CT SCAN basics-1

Source: CTC/CTM© Siemens AG, 1998Medical Engineering Group, Computed TomographyCT Basics 1

Siemens Medical Systems

Computed Tomography (Basics)


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 thefirst time to be able to viewthe anatomy of the human

body noninvasively


In 1972,

two scientists -

Hounsfield and Ambrose-

presented the first

clinical CT image ...

CT Broke the Barrier...


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

♠ And the resolution needed to be improved

So we couldsee

tomographic anatomy&

density differences

The Human Body Slice by Slice...


What Does a CT Look Like?


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 (1974)

SOMATOM Plus 4 UFC(1996)


What Does a CT Look Like?

♣ Gantry♣ Table♣ Generator♣ Console♣ Computer

From the outside...

Console

Computer

Table

Gantry

Generator


What Does a CT Look Like?From the inside...

♣ Tube♣ Detector♣ DAS*

Tube

Detector

DAS

* Data Acquisition System


How Does CT Work?

Recon & postprocessingRecon & postprocessing

Data acquisitionData acquisition

X-ray generationX-ray generation


Image Generation - The “Slice”X-rays pass through a collimator therefore onlypenetrating an axial layer of the object, calleda "slice"


The slice is artificiallydivided into small volumeelements called "voxels"with a square base,inside which theattenuation is measuredas a constant value.And in plane, the pictureelements are called “pixels”

Image Generation - The “Voxel”


The attenuation of radiation values is measured,encoded and transferred to a computer.

Image Generation - The “Matrix”

35 36 3934 3331 34 33 35 3231 80 85 9078


The numerical matrix is converted into a black andwhite image in a corresponding gray scale.

CT Image Generation - A/D/A*

35 36 3934 3331 3433 35 3231 80 85 9078

*Analog - Digital - Analog


Criteria for CT Image Quality

CT Image Quality

Spatial resolution

Contrast Detectability

Artifacts


Spatial Resolution...The ability to resolve High Contrast Objects,(also called “High Contrast Resolution”)

This is influenced by system geometry,and determines image definition


Definition

Scan time

AlgorithmOperating mode

Slice thickness

Image display

Definition...

Image definition means the sharpness of anobject relative to surrounding tissue. Itdepends on:


When Small Contrast Differences are crucial(also called “Low Contrast Resolution”)

This is influenced by image definition & noise

Contrast Detectability...


Noise is superimposed on the image and results ina "grainy" impression, as is the case with poor TVreception.

Noise?

206 mA 60 mA


Noise...

Noise

mAs

kV

Algorithm

Slice thickness

Patient size

Operating mode

Image display

Image “noise” is determined by the number ofx-ray quanta that reach the detector and thencontribute to the image. It depends on:


Artifacts...The various structures or patterns that appear in aCT image, but are not found in the original object.

Artifacts

Scan time

Operating modeSlice thickness

They depend on: ...

Patient


Influences on CT Image Quality?

User

PatientSystem

CT Image Quality


System Efficiency...

100 mAs 100 mAs

The complete system design determines howefficiently x-rays are finally converted toelectrical signal as the detector output, afterpassing through the patient.

The generator, tube, geometry, filtration,collimation and detector design all play a role.


mAs...

Noise

high

low

highlow

Rule of thumb:The higher the dose,the lower the noise.

Rule of thumb:The higher the dose,the lower the noise.

mAs

noisedose

1

* Noise reduced by factor 1.4while dose is doubled.

Tube current and scan time, determine dose.


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.

Recommendations for mAs...


Tube Voltage - kV

The dose level, depends very strongly on thevoltage 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 - kVThe higher the voltage, the more the radiationspectrum is shifted to a higher energy level,resulting in decreased radiation attenuation. Thisis most noticeable in bone and contrast media.140 kV 80 kV


Algorithms...

Image definition

Sharp

Smooth

low HighNoise

ULTRA HIGH

HIGH

STANDARD

SOFT

SOFT DETAIL

... provide the recipe for mathematical imagecalculation


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.

Algorithms


Soft Algorithm HighRes Algorithm

Algorithms

Soft algorithms provide better contrastdetectability with less noise.


Standard Algorithm HighRes Algorithm

Algorithms

HighRes algorithms provide better spatialresolution, but with more noise.


Selecting Slice Thickness

A thick slicemeans:A thick slicemeans:

A thin slicemeans:A thin slicemeans:

Selecting a suitable slice thickness is a balancebetween edge definition and noise because oftheir mutually offsetting effects.

low noise better contrast resolution poorer edge definition partial volume artifacts

high noisepoorer contrast resolutionbetter edge definitionno partial volume artifacts


Slice Thickness...

Slice thickness

Noise

10 mm

High

Low

1 2 3 5

Slice thickness

Image definition = spatial resolution

High

Low

10 mm1 2 3 5

slice thickness 1/2 noise level 1.4no. of x-ray quanta 50%


3 mm Slice 10 mm Slice

Slice Thickness...

