Functional cardiac imaging
Ultrafast gradient echo sequences have made MRI the technique of choice for the dynamic study of cardiac motion and cardiac contractile function.
The sequences currently used in the study of cardiac cinetics are of the steady state gradient echo type with balanced gradients. They have the advantage of being very fast, with a high signal-to-noise ratio and T2/T1 contrast clearly differentiating between blood (as a hypersignal), endocardium and epicardium (as an isosignal) and fat (as a hypersignal). These acquisitions are preferably made with retrospective gating to improve temporal resolution.
A comparative analysis of the images in telesystole and telediastole, with an estimate of ventricular volume, gives the value of the ventricular ejection fraction. It is also possible to estimate left ventricle mass and the segmented cinetic parameters.
Real-time cardiac imaging
By combining these sequences with parallel imaging techniques, and reducing the size of the matrix for single-shot acquisition, acquisition time can be shortened by at least 100 ms, resulting in real-time imaging.
This gain in speed comes at the cost of a loss in spatial resolution and number of images (phases) per cardiac cycle (loss of temporal resolution). The new time-correlated parallel acquisition techniques (k-t BLAST, k-t SENSE) show promise for real-time imaging.
The main interest of real-time imaging is their capacity to study cardiac cinetics in arrhythmia patients or those unable to practice adequate breath-hold.
Tagging consists in tattooing the myocardium with a geometrical pattern (lines or grid), using selective spatial presaturation pulses (SPAMM: SPAtial Modulation of Magnetization). Myocardial preparation is conducted prior to a cine imaging sequence, to make an accurate study of myocardial contraction: modifications to the initial tattoo will enable the visualization of inframillimetric deformations.
Analysis of the tattooed images calls for techniques like HARP (HARmonic Phase MRI) that quickly and automatically extract the deformations consecutive to myocardial contraction.
As in ultrasonography the dynamic study of myocardial contraction can be sensitized by “stress” tests, i.e. by bringing coronary reserve capacity into play. The stress test can be provoked by physical exercise (hard to set up during an MRI examination) or via pharmacodynamic action.
The agents used to achieve this are dobutamine or adenosine/dipyridamole. At low doses, dobutamine can be used to study myocardial viability while adenosine/dipyridamole enhances accuracy in studying myocardial perfusion (vasodilatory action).
At high doses, dobutamine increases myocardial oxygen consumption through positive inotropic and chronotropic effects (increased contractility and heart rate) thereby unmasking significant coronary lesions by demonstrating myocardial dyskenesis.
Given the risks of severe ischemia and heart rhythm disorders of high doses of dobutamine, these examinations must be strictly monitored in a specialized setting.