|Year : 2018 | Volume
| Issue : 3 | Page : 219-222
MRI: an insight
Tim Peter1, Deepthi Cherian2
1 Department of Oral Medicine and Radiology, KMCT Dental College, India
2 Department of Periodontics, Anjaneya Dental College, Calicut, Kerala, India
|Date of Submission||23-Jun-2017|
|Date of Acceptance||04-Feb-2019|
|Date of Web Publication||15-Apr-2019|
Department of Oral Medicine and Radiology, KMCT Dental College, Calicut, Kerala, 673602
Source of Support: None, Conflict of Interest: None
MRI has been a potential tool for lesion diagnostics in the human body. It is accurate owing to the sound biological principle behind it. It has an array of unique features but is not free from certain constraints that hinder the smooth progression of its course. This article throws light on the salient features and constraints pertaining to MRI, with an insight towards the future prospects of the same.
Keywords: dentistry, MRI, Radiofrequency (RF)
|How to cite this article:|
Peter T, Cherian D. MRI: an insight. Al-Azhar Assiut Med J 2018;16:219-22
| Introduction|| |
MRI is a noninvasive method of mapping the internal structure and certain aspects of function within the body. It uses nonionizing electromagnetic radiation and appears to be without exposure-related hazards. It employs radiofrequency (RF) radiation in the presence of carefully controlled magnetic fields to produce high-quality cross-sectional images of the body in all planes. The MRI is constructed by placing a patient inside a large magnet, which induces a relatively strong external magnetic field. This causes the nuclei of many atoms in the body, including hydrogen, to align them with the magnetic field, and later, on application of RF signal, energy is released from the body, detected and used to construct the MRI by computer .
The MR system comprises two main groups of equipment. The first is the control centre, which is positioned where the operator sits. The control centre houses the ‘host’ computer with its graphical user interface. Its associated electronics and power amplifiers are usually situated in an adjacent room and connect to the second equipment group. This second group of equipment is housed within the machine in which the patient lies. It contains the parts of the MR system that generate and receive the MR signal and include a set of main magnet coils, three gradient coils, shim coils and an integral RF transmitter coil .
The MR system uses a set coordinates to define the direction of the magnetic field. Gradient coils representing the three orthogonal directions (x, y and z) lie concentric to each other within the main magnet ([Figure 1]). They are not supercooled and operate relatively close to room temperature. Each gradient coil is capable of generating a magnetic field in the same direction as B0, but with a strength that changes with position along the x, y or z directions, depending on which gradient coil is used.
- 1857–1952: Larmor relationship − Sir Joseph Larmor: The rate of frequency of precession of proton is termed Larmor frequency .
- 1930: Isidor Isaac Rabi succeeded in detecting single state of rotation of atoms and molecules and in determining the mechanical and magnetic moments of the nuclei .
- 1946: MR phenomenon − Bloch and Purcell .
- 1952: Nobel Prize − Bloch and Purcell 
- 1950–1970: Nuclear MRI developed as an analytical tool .
- 1972: Computerized tomography was introduced .
- 1973: Back projection MRI − Lauterbur .
- 1975: Fourier imaging − Erust .
- 1977: Echoplanar imaging − Mansfield .
- 1980: FT MRI demonstrated − Edelstein .
- 1986: Gradient echo imaging: NMR microscope .
- 1987: MR angiography − Dumoulin .
- 1991: Nobel Prize − Ernst .
- 1992: Functional MRI .
- 1994: Hyperpolarized 129Xe imaging .
- 2003: Nobel Prize: Lauterbur and Mansfield for contributions in MRI .
The salient features of MRI are as follows:
- No ionizing radiation: RF pulses used in MRI do not cause ionization and have no harmful effects of ionizing radiation. Hence, it can be used in child-bearing women and children.
- Noninvasive: MRI is noninvasive.
- Contrast resolution: it is the principal advantage of MRI, that is, ability of an image process to distinguish adjacent soft tissue from one another. It can manipulate the contrast between different tissues by altering the pattern of RF pulses.
- Multiplanar image: with MRI, we can obtain direct, sagittal, coronal and oblique image which is impossible with radiography and computed tomography.
- It could differentiate between acute and chronic transit and fibrous phases parallel with histopathological changes.
- There is absence of significant artifact associated with dental filling.
- No adverse effect has yet been demonstrated.
- Image manipulation can be done.
- It is useful in determining intramedullary spread.
Applications in dentistry
Signal intensity for each tissue
- Fat tissues: fat tissue appears as high signal intensity on T1-weighted images and low signal intensity on T2-weighted images with fat suppression.
- Muscle tissue: muscle commonly appears as low signal intensity on both T1-weighted and T2-weighted images with fat suppression except lingual muscles, which have intermediate signal intensity on T1-weighted images owing to their relatively high fat component compared with other muscles.
- Cortical bone tissue: cortical bone tissue is indicated as a signal intensity void on T1-weighted and T2-weighted images. Cancellous bone tissue demonstrates high intensity on T1-weighted images and low intensity on T2-weighted images with fat suppression.
- Lymph nodes and tonsils: lymph nodes and tonsils have low intensity on T1-weighted images and intermediate − high signal intensity on T2-weighted images with fat suppression.
- Teeth: the teeth, except pulp tissue, appear as a signal void on T1-weighted and T2-weighted images; pulp tissue has intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images with fat suppression. The dental follicle of an unerupted tooth has signal intensity on T1-weighted images and high signal intensity on T2-weighted images with fat suppression.
