Diffusion MRI measures the Brownian motion of water molecules in brain tissue on length scales ranging from 10 nm to 100 µm, and on time scales ranging from 1 ms to 1 s (1
). Given that the average size of neurons in the brain is approximately 10 µm, diffusion MRI provides a sensitive probe of tissue microstructure. Diffusion MRI has emerged as a powerful tool to investigate a wide range of neuropathologies such as stroke, Parkinson’s disease, HIV dementia, schizophrenia, cocaine addiction, normal aging, Alzheimer’s disease, chronic alcoholism, multiple sclerosis, epilepsy, and amyotrophic lateral sclerosis (2
). More recently, diffusion MRI has also been proposed as a promising technique to identify anatomical white-matter fiber tracts in vivo (4
) and even to measure neural activity (8
In clinical diffusion MRI, the two scalar measures of diffusion in cerebral tissue are the apparent diffusion coefficient (ADC) and fractional anisotropy (FA). The ADC represents the magnitude of diffusion, whereas the FA is a measure of its anisotropy. Despite the widespread applications and usefulness of diffusion MRI in basic and clinical neuroscience, the underlying biophysical mechanisms which affect diffusion contrast (ADC and FA) remain largely unknown (9
Initially, it had been assumed that FA in cerebral white matter was caused by the diffusion barrier presented by the myelin sheath. However, diffusion anisotropy has been observed in de-myelinated garfish nerve (10
), premyelinated newborn white matter (11
), cortical gray matter in rat (12
), and the thalamus (13
). Furthermore, FA changes in ischemia studies have been shown to occur over longer time periods (hours to days) and after lesions causing anterograde and retrograde secondary white matter degeneration (14
A recent study suggested that ADC depends on neuronal activity (8
). It has also been suggested that the ADC of water decreased during the inhibition of fast axonal transport (10
). Another study in which microtubule concentration was depleted found no affect on the ADC (16
). Yet, the latter experiments were confounded by crystal formation during the artificial breakdown of microtubule, which could have affected the ADC. Because of the immediate decrease in ADC during (i) ischemia, (ii) spreading depression, (iii) inhibition of fast axonal transport (10
), (iv) and changes during neuronal activity (8
), we hypothesized that processes directly dependent on cerebral metabolism affect the ADC.
The purpose of this study is to describe the methodology that we developed to focally reversibly deactivate cerebral metabolism and measure the diffusion MRI signal. By focally deactivating cerebral metabolism in a reversible manner, the relationship between the diffusion MRI signal and one aspect of neural activity, the fMRI response, can be quantified throughout the brain. A later study will discuss the application of this methodology to study the effect of reversible deactivation on the hemodynamic response. Monkeys were chosen instead of rats because of the future desire to compare the ADC with the hemodynamic response in visual cortex.
To reversibly deactivate the cortex, we developed an MRI-compatible cortical cooling probe. Indeed, it is well-established that cooling cortical tissue below 20°C blocks metabolism, hence also neuronal activity (17
). By reversibly deactivating cerebral metabolism, rather than making permanent lesions or inducing stroke, we avoided confounding mechanisms that could contribute to the diffusion signal as observed during neuropathology, such as functional and anatomical reorganization (plasticity), blood accumulation, macrophage infusion, gliosis, and necrosis. Marked cooling could also have an ischemic effect and ADC decreases would be expected in the deactivation region. In deoxyglucose (DG) studies, DG uptake decreased in foci connected with the deactivated site which would could also cause ADC decreases outside the deactivation region (21
Hitherto, it has been difficult to quantify in vivo the degree and three-dimensional (3D) extent of brain tissue affected during reversible deactivation experiments, irrespective of the method used (e.g., cortical cooling or drug injection). To overcome this problem, we measured the temperature of the brain inside the MR-scanner using proton resonance frequency shift thermometry (PRFST) (22
). To assess the accuracy and precision of the PRFST method, we compared ex vivo MR thermometry measurements with concurrent temperature measurements using MRI-compatible thermocouples. The PRFST method allowed us to calculate 3D temperature maps (isothermals) at a temporal resolution of four seconds and infer the amount of affected cortex during a cooling experiment. A potential confound during combined cooling-DTI experiments is the temperature dependence of ADC, such that a change in the ADC may simply reflect a change in temperature. However, the in vivo 3D temperature maps allowed us to quantify precisely ADC changes in voxels where the temperature did not change.
Another potential confound of this study is that cooling will induce a physiological response in the local vasculature (i.e., blood flow) so that a change in ADC could not be attributed solely to changes in metabolism (24
). Although the authors believe that the small volume fraction of blood in cerebral tissue, ~5% (25
), could not be responsible for the changes in diffusion observed, the quantification of changes in blood flow during deactivation was not measured directly and warrants investigation.
In summary, we were able to reversibly deactivate cerebral metabolism in a small portion of macaque V1 in a MRI-compatible manner. Moreover, we were able to define dynamically the extent of this deactivation. Short-term deactivations (~30 min) led to pronounced changes in ADC, but not FA, in voxels at distant sites from the deactivated region where the temperature had not changed. These data suggest that a considerable fraction of the ADC in the brain depends on normal cerebral metabolism.