MRI from A to Z: A Definitive Guide for Medical Professionals

by Gary Liney

Overview

From 'AB systems' to 'Zipper artefact' - even for the experienced practitioner in MRI, the plethora of technical terms and acronyms can be daunting and bewildering. This concise but comprehensive guide provides an effective and practical introduction to the full range of this terminology. It will be an invaluable source of reference for all students, trainees and medical professionals working with MRI. More than 800 terms commonly encountered in MR Imaging and Spectroscopy are clearly defined, explained and cross-referenced. Illustrations are used to enhance and explain many of the definitions, and references to further reading will point the reader to more in-depth coverage. As well as being a compendium of terms from A to Z, the volume concludes with a useful collection of appendices, which tabulate many of the key constants, properties and equations of relevance.

Product Details

ISBN-13:	9780521606387
Publisher:	Cambridge University Press
Publication date:	02/17/2005
Pages:	270
Product dimensions:	4.84(w) x 7.28(h) x 0.47(d)

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Cambridge University Press
0521606381 - MRI from A to Z - A Definitive Guide for Medical Professionals - by Gary Liney
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Main glossary

Aa

AB systems

Referring to molecules exhibiting multiply split MRS peaks due to spin-spin interactions. In an AB system, the chemical shift between the spins is of similar magnitude to the splitting constant ( J ). A common example is citrate (abundant in the normal prostate). Citrate consists of two pairs of methylene protons (A and B, see Appendix VI) that are strongly coupled such that:
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where ν_A, ν_B are the resonating frequencies of the two protons. A tall central doublet is split into two smaller peaks either side, which are not usually resolved in vivo at 1.5 tesla. Citrate exhibits strong echo modulation.

See also J-coupling and AX systems.

Reference R. B. Mulkern & J. L. Bowers (1994). Density matrix calculations of AB spectra from multipulse sequences: quantum mechanics meets spectroscopy. Concepts Magn. Reson. 6, 1-23.

Absolute peak area quantification

MR spectroscopy method of using peak area ratios where the denominator is the water peak. The areas are adjusted for differences in relaxation times, and the actual concentration of the metabolite is determined from:
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where [w] is the concentration of water and S₀ are the peak area amplitudes of the metabolite and water signals at equilibrium, i.e. having been corrected for relaxation, which has occurred at the finite time of measurement. The factor 2/n corrects for the number of protons contributing to the signal (here 2 is for water).

Note: [w] is taken as 55.55 Mol/kg.

Reference P. B. Barker, B. J. Soher, S. J. Blackband, J. C. Chatham, V. P. Mathews & R. N. Bryan (1993). Quantitation of proton NMR spectra of the human brain using tissue water as an internal concentration reference. NMR Biomed. 6, 89-94.

Acoustic noise

The audible noise produced by the scanner. Caused by vibrations in the gradient coils induced by the rapidly oscillating currents passing through them in the presence of the main magnetic field. Ear protection must by worn by patients because of this noise. Gradient-intensive sequences, e.g. 3-D GRE, EPI, produce the highest noise levels. Typically, the recorded noise level may be weighted (dB (A) scale) to account for the frequency response of the human ear. Values of 115 dB (A) have been recorded with EPI. The Lorentz force, and therefore noise level, increases with field strength (typically a 6 dB increase from 1.5 to 3.0 tesla). Current methods to combat noise include mounting the gradient coils to the floor to reduce vibrations and lining the bore with a vacuum. More sophisticated measures include active noise reduction.

See also bore liner and vacuum bore.

Reference F. G. Shellock, M. Ziarati, D. Atkinson & D. Y. Chen (1998). Determination of gradient magnetic field-induced acoustic noise associated with the use of echoplanar and three-dimensional fast spin echo techniques. J. Magn. Reson. Imag. 8, 1154.

Acquisition time

Time taken to acquire an MR image. For a spin-echo sequence it is given by:

N_p × N_A × TR

where N_p is the number of phase encoding steps, N_A is the number of signal averages, and TR is the repetition time. Shorter scan times means a trade-off in image quality in terms of resolution (N_p), SNR (N_A) and contrast (TR). Scan times may also be reduced by using parallel imaging.

In gradient-echo sequences with very short TR times, the above equation includes a factor for the number of slices acquired.

Active noise reduction

Advanced method of reducing gradient noise produced from the scanner. Utilises force-balanced coils, which are designed so that the Lorentz forces act in a symmetrical manner to counteract the vibrations. May offer up to 30 dB improvement.

