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MAXILLOFACIAL IMAGING

Author: akil, Posted on Tuesday, September 14 @ 15:14:57 IST by RxPG  

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Dental


OCCLUSAL RADIOGRAPH:

" Detecting sialolith of submandibular Gland.
" Evaluate buccal and lingual cortex for perforation erosion or expansion
" Localised lesion or foreign bodies


INTRA ORAL PERIAPICAL RADIOGRAPH

" Fine detail in visualisation of teeth and supporting teeth.

PANORAMIC RADIOGRAPH (OPG):

Wide view of maxilla and mandible and surrounding structures including the TMJ, Zygomatic arches, maxillary sinuses, nasal cavity and orbits.(but less sharp and less details than the intraoral views)

" Excellent of comparison of Rt and Left side structures.
" Initial view of the osseous structure of TMJ
" Sinus floor integrity
(Some allow additional projections like lateral, oblique lateral, A-P, P-A and sub mento vertex)

DIGITAL IMAGING:
Indirect: scanning the conventional radiograph

" Direct digital image acquisition:
" Need conventional radiographic unit.
" The conventional radiographic film is replaced by sensors.
" NO better diagnostic capabilities than conventional radiography.
" Both intraoral and panoramic radiographs can be taken.

The main advantages of digital image acquisition are:
" REDUCED RADIATION DOSAGE.
" Reduced time for acquisition of image.
" Transmit images in electronic form.
" Can be used with lots of image analysis tools.
" Colouring of important features like for some tumours.
" Digital subtraction technique for evaluating the loss of bone ht. and density in periodontitis and judging the degree of healing and remineralisation of periapical lesions after endodontic therapy. (The technique is standardized only for small intraoral views and not yet for the larger extraoral radiographs)

SINGLE STEP WIRED SYSTEM:
" Using CCD (charged coupled device) or CMOS(C??? metal oxide semiconductor)
" On CCD or CMOS, the ionising radiation that falls are converted to electrical signals which are displayed on computer screen in few seconds. The sensor in rigid plastic case is attached to a computer.

TWO STEP WIRELESS SYSTEM:
" PSP (phosphate storage phosphor) plates are used in place of conventional film.
" Imaging plate is thinner and more flexible than that in previous technique. After exposure, the plate is inserted into a machine that scans this with laser and converts the latent image into visual image.
" Image is got in 30 secs - 2.5 minutes depending on the speed of the system.
" The sensor plate is discharged before reuse.

LATEST DEVELOPMENTS:

TUNED APERTURE COMPUTED TOMOGRAPHY:
Several digital images are taken at slightly different angles and the computer recombines the resultant data to provide data about depth. The various layers are brought into focus and depth of lesions and the relationship between structures.

SCANORA PANORAMIC RADIOGRAPHY:
Special image intensifier to produce 3D data of cylindrical volume of tissue. It is called as "ortho-cubic CT"
IT HAS THE ADVANTAGE OF VERY LOW RADIATION DOSE.

CONVENTIONAL OR PLAIN TOMOGRAPHY:
Image of thin layer of tissues. Tissues superficial and deep to the layer under scrutiny are blurred out due to movement of X-ray tube and film.

PRIMARY USE:
" Detailed evaluation of osseous structures of TMJ.
" Assessment of bone prior to implant placement.
" Cross Sectional view of jaws(determines the relationship between lesion and apex of the tooth, evaluation of integrity of the buccal or lingual cortical bone)
Note:For large and complex lesions like facial fractures or tumours CT or MRI is used.

COMPUTED TOMOGRAPHY:
" Imaging thin thickness of tissues in various planes. Mostly in axial view but coronal view is also used.
" X-ray source and scanner move around the region to be scanned with the patient lying on the table.

Slice thickness:
" 10mm - body and brain.
" 5mm - through head and neck.
" 1-2 mm-TMJ*
" 3D reconstruction will require 1.0mm to 1.5 mm.
uses:

Bone windowing refers to the technique of viewing the bone structures more clearly.
Soft tissue windowing refers to the technique of viewing the soft tissues more clearly in the scan. (The bone structures appear uniformly white)
These methods DO NOT require rescanning.

MAGNETIC RESONANCE IMAGING:
" This is a non-invasive virtually safe (there is evidence of teratogenicity in pregnant mice), technique that uses electrical signals, magnetic field and radio frequency.
" When a magnetic field is applied, the protons are aligned along the magnetic field.
" Then RF pulse is applied and the equilibrium is destabilised; altered according to orientation and magnetic moment.
" Then RF removed. Protons gradually return to equilibrium giving excess energy as radio signals that can be detected and converted to visible image. This return of equilibrium is called as RELAXATION TIME.
The sequence is repeated many times in forming an image. The time after which the radio pulse is repeated is TR (time of repetition)
" TE (time of echo) is the time of signal detection depending on the various tissues to be highlighted.
T1 relaxation-energy release from proton to immediate environment.
" TR and TE short - T1 weighted image
" Fat- bright
" Fluids and muscle-intermediate.

