Gradient Coils, Creating Secondary Magnetic Field and Using it in MRI - Research Paper Example

Gradient coils in MRI Gradient coils in MRI Abstract Gradient coil is among the most advanced technology that has changed the nature of scanning and imaging within the healthcare sector. The magnetic Resonance Imaging is one of the latest imaging techniques that have been considered useful in the current age. Gradient coils have enhanced the nature of imaging achieved in MRI and thus is among the greatest breakthrough that has been made in the current age. In the operation of the MRI machine, the gradient coils play a very crucial role in the functioning of the scanner. This paper will thus look at some of the major findings regarding the use of gradient coils, and will specifically focus on the gradient coil properties, the action of the gradient, the design, performance, the types and the impacts that gradient coils have on imaging. Introduction Magnetic resonance imaging (MRI) is among the most current and sufficient imaging modalities can be accurately taken for analysis and diagnosis the body of human and animals. MRI makes the use of magnetic field as well as radio wave energy pulses to generate images of structures and organs within the body. The MRI system consists of many parts, the important part of them is the gradient coil which comprises wire coils that used to generate electromagnetic through turning on and off. Moreover, gradient coil creates controlled spatial variation on the magnetic field strength (Buszko, 2010). The gradient coils are designed to offer linear magnetic fields on three directions standing perpendicular to the volume of interest. This therefore makes it easy to capture the imaging from all dimensions thus increasing the accuracy of the MRI process. Fig. Gradient coil (Crozier & Doddrell, 2009) Gradient coil properties The gradient coil has a number of properties, most of which are considered desirable and making it more efficient in its functioning. One of the most desirable properties is that it is can undergo rapid pulsing; the recommended rise time for gradient coil properties is 100-200 µs. Moreover, the gradient coil has low resistance which helps in minimizing power dissipations (Harvey & Katznelson, 2009). Relatively, the gradient coil is often shielded to prevent induction of eddy currents on the magnetic structure when the pulse is released (Figure 2).The eddy current that generated by pulsed field gradient can be combat by two ways which they are placing the gradient coil in an additional shielding layer and of coil in the opposite direction to the primary coil (Figure 3) and shaping the gradient waveform (Vegh, 2014). The gradient coil equally has the quality of minimizing the torque forces as well as the acoustic output and vibrations (Legget, Crozier, & Bowtell, 2003). Gradient design A number of gradient coil designs are often used in clinical setting; these include Maxwell pair and the Golay pair (Knopp et al. 2010). Each of these designs has a number of attributes that make them unique and distinctively capable of producing the required image. The Maxwell pair design The Maxwell pair design is a special kind of gradient coil that is used commonly to create magnetic field gradient on the direction of the major magnetic fields. Maxwell coil comprise of pair of coils that are separated along z-axis by approximately 1.73 times of their radius. Thus, current flow takes place in the opposite direction of the coils thereby resulting into linear gradient (Ha, Han, & Lee, 2010) Advantages One of the most common advantages is that the Maxwell pair coil releases uniform gradient for the body imaging. Through this high image, clarity is obtained and the internal organs under study are checked and observed clearly (Liu, 2008) The Maxwell pair designs can also be wound over the surface: they enhance the imaging under the magnetic field of view. As well, imaging planes such as axial or coronal can also be achieved. This means that it is flexible and can be used to acquire information about any part of the body (Ha, Han, & Lee, 2010). The use of Maxwell pair design helps in generation of high field NMR that provides high-resolution image clarity hence making the various components visible. This advantage makes MRI to be the best choice in terms of the scan quality among other methods (Liu, 2008) Golay coil Golay coils are mainly used in creating magnetic field gradients that are perpendicular to the main magnetic field. Golay coil produces linear gradient on the x and y-axis hence requiring wires to run on the bores of the magnet. Coils of this nature often produce linear field, however, this linearity is lost rapidly as it moves away from the central plane. This non-uniformity can be removed by introduction of other pairs that have different axial separations to enhance equal distribution (While, Forbes & Crozier, 2010). Advantages The golay coils have a number of advantages some of which include reduced acoustic response, reduced image fold back and increased continuous gradient (Wu et al., 2014). All these qualities enable the creation of perfect and clear images for analysis. Modern gradient coils The contemporary gradient coils have been improved through the use of multiple turns to ensure increase sensitivity (MT/M/A) of the coil. The gradient coils have further been designed to enable the coil to rapidly switch and allow for a high cycle of pulsing. The process can be likened to a istributed Gy coil, then slicing it along the length and rolling out the conductors in a flat sheet.The modern Z-gradient coils mainly comprise of multiple distributed loops. The gradient is further optimized for linearity and the impedance of the coil minimized to some level (Vegh, 2014). Gradient coil action Gradient coils act by producing linear variation in magnetic field intensity in the direction within the space. The variation in magnetic field is then added to the main magnetic field (static field), which by far has more power. The variations result from the pairs of coils, which are placed, in each of the spatial direction. As well, the direction of the magnetic field is never modified. Adding them to B0 results into the creation