MRI & CT basics/ oral surgery courses

• Protons have a +ve charge , moving constantly
• This moving electrical charge is electrical current and it
induces a magnetic field.
• So every proton has its magnetic field , thus becoming a
tiny bar magnet.
• When the patient is put in the magnet , the protons
align with the external magnetic field . They do it in two
ways, Parallel and anti parallel . The state of less energy
is preffered. *walking on their feet.
Time to take a break
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacdemy.com

when the patient is in the
MR magnet , his own
magnetic field is
longitudinal to external
magnetic field of MR
machine magnet.
Because it is longitudinal it
cannot be measured.

• Protons are like little magnets.
• In an external magnetic field they align parallel or anti
parallel.
• The low energy state(parallel) is preferred , so a few more
protons align this way.
• The protons perform a motion that resembles the
wobbling of a spinning top , that was hit . This is called
precession.
As there are more protons aligned parallel to the external
field there is net longitudinal magnetization.
So time to review

• A radio frequency pulse that has the same frequency as the
precising protons , can cause resonance , ie transfer energy
to protons . This results in more protons being anti parallel
leading to decrease in longitudinal magnetization.
• The RF pulse results in a new magnetic vector , the
transversal magnetization.
• When the RF pulse is switched off …….
There is an increase in longitudinal magnetization…… Long .
relaxation ( T 1 )
And the transversal magnetization decreases …
Transversal relaxation (T 2)

Long . relaxation Transversal relaxation
T1 T2

Implants cardiac pacemakers; defibrillators.
CNS aneurysm clips
Ocular foreign body e.g. metal shavings.
Absolute contraindications to MRI
Relative contraindications
Lead wires or similar wires.
Non-ferromagnetic stapedial implants,
Cochlear implants.
Claustrophobia.
Pregnancy

MRI Technique
T1 and T2 images
Fat saturated images (STIR)
Diffusion imaging

? MRI
? MRIwww.indiandentalacdemy.com

FAT SATURATED IMAGES

NORMAL
FAT SUPRESSED

Mandible Marrow

Diffusion MRI
Aims to analyze human brain through diffusion of water molecules
Measures the mobility of water within tissues
and, may function as a marker for both tissue
cellularity.

MRI techniques are currently being used in dentistry
For evaluation of temporomandibular joint diseases
Examination of salivary glands, maxillary sinuses, masseter
muscles, & facial skeleton
Detection of early bone changes in tumors, fractures,
inflammatory conditions and hematoma.
Evaluation of bone before applying dental implants

Enamel and dentine appeared black
due to a lack of unbound protons.
Dental pulp chamber,, appeared
white or grey on T1 weighted and
STIR imaging.
Cortical bone was seen as a black
zone and high signal internal fatty
marrow
Moderate signal from external soft
tissues
T1 weighted scans.
Slices taken at 3 mm intervals

The two buccal roots (B) of the upper first molar
The inferior dental (ID) canal A
The ID canal containing the ID
nerve is seen as two parallel
grey lines running in the ramus
and body of the mandible,
Between the two lines the
neurovascular bundle and
surrounding tissue and fat are
seen as higher signal (whiter).
crowns and roots of premolar and molar teeth are clearly seen

Root apices of first and second
molars in close proximity to the
floor of maxillary antrum.
unerupted upper third molar
(wisdom tooth) (G)
White pulp chambers (F)
The root apex of the lower third
molar (H) is in close proximity to
the inferior dental canal (A).
Extensions of the ID neurovascular bundle contents are seen as
thin grey lines passing into the pulp chambers of the mandibular
teeth via their respective apical foramina (R)www.indiandentalacdemy.com

ID canal is seen to connect
to the mental foramen (C)
and through this the mental
branches of the ID nerve
innervate the lower lip and
skin of the chin.
The cortex of the mylohyoid
ridge (J) (into which the
mylohyoid muscle is inserted)
is seen as a black line.
Maxillary antrum there are two large mucous retention cysts.

The crowns of the unerupted
upper wisdom teeth (G)
small black circles of the apices of
the second molars just visible
anterior to them.
The root apices of teeth are seen
as black circles, and pulp chamber
(F) seen centrally as a white dot
ID nerve (N) enters inferior dental
canal via the mandibular foramen.
T1 weighted image.

Coronal T1 weighted images through the canine and incisor teeth
showing nasopalatine suture (S) and pulp chambers (F)

The unerupted lower wisdom teeth (H) their crowns angled
lingually and roots angled buccally
There is high signal from their pulp chambers and a band of high
signal immediately surrounding the crown
This band or “halo” enveloping the crown is an eruption follicle (Q)
developed from the remnants of cells left over from the formation
of the enamel and crown of the tooth, and usually disappears on
eruption of the tooth into the mouth.
Coronal T1 scans

shows pulp chambers (F) of the teeth to have high signal due to a
relatively high water content.
High signal around the un erupted wisdom teeth due to high water
content in the dental follicles (P), formed during development of
the tooth crown.
STIR ……

Right Warthin duct …… There are segmental dilatations and
stenosis consistent with chronic inflammatory change of duct.

STRENGTH ………..
No iodinated contrast
• No radiation
• Dental artifact is not that big a problem
• Soft tissue contrast
• Perineural extent of disease
• Medullary cavity invovlement
WEAKNESS ………
Time to acquire images (20-45 min)
• Small bore/claustrophobia
• Motion

A fish that keeps its mouth shut,
Never gets caught.

