x-ray planar imaging

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					Projection Imaging
               Projection Imaging with X Rays




                     I0             I
X-Ray Source



                          Patient
                                         Detector
Absorbed dose and the x-ray
spectrum

                         incident spectrum
  contribute
 to dose, but
  not image                            exit
                                     spectrum
                absorbed x rays




                   detected x rays
X-ray beam filtration

 So-called “added filtration” is used to harden
 the x-ray beam in some imaging procedures.
 Purpose of the added filtration is to remove the
 lowest energy photons that contribute little to
 an image.
 As filtration is added the x-ray tube must
 produce increased output, i.e., tube “loading” is
 increased.
 Less added filtration is used for mammography
 than for other exams.
           Primary vs. scatter “image”




                    sig      P
CP=sig/P
CP=sig/P                                                  CS=sig/(P+S)
                                                          CS=sig/(P+S)
                                                      P+S    < CP
                                                sig           < CP

           primary “image”       primary + scatter “image”
Scattered radiation properties

 is the dominant type of radiation that exits a
 patient during a radiographic exam
 deviates in direction from the direction of the
 primary radiation
 does not carry much image “information”
 because it is diffuse
 reduces primary image contrast, sometimes
 significantly
  Scatter removal




                                                        grid




primary + scatter “image”   primary + scatter “image”
Scatter-reduction grid properties

a scatter reduction grid has direction-dependent
transmission, which preferentially attenuates
the scattered radiation
grids are comprised of parallel strips of lead,
intersperced with strips of low-Z material
the thickness of the lead strips relative to their
spacing is called the grid “ratio”
the higher the grid ratio, the better the scatter
reduction - 8:1 to 12:1 ratios are common
whenever a grid is used, x-ray exposure to the
patient must be increase for same number of
detected x-ray photons
Focused grid geometry
                 focal spot




                      x-ray beam




                              patient



                              focused grid
                                  detector
          Grid misalignment
   backwards          wrong distance     left-right shift




severe edge cut-off    edge cut-off    uniform attenuation
               Air gap for scatter removal

                                         scatter


                                                   primary


                      I0             I
X-Ray Source



                           Patient
                                                             Detector
    Radiographic magnification
                       focal spot

                       A = source-image-distance
                       B = source-object-distance
        B
                        magnification = m = A/B
                        magnification = m = A/B


A               I0           object


                          x-ray intensity (# / area)




              I0 /m2                  image (shadow)
Projection radiography properties

 similar to “pin-hole” camera
 images of objects are always larger than the
 objects themselves, i.e., magnification m > 1
 magnification = ratio of image distance to
 object distance
 three-dimensional anatomy is unavoidably
 distorted due to variable magnification
 intensity of x-ray beam proportional to 1/m2
    Point spread function
                         focal spot (size = f)



         B
                     point



A                            object

                     point spread (blur, penumbra):
                      point spread (blur, penumbra):
                        same shape as focal spot
                         same shape as focal spot
        A-B

                         blur = (A-B)/B*f = (A/B --1)f
                          blur = (A-B)/B*f = (A/B 1)f
                              = (m --1)f
                               = (m 1)f
Focal spot blur

finite size of focal spot introduces image
blurring
the shape of the blur reflects the distribution of
x-ray intensity across the focal spot
this “geometric” blur increases with
magnification
blur “softens” the edges of large objects, but
leaves their maximum contrast unaltered
blur reduces the contrast of small objects
blur limits “spatial resolution”
             Modulation transfer function
             (MTF)

objects



                     blur dimension
projection
 images


                     decreasing size
                   decreasing contrast
Spatial frequency
      1 line pair




              Increasing spatial frequency
              Decreasing contrast with blur
    Spatial frequency measured in “line pairs per mm”
                    Modulation transfer function
                    (MTF)
      1.0
Relative Contrast




                                            0.6 mm focal spot



                    1.0 mm focal spot




              0
                                 spatial frequency (lp/mm)
                                   increasing object size

				
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posted:8/18/2012
language:English
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