A. Reykowski, PhD
Invivo Corporation, 3545 SW 47th Ave, Gainesville, FL 32608, USA
C O N T E N T S
13.1 Introduction 155
13.2 Array Coils and Parallel Imaging 155 13.3 Data-Reduction Concepts 156 13.4 Stationary Coils and Moving Coils 157 13.5 Dedicated Parallel-Imaging Systems 157
References 158
13.1
Introduction
This chapter discusses the requirements for modern MRI systems with parallel imaging capability. It explains the original motivation that led to the introduction of array coils and how this concept was later extended to parallel imaging. Data compression schemes, such as the Eigencoil or Matrix Coil (Mode Matrix) concepts, are introduced using a three-chan- nel Mode Matrix as an example. Also, the differences between stationary and moving coil arrays are dis- cussed. Finally, the necessary hardware and software components for a dedicated parallel imaging system are laid out in detail.
13.2
Array Coils and Parallel Imaging
The use of multiple decoupled coil elements was fi rst proposed by Roemer et al. in 1988 (Roemer et al.
1988; Roemer et al. 1990; Hayes and Roemer 1990).
Although offering high signal-to-noise ratio (SNR), single surface coils are limited in their fi eld of view (FOV). This limitation can be overcome by using sev- eral surface coil elements and arranging them in an array. The signals detected by the individual elements are not combined in hardware, e.g., linear polarized (LP) signals are combined to form circular polarized (CP) signals. Instead, each coil element in the array is independently fed into its own receiver channel and the fi nal signal combination is done on a pixel- by-pixel basis in the image domain. This way, the high SNR of each individual coil element is preserved while the combined FOV of the array is much larger than the FOV of a single coil element (Fig. 13.1).
The fi rst ideas to use the spatial information from coil arrays in order to accelerate image acqui- sition date back to 1988, as described in Chapter 2 (Hutchinson and Raff 1988; Kwiat and Einav 1991; Kelton et al. 1989; Ra and Rim 1993). It was, however, not before 1997 until feasibility was fi rst demonstrated by experiments (Sodickson and Manning 1997; Pruessmann et al. 1999; Griswold et al. 2000; Griswold et al. 2002).
The use of spatial information for parallel imag- ing had a strong infl uence on array design and MRI system architecture. Typical arrays for non-parallel imaging consist of anterior and posterior groups of elements which are organized in headfeet direction.
Most of these arrays have no independent elements in leftright direction. This is due mainly to the fact that arrays with independent elements in leftright direc- tion do need a signifi cant larger number of radio fre- quency (RF) channels but do not provide a major SNR improvement in the center region of the image.
In parallel imaging applications, however, maxi- mizing the SNR in the image center is only one of sev-
eral design criteria. In addition to that, the elements of the array have to be organized along the phase- encoding direction in order to generate suffi cient information for accelerated imaging. For a maximum degree of freedom, arrays for parallel imaging need to have elements in all three spatial directions, includ- ing leftright. This requirement leads to a signifi cant larger number of array elements and RF channels for parallel imaging than for non-parallel imaging.
Before parallel imaging methods became a stand- ard for clinical procedures, scanners were equipped with a maximum of eight RF channels. In contrast to that, recent MR systems which support parallel imag- ing in all spatial directions have up to four times as many RF channels.
13.3
Data-Reduction Concepts
The increase in the number of RF channels in com- bination with parallel imaging methods, such as SENSE or GRAPPA, leads to a more than propor- tional increase in reconstruction time. It is therefore prudent to keep the number of parallel RF channels to a necessary minimum. This can be accomplished by applying hardware- or software-based data com- pression schemes. Two hardware-based examples
are the so-called Eigencoil and Matrix Coil concepts (King et al. 2003; Reykowski and Blasche 2004).
Both concepts allow a scalable and effi cient use of the number of receiver channels by transforming a given set of coil signals to an equal number of mode signals. While each of the original signals only covers a part of the FOV, each of the mode signals covers the entire FOV.
In the case of the Matrix Coil concept, the center piece is a signal combiner network which is called
“Mode Matrix.” A good analogy for this Mode Matrix combiner comes from broadcasting: fi rstly, radio transmissions used single-channel mono broad- cast signals. Such a mono signal contains all neces- sary information to listen to a broadcast. It can be described as the sum of the signals for left and right audio or simply L+R (left plus right).
With the advent of stereo broadcasting, a trans- mission format had to be designed which ensured compatibility with existing mono radio receivers;
therefore, stereo broadcast transmissions still trans- mit the mono L+R signal on a main channel. On a second channel, a differential signal LR is transmit- ted. This is identical to transmitting L and R on two individual channels but at the same time ensures full compatibility with mono receivers (Fig. 13.2).
The Mode Matrix works in a similar fashion. All input signals are combined to a circular-polarized (CP) output signal on the main output channel. This CP Mode signal is the MR signal equivalent of the L+R
Fig. 13.1. Array technology allows maintaining the high signal-to-noise ratio of local coils while at the same time the fi eld of view (FOV) of the array is much larger than the FOV of a single coil element.
Fig. 13.2. Stereo broadcast consists of a mono broadcast signal plus a differen- tial signal.
(for primary, secondary, and tertiary) contains the same information as the original signals R, M, and L (for right, middle, and left) from the coil elements (Fig. 13.3).
13.4
Stationary Coils and Moving Coils
Frequently, a discussion arises in the context of move- during-scan applications, whether it is more advanta- geous to use a stationary or a moving coil array for data acquisition. A stationary coil array is at a fi xed position inside the magnet bore and does not move with the table. A moving coil array is attached to the table and thus moves with patient and table.
