Reference: Rinck PA: The field-strength war. Hospimedica
1991; 9,4: 16-18.
Reprinted
and updated several times, last printed version 2003. Translated into Italian,
Portuguese, Russian, German, and Chinese.
Key Words: Magnetic resonance imaging;
magnetic field strength; spectroscopy; signal-to-noise.
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almost everything in this world, MR machines come in different sizes: extra-small,
small, medium, large, and extra-large. The technical terms in MR lingo for these
sizes are ultralow, low, medium, high, and ultrahigh field machines. These terms
refer to the magnetic field strength of the respective machine. The field strength
is measured in Tesla (T), a unit that replaced the former unit of Gauss (G) some
years ago, although Gauss is still used sometimes (10,000 G = 1 T).
Ultralow-field
machines operate at a field strength below 0.1 T, low field between 0.1 and 0.5
T, medium field between 0.5 and 1 T, high field between 1 and 2 T, and ultrahigh
field machines above 2 T. In
clinical surroundings, the national radiological protection boards used to allow
machines as high as 2.0-2.5 T. Everything above this limit was considered potentially
hazardous and thus should only be admitted to research facilities – particularly
if fast gradient-switching was used. Today,
ultrahigh fields are considered safe for research and, partly, for clinical routine
– at least in some countries. In
describing MR machines, natural scientists prefer to talk about frequencies instead
of field strengths. This is because different nuclei in the periodic system possess
different resonance frequencies. At 1 T, for instance, protons resonate at 42.58
MHz, whereas at the same field strength, phosphorus nuclei resonate at 17.23 MHz.
For clinical imaging purposes in medicine, this is of no importance because only
proton MR imaging is used. 
Strolling
down the aisles of the world’s biggest commercial exhibition of medical imaging
equipment at the annual meeting of the Radiological Society of North America,
one could find small machines operating at 0.06 T and huge machines operating
at 4.0 T or even higher fields.
Their magnets are different: below approximately
0.3 T, the magnets are permanent and resistive or electromagnetic, but above this
field the magnets are superconductive. All these magnet types have their pros
and cons. Why
does one find small ultralow field MR imagers and high field machines operating
at fields 100 times stronger? Why are there not only low or high field machines?
The
field-strength question has divided the MR community since the early 1980s. At
that time, all MR machines operated at low fields, and many of the prototypes
had strengths of approximately 0.15 T. Researchers did not believe that imaging
at higher field would be possible because higher radio frequencies would not be
able to penetrate the human body. Like many other predictions in MR imaging, this
prediction was wrong. MR
images at that time were crude, blurry, and generally worse than CT images. Scientists
working for the R&D divisions of companies producing MR equipment were asked:
“How
do you get better image quality?” They had a simple answer: “Increase
field strength.” From
analytical applications of MR, it was known that the signal-to-noise ratio increases
when you increase the field strength. The better your signal-to-noise, the better
your image will be. Higher fields also require higher gradient strength to reduce
the chemical-shift artifacts created by these fields. In turn, this led to better
spatial resolution. So some manufacturers, driven by their research and marketing
people, moved to high-field superconductive magnet systems. These systems were
(and in some instances still are) huge, dinosaurlike machines. They were expensive,
difficult to produce, and costly to maintain, but image quality suddenly became
better. Another
argument supported the development of high field machines; only these machines
are able to produce in vivo MR spectra for phosphorus or proton spectroscopy.
At this time, one of the aims in the development of MR in medicine was to combine
imaging and spectroscopy to acquire morphological and metabolic information about
the human body. The higher the field, the more detailed spectroscopic information
will be. However,
in vivo spectroscopy did not take off, whereas the popularity of MR imaging
exploded. Dedicated imaging machines became the rule, combined imaging and spectroscopy
the exception.
Even
for imaging, it became an ideology to plead for high fields. There is no rational
scientific reason for this development; image quality and spatial resolution of
low and medium field machines became as good as, and in some instances even better
than, that of high or ultrahigh field equipment. Additional research revealed
that the most important factor in medical imaging, tissue contrast, at least for
certain diagnostic questions in the central nervous system, seems to be best at
medium fields and, in some instances, even decreases with higher fields [2,3]. There
was still no rational approach to the problem. At a 1983 magnetic resonance conference
in San Francisco, a debate on field strength that had started on the platform
was continued in the corridor of the conference center. The discussion nearly
ended in a fist fight between the proponent of the high field ideology, whose
company had put all its efforts into 1.5 T machines, and the proponent of low
fields, whose company advocated MRI systems at 0.35 T. The
front lines in this war were mighty and the trenches deep. You were either part
of one camp or the other. All large companies jumped on the high field side and
promoted high fields with all the ammunition their marketing departments could
provide. In some countries, millions of dollars of taxpayers’ money were
channelled into subsidies for the development of high field systems.
