Download Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging
(MRI)
The Components:
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A magnet which produces a very powerful
uniform magnetic field.
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Gradient Magnets which are much lower in
strength.
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Equipment to transmit radio frequency (RF).
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A very powerful computer system, which
translates the signals transmitted by the coils.
The Magnet
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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.
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Earth magnetic field: 0.5-gauss
The Magnet (cont.)
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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).
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The most commonly used imaging planes in MRI
are axial, sagittal and coronal.
The main uses of MRI
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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
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.
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Disadvantages
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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
Contraindication to MRI
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Cardiac pacemaker
Implanted cardiac defibrillators
Cochlear implants
Pregnancy
marked obesity
Tissue contrast
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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.
T1 weighted images
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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
T2 weighted images
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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.
Low signal or no signal tissues
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Cortical bone
Calcification
Tendons/ligaments
Metal
Gas