Single-Shot Pulse Sequences 10

O. Dietrich, PhD

Department of Clinical Radiology, University Hospitals – Grosshadern, Ludwig Maximilian University of Munich, Marchioninistr. 15, 81377 Munich, Germany

C O N T E N T S

10.1 Introduction 119

10.2 Single-Shot Spin-Echo Techniques:

RARE and HASTE 120 10.2.1 General Properties 120 10.2.2 Parallel Imaging 121

10.2.3 Acquisition of Coil Sensitivity Information 122 10.3 Single-Shot Gradient-Echo Techniques: EPI 121 10.3.1 General Properties 121

10.3.2 Parallel Imaging 122

10.3.3 Acquisition of Coil Sensitivity Information 122 10.4 Conclusion 123

References 124

10.1

Introduction

Single-shot MRI pulse sequences acquire the com- plete k-space data required for image reconstruc- tion after a single RF excitation. The most important single-shot techniques are single-shot gradient-echo techniques known as echo-planar imaging (EPI) and single-shot spin-echo techniques; the former were already suggested by Mansfi eld in 1977. In EPI a large number of gradient echoes are rapidly acquired either immediately following the excitation (called free-induction-decay EPI) or after one or several refo- cusing RF pulses (spin-echo EPI or stimulated-echo EPI). Single-shot spin-echo techniques are based on an echo train of multiply refocused spin echoes, i.e.

the spins are refocused between successive echoes using, for example, 180° RF pulses. Spin-echo trains can be used in multi-shot and single-shot sequences.

In both cases the generic technique is referred to as fast-spin-echo (FSE) or turbo-spin-echo (TSE) sequences. The single-shot spin-echo technique was fi rst proposed by Hennig et al. (1986) under the acronym RARE (rapid acquisition with relaxation enhancement). Single-shot spin-echo sequences are frequently combined with half-Fourier techniques to reduce the length of the spin-echo train. This com- bination is sometimes called HASTE (half-Fourier- acquisition single-shot turbo-spin-echo) sequence.

The main diffi culty of the single-shot techniques described above is the signal decay during the echo train due to transversal (T2 or T2*) relaxation. This signal decay limits the maximum duration of the echo train since, after a certain interval after excitation (e.g. 200 ms), no magnetization is left in the transversal plane (Fig. 10.1);

thus, it does not make sense to use echo trains with an arbitrarily long duration, and the maximum number of echoes (i.e. the matrix size or image resolution) is restricted. A second problem arising from transversal relaxation is that echoes contributing to the same image are acquired with different signal intensities depending on their individual echo time. As a consequence, image artefacts arise, in particular blurring in phase-encod- ing direction is frequently observed. As discussed below, parallel imaging is a valuable method to overcome these disadvantages of single-shot sequences.

Most single-shot techniques acquire two-dimensional

image data; however, three-dimensional techniques based

on gradient echoes have also been proposed (Mansfi eld

1977; Song et al. 1994; Mansfi eld et al. 1995). Since the

number of k-space lines required for a three-dimensional

acquisition is typically substantially larger than for a two-

dimensional image, echo trains tend to become very long

resulting in very low signal of the majority of echoes

because of transversal relaxation; however, with short-

ened echo trains due to parallel imaging, three-dimen-

sional single-shot techniques might become much more

attractive and applicable in the future.

10.2

Single-Shot Spin-Echo Techniques:

RARE and HASTE

10.2.1

General Properties

In single-shot spin-echo techniques, the transversal magnetization decays exponentially with the time constant T2. After 3 × T2, the signal is decreased to about 5% of its initial intensity and, thus, becomes too low to acquire further spin echoes; hence, assuming typical T2 relaxation times between 35 and 130 ms depending on the examined tissue, the maximum duration of the echo train is restricted to 100 to 400 ms. Only special applications, such as MRI of liquids with very long T2 in MR cholangio-pancreati- cography, is feasible with longer echo trains of several hundred milliseconds (cf. Chap. 22).

