7.  Extraoral radiographic techniques

Q. 2. Write in brief the radiographic technique used for viewing the maxillary sinus.

Ans. The various radiographic techniques used for viewing the maxillary sinus are:

i. Standard occipitomental projection (0° OM)

ii. Modified method (30 ooccipitomental projection)

iii. PA Waters’ view

iv. Bregma–menton view.

The various radiographic techniques used for viewing the maxillary sinus are described in detail below:

Standard occipitomental projection (0° OM)

This projection shows the facial skeleton and the maxillary antra, and avoids superimposition of the dense bones of the base of the skull. It is especially useful to detect middle third fractures (Le Fort I, II, III, zygomatic complex, nasoethmoidal complex, orbital blowout) and coronoid fractures.

The cassette is placed perpendicular to the floor with its long-axis of cassette positioned vertically.

The patient should be positioned such that the midsagittal plane should be vertical and perpendicular. Only the nose and chin should touch the cassette. The head is tipped back so that the radiographic baseline is at 45° to the film.

The central ray is directed horizontally through the occiput.

Exposure parameters are kVp—65, mA—10, seconds—2–3.

ii. Modified method (30 ooccipitomental projection)

This projection shows the facial skeleton, from a different angle enabling certain bony displacements to be detected. It is useful in detecting middle third fractures (Le Fort I, II III) and coronoid process fractures.

The cassette is placed perpendicular to the floor in a cassette holding device. The long-axis of the cassette is positioned vertically.

The patient is positioned such that the midsagittal plane is vertical and perpendicular to the cassette and the head is centred so that the nasion is in the centre of the cassette. Only the nose and chin touch the cassette; the head is tipped back so that the radiographic baseline is at 45° to the film.

The central ray is directed 30° to the horizontal, centred through the lower border of the orbit.

Exposure parameters are kVp—65, mA—10, seconds—2–3.

iii. PA waters’ view

This projection is primarily used to demonstrate the maxillary sinus, frontal, and ethmoidal sinuses. The sphenoidal sinuses can be seen if the patient is asked to open his mouth, where by the sphenoidal sinuses are projected on the palate.

The orbit, frontozygomatic suture, nasal cavity, coronoid process of the mandible and the zygomatic arch are also seen.

The cassette is placed perpendicular to the floor in a cassette holding device. The long-axis of the cassette is positioned vertically.

The patient is positioned such that the midsagittal plane should be vertical and perpendicular to the plane of the film and the patient’s head is extended so that only the chin touches the cassette. The cassette is centred around the acanthion (anterior nasal spine).

The canthomeatal line should be at 37° to the plane of the film and the line from the external auditory meatus to the mental protuberance should be perpendicular to the film.

Waters (1915) specified that the tip of the nose should be 0.5 to 1.5 mm away from the cassette. Mahoney (1930) found that the petrosal shadows can be correctly placed by adjusting the orbitomeatal line at 37° to the horizontal.

The patient’s head is extended as far as comfortable, to make the lower border of the mandible as parallel to the cassette as possible. Only the chin touches the cassette. The canthomeatal line should also be approximately parallel to the plane of the film.

The central ray enters at the bregma and exits at the menton.

Exposure parameters are kVp—65, mA—10, seconds—2–3

iv. Bregma–menton view

This projection is primarily used to demonstrate the walls of the maxillary sinus (especially in the posterior areas), the orbits, the zygomatic arches and the nasal septum. It also demonstrates medial or lateral deviations of any of the mandible.

The cassette is placed perpendicular to the floor in a cassette holding device. The long-axis of the cassette is positioned vertically.

The patient is positioned such that the midsagittal plane should be vertical and perpendicular to the plane of the film.


Q. 3. Give the radiographic diagnosis of important pathological entities involving the antrum.

Ans. The various pathologies involving the antrum are:

i. Inflammatory changes thickened mucosal sinusitis

a. Acute

b. Chronic

ii. Empyema–fluid levels

iii. Polyps, mucosal retention cysts, and mucocele

iv. Carcinoma

v. Postoperative maxillary cysts

vi. Foreign objects within maxillary sinus, and

vii. Soft tissue calcification like lymph nodes and sialoliths.