of linear variation on the total magnetic field amplitude on the direction through which they are applied (Crozier & Doddrell, 2009). The action of the coils is often considered homogeneous on the plane perpendicular to the direction in which they are applied. Figure 4. Illustrating gradient coil functioning (Crozier & Doddrell, 2009) From this, resonance frequency is modified proportionate to the magnetic field intensity through which they are submitted. This is based on the Larmor’s equation, which states that the stronger field, the faster the processes. Through the variation in Larmor’s frequency, the variation and dispersion of the spin phases is done. Larmor precession frequency refers to the rate of precession of the spin packet due to the influence of the magnetic field. The larmor equation gives the frequency of the RF signal that will result in change in nucleus spin energy level. The frequency is determined through gyro magnetic ratio of atoms as well as magnetic field strength. Thus, the stronger the magnetic field, the greater the precessional frequency; if the RF pulse is applied to nucleus of an atom, protons tend to alter their alignment from the magnetic field direction to the opposite direction of the magnetic field. Therefore, as the proton attempts to realign with the major magnetic field, it emits energy on the larmor frequency. Through the variation of magnetic field across the body using a magnetic field gradient, the matching variation of larmor frequency can be used in encoding position. The larmor frequency is approximated to be 42.58MHz/T Gradient performance It is equally worth noting that the gradient performance is mainly linked to their maximal amplitude. This is the magnetic field variation, which determines the maximal spatial resolution. The performance also depends on the slew rate of the gradient coils, which corresponds to their switching speed. High slew rates and lower rise time is often required to switch the gradients faster and allow for ultra fast imaging sequences e.g. the echo planar. Furthermore, the coil performance depends on the linearity, which is needed to be perfected within the area of scan (Shvartsman & Steckner, 2007).  Through the rapid switching of gradients, current is induced on the conducting materials within the vicinity of the gradient coils. The induced eddy currents then act to oppose the gradient field resulting into decay in the profile. The effects of the induced currents can also be reduced through a number of ways; these may include the use of active gradient coil shielding, optimization of the electrical current profile, which is sent to the gradient during the rising, and falling of the eddy currents (Ha, Han & Lee, 2010). There are also three gradient coils which include Gx, Gy and Gz. Thus, a magnetic field magnitude located on x, y, z planes takes the following form: M(x,y,z) = Mo + xGx +yGy +zGz This theoretical formulation encompasses constraint function between the required field within a particular region and an arbitrary defined surface which perfoms the current configuration in consideration to the Biot-Savart’s integral equation. the weight function alongside the linear approximation function enables the transformation of the problem to linear matrix equation which yields the discrete current elements in regards to direction and magnitude within the specified coil surface. Numerical comparisons and predictions using practical measurements of the Gx, Gy, Gz gradient coils highlight the success of the approach (Aldefeld, Ãrnert & Keupp, 2010). Steep slope gradient During the switching on the gradient coil, magnetic field strength is either added or subtracted from B0 relative to the iso-centre. The resulting slope from the magnetic field gives the amplitude of the magnetic field strength on the gradient axis. Steep gradient slopes have the tendency of altering the magnetic field strength between two given points as compared to the shallow gradient slopes. Relatively, the steep gradient slopes acts to alter the precesional frequency of the nuclei between two different points through higher intensity as compared to the shallow gradients slopes (Aldefeld, Ãrnert & Keupp, 2010). Furthermore, steep slope is selected to achieve thin slices. Gradient functions: Signals from a patient mainly contain information from the entire part that is under examination. They do not contain any spatial information, i.e. the specific point of origin of each signal component cannot be determined. Thus the function of the gradients, in which case one of the gradient is needed in each of the points x, y and z in order to obtain spatial information on that direction. Relative to their functions, the gradients can be called phase encoding gradient, slice select gradient and the read out gradient (Callaghan, 2012). Each of these can be used based on their orientation to the axis and depending on the slice orientation, they can be classified. Slice selection During slice selection, when the patient lay on the table and a slide is to be selected at a given level and within certain thickness. Here the patient lay on the external magnetic field Bo which is on the X-axis. When RF pulse is transmitted, and an echo is got back, the signal would be from the entire patient’s body. As well, transmission of RF pulse that is inconsistent with the Larmor frequency, no protons would be excited on the patient’s body hence there will not be any results (Crozier & Doddrell, 2009). However, if the magnetic field is varied from point to point then each of the positions will have their resonant frequency. The magnetic field strength increases on right side of magnet and decreases on the other side through the use of gradient coil (Figure 5). Figure 5: the changing of field strength by gradients . Frequency encoding After the selection of a slide, the signals emanating from it have to be spatially located along the image axes. Location of signals on the long axis of the anatomy is referred to as frequency encoding. A gradient is also applied on the selected axis, and the precessional signal frequency along the axis is altered in linear fashion (Aldefeld, Ãrnert & Keupp, 2010). The signal can then be located on the