Neuroimaging or brain imaging is the use of various techniques to directly or indirectly describe the structure, function/pharmacology of the nervous system. This is a relatively new discipline in medicine, neuroscience, and psychology. Doctors who specialize in the performance and interpretation of neuroimaging in a clinical setting are neurologists.
Neuroimaging falls into two broad categories:
- Structural imaging, which deals with the structure of the nervous system and diagnosis of large-scale (large) intracranial diseases (such as tumors) and injuries.
- Functional imaging, used to diagnose metabolic diseases and lesions on a finer scale (such as Alzheimer's disease) and also for neurological and cognitive psychological research and to build brain-computer interfaces.
Functional imagery enables, for example, information processing by centers in the brain to be visualized directly. Such processing causes the areas of the brain involved to increase metabolism and "light up" on scanning. One of the more controversial uses of neuroimaging is the study of "mind identification" or mind reading.
Video Neuroimaging
Histori
The first chapter of the history of neuroimaging traces back to the Italian neuroscientist Angelo Mosso who discovered the 'balance of human circulation', which could non-invasively measure the redistribution of blood during emotional and intellectual activity. However, although only briefly mentioned by William James in 1890, the exact details and workings of this equilibrium and Mosso's experiments with it remain unknown until the recent discovery of the original instrument as well as Mosso's report by Stefano Sandrone and his colleagues..
In 1918 American neurosurgeon Walter Dandy introduced ventriculographic techniques. An X-ray image of the ventricular system in the brain is obtained by injecting air filtered directly into one or both of the cerebral ventricles. Dandy also observed that the air entering the subarachnoid chamber via lumbar spine puncture can enter the cerebral ventricle and also shows the cerebrospinal fluid compartment around the base of the brain and above its surface. This technique is called pneumoencephalography.
In 1927 Egas Moniz introduced cerebral angiography, where normal and abnormal blood vessels in and around the brain can be visualized with high accuracy.
In the early 1970s, Allan McLeod Cormack and Godfrey Newbold Hounsfield introduced a computer axial tomography (CAT or CT scan), and an increasingly detailed image of the brain anatomy became available for diagnostic and research purposes. Cormack and Hounsfield won the 1979 Nobel Prize for Physiology or Medicine for their work. Immediately after the introduction of CAT in the early 1980s, the development of radioligand allowed single photon emission tomography (SPECT) and positron emission tomography (PET) brain.
More or less simultaneously, magnetic resonance imaging (MRI or MR scanning) was developed by researchers including Peter Mansfield and Paul Lauterbur, who was awarded the Nobel Prize for Physiology or Medicine in 2003. In the early 1980s MRI was introduced clinically, and during 1980s is a real explosion of technical improvements and MR diagnostic applications occur. Scientists soon learned that changes in blood flow measured by PET can also be imaged by the correct type of MRI. Functional functional magnetic resonance imaging (fMRI) is born, and since the 1990s, fMRI has dominated the field of brain mapping due to low levels of invasion, lack of radiation exposure, and relatively wide availability.
In the early 2000s, the field of neuroimaging reached a stage where the limited practical application of functional brain imaging has become feasible. The main application area is a rough form of the brain-computer interface.
Maps Neuroimaging
Indication
Neuroimaging follows a neurological examination in which a doctor has found a cause to investigate deeper patients who have or may have neurological disorders.
One of the more common neurological problems that a person may experience is simple syncope. In the case of simple syncope in which the patient's history has no other neurological symptoms, the diagnosis includes neurological examination but routine neurologic imaging is not indicated because the probability of finding the cause in the central nervous system is very low and the patient is unlikely to benefit from the procedure.
Neuroimaging is not indicated for patients with stable headaches who are diagnosed as migraines. Studies show that the presence of migraine does not increase the patient's risk for intracranial disease. The diagnosis of migraine recording in the absence of other problems, such as papilledema, does not indicate a need for neuroimaging. In the process of making a careful diagnosis, the doctor should consider whether the headache has a cause other than migraine and may require neuroimaging.
Other indications for neuroimaging are CT-, MRI and PET-guided stereotactic surgery or radiosurgery for the treatment of intracranial tumors, arteriovenous malformations and conditions that can be treated with other operations.
