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Captcha Image Reload image challenge


Oral Medicine

Authors: Dr. Suraj Agarwal

INTRODUCTION

Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the soft tissues of the human body.It is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique to obtain microscopic chemical and physical information about molecules.MRI has advanced beyond a tomographic imaging technique to a volume imaging technique.
 
French Mathematician &Engineer. Developed Mathematical transformation: analysis of heat transfer b/w solid bodies.Rapidly process the frequency signals of NMR data & utilize this for image reconstruction. Started studying the magnetic . Properties in1930. He succeeded in detecting and Measuring single states of rotation of atoms and molecules, and in determining the magnetic moments of the nuclei.


TOMOGRAPHIC IMAGING:

Started out as a tomographic imaging modality for producing NMR images of a slice though the human body. Each slice is composed of several volume elements or voxels.The volume of a voxel is 3 mm3. The computer image is composed of several picture elements called pixels. The intensity of each pixel is proportional to the NMR signal intensity.
 
Invented tesla coil in 1891. Had the idea of applying magnetic gradient in 3 spatial dimension & used computer to create 2D NMR Images c/d “ZEUGMATOGRAPHY”


Basic Physics of MRI:

All magnetic fields are the result of charge in motion. Nucleus of an atom has a magnetic moment when it has an odd number of protons (or neutrons). Single proton in Hydrogen yields strongest magnetic effect.
 
Raymond Damadian 1977. Produced the MR image of the body.


Model of spin as motion

The orientation of nuclear magnetic moments are affected by an external magnetic field (that not due to the local nuclear magnetic moments).
Nuclei line up with magnetic moments either in a parallel or anti-parallel configuration.In body tissues more line up in parallel creating a small additional magnetization M in the direction of B0.

Microscopic Principles

The composition of the human body is primarily fat and water. Fat and water have many hydrogen atoms . Hydrogen nuclei have an NMR signal. MRI uses hydrogen because it has only one proton and it aligns easily with the MRI magnet. The Hydrogenatom’s proton, possesses a property called spin
  1. A small magnetic field
  2. Will cause the nucleus to produce an NMR signal



Magnetic Principles
 
  • The spinning hydrogen protons act like small , weak magnets.
  • They align with an external magnetic field (Bø).
  • There is a slight excess of protons aligned with the field. ~6 million billion/voxel at 1.5T
  • The # of protons that align with the field is so very large that we can pretty much ignore quantum mechanics and focus on classical mechanics.
  • The spinning protons wobble or “precess” about that axis of the external Bø field at the precessional, Larmor or resonance frequency.
  • Magnetic resonance imaging frequency n = g Bo where g is the gyromagnetic ratio
  • The resonance frequency n of a spin is proportional to the magnetic field, Bo.
  • Now if an electromagnetic radio frequency (RF) pulse is applied at the resonance (Larmor, precession, wobble) frequency, then the protons can absorb that energy, and (at the quantum level) jump to a higher energy state.
  • At the macro level, the magnetization vector, Mø, (6 million billion protons) spirals down towards the XY plane.
 
No external magnetic field. Orientation is random. External magnetic field B0. Orientation follows direction of external magnetic field


Stages in Magnetic Resonance:
Once the RF transmitter is turned off three things happen simultaneously.
  1. The absorbed RF energy is retransmitted (at the resonance frequency).
  2. The excited spins begin to return to the original Mz orientation. (T1 recovery to thermal equilibrium).
  3. Initially in phase, the excited protons begin to dephase (T2 and T2* relaxation)
 
NNuclei spin axis not parallel to B0 field direction. Nuclear magnetic moments precess about B0.


Frequency of precession of magnetic moments given by Larmorrelationship. f = ×B0
 
f = Larmor frequency (mHz)  = Gyromagnetic ratio (mHz/Tesla) B0 = Magnetic field strength (Tesla ~ 43 mHz/Tesla Larmor frequencies of RICs MRIs 3T ~ 130 mHZ 7T ~ 300 mHz 11.7T ~ 500 mHz
 
A SCHEMATIC REPRESENTATION OF THE MAJOR SYSTEMS ON A MAGNETIC RESONANCE IMAGER
 
 
GRADIENT COILS RF Coil
 


MRI SCANNER COMPONENTS
 


MAKING IMAGES OF THE NMR SIGNAL
 
  • Uniform magnetic field to set the stage (Main Magnet)
  • Gradient coils for positional information
  • RF transceiver (excite and receive)
  • Digitizer (convert received analog to digital)
  • Pulse sequencer (controls timing of gradients, RF, and digitizer)
  • Computer (FFT to form images, store pulse sequences, display results, archive, etc.)

