Medical Imaging Technology

135 Pages·2013·4.382 MB·English

by  Haidekker, Mark A

SPRINGER BRIEFS IN PHYSICS
Mark A. Haidekker
Medical Imaging
Technology
SpringerBriefs in Physics
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Egor Babaev, University of Massachusetts, USA
Malcolm Bremer, University of Bristol, UK
Xavier Calmet, University of Sussex, UK
Francesca Di Lodovico, Queen Mary University of London, UK
Maarten Hoogerland, University of Auckland, New Zealand
Eric Le Ru, Victoria University of Wellington, New Zealand
James Overduin, Towson University, USA
Vesselin Petkov, Concordia University, Canada
Charles H.-T. Wang, University of Aberdeen, UK
Andrew Whitaker, Queen’s University Belfast, UK
For furthervolumes:
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Mark A. Haidekker
Medical Imaging
Technology
123
MarkA.Haidekker
College ofEngineering
Universityof Georgia
Athens,GA
USA
ISSN 2191-5423 ISSN 2191-5431 (electronic)
ISBN 978-1-4614-7072-4 ISBN 978-1-4614-7073-1 (eBook)
DOI 10.1007/978-1-4614-7073-1
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Preface
Amongmanymajordevelopmentsinthemedicalfieldoverthepasttwocenturies,
I personally consider three of them of particularly outstanding importance: The
discovery of antibiotics inthe late nineteenth century, the discovery of anesthesia
during a period from the late eighteenth century to the mid-1800s, and the dis-
covery of methods to look inside the human body without surgery. Out of these
three,biomedicalimagingistheyoungestdevelopment,anditsbeginningscanbe
pinpointedtoaveryprecisepointinhistory—thediscoveryoftheX-raybyC.W.
Röntgen in 1895.
The history of medical imaging is a fascinating topic in itself, and it is briefly
coveredinChap.1.Mostnotably,thehistoryofmedicalimagingiscloselylinked
to the evolution of digital data processing and computer science, and to the evo-
lution of digital electronics and the microprocessor. Medical imaging is truly
interdisciplinary as it relies on physics, mathematics, biology, computer science,
andengineering.Thisbooktriestoprovideasolidfoundationoftheprinciplesthat
lead to image formation. Specifically, many books on the same subject are
intended for a medical or more general audience and treat the image formation
process to some extent as a black box.
In this book, the image formation process can be followed from start to end,
beginning with the question of how contrast is achieved. To this end, the source/
detector systems that probe the tissue and provide the data necessary for image
formation are explained. For each modality, this book explains the type of data
collected, and how it is converted into an image. In addition, engineering aspects
of the imaging devices, and a discussion of strengths and limitations of the
modality can be found. In the first chapter, the basic concepts of contrast and
resolutionareintroduced,andsomeconceptsthatarecommontoallmodalitiesare
explained. Subsequent chapters cover specific modalities.
We can broadlydivide the imaging modalities intotwo groups,thosewith and
those without the use of ionizing radiation. Unlike visible light, high-energy
photons undergo only moderate scattering in tissue and can be used to penetrate
the entire body. Most of the high-energy photons (X-ray and gamma radiation)
follow a straight path, and certain geometrical assumptions are allowed that give
rise to projection imaging (Chap. 2) and X-ray-based tomography (Chap. 3).
Emission tomography, based on radioactive compounds that emit radiation from
v
vi Preface
inside the body (Chap. 4) also belong to this category, since gamma radiation is
used for the actual image formation. Imaging modalities with ionizing radiation
sharemanycommonprinciples,andChaps. 2through4partlybuildoneachother.
Magnetic resonance imaging and ultrasound imaging both use fundamentally
different physical phenomena that are covered in Chaps. 5 and 6, respectively.
Finally,Chap. 7deals withrecentdevelopmentsboth inthe traditional modalities
and in new modalities that are not yet widely used in clinical practice.
Since this book places considerable emphasis on the mathematical description
of image formation, the reader should be familiar with calculus and have a basic
understanding of differential equations. Although the Fourier transform is intro-
duced in Sect. 1.4, some familiarity with the Fourier transform is helpful. Prior
understanding of digital signal processing is helpful, too, although not a pre-
requisite for the topics covered in this book.
