01 amols ct talk for iaea china course

7/29/2019 01 Amols Ct Talk for Iaea China Course

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IAEA Regional Training Course on Radiotherapy

Techniques with Emphasis on Imaging

and Treatment Planning

Tuesday Sept 4, 2012, Beijing, China

Imaging for Radiation Treatment Planning II:

CT: Principles & applications.

Howard Amols, Ph.D.

Memorial Sloan Kettering

Cancer CenterNew York, USA

1.0 hr


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Acknowledgements

Many slides kindly provided by

Dr. Lawrence N. Rothenberg,Member Emeritus,

Memorial Sloan Kettering Cancer Center


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Talk Outline

1.History and Properties of CT Scanning2.Commissioning and Quality Assurance

3.Using CT for 3D treatment planning


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RSNA/AAPM On-Line

Physics Modules Computed Tomography

CT Image Quality and Protocols

CT Systems

Radiation Dose in CT
http://physics.rsna.org/enroll.asp?id=PHYS2809http://physics.rsna.org/enroll.asp?id=PHYS1809http://physics.rsna.org/enroll.asp?id=PHYSICS_01http://physics.rsna.org/enroll.asp?id=PHYSICS_01http://physics.rsna.org/enroll.asp?id=PHYS1809http://physics.rsna.org/enroll.asp?id=PHYS2809


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Radiography - Disadvantages

Two-dimensional image of three-dimensional object

Poor low contrast performance

However, Doses are low (entrance surface): PA Chest-0.1 mGy, Skull-2 mGy,

Abdomen-4mGy, Hand-0.3 mGy

CT doses are several cGy(10 mGy = 1cGy = 1 rad = 1000 mrad)


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Two Images

Radiography Computed Tomography


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X-ray CT is a cross-sectionalimaging modality that derivesa two dimensionaldistribution of x-rayattenuation from one

dimensional projections In x-ray CT, the primary

quantity is attenuation,derived from transmission

measurements Using a stack of relatively

thin slices, the threedimensional problem is

reduced to a two dimensionalone

What is CT?


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Thus, for each slice, we

make X-ray transmissionmeasurements with manyrays covering the full widthof the patient at each ofmany angles

This gives us sufficient datato reconstruct the twodimensional cross-section

The reconstruction is done

using filtered backprojection

The CT measurement process

Axial CT: Scanner rotates, then patient/couch translates to measure the

next slice, rotate, translate, etc.

Helical or spiral CT: continuous motion of couch and gantry rotation


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Computed Tomography Image shows gray levels for Hounsfield units associated

with each of 5122pixels

Typical settings: 120 - 140 kV, 200 - 300 mA

(Note: 80 or 100 kV being used to reduce dose and/or

enhance iodine contrast, lower mA for screening orpediatric exams to reduce dose)

Gantry rotation times: 0.33s to 2.0 sdepends on CT

scanner design and type of exam


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CT Numbers-Hounsfield Units (HU)

11

water

water-y)(x,1000y)(x,CT#

CT# (water) = 0

CT# (air) = -1000

CT# (soft tissue) = -300 to +100CT# (bone, I) = up to +3000


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History of CT1895 Roentgen discovers X-rays

1917 Radon solves mathematical problem of determining a 2D distribution

from its 1D projections (or line integrals)1958 Soviet scientists develop plans for x-ray CT scanner. Work unknown

outside of USSR until many years later

1960s Oldendorf, Cormack, and Kuhl independently investigated this concept

in medical imaging

1967 Hounsefield initiates development of an x-ray brain CT scanner for

clinical use at EMI Ltd. (some money came from the Beatles!)

1971 First CT scanner installed at Atkinson Morely Hospital, London

1972 First scanners installed in US

1975+ CT scanners first used for radiation therapy 3D treatment planning

1979 Hounsfield and Cormack share Nobel Prize for Medicine

1989 First Spiral CT scanners1998 First Multi-slice CT scanners

2000 > 3000 clinical CT installations


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Before CT:

Conventional Tomography

Transverse Axial Tomography (TAT)

Film cassette parallel to beam direction, no

mathematical reconstructionpurely optical!


