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
9.1 Introduction 113
9.2 Spin-Echo and Gradient-Echo Sequences with Long Repetition Times 113 9.3 Turbo-Spin-Echo Sequences 114 9.4 Fast Sequences for Dynamic MRI 116 9.5 Conclusion 116
References 117
Conventional Spin-Echo and 9
Gradient-Echo Pulse Sequences
Olaf Dietrich
9.1
Introduction
The basic idea of parallel imaging is to reduce the number of acquisition steps (i.e. phase-encoding steps) by decreasing the sampling density in k- space. Artefacts, in particular aliasing artefacts, that would result from the reduced sampling density are compensated by employing image data from several independent coil elements with different spatial sen- sitivity profi les for reconstruction. This basic con- cept of parallel imaging, i.e. reducing the sampling density and increasing the number of coil elements, can be combined with almost all types of MR pulse sequences. The only precondition with respect to the pulse-sequence design is that the phase-encod- ing sampling density in k-space can be varied, and this is true for virtually all pulse sequences that acquire at least two-dimensional k-space data,
i.e. for all sequences apart from exotic techniques such as column-selective MRI or line scan imaging (Maudsley 1980; Gudbjartsson et al. 1996). The latter sequence types can, however, benefi t from some more general parallel-imaging approaches, e.g. by acquiring data from two separate volumes at once.
In this case more complex pulse-sequence modifi ca- tions, such as the insertion of special RF pulses, are required (Larkman et al. 2001; Breuer et al. 2005).
Parallel imaging can be combined with conven- tional Cartesian k-space trajectories as well as with non-Cartesian sampling strategies such as radial or spiral trajectories; however, the complexity of the image reconstruction differs substantially depending on the k-space sampling strategy. Parallel imaging with non-Cartesian k-space trajectories requires very complex and time-consuming reconstruction algo- rithms and, thus, has not yet found general accept- ance in clinical routine. More details about parallel imaging with non-Cartesian k-space techniques can be found in Chap. 6. The following sections discuss the application of parallel imaging in several basic pulse sequence types such as spin-echo or gradient- echo techniques. Generally, Cartesian k-space sam- pling is assumed, although many conclusions remain valid for other trajectories as well.
9.2
Spin-Echo and Gradient-Echo Sequences with Long Repetition Times
Spin-echo pulse sequences were historically the fi rst sequences applied for MR imaging. They are still in general use in certain areas of MRI, in particular in T2- and T1-weighted imaging of the brain, because of their well-established contrast properties which are not fully reproduced by alternative techniques such as the faster turbo-spin-echo sequences (Henkelman et al. 1992; Stables et al. 1999). The main disadvan-
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tage of the spin-echo technique is the long acquisition duration of this sequence type: only a single line of k-space is acquired after each excitation, and excita- tions are separated by the repetition time, TR; thus, the total duration of acquisition is the product of the number of k-space lines and TR. The repetition times vary between about 600 ms for T1-weighting and 3000 ms for T2-weighting, resulting in acquisi- tion durations between about 2:30 and 13 min for raw data with 256 lines of k-space. Fortunately, the long TR can be used to interleave the acquisition of several slices such that the acquisition of a complete multi- slice data set (and not only a single slice) is possible within the acquisition duration mentioned above.
To restrict the acquisition time of conventional spin-echo sequences, the number of phase-encoding steps is usually decreased by choosing a rectangular fi eld of view. A further acceleration could be achieved by partial-Fourier techniques (MacFall et al. 1988;
McGibney et al. 1993), however, at the cost of reduced image quality; thus, parallel imaging becomes highly attractive to either reduce the acquisition duration or to increase the maximum spatial resolution achiev- able within a given scan time. The main disadvan- tage of parallel imaging, the increased image noise, is often acceptable in spin-echo techniques because of the originally high signal-to-noise ratio of these techniques.
Parallel imaging always requires information about the spatial coil sensitivities that can be acquired either in a separate scan or integrated in the pulse sequence to be accelerated (cf. Chap. 8 about coil-sensitivity measurements). Whereas the choice when and how to acquire these data can be crucial in the design of fast sequences as discussed below, this question is relatively uncritical for conventional spin-echo tech- niques. In general, the acquisition of the actual image data takes much longer than the measurement of the coil sensitivity information, since the latter can be calculated from data with low spatial resolution;
hence, the acceleration achieved by parallel imaging is almost not infl uenced by the acquisition of this ref- erence data.
Consider, for example, the acquisition of 224 k- space lines with a TR of 3 s for T2-weighted spin-echo imaging. The acquisition duration of 224×3=672 s (11:12 min) can be reduced to 336 s (5:36 min) by applying parallel imaging with an acceleration factor of R=2 (Fig. 9.1). This acceleration is only slightly reduced by the additional acquisition of, for example, 16 auto-calibration or “reference” lines to determine the coil sensitivities. The reference lines prolong the
total acquisition time to 384 s (6:24 min) which is still a substantial acceleration by more than 5 min (Fig. 9.1c).
