6 MedPhys CT 2 - uni-heidelberg.de · 2 Seite 2 RUPRECHT-KARLS-UNIVERSITY HEIDELBERG Computer...

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RUPRECHT-KARLS-UNIVERSITY HEIDELBERG

Computer Assisted Clinical MedicineProf. Dr. Lothar Schad

12/9/2008 | Page 1Master‘s Program in Medical Physics

Chair in Computer Assisted Clinical MedicineFaculty of Medicine Mannheim University of HeidelbergTheodor-Kutzer-Ufer 1-3D-68167 Mannheim, GermanyLothar.Schad@MedMa.Uni-Heidelberg.dewww.ma.uni-heidelberg.de/inst/cbtm/ckm/

Physics of Imaging Systems

Basic Principles of Computer Tomography (CT) II

Prof. Dr. Lothar Schad



12/9/2008 | Page 2

Spiral CT

Spiral CT

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12/9/2008 | Page 3Spiral CT 1989

Kalender et al. Radiology 1989Vock et al. Radiology 1989



12/9/2008 | Page 4Spiral CT: Principle I

source: Kalender. Computertomographie, Publicis MCD Verlag 2000

direction of patient transport

starting point of spiral scan

trajectory ofrotating tubeand detector

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12/9/2008 | Page 5CT Artifacts: Patient Movement


- critics to spiral CT 1989:“spiral CT is a method to create image artifacts”

- solution: z-interpolation



12/9/2008 | Page 6Spiral CT: Z – Interpolation I


conventional CT spiral CT

measurement

preprocessing

intermediate step

reconstruction

result

n scans over 360°at positions

z1 to zn

1 scan over n·360°at positions

z1 to zn

data correction data correction

z-interpolation

convolution and backprojection

convolution and backprojection

n images at fix positions z1 to zn

> n images at any positions z1 to zn

ima 1ima 2

ima 3ima n

ima 1ima 2

ima 3ima 4

ima i

any defined sliceat position z

z-axis

time

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12/9/2008 | Page 7Spiral CT: Z – Interpolation II


filter width W

- data rate spiral CT (64 rows): 600 MB / sec → ~ 10 GB / patient !



12/9/2008 | Page 8

B

volume data

continuous data acquisition

A

• slices can be reconstructed atany position within the volume

• distance between the slices iscalled “increment”

Spiral CT: Slice Reconstruction

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12/9/2008 | Page 9

• no misregistration of lesion

• fast imaging• better patient’s comfort• less motion artifacts• less contrast medium• less radiation dose• higher throughput

volume scanning in a single breathhold

Spiral CT: Clinical Advantage



12/9/2008 | Page 10

pitch = table feed per rotationcollimation

TT aa bb llee FF ee ee dd CC oo ll ll iimm aa tt iioo nn PP ii tt cc hh11 00 mm mm // rr oo tt 11 00 mm mm 11 ..0022 00 mm mm // rr oo tt 11 00 mm mm 22 ..00

Spiral CT: Pitch - Factor

- example: spiral CT64 rows, slice thickness 0.5 mm, table feet 32 mm per rotationcollimation = 64 x 0.5 mm = 32 mmpitch = 32 mm / 32 mm = 1.0

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12/9/2008 | Page 11

PITCH 2PITCH 1

pitch 2 produces a broader image slice

Pitch and Image Quality



12/9/2008 | Page 12Spiral-CT: Slice Sensitivity Profile (SSP)


SSP atconventional CT

table movingfunction

SSP atspiral CT

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12/9/2008 | Page 13Spiral-CT: Partial Volume Effect

best case worst caseobject

image

object

image

best case worst case



12/9/2008 | Page 14

pitch 1.0 10 mm/rot

pitch 2.020 mm/rot

30 s 15 s

same volume in a shorter time!

