The Major Concepts of Magnetic Resonance Imaging - Report Example

MR Gradient Coil: Action and Performance Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Name Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Course Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Instructor Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Date Abstract The major concepts of magnetic resonance imaging (MRI) are reviewed while its application to biological and medical systems is described. Gradient coil is a major component of Magnetic Resonance Imaging. MRI gradient coil is defined as a set of wires that carry current and produces a specific magnetic field which is referred to as gradient field The gradients coils may be broadly be classified into two categories. In spatial encoding, they are three of them which are commonly used. These are: selective excitation, (z gradient) frequency encoding (x gradient) and phase encoding (y gradient). Gradients coil have achievements and limitation in MRI. MRFM and fMRI are example of the recent development in MRI. In addition, reviews of the most utilized imaging techniques are given. The PNS is highly affected by the MRI as seen in the paper. Eddy currents are known to affect the gradient coils. Introduction A breakthrough in magnetic resonance (MRI) experiment in matter which is condensed was done in the laboratories of Purcell and Bloch, 58 years ago. This made them share a Nobel Prize in physics. Mansfield and Lauterbur published a paper that proposed the use of NMR in formation of images. NMRI was the name given to this imaging modality which was however changed into MRI due to the widespread use of the phrase nuclear (Westbrook & Kaut 1998). Magnetic Resonance Imaging techniques are on the increase in the field of science. It has been seen to have major contribution in cardiovascular, oncology and musculoskeletal (Brown & Semelke 1999). It is an effective instrument as it scans from any angle. It has no pain and noninvasive. Gradient coils are a major component of the Magnetic Resonance Imaging design. They are designed in away that they generate spatially magnetic fields and are highly effective the imaging process. For a given time, the gradient coils and the components of the MR that are related have increased their performance rapidly. Definition of gradient coils Gradients are said to be coils of wire that alter the applied magnetic field force of the magnet when a current is passed trough. The following occurs when a gradient is applied. The field force remains unchanged at isocentre. The field strength will either decrease or increase while at a distance from the isocentre. The changing rate of the magnetic field strength is determined by the slope of the gradient. The amplitude of this magnetic field strength is highly determined by the current flow. Polarity of the gradient highly determines the point of the gradient that generates a higher observable field than isocentre, i.e., positive or lower field strength, negative. Rise time and slew rate refer to the speed that the gradients are switched on and off. Figure1. Gradients A Magnetic Resonance Imaging gradient coil is defined as a set of wires that carry current and produce a specific magnetic field is referred to as gradient field. The gradient coils are mainly used for localization of the signals (Haacke 1999). They actually provide the required spatial information which is vital in constructing of a MR image. The gradients coil are made up of three pairs coils that are used to make magnetic field gradients in the directions of x, y and z. The amplitude of this magnetic field generated by a given pair of gradient coils mainly depends on the exact amount of the current which is passing through every coil. The gradient is said to be centered when the current has passed through every coil while the magnetic field remains at 1.5T when it is eventually halfway through the two coils. The magnetic field is stronger when the current is strong in every gradient coil. Reversing the current flows leads to the change of the operating gradient to the exact opposite direction. The gradients coil may fall into tall broad categories: local gradient coils which are much smaller as compared to the whole body gradient coils (Kuperman 2000). However, the local gradient coils do achieve higher gradient slew rate as compared to the other. This has been as a result of advancement and progression of technology associated with MRI. It encompasses advancement in water-cooling, active-shielding and amplifier of high switch mode gradient. These advancements have played a major role in enhancing performance (Schempp1998). Figure 2. X, Y and Z gradient Spatial encoding in MRI When a receiver is placed next to the object, the magnetization that is transverse will induce current immediately in the current. This signal seen is the transverse over the given volume. (1) The main task of spatial encoding is to vary the phase and frequency over the volume for each measurement in a way that the original and main distribution of magnetization will be recovered by integrals already set. This is particularly achieved when a spatial variable, which is stationery in time, magnetic field that has to induce spatial distribution of the so called Larmor frequencies which is over the volume, is applied. Derivatives spatial of will mainly determine the resolution of the image. Linear magnetic fields are used in MRI as the constant gradient is seen to yield a well homogeneous resolution over the volume and the most favorable bandwidth properties of the current flow in the gradient coils. There are three commonly used spatial encoding that are different (Haacke et al 1999).These are: selective excitation, (z gradient) frequency encoding (x gradient) and phase encoding (y gradient). Selective excitation It