INDICATIONS OF MRI IN HEAD & NECK
  1. Assessment of intra cranial lesions involving posterior cranial fossa, pituitary gland & spinal cord.
  2. Tumour staging- size, extent, site of tumours including salivary glands, pharynx, larynx, sinuses, orbit.
  3. TMJ investigations to show bony and soft tissue component -internal derangement of disc - post op assessment before disc surgery



CONTRA INDICATIONS OF MRI
  1. Cardiac pace makers.
  2. Ear prosthesis, cerebral aneurysm clips.
  3. Metallic implant and dental filling- relative contra indication.
  4. In pregnancy, MRI is indicated if other non ionizing forms of imaging are inadequate or if MRI provides information that would otherwise require exposure to ionizing radiation.

ADVANTAGES OF MRI
  1. Ionizing radiation not used
  2. Non invasive
  3. High contrast sensitivity excellent soft tissue imaging
  4. Images in all planes obtained without repositioning the patient
  5. High resolution images constructed in all planes
  6. No bone or air artifacts
  7. Equipment contains no moving objects

DISADVANTAGES OF MRI
  1. Expensive
  2. Long scanning time & available only in large setups
  3. Equipment tends to be claustrophobic, noisy
  4. Needs trained staff
  5. Hard tissue like bone does not give an MR signal. Signal obtained only from the bone marrow

TYPES OF MRI IMAGINGS
  1. T1W1
  2. T2W1
  3. FLAIR
  4. STIR
T 1 RELAXATION TIME ( SPIN LATTICE RELAXATION TIME)

Time constant that describes the rate at which net magnetization returns to equilibrium by transfer of energy to the surrounding molecules ( lattice). Varies with different tissue- ability of nuclei to transfer excess energy to the environment.

T 1 WEIGHTED IMAGES

Short repetition time ( 500msec) between R F pulse and a short signal recovery time ( 20 msec). Repetition time.Period of time between beginning of a pulse sequence and beginning of succeeding pulse sequence.

T 1 weighted shows anatomy.
T 1 weighted images are called fat images- high intensity signal

T 2 relaxation time( Transverse relaxation time)
  • Time constant that describes the rate of loss of transverse magnetization by interaction of hydrogen nuclei with one another which causes the nuclei to dephase (with resultant loss of transverse magnetization).
  • Long repetition time ( 2000 msec) between R F pulse and a long signal recovery time ( 80 msec)
  • Also called Water images- high intensity signal. Water, saliva, CSF T 2 long High intensity signal Appear bright Fat T2 short low intensity signal Appear Dark

T2 WEIGHTED IMAGE USED TO DETECT DISEASE

M R signal depends on degree to which H+ is bound within a molecule

Bones, teeth-                          H+ tightly bound – no signal- dark

Soft tissue, liquids-        H+ loosely bound- signal- bright




Proton Density or Spin Density

Measure of concentration of loosely bound hydrogen nuclei available to create a signal

Spin Echo

Once signal is got in the form of free induction decay, it is necessary to refocus the dephased protons with a second stronger R F pulse ( 1800) signal received after refocussing is Spin echo.

Echo Time

Time between excitation pulse and spin echo.

MR Contrast Agents

Signal emitted by tissue can be altered by injecting MR contrast agents. Contrast enhancement is determined by vascularity and interstitial vascular space of tissue involved. In jaw cysts and tumours- to study the margins of the lesions. e.g. Lanthamide- Gadolinium diethelenetriamine pantothenic acid ( Gd- DTPA)
Dark on T1- Edema,tumor,infection,inflammation,hemorrhage(hyperacute,chronic) Bright on T1 - Fat,subacutehemorrhage, melanin,protein rich fluid, Slowly flowing blood, Paramagnetic substances (gadolinium,copper, manganese)

FLAIR & STIR

-180° preparatory pulseis applied to flip the net magnetization vector 180° andnull the signal from a particular entity (eg, water in tissue).When the RF pulse ceases, the spinning nuclei begin to relax.When the net magnetization vector for water passes the transverseplane (the null point for that tissue), the conventional 90°pulse is applied, and the SE sequence then continues as before. The interval between the 180° pulse and the 90°pulse is the TI ( Inversion Time).
At TI, the net magnetization vector of water is very weak, whereas that for body tissues is strong. When the net magnetization vectors are flipped by the 90° pulse, there is little or no transverse magnetization in water, so no signal is generated (fluid appears dark), whereas signal intensity ranges from low to high in tissues with a stronger NMV.

Two important clinical implementations of the inversion recovery concept are:
  • Short TI inversion-recovery (STIR) sequence
  • Fluid-attenuated inversion-recovery (FLAIR) sequence.




  •  
  • STIR

    In STIR sequences, an inversion-recovery pulse is used to nullthe signal from fat (180° RF Pulse).When NMVof fat passes its null point , 90° RF pulse is applied. As little or no longitudinalmagnetization is present and the transverse magnetizationis insignificant. It is transverse magnetization thatinduces an electric current in the receiver coil so no signal is generated from fat. STIRsequences provide excellent depiction of bone marrow edema which may be the only indication of an occult fracture.Unlikeconventional fat-saturation sequences STIRsequences are not affected by magnetic field inhomogeneities,so they are more efficient for nulling the signal from fat.