The chapters in this book can serve as an entry point for the in-depth study of
individual modalities by providing the essential basics of each modality in a
comprehensive and easy-to-understand manner. As such, this book is equally
suitable as a textbook for undergraduate or graduate biomedical imaging classes
andasareferenceandself-studyguidethatpreparethereaderformorespecialized
in-depth studies.
However,anyoneimagingmodalitycouldfillawholebookonitsown,andfor
advanced study of a specific modality, more specialized books are also available.
In-depth coverage of all modalities with an emphasis on the clinical application
(butlessemphasisonthemathematics)isprovidedinthebookbyBushbergetal.
[1]. A practice-oriented, user-friendly, yet highly detailed view of computed
tomography can be found in [2], although it does not provide the mathematical
details of CT image reconstruction. For readers who are interested in image
reconstruction algorithms and their mathematical foundation, the books by Kak
and Slaney [3] or by Herman [4] are recommended. MRI is a comprehensive
subject, and in-depth coverage is provided in the work by Haake et al. [5].
AreferenceforultrasoundimagingcanbefoundinthebookbyHedricketal.[6].
Among many individuals and colleagues who helped to shape this book with
comments and discussions, I would particularly like to thank Erin E. Roberts for
the help I received with some figures, Richard Speir and Dr. Adnan Mustafic for
detailed revisions of the manuscript and helpful ideas how to improve the text,
and to Prof. Qun Zhao and Prof. Geoff Dougherty for reviewing the book.
Furthermore, I would like to express my gratitude toward the team at Springer:
ChristopherCoughlinandHoYingFan,aswellastheproductionteamatSPSwho
ensured a smooth path from concept to publication.
Athens, January 2013 Mark A. Haidekker
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 A Brief Historical Overview. . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Image Resolution and Contrast . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Systems and Signals: A Short Introduction. . . . . . . . . . . . . . . . 6
1.4 The Fourier Transform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 X-Ray Projection Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1 X-Ray Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 The X-Ray Tube. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1.2 A Focus on Geometry. . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2 X-Ray Attenuation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.1 Photon-Matter Interaction. . . . . . . . . . . . . . . . . . . . . . . 20
2.2.2 Macroscopic Attenuation and Lambert-Beer’s Law. . . . . 22
2.2.3 Lambert-Beer’s Law in Inhomogeneous Materials. . . . . . 25
2.2.4 Dual-Energy X-Ray Absorptiometry . . . . . . . . . . . . . . . 26
2.3 X-Ray Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.1 Film-Based Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.2 Fluoroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3.3 Semiconductor Detectors . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.4 Photomultiplier Tubes . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.4 Factors that Determine X-Ray Image Quality . . . . . . . . . . . . . . 34
3 Computed Tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.1 CT Image Formation Principles. . . . . . . . . . . . . . . . . . . . . . . . 37
3.1.1 The Radon Transform and the Fourier Slice Theorem. . . 39
3.1.2 Practical Image Reconstruction. . . . . . . . . . . . . . . . . . . 42
3.2 Engineering Aspects of CT Scanners. . . . . . . . . . . . . . . . . . . . 49
3.3 Quantitative CT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.4 Image Quality and Artifacts . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4 Nuclear Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.1 Radiopharmaceuticals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.2 Production of Short-Lived Radioactive Tracers. . . . . . . . . . . . . 56
vii
viii Contents
4.3 Detector Systems and the Anger Camera . . . . . . . . . . . . . . . . . 57
4.4 Single Photon Emission Computed Tomography. . . . . . . . . . . . 59
4.5 Positron Emission Tomography. . . . . . . . . . . . . . . . . . . . . . . . 64
4.6 Multi-Modality Imaging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5 Magnetic Resonance Imaging. . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.1 Proton Spins in an External Magnetic Field . . . . . . . . . . . . . . . 67
5.2 The Spin-Echo Experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.3 The Spin-Echo Pulse Sequence. . . . . . . . . . . . . . . . . . . . . . . . 76
5.3.1 Measurement of T . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
2
5.3.2 Measurement of T Through Incomplete Recovery . . . . . 77
1
5.3.3 Measurement of Proton Density . . . . . . . . . . . . . . . . . . 78
