Chapter 3

Functional Magnetic Resonance Imaging

\label{ch:chapter3}

Introduction

It has been the goal for centuries now to understand functional organization of the human brain, but experimental tools to so have only been developed in a later period. As one of the most vital organs for the human, the brain is an extremely complex machine serving as the center control of all the nervous system. Hence, still today, the phrase ”little is known about the human brain” is valid and contemporary.

In order to detect neural activity, a highly precise method requires the placement of a microscopic electrode directly near or within a neuron of interest. These single-unit recordings detect the rate of change in voltage if the neuron generates an action potential. However, it is an invasive technique, not applicable to healthy human subjects in a gross population. Electrophysiogical monitoring non-invasive devices came about in the late 1800s with electroencefalography and in the mid-19s with magnetoencefaloraphy. These electromagnetic recoding methods measured at the scalp or hear the head provide information of the rapid changes in electric potentials and magnetic flux. From these data the location of the source of the activity can be estimated, but solution to this inverse problem is not sufficient to produce a detailed map of the pattern of activation.

More feasible approaches that from the basis of functional neuroimaging techniques nowadays rely either on positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Given the non-invasive nature of the technique, fMRI has been used to produce spatial maps of brain activation applied in both basic clinical neuroscience (Matthews et al., 2006; Detre, 2006) and even in pre-surgical planning (Krings et al., 2001; Bookheimer, 2007). In this chapter the physiological principles and analysis of functional MRI data will being reviewed.

The BOLD effect

As introduced in section LABEL:Section_2.5, in BOLD fMRI the deoxyhaemoglobin (dHb) acts as as an endogeneous paramagnetic contrast agent altering signal intensity in magnetic resonance images. It is thus important to understand the chain of events that characterize the signal changes in BOLD imaging.

The working brain requires a continuous supply of glucose and oxygen, which must be supplied via the cerebral blood flow (CBF). Although the brain accounts for only about 2% of the total human mass, it receives more than 15% of the total cardiac output. In addition, the distribution of blood in the brain is heterogeneous, where gray matter receives most of the blood volume per cycle than white matter.

Early in 1890, Roy and Sherrington were the first to report that cerebral blood flow (CBF) reflected neuronal activity by performing experimentations in the brain of dogs (Roy et al., 1890). In fact, task and/or stimulation induce synaptic electric activities at localized regions in the brain, which will trigger the increase of CBF, cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMR\({}_{O_{2}}\)) and cerebral metabolic rate of glucose (CMR\({}_{glc}\)).