See also acoustic noise.

Reference R. W. Bowtell & P. M. Mansfield (1995). Quiet transverse gradient coils: Lorentz force balancing designs using geometric similitude. Magn. Reson. Med. 34, 494.

Active shielding

Refers to either shielding of the main magnetic field or the gradient coils. The fringe field may be actively shielded using an additional set of coil windings around the main set, with a current of opposite polarity passing through it. An unshielded 7 tesla scanner has a 5 gauss fringe field of 23 m.

See also passive shielding.

Actively shielded gradients are now standard on all systems. This reduces eddy currents in the cryogen and other conducting structures.

Active shimming

Improving the homogeneity of the main magnetic field (the shim) by passing current through additional sets of coils within the scanner to augment the field. Typically, 12 to 18 sets of coils are used which affect the field in each orthogonal direction. A first-order shim changes the field in a linear fashion, a second-order shim produces field changes that vary with the square of distance and so on (higher-order shims). The shim coils themselves may be resistive or superconducting.

See also passive shimming.

ADC

Apparent diffusion coefficient. Refers to the measurable value of diffusion rather than the actual value due to the effects of cell boundaries, etc. The signal attenuation observed in a diffusion-weighted image due to the apparent diffusion coefficient, D, is:
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where b is the gradient factor (see b-factor).

The ADC value for water is approximately 2.0 × 10^-3 mm² s^-1.

Not to be confused with the analogue-to-digital converter (ADC) which digitises the measured MR signal before further processing.

Reference B. Issa (2002). In vivo measurement of the apparent diffusion coefficient in normal and malignant prostatic tissues using echo-planar imaging. J. Magn. Reson. Imag., 16, 196-200.

ADC map

A parameter map in which the pixel intensity is equal to the value of ADC. The map may be obtained from images acquired at several different values of b-factor. Care must be taken in selecting a minimum b value as flow effects dominate at very low b. Alternatively, a two-point method may be used typically acquiring a b = 0 image and a second image at a high b value. ADC maps have proved useful in diagnosing stroke but do not provide any directional information.

See also DTI.

Adiabatic pulse

Specific use of a variable frequency excitation pulse which is swept through the Larmor frequency. These pulses are less sensitive to B₁ inhomogeneities than conventional pulses but take longer to apply. Used in continuous wave NMR.

Agarose gel

Common material used in the construction of phantoms. Its T₂ relaxivity (10 mM^-1 s^-1) is much higher than corresponding values for T₁ (0.05 mM^-1 s^-1), which means T₂ values can be made to vary considerably with little alteration in T₁. The material is often mixed with Gd-DTPA to produce phantoms with a range of T₁ and T₂ values.

See also gel phantom.

Reference M. D. Mitchell, H. L. Kundell, L. Zxel & P. M. Joseph. (1986). Agarose as a tissue equivalent phantom material for NMR imaging. Magn. Reson. Imag., 4, 263-266.

AIF

Arterial input function. This is the signal-time characteristic of the contrast agent bolus in the blood, and may be used to model uptake in other tissues. For best results, an artery near to the site of interest needs to be selected for the appropriate AIF. This function can be deconvolved from tissue or tumour enhancement to quantitate perfusion.

See also dynamic scanning and perfusion imaging.

Alanine

Proton spectroscopy peak with a resonance at 1.48 ppm. It is often seen to increase in meningiomas.

Aliasing

Image artefact caused by anatomy extending beyond the imaging field of view but within the sensitive volume of the RF coil. It results in the offending part of the anatomy being incorrectly mis-mapped onto the opposite side of the image. Frequency oversampling usually ensures aliasing is only possible in the phase direction and can be avoided by swapping the direction of encoding. It is also referred to as wrap and foldover.

See also no phase wrap, frequency wrap and nyquist frequency.

Figure 1. Phase wrap or aliasing results in the top of the hand appearing at the bottom of this image.
Image not available in HTML version

Alignment

Referring to the direction of the net magnetisation vector when it is parallel to B₀, i.e. the situation prior to the first excitation pulse.

Angiogenesis

Phenomenon typical of tumours where new blood vessel growth is induced (mediated by angiogenic factors) to meet the increased oxygen demand required for rapid development. This is utilised in contrast enhanced scanning in cancer, where the preferential uptake of contrast agent by tumours improves its differentiation from normal tissue.