The maximum image resolution is restricted by the number of acquired echoes and therefore by the maximum echo-train duration, e.g. a typical echo spacing of 4 ms limits the number of echoes to about 100. These echoes can be used to fi ll, e.g. the k-space of a 256×96 acquisition (with 96 phase-encoding steps and anisotropic pixel size) or, using half-Fou- rier acquisition, of a 256×192 acquisition. The echo spacing can be reduced by increasing the receiver bandwidth or applying shorter RF pulses; however, this either decreases the signal-to-noise ratio of the acquisition or deteriorates slice profi les and increases the specifi c absorption rate (SAR).

A second point to consider is the image contrast, which is infl uenced by the effective echo time of the pulse sequence, i.e. the interval between excitation and the acquisition of the central k-space line. The effective echo time depends on the echo spacing and the selec- tion and ordering of acquired k-space lines. The two most important k-space sampling schemes are centri- cally ordered and linearly ordered sampling (Fig. 10.2).

Both schemes fulfi l the condition that the time inter- val between the acquisitions of two adjacent k-space lines is constant for (almost) all lines; this condition prevents strong variations of signal intensity between neighbouring lines due to T2 relaxation. Obviously, with centrically ordered sampling, only very short echo times below 10 ms can be obtained, while with linearly ordered sampling of the full k-space, the echo time is half of the echo-train duration, e.g. about 200 ms.

Using partial-Fourier or half-Fourier techniques, the echo time of linearly ordered sampling can be reduced down to a minimum of four to eight echo spacings, since a minimum of four to eight readouts before the k-space centre is required for phase correction.

Centrically ordered k-space sampling provides the shortest effective echo times as required, e.g. for proton-density-weighted or T1-weighted imaging.

Nevertheless, linearly ordered k-space sampling is frequently preferred because of the shorter interval between the acquisitions of adjacent lines, which reduces image blurring; hence, reducing the mini- mal effective echo time of linearly ordered HASTE sequences is an important design issue for many imaging applications.

This effect restricts the

maximum length of the

echo train and causes

blurring of the resulting

image.

10.2.2

Parallel Imaging

Parallel imaging can be used to overcome several limi- tations of RARE or HASTE sequences: their maximum resolution can be increased, blurring can be decreased, and the minimal effective echo time can be reduced.

Application of parallel imaging is relatively straight- forward if the coil sensitivity information is acquired in a separate measurement independent of the single- shot application. In this case, the k-space distance, 'k, between adjacent lines is simply increased (by acquiring, for example, only every other line or every third or fourth line) as illustrated in Fig. 10.3). As a consequence, either the echo-train length can be reduced resulting in decreased T2 relaxation during readout and thus in decreased blurring (Fig. 10.4), or the sampled k-space range can be increased to acquire data of higher spatial resolution within the same total readout time as before. If image quality without parallel imaging is already substantially com- promised by blurring, the former alternative is gener- ally more attractive. An additional advantage is the reduced total duration of the acquisition enabling the acquisition of more slices, e.g. during one breath hold.

It shouls be noted that the effective echo time is also substantially shortened due to parallel imaging, if the sequence samples the full k-space linearly ordered.

The usually heavy T2 weighting is reduced, which might not be desired in all applications.

Alternatively, the usually low matrix size of single- shot spin-echo sequences can be increased, e.g. from 256×128 without parallel imaging to 256×256 with parallel imaging (Heidemann et al. 2003); thus, T2- weighted images with spatial resolution similar to much slower fast-spin-echo techniques (cf. Chap. 9) can be obtained by single-shot imaging.

A further important consequence of parallel imag- ing in single-shot applications in comparison with non-accelerated acquisitions with identical resolu- tion is that fewer k-space lines with very low signal- to-noise ratio are acquired; thus, the average signal- to-noise ratio of all acquired k-space data is higher.

This effect depends on the duration of the echo train and the T2 relaxation time of the imaged tissue and can partially compensate the intrinsically reduced signal-to-noise ratio of parallel imaging.

