CT SCAN basics-1

Siemens Medical Systems

The Discovery of X-Rays...

100 years ago,

Wilhelm Conrad Roentgen,

a German scientist,

discovered x-rays...

Look Inside The Human Body...

«

But anatomic structures were superimposed

«

And soft tissue couldn’t be differentiated

This allowed people for the

first time to be able to view

the

anatomy

of the human

In 1972,

two scientists

-Hounsfield

and

Ambrose-presented the first

clinical CT image ...

♠

But it was time consuming (10 min. / image)

♠

And the resolution needed to be improved

So we could

see

tomographic anatomy

&

density differences

The First Siemens CT Scanner...

Acquisition time 7 min., image matrix 80x80 pixels,

scan field 25 cm , spatial resolution 1,3 mm (4LP/cm)

SIRETOM

(in 1974)

The Progress in Image Quality...

SIRETOM

What Does a CT Look Like?

♣

Gantry

♣

Table

♣

Generator

♣

Console

♣

Computer

From the outside...

Console

Computer

Table

Gantry

What Does a CT Look Like?

From the inside...

♣

Tube

♣

Detector

♣

DAS*

Tube

Detector

DAS

How Does CT Work?

Recon & postprocessing

Recon & postprocessing

Data acquisition

Data acquisition

X-ray generation

X-ray generation

Image Generation - The “Slice”

X-rays pass through a collimator therefore only

penetrating an axial layer of the object, called

a "

slice

"

The slice is artificially

divided into small volume

elements called "

voxels

"

with a square base,

inside which the

attenuation is measured

as a constant value.

And in plane, the picture

elements are called “

pixels

”

The attenuation of radiation values is measured,

encoded and transferred to a computer.

Image Generation - The “Matrix”

35 36

34

39

33

31 34 33 35 32

31

78

80 85 90

The numerical matrix is converted into a black and

white image in a corresponding gray scale.

CT Image Generation - A/D/A*

35 36

34

39

33

31 34 33 35 32

31

78

80 85 90

Criteria for CT Image Quality

CT Image Quality

Spatial resolution

Contrast Detectability

Spatial Resolution...

The ability to resolve

High Contrast

Objects,

(also called “High Contrast Resolution”)

This is influenced by

system geometry,

Definition

Scan time

Algorithm

Operating mode

Slice thickness

Image display

Definition...

Image definition means the

sharpness

of an

object relative to surrounding tissue. It

When

Small Contrast Differences

are crucial

(also called “Low Contrast Resolution”)

This is influenced by image

definition

&

noise

Noise is superimposed on the image and results in

a "grainy" impression, as is the case with poor TV

reception.

Noise?

Noise...

Noise

mAs

kV

Algorithm

Slice thickness

Patient size

Operating mode

Image display

Image “noise” is determined by the number of

x-ray quanta that reach the detector and then

contribute to the image. It depends on:

Artifacts...

The various structures or patterns that appear in a

CT image, but are not found in the original object.

Artifacts

Scan time

Operating mode

Slice thickness

They depend on:

_...

Influences on CT Image Quality?

User

Patient

System

System Efficiency...

100 mAs

100 mAs

The complete system design determines how

efficiently x-rays are finally converted to

electrical signal as the detector output, after

passing through the patient.

The generator, tube, geometry, filtration,

mAs...

Noise

high

low

Rule of thumb:

The higher the dose,

the lower the noise.

Rule of thumb:

The higher the dose,

the lower the noise.

mAs

noise

dose

1

* Noise reduced by factor 1.4

while dose is doubled.

Low mAs

High mAs

mAs...

Image 1:

Low mAs value -

high noise level

Image 2:

4 times the mAs value-

half the noise level

In the case of a

soft tissue study

, it is

most important to keep noise to a minimum

by using higher mAs. The lower the noise

level, the easier it is to recognize structures

with minute differences in density. But for

bone or lung studies, higher mAs is not

necessary.

Tube Voltage - kV

The dose level, depends very strongly on the

voltage applied to the tube.

