Magnetic Resonance Imaging Technology Research Paper

Abstract

The use of Magnetic Resonance Imaging (MRI) opened new perspectives for medicine and neuroscience. MRI has been used for functional imaging. Current imaging methods allow the assessment of the properties of brain tissue to obtain information of brain mechanisms. Nonetheless, areas exist wherein clarification is needed. The processes or principles involved in the production of images from MRI have not been clearly understood by most people, thus producing more issues and concerns. In this study, the principles of MRI, with emphasis on the theories and technologies involved, are discussed. MRI involves numerous theories for its application in various fields of medicine, most especially in brain imaging. It is also composed of hardware components that in work together to allow the MRI scanner to work. Images may be produced through point and line, two-dimensional and three-dimensional methods. MRI technologies include high-field, ultra-low-field and magnetoencephalography. MRI holds a lot of potential, thus its principles and processes should be clearly understood. The use of MRI is increasing in number. Nonetheless, regarding the technology, there are still areas for improvement that need further study.

Table of Contents

List of Figures. v

List of Tables. vii

1.Introduction. 1

1.1 Scope. 2

1.2 Background. 2

2. Functional Magnetic Resonance Imaging. 3

3. Theories. 4

3.1. Nuclei of Atoms. 4

3.2. Larmor Precession. 5

3.3. Image Production. 7

4.Imaging Hardware. 9

4.1. Magnets. 9

4.2. Gradients. 10

4.3. Transmission and Reception. 11

4.4. Control and Processing. 12

5. Imaging Methods. 13

5.1. Point and Line. 13

6. Technologies………………………………………………………………………………..16

6.1 High-field MRI……………………………………………………………………………16

6.2 Ultra-low-field MRI………………………………………………………………………17

6.3 Magnetoencephalography (MEG)……………………………………………………..…18

7. Advances. 18

8. Conclusion. 20

9. References. 21

List of Figures

Figure 1. Finding the optimal value of TE for the maximum percentage

signal change with the BOLD effect (Stuart,1997)……………………………………………….4

Figure 2. The allowed energy levels of a spin 1/2 in a magnetic field

(Rodriguez, 2004)…………………………………………………………………………………5

Figure 3. Larmor precession of a nucleus (Kauppinen and Partanen, 2004)…….……..…….…..6

Figure 4. The Larmor precession of a magnetic moment in a uniform

 magnetic (Rodriguez, 2004)……………………………………………………………..………7

Figure 5. (Top) Application of a linear gradient field to a sample, (middle)

projection of the sample spin distribution, and (bottom) selective excitation

 of a plane of spins in a cylindrical sample (Rodriguez, 2004)…………..………………………..9

Figure 6. Magnetic resonance imager (Armstrong and Keevil, 1991)……………………………9

Figure 7. Maxwell coils produce a linear field gradient in Bz along the

z-axis (Stuart, 1997)……………………………………………………………………………..11

Figure 8. Golay coil for producing linear field gradients in Bz along the

x or y axes. l=3.5a, d=0.775a and f=120 degrees (Stuart, 1997)………………………..……….12

Figure 9. Surface r.f. coil which is tuned to resonance with the tuning capacitor

 CT and matched to 50Ω with a matching capacitor CM (Stuart, 1997)……………….………..12

Figure 10. A schematic diagram of the MRI scanner (Stuart, 1997)……………………………..13

Figure 11. Magnetic field gradient applied in the z-direction causes the

 resonant frequencies to vary by a few thousand Hz from slice to slice (Elster, n.d.)……………….15

Figure 12. MRI procedures in developed countries, per modality in the

 year 2007 (Cosmus and Parizh, 2010)…………..……………………………………………….15

Figure 13. Delivered superconducting MRI systems by field strength in the

 year 2008 (Cosmus and Parizh, 2010…………………..………………………………………..20

List of Tables

Table 1. Properties of some NMR-active nuclei (Rodriguez, 2004)……………….……………..5

Table 2. Image orientation and the gradient coils used (Rodriguez, 2004)………………………..8

1.Introduction

The use of Magnetic Resonance Imaging (MRI) opened new perspectives for medicine and neuroscience (Minati and Weglarz, 2007). MRI is a medical application of Nuclear Magnetic Resonance (NMR). Bloch and Purcell made the first successful NMR experiment in condensed matter in 1946. This experiment became one of the foundations of MRI being applied to medicine. Few years later, the utilization of NMR to form an image was proposed independently by scientists. This method was viewed as very valuable due to its flexibility and sensitivity to various properties of tissues. It is non-invasive, making diagnosis more efficient (Rodriguez, 2004). MRI has been used for functional imaging. Current imaging methods allow the assessment of the properties of brain tissue in order to gather information of brain mechanisms from the macroscopic to microscopic levels (Volkow et al., 1997).