The described advantages of parallel imaging apply for centrically and linearly ordered k-space sampling as well as for full and partial-Fourier sampling; however, the reduction of effective echo time is a feature particu- larly interesting for half-Fourier sequences with linear k-space sampling. Since these sequences are frequently used for proton-density-weighted or T1-weighted MRI (using a magnetization preparation such as an inver- sion-recovery pulse), the shortest possible echo time is often desired. The effective echo time of these sequences is limited by the number of k-space lines acquired before the central line. These lines are required for phase correc- tion. With k-space-based parallel-imaging techniques,

Fig 10.2a–c. Single-shot spin-echo sequences can acquire the lines in k-space (illustrated as fi lled blue circles) in different order;

the most important schemes are a centrically ordered k-space sampling and b linearly ordered k-space sampling. c To reduce the minimal effective echo time, TE, (i.e. the interval from excitation to the readout of the central k-space line), linearly ordered k-space sampling is frequently combined with half-Fourier acquisition.

a b c

10.2.3

Acquisition of Coil Sensitivity Information

In contrast to conventional spin-echo sequences (cf.

Chap. 9), the integrated acquisition of auto-calibra- tion signals containing coil sensitivity information may be disadvantageous in single-shot sequences. On the one hand, the echo train is prolonged by inserting

Fig 10.3a–c. Parallel imaging applied to a linearly ordered RARE sequence. a Acquisition without parallel imaging; all k-space lines (fi lled circles) are acquired. b Acquisition with parallel imaging (acceleration factor R=2) yielding the same spatial resolu- tion as in a; only every other k-space line is acquired (i.e. the distance between lines in k-space, 'k, is doubled). The effective echo time (TE), the echo-train length, and, thus, image blurring, is reduced. c Acquisition with parallel imaging and the same echo-train length as in a in order to double the spatial resolution.

Fig 10.4. Phantom images demonstrate the reduction of blurring with increasing parallel-imaging acceleration factors, R, between 1 and 4. The images were acquired with a HASTE sequence at 3 T using a 12-element head coil, a 384×384 matrix, and phase encoding from left to right. The liquid in the large sphere has a shorter T2 relaxation time than the one in the small phantoms and, thus, exhibits more obvious blurring in phase-encoding direction (arrows).

such as GRAPPA, some of these lines can be generated synthetically, i.e. fewer lines have to be acquired before the central k-space line and the minimal effective echo time is reduced. The higher tissue signal at shorter effec- tive echo times can also partially compensate the intrin- sically reduced signal-to-noise ratio of parallel imaging (in addition to the effect described above which is caused by the reduced echo-train length).

a b c

time of HASTE sequences cannot be reduced com- pared with non-accelerated imaging since the refer- ence lines are acquired around the centre of k-space, i.e. at the very beginning of the echo train (Fig. 10.5);

thus, the separate acquisition of either the spatial coil profi les or the auto-calibration signals required for k-space-based reconstruction techniques, such as GRAPPA, is frequently preferable for single-shot spin-echo sequences.

10.3

Single-Shot Gradient-Echo Techniques: EPI

10.3.1

General Properties

In single-shot gradient-echo techniques, the trans- versal magnetization decays rapidly with the time constant T2*. T2* is in general considerably shorter than T2 and in particular includes spin dephasing due to static magnetic fi eld inhomogeneities. Typi- cal T2* relaxation times range between 8 and 50 ms

single-shot gradient-echo sequences must be kept substantially shorter than of single-shot spin-echo techniques. Luckily, the echo spacing of gradient- echo techniques is also much smaller than that of spin-echo techniques, since no RF pulses and only short gradient pulses need to be applied between the readouts of successive lines; thus, a similar total number of echoes (about 100) can be acquired in single-shot gradient-echo MRI as in spin-echo MRI before the signal intensity has decreased too much for data acquisition.

The EPI sequences usually acquire the k-space lines linearly ordered, since, in this case, phase encod- ing during the echo train can be performed most effi ciently by very small “blipped” gradients; thus, the minimal effective echo time depends (apart from matrix size and echo spacing) on the application of partial-Fourier techniques which allow reducing the number of phase-encoding steps before the acquisi- tion of the central k-space line.

A major limitation of single-shot EPI sequences are severe image artefacts caused by chemical-shift and susceptibility effects. Spins with different Larmor frequencies are not rephased for each echo as in spin- echo techniques, but are allowed to evolve during the

Fig 10.5a–d. Coil sensitivity information can be acquired either independent of the accelerated imaging scan or integrated in the actual acquisition as auto-calibration signals (ACS) shown in red. a Accelerated acquisition without ACS. b Accelerated acquisi- tion with additional ACS. The total echo-train length is increased and the benefi t of parallel imaging is reduced. c Accelerated half-Fourier acquisition without ACS. d Accelerated half-Fourier acquisition with additional ACS which increases the minimal effective echo time (TE).

a b c d

the receiver bandwidth).

