Advanced CT systems and

Their Performance

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DisadvantagesGenerations sourceSource

collimation detectorDetector

collimation

Source- Detector movement

Advantages

single

single

single

single

single

single

multiple

Pencil beam

Fan- beamlet

Fan- beam

Fan- beam

Fan- beam

Fan- beam

Narrow

cone- beam

single

multiple

many

Stationary ring

many

Stationary ring

Multiple arrays

1st Gen.

2nd Gen.

3rd Gen.

4th Gen.

5th Gen.

6th Gen.

7th Gen.

no

yes

no

no

no

yes

yes

Trans.+Rotates Trans.+Rotates Rotates together

Source Rotates only

No movement

3rdGen.+

bed trans.3rdGen.+

bed trans.

No scatter

Faster than 1G

Faster than 2G

Higher efficiency than 3G

Ultrafast for cardiac

faster 3D imaging

faster 3D imaging

slow

Low efficiency

High cost and Low

efficiency

high scatter

high cost

higher cost

higher cost

8th Gen. singlewide

cone- beamFPD no 3rd Gen. Large 3D Relatively

slow

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4th Generation CT ScannersRotate/Stationary

• Fan beam geometry• More than 4800 detectors

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5th generation: Electron Beam CT (EBCT)

- x-ray source is not x-ray tube but a focused, steered, microwave-accelerated EB incident on a tungsten target.

- It has no moving parts .- Target covers one-half of the imaging circle; detector array

covers the other half.- Images in - less than- 50ms.

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Electron Beam CT (EBCT)

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EBCT(CONT’D)

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Spiral (Helical) CT: Reciprocating rotation (A) versus fast continuous rotation using

slip-ring technology (B)

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(A) Pitch =1 (B) Pitch = 2

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MULTISLICE SPIRAL CT

• Introduced at the 1998.• They are based multiple detector.

rows ranging between 8, 16, 24, 32 and 64 depending on the manufacturer.

• The overall goal is to improve the volume coverage speed performance.

• Complete x-ray tube/detector array

rotation in less than 1s.

• Partial scan images can be obtained in approximately 100ms.

256-slice cone-beam CT detector

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REAL-TIME CT FLUOLOROSCOPY

• CT fluoroscopy acquire dynamic images in real time.

• Fast continuous imaging, fast image reconstruction & continuous image display.

• Patient movement is low during Tube rotation.

• Fast image Reconstruction algorithm is required.

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CT ANGIOGRAPHY (CTA)

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CT VIRTUAL REALITY IAMAGING

• The use of virtual reality is the creation the inner views of tubular structures.

• Offers both endoluminal and extra luminal information.

• It reduces complication (eg. infection and perforation).

• Four requirements:– data acquisition

– image processing

– 3D rendering

– image display and analysis.

What is displayed in CT images?

HU1000CT# T

water

water

Water: 0HU Air: -1000HU

Typical medical scanner display:

[-1024HU,+3071HU],

Range: 1224096 12 bit per pixel is required in display.

Hounsfield scales for typical tissues

For most of the display device, we can only display 8 bit gray scale. This can only cover a range of 2^8=256 CT number range. Therefore, for a target organ, we need to map the CT numbers into [0,255] gray scale range for observation purpose. A window level and window width are utilized to specify a display.

WL

-1024

+3071

0255

2/#

2/#2/

2/#

)2/(#

0

scaleGray Displayed

max

max

WLCT

WLCTWL

WLCT

I

IW

WLCT

Windowing in CT image display

Scintillator Properties of CT

• Transparency

• X-ray stopping power

• Light output and efficiency for lower dose

• Primary speed (Fast decay time or quicker response)

• Luminescent afterglow for quicker speed response

• Radiation damage

PRIMARY SPEED• Primary speed is the rise time of the output signal in response to

a constant x-ray input (~10-3-10-6seconds supporting 0.5 second scanning time).

• It is also the time constant for the first component of exponential decay of that output after the input is turned off.

Clinical Significance: • Primary speed is critical to maintaining high resolution during

sub-second scanning. It must be fast enough to prevent blurring especially at the perimeter of the scan FOV.

