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Role of Magnetic Resonance Imaging In Dentistry --------------------------------------------------- C.D. Nayak et al

Scientific Journal Vol. III - 2009

Westesson et al. performed MRIs 2 yearspostoperatively after vertical ramus osteotomyin 10 patients and found imaging to be an

excellent method to study morphological

changes of the muscles of mastication for and

osseous fragments when compared to controls.Traxler et al. studied the relevance of MRI fordiagnosing tumors of the parotid gland. MRI is

more superior as compared to CT and USG indiagnosing muscle hypertrophy.

In clinical endodontics there is need for

improved diagnostic methods for accurate pulppathosis and configuration of root canals.Lockhart et al. had imaged extracted human

teeth to establish possible future applications

of MRI in pulp pathosis. They reported highaccuracy of MRI in images of periodontal

membrane and gross pulpal anatomy.

However, due to limitation of slice thickness,detailed resolution was poor. Bauman et al.used stray field imaging (STRAFI), a newly

developed and as yet experimental magneticresonance microscope , for imaging carious

mandibular molar root canals and pulp

chamber. Magnetic fields used were high andtime taken for data collection was 8-10 hrswhile image reconstruction took 1 hour.

Inspite of drawbacks, MR microscopy may

open a new dimension of non-destructiveimage reconstruction.

Maxillofacial Disorders

MRI has the ability to differentiatebetween solid and cystic lesions which isespecially important in the evaluation of jaw

tumours6. It is the only imaging modality

capable of demonstrating perineural spread of

a tumour by branches of trigeminal and facialnerves and can also detect early corticalerosion and marrow involvement. MR

evaluation of maxillary pathologicalconditions includes both axial and coronal T1

weighted images to assess the distribution of

the lesion and its extension beyond the cortexof the bone of origin.

Coronal images ideal for assessing the

transcortical extension of maxillary lesions

into orbits, alveolus, palate or more whereasaxial images facilitate the detection of tumour

spread into the pterygopalatine regions and

soft tissue regions of the cheek.T2 weighted images demonstrate

extraosseous spread of tumor, are useful in

differentiating tumor from sinus mucosa orretained sinus secretions. In the mandible,

axial images are particularly useful for

assessing the symphyseal region. Coronalimages are more valuable for the evaluation ofthe tumor spread to or through the floor of the

mouth and assessment of the tumor to the

inferior dental nerve.

Fig 1: T1-Wt Image Shows Hypointensity That

Encroaches On The Left Pterygopalatine Fossa.Fig 2: Contrast Enhanced T1-Wt Images Show

Tumoral Enhancement

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Fibrous Dysplasia

Fibrous dysplasia4 is an idiopathic

disease in which medullary bone is replaced

by disorganized fibroosseous tissue. An MR

signal is hypointense on all sequences.Foci ofbright signal intensity are intermixed with themore typical hypointense signal of

fibroosseous tissue on both T1 and T2 weighted images (Fig 1).

Osteosarcoma

On MR examinations, osteosarcomas5

typically present as a soft tissue massesassociated with osseous destruction.The soft

tissue mass exhibits low to intermediate signal

intensity on T1 weighted images (Fig 2) and

high signal intensity on T2 weightedimages.MR imaging usually permits

demarcation of the tumor from the adjacentbone marrow in T1-weighted images orextraosseous soft tissue in T2 weighted

images.

Dentigerous Cyst

On MR examination, the cystic fluid exhibitslow to intermediate signal intensity on T1-wtimages and high signal intensity on T2- wt

images (Fig 3), whereas the partiallycompleted crown appears as an area devoid of

signal or of low signal because of its low

mobile proton density. The cyst wall is of

intermediate intensity on T2-wt images (not asbright as sinus mucosa).

Ameloblastoma

MRI can provide important informationconcerning ameloblastomas

7. This technique

can help differentiate between cystic and

mixed cystic solid lesions. The nature of thelining epithelium can be studied, and earlyextension into the adjacent soft tissues can be

detected. Solid and cystic components are

usually hypointense on T1-wt images withsolid tumors demonstrating a more

homogeneous appearance. Purely cystic areashave been noted to exhibit low signal intensity

on T1-wt images (Fig 4) and high signalintensity on T2-wt images. Cortical expansion

and displacement of adjacent structures mayalso be often evident. After surgery, MRI can

demonstrate the presence of early recurrences,

which appear bright on T2-wt images.

