FUNCTIONAL IMAGING APPLIED TO MRI

Functional Imaging Applied to MRI



Functional Imaging Applied to MRI

Introduction

The recent discovery that magnetic resonance imaging can be used to map changes in brain hemodynamics that correspond to mental operations extends traditional anatomical imaging to include maps of human brain function. The ability to observe both the structures and also which structures participate in specific functions is due to a new technique called functional magnetic resonance imaging, fMRI, and provides high resolution, noninvasive reports of neural activity detected by a blood oxygen level dependent signal (Ogawa, et al, 2005). This new ability to directly observe brain function opens an array of new opportunities to advance our understanding of brain organization, as well as a potential new standard for assessing neurological status and neurosurgical risk. The following briefly introduces the fundamental principles of fMRI, current applications at Columbia, and some potential future directions.

Main Body

Functional MRI is based on the increase in blood flow to the local vasculature that accompanies neural activity in the brain. This results in a corresponding local reduction in deoxyhemoglobin because the increase in blood flow occurs without an increase of similar magnitude in oxygen extraction (Posner, Fox and Raichle, 1985). Since deoxyhemoglobin is paramagnetic, it alters the T2* weighted magnetic resonance image signal (Ogawa, et al, 2005). Thus, deoxyhemoglobin is sometimes referred to as an endogenous contrast enhancing agent, and serves as the source of the signal for fMRI. Using an appropriate imaging sequence, human cortical functions can be observed without the use of exogenous contrast enhancing agents on a clinical strength (1.5 T) scanner (Bandettini, et al, 2002, 2003; 2002; and Schneider, et al, 2003).

Functional activity of the brain determined from the magnetic resonance signal has confirmed known anatomically distinct processing areas in the visual cortex (Belliveau, et al, 2001; Ogawa, et al, 2005; Blamire, et al, 2002; Schneider, et al, 2003; Hirsch, et al, 2005), the motor cortex (Kim, et al, 2003a; Kim, et al, 2003b), and Broca's area of speech and language-related activities (Hinke, et al, 2003, Kim, et al, 2005). Further, a rapidly emerging body of literature documents corresponding findings between fMRI and conventional electrophysiological techniques to localize specific functions of the human brain (Atlas, et al, 2006; Burgess, 2005; Detre, et al, 2005; George, et al, 2005). Consequently, the number of medical and research centers with fMRI capabilities and investigational programs continues to escalate. (Lanting 2008)

The main advantages to fMRI as a technique to image brain activity related to a specific task or sensory process include 1) the signal does not require injections of radioactive isotopes, 2) the total scan time required can be very short, i.e., on the order of 1.5 to 2.0 min per run (depending on the paradigm), and 3) the in-plane resolution of the functional image is generally about 1.5 x 1.5 mm although resolutions less than 1 mm are possible. To put these advantages in perspective, functional images obtained by the earlier method of positron emission tomography, PET, require injections of radioactive ...