Transcript
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
BH+ longitudinal returned to
original
Dephase
RELAXATIONRF turned off
BH+ Transverse- DECAY
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.
BH+ longitudinal returned to
original
Dephase
RELAXATION
BH+ Transverse- DECAY
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
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