Cardiovascular Magnetic Resonance

Abstract

Cardiovascular magnetic resonance (CMR) creates images from atomic nuclei with uneven spin using radiowaves in the presence of a magnetic field. Full details of the physical principles can be found elsewhere.1 For clinical purposes, MR is performed using hydrogen-1, which is abundant in water and fat. Radiofrequency waves excite the area of interest to create tissue magnetization that decays (relaxation) and after a short period is induced to release energy as a radio signal. These echoes are converted with Fourier transformation into images of spatially resolved radio signals. Relaxation is quantified in spatially orthogonal directions as T1 and T2, which allows tissue characterization to serve as a powerful clinical tool. A CMR scanner consists of a superconducting magnet, a radiofrequency transmitter and receiver connected to radio aerials, and gradient coils driven by powerful pulses of electricity to create transient magnetic fields. The imaging computer triggers to the ECG and runs scanning sequences that coordinate the complex processes. The spin-echo sequence yields static images with black blood, whereas the gradient-echo sequence usually acquires multiple images through the cardiac cycle that display cardiovascular function as cine loops. Preparation pulses can be used to display T1 or T2 contrast for tissue characterization. Fast and real-time sequences include fast low-angle shot, steady-state free-precession, spiral, and echo-planar imaging. Velocity and flow can be quantified with gradient-echo sequences with encoding of velocity in the signal phase.2 CMR is extremely safe, which is unequivocally advantageous compared with x-rays, but special cautions apply. Ferromagnetic items become potentially dangerous projectiles in the scanner room. Although most metallic medical implants, including all prosthetic cardiac valves and coronary stents, are MR compatible, care is required with cerebrovascular clips. Pacemakers and defibrillators present problems, in particular related to wire heating, and are a contraindication for CMR, although specialist centers have …