Brain imaging
Brain imaging is a fairly recent discipline within medicine and neuroscience. Brain imaging falls into two broad categories -- structural imaging and functional imaging. The former deals with the overall structure of the brain and diagnosis; the latter involves monitoring of neural processes in the brain.Functional imaging is used for neurological and cognitive science research and building brain-computer interfaces. It includes sensing information coming from the brain and commands going from the brain to the organism.
Structural imaging began with early radiographic techniques to image the human brain. Unfortunately, because it is largely composed of soft tissue, the brain and remained largely invisible. This is also true of brain abnormalities. Crude images of the ventricular system within the brain were obtained by air injection -- a painful procedure.
With the advent of computerized axial tomography (CAT), detailed anatomic images of the brain became available for diagnostic and research purposes. Soon after, the development of radioligands started the functional imaging revolution. Radioligands either remain within the blood stream or enter the brain and bind to receptors. Radioligands are either single photon or positron emitters. This is how single photon emission computerized tomography (SPECT) and positron emission tomography (PET) got their names.
Early techniques such as xenon inhalation provided the first blood flow maps of the brain. Functional imaging took a large step forward with the development of oxygen-15 labelled water (H215O, or H20-15) imaging. H20-15 emits positrons and creates images based on regional blood flow within the brain. Since active neurons recruit a robust blood supply, H20-15 PET allowed investigators to make regional maps of brain activity during various cognitive tasks.
Concurrently, magnetic resonance imaging (MRI) was developed. Rather than using radiation, MRI uses the variation in signals produced by protons in the body when the head is placed in a strong magnetic field. At first, structural imaging benefited more than functional imaging from the introduction of MRI.
However, scientists soon learned that the large blood flow changes measured by H20-15 PET were also imaged by MRI. Functional magnetic resonance imaging (fMRI) was born. Since the 1990s, fMRI has come to dominate the brain mapping field due to its low invasiveness, lack of radiation exposure, and relatively wide availability.
Physicists have also developed other MRI-based techniques such as magnetic resonance spectroscopy (for measuring some key metabolites such as n-acetylaspartate and lactate within the living brain) and diffusion tensor imaging (for mapping white matter tracts within the living brain). Structural MRI and CAT scanning have a large place in medicine, however fMRI and its brethren are still largely devoted to neuroscience research.
The improvement of neuroscience is the main desire of many researchers in cognitive science and philosophy today. For this to happen, our ability to develop and understand brain mapping and imaging technologies is absolutely crucial. In order to survey some of the brain imaging and recent breakthroughs in brain mapping, this article will look in detail at a brief history of brain imaging technology, the currently established technologies, recent breakthroughs, and implications of developing a more completed neuroscience.
The desire to understand the human mind has been one of the main desires of philosophers throughout the ages. Questions about thoughts, desires, etcetera have drawn psychologists, computer scientists, philosophers, sociologists and the like together into the new discipline of cognitive science. Non-invasive imaging of the human brain has proven invaluable in this context. Brain imaging, as we know it today, began in the 1970s with a technique called pneumoencephalography. This process required air to be put into the ventricles of the brain for an air-contrasted skull x-ray and was considered to be incredibly unsafe for patients (Beaumont 8). A form of magnetic resonance imaging (MRI) and computerized tomography (CT) were formed slightly more than a decade later. The new MRI and CT technologies were considerably less harmful and are explained in greater detail below. Next came SPECT and PET scans, which allowed scientists to map brain function because, unlike MRI and CT, these scans could create more than just static images of the brain's structure. Learning from MRI, PET and SPECT scanning, scientists were able to develop functional MRI (fMRI) with abilities that opened the door to direct observation of cognitive activities. To understand these techniques better, each will now be explained more thoroughly.
Positron Emission Tomography (PET) measures emissions from radioactively labeled chemicals that have been injected into the bloodstream and uses the data to produce two or three-dimensional images of the distribution of the chemicals throughout the brain (Nilsson 57). PET scans involve the use of a machine called a cyclotron to label chemicals with small amounts of radioactivity. The labeled compound, called radiotracer, is injected into the bloodstream and eventually makes its way to the brain. Sensors in the PET scanner detect the radioactivity as the compound accumulates in different regions of the brain. A computer uses the data gathered by the sensors to create multicolored two or three-dimensional images that show where the compound acts in the brain.