Clinical Advantage: High Pitch - Factor

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12/9/2008 | Page 15

30s with 1.0s rotation

22s with 0.8s rotation

40cm 30cm

0.8s Spiral 1.0s Spiral

10mm30s

8mm28s

7mm32s 0.75s SPIRAL 1s SPIRAL

• same coverage in less time• more coverage in the same time• thinner slices in the same time

Clinical Advantage: Subsecond Rotation



12/9/2008 | Page 16

4 rows instead of 1

• more volume more detector rows

• better detailssubmillimeter slices (0.5 mm)

• more speed0.5 sec gantry rotation8 slices per second

Multi - Row Spiral CT

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12/9/2008 | Page 17Future: Cone Beam Spiral CT




12/9/2008 | Page 18Spiral CT with 256 Rows

Toshiba

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12/9/2008 | Page 19Cone Beam CT I

source: Dössel. “Bildgebende Verfahren in der Medizin” 2000

X-raysource

trajectory of X-ray source

detectormatrix

object



12/9/2008 | Page 20Cone Beam CT II


detailsimage points

cone angle κ

plane through detail structures

central plane

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12/9/2008 | Page 21Cone Beam CT II

- cone beam experimental setup



12/9/2008 | Page 22Electron Beam CT


deflecting coils

focusing coils

electron generationelectron beam

data acquisition

detector ring

patient couch

target ring

X-ray fancy beam

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12/9/2008 | Page 23

Esprit Balance Emotion EmotionDuo

VolumeAccess

VolumeZoomSmile

single slice spiral scanners multi-slice spiral scanners2.0 s 1.5 s 1.0 s 0.8 s 0.8 s 0.5 s 0.5 s

ultrafast ceramic detectorsSyngo user interface

Siemens Somatom CT Family



12/9/2008 | Page 24Slices Within a Typical Breathhold (20 s)

Emotion EmotionDuo Option

VolumeAccess

VolumeZoomBalance

80 slices

25 slices

160 slices

50 slices

20 slices3s

1 s1.5 s

0.7 s0.7 s

0.8 s 0.8 s 0.5 s 0.5 s

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12/9/2008 | Page 25Volume: Routine Protocol 5 mm, Pitch 1.5, in 20 s


VolumeAccess

VolumeZoomBalance

60 cm

19 cm

120 cm

38 cm

15 cm

80 s spiral 100 s spiral 80 s spiral 100 s spiral

50 cm / P 2.0



12/9/2008 | Page 26Chest (30 cm) Scan Time with 5 mm, Pitch 1.5


VolumeAccess

VolumeZoomBalance

10 s

32 s

5 s

16 s

40 s

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12/9/2008 | Page 27


VolumeAccess

VolumeZoomBalance

2*2.5 mm / 20 s2*1 mm / 38 s?

8 mm / 20 s5 mm / 32 s

4*1 mm / 25 s4*2.5 mm / 10 s

2*5 mm / 16 s2*2.5 mm / 32 s

8 mm / 25 s

Chest (30 cm): Breathhold



12/9/2008 | Page 28


scanogram:

• exact anatomic positioning• X-ray tube and detector are fix• patient is moved continuously

through the scanner• reconstruction of single rows

Spiral CT: Scanogram IX-ray tube

detectors

patient couch

moving table

X-ray scanscanogram

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12/9/2008 | Page 29

- scanogram are measured at low doseand low spatial resolution

- projection direction (here a.p.) is arbitrary

- based on the scanogram the slice positioningand gantry tilting of a single slice or wholevolume can be anatomically exact defined


Spiral CT: Scanogram II

detector (fix)

tube (fix)



12/9/2008 | Page 30


xenon high pressureionization chamber

scintillator crystals (CsI)with photo diode

CT DetectorsX-ray quant’s

xenon gas fillingabout 20 bar

anode cathode

current signalisolation

pressure chamber

about10 cm

photodiode

about1 cm

voltage signal

scintillation crystal

lightquant’s

X-ray quant’s

+ anode &cathodecan beused ascollimators

+ lowdeadtime108 cps

- lowefficiency

- large size

+ highefficiency

+ smallsize

- highdeadtime

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12/9/2008 | Page 31


Spiral CT: Multi – Slice Detector Systems

“adaptive array detector”

variable slice collimationby near-focus fade in

and electronic signal combination



12/9/2008 | Page 32

1.25 mm

1.25 mm

1.25 mm

1.25 mm

2.5 mm

2.5 mm

5 mm

X-ray collimation 5 mm

X-r

ay tu

be

z -a

xis

Spiral CT: Multi – Slice Scanning Principle

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12/9/2008 | Page 33Spiral CT: Variable Slice Thickness

1 mmLF

5 mmWTF

1 mmWTF

5 mmLF



12/9/2008 | Page 34Spiral CT: Multi – Planar Reconstruction

transversal(axial)

sagittal

frontal(coronal)

oblique

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12/9/2008 | Page 35Spiral CT: Coronal Reconstruction



12/9/2008 | Page 36Spiral CT: Summary

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12/9/2008 | Page 37

CT Image Quality

CT Image Quality



12/9/2008 | Page 38CT Image Quality Criteria

- dependent on measuring parameters !

