BMED4800ECSE4800 Introduction to Subsurface Imaging Systems

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BMED-4800/ECSE-4800Introduction to Subsurface
Imaging Systems

• Lecture 5 X-ray Imaging (cont.)
• Kai E. Thomenius1 Badri Roysam2
• 1Chief Technologist, Imaging Technologies,
• General Electric Global Research Center
• 2Professor, Rensselaer Polytechnic Institute

Center for Sub-Surface Imaging Sensing
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Review of Last Lecture

• Quick historical review of X-rays was given.
• Block diagrams, key components defined.
• Brief discussion of x-ray scattering
• An X-ray beam, traversing through an object, is
  attenuated by the exponential Lambert-Beer Law.
• The product of the attenuation coefficient and
  the path length of the x-ray beam in such a
  target is critical in establishing
  detectability.
• Today
• Digital Detectors, X-ray Metrics

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Outline of Course Topics

• THE BIG PICTURE
• What is subsurface imaging?
• Why a course on this topic?
• EXAMPLE Projection Imaging
• X-Ray Imaging
• Computer Tomography
• COMMON FUNDAMENTALS
• Propagation of waves
• Interaction of waves with targets of interest 

• PULSE ECHO METHODS
• Examples
• MRI
• A different sensing modality from the others
• Basics of MRI
• MOLECULAR IMAGING
• What is it?
• PET Radionuclide Imaging
• IMAGE PROCESSING CAD

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www.aapm.org/meetings/amos2/pdf/26-5959-83142-414.
pdf
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Digital Detector Front End
www.aapm.org/meetings/amos2/pdf/26-5959-83142-414.
pdf
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Detector Details
www.aapm.org/meetings/amos2/pdf/26-5959-83142-414.
pdf
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Selenium based Detector
www.aapm.org/meetings/amos2/pdf/26-5959-83142-414.
pdf
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Performance Metrics
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Signal-to-noise Ratio (SNR)

• SNR determines the detectability of an object
• Signal derived from x-ray quanta
• Noise comes from a variety of sources
• X-ray quantum statistics, Poisson distribution
• Electronic noise
• Sampling noise
• Anatomical noise
• Signal processing steps critical to image quality
• Correction for detector variability, defects
• Post-process filtering

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Quantum noise

• For a digital x-ray detector system with square
  pixels
• if the average number of x-rays recorded in each
  pixel is N,
• then the noise (per pixel) will be
• Statistical distribution associated with x-rays
  is the Poisson distribution.
• The above relation falls out directly from this
  fact.

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Poisson Distribution

• Poisson Distribution is a probability
  distribution given by

If the expected no. of occurrences in a space is
l, then the probability that there are exactly k
occurrences is given by f(k,l)
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Signal-to-noise ratio

• The signal-to-noise ratio (SNR) is given by
• When the number of x-rays, N, is increased, the
  radiation dose also increases.
• To double the SNR, the dose to the patient needs
  to be increased by a factor of 4
• Contrast-to-noise ratio (CNR) for any two
  intensities (I1 and I2) at a detector is given by
• Here N is the nominal value of photons reaching
  the detector.

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Other Measures of Image Quality

• Limiting Spatial Resolution (LSR)
• The highest frequency that can be visualized
• Modulation Transfer Function (MTF)
• Measures how the detector passes signal, as a
  function of spatial frequency

MTF
1.0
0.03 - 0.05
LSR
0
Spatial Frequency (cycles/mm)
Modulation at detector output
MTF
Modulation at detector input
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MTF1
in
out
Source http//203.64.251.39/info/download/etc/br
eastx/93/93-04.ppt
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MTF0.5
in
out

• Modulation Transfer Function (MTF) / Spatial
  resolution
• An imaging systems ability to render the
  contrast of an object as a function of object
  detail.

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MTF
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Modulation Transfer Function

• LSR
• Screen-film has LSR 20 lp/mm
• corresponds to 25 mm pixel
• Digital (GE) 100 mm pixel
• Sources of MTF degradation
• Lateral spread of light in scintillator
• limited by CsI needles
• increases with scintillator thickness
• Lateral spread of secondary x-rays
• not significant away from k-edges of Cs and I
• Sampling aperture of pixel sinc(fxa)sinc(fya)

Digital Imager
M T F
Film-Screen
Spatial Frequency (cycles/mm)
If films LSR is better than digital why do we
see improved performance in digital?
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MTF For Direct, Indirect, and Screen Film
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Measures of Image Quality-DQE

• Detective Quantum Efficiency, DQE
• SNR gives the transfer function of both signal
  and noise

SNR2 at detector output
SNR2 at detector output
µ
DQE
SNR2 at detector input
Patient Dose
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DQE
1.0
The higher the DQE, the higher the SNR, and the
greater the probability of detection.
Digital Imager
DQE
Film-Screen
Spatial Frequency (cycles/mm)
High DQE in low-to-mid frequencies aids
detection. High DQE in high frequencies aids
characterization.
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DQE Definition
where f is the spatial frequency (lp/mm), X is
the exposure (mR) and S Median Signal Level
(cts), i.e. amplitude of information MTF
Modulation Transfer Function NPS Noise Power
Spectrum (cts2 mm2) C Incident Xray
Fluence (xrays / (mm2 mR) DQE describes the
measured SNR in relation to an ideal
detector. SNR2 is deduced from the ratio of MTF2
(signal2) to the NPS (noise2)
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www.aapm.org/meetings/amos2/pdf/26-5959-83142-414.
pdf
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Calibration of Digital Detector

