MRI
Magnetic Resonance Imaging
History
Year Event
1946
MR phenomenon - Bloch and Purcell. Back projection MRI Lauterbur
1973
1992
Functional MRI
2003
Nobel Prize - Lauterbur and Mansfield
Overview
Nonionising radiation imaging modality MRI is recently developed imaging modality that can totally replace conventional X-ray generating equipment and film. Essentially it involves the behavior of protons (positively charged nuclear particles) in a magnetic field.
Overview
Steps of an MR examination involve 1. 2. 3. 4.
The patient is placed in a magnetic field, A radio wave is sent in, The radio wave is turned off, The patient emits a signal, which is received and used for Reconstruction of the picture
4
5.
Components of MRI system
1. 1. 2. 2. 3. 4. 5. 6.
Magnet - also includes
Gradient Coil RF Coil
Power supplies Computer system A documentation system Cooling system Monitoring camera
Screening before MRI
Patient is asked to remove Glasses RPD Hearing aid Jewellery Watch Wallet or money clip Pens or pencils Keys Coins Pocket knife
Belt buckle Shoes Magnetic strip cards Credit and bank cards Hair pins Metal hooks Sanitary belt Safety pins
Contraindications
Due to high magnetic field used in MRI examination, for certain patients MRI is contraindicated and is unsuitable for imaging Patient having
Aneurysm clips (ferromagnetic types) Cardiac pacemakers Otologic and ocular implants Cochlear implants Metallic foreign body
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Physical Fundamentals
AXIS
When an external magnetic field is applied, the N and S poles of H atoms (protons) do not align exactly with the direction of the magnetic field. The axes of spinning protons oscillate or wobble with a slight tilt from a position which was parallel with the flux of external magnet.
This tilting or wobbling is called precession.
The rate or frequency of precession is called the Resonant or Larmor frequency, which is proportional to the strength of the applied
AXIS
Magnetism
The Larmor frequency of hydrogen is 42.58 MHZ in a magnetic field of 1 Tesla. 1 Tesla is 10,000 times the earths magnetic field. The magnetic field strengths used for MR imaging range from 0.1 to 4.0T.
T1 and T2 Weighted Images
The time constant that describes the rate at which
the net magnetization returns to equilibrium by this
transfer of energy is called T1.
The time constant that describes the rate of loss of transverse magnetization is called T2 relaxation
time / transverse (Spin) relaxation time.
T1 and T2 Weighted Images
T1 weighted images are called fat images.
1. because the fat has the shortest T1 relaxation
time. 2. emits the lightest signal relative to other tissues and thus appear bright in the image. 3. High anatomic detail is possible in this type of
image because of good image contrast.
4. Useful for depicting small anatomic regions (eg: TMJ)
T1 and T2 Weighted Images
T2 Weighted images are called water
images.
1. water has the longest T2 relaxation tine and
thus appear bright in the image.
2. the T2 time of abnormal tissues is longer than that of normal tissues. 3. Images with T2 weighting are most commonly used when looking for inflammatory changes
and tumors.
Tissue characteristics
Liquids have long T1 and a long T2
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Tissue characteristics
Fat has a short T1 and a short T2
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Fluids have a long T1
i.e. they take a long time to reestablish their longitudinal magnetic moment, produce a weak signal & appear dark on T1-weighted images
Fat behaves in the opposite manner and has a short T1,
produces a strong signal and appears white on T1 weighted images.
T1 weighted image
Fluid dark Fat- white
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Fluids such as CSF have a long T2
give a strong transverse signal for a long time and appear white on T2- weighted images.
Fat on the other hand has a short T2,
produces a weak signal and appears dark.
T2 weighted image
Fluid- white Fat - dark
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Indications for MRI in the Head & Neck
Assessment of intracranial lesions involving particularly the posterior cranial fossa, pituitary & spinal cord Tumor staging evaluation of the site, size and extent of all soft tissue tumors including nodal involvement, involving all areas in particular:
Salivary glands Tongue and floor of mouth Pharynx / larynx Sinuses Orbits
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Indications for MRI in the Head & Neck
Investigation of the TMJ to show both the bony and
soft tissue components of the joint including the
disc position.
MRI may be indicated:
When diagnosis of internal derangement is in doubt
As a preoperative assessment before disc
surgery
Implant assessment
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Advantages
1. 2. 3. 4.
Ionizing radiation is not used No adverse effects have yet been demonstrated Image manipulation available. High-resolution images can be reconstructed in all
planes (using 3D volume techniques ).
5.
Excellent differentiation between different soft tissues is possible & between normal and abnormal tissues enabling useful differentiation between benign and malignant diseases.
6.
Useful in determining intra-medullary spread.
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Disadvantages
1.
Bone does not give an MR signal, a signal is only obtainable from bone marrow.
2.
Scanning time can be long and is thus demanding
on the patient.
3. 4.
Equipment tends to be claustrophobic and noisy. Metallic objects, e.g. endotracheal tubes need to be replaced by non-ferromagnetic alternatives.
5. 6.
Equipment is very expensive.
Bone, teeth, air and metallic objects all appear black, making differentiation difficult.
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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 .
While not routinely applicable in dentistry,
appropriate use of MRI can enhance the quality of
patient care in selected cases.