- quantitative criteria are influencing each other !

quantitative criteria:

- spatial resolution

- contrast resolution

- noise and homogeneity

- artifacts

• metallic implants• patient movement• partial volume effects• beam hardening• scattering• raster errors• exceeding field-of-view

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12/9/2008 | Page 39

There are 7 or 8 mice...

No, there are10 of them!

- the ability to resolve high contrast objects→ high contrast resolution

Spatial Resolution I



12/9/2008 | Page 40Spatial Resolution II

edge enhancing reconstruction kernelnoise ↑

smoothing reconstruction kernelnoise ↓

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12/9/2008 | Page 41

Is therea cat? !

There could bea cat!

- when small contrast differences are questioned → low contrast resolution

A cat !

Contrast Resolution I

- more photons → noise ↓ but dose ↑



12/9/2008 | Page 42Contrast Resolution II

edge enhancing reconstruction kernel smoothing reconstruction kernel

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12/9/2008 | Page 43Contrast – Detail - Diagram

relationship between spatial resolution and contrast resolution



12/9/2008 | Page 44Noise

I = 50 mAd = 4 mm

I = 200 mAd = 4 mm

I = 200 mAd = 16 mm

- Poisson statistic of photons is dominating- dependent on X-ray tube current I and slice thickness d- in principle more photons → noise ↓ but dose ↑- today CT noise is defined by physical limit of quantum statistics of X-ray tube

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12/9/2008 | Page 45Modulation Transfer Function (MTF)


CT image of a thin wirepoint spread function

(PSF)modulation transfer

function (MTF)

convolution kernel

high resolution

standardsmoothing

2% value

quarter jumping high res. 2% valueoffset focus comb

Yes Yes Yes 15.8

Yes Yes No 13.7Yes No No 11.7

No No No 8.7



12/9/2008 | Page 46CT Artifacts: Metallic Implants

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12/9/2008 | Page 47CT Artifacts: Patient Movement




12/9/2008 | Page 48

CT slice

bonesoft tissue

CT Artifacts: Partial Volume Effect Principle

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12/9/2008 | Page 49

bonesoft tissue


CT slice



12/9/2008 | Page 50

CT slice

bonesoft tissue


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12/9/2008 | Page 51


CT Artifacts: Partial Volume Effect I



12/9/2008 | Page 52

values of ln(J0/J) of different projection directions are not in agreement !


CT Artifacts: Partial Volume Effect II

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12/9/2008 | Page 53


CT Artifacts: Partial Volume Effect III



12/9/2008 | Page 54

- in general: µ = µ(x,y,E) – attenuation of radiation

dEdE

EdJJE

E

⋅= ∫)(0

0

max

min

dEedE

EdJJdlEyx

E

E

⋅∫⋅=⋅−

∫),,(0 )(max

min

μ

- beam hardening artifacts:

dependent on the energy the polychromatic spectrum of X-ray radiation is differently attenuated and the mean energy of the spectrum increases in thick objects

CT Artifacts: Beam Hardening I

source: Dössel. “Bildgebende Verfahren in der Medizin”, 2000

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12/9/2008 | Page 55

“Hounsfield balk”(corpus callosum)

CT Artifacts: Beam Hardening II




12/9/2008 | Page 56CT Artifacts: Scattering


scatterintensity

primaryintensity

primarybeam

scatterintensity

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12/9/2008 | Page 57CT Dose Aspects I


frequency of radiological examinations

fraction to collective effective dose

- dose ~ mAs (X-ray tube current)- SNR ~ √number of photons- dose@CT = doseCT / pitch



12/9/2008 | Page 58CT Dose Aspects II


a.p.-projection a.p.-projection

intensitynoise

- anatomy adapted X-ray tube current

- dose reduction ~ 50%

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12/9/2008 | Page 59CT Dose Aspects III


induction of radiation damage

effective dose at CT:typical value 1-10 mSv

biological effective cell damage

low dose area

activation of cell prevention factors

dose [mSv]

effect

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