Projection Radiography

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Projection Radiography

• Miguel A. RichardECE 5259

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Definition

• A radiograph is the projection, by means of
  x-rays, of a 3-D object into a 2-D image
• When the body undergoes x-rays, different parts
  of the body allow varying amounts of the x-rays
  beams to pass through
• The soft tissues in the body (blood, skin, fat,
  muscle) allow most of the x-ray to pass through
  and appear dark gray on the film or digital media
• Dense tissue (bones, tumors) allows few of the
  x-rays to pass and appears white
• A scintillator converts x-rays to visible light
  that can be captured with photographic film,
  camera or solid-state detectors

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Modalities

• Routine diagnostic radiography, chest x-rays,
  fluoroscopy, mammography, motion tomography
• Digital radiography, images are stored digitally
• Angiography, the systems are specialized for
  imaging the bodys blood arteries and vessels
• Neuroradiology, x-rays systems for precision
  studies of the skull and cervical spine
• Mobile x-ray systems, small x-ray units for
  operating rooms or EM vehicles

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X-rays

• EM waves with wavelength in the range of 10 to
  0.01 nm, corresponding to frequencies in the
  range of 30 PHz to 30 EHz
• Form of ionization radiation, capable of ejecting
  electrons from atoms

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X-Ray Rotating Anode Tube

• Electrons are boiled off the hot filament which
  glows just like a light bulb
• They are accelerated by the anode voltage
• They hit the target, giving off energy mostly as
  heat, but 1 is given as X-rays
• The target would rapidly melt, so it is turned by
  an AC induction motor. The rotor is in the
  evacuated glass bulb, while the stator (the coils
  of wire) is on the outside. The cathode spins at
  3000 rpm

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X-Ray Rotating Anode Tube
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Advantages

• Short time exposure (0.1 second)
• The production of a large area image(14x17
  inches)
• Low cost
• Low radiation exposure (30mR for a chest
  radiograph, equivalent to about one-tenth of the
  annual background dose)
• Excellent contrast and spatial resolution

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Limitations

• Lack of depth resolution- superimpositions of
  shadows from overlaying and underlaying tissues
  sometimes hide important lesions, which limit
  contrast)

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Medical Applications

• Screen for pneumonia
• Heart disease
• Lung disease
• Bone fractures
• Cancer
• Vascular disease

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Noise and Artifacts

• Geometric Effects, images are created from a
  diverging beam of x-rays
• Inverse Square Law, photons per unit area
  decrease as 1/r2, could be falsely interpreted as
  object attenuation
• Obliquity, detector surface not being orthogonal
  to the direction of x-ray propagation
• Anode Heel Effect, the beam generated from the
  x-ray tube is not uniform within its cone of
  radiation
• Path Length, x-rays away from the source center
  are attenuated more by the object that the ones
  traveling closer to the center

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Noise and Artifacts

• Blurring Effects
• Extended sources
• Film-Screen Blurring
• Noise and Scattering
• SNR, background noise
• Quantum Efficiency, probability that a single
  photon incident at the detector will be detected
• Compton Scattering

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Signal Processing

• Spectral Noise Attenuation, noise attenuation is
  carried out in the spectral domain
• Wiener filter-based estimation
• Minimum mean square error (MMSE) estimation
• Wavelet Transform

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References

• Prince, J. L., Links, J. M., Medical Imaging
  Signal and Systems, Person Education Inc, New
  Jersey, 2006
• Aach, T., Kunz, D., Spectral Estimation Filter
  Reduction in X-ray Fluoroscopy Imaging,
  Proceedings EUSIPCO-96, 2006
• Okamoto, T., Furui, S., Ichiji, H., Noise
  reduction in digital radiography by wavelet
  packet using nonlinear correlation threshold, M.
  Proceedings of the SPIE, Volume 5747, pp.
  1054-1065, 2005

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References

• Rantala, M., Vänskä S., Järvenpää, S.,
  Wavelet-Based Reconstruction for Limited-Angle
  X-Ray Tomography, IEEE Transaction on Medical
  Imaging, Vol 25, No. 25, February 2006, pp.
  210-217