MR Imaging Abbreviations ,Definitions.
Radiology, IRM abréviations description de séquences, physique .
MR Imaging Abbreviations, Definitions, and Descriptions: A Review1
Mark A. Brown, PhD and Richard C. Semelka, MD
1 From the Department of Radiology, University of North Carolina Hospitals and School of Medicine, 101 Manning Dr, Chapel Hill, NC 27599-7510 (M.A.B., R.C.S.), and the Siemens Training and Development Center, Cary, NC (M.A.B.). Received September 2, 1998; revision requested October 19; final revision received February 26, 1999; accepted April 5. Address reprint requests to R.C.S. (e-mail: firstname.lastname@example.org).
Index terms: Magnetic resonance (MR), echo planar, **.1214122 • Magnetic resonance (MR), fat suppression, **.121415 • Magnetic resonance (MR), gradient echo, **.121412 • Magnetic resonance (MR), motion correction, **.121419 • Magnetic resonance (MR), pulse sequences • Magnetic resonance (MR), spectroscopy, **.12145 • State-of-art reviews
Magnetic resonance (MR) imaging is a powerful imaging modality that combines excellent soft-tissue contrast and spatial resolution. One of the strengths of MR imaging is the myriad of measurement techniques, known as pulse sequences and parameter modifications, available for use. The variety of MR imaging methods has also introduced complexity, as there exists no standard for technique nomenclature that can be used by manufacturers. Early in the development of MR, basic MR imaging terminology was standardized by the American College of Radiology (1). However, many imaging methods and variations have since been developed. As manufacturers implement new techniques, they develop vendor-specific nomenclature for them. This has made it extremely difficult for the radiologist or technologist to compare results acquired with different MR systems or to ensure that the proper images are obtained.
This review is intended to provide the reader with a brief description of the general categories of MR pulse sequences and measurement techniques as implemented on MR imaging systems from some of the major manufacturers. Our assumption in writing this article is that the reader is familiar with the basic concepts of MR imaging as used on commercially available MR systems. There are many introductory books currently available that describe the fundamental principles of MR imaging in more detail (2–6). We used the applications guides as supplied by the individual manufacturers as the primary references for vendor-specific implementation of the pulse sequences (7–14). This provided a nonproprietary, public, and accurate source of the particular techniques as implemented by the manufacturers. The applications guides that were used provided the most detailed information on pulse sequence definitions, even though they were not necessarily the most recent guides. Our goal is not to critique the particular implementation of a sequence or technique by any specific manufacturer. Rather, this review is an attempt to provide both a general description of some common pulse sequences, modifications, and terminology and a reference to the practicing radiologist or technologist when comparing studies from different MR systems.
PULSE SEQUENCES AND TIMING DIAGRAMS
The heart of an MR measurement is the technique used for data collection, known as a pulse sequence. Pulse sequences are computer programs that control all hardware aspects of the measurement process. Comparisons of hardware activity between different pulse sequences can be easily made through the use of timing diagrams. These are schematic figures often used to illustrate the basic hardware steps that are incorporated into a pulse sequence. Although there may be stylistic differences between diagrams by different authors, the general features are the same for all diagrams. Time during sequence execution is indicated along the horizontal axis. Each line corresponds to a different hardware component. The vertical separation between each line is employed only for visualization. At a minimum, four lines are needed to completely describe any pulse sequence: one for the radio-frequency (RF) transmitter and one for each gradient (indicated as Gx, Gy, Gz, or Gsection, Gphase, Greadout). Additional lines may be added to indicate other activity such as analog-to-digital converter, or ADC, sampling. Activity for a particular component such as a gradient pulse is shown as a deviation above or below the horizontal line. Simultaneous activity from more than one component such as the RF pulse and section-selection gradient is indicated as nonzero activity from both lines at the same horizontal position. Constant amplitude gradient pulses are shown as simple deviations from zero. Gradient amplitudes that change during the measurement such as for phase encoding are represented as hatched regions.
The hardware steps of a pulse sequence are performed in repeated blocks of instructions known as loops. The total time illustrated by a timing diagram is generally the smallest such block, known as the section loop or sequence loop. This is the minimum repeat time required to complete one loop through all hardware steps per the given repeat unit (section, phase-encoding step, acquisition, etc). The order in which the steps are incremented is sequence dependent, but there are three loop structures that are typically used: two-dimensional (2D) multisection, 2D sequential section, and three-dimensional (3D). Two-dimensional multisection (or multislice) is the most common loop structure. Narrow regions of tissue (~2–10 mm) are excited with each excitation pulse, and the measured signal for each image is produced from only this volume of tissue. Each section is excited once per repetition time (TR) before signal averaging or changing the phase-encoding gradient amplitude (Fig 1).
The 2D multisection loop structure provides the most time-efficient method for acquiring information from individual sections in that data from each section are acquired every TR. The measurement time (also referred to as "scan time"), T, is calculated as follows: Tmultisection = TR x (no. of acquisitions) x (no. of phase-encoding steps).
The 2D sequential section looping uses narrow volume excitation and detection like that of the 2D multisection loop but acquires all information from a single section (eg, phase encoding, acquisitions) before advancing to the next section position. This is used for special applications and with very short TR. The measurement time for sequential section looping is Tsequential = TR x (no. of acquisitions) x (no. of phase-encoding steps) x (no. of sections).
