CBCT Principles

Question 1

Hounsfield Units HU

Answer 1

CT numbers
represent tissue density
-proportional to the degree to which the material in the voxel has attenuated the X-ray beam
-standardized to the CT number of water

Question 2

HU of bone

Answer 2

above 400

Question 3

HU of soft tissue

Answer 3

40-80

Question 4

HU of water

Answer 4

0

Question 5

HU of fat

Answer 5

-60-100

Question 6

HU of lung

Answer 6

-400 to -600

Question 7

HU of air

Answer 7

-1000

Question 8

Field of View

Answer 8

Scan Volume
width of image
depends on the detector size, shape, beam projection geometry
-cylinder or spherical 
-limiting FOV is based on pt needs

Question 9

Pixel

Answer 9

2d picture element

• discrete unit of info
• id’d by row and column coordinate
• value assigned to a pixel represents the intensity of the image at that location

Question 10

Voxel

Answer 10

3D volume element
-isotropic in CBCT
-3d pixel
size dependent on
focal spot size, X-ray geometric configuration, matrix and pixel size of solid state detector

Question 11

Large Voxel

Answer 11

.4 .3 mm

• lower dose
• smaller data set
• reconstruction time is shorter
• less resolution and detail
• not as sensitive to pt movement due to short scan time
• used for big imaging not small details

Question 12

Small voxel

Answer 12

.1,.15,.2 mm
higher dose
larger data set
reconstruction time is longer
better resolution
sensitive to pt movement
used for limited FOV

Question 13

Axial

Answer 13

Transverse section
perpendicular to both coronal and sagittal sections
parallel to Frankfurt plane
(lower margin of orbit and upper edge of external auditory meatus

Question 14

Coronal

Answer 14

Frontal section
any vertical section perpendicular to the median section
parallel to plane of coronal suture
gives visualization of ostiomeatal complex and nasal cavities

Question 15

Sagital

Answer 15

Median section divides body in two symmetrical halves
any section parallel to median
(oblique sagittal is non symmetric, can be used to look at TMJ)

Question 16

Low density

Answer 16

3d term for radiolucency

“hypo density)

Question 17

High density

Answer 17

3d term for radio opacity

hyperdense

Question 18

Multiplayer reformation

Answer 18

Projected in software as 2d image
can be thickened by increasing numbers of adjacent voxels
different modes including oblique, curved planar, and serial transplanar

Question 19

Curved planar reformat

Answer 19

Long axis of image is aligned with specific structure. Think taking a pan.

Question 20

Serial transplant reformat

Answer 20

Giving multiple image slices as travel down an axis

imagine cutting bread and laying the slices out in order

Question 21

Maximum energy projection

Answer 21

Taking a specific HU and amplifying those tissues

Question 22

2D ray sum reformation

Answer 22

example, creating a cephalopod slab from the cbct

Question 23

Beam Hardening

Answer 23

type of artifact
dark band
think burnout absorption

Question 24

Streak artifact

Answer 24

beam scattered by two dense objects

white streaks

Question 25

Correcting streak ad beam hardening artifacts

Answer 25

smaller FOV
mod pt position
separate dental arches with cotton

Question 26

Motion artifact correction

Answer 26

shorter scan time and head restraint

Question 27

Scanner related artifacts

Answer 27

circular or ring streaks from imperfections in the scanner detection or poor calibration
-persistant circular artifact
correct by callibration

Question 28

Partial Volume averaging

Answer 28

CB related artifact
voxel size is larger than object being imaged
weighted average of the objects in the voxel
lose detail

Question 29

Undersampling

Answer 29

too few basis projections used
misregistration, sharp edges, noising images as a result of aliasing (fine striations)
fix by having more basis projections

Question 30

Cone Beam effect

Answer 30

artifacts in the peripheral portions of the FOV
-due to divergences of the X-ray beam as it rotates in the horizontal plane
noise in the top and bottom

Question 31

Monitor displays

Answer 31

8 bit=256 shades of gray

no matter how good the detector

Question 32

Window width and level

Answer 32

width=range of CT umbers optimized
level is the mean of the range.
by setting it to the appropriate level you can see specific tissues better

Question 33

CBCT

Answer 33

• cone shaped beam
• isotophic voxel
• low effective dose
• sit or stand
• 180-360 rotation arc
• 1 or 2 rotations
• 8-30 sec scan time
• poor soft tissue visualization
• small facial skeleton FOV
• more scatter

Question 34

Medical CT

Answer 34

• fan shaped beam
• anisotropic voxel
• higher effective dose
• supine position
• 360 rotation arc
• multiple rotations
• .5 - 3 sec scan time
• excellent soft tissue visualization
• large body part FOV
• less scatter