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As one recalls the 1970s and some of the first steps in magnetic resonance imaging [ 1 ]–[ 4 ], it is easy to discern the great strides that have been made in this discipline over the past 30 years [ 5 ]–[ 7 ]. Early coarse and grainy results [ 4 ] have given way to exquisite anatomical and functional images [ 5 ]–[ 7 ]. The availability of MRI is now synonymous with quality of medical care, even within the rural hospital setting, and the 1.5 Tesla scanner has become a workhorse of the modern radiological exam. With the exception of CT, and this primarily in the abdomen, no other radiological modality can compete with MRI, not only in terms of the breadth of exams currently possible, but also in the future promise of the technique. Indeed, it seems that every year new clinical applications join the arsenal of MRI exams. Soon, it is anticipated that MRI will be able to fully scan the entire body [ 8 ] in great detail, including the most difficult thoracic [ 9 ]–[ 17 ] and abdominal locations [ 18 ]–[ 22 ]. Technical advancements forged and tested in the research laboratories of the world [ 23 ]–[ 40 ] continue to add to the versatility and power of MRI scanners. Nonetheless, what is perhaps most fascinating relative to the evolution of MRI is the seemingly untapped potential that remains. The spawning of new techniques may well open up tremendous venues for MRI in the coming decades. Thus, the clinical horizon is imperceptible. One is left only with the realization that future progress may well surpass all contributions to date.

Magnetic field strength has always been an important parameter to consider for Magnetic Resonance Imaging (MRI). It is generally agreed that the Signal to Noise Ratio (SNR) is approximately proportional to magnetic field strength [ 1 , 2 ], although other more subtle effects, such as chemical shift dispersion and susceptibility, also scale with field strength and can cause problems for good anatomical imaging. However, it is no surprise to learn that the engineering challenges presented by the commercial construction of higher field MRI systems are formidable. This chapter is an opportunity to introduce these to a wider audience.

Ultra high field MRI systems present a number of unique challenges to the system designer and integrator beyond simply scaling up the performance of a lower field system. The primary areas of concern are the magnet, gradient coils and drivers, and RF coils and coil interface. The art of system integration lies in identifying sufficiently clear performance targets for each of the subsystems and ensuring that those targets are met in a way that preserves the overall performance of the system. The following discussion identifies for each of these areas the key performance requirements that are changed at higher field strengths, methods to address those requirements, and how those methods affect the rest of the system. As this is an area of ongoing research and development, many of the specific solutions presented here are likely to be superseded in the future, but the general approach to the problem should remain valid. While a complete description of every aspect of system design and integration of UHFMRI systems is beyond the scope of this chapter, the following is intended to provide practical guidance in addressing the more common problems in siting or operating a UHFMRI system.

The intrinsic improvements in signal-to-noise ratio, spectral dispersion, and susceptibility contrast with increasing static magnetic field strength, B _0, has spurred the development of MR technology from its very first application to clinical imaging. With maturing magnet, RF, and gradient technology, the clinical community has seen the static magnetic field of clinical systems increase from 0.2 to 1.5 to 3.0 T. Today, the “high field” label for human MR research describes initial experiences with 7, 8, and 9.4T systems. While currently primarily research instruments, this technology is bound to cross the boundary into the clinical diagnostic arena as key technical issues are solved and the methodology proves itself for addressing clinical issues. In this chapter we discuss the particular advantages and disadvantages of ultra high field systems for clinical imaging as well as some of the immediate technological challenges that must be solved to derive the full benefit of the extraordinary sensitivity of these systems, which has been glimpsed from their research use.

The increasing appreciation of neuroradiologists, other healthcare professionals, neuroscientists, and cognitive scientists for the exquisite detail of anatomical, physiological, and functional magnetic resonance imaging (fMRI) of the human brain has encouraged increasing use of MRI in medical care and research. As MRI has no adverse biological effects when performed within FDA guidelines, longitudinal studies of development and aging and detailed studies through repetitive measurements on single subjects can be undertaken with insignificant risk. Scanner performance for clinical MR scanners has been enhanced as field strengths have migrated upward to 3.0 Tesla. It is appropriate to consider the technical challenges of further improving sensitivity by moving from 3.0 to 9.4T, the highest magnetic field scanner now available for human MRI that became operational in 2004.