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Toshiba TAT


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TAT-Transverse Axial Tomography:

Toshiba Unit in Radiation Oncology

E l d l


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EMI Mk1 head onlyscanner, introduced 1971

1st generation, i.e.translate-rotate geometry,parallel rays, pencil beam,one NaI detector per slice

180 rotation, 5 mins peracquisition (2 slices), 5mins per reconstruction

Only 160 x 160 matrix

Water bag/box

Early CT development

EMI


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Phillips Big Bore80cm bore diameter,

flat table top

EMIMk11972

GE VCT 64Slice2006-Present


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PET/CT SimulatorGE Discovery ST

w LightSpeed Ultra CT (8 Slice)

Front View Rear View

4 G i f CT S


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First generation: pencil x-ray beam

and a combination of translation and rotation

Second generation: fan x-ray beam, multiple detectors and

a combination of translation and rotation

Third generation: fan beam and a combined rotational

motion of the x-ray source and ~ 500 to 900 detectors

Fourth generation: rotational motion of the x-ray tube

and a stationary array of ~ 1200 detectors

4 Generations of CT Scanners

H li l (S i l) i


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Helical (Spiral) scanning

Note: Tomotherapy is based on the concept of helical CT, except kV x-ray

tube is replaced by 6MV Linac

Tube and detector

continuously rotate

Patient couch continuously

translates


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Helical CT (Also called Spiral)

21

isocenteratmmwidthcollimator

rotationpermmmovementtablePitch

)(

)(

Note: Pitch < 1.0 means slices overlap. Gives better image quality, but

higher patient dose and longer scan times. Required for respiratory gated scans.

l l h l l


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Single slice helical scanning


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Improved high voltage

generator technologymade units smaller

Slip ring technologyenabled them to beplaced on the rotatingpart of gantry

This permittedcontinuous rotationwithout interscandelays

Set the stage for spiralscanning

Size reduction & continuous rotation slip rings

Key: 1. Tube, 2. Collimator, 3. Tube Controller, 4. HV Gen (-),

5. Detector, 6. DAS, 7. HV Gen (+), H. OB Comp., 9. Stat Comp.


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X-ray tubes


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Note: detector response time must be < 1ms:

>1000 projection images acquired during


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Multi-slice ormulti-detectorrow CT

Driven by

X-ray tubeheat loading

Faster scans

Morepractical thinslices

Improvedspiral

interpolation

Multi-slice or multi-detector row CT

Note: Modern CT

scanners have 8-256

rows of detectors


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Computed Tomography Summary

1st Generation: Rotate - Translate - 5 min 2nd Generation: Multi-detector - 20 sec

3rd Generation: Detectors and source rotate -

0.33 sec or less per rotation 4th Generation: Source Rotates, detectors

fixed

5th Generation: Sweeping electron beam -cardiac studies - ms per image

Detectors: Solid State or xenon

T h i l d l t 1972 2000


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Technical developments 1972 - 2000

Note: Massive amounts of imaging data acquired by a busy

DepartmentTerabytes/yr. >40% of all digital data in the

entire world will soon be medical images! Need PACS.

(decreases as more x rays interact in slice


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(decreases as more x-rays interact in slice

being imaged)


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(produced by high-Z objects)

Note: Results from fact that linear attenuation coefficient is proportional

to electron density AND atomic number (Z3)


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CT reconstruction


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CT reconstruction

1. Each slice in the patient

consists of 512x512 pixels

2. We make thousands of

individual attenuation

measurements thru the slice

from many directions

3. Essentually, `x unknowns(voxel attenuation values)

and `x equations (attenuation

measurements

4. Can in principle be solved as`x simultaneous equations

h


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Transmission through a uniform block of material

I = Io exp(-X) , where (linear attenuation coefficient)depends on r, Z, Ex

Transmission through a heterogeneous block of material

I = Io exp(-(1x1 + 2x2 + 3x3 + 4x4 + 5x5 ++ nxn))

Or, if we define the pixel size

I = Io exp(-x(1 + 2 + 3 + 4 + 5 ++ n))

Rearranging with measurable or known quantities on the left,and the unknowns to be determined on the right

-(1/x)ln(I/Io) = (1 + 2 + 3 + 4 + 5 ++ n)

The basic transmission attenuation equations

CT t ti


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CT reconstructionNote: Although

mathematically correct,

solving thousands of

simultaneous equations

is not a practical way

to calculate image

reconstructions.


