Two forms of x ray CT -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

CBCT Flashcards by Caleb Houghton

CBCT is what form of image processing?

image reconstruction

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is CBCT a direct image capture

no, indirect
many images captured and used by software to create 3d

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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.

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forms of xray CT

Two forms of x ray CT
-CBCT (Cone Beam Computed Tomography)
–MDCT (Multi Detector Computed Tomography)

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when did CBCT development begin?

around 2000, EU and japan

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pt position options of CBCT

standing or sitting

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CBCT machine developed in US

i-CAT FLX
V8, V10, or V17 models (sit down system)

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pros of a stand up machine

faster, less space taken up

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cons of stand up machine

more likely pt will move

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benefit of sit down machine

less pt movement

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cons of sit down machine

more space/time

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CBCT units quality

Nearly all CBCT units are good quality high-end
imaging equipment made for a very competitive market
place

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why do CBCT unit become outdated

Become outdated due to competition from competitor brands technical innovations: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.

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lifetime of typical CBCT unit

6-8 yrs

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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

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CBCT effective dose compared to PSPP,CMOS,CCD

usually more but wide range

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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)

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facotrs affecting CBCT dose

FOV
Exposure
Voxel
Sensors

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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

1. Image Quality, more = increased detail/quality 2. 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)