• A slower primary speed can be seen as shading across high contrast edges, such as the skin-air interface.

AFTERGLOW

• Afterglow is the second time component of the exponential decay of the output after the x-ray source is turned off.

• Clinical Significance: Afterglow will result in arcing artifacts

extending from low attenuation anatomy into areas of higher attenuation. It also decreases in plane spatial resolution.

RADIATION DAMAGE• Radiation damage is the darkening of the material with radiation

exposure over time.

• It results in a gain or a shift in output for a given x-ray exposure.

• It can also cause changes in Z- Axis uniformity. This is especially true for translucent materials.

Clinical Significance: • Radiation damage causes changes in gain that require frequent re-

calibration.

• It can result in changes in Z-axis uniformity which are more severe. These can cause rings or spots especially when scanning anatomy that changes rapidly along Z.

TRANSPARENCY

• A transparent material allows light to be transmitted with very little scatter. Most light rays can pass through with a shorter more direct path for light.

• In a translucent material light rays scatter many times as they travel from the creation site to the photodiode. This longer path length can result in more self absorption, lower net light output, and greater susceptibility to radiation damage.

Clinical Significance:• Transparency results in higher light output, better signal to

noise, better Z-axis uniformity and reduced radiation damage.

X-RAY STOPPING POWER

• Stopping power is defined as the thickness of material needed to stop 98% of incident x-rays in the typical 140 kVp CT beam (~2-3 mm).

Clinical Significance: • The thinner the material needed to stop 98% of the

x-rays, the greater the light output at the diode.

LIGHT OUTPUT

• Relative light signal at the diode for a given x-ray input (~70% at 610nm).

Clinical Significance:• Low light output can result in performance due to

less electronic noise vs. quantum noise for thin slice, large patient, low dose application.

• It can also result in low signal artifacts such as streaking at the shoulders and hips.

EMMISSION SPECTRUM

• The emission spectrum of a scintillator is the relative intensity of light output at a given wavelength.

Clinical Significance: • In addition to diode matching for optimal electronic signal

output, the emission spectrum of a scintillator can impact the design flexibility of detector systems and its long term stability.

– In addition to x-ray radiation, scintillator emission in the photo active range can impact detector aging.

DIODE MATCHING andRELATIVE OUTPUT

• Total signal output is a function of how well the emission spectrum of the detector material and sensitivity spectrum of photo diode match.

• The closer the output spectrum of the detector matches the sensitivity profile of the photo diode, the higher the resultant electrical signal.

Clinical Significance: • Can reduce effective light output with the expected low

signal impacts when scanning large patients with thin slices.

Relative Diode Sensitivity

• HiLight: 60% @ 610nm• Gadolinium Oxysulfide: 40% @510nm• Cadmium Tungstate: 42% @530nm

• Relative Output• HiLight: 60% x 70% = 42%• Gadolinium Oxysulfide: 40% x 80% = 32%• Cadmium Tungstate: 42% x 30% = 13%

Quality criteria for CT images

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The slice sensitivity profile (SSP) 1) For conventional CT and spiral/helical CT.

2) SSP is wider for 360-degree linear interpolation algorithms.

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Scanner performance: technical parameters (I)

• CT Number Accuracy– CT number depends on tube voltage, filtration, object thickness

– CT number of water is by definition equal to 0

– Measured CT number should be < ± 4 HU in the central ROI

• CT Number Linearity– It concerns the linear relationship between the calculated CT number and the

linear attenuation coefficient of each element of the object

– Deviations from linearity should be < ± 5 HU

• CT Number Uniformity– It relates to the fact that a CT number of each pixel in the image of an

homogeneous object should be the same over various regions

– The difference in the CT number between a peripheral and a central region of an homogeneous test object should be < 8HU

– Differences are largely due to beam hardening phenomenon

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• Spatial Resolution– The high contrast resolution determines the minimum

size of detail visualized in the plane of the slice with a contrast >10%.

It is affected by:• the reconstruction algorithm• the detector width• the effective slice thickness• the object to detector distance• the X-ray tube focal spot size• the matrix size.

