CT Basic Principles Modified

ImPACT Day: 23rd July 2002

Basic Principles of CT Scanning

ImPACT Day: 23rd July 2002 - Basic Principles of CT Scanning 2

Basic principles of CT scanning

• Basic theory of CT

– What’s inside the gantry?

– Data acquisition

– Image reconstruction

• Variations in scanner design

– Generation

– Detector type

– Number of projections and detectors

– Slip-ring scanning


Why CT?

• Conventional radiography suffers from the

collapsing of 3D structures onto a 2D image

• Although resolution is lower in CT, 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








– Generation

– Detector type




Construction of a CT scanner

Whizzo CT Company

x-ray tube

x-ray detectors

slip rings


TUBE

DETECTORS

APERTURE

In practice









– Generation

– Detector type




Data acquisition

X-ray tube

Slice width Fan beam

Detectors

Collimators

X-ray emission in all directions


What are we measuring?

• The average linear attenuation

coefficient, µ, between tube and detectors

• Attenuation coefficient

reflects the degree to

which the x-ray

intensity is reduced

by a material

x-ray tube

detector

attenuation


Projections

• 2D views - ‘projections’ at angles all the

way round the patient

– sample µ at each

detector to generate

a projection

– rotate tube and

detectors a small

amount and repeat

the measurements atte

nu

ation

detector








– Generation

– Detector type




Back projection

• Reverse the process of measurement of

projection data to reconstruct an image

• Each projection is ‘smeared’ back across

the reconstructed image


Back projection

2 projections 4 projections

8 projections 16 projections 32 projections

Original object


Filtered back projection

• Back projection produces blurred trans-axial images

• Projection data needs 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 imaging

• Back projection process same as before



• Filter applied to projection data

-15

-10

-5

0

5

10

15

20

25

30

-15

-10

-5

0

5

10

15

20

25

30

0 2 4 6 8 10

Frequency (lp/cm)

Am

pli

ficati

on

smooth

sharp



16 projections

4 projections 2 projections 8 projections

64 projections 64 projections

(not filtered)



original

back

projected

image

filtered back

projected

image

Profile

Filtered profile


m to CT number

• Originally measured was the distribution of m

• m values are scaled to that of water to give the

CT number

mtissue - mwater

• CT number = x 1000

mwater

• Water = 0; Air = -1000; Bone = ~1000


CT number flexibility

• We can change the appearance of the image

by varying the window width and level

• This spreads a small range of CT numbers over

a large range of grayscale values

• This makes it easy to detect very small

changes in CT number


CT number window

• Same image data at different WL and WW

WL -593, WW 529 WL -12, WW 400 -1000 HU

4000+ HU

WW WL

0 HU

-1000 HU

4000+ HU

WW WL


CT image

Slice

Width

Picture

Element

(PIXEL)

512

PIXELS

Volume

Element

(VOXEL)








– Generation

– Detector type




Scanner generation

3rd generation

4th generation








– Generation

– Detector type




Detector type

Pressurised xenon gas

Ionisation

Signal

Scintillation

Photon capture

Light

Photo-diode

Signal

XENON SOLID STATE


Xenon detectors

_

+

Xenon Atom

X-ray

Electrical signal

Negative electron Positive xenon ion + -


Electrical signal

Ceramic scintillators

Scintillator

Photo-diode

Visible photon

X-ray


• Xenon - Single detector chamber

sub-divided by electrodes

• Solid state - Detector array made up of

individual elements

Xenon vs. solid state








– Generation

– Detector type




• Lower range scanners:

400 - 600 detectors

• Top range scanners:

650 - 900 detectors (per row)

Typically, for 3rd generation scanners:

Number of detectors


• Lower range scanners:

600 - 1000 per image

• Top range scanners:

1100 - 1800 per image

Number of projections








– Generation

– Detector type




Conventional CT Systems

• • Power to X-Ray Tube via Cord

• • Scan CW and CCW to Wind/Unwind

Cord

• • Tube Rotates Around Stationary Patient

• (Table Position is Incremented Between

Acquisitions)

• • Interscan Delays:

3.5 Seconds Between Slices


Power

supply

Projection

data

Slip ring scanning

Slip Rings Slip-rings


Low voltage

AC

mains

Low

voltage

slip ring

High

voltage

generator X-ray

tube

High voltage

AC

mains

High

voltage

generator

High

voltage

slip ring X-ray

tube

High voltage v low voltage slip rings


• Faster conventional axial scanning

(stepwise table feed)

– Interscan delay governed only by time

taken for table to move to new position

(~ 1 sec.)

