Basic Principles of Magnetic Resonance Imaging for Beginner Oral and Maxillofacial Radiologists
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Oral Radiol (2017) 33:92–100 DOI 10.1007/s11282-017-0274-z REVIEW ARTICLE Basic principles of magnetic resonance imaging for beginner oral and maxillofacial radiologists Toyohiro Kagawa1 · Shoko Yoshida1 · Tomoko Shiraishi1 · Marie Hashimoto1 · Daisuke Inadomi2 · Mamoru Sato2 · Takashi Tsuzuki3 · Kunihiro Miwa1 · Kenji Yuasa1 Received: 24 July 2016 / Accepted: 13 December 2016 / Published online: 27 March 2017 © Japanese Society for Oral and Maxillofacial Radiology and Springer Japan 2017 Abstract The basic principles and diagnostic methods of Keywords Diagnosis · Magnetic resonance imaging · magnetic resonance imaging (MRI) for beginning surgeons Maxillofacial are described in this review. MRI is an important technique that is essential for diagnoses in the maxillofacial area. It is a scanning method that obtains tomographic images of Introduction the human body using a magnetic feld. In contrast to com- puted tomography, it does not utilize X-rays and, therefore, Magnetic resonance imaging (MRI) began with the dis- represents a noninvasive test that lacks radiation expo- covery of nuclear magnetic resonance by Bloch [1], fol- sure. It is particularly efective for soft-tissue diagnoses. lowed by the world’s frst imaging using nuclear magnetic MRI involves imaging protons in vivo. Protons emit a sig- resonance by Lauterbur [2]. At present, MRI and computed nal when a radio frequency pulse is applied in a magnetic tomography (CT) are essential techniques in modern medi- feld; the MRI device then forms an image from these sig- cine. MRI is an important testing method essential for diag- nals. The basic images produced are T1- and T2-weighted noses in the maxillofacial area. This technique utilizes a images; comparison of these images is the frst step of magnetic feld to obtain tomographic images of the human MRI-based diagnosis. Short-T1 inversion recovery images, body. In contrast with CT, MRI does not utilize X-rays and which eliminate the signal from fat, are also useful for is, therefore, a noninvasive test that lacks radiation expo- diagnosis. Gadolinium is used as a contrast agent for MRI. sure. It is particularly efective for soft-tissue diagnoses, Taking sequential images at fxed intervals while inject- even in the oral cavity, including benign and malignant ing the contrast agent and then graphing the contrast efect neoplasms, infammation, and temporomandibular joint along the time axis produces a time–signal intensity curve, disorders. This review describes the basic principles and which is useful for identifying features such as malignant diagnostic methods of MRI for beginner oral and maxillo- neoplasms based on the graph pattern. facial radiologists. MRI machines * Toyohiro Kagawa MRI machines are based on the same principle as are [email protected] electromagnets, which produce a magnetic feld by pass- ing an electrical current through a massive coil. To elimi- 1 Section of Image Diagnosis, Department of Diagnostics and General Care, Fukuoka Dental College, 2-15-1 Tamura, nate electrical resistance, the coil is enveloped by liquid Sawara-ku, Fukuoka 814-0193, Japan helium (−273 °C) to bring it into a superconducting state. 2 Department of Radiology, Fukuoka Dental College Medical The strength of the magnetic feld of the superconduct- and Dental Hospital, Fukuoka, Japan ing magnets in mainstream MRI machines today is 1.5 or 3 Section of Removable Prosthodontics, Department of Oral 3.0 T (T; a unit quantifying magnetic feld density). The Rehabilitation, Fukuoka Dental College, Fukuoka, Japan type of superconducting magnet used in MRI machines Vol:.(1234567890)1 3 Oral Radiol (2017) 33:92–100 93 magnets have been developed to address this drawback. Nevertheless, their application is limited, because the mag- netic feld that can be obtained using permanent magnets is only approximately 0.5 T. Cautionary notes before MRI imaging Because all MRI devices constantly utilize a powerful mag- netic feld, bringing magnetic materials into the examina- tion room is prohibited (Fig. 1). Medical equipment such as stretchers, wheelchairs, scissors, and gas cylinders used in the same room as the MRI device must be made from special-purpose nonmagnetic materials. In addition, MRI examinations are contraindicated for patients with cardiac pacemakers, implantable cardioverter defbrillators, and artery clips [3]. Caution is also required when examin- ing patients with tattoos or those wearing colored contact lenses, mascara, or eye shadow, because all of these materi- Fig. 1 Metal scissors brought in close proximity to a magnetic reso- als include minute iron particles that cause image artifacts nance imaging machine. Because the machine contains a powerful and can become heated due to the magnetic feld, poten- magnet, magnetic objects are not permitted in the examination room tially resulting in patient burns [3–6]. Burns resulting from loops in the coiled cord have