Scanner performance: technical parameters (II)

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• Spatial Resolution– The low contrast resolution determines the size of

detail that can be visibly reproduced when there is only a small difference in density relative to the surrounding area

• Low contrast resolution is considerably limited by noise.

• The perception threshold in relation to contrast and detail size can be determined, for example, by means of a contrast-detail curve.

Scanner performance: technical parameters (III)

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Scanner performance: technical parameters (IV)

• Slice Thickness

– The slice thickness is determined in the center of the field of view.

– The use of post-patient collimation to reduce the width of the image slice leads to very significant increases in the patient dose

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Axial image of Axial image of an an homogenous homogenous phantomphantom

CT number uniformity

can be assessed at the same time as measuring noise, by placing four additional ROI (N, E, S and W) at positions near the edge of the image of a uniform phantom

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CT number linearity

CT number linearity is assessed using a phantom containing inserts of a number of different materials (materials should cover a wide range of CT numbers

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Low contrast resolution

Typical image Typical image of the Catphan of the Catphan LCR insertLCR insert

Low contrast resolution (or low contrast detectability) is often quoted as the smallest visible object at a given contrast for a given dose

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Spatial resolution (high contrast)

The number of line pairs per cm just visible in the image is approximately equivalent to the 2% value of the MTF

This result can then be

compared with the 2% MTF

The resolution is quoted as the spatial frequency (in line pairs / cm) at which the modulation falls to 50%, 10% or 2% MTF.

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Z-Sensitivity (Imaged slice width)

Plan view of a test object used to measure imaged slice widths for axial scans, to assess the accuracy of post patient collimation, and to calculate the geometric efficiency for the scanner

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Dosimetry - CTDI in air (helical)

Axial slice positionsAxial slice positions

Helical scan (pitch 1)Helical scan (pitch 1)

The Computed Tomography Dose Index (CTDI) in air can be The Computed Tomography Dose Index (CTDI) in air can be measured using a 10cm pencil ionization chamber, bisected measured using a 10cm pencil ionization chamber, bisected by the scan plane at the isocentre.by the scan plane at the isocentre.

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Dosimetry - CTDI in Perspex Phantoms

Head phantomHead phantom Body phantom Body phantom (or annulus (or annulus to fit over haed phantom)to fit over haed phantom)

Insert to plug holesInsert to plug holes

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Dosimetry - CTDI in Perspex Phantoms

Central and peripheral CTDI’s are used to calculate weighted CTDI, CTDIw:

CTDIws can be compared against diagnostic reference levels for standard scan examinations

)( CTDI3

2 + CTDI

3

1

C

1 = CTDI p100,c100,wn

CT Noise Characteristics

• For low mAs values– standard deviation decreases with increasing

mAs

• For higher mAs values– standard deviation stays fairly constant

mAs

StandardDeviation

Transition point mAs should not increase throughout scanner life

CT Noise Characteristics

• Excessive noise can be caused by– detector sensitivity– electronic noise in detector amplifier circuits– reduced output per mAs

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Imaging performance (Noise)

Noise Noise isis generally assessed generally assessed usingusing cylindricalcylindrical phantoms, which are either phantoms, which are either filled with filled with waterwater or made of a tissue or made of a tissue equivalent materialequivalent material

Once an axial image of the phantom has Once an axial image of the phantom has

been acquired, noise is obtained from the been acquired, noise is obtained from the standard deviation in CT number in a standard deviation in CT number in a region of interest (ROI) placed centrally region of interest (ROI) placed centrally within the imagewithin the image

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Imaging performance (Noise)

Region of Region of interest interest (ROI)(ROI)

SNR is dependent on dose, as in X-ray.Notice how images become grainier and our ability to see small objects decreases as dose decreases.

There are some similarities with X-ray. But we also see some important differences.

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Actions that can influence image quality

Avoid bad viewing conditions (e.g. lack of monitor brightness or contrast, poor spatial resolution, etc)

Improve insufficient skill to use the workstation capabilities to visualize images (window level, inversion, magnification, etc).

Reduce artifacts due to incorrect digital post-processing (creation of false lesions or pathologies)

Compromise between image quality and compression level in the images