Advantages of slip ring scanners


• Cine scanning

(no table feed)

– continuous series of images at one position

• “CT fluoroscopy”

– new image reconstructed several times

during one rotation

Advantages of slip ring scanners

• Spiral scanning

(continuous table feed)


Spiral (Helical) CT Systems

• Power to X-ray Tube via Slip Ring -

Allows Continuous Rotation, No InterScan Delays

• Table Moves as Tube Rotates

• Synthesizing Projection Data via Interpolation


0

z, mm

t, sec

Direction of Continuous Patient Transport

Center of Spiral Path

Width of Spiral Path (From Collimation)

Pitch=1; Contiguous Spiral


0

z, mm

t, sec

Center of Image

Width of Image (From Collimation)

Contiguous Reconstruction - No Overlap

Image Number 1 2 3


0

z, mm

t, sec


Center of Spiral Path

Width of Spiral Path (From Collimation)

Pitch=2; Extended Spiral


0

z, mm

t, sec

Overlapping Reconstruction - 50% Overlap

Image Number 1 2 3


Image Formation

u Collect Projection Data

u These Data are NOT all at same table

position, so:

u Synthesize a set of Planar Projection Data

via Interpolation (interpolate between

views taken at same projection angle, but

different table positions), then

u Use Filtered Back Projection on

Synthesized Planar Data


Selected Image Plane (Arbitrary)

Path of Continuously Rotating X-ray Tube (and Projection Data)

0

z, mm

t, sec



Selected Image Plane (Arbitrary)

Path of Continuously Rotating X-ray Tube (and Projection Data)

0

z, mm

t, sec



Image Formation

u Because a VOLUME of Data is Acquired and

Interpolation is used, Images can be formed

ANYWHERE -->

• Slice Location is Arbitrary

u Therefore, Overlapping Images can be created – (e.g. 10 mm thick, 5 mm apart).

u Slice Thickness Is Determined by Collimation


Index

• Index = Interval at which images are

reconstructed

• (e.g. image collimation of 10 mm with

index of 5 mm means that images are

reconstructed every 5 mm).

• % Overlap = Percent of image that

overlaps with adjacent image. (e.g. 10

mm collimation with 5 mm reconstruction

interval is a 50% overlap).


Data Acquisition

Pitch = Table Movement per Rotation

X-ray Beam Collimation

• Contiguous Spiral

Pitch = 1 (10 mm / 10 mm)

• Extended (Non-Contiguous) Spiral

Pitch = 2 (20 mm/ 10 mm)

• Overlapping Spiral

Pitch = 1/2 ( 5 mm / 10 mm)


Spiral CT - Differences From Axial

Image Quality (Assuming 180 reconstruction

algorithm):

• Effective Slice Thickness Increases with Pitch

(~10% increase for pitch 1; ~ 30% for pitch 2;

~ 70% for pitch 3)

• Hence volume averaging increases with pitch

• Noise is greater than Conventional Axial under

same conditions, but does not change with pitch


Spiral CT - Differences From Axial

Radiation Dose:

For Pitch of 1 ~ Same as Comparable Contiguous

Conventional Scans

For Pitch 1.5 - Approximately 2/3 that of

Contiguous Scans

For Pitch 2 - Approximately 1/2 of Contiguous

Scans

• Radiation Dose Proportional to 1/pitch


Spiral CT Systems

System Requirements:

• High Heat Unit Capacity Tube to Sustain

Continuous Scanning- Now 5 and 6 Million

Heat Units

• Faster Rotation Times to Get Through a

Volume Even Quicker; < 0.5 second rotation

times available

• Faster Data Transfer Systems to Get Data

From DAS - Fiber Optics

• Faster Computers to Reconstruct

- 0.1 to 0.5 seconds per image


Spiral CT Systems

Advantages over Conventional:

• Faster Acquisition

• Follow Contrast Quicker

• Begin to Image Physiology/Angiography

• Reduce Breathing Artifacts/Misregistration


Spiral CT Systems

Limitations:

• Motion Blurring (Effective Slice is Thicker)

• Data/Image Overload

• Memory and Data Transfer May Be

Limiting Factors

CT Basic Principles Modified

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