gradient axis depending on its frequency. Figure 6:shows the frequency encoding . Phase encoding This is the second step in spatial encoding. It involves the application of phase gradient, which is chosen on the vertical direction. Phase encoding gradient often intervenes for a limited period of time, During its application, the phase encoding gradient modifies the spin resonance frequency, and induce dephasing which stays even after the interruption of the gradient. Because of this, the protons precess in similar frequency but on different phases. The protons stays in a similar row, and perpendicular to gradient direction and these all have similar phase. Figure7: illustrate the phase encoding . Gradient impacts The gradient impact, results into peripheral nerve stimulation as well as acoustic noise (Dean, 2009). Below is the description of each of the following. Peripheral Nerve stimulation Applying high gradient amplitudes as well as switching the rates for the MRI and spectroscopy results in short rise times on the gradient field, and causes high changes on the patient’s magnetic flux density. This is known to induce peripheral nerve stimulation on patients. If the electric fields have sufficient magnitude and duration, excitation of the peripheral nervous system can occur (Liu, 2008). The Food Grag Asusation (FDA) has equally been observed to have considerable control on the upper limit of the MR gradient, which is 200T/M/S. There is a lot of debate regarding the mechanism of this phenomenon. It is evident that the greatest temporal field generated by the gradient coil is of great significance since its the volumetric exposure of the patient. There is thus a trend to reduce the length of the gradient coils and then reduce the maximum field produced per volumetric exposure as well as the gradient strength (Vegh, 2014). The inclusion of these factors in gradient coil design is an area of active current research. The subjects would then report spatial sensations of tingling, muscle twitching and pressure. It is worth noting that the MRI gradient fields operate in that they switch at frequencies under which the nervous system is sensitive and likely to respond to. Acoustic Noise Acoustic noise causes discomfort for the patients as well as the members of staff working within the MR environment. This is attributed to the loud staccato noise experienced during the scanning process. This is as a result of gradient coils experiencing the magnetic forces as well as the torques and at the same time being pulsed. Due to the physical restraining of the energy associated with the magnetic forces, acoustic noise is released in the form of loud pulses (Vegh, 2014). Conclusion The gradient coils play a very important role in MRI scanning process. Most property of the gradient coils is deemed reliable and offers solution to number of problems during imaging. The gradient coils enhance the levels of efficiency in image acquisition. Moreover, the rapid pulsing and the low resistance are among the properties of gradient coils, which make them efficient in their functioning (Frahm, 2011). Other factors such as the acoustic noise are some of the few limitations that the gradient coils experience, however, this problem has been solved in the new MRI machines and the acoustic noise is no longer a problem. References Aldefeld, B., Ãrnert, P., & Keupp, J. (2010). Continuously moving table 3D MRI with lateral frequency-encoding direction. Magnetic Resonance in Medicine, 55(5), 1210-1216. Bauer, B. B. (2007). Moving-Coil Pressure-Gradient Hydrophone. The Journal of the Acoustical Society of America, 39(6), 1264. Buszko, M. (2010). Magnetic-Field-Gradient-Coil System for Solid-State MAS and Cramps NMR Imaging. Journal of Magnetic Resonance, Series A, 107(2), 151-157. Callaghan, P. (2012). Correlated Susceptibility and Diffusion Effects in NMR Microscopy Using both Phase-Frequency Encoding and Phase-Phase Encoding. Journal of Magnetic Resonance, Series B, 104(1), 34-52. Crozier, S., & Doddrell, D. (2009). A simple design methodology for elliptical cross-section, transverse, asymmetric, head gradient coils for MRI. IEEE Transactions on Biomedical Engineering, 45(7), 945-948. Dean, D. (2009). Method and apparatus for reducing acoustic noise in MRI scanners. The Journal of the Acoustical Society of America, 114(5), 2539. Frahm, J. (2011). The Influence of the Slice-Selection Gradient on Functional MRI of Human Brain Activation. Journal of Magnetic Resonance, Series B, 103(1), 91-93. Ha, Y. H., Han, B. H., & Lee, S. Y. (2010). Magnetic Propulsion Of A Magnetic Device Using Three Square-Helmholtz Coils And A Square-Maxwell Coil. Medical & Biological Engineering & Computing, 48(2), 139-145. Harvey, P. R., & Katznelson, E. (2009). Modular gradient coil: A new concept in high-performance whole-body gradient coil design. Magnetic Resonance in Medicine, 42(3), 561-570. Knopp, T., Erbe, M., Sattel, T. F., Biederer, S., & Buzug, T. M. (2010). Generation of a static magnetic field-free line using two Maxwell coil pairs. Applied Physics Letters, 97(9), 092505. Leggett, J., Crozier, S., & Bowtell, R. (2003). Actively shielded multi-layer gradient coil designs with improved cooling properties. Journal of Magnetic Resonance, 165(2), 196-207. Liu, H. (2008). Finite size bi-planar gradient coil for MRI. IEEE Transactions on Magnetics, 34(4), 2162-2164. Shvartsman, S., & Steckner, M. C. (2007). Discrete design method of transverse gradient coils for MRI. Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering, 31B(2), 95-115. Simonetti, O. P., Duerk, J. L., & Chankong, V. (2009). MRI gradient waveform design by numerical optimization. Magnetic Resonance in Medicine, 29(4), 498-504. Westbrook, C., & Roth, C. K. (2013). MRI in Practice Retrieved from http://UQL.eblib.com.au/patron/FullRecord.aspx?p=693813 While, P. T., Forbes, L. K., & Crozier, S. (2010). 3D gradient coil design for open MRI systems. Journal of Magnetic Resonance, 207(1), 124-133. Wu, Z., Kim, Y., Khoo, M. C., & Nayak, K. S. (2014). Evaluation of an independent linear model for acoustic noise on a conventional MRI scanner and implications for acoustic noise reduction. Magnetic Resonance in Medicine, 71(4), 1613-1620. Read More

