mri in dentistry

MRI(MAGNETIC RESONANCE

IMAGING)Prepared by-DIVYA PANDAYUnder the guidance of –

• Dr. Sunita• Dr. Sujoy Ghosh• Dr. Meera chaudhary • Dr. khushboo

CONTENTS Introduction Protons Precession Resonance Magnetic resonance signal T1 and T2 relaxation Radiofrequency pulse sequence Tissue contrast Scanner gradient Advantages Disadvantages Applications references

PROTONS

Proton – spin – angular momentum

Spin – associated with charge – B generated in nuclei – act as magnet– magnet dipole has magnetic moment.

Magnetic dipoles randomly oriented –Bext -

SPIN UP SPIN DOWN

PRECESSION

Magnetic moment of H+ in a Bext – do not align exactly with the direction of Bext

– TILT from a position exactly parallel to Bext

Bext – axis of spinning of H+ to wobble(precess) around the lines of B ext.

This frequency of precession • PRECESSION FREQUENCY/• RESONANCE FREQUENCY/ • LARMOR FREQUENCY

Depends on • Atom• Strength of Bext- 0.1-4 Tesla

RESONANCE Energy to energy – provided by radiofrequency(RF) spectrum of EM spectrum.

When RF = larmor frequency resonance.

COIL PATIENT RESONANCE

Spin up to spin down

BH+ longitudinal ses

RF

Causes the H+ to

precess in phase

BH+ transverse

Longer RF pulse –BH+ longitudinal BH+ transverse

90 degree RF pulse/flip angle of 90 degree Significant intensity and duration – BH+ longitudinal sed to 0

MAGENTIC RESONANCE SIGNAL

Current in a receiver coil

Precession of H+ in phase MR signal

Frequency of this AC current = RF pulse = larmor frequency.

Magnitude of MR signal 1. Density of H+

2. Loosely bound H+

BH+ longitudinal returned to

original

Dephase

RELAXATIONRF turned off

BH+ Transverse- DECAY

MR signal

FREE INDUCTION

DECAY SIGNAL


original

Dephase

RELAXATIONRF turned off


T2 - magnetic

moment of H+ interact with each otherSPIN SPIN

RELAXATION TIME

T1 – energy absorbed by

latticeSPIN –

LATTICE REALAXATIO

N TIME

T1 and T2 RELAXATION

T1 RELAXATION TIME It is an exponential process and the time

required for 63% of the magnetization to return to equilibrium (the time constant) by this transfer of energy.

Varies with tissue

Ability to transfer their excess energy to lattice

fluid content

TI TI

fat content

Inherent energy of

water- Inhibit

transfer of energy

Inherent energy-

quickly transfer energy

T2 RELAXATION TIME Time constant that describes the

exponential rate of loss of transverse magnetization.

Varies with tissue

Depend on packing of tissues.

fluid content

T2 T2

fat content

Closely packed - interaction

Loosely packed - interaction

T2 occurs more rapidly then T1

RF PULSE SEQUENCE

Components of RF pulse sequence are set by the operator.

Can be used to emphasize various features of tissue being examined

TR and TE TR(REPETITION TIME) : duration

between repeat RF pulses.

TE (ECHO TIME) : time after application of the RF pulse when the MR signal is read.


original

Dephase

RELAXATION


Controlled by TE

Controlled by TR

IMAGE CONTRAST

INTRINSIC-Features of

tissues.

EXTRINSIC-Parameters of a given pulse

• H+ density• T1• T2

• TR• TE

T1 –Weighted ImageEmphasizes differences in T1 values of

tissues

Accomplished by1. short TR(300-700ms)

2. short TE(20ms)

Tissue with short T1

Tissue with long T1

BRIGHT

DARK

• anatomy

USE

T2 –Weighted ImageEmphasizes differences in T2 values of

tissues

Accomplished by1. Long TR(2000ms)2. Long TE(≥60ms)

Tissue with short T2(eg-fat)

Tissue with long T2

(eg-TMJ fluid CSF fluid)

DARK

BRIGHT

USE• Pathology- since pathologic tissue has more water.