Bones produce dark signals in MRI due to relative paucity of hydrogen protons (nevertheless some information like alteration in bone marrow can be obtained)

T2 relaxation-there is interaction between adjacent protons.
" TR and TE long- T2 weighted image
" Fluids-bright
" Fats-darker.

Contrast agents like gadolinium diethlenetriamine penta acetic acid (DTPA) allows even more tissue differentiation because certain tumours to enhance (produce bright signal) in characteristic way due to increased blood flow.

USES:
" Evaluation of soft tissues.
" Due to signals from flowing blood MRI is used to evaluate the blood vessels.
" 3D MRI angiography rivals the conventional angiography with the advantage of NO CONTRAST MEDIUM,
" Assessment of joints-including ligaments (normal and torn), menisci, and synovial membrane proliferation can be studied by MRI.

USES IN DENTISTRY:
" Evaluation of various pathologic lesions and TMJ assessment.MRI can accurately depict location, morphology and function of articular disc. So internal derangements can be assessed.
" Information on joint effusion and pannus formation can be obtained and also some osseous changes evaluated.
" TMJ MRI is done in THIN 3MM SLICES.*

TMJ MRI technique:
Closed and open mouthed view- oblique sagittal plane. Section oriented perpendicular to long axis of condyle.
The sagittal images - evaluate disc position with respect to head of condyle.

On the sagittal view, with the mouth closed the posterior band is located at the 12 o'clock position, directly on top of the condyle (Harms, 1999; Kaplan et al., 2001).*

The disc is considered to be in normal location when the posterior band is superior to the condyle (12'o'clock position) when the mouth is closed. There is no complete agreement about how far the disc must be before the anterior displacement is diagnosed.
In open mouth view the disc is between the condyle and articular eminence (normal or reducing) or remain anterior to condyle (non reducing)

ADVANTAGES:
" Imaging soft tissue in any plane.
" NO ionising radiation and VITUALLY SAFE (there is evidence of teratogenicity in pregnant mice).

DISVANTAGES:
" Trained technologist or radiologist.

CONTRAINDICATIONS:
" Demand type cardiac pace maker.
" Ferromagnetic metallic objects in strategic places (aneurismal clips in brain, metallic fragments in eye)

PROBLEMS:
" Claustrophobic in the magnet and may need to be sedated.
" Patient who cannot remain motionless for long periods are not suitable candidates for MRI.

ULTRASOUND (US)
Non invasive and inexpensive imaging for superficial tissues.
Great deal of interest in imaging salivary gland.

Research is on standardising the various tumours and other pathologic conditions of parotid glands.
" Categorise lymph nodes in neck as normal, reactive or metastatic
" Carotid artery stenosis evaluation.
" For evaluation of some joints of body for inflammation, tear in ligaments and tendons and other abnormalities.
" But US is not useful for TMJ derangements as of now.

RADIONUCLIDE IMAGING:
Radioactive isotope injected IV depending on material, specific tissue takes it up and can be assessed.
" Technetium [Tc] 99m labelled iodine- thyroid
" Technetium 99m pertechnetate- salivary glands.
" Tc 99m methylenediphosphonate [MDP] - bone.
" Gallium 67 citrate- inflammation and infection of bone.

Gamma camera is used at various times after injection. High concentrations of isotopes are shown as "hot spots". And are indications of high metabolic activity. This can also detect the metastasis of bone or fibrous dysplasia in active phase.
Periapical and periodontal infection also take up tracer and show as hot spots and must be differentiated from various pathology of the jaws.

A variation of bone scintigraphy can be used to quantify bone called as single- photon emission computed tomography(SPECT). Gamma rays given out by Tc MDP are detected by rotating camera and data processed and in one plane and then reconstructed in other planes. Volumetric measurement can be used to assess the metabolic function and tissue activity.

CONTRAST ENHANCED RADIOGRAPHY:
Major use in dentistry:-
" arthrography
" sialography.
ARTHROGRAPHY:
" Radio opaque material injected in lower or upper joint space under fluoroscopic guidance. Once dye in place-fluoroscopic assessment of joint in motion to locate shape and function of disc.
" Then X ray and Tomography is performed.
" Arthrography is virtually replaced by MRI since arthrography is difficult to perform and is invasive.

