Published online October 6, 2006, 10.1148/rg.e24
RadioGraphics 2006;26:e24
© RSNA, 2006
An Interactive Taxonomy of MR Imaging Sequences1
Gerard E. Boyle, PhD,
Mary Ahern, BRad, DCR,
Jennie Cooke, MSc,
Niall P. Sheehy, MD and
James F. Meaney, MD
1 Departments of Medical Physics and Bioengineering (G.E.B., J.C., N.P.S.) and Diagnostic Imaging (M. A., J.F.M.), St James’s Hospital, James’s Street, Dublin 8, Ireland. Presented as an education exhibit at the 2005 RSNA Annual Meeting. Received May 5, 2006; revision requested July 21; revision received and accepted August 4. J.F.M. is a holder of patents related to moving table MR angiography. All other authors have no financial relationship to disclose.
Address correspondence to G.E.B. (e-mail: gboyle{at}stjames.ie).
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Abstract
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Discerning an underlying structure in the array of magnetic resonance (MR) imaging sequences and acronyms available is a bewildering task. Attempts have been made to present standard taxonomies of MR imaging sequences, primarily on the basis of their underlying physics. Despite this, it is difficult to usefully incorporate given taxonomies into routine clinical knowledge. The links, differences, and similarities among sequences are multidimensional and too complex for tabular presentation on the printed page. The authors present an interactive taxonomy of MR imaging sequences. With this graphical interface, the user can explore the changing relationships among a wide range of sequence types as they are viewed from different perspectives and through different associations, working through a natural learning process.
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Introduction
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The range and complexity of magnetic resonance (MR) imaging sequences available presents the radiology student or clinical user with a difficult learning task. Each sequence has its own physics, characteristics, and typical applications. Sequences do not sit in isolation but are part of a seemingly ever growing evolutionary tree, or "taxonomy." This exhibit allows the user to attain an overview of the range of sequences available while allowing in-depth study of sequence physics. The core of the exhibit is a graphical representation of a sequence taxonomy. The online presentation allows the user to move simply from overview to physics to applications for any sequence in the graphic. The physics pages present animated and interactive demonstrations of the underlying physics. The exhibit is designed to encourage a "nonlinear" approach to the study of MR imaging sequences. The student can enter the taxomomy at any point and work through sequence overviews, physics, and applications in any order. The flexibility of usage and the high level of interactivity provide a learning environoment not possible with a textbook approach.
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Taxonomy of Sequences
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A taxonomy of the major MR imaging sequences in use is presented in the Figure (see link below). The "nodes" in the schematic represent sequences. Each sequence is linked to seqeunces that are a development of or a precursor of that sequence. Clicking on any sequence node leads to a single-page overview of that sequence, detailing its major features and the equivalent acronyms used by several manufacturers. The overview page provides links to detailed physics and application pages for that sequence. The physics pages are highly interactive, and the user is encouraged to make use of this facility.
The concept of a taxomony of sequences presented here is a development of similar taxonomies elsewhere (1,2). Here we present the taxonomy with two starting nodes: one at conventional spin echo, the other at gradient echo. The spin-echo track splits into RARE (rapid acquisition with relaxation enhancement) and inversion recovery (IR) sequences, with RARE leading to the HASTE (Siemens terminology for half-Fourier acquisition single-shot turbo spin echo) sequence and IR leading to the FLAIR (fluid-attenuated inversion recovery) and STIR (short inversion time inversion recovery) sequences. As IR sequences may often make use of a RARE "host," a track links RARE back to IR. The gradient-echo track splits into single-echo spoiled (or incoherent) sequences and unspoiled (or coherent) sequences and the multiecho EPI (echo-planar imaging) sequence. EPI links back to the spin-echo family through the hybrid spin-echo/gradient-echo sequence. Coherent sequences further differentiate into balanced and unbalanced sequences.
Each sequence links to pages that include animations demonstrating the underlying physics. Timing diagrams presented are adaptations from a range of sources (1–6). The mathematics driving these animations is derived from texts describing the physics of these sequences in detail (3,4). Animations and illustrations on balanced SSFP (steady-state free precession) are based primarily on a description of the physics of balanced sequences by Scheffler and Lehnhardt (6). Each sequence also links to images demonstrating the typical applications of each sequence. Images are from a 1.5-T MR imager (Symphony; Siemens Medical Solutions, Erlangen, Germany).
Go to Taxonomy of MR Imaging Sequences
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Footnotes
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Abbreviations: ADC = apparent diffusion coefficient, EPI = echo-planar imaging, FFE = fast field echo, FID = free induction decay, FISP = fast imaging with steady-state precession, FLASH = fast low-angle shot, FOV = field of view, IR = inversion recovery, MIP = maximum-intensity projection, PSIF = mirrored FISP, RARE = rapid acquisition with relaxation enhancement, RF = radiofrequency, SPGR = spoiled gradient-recalled acquisition in the steady state, SSFP = steady-state free precession, TE = echo time, TI = inversion time, TOF = time of flight, TR = repetition time, 2D = two-dimensional, 3D = three-dimensional
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References
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- McRobbie DW, Moore EA, Graves MJ, Prince MR. In: MRI from picture to proton. Cambridge, England: Cambridge University Press, 2003.
- Nitz WR. MR imaging: acronyms and clinical applications. Eur Radiol 1999; 9(5):979–997.[CrossRef][Medline]
- Liang ZP, Lauterbaur PC. In: Principles of magnetic resonance imaging: a signal processing perspective. Hoboken, NJ: Wiley-IEEE Press, 2000.
- Bernstein MA, King KF, Zhou, XJ. In: Handbook of MRI pulse sequences. Burlington, Mass: Elsevier Academic Press, 2004.
- Elster AD, Burdette JH. In: Questions and answers in magnetic resonance imaging. St Louis, Mo: Mosby, 2001.
- Scheffler K, Lehnhardt S. Principles and applications of balanced SSFP techniques. Eur Radiol 2003;13(11):2409–2418.[CrossRef][Medline]
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