A technologist, the individual that administers MRI services to patients, put the patient into the machine in order to start the examination. The computer is then utilized in order to establish the different sections of the human body that need assessing or screening. This would also helps in determining the time needed for hydrogen atoms to release the energy absorbed from the radio-frequency waves. Originally, MRI generates information in the form of numbers. These are subsequently converted into “a series of anatomical pictures” through the computer  (Joyce, 2005).

The procurement of images of the brain has been utilized in the field of medicine for years. Nonetheless, areas exist wherein clarification is needed. The processes or principles involved in the production of images from MRI have not been clearly understood by most people, thus producing more issues and concerns. Functional MRI is highlighted in the beginning. Select theories related to MRI, imaging hardware, imaging methods, technologies and current advances in the field of MRI are subsequently discussed.

1.1 Scope

The physics principles of nuclear magnetic resonance (NMR) is the basis of instrumentation behind MRI technology. NMR involves the interaction of the radiofrequency pulses, atomic nucleus (mainly hydrogen) and the magnet. Magnetic Resonance (MR) provides detailed three-dimensional images, especially of soft tissues that other modalities like CT cannot easily image. MRI’s ability to detect small changes (contrast) within soft-tissues is one of its strength, and MRI soft tissue contrast is considerably better than that found on CT images and nuclear medicine radiographic images. MRI techniques can be used in the field of functional neuroimaging to generate real-time images of the brain, for educational or clinical purposes.

1.2 Background

Magnetic resonance imaging (MRI) is based on the principles of rotating magnetic field discovered by Nikola Tesla in 1892 in Budapest, Hungary as one researcher reported (Saltzman, 2015, p. 524). The principle of nuclear magnetic resonance (NMR) was discovered by Felix Bloch and Edward Purcell in 1946. The two were awarded the Nobel Prize in physics for this discovery in 1952. The use of MR phenomenon in imaging, later at the University of Nottingham. Their work later matured to clinical applications in the 1980s (Webster, 2006, p.283). Magnetic resonance imaging (MRI) was first used in Britain in the early 1990s. MRI had its beginning as NMR. NMR imaging was later renamed to magnetic resonance imaging (MRI) to remove the word nuclear, which the public associated with ionizing radiation.  Magnetic resonance (MR) provides exquisitely detailed images of various soft tissues that cannot easily be imaged in other imaging techniques like CT and nuclear medicine. Saltzman (2015) indicated that “MRI provides excellent structural images of the joints, the brain, and the abdomen with the resolution of less than a millimeter” (p. 524). 

2. Functional Magnetic Resonance Imaging

Functional Magnetic Resonance Imaging (fMRI) is a neuro-imaging device that utilizes MRI in order to produce images that show the changes in brain tissue, which are the results of neural metabolism (Chen and Glover, 2015). The process of oxygenation by which oxygen is reversibly bound to the ferrous ion of haemoglobin in red blood cells is called as Blood Oxygenation Level Dependent contrast (BOLD). Functional MRI utilizes the magnetic features haemoglobin. The BOLD contrast in fMRI can be explained by the varying magnetic vulnerability of haemoglobin in its oxygenation states. The neurons involved in cognitive processes increase the amount of oxygen-carrying haemoglobin and the MRI signal. The fMRI can give valuable temporal information of the MRI signal in every image. Functional MRI may have a decreased spatial resolution than anatomical MRI and a decrease temporal resolution than direct techniques used for brain processes (Seixas et al., 2013).

