3-T MR Imaging: Ready for Clinical Practice

Lawrence N. Tanenbaum

Having more than a year’s worth of clinical experience on a first-generation, whole-body, 3-T MR system and approximately 12 months using a second-generation, short-bore, whole-body machine in the community setting, I read Dr. Ross’ February 2004 editorial, “The High-Field-Strength Curmudgeon,” with great interest and some consternation. Although I assume his “musings” are valid for the (head-only) 3-T system in use at his institution, I am concerned that they do not accurately reflect the strengths and limitations of systems currently being installed and thus may mislead those in the process of assessing the feasibility of higher-field-strength whole-body MR imaging. Those considering acquisition of a higher-field-strength MR system should understand that many of the stated limitations are characteristic of older systems, and through advances in hardware and software, have already been overcome.

Over the past several years, systems operating at higher field strengths have become more prevalent, particularly at research centers. Of late, there has been increasing interest in 3-T MR imaging in the community setting for whole-body imaging purposes. Fueling the shift in interest from 1.5 T to 3 T and from primarily research to clinical practice is the validation of what was once considered to be very high-field-strength MR (3 T) as feasible and indeed now or potentially superior to 1.5-T for clinical indications throughout the body. Reduced concerns over surface coil availability, radio-frequency (RF) deposition limits, higher ambient noise, system homogeneity, increased magnetic susceptibility, chemical shift effects, and reduced tissue contrast as well as demonstration of the incremental benefits of 3-T over 1.5-T imaging with respect to image quality and efficiency is driving this increased penetration of 3-T systems into the clinical setting.

There are a number of fundamental differences in later-generation 3-T devices that impact on clinical feasibility, the most important of which are new system designs that are inherently more SAR (specific absorption ratio) efficient. Because SAR scales with the square of field strength, RF deposition is more limiting at higher field strengths. Older less SAR-efficient system designs were so RF intense that “patient cooling” delays between sequences were often required. With today’s SAR-efficient modern MR systems and appropriate protocol design, intersystem delays should no longer occur. Limits on the rate of RF energy deposition continue to place minor restrictions on the number of sections that can be acquired per TR period, sacrificing some of the potential efficiency boost afforded by the potentially doubled signal intensity at 3 T. The situation is much less severe with newer systems and section reduction is currently a relatively minor concern. In addition, pending modifications in pulse sequence design from several manufacturers (VERSE, TRAPS, hyperechoes) should, in the very near future, lead to RF limitation and section acquisition efficiency equal to or slightly greater than those currently in place at 1.5-T.

Today’s short-bore whole-body MR systems do pose certain challenges. To maintain clinically acceptable static field homogeneity more coil windings are required and thus the magnets are much heavier, potentially affecting the site in which they are installed. However, inherent shielding maintains a similar fringe field and footprint to that of a 1.5-T system, and many sites with 1.5-T systems can easily accommodate a swap for a 3-T system. On the other hand, a modern 3-T system does not suffer from the distortion and limited cephalocaudal coverage that Dr. Ross laments. Long z- and off-center field-of-view (FOV) imaging, even with fat suppression, easily matches or surpasses the best of 1.5-T performance (Fig 1).

The broadband acquisition and reconstruction architecture of today’s MR systems has an enormous impact on the quality and efficiency of imaging at 3-T. Today’s eight channel coils deliver a significant boost in signal-to-noise ratio (SNR) over older designs and are designed for use with parallel imaging (PI) techniques. The use of PI leads to a lower image duty cycle load (proportional to SAR) by reducing the number of phase encoding steps that are performed. The resulting SNR drop on a routine imaging sequence (a factor of 2 is associated with a 40% reduction in SNR and a 50% reduction in imaging time) is better tolerated at higher field strength, particularly with higher SNR coils and thus PI techniques used routinely (Fig 2).

