New Horizons in MR Technology: RF Coil Designs and Trends
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Magn Reson Med Sci, Vol. 6, No. 1, pp. 29–42, 2007 REVIEW New Horizons in MR Technology: RF Coil Designs and Trends Hiroyuki FUJITA1,2,3* Departments of 1Physics, Case Western Reserve University, and 2Radiology, University Hospitals of Cleveland, 10900 Euclid Avenue, Cleveland, Ohio 44106-7079, USA 3School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane Qld 4072, Australia (Received January 9, 2007; Accepted February 14, 2007) Parallel imaging techniques have developed very rapidly, and realization of their full potential has required the design of magnetic resonance (MR) scanners with ever-increasing numbers of receiver channels (32 to 128). In particular, 1.5- and 3-Tesla fast MR imaging applications are now used in everyday clinical practice. Both strengths require maximum achievable signal-to-noise ratio (SNR) and multi-detector array coil optimization within the framework of the parallel imaging scheme for more advanced and faster clinical MR scanning. Preampliˆers are key components in the detector array coils and serve many functions beyond mere signal ampliˆcation. One critical function is to aid in the decoupling of individual coils, which is essential for optimal SNR and the performance of parallel imaging. To support a large number of detector array coils for parallel imaging, preampliˆers must be physically very small so that they may be tightly packed together to form an optimized detector array. The author herein reviews the state-of-the-art work reported by those skilled in the art to consider the rationale for determining how many channels are enough and how fast we can go. The paper explores the important and fundamental principles of RF array coils for MR imaging and reviews cutting-edge array coils, including those for transmit-SENSE or parallel transmission applications. The future of radiofrequency (RF) coil technology is also considered. Keywords: phased array RF coils, preampliˆer decoupling, parallel imaging, parallel transmission, wireless RF coils today's advanced and sophisticated clinical appli- Introduction cations. Modern radiofrequency (RF) array coil design is In 1978, in his historic paper, Hoult detailed and complex, and its understanding requires broad discussed the nuclear magnetic resonance (NMR) knowledge of magnetic resonance (MR) physics receiver;1 in 1990, Roemer and associates proposed and electronics. Today's coils are very intricate and the phased-array coil technology that underlies the delicate. For example, as many as 400 to 1500 design of most of today's RF receive coils;2 and in individual componentsWparts are used in the con- 1991, Hayes's group discussed the application of struction of 8-channel head coils and 16-channel that technology to volume imaging.3 Original and brain-spine combo coils, both of which are typical general phased-array radar technology4,5 and array detector array coils. Every component must be coil theory and application6 are reviewed in detail correctly integrated; if even one component is elsewhere. broken or damaged, the coil does not function A small surface coil is well known to yield higher optimally. The author attempts to explain the SNR nearer the surfaces of the coil than does a critical components that constitute an RF array volume coil, such as the birdcage coil,7 but the coil, especially emphasizing principles and methods B1-sensitive region of a small surface coil is (much) for decoupling array coils, which is essential to smaller than that of a volume coil. It is remarkable that in an approach utilizing phased-array coils, the *Corresponding author, Phone: ＋1-216-368-3894, Fax:＋1- high signal-to-noise ratio (SNR) associated with a 216-368-4671, E-mail: hiroyuki.fujita＠case.edu small surface coil can be achieved and maintained 29 30 H. Fujita Fig. 1. The mechanism of electromagnetic coupling between the patient and the magnetic resonance (MR) radiofrequency (RF) coils, due to the stray capacitances present, is illustrated. Baluns are added to block the common mode current and allow the diŠerential mode current to ‰ow. over an extended ˆeld of view (FOV) by using a set author uses state-of-the-art examples to address the of RF coils. This is possible because of the develop- questions of what constitutes a su‹cient number of ment of various array coil designs for imaging channels and how coil elements in an array are applications of interest and of techniques to decou- decoupled and to consider how array detectors can ple the mutual inductances and thereby elimi- be optimized. nate cross-talk among the elements of the array coils. Adjacent coil elements are often partially Signal-to-Noise Ratio overlapped to cancel the mutual inductance be- tween the elements. The next nearest and other Signal-to-noise ratio (SNR), one of the most coil elements can be mutually decoupled using a important parameters to be optimized in MR low-input impedance (typically less than 5 Ohms) imaging applications, was detailed by Hoult and preampliˆer decoupling circuit. Other decoupling Richards.14 Here, it is essential to understand how techniques and methods are brie‰y discussed later; SNR relates to the coil-related parameters. The detailed discussions of low noise ampliˆers may be signal is written as found elsewhere.8 S1g3B 2 B xy(r)Eq.