Stuart (1997) highlighted that the most essential part of an image sequence is that T2 weighted images must be generated. A gradient echo, therefore, is typically used. Nonetheless, BOLD contrast is still exhibited by spin echo sequences due to the results of diffusion. Figure 1 describes the way of determining the optimal value of echo time, the time between the initial 90 degrees RF pulse and the echo, for the maximum percentage signal change with the BOLD effect.

Figure 1. Finding the optimal value of TE for the maximum percentage signal change with the BOLD effect (Stuart, 1997).

3. Theories

3.1.  Nuclei of Atoms

Atomic nuclei having 99.9% of the mass of all matter is contained within them. In atoms, each electron is situated away from other electrons and other atomic nucleus. According to Rodriguez (2004), “nuclei of atoms exhibit proportionality between their total magnetic moment μand total angular momentum J”. The relationship is established by the equation μ = γJ where γ is the gyromagnetic ratio of the nucleus that is nucleus-dependent. This ratio is an unchanging feature of a nucleus. The properties of some NMR-active nuclei are shown in Table 1.

Nucleus Spin Relative sensitivity Gyromagnetic ratio  [MHz/T]
1H ½ 1.000 42.58
13C ½ 0.016 10.71
19F ½ 0.870 40.05
31P ½ 0.093 11.26

Table 1. Properties of some NMR-active nuclei (Rodriguez, 2004).

The allowed energy levels of a spin 1/2 in a magnetic field are described in Figure 2.

Figure 2. The allowed energy levels of a spin 1/2 in a magnetic field (Rodriguez, 2004).

The extent of the bulk magnetization vector points in the positive direction of the Z-axis at equilibrium is described in equation . This emphasizes that MRI has low sensitivity for 1 Tesla of magnetic field (Rodriguez, 2004).

3.2.  Larmor Precession

The Larmor precision frequency is the rate of precission of spin packet under the affect of magnetic field. There is change in nucleus spin energy level due to frequency of an RF signal, is given by Larmor precision. The frequency is calculated by the gyromagnetic ratio of the atoms and strength of magnetic field. The stronger the magnetic field, the higher the precessional frequency. According to Kauppinen and Partanen (2011), Larmor precession refers to the “precession of the nuclear magnetic moment about the external magnetic field direction”. Its angular frequency is the Larmor angular frequency. In Figure 3, the Larmor precession of a nucleus is shown. B refers to the external static magnetic field while µ refers to the magnetic moment of the nucleus. The precession angle describes the change in of the magnetic moment.

Figure 3. Larmor precession of a nucleus (Kauppinen and Partanen, 2011).

The Larmor precession of a magnetic moment in a uniform magnetic field is shown in Figure 4.

Figure 4. The Larmor precession of a magnetic moment in a uniform magnetic field (Rodriguez, 2004).

3.3.  Image Production

Spatial localization of MR signals is needed to produce an image. This involves two steps, namely, the selection of a slice of the body for imaging and the application of a magnetic field gradient along to any combination of directions. The measurement of spatial variation of MR parameters like, spin density, is important in the production of images. These are dependent on the spatial coordinates of the spin system. The uniformity of the field B0 may be changed by using the linear magnetic field gradients. To generate an image that is in one dimension only, the NMR signal can be obtained “in the presence of a spatially varying magnetic field which is added to the uniform field”. It must be emphasized that the gradient provides a linear variation of frequency due to location. Figure 5 shows the possible results of the application of a linear gradient across a two-dimensional material. Armstrong and Keevil (1991) emphasized than an MRI image can only be obtained under certain situations. One of the most important situation is that a magnetic field must exist around the body being examined. The strength of this magnetic field should be determined in order to correctly select the frequency content of the radiofrequency pulse. The amount of magnetic field and the proportion of nuclei are directly related and   line up in the direction of the field. Figure 6 shows a summary of how an image is produced in MRI. In the figure, it may be imagined that a patient is placed inside a cylindrical-shaped material created by the magnet and radiofrequency coils. Thereafter, a magnetic field is created due to the high-field magnet and gradient coils. Radiofrequency coils then begin the transmission and reception of energy pulses. The signal from the selected slice of the patient is then “measured, localised, and used to create the final image”.

Table 2. Image orientation and the gradient coils used (Rodriguez, 2004).