10.3.2

Parallel Imaging

The benefi ts of parallel imaging for single-shot gradi- ent-echo techniques include most of the advantages that are discussed for spin-echo-based techniques in section 10.2.1. In particular, either the echo-train length can be shortened or the spatial resolution (matrix size) can be increased with parallel imag- ing as illustrated in Fig. 10.3, e.g. T2*-induced blur- ring in the phase-encoding direction can be reduced

Fig 10.6. Phantom images demonstrate the effect of parallel imaging with different acceleration factors, R, between 1 and 6. In echo-planar imaging (EPI), severe distortion artefacts occur without parallel imaging and are reduced at increasing accelera- tion. The EPI acquisitions were performed at 3 T with a 12-element head coil, acquiring a 128×128 matrix with phase encoding from left to right. The non-distorted reference image was acquired with a turbo-spin-echo sequence with a 320×320 matrix and an echo-train length of 15.

using parallel imaging with an acceleration factor of R=2 or higher, respectively; thus, images with reduced distortions can be acquired as demonstrated in Fig. 10.6. Reducing image distortions is certainly one of the main motivations to apply parallel imaging to EPI sequences (Schmidt et al. 2005; Yang et al. 2004).

10.3.3

Acquisition of Coil Sensitivity Information

Integrated acquisition of coil sensitivity informa-

tion is even less feasible for echo-planar techniques

than for single-shot spin-echo sequences. The main

Fig 10.7a,b. Acquisition of auto-calibration signals (ACS) containing coil sensitivity information for parallel echo-planar imag- ing. a If all ACS lines are acquired in a single EPI scan, the line distance in k-space, 'k, is much smaller for the ACS scan than for the actual imaging scan with acceleration factor R, 'k

<< 'k

. As a consequence, geometric distortions of the coil sensi- tivity data are not consistent with the distortions of the imaging data. b To obtain consistent line spacings in k-space and, thus, consistent distortion, ACS data can be acquired interleaved in multiple scans covering the same k-space range as in a.

low-resolution ACS acquisition with small 'k and by the under-sampled accelerated data set with large 'k;

thus, the coil position information contained in the ACS is not consistent with the coil positions required for reconstruction of the under-sampled raw data resulting in reconstruction artefacts.

Similar problems can occur, however, when coil sensitivity information is taken from completely undistorted reference data such as obtained by short- TE gradient-echo scans. Care must be taken that the more or less distorted geometries of both the coil profi les and the parallel-imaging acquisition are consistent (Fig. 10.7). A possible way to acquire coil sensitivity profi les with the same distortion proper- ties as the actual image acquisition is to acquire echo- planar interleaved k-space data that can be combined to a single full-fi eld-of-view data set as illustrated in Fig. 10.7b. In this case, the line spacing, 'k, is the same for the ACS and the imaging data. An obvious disadvantage of this approach is the relatively long acquisition duration for ACS data; however, if the actual image acquisition is repeated several times, which is frequently the case in EPI applications such

10.4 Conclusion

In single-shot sequences, parallel imaging does not only reduce the number of required phase-encod- ing steps and, thus, accelerates the acquisition, but can also substantially improve the image quality: By shortening the echo train, image blurring caused by decaying transversal magnetization can be reduced, and in addition, EPI sequences show less distortion due to the increased step size in k-space. As a positive side effect, parallel-imaging-induced signal-to-noise losses are partially compensated by obtaining shorter effective echo times and by acquiring fewer echoes of very low amplitude; thus, single-shot sequences particularly benefi t from the application of parallel- imaging methods.

Although only conventional Cartesian k-space techniques are discussed in this chapter, similar con-

a b

are echo-planar brain imaging (discussed in Chap- ters 18, 33, and 34), echo-planar abdominal imaging (Chap. 21), and single-shot spin-echo imaging such as MR cholangio-pancreaticography (Chap. 22) or MRI of the lung (Chaps. 20 and 38).

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