Fundamentals of Magnetic Resonance Imaging

Abstract

Magnetic resonance imaging (MRI) is a technology that has revolutionized the medical field. With this technology, high-resolution images of internal body parts are produced without the need for x-rays. The use of radiofrequency light during this procedure is said to be safe since it does not pose any known risks (Jost & Kumar, 1998). This procedure is arguably the best when it comes to the examination of the brain and spinal cord (nervous system). However, it may not be proper to use it with individuals with metal implants such as pacemakers. Other risk factors include the powerful magnetic field, loud noise, radio waves, and cryogens (Price, & Wilde, Papadaki, Curran, & Kitney, 2001).

Introduction

Magnetic resonance imaging is a technique employed in medicine and is used in the viewing of the internal body structures (Snopek, 2006). It is mainly used in radiology where images of the body are taken for medical purposes. This process differs from pathology, which also involves imaging parts of the body for medical purposes. The difference is the fact that pathology involves taking images of the organs that are already removed from the body. Magnetic resonance imaging employs the technology behind nuclear magnetic resonance (NMR). It provides the ability to view a nucleus of an atom within the body (Carr, 2004).

This technology is employed in the MRI scanner. This is the structure made of a large, strong magnet where the patient lies within. Radiofrequency magnetic fields are used in the alteration of magnetization. Consequently, the magnetic field of the nucleus is caused to rotate. The scanner can detect the magnetic fields from the nuclei, record, and produce images of the particular area. The device also produces different magnetic field gradients. This causes the nucleus to move at different speeds depending on its location. Therefore, this provides spatial information that is necessary to provide 2D and 3D imaging.

Magnetic Resonance Imaging can differentiate between the soft tissues of body organs. This makes it possible to imagine the heart, muscle, and brain. It is also possible to detect cancers using this technology (Damadian, Goldsmith, & Minkoff, 1977). Other technologies that can be used for the same purpose include X-rays and computed tomography (CT) scans. However, the difference is that these two use ionizing radiation.

Literature Review

History

The technology behind magnetic resonance imaging was present even as early as the 1950s when Herman Carr was able to produce one-dimensional MRI images (Carr, 2004). Vladislav Ivanov was also reported to have invented an MRI device but this was not approved until ten years later (MacWilliams, 2003). In the early 1970s, Raymond Danadian suggested that tumours and normal body tissues could be distinguished using NMR (Damadian, 1971).

Therefore, he argued that cancers could be detected using this technology. However, research later showed that the difference detected between tumours and normal tissue was too variable to be used for diagnosis. Therefore, Raymond’s initial methods could not be used practically in medicine. Raymond went on to research the properties of magnetic resonance to get a full understanding of this concept. This led him to create the first-ever Magnetic Resonance Imaging machine. He was issued with a patent two years after he created it (in 1972).

Advancements and more discoveries were made concerning magnetic resonance imaging. Paul Lauterbur and Peter Mansfield made great contributions concerning the same. Their significant contributions were rewarded since they were awarded the 2003 Nobel Prize in Physiology. Lauterbur’s main contribution was the idea of the spatial localization of protons. The gradients of the magnetic fields were used for this procedure. This discovery enabled the production of 2D images during MRI scans. Mansfield, on the other hand, came up with mathematical formulas and techniques for quickening the imaging process using Lauterbur’s techniques.

The mechanism behind MRI machines

These machines take advantage of the fact that the human body and the tissues contain huge volumes of water. Therefore, the protons can be easily aligned. A radio frequency is then introduced to produce different electromagnetic fields. The frequency of this field is referred to as the resonance frequency. The radiofrequency is then switched off to allow the protons to return to equilibrium. This process defines their relaxation rate. They become aligned with the static magnetic field. During this process, radio frequency signals are generated. They can then be measured by the receivers.

If information about their positions in 3D space is required, more magnetic fields are introduced. Physicians can use 3D images to detect small changes in structures within the human body. These rates vary depending on the type of tissue. Therefore, it is possible to differentiate between the tissues. To make the internal organs and tissues more visible, contrast agents are added. They are injected into the area of investigation to make images easily distinguishable.

For instance, they may be used for producing images of the joints. MRI is usually considered safe as compared to CT scans and x-rays. This is mainly since it does not use ionizing radiation. However, this procedure may not be very safe for individuals with metal implants. These implants may be in the form of cardiac pacemakers. In the case of individuals with cardiac pacemakers, MRI scans are strongly discouraged since they can lead to death. The strong magnetic field generated by the device affects the functioning of such equipment.