Water

20 cm

kV

Relative dose

140

100%

120

58%

80

12%

kV

Relative dose

140

100%

120

58%

80

12%

Tube Voltage - kV

The higher the voltage, the more the radiation

spectrum is shifted to a

higher energy level

,

resulting in decreased radiation attenuation. This

is most noticeable in bone and contrast media.

Algorithms...

Image definition

Sharp

Smooth

Noise

ULTRA HIGH

HIGH

STANDARD

SOFT

SOFT DETAIL

... provide the recipe for mathematical image

calculation

An edge-enhancing (HighRes) algorithm

produces good

edge definition

, but also a

high

noise level

, while a smoothing

algorithm produces a low noise level, but

also poorer edge definition. For routine

studies, a standard algorithm is normally

recommended.

Soft Algorithm

HighRes Algorithm

Algorithms

Soft algorithms provide better contrast

detectability with less noise.

Standard Algorithm

HighRes Algorithm

Algorithms

HighRes algorithms provide better spatial

resolution, but with more noise.

Selecting Slice Thickness

A thick slice

means:

A thick slice

means:

A thin slice

means:

A thin slice

means:

Selecting a suitable slice thickness is

a balance

between edge definition and noise

because of

their mutually offsetting effects.

low noise

better contrast resolution

poorer edge definition

partial volume artifacts

high noise

poorer contrast resolution

better edge definition

Slice Thickness...

Slice thickness

Noise

10 mm

High

Low

1 2

3

5

Slice thickness

Image definition = spatial resolution

High

Low

10 mm

1 2 3

5

3 mm Slice

10 mm Slice

Slice Thickness...

Thicker slices give less noise & better

contrast detectability for soft tissue

5 mm Slice

_{1 mm Slice}

Slice Thickness

Thinner slices give better spatial resolution

for bony structures.

Patient Size...

Noise

400%

300%

200%

100%

30

34

38

42

46

cm

Patient diameter

Rule of thumb:

The noise level doubles for every 8 cm

increase in patient diameter.

Rule of thumb:

The noise level doubles for every 8 cm

increase in patient diameter.

Patient Size...

45 cm

_{28.8 cm}

An attenuation by a factor of 2 results from

each 4 cm increase in patient thickness, thus

increasing the pixel noise.

Water

Mamma

Air

Bone

Spleen

Fat

Pancreas

Lung

Kidneys

Adrenal

Gland

Blood

Heart

Liver

Intestine

Tumor

Bladder

3000

60

40

0

-100

-200

-900

-1000

Image Display - Windowing

This is the so-called CT number in Hounsfield unit (

HU

)

Rule of thumb:

The CT value of water is 0 and

air -1000. The relative values of

the other tissues are calculated

relative to that of water

Rule of thumb:

The CT value of water is 0 and

air -1000. The relative values of

the other tissues are calculated

relative to that of water

The range of CT density values is defined from -1000 to

+3000, but the human eye can distinguish only 30 - 40 gray

scales at best.

So, the window settings must be in accordance

with the

structures to be visualized

Lung Window

_{Mediastinum Window}

Window width

(W): the density range represented within the gray scale.

Window center

(C): the center of the density range.

Image Display - Windowing

Hounsfield

unit

+3000

-1000

0

Window

width W

Window

center C

Gray scale

display

White

Black

CT Windowing

Narrow Window Width

Broad Window Width

Narrow

window width:

High-contrast image, but

structures outside that window

range may be inadequately

represented or overlooked.

Broad

window width:

Minor density differences

appear homogeneous and

may be masked.

Hounsfield

unit

+3000

-1000

0

Window 1

Gray scale

display

White

Black

CT Windowing

Window 2

Double Window Technique

This is used for the display of two tissue types

differing greatly in their density values, such as

the lung & the mediastinum.