(1), In conjunction with a coil matchingWdecoupling 0 1 circuit, the low-input impedance preampliˆer where g is the nuclear gyromagnetic ratio, B0 is the xy eŠectively eliminates current ‰ow in the coil ele- static magnetic ˆeld, and B1 (r)istheRFmagnetic ment loop, thereby eliminating the magnetic ˆelds ˆeld produced by a coil with unit current 1A. In xy induced in neighboring coil elements (shown later). designing a coil, B1 (r) must be optimized, which Baluns and considerations pertaining to cable requires the appropriate choices of coil size over a routing are also important factors in array coil target FOV and of distance from the FOV. Size design. Figure 1 illustrates how cables couple with choice depends generally on the number of availa- the patient and why baluns need to be implement- ble receiver channels. ed; details are discussed elsewhere.9 On the other hand, NMRWMR imaging noise is RF coil technologies are driven by the rapid thermal noise, and the noise generated from the advances in development of MR scanners with coil is given by greater numbers of receiver channels as well as in N＝ 4kTDfR Eq. (2), development of parallel imaging techniques;10–12 the 96-channel head array coil is one such example.13 where k is the Boltzmann constant, T is the temper- Whatever the application at any ˆeld strength, ature in K, Df is the bandwidth, and R＝RC＋RS. maximum achievable SNR is key. Because SNR is RC isthecoilresistanceandRS thesampleenergy destined to be degraded or lowered from the loss, i.e., the equivalent series resistance resulting starting point, i.e., the signal source, to the end from the induced eddy current losses in the conduc- point of the receiver chain, minimizing its loss in tive sample. Combining Eqs. (1) and (2), SNR can the process is of primary concern. In this paper, the be expressed using the coil-related parameters as components of an array coil are explained, and issues requiring special attention are noted. The Magnetic Resonance in Medical Sciences RF Coil Designs and Trends 31 Table 1. Relationship between dB expression and common factor Factor 1W100 1W10 1W51W31W223456 7 10100 dB －20 －10 －7 －4.8 －3 3 4.8 6 7 7.8 8.45 10 20 S B xy(r) 1 1 Eq. (3). N RC＋RS xy To optimize SNR, B1 (r) can be maximized by having the coil closer to the sample, and RS can be minimized by matching coil size to the target FOV. In fact, Wright and Wald compared an N-element Fig. 2. Depiction of a preampliˆer noise model. V array to a single coil with respect to the resultant denotes the signal; NS, the noise generated by the resistance, r, of the input signal source; and N ,the SNR while overall coil dimension remained un- P noise generated inside the preampliˆer. changed.6 They showed, for example, that when comparing a single coil with arrays measuring 8×8, 4×4, and 2×2, improvement in SNR disappears where.1,16 Figure 2 depicts a simple model for at a depth equal to the diameter of the array. preampliˆer noise. V denotes the signal; NS,the However, for distances in between, SNR curves noise generated by the resistance, r, of the input vary according to the number of array (i.e., coil) signal source; and NP, the noise generated inside elements. If each coil is made too small, the coil the preampliˆer. Using these quantities, NF is resistance loss dominates the sample noise. This deˆned as implies that optimization is required for a given N 2 ＋N 2 speciˆc application, e.g., target depth and available NF＝10 log P S where N ＝ 4kTDfr 10 Ø N 2 » S number of receiver channels. R is minimized by S C Eq. (4). making the unloaded coil Q high. It is well known that at low-B0 ˆeld systems (Ã0.3T), coil resistance The industry standard for preampliˆer NF is less is dominant or at least comparable to sample noise, than 0.5 dB, and the ˆrst NF and gain are well but at 1.5T and above, sample noise dominates known to have the most signiˆcant impact in the (i.e., RS:RC ). Furthermore, it is useful to be aware entire electronics circuit, which is often cascaded. that if each of 2 coils yields the same relative Kucera, Lott, Horowitz and Hill demonstrate this sensitivity in free space and is dominated by sample in their respective chapters on the design of low- noise, the two have the same absolute sensitivity.15 noise ampliˆers.8,16 Because noise is generated in (The detailed discussion of the coil unloaded Q, any passive elements that dissipate power, includ- loaded Q, and sensitivity in this reference should be ing cables, the design of most detector array coils useful to the coil engineer.) integrates preampliˆers so as to minimize undesira- ble noise.