CBCT Flashcards
Image reconstruction
Data is utilized by other software programs
and produces different image presentations
Image reconstruction
Data is utilized by other software programs
and produces different image presentations
Computed Tomography (CT)
* Use of computers to generate multiple
images of an object from digitized density
information obtained from various signals;
(3)
x radiation (MDCT, CBCT), magnetic fields
(MRI), sonar radiation (ultrasound), etc.
Two forms of x ray CT
(2)
-CBCT (Cone Beam Computed Tomography)
–MDCT (Multi Detector Computed Tomography)
Nearly all CBCT units are
good quality high-end
imaging equipment made for a very competitive market
place
Become outdated due to competition from
competitor brands
technical innovations:
(5)
- memory
- bit depth
- FOV options
- operating systems
- compatibility with
software technologies, etc.
The lifetime of a CBCT unit is
~6 - 8 years
Benefits of Cone Beam Imaging
Less radiation than other forms of CT (i.e. MDCT) for hard
tissue imaging because of:
(2)
less scanning of X-radiation
lower radiation
1 day of background radiation ~
8.5μSv
skipped
CBCT Dosimetry Study Values Vary
because
Studies are not Directly Comparable!!!
(6)
- 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
Factors Affecting Dose
(4)
- FOV
- Exposure
- Voxel
- Sensors
CBCT FOV
Flat Panel Detector
Cylindrical shape and measurement characteristics
Diameter (mm) x Height (mm)
CBCT FOV
Image Intensifier Detector
Cylindrical shape and measurement characteristics
Diameter (mm) only
Multiple FOV Options
CBCT units have capabilities for
scanning a range of
FOV sizes
Exposure Factors
* kV ranges between various units
kV
mA
75 kV – 120 kV
7 mA – 20 mA
• Time – affected by
basis image data
Multiple basis image projections form the
projection data.
A —- is then constructed by an imaging software program which then
displays various image reconstructions. The latter is dependent on the power of the
programs as specified by the
volumetric data set
software manufacturers.
CBCT Basis-image Capture
Two basis-image capture sequences as the machine
rotates — from
counterclockwise
Position 1 to Position 2
CBCT Image Production
Number of basis images affects image
(2)
- Image Quality
- Dose
skipped
Resolution: Voxel size
0.4mm3 (400um)
0.30mm3 (300 um)
0.20mm3 (200 um)
0.08mm3 ( 80 um)
0.076mm3 ( 76 um)
0.075mm3 ( 75 um)
0.050mm3 ( 50 um)
Voxel size
* 0.3mm = 300μm
* 0.075mm = 75 μm
- small voxel yields —
resolution - small voxel yields
— signal to noise
ratio, i.e., - small voxel yields —
radiation dose
high
LOWER, degraded
image quality
high
Voxel size
* —- – for larger FOVs
* —- – for smaller FOVs
~ < 5cm x 5cm
> 0.200mm = 200μm
<0.200mm = 200 μm
- small voxel yields high resolution but requires more
x-ray photons (higher exposure) to capture signal in
the greater number of the smaller voxels - Current scanners are not configured for the longer
scan times and longer processing times for a large
FOV with a small voxel size - Smaller voxels need more photon signal which
yields a higher radiation dose
CBCT Image Detectors
(2)
charge-coupled image
intensifier detector.
flat-panel detector
Cone Beam terminology
- CBCT
*CBVT - DCT
- MCT
- 3DI
