Advanced Imaging Techniques - umm.uni-heidelberg.de · 14.11.2018 1 Advanced Imaging Techniques...

14.11.2018

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Advanced Imaging TechniquesDual Energy Computed Tomography

Prof. Dr. Frank G. Zöllner

(Today: Dominik Bauer)

Computer Assisted Clinical Medicine

Medical Faculty Mannheim

Heidelberg University

Theodor-Kutzer-Ufer 1-3

D-68167 Mannheim, Germany

[email protected]

www.ma.uni-heidelberg.de/inst/cbtm/ckm

Prof. Dr. Zöllner I Slide 2I 11/14/2018

Learning Goals

� introduction to CT imaging

� basic CT principles -> Physics of Imaging Techniques

� Goals:

1. How does the technique work?

2. What kind of images do we receive?

3. Where is this applied to?

� Slides of the lectures at

https://www.umm.uni-heidelberg.de/inst/cbtm/ckm/lehre/index.html

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References

� Johnson, “Dual-Energy CT: General Principles”, 2012.

� K. Cranley et al., “Catalogue of diagnostic x-ray spectra and other data”, 1998.

� Schlegel, Bille, “Medizinische Physik II”, 2002.

� McCollough et al., “Dual- and Multi-Energy CT: Principles, Technical Approaches, and Clinical Applications”, 2015.

� Hansmann et al., “Correlation analysis of dual-energy CT iodine maps with quantitative pulmonary perfusion MRI”, 2013.

� Goo and Goo, “Dual-Energy CT: New Horizon in Medical Imaging”, 2017.

� NISTIR 5632: “X-Ray Mass Attenuation Coefficients”, https://www.nist.gov/pml/x-ray-mass-attenuation-coefficients, July 2017.

� Slavic et al, Technology White Paper, GSI Xtream on RevolutionTM CT, 2017

� Ravi et al, Dual-Energy CT with Single- and Dual-Source Scanners: Current Applications in Evaluating the Genitourinary, RadioGraphics2012 32:2, 353-369


Motivation

80 kVp

140 kVp

Max. contrast

Dual-Energy CT

Iodine map

Dual-Energy CT

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X-Ray Properties: Attenuation

X-ray source Attenuation Projection

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X-Ray Properties: Attenuation

X-ray source Attenuation Projection

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Photon Attenuation Effects

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Photoelectric effect Compton effect


Photoelectric Effect

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Photon Attenuation Cross-Section

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Cortical Bone Iodine


Photon Attenuation Cross-Section

Cortical Bone Iodine

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Photon Attenuation Ambiguity

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Photon Attenuation Ambiguity

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Physics

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Dual-Energy CT

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DECT: Components

Detector:

�Spectral sensitivity

X-ray tube:

�Voltage

�Current

Image quality

Filter/Collimation:

�Hardening

�Filtering

bandwidth energy throughput

efficiency

Spectral separation

attenuation

Dose

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DECT: Implementations

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DECT: Sequential Scanning

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DECT: Rapid Voltage Switching

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DECT: Dual Source

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Dual Source CT

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Comparison of Dual Energy CT Imaging Devices

Ravi et al, RadioGraphics 2012


Dual-Energy CT

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DECT: Image Processing

� Dual energy CT offers two image informations of the same material

� allows for material decomposition

� blended images

� virtual unenhanced images

� monocromatic images

� Processing of DE images in

� projection space

� image space


DECT: Image Processing in Projection Space

� conventional X-ray source have a broad energy spectrum

� separate the attenuation coefficient into the contributions

� from photoelectric effect and Compton scattering

� can be approximately modeled using

� a material’s effective atomic number ( Zeff)

� effective mass density (reff )

� knowledge of the X-ray energy spectra ( E )

� Alternative: attenuation coefficient, @A , can be expressed with sufficient

accuracy as a linear combination of the photoelectric and Compton

attenuation coefficients

B C � � DEF)1D�GH9 C ( @

A IDJGKHI C

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DECT: Image Processing in Projection Space

� DE signal can be written

with L9 � ��MH9 DCG and LI � ��MHI DCG represent the attenuation

density line integrals for the two basis materials


DECT: Image Based Processing

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� simplest way to process dual-energy data

� Perform a weighted subtraction or addition for the separately

reconstructed images at different beam energies (i.e., image

blending)

� low-voltage images (typically 80 kV) are multiplied by a weighting

factor and subtracted from or added to the high-voltage images (140

kV) to obtain material-specific information



� simplest way to process dual-energy data

� Perform a weighted subtraction or addition for the separately

reconstructed images at different beam energies (i.e., image

blending)

� low-voltage images (typically 80 kV) are multiplied by a weighting

factor and subtracted from or added to the high-voltage images (140

kV) to obtain material-specific information

14

0 k

Vp

80

kV

p

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DECT: Image based processing – Types of Images

� Blended Image

� nonmaterial-specific images generated from the dualenergy data

to provide images for the purpose of routine diagnostic

interpretation

� Material-Selective Image

� differentiation between different atomic elements and therefore

chemical compositions

� Energy-Selective Image

� a pseudo monoenergetic image created at any desired energy


Blended Image

� combining the low- and high energy images, mixed images utilize all

of the radiation doses delivered by the dual-energy scan

� better image quality

� Generating the highest iodine contrast to noise ratio (CNR)

� two types of image blending methods

� linear and nonlinear

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Blended Image

� Linear blending

with wL + wH = 1.

� Increasing the blending ratio more toward the 80 kV image

� Iodine contrast enhancement but more noise

� Increase towards high energy image

� higher CNR and less noise

� Optimal settings for best settings exist


Blended Image

� Non linear blending

� combine images by

various weighting

functions

� e.g. modified sigmoidal

blending

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Blended Image


Material Decomposition / material selective image

� DE CT offers decomposition into two materials

� total mass attenuation coefficient of an object containing two

constituent elements is the weighted summation of the two element’s

mass attenuation coefficients

� assume the dual-energy measurements are made with two

monochromatic X-ray beams of high energy EH and low energy EL

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� effective linear attenuation coefficients µeff can be obtained from the

dual-energy CT image data:

� perform chemical identification (i.e., material decomposition) based on

previous equations using the reconstructed dual-energy images


DECT: Two Material Decomposition

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DECT: Three “Material” Decomposition

� DE CT can quantify 2 mass fraction -> 2 independent measurements

� for 3 materials and 2 spectral measurements one need additional

condition

� assume sum of volumes of the 3 materials is equivalent to the mass of

ist mixture

N9 ( NI ( N> � �

� therefore effective density �eff

H5OO � N9H9 ( NIHI ( D� � N9�NIGH>

B� � $9B9 ($I BIP$>B> �K�H5OODN9H9B9 ( NIHIBI ( N>H>B>G


DECT: Three “Material” Decomposition

Iodine slope Virtually Un-enhanced

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Application - Angiogram Bone-Removal

Two material decomposition

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Contrast material map


Application - Pulmonary Perfusion

Contrast Agent Map(Iodine based)

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Application - Gout Diagnosis

calcium pyrophosphate crystal

uric acid


Virtual noncontrast- enhanced Images

Axial CE DE scan, mixed images shows subcapsular menatoma (*)

and low attenuation lesions (arrow)

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Iodine overlay demonstrate idodine within one lesion

indicating a metastasis, the other a hematoma

Virtual noncontrast- enhanced Images


Atherosclerotic Plaque Removal

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Dual-Energy CT Summary

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Outlook/Honorable Mentions

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