Sokoloff responded to an inquiry on the future of PET in research, noting that the MRI enthusiasts have been premature in
their claims for that technology.
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1992-10-22 (October 22, 1992)
National Institute of Mental Health (U.S.)
Clatterbridge Hospital (U.K.)
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Positron Emission Tomography Scanning and Beyond, 1979-2004
Box Number: 1
Folder Number: 88
October 22, 1992
This letter is in regard to your recent inquiry about my opinions on the future of PET in functional imaging of cognitive
functions in the human brain, particularly in regard to its possible replacement by functional MRI imaging of the same processes.
I can assure you that PET continues to have a bright future despite the sometimes extravagant claims of the MRI enthusiasts.
The PET activation studies make use of established methods that measure quantitatively functions that are known to be related
to true functional activity in the brain. These functions are blood flow and energy metabolism, both of which increase with
increased functional activity. Changes in blood flow and/or energy metabolism that PET measures are used to localize the sites
of altered functional activity in the nervous system.
Recently, the MRI has been used to detect and image changes in MRI signals in specific regions of the brain that are known
to be associated with specific functions when the functional activity is these regions is altered. The physiological mechanisms
that are responsible for these changes in the MRI signals are being misrepresented. The MRI enthusiasts prematurely attribute
the changes in MRI signals to changes in blood flow, but I believe that they are wrong. There is a strong physiological basis
for the belief that the changes they see are due to changes in the hematocrit in the regions of interest and are not specifically
and directly related to changes in blood flow. As you know, the hematocrit in the brain is considerably less than that in
the peripheral blood, and anything that would dilate the arterial resistance vessels would bring in more oxygenated blood.
Because the brain is contained within a rigid container, blood in the low pressure vessels, i.e., the venules and veins would
be expressed from the region of interest and from the cranial cavity. The net effect would be a fall in the amount of reduced
hemoglobin in the field of interest and, therefore, an enhancement of the proton MRI signal. These changes in hematocrit may
correlate with blood flow changes, but not always necessarily so. Local hematocrit can change with other factors. The uncertainties
in the mechanisms underlying the changes in the MRI signals serves to limit the usefulness of MRI for studies of physiologic
I do not mean to imply that MRI imaging of functional activity in the nervous system has no future. I think that there is
potentially a very great future for it, but it will take years to understand what they are looking at and why the changes
occur. With PET we already know the physiologic basis for the changes.
When the relationship of MRI signals to physiological mechanisms in the nervous system are better understood, then MRI imaging
may achieve a status equal to or maybe even greater than that of PET in activation studies.
You can feel secure that there is a future for PET for the next 5, 10 or more years in studies of cognitive functions, and
I would recommend that you continue those elegant studies which you have already been carrying out with PET.
I am enclosing a couple of photocopies that I believe are very relevant to the current excitement about MRI. The cartoon applies
particularly to the current propaganda in the MRI field. So also does the copy of the article from the Washington Post.