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Computed Tomography (CT):Physics and Technology

Clinical Applications

Computed Tomography (CT):Physics and Technology

Clinical Applications

Ontario Cancer Institute

Princess Margaret HospitalUniversity Health Network

Medical BiophysicsMedical ImagingIBBME

JH Siewerdsen PhD

Dept. of Medical Biophysics, University of TorontoOntario Cancer Institute, Princess Margaret Hospital

[email protected]

JH Siewerdsen PhD

Dept. of Medical Biophysics, University of TorontoOntario Cancer Institute, Princess Margaret Hospital

[email protected]

M OMalley MD

Dept. of Medical Imaging, University of TorontoDept. of Medical Imaging, University Health Network / Mt. Sinai Hospital

[email protected]

M OMalley MD

Dept. of Medical Imaging, University of TorontoDept. of Medical Imaging, University Health Network / Mt. Sinai Hospital

[email protected]

Computed Tomography (CT)

- Basic principles of CT

Natural history of scanner technologies (generations)

- CT reconstruction

Fourier slice theoremFiltered backprojection

Other techniques

- Image quality / artifacts

Physical factors

Performance metrics

- Radiation dose

Magnitude and risk (in context)

- ApplicationsDiagnostic imaging IG interventions Radiation therapy

OverviewOverview

Circa 1895

Projection radiographyI0

I

I = Ioe-(x,y)dy0

d

Computed Tomography

P = ln(Io/I)= (x,y)dy

Sir Godfrey Hounsfield

Nobel Prize, 1979

source

9-day acquisition 2.5-hr recon

Detector

Turntable

and linear track

Hounsfields CT Scanner

First Generation CT

x

Scan and Rotate:

Linear scan of source and detector

Line integral measuredat each position: P(x)

Rotate source-detector

Repeat linear scan

Projection data: P(x;)

x x x x x x x

P(x)


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CT Generations

WA Kalender, Computed Tomography, 2nd Edition (2005)

1st Generation (1970)

Pencil Beam

Translation / Rotation

2nd Generation (1972)

Fan Beam

Translation / Rotation

CT Generations

3rd Generation (1976)

Fan Beam

Continuous Rotation

4th Generation (1978)

Fan Beam

Continuous Tube RotationStationary Detector

The Fourier Transform of a projection of an object at a given angle

yields a slice of the Fourier Transform of the objectat the corresponding angle in the Fourier domain.

Fourier Slice Theorem

f(x,y)

y

x

v

u

FT

F(u,v)

CT Image Reconstruction

Fourier Slice Theorem

v

u

f(x,y)

y

x

p( )

X-rays

F(u,v)

[p( )]


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y

x

v

u

f(x,y) p( ) F(u,v)

-1[F(u,v)]

Backprojection

Simple Backprojection:

Trace projection data P(x;)through the reconstruction matrix

from the detector (x) to the source

Simple backprojection yields

radial density (1/r)

Therefore, a point-object is

reconstructed as (1/r)

Solution: Filter the projection databy a ramp filter |r|

P(x;)

X-ray source

Sinogramp(x,)

Sinogram:

Line integral projection: p(x)

measured at each angle ()

Projection data (sinogram): p(x;)

x

p(x;)

Sinogram

p(x)= ln(Io/I)= (x,y)dy

Sinogram

Filtered Sinogram


Filtered Back-Projection

Object

Projection p(,)


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Back

-Proje

ct



Filtered SinogramObject Space

Back-P

roject



Filtered SinogramObject Space

Reconstructed

Image

Filtered Backprojection: Implementation

Projection at angle

p(,)

Filtered Projection

g(,)

Backproject g(,).Add to image (x,y)

(x,y)

Loopoverallviews(all)

Helical CT

WA Kalender, Computed Tomography, 2nd Edition (2005)

Slip ring gantry

Continuous gantry rotationContinuous couch translation

Pitch =Table increment / rotation (mm)

Beam collimation width (mm)

Pitch 1:Non-overlap

Lower z-resolutionLower patient dose


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Recent Advances:Dual-Source CT

Two complete x-ray and data acquisition systems on one gantry.330 ms rotation time

(effective 83 ms scan time)

Siemens Medical Solutions Somatom Definition

Recent Advances:Multi-DetectorCT

Multiple slices acquired in

each revolution

Higher speed

Reduced slice thickness

(Improved axial resolution)

GE Light Speed multi-row CT detector

4x

1.25 mm

4x

2.5 mm

4x

3.75 mm

4x

5.0 mm

From Fan to Cone

Recent Advances:Multi-DetectorCT

Fast (whole-body) scans

at high resolution (thin slices)Dynamic (4D) imaging


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Recent Advances: Cone-Beam CT

Fully 3-D Volumetric CT

Conventional CT:

Fan-Beam

1-D Detector Rows

Slice Reconstruction

Multiple Rotations

Cone-Beam CT:

Cone-Beam Collimation

Large-Area Detector

3-D Volume Images

Single Rotation

Cone-Beam CT

Projection data (2D)

200 2000 projectionsover 180o 360o

Volume reconstruction

~1 mm spatial resolution+ soft tissue visibility

CT Detectors

K. Kanal, University of Wisconsin

Gas (Xenon)

Conventional (old)

Single-slice CT only

Scintillator / Semiconductor

State of the art

Well-suited to MDCT

Single-Slice CT vs Multi-Detector CT

K. Kanal, University of Wisconsin


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Cone-Beam Filtered Backprojection

Weight Filter2D

Interpolation

Geometry

# of voxels

# of projectionsRepeat

Reconstruction

Volume

CT Image Reconstructions

1975

Liver

GB

Spine

Spleen

AO

Pancreas

2000

282

237

Contrast

Contrast =I1 I2

(I1 + I2)/2

CT Radiograph

6325 25

252524182219251920 40

20214022 17 3019

Why CCT >> Crad?

