What Does the Term “Field Strength” Mean?
The strength of a magnetic field is measured in units of either Gauss or Tesla, where 10,000 Gauss units equal 1 Tesla. Based on these measurements, MRI scanners are often categorized as low-, mid- or high-field as follows:
Low-field MRI:
Under 0.2 Tesla (2,000 Gauss)Mid-field MRI:
0.2 to 0.6 Tesla (2,000 – 6,000 Gauss)High-field MRI:
1.0 to 1.5 Tesla (10,000 – 15,000 Gauss)
For years, there was a debate among radiologists as to which range of field strengths was more effective diagnostically. High-field strength proponents would point to the fact that, other things being equal, the stronger the field, the stronger the amount of usable radio signal which can be elicited from the body’s atoms and, therefore, the higher the quality of the MRI image. Low- and mid-field proponents pointed out, on the other hand, that though it was true that higher field strength meant more signal, that single advantage was offset by a number of disadvantages. With time and further research, that debate has largely become history. Of particular significance was a carefully-conducted relaxometry study which found that optimum image contrast was to be found in the mid-field range. [Source: P.A. Rinck, et al., Radiology 168, (1988), 843-849.] Since differentiation between signals in an image is particularly important in arriving at a diagnostic conclusion, this study did much to confirm the arguments of those favoring mid-field scanners. A recent editorial in Applied Radiology by noted radiologist Dr. David Stark stated: “The great field strength debate lasted one decade. . . . Increasing field strength was an obvious, and expensive, approach to improve image quality. Although it is unarguable that increasing field strength increases image quality by increasing image signal-to-noise ratios (SNR) achievable during a given scan time, over the past few years it has become apparent that increasing field yields only fractional gains in SNR, not the exponential bonanza touted in the 1980s.” Fortunately for MRI patients, there is a best-of-both-worlds solution to the field-strength question. First, however, it should be pointed out that, because of their inability to harvest much of the available signal, low-field MRIs are largely deficient in image quality, a significant shortcoming when it comes to making an accurate diagnosis. Unfortunately, most so-called “open” environment MRI scanners fall into the low-field category. Secondly, all FONAR scanners fall within the mid-field range when measuring the actual field strength. As a result, every one of them produces good image contrast, as proven by the relaxometry study cited earlier. Furthermore, because of their vertical-field orientation, FONAR scanners have a distinct advantage-an inherent advantage based on physics-over their horizontal-field competitors. That’s because a vertical-field scanner is capable of using a particular type of antenna, known as a solenoidal surface coil, an antenna which a horizontal-field scanner cannot utilize. Together, the vertical-field magnet combined with the solenoidal surface coil double the signal-gathering capability of a horizontal-field scanner of the same field strength. Thus, a FONAR 0.3 Tesla scanner, the field strength at which most FONAR scanners operate, has the effective field of a 0.6 Tesla scanner. FONAR is developing a 0.6 Tesla, open-environment scanner called the QUAD 12000 which has a measured field strength of 0.6 Tesla. Since it has a vertical field, however, and is thus capable of utilizing FONAR’s arsenal of super-efficient solenoidal surface coils, it is capable of achieving an effective field of 1.2 Tesla. This latest scanner is thus provides not only the 0.6 Tesla field strength found to provide maximum image contrast, it also provides the high-field strength capable of eliciting maximum signal from the human body-but with none of the detrimental drawbacks associated with a measured high field. When it is introduced, it truly will provide the best of both worlds.