Introduction
Since the initial years of studies in proton magnetic resonance imaging (MRI), the field’s pioneers have made efforts to create stronger systems using higher magnetic field strengths despite arguments made by theoreticians that the maximum field strengths had already been attained. The first 8 Tesla (340MHz) MRI system created for use in the whole body of human beings was created at Ohio State. The researchers in charge of this system encountered numerous challenges during the production and operation of this system. The issues identified by the researchers have investigated in later years thanks to availability of huge bore magnets at ever increasing field strengths. Although substantial comprehension achievements were made in the course of the creation of 1.5 Tesla systems and later 4 Tesla systems, the mastery of the challenges facing ultra high field (UHF) imaging is still unrealized even in the midst of alleged experts in ultra high field.
Burgess (2004) argues that “despite the very real issues raised by critics, the highest field attainable for MRI progressed from under 5 MHz in the late 1970s, to 63MHz in the mid 1980s, 200MHz in the early 1990s, and 340MHz at the end of the 20th century” (14). The achievement of high field strength of MRI was constrained by difficulties in the design and construction of magnets as was the case for analytical spectroscopy systems. Because of the challenge of producing a magnetic field with sufficient homogeneity to cover a huge portion of interest relating to the width of the human body, there are challenges of operating at higher field strengths in human body as compared to the physical sciences and small animals.
Potential Benefits of Ultra High Field Magnetic Resonance Imaging
Ultra high field MRI at 7-9.4 T in human-sized whole body systems has become a new forefront of MRI. After the original illustrations of the viability of ultra high field MRI at educational institutions, the chief MRI traders have now taken into consideration the perspective of these systems, and have set out on the creation of ultrahigh field systems for sale. The major benefit of higher magnetic field strength is the increase in the signal-to-noise ratio (SNR) as the field strength is increased. This high SNR may, in turn, be utilized to either enhance the spatial or temporal motion in proton MRI, or to considerably enhance the responsiveness of MRI to other nuclei. In addition, high field strength increases the chemical shift spectral dispersion, which in turn enhances the detection of metabolites by enabling for improved differentiation of spectral lines in MR spectroscopy.
Lastly, the consequences of magnetic susceptibility from paramagnetic material increase with higher field strength, and this creates greater contrast means for naturally occurring paramagnetic material such as deoxyhemoglobin and tissue iron. Andra and Nowak (2007) argue that “increased sensitivity to deoxyhemoglobin content presents a mechanism for improved depiction of venous vasculature and is the basis for blood oxygen level dependent (BOLD) neurofunctional MRI” (37). The amount of tissue iron may permit the nonintrusive evaluation of numerous situations that change the iron content in the brain, for instance, the normal aging and neurodegenerative illnesses. These potential benefits of ultra high field MRI are however limited not only by the high costs incurred in large high-field magnets, but also by numerous essential physical issues that currently create serious problems for ultrahigh field MRI. These challenges include “artefacts from inhomogeneous b0 and b1 fields, different tissue relaxation tissues, and safety concerns” (Brown and Semelka 2010, 113).
Issues Facing Ultra High Field Magnetic Resonance Imaging
Difficulties in capturing images due to high specific absorption rate
The inception of ultra high field MRI systems has brought the MRI technology nearer to the physical challenges and as a result, more development efforts are needed to attain suitable sequences and images. Hypothetically, the signal-to-noise ratio (SNR) at high field MRI should portray a direct increase with higher magnetic field strength. However, the relations between the magnetic field and other causal variables such as relaxation times, radio frequency (RF) pulses and coils performance in the course of image acquisition, is extremely complicated. One essential aspect is the alteration of RF pulses in higher magnetic field strengths. Ulmer and Jansen (2010) argue that “changing the field strength from 1.5 to 3 T results in a fourfold increase in the required energy, increasing specific absorption rate (SAR),” (36). The augmentation of SAR in turn creates difficulties in the capturing of images because the amalgamation of energy in the tissue is not permitted to surpass some specific limits. Thus, limitations in the amount of slices and the attainment of homogenous excitation of the nuclei are greater in higher field strengths.
