MAGNETOM Trio-3T whole body MR scanner, Siemens Medical Solutions, Iselin, NJ.

The early years of MRI were marked by debates over field strength. What was best for a particular indication? Eventually, 1.5T MRI came to be viewed as the standard, but is 1.5T always enough? As Yuri Wedmid, PhD, manager of Ultra-high Field MRI at Siemens Medical Solutions USA, Iselin, NJ, says, “At 1.5T, we are signal starved in some applications.” For those applications, there now are 3T scanners, some designed specifically for use on the head and others for whole-body applications.

Why 3T MRI Is Needed

The image generated by an MRI scanner is formed using the signal picked up by the receiver. To various extents, this signal, and therefore the quality of the image, is degraded by noise (systematic and random errors arising from various sources, such as patient motion and imperfections in the MRI system). The more the signal can be strengthened and the noise can be reduced, the clearer the image becomes. The relationship between these two image elements is referred to as the signal-to-noise ratio (SNR).

The spatial resolution of an image is a measure of the smallest element of the target that can be identified. The spatial resolution is determined by the size of the volume elements, or voxels, that make up the image. The image is clearest (has the best SNR) when the signal from each voxel is much greater than the noise from the same volume.

Clearly, then, there is a relationship between SNR and spatial resolution: as voxels are made smaller, noise will assume a proportionally greater role in the data captured by the receiver. The easiest way to maintain SNR while increasing resolution is to obtain data repeatedly from the same voxel so that the variations in noise will cancel each other. This method, however, lengthens image-acquisition time. Longer acquisitions increase the extent of artifacts, as well as making scanning less pleasant for the patient and reducing the number of patients who can be scanned in a given time.

The SNR also can be improved by increasing signal intensity, which can be accomplished by raising the field strength of the magnet. The higher field also makes it possible to improve spatial resolution, reduce the acquisition time, or both. Anything that a 1.5T scanner can do, a 3T scanner can do more quickly. Moreover, the higher-field scanners can depict smaller objects, including blood vessels as small as 0.2 ?m, and localize them more accurately. In addition to SNR, which increases visible signal size, the higher field also offers other benefits. For example, 3T magnets double the separation between different signals. This doubling of signal resolution for MR spectroscopy is of immense importance. Whereas spectroscopy at 1.5T focuses on hydrogen, 3T MR spectroscopy also can measure isotopes of common body elements such as carbon, oxygen, sodium, and phosphorus, along with tracers such as fluorine, helium, lithium, and xenon.

Technological Challenges

Bringing the 3T MR system to market presented some significant engineering challenges. The more powerful 3T magnets, for example, are more complex in terms of greater size and out-reaching fringe fields, resulting in higher construction costs in order to site the magnets in crowded hospital locations. New magnet designs have addressed these issues to nearly the level of 1.5T magnets, without compromising any of the critical magnet performance parameters, such as field of view. Another challenge faced in creating advanced MR technology was the development of gradient systems with high slew rates and, therefore, generally greater performance.

Now that the scanner hardware has been considerably optimized, the challenge becomes modification of the imaging protocols. For example, protocol changes can be made to control the specific absorption rate, which is a measure of the heat-creating energy affecting the body. Higher-field scanners also require longer-to-acquire T1-weighted images. Doses of contrast medium can be reduced, but radiologists will need to adapt to the greater inherent contrast of the images.

Clinical Applications

The value of 3-T scanners is not limited to imaging the brain, although that is now their principal use (see page 2). Considerable work is being done on 3T cardiac MRI, as well as on abdominal and orthopedic imaging. In the abdomen, 3T scanners offer the possibility of obtaining images without requiring the patient to hold his or her breath. In orthopedics, the high spatial resolution and SNR of 3T MRI improve the visibility of small structural details, such as those seen in the cartilage of the knee. As Peterson et al1 recently noted, high-resolution imaging has clinical implications, facilitating treatments such as cellular implantation as they become available for the correction of the early degeneration of cartilage.


The 3T scanners are only now beginning their employment as clinical tools. It is certain that many more applications will be found for which they have much to offer. It is not unrealistic to believe that, in the not-too-distant future, they will displace 1.5T scanners as standard equipment for MRI and MR spectroscopy. Generally, 3T provides clinicians with a better ability to identify smaller pathologies at earlier stages when they can be better treated.

Judith Gunn Bronson, MS, is a contributing writer for Decisions in Axis Imaging News.


  1. Peterson DM, Carruthers CE, Wolverton BL, et al. Application of a birdcage coil at 3 tesla to imaging of the human knee using MRI. Magn Reson Med. 1999;42:215-221.