Lecture 6 - Extraoral Imaging

Question 1

When should a CBCT or a Pano be taken?

Answer 1

To evaluate the teeth when there is no need for high resolution and/or sharp detail

• Trauma
• Third molars
• Disease
• Anomalies
• TMJ
• Pt can’t tolerate intramural radiographs
• Sinuses

Question 2

Advantages of panos vs FMX?

Answer 2

Broad coverage of facial bones and teeth

Low radiation dose - ~16 microSV vs 35-170 microSV

Quick and convenient

Question 3

Disadvantages of panos.

Answer 3

Lower resolution

Magnification is unequal

Some image superimposition

Question 4

What is the focal trough?

Answer 4

A zone of sharpness

• The nearer a structure is to the middle of the focal trough, the clearer the resulting image
• Structures outside the focal trough are blurred, magnified, reduced in size, or distorted

Question 5

T/F - Pano images are inherently distorted in both size and shape, making measurements highly unreliable.

Answer 5

TRUE

• Influenced by
• Beam angulation
• X-ray source to object distance
• Path of rotational center
• Poistion of object w/in the focal trough

Question 6

What are ghost images?

Answer 6

Objects that are located b/t X-ray source and center of rotation

*Typically blurred, magnified, located opposite the anatomic structure on the film

Question 7

Tell me about pano positioning for a patient.

Answer 7

Incisal edges into notched positioning block

Center mid-sagittal plane

Upright, centered head

Infraorbital foramen line parallel to floor

Spine erect

Tongue to roof

Question 8

Why take intraoral bitewings over extraoral bitewings?

Answer 8

The fine details are still hard to make out/resolution isn’t as good still

Question 9

What is a cephalometric?

Answer 9

Lateral skull film

Question 10

T/F - MRI has NO radiation.

Answer 10

TRUE

Question 11

MRI?

PET?

CT?

Answer 11

MRI - Magnets - NO RADIATION

PET - Nuclear - Gamma rays emitted by a tracer

CT - A lot of radiation - X-ray moving on a fixed axis

Question 12

CBCT. What is it?

Answer 12

Cone beam CT - X-ray beam shaped like a cone rather than a fan

Question 13

What’s a voxel?

Answer 13

3D pixels

Question 14

.4 voxel needs 1 photon to turn it to a latent image.

.2 voxel needs 8 photons to turn it to a latent image (resolution is better though)

.1 voxel needs _____ photons to turn it to a latent image.

Answer 14

64

Question 15

T/F - You can change the size of the X-ray you are taking with CBCT

Answer 15

TRUE

Question 16

T/F - Larger the volume, the higher the dose to the pt.

Answer 16

TRUE

Question 17

T/F - The smaller the voxel size, the higher the dose t often pt.

Answer 17

TRUE

Question 18

T/F - You should use the smallest volume and largest voxel size that will generate the diagnostic image you need in order to keep the radiation dose to the pt as low as possible.

Answer 18

TRUE

Question 19

When do we use a cone beam?

Answer 19

Assessment of the jaws

Facial structures for ortho

TMJ

3rd molars

Question 20

What is the effective dose from CBCT?

Answer 20

6 - 477 microSV

*On average ~200 microSV