Principle of Panoramic

Radiography

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• The first to describe the principles of panoramic radiography were Paatero and, working independently, Numata.

• Two adjacent disks rotate at the same speed in opposite directions as an xray beam passes through their centers of rotation . Lead collimators in the shape of a slit, located at the x-ray source and at the image receptor, limit the central ray to a narrow vertical beam. Radiopaque objects A, B, C, and D stand upright on disk 1 and rotate past the slit. Their images are recorded on the receptor, which also moves past the slit at the same time..

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Movement of the film and objects (A,B ,C, andD) about two fixed centers of rotation. Pb, Lead collimator

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• The objects are displayed sharply on the receptor because they are moving past the slit at the same rate and in the same direction as the receptor. This causes their moving shadows to appear stationary in relation to the moving receptor.

• Other objects between the letters and the center of rotation of disk 1 rotate with a slower velocity and are blurred on the receptor. Any objects between the x-ray source and the center of rotation of disk 1 move in the opposite direction of the receptor, and their shadows are also blurred on the receptor.

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Movement of the film and x-ray source aboutone fixed center of rotation. Pb, Lead collimator.

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• The same relationship of moving film to image is achieved if disk I is held stationary and the x-ray source is rotated so that the central ray constantly passes through the center of rotation of disk 1 and, simultaneously, both disk 2 and the lead collimator (Pb) rotate around the center of disk 1.

• In this situation, as before, the objects A through D move through the x-ray beam in the same direction and at the same rate as the receptor.

• To obtain optimal image definition, it is crucial that the speed of the receptor passing the collimator slit (Pb) be maintained equal to the speed at which the xray beam sweeps through the objects of interest

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• A patient in the place of disk 1, and objects A through D represent teeth and surrounding bone.

• In practice, the center of rotation is located off to the side, away from the objects being imaged. During the exposure cycle, the machine automatically shifts to one or more additional rotation centers. The rate of movement of the receptor behind the slit is regulated to be the same as that of the central ray sweeping through the dental structures on the side of the patient nearest the receptor.

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Movement of the film and x-ray source about a shifting center of rotation. Pb, Lead collimator.

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• Structures on the opposite side of the patient (near the x-ray tube) are distorted and appear out of focus because the x-ray beam sweeps through them in the direction opposite that in which the image receptor is moving.

• In addition, structures near the x-ray source are so magnified (and their borders so blurred) that they are not seen as discrete images on the resultant image. These structures appear only as diffuse phantom or ghost images.

• Because of both these circumstances, only structures near the receptor are usefully captured on the resultant image.

• Structures located more centrally in the body relative to the jaws, such as the hyoid bone and epiglottis, appear on the right, left, and sometimes central areas of the final image.

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• The Image layer is a three-dimensional curved zone (or focal trough) in which the structures lying within the layer are reasonably well defined on final panoramic image.

• The structures seen on a panoramic image are primarily those located within the image layer.

• Objects outside the image layer are blurred, magnified, or reduced in size and are sometimes distorted to the extent of not being recognizable.

• The shape of the image layer varies with the brand of equipment used.

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Focal trough--- The closer to the center of the trough (dark zone) an anatomic structure is positioned, the more clearly it is imaged on the resulting radiograph.

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Errors in panoramic radiography

Common errors in panoramic radiography

Due to faults in technique:-

Errors in patient positionening ,

Errors in film exposures and processing errors.

Errors in handling the film .

Errors in patient positioning

1) Anterior teeth positioning errors

2) Midsagittal plane positioning errors

3) Occlusal plane positioning errors

4) Spinal column positioning errror

5) Patient’s shoulder touching the cassette holder during the movement

6) Distortion due patient movement

Anterior positioning • For anterior teeth positioning, insert a bite block

between incisal edges of maxillary and mandibular teeth

• Advantages of using bite block :

a.Places the teeth in image layer

b.Prevents overlapping vertically

c. Prevents undue unsharpness,widenening or narrowing of anterior teeth

Patient positioned too far forward:

Features:- Narrow blurred anterior teeth, superimposition of spine over the ramus.