Thicker slices give less noise & bettercontrast detectability for soft tissue


5 mm Slice 1 mm Slice

Slice Thickness

Thinner slices give better spatial resolutionfor bony structures.


Patient Size...

Noise

400%

300%

200%

100%

30 34 38 42 46 cmPatient diameter

Rule of thumb:The noise level doubles for every 8 cmincrease in patient diameter.

Rule of thumb:The noise level doubles for every 8 cmincrease in patient diameter.


Patient Size...

45 cm 28.8 cm

An attenuation by a factor of 2 results fromeach 4 cm increase in patient thickness, thusincreasing the pixel noise.


Water

Mamma

Air

Bone

Spleen

Fat

Pancreas

Lung

Kidneys

AdrenalGland

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 andair -1000. The relative values ofthe other tissues are calculatedrelative to that of water

Rule of thumb:

The CT value of water is 0 andair -1000. The relative values ofthe other tissues are calculatedrelative 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 grayscales at best.

So, the window settings must be in accordancewith 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 - WindowingHounsfieldunit

+3000

-1000

0

Window width W

Window center C

Gray scaledisplay

White

Black

CT Windowing


Narrow Window Width Broad Window Width

Narrow window width:High-contrast image, butstructures outside that windowrange may be inadequatelyrepresented or overlooked.

Broad window width:Minor density differencesappear homogeneous andmay be masked.



Hounsfieldunit

+3000

-1000

0

Window 1

Gray scaledisplay

White

Black

CT WindowingWindow 2

Double Window Technique

This is used for the display of two tissue typesdiffering greatly in their density values, such asthe lung & the mediastinum.


Double Window Technique*

To see bothlungs andmediastinumwithin imagesimutanously

*Double window is not recommended for diagnosis.


Lung Window Mediastinum WindowDouble Window


Image 2:Only lungvisible

Image 1:Both lung,thorax wall &mediastinumvisible

Image 3:Only thorax wall& mediastinumvisible


Extended CT ScaleNormally, CT values can be measured from -1024 to +3071,but with SOMATOM Plus 4, this can be extended from -10240to +30710 to visualize metals of high attenuation. Therefore,it is always possible to measure the real CT value no matterwhere and how the window is positioned.

Post operative femoralhead replacement - theCT value is 6000 HU


Reviewed Magnified

Review vs. Magnification

Magnification:a purely opticalmagnification of imagedata which may result inblurred appearance

Review:A zoom reconstruction fromraw data to enhancesharpness of details


Image Artifacts - Origins

Image Artifacts

Motion

Technicaldefects

Partial volume Beam hardening

Metal

Operatorerror

...


Image Artifacts - Appearance

Image artifacts

Streak Cupping

Dark bar Ring


Motion Artifact & Scan TimeMotion artifacts

Severe

Moderate

Short LongScan time (s)

Rule of thumb:The shorter the scan time, theless likely motion artifacts areoccur.

Rule of thumb:The shorter the scan time, theless likely motion artifacts areoccur.


Motion Artifact & Correction

w/o correction w/ correction

Motion artifacts can be compensated for bythe Motion Correction Algorithm (MCA)


Metal Artifact

Metals, such as gold, absorbx-radiation almost completely,thus producing

“radiation shadows”,leading to pronounced streakartifacts over the entirereconstructed image

This can only be avoided via a gantry tilt that excludesthe 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 structuresat the base of the skull and the petrous bone region.

That is because thevery dense structures(bones) are onlypartially included in theslice, resulting in highcontrast errors.


5 mm 2 mm

Selecting a thinner slice prevents such artifacts fromoccurring, since high contrast structures are lessfrequently partially included, but this inherentlyincreases the noise level, thus degrading contrastresolution.

Partial Volume Effect


VAR - Volume Artifact Reduction2 x 2 mm

Combines several thin slices (which reduces the partialvolume artifact) to provide a thicker slice (which reducesthe pixel noise and offers good soft tissue discrimination).

5 mm


Beam Hardening EffectThe x-ray photons emitted from the x-ray tube donot all have the same energy. As they penetratethe irradiated object, the spectrum is shifted tohigher energies - called “beam hardening”. In theimage, streak artifacts or the so-called “cuppingeffect” can be seen.


Beam Hardening & Correction



Beam Hardening & CorrectionThe “cupping” effect can be compensated forby means of “beam hardening correction”


severe cupping homogenuous CT values


Technical DefectsThe individual detector elements of a detectorsystem may not produce the same signal for thesame irradiation.

When a detector elementoutputs an erroneous signal,ring artifacts appear.

This can be eliminated bycalibration, if not, calltechnical service!


Adaptive Filter

w/o A.F. w/ A.F.

For a non-circular object, x-ray attenuation isgreater along the long axis than along the shortaxis, therefore directional noise is seen.


