The definitions of low-field, mid-field, and high-field have changed over the last four decades and there is no official dividing line between them.
In the 1980s and 1990s, several vendors produced scanners with field strengths at 0.2T or below and these were considered low field. During that era, high-field magnets were anything greater than or equal to 1.0T and those in between were considered intermediate- or mid-field.
Today, as the marketplace has shifted toward higher and higher fields, the characterizations of magnetic field strength have also changed. Although some would argue (especially those who like to refer to 7.0T scanners as UHF), I would suggest the following terms are reasonable in 2020:
Low field → 0.1T to 0.3T
Mid-field → 0.3T to 1.0T
High field → 1.0T to 3.0T
Very high field → 3.0T to 7.0T
Ultra high field → 7.0T and above
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How did we end up with the various popular field strengths we see today, such as 1.5T, 3.0T, and 7.0T?
My first experience in the early 1980s was with both a 0.15T Picker resistive scanner as well as a 0.35T superconductive Diasonics scanner. Over the next few years superconducting magnet production was dominated by Oxford Instruments, who supplied magnets to Picker, Philips, GE and Siemens.
GE finally and fairly quickly locked onto 1.5T as their primary field strength by the mid-1980's (although they experimented with scanners at 0.5T, 1.0T, 1.3T and 1.4T). They heavily marketed 1.5T as “high field”. The other big vendors eventually caught up and unveiled 1.5T scanners themselves.
For a good portion of the 1980s and early 1990’s there were many superconducting 0.5T and 1.0T scanners manufactured by several vendors including the big guys plus Technicare and Elscint; these were commonly referred to as “mid-field”. But gradually it was realized that if you had to go to the trouble of supercon with helium etc, you might as well just go with 1.5T instead of intermediate field scanners.
Oxford never really capitalized on their initial success to the extent that they should have. Ultimately they had to agree to let GE and Philips produce their own magnets and were finally were acquired by Siemens.
Concerning 3.0T, I do suspect this was more marketing than anything else. In the early 1990s Siemens, Philips and GE installed 4T scanners at UAB, Minnesota, and the NIH (maybe others elsewhere in the world). But 3.0 ultimately prevailed.
The jump to 7.0T and higher for clinical scanners and research devices likely took origin from the desire for “number rounding”. It turns out the Larmor frequency for 7.0T is conveniently very close to 300 MHz. Popular field strengths for small bore animal and analytical magnets are 9.4T (400 MHz), 11.7T (500 MHz), and 14.1T (600 MHz).
There seem to be many different MRI magnets and designs. Can you explain?
How many brands of scanners are there?