The greatest benefit of PET scanning is that different compounds can show blood flow and oxygen and glucose metabolism in the tissues of the working brain. These measurements reflect the amount of brain activity in the various regions of the brain and allow us to learn more about how the brain works. PET scans were superior in terms of resolution and speed of completion (as little as 30 seconds) when they first came online. The improved resolution permitted better judgments to be made as to the area of the brain activated by a particular task. The biggest drawback of PET scanning is that because the radioactivity decays rapidly, it is limited to monitoring short tasks (Nilsson 60). Before fMRI technology came online, PET scanning was the preferred method of brain imaging, and it still continues to make large contributions to neuroscience.
Similar to PET, Single Photon Emission Computed Tomography (SPECT) uses radioactive tracers and a scanner to record data that a computer uses to construct two- or three-dimensional images of active brain regions (Ball). SPECT tracers are considered to be more limited than PET scanners in the kinds of brain activity they have the ability to monitor. The tracers of SPECT are longer lasting than those of PET, which allows for different, longer lasting brain functions to be examined, but this also requires more time for the SPECT to be completed. The resolution of a SPECT is poor (about 1 cm) compared to that of PET. SPECT is often chosen over PET simply as a cost issue, for less equipment is involved and fewer staff is required to perform the tests.
Electroencephalography (EEG) is the oldest of the modern brain imaging techniques and uses electrodes placed on the scalp to detect and measure patterns of electrical activity coming from the brain. There have been many recent developments regarding EEG's ability to read brain activity data from the entire head simultaneously (Thompson, Bioinformatics). Using scale electrodes, EEG can determine the relative strengths and positions of electrical activity in different brain regions by measuring electrical activity on the outside of the brain. EEG records timing of activity very precisely but resolution is poor and does not directly record interior brain activity. As a result, researchers often use EEG images of brain electrical activity in combination with MRI scans to better pinpoint the location of the activity in the brain.
Magnetic Resonance Imaging (MRI) uses magnetic fields and radio waves to produce high quality two- or three-dimensional images of brain structures without injecting radioactive tracers. During an MRI, a large cylindrical magnet creates a magnetic field around the head of the patient through which radio waves are sent. When the magnetic field is imposed, each point in space has a unique radio frequency at which the signal is received and transmitted (Preuss). Sensors read the frequencies and a computer uses the information to construct and image. The detection mechanisms are so precise that changes in structures over time can be detected. Using MRI, scientists can create images of both surface and subsurface structures with a high degree of anatomical detail. MRI scans can produce cross sectional images in any direction from top to bottom, side to side, or front to back. The problem with original MRI technology was that while it provides a detailed assessment of the physical appearance of the brain, it fails to provide information about how well the brain is working at the time of imaging. The distinction is now made between MRI imaging and functional imaging since the brain's function rather than the brain's structure is of interest.
Functional MRI (fMRI) relies on the magnetic properties of blood to enable scientists to see images of blood flow in the brain as it occurs. This mapping of blood flow allows for dynamic brain mapping to take place (Shorey). During the test, the subject is normally asked to perform a repetitive motion like tapping a finger or tapping a foot. FMRI has taken the place of PET scanning as the king of brain imaging because fMRI can produce images of the brain every second, and scientists can determine with great precision when brain regions become active and for how long. Also, fMRI has such high resolution that it can distinguish structures less than a millimeter apart. This allows scientists to know exactly which areas of the brain are being activated. PET, however, retains the significant advantage of being able to identify which brain receptors are being activated by neurotransmitters, abused drugs, and potential treatment compounds.
Drawbacks of fMRI are few but substantial at this point. First, it takes quite a bit of time to perform the procedure and the patient needs to be completely still for often more than twenty minutes at a time. Second, and more importantly, interpretations of fMRI results are still vague. It is difficult to determine if the subject was thinking about something that caused certain parts of the brain to activate, if the scanner picked up real data or noise, and so on (Shorey). For these and other reasons, fMRI technology has begun to be combined with EEG technology.
