The road to functional imaging and ultrahigh fields

Abstract

The Center for Magnetic Resonance (CMRR) at the University of Minnesota was one of the laboratories where the work that simultaneously and independently introduced functional magnetic resonance imaging (fMRI) of human brain activity was carried out. However, unlike other laboratories pursuing fMRI at the time, our work was performed at 4T magnetic field and coincided with the effort to push human magnetic resonance imaging to field strength significantly beyond 1.5T which was the high-end standard of the time. The human fMRI experiments performed in CMRR were planned between two colleagues who had known each other and had worked together previously in Bell Laboratories, namely Seiji Ogawa and myself, immediately after the Blood Oxygenation Level Dependent (BOLD) contrast was developed by Seiji. We were waiting for our first human system, a 4T system, to arrive in order to attempt at imaging brain activity in the human brain and these were the first experiments we performed on the 4T instrument in CMRR when it became marginally operational. This was a prelude to a subsequent systematic push we initiated for exploiting higher magnetic fields to improve the accuracy and sensitivity of fMRI maps, first going to 9.4T for animal model studies and subsequently developing a 7T human system for the first time. Steady improvements in high field instrumentation and ever expanding armamentarium of image acquisition and engineering solutions to challenges posed by ultrahigh fields have brought fMRI to submillimeter resolution in the whole brain at 7T, the scale necessary to reach cortical columns and laminar differentiation in the whole brain. The solutions that emerged in response to technological challenges posed by 7T also propagated and continues to propagate to lower field clinical systems, a major advantage of the ultrahigh fields effort that is underappreciated. Further improvements at 7T are inevitable. Further translation of these improvements to lower field clinical systems to achieve new capabilities and to magnetic fields significantly higher than 7T to enable human imaging is inescapable.

FIGURE 1

(A) and (B): Early 4 Tesla brain images published from Siemens (Barfuss et al. 1988; Barfuss et al. 1990). (C) and (D): 4 Tesla MDEFT images of human brain obtained in CMRR (Ugurbil et al. 1993; Lee et al. 1995).

FIGURE 2

The “Heineken” coaster that I and others signed 1-April-1987 as we enjoyed a beer on the occasion of a visit to Philips by me, Gerald Pohost and some of his colleagues from the University of Alabama to discuss 4 Tesla.

FIGURE 3

Image of two slices obtained in intact anesthetized porcine torso 9.4 Tesla with four loops, somewhat mimicking a multichannel transmits and receive coil (built by Hellmut Merkle in CMRR); the images were shown in a plenary talk at the 1995 annual meeting of the RSNA to demonstrate the feasibility of (and our intent to pursue) imaging human heads at such high field strengths.

FIGURE 4

Two Faxes from David Rayner dated 03 August 1995 and 01 April 96; discussing the 7T initiative. The approximate pricing supplied is blanked out. The color markings in the 1996 Fax identify our deliberations in CMRR at the time. Ultimately we decided on the 7.0/900 magnet; this design was employed on all 7T systems installed until 2011 when two new, actively shielded 7T magnets with 830 and 900 cm bore diameters were recently installed.

FIGURE 5

Number of 7T human systems ordered as of April 2010. Data compiled from information obtained from Magnex/Varian (now Agilent) and Siemens.