The BOLD consequence at higher field strength grows at a lower rate in vessels that are bigger than the size of the voxel and is therefore more evident in vessels with smaller size than the voxel. By making use of small-sized voxels at higher field strength in comparison with 1.5T, the blood oxygen level dependent (BOLD) signal can be made more precise and dependable. As a result, alterations in the signal ought to be more closely associated with the cortical activity. Wattjes and Barkhof (2009) argue that “with the increasing signal and enhanced stability of the BOLD signal in higher field strength, the repetition of events can be reduced,” (281).
Magnetic susceptibility and b0 inhomogeneity
Differences in susceptibility between diamagnetic tissue water and paramagnetic material lead to changes in local magnetic fields. These alterations take place on a subvoxel spatial scale, for instance with red blood cells, ferritin and MRI contrast agents, and create a crucial contrast system. On the other hand, susceptibility inhomogeneity on a spatial scale of numerous voxels near air/tissue interfaces generates grave image artefacts, such as geometric deformations and loss of signal. Zivadinov and others (2008) argue that “susceptibility field inhomogeneity can be described analytically for simple geometries, whether numerically modelled or experimentally measured,” (67). B0 inhomogeneity is particularly a challenge for ultra high field MRI because field changes scale linearly with the magnetic field strength. Moreover, susceptibility inhomogeneity leads to severe loss of signal in gradient echo images, where the size of the signal void increases with increasing echo time and higher field strength.
B1 inhomogeneity
Andra and Nowak (2007) argue that “another fundamental challenge with ultrahigh field MRI arises from the fact that presently b1 fields with uniform amplitude are not easily achievable for human head or larger-sized samples,” (39). Instinctively, this can be illustrated as follows: the wavelength at 300-340 MHz in tissue is approximately 6cm, which is lesser than the size of a human head. As a result, the field distribution of radio frequency symbolizes complex patterns of standing and proliferating waves with local amplitude minima and maxima in three-dimensional space. This depends on the form, direction, dialectic, and conductive nature of the anatomic region to be imaged instead of on the sole design of the coil. On the other hand, at lower field strengths, the wavelength is bigger than anatomy and b1 homogeneity can be attained. High-field b1 field distribution cannot be forecast instinctively, and some researchers have made use of numerical simulations to measure b1 fields. The consequential complex highly inhomogeneous patterns of radio frequency intensity distribution support the experimentally calculated maps. B1 inhomogeneity creates uneven radio frequency flip angles and has great sensitivity, thus negatively affecting the image contrast, SNR and general image quality (Newton and Jolesz 2008).
The challenge of radio frequency inhomogeneity ought to be tackled and solved to reap maximum benefits of ultrahigh field MRI. This essential problem remains to be tackled, although several potential propositions have been made by experts in the field. Conventional radio frequency coil design can solve this predicament only partly, because the distribution of radio frequency is under the control of the patient. The propositions suggested by experts include the utilization of shaped radio frequency pulses, and multi-port excitation of TEM coils. The latter suggestion can be expanded by making use of numerous parallel transmit coils and RF amplifier chains. This strategy used solely or in combination with numerous autonomous receive coils – can be the most potent solution to the problem and may enable not only the enhancement of radio frequency and image homogeneity but also the speeding up of imaging, similar to present receive coil parallel imaging techniques. However, for this solution to be put into effect, complicated and expensive hardware-software systems need to be installed, but these are currently being theoretically investigated and executed (Robitaille and Berliner 2006).
Changes in relaxation times
A third essential problem with ultrahigh field MRI originates from the fact that relaxation times change significantly in comparison with lower field strengths, thus substantially changing the appearance of the images. Bottomley used data obtained at lower field strengths, and forecast greater tissue T1 relaxation times with higher field strength, whereas T2 relaxation times were forecast to remain roughly the same (Stippich and Blatow 2007). As a result of susceptibility consequences, T2 relaxation times are anticipated to become substantially shorter at ultra high field MRI. Because all T1 and T2 relaxation time measurement techniques are influenced by flip angle inconsistency, empirical calculations of relaxation times are hampered by b1 inhomogeneity. Therefore, initial T1 and T2 measurements at 8 T only made use of data from image portions where flip angles did not diverge more than 20 degrees from the nominal 90/180 degrees.