SIALOGRAPHY:
" Dye injected into duct and the ductal system is assessed for obstructions and other pathosis.
" This is the gold standard for assessment of salivary component of Sjogren 's syndrome. (It is rivalled by US now the protocol is US followed by sialography.)

Note: CT, MRI or US is used more than sialography for evaluation of salivary glands.

IMAGING PROTOCOLS
OROFACIAL PAIN
1) Rule out tooth pain-IO or OPG

2) Maxillary sinus -
Discontinuity of bony margins, thickening of mucous membrane, partial or total opacification of antrum and presence of mucous retention cyst - OPG (best)
Full imaging of sinuses is by CT

3) TMD-clinical exam enough. But when
a) Bone pathosis of TMJ is suspected or when the pain is refractory to Rx then osseous structure evaluation by
1. OPG(only gross abnormalities evaluated)
2. Conventional tomography(ob sagittal view corrected for condylar angle)
3. Coronal or frontal tomography.
b) Internal derangement evaluated by MRI (non invasive) or arthrography (invasive)

4) Cortical lesion- CT or MRI of skull.

SALIVARY GLAND DISEASES:
1) Obstructive diseases-
" Plain radiograph. But 20%of sialolith in submandibular gland and 40% of sialolith in parotid is not calcified fully and are radiolucent.
" Occlusal radiograph-submandibular gland duct sialolith is diagnosed(CS view)
" Need less exposure time because Sialolith are less calcified than teeth and bone.???
" Periapical radiograph with film in vestibule is used for sialolith in parotid gland.
" Sialography-thorough evaluation of the ductal pattern. Main indications for inflammatory diseased and ductal pathosis. Space occupying lesions are evaluated by other means.

2) Tumours of salivary glands.
" CT,MRI and US
" CT is choice-evaluation of extent of mass.
" MRI delineates the internal structure of tumour and demonstrates extension into adjacent tissue.
" US-used to differentiate solid lesions from cystic lesions.

3) Salivary component of auto immune disease Sjogren's syndrome- sialography
The collection of contrast medium is called as sialectasis and is termed as punctuate (<1mm) and globular (1-2mm).
" Advanced gland destruction can be known by the extraductal contrast medium.
" (As mentioned earlier US is rivalling the sialography as its non invasive and inexpensive. Not high accuracy; but useful when sialography cant be done)
" The current recommendation is to do US followed by sialography.

4) Salivary gland scintigraphy using Tc 99m pertechtenate for evaluation of function of all salivary glands simultaneously.
" Not known how good to evaluate xerostomia.
" Not useful to determine if the patient will respond to pilocarpine after radiotherapy induced salivary dysfunction.


JAW LESIONS:

" Well defined lesion-std X-rays followed by biopsy.
" Large lesions, causing jaw expansion, indistinct and irregular margins-require additional information on the extent and relation to adjacent tissues.
" If malignant then evaluation for metastasis needed.-CT for lymph nodes of neck. (US is said to differentiate metastatic from normal and reactive nodes)

CT and MRI in some cases needed.
CT and MRI are of more value in treatment planning than in diagnosis since appropriated Treatment is predicted based on extension including invasion.

RISK AND BENEFIT:
Selection of imaging technique that has low dose radiation still capable of providing required information.

1) Expansile lesion-in mandible
OPG, supported with occlusal view. Also be visualised by plain film radiograph, CT, MRI or combination.
2) If lesion is contained and defined-OPG adequate.
Large and poorly defined-CT mandatory.

If CT or MRI will not affect diagnosis or Tx planning then it is waste of money and time to ask for it.

All in except US and MRI (perhaps) there is risk of radiation

Current practice is to covert absorbed dose from the radiation of effective dose which is quantity converted for radiation type and dose and for radio sensitivity of tissues. So expressed to limited portion of body, equivalent in terms of detriment to a smaller dose to entire body.

For different techniques the effective dose(ED) in mSv and the equivalent normal dose (END) in days are given below.
The first value gives the effective dose (ED) in mSv and the second value gives the equivalent normal dose (END) in days
1) Full mouthed IO survey
a) Round collimated d-speed- 150 mSv = 18.8 days
b) Rectangular collimated e-speed- 33 mSv = 4.1 days

2) OPG - 26 mSv = 3.3 days

3) CT
a) Maxilla - 104-1202 mSv = 13.0- 150.3 days
b) Mandible -761-3324 mSv = 95.1-415.5 days
c) Skull - 220 mSv = 27.5 days

Note:
The points followed by the * mark are NOT from the Burkett's Oral medicine and diagnosis 10 Ed text. . They have been included to facilitate a better understanding.


Note: The above notes are first in the series of revision notes from Burket’s Oral Medicine Textbook 10ed, prepared by Akilesh Ramasamy, RxPG dental editor.



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