Robert Speller Medical Physics & Bioengineering Department UCL, London, UK.

absorption

Phase contrast

•  Background to X-ray phase contrast imaging •  Absorption & phase techniques

•  Advantages and disadvantages •  Possible options for clinical application

•  Successes so far

•  The Coded Aperture approach •  An in-vitro trial •  Results

•  Absorption based mammography is the current technique for screening for and assessing breast cancer

•  There are some known limitations

•  X-ray phase contrast has been shown to provide improved definition of important structures

•  …but there are some problems..

Absorption •  All current clinical systems

record X-ray attenuation differences to create the image

•  Differences in the attenuation coefficients and thicknesses of tissues create contrast

Phase contrast •  Phase effects are based on

refraction •  The general form of the

refractive index is: Absorption

component Phase component

Energy (keV)

η

η = 1 – δ + iβ

NB: magnitude & energy dependence of δ

Phase contrast imaging – Arises due to

refraction – change of velocity when an X-ray beam enters different materials

Conventional transmission

Phase contrast imaging Recorded intensity

Methods for creating X-ray Phase Contrast images

sensor

coded apertures

X-ray tube

Free-space propagation

Coded Aperture Approach

• Conventional polychromatic source

• Divergent cone beam

• Dose reduction

1

Analyser crystal

DEI sensor

Synchrotron beam

2

4

Interferometric

XPCi sensor 3

Synchrotron or micro-focus X-ray tube

Synchrotron or X-ray tube

object

crystal

gratings

Clinical potential of Phase Contrast imaging

Systems •  1. Free space propagation

–  Synchrotron –  X-ray tubes

•  3. Interferometry –  X-ray tubes

•  4. Coded aperture –  X-ray tubes

Work

–  Elletra –  Konica Minolta

–  Franz Pfeiffer (Technical University of Munich) + others

–  Sandro Olivo & Robert Speller (UCL)

NB: there are no type 2 systems proposed for clinical work

• A total of 20 beamlines are available.

• SYRMEP is devoted to medical physics applications

Conventional Phase contrast

Phase contrast Conventional

Conventional Phase contrast

1b. Free space propagation with X-ray tubes

•  Generally uses micro-focal X-ray sources in the laboratory

However…. •  Konica Minolta

system uses ~100µm focal spot but few results to date

(http://www.konicaminolta.com/healthcare/technology/phasecontrast)

Phase contrast absorption

3. Phase contrast imaging use interferometric techniques (TUM, Franz Pfeiffer)

Pfeiffer. F., et al., Nature Physics (2006), 2(4), 258.

•  System set up as a Talbot Lau Interferometer

•  G--0 is the source grating used to make the incoherent source into a series of independent coherent sources

•  G--1 is a phase grating

4. Phase contrast imaging using coded apertures (UCL)

Coded aperture system

aperture 1

aperture 2

detector

Beam

Detector pixels Aperture 1

Beam

tissue

Olivo, A.; Speller, R.,2006, Phys. Med. Biol., 51, 3015-3030

phase

absorption

An in-vitro trial •  98 samples were imaged in absorption and

phase contrast modes at clinical dose levels •  3 experienced breast radiologists viewed the

images and made assessments

•  3-way ANOVA analysis was carried out

Phase vs absorption

Histograms of the differences in the scores between phase and absorption images

+ve +ve -ve +ve +ve -ve +ve

Unpublished results

Conclusions •  Phase contrast has been shown to provide

enhanced images in mammography •  Translation of the coded aperture approach to

phase contrast imaging to the clinic should be possible but further work required –  Improved apertures/detector –  Improved source power to reduce exposure time

Acknowledgements: The UCL/Dundee/Barts team Sandro Olivo, Peter Munro, Konstantin Ignatyev, Charlotte Hagen, Magdalena Szafraniec, Endrizzi, Paul Diemoz, Spyros Gkoumas, Sarah Vinnicombe, Janet Litherland, Andy Evans, Louise Jones The funding The Wellcome Trust, EPSRC, Home Office