Applications of MRI

Since MRI can differentiate between normal tissues and pathologic tissues, it may be applicable for detecting brain tumours. Coupled with the safety associated with this process, MRI is widely used in medicine. It also produces better contrast resolution when compared to the CT scans and the traditional X-rays. This technique may also be used to detect multiple sclerosis. Although MRI is used to provide imaging of the soft tissue, it may also be used to produce images of teeth and bones.

MRI may generally be administered to individuals suffering from the following ailments:

1. Inflammations and infections in organs
2. Stroke
3. Degenerative diseases
4. Tumours
5. Musculoskeletal disorders
6. Measuring volumes of brain structures
7. Detection of cancers of the breast, colorectal, liver, and prostate
8. Various other irregularities within tissues or organs

Problems within the nervous system are best investigated using MRI imaging. This is especially because the spinal cord and brain might need to be examined and other methods such as x-rays may not be safe. Functional MRI may be done to determine the functions of different parts of the brain. Certain parts control different functions of the human body. This technology is used to measure changes in brain activity.

The technology behind MRI may be used to burn out disease tissues. This procedure is referred to as magnetic resonance-guided focused ultrasound (MRgFUS) therapy. It is whereby ultrasound beams are focused on the target. The beam is guided to the target tissue by the use of MR thermal imaging. Using 3D imaging, the beam may be directed with precision. When the beams have been focused, the temperatures within the area rise to high temperatures and eventually destroy the tissue.

Risk factors associated with MRIs

Despite the many advantages of the use of MRI in medicine, there are several risks associated with its use and they are brought about by the following:

1. The strong magnetic fields
2. Loud noise produced by the magnetic forces
3. Cryogenic liquids
4. Radio waves
5. Cryogenic liquids
6. MRI contrasting agents

• Strong magnetic field – it has been determined that patients with certain medical implants should not undergo MRI examinations (Jost & Kumar, 1998). If such patients must undergo the examinations, the procedure must be done under certain strict conditions. This explains why patients are required to provide complete information about any implants within their bodies before they can enter the examination room. It has been reported that several patients with pacemakers have died while undergoing MRI scanning (Jost & Kumar, 1998). Research indicates that the appropriate precautions were not taken during those scans. However, scientific advancements have enabled the development of implants that can be scanned safely.
• Radio waves – the waves produced in the scanners usually cause a heating effect. A temperature rise occurs when the energy is absorbed by the body. When temperatures increase beyond certain limits, it may be fatal. Therefore, the rate of absorption must be limited.
• Acoustic noise – the noise caused by the MRI scanners is due to the switching of the field gradients. It usually causes changes in Lorentz force. The sound caused by these machines can reach 120 decibels (Price, & Wilde, Papadaki, Curran, & Kitney, 2001). This is equivalent to sound caused by a jet engine during take-off. Therefore, everyone in the room requires ear protection.
• Cryogenic liquids – the properties of the liquids used for enhancing superconductivity of the coils may be hazardous. The release of helium during ‘quenching’ may cause displacement of oxygen in the room. This may cause asphyxiation.
• Discomfort – some individuals find lying inside a scanner uncomfortable. Claustrophobic individuals also find it hard to tolerate the long and narrow tunnel in the scanner. Therefore, modern scanners have been designed in such a way as to accommodate such individuals.

Conclusion

Magnetic Resonance Imaging is a useful technology that has transformed the field of medicine. MRI scanners have several applications including detecting tumours, musculoskeletal disorders, degenerative diseases, and cancers, among others. The functionality of the MRI revolves around the use of a magnetic field to align the nuclei in the human body. MRIs are advantageous in that they do not use ionizing radiation. Therefore, it is the safest method when compared to x-rays and CT scans. However, there are several risks associated with it due to the strong magnetic field, noise, radio waves, and cryogenic fluids.

References

Carr, H. (2004). Field Gradients in Early MRI. Physics Today, 57(7), 231-242.

Damadian, R. (1971). Tumor Detection by Nuclear Magnetic Resonance. Science, 171(1), 1151–1153.

Damadian, R., Goldsmith, M., & Minkoff, L. (1977). NMR in cancer: XVI, Fonar image of the live human body. Physiological Chemistry and Physics, 9(1), 97–100.

Jost, C., & Kumar, V. (1998). Are Current Cardiovascular Stents MRI Safe. The Journal of invasive cardiology, 10(8), 477–479.

MacWilliams, B. (2003). News & Views: Russian claims first in magnetic imaging. Nature, 426(6965), 375.

Price, D., & Wilde, J., Papadaki, A., Curran, J., & Kitney, R. (2001). Investigation of acoustic noise on 15 MRI scanners from 0.2 T to 3 T. Journal of Magnetic Resonance Imaging, 13(2), 288–293.

Snopek, A. (2006). Fundamentals of special radiographic procedures (5^th ed.). United Kingdom: Elsevier Health Science.