Artifacts & CorrectionsThe reasons for artifacts are quite diverse. Whatwe do is to perform corresponding corrections inorder to avoid them. But sometimes artifacts cannot be compensated for completely.

Nothing is perfect...

But we keep on working to reducethem as much as possible!


Since the influence on HC resolution & LCresolution by changing parameters can becontradictory, it is necessary to differentiatebetween:

For the Clinical Routine...

¨ A Soft tissue study (contrast detectability) ( >90% of routine studies, normally)

¨ A Bone study (spatial resolution) (<10% of routine studies, normally)


CT is really great!

But there are still some problems...

Clinical Requirements...


Conventional CT...

I.S.D* I.S.D

I.S.D I.S.D 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...

Deep inspiration


Breathing Was the Problem...

Sorry, my dear patient, thelesion was missed becauseyou did not keep the sameinspiration level for eachscan...


Best case

Worst case

Problems of Conventional CT...Partial Volume Effect and slice location causemisregistration and/or misdiagnosis


Location Was the Problem...

Sorry, the lesion wasmissed because its locationdid not fit my sliceposition...

One mouse...


Deep inspiration Moderate inspiration

l Scan influenced by respiration

l Image can’t be reconstructed anywhere as desired

Problems of Conventional CT...


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 onebreathhold

Change "slice" into "volume"


w/o I.S.D w/ I.S.D

2D Slice 3D Volume

Conventional vs. Spiral CT ...


Continuously rotating tube/detector system

Continuous radiation

Continuous data acquisition

Continuous table feed

Spiral CT = Volume scan

The 4 “C”s?


“You said that images will contain artifacts if thereis any motion during the scan, but a spiral scan isperformed during constant table movement!”

Yes, therefore we need special techniques forimage reconstruction:

Spiral AlgorithmsWIDE, SLIM & SLIM 2

Spiral CT...


Wide (360°) Algorithm2 x 360° (2 full rotations) data acquisition is usedfor 1 image reconstruction. Both are measured data.

measureddata

table positionslice


Slim/Slim 2 (180°) Algorithm2 x 180° (2 half rotations) data acquisitions areused for 1 image reconstruction. Measured dataare weighted for Slim, and interpolated for Slim 2.

measureddata

table positionslice

complementarydata


Different Interpolations?

♣ The in-plane resolution for a homogenousobject in Z-direction is the same for WIDE,SLIM & SLIM 2

♣ SLIM & SLIM 2 give better Z-directionresolution than WIDE when pitch 0.5.


Effective Slice Thickness (FWHM*)

WIDE makes FWHM wider, while SLIM/SLIM 2 keepFWHM narrow.

0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

2.5

3

3.5

WIDE

SLIM

Pitch

Slic

e T

hic

knes

s (r

elat

ive)

* Full Width Half Maximum


Noise AmplitudesCompared with Sequence scans, WIDE gives less noise, andSLIM/SLIM 2 give more. But for all of them, noise amplitudedoes not depend on pitch.

0 0.5 1 1.5 2 2.5 30

0.2

0.4

0.6

0.8

1

1.2

Pitch

No

ise

(rel

ativ

e)

WIDE

SLIM / SLIM 2SEQ.


Noise Amplitudes

WIDE

Image reconstructed with WIDE algorithm shows less noisethan those with SLIM & SLIM 2, but for SLIM & SLIM 2they are about the same.

SLIM 2SLIM


Slim: use it with HighRes algorithm when onlyspatial resolution is most important (e.g. bone andlung study)

Slim2: use it with algorithms from smooth tostandard when the contrast detectability is mostimportant (e.g. soft tissue study)

Wide: use it to achieve low noise only if z-directionresolution is not important.

Which Algorithm to Choose?


Spiral ParametersPitch: Table feed per rotation divided by slice thickness

Pitch =Table feed / rotation

Slice thickness

It is physically not possible to scan a volumewith gaps because x-rays always irradiate thewhole volume.


Increment:Determines degree of overlap between successiveimages.

Increment = 5 mmIncrement = 3 mm

*slice = 10 mm

Spiral Parameters

Increment = 10 mm

The smaller the increment, the morethe 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

Advantages of Spiral CT...


Why Perform Spiral CT?u Fast scanning of large anatomical volumesu Gapless data acquisition during one breathholdu Optimum utilization of contrast mediumu Retrospective reconstruction with arbitrary slice increments

Overlapping reconstruction givesbetter z-axis resolution

Spiral CT ScanConventional CT Scan


u Use Spiral CT for contrast studiesu Use Spiral CT for all regions of the bodyu Use it for pediatrics and trauma patients, who

require quick scanningu Use it for long anatomical ranges

Always use Spiral if you intend toperform 3D postprocessing (e.g. CTA) !

When to Use Spiral CT?

CT SCAN basics-1

Documents