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Backprojection


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Filtered Backprojection


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Digital Display: Window/Level

N b H


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CT Numbers - HU

Hounsfield units = 1000 x ( - water) / water


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CT Numbers vs. Electron Density

Because the CT numbers bear a linear relationship withthe attenuation coefficients, it is possible to infer

electron density (electrons cm-3) as shown in Figure 12.4

from Khan. Although CT numbers can be correlated

with electron density, the relationship is not linear in theentire range of tissue densities. The nonlinearity is

caused by the change in atomic number of tissues,

which affects the proportion of beam attenuation by

Compton versus photoelectric interactions. Figure 12.5shows a relationship that is linear between lung and soft

tissue but nonlinear between soft tissue and bone.

CT Number (HU) vs Electron Density


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CT Number (HU) vs. Electron Density

Fig. 12.5 Khan

4th Ed.


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Why is this so Important?

With CT we are measuring linear attenuation coefficient with

x-rays of 1MeV for whichCompton Effect is dominant interaction, for which the linear

attenuation coefficient depends mostly on electron density


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47

33 cm


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48


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Relative Electron Density

1.0 for hydrogen

0.5 for helium thru calcium (Z = 20, A = 40))

0.4 for uranium (z=92)

Photoelectric cross section proportional to Z3

kV fan beam


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50

Air

Brass

Aluminium

Steel

120 kV

PMMA

PMMA

974.140.53%

18.76195%

39760 (saturated?)

2366.4422.6%

39760 (saturated?)

838.220.92%

2x MVCB, 0.005


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51

,

MU/frame

2.6 MU total

Air

Brass

Aluminium

Steel

PMMA

PMMA

2228.947.95%

577.5729.7%

80000 (saturated?)

4068.57.95%

80000 (saturated?)

2012.667.75%


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Patient doses from CT


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NCRP

Report

No. 160

Operating parameters


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X-ray Tube Voltage (kVp)

X-Ray Tube Current (mA)Scan Time (sec)

Scanner Rotation Angle

Beam On - Start Angle

Filtration

Field Size - Scan Diameter

Patient Position within Field

Patient OrientationSource Collimation

Slice Thickness

Slice Spacing - Pitch

Number of Adjacent Slices

CT il i i ti h b


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CT pencil ionization chamberFor CT Dose Measurements

NCRP R t N 160


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NCRP Report No. 160

56

Typical CT scattered dose levels


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Typical CT scattered dose levels

GE Medical Systems


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Effective Dose (E) Factors

E for adults can be calculated from product

of DLP and normalized effective dose

factors from (Shrimpton et al, BJR 2006)

Head 0.0021 (mSv / mGy-cm)

Neck 0.0059

Chest 0.014

Abdomen 0.015

Pelvis 0.015


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Effective Dose, E(Prev. Effective Dose Equivalent, H

E)

Same Probability of Occurrence of Cancer and

Genetic Effects as for Whole Body Uniform

Dose

Thorax: 11 - 15 mSv (Gelieijns)

Abdomen: 15 - 20 mSv

Head: 1 - 2 mSv

Note: CTDIvol Higher for Head

Higher for Children

CT Simulation


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CT simulator combines some of the functions of imaging for radiation

therapy planning, the computerized treatment planning system, and the

conventional simulator

CT Simulator contains:

CT Scanner

Patient couch that simulates Linac treatment couch

Laser localization System similar to Linac treatment room lasers

Computer graphics workstation image manipulation, target volume

and normal tissue delineation, beam geometry display

Interface to treatment planning system: scanner can export images,

contours, plus isocenter coordinates to treatment planning system viaelectronic network or via `sneaker net