Brain imaging techniques
Computed axial tomography
Computed tomography (CT) or Computed Axial Tomography (CAT) scanning uses a series of x-rays from heads taken from various directions. Usually used to quickly see brain injury, CT scans use computer programs that perform numerical integral calculations (reverse Radon transformation) on a measured x-ray series to estimate how much x-ray light is absorbed in the small volume of the brain. Usually the information is presented as a cross section of the brain.
Diffuse optical imaging
Diffuse optical imaging (DOI) or diffuse optical tomography (DOT) is a medical imaging modality that uses near infrared light to produce body images. This technique measures the optical absorption of hemoglobin, and relies on a spectrum of hemoglobin absorption that varies with its oxygen status. High-density diffuse optical tomography (HD-DOT) has been compared directly with fMRI using a response to visual stimuli in the subjects studied with both techniques, with similar results convincing. HD-DOT has also been compared with fMRI in terms of language tasks and functional state connectivity breaks.
The event-related optical signal (EROS) is a brain scanning technique that uses infrared light through optical fibers to measure changes in the optical properties of the active area of ​​the cerebral cortex. While techniques such as diffuse optical imaging (DOT) and near infrared spectroscopy (NIRS) measure the absorption of hemoglobin optics, and thus based on blood flow, EROS takes advantage of the scattering properties of the neurons themselves, and thus provides more directly the size of cellular activity. EROS can show activity in the brain in millimeters (spatial) and in milliseconds (temporal). The biggest drawback is the inability to detect activity more than a few centimeters. EROS is a relatively inexpensive new technique that is non-invasive to the test subjects. It was developed at the University of Illinois at Urbana-Champaign where it is now used in the Cognitive Neuroimaging Laboratories Dr. Gabriele Gratton and Dr. Monica Fabiani.
Magnetic resonance imaging
Magnetic resonance imaging (MRI) uses magnetic fields and radio waves to produce images of two- or three-dimensional high-quality brain structures without the use of ionizing radiation (X-rays) or radioactive tracers.
Functional magnetic resonance imaging
Functional magnetic resonance imaging (fMRI) and arterial spin labeling (ASL) depend on the paramagnetic properties of oxygenated and deoxygenated hemoglobin to see images of changes in blood flow in the brain associated with neural activity. This allows images to be generated that reflect which brain structures are activated (and how) during different task performance or at rest. According to the oxygenation hypothesis, changes in the use of oxygen in regional brain blood flow during cognitive or behavioral activity can be attributed to regional neurons as being directly related to cognitive or behavioral tasks attended.
Most fMRI scanners allow subjects to be presented with different visual images, sounds and touch stimuli, and to perform different actions such as pressing a button or moving a joystick. As a result, fMRI can be used to reveal the structures and processes of the brain associated with perception, thought, and action. The fMRI resolution of about 2-3 millimeters is currently limited by the spatial spread of the hemodynamic response to neural activity. It has replaced PET for the study of brain activation patterns. PET, however, retains a significant advantage by being able to identify certain brain receptors (or carriers) associated with certain neurotransmitters through its ability to describe radiolabelled receptors "ligands" (receptor ligands are chemicals attached to receptors).
As well as research on healthy subjects, fMRI is increasingly used for medical diagnosis of diseases. Because fMRI is very sensitive to the use of oxygen in the bloodstream, fMRI is very sensitive to early changes in the brain due to ischemia (abnormal low blood flow), such as changes following a stroke. Early diagnosis of certain types of stroke is increasingly important in neurology, since substances that dissolve blood clots can be used within the first few hours after some type of stroke occurs, but are harmful to use afterwards. The brain changes seen in fMRI can help make the decision to treat with these agents. With an accuracy of between 72% and 90% where the odds will reach 0.8%, the fMRI technique can decide which of a series of known images the subject views.
Magnetoencephalography
Magnetoencephalography (MEG) is an imaging technique used to measure magnetic fields generated by electrical activity in the brain through highly sensitive devices such as superconducting quantum interference devices (SQUIDs) or free-spin exchange swim magnetometers (SERF). MEG offers very direct measurements of neural electrical activity (compared to fMRI for example) with very high temporal resolution but relatively low spatial resolution. The advantage of magnetic field measurements produced by neural activity is that they tend to be less distorted by the surrounding tissues (especially the skull and scalp) compared to the electric field measured by electroencephalography (EEG). In particular, it can be shown that the magnetic field generated by the electrical activity is not affected by the surrounding head tissue, when the head is modeled as a set of concentric spherical shells, each of which is an isotropic homogeneous conductor. The real head is non-spherical and has anisotropic conductivity (mainly white matter and skull). While skeletal anisotropy has a negligible effect on MEG (unlike EEG), white matter of anisotropy greatly affects MEG measurements for radial sources and deep sources. Note, however, that the skull is assumed to be an anisotropic uniform in this study, which is not true for a real head: the absolute and relative thickness of the diploÃÆ' and the table layers vary between and within the skull bone. This allows MEG is also affected by skull anisotropy, although it may not be at the same level as the EEG.
