coronary pressure measurements, quantitative coronary angiography and intravascular ultrasound. The non-invasive modalities consist of coronary Doppler echocardiography, positron emission tomography, myocardial scintigraphy and magnetic resonance imaging. Because of the different technical and physiological principles involved, these methods are complementary by providing independent access to different aspects. The combined invasive functional testing as employed in the cardiac catheterization laboratory allows for a simultaneous synopsis of high-resolution coronary imaging and direct measurement of physiologic parameters during local application of defined pharmacologically active substances. However, the demands in terms of equipment, time and operator skills are high and limit this combined invasive approach to specialized centers. Besides these research purposes, a number of functional methods has entered the clinical arena. They are employed to evaluate the hemodynamic significance of coronary lesions and to assess functional outcome of therapeutic interventions in the catheterization laboratory. The underlying principles and applications of the different methods are described and an overview of selected results is presented.