Applied slice select gradient Name Slice plane orientation
Gx sagittal parallel to y-z plane
Gy coronal parallel to x-z plane
Gz transverse parallel to x-y plane

Table 2 further shows the connection between the gradient that is specific for the slice used and the orientation of the produced slice plane (Rodriguez, 2004).

Figure 5. (Top) Application of a linear gradient field to a sample, (middle) projection of the sample spin distribution, and (bottom) selective excitation of a plane of spins in a cylindrical sample (Rodriguez, 2004)

Figure 6. Magnetic resonance imager (Armstrong and Keevil, 1991)

4.Imaging Hardware

4.1.  Magnets

The magnet is the primary component of the MRI system. It is considered as the largest and most costly part of the system. According to Steckner (2006), the ideal magnet for MRI “produces a perfectly homogeneous magnetic field”. Theoretically, this may be attained by making a surface current with a spherical shape that changes with a sinusoidal spatial distribution. This is not ideal for the users of the MRI. Recently, magnets of other shapes with special current density distributions have beendeveloed. Examples of these are the cylinders and parallel plates.

The maximum homogeneity of the field is a vital requirement NMR. Tolerances may reach to the low point of 1 ppm needed over the interested volume. In order to address this, the field is evened out as much as possible during installation with the use of ferromagnetic blocks put in the bore of the magnet. Shim coils are also put inside the bore. Fields that change with a particular function of position are subsequently produced. With the simultaneous utilization of these, the intrinsic homogeneity of the magnet may be improved and the field effects caused by susceptibility differences in the scanned materials may be decreased (Stuart, 1997).

The real obstacle in an MRI is decreasing the magnet package to decrease also the cost but increase the quality of siting features. A common method used to increase the quality of siting features is the placement of the magnet into a “counter-acting” magnet. However, a “missile effect” may be experienced since a force on ferromagnetic objects is produced due to the magnetic field gradient between the inside and outside portions of the magnet. In the current industry, there are magnets that are designed for specific, specialized functions (Steckner, 2006).

4.2.  Gradients

Gradient coils are required to generate a linear field variation in one distinct direction and to possess high efficiency, low resistance and low inductance. These are necessary in order to decrease the requirements for the current and the deposition of heat. The Maxwell coil normally produces a linear field variation along the field’s direction, also known as the z-axis. A pair of coils such as the one shown in Figure 7 is involved. Current flows in opposite directions, generating a linear gradient (Stuart, 1997).

Figure 7. Maxwell coils produce a linear field gradient in Bz along the z-axis (Stuart, 1997).

Wires connected along the magnet’s bore are needed for the production of a linear field gradient in the other axes. A saddle coil, like the Golay coil in Figure 8, is best used for this. Four saddles are placed along the bore. This generates a linear variation in Bz along the axis of orientation. A very linear field is then generated at the central plane. The linearity, however, diminishes. A number of pairs with varying axial partitions may be utilized to improve the linearity. If the axis desired is not along x, y or z, then current must be sent proportionally to Gx, Gy and Gz coils (Stuart, 1997).

Figure 8. Golay coil for producing linear field gradients in Bz along the x or y axes. l=3.5a, d=0.775a and f=120 degrees (Stuart, 1997).

4.3. Transmission and Reception

The radiofrequency coil is the third primary component of an MRI scanner. It has two types, namely, the surface coils and the volume coils. A surface coil is placed on the surface of the material being scanned. It is basically a coil of wire that is in parallel with a capacitor. A resonant circuit is formed and tuned to obtain a resonant frequency that is similar to the spins being imaged. Because the coil is connected to a power amplifier with an output impedance of 50Ω and the order of kilo-ohms is the input impedance of the coil, a very large amount of power is mirrored back. A second capacitor is therefore added in series with the coil, as shown in Figure 9, in order to address the issue. Simple surface coils generate small homogeneous fields.

Figure 9. Surface radiofrequency coil which is tuned to resonance with the tuning capacitor CT and matched to 50Ω with a matching capacitor CM (Stuart, 1997).

Volume coils are need to image whole bodies or areas that are far from the surface (Stuart, 1997). Volume coils normally have cylinder-like shapes. Rodriguez (2004) stated that the bird-cage coil remains as the most efficient volume coil in recent times.