Computerized Tomography (CT) scanning uses a series of x-rays of the head taken from many different directions. Typically used for quickly viewing brain injuries, CT scanning has a computer program that uses a set of algebraic equations to estimate how much x-ray is absorbed in a small area within a cross section of the brain (Jeeves 21). In the final analysis, the harder a material is, the whiter it will appear on the scan. CT scans are primarily used for evaluating swelling from tissue damage in the brain and in assessment of ventricle size. Modern CT scanning exposes the subject to about as much radiation as a single x-ray and can provide reasonably good images in a matter of minutes.
Recent breakthroughs in non-invasive brain imaging have been somewhat limited because most of them have not been completely novel; rather, they are simply refining existing brain imaging techniques. FMRI is a perfect example of this from the early 1990s, and it still remains the most accurate brain imaging technique available today. Advances have been made in a number of ways regarding neuroimaging, and this section will cover some of the more prominent improvements including computational advances, transcranial magnetic stimulation, and nuclear magnetic resonance.
To begin with, much of the recent progress has had to do not with the actual brain imaging methods themselves but with our ability to utilize computers in analyzing the data. The UCLA Laboratory of Neuro Imaging (LONI), lead by Dr. Paul Thompson, has made substantial discoveries in the growth of human brains from age three months to the age of fifteen (Thompson, UCLA). MRI technology was used to create high-resolution maps and computer technology was used to analyze the maps over various periods of time and growth. This type of breakthrough represents the nature of most breakthroughs in neuroscience today. With fMRI technology mapping brains beyond what we are already understanding, most innovators time is being spent trying to make sense of the data we already have rather than probing into other realms of brain imaging and mapping.
This can be seen more clearly in the fact that brain imaging archives are catching on and neuroinformatics is allowing researchers to examine thousands of brains rather than just a few (Lynch). Also, these archives are universalizing and standardizing formats and descriptions so that they are more searchable for everyone. For the past decade we have been able to get data and now our technology allows us to share findings and research much easier. This has also allowed for "brain atlases" to be made. Brain atlases are simply maps of what normal functioning brains look like (Thompson, Bioinformatics).
Another example of technology allowing relatively older brain imaging techniques to be even more helpful is our ability to combine the different techniques to get one brain map. This happens quite frequently with MRI and EEG scans. The electrical diagram of the EEG provides split-second timing while the MRI provides high levels of spatial accuracy.
Transcranial magnetic stimulation (TMS) is the most interesting, and perhaps the most obscure, of all the recent innovations in brain imaging. With TMS, a coil is held near a person's head and the coil generates magnetic field impulses that stimulated underlying brain cells to make someone perform a specific action (Leventon). Using this in combination with MRI, the researcher can generate maps of the brain performing very specific functions. Instead of asking a patient to tap his or her finger, the TMS coil can simply "tell" his or her brain to tap his or her finger. This eliminates many of the false positives received from traditional MRI and fMRI testing. The images received from this technology are slightly different from the typical MRI results, and they can be used to map any subject's brain by monitoring up to 120 different stimulations. This technology has been used to map both motor processes and visual processes (Potts link at bottom of TMS).
Nuclear magnetic resonance (NMR) is what MRI and fMRI technologies were derived from, but recent advances have been made by going back to the original NMR technology and revamping some of its aspects. NMR traditionally has two steps, signal encoding and detection, and these steps are normally carried out in the same instrument. The new discovery, however, suggests that using laser-polarized xenon gas for "remembering" encoded information and transporting that information to a remote detection site could prove far more effective (Preuss). Separating the encoding and detection allows researchers to gain data about chemical, physical, and biological processes that they have been unable to gain until now. The end result allows researchers to map things as big as geological core samples or as small as single cellss.