This chapter reviews RF volume, array, and surface coil modeling, design, construction, control, safety, and human in-vivo application examples for field strengths from 4 to 9.4 T. While a comprehensive variety of coils is included, focus is on the transmission line (TEM) technology head, body, surface, and array coils developed by the author over the past 16 years. References provide a supplement to this material for the many details that cannot be covered in a single book chapter.

The advancement of MRI as a radiological instrument has been associated with a constant drive toward higher magnetic field strengths, resulting in higher operational frequencies. More powerful magnets bring the promise of enhanced signal to noise ratio resulting in exquisite resolution, and reduced scan times. At the same time, however, operating MRI at higher frequencies adds significant physical and engineering complexities to the MRI experiment, most notably in designing safe, versatile, and high-performance radiofrequency (RF) coils. This chapter provides RF coil studies that span frequencies ranging form 1.5 to approx. 12 T. The results and conclusions are based on experimental findings using 8 and 1.5T whole-body MRI systems, computational electromagnetics using the finite-difference time-domain method, and analytical derivations using electromagnetic theory. The outcome of these studies is then utilized to provide new avenues and techniques to improve the performance of RF head coils for human MRI at very high fields.

In MRI, increasing radiofrequency magnetic ( B _1) field frequency is a consequence of employing higher static magnetic ( B _0) field strengths in the drive to improve signal-to-noise ratio (SNR). Due to the direct proportionality between B _0 field strength and B _1 field frequency in MRI, B _1 field distributions become more complex at higher B _0 fields due in part to shorter wavelengths and penetration depths. Consequently, it becomes both more difficult to calculate RF field behavior and more important to do so accurately for high-field MRI. In this chapter the basics of electromagnetic properties of tissue, the method of radiofrequency field calculation currently most prevalent in high-field MRI (the FDTD method), and methods for relating calculation results to MRI are covered briefly before results from calculations are used to discuss current challenges in high-field MRI including central brightening, SNR, power absorption by tissue, and image homogeneity.

In high-magnetic-field MRI, both valuable image contrast and undesirable artifacts associated with the magnetic susceptibility effects are significantly increased. The magnetic field distortion in and by the human body is described with computer modeling methods in the human head. The manifestations of the resultant image artifacts include signal loss, blurring, and geometric distortion and are dependent on imaging methods. The treatments of the artifacts in the specific imaging sequences are described and demonstrated with human studies at 3 and 7 Tesla and animal studies at field strengths as high as 14 Tesla. With these in vivo studies, the enhanced image contrast produced by the increased field strength and the improved image quality by the artifact reduction methods provide strong and stimulating evidence for exciting potential scientific applications of high field MRI.

In the last two decades, magnetic resonance imaging (MRI) instruments operating at a magnetic field strength of 1.5 Tesla have emerged as the most commonly employed high-end platform for clinical diagnosis. Despite the dominant position enjoyed by this field strength, even its promotion as the “optimum” field to work for human applications, the late 1980s witnessed the beginnings of an interest in substantially higher magnetic fields. After brief and cursory explorations, however, high field strengths were virtually abandoned by industry leaders while their efforts were focused on further refinements of the 1.5T or even lower field platforms. Nevertheless, a handful of 3 and 4-Tesla instruments were established in academic research laboratories by about 1990. Since these early beginnings, work conducted in these academic sites has demonstrated that magnetic fields substantially beyond 1.5 Tesla provide numerous advantages in aspects of magnetic resonance imaging and spectroscopy (MRS) applications in humans, even though such high fields also pose serious challenges. In considering these accomplishments, however, it is imperative to recognize that, to date, virtually all of the research at high magnetic fields, especially at field strengths greater than 3 Tesla, has been carried out only in a few laboratories and using instruments that are definitely far less than optimized; as such, the amount of man-hours and talent dedicated to this effort has been minuscule compared to the clinical uses of MR and, even then, this effort has been hampered by suboptimal instrumentation. Therefore, any positive conclusions obtained thus far, and there are many, can only be interpreted as harbingers of potential gains and definitely not as what can be ultimately achieved.