MRI at a Glance

Catherine Westbrook

• Publisher: Wiley-Blackwell
• Year: 2015
• Print ISBN: 9781119053552
• eBook ISBN: 9781119053538
• Edition: 3
• Topic: Medicina
• Subtopic: Tecnologia e forniture mediche

Highly Commended at the British Medical Association Book Awards 2016

MRI at a Glance encapsulates essential MRI physics knowledge. Illustrated in full colour throughout, its concise text explains complex information, to provide the perfect revision aid. It includes topics ranging from magnetism to safety, K space to pulse sequences, and image contrast to artefacts.
This third edition has been fully updated, with revised diagrams and new pedagogy, including 55 key points, tables, scan tips, equations, and learning points. There is also an expanded glossary and new appendices on optimizing image quality, parameters and trade-offs.

A companion website is also available at www.ataglanceseries.com/mri featuring animations, interactive multiple choice questions, and scan tips to improve your own MRI technique.

MRI at a Glance is ideal for student radiographers and MRI technologists, especially those undertaking the American Registry of Radiation Technologist (ARRT) MRI examination, as well as other health professionals involved in MRI.

Magnetism and electromagnetism

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Figure 1.1 Paramagnetic properties.

Figure 1.2 Diamagnetic properties.

Figure 1.3 Ferromagnetic properties.

Figure 1.4 The right-hand thumb rule.

Figure 1.5 A simple electromagnet.

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Magnetic susceptibility

The magnetic susceptibility of a substance is the ability of external magnetic fields to affect the nuclei of a particular atom, and is related to the electron configurations of that atom. The nucleus of an atom, which is surrounded by paired electrons, is more protected from, and unaffected by, the external magnetic field than the nucleus of an atom with unpaired electrons. There are three types of magnetic susceptibility: paramagnetism, diamagnetism and ferromagnetism.

Paramagnetism

Paramagnetic substances contain unpaired electrons within the atom that induce a small magnetic field about themselves known as the magnetic moment. With no external magnetic field, these magnetic moments occur in a random pattern and cancel each other out. In the presence of an external magnetic field, paramagnetic substances align with the direction of the field and so the magnetic moments add together. Paramagnetic substances affect external magnetic fields in a positive way, resulting in a local increase in the magnetic field (Figure 1.1). An example of a paramagnetic substance is oxygen.

Super-paramagnetism

Super-paramagnetic substances have a positive susceptibility that is greater than that exhibited by paramagnetic substances, but less than that of ferromagnetic materials. Examples of a super-paramagnetic substance are iron oxide contrast agents.

Diamagnetism

With no external magnetic field present, diamagnetic substances show no net magnetic moment, as the electron currents caused by their motions add to zero. When an external magnetic field is applied, diamagnetic substances show a small magnetic moment that opposes the applied field. Substances of this type are therefore slightly repelled by the magnetic field and have negative magnetic susceptibilities (Figure 1.2). Examples of diamagnetic substances include water and inert gasses.

Ferromagnetism

When a ferromagnetic substance comes into contact with a magnetic field, the results are strong attraction and alignment. They retain their magnetization even when the external magnetic field has been removed. Ferromagnetic substances remain magnetic, are permanently magnetized and subsequently become permanent magnets. An example of a ferromagnetic substance is iron.

Magnets are bipolar as they have two poles, north and south. The magnetic field exerted by them produces magnetic field lines or lines of force running from the magnetic south to the north poles of the magnet (Figure 1.3). They are called magnetic lines of flux. The number of lines per unit area is called the magnetic flux density. The strength of the magnetic field, expressed by the notation (B) – or, in the case of more than one field, the primary field (B₀) and the secondary field (B1) – is measured in one of three units: gauss (G), kilogauss (kG) and tesla (T). If two magnets are brought close together, there are forces of attraction and repulsion between them depending on the orientation of their poles relative to each other. Like poles repel and opposite poles attract.

Electromagnetism

A magnetic field is generated by a moving charge (electrical current...

Table of contents

• Cover
• QR code
• TitlePage
• Copyright
• Preface
• Acknowledgement
• How to use your textbook
• About the companion website
• 1 Magnetism and electromagnetism
• 2 Atomic structure
• 3 Alignment
• 4 Precession
• 5 Resonance and signal generation
• 6 Contrast mechanisms
• 7 Relaxation mechanisms
• 8 T1 recovery
• 9 T2 decay
• 10 T1 weighting
• 11 T2 weighting
• 12 PD weighting
• 13 Conventional spin echo
• 14 Fast or turbo spin echo – how it works
• 15 Fast or turbo spin echo – how it is used
• 16 Inversion recovery
• 17 Gradient echo – how it works
• 18 Gradient echo – how it is used
• 19 The steady state
• 20 Coherent gradient echo
• 21 Incoherent gradient echo
• 22 Steady-state free precession
• 23 Balanced gradient echo
• 24 Ultrafast sequences
• 25 Diffusion and perfusion imaging
• 26 Functional imaging techniques
• 27 Gradient functions
• 28 Slice selection
• 29 Phase encoding
• 30 Frequency encoding
• 31 K space – what is it?
• 32 K space – how is it filled?
• 33 K space and image quality
• 34 Data acquisition – frequency
• 35 Data acquisition – phase
• 36 Data acquisition – scan time
• 37 K space traversal and pulse sequences
• 38 Alternative K space filling techniques
• 39 Signal to noise ratio
• 40 Contrast to noise ratio
• 41 Spatial resolution
• 42 Chemical shift artefacts
• 43 Phase mismapping
• 44 Aliasing
• 45 Other artefacts
• 46 Flow phenomena
• 47 Time-of-flight MR angiography
• 48 Phase contrast MR angiography
• 49 Contrast-enhanced MR angiography
• 50 Contrast agents
• 51 Magnets
• 52 Radiofrequency coils
• 53 Gradients and other hardware
• 54 MR safety – bio-effects
• 55 MR safety – projectiles
• Appendix 1(a): The results of optimizing image quality
• Appendix 2: Artefacts and their remedies
• Appendix 3: Main manufacturers’ acronyms
• Glossary
• Index
• Eula