Similarly, the matrix size in T1-weighted spin-echo imaging can be doubled without increase of scan time using parallel imaging. With a TR of 600 ms, the original acquisition with 224 phase-encoding steps takes 134 s. Parallel imaging with an acceleration factor of R=2 can be used to increase image resolu- tion to 448 lines in phase-encoding direction in the same time. The additional acquisition of 16 reference lines increases the imaging duration by only 10s to a total of 144 s.
The integrated acquisition of auto-calibration lines is, on the one hand, relatively ineffi cient in theses cases because of the long TR while fast gradient-echo sequences could be employed much more effi ciently to determine the coil sensitivities. This disadvantage does not, on the other hand, result in critical time penalties; thus, applying parallel imaging in conven- tional single-echo sequences with long TR provides substantial benefi ts because of the originally very long acquisition durations independent of the used implementation details.
Gradient-echo methods with large (90°) fl ip angles and long repetition times can provide T1-weighted or T2*-weighted images. With respect to parallel imag- ing, these sequences are very similar to conventional spin-echo techniques with comparable timing, and the arguments considered above apply as well.
9.3
Turbo-Spin-Echo Sequences
Turbo-spin-echo sequences, i.e. sequences that acquire a series of spin echoes after a single excitation, have become one of the most important techniques for reliable and high-quality morphological MRI. These sequences, which are also known as fast-spin-echo sequences, have properties similar to those of the original spin-echo sequences but are substantially faster, e.g. the acquisition time of a turbo-spin-echo sequence with an echo train length of fi ve is only a fi fth compared with a corresponding spin-echo sequence.
Most of the considerations about spin-echo tech- niques apply to turbo-spin-echo sequences as well.
Typical acquisition times are still in the range between 1 and 4 min since a part of the reduced acquisition
duration is usually spent for higher spatial resolu- tion; hence, combination with parallel imaging is still attractive to further accelerate the acquisition or to increase the matrix size.
A specifi c property of turbo-spin-echo sequences is that the echo train consists of spin echoes acquired after several different individual echo times. The effective echo time of a turbo-spin-echo sequence is defi ned by the echo time of the central k-space lines.
A consequence of varying echo times for different parts of the k-space is that objects can appear slightly blurred in the image, i.e. the point-spread function has a larger width than in spin-echo sequences (Constable and Gore 1992; Chien and Mulkern 1992). Parallel imaging can be applied to reduce these effects by decreasing the echo-train length instead of shortening the total acquisition time. For example, the total scan time of a non-accelerated T2-weighting turbo-spin-echo sequences with an echo train length of 17, a TR of 5 s, and a resolution of 320 lines in phase- encoding direction is about 320/17×5 s=1:35 min.
Applying parallel imaging with an acceleration
factor of R=2, the acquisition time could be reduced by about 50%; however, it may be more benefi cial in terms of image quality to combine parallel imaging with a reduction of the echo-train length from 17 to 9 echoes; thus, the scan time remains practically unchanged, but the turbo-spin-echo-related blurring will be reduced.
Since turbo-spin-echo sequences are consider- ably faster than spin-echo sequences, the averaging of several acquisitions becomes feasible as a means to increase the signal-to-noise ratio. In combination with parallel imaging, averaging can also be useful to reduce the sensitivity to motion artefacts. If the time saved by using parallel imaging is spent for averag- ing, the signal-to-noise ratio remains practically unchanged and the sensitivity to motion is reduced.
In this case it is advantageous to acquire coil-sensi- tivity profi les only once and not repeatedly for each acquisition.
By increasing the length of the spin-echo train, the acquisition can be further accelerated to the point where all k-space lines are acquired after a single
Fig 9.1a–c. Acceleration of conventional T2-weighted spin-echo imaging with TR of 3000 ms and 224 phase-encoding lines (each acquired line in k-space is illustrated as fi lled circle). a Non-accelerated acquisition with total scan time of 224×3s=672 s.
b Scan with parallel-imaging acceleration factor R=2 and half scan time of 336 s. c Scan with acceleration factor R=2 and inte- grated acquisition of 16 additional auto-calibration signals (ACS) shown in red. The scan time is increased to 384 s; however, the acquisition is still about 5 min shorter than the original protocol without parallel imaging. (For better illustration, only every other k-space step is shown.)
a b c
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excitation. These single-shot measurements and their special properties with respect to parallel imaging are discussed in Chapter. 10. Another technique becom- ing feasible with parallel imaging is three-dimen- sional turbo-spin-echo MRI. By combining strate- gies, such as variable fl ip angles, which allow very long echo trains (Mugler et al. 2000), and driven- equilibrium techniques to recover the longitudinal magnetization after the readout (Hargreaves et al.
