Research Techniques
Magnetic Resonance Imaging (MRI)
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| T1-weighted MRI of the brain |
Clinical Magnetic Resonance Imaging (MRI) uses the magnetic properties of hydrogen and its interaction with both a large external field and radio waves to produce highly detailed images of the human body. We will discuss some basic principles of magnetism, the magnetic properties of the hydrogen nucleus, and its interaction with the externally applied magnetic field.
In order to perform MRI, we first need a strong magnetic field. The field strength of the magnets used for MR is measured in units of Tesla. The magnet used at the CIR has a strength of 4 Tesla.
Certain nuclei that are found in the body exhibit magnetic properties. Because a proton has mass, a positive charge, and spins, it produces a small magnetic field much like a bar magnet. This small magnetic field of the proton is referred to as the magnetic moment. The magnetic moment is also a vector quantity having both magnitude and direction and is oriented in the same direction as the angular momentum. There are several nuclei which are magnetically active, but hydrogen has a significant magnetic moment and is very abundant in the human body.
For these reasons, we use only the hydrogen proton in routine clinical imaging. Although MRI is used for medical diagnosis, it utilizes a physics phenomenon discovered in the 1940s called nuclear magnetic resonance in which fields and radio waves, both harmless, cause atoms to give off tiny radio signals. Physicists found that the length of time these response signals are emitted after an atom is stimulated by radio waves varies widely depending upon the substance being examined. This amazing phenomenon also holds true for biological tissue. Different kinds of tissue emit response signals, normally called relaxation times, which vary in length.
There are two fundamental relaxation times that can be detected, which are known as T1 and T2. When a patient is being scanned with MRI, the response signals emitted by the atoms in the patient’s body are picked up by a very sensitive antenna and forwarded to a computer for processing. When the processing of these signals is complete, a two-dimensional, cross-sectional pattern is created on a monochrome monitor that looks very much like what you would expect if you took a black-and-white TV picture of that particular cross-section. In other words, the intensity of each pixel of the image reflects the T1 and T2 of the tissue from that particular location. A typical image is made up of 65,000 tiny rectangles that are either white, black or one of a wide range of gray tone values that fall somewhere between black and white.
More information about MRI can be found at the following sites:
Functional Magnetic Resonance Imaging (fMRI)
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| Brain activation during passive viewing of pictures of houses |
Functional Magnetic Resonance Imaging is an imaging technique that is used to determine the brain regions that are activated during the performance of a task, such as looking at pictures or trying to remember words. When a particular brain region is active during a task, there is an increase in local blood flow to provide oxygen to the working neurons. The magnetic properties of water are different between areas that are near freshly oxygenated blood and areas where blood oxygen levels are relatively depleted. The MRI system is able to use the different blood oxygen levels to indirectly measure the degree of neuronal firing. The functional MRI data can then be overplayed on the structural image. The CIR focuses on fMRI of healthy subjects and patients with bipolar disorder, substance abuse, epilepsy and stroke.
More information about how fMRI works can be found at the following site:
Electroencephalogram (EEG)
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| The first EEG recording, which was obtained by Hans Berger in 1929 |
Electroencephalograms are preformed by placing electrodes over many areas of the scalp in order to record patterns of electrical activity created by neurons in the brain. The EEG machine amplifies the size of the electrical signals and can record them either on paper or on a computer. These electrical signals are often called “brain waves. The EEG is has a lower anatomical resolution than fMRI, but is able to measure the time course of neuronal events more specifically. At the CIR, EEG is typically done in concurrently with a MRI study.
Diffusion Tensor Imaging (DTI)
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| A slice of DTI knee imaging |
Diffusion Tensor Imaging measures the directions and rates of water diffusion across different types of brain tissue. White matter (made up of mylenated neurons) and grey matter (unmylenated neurons) show different patterns of water displacement.
Because brain injury tends to reduce water displacements, DTI is particularly useful for the early detection of brain injury and stroke.
The CIR uses Diffusion Tensor Imaging for knee ligament reconstruction, and neuroimaging studies.
Magnetic Resonance Angiography (MRA)
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| 4T MRA without contrast agent |
Magnetic Resonance Angiography uses MRI technology in order to study blood vessel pathology. It is commonly used to diagnose and plan treatment of blood vessel diseases including heart disorders and stroke.
MRA can be conducted with or without the use of contrast material.
Magnetic Resonance Angiography is used at the CIR in investigations of the detection of unruptured intracranial aneurysms.
Magnetic Resonance Spectroscopy
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| The first CIR’s P-31 MRS |
Magnetic Resonance Spectroscopy is used to measure the chemical composition of different molecules in the brain without taking any tissue or blood samples and without using radioactive tracers. MRS is based on the fact that different molecules are revealed at different frequencies when subjected to a magnetic field. Each molecule creates a specific signature which can be used to determine what molecules are present in the brain, and in what amounts.
The CIR works on refining proton and phosphorus MRS for brain bioenergenics of bipolar and other neural disorders.