- Parotid gland: signal intensities differ among the tissues of the salivary glands. The parotid glands have relatively high signal intensity on T1-weighted images and low signal intensity on T2-weighted images with fat suppression. However, the parotid ducts have high signal intensity on T2-weighted images with fat suppression and low signal intensity on T1-weighted images.
- Submandibular gland: the submandibular glands have intermediate signal intensity on T1–weighted images and low signal intensity on T2-weighted images with fat suppression. Ducts have high signal intensity on T2-weighted images with fat suppression and low signal intensity on T1-weighted images.
- Sublingual gland: the sublingual gland has intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images with fat suppression.
- Temporomandibular joint (TMJ): the discs of the TMJ have low signal intensity on T1-weighted and T2-weighted images. TMJ effusion appears as low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.
- Cavities: the cavities (maxillary sinus and nasal cavities) appear as void signal on T1-weighted and T2-weighted images.
- Blood vessels: blood vessels usually have void signal intensity owing to blood flow, termed ‘signal void’, on both T1-weighted and T2-weighted images; however, some vessels with lower flow rate appear with high signal intensity on T2-weighted images with fat suppression and low intensity on T1-weighted images, like the signal from water ,.
Indications of MRI in the oral and maxillofacial region are as follows:
- For the diagnosis and evaluation of benign and malignant tumours of jaws.
- Tumour staging evaluation of the site, size and extent of all soft tissue tumours and tumour-like lesions, involving all areas, including the following:
- The salivary glands.
- The pharynx.
- The sinuses.
- The orbits.
- To evaluate structural integrity of trigeminal nerve in trigeminal neuralgia.
- In surgery of parotid gland, MRI can detect the cause of facial nerve within the glandular tissue and help lessen the likelihood of post-operative facial nerve palsy.
- For the assessment of intracranial lesions involving particular posterior cranial fossa, the pituitary and the spinal cord.
- For noninvasive evaluation of the integrity and position of articular disk within the TMJ.
- Investigation of the TMJ to show both the bony and soft tissue components of joint including disc position:
- When diagnosis of internal derangement is in doubt.
- As a preoperative assessment before disc surgery.
- Implant assessment ,.
The following are the limitations of MRI:
- Claustrophobia, that is, morbid fear of closed places because the patient is within the large magnet up to 1 h.
- MRI equipment is expensive to purchase, maintain and operate. Hardware and software are still being developed.
- Because of the strong magnetic field used in patient electrically, magnetically or mechanically activated implants such as cardiac pacemakers, implantable defibrillators and some artificial heart valves may not be able to have MRI safely.
- The MRI becomes distorted by metal, so the image is distorted in patients with surgical clips or stents, for instance.
- Bone does not give MR signal; a signal is obtained only from the bone marrow. Long scanning time and requires patient cooperation.
- The very powerful magnets can pose problems with sitting of equipment, although shielding is now becoming more sophisticated.
- MRI scanners are noisy.
- Patient could develop an allergic reaction to the contrasting agent or that a skin infection could develop at the site of injection.
- MRI cannot always distinguish between malignant tumours or benign disease, which could lead to a false-positive result.
- Facilities are not widely available, but there has been development of small open systems suitable for district general hospitals.
- Bone, teeth, air and metallic objects all appear black, making differentiation difficult.
- Volume imaging − 3D imaging: volume imaging is the requisition of magnetic resonance data from a volume rather than a tomographic slice. It can be thought of as collecting several contiguous slices through a region of imaged object.
- Flow imaging [MRI angiography (MRA)]: angiography is the imaging of flowing blood in the arteries and veins of the body. MRA produces images of flowing blood. The intensity in these images is proportional to the velocity of the flow. There are three general types of MRA − time of flight, phase contrast angiography and contrast enhanced angiography.
- Fast spin: echo imaging is a multiecho spin echo sequence where different parts of space are recorded by different spin echoes. The benefit of the technique is that a complete image can be recorded in one-fourth of the time.
- Chemical shift imaging (fat suppression): it is the production of an image from just one chemical shift component in a sample.
- Echoplanar imaging (functional MRI): it is a rapid MRI technique which is capable of producing tomographic images at video rates. Its greatest application appears to be in the area of functional MRI of the brain. Functional imaging is the imaging which relates body function or thought to specific locations in the brain.
- Magnetization transfer contrast: it is a method of increasing the contrast between tissues by physical rather than chemical means.
- MR elastography: it is the imaging of shear waves using Magnetic Resonance Radiation (MRR). Contrast in MR elastography is related to the elastic modulus of the tissue. MRI is recorded while ultrasound waves are being sent into the imaged volume. This technique is expected to find applications in locating pathology in soft tissue based on difference in the elastic modulus of tissues. Hence, it is referred to as ‘magnetic resonance palpation’.
- Electron spin resonance (ESR) or electron paramagnetic resonance: ESR is based on the spin, rather than the volumetrics of the nucleon. ESR imaging is the study of the spatial distribution of ESR signal-bearing substance. Very few substances can be studied with ESR.Nitroxide spin probes and some transition metals have an ESR signal. These substances have been studied directly by ESR but are commonly used to probe biologic process with ESR ,.
| Conclusion|| |
MRI is a complex but effective imaging system that has a variety of clinical indications directly related to the diagnosis and treatment of oral and maxillofacial abnormalities. Although not routinely applicable in dentistry, appropriate use of MRI can enhance the quality of patient care in selected cases. Further advances in 3D imaging and dynamic scanning will enhance the use of this imaging technique even further.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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