4.4. Control and Processing

A computer handles the control of the MRI scanner (Stuart, 1997). According to Rodriguez (2004), the computer system is where the user begins the measuring system functions such as system tests, display images, and is where the images are obtained. A form of Fourier transform is needed in the computer system process. The fast Fourier transform (FFT) is considered as the best algorithm since it may be utilized to obtain two-dimensional or three-dimensional images

 Figure 10 shows a schematic diagram of the overall MRI system.

Figure 10. A schematic diagram of the MRI scanner (Stuart, 1997).

5.  Imaging Methods

5.1. Point and Line

The point methods usually do not need various hardware components and the field of homogeneity is only required on a small area. One example of a point method is the Field Focused Nuclear Magnetic Resonance (FONAR) method. It has been determined that the Free Induction Decay (FID), the observable NMR signal, will quickly decay when acquired from an area of inhomogeneous magnetic field. As a result, the signal has a wide range of frequency components that dephase in an increased rate, shortening the FID. Also, an extensive line is formed on Fourier transformation. A sample large enough to go beyond the homogeneous area of the magnet will produce a signal that is “long-lived from the central homogeneous region” and that is “short-lived from the surrounding non-homogeneous region”. To trigger spins inside the area that generates signals, a selective pulse with enough bandwidth may be utilized. An image can then be generated point by point with the movement of the sample in relation to the magnet or with the relocation of the area that generates signals. Another example of a point method utilizes three different sinusoidal oscillating orthogonal field gradients. The spins within the sensitive point, the small region of intersection, are the only ones in a time-constant magnetic field. This method does not require the formation of an image to make NMR measurements. Its major detriment, however, is its very long imaging time duration. This is generally an extension of the point methods since its primary principle is the isolation of a line within a three-dimensional object (Rodriguez, 2004).

5.2. Two-dimensional or Planar

Sequential plane methods comprise most of the investigations on MRI. In these, before gathering data or information, a slice of magnetization is energized by utilizing selective irradiation. Imaging sequences may be divided into two groups, namely, frequency encoding of the spin system and phase encoding (Rodriguez, 2004). According to Mansfield and Maudsley (1977), frequency-encoding may be used to “define the position of entire slices in two-dimensional imaging”. The primary concept is similar to frequency-encoding in a plane. However, the gradient is already placed along the “slice-select direction”.

As a function of position, the center resonant frequencies of whole slices differ as shown in Figure 11. An RF pulse may be tuned to a distinct slice for stimulation.

Figure 11. Magnetic field gradient applied in the z-direction causes the resonant frequencies to vary by a few thousand Hz from slice to slice (Elster, n.d.).

In phase encoding, the maximum MR signal is obtained in the center line, the area wherein the phase encoding gradient is nearly zero. The raw data or information is the k-space. Phase encoding should be repeated in order to produce two-dimensional information about the material of interest. The spins should be excited more times. Hence, the RF pulses, 90 degrees and 180 degrees, and the gradients shall be applied numerous times. The phase encoding gradient, however, varies always (Salzer, 2012).

5.3. Three-dimensional

According to Rodriguez (2004), two-dimensional methods may be extended to generate three-dimensional images. Fourier techniques may be used to generate three-dimensional data sets. For example, images may be sliced multiple times of additional phase encoding may be utilized. The slicing method can produce images of varying sections. In addition, according to Naraghi and White (2012), three-dimensional isotropic MRI with appropriate acquisition times is possible through the use of various technical advances that include higher-field-strength in MRI processes, stronger performance gradients, high-resolution coils for multiple channels, and “pulse sequences with shorter acquisition times”. Furthermore, three-dimensional isotropic acquisitions offer numerous benefits that include better spatial resolution in a plane and generation of “high-quality reformats” to subsequently produce images for multiple planes from the original set of data or information. Nonetheless, it is highlighted that the long procurement and post-processing time duration have limited the utilization of pulse sequences into medicinal applications even if the production of three-dimensional magnetic resonance images has been possible for a long time.

