The challenges of modifying the antenna of imaging systems, e.g., MRI, are enormous. The electromagnetic principles for the non-ionizing radiation technique to view internal structures in the human body depend on many factors such as the ratings of the magnetic field, computer, digitizer, RF source, and electrical field. An incorporation of the Bloch NMR flow equation alongside the electromagnetic principles is quite complex. However, the modality was successfully developed to predict the radiofrequency appropriate for the successful imaging session. It was observed that the patient is currently under severe danger of excess exposure to electromagnetic fields.
Keywords: Antenna, Bloch NMR flow equations, Maxwell’s equation, MRI
|How to cite this article:
Emetere M E, Sanni E S. The challenges of antenna modification in medical practice: The MRI machine. Ann Trop Med Public Health 2017;10:726-30
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Emetere M E, Sanni E S. The challenges of antenna modification in medical practice: The MRI machine. Ann Trop Med Public Health [serial online] 2017 [cited 2021 Mar 6];10:726-30. Available from: https://www.atmph.org/text.asp?2017/10/3/726/213173
Electromagnetism is a phenomenon that occurs when electric currents are influenced by the presence of magnetic fields and vice-versa. It possesses very wide application in electronics.,, However, in medicine, the application of electromagnetism has begun to gain prospect. The physics behind the application of electromagnetism is in the incorporation of alternate current (AC). In AC, the electricity moves back and forth which produces a dynamic magnetic field. Increased current initiates a greater magnetic field. Hence, electromagnetism is a dynamic magnetic field which contains both electric and magnetic fields. These fields form unified electric and magnetic waves that are perpendicular to each other moving in streams along a straight path in a vacuum. This phenomenon is used to explain the mode of operation of bio-medical devices supported by or hinged on this principle. For example, the non-inductive non-thermal devices work on the principle of applying low intensity-low frequency electromagnetic fields to biological systems and not necessarily by inductive electromotive force (emf) generated by thermal means. The simple chart [Figure 1]below illustrates the down chain of electromagnetic medicine. The chart highlights the three basic applications of electromagnetic medicine which include diagnosis (for detecting defects without opening the body of a patient) and therapy (the use of x-rays in destroying cancerous cells), for maintaining good hygiene and for research. Electromagnetism can be applied to three main aspects of medicine, that is, magnetic resonance imaging (MRI)-diagnostic imaging;,,,, radiofrequency thermal ablation-cardiology and cancer (tumor) therapy; localized dielectric heating (shortwave diathermy)-physiotherapy. This paper review is a discourse on the application of MRI.
|Figure 1: MRI antenna configuration for higher performance (http://ece.wpi.edu)
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In medicine, electromagnetic fields (EMFs) have their side-effects when used but the advantages and achievements far outweigh the disadvantages. For example, the therapy for malignant growth is possible via the destruction of cancer cells, and according to Leitgeb et al., MRI imaging finds application in medicine in relation to its use as a diagnostic tool which concentrates magnetic flux from an electromagnetic source of increasing radio frequencies. Due to the high radiation intensity, tissues could be excessively heated and damaged during metallic implants. Furthermore, in the paper, an investigation was carried out to determine whether patients with metallic implants can be accepted for MRI taking into consideration possible limitations with regard to static magnetic fields. A numerical-anatomical and thermal modeling of heated tissues was done using different flux densities in the range of 1.5-7 T. The findings show that the overall tissue temperature increased with increasing RF EMF frequency. Also, setting the MRI at the maximum permissible level still had a marginal effect on the heated body. Hence, they concluded that from a heating point of view, for thorax surgery, metallic sutures used to fix the stenum after thorax surgery have no contraindication for MRI with static magnetic flux up to 7T. The report by Chadwick  deals with assessment of the exposure level of operators of MRI equipment to static and switched gradient fields of the equipment. The work also involves computational modeling and measurements of exposure levels using magnetic field dosemeters. The model has three strands which include a model for static and switched magnetic fields with 1.5, 4, and 7T magnets, a model of induced current densities and internal electric field strengths arising from motion through static field spatial gradients, and a model for induced current densities and internal electric field strengths from time-varying switched gradient fields. The findings show that guidelines for exposure levels can be exceeded by movement through the static field or by direct interactions with switched static fields, that is, faster movement through the static field above 1 m/s would increase the exposure levels and versa. The work of Turan and Çömlekçi  finds application in the field of bio-electromagnetics. It involves the development of biological voxel-based computational models which were constructed for use in dosimetry calculations for probing the effects of electromagnetic interaction between electronic equipment and biological structures. The numerical models adopt the DICOM format (medical image file format) as its input files; hence, the adoption of a developed DFP (DICOM File Processing) is necessary for processing the so-called files. In addition, a voxel-based computational female adult human head was developed which contained gray and white matters, skin, internal air of head, and external air from MRI images in the DICOM files. According to them, the DFP is a medical program that helps provide easy access to medical image, modalities, metadata, and patient information by increasing the resolution of the medical images within the DICOM files. A survey was conducted by Schaap et al. where a questionnaire was distributed in some clinical and research departments in Nertherlands to collect useful information from workers on exposure, work procedures, historical developments, and employee working with or closely to MRI scanners. Based on the survey, about 7000 individuals were reported to be working in an MRI scanning room and were however confirmed to have high risks of exposure to static magnetic fields; 54% of the occupationally exposed group had a day per month exposure. The most exposed group investigated constituted radiographers which were approximately 1700 persons. The findings also reveal that 9% of 7000 were regularly present in the scanning room during image acquisition, when exposure to additional types of EMF is considered a possibility. In essence, their claims show that type and frequency of potential exposure to MRI depend on the nature of the job, as well as the immediate work environment.