AngioMARK

Commercial name of a blood-pool agent undergoing clinical trials (Epix, Cambridge, MA). Also known as MS-325, it binds to albumin to extend its vascular half-life. The T₁ relaxivity is approximately ten times that of Gd-DTPA.

Anisotropy

Diffusion that is not the same in each direction, i.e. not isotropic. Usually implies some preferred diffusion direction and therefore can be used to elucidate structural information, e.g. white matter fibre tracts in the brain.

See also tensor, tractography and fractional anisotropy.

Anisotropic resolution describes spatial resolution that is not similar in each direction, e.g. in 2-D imaging where slice thickness is much greater than the in-plane resolution.

Anterior

Referring to the front side of the patient anatomy. It is at the top of an axial image and on the left of a sagittal image (see Figure 2).

See also posterior.

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© Cambridge University Press

Table of Contents

Main Glossary (A-Z); Boxed Asides; Figures; Appendices (I - VIII).

Editorial Reviews

Reviewer: Brian F. DeCesare, MD (University of Colorado Health Sciences Center)
Description: This book provides an alphabetical listing of definitions and explanations of various terms and phrases used in magnetic resonance imaging (MRI).
Purpose: The purpose is to provide a quick and easy reference of MRI terminology for a broad spectrum of medical professionals. The book, easy to use and portable, provides concise definitions and explanations of topics ranging from the most basic to most complex. It is useful for professionals who have any contact with MRI. The author provides a quick reference, easy to use with alphabetical listings, and portability, all consistent with his objectives.
Audience: The book is written for medical professionals who have varying degrees of exposure to MRI. The author is a medical physicist, and therefore most credible on MRI topics.
Features: This book covers all the terms and phrases used when working with MRI. The best features include practicality, ease of use, portability (size), and broad coverage of MRI. The book includes some figures, images, and equations. These are particularly helpful, and more should be considered to improve the book.
Assessment: I enjoyed the book because it is easy to use, practical, portable, and covers a wide range of topics. I am much more likely to use this book instead of carrying large textbooks for quick questions regarding MRI terminology.

Doody's Review Service

3 Stars from Doody

From the reviews of the second edition:

“The book is a pocket-friendly manual, easy to consult, that provides a wide range of theoretical and practical information through the analysis of the specific terminology used in MRI. … this is a very practical text, representing an invaluable reference tool for radiologists, physicists, technologists and trainees who work with MRI and, more generally, in diagnostic imaging. The publication may be of fundamental utility in daily clinical practice especially for those who want a quick understanding of key concepts.” (Mariarosaria Prisco and Luigi Mansi, European Journal of Nuclear Medicine and Molecular Imaging, Vol. 40, 2013)

Praise for the 1st edition: "This slim paperback...is written almost in note form with concise, sharp, sentences that pack in a lot of information. It deals with all the MRI terms and common phrases that are heard in and around an MRI practice in alphabetical order. [It is] a handy quick reference guide, a good time-filler while you idle by the control desk waiting for that sequence to finish. Regardless of how long you have worked with MRI, this book will hold gems that you have long since forgotten you once knew, many you have not even wondered about, and others that, although familiar, have become vague...Between the obvious and the obscure there are several entries that will be useful to radiologists, including some nice explanations of common MRI artefacts. There are clear diagrams illustrating common sequences, magnet and coil design, and concepts such as “relaxation”. Usefully this book includes an explanation of all those hideous acronyms so beloved by industry and by those “in the know”. If you thought DIET and TONE were the obsessions of fashion models or have not yet made SENSE of DESSA or GRAPPA, it’s all explained here." A. Gillams, in Clinical Radiology (2005) 60, 1219–1220 "Gary Liney, a renowned MRI physicistis the author of this short, yet comprehensive guide... All terms are explained in short and easily understandable paragraphs. The text is written in a clear and concise form, making it even for the inexperienced reader easy, to understand the full range of physical terms used in MR imaging...The author manages very well to explain this rather complicated field in an easily digestible form, without sacrificing any necessary information. Thus, this pocket-sized book is a truly valuable companion to answer all technical questions regarding MR imaging. In summary, I can fully recommend ‘MRI from A to Z’ by Gary Liney, to any physician dealing with MRI in their daily clinical routine" B. Halpern, in European Journal of Radiology (2005), 56, 270-271

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