However,
one morning in the early 1990s MR customers woke up and found that a gap was emerging.
One company had decided to enter the mid-field market, another followed suit,
and a third decided to compromise by offering an MR machine operating at a field
strength in between the others. The
reasons for these steps were never publicly discussed, but people had realized
that the signal-to-noise increase expected from the results in analytical NMR
did not occur in the same way in whole-body MR imaging. In
whole-body MR imaging, signal-to-noise increased to a certain extent, and then
the human body created additional noise that led to a flattening of the signal-to-noise
curve at high fields. In addition, nobody had foreseen the new problems faced
by users at higher fields, among them being the worsening of motion and susceptibility
artifacts. Cost and hazards also increased with higher fields. At the same time,
low and medium field machines became smaller, the quality of their diagnostic
output better, and interventional MR became feasible. The
market for 1.5 T high field equipment is nearly unbroken because they are good
diagnostic machines. They
also have some advantages over low field equipment; for instance, ultrafast imaging,
where scan time is reduced at the expense of signal-to-noise ratio, is generally
more effective at higher fields. This facilitates another ‘sexy’ research
area: functional imaging of the brain. However,
the new generation of buyers, the smaller hospitals and private practices, will
prefer cost-efficient MR systems that they can use for most of the daily routine
examinations. Bigger hospitals, and in particular those interested in spectroscopy
and research in functional imaging, will go for high field systems, but for them
also the second and third system will be medium or low field.
"Definitions
always seem to be in the eye of the beholder."
This
has been realized by the marketing department of the biggest US-American manufacturer.
Its marketing people had pushed for high field (1.5 Tesla) in the 1980s. Fifteen
years later, it postulated that its new mid-field equipment (0.7 Tesla) is also
high field. Definitions always seem to be in the eye of the beholder.
Derek
Shaw worked for Varian, later for Oxford Instruments, and since 1983 until his
retirement for General Electric Medical Systems. He is one of the leading MR scientists
in Europe. In 1996, he wrote the following statement in a book chapter: “The
early period of MRI ... was dominated by the 'field-strength war'. What was the
best field strength for MRI? These battles were essentially commercial, science
being used to justify the company’s competitive position ... “Our
pawn in the field strength battle was in vivo spectroscopy... As it became apparent
that there was not going to be sufficient specificity available via T1 and T2
determinations, MRS ... was seen as a potential alternative ... MRS needed the
highest field possible ... “This
need, along with the higher signal-to-noise ratios achievable at higher field
strengths ... led, despite their extra costs, to the use of 1.5 T magnets ...
Without this push to high field, MRI systems might be quite different today, probably
lower down on the cost/performance scale.”[4]
However,
the trend was towards high field. If all of this had been known or taken into
consideration 15-20 years ago, more patients would have had access to MR imaging,
and medical MR equipment might have been less expensive than it is today. This
is reflected in equipment sales worldwide and in the sales revenue of MR equipment
according to field strength. High field makes higher profit, which is a recurrent
theme not only in medical technology. Recently
the field strength quarrel has flared up again. This time it is 1.5 Tesla versus
3.0 Tesla. However,
this time it is not MR spectroscopy, but functional MR imaging pushing up field
strength. The results are the same. In
research – and in some places even in routine imaging – 3-Tesla system
have become the sine qua non. It has become fashionable to buy them. Once
again, people claim that the signal-to-noise ratio in MR imaging increases linearly
with field strength. Some researchers state that signal-to-noise between 1.5 T
and 3.0 T increases by 200%, even 300%. There are papers indicating that this
might be correct for functional MR imaging using the BOLD technique. However,
comparing the BOLD technique and MR imaging is like comparing apples and oranges.
On the other hand, for MR spectroscopy a 20% increase in sensitivity, but
the same signal-to-noise ratio has been shown in comparative studies between 1.5
Tesla and 3.0 Tesla [1].
For
MR imaging itself, reliable comparative studies do not exist – to make a
valuable comparison, the total amount of signal acquired during the same time
should be taken into account and this is not in favor of ultahigh fields since
T1 increases. Without
any doubt, signal-to-noise and spatial resolution can be better at 3 Tesla, coronary
arteries are better seen, small brain structures better delineated. However, the
cost/benefit ratio remains unknown.
Next
stop: 7 Tesla, perhaps 9. |
April 1991
_