Work on the neural correlates of human behaviors with PET began with studies of brain glucose metabolism and the tracerF-2-fluoro-2-deoxy--glucose (FDG). This was an extension of the autoradiographic deoxyglucose technique ( Box 3 ) developed by Sokoloff and his colleagues (National Institutes of Health) for studies in laboratory animals []. Sokoloff had demonstrated the sensitivity of this technique to functional changes in neuronal activity in a wide-ranging group of animal experiments (for an excellent early summary see Ref. []).

This involves the systemic administration of a radiolabeled compound of interest. When the compound has reached the organ of interest the animal is sacrificed and the organ is removed, sliced and placed on X-ray film enabling researchers to assess the distribution of the radiolabeled compound. In the case of C-deoxyglucose (DG), an analog of the primary fuel of the brain, glucose, it enters the brain where it is taken up by cells in proportion to their metabolism. On entry into the cell it is phosphorylated. Once this has occurred the DG is trapped in the cell because its further metabolism is blocked because of its structure and exit from the cell is blocked because it is phosphorylated. Exquisite maps of brain metabolism in animals resulted from the extensive use of this technique in neuroscience research. This technique was extended to PET using deoxyglucose labeled with fluorine [] rather than carbon [].

The [ 14 C]deoxyglucose method for the measurement of local cerebral glucose utilization: theory, procedure, and normal values in the conscious and anesthetized albino rat.

With FDG PET, 30–40 min was used typically for image acquisition. Because of this long data-acquisition time, functional brain mapping as we know it today would not have developed. Needed were speed of data acquisition and measurement repeatability to assess brain function. Because of this, blood flow became the technique of choice with PET because it could be measured quickly (<1 min) using an easily produced radiopharmaceutical (H 2 15O) whose short half life (123 s) enabled repeat (up to 12) measurements in the same subject.

Although clinical imaging with MRI flourished after its introduction, there also existed in the scientific community the knowledge that MRI could also tell us much about tissue chemistry, perfusion and metabolism. This view was the result of many years of basic NMR research indicating that intrinsic signals from tissue in addition to signals from exogenously administered MRI contrast agents could be used to extract such information. Functional brain imaging with MRI was able to tap this knowledge base with remarkable results. It was an interesting convergence of research in MRI techniques, studies of the magnetic properties of deoxyhemoglobin and research on circulatory and metabolic correlates of changes in brain activity (see earlier) that led to the development of this approach.

In 1973, Paul Lauterbur [] came up with a strategy in which the NMR signals could be used to create cross-sectional images in much the same manner as CT (for a review, see Ref. []). Interest was immediate not only because the technique was free of any ionizing radiation but also because it produced superb images of the human body with much greater detail and variety than CT because of its sensitivity to soft tissues. Although imaging with this technique initially retained the name nuclear magnetic resonance or NMR, it was changed after a short time to magnetic resonance imaging or MRI to eliminate the term ‘nuclear’, which some thought might detract from its clinical acceptance.

The physical principles associated with MRI were discovered independently by Felix Bloch (Harvard University) and Edward Purcell (Stanford University) and his colleagues in 1946 (for detailed early reviews of this work see Refs []). Many years of research followed, in which the technique was used for basic research in chemistry. During this time it was known as nuclear magnetic resonance (NMR).

Finally, another technology emerged contemporaneously with PET and CT. This was MRI. MRI is based upon yet another set of physical principles associated with the behavior of atoms in water (often described by their nuclei or protons) in a magnetic field. When placed in a strong magnet field, these protons behave like tiny bar magnets by lining up in parallel with the magnet field. When these protons are disturbed from their equilibrium state by radio frequency pulses, a voltage is induced in a receiver coil that can be characterized by its change in magnitude over time. Because these time-dependent changes in voltage are a function of the local environment of the protons, many important deductions can be made about the tissue being examined.

The emergence of functional MRI

38 Villringer A.

et al. Dynamic imaging with lanthanide chelates in normal brain: contrast due to magnetic susceptibility effects. 39 Belliveau J.W.

et al. Functional cerebral imaging by susceptibility-contrast NMR. 40 Belliveau J.W.

et al. Functional mapping of the human visual cortex by magnetic resonance imaging. A important first step in the development of fMRI was the work of group of researchers at the Massachusetts General Hospital working on the use of exogenously administered MRI contrast agents designed to produce transient changes in the MRI image as the agent passed through the brain after its intravenous administration. Work in rodents [] and dogs [] using contrast agents confined to the vascular compartment and novel rapid data acquisition strategies demonstrated for the first time with MRI that it was possible to measure changes in brain blood volume produced by physiological manipulations of brain blood flow. This approach was extended to normal human volunteers for task activation brain mapping by the same group in 1991 [] in a much heralded study demonstrating for the first time that MRI was to be a serious player in the functional mapping of the human brain.