Double Window Technique*

To see both

lungs and

mediastinum

within image

simutanously

Lung Window

Mediastinum Window

Double Window

Image Display - Windowing

Image 2:

Only lung

visible

Image 1:

Both lung,

thorax wall &

mediastinum

visible

Image 3:

Only thorax wall

& mediastinum

visible

Extended CT Scale

Normally, CT values can be measured from -1024 to +3071,

but with SOMATOM Plus 4, this can be extended from -10240

to +30710 to visualize metals of high attenuation. Therefore,

it is always possible to measure the real CT value no matter

where and how the window is positioned.

Post operative femoral

head replacement - the

CT value is 6000 HU

Reviewed

_Magnified

Review vs. Magnification

Magnification:

a purely optical

magnification of

image

data

which may result in

Review:

A zoom reconstruction from

raw data

to enhance

sharpness of details

Image Artifacts - Origins

Image Artifacts

Motion

Technical

defects

Partial volume

_{Beam hardening}

Metal

Operator

error

Image Artifacts - Appearance

Image artifacts

Streak

_Cupping

Motion Artifact & Scan Time

Motion artifacts

Severe

Moderate

Short

Long

Scan time (s)

Rule of thumb:

The shorter the scan time, the

less likely motion artifacts are

occur.

Rule of thumb:

The shorter the scan time, the

less likely motion artifacts are

occur.

Motion Artifact & Correction

w/o correction

w/ correction

Motion artifacts can be compensated for by

the Motion Correction Algorithm (

MCA

)

Metal Artifact

Metals, such as gold, absorb

x-radiation almost completely,

thus producing

“

radiation shadows

”,

leading to pronounced streak

artifacts over the entire

reconstructed image

This can only be avoided via a gantry tilt that excludes

the disturbing metallic objects from the slice plane.

Partial Volume Effect

5 mm

Streak-like artifacts, also called partial volume artifact,

occur most frequently in the bony structures

at the base of the skull and the petrous bone region.

That is because the

very dense structures

(bones) are only

partially included in the

slice, resulting in high

contrast errors.

5 mm

2 mm

Selecting a thinner slice prevents such artifacts from

occurring, since high contrast structures are less

frequently partially included, but this inherently

increases the noise level, thus degrading contrast

VAR - Volume Artifact Reduction

2 x 2 mm

Combines several thin slices (which reduces the partial

volume artifact) to provide a thicker slice (which reduces

the pixel noise and offers good soft tissue discrimination).

Beam Hardening Effect

The x-ray photons emitted from the x-ray tube do

not all have the same energy. As they penetrate

the irradiated object, the spectrum is shifted to

higher energies - called “beam hardening”. In the

image,

streak artifacts

or the so-called “

cupping

effect

”

can be seen.

Beam Hardening & Correction

Beam Hardening & Correction

The “cupping” effect can be compensated for

by means of “

beam hardening correction

”

w/o correction

w/ correction

Technical Defects

The individual detector elements of a detector

system may not produce the same signal for the

same irradiation.

When a detector element

outputs an erroneous signal,

ring artifacts appear.

This can be eliminated by

calibration

, if not, call

technical service!

Adaptive Filter

w/o A.F.

w/ A.F.

For a non-circular object, x-ray attenuation is

greater along the long axis than along the short

axis, therefore directional noise is seen.

Artifacts & Corrections

The reasons for artifacts are quite diverse. What

we do is to perform corresponding corrections in

order to avoid them. But sometimes artifacts can

not be compensated for completely.

Nothing is perfect...

But we keep on working to reduce

them as much as possible!

Since the influence on HC resolution & LC

resolution by changing parameters can be

contradictory, it is necessary to differentiate

between:

For the Clinical Routine...

¨

A

Soft tissue study

(contrast detectability)

( >90% of routine studies, normally)

¨

A

Bone study

(spatial resolution)

CT is really great!

But there are still some problems...

Conventional CT...

I.S.D*

I.S.D

* Inter Scan Delay

Standard Scan:

- Longer cycle time

- but instant display

Dynamic Scan:

- Fast acquisition

- but delayed display

Moderate inspiration

Misregistration due to different levels of respiration

Problems of Conventional CT...

Breathing Was the Problem...

Sorry, my dear patient, the

lesion was missed because

you did not keep the same

inspiration level for each

scan...