mainly occurs along the axis of specific magnet. It causes non-resonance to the tissues. When an rf pulse which contains a set of selected frequencies follows the spatially varying magnetic field, magnetic resonance occurs if the sub-volume whose the Larmor frequencies are all included in the specific rf pulse ((Westbrook & Kaut 1998). The technique is applied in slice selection of MRI. Therefore, together with rf have a single frequency The pulse only affects spins in a slice and is defined as, (2) Hence, to select a slice of homogenous thickness, rf pulses are applied. The signals produced immediately after the pulse is said to be integration above the specific selected slice. Figure 3 a) Shows application of a linear gradient field to a given sample b) selective excitation in a cylindrical sample. Phase encoding It occurs when the gradient is mainly applied in y direction in order to encode the image obtained in the given remaining direction (Brown & Semelke 1999). The distribution of the oblique magnetization in the given sample is 2D, immediately after excitation pulse. When a linear field of gradient Gy is applied, this will definitely imply that the Larmor frequency distribution is also linear in y. It will then result to variation in the magnetization phase. Phase of point will be determined by, immediately after time. (3) The precession of the frequency will return to constant value over the selected slice when the gradient is turned off as the phase will definitely remain proportional to y (Liang & Lauterbur 2000). Frequency encoding Refers to the superposition of the given x gradient on the body. It enhances spatial localization in direction of L-R only. In the sample, when a constant gradient is applied; the precession has to change with the location linearly: (4) In case a signal is read off as the gradient is on, voxels which are at different locations will eventually have very different frequencies. The spatial encoding is mainly accomplished by specifically superimposing gradient fields. As the x gradient will alter the frequencies in the signal received, the y gradient will have to alter that phase of the signal, according to spatial location. Achievements One the latest achievements is the of use of MRI gradient coils, is in the development of magnetic-resonance force microscope (MRFM) which is a new scanned probe instrument that does combine the 3-D, three-dimensional imaging capabilities of MRI that is said to have high resolution and sensitivity of atomic-force microscopy (Schempp 1998). The MRFM will create a nondestructive, microscopy studies that will have a high resolution, chemical-specific and imaging of subsurface characteristics of a wide range of materials. Another advance in technology in MRI is noted on the imaging functional magnetic resonance (fMRI) which is applied in the brain scanning technology. This MRI uses radio and a powerful magnet to noninvasively and safely produce images of structures in body or the brain (Liang & Lauterbur 2000). The MRI scanner monitors blood flow to the brain and to some extent the body regions. While fMRI gives some show starring the brain, convectional MRI produces snaps of exactly what is found in the body (Liang & Lauterbur 2000). FMRI is now being applied in various treatments which include preventing disability, treating disorder, studying autism and taming addictions (Liang & Lauterbur 2000). MRI has been applied in various usages in the medical field, for example, in sports medicines it is being used in examination of sports injuries (Midzor 2000). General, the use of MRI is advancing in the biomedical field as it is improving to be well effective. Limitations Components of the gradient coils play a major role in the limitation of the system. For instance, the gradient amplifiers that if overheated may decrease the effectiveness of gradient coil. This is an example of technology limitation. Safety issues have also been a major limitation and include: ferromagnetic projectiles, high increase of torques on implants and medical devices and augmented electromagnetic effects (Brown & Semelke 1999). In addition patients who are exposed to high fields have been reported to been having headaches, nausea numbness and visual disturbances. However, the effects disappear as they leave the magnet. The gradient coils are also expensive and they need specialized personnel to operate them. Peripheral Nerve Stimulation Magnetic fields of gradients of coils are seen to induce electric fields found in human tissues and thus causing rapid stimulation of the peripheral nerve system. Latest advancement in technology has seen the introduction of sub-systems that are inducing the peripheral nerve stimulation found in humans. In actual fact, PNS is not seen as to be a significant threat in MRI. At verge level, PNS is usually experienced as a gentle sensation such as poking of the sun. Exposure to levels above the threshold will lead to pain (McRobbie et al. 2003) .Thus; it will lead a painful response towards the exposure. Stimulation of important organs, for instance, the heart is much possible at higher levels. Peripheral nerves system does set an essential limit for the performance of gradient for magnetic resonance imaging. Eddy Currents and inductors In the scanning MRI coils, eddy currents effects are increasingly playing an important role. The operation and design of electrical devices do require a lot of knowledge of the phenomena of electrical. Most devices are now based on the increasingly development of eddy currents. This includes: flaw detectors, generators which are of very high magnetic fields, material separators, devices which are use to form magnets. In fact in these applications, the eddy currents are exploited and desirable. However, these currents are undesirable as they may cause the loss of