Digital Xray Mammography

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Digital Xray Mammography
THURSDAY MEETING
2.11.2000
Xavi Espinal Curull
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Need for Digital Mammography

• Women has 1/8 chance of developing breast
  cancer.
• Best hope early detection through mammography.
• Detect tumours 2 years before a lump can be felt
  ( recover chance nearly 100 ).
• 20 tumours actually missed in conventional
  mammographys, even more with young women.
• DM advantages store and exchange pictures, play
  with contrast, less radiologists fails.
• 90 dose reduction. There is a correlation
  between several exposures and future cancer
  development.

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Xray Project

• Low dose Xray Mammography system
• CdZnTe Detector
• Medipix II Chip Deep sub-micron CMOS process
• Improved spatial resolution than Medipix pixel
  size 55um2
• Preamplifier, comparator and 13 bit counter.
• Non-sensitive area as small as posible in order
  to obtain large areas by tilling the detectors.

• Medipix I (PCC)
• 32000 counts before threshold
• 170 um2

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Comparision CCD PCC

• Top Image taken by a standard Xray source and a
  screen CCD system. ( Indirect Capture )
• Bottom Image, taken by 109Cd source and the PCC.
  
• (Direct Capture)
• Larger Pixel size in PCC
• Inserted needle distinguished in both.
• Density differences much more clear in PCC

30 TIMES LOWER DOSE IN PCC IMAGE
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Absorption Coefficients

• CdZnTe has wide range of good absorption for low
  energies.

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Cd(1-x)Zn(x)Te Detector

• 4000 electron hole pair per 20KeV ?
• Good behaviour at room temperature

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Cd(1-x)Zn(x)Te Detector Growth

• Growing High Pressure Bridgman
• impurities lt 1016 cm -3
• Te Rich
• 10-150 bar internal pressure Argon

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CdZnTe Properties
Grown Crystal
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CdZnTe Properties

• High Density 5.8 g/cm3 and high atomic number.
• Provides an excellent absorption coefficient
  even for thin detectors ( 98 absorption at 20
  Kev for 0.4mm thickness ).

• High resistivity ( 10-100 Gigaohm per cm )
• Low Dark Currents

• High gain 400 e-h pair per 20KeV photon
• Excellent signal to noise ratio

• Design limitation Small hole mobility
• The Detector must be as thin as possible and
  biased correctly to provide the shortest distance
  for the holes to travel.

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Imaging Process
Xray production
Colimator

• Xray Production
• Beam Collimator
• Interaction with Breast
• Energy deposition in the detector
• Readout system

BREAST
Air GAP
DETECTOR
Bump Bonding
Chip Array and Readout electronics
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Processes at Breast Level
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The tungsten plane

• Tungsten plane is inserted in order to focus the
  beam in region of 150 um.
• Tungsten absorbs all the radiation with a
  thickness of 0,1mm.

• The tungsten Plane / Hole reduces the scattering
  effects ,which we will discuss later, improving
  the shape of the image.
• Less dose received by the patient as we are
  focusing only into small regions.

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Tungsten plane imaging
Scattered particles high angle
Xray Source
Non Scattered particles, and low Angle scattered
particles
Hole
Tungsten
Breast
Detector

• The idea is to take the image in 1 second by
  making a shift of the hole through all the
  breast.
• So the exposition to the radiation will be low,
  and the dose will decrease, and this is less
  invasive for the breast tissue.

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Contribution to the imaging

• Non scattered particles that outcome from the
  breast with no scatter.
• Scattered particles ( roughly speaking ) with low
  scattering angle.
• Scattered particles at CdZnTe detector level.

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Microcalcifications I

• What we are focusing on is to recognise the
  calcium microcalcifications inside the breast, of
  about 150um in the early stage.
• These are the most common sources of cancerous
  tumours in women, this microcalcifications are
  produced by the milk lobuls, in the sacs.

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Microcalcifications II
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Photon counting

• With the chip Medipix2 we are able to count
  photons in each pixel with an excellent
  precission.
• Simulation results gives the ratio
• 0.71 between events beneath Calcium box
  150um width, and free space under the breast.
• So we have aprox 29 less events under the
  Calcium phantom.

