A scan-rotate geometry for efficient equalization mammography.
Published: 1996
Language: eng
Number of pages: 127
First sentence: Cancer of the breast is the most common cancer and the second leading cause of cancer death in women, in both the United States and Canada.
Description: Mammographic screening is acknowledged as the best
method for the reduction of breast cancer mortality.
However, breasts containing a significant fraction of dense,
fibroglandular tissue produce a range of exposure which
exceeds the dynamic range of conventional mammography.
Equalization radiography involves the modulation of the
incident x-ray exposure distribution to compensate for
variations in x-ray transmission within the patient, ensuring
optimal contrast throughout the image. It has been shown
that equalization radiography offers the potential for doseefficient,
improved lesion detection in the dense breast.
However, current equalization geometries are not practical
due to tube loading and scan duration inefficiencies. I
propose a scan-rotate geometry for equalization
radiography (rotary scanning equalization radiography,
RSER) in which the image receptor is exposed by repeated
scans of a modulated fan beam, oriented at a variety of
angles with respect to the patient. The superposition of
exposure from appropriately modulated, rotated fans beams
will produce an entrance exposure distribution that will
efficiently equalize the film exposure.

In this thesis, it is shown that less than half of the area of a
dense breast is imaged conventionally with high contrast.
The RSER geometry is described and its imaging
characteristics are shown theoretically to be similar to
current equalization geometries. Furthermore, RSER is
resistant to exposure artefacts in typical mammographic
imaging tasks. Numerical simulations which compared the
imaging performance of RSER, current equalization
geometries and conventional mammography show that
RSER produces images with the same degree of
equalization as current equalization geometries, with
minimal tube loading, and only four scanning angles.
Finally, an experimental prototype RSER system is
described and characterized. Experimental images confirm
the theoretical and numerical predictions. High contrast,
artefact-free images of anthropomorphic breast phantoms
can be obtained with minimal scan times and tube loading.
The prototype system increases the fraction of the breast
imaged with high contrast from 46 %to 80%.

These results indicate that RSER is an efficient, simple, and
practical means of overcoming the latitude limitations of
film screen mammography, and improving the detection of
breast cancer.
Table of contents: value: ABSTRACT	i ACKNOWLEDGMENTS	i  TABLE OF CONTENTS	ii  LIST OF FIGURES	iv  LIST OF TABLES	vii  LIST OF SYMBOLS AND ABBREVIATIONS	viii  1. IMAGING THE RADIOGRAPHICALLY DENSE BREAST	1  1.1 BREAST CANCER	1  1.1.1 Epidemiology and Etiology of Breast Cancer	1  1.1.2 Mammographic Screening	2  1.2 THE RADIOGRAPHICALLY DENSE BREAST	3  1.2.1 Prevalence of Radiographic Density	4  1.2.2 Radiographic Density and Breast Cancer Risk	5  1.3 MAMMOGRAPHY OF THE RADIOGRAPHICALLY DENSE BREAST	5  1.4 ALTERNATE TECHNOLOGIES FOR IMPROVED IMAGING OF THE RADIOGRAPHICALLY DENSE BREAST	7  1.4.1 Non-Film Detectors for Breast Imaging	7  1.4.1.1 Breast Imaging Without X-rays	7  1.4.1.2 Digital Mammography	9  1.4.2 Equalization Radiography	10  1.4.2.1 Source Modulation Equalization Geometries	12  1.4.2.2 Attenuator Modulation Equalization Geometries	14  1.4.2.3 Rotary Scanning Equalization Radiography	16  1.5 SUMMARY	17  1.6 OUTLINE OF THESIS	18  2. LATITUDE LIMITATIONS OF FILM-SCREEN MAMMOGRAPHY	20  2.1 INTRODUCTION	20  2.2 METHODS	21  2.3 RESULTS	25  2.4 DISCUSSION	27  2.5 CONCLUSIONS	29  3. ROTARY SCANNING EQUALIZATION RADIOGRAPHY:  THEORETICAL ANALYSIS	31  3.1 INTRODUCTION	31  3.1.1 RSER, Computed Tomography, and Inverse Conformal Radiotherapy Treatment Planning:	32  3.1.2 Characterization of the Linear and Non-linear Behavior of RSER:	36  3.2 LOW CONTRAST (LINEAR) BEHAVIOUR OF RSER	37  3.2.1 The Transmitted Exposure in a Scan-Rotate Geometry	37  3.2.2 Low Contrast Imaging Characteristics of RSER Projections:	41  3.2.3 The Two-dimensional Low Contrast Transfer function for RSER:	43  3.2.4 Comparison of RSER and SER Low Contrast Transfer Functions:	45  3.3 THE HIGH CONTRAST (NON-LINEAR) BEHAVIOUR OF RSER	48  3.3.1 A Representative High Contrast Object:	49  3.3.2 Filtered Projections of Representative High Contrast Object:	51  3.4 IMPLICATIONS FOR MAMMOGRAPHY	54  3.5 CONCLUSIONS	56  4. RSER IMAGING OF THE RADIOGRAPHICALLY DENSE BREAST:  NUMERICAL SIMULATIONS	58  4.1 INTRODUCTION	58  4.2 DETERMINATION OF THE RSER EXPOSURE PROFILES	59  4.3 SIMULATION METHODS	63  4.3.1 Images	64  4.3.2 Simulation Techniques	65  4.3.3 Contrast and Heat Loading Calculation	67  4.3.4 Results	69  4.4 DISCUSSION	74  4.5 CONCLUSION	76  5. A PROTOTYPE RSER IMAGING SYSTEM	77  5.1 INTRODUCTION	77  5.2 MATERIALS AND METHODS	78  5.2.1 Prototype system description	78  5.2.2 Calculation of the exposure profiles	81  5.2.3 Fraction of image with high contrast	82  5.2.4 Tube loading comparison calculation	83  5.2.5 Artefacts and exposure error characterization	84  5.3 RESULTS	85  5.3.1 Prescan and desired entrance exposure images	85  5.3.2 Exposure profile calculation	85  5.3.3 Anthropomorphic phantom images	85  5.3.4 Fraction of breast imaged with high contrast	85  5.3.5 Tube loading	85  5.3.6 Scanning artefacts and exposure errors	85  5.3.6.1 Scanning artefacts	85  5.3.6.2 Exposure errors	85  5.4 CONCLUSIONS	85  6. CONCLUSIONS AND FUTURE DIRECTIONS	85  6.1 SUMMARY	85  6.2 A CLINICAL RSER PROTOTYPE	85  6.3 SECOND GENERATION RSER GEOMETRIES	85  6.4 EQUALIZATION AND DIGITAL MAMMOGRAPHY	85  6.5 EQUALIZED IMAGES AND THE OBSERVER	85  6.6 OTHER APPLICATIONS FOR EQUALIZATION RADIOGRAPHY	85  6.7 CONCLUSIONS	85  7. APPENDIX A: DERIVATION OF THE LOW CONTRAST TRANSFER FUNCTION IN THE SPATIAL DOMAIN	85  8. APPENDIX B: RHO FILTERING WITH THE DERIVATIVE OF THE HILBERT TRANSFORM IN THE SPATIAL DOMAIN	85  9. APPENDIX C: ENERGY WEIGHTED CALCULATION OF THE X-RAY TRANSMISSION OF BREAST TISSUE OF VARIABLE GLANDULAR TISSUE CONTENT	85  REFERENCES	85