• Dark Image? Offset
• Diode leakage
• FET charge retention
• Electronic noise

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Calibration of Digital Detector

• Offset Corrected Dark Image
• Electronic Noise

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Calibration of Digital Detector

• Offset Calibrated
• Amplifier gainvariation
• Pixel-to-pixel gain variation

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Calibration of Digital Detector

• Offset and Gain calibratedFlood exposed image
• Poisson statistical x-ray noise
• Electronic noise

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Apply Corrections

• Low dose before and After Offset Correction

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Apply Corrections

• High dose

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Tomosynthesis
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Advanced Applications

• Tomosynthesis- 3D X-ray

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3D Breast Imaging - Tomosynthesis

• 3D imaging addresses the major problem with
  mammography today superimposed tissue
• 3D imaging may enable compression reduction
• Tissue immobilization vs. compression
• Compliance with screening protocols
• Single tomo exam in MLO position may replace
  conventional mammography, potentially enabling
  dose reduction

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Tomosynthesis Concept
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Prototype System Parameters

• Prototype based on GEMS Senographe DMR,
  Revolution flat panel detector, motorized tube
  motion assembly
• 11 projections over /- 25 degrees
• 7.5 sec patient exam time
• Total dose
• 1.5x a single mammographic view
• 0.75x a standard mammographic screening exam
• 100 micron pixels
• 1 mm (3d) slice separation

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Tomosynthesis

• Goal
• Limited 3-D reconstruction to remove
  overlying/underlying structure
• All image planes visualized using a single
  acquisition
• Acquisition
• Vertical tube motion
• Total tube angle 5 -15
• Number of Projected Images 15 25
• Exam length 5 -10 sec (single breath-hold)
• Slice thickness 1 cm
• Enabled by GE Revolution detector

Rotational Axis
Tube vertical motion
Courtesy of Duke University and Wake Forest
Medical Center
Small Changes to Rad System allows for 3D Imaging!
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Image Reconstruction in Tomo

• Data incompleteness
• From a CT perspective, data is very sparse
• Limited angular range (z-resolution)
• Insufficient angular sampling (streaks)
• Truncated projections (inconsistency)

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Reconstruction Concept Shift and Add
Vertical slice through object
Reconstruction of single plane
Reconstruction of vertical slice through object
Projections at different angles
Add
Shift
Artifacts Out-of-plane structures appear as N
low-contrast copies (N of projections).
Contrast / blurring of artifacts depends on N,
projection angles / tube trajectory, etc.
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An ExampleStandard 2D x-ray
Images courtesy of Dr. Dan Kopans- MGH
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Tomosynthesis Missed Cancer
Standard Mammogram MLO
Tomo Slice MLO
Spiculated Lesion
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Tomosynthesis
Images courtesy of Dr. Dan Kopans- MGH
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An Example3D Tomosynthesis
Images courtesy of Dr. Dan Kopans- MGH
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Rad Tomo Example
Low Dose 3D Imaging!
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Receiver Operating Characteristics
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Receiver Operating Characteristic (ROC) curves

• Most basic task of the diagnostician is to
  separate abnormal subjects from normal subjects
• In many cases there is significant overlap in
  terms of the appearance of the image
• Some abnormal patients have normal-looking films
• Some normal patients have abnormal-looking films
• ROC curves are a tool for assessing the
  performance of a hypothesis testing algorithms.

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2 x 2 Decision Matrix
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ROC curves (cont.)

• For a single threshold value and the population
  being studied, a single value for TP, TN, FP, and
  FN can be computed
• The sum TP TN FP FN will be equal to the
  total number of normals and abnormals in the
  study population
• True diagnosis must be determined
  independently, based on biopsy confirmation,
  long-term patient follow-up, etc.

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Summary

• Design of digital x-ray detectors was described.
• Performance metrics (MTF, DQE) for x-ray
  performance were given.
• Justification for digital detectors was based on
  these.
• Tomosynthesis concept introduced.
• Brief review of ROC methods for hypothesis
  testing was given.
• Next time Introduction to CT Scanners

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Homework

• Using web resources (or sources given below),
  describe the key steps of the direct conversion
  process with amorphous Selenium. How are x-rays
  converted to electrons?
• What is the relative performance (MTF or DQE)
  with respect to the CsI-Photodiode approach?
• Which would you buy and why?
• http//www.dondickson.co.uk/download/Challenges_of
  _Direct_Digital_Radiology.pdf
• Hoheisel et al., Modulation transfer function of
  a selenium-based digital mammography system,
  IEEE Proc. Nuclear Science Symposium, 2004,
  3589-3593

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Instructor Contact Information

• Badri Roysam
• Professor of Electrical, Computer, Systems
  Engineering
• Office JEC 7010
• Rensselaer Polytechnic Institute
• 110, 8th Street, Troy, New York 12180
• Phone [phone]
• Fax [phone]
• Email roysam_at_ecse.rpi.edu
• Website http//www.ecse.rpi.edu/roysabm
• Secretary Laraine Michaelides, JEC 7012, (518)
  276 8525, michal_at_rpi.edu

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Instructor Contact Information

• Kai E Thomenius
• Chief Technologist, Ultrasound Biomedical
• Office KW-C300A
• GE Global Research
• Imaging Technologies
• Niskayuna, New York 12309
• Phone [phone]
• Fax [phone]
• Email thomeniu_at_crd.ge.com, thomenius_at_ecse.rpi.edu
  
• Secretary TBD