6. Technologies

6.1. High-field MRI

High-field MRI can generate millimetre-resolution images of the human body. Because of this, it is being used in whole body imaging and functional brain imaging. It is based on a homogeneous magnetic field B0 = B0ez that is static and capable of producing “a strong thermal equilibrium magnetization”. A conducting magnet that should not be disturbed during the measurement generates a tesla-range B0 field. The magnetization may be tossed to the xy-plane by a 90 B1 AC pulse at the Larmor frequency in order to stimulate a signal. The magnetic field generated may thereafter be calculated by induction coils that are tuned to the Larmor frequency.

The benefits of high-field MRI include higher signal-to-signal noise ratio and contrast-to-noise ratios. Technological issues however focus on the homogeneity of the B0 and B1 magnetic fields and the design of radiofrequency array coils for signals to be received. There may also be problems or concerns on the relaxation kinetics and susceptibility effects. Specifically, there is a “larger spectral separation between different chemical species” and this further generates higher spectral resolution. Applications for magnetic resonance spectroscopy gain an advantage from this and from the higher signal-to-noise ratio (Stafford, n.d.).

6.2. Ultra-low-field MRI

Ultra-low-field (ULF) MRI is a new method wherein microtesla-range fields are used to obtain images. The signals in ULF MRI are primarily quantified by superconducting quantum interference devices (SQUIDs) sensors with “untuned input circuits”.

SQUIDs are being utilized to replace the method of detecting signals since they are able to sense the magnetic flux promptly. With this, the factor for signal strength is the polarizing field strength. The advantages of ULF MRI include relaxed homogeneity requirements, lightness and cost-efficiency. Since the magnetic fields used are weaker, ULF MRI is safer due to the lack of danger from the use of ferromagnetic projectiles and the like, and is more silent since the Lorentz forces in the coils are weaker. The signal is therefore dependent on the polarizing field strength. The ULF-MRI device is commonly placed in a magnetically shielded room in order to avoid magnetic field noise from the surroundings as much as possible (Vesanen, 2013). The biggest disadvantage of current ULF MRI is its lower signal to noise ratio, when compared to high-field MRI.

6.3. Magnetoencephalography (MEG)

The magnetic field produced by synchronous neuronal activity in the brain is primarily measured using magnetoencephalography (MEG). Single induction coils were traditional materials for MEG. Present-day techniques, however, utilize helmet-shaped arrays of up to 306 SQUIDs in order to detect the magnetic field. According to Braeutigam (2013), the technology of MEG revolves around the determination of magnetic field produced by currents in the neurons. The signals in MEG are also “reference-free” and are not influenced by changes in the conductivity on the magnetic flux. As a result, the image of the brain is much clearer. Any MEG device should be operated inside a magnetically shielded room in order to limit magnetic noise from the surrounds. One advantage of MEG is its temporal resolution of up to numerous kilohertz. One application of MEG is in the assessment of epilepsy and brain tumor conditions prior surgeries (Vesanen, 2013). MEG is now considered as one of the most modern imaging tools for the field of radiology.

7. Advances

MRI has been continuously used in countries around the world more frequently than ever as shown in Figure 12. This is primarily due to the needs of the healthcare industry that include higher efficiency, lower costs and more reliable systems. Magnet designs better than the previous are being bulk produced (Cosmus and Parizh, 2010).

Figure 12. MRI procedures in developed countries, per modality in the year 2007 (Cosmus and Parizh, 2010)

Numerous whole-body systems of 1.5 tesla and 3 tesla are produced annually. Currently, higher-field MRI systems are desired since they may provide better images for the identification of symptoms and illnesses. Figure 13 displays the trend in the use of MRI systems by field strength in the year 2008 (Cosmus and Parizh, 2010). According to Stafford (n.d.), higher fields mean higher signal-to-noise ratios and contrast changes that could possibly ease certain applications. There are still some technical issues that must be addressed prior the full utilization of MRI systems with 3 tesla. It is emphasized, however, that image quality is improved with higher field systems.

Figure 13. Delivered superconducting MRI systems by field strength in the year 2008 (Cosmus and Parizh, 2010).

