A comprehensive look at magnetic resonance imaging: A review :Mahapatra Subharina, Hebbale Manjula, Mhapuskar Amit, Singh Simran, SRM Journal of Research in Dental Sciences

REVIEW ARTICLE
Year : 2022 | Volume : 13 | Issue : 4 | Page : 174--178

A comprehensive look at magnetic resonance imaging: A review

Mahapatra Subharina, Hebbale Manjula, Mhapuskar Amit, Singh Simran
Department of Oral Medicine and Radiology, Bharati Vidyapeeth (Deemed to be University) Dental College and Hospital, Pune, Maharashtra, India

Correspondence Address:
Dr. Mahapatra Subharina
Ambegaon BK, Pune, Maharashtra
India

Abstract

Introduction: High-resolution pictures of soft tissue may be obtained using magnetic resonance imaging (MRI). Sophisticated chemical and physical data may be gathered using this spectroscopic approach, which depends on the magnetic resonance principle as its base. Aim: The purpose of this article was to provide a short overview of MRI, different contrast agents in MRI, its use in oral medicine, its contraindications, and the most recent MRI discoveries and artifacts related to MRI. Methods: An online search was conducted using databases such as PubMed, Embase, Web of Science, Scopus, and Google Scholar to locate articles that dealt with MRI, oral medicine, recent advancements, dental applications, and contrast agents. The search was limited to studies that were published before March 2022. Results: A total of 31 articles were included and analyzed for this review. Conclusion: MRI is more sensitive than other imaging modalities in the diagnosis of soft tissue in the head-and-neck area; the use of three-dimensional imaging will expand further advancements in this imaging modality.

How to cite this article:
Subharina M, Manjula H, Amit M, Simran S. A comprehensive look at magnetic resonance imaging: A review.SRM J Res Dent Sci 2022;13:174-178

How to cite this URL:
Subharina M, Manjula H, Amit M, Simran S. A comprehensive look at magnetic resonance imaging: A review. SRM J Res Dent Sci [serial online] 2022 [cited 2023 Mar 26 ];13:174-178
Available from: https://www.srmjrds.in/text.asp?2022/13/4/174/363801

Full Text

Introduction

Magnetic resonance imaging (MRI) is a non-ionizing, non-invasive method for detecting body structures, discriminating hard and soft tissues, and evaluating physiological processes. The Shepherd Magnes, who is credited with discovering magnets circa 1000 B. C., is thought to have given the word "magnet" its name. He discovered magnetite, often known as lodestone or lead stone.[1] William Gilbert released the first significant scientific work on the lodestone, magnetic bodies, and on the Great Magnet – the Earth. Hans Christian Østed was the first to explain how an electric current and a magnetic field are related in 1820. In 1992, functional MRI was introduced. The Nobel Prize in Physiology or Medicine 2003 was awarded jointly to Paul C. Lauterbur and Sir Peter Mansfield for their discoveries concerning MRI.[2],[3]

A high-quality cross-sectional image of the body is produced by MRI using radiofrequency electromagnetic radiation that is not ionizing and that is subject to a regulated magnetic field.[4] Protons release a signal when a radiofrequency (RF) pulse is introduced to a magnetic field. The MRI machine uses these signals to create an image. Gadolinium is a contrast agent used in MRI. Deep neural networks, compressed sensing algorithms, and artificial intelligence are being integrated to improve rapid MRI. Deep learning-based MRI approaches have gotten more complex, diversified, and effective in recent years.[5]

The purpose of this article is to provide a short overview of MRI, different contrast agents in MRI, its use in oral medicine, its contraindications, the most recent MRI discoveries, and artifacts related to MRI.

Methods

An online search was conducted using databases such as PubMed, Embase, Web of Science, Scopus, and Google Scholar to locate articles that dealt with MRI, oral medicine, recent advancements, dental applications, and contrast agents. The search was limited to studies that were published before March 2022.

Results

A total of 31 articles were included and analyzed for this review.

