CBCT Flashcards
CBCT is what form of image processing?
image reconstruction
is CBCT a direct image capture
no, indirect
many images captured and used by software to create 3d
Computed Tomography (CT) defined
Use of computers to generate multiple images of an object from digitized density information obtained from various signals; x radiation (MDCT, CBCT), magnetic fields (MRI), sonar radiation (ultrasound), etc.
forms of xray CT
- Two forms of x ray CT
-CBCT (Cone Beam Computed Tomography)
–MDCT (Multi Detector Computed Tomography)
when did CBCT development begin?
around 2000, EU and japan
pt position options of CBCT
standing or sitting
CBCT machine developed in US
i-CAT FLX
V8, V10, or V17 models (sit down system)
pros of a stand up machine
faster, less space taken up
cons of stand up machine
more likely pt will move
benefit of sit down machine
less pt movement
cons of sit down machine
more space/time
CBCT units quality
- Nearly all CBCT units are good quality high-end
imaging equipment made for a very competitive market
place
why do CBCT unit become outdated
- Become outdated due to competition from competitor brands technical innovations:
- memory
- bit depth
- FOV options
- operating systems
- compatibility with software technologies, etc.
lifetime of typical CBCT unit
6-8 yrs
radiation of CBCT compared to other CT, why?
Less radiation than other forms of CT (i.e. MDCT) for hard tissue imaging because of:
less scanning of X-radiation
lower radiation
CBCT effective dose compared to PSPP,CMOS,CCD
usually more but wide range
why do CBCT dosimetry studies values vary
because Studies are not Directly Comparable!!!
* Variations in experimental methodologies
* Different devices and settings
* Different size FOVs including shapes of FOV
* Differences in high and low resolution scans
* Dosimetry phantoms
* ICRP E1990 vs E2007 (dif calculations)
facotrs affecting CBCT dose
- FOV
- Exposure
- Voxel
- Sensors
CBCT FOV
Flat Panel Detector shape
Cylindrical shape and measurement characteristics
Diameter (mm) x Height (mm)
CBCT FOV
Image Intensifier Detector
Sphericalshape and measurement characteristics
Diameter (mm) only
can FOV be selected? why?
yes, based on diagnostic task
CBCT units and FOV options
CBCT units have capabilities for
scanning a range of FOV sizes
exposure factors of CBCT
- kV ranges between various units 75 kV – 120 kV
- mA 7 mA – 20 mA
- Time – affected by basis image data (# images)
CBCT Image Production
CBCT image capture
rotation of source and receptor around head with many scans occurring each capturing a basis image used for projection data to form volumetric data set
how many basis images are captured
can be hundreds
effect of # of basis images
number of basis images/projection angles increased will improve image reconstruction
CBCT quick scans
can take decreasaed images with decreased image quality when not an issue, allows for decresed dose (ortho)
ways # basis images affects the image
- Image Quality, more = increased detail/quality
- Dose, more = increased dose
voxels
smallest imaging component
smallest area of info we can obtain
make up the FOV
effect of voxel size on res/sharpness
smaller voxels yield greater resolution and sharpness
issues with small voxels
- small voxel yields LOWER signal to noise ratio, i.e., degraded image quality due to wider scatter effect
- small voxel yields high radiation dose
why do small voxels have a lower signal to noise ratio?
more scatter occurs resulting in scatter photons at receptor= blurred image
why do small voxels increase exposure
small voxel yields high resolution but requires more
x-ray photons (higher exposure) to capture signal in
the greater number of the smaller voxels
voxel size for larger FOVs (>5x5cm)
- > 0.2mm = 200μm – for larger FOVs
voxel size for smaller FOVs (less than or = to 5x5cm)
- <0.200mm = 200 μm – for smaller FOVs
current scanners and long scans/processing times for large FOV with small voxels?
Current scanners are not configured for the longer scan times and longer processing times for a large FOV with a small voxel size
how do small voxels increase radiation dose
Smaller voxels need more photon signal which
yields a higher radiation dose
CBCT image detector types
flat panel and image intensifier
cons of the image intensifier
bulkier, heavier, more money to make
pro of image intensifier
decreased dosage
Image Detector Distortion Patterns
image intensfier will have peripheral distortion
not seen in flat panel
peripheral distortion of image intensifier
Distortion of the image grid when moving away
from the center.
Flat Panel Detector
Distortion Patterns
Image receptor area receiving the signal from the
flat-panel detector’s scintillator is flat. Even at more
distant areas from the center of the grid, there is
minimal to no distortion of the grid pattern.
flat panel image vs image intensifier
flat panel with improved quality and higher signal to noise ratio (more sharp)
other names used?
Cone Beam terminology
- CBCT
*CBVT (volume tomograhpy) - DCT (dental)
- MCT (microCT)
- 3DI (3D image)
