Computed Tomography

Magdalena Bazalova

CT, CATtomos = slice, graphein = to write

1. What is a CT scanner?

• an X-ray device capable of cross-sectional imaging

• creates images of slices through the patient

What is a CT scanner?

• doughnut shaped gantry with moving patient table

Why CT?

• conventional radiography suffers from collapsing of 3D structures onto a 2D image

• although the resolution of CT is lower, it has extremely good low contrast resolution enabling the detection of very small changes in tissue type

• CT gives accurate diagnostic information about the distribution of structures inside the body

CT scanning applications

• very wide ranging – good for imaging bone and soft tissue – diagnostic imaging– radiotherapy planning

• 3D applications

CT imaging applications

CT imaging applications

CT imaging applications

Why CT for radiotherapy?

• Radiation therapy planning is done on the basis of patient CT images and is therefore patient specific– the target and organs at risk are delineated in CT

images (possibly with help of other imaging modalities – PET)

– dose calculation algorithms use CT images for determination of dose delivered to the patient during treatment

Why CT for radiotherapy?

• Tissue inhomogeneities can be taken into account in most treatment planning systems

• Dose to soft tissue is different than dose to cortical bone - mass density variations between tissue types are the most important factor

• Therefore, mass densities of tissues have to be known for an accurate dose calculation

• CT images do not represent mass densities of patient body directly but they can be converted into mass densities using a calibration curve

2. CT scanner components

detector ring

X-ray tube

X-ray beam

X-ray journey

X-ray tube

Beam shaping filter

Detectors

Detector arrangement

Philips CT simulator

Questions on CT apparatus

• How do we call the device that produces X-ray beam? – (X-ray tube )

• What have the X-rays pass through on their way to the detector ring? – (beryllium window, Al filters, bow-tie filter,

patient, anti-scatter grid)

3. CT image definition and formation

What are we measuring in CT?

• the linear attenuation coefficient, µ, between the X-ray tube and

the detector• the linear attenuation

coefficient is a measure

of how rapidly are X-ray

attenuated

)exp(0 xII

p

-d d

d

-d

• projections

0

lnI

Ip • I, I0 - intensities

x-ray source

2D-projection data set - sinogram

Projection angle

Reconstruction algorithms

• Computer based

• simple back-projection

• filtered back-projection

• iterative techniques

Simple back-projection

• reverse the process of measurement of projection data to reconstruct an image

• each projection is uniformly distributed across the reconstructed image

Simple back-projection

1/r blurring

Filtered back-projection

• simple back-projection produces blurred images

• projection data need to be filtered before reconstruction

• different filters can be applied for different diagnostic purposes– smoother filters for viewing soft tissue– sharp filters for high resolution images

• back-projection is the same as before

Filtered back-projection

Image reconstruction

Simple back-projection

Filtered back-projection FBP

Patient image reconstruction

Patient filtered back-projection

CT number scale

HU represents the linear attenuation of a material.

CT number window

CT number window

CT for radiotherapy – calibration, HU to mass density conversion

• HU do not represent mass density, needed for dose calculation, directly. To obtain mass densities of each voxel:

• A set of tissue equivalent

materials with known

mass densities is scanned

and a calibration curve

is created

Mass density calibration curve

y = 1.03E-03x + 1.03E+00

y = 8.84E-04x + 9.83E-01

y = 5.78E-04x + 1.05E+00

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

1.800

-1000 -500 0 500 1000

HU

mas

s de

nsity

[g/c

m3 ]

Mass density calibration curve

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

1.800

-1000 -500 0 500 1000

HU

mas

s de

nsity

[g/c

m3 ]

Calibration curve for treatment planning

Questions on reconstruction

• How do we call picture and volume elements?– (pixels and voxels)

• What do CT images represent?– (linear attenuation coefficients of voxels)

• How do we call raw detector data?– (a sinogram)

• Name two reconstruction techniques? – (simple and filtered back-projection)

4. CT technology

Third generation CT scanners

Fourth generation CT scanners

Fifth generation CT scanners

Helical CT scanning

Advantages of helical mode

Questions on CT technology

• How many CT generations exist?– 5 (maybe more)

• Which one is the third one?– rotate/rotate

• What are the advantages of helical scanning?– arbitrary image position, faster scanning

5. CT image quality

Image noise

Image noise

Image contrast

Image contrast

Factors affecting image noise

Reconstruction filters

Factors affecting detector signal

Radiation dose

Questions on image quality

• Name three factors that influence image quality.– kVp, mA, time, filteration of the beam, slice thickness,

reconstruction filter, pitch

• Name three parameters that describe image quality– spatial resolution, contrast, noise

• What is noise?– variation in HU in a uniform image

• What is contrast– ability to resolve details without blurring

6. Artefacts in CT

If not recognized, CT artifacts can cause misdiagnosis and incorrect outcomes of radiotherapy treatment planning.

Definition of CT artefacts

Types of CT artefacts

Origin of artefacts

Beam hardening: cupping

Beam hardening: correction

Beam hardening: correction

Reduction of streaks and bands

Partial volume artefacts

Partial volume artefacts

Minimizing partial volume artefacts

Photon starvation

Avoidance using mA modulation

Metal artefacts

Metal artefact reduction

Patient motion artefacts

• Voluntary and involuntary motion cause artefacts in the reconstructed image

Minimizing motion artefacts

Motion artefact correction

Detector sensitivity: ring artefacts

Ring artefacts

CT artefacts: summary

Questions on CT artifacts

• Remedy for photon starvation?– mA modulation, adaptive filtering

• What can cause metal artifacts in patients?– dental implants, surgical clips, electrodes,

prostheses

• In which CT generation can occur ring artifacts?– in the 3rd where detector ring rotates with X-ray

tube