Best

case

Worst

case

Problems of Conventional CT...

Partial Volume Effect and slice location cause

misregistration and/or misdiagnosis

Location Was the Problem...

Sorry, the lesion was

missed because its location

did not fit my slice

position...

l

Scan influenced by respiration

l

Image can’t be reconstructed anywhere as desired

How can we improve this?

l

Scan influenced by respiration

l

Image can’t be reconstructed anywhere as desired

Make it as fast as possible within one

breathhold

w/o I.S.D

w/ I.S.D

2D Slice

3D Volume

C

ontinuously rotating tube/detector system

C

ontinuous radiation

C

ontinuous data acquisition

C

ontinuous table feed

Spiral CT = Volume scan

“You said that images will contain artifacts if there

is any motion during the scan, but a spiral scan is

performed during constant table movement!”

Yes, therefore we need special techniques for

image reconstruction:

Spiral Algorithms

WIDE

,

SLIM

&

SLIM 2

Wide (360°) Algorithm

2 x 360° (2 full rotations) data acquisition is used

for 1 image reconstruction. Both are measured data.

measured

data

table position

slice

Slim/Slim 2 (180°) Algorithm

2 x 180° (2 half rotations) data acquisitions are

used for 1 image reconstruction. Measured data

are weighted for Slim, and interpolated for Slim 2.

measured

data

table position

complementary

data

Different Interpolations?

♣

The in-plane resolution for a homogenous

object in Z-direction is the same for WIDE,

SLIM & SLIM 2

♣

SLIM & SLIM 2 give better Z-direction

resolution than WIDE when pitch 0.5.

Effective Slice Thickness

(FWHM*)

WIDE makes FWHM wider, while SLIM/SLIM 2 keep

FWHM

narrow.

0

_0.5

₁

_1.5

₂

_2.5

₃

0

0.5

1

1.5

2

2.5

3

3.5

WIDE

SLIM

Pitch

Noise Amplitudes

Compared with Sequence scans, WIDE gives less noise, and

SLIM/SLIM 2 give more. But for all of them, noise amplitude

does not depend on pitch.

0

0.5

1

1.5

2

2.5

3

0

0.2

0.4

0.6

0.8

1

1.2

Pitch

Noise (relative)

WIDE

SLIM / SLIM 2

SEQ.

Noise Amplitudes

WIDE

Image reconstructed with WIDE algorithm shows less noise

than those with SLIM & SLIM 2, but for SLIM & SLIM 2

they are about the same.

SLIM 2

SLIM

Slim

: use it with HighRes algorithm when only

spatial resolution is most important (e.g. bone and

lung study)

Slim2

: use it with algorithms from smooth to

standard when the contrast detectability is most

important (e.g. soft tissue study)

Wide

: use it to achieve low noise only if z-direction

resolution is not important.

Spiral Parameters

Pitch

:

Table feed per rotation divided by slice thickness

Pitch =

Table feed / rotation

Slice thickness

It is physically not possible to scan a volume

with gaps because x-rays always irradiate the

whole volume.

Increment

:

Determines degree of overlap between successive

images.

Increment = 5 mm

Increment = 3 mm

*slice = 10 mm

Spiral Parameters

Increment = 10 mm

The smaller the increment, the more

the images are overlapped.

u

Scan a whole volume in one breath hold

u

Reduce partial volume effects

u

No gaps

u

Overlapping images can be reconstructed

without additional dose

u

High quality data for 3D-rendering

Why Perform Spiral CT?

u

Fast scanning of large anatomical volumes

u

Gapless data acquisition during one breathhold

u

Optimum utilization of contrast medium

u

Retrospective reconstruction with arbitrary slice

increments

Overlapping reconstruction gives

better z-axis resolution

Spiral CT Scan

Conventional CT Scan

u

Use Spiral CT for contrast studies

u

Use Spiral CT for all regions of the body

u

Use it for pediatrics and trauma patients, who

require quick scanning

u

Use it for long anatomical ranges

Always use Spiral if you intend to

perform 3D postprocessing (e.g. CTA) !