Glossary
Nuclear Magnetic Resonance (NMR)
Nuclei are found at the centre of every atom and contain protons and neutrons, but only certain nuclei have the correct combination of protons and neutrons to allow us to study them with MRI. Luckily for MRI, one of these NMR sensitive nuclei is hydrogen. This is lucky, because water of course consists of oxygen and hydrogen, and there is a huge amount of water in our bodies.
When we put a person in a magnet, the hydrogen nuclei in their bodies prefer to be aligned with the field. We use RF (radio frequency) energy to knock them out of alignment, and then as they return back to alignment they emit a signal which we can detect. However the RF energy needs to be exactly matched to the energy the spins loose when they align with the field; this is resonance (like an opera singer singing at exactly the right pitch to break a glass). A number of different properties can be measured with NMR, allowing us to find out a lot about a given sample.
By the way, the only connection bewteen NMR and nuclear power is the word nuclear. In NMR we make nuclei wobble in a magnetic field and they then whisper a tiny signal back to us. In nuclear energy nuclei must be smashed appart, releasing a huge amount of energy, which is used to produce electricity, but which can also be dangerous to our cells.
Magnetic Resonance Imaging (MRI)
By using a magnetic field that varies with position, it is possible to obtain information about where the NMR signal is coming from. MRI scanners requie:
- A large magnet (100,000 x earth's magnetic field).
- Radio-frequency excite the nuclei and recieve the signal back..
- Gradient Coils allow us to get information about position from NMR (and hence make pictures). The gradient coils are used to produce a known variation in the constant magnetic field of the main magnet. This means that at some points the magnetic field is higher than others. This provides a way of knowing where in the sample the signal is coming from.
We have a several whole body scanners, ranging from a 1.5 T (15000 G), which is roughly 120000 times stronger than the Earth's magnetic field, to a 7T.
Echo-Planar Imaging (EPI)
EPI can produce MRI images in a 'snapshot' . Traditional MRI works by taking pictures one line at a time; that is for each stimulating pulse, information can be read along one line. In order to make up an image, a series of these lines must be scanned and assembled into a picture. EPI allows an image to be scanned all at once. This speeds up scanning immensely, overcoming the problems that normally result from subject motion. However perhaps more importantly EPI opens up a the possibility of using MRI for 'dynamic imaging', (studying processes in the body that are changing) for instance of the activation of the brain, the emptying of the stomach or the movement of the fetus. However EPI is a relatively low signal technique (although the signal can be recovered by averaging- the signal to noise per unit time is good), and sensitive to susceptibility artefacts.
functional Magnetic Resonance Imaging (fMRI)
fMRI is a special form of MRI used for studying the behaviour of the brain. Normally MRI is used on the brain, as in the rest of the body, to give structural information, however fMRI gives us pictures of brain activity. When you click your fingers, the brain cells in the region of your brain known as your motor cortex start working harder to send the signals to the muscles in your fingers to tell them to contract. The brain then responds by increasing the blood flow to the motor cortex to provide the food and oxygen needed to create these signals. To ensure sufficient supply the oxygenation of the blood in the releavnt part of the brain (the motor cortex) actually increases. It turns out that oxygenated and deoxygenated blood have different magnetic susceptiblities so that they show up differently on the images. This is known as the BOLD effect and it leads to the HDR that is detected with MRI.
Cerebral Blood Flow(CBF)
the amount of blood flowing through a region of the brain (units usually ml/100g/minute). MRI provides several methods of measuring this including some which are entirely non-invasive. Cerebral Blood Volume is the fraction of brain tissue occupied by blood.
HDR
Hemodynamic Response. This term is gernally used to describe the time course of the signal change that occurs in the MR images in response to brain activity. More specifically it describes the time courses of the changes in blood flow, blood volume and blood oxygenation that occur in the brain in response to brain activity.
Contrast agents
Contrast agents are chemicals that will change the MRI signals. They are injected into the blood stream and generally stay within the blood stream. Therefore they then highlight regions of the images that contain more blood compared to other body tissues.
Magnetoencephalography (MEG)
An electric current produces a magnetic field. Thus the electrical signals flowing in your brain create tiny magnetic fields around your head. MEG measures these fields to study brain activity.
Magnetic Resonance Spectroscopy (MRS)
The data gathered in MRS is presented as a spectrum. (i.e. the strength of the magnetic resonance signal is plotted as a function of resonant frequency). Because of the way magnetic resonance works, the chemical environment of the nucleus being scanned will vary its resonant frequency. Hence, by observing the position of peaks in MR spectroscopic data it is possible to determine some of the molecules present in the sample.
RF
RF Pulses are used in Magnetic Resonance experiments to excite the nuclei. The pulses are created using RF coils. The pulses disturb the equilibrium alignment of the tiny nuclear magnetization present in the sample. This magnetization then rotates ("precesses"), and this precession can be detected in the RF coils.Susceptibility artefacts
The magnetic susceptiblity of a material is a measure of how much magnetization is produced within it when it is placed in a magnetic field. Susceptibility differences between tissues can lead to signal loss in MR scans, especially in EPI scans. The susceptibility difference between deoxygenated and oxygenated blood is the the basis of the BOLD effect used to detect fMRI signals.
T1/T2 relaxation
- These are the 'longitudinal' and 'spin-spin' relaxation times, which describe the rate at which nuclei return to their equilibrium position once they have been disturbed. They depend on the state of the water in the sample: broadly speaking the more bound the water (the more jelly-like the sample) the shorter the relaxation times. T2 can be particularly affected by the presence of large molecules also containing hydrogen, eg protein molecules. Finally T2 and T1 can be affected by the presence of magnetic materials, such as contrast agents, or oxyhaemoglobin.