It is interesting to see how advances are split between those seeking a completely mapped brain by utilizing single neuron imaging and those utilizing images of brains as subjects perform various high-level tasks. Single neuron imaging (SNI) uses a combination of genetic engineering and optical imaging techniques to insert tiny electrodes into the brain for the purpose of measuring a single neuron's firing. Due to its damaging repercussions, this technique has only been used on animals, but it has shed a lot of light on basic emotional and motivational processes. The goal of studies in higher-level activities is to determine how a network of brain areas collaborates to perform each task. This higher-level imaging is much easier to do because researchers can easily use subjects who have a disease such as Alzheimer's. The SNI technology seems to be going after the possibility for AI while the network-probing technology seems to be more for medical purposes.
In early 2000s the field of brain imaging reached the stage where limited practical applications of functional brain imaging became feasible. The main application area is crude forms of brain-computer interface.
In order to best present the implications of new and better developed brain imaging technologies, it will be easiest to evaluate the implications under the categories of medicine, law, and education.
When the increased possibility of mapping brains at the neuronal or cellular level is coupled with the increased possibility for nanotechnology, the results can be very striking. Our ability to understand the brain could both be aided by and of aid to nanotechnology. Autonomous nanotech devices could disperse to defined locations in the brain and could be used as sensors for reporting back new information, or they could be used to eliminate unwanted cell types such as tumors.
Tumors of the brain often have a tendency to grow between normal cells and are thus quite invasive and difficult to fully eradicate. Devices can be designed to recognize tumorous cells and selectively destroy them with some sort of toxin. They can be guided to the tumor mass by using information gathered from fMRI technology, and as mentioned above, they could remain in place to act as sensors for reporting back new information. People with seizures are another good example of something that could be helped with this technology. Blindness resulting from eye disease may be correctable by replacing the eye with a nanotechnological equivalent. Optobionics Corporation has already successfully tested its silicon retina on patients with damaged retina cells (Hook). The scenarios where this could come into play seem almost endless. FMRI technology is already allowing the completion of surgeries that were once thought unimaginable (Schulder). Many risky brain surgeries are performed via computerized images of brains by the surgeons, and then carried out on the patient by means of smaller, less invasive mechanisms.
Closer on the horizon than the above-mentioned nanotechnologies would be the possibility for researchers to locate specifically where various diseases are located in the brain and to figure out exactly what is going on. Scientists and doctors are only now beginning to understand where exactly Alzheimers takes effect and what it is that Alzheimers does. Other examples of diseases helped by brain imaging are Parkinsons disease, Lyme disease, schizophrenia, and depression.
The way we teach children may also be impacted by new brain imaging technology. Imaging has confirmed the theory that loving parents make smarter, happier babies, and that talking and playing with a baby and letting him or her see, touch, smell and hear new things will help develop the brains hardwiring (Zwillich). New teaching techniques could be developed based upon what brain imaging research tells us about how the brain reacts to learning various types of things. Also, if we understand all the functions of the various sections of the brain, it is very possible that we could augment the brain through surgery or nanotechnology to give people special abilities in practically any area.
With everything new in society there are always legal issues and the possibilities stemming from brain imaging are no exception. For starters, imaging has shown that multiple brain areas are often dysfunctional in criminals. In psychopaths, the amygdala is not activated by emotional stimulus (Swiercinsk). Many criminal defense lawyers now use brain imaging in the defense of their client. There are many questions brought up by this including the question of whether or not we should force someone to submit to a brain scan if we can stimulate his or her brain in such a way that we will be able to tell what, if any, crimes that person is guilty of. Of course the scenarios like this are unlimited in number. Questions of how the law should limit TMS activity and the ethical implications of that are sure to arise as well.
The expediency with which scientists and researchers are able to fully understand a completely mapped human brain will directly determine the arena of discussion in neuroscience, philosophy, and all of cognitive science for years to come. A completed neuroscience would have far reaching potential implications, including direct mind-computer interface, technologically assisted telepathy and mind transfer. By looking at some current technologies, recent breakthroughs in brain imaging, and some of the implications of brain imaging, this article has barely begun to tip what is building up to be an enormous iceberg.
History, breakthroughs, and implications
History
Recent breakthroughs
Practical achievements of functional brain imaging
Implications of a highly developed neuroscience
See also
Works cited