    FLAIR

    First described in 1992 and has become one of the corner stones of brain MR imaging protocols. An IR sequence with a long TR and TE and an inversion time (TI) that is tailored to null the signal from CSF.In contrast to real image reconstruction, negative signals are recorded as positive signals of the same strength so that the nulled tissue remains dark and all other tissues have higher signal intensities.
    Most pathologic processes show increased signal intensity on T2-WI, and the conspicuity of lesions that are located close to interfaces b/w brain parenchyma and CSF may be poor in conventional SE or FSE T2-WI sequences.FLAIR images are heavily T2-weighted with CSF signal suppression, highlights hyperintense lesions and improves their conspicuity and detection, especially when located adjacent to CSF containing spaces.
    In addition to T2- weightening, FLAIR possesses considerable T1-weighting, because it largely depends on longitudinal magnetization. As small differences in T1 characteristics are accentuated, mild T1-shortening becomes conspicuous. This effect is prominent in the CSF-containing spaces, where increased protein content results in high SI (eg, associated with sub-arachnoid space disease) .High SI of hyperacute SAH is caused by T2 prolongation in addition to T1 shortening.

    Clinical Applications:
     
    • Used to evaluate diseases affecting the brain parenchyma neighboring the CSF-containing spaces for eg: demyelinating disorders.
    • Unfortunately, less sensitive for lesions involving the brainstem & cerebellum, owing to CSF pulsation artifacts
    • Useful in evaluation of gliomatosiscerebri owing to its superior delineation of neoplastic spread
    • Useful for differentiating extra-axial masses eg. epidermoid cysts from arachnoid cysts.
    • Mesial temporal sclerosis: m/c pathology in patients with partial complex seizures.Thin-section coronal FLAIR is the standard sequence in these patients & seen as a bright small hippocampus on dark background of suppressed CSF-containing spaces. However, normally also mesial temporal lobes have mildly increased SI on FLAIR images.
    • Focal cortical dysplasia of Taylor’s balloon cell type- markedly hyperintense funnel-shaped subcortical zone tapering toward the lateral ventricle is the characteristic FLAIR imaging finding.
    • In tuberous sclerosis- detection of hamartomatous lesions, is easier with FLAIR than with PD or T2-W sequences.
    • Embolic infarcts- Improved visualization
    • Chronic infarctions- typically dark with a rim of high signal. Bright peripheral zone corresponds to gliosis, which is well seen on FLAIR and may be used to distinguish old lacunar infarcts from dilated perivascular spaces.

    MR IMAGES IN HEAD & NECK REGION

    Hyper intense on T1 weighted images Melanin, haemorrhage, hyperproteinaceous secretions, cholesterol crystals, fat

    Lesions of head & neck
    • Fat containing lipomas and dermoids
    • High protein cyst-mucocele&lymphoepithelial cyst
    • Blood containing traumatic cyst
    • High melanin containing melanomas
    • Fibro chondro osseous bone lesion
    • Squamous Cell Carcinoma
    • Inflammatory lesions resulting from edema, sinusitis, abscesses
    • Benign and malignant salivary gland tumours

    MRI OF TMJ

    Depict location, morphology, function of articular disc, thus allowing diagnosis of internal dearrangement. Cases of bone marrow edema, joint effusion, fibrous adhesion and certain tumours.Osseous changes evaluated

    MRI of Salivary Glands
  • Parotid gland compared to submandibular gland. – high fat content - bright signal on T1 weighted images
  • Abnormalities of gland, gross dilatation of duct, cysts with fluid collection can be diagnosed.
  • All benign tumours are sharply defined with bright signal on T1 & T2
  • The internal structure and regional extension of the lesions into adjacent tissue from deep lobe of the parotid can also be demonstrated.
  • Uses of IV contrast agents – distinguish between cystic and solid masses and evaluation of perineural spread of malignant tumour.

Other Uses OfMRI In Head & Neck Region
 
  1. To evaluate lymph nodes for possible tumour invasion; larger than 1 cm in size or with central in homogeneity – malignant. Central cavitation greater than 3 mm surrounded by contrast enhancement. Tumour necrosis.
  2. To detect diseases of paranasal sinuses & nasal cavities.
    - Normally contains air- no signal
    - Edema – altered signal
  3. To assess bone marrow changes in skull, zygoma & other facial bones.
  4. Detection of cavernous sinus thrombosis.
  5. To detect any abscess in orbit or intra cranial cavity.
 


 
 
 


CONCLUSION

The clarity & superiority of the images produced by MRI over other conventional imaging techniques can be of great advantage to the oral physician to correctly diagnose various diseases and initiate the most appropriate treatment to cure the patient