Diagnostic MRI in Dogs and Cats

Wilfried Mai, Wilfried Mai

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351647373
• Edition: 1
• Topic: Medicine
• Subtopic: Veterinary Medicine

Diagnostic MRI in Dogs and Cats makes the vast and increasingly complex topic of clinical MRI in small animals accessible to all veterinarians. With the increasing availability of MRI technology, there is also a pressing need for expertise in interpreting these images. This is the first reference textbook to provide a well-illustrated and comprehensive overview of the current knowledge, focusing on imaging appearance rather than on clinical signs or treatment. With chapters on MRI physics and technology as well as sections on specific anatomical regions, the book functions as a stand-alone reference for the reader, whether they be a radiology/neurology resident in training or a practitioner with a need to learn about veterinary clinical MRI.

• Includes both evidenced-based material and the authors' personal experience, providing an excellent overview of current knowledge in the field.
• Contributors are international leaders in the field.
• Bullet points format and table summaries throughout the book keep the concepts concise and organized.
• Richly illustrated with over 650 annotated images showcasing the main features of the disease processes.
• Images are obtained at all magnet field strengths, so as to reflect the current reality of veterinary MRI, which uses low-, mid- and high-field magnets.

The chapters on physics and MRI technology are concise and accessible, using many visual aids and diagrams, and avoiding abstract concepts and equations whenever possible. Within each anatomical section, each chapter focuses on a disease category of that body region. When it is important to understand the imaging appearance, the pathophysiology is reviewed and imaging features of prognostic relevance are detailed.

This practical yet thoroughly comprehensive book is primarily an evidence-based learning resource for trainees, but will also aid practising veterinarians who have less MRI experience.

SECTION 1

PHYSICS AND TECHNICAL CONSIDERATIONS – IMAGE OPTIMIZATION

CHAPTER 1 General principles of magnetic resonance imaging

CHAPTER 2 Image characteristics in MRI and principal pulse sequences

CHAPTER 3 MRI artifacts

CHAPTER 4.1 Optimized technique: brain

CHAPTER 4.2 Optimized technique: spine

CHAPTER 4.3 General features and optimized technique for the musculoskeletal system

CHAPTER 4.4 Technical particularities with low-field imaging

CHAPTER 1

GENERAL PRINCIPLES OF MAGNETIC RESONANCE IMAGING

Wilfried Mai

CONTENTS

Atomic and Nuclear Structure

What is Magnetism?

Spins – Magnetic Moment

Spin Angular Momentum

The Net Magnetization (M₀): Spin-up and Spin-down

Transverse (M_xy) and Longitudinal (M_z) components of the Net Magnetization

Measuring the net magnetization: magnetic resonance

The Return to Equilibrium: Spin-Lattice and Spin-Spin Relaxation

The Longitudinal Relaxation Time: T1

The Transverse Relaxation Time: T2

The Free Induction Decay

T2* Versus T2

A Basic Pulse Sequence: The Spin Echo, or how to Measure the True T2

What About the Repetition Time?

The Mr Image: Field of View, Matrix, Pixel, and Voxel

From image to spatial frequencies: notion of Fourier transform

Spatial Encoding: Slice Selection

Spatial Encoding: Notions of Frequency-Encoding and Phase-Encoding

A Bit More About Phase-Encoding

The Frequency Domain, or k-Space, and its Relationship to the Mr Image

Further reading

It is beyond the scope of this textbook to provide in-depth details regarding the physics of magnetic resonance imaging (MRI). There are a large number of textbooks, scientific articles, and free online resources available for readers who are interested in a more detailed description. However, we will explain the basics and go over the material that is important for radiologists to understand:

• The basic principles of nuclear magnetic resonance and the phenomenon of relaxation (longitudinal and transverse).

• The concepts of image formation, and how they influence image quality such as signal-to-noise ratio, image contrast, and spatial resolution.

• The mechanisms and pros/cons of the essential pulse sequences often used in diagnostic imaging.

• Common image artifacts and prevention/correction.

ATOMIC AND NUCLEAR STRUCTURE

• Atoms are made of a nucleus and orbiting electrons.

• The nucleus itself consists of protons and neutrons.

• The electrons have a negative charge and weigh about 2,000 times less than protons and neutrons; protons have a positive charge while the neutrons have no charge (Table 1.1).

• The number of protons in a nucleus defines the identity of the element: for example, all hydrogen atoms contain 1 proton, all carbon atoms contain 6 protons. This number is called the atomic number, Z.