energy and thus should be highly minimized. A major problem in the application of gradient coils is that interaction of conducting structures and the fast changing fields in the MRI which includes the gradient coils. This result to the eddy currents being on the surface of these used conductors. Eddy currents create force in the conductor which then results to reduction of lifetime and noise. In addition, they also cause perturbations n the specific gradient fields which are expected. The main results of this situation is MR pictures which are blur and ghostly. As the signals are detected outside the field of view, they end up having the same value of magnetic field. Therefore, the eddy currents need to be controlled. Generally, the appearance of these currents is vital when designing the gradients coils. Different ways of reducing the eddy currents include: making slit in the conductors used, using pre-emphasis currents and by active shielding. It has proven hard to predict eddy currents inside the gradients coils due to intricate geometries of the gradients coils and those mutual inductances that are found between he different parts. For instance, a given z-coil will be highly influenced by the y and x-coil and also the cryostat and shielding coils (McRobbie et al. 2003). The main function of a cryostat is to cool the superconductors found in the main magnet. Frequencies mainly affect the response of the eddy currents. In addition, electrical conductivity will directly affect the eddy currents: on the surface, the high conductivity the greater the flow is seen in the eddy currents. Curvature, grooves, edges are geometrical features that also eddy currents together with the distance found between the different conductivity. Inductor is an electrical component that is used in storage of energy in any magnetic field. Its capability to store energy is measured by the inductance and the units used are henries. Examples of inductor include laminated core, ferrite-core and ferromagnetic core. ` Way formed Self-inductance, linearity, stores energy, dissipated, external fields, resistance and region of main interest are considered when forming a gradient coil (McRobbie et al. 2003). They are all related to image quality, costs, image processing and functionality. Issues of performance include the following: An image distortion is a resultant of spatial non-linearity of the specific gradient. Nuclei that do not belong to the specific slice are excited. The system is seen to detect resonance signal that are from different positions and are processed as they enter from the slice. The energy that is stored in a gradient coil may be defined as the squared amplitude of the field magnet, when integrated over the volume found in the coil. Stored energy will therefore, increases with increase volume of the coil and magnetic field. The dissipated power of the coil is said to be proportional to the resistance of Ohmic in a given current. To reduce the energy cost, minimization of the dissipated power is necessary. A coil always creates both internal and external fields. Eddy currents are induced by the external fields in other conductors. This has an effect on the total field. This, therefore, calls for a shielding coil to be included. Region of interest may not be enlarged anyhow; this would affect the viability of the requirements of the points previously given. Figure 3. Using a higher gradient, to select a thinner slice. Gradient coils are mainly manufactured by using copper strip. Wires are not commonly used. Design may be formulated as a problem optimizer (McRobbie et al. 2003). The most efficient electromagnetic or electrical constraints, conductor shape and geometrical should be selected. The total energy that is delivered should be minimized. In designing, a gradient coil, thin strips of the copper used are positioned along the streamlines. Assuming that the strips are uniform and current density constant, the magnetic field that is induced by the currents is computed. Magnetic induction is seen to be related to gradient coils. The gradient coil is mainly manufactured using copper that are kept in the epoxy resin while bonded onto fiberglass (Haacke et al 1999). X-coil Z-coil Figure 4. Possible designs of coils. Conclusion The MRI is quite vital in image improvement. The principles of magnet resonance imaging have made it easier for the MRI technology to be applied in the medical field. Nevertheless, advances are needed in order to successful make the technique useful in the biomedical sector. Spatial encoding is an important aspect in MRI gradient coils. In the scanning MRI coils, eddy currents effects are increasingly playing an important role. Functional magnet resonance imaging is now being used by psychologists in research. The gradient coils have both achievements and limitations as the technology is advancing. However, as it is being applied in biomedical sector, it is highly recommended that the technology be advanced further. References Brown,M and Semelke,R. 1999, Principle and Application, 2ed. New York: Wiley-Liss Haacke, E. et al, 1999, Magnetic Resonance Imaging: Physical Principle and Sequence Design.New York: John Wiley & Sons. Kuperman, V. 2000, Magnetic Resonance Imaging: Physical Principle and Application. SanDiego: Academic Press. Liang, Z and Lauterbur, P. 2000, Principle of Magnetic Resonance Imaging: A signal Processing Perspective. New York: IEEE Press. McRobbie, D.,et al. 2003, MRI From Picture to Proton. New York: Cambridge University Press Midzor, M. 2000, “Ferromagnetic resonance force microscopy,” Ph.D. dissertation, Calif. Ins Technol., Pasadena, CA Schempp, W. 1998, Magnetic Resonance Imaging: Mathematical Foundation and Application.New York, Wiley-Liss. Westbrook, C and Kaut, C. 1998, MRI in Practice, 2ed. Maiden: Blackwell science. Read More