CBCT Principles Flashcards
Hounsfield Units HU
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
HU of bone
above 400
HU of soft tissue
40-80
HU of water
0
HU of fat
-60-100
HU of lung
-400 to -600
HU of air
-1000
Field of View
Scan Volume width of image depends on the detector size, shape, beam projection geometry -cylinder or spherical -limiting FOV is based on pt needs
Pixel
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
Voxel
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
Large Voxel
.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
Small voxel
.1,.15,.2 mm higher dose larger data set reconstruction time is longer better resolution sensitive to pt movement used for limited FOV
Axial
Transverse section
perpendicular to both coronal and sagittal sections
parallel to Frankfurt plane
(lower margin of orbit and upper edge of external auditory meatus
Coronal
Frontal section
any vertical section perpendicular to the median section
parallel to plane of coronal suture
gives visualization of ostiomeatal complex and nasal cavities
Sagital
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)
Low density
3d term for radiolucency
“hypo density)
High density
3d term for radio opacity
hyperdense
Multiplayer reformation
Projected in software as 2d image
can be thickened by increasing numbers of adjacent voxels
different modes including oblique, curved planar, and serial transplanar
Curved planar reformat
Long axis of image is aligned with specific structure. Think taking a pan.
Serial transplant reformat
Giving multiple image slices as travel down an axis
imagine cutting bread and laying the slices out in order
Maximum energy projection
Taking a specific HU and amplifying those tissues
2D ray sum reformation
example, creating a cephalopod slab from the cbct
Beam Hardening
type of artifact
dark band
think burnout absorption
Streak artifact
beam scattered by two dense objects
white streaks
Correcting streak ad beam hardening artifacts
smaller FOV
mod pt position
separate dental arches with cotton
Motion artifact correction
shorter scan time and head restraint
Scanner related artifacts
circular or ring streaks from imperfections in the scanner detection or poor calibration
-persistant circular artifact
correct by callibration
Partial Volume averaging
CB related artifact
voxel size is larger than object being imaged
weighted average of the objects in the voxel
lose detail
Undersampling
too few basis projections used
misregistration, sharp edges, noising images as a result of aliasing (fine striations)
fix by having more basis projections
Cone Beam effect
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
Monitor displays
8 bit=256 shades of gray
no matter how good the detector
Window width and level
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
CBCT
- 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
Medical CT
- 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