There are many uses for MEG, including helping surgeons in localizing pathology, helping researchers in determining the functioning of different parts of the brain, neurofeedback, and others.
positron emission tomography
Positron emission tomography (PET) and positron brain emission tomography, measure emissions from radioactively labeled active metabolic chemicals that have been injected into the bloodstream. Emission data are processed by the computer to produce 2 or 3-dimensional images of the distribution of chemicals throughout the brain. Positron-emitting radioisotopes used are produced by cyclotrons, and chemicals are labeled with these radioactive atoms. The labeled compound, called radiotracer , is injected into the bloodstream and eventually into the brain. Sensors in PET scanners detect radioactivity when compounds accumulate in different areas of the brain. Computers use data collected by sensors to create 2-dimensional or multicolored images showing where the compound works in the brain. Particularly useful are the various ligands used to map various aspects of neurotransmitter activity, by far the most commonly used PET tracer becomes a form labeled glucose (see Fludeoxyglucose (18F) (FDG)).
The greatest benefit of PET scanning is that different compounds can show the flow of blood and oxygen as well as the metabolism of glucose in the brain tissue that works. These measurements reflect the amount of brain activity in different areas of the brain and make it possible to learn more about how the brain works. PET scans are superior to all other metabolic imaging methods in terms of resolution and speed of completion (as little as 30 seconds), when they are first available. Enhanced resolutions allow for better research to be performed on areas of the brain that are activated by specific tasks. The biggest drawback of PET scanning is because radioactivity decays rapidly, limited to monitor short tasks. Before fMRI technology came online, PET scanning was the preferred method of functional brain imaging (compared to structural), and continued to make major contributions to neuroscience.
PET scans are also used for the diagnosis of brain diseases, especially because brain tumors, strokes, and disease-causing neurons that cause dementia (such as Alzheimer's disease) all cause major changes in brain metabolism, which in turn leads to detectable changes in PET scans. PET may be most useful in certain early cases of dementia (with classic examples of Alzheimer's disease and Pick disease) in which premature damage is too widespread and makes too little difference in brain volume and gross structure to alter CT and standard MRI images sufficient to being able to distinguish reliably from the "normal" range of cortical atrophy that occurs with aging (in many but not all) people, and which does not cause clinical dementia.
Single-photon-counted photon emission tomatography
Single-photon emission computed tomography (SPECT) is similar to PET and uses gamma ray-beam radioisotopes and gamma cameras to record the data the computer uses to create two or three dimensional images of the active brain region. SPECT relies on a radioactive tracer injection, or "SPECT agent," which is rapidly picked up by the brain but not redistributed. The absorption of the SPECT agent is almost 100% complete within 30 to 60 seconds, reflecting the cerebral blood flow (CBF) at the time of injection. The SPECT properties make it particularly suitable for epilepsy imaging, which is usually made difficult by problems with patient movement and variable seizure types. SPECT provides a "snapshot" of brain blood flow because scans can be obtained after cessation of seizures (as long as the radioactive tracer is injected during seizures). A significant limitation of SPECT is its poor resolution (about 1 cm) compared to MRI. Currently, SPECT engines with Dual Detector Heads are commonly used, although Triple Detector Head machines are available in the market. Tomographic reconstruction, (especially used for functionally functional "snapshots" of the brain) requires multiple projections of detector heads rotating around the human skull, so some researchers have developed 6 and 11 SPECT Detector Head machines to cut the imaging time and provide higher resolution.
Like PET, SPECT can also be used to differentiate different types of disease processes that produce dementia, and is increasingly used for this purpose. Neuro-PET has a disadvantage because it requires the use of a tracker with a half-life of at most 110 minutes, such as FDG. It should be made in a cyclotron, and is expensive or even unavailable if the required transport time is extended over some half-life. SPECT, however, is capable of utilizing trackers with longer half-life, such as technetium-99m, and as a result, is much more available.