Discussion

The main magnet of the MRI machine, which is made up of superconducting coils, gradient coils, RF coils, and computer systems, is one of the four primary parts of MRI system. There are safety considerations for each component.[6] To eliminate the electrical resistance, the coil is made superconducting by enclosing it using liquid helium (273°C). Modern MRI equipment have superconducting magnets with magnetic fields of 1.5 or 3.0 T or greater.[7],[8]

Working principle

Body tissue is mostly water and fat and contains several hydrogen atoms. The magnet of the MRI scanner can induce positively charged hydrogen ions (H+ ions/protons) to "spin" in the same way. Once the magnet is turned off, protons return to their initial state. An RF pulse applied to protons in this state causes a 90° fault. Nuclear magnetic resonance describes this phenomenon. The RF pulse causes a loud MRI-specific noise. Defective protons store RF pulse energy.[8] If the RF pulse is interrupted, faulty protons reorient and release their energy. How long protons stay coupled depends on the tissue (muscle, blood, fat, etc.). Analyzing the signal and return time creates an image. As protons depart the MRI scanner, they are reoriented. Varying body tissues regenerate at different speeds, allowing us to recognize and visualize them.[9] Protons are commonly oriented differently in the body. In a strong magnetic field, they change from dispersed to align. Spin echo and inversion recovery have enhanced signal-to-noise ratio, contrast resolution, and imaging duration. Primary MRI approach for clinical imaging is multi-slice, multi-echo spin-echo imaging and a typical spin-echo examination consists of spin–lattice relaxation time (Tl)-weighted images, short repetition time (TR)-weighted images, and spin–spin relaxation time (T2)-weighted images (long TR).[10] The magnetic field is created by three electromagnetic coils. The magnetic fields of the coils oppose and redirect magnetic flux in three right-angled directions to define tissue vowels. Selective excitation aligns hydrogen protons at selected pixels to the same resonant frequency. An RF pulse with several frequencies stimulates a pixel array of tissue at a specific frequency, which detects and localizes it. Slice thickness is RF pulse bandwidth. The computer scans the picture and then prints it.[11] MRI diagnoses soft tissue, vascular, intracranial, and inflammatory skull base illnesses. Hearing loss, facial nerve paralysis, and vertigo are MRI indicators.[12]

Contrast agents

Before or during the MRI, a patient may receive intravenous contrast agents (typically including the element gadolinium) to hasten the rate at which protons realign with the magnetic field. The image becomes brighter as the protons realign more quickly. By injecting contrast chemicals, it is possible to change the signal intensity that a tissue emits. By changing the T1 or T2 relaxation duration, these substances modify the signal intensity of tissues. The interstitial vascular space and vascularity of the tissue are the primary determinants of contrast enhancement. Contrast agents can be categorized in a variety of ways, as discussed in [Table 1]. The greatest relaxation occurs when water molecules contain protons that are magnetic contrast agents. The gadolinium-diethylene is the sole paramagnetic substance.[13],[14] MRI contrast agents can be given intravenously or orally. Depending on the topic of interest, a different administration method may be used. The extracellular space is distributed using extracellular fluid agents, often known as intravenous contrast agents. A larger temporal window for imaging blood vessels is made possible by blood-pool contrast agents.{Table 1}

Signal intensity

The signal intensity refers to the signal strength of each tissue on MR images. The interpretation of signal intensity is shown in [Table 2]. The appearance of various structures in MRI is given in [Table 3].{Table 2}{Table 3}

Essential MRI images

T1- and T2-weighted imagesT1-weighted and T2-weighted diagnosticsImages that suppress fatMRI with contrastImage weighted by diffusion.

Advantages of magnetic resonance imaging

RF pulses utilized in MRI is non-ionizing. The primary benefit of MRI is contrast resolution, which refers to the capability of image processing to distinguish nearby soft tissue. By altering the RF pulse pattern, it can alter how different tissues contrast with one another.[15] Unlike radiography and CT, MRI is a non-invasive technology which can produce direct, sagittal, coronal, or oblique images. Artifacts with dental fillings are not seen.[16]

Disadvantages of magnetic resonance imaging

MRI scanners are noisy. MRIs are expensive to buy, maintain, and run.[17] Images are affected by metal, such as surgical clips or stents. Only bone marrow gives off an MR signal. Long scans require patient cooperation. The patient spends an hour in the large magnet, producing claustrophobia. The patient might have an adverse reaction to a contrasting substance or injection site infection. Powerful magnets can damage technology, although shielding is improving. Black bone, teeth, air, and metal make distinguishing impossible. MRI cannot always differentiate between malignant and benign tumors, leading to false positives.

Uses of magnetic resonance imaging in several dental specialties

MRI is the best imaging modality due to its superior soft-tissue resolution and lack of ionizing radiation. In MRI, the normal structure of the oral cavity, muscles, lymph nodes, salivary glands, temporomandibular joint (TMJ), and pathology associated with all the structures can be seen.[18],[19] Disease processes ranging from tooth decay to gum disease can cause damage to the floor of the mouth, so the abnormalities can only be seen and characterized at imaging.