Antral diseases and their radiographic appearances are as follows:

I. Inflammatory diseases

a. Acute sinusitis

Acute sinusitis can be caused by:

Upper respiratory tract infection, e.g., common cold.

Trauma, e.g., oroantral communication or a tooth fragment being pushed into the sinus.

Periapical infection of posterior teeth. A single maxillary posterior tooth with chronic apical periodontitis may produce a localized inflammatory response. It is known as ‘periapical mucositis’.

The radiographic picture would be:

Periapical picture depicting antral halo because of resorption and remodelling of antral floor.

A periapical lesion that has resulted in an inward bulging of the sinus floor is characterized by a periapical radiolucency surrounded by a thin opaque line of bone. The radiographic appearance has been called the ‘halo effect’.

Opaque zone at the base of the sinus because of fluid collected in it.

Total opacity of sinus is because of mucosal hypertrophy and fluid in sinus.

Evidence of foreign body when applicable.

b. Chronic sinusitis

Chronic sinusitis can be caused by:

Persistent infection of the sinus.

Continued presence of a foreign body or communication.

The radiographic changes would be:

Irregular thickening of the radiopaque lining on the inner side of sinus because of mucosal hypertrophy.

Shrinkage of the radiolucent cavity of the sinus.

Radiopacity at the base of the sinus cavity due to collection of the fluid.

Round dome-shaped radiopacity seen in the cavity may be because of a mucosal polyp.

Appearance of multiple, smooth, rounded opacities on the sinus walls and floor is common with patients suffering from allergic sinusitis.

Increased thickness of the radiopaque lining of the sinus i.e., thickness of boundary walls.

II. Trauma

Fractures are commonly demonstrated by conventional radiographic techniques but CT is often necessary to show the fracture lines.

a. Nasal fracture

Most injuries affect the paired nasal bones, which are best seen in the lateral skull view.

b. Orbital blowout fracture

In pure ‘blowout’ fractures, the orbital rim is intact with no injury to the globe.

On plain films, the bone fragments are displaced into the superior aspect of the maxillary sinus and/or one end of the single fragment may be in contact with the remaining walls, the so-called ‘Trap door’ appearance, which is represented by a linear radiopacity that extends into the superior aspect of the maxillary sinus. This ‘trap door’ is a hallmark feature of the orbital blowout fracture.

Waters’ view best demonstrates the intact orbital rim together with herniation of soft tissue contents into the maxillary sinus.

Coronal CT scans are the most favoured imaging modality for identifying blowout fracture and evaluating involvement of adjacent tissues.

c. Orbital rim fractures

The Waters’ or Caldwell’s views are usually adequate to demonstrate the integrity of the orbital rims.

Coronal CT may also be used, though the former can also be used to see the frontal sinuses. Besides, an axial CT may be used to evaluate the integrity of the anterior cranial fossa.

d. Zygomatic arch fractures

Zygomatic arch fractures may occur singly or may be associated with either a tripod fracture or a Le Fort III fracture.

The plain film study of choice is the ‘soft tissue’ or low kVp submentovertex or ‘Jug handle’ view.

Axial CT may be of use particularly in complex fractures.

Three-dimensional CT scans have proved helpful in evaluating degrees of displacement.

e. Tripod fractures or zygomatic maxillary complex fracture

The fracture of suggestive bone usually results in radiopacity of maxillary antrum because of the presence of blood.

Type I or nondisplaced or minimally displaced fractures can be visualized in plain films or in Waters’ view.

Type II or segmented zygomatic arch or orbital rim fractures result in subtle rotation of the fragment. The coronal CT is the radiographic technique of choice.

Type III or fractures with substantial rotation or displacement of the fracture fragment are best seen on a coronal CT scan.

Type IV or comminuted fracture with gross rotation or displacement is best seen by coronal CT scan. Three-dimensional CT scans have proved helpful in evaluating degrees of displacement.

f. Transfacial fracture (Le fort fracture)

Le Fort fractures are complex fractures, Le Fort I and Le Fort II involve the maxillary sinus and Le Fort III is a craniofacial disjunction. For such fractures, plain film radiograph is inadequate and scans are the modality of choice for evaluating all transfacial injuries.