1999; Melhem et al. 2001) with parallel imaging to reduce the acquisition duration, three-dimensional high-resolution turbo-spin-echo data sets with iso- tropic voxel size can be acquired in clinically accept- able scan times.
9.4
Fast Sequences for Dynamic MRI
Gradient-echo sequences are used for very differ- ent MRI applications. Apart from the gradient-echo sequences with long TR mentioned previously, there is a distinct group of gradient-echo sequences for fast data acquisition with small fl ip angles and short or very short repetition times. If these sequences are used for dynamic imaging, i.e. for repeated acquisi- tions of the same anatomical volume, their timing is usually carefully optimized to provide maximal temporal resolution. This can include the applica- tion of relatively low spatial resolution, e.g. 128×128 matrices, partial-Fourier techniques, or interpolation of slices in three-dimensional acquisitions. The pur- pose of all these approaches is to reduce the number of acquired k-space lines (i.e. phase-encoding steps) and, thus, to minimize the acquisition time for a single data set.
Obviously, parallel imaging is an additional important technique to accelerate these acquisitions.
In contrast to the sequences discussed in the previous sections, now the strategy to acquire coil-sensitivity information becomes crucial, since the number of k- space lines acquired for imaging is relatively low and, thus, frequently of the same order of magnitude as the number of reference lines. Generally, integrated acquisition of reference lines, i.e. acquiring with a higher k-space sampling density in the centre of k- space than in the periphery, is relatively ineffi cient if data acquisition is either frequently repeated for dynamic MRI or if relatively small matrix sizes are acquired (Fig. 9.2).
Multi-slice perfusion imaging of the kidney or the myocardium, for instance, can be performed with a fast two-dimensional gradient-echo sequence.
Assuming a 128×128 matrix and a 6/8 partial-Fourier acquisition, 96 k-space lines are acquired for each slice. With a repetition time of 5 ms, the acquisition of six slices requires 6×96×5 ms=2880 ms. Apply- ing parallel imaging with an acceleration factor of 3, only 32 k-space lines are acquired for each slice; thus, six slices can be acquired in 6×32×5 ms=960 ms, i.e.
the temporal resolution is substantially increased to slightly more than one data set per second. If 16 addi- tional phase-encoding steps are performed to meas- ure coil profi le data (corresponding to a low resolution image consisting of 24 k-space lines), the acquisition time is increased to 1440 ms, i.e. an arterial bolus passage of 15 s is sampled by only 10 instead of 15 points. Thus, data describing the coil-sensitivity pro- fi les should be acquired only once at the beginning of the measurement to obtain the highest possible tem- poral resolution. An alternative is fast auto-calibrat- ing techniques such as the TSENSE acquisition (c.f.
Chap. 12). It should be noted, however, that spoiled gradient-echo sequences are somewhat limited in their signal-to-noise ratio such that parallel imaging with acceleration factors >2 typically requires at least fi eld strengths of 3 Tesla or above.
A considerably higher intrinsic signal-to-noise ratio is provided by steady-state free-precession (SSFP) sequences that are also frequently used for dynamic MRI with acquisition parameters similar to those of fast spoiled gradient-echo sequences. A typical application is imaging of the cardiac func- tion. Due to their higher signal-to-noise ratio, they are generally much better suited for parallel-imag- ing applications with high acceleration factors, par- ticularly at fi eld strengths below 3 Tesla. Since the sequence timing is comparable to the one of gradi- ent-echo sequences, all considerations discussed above apply and the integrated acquisition of refer- ence lines should be avoided as well. More details about fast MR sequences for dynamic imaging are presented in Chapter11.
9.5
Conclusion
The main motivation for the application of paral- lel imaging in “conventional” pulse sequences is the
acceleration of acquisition. This is relevant both for sequences with very long and for sequences with extremely short acquisition times. In the former case, the acquisition time can be easily reduced by several minutes, while in the latter, the temporal resolution for dynamic imaging can be substantially increased. The implementation of parallel imaging in these pulse sequences is relatively uncomplicated. In Cartesian k-space sampling, reducing the sampling density simply corresponds to choosing a smaller fi eld of view. Only if the acquisition of coil-sensitivity profi les is to be integrated into the sequence, addi- tional considerations are required to avoid ineffi cient sequence designs especially in dynamic MRI.
Fig 9.2a,b. Acquisition of auto-calibration signals (ACS) in fast pulse sequences for dynamic MRI.
This example illustrates the repeated 6/8 partial-Fourier acquisition of a 128×128 matrix with a parallel-imaging acceleration factor of R=3. The ACS scans (shown in red) are either acquired integrated in each repeated measurement (a) or only once at the beginning of the dynamic imag- ing series (b). In the latter case, seven repetitions can be acquired in the same scan time required for fi ve repetitions with integrated ACS scans.
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