Mri Scans: Definition, Uses, And Procedure

Discuss the principle, technique, indications, merits, and demerits of MRI in detail.
Answer:

• MRI Principle: The technique is based on the magnetic properties of an atom.MRI uses nonionizing radiation from the radiofrequency (RF) band of the electromagnetic (EM) spectrum. RF pulses transmitted through the patient are stimulating the atoms in tissue to produce a signal.
• Atoms consist of nuclei and electrons. In the nucleus, there are positively charged protons are rotating around an axis, i.e. possessing a spin. In the natural state magnetic dipoles are randomly oriented resulting in net magnetization of zero.
• When the patient is placed in EMF, protons align with it. They do this in two ways: Spin up in a lower energy state and spin down in a high-energy state.

Magnetic Imaging Resonance

MRI Terms:

• Resonance: Transition of nuclei from one energy state to another.
• Precession: Oscillation of spinning protons with a slight tilt from parallel.
• Larmor frequency: Rate or frequency of precession of protons
• T1weighted image
• T2 weighted image.

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MRI Basic Steps:

• An external magnetic field is applied which causes the alignment of protons.
• Radiofrequency pulse sent.
• RF pulse switched off, and protons realign.
• During realignment, a signal can be detected.
• A signal is used for image reconstruction in the computer.

MRI Interpretations:

• Hyperintense
• Hypointense

Units of Magnetic Field:

• Tesla
• The magnetic field of MR imaging: 0.1-4.0 T.

Indications of MRI in Head and Neck

• Assessment of intracranial lesions
• Tumor staging
• TMJ investigations to show bony and soft tissue component-internal derangement of the disc, disc perforation
• Diagnosis of salivary gland diseases.

MRI Contraindications:

• Cardiac pacemakers
• Ear prosthesis, cerebral aneurysm clips
• Metallic implant and dental filling relative contraindication
• Pregnancy

MRI Advantages:

• Ionizing radiation is not used
• Noninvasive
• High contrast sensitivity and is an excellent soft tissue imaging modality.
• Images in all planes obtained without repositioning the patient
• Exposure of the patient to a magnetic field of less than 2.5 T has no adverse effects.

MRI Disadvantages:

• Expensive
• Long scanning time
• Equipment tends to be claustrophobic, noisy
• Needs trained staff
• Hard tissue like bone does not give an MR signal and the signal is obtained only from the bone marrow.