A stationary array has the advantage that it requires less coil elements, because it only has to cover a single FOV. Typically, the array is attached to the magnet bore and there is no need to place coils on top of the patient. The main disadvantage of such a stationary coil array is the reduced SNR, due to the fact that it is at a larger distance from the patient.
On the other hand, a moving coil array has the advan- tage of providing signifi cantly higher SNR, especially in the periphery of the FOV. This SNR can be utilized to increase acceleration or resolution. In addition, dedi- cated surface coils with small FOVs can be easily added, for example, to provide high-resolution, high-SNR data of vessel walls or the bulbus. The disadvantage of a moving coil array is the higher number of array ele- ments since it has to cover more than a single FOV.
13.5
Dedicated Parallel-Imaging Systems
A dedicated MRI system for parallel imaging has to contain all the necessary hardware and software to enable parallel data acquisition and processing in various regions of the human anatomy. Such a system should also support more recent trends such as high- resolution whole-body imaging with local coil arrays and move during scan techniques.
The parallel acquisition hardware should there- fore consist of the following components:
One or several arrays with multiple decoupled coil elements
An RF switching matrix which allows selection of alternating subsets of coil array signals for acqui- sition to reduce the number of required receiver channels
Means for array identifi cation in order to deter- mine which coil arrays are plugged into the system and where these arrays are located relative to the patient
Static/dynamic detune signals for individual ele- ments or array subsets
Multiple independent receivers Fast image-processing system
For scanning more than a single FOV within one examination, the number of coil elements which can be connected to the system should ideally be substan- tially greater than the number of receiver channels.
The system has to support both, static and dynamic detuning:
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Fig. 13.3. In analogy to stereo broad- cast, the Mode Matrix delivers a high SNR circular-polarized (CP) signal plus additional differential signals for iPAT applications.
Non-active coil elements, i.e., those which are not within the active FOV, are statically detuned. This means that they are detuned during the transmit phase as well as receive phase of the experiment.
Active coil elements, i.e., those which are in the active FOV, are dynamically detuned. This means that they are detuned during the transmit phase and tuned during the receive phase of the experi- ment.
The fi rst versions of local coils were connected to the MRI system via a plug in the magnet covers;
however, placing the coil plug in the covers has the disadvantage of relative movement between coil and coil plug. The connecting cable can create unwanted loops during patient table movement or can get caught between patient table and magnet covers. The operating personal has to be trained to observe cable movements and, if necessary, interact manually in order to prevent damage or injury; therefore, systems with such a confi guration do not allow a 100% remote operation, especially not in case of move during scan applications with continuous table movement. For this reason, the majority of present-day MRI systems have the coil plugs designed so that they are moving with the patient table. The plugs can be either found distributed along the table or within a docking sta- tion on the magnet side of the table.
Figure 13.4 shows how matrix coils can be com- bined to form a “Total imaging matrix” (Tim) for whole-body coverage. In this specifi c example, up to 76 local coil elements can be connected to the system.
From these 76 elements, a subset of up to 32 elements in the active FOV can be selected for image acquisi- tion via a coil channel selector.
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In the past, an exam was based on selecting ele- ments of a single array of coils within a single FOV.
Modern MRI systems, however, allow selection of a multitude of coil elements from a multitude of coil arrays, which extend over an area that is larger than a single FOV. The user interface of such a system should therefore aid in simplifying the exam setup procedure. The MRI system should be capable of detecting which coil elements are within the present FOV and choose the appropriate hardware settings based on easy-to-defi ne protocol parameters such as FOV, acceleration factor, phase-encoding direction, and others.
References
Griswold MA, Jakob PM, Nittka M, Goldfarb JW, Haase A (2000) Partially parallel imaging with localized sensitivi- ties (PILS). Magn Reson Med 44:602–609
Griswold MA, Jakob PM, Heidemann RM, Nittka M, Jellus V, Wang J, Kiefer B, Haase A (2002) Generalized autocalibrat- ing partially parallel acquisitions (GRAPPA). Magn Reson Med 47:1202–1210
Hayes CE, Roemer PB (1990) Noise correlation in data simul- taneously acquired from multiple surface coil arrays. Magn Reson Med 16:181–191
Hutchinson M, Raff U (1988) Fast MRI data acquisition using multiple detectors. Magn Reson Med 6:87–91
Kelton JR, Magin RL, Wright SM (1989) An algorithm for rapid image acquisition using multiple receiver coils. Proc SMRM 8th Annual Meeting, Amsterdam, p 1172
King SB, Varosi SM, Huang F, Duensing GR (2003) The MRI Eigencoil: 2N-Channel SNR with N-Receivers, Proc. ISMRM 11th Annual Meeting, Toronto, p 712
Fig. 13.4. Total imaging matrix for whole-body scans. In this example, a subgroup of up to 32 coil elements can be selected from a total of 76 coil ele- ments.
signal combiner for parallel imaging arrays. ISMRM 12th Annual Meeting, Kyoto, p 1587
Roemer PB, Edelstein WA (1987) Ultimate sensitivity limits of surface coils. Proc SMRM 6th Annual Meeting, New York, p 410
48:418–428
Sodickson DK, Manning WJ (1997) Simultaneous acqui- sition of spatial harmonics (SMASH): ultra-fast imag- ing with radiofrequency coil arrays. Magn Reson Med 38:591–603
