Cone Beam Computed Tomography Introduction

May 13

How should a patient be positioned when taking a CBCT?

They sit, stand or lay with their head between x-ray source and detector (head stabilization/immobilization is key)

How does a CBCT rotate?

There is a rotating gantry with x-ray source and reciprocating detector. There is a single rotation around a single fixed axis or rotation center

How does a CBCT capture images?

There are many sequential planar (2D) projection images captured. All several hundred of these basis/fram/raw images constitute the projection data

How do CBCTs get their image generated?

A computer algorithm generates image volume (volumetric data set). They take images by secondary reconstruction into orthogonal planes (axial, sagittal, coronal)

How are x-rays generated in beam geometry

• They are transmitted from the central ray of an x-ray beam at a perpendicular angle

• Non-central rays are incident on detector at non-perpendicular angles

• Image quality is optimal at the center of the beam

  • Anatomy positioned at the beam center will be images best

  • Position region of interest (ex. tooth #8) at center of scan volume/field of view (ex. anterior maxilla)

  • Quality decreases toward the edges as beam diverges

X-ray generation

The fundamental concepts of x-ray production in CBCT are the same as PAs:

• The beam is filtered and collimated

• Parameters depend on machine-specific, variably adjustable presets

Define focal spot

Area on anode struck by elections from cathode

How can you adjust focal spot?

Fixed/non-adjustable

What is the impact of adjusting focal spot?

Decrease spot size will increase image sharpness

Define tube voltage (kVp)

Voltage applied between anode and cathode

How do you adjust tube voltage? (kVp)?

Preset/adjustable

What is the impact of adjusting tube voltage (kVp)?

• Increase kVp = increase beam energy and penetrability

• Decrease image contrast, x-ray exposure inversely proportional to kVp²

Define tube current mA

Current applied to the cathode filament

How do you adjust mA?

Preset/adjust

What is the impact of adjusting tube current (mA)?

• Increasing the mA increases the number of x-rays

• Increases image low-contrast resolution

• Decreases image noise

Define exposure time (s)

Duration of the current applied to the filament

How do you adjust exposure time (s)?

Present/adjustable

What is the impact of adjusting exposure time (s)?

• Increase exposure time = increase the number of x-rays

• Expressed with mA as mA*s

• x-ray exposure is inversely proportional to mAs

In field of view, what do dimensions depend on?

• Detector size/shape

• Beam projection geometry

• Beam collimation

How is the beam collimated in field of view?

To cover the region of interest —> it limits radiation exposure (dose reduction of 25 to 66%)

What does a smaller field of view mean?

It is associated with better image quality

• Reduced scattered radiation per detector area

• Less beam divergence at scan edges (cone beam effect)

• Increase image resolution (smaller voxel size)

What are the scan parameters for trajectory arc?

180 to 360 degrees. Combined panoramic units often have arcs <360

• Sometimes it is adjustable

• Limited arc is associated with

  • Decreased scan time

  • Decreased radiation dose

  • Decreased projection data (fewer basis images)

  • Decrease image quality (increased noise and artifact)

What are the parameters for scan time?

• Limiting scan time helps reduce motion artifact

• Reduced scan time is associated with

  • Decreased projection data

  • Decreased image quality

How are x-ray signals collected in their detector function?

They record photon attenuation for each basis/projection image

How do x-rays read out?

They send a signal for each basis/projection image to the computer

What is the detector frame rate?

The number of basis images acquired per second. It is limited by detector capacity, not directly selectable or controllable by operator. A high frame rate is associated with

• Increased projection data

• Increased image quality (increased signal-to-noise ratio and decreased artifact)

• Increased radiation dose

• Increased image/volume reconstruction time

Increasing frame rate could be used to reduce scan time

More data (x-rays) =

Better image quality