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Cranial ultrasound is usually used only in infants, whose open fontanela provides an acoustic window that allows brain ultrasound imaging. Advantages include the absence of ionizing radiation and possible bedside scans, but lack of soft tissue detail means MRI may be preferred for some conditions.
Advantages and Concerns of Neuroimaging Techniques
Functional Magnetic Resonance Imagery (fMRI)
fMRI is generally classified as minimal-to-moderate risk because it is non-invasive compared to other imaging methods. fMRI uses the level of oxygenase-dependent blood (BOLD) -contrast to produce the imaging form. BOLD contrast is a natural process in the body so fMRI is often preferred over imaging methods that require radioactive markers to produce the same imaging. The concern in the use of fMRI is its use in individuals with implants or medical devices and metal items in the body. Magnetic resonance (MR) emitted from the apparatus may cause medical device failure and attract metal objects in the body if not filtered properly. Currently, the FDA classifies implants and medical devices into three categories, depending on MR compatibility: MR-safe (safe in all MR environments), MR-insecure (unsafe in MR environment), and MR-conditional (MR-compatible) in certain environments, need more information).
- FDA FDA security labels for implants and devices
Computed Tomography (CT) Scan
CT scans were introduced in 1970 and quickly became one of the most widely used imaging methods. CT scans can be done in less than a second and produce quick results for doctors, with ease of use leading to an increase in CT scans performed in the United States from 3 million in 1980 to 62 million in 2007. Doctors often take multiple scans , with 30% of individuals who underwent at least 3 scans in one study using CT scans. CT scans can expose patients to radiation levels 100-500 times higher than traditional x-rays, with higher radiation doses resulting in better resolution imaging. Although easy to use, an increased use of CT scans, especially in asymptomatic patients, is a topic of concern because patients are exposed to very high levels of radiation.
In PET scans, imaging does not depend on intrinsic biological processes, but depends on foreign substances injected into the bloodstream leading to the brain. Patients are injected with radioisotopes that are metabolized in the brain and emit positrons to produce visualization of brain activity. The amount of radiation of patients exposed to PET scans is relatively small, comparable to the amount of environmental radiation a person experiences throughout the year. PET radioisotopes have limited exposure time in the body because they usually have a very short half-life (~ 2 hours) and quickly rot. Currently, fMRI is the preferred method for brain activity imaging compared to PET, since it involves no radiation, has a higher temporal resolution than PET, and is more readily available in most medical settings.
Magnetoencephalography (MEG) & amp; ; Electroencephalography (EEG)
The high temporal resolution of MEG and EEG allows this method to measure brain activity up to milliseconds. Both MEG and EEG do not require patient exposure to radiation to function. EEG electrodes detect electrical signals generated by neurons to measure brain activity and MEG uses oscillations in the magnetic field generated by these electric currents to measure activity. The obstacles in using MEG widely are due to pricing, because the MEG system can spend millions of dollars. EEG is a much more widely used method for achieving temporal resolution as the EEG system is much cheaper than the MEG system. The disadvantage of EEG and MEG is that both methods have poor spatial resolution when compared to fMRI.
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Criticism and attention
Some scientists have criticized claims based on brain images made in popular scientific and press journals, such as the discovery of "responsible brain parts" for functions such as talent, special memory, or evoking emotions like love. Many mapping techniques have relatively low resolution, including hundreds of thousands of neurons in a single voxel. Many functions also involve several parts of the brain, which means that this type of claim may not be verified with the equipment used, and is generally based on false assumptions about how brain functions are divided. Probably most brain functions will only be described correctly after being measured with finer measurements that are not visible in large areas, but on a large number of very small individual brain circuits. Many of these studies also have technical problems such as small sample sizes or poor equipment calibration which means they can not be reproduced - sometimes neglected considerations for generating sensational journal articles or news headlines. In some cases brain mapping techniques are used for commercial purposes, lie detection, or medical diagnosis in a way that has not been scientifically validated.
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See also
src: www.decisionneurosciencelab.org
References
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External links
- All Brain Atlas @ Harvard
- Lecture notes on the mathematical aspects of neuroimaging by Will Penny, University College London
- "Transkranial Magnetic Stimulation". by Michael Leventon in collaboration with MIT AI Lab.
- NeuroDebian - a full operating system that targets neuroimaging
Source of the article : Wikipedia
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