also been reported [7, 8]. can produce a very strong magnetic feld; however, associ- ated disadvantages include the need for a very large device as well as periodic helium replenishment. In addition, the Basic principles of MRI space in which the patient reclines (the gantry) is a narrow tube, leading to a signifcant feeling of restriction during More than 80% of bodily tissue are composed of water scans. Open gantry style MRI machines using permanent and fat. As can be seen in the chemical formulas for water (H2O) and fat [CH2(OCOR)–CH(OCOR′)–CH2(OCOR″)], Fig. 2 Protons inside the human body. The inside of the human body contains a vast amount of protons. The proton is a small magnet in itself. Because protons are oriented in various directions in the body, magnetization is not manifested in the normal state 1 3 94 Oral Radiol (2017) 33:92–100 Fig. 3 Conceptual diagram of obtaining signals from protons in tons become defected by 90°. The defected protons then continue to the body. a Protons are generally oriented in a variety of directions store the energy of the RF pulse. d If the RF pulse is suspended, the within the body. b When entering a powerful magnetic feld, they defected protons return to their previous orientation while emitting change from having a scattered orientation to being aligned within their stored energy. e After the scan, when exiting the MRI machine, the magnetic feld. c If an RF pulse is applied in this state, the pro- the protons become reoriented in scattered directions within the body these tissues contain many hydrogen atoms. MRI machines (Fig. 3b). If a radio frequency (RF) pulse is applied in this produce images based on the nuclei of these hydrogen state, the protons become defected by 90° (Fig. 3c). This atoms (also known as protons). The hydrogen nuclei have a phenomenon is known as nuclear magnetic resonance. magnetic feld; therefore, they can each be considered as a When the RF pulse is applied, a large sound unique to the tiny magnet (Fig. 2). MRI scan is emitted. The defected protons then continue Protons are generally oriented in a variety of direc- to store the energy of the RF pulse. If the RF pulse is sus- tions within the body (Fig. 3a). However, when entering a pended, the defected protons return to their previous ori- powerful magnetic feld, they change from having a scat- entation while emitting their stored energy. The duration tered orientation to being aligned within the magnetic feld of this return difers depending on the type of tissue (mus- cle, blood, fat, etc.) in which the protons are bonded. This energy is received by a receiver as a signal, and an image is formed by analyzing this signal and the time duration of the return (Fig. 3d). After the scan, when exiting the MRI machine, the protons become reoriented in scattered direc- tions within the body (Fig. 3e). Advantages and disadvantages of MRI The advantages of MRI include its (1) noninvasiveness and lack of radiation exposure, (2) ability to produce any given tomographic image, and (3) ability to display blood ves- sels without using a contrast agent. In addition, MRI scans provide higher tissue resolution and a lower temporal reso- lution than do CT scans, which also produce tomographic images (Fig. 4). The disadvantages of MRI include the (1) long scan time (approximately 30–60 min), (2) inability to obtain a signal from cortical bone and calcifcations, (3) inability to perform the test when metal is present in the body, and (4) difculty in scanning claustrophobic patients. Fig. 4 Computed tomography (CT) and magnetic resonance imaging MRI artifacts caused by metal (MRI) of the same slice. MRI has a lower spatial resolution than CT; therefore, tissue boundaries are difcult to ascertain. In contrast, MRI Because MRI examinations utilize a magnetic feld, arti- has a high ability to resolve tissues. While CT scans do not indicate density diferences among tissues, the diferences in density among facts can occur due to the presence of magnetic metals in tissues are clear in MRI scans the imaging area. Even if the magnetic metals themselves 1 3 Oral Radiol (2017) 33:92–100 95 Fig. 5 Conception diagram of a metal artifact. Artifacts caused by metals in computed tomography scans appear only within slices, but they appear in three-dimensional directions in magnetic resonance imaging scans Fig. 6 Proton phase and vector quantity. When defected protons vector quantity in the longitudinal direction is known as the T1 value, revert to their original states, the longitudinal and transverse vector and that required for recovery of the vector quantity in the transverse quantities change. The duration of time required for recovery of the direction is the T2 value do not exhibit magnetism, they become magnets in the quantity in the transverse direction is the T2 value (Fig. 6). magnetic feld. As a result, they form their own magnetic Graphs of vector quantity changes in the longitudinal and felds that cause the local magnetic feld to become nonu- transverse directions over time are called the T1 curve and niform.