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Digital Xray Mammography

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Tungsten plane imaging. Xray Source ... Simple Algorithm to detect the image of an Object at the statistical limit of Xray photons flux ... – PowerPoint PPT presentation

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Title: Digital Xray Mammography


1
Digital Xray Mammography
THURSDAY MEETING
2.11.2000
Xavi Espinal Curull
2
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.

3
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

4
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
5
Absorption Coefficients
  • CdZnTe has wide range of good absorption for low
    energies.

6
Cd(1-x)Zn(x)Te Detector
  • 4000 electron hole pair per 20KeV ?
  • Good behaviour at room temperature

7
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

8
CdZnTe Properties
Grown Crystal
9
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.

10
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
11
Processes at Breast Level
12
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.

13
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.

14
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.

15
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.

16
Microcalcifications II
17
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
18
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.

19
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.

20
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.

21
Fluka simulations I
  • Several geometries can be implemented, and all
    the materials and compounds are allowed.

22
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
    ?.

23
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
24
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.

25
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.

26
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)

27
Low contrast objects
28
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

29
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.
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