Gradient design A number of gradient coil designs are often used in clinical setting; these include Maxwell pair and the Golay pair (Knopp et al. 2010). Each of these designs has a number of attributes that make them unique and distinctively capable of producing the required image. The Maxwell pair design The Maxwell pair design is a special kind of gradient coil that is used commonly to create magnetic field gradient on the direction of the major magnetic fields. Maxwell coil comprise of pair of coils that are separated along z-axis by approximately 1.

73 times of their radius. Thus, current flow takes place in the opposite direction of the coils thereby resulting into linear gradient (Ha, Han, & Lee, 2010) Advantages One of the most common advantages is that the Maxwell pair coil releases uniform gradient for the body imaging. Through this high image, clarity is obtained and the internal organs under study are checked and observed clearly (Liu, 2008) The Maxwell pair designs can also be wound over the surface: they enhance the imaging under the magnetic field of view.

As well, imaging planes such as axial or coronal can also be achieved. This means that it is flexible and can be used to acquire information about any part of the body (Ha, Han, & Lee, 2010). The use of Maxwell pair design helps in generation of high field NMR that provides high-resolution image clarity hence making the various components visible. This advantage makes MRI to be the best choice in terms of the scan quality among other methods (Liu, 2008) Golay coil Golay coils are mainly used in creating magnetic field gradients that are perpendicular to the main magnetic field.

Golay coil produces linear gradient on the x and y-axis hence requiring wires to run on the bores of the magnet. Coils of this nature often produce linear field, however, this linearity is lost rapidly as it moves away from the central plane. This non-uniformity can be removed by introduction of other pairs that have different axial separations to enhance equal distribution (While, Forbes & Crozier, 2010). Advantages The golay coils have a number of advantages some of which include reduced acoustic response, reduced image fold back and increased continuous gradient (Wu et al., 2014). All these qualities enable the creation of perfect and clear images for analysis.

Modern gradient coils The contemporary gradient coils have been improved through the use of multiple turns to ensure increase sensitivity (MT/M/A) of the coil. The gradient coils have further been designed to enable the coil to rapidly switch and allow for a high cycle of pulsing. The process can be likened to a istributed Gy coil, then slicing it along the length and rolling out the conductors in a flat sheet.The modern Z-gradient coils mainly comprise of multiple distributed loops. The gradient is further optimized for linearity and the impedance of the coil minimized to some level (Vegh, 2014).

Gradient coil action Gradient coils act by producing linear variation in magnetic field intensity in the direction within the space. The variation in magnetic field is then added to the main magnetic field (static field), which by far has more power. The variations result from the pairs of coils, which are placed, in each of the spatial direction. As well, the direction of the magnetic field is never modified. Adding them to B0 results into the creation of linear variation on the total magnetic field amplitude on the direction through which they are applied (Crozier & Doddrell, 2009).

The action of the coils is often considered homogeneous on the plane perpendicular to the direction in which they are applied. Figure 4. Illustrating gradient coil functioning (Crozier & Doddrell, 2009) From this, resonance frequency is modified proportionate to the magnetic field intensity through which they are submitted. This is based on the Larmor’s equation, which states that the stronger field, the faster the processes. Through the variation in Larmor’s frequency, the variation and dispersion of the spin phases is done.

Read More