III. Benign lesions of the maxillary sinus (Cysts and tumours)

Cysts and tumours of the maxilla and maxillary antrum are space-occupying lesions which increase in size gradually to encroach on the contiguous structures such as walls of sinus or the ostium.

The signs and symptoms then follow. Radiographic analysis provides an immense database to aid in the diagnosis of the sinus lesions.

A panoramic radiograph is useful as a beginning investigation. Maxillary occlusal radiographs and periapical radiographs are also useful in addition to the more sophisticated modalities such as CT and MRI.

A. Cysts

Cysts that develop outside the sinus may expand to produce a bowing inward of the sinus wall.

i. Radicular cysts

The radicular cysts are most common of all cystic lesions and are most prevalent in the anterior maxilla and appear as a rounded or ovoid radiolucency at the root end of a tooth, often demarcated by marginal bone sclerosis.

ii. Odontogenic keratocyst

The odontogenic keratocyst radiographically presents as well-circumscribed radiolucency with smooth margins and then radiopaque borders.

Most of the lesions are unilocular, but larger lesions may be multilocular. They produce buccal expansion rather than palatal. Large maxillary lesions are destructive, may be expansile and usually involve the sinus.

iii. Dentigerous cysts

The dentigerous cysts appear as well-corticated pericoronal radiolucencies exceeding 3.0 mm.

The margins are well-corticated, thin, and smoothly curved. A tooth is an integral part of the dentigerous cyst.

iv. Calcifying odontogenic cyst (Gorlin cyst)

The most common radiologic appearance is of a cystic radiolucency, which may be unilocular or multilocular. Expansion and perforation can be well-demarcated or irregular with characteristic calcifications. The radiopaque foci often are clustered around the occlusal or incisal surfaces of an impacted tooth.

CT and MRI complement conventional radiographs and show that calcifying odontogenic cyst originates as unilocular lesion that may become multilocular with time as CT and MRI display incomplete bony system.

B. Tumours

i. Ameloblastoma

Ninety percent of the maxillary lesions involve the premolar–molar region.

On plain films and CT, the lesion appears as a multilocular (soap bubble) lytic lesion without mineralized components. Sometimes the sinus wall may be destroyed.

ii. Odontoma

Two-thirds of odontomes are found in the anterior and posterior aspects of the maxilla.

Radiographically, the compound composite odontome resembles an accumulation of small, fully formed teeth; whereas, the complex composite odontome appears as an amorphous radiopacity.

iii. Squamous odontogenic tumour (benign epithelial odontogenic tumour)

This rare, benign odontogenic tumour occurs more often in the maxillary lateral canine region presenting as a triangular or semi-circular radiolucency with in the alveolar bone between the roots of several teeth. Additionally, there is displacement of one or both the adjacent roots, destruction of crestal bone and a sclerotic rim at the margin of the lesion.

iv. Cementoma or periapical cemental dysplasia

These are benign lesions that arise from cementum that surrounds the tooth root.

Periapical cemental dysplasia begins as a radiolucent lesion but gradually calcifies to appear as a radiopaque mass separated from tooth root by a radiolucent zone.

The ‘gigantiform cementoma’ appears as nodular, irregular-shaped radiopacities in multiple locations.

v. Benign cementoblastoma or true cementoma

Radiographically, benign cementoblastoma appears as well-defined radiopacity attached to the tooth root with loss of outline of the affected root.

vi. Odontogenic myxoma

The radiographic appearance of myxoma is variable. The lesion may have a ‘mottled’ or a ‘honey comb’ appearance, or it may present as an expanding radiolucency with an occasional multilocular pattern.

IV. Malignancy of maxillary sinus

Squamous cell carcinoma

A sinus opacity and in most cases, antral wall destruction with adjacent bony involvement is pathognomic of maxillary sinus carcinoma.

Besides the conventional views, 3–5 mm contiguous section of CT scan permits accurate evaluation of tumour extension.

The primary pathologic and imaging feature of squamous cell carcinoma is the propensity to destroy bone even in the presence of a relatively small mass.

V. Antroliths

An antrolith is a calcified mass in the maxillary sinus or antrum.

