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Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI). The Components:. A magnet which produces a very powerful uniform magnetic field. Gradient Magnets which are much lower in strength. Equipment to transmit radio frequency (RF).

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Magnetic Resonance Imaging (MRI)

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  1. Magnetic Resonance Imaging (MRI)

  2. The Components: • A magnet which produces a very powerful uniform magnetic field. • Gradient Magnets which are much lower in strength. • Equipment to transmit radio frequency (RF). • A very powerful computer system, which translates the signals transmitted by the coils.

  3. The Magnet • The most important component of the MRI scanner is the magnet: • The magnets currently used in scanners today are in the .5-tesla to 2.0-tesla range (5,000 to 20,000-gauss). Higher values are used for research. • Earth magnetic field: 0.5-gauss

  4. The Magnet (cont.) • There are three types of magnets used in MRI systems: • Resistive magnets • Permanent magnets • Super conducting magnets (the most commonly used type in MRI scanners). • The most commonly used imaging planes in MRI are axial, sagittal and coronal.

  5. The main uses of MRI • NEUROLOGY • Brain (early detection of cerebral white matter disease, posterior fossa and brain stem pathology, intracranial arterial and venous abnormalities ) • Spinal cord (spinal cord lesions) • Brachial plexus • ORTHOPAEDICS • Lumber spine • Knee • Shoulder

  6. Advantages of MRI • No ionizing radiation. • Superior soft tissue contrast than CT. • Can select any plane e.g coronal,sagittal,oblique. • More sensitive to tissue changes • No bone artifacts • No iodinated contrast media used. IV contrast (gadolinium) has a lower risk of death compared with iodinated contrast used with CT.

  7. Disadvantages • Limited slice thickness 3mm(CT-1mm) • Bone imaging limited to display of marrow • Claustrophobia(due to the confined space in the scanner). • Cannot use with pacemaker or ferromagnetic implant • Longer imaging times 20-30 mins • Cardiac scan upto 1hr. • MRI equipment is expensive to purchase, maintain, and operate

  8. Contraindication to MRI • Cardiac pacemaker • Implanted cardiac defibrillators • Cochlear implants • Pregnancy • marked obesity

  9. Tissue contrast • The relaxation times T1 and T2 are times related to the nuclei returning to their original alignment in the longitudinal axis (T1) and in the transverse axis (T2) of the magnetic field. • There are 2 basic types of relaxation T1 and T2. • To define a tissue or pathology accurately a radiologist often compare its appearance on both T1 and T2.

  10. T1 weighted images • T1 MRI is known as Spin-lattice relaxation time. • T1 shows anatomy very well. • Water is black on T1.-low signal • Fat subacute blood melanin proteinaceous fluid and contrast media- high signal

  11. T2 weighted images • T2 MRI is known as spin-spin relaxation time • T2shows pathology or inflammation well • Water and inflammation are white-high signal • Fat is high signal on T2.

  12. Low signal or no signal tissues • Cortical bone • Calcification • Tendons/ligaments • Metal • Gas

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