Over the past 30 years, cardiovascular magnetic resonance (CMR) imaging has evolved from a technique used to acquire static images of the heart and chest into a versatile imaging modality for assessing many physiologic variables pertinent to the evaluation of patients with cardiovascular (CV) disease. CMR-based methods accurately measure left and right ventricular (LV and RV, respectively) volumes, mass, and function and are able to characterize the presence and extent of myocardial infarction (MI) and viability. Hence, they now serve as “gold standard techniques” for assessing these metrics. Increasingly, CMR enables (1) assessments of myocardial perfusion in the investigation of ischemic heart disease; (2) differentiation of the etiology of nonischemic cardiomyopathies; and (3) characterization of congenital heart syndromes, pericardial disease, and cardiac masses. The purpose of this chapter is to review the use of CMR in these clinical situations. Also, several advantages (lack of ionizing radiation, ability to image in multiple tomographic planes, high spatial, temporal and contrast resolution, and reproducibility),1 as well as disadvantages (primarily need for availability/operator-staff experience, less feasibility in claustrophobic patients, and potential hazards associated with the magnetic field) are discussed.
PHYSICAL PRINCIPLES AND INSTRUMENTATION
For the majority of magnetic resonance imaging (MRI)–related studies, image formulation relies on principles related to signals detected from hydrogen nuclei within the body. In a strong magnetic field, these hydrogen nuclei (or protons) align and “precess” (spin) about the z-axis of the field (along the bore of the scanner). Pulse sequences or small magnetic field pulses alter this precession by delivering nonionizing electromagnetic radiation that transfers energy to (“excites”) protons and tilts them into the x-y plane to a degree determined by the flip angle. The subsequent relaxation of the protons to their original orientation within the magnetic field creates a signal that is detected by a radiofrequency receiver (or coil). Spatial variations in the magnetic field (magnetic gradients) enable identification of the physical location of these proton signals, which can then be processed into an image of their spatial distribution (Fig. 16–1). After excitation, relaxation of protons within the magnetic field is governed by the field strength and the T1 and T2 magnetic times, which are intrinsic properties of the tissue. T2* time refers to T2 after accounting for inhomogeneity of the magnetic field. Quantification (“mapping”) of these times can be performed with appropriate sequences.
Basic operations of the magnetic resonance imaging scanner. A. The protons align parallel or antiparallel to the static magnetic field (Bo), creating a small net magnetization vector. While aligned to the magnetic field, the protons precess at a frequency proportional to the strength of the magnetic field (the Larmor frequency). B. Transmission of radiofrequency (RF) energy. Energy is transmitted to the rotating protons by an RF pulse at the Larmor frequency. RF pulses that result in a flip angle of 90 and 180 degrees are shown (top and bottom, respectively). ...
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