Temporomandibular Joint

Temporomandibular joint is ideally suited forinvestigation with MRI because it allows

simultaneous bilateral imaging, both joints canbe examined efficiently

8(Fig 5 A, B). A

routine MRI evaluation of TMJ consists of a

series of acquisitions. Images are obtained in

the axial plane to define the location of thejoints and provide a global view of the

surrounding anatomy. Coronal images are

routinely obtained because they provideinformation about mediolateral relationships at

the TMJ. The sagittal images are assignedfrom the axial in an oblique planecorresponding to the axis of the condyle andbody of the mandible10. These oblique sagittal

images provides the greatest diagnostic detail

for anatomical diagnosis.The TMJ9

can beexamined with all clinically available magnet

strengths. Although early reports of TMJ MRIused midfield imagers (0.3 to 0.6 Tesla), morerecent reports have emphasized high field

imaging (1to2 Tesla)9.

Fig 3: Dentigerous Cyst On T1 And T2- Wt

Images

Fig 4: Unicystic Ameloblastoma OnT1-Wt

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Fig 5 (A,B): Left TMJ Region The Anterior

Displacement Of Articular Disc Without

Reduction Can Be Observed At

(A) A Proton Density-Weighted Imaging

( PDWI ) Slice At Closed Mouth and

(B) A PDWI Slice at Open Mouth.

Sjogrens Syndrome

MR images of Sjogrens syndrome11,12

demonstrate an enlarged parotid gland with aninhomogeneous speckled or nodular pattern

(salt and pepper appearance) on T2-wt images

(Fig 6). Overall signal intensity decreases on

T2-wt images as compared with normalcontrols.

Discussion

The benefits of MRI are substantial because

of its excellent soft tissue contrast, lack of

artefact from adjacent tissue, multiplanarcapabilities and lack of ionizing radiation. The

chief strengths of MRI are its ability to

provide cross-sectional images of anatomicalregions in any plane and its excellent soft

tissue contrast. New applications arecontinually being developed as the techniqueand equipment undergo refinement. Inspite ofits present selective and restrictive uses due to

its costs, MRI quality has already set it apart

from other imaging modalities and it is only amatter of time before its use in dentistry is an

everyday occurrence.

Summary and ConclusionOf many major technological advances

such as Computed Tomography, Ultrasound

that have been applied in clinical research,

Magnetic Resonance Imaging has proved tobe the most versatile. The clinical significanceof MRI is reflected by the extra ordinary

growth of MR scanner worldwide,(worldwide-9000 scanners ; North America-

45000 scanners , Japan-3400 scanners). MRI

has the ability to generate high resolution

multiplanar (3D) images in addition toproviding superior contrast resolution . ThusMRI is a pre- eminent soft tissue diagnostic

modality for many neurological, musculo-skeletal, cardiac and maxillo-facial

pathologies.

Fig 6: T2-Wt Coronal Image:

Sjogrens Syndrome

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References

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David D. Stark, William G. Bradley ,MAGNETIC RESONANCE IMAGING, 3RD Edition

3. Wu, Y.; Chesler, D.A.; Glimcher, M.J.;Garrido, L.; Wang, J.; Jiang, H.J.;

Ackerman, J.L. "Multinuclear solid-state

three-dimensional MRI of bone andsynthetic calcium phosphates". Proc

Natl Acad Sci US 1999; 4: 15741578

4. Fibro-osseous lesions : the role ofimaging. Radiol Clin North America1993; 31: 121.

5. Seeger L L, Widoff, Basset et al :Preoperative evaluation of

osteosarcoma: value of gadopentatedimeglumine enhanced MR imaging ,

American J Roentgenology 1991;

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6. Van Resenburg LJ, Nortje CJ et al:Magnetic resonance imaging and

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7. Manabu Minami,Takashi Kanedaet al Ameloblastoma in the

Maxillomandibular Region: MR

Imaging. Radiology 1992; 184:389-393

8. Donlon WC and Moon KL :Comparison of MRI , arthrogragphy andclinical and surgical findings in TMJderangements , Oral surgery, oral

medicine , oral radiology ,endod 1987;64:2.

9. Steven E. Harms, M.D.Randall M.Wilk et al, The TemporomandibularJoint:Magnetic Resonance Imaging

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10. Richard W. Katzberg, Per-LennartWestesson et al TemporomandibularJoint: MR Assessment of Rotational andSideways Disk Displacements

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