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									Image Quality, Digital
Technology and Radiation

       Chapter 2- Bontrager
Image Quality in Film-Screen

  Film screen (the traditional method) is
  rapidly being replaced by digital
  Because you will work in departments that
  may utilize one method or the other, we
  must understand both
Film Images
 2 dimensional image of anatomic
 Develop, Fixing, Washing and Drying
 are the 4 steps of processing a film
 Various shades of gray are
 representative of densities and atomic
 numbers of the tissues
 Radiograph is known as the „hard-
Exposure Factors for Film-
 Selected on the control panel
 Kilovoltage, milliamperage and time
 Each has a specific effect on the quality
 Remember: highest quality and lowest
 radiation dose as possible
 AEC: automatic exposure control; provide
 auto. Termination when sufficient radiation
 or exposure is received by the selected
Image Quality Factors

Quality Factor #1: Density

 Density is the amount of blackness on
 the film
 Primary controller of density is mAs
 (mA x s= mAs)
 SID also can effect density according
 to the inverse square law (SID
 doubled, intensity reduced to ¼ which
 reduces density by ¼)
Adjusting Density

 Underexposed film will require a
 minimum change in mAs of 25-30% to
 make a visible difference
 Some may require a greater change
 See images on page 43
Density and the Anode Heel Effect
 Intensity is the greatest on the cathode end of
 the x-ray tube
 This phenomenon is known as the anode heel
 All to do with attenuation of the beam on the
 anode side
 Also has to do with angle of the anode
 Applying this principle will assist in quality
 radiographs of structures with high thickness
 variances (see summary chart on p. 44)
 Not always possible or practical to utilize the
 principle; depends on patient condition and the
 x-ray room
Compensating Filters and
 Wedge filters can have the same
 effect as the correct use of the anode
 heel effect
 A good example is the foot
 We are looking for close uniformity of
 tissue densities
 A lot of times, technique that is good
 for the thick part of the foot burns the
 toes out (too dark)
Quality Factor #2: Contrast
 Contrast is the difference in adjacent densities
 The greater the difference the higher the contrast
 and vice-versa
 Long scale, short scale (review)
 Controlling factor is kV or the penetrating power of
 the beam
 Attenuation: differential absorption
 kV is a secondary controlling factor of density-
 15% rule says that 15% increase in kV will
 increase film density (similar to doubling the mAs)
 Scatter, collimation and grids
 Highest kV and lowest mAs that yields a sufficient
 diagnostic x-ray should be used each time
Quality Factor #3: Resolution
 Resolution is the recorded sharpness
 of structures on the image
 AKA detail, recorded detail, image
 sharpness or definition
 Lack of resolution is known as blur or
 Controlling factors for resolution are
 geometric factors, film-screen system
 and motion
Geometric Factors
 Geometric factors that control resolution
 are focal spot size, SID and OID
 Penumbra: unsharp edges of the projected
 Small focal spot results in less
 unsharpness and decreased penumbra
 Most tubes are dual focus: they have a
 small and large focal spot (the focal spot is
 on the anode)
Film Screen System

 Consists of the film speed and the
 screen speed
 Faster film-screen system allows
 shorter exposure times which is
 helpful in preventing patient motion
 and lessening the dose; however, the
 image is less sharp than if a slower
 speed had been used (trade-off)

 2 types of motion: voluntary and
 Voluntary: patient can control;
 motion from breathing or movement
 of body parts
 Involuntary: cannot be controlled by
 the patient. Peristalsis, tremors,
 chills, heartbeat.
Difference in Voluntary and
 Voluntary motion can be minimized through
 the use of a high mA and short exposure
 time, also by patient cooperation and clear
 Involuntary motion is identified by localized
 blurring- the only way to minimize motion
 blur from this type is by utilizing an
 increased mA and decreased time
 See examples on page 46
Quality Factor #4: Distortion
 Distortion is defined as the
 misrepresentation of object size or shape
 Two types: size and shape
 *No radiograph reproduces the exact size
 of the body part being radiographed.
 There is always a degree of magnification
 and/or distortion as a result of the OID and
 the divergence of the beam
X-ray Beam Divergence

 When using a perpendicular CR, the
 center point of the beam strikes the