Cardiovascular Magnetic Resonance

A Companion to Braunwald's Heart Disease E-Book

Warren J. Manning, Dudley J. Pennell

• Publisher: Elsevier
• Year: 2018
• ISBN:
  9780323497374
• Edition: 3
• Topic: Medicina
• Subtopic: Cardiología

• Provides state-of-the-art coverage of CMR technologies and guidelines, including basic principles, imaging techniques, ischemic heart disease, right ventricular and congenital heart disease, vascular and pericardium conditions, and functional cardiovascular disease.
• Includes new chapters on non-cardiac pathology, pacemaker safety, economics of CMR, and guidelines as well as new coverage of myocarditis and its diagnosis and assessment of prognosis by cardiovascular magnetic resonance, and the use of PET/CMR imaging of the heart, especially in sarcoidosis.
• Features more than 1, 100 high-quality images representing today's CMR imaging.
• Covers T1, T2 and ECV mapping, as well as T2* imaging in iron overload, which has been shown to save lives in patients with thalassaemia major
• Discusses the cost-effectiveness of CMR.

Section I

Basic Principles of Cardiovascular Magnetic Resonance

Outline

• 1 Basic Principles of Cardiovascular Magnetic Resonance
• 2 Techniques for T1, T2, and Extracellular Volume Mapping
• 3 Cardiovascular Magnetic Resonance Contrast Agents
• 4 Myocardial Perfusion Imaging Theory
• 5 Myocardial Perfusion Cardiovascular Magnetic Resonance Advanced Techniques
• 6 Blood Flow Velocity Assessment
• 7 Use of Navigator Echoes in Cardiovascular Magnetic Resonance and Factors Affecting Their Implementation
• 8 Cardiovascular Magnetic Resonance Assessment of Myocardial Oxygenation
• 9 Cardiac Magnetic Resonance Spectroscopy
• 10 Special Considerations for Cardiovascular Magnetic Resonance Safety, Electrocardiographic Setup, Monitoring, and Contraindications
• 11 Pacemaker and Implantable Cardioverter-Defibrillator Safety and Safe Scanning
• 12 Special Considerations Cardiovascular Magnetic Resonance in Infants and Children
• 13 Human Cardiac Magnetic Resonance at Ultrahigh Fields Technical Innovations, Early Clinical Applications and Opportunities for Discoveries
• 14 Clinical Cardiovascular Magnetic Resonance Imaging Techniques
• 15 Normal Cardiac Anatomy

1

Basic Principles of Cardiovascular Magnetic Resonance

Robert S. Balaban, Dana C. Peters

Introduction

This introduction to the basic principles of cardiovascular magnetic resonance (CMR) describes the concepts of magnetization, T1, T2, T2*, and image formation, and describes some common CMR pulse sequences and parameters. These basic ideas will require some time and rereading to fully appreciate, but each new concept will add to the understanding of CMR.

The human body is composed of mostly water, and a lot of fat. The water (H₂O) and fat contain many hydrogen atoms. Hydrogen atoms in turn are made up of a proton (¹H, the hydrogen nucleus) and an electron. Hydrogen protons are in very high concentration in the body, roughly ~100 molar.^a These hydrogen protons within the body can be imaged through magnetic resonance imaging (MRI). MRI is possible because hydrogen protons have “spin” and all nuclei with spin interact with magnetic fields. Spin is called the “intrinsic angular momentum,” and it is a fundamental property of protons. MRI physics is often called “spin physics” because it describes how the proton spins are used to image the body. In this chapter we will use the term spin to indicate the hydrogen proton's spin. MRI images these spins, and in this way it images the tissues in which they are embedded.

In the absence of a magnetic field, the spins are randomly oriented (Fig. 1.1A). But if placed in a large magnetic field (called B₀; i.e., the CMR scanner), the water spins partly align in the same orientation of this applied magnetic field much like iron filings (Fig. 1.1B), with larger magnetic fields causing greater alignment of the spins. In fact, the bulk magnetization, M_z, is proportional to the strength of the applied field. Unlike iron filings, however, the interaction of the spins with the B₀ field results in the spins rotating around the axis of the magnetic field (Fig. 1.1C). This is why the hydrogen nuclei in MRI are also referred to as spins: they spin (precess) around the B₀ field. The frequency of precession (v) is a very important property of the spins in a magnetic field and is defined by the Larmor equation:

FIG. 1.1 Orientation of molecular spins. (A) With no magnetic field, the spins are oriented randomly, producing no net magnetization. (B) The spins orient with an applied magnetic field (B₀), producing a net magnetic field aligned along B₀. (C) The magnetized spins rotate around the B₀ field (the Z axis) in a random way, resulting in no net magnetization in the X-Y plane.

$$\nu =\frac{\gamma }{2\pi }B$$

Eq. 1.1

where B is the magnetic field strength experienced by the spin, which is approximately B₀, but includes any variations as described below, v is the spin's precessional frequency in cycles/s or hertz (Hz), and γ (gamma) is the gyromagnetic ratio, which is a constant related to the mass and charge of the water proton, γ/2π = 42.58 × 10⁶ Hz Tesla for water protons. The precessional frequency for water protons at 1.5 Tesla (T; a common CMR field strength) is ν = 63.87 × 10⁶ Hz, or roughly 64 MHz (about the frequency of an FM radio station and one of the reasons the CMR environment must be shielded from FM radio waves). Thus, at 3 T, the precessional frequency is twice this, ν = 127.74 × 10⁶ Hz (Eq. 1.1).

The Larmor equation (see Eq. 1.1) looks simple, but it is the basis of CMR imaging. To determine the location of different magnetic spins, the magnetic field (B) is made to vary (linearly) with position using specialized coils of wire (called “gradient coils”) inside the magnet. This results in a precessional frequency (v) that depends on a spin's location in the scanner. By measuring the number of spins precessing at each location (i.e., frequency), a magnetic resonance (MR) image is created¹ as will be discussed below.

Detection of the MRI Signal

Alignment With the Main Magnetic Field

How can the MRI signal from water spins be detected in order to image the sample? Remember, when a sample (i.e., a patient) is placed in the main magnetic field (i.e., inside the bore of the magnet), the hydrogen spins align with that field (see Fig. 1.1B) and begin to precess at the Larmor frequency. We call the direction of the main magnetic field, B₀, the Z-axis direction or the longitudinal direction. Since the spins partially align with the B₀ field, they create a net magnetic field along the Z axis (oriented parallel with B₀). However, the spins precess or rotate around the Z axis in a random, incoherent fashion, resulting in no net magnetization in the X-Y plane (also called the transverse plane) (see Fig. 1.1C). Magnetization by the B₀ field (i.e., M_z) is not enough to generate an MR signal. The MR signal is measured as the magnetization on the X-Y plane (called M_xy). The MR signal is detected by placing a receiver coil (loops of wire tuned to receive signals oscillating at the Larmor frequency) near the subject to detect any spin precession in the X-Y plane. A receiver coil, placed to detect the precession of spins in the X-Y plane, will not detect any signal if M_xy is zero (see Fig. 1.1C).

Radiofrequency Excitation

To create an MR signal, the water spins must be precessing in a coherent manner in the X-Y plane. To accomplish this task, another (less powerful) magnetic field, with a strength that oscillates at the Larmor frequency is applied perpendicular to the main magnetic field (Fig. 1.2A). This process is called radiofrequency (RF) excitation, and the field is called an RF pulse or a B₁ field, since it uses a magnetic (B₁) field rotating at a high (radio wave) frequency. The frequency of this perpendicular B₁ field has to precisely match the Larmor frequency of the water to excite the spins in the presence of the stronger B₀ field. (This concept is analogous to exciting a piano string with a tuning fork. Only the string with the same resonant frequency as the tuning fork will efficiently absorb the energy from the fork and resonate. In fact, magnetic resonance imaging gets its name from the resonant frequency used for RF excitation.) The excitation field (B₁) has a strength of only a few gauss, compared to the B₀ field, which has a strength of 15,000 gauss at 1.5 T (1 T = 10,000 gauss). In addition, the B₁ field is only applied transiently (for milliseconds), long enough to temporarily deflect the spins into a plane perpendicular to B₀. This effect on the net magnetic field is illustrated in Fig. 1.2A, which shows a B₁ pulse that drives the magnetization completely into the transverse plane (X-Y plane). This is called a 90-degree pulse (or saturation pulse), referring to the angle through which the initial magnetization (M_z) moves relative to the Z axis. However, in practice, the B₁ pulse can be used to flip the magnetization by any angle from 0 to 180 degrees. A small B₁ pulse (<90 degrees) is called an alpha pulse, while a 180-degree pulse is called an inversion pulse. A 180-degree flip is achieved by applying the B₁ field for twice as long or with tw...