5.3.4 The Significance of T and T . . . . . . . . . . . . . . . . . . . 78
E R
5.4 From NMR to MRI: The Gradient Fields. . . . . . . . . . . . . . . . . 79
5.4.1 The Slice Encode Gradient. . . . . . . . . . . . . . . . . . . . . . 81
5.4.2 Fourier-Encoding with the Gradient . . . . . . . . . . . . . . . 84
5.4.3 The Frequency Encode Gradient. . . . . . . . . . . . . . . . . . 85
5.4.4 The Phase Encode Gradient . . . . . . . . . . . . . . . . . . . . . 86
5.5 Putting Everything Together: Spatially-Resolved
Spin-Echo Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.6 Other Imaging Sequences. . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5.6.1 Gradient-Recalled Echo Sequences . . . . . . . . . . . . . . . . 89
5.6.2 Inversion Recovery Sequence. . . . . . . . . . . . . . . . . . . . 90
5.6.3 Echo Planar Imaging. . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.7 Technical Realization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.7.1 B Magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
0
5.7.2 Gradient Subsystem. . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.7.3 RF Subsystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6 Ultrasound Imaging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.1 Sound Propagation in Biological Tissue. . . . . . . . . . . . . . . . . . 97
6.2 Ultrasound Image Formation. . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.2.1 Ultrasound Generation and Echo Detection . . . . . . . . . . 101
6.2.2 A-Mode Scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.2.3 B-Mode Scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.2.4 M-Mode Scans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.2.5 Volumetric Scans and 3D Ultrasound . . . . . . . . . . . . . . 108
6.3 Doppler Ultrasound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
7 Trends in Medical Imaging Technology . . . . . . . . . . . . . . . . . . . . 111
7.1 Progress in Established Imaging Modalities . . . . . . . . . . . . . . . 112
7.1.1 X-ray and CT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
7.1.2 Magnetic Resonance Imaging. . . . . . . . . . . . . . . . . . . . 113
7.1.3 Ultrasound Imaging. . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Contents ix
7.1.4 PET and Multi-Modality Imaging. . . . . . . . . . . . . . . . . 114
7.1.5 Molecular Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
7.2 Optical Tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
7.3 Advanced Image Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 118
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Chapter 1
Introduction
Abstract “Medicalimagingreferstoseveraldifferenttechnologiesthatareusedto
viewthehumanbodyinordertodiagnose,monitor,ortreatmedicalconditions”.All
imagingmodalitieshaveincommonthatthemedicalconditionbecomesvisibleby
someformofcontrast,meaningthatthefeatureofinterest(suchasatumor)canbe
recognized in the image and examined by a trained radiologist. The image can be
seenasamodeloftheimagedtissue.Imagesinthecontextofthisbookaredigital.
Thisimpliesafiniteresolutionwiththepixelasthesmallestelement.Furthermore,
all imaging modalities lead to some degradation of the image when compared to
the original object. Primarily, the degradation consists of blur (loss of detail) and
noise(unwantedcontrast).Someunderlyingprinciplesarecommontoallimaging
modalities,suchastheinterpretationasasystemanditsmathematicaltreatment.The
image itself can be seen as a multidimensional signal. In many cases, the steps in
imageformationcanbeseenaslinearsystems,whichallowsimplifiedmathematical
treatment.
“Medicalimagingreferstoseveraldifferenttechnologiesthatareusedtoviewthehuman
bodyinordertodiagnose,monitor,ortreatmedicalconditions.Eachtypeoftechnology
givesdifferentinformationabouttheareaofthebodybeingstudiedortreated,relatedto
possibledisease,injury,ortheeffectivenessofmedicaltreatment”.
This concise definition by the US Food and Drug Administration illuminates the
goal of medical imaging: To make a specific condition or disease visible. In this
context, visible implies that the area of interest is distinguishable in some fashion
(forexample,byadifferentshadeorcolor)fromthesurroundingtissueand,ideally,
fromhealthy,normaltissue.Thedifferenceinshadeorcolorcanbegeneralizedwith
thetermcontrast.
Theprocessofgatheringdatatocreateavisiblemodel(i.e.,theimage)iscommon
toallmedicalimagingtechnologiesandcanbeexplainedwiththesimpleexample
of a visible-light camera. The sample is probed with incident light, and reflected
light carries the desired information. For example, a melanoma of the skin would
reflect lesslightthan thesurrounding healthy skin.The camera lens collects some
of the reflected light and—most importantly—focuses the light onto the film or
M.A.Haidekker,MedicalImagingTechnology,SpringerBriefsinPhysics, 1
DOI:10.1007/978-1-4614-7073-1_1,©TheAuthor(s)2013