Biomedical Engineering Reference
In-Depth Information
structural problems. The two most significant disadvantage of MRI are that the technique is expensive
and requires very specialized equipment.
Functional MRI (fMRI) is a reinterpretation of traditional MRI that allows for regions with active
neurons to be highlighted. Active regions of the brain will demand more O 2 , causing oxyhemoglobin
(HbO 2 ) to become deoxyhemoglobin (Hb). Deoxyhemoglobin is paramagnetic and so has a significant
impact on spin interactions and T 2 times. Regional changes in Hb concentration, as a result of neuronal
activity, can therefore be detected as a change in the T 2 times.Again, real-time 3Dmaps can be created.The
disadvantage of fMRI is that neurons must be firing for many seconds before the drop in deoxyhemoglobin
can be detect.
10.2.3 DiffusionTensor Imaging
A technique that also uses MRI technology is Diffusion Tensor Imaging (DTI). The same physics and
equipment are used and the images are similar. But, more information can be added by taking several
closely timed images. When the images are averaged together, regions that are static will be reinforced
while regions that are in motion will be blurred. Since the brain is not moving, the blurring is caused
by the diffusion of water molecules. It has been found that water diffuses much more easily inside the
cytoplasm of the axons and less easily across the membrane. In this way, the pathways of axon bundles
(tracts) can be identified using DTI as the directions along which the most blurring occurs.
10.3 NEURAL STIMULATION
10.3.1 Cortical Stimuli during Surgery
During neurosurgery, a surgeon will do their best to avoid removing or damaging regions that are vital
to the patient's normal cognitive functions. Unfortunately, these areas are not in the same location from
one person to the next so the surgeon will often create a functional map before beginning the surgery.
Creating a function map is often performed by electrically stimulating a region of the cortex and then
asking the patient what they experienced. Remember that neurosurgery is often performed on conscious
patients who feel nothing because the brain has no sensory neurons.
10.3.2 Deep Brain Pacing
Recently, the symptoms of Parkinson's Disease and some forms of chronic pain have been reduced by
injecting current into specific sites deep in the brain. It has also been suggested that repeated pacing
may help depression and prevent epileptic seizures. Furthermore, patient controlled stimulation of select
locations in the brain stem or spinal cord could lead to better control of basic bodily functions. For
example, stimulation of the sacral nerve is an effective means of bladder control.
Building upon the technology of cardiac pacemakers and defibrillators, some companies have
already begun to offer deep brain stimulating devices. There are two potential problems with deep brain
pacing. First, the blood brain barrier must be compromised to insert the electrodes, potentially opening
the way for dangerous bacterial infections and viruses. Second, in some patients an external stimulus can
trigger an epileptic seizure.
10.3.3 Magnetic Recordings and Simulation
We have so far only considered electric fields. Currents in the brain, however, generate extremely small
magnetic fields (10 9
10 6 T) that can be recorded on the scalp by a Superconducting Quantum Inter-
ference Device (SQUID). An individual SQUID element uses a phenomenon known as the Josephson
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