Normal anatomical structure of the oral cavity: The floor of the mouth and tongue muscles can be seen in MRILudwig angina: Ludwig angina is a potentially fatal illness, with a fatality rate of up to 10%. On MRI, the accumulations of serosanguinous fluid in the oral cavity may be detected, which shows inflammatory changes and tiny cystic cavities[20]Lipoma: Lipomas of the oral cavity can be seen in the floor of the mouth. On imaging, lipomas are readily apparent. Fatty lesions show increased signal on T1-weighted images, but they have less signal on fat-saturated imaging[20]Arteriovenous malformation involving the tongue: The occurrence of a vascular anomaly in the floor of the mouth is very unusual among the numerous forms of head-and-neck vascular abnormalities. Images from MRI show the lesions to be round masses with many flow voids, as well as an extensive feeding vein and/or drainage artery[20]Lymph nodes: In patients with head-and-neck squamous cell carcinoma, the presence of cervical lymph node metastases is an important prognostic marker since it greatly affects the treatment outcome. Treatment choices are guided by MRI, although MRI has limited capacity to distinguish between lymph nodes with and without metastases. The short axial diameter on MRI is often used to assess the involvement of cervical lymph nodes. Many researches have been conducted to determine the appropriate size of the short axial diameter to distinguish between the metastatic and nonmetastatic lymph nodes[16],[21]Salivary glands: The parotid, submandibular, and sublingual glands are the three largest pairs of salivary glands. Imaging techniques for the salivary gland include classic sialography, simple X-rays, computed tomography (CT), and MRI. MRI is better than CT at showing how far a lesion has spread perineurally or along the meninges because it can show more than one plane at once and has a better soft-tissue resolution. MRI can assist in identifying all of the inflammatory, neoplastic, systemic, and congenital diseases that can affect the major salivary glands[22]Periapical pathology: When planning periapical surgery and learning more about the nature of endodontic lesions, MRI may be helpful. Metallic extra-coronal restorations like amalgam do not generate artifacts that interfere with MRI scans. However, MRI does have a few limitations. These include low resolution compared to standard radiographs, lengthy scanning durations, higher hardware costs, and restricted access to radiology units only. Enamel and dentin, which all appear radiolucent, are dental hard tissues that cannot be separated from metallic restorations. Due to these factors, MRI has a limited role in the treatment of endodontic illness.[23],[24]TMJ disorders: MRI is considered the reference approach for imaging TMJ soft tissues such as articular disc, synovial membrane, and lateral pterygoid muscle. It evaluates the morphology of the articular disc and its placement relative to the condyle in closed- and open-mouth postures. Disc position is crucial because a misplaced disc indicates TMJ dysfunction. MRI will help to clarify TMJ discomfort and discrepancies between imaging findings and patient symptoms. MRI is the best imaging modality for identifying disc displacements in closed-mouth and open-mouth positions.[25]

Magnetic resonance imaging artifacts

Metal-related MRI artifacts can appear when magnetic metals are present in the imaging area since MRI examinations use a magnetic field. Even if the magnetic metals do not naturally exhibit magnetism, the magnetic field causes them to do so. They consequently generate their own magnetic fields, which result in a nonuniform local magnetic field. As a result, whereas artifacts only occur along the direction of the slices in CT scans, they show up in MRI tests as three-dimensional (3D) missing signals.[26],[27]

Biotherapies and safety implications

Inducing electric currents, heating, misinterpreting artifacts, and the potential for implant movement or dislodgment are all potential concerns associated with doing MRI on patients who have ferromagnetic implants or materials.[28] Pregnant women should use care even though MRI is not believed to be detrimental to the fetus at this time. It is recommended to get an MRI when other non-ionizing diagnostic imaging methods, such as ultrasound, are unsuitable for pregnant patients.

Recent advances in magnetic resonance imaging

MRI technology is still developing, with new system designs that produce a better image quality and yield larger throughput. Some of the recent advances in MRI technology are flow imaging (MRI angiography MRA), fast spin, chemical shift imaging (fat suppression), echo-planar imaging (functional MRI), magnetization transfer contrast, and MR elastography.[29]

Conclusion

The usage of 3D imaging will increase as more advances are made in the field since MRI is more sensitive than other imaging modalities in the diagnosis of soft tissue in the head-and-neck region.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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