Correction :-Use bite block. Line up the incisal edge of the teeth with

notch, Ask edentulous patients to bite about 5mm

behind the groove on the block

Patient positioned too far back Feature:-Wide blurred anterior teeth

Correction ;-Use bite block. Line up the incisal edge of the teeth with notch

Occlusal plane positioning :-

The patients head should be tilted downwards so that the ala tragus line is about 5 degree downwards,this will position occlusal plane slightly downwards towards the floor.

In this plane patients franfurt’s horizontal plane , the plane intersecting the lower orbit of the eyes and ear opening ,is alighned horizontally with floor level.

Chin positioning too low

Feature :-• Excessive curving of the occlusal plane:loss of image

of roots of lower anterior teeth • Narrowing of intercondylar distance and loss of head

of condyle at top of film.

Correction;-Tip chin down , but ala-tragus line should not exceed

5-7 downwards. Use chin rest

Chin positioned too high

Features • Flattening or reverse curvature of occlusal plane.• Loss of image of the roots of upper anterior teeth • Lenghtening of intercondylar distance and loss of

head of condyles at the edge of the film hard palate shadow superimposed on apices of maxillary teeth.

Correction Tip chin down 5-7degrees Use chin rest

Midsagittal plane positioning error • When the midline of the head is not in center of the

unit, the rami and posterior teeth are unequally magnified .

• The image of the structures farthest from the film are magnified ,whereas,on the oppposite,the image of the structures closest to the film are decreased in size.

Patient twisted Features• Unequal right –left magnification • Severe overlap of contact points and blurring

Correction • Line up the patient’s midline with middle of

incisal bite guide. • Close side guides

Slumped position

Features :-• Ghost image of cervical spine superimposed on

midline of the film

Correction :-Stand up machine –have patient step forward ,or

place feet on markers.

Spinal column positioning errors

Incorrect vertebral column positioning results in underexposed area in middle portion of film .

This happens because if spine is not kept erect,the radiation is excessively absorbed by the spinal column and surrounding soft tissue sof the neck, resulting in low density area around the lower center of the film.

spine-shadow ghost

Chin not on the chin rest

Features :-• Sinus not visible on the film• Top of condyle is cut off• Excessive distance between inferior border of

mandible and lower edge of the film.

Correction :- Position chin on chin rest

Tongue not on palate

Features :-• Relative radiolucency obscuring the apices of

maxillary teeth• Palatoglossal air space

Correction :- Ask the patient to swallow or suck on the tongue and

cheek during exposure

Lips open

Features• Relative radiolucency on coronal portion of upper and

lower teeth.

Correction Ask the patient to swallow or suck on the tongue and

cheek during exposure.

Patient movementFeatures :-• Wavy outline of the cortex of inferior border of the

mandible• Blurring of image above the wavy cortical outline

Correction :-Ask the patient to hold still and explain the function of

the machine to avoid startling the patient

Errors in handling the film

Static electricityFeatures• Lightning like radiolucency.• Dot like radiolucencies.• Other patterns are herring bone, star burst or smudge.

Correction :- Dry air in darkroom can be humidified with humidifier

or large bowl of water.Avoid rapid pulling of film from envelope type cassette.Use antistatic carpet.

Panoramic Machines • A number of companies manufacture high-quality film

based and digital panoramic machines. • The Orthopantomograph 100 (Instrumentarium), the

Orthophos Plus (Sirona), the Orthoralix S (Gendex) Division, Dentsply International), and the ProMax (PLANMECA) .

• Panoramic machine are all highly versatile. In addition to producing standard panoramic images of the jaws, they have the capability of adjusting to patients of various sizes as well as making frontal and lateral images of the TMJs. These machines also are capable of producing tomographic views through the sinuses and cross-sectional views of the maxilla and mandible.

References

• Panoramic radiology,2nd edition, langland and langlais

• Oral radiology ,white and pharaoh,5th edition • Oral radiology ,white and pharaoh,3rd edition • Exercises in oral radiology and interpretation,4th

edition,R.P.Langlais