• The total number of protons and neutrons in an atom is called mass number, A.

• Two atoms with the same number of protons but a different number of neutrons are called isotopes.

• It is customary to represent an element X as ^AX: X defines the element (i.e., its number of protons) and A defines which isotope of the element is considered. For example, ¹³C (six protons) is the isotope of carbon that contains seven neutrons; the most abundant isotope of carbon in nature is ¹²C (six protons and six neutrons).

Table 1.1 Mass and charges of atomic particles.

PARTICLE	MASS (g)	CHARGE
Proton	1.6727 × 10^−24	+1
Neutron	1.6750 × 10^−24	0
Electron	9.110 × 10^−28	−1

WHAT IS MAGNETISM?

• Magnetism and displacement of an electric charge can be related to each other: an electrical current running through a cable generates a magnetic field that one can detect by placing a compass in the vicinity. Conversely, an electrical current can be induced by placing a bar magnet in the center of a solenoid cable.

• Magnetism is a fundamental property of matter, caused by the orbiting electrons at the atomic level. These orbiting electrons can cause atoms to possess a small magnetic field called ‘magnetic moment’.

• In a nucleus, the nucleons (protons and neutrons) rotate around their axis and, given the existence of electric charges, you can already see how the nucleus of an atom also has the potential of intrinsic magnetic properties.

• The units to measure magnetic fields are the Gauss (G) and Tesla (T), the official SI unit being the Tesla. The Tesla is defined as such: a particle carrying a charge of 1 Coulomb and passing through a magnetic field of 1 Tesla at a speed of 1 meter per second perpendicular to said field experiences a force with magnitude 1 Newton.

$1\text{Tesla}\approx 10,000\text{Gauss}$

• The Earth’s magnetic field is about 0.5 Gauss or 5 × 10⁻⁵ Tesla; compare this with the magnetic field strengths used in high-field MRI, typically 1.5 to 3 Tesla. This is about the strength of electromagnets used to pick up cars in junk yards.

• Magnetic properties of materials vary depending on their composition, and in MRI four different types of magnetic properties are typically encountered:

• Ferromagnetism: materials that have a large positive magnetic susceptibility. When they are placed in a magnetic field, the field strength is much stronger inside the material than outside; such materials typically contain iron, nickel, or cobalt, and include magnets and various objects that can be found in veterinary patients such as identification microchips, surgical implants, and ballistic projectiles.

• Paramagnetism: refers to materials with some ions with unpaired electrons such as ions of various metals like Fe (iron), Mg (magnesium), and Gd (gadolinium). The electronic magnetic moment due to the unpaired electrons confers on these ions a positive magnetic susceptibility capable of effecting magnetization of other structures around them. This magnetic susceptibility is much less than that of ferromagnetic materials, but sufficient to cause effects on MR images. This is exploited with some of these ions being used as MRI contrast agents (gadolinium).

• Super-paramagnetism: these are materials made of discrete individual domains of elements that, when in bulk, have ferromagnetic properties. This results in a positive magnetic susceptibility that falls somewhere between that of ferromagnetic and paramagnetic materials; some of these can be used as contrast agents in MRI, such as super-paramagnetic iron particles.

• Diamagnetism: refers to materials that do not possess intrinsic atomic magnetic moment but, when placed in a strong external magnetic field, slightly repel the field, resulting in a negative magnetic susceptibility. Water and most biologic tissues are diamagnetic.

SPINS – MAGNETIC MOMENT

• All fundamental particles (electrons, protons, neutrons) spin around their own axis and, therefore, as a result of electromagnetism, they have an associated magnetic field called ‘magnetic moment’ or ‘magnetic dipole moment’ (Fig. 1.1). This property is called ‘spin’.

• Spin is a fundamental property of nature, like mass or charge. It comes in multiples of ½ and can be positive or negative. Individual unpaired electrons, neutrons, and protons have a spin of +½ or −½.

• Note that neutrons, although devoid of a net charge, do possess a magnetic moment (non-zero spin). This is because at the sub-particle level, they are made (like protons) of quarks, which are electrically charged; the total charge of a neutron’s quarks is zero, but their spatial distribution within the neutron generates a magnetic moment when the neutron spins. The magnetic moment of a neutron is equal to about two-thirds that of a proton.

• Particles with similar spin can pair up, with their magnetic moments facing opposite directions, in the same way that two identical little magnets would. This in turn eliminates the observable m...