Cerebral [¹⁵O] Water Clearance in Humans Determined by Positron Emission Tomography: II. Vascular Responses to Vibrotactile Stimulation

Abstract

When used to measure blood flow, water leaves a residue in the vascular bed, which may contribute to the calculation of increased blood flow during functional activation of brain tissue. To assess the magnitude of this contribution with the two-compartment positron emission tomography (PET) method, we mapped the water clearance (K1) of the brain as an index of cerebral blood flow (CBF) and the apparent vascular distribution of nonextracted H2 15O (Vo). The latter map represented mainly the cerebral arterial and arteriolar volume. We also prepared subtraction maps (delta K1, delta Vo) of the response to vibrotactile stimulation of the fingertips of the right hand of six normal volunteers. Using magnetic resonance (MR) images of all subjects, the data were rendered into Talairach's stereotaxic coordinates and the averaged subtraction images (activation minus baseline) merged with the corresponding averaged MRI image. The delta K1 map revealed the expected response in the primary sensory hand area; the delta Vo response was located about 13 mm more anteriorly, close to the central fissure, most likely reflecting changes of the arteries feeding the primary sensory hand area. We conclude that cerebral perfusion and cerebrovascular responses to vibrotactile stimulation may occur in disparate locations that can be identified separately by using the two-compartment method.

Tactile‐associated fMRI recruitment of the cervical cord in healthy subjects

“…This effect, called ''signal enhancement by extravascular protons'' (SEEP) [Stroman et al, 2002c[Stroman et al, , 2003b[Stroman et al, , 2005b, is thought to arise from a local change in fluid balance, which in turn may result from changes in perfusion pressure, production of extracellular fluid, neurons and glial cells swelling, and maintenance of ion and neurotransmitter concentrations at sites of neuronal activity [Fujita et al, 1997;Ohta et al, 1996;Stroman and Andrew, 2007;Stroman et al, 2002cStroman et al, , 2003bStroman et al, , 2005b. Previous studies showed that both the BOLD and the SEEP effects give a contribution to the signal change observed during spinal fMRI activity, with different relative proportions depending on field strength and TE value used [Stroman, 2005;Stroman et al, 2002c].…”

Magnetic Resonance Imaging of Neuronal Function in the Spinal Cord: Spinal fMRI

“…Positron emission tomography (PET) studies using a radiolabeled water tracer have demonstrated that there is increased unidirectional clearance of water from the blood into the tissues at sites of neuronal activity in the brain. 17,18 Another factor influencing the local fluid balance is cellular swelling at sites of activity. Astrocytes make contact with both blood vessels and neurons and play a role in providing metabolites to neurons and in maintaining the extracellular concentration of glutamate.…”

Detection of the neuronal activity occurring caudal to the site of spinal cord injury that is elicited during lower limb movement tasks

“…[7][8][9] In addition, the increased blood flow to the active tissues has been shown to be accompanied by increased intravascular pressure, and increased production of extracellular fluid at sites of neuronal activity. 10,11 The net effect is a local increase in water content near active neural tissues which causes a higher MR signal intensity. Thus, spinal fMRI indirectly detects neuronal activity in spinal cord gray matter, and allows for mapping of spinal cord responses to sensory and motor stimulation.…”