One essential finding from research is that at 8 T multi echo T2 are approximately twice as long as single echo T2. Faro (2010) argues that “the fundamental difference between single and multi spin echo sequence is that the signal decay observed with single echoes with long times between RF pulses is significantly affected by water molecule diffusion in magnetic field gradients,” (53). This implies that water protons obtain different numbers of phase shifts as they pass through magnetic field gradients generated by small dimensional magnetic field perturbers.
Dielectric effects
The basis of dielectric effects originates from relations between the radio frequency b1 fields and the dielectric nature of the body, for instance the permittivity and conductivity of the human tissue. The wave property of the RF electromagnetic field and the dielectric and conductive boundaries of the body cannot be taken for granted. Consequences of the wavelike property of the RF fields substantially change the anticipated b1 homogeneity for clinical imaging. As a result, manufacturers of RF coils encounter numerous difficulties when creating coils for ultra high field MRI. Robitaille and Berliner (2006) state that “with a typical permittivity of about 60, the effective half wavelength dimension inside the human body shrinks to about 10 cm at 300 MHz, which corresponds to a fraction of a human head,” (84). Thus, the head can perform the role of RF resonator and can produce standing wave patterns but this depends on the local dielectric nature of human tissue. In general, these difficulties can be solved at 3T, and there are justifying aspects to offset them for instance by making use of saline bags and/or oscillating RF coils. Even though it is a big challenge to attain homogeneous b1 fields in the body at 7T, there have been recent demonstrations of human body imaging at 7T. Technologies that include RF shimming and transmit arrays may further aid in addressing these challenges.
Safety concerns
Patient comfort is an essential concern at all field strengths, and more so at higher field strengths. This is because certain physiological consequences increase with higher magnetic field strength. Some patients complain of metallic taste on the tongue. This may arise when the patient is talking when inside the magnet or when being moved into the magnet, but the feeling is also temporary and often goes away when the patient comes out of the magnet. Hydrodynamic consequences can lead to distressing feelings when the patient is passed through the magnet. Other patients complain of giddiness and queasiness as a result of the Db/Dt exposure when passed through the strong fringe field gradients (dB/dr). This is normally limited to approximately 1 m in 10-15 seconds. These negative consequences can be minimized by the use of motorized patient table drives when passing patients through the magnet. The consequences can also be controlled by changing the speed of the patient table when passing the patient through the fringe field. Feelings of giddiness and queasiness are temporary and patients can become accustomed to them. However, patient comfort and safety should not be taken lightly and therefore more research is needed to wholly understand these adverse consequences (Newton and Jolesz 2008).
Magnetic forces
The most danger from an ultra high field MRI arises from flying objects. Because of the nonlinear property of magnetic fringe fields, it is easy to take too lightly the power of a magnetic field particularly at an ultra high field. Caution should be taken to make certain that no one enters the examination room with magnetic objects, particularly when scanning is done on the patient while in the magnet. This can be achieved using metal detectors but unfortunately, they are not 100 percent effective because healthcare workers can become negligent. It is thus a necessity to permit access to the 7T magnet only to authorized and highly trained professionals (Brown and Semelka 2010).
Conclusion
Ultra high field (UHF) magnetic resonance imaging (MRI) has become more desirable in the recent past. This is attributed to several factors chief of which is the better detection of illnesses. However, UHF MRI is laden with several challenges including: difficulties in capturing images due to high specific absorption rate, magnetic susceptibility and b0 inhomogeneity, b1 inhomogeneity, dielectric effects, changes in relaxation times and patient safety concerns including adverse effects of magnetic forces. However, all is not lost as there are some potential propositions which have been made by experts in this field to address these challenges.
Reference List
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Burgess, Richard. 2004. Magnetic resonance imaging at ultra high field: Implications for human neuroimaging. Ohio: Ohio State University.
Faro, Scott. 2010. BOLD FMRI: A guide to functional imaging for neuroscientists. New York: Springer.
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