CCT =6325

(63+25)/2=86%

Crad =282237

(282+237)/2=17%

CT Number (Pixel Value)

Hounsfield Units (HU)

The CT image pixel values have units of

the attenuation coefficient, (cm-1 or mm-1)

Commonly converted to a convenient scale:Hounsfield Units (HU)

HU =- waterwater

1000 (+1000)(sometimes)

Brain (8)

Fat (-100)

Liver (+85)

Breast (-50)

Water (0)

Polyeth (-60)


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NoiseNoise:

Standard deviation in voxelvalues in an otherwiseuniform region of interest

(4.6 3.2)(5.6 2.4) (-1.3 6.2)

(3.8 4.2)

(4.4 4.2)

cf

winTTdf0

2interp

2

Bandwidth Integral

xyK

zxyo

Evox

aaD

k3

2

= xy

K

(Fourier domain integral over the

low-pass smoothing filters)

Minimum resolvableline-pair group

Spatial ResolutionFactors affecting spatial resolution:Focal spot sizeDetector pixel sizeSlice thickness

Pitch

Number of projectionsReconstruction filter (kernel)Field of viewPatient motion

Metrics of spatial resolution:Minimum resolvable line-pairPoint-spread function (psf)Modulation transfer function (MTF)

www.impactscan.org

Smooth Sharp

Reconstruction Filter

Reduced Spatial ResolutionLower Noise

Improved SNRImproved Soft-Tissue Visibility

Improved Spatial ResolutionHigher NoiseReduced SNR

Reduced Soft-Tissue Visibility

Artifacts

Rings Shading

Lag

Motion

Metal

Streaks

Cone-BeamTruncation


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Dosimetrics

Measure Common Units SI UnitsActivity Ci Bq (disintegrations / sec)Exposure R C/kg (ionization in air)Absorbed Dose rad Gy (1 Gy = 1 J/kg = 1 Rad)

Effective Dose rem Sv (1 Sv = 100 rem)

Some forms of radiation more efficient than others at transferring energy to the cell.

To level the playing field, multiply dose (Gy) by a quality factor (Q).

Q compares biological damage to that associated with the same dose of X rays

(photons). The resulting unit is the Sv (seivert). Thus, Sv = Gy x Q.

1 Sv is the amount of (any type of) radiation which would cause the same amount of

biological damage as would result from 1 Gy of X rays.

CT Dose Measurement (CTDI)

Kanal, University of Wisconsin

Dose estimate from a single scan:CT Dose Index (CTDI)

CTDI =f X

T L

f = exposure-to-dose factor (mGy/R)X = exposure (R)

L = length of ion chamber (100 mm)T = slice thickness (mm)

Standard (Cylindrical) Phantoms:Head (16 cm diameter acrylic)Body (32 cm diameter acrylic)

Radiation Dose

Bushberg, The Essential Physics of Medical Imaging, 2ndEd.

CTDIw =

Surface dose > Central doseHead: (Dsurf/ Dcenter) ~1Body: (Dsurf/ Dcenter) ~2

CTDIw combines:

Peripheral dose: CTDIperiphCentral dose: CTDIcenter

(2/3 CTDIperiph ++1/3 CTDIcenter)

Electrometer (mGy / C)

Ion Chamber

16 or 32 cm DiameterAcrylic Cylinder

center

periphery

Factors Affecting Radiation Dose

Typical Skin Dose:Head ~ 20 mGyBody ~ 40 mGy

(induction of erythema: ~2 Gy)

kVp

mAs

Kanal, University of Wisconsin

Dose

~(kVp)2

Dose mAs


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Effective Dose

Region FactorHead 0.0023

Neck 0.0054Chest 0.017Abdomen 0.015Pelvis 0.019

(mSv/mGy.cm)

30 mGy x 30 cm = 900 mGy.cm

EffectiveDose

(mSv)

2-8

10-2010-20

20 mGy x 50 cm = 1000 mGy.cm

Effective Dose

ExamSkullChest (PA)

AbdomenPelvisBa swallowBa enema

HeadChestAbdomenPelvis

EffectiveDose (mSv)

0.070.021.00.7

1.57

28

10-2010-20

Equivalent# CXR

3.515035

75350

100400500500

Approx. PeriodBackroundRadiation

-3 days

6 months4 months

--

3.6 yrs4.5 yrs4.5 yrs

(typical background

= 3 mSv / yr)

Radiography

CT

Key to numerous areas of medical imaging

- ScreeningE.g., low-dose CT screening of early-stage lung cancer

- DiagnosisE.g., almost everything

- Staging and prognosisE.g., PET-CT

- Treatment planningE.g., Dose calculation in radiation therapy

- Image guidance

E.g., CT-guided biopsy, interventions, surgery, and RT- Response assessment

E.g., Tumor regression; perfusion changes

- Pre-clinical imagingE.g., Micro-CT of mice (drug development, etc.)

Computed TomographyComputed Tomography

Remaining Challenges

- Reduced imaging doseE.g., pediatrics mA modulation Low-dose protocols

- Imaging speed

Cardiac imaging 4D CT-fluoroscopy- Image quality

E.g., Improved SNR Artifact management

Computed TomographyComputed Tomography

Ongoing Developments

- Multi-detector CT (The Slice Wars)Single-slice 8 16 64 256 slice Volume CT

- Alternative source configurations (The Source Wars)Dual-source Multiple-source No moving parts

- CT imaging functionality and applications

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