Susceptibility effects scale with field strength and are exploited in improving the sensitivity of fast spin-echo (FSE) techniques to the presence of hemorrhage and mineralization at 3 T. Clinical BOLD imaging is also more practical and robust as a result. These same effects have been cited as quality limiting on older 3-T systems with single-shot echo-planar techniques employed for diffusion and perfusion imaging. By reducing effective echo spacing and TE, at the expense of some drop in SNR, PI results in images with artifact severity similar to that seen at 1.5 T. Although susceptibility effects might be expected to be prohibitive and limiting for patients with spine hardware, the combination of efficient coil designs and high bandwidth techniques keeps artifact manageable (Fig 3).

Chemical shift effects also scale with field and have been cited as providing a boost in metabolite peak separation and resolution for spectroscopy at 3 T. Alternatively, an increase in chemical shift artifact at 3 T has been cited a significant limiting factor in routine anatomic imaging. The SNR inherent to 3-T and late-generation multichannel coils are now routinely leveraged via the routine use of higher bandwidths (32–125 KHz) for spin-echo (SE) and FSE imaging, managing susceptibility issues and alleviating concerns over chemical shift artifact (Fig 4).

The longer T1 of background (brain) tissue at 3 T has been exploited to produce superior time-of-flight MR angiography. This same effect leads to somewhat unsatisfactory results with conventional T1-weighted SE imaging in the brain and spine. Fortunately, techniques that are in wide clinical use at 1.5 T, such as inversion recovery FSE (T1-weighted fluid-attenuated inversion recovery [FLAIR]) (Figs 2 and 4) and RF-spoiled gradient echo (MP SPGR), produce superior contrast resolution to that provided by T1-weighted SE imaging and are equally well suited to use at 3 T. In addition, in contrast to a situation whereby T1-weighted studies have traditionally been less satisfactory and take longer to perform, a typical T1-weighted FLAIR study of the brain or spine, coupled with the use of PI on a modern 3-T system, allows a higher spatial resolution protocol with a shorter imaging time than at 1.5 T (Fig 2). Although the strength of these novel sequences encourages a shift away from conventional T1-weighted SE imaging, we view this as just another incremental step in progress and quality improvement, like many others that have occurred in the roughly 20 years of clinical MR imaging.

Our 3-T system replaced a 1.5-T system and currently works in tandem with a late-generation, gradient-enhanced, broadband 1.5-T system. The 3-T system looks almost identical to the 1.5-T system with a similar form factor and a similar fringe field. The 3-T system is no more difficult for the technologists (or me) to operate than our lower-field-strength system and in our high-demand, competitive clinical setting the high-field-strength system is our preferred choice for whole-body applications. With a variety of currently available coils of various levels of sophistication ranging from a quadrature extremity coil through an eight-channel spine coi, we easily create a recognizably better imaging examination in the same or slightly less time. As coils match or exceed the capabilities of those available at 1.5 T and current SAR limitations are circumvented, both quality and efficiency will advance.

Decisions made about hardware purchases have ramifications for many years, and readers need to know that 3-T is ready to perform “bread and butter” as well as advanced clinical applications today. Referring physicians from neurologic and non-neurologic specialties, imaging technologists, and interpreting radiologists are highly enthusiastic about using 3-T systems for applications throughout the body. Only cost considerations will prevent our practice from making 3-T the selection for each of our next high-field-strength systems. Users in a busy, competitive clinical setting should have little difficulty leveraging the power and unique capabilities of higher field strength to generate an incremental boost in demand to justify the higher base cost of a 3-T system.

Fig 1.

A sagittal T2-weighted image (FOV, 1024 × 384 mm; section thickness, 3-mm) obtained in a patient with a suprasellar dermoid. Note the z-axis uniformity of signal intensity from convexity through the foramen magnum on this image obtained with an eight-channel head coil

Fig 2.

Residual low-grade glioma. Twenty 5-mm-thick sections were obtained with a FOV of 20 mm at 288 × 192, imaging time of 54 seconds, with a parallel imaging acceleration factor of 2.

Fig 3.

Anterior decompression, fusion, and instrumentation. High-bandwidth techniques are facilitated by the combination of 3-T signal intensity and a high SNR, and an eight-channel spine coil effectively manages susceptibility artifact.