A conventional X-ray image is basically a shadow . Shadows give
you an incomplete picture of an object's shape.

The X-ray beam moves all around the patient, scanning from
hundreds of different angles. The computer takes all this
information and puts together a 3-D image of the body.
In the CT machine patient lies down on a platform, which slowly
moves through the hole in the machine.

Dental CT programs , use 1-mm
transverse images of the jaw. The
transverse images are scanned
parallel to the alveolar ridge.
Transverse image shows where the cursor is deposited (curved
arrows) for the program to produce a curved line (straight
arrow) that defines the location for reformatting image in
Perpendicular numbered lines (arrowheads)

Cross-sectional views show
relation of the periapical radiolucency to
the buccal and lingual cortex
Panoramic image.
Transverse image

Investigation of jaw pathology including cysts, tumours and fibro-
osseous lesions.
Investigation of the paranasal sinuses.bony components of the TMJ.
Pre- and post-implant assessment . Orthodontic assessment, both
dental development and skeletal base relationship.
Assessment of wisdom teeth, in particular their relationship to the
inferior dental canal
Indications

CBCT ……………… have 2 major differences .
First, CBCT uses a low-energy fixed anode tube, similar to that used
in dental panoramic x-ray machines.
Second, CBCT machines rotate around the patient only once,
capturing the data using a cone-shaped x-ray beam.
These changes allow for a less expensive, smaller machine that
exposes the patient to approximately 20% of the radiation of a
helical CT .

All CBCT scanners use the same technology.
The difference is in detector , either an amorphous silicon flat-panel
detector or a combination of an image intensifier and a charge-
coupled device (CCD) camera.

TMJ
Surface modeRadiographic mode.
Cross sectional
view

Advantages in Dental Imaging
Dose:
Panoramic: 6-20 µSv
CBCT: 20-70 µSv
CT 314 µSv
Lower dose than helical CT
Compact design
Superior images to Panoramic
Low cost
Low heat load

Implant Dentistry
Advantages of CBCT …..
Alveolus in 3 dimension
Precise measurements before surgery.
Measurements of bone height, width, nerve position,
Objective measures of bone quality.
Traditional panoramic radiography ……25% magnification

Dental implant planning.
Mandibular nerve marked

Oral and Maxillofacial Pathology
CBCT is replacing conventional CT as it has higher resolution,
lower radiation dose, and lower cost.
Three dimensional imaging of cysts and tumors of the
maxillofacial region can give the surgeon the vital information
necessary for planning surgery.

cross-sectional view of
maxilla and mandible.
CBCT images of a Mandibular cyst
Anterior view
surface mode
Lingual view

Gold standard for imaging the intra-articular components of the
TMJ .
Panoramic radiographs provides
2 dimensional image,
low sensitivity in evaluating changes in the condyle,
poor reliability
low accuracy in evaluating the temporal components of the joint.
CBCT The resulting images are of high diagnostic quality.
With significantly reduced radiation dose and low cost.
Temporomandibular Joint Disorders

TMJSurface modeRadiographic mode
Cross sectional
view

Craniofacial Surgery
Treatment planning for patients with cleft lip and palate .
Young age of patients ……. radiation exposure.
CBCT allow better evaluation of dental age, arch segment
positioning, and cleft size .Better prediction in terms of the
morphology of the defect, as well as the volume of graft material
necessary for repair

Occlusal view of maxilla in surface mode. & radiographic mode.
CBCT images of cleft palate
Anterior view of maxilla in surface mode & radiographic mode.

Lateral cephalography has been the standard modality
for diagnosing skeletal and dental deformities, as well as
for use in surgical prediction and treatment planning.
Orthognathic Surgery
Clinicians have long evaluated the usefulness of 3-
dimensional imaging in orthodontics and
orthognathic Surgery .

Identification, treatment planning, and evaluation of potential
complications of impacted teeth are greatly improved by adding
the third dimension through CBCT.
Impacted Teeth

CBCT in impacted
supra numerary
tooth
Anterior view of maxilla in radiographic mode.
surface mode Occlusal view in radiographic modewww.indiandentalacdemy.com

Shortcomings
• Metal artifacts ?
• Worse low contrast detectability
• Long scan times = motion artifacts
• Slightly Inferior quality to conventional CT
Periodontal ligament spaces easily recognizable
in the dental CT but not satisfactory in the CBCT
CBCT

• CBCT offers less dose than conventional CT
• CBCT offers superior images and diagnosis than
panoramic
• More practical than a conventional CT
Advantages

Slow and steady wins the race.
THE STORY DOESN’T END HEREwww.indiandentalacdemy.com

The hare was disappointed at losing and he did some soul-searching.
If he had not taken things for granted, there's no way the tortoise
could have beaten him. So he challenged the tortoise to another
race. The tortoise agreed.
Fast and consistent will always beat the slow and steady.
It's good to be slow and steady; but it's better to be fast and reliable.
THE STORY DOESN’T END HERE
He realized that he'd lost the race
only because he had been over
confident , careless and lax

THE STORY STILL HASN’T ENDED
The tortoise did some thinking this time . He
thought for a while, and then challenged the hare
to another race, but on a slightly different route.
change the playing field to suit your core
competency.

Teamwork
They reached the finishing line together. They both felt a greater sense of
satisfaction than they'd felt earlier.

MRI & CT basics/ oral surgery courses

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