Cardiovascular magnetic resonance physics for clinicians: part II

Abstract

This is the second of two reviews that is intended to cover the essential aspects of cardiovascular magnetic resonance (CMR) physics in a way that is understandable and relevant to clinicians using CMR in their daily practice. Starting with the basic pulse sequences and contrast mechanisms described in part I, it briefly discusses further approaches to accelerate image acquisition. It then continues by showing in detail how the contrast behaviour of black blood fast spin echo and bright blood cine gradient echo techniques can be modified by adding rf preparation pulses to derive a number of more specialised pulse sequences. The simplest examples described include T2-weighted oedema imaging, fat suppression and myocardial tagging cine pulse sequences. Two further important derivatives of the gradient echo pulse sequence, obtained by adding preparation pulses, are used in combination with the administration of a gadolinium-based contrast agent for myocardial perfusion imaging and the assessment of myocardial tissue viability using a late gadolinium enhancement (LGE) technique. These two imaging techniques are discussed in more detail, outlining the basic principles of each pulse sequence, the practical steps required to achieve the best results in a clinical setting and, in the case of perfusion, explaining some of the factors that influence current approaches to perfusion image analysis. The key principles of contrast-enhanced magnetic resonance angiography (CE-MRA) are also explained in detail, especially focusing on timing of the acquisition following contrast agent bolus administration, and current approaches to achieving time resolved MRA. Alternative MRA techniques that do not require the use of an endogenous contrast agent are summarised, and the specialised pulse sequence used to image the coronary arteries, using respiratory navigator gating, is described in detail. The article concludes by explaining the principle behind phase contrast imaging techniques which create images that represent the phase of the MR signal rather than the magnitude. It is shown how this principle can be used to generate velocity maps by designing gradient waveforms that give rise to a relative phase change that is proportional to velocity. Choice of velocity encoding range and key pitfalls in the use of this technique are discussed.

The Role of Left Atrial Imaging in the Management of Atrial Fibrillation

“…Following restoration of longitudinal magnetization of the protons, tissue-specific T 1 and T 2 relaxation properties can be determined. 18,19 The T 1 relaxation time reflects the time decay constant for 63% recovery of the longitudinal magnetization equilibrium value of a proton, while T 2 relaxation time represents the decay of the transverse magnetization signal to 37% of its original value. Different cardiac tissues exhibit varying relaxation times depending on the molecular environment of water molecules, and these qualities are used to construct pixel-based images.…”

“…Reproduced with permission from Journal of Cardiovasc Electrophysiol. 2011;22:[16][17][18][19][20][21][22].…”

Cardiovascular magnetic resonance imaging: what the general cardiologist should know

“…These are used in phase-contrast GRE sequences, also called velocity-encoded sequences, which are used to quantify blood flow velocity. A velocity image is by generated, known as a phase map, in which pixel intensity depends upon the phase of the transverse magnetisation, rather than its magnitude 19. Pixels are displayed as either dark (moving away from the phase-encoding direction), bright (towards) or mid-grey (stationary).…”

Blood flow specific assessment of ventricular function: Visualization and quantification using 4D flow CMR

“…As B 0 often is not completely homogenous, phase offsets will occur during measurement. By acquiring a reference image without the gradients and subtracting the reference image from the velocity map, these background offsets can be removed (121).…”

“…The velocity encoding (VENC) is given in cm/s, meaning that a VENC of 100 cm/s describes a measurable range of flow velocities between +100 and -100 cm/s. If velocities exceed this level (VENC is too low), aliasing occurs and conversely, if VENC is set too high, noise and lowered velocity-to-noise ratio (the ability to discern real velocity from background noise, VNR) occurs (121).…”