10.07 Extraoral plain skull

Question 1

for an extraoral film cassette what is the front surface made of?

Answer 1

the front surface is the tube side, and should be made of thin material with a low atomic number (plastic, Bakelite) to allow as much radiation through as possible

Question 2

what is attached to each cover of an extraoral film cassette, and why?

Answer 2

an intensifying screen with a compression device such as foam between the screen and cover to maintain close contact between the screen and film

Question 3

for an extraoral film cassette, what is the back cover made of, and why?

Answer 3

should be made of heavy metal (lead-lined aluminum) to minimize backscatter

Question 4

intensifying screens are coated with…

Answer 4

plastic intensifying screens are coated with a layer of flat crystals of lanthanum or gadolinium (rare earth screen)

Question 5

what is the purpose of the crystal in the intensifying screens?

Answer 5

when ionizing radiation strikes these phosphors, they absorb the energy which has penetrated through the patient and emit it as a flash of visible light in the green portion of the visible light spectrum
-thus the inten. screen allow for a significant reduction in exposure to radiation bc it is a combo of radiation and visible light which exposes the film

Question 6

why would you take an extraoral plain skull projection?

Answer 6

-when pathology is encountered and you need to get better 3D visualization

Question 7

which extraoral plain skull projection would be best to visualize the medial and lateral wall of the maxillary sinuses? (including the medial wall of the orbit free of superimposition of the petrous ridges of the temporal bone)

Answer 7

-Water’s view (occipitomental)

Question 8

what does Water’s View (occipitomental) permit assessment of?

Answer 8

the zygomatic buttresses, orbital rims, nasal bones, anterior lamina papyracea and ethmoid air cells and frontal sinusus
-only view which projects the anterior and posterior ethmoid air cells separately

Question 9

which is the only view which projects the anterior and posterior ethmoid air cells separately

Answer 9

-Water’s view (occipitomental)

Question 10

Positioning for Water’s View (extraoral plain skull projection)

Answer 10

• sagittal plan perpendicular to film
• chin raised high so canthomeatal line is 37 degrees above horizontal
• central ray should be perpendicular to the film through the midsagittal plane at the level of the maxillary sinus
• source-film distance 36-40’’

Question 11

which projection best demonstrates the midline and posterior facial structures?

Answer 11

Posteroanterior projection

Question 12

projection of choice for demonstration of orbits, frontal and ethmoid sinusues and nasal fossa,

• lamina papyracea well visualized
• good for detecting mediolateral changes in the skull including asymmetry

Answer 12

posteroanterior projection

Question 13

if you wanted to visualize the lower half of the maxillary sinuses, what projection would NOT be used and why?

Answer 13

posteroanterior projection, due to superimposition of the petrous ridges

Question 14

positioning for posteroanterior view/projection

Answer 14

• head is centered in front of cassette with canthomeatal line parallel to the floor, placing the occlusal plane in a horizontal position
• central ray is directed perpendicular to the plane of the film, with a source-film distance of 36-40’’
• source should be coincident with the midsagittal plane of the head at the level of the bridge of the nose

Question 15

which projection would be used to evaluate the anterior and posterior walls of the maxillary, frontal, ethmoid and sphenoid sinuses and the pterygomaxillary buttress?

Answer 15

lateral projection

Question 16

what is the best projection for evaluating an air-fluid level in the maxillary sinuses?

Answer 16

lateral projection

Question 17

which view demonstrates the relationship of the sphenoid sinus to the sella turcica and the nasopharyngeal soft tissues and airway?

Answer 17

lateral projection

Question 18

positioning for lateral projection

Answer 18

• left side of face adjacent to cassette and midsagittal plane parallel with the film
• central ray is oriented toward the external auditory meatus and perpendicular to the midsagittal plane and the plane of the film
• source-film distance is 36-40’’ (standard)

Question 19

what is different for cephalometric projections than lateral projections?

Answer 19

the source-film distance is 60’’ insctead of 36-40’’ and a wedge filter is placed over the anterior side of the beam at the tube head to reduce the intensity of radiation in the region and help reveal soft tissue profile

Question 20

which projection demonstrates the base of the skull, the position and orientation of the condyles, the curvature of the mandible, the lateral wall of the maxillary sinuses, and the pterygoid plates?

Answer 20

the Submentovertex projection (SMV)

Question 21

which view best demonstrates fractures of the zygomatic arch?

Answer 21

Submentovertex projection (SMV)

Question 22

which is the only projection that demonstrates the left and right sphenoid sinuses separately

Answer 22

the submentovertex projection (SMV)

Question 23

positioning for the submentovertex view

Answer 23

• patients head is extended backward with the vertex on the center of the cassette
• the midsagittal plane must remain perpendicular to the floor
• the canthomeatal line should extend 10 degrees past vertical
• the central ray is projected from below the mandible up toward the vertex of the skull, to pass about 2 cm. in front a line connecting the left and right condyles
• fro viewing the sygomatic arches specifically, the exposure time is ruduced to 1/3 that used to visualize the skull.

Question 24

-for viewing the zygomatic arches specifically, what is done compared to a normal SMV projection?

Answer 24

the exposure time is reduced to 1/3 that used to visualize the skull.

Question 25

which projection is the most important plain film for assessing subcondylar fractures as it is the only one which optimally demonstrates the angulation and/or displacement that occurs

Answer 25

Reverse-Towne’s projection

Question 26

Reverse-Towne’s projection

Answer 26

• the most important plain film for assessing subcondylar fractures as it is the only one which optimally demonstrates the angulation and/or displacement that occurs
• alos permits evaluation of the petrous ridges and mastoid air cells and the posterolateral wall of the maxillary antrum

Question 27

positioning for the Reverse-Towne’s view

Answer 27

• head in front of the cassette with the canthomeatal line oriented downward 25-30 degrees. The condylesare better visualized if the patient’s mouth is fully opened
• the central ray is directed toward the film in the sagittal plane through the occipital bone the beam is collimated to the areas of interest to reduce patient exposure and film fog