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Q:
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.
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