17-04-2009, 06:39 PM
Nuclear Magnetic Resonance Imaging
INTRODUCTION
Nuclear Magnetic Resonance is better known as NMR.Generally used for Imaging of the human body.The device which Images the human body using this principle is called a MRI or Magnetic Resonance Imaging Device.Principles of NMRAccording to the electromagnetic theory, any nucleus such as a hydrogen proton which possesses a magnetic moment attempts to align itself with the magnetic field in which it is placed. This results in wobbling of the magnetic moment about the applied magnetic field with a resonant angular frequency, called the Larmor frequency determined by a constant called the Magnetogyric ratio and the strength of the applied field. This relation is given by wo = yBo Another important phenomenon of NMR is that the applied exteal field creates an energy absorption state from statistical point of view. When a nucleus with a magnetic moment is placed in a magnetic field, the energy of nucleus is split into lower (moment parallel with the field.) and higher (antiparallel) energy levels. The energy difference is such that a proton with a specific frequency (energy) is necessary to excite a nucleus from lower to higher state. The excitation energy is given by Planckâ„¢s equation E = hwo In NMR, at room temperature, more protons are in low energy state than in high energy state. The excited proton tends to retu to its lower energy state with spontaneous decay and re-emissions of energy at a later time Ëœtâ„¢ in the form of radio wave photons. This decay is exponential in nature and produces a free induction decay (FID) signal.
THE VECTOR MODEL
Let us consider that the material or tissue in the magnetic field (B0 along z-axis) is now subjected to another magnetic field, say a bar magnet placed along the y-axis, this would cause net magnetization to shift slightly from the z-axis, through an angle ?.nAn alteative technique to accomplish the same result would be to apply an RF pulse at the resonant frequency of the protons in the tissue. When the RF pulse is tued off, the net tissue magnetization swings back towards the z-axis (direction of B0), inducing an NMR signal in the receiver coil placed perpendicular to the moving magnetic vector.
FUNCTIONAL MRI OF THE MOTOR FUNCTION
Motor functions are those functions of the brain which help us to control our muscles very accurately. Motor function is controlled by four major regions in the frontal lobes; the primary motor cortex, supplementary motor area (SMA), lateral premotor cortex and the cingulate motor area. The primary motor cortex is responsible for the direct production of movements via it's outputs to the pyramidal tract, and damage to these areas resulting from stroke produces a weakness in the corresponding part of the body. The other three areas are responsible for higher order control of motor function.STIMULUS PRESENTATIONFor most paradigms involving the motor cortex, it is necessary to provide some form of cue to the subject to move and, in most cases, a measurement of the movement is made. All our studies involve hand movements since these are the simplest to measure. The cue to move was given from a 6 cm single digit LED display.This allowed the presentation of symbols, individual bars or the dot as well as numbers. To measure response times the subject was given two hand held buttons, one for the left hand and one for the right hand, operated by a thumb press. A schematic diagram of the stimulus rig is shown in Figure.The experiment was controlled by a PC fitted with an interfacing card . This card had a number of analogue input and output ports.
RESULTST
he following Figure shows the activation at 0 s, 3 s, 6 s and 9 s following the stimulus, for one subject in the ', no-' experiment. Maximum activation occurs at 3-6 seconds from the delivery of the stimulus. During the 'no-' task, activation was seen in the medial premotor cortex for all the subjects. This region was rostral to the anterior commissural (AC) line. Activity was observed in these regions during the '' task as well. During the '' task activation was also seen in a region of the premotor cortex caudal to the AC line. The mean value of the percentage change upon activation of the areas involved in the motor task are shown in Figure. This shows the rostral part of the premotor cortex (labelled pre-SMA) to be active in both '' and 'no-' tasks, with the caudal part (labelled SMA) being only active in the '' task. This suggests that the pre-SMA is involved in the movement decision making aspect of the task, whereas the SMA proper is more intimately involved in the movement itself.Figure 9:
ADVNATAGES OF NMR
The NMR image provides substantial contrast between soft tissue that are nearly identical in existing techniques. Cross-sectional images with any orientation are possible in NMR imaging systems. The alteative contrast mechanisms of NMR provides promising possibilities of new diagnostics for pathologies that are difficult or impossible with present techniques. NMR uses no ionizing radiation and has minimal if any, other hazards for operators of the machines and for patients. Unlike CAT, NMR imaging requires no moving parts, gantries or sophisticated crystal detectors.With the new techniques being developed, NMR permits imaging of entire three dimensional volumes simultaneously instead of slice by slice, employed in other imaging systems.
DISADVANTAGES OF NMMR
signal is inherently of very low intensity because it is based on a very weak , non-ionizing radio-frequency phenomenon. Unlike CAT scanning, where the signal-to-noise ratio per unit time can, in principle, be increased by increasing the x-ray beam intensity, there is no possibility of increasing the NMR signal because its size depends on the net concentration of atoms being imaged.NMR takes much longer time to construct an image. Efforts to produce multiple slices during a single NMR exposure have been made to reduce the scanning time. The collection of data from the whole sample, so called volume scanning, allows a very large number of contiguous slices to be generated which would take many times longer to produce if collected as single slices.
RISKS INVOLVED IN A MRI SCAN
Patients who have any metallic materials within the body must notify their physician prior to the examination or inform the MRI staff. Metallic chips, materials, surgical clips, or foreign material (artificial joints, metallic bone plates, or prosthetic devices, etc.) can significantly distort the images obtained by the MRI scanner. Patients who have heart pacemakers, metal implants, or metal chips or clips in or around the eyeballs cannot be scanned with an MRI because of the risk that the magnet may move the metal in these areas.n Similarly, patients with artificial heart valves, metallic ear implants, bullet fragments, and chemotherapy or insulin pumps should not have MRI scanning. Patients with any history of claustrophobia should relate this to the practitioner who is requesting the test, as well as the radiology staf