 agnetic
resonance imaging has taken off like a rocket and become the diagnostic runner
of the last twenty years, but MR spectroscopy has stayed in the back rooms of
the researchers. There are two main reasons for this development: there are not
many clinical applications for MR spectroscopy, and there is no reimbursement
for such examinations in most countries. This makes the method unattractive for
physicians, hospitals, and in particular for private practices.
One
of the first papers on medical MR spectroscopy applications was published in the
New England Journal of Medicine in 1981 by Ross and his collaborators. They described
spectroscopic changes of phosphorus in McArdle's syndrome [3]. McArdle's
syndrome is not a major global disease, nor are other muscular diseases in which
MR spectroscopy has shown changes of phosphorus or proton spectra.
Thus,
it is understandable that both the clinical users and the manufacturers of MR
machines have reduced or even ceased to use whole-body MR spectroscopy machines.
In 1990, a spokesman for one of the major manufacturers of whole-body MRI/MRS
equipment stated that there are no clinically efficient applications for MR spectroscopy.
Therefore, his company and other producers of high-field equipment have limited
their investments in whole-body MR machines below 2 Tesla although in recent years
some higher field machines have reached the market. However,
this trend is not reflected by the research output – MR spectroscopy research
is thriving. In 1982, at the first meeting of the Society of Magnetic Resonance
in Medicine in Boston only two papers dealt with MRS. In 1983 less than 100 papers
were published about MRS, in 1991 MedLine counted 500, and in 1999 700 publications.
As well as there being more papers, there was also an increase in complexity.
The
following statement is typical of many articles dealing with MR spectroscopy and
its applications: “It
is hoped that the new information provided by (in this case) multidimensional
spectroscopic imaging of metabolites in vivo will further enhance the clinical
and scientific value of this technology [1].” The
overwhelming majority of publications about MRS either focus on anecdotal clinical
cases, in which some changes in spectra were (or were not) seen, or they discuss
improvements of MRS technology. It is always 'hoped' that one day MRS will enhance
the horizons of medicine.
MR spectroscopists sometimes claim that whole-body
MRS is not accepted by clinicians because the latter cannot read and interpret
the spectra. They postulate that: “The
arrogance of the ignorants hinders the development of spectroscopy.” This
might be partly true because radiologists are not trained in biochemistry or in
reading and recalculating spectra. However,
the ball is played back into the spectroscopists' court by the physicians. The
latter underline that spectroscopists, with a background in chemistry or physics,
have no idea of the possible medical relevance of spectra and are, by and large,
only interested in playing scientific games. They also claim that spectroscopists
create a sea of irrelevant data in which potentially useful information is drowned. Another
important argument is that spectroscopy is insensitive. Phosphorus spectroscopy
is sometimes dubbed 'the Twin Peaks of MR', although in reality there are three
main peaks in in vivo phosphorus spectra.
The
technique of phosphorus spectroscopy suffers because of the large volumes (50-100
ccm) that are necessary to acquire decent spectra within the time period a patient
can remain motionless in the magnet. However, tissue volumes of 50-100 ccm are
of no relevance to clinical diagnosis. When examining brain tumors, an MRS examination
volume usually includes vital tumor tissue, the necrotic tumor center, edema,
perhaps hemorrhage, and also normal non-involved tissue. This type of volume is
too inhomogeneous to clarify or even grade such a tumor. Follow-up examinations
may reveal whether a tumor responds to therapy, but even this is doubtful. However,
proton spectroscopy has a greater sensitivity and possesses a wider range of metabolic
information than phosphorus MRS. It saves between a half and two-thirds of the
time necessary to acquire a similar phosphorus spectrum at 1.5 T. Spectroscopic
data usually require spectral analysis to indicate the metabolite concentration,
ratios, and tissue pH. These data give a momentary picture of macroscopic local
metabolism and the distribution of metabolites. To date, both time and space resolution
are restricting factors of MRS, and therefore MRS examinations cannot compete
directly with single photon emission computed tomography (SPECT) or positron emission
tomography (PET). However, MRI can now begin to compete with these radioisotope
technologies. It
is also possible to convert the spectroscopic result into metabolic maps. Thus,
images can be created that reflect the concentrations of certain metabolites on
an anatomical background. Maps
of phosphates or other metabolites can deliver spectroscopic information as pictures
that can be more easily understood by radiologists. Proton spectra might become
the solution for creating such maps because numerous metabolites such as creatine,
choline, and lactate can be depicted. The
interpretation of such maps still requires considerable knowledge of diagnostic
biochemistry. Because today's radiologists are not trained in this field, this
is a job for skilled spectroscopists. Worldwide, there are few scientists with
such knowledge, and training is limited because of financial restrictions.
The
question remains as to how MRS can be accepted by clinicians using whole-body
MR machines. First,
relevant clinical and diagnostic applications have to be found. These applications
must be better than competing techniques, and if possible, the MRS examinations
must become faster and cheaper than comparable diagnostic methods. In addition,
for its implementation in clinical routine, there should be a therapy for the
patient's disease. MRS must be able to exclude certain differential diagnoses
better than other diagnostic techniques, and/or MRS must be superior to other
diagnostic methods in the follow-up period. Second,
MR spectroscopy must be easy-to-use and accepted by radiologists, otherwise it
will stay a research tool. On
the other hand, there is no doubt that MRS has already contributed greatly to
the furtherance of medical knowledge and the understanding of certain aspects
of human physiology and pathophysiology. MRS examinations of muscle metabolism,
tumors, tissue damage caused by ischemia and infarction, and transplant rejection
have added to the understanding of these diseases.
Still,
to date, most examinations have not proved useful for daily medical routine. And,
what makes it even more difficult for medical spectroscopy, functional and dynamic
magnetic resonance imaging have become possible during the last few years. Functional
imaging allows users to depict some of the working mechanisms in the body such
as the response of the visual cortex of the brain upon light, enabling almost
direct assessment of neuronal function. However,
unrelated events can influence and boost medical techniques – such as diseases
of presidents or monarchs or wars. MR spectroscopy of the brain, for instance,
hit the frontpages of newspapers when a research group was able to show brain
abnormalities in veteran military personnel after the Gulf War [2]. | 
August 1991
PREVIOUS
Reference: Rinck PA: The slow life of clinical spectroscopy.
Hospimedica 1991; 9,9: 16-18.
_