Specific application of electromagnetism to magnetic resonance imaging machine
The magnetic resonance imaging is aided by electromagnetic principles for the non-ionizing radiation technique to view internal structures in the human body.
The type and magnitude of radio waves spreading from the antenna make it possible to form images of the body via structured processes.
The science of the physics of RF sources has been discussed by scientists.,, The RF magnetic field is derived from antenna coils whose frequency is frequently higher than an amateur radio. The electromagnetic field from the antenna coils is governed by the Maxwell equations which deal with the interactions of electric and magnetic fields. Emetere ,, derived the governing equations of the RF sources
where β is the frequency of excited power; j is the radio frequency current; r represents the radius or horizontal component of the antenna; z represents the vertical component of the antenna; m represents the number of the protons; ξ represents the electrical permeability; μo represents the magnetic permeability; er is the spin factor which determines the protons spin along the horizontal component of the MRI; ez is the spin factor which determines the protons spin along the vertical component of the MRI transmitting antenna; fr is the spin factor which determines the protons spin along the horizontal component within the magnetic field of the MRI receiving antenna.
Equations 5, 7-9 express the dynamism of the frequencies of the electromagnetic field for diagnosis. Hence, when radio waves at a particular frequency are radiated toward the magnetic field tissue, the nuclei scatter from the original position and release weak electromagnetic waves during the period. These electromagnetic waves produced at the antenna/coil [Figure 1] are received as signals, which are used by the computer to produce clear images of the tissue.
When radiofrequency energy is applied to the body of a patient, it is released by the body in different forms and at different intervals. The released energy is accepted by the coil (antenna) in form of reflected rays or magnetic resonance signal. The mathematical expression for the solid antenna has been documented in few literature.,,,,
where Δω = ω1 − ω0 is the difference between Larmor frequency and the reference frequency, ω1 = −γB1 is the Rabi frequency, ω0 = −γB0 is the Larmor frequency, Mx, My are the transverse magnetization in the x and y directions, respectively, Mz is the longitudinal magnetization, and Mo is the equilibrium magnetization. Theoretically, many mathematical methods have been used to maximize the magnetic resonance output. Experimentally, the magnetic resonance signals are transformed into a single channeled color image. Hence, the theoretical principles highlighted in equation  may be used to estimate, control, and predict the shortcomings of the MRI such as the heavy absorption of radio frequency around the human body. Excessive RF absorption may be in form of heat effects which affects vulnerable parts of the human body, for example, the eyes, testes, body, and so on.
Virtual experimentation of the radiofrequency features
The first experimental experience is when the radiofrequency absorption in the human body is transient [Figure 2] and [Figure 3]. The mathematical representation is given as
|Figure 2: The radio frequency transmission equipment (users.fmrib.ox.ac.uk)
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|Figure 3: Cross-section of the MRI scanner (magnet.fsu.edu)
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This regime is known as the Stokes regime because the proton responses of the soft tissue compared with its immediate external boundaries are directly proportional to the size of the tissues. Hence, the estimation of radiofrequency effect in relation to the expected RF energy administered to the body can be approximately adduced as shown in [Figure 4] and [Figure 5].
|Figure 4: Linearly dependence of the RF pulse, loss path, and cumulative spin precession. A decrease in frequency translates in an increase in the RF pulse
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|Figure 5: Distribution of RF for deep tissue analysis
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|Results and Discussion|
[Figure 4] shows that the current radiofrequency setting of the MRI machine is exposing the patients to excess RF exposure. However, the appropriate RF pulse when viewed via deep tissue analysis [Figure 5]. Magnetic resonance imaging produces non-ionizing radiation and is being considered to be safer than x-ray imaging which makes it preferable to some x-ray diagnostic procedures. An example is the imaging of children and pregnant patients. MRI is also best for viewing soft tissues which make it easy to view some useful anatomical structures, for example, the brain, muscles, and the heart. It is a very easy and painless procedure. The radio frequencies in MRI scanners can result in heavy absorption of radio frequency around the human body, which affects the eyes and testicles which are vulnerable to heat of radiation. Just as tattoos are metal-based pigments that increase chances of skin burns so also is the effect of metallic implants and this is due to the production of the magnetic fields in the machine.
In MRI scanning, the time-varying magnetic field gradients at certain frequencies cause heart problems to patients if the induced-current-density is more than the cardiac threshold of 1.2 Amperes/m2. In addition, symptoms such as vertigo and nausea may occur in humans due to sensitive effects that occur with fast patient movement in the MRI machine. Furthermore, the dyes used in MRI do not contain iodine and so may cause some reactions such as rashes and hives on the body. From research, patients examined by MRI have been found to be linked with possible temporarily increased traces of DNA damage during the MRI diagnostic scans.
There is little doubt that weakly energetic electromagnetic fields are biologically interactive to the point where they can be usefully applied in medically relevant therapeutic procedures. Not only does this fact suggest a bright future for the role of electromagnetism in medicine, but also underscores the need for caution when examining the effects of low-level electromagnetic fields on patients.
The authors acknowledge the sponsorship of Covenant University, Ota, Nigeria.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]