Radiographic features

These are the small opaque bodies of varying sizes generally found in the bases of the sinus.

Generally, the antroliths are of homogeneous density, and rarely, they may have a more radiopaque area around. They usually have an irregular border.


Q. 4. Describe the principle, procedure, indications, and limitations of panoramic radiography.

Or

Discuss the procedure, advantages, and disadvantages of panoramic imaging.

Ans. Panoramic radiography is a radiographic procedure that produces a single tomographic image of the facial structures including both maxillary and mandibular arches and their supporting structures.

Principle

If the film moves at a speed that follows the moving projection of a certain point, this point will always be projected on the same spot on the film and will not appear unsharp.

In the OPG, the film is attached to a rotating system and moves in the same direction as the beam. The film is given the correct speed by opposing this movement with a contrary movement relative to the beam.

Procedure

Explain the procedure to the patient.

Make the patient wear a lead apron without a thyroid collar, and remove all objects from the head which will interfere with film exposure. Also have the patient remove jacket or bulky sweater; this allows more room between the bottom of the cassette holder and the patient’s shoulder.

Load the panoramic film in the darkroom and cover the bite block with a disposable plastic cover slip.

Set the exposure factors and adjust the height of the machine to accommodate the patient.

Instruct the patient to sit or stand with the back straight and erect, and ask him to bite on the plastic bite block. The upper and the lower front teeth must be placed in an end-to-end position in the groove of the bite block.

The midsagittal plane should be perpendicular to the floor and aligned with the vertical centre of the chin rest, and the Frankfort plane should be parallel to the floor, thus obtaining the correct position for the occlusal plane. The patient’s head is tilted downwards so that the tragus ala line is 5° down and forward.

If the patient has a low palatal vault, increase the occlusal plane angulation slightly. If the patient has a high palatal vault, decrease the occlusal plane slightly. The indicator lights in the machine help as a guide and the patients head should be immobilized by the head band.

Centre the lower border of the mandible on the chin rest and equidistant from each side.

Instruct the patient to position the tongue on the palate and ask him to remain still while the machine is rotating during exposure. Also explain that the cassette holder will not strike him, although it may gently rub his ear and head at the limits as of the excursion.

After the exposure is complete the film is subjected to routine processing.

Indications

As a substitute for full mouth intraoral periapical radiographs.

For evaluation of developmental anomalies and tooth development for children during the mixed dentition period as well as TMJ dysfunctions.

To assess the patient for and during orthodontic treatment.

To establish the site and size of lesions such as cysts and tumours in the body and ramus of the mandible.

For progress of pathology and follow-up of treatment, or postoperative bony healing.

Prior to any surgical procedures such as extraction of impacted teeth, enucleation of a cyst etc.

For detection of fractures of the middle third and the mandible following trauma.

In case of periodontal disease for an overall view of the alveolar bone levels.

Assessment for underlying bone disease before constructing complete or partial dentures.

Evaluation of the vertical height of the alveolar bone before inserting osseo-integrated implants.

Advantages

OPG is an extraoral procedure, which is convenient for the patient and requires a minimal amount of patient’s cooperation

Useful in patients with trismus and gagging problems. Most units can be operated without radiation to demonstrate to the patient what the procedure will be like before the actual exposure will be made. It virtually eliminates problems with gaggers, patient with trismus, and fearful or uncooperative children.

Time required is minimal compared to a full mouth intraoral periapical radiographs.

Radiation dose to the patient is relatively low when compared with conventional full mouth intraoral radiography.

Patient education: OPG films are a valuable aid in patient education and case presentation.

Conditions such as impactions, eruption patterns of teeth, the need for replacement of missing teeth, and fractures are more easily illustrated on panoramic views.

Size of the area radiographed: A broad anatomic region is imaged. The OPG covers an area that includes the entire mandible from condyle to condyle and maxillary region extending superiorly to the middle third of the orbits. Areas such as condyles, inferior border, angle and ascending ramus of the mandible, and entire maxillary sinus that are not visualized in intraoral surveys are seen routinely on OPG.

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Apr 11, 2016 | Posted by in Orthodontics | Comments Off on 7.  Extraoral radiographic techniques
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