OTHER METHODS

STIR(short tau inversion recovery) – dec signal from fat(FAT SATURATION) – better visualization of adjacent structures.

FLAIR(fluid attenuation inversion recovery) - dec signal from fluid – better visualization of pathology.

CONTRAST AGENT – gadolinium. It dec T1 of enhancing tissue – they appear bright.

SCANNER GRADIENT 3 gradient coils1. X-axis : left to right2. Y-axis : anterior to posterior3. Z-axis : head to toe

Intensity of Bext can be modified when gradient applied

Z gradient turned on precessional frequency of H+ varies linearly along Bext RF pulse applied – nuclei with RF = PF – resonate Hence, desired slice selected.

Z gradient coil turned off – X(phase encoding) and Y(frequency encoding) gradients are turned on – location of signal within xand y is determined.

ADVANTAGES

TimePresence of ferromagnetic materialClaustrophobic patient

Best contrast resolution of soft tissuesNo ionizing radiationMultiplanar image without reorienting the patient

DISADVANTAGES

APPLICATIONS Soft tissue condition – position and integrity of

disk of TMJ

Soft tissue disease – tongue, cheek, salivary gland, neck – neoplasia lymph nodes - malignant involvement Perineural invasion - by malignant neoplasia Extent of penetration of carcinoma- in cortex of

mandible – SWIFT Neoplasia - Gadolinium enhanced

Osteomyelitis – edematous change in marrow and surrounding tissue

Location of mandibular nerve

MR angiography

closed

NORMAL TMJ

Anterior disk displacement with reduction

closed

Lateral displacement

Open-Normal

T2 image - effusion

Anterior disk displacement without reduction

T2 image- effusion

T1 image- anterior

displacement

T1 image- remains

anterior to condyle

a. T1 image- bow tie

appearance

b. T2 image- inflammatory

effusion

c. Anterior disk displacement

TMJ

DEGENERATIVE JOINT DISEASE

a. T1 image

b. T1 post gadolinium

c. T2 image

RHABDOMYOSARCOMA- soft tissue of right face

MULTIPLE MYELOMA- right mandible and right carotid space

T1 post-contrast(fat saturation)

T1 image

T2 post-saturation(fat

saturation)

T1 image post-

gadolinium,Fat

saturation

T2 image

T1 image

ADENOID CYSTIC CARCINOMA in submandibular gland

T2 image – hyperintense

to muscle

T1 image –isointense to

muscle

PLEOMORPHIC ADENOMA

CYST IN LEFT PAROTID GLAND

SIALECTASES of parotid gland

T2 image

ANGIOGRAPHY

1. RF pulse causesa. B transverseb. B longitudinalc. Bothd. None

2. T1 weighted image is used for studyinge. Anatomyf. pathology

3. T1 weighted image usesg. Short TR, short TEh. Long TR, long TEi. Short TR, long TEj. Long TR, short TE

4. In T2 weighted image, tissue with long T2 (eg- TMJ fluid) appearsa. Bright b. Darkc. Both d. None

5. Which diagnostic method would you prefer for diagnosing collection of effusion in TMJe. T1f. T2g. Bothh. None

6. Which diagnostic method would you prefer for diagnosing disk displacement in TMJ?i. T1j. T2k. Both l. none

7. This is a case of lymphoepithelial cyst in right parotid space, which is T2?a. Ab. B

8. MRI usesa. Ionizing radiationb. Non-ionizing radiationc. Bothd. None

9. MRI is used for-e. Neoplasia of soft tissuef. Mallignant involvement of lymph nodesg. Bothh. none

10. Contrast agent used is ________?

REFERENCES White and pharaoh – 1st South Asian

edition

THANKYOU

mri in dentistry

Health & Medicine