One of the first digital images used to detect
breast cancer
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Detecting Microcalcifications

• A Simulation of (150?m)3 calcium box has been
  made.
• The Calcium at 150 um depth stops nearly 26 of
  the radiation, theoretically.

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Detecting Microcalcifications

• So there is a difference in the event density
  between the region beneath calcium phantom, and
  the same region placed in free space ratio 0.7
• This means that if we can control the scattering
  effects and they are not bigger than 10 we
  will be able to detect early microcalcifications.
• Moreover we can cut non desired scattering events
  with energy less than 19 KeV by the resolution of
  the detector, assuming that we will lose 16 of
  valid events . ( 18KeV ?2.5 lose. )
• Ca Microcalcifications are aprox. Spherical in
  shape, and the most common are lobular or low
  grade ductal carcinoma.

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Fluka simulations I

• Fluctuating kaskad is a Monte Carlo simulation
  Software developed at CERN by Alfredo Ferrari et
  al.
• This software allows to define several regions to
  count the energy deposited breast, detector or
  whatever at the same run i.e. For each event.
• This allows us to track a single photon, and get
  the number of scatterings that this photon has
  had, and to know the energy loss in each of the
  scatterings.
• One can make a fine binning of the regions, in
  order to know with high precission where the
  photons interact.
• We are working with bins of 50um because this is
  the size of the chip, and there is no need to go
  to higher binning.
• A big trick has been made because for each event
  we had 8MBytes, so now we are able to generate as
  many events as we want with no disk quota
  matters.

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Fluka simulations I

• Several geometries can be implemented, and all
  the materials and compounds are allowed.

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Simple Algorithm to detect the image of an Object
at the statistical limit of Xray photons flux

• Naked eye is limited by nature in detecting
  digital radiology image, because the statistical
  noise is dominant.
• It Is posible to obtain image with low dose by
  using a simple algorithm
• Sliding-Window Pixel
  Integration Method
• Detection implies a contrast between the N?
  detected beneath the object, and the N?
  elsewhere.
• N? that reachs the detector underneath the
  object (1-C) N?
• N? seen elsewhere A0N?
• By computing the difference, we get A0N?C.
• At the end of the day , and adding the
  efficience, ?, and the error
• and this formula gives the number of photons
  needed to make the imaging of an object in order
  to achieve a certain resolution specified through
  ? and given by multiples of standard desviation
  ?.

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Imaging low contrast objects
Signal to Contrast Ratio
SCR
SCR
Incident Photons
Signal to Noise Ratio
SNR
SNR
Contrast
Low contrast detection means
Earlier tumor visibility
Patient dose reduction
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Study of scattering effects in mammography

• We are able to determine scattering effects in
  the process of radiological imaging.
• Breast produces scattering at low and high
  angles.
• Simulations have been made in order to quantify
  the ratio of scattered photons that produce a bad
  shape of the image.
• A box of 150?m has been inserted inside the
  breast tissue , this box was filled with W and
  Ca.
• W at 150 ?m has a transmission of 0,and Ca with
  the same width has a transmission of aprox. 0.70.
• So, by filling the box with W, we should have no
  events under the box. But unfortunately we have
  events.
• W absorbs almost all the radiation outcoming from
  the source. As we have seen in transparence 12.

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Study of scattering effects in mammography II

• Some Results
• There is a dependence on the distance between
  the object and the detector, the longer the
  distance, the greater the scattering.

Beam
W
Breast
W Shadow

• If no scattering we will get no events in the
  shadow.

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Study of scattering effects in mammography III

• Simulation inputs
• 107events generated.
• 3cm between W and detector.
• 1cm radiation covered
• Box Material W

• We cant see anything with contrast less than 11
• Box Material Ca (150um width)

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Low contrast objects
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Future Semiconductor Detectors

• Mercuric Iodide Detector ( HgI2)
• Better performance over a broader energy range
  than CdZnTe.

• Resistivity is 1000 times bigger than CdZnTe.
• Even less dark currents than CdZnTe

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Conclusions

• Xray mammography with 90 dose reduction.
• Able to detect microcalcifications at early
  stages.
• Image can be software optimized.
• Reducing the scattering we can improve the image
  quality low contrast objects can be imaged.