Theory and Application of Array Coils in MR Spectroscopy Steven M. Wright  and Lawrence L. Wald† Departments of Electrical Engineering and Radiology, Texas A&M University, College Station TX, USA and † Brain Imaging Center, McLean Hospital and Consolidated Department of Psychiatry, Harvard Medical School, Belmont, MA, USA The theory and application of array coils are reviewed in the context of phased array spectroscopy. The optimization of the signal-to-noise ratio from an array of coils is developed by considering the efficiency of a phased array transmit coil. This approach avoids the need to consider noise correlation, and should be useful in future considerations of transmit phased array coils for MR spectroscopy. Methods to characterize array coil performance, including fields and coupling are briefly summarized, along with methods to minimize the effects of mutual inductance. The signal-to-noise advantages of array coils over single coils are examined for both planar and cylindrical arrays. Numerical simulations of planar arrays of 2  2, 4  4 and 8  8 elements and constant overall dimension are compared to a single coil of the same size. The results demonstrate a significant improvement in sensitivity near the array coil. Although the benefits of the array decrease as a function of distance from the array, the array sensitivity never drops below that of a single coil with the same overall dimensions, or that of a single element of the array. Similar results are obtained for a sixteen element cylindrical array, which is compared to a standard quadrature birdcage coil using both computational methods and phantom measurements. The phased array techniques reviewed are demonstrated with proton spectroscopic images of the brain. © 1997 John Wiley & Sons, Ltd. NMR Biomed. 10, 394–410 (1997) No. of Figures: 17 No. of Tables: 1 No. of References: 89 Keywords: CSI; spectroscopic imaging; RF coils; phased arrays; SNR Received 4 August 1997; accepted 25 August 1997 INTRODUCTION Surface coil arrays were first implemented in MR imaging as switched coils, allowing different coils or combinations of coils to be used for different slices. 1–4 Examples include multislice axial imaging of the spine and sagittal imaging of the temporomandibular joints. Although a single coil can be optimized for a given observation point, 5 optimizing for large or multiple volumes without significantly sacrificing signal-to-noise ratio (SNR) requires multiple coils. Hyde 1 proposed using multiple receivers to independently and simultaneously obtain signals from multiple coils, achieving improved SNR from multiple regions with no increase in time. Roemer implemented a system using four receiver coils and four receiver channels, calling it an NMR phased array. 6 Additionally, Roemer described methods for combin- ing the data from the multiple receivers to optimize the SNR in every voxel. Over the last few years significant effort has gone into the development and application of array coils in MR imag- ing. 6–31 More recently several groups have domonstrated the potential for array coils and multiple channel receivers in MRS. 10, 25, 32, 33 This paper will review the theory, imple- mentation and application of array coils in MRS. The general theory of the array coil will be developed from the context of a phased array transmit coil, rather than a receiver coil as initially described in detail by Roemer. 6 This approach was chosen because it avoids the issue of noise correlation, which has been the subject of some discus- sion. 6, 34–39 Additionally, this approach should be more applicable to the development of transmit array coils, a logical extension of array coil technology. Section III provides a summary of some of the details helpful in implementing array coils. For completeness, quasistatic methods for calculating flux maps and coil self and mutual- impedances are included. Additionally, methods for measuring and minimizing the coupling between antennas are briefly discussed. Section IV includes several examples of the application of array coils in chemical shift imaging, along with new theoretical and phantom results demonstrat- ing the SNR gain obtained by planar and cylindrical array coils. THEORY AND IMPLEMENTATION OF PHASED ARRAYS FOR NMR SPECTROSCOPY A generalized phased array receiving antenna is illustrated in Fig. 1(a). In order to optimize the signal-to-noise ratio obtained from the chosen observation point, the relative amplitudes and phases of the signals from each element are adjusted before combining. The relative amplitudes and phases form a set of complex weighting coefficients [w], where w i =|w i |e j i . When multiple receiver channels are available, as in MRI/MRS applications, it is of great advantage to simply record the N independent signals and  Correspondence to: S. M. Wright. Contract grant sponsor: National Science Foundation; contract grant number: BCS-9308921. Abbreviations used: NAN, N-acetyl aspartate; RF, Radio-frequency; S/m, Siemens per meter; SNR, Signal-to-noise ratio; Z-parameters, impedance parameters; S-parameters; scattering parameters. © 1997 John Wiley & Sons, Ltd. CCC 0952–3480/97/080394–17 $17.50 NMR IN BIOMEDICINE, VOL. 10, 394–410 (1997)