This study caused much excitement, especially in the MRI community, which was anxious to join the rapidly expanding cognitive neuroscience revolution. What this study demonstrated clearly was that, within one imaging modality, superb anatomical images and physiology relevant to brain function could be combined. The primary limitation of the approach was the need to administer a contrast agent, which is something that could only be done a limited number of times. However, physiology ( Figure 2 a) and the ingenuity of the MRI researchers came to the rescue. The answer came from the property of deoxyhemoglobin in a magnetic field.

41 Faraday M. Faraday's Diary. Being the Various Philosophical Notes of Experiment Investigation During the Years 1820–1862. 23 Pauling L.

Coryell C.D. The magnetic properties and structure of hemoglobin, oxyhemoglobin and carbonmonoxyhemoglobin. It was Michael Faraday who first studied the magnetic properties of hemoglobin. In experiments on the 8 November 1845 he noted (to his surprise because hemoglobin contains iron) that dried blood was not magnetic, writing ‘Must try fluid blood’ []. Remarkably, 91 years later his obscure laboratory note somehow caught the attention of Linus Pauling and Charles Coryell [] who found that the magnetic susceptibilities (i.e. the ability to interact with a magnetic field) of oxygenated and deoxygenated hemoglobin differed significantly. Deoxyhemoglobin was paramagnetic and, hence, equivalent to an MRI contrast agent, whereas oxyhemoglobin was not ( Box 1 ). Of course, the role of this observation in the development of fMRI did not occur to Pauling and Coryell.

42 Thulborn K.R.

et al. Oxygenation dependence of the transverse relaxation time of water protons in whole blood at high field. In 1982, Keith Thulborn took the story one step further while seeking to exploit the difference in magnetic susceptibility of oxy- and deoxyhemoglobin for the measurement of brain oxygen consumption with MRI []. In this often overlooked work he clearly demonstrated the feasibility of measuring the state of oxygenation of blood in vivo with MRI, another crucial step on the road to fMRI BOLD imaging as we know it.

43 Ogawa S.

et al. Brain magnetic resonance imaging with contrast dependent on blood oxygenation. 43 Ogawa S.

et al. Brain magnetic resonance imaging with contrast dependent on blood oxygenation. Experiments performed by Sieji Ogawa and colleagues (AT&T Bell Laboratories) were a crucial next step in establishing the basis of fMRI BOLD imaging. In their experiments, the concentration of deoxygenated blood in the brains of living rodents was manipulated by alternately breathing his animals on room air and 100% oxygen. On room air, detailed anatomy of venules and veins were easily visible throughout the rat brain as dark structures ( Figure 2 b). This was owing to the loss of MRI signal in the presence of deoxyhemoglobin ( Box 1 ). On 100% oxygen, the venous structures disappeared. Ogawa labeled his finding ‘blood oxygen level dependent contrast’ or BOLD contrast [] and noted that ‘BOLD contrast adds an additional feature to magnetic resonance imaging and complements other techniques that are attempting to provide positron emission tomography-like measurements related to regional neural activity’ [].

44 Kwong K.K.

et al. Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation. 45 Bandettini P.A.

et al. Time course EPI of human brain function during task activation. 46 Ogawa S.

et al. Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging. 44 Kwong K.K.

et al. Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation. 46 Ogawa S.

et al. Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging. 47 Raichle M. A brief history of human functional brain mapping. The potential of BOLD fMRI was soon realized ( Figure 2 c) with publications from three groups in 1992 []. The events leading up to these publications (particularly from the group at the Massachusetts General Hospital led by Ken Kwong [] and the group that represented the combined resources Sieji Ogawa and David Tank from the AT&T Bell Laboratories and Kamil Ugirbil, Ravi Menon and their colleagues at the University of Minnesota []) provide insight into how the research actually unfolded. Space here does not permit a detailed recounting of these fascinating events. Interested readers might wish to read a more detailed account, based on my interviews with the key participants [].

44 Kwong K.K.

et al. Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation. 48 Kim S.G.

et al. Determination of relative CMRO2 from CBF and BOLD changes: significant increase of oxygen consumption rate during visual stimulation. Since the introduction of fMRI BOLD imaging, the growth of functional brain imaging has been nothing short of spectacular. Although MRI also offers additional approaches to the measurement of brain function [], it is BOLD imaging that has dominated the research agenda thus far.

However, the success of the human brain imaging was the product not only of relevant physiology that could be imaged and the scanning devices that could accomplish this but also of the behavioral paradigms that approached human behavior in a principled and quantitative manner while accommodating the constraints of the imaging environment and strategies to process the resulting data. I turn to these important issues next.