More specifically, according to Seixas et al. (2013), for fMRI alone, the advances include hardware development, image processing and brain stimulation for data gathering. These are still under development since fMRI is desired to have more spatial and temporal resolution, specificity, sensitivity and durability. In addition, approximately 95% of superconducting magnets are cylindrical in shape

8. Conclusion

The theories and technologies involved in MRI were thoroughly discussed and tackled. MRI involves numerous theories for its application in various fields of medicine, most especially in brain imaging. It is also composed of hardware components, namely, magnets, gradients, transmission and reception devices, and computer system that in work together to allow the MRI scanner to work. Images may be produced through point and line, two-dimensional and three-dimensional methods. MRI technologies include high-field, ultra-low-field and magnetoencephalography. MRI is currently being widely used in medicine and has been continuously developed to attain higher field systems. MRI holds a lot of potential, thus its principles and processes should be clearly understood by its operators and users. The use of MRI is increasing in number. Based on the advances regarding MRI technology, it may be concluded that people will always seek to attain improved efficiency in the forms of greater speed, increased accuracy and less cost. Nonetheless, regarding the present technology, there are still areas for improvement that need further study.

9.References

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Braeutigam, S. (2013). Magnetoencephalography: Fundamentals and established and emerging clinical applications in radiology. ISRN Radiology.

Chen, J.E. and Glover, G.H. (2015). Functional magnetic resonance imaging methods. Neuropsychol Rev, 25, 289-313.

Cosmus, T.C. and Parizh, M. (2010). Advances in whole-body MRI magnets. European Superconductivity News Forum, 14.

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Joyce, K. (2005). Appealing images: Magnetic resonance imaging and the production of authoritative knowledge. Social Studies of Science, 35(3), 437-462.

Kauppinen, J., and Partanen, J. (2011). Fourier transforms in spectroscopy. Berlin: Wiley-VCH.

Mansfield, P., and Maudsley, A. A. (1977). Medical imaging by NMR. The British Journal of Radiology50(591), 188-194. doi:10.1259/0007-1285-50-591-188

Minati, L., and Weglarz, W.P. (2007). Physical foundations, models, and methods of diffusion magnetic resonance imaging of the brain: A review. Concepts in Magnetic Resonance Part A, 30A(5), 278-307.

Naraghi, A., and White, L.M. (2012). Three-dimensional MRI of the musculoskeletal system. American Journal of Roentgenology, 199(3).

Rodriguez, A.O. (2004). Principles of magnetic resonance imaging. Revista Mexicana de Fisica, 50(30), 272-286.

Salzer, R. (2012). Biomedical imaging: Principles and applications. Hoboken, NJ: John Wiley & Sons.

Seixas, D., Ebinger, G., Mifsud, J., von Koch, J.S., and Alpert, S. (2013). Functional magnetic resonance imaging. European Commission Publication.

Stafford, R.J. (n.d.). High field MRI: Technology, applications, safety, and limitations.

Steckner, M. (2006). Advances in MRI equipment design, software, and imaging procedures. Medical Physics33(6), 2156-2157. doi:10.1118/1.2241397

Stuart, C. (1997). Functional MRI: Methods and applications (Doctoral dissertation, The University of Nottingham).

Vesanen, P. (2013). Combined ultra-low-field MRI and MEG: Instrumentation and applications (Doctoral dissertation, Aalto University).

Volkow, N.D., Rosen, B., and Farde, L. (1997). Imaging the living human brain: Magnetic resonance imaging and positron emission topography. Proc. Natl. Acad. Sci., 94, 2787-2788.

Zotev, V.S., Matlashov, A.N., Volegov, P.L., Urbaitis, A.V., Espy, M.A., and Kraus, Jr., R.H. (n.d.). SQUID-based instrumentation for ultra-low-field MRI.

The terms offer and acceptance. (2016, May 17). Retrieved from

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[Accessed: March 29, 2024]

freeessays.club (2016) The terms offer and acceptance [Online].
Available at:

[Accessed: March 29, 2024]

"The terms offer and acceptance." freeessays.club, 17 May 2016

[Accessed: March 29, 2024]

"The terms offer and acceptance." freeessays.club, 17 May 2016

[Accessed: March 29, 2024]

"The terms offer and acceptance." freeessays.club, 17 May 2016

[Accessed: March 29, 2024]

"The terms offer and acceptance." freeessays.club, 17 May 2016

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