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Simulation Procedure

Patient Positioning and immobilization

Scouts for patient alignment

CT Scan Isocenter definition

Isocenter Marking using laser localization

system

Isocenter tattoos and bi-angulation or tri-angulation tatoos, cast lines


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Special Issues for RT Treatment Planning


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Special Issues for RT Treatment Planning

Body casts, frames, masks, and immobilizers

Couch top sag

Alignment lasers

Slice thickness and number of slices

Generating DRRs and 3D reference images for OBI

Sometimes thinner slices needed to generate DRRs thanfor treatment planning

CT scanner images do NOT look the same as CBCTimages!

Fusion with MR or PET

Converting CT-numbers to electron density

Respiratory gating

Structure contouring


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Digitally Reconstructed Radiographs (DRR)

and reference images for kVCBCT


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(Contouring)


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Third Generation


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Helical Scanning


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Helical Scanning


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ResolutionRays and Views


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Rays


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Views

Multi-slice CT pitch


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p

P' = Table travel per rotation / Nt

WhereN= number of data channels

t = the z-axis width of one data channel of an N-channel multi-slice detector.

Multi-planar Reconstruction


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(not just axial views)

GE Lightspeed 16


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g p

Pitch: Single Slice CT vs MDCT


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Pitch: Single Slice CT vs. MDCT

85

isocenteratwidth(mm)Collimator

gantryofrotationdegree-360per(mm)movementTablePitchCollimator

isocenteratwidth(mm)Detector

gantryofrotationdegree-360per(mm)movementTablePitchDetector

NPitchDetectorPitchCollimator

For Single Slice:

For MDCT:

Use collimator pitch for MDCT to be consistent with pitch

for single slice

Computed tomography dose index - CTDI


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CTDI Ideal =

1

T

D1(z) dz-

CTDIReg = 1nT

D (z) dz

-7 T

+7 T

CTDI100=1

nT Da(z) dz

-50 mm

+50 mm

CTDIw =

(2/3) x CTDI100-peripheral

+ (1/3) x CTDI100-axial

(New IEC CT Dose Quantity)

Note: Use f-factor for air (8.69 mGy/R),not PMM (7.8 mGy/R)

(New IEC CT Dose Quantity)


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Volume CTDI

Spiral: CTDIvol = CTDIw / Pitchor

Axial: CTDIvol = CTDIw * NT/I



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p g p

Bolus

Patient Position Within FieldPatient Orientation

Repeat Scans

Image ParametersSpecial Techniques

Other Factors

The Patient Size and TissueComposition

Anatomy Being Imaged

Generally: Increased Dose Provides BetterLow Contrast Performance


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Normal tissue damage vs. radiation dose for organs

with large volume effect


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with large volume effect

(e.g., liver, lung, kidney)

0 50 100 150 2000

0.2

0.4

0.6

0.8

1

1.2

Dose (Gy)

NTCP

D50 = 29.5 Gy, m = 0.18

1 2/3 1/3 1/6

Normal tissue damage vs. radiation dose for organs

with small volume effect


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with small volume effect

(e.g., spinal cord, optic chiasm)

0 50 100 150 2000

0.2

0.4

0.6

0.8

1

1.2

Dose (Gy)

NTCP

D50 = 29.5 Gy, m = 0.18


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Tube Motion

Principles

ReappearingIn Digital

Tomosynthesis

Conventional Radiography


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Helical, or Spiral CT

The Patient couch advances at a constant


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The Patient couch advances at a constant

speed through the gantry while the x-ray

tube rotates continuously around the

patient (slip ring technology)

The acquired transmission data can be

reconstructed to provide images at any

point along the patients axis during scan and

slices as thin as 1 mm can be obtainedquickly

Reduces conventional scan times of 20-30

min to 5-10 min

Pitch distance, in mm, the couch movesduring one revolution of the x-ray tube

Pitch Factor pitch divided by the

collimated slice thickness (range between 1

and 2)

Single Slice vs. Multi-row


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g

Multi-row (slice) CT


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( )

01 amols ct talk for iaea china course

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