The third loop structure is 3D or volume imaging. For 3D imaging, large volumes of tissue (typically 30–200 mm or larger) are excited by each excitation pulse. The excitation volume is subsequently phase encoded in directions perpendicular (phase encoding) and parallel (partitions or section encoding) to the plane of excitation. In contrast to 2D imaging in which there is only a single gradient table in the phase-encoding direction, 3D imaging uses two independent gradient tables in the section and phase-encoding directions (Fig 2). The section encoding of the volume enables thin, contiguous sections with excellent signal-to-noise ratio to be produced on the basis of the signal from the excited volume. The measurement time for volume excitation techniques is T3D = TR x (no. of acquisitions) x (no. of phase-encoding steps) x (no. of 3D partitions).
Techniques for reducing the measurement time for a particular measurement technique involve altering one or more of the measurement parameters that appear in the appropriate equation. There are practical limits to the amount of measurement time reduction possible, since the image contrast-to-noise (signal difference-to-noise) ratio and spatial resolution as well as the measurement time will be affected by the various parameters. For example, reduction of TR increases the amount of T1 saturation contributing to the contrast, as well as decreases the number of sections that can be obtained by using a multisection loop. Reducing the number of acquisitions does not affect the intrinsic signal intensity or contrast on the image, but it does increase the relative amount of noise in each line of data. In addition, for standard imaging techniques, there is a minimum of one acquisition required for each image. The most common parameter that is adjusted to reduce the measurement time is the number of phase-encoding steps. In a similar fashion, the number of partitions for a 3D acquisition may be adjusted.
Reducing the number of phase-encoding steps for the measurement can be done in two ways, both of which maintain the field of view for the image. One approach increases the size of each volume element (voxel) within the image. This is accomplished by acquiring a narrower range of phase-encoding steps while keeping constant the change from one step to the next, acquiring the raw data set in a coarser fashion. The "missing" data are replaced with zeroes prior to reconstruction. The signal-to-noise ratio is increased in each voxel since more tissue is included, but the spatial resolution is decreased. The particular clinical application will dictate the amount of resolution loss that is acceptable. Alternatively, the number of phase-encoding steps can be reduced while maintaining the spatial resolution by keeping the maximum phase-encoding gradient amplitude equal to that of the full matrix. Reducing the number of phase-encoding steps reduces the number of independent measurements, allowing the noise to be a greater percentage of the total result. The raw data matrix will be incomplete and asymmetric, necessitating corrections to be made prior to image reconstruction. These corrections generally make the images more sensitive to motion than is the corresponding full acquisition. This is the basis for the class of techniques known as partial Fourier or partial k-space acquisition approaches (15).
CLASSES OF PULSE SEQUENCES
Two of the most confusing aspects of MR imaging are the large number of pulse sequences that are possible and the different manners in which different vendors have implemented and named the same technique. In spite of this, there are similarities between pulse sequences that make a general categorization possible. First, some form of echo signal is detected. Each measurement will have an operator-selectable parameter, the echo time (TE), defined as the time from the excitation pulse to the echo maximum. The second common feature of pulse sequences is that the basic aspects of spatial localization are used in all imaging sequences currently used in MR imaging. Some form of region-selective excitation is used. Localization of the RF energy to a volume of tissue is accomplished by using frequency selective, narrow bandwidth pulses in conjunction with a gradient called the section-selection gradient. All signals are detected in the presence of a gradient known as the readout or frequency-encoding gradient, which is oriented perpendicular to the section-selection gradient. There will be some form of residual phase advancement or retardation based on the position of the tissue in the third direction, the phase-encoding direction, which is perpendicular to the other two directions. Finally, gradient pulses are usually applied in complementary pairs. This allows any phase variation induced by the gradient to be reversed. For most MR pulse sequences, these complementary pulse pairs are applied in the section-selection and readout directions. The following sections present the major classes of pulse sequences that are currently used in MR imaging and their defining characteristics.
At the end of the text are tables that describe common measurement techniques and acronyms, and the manufacturers that use them. Certain generic terms are used by all manufacturers such as magnetization transfer, or MT. Other terms are used by specific manufacturers to identify certain aspects or features of sequences. In addition, depending on the software of the manufacturer, certain techniques may be identified as separate techniques or as options under more general techniques. Also, no indication is made regarding 2D and 3D acquisition, nor between sequential versus multisection mode. Specific questions about particular sequences should be addressed to the individual manufacturer. Although each table is cited within the following sections, they are grouped together at the end of the text to provide a stand-alone reference document for the reader.
The most common class of pulse sequences is based on detection of a spin or Hahn echo. Spin-echo pulse sequences are characterized by the use of an excitation pulse (often called the alpha  pulse) and one or more 180° pulses following excitation to refocus the transverse magnetization (16). The refocusing pulses generally excite the same volume of tissue as the excitation pulse. Each refocusing pulse produces an echo known as a spin echo. The differences between the types of spin-echo sequences are in the number of refocusing pulses and the number of phase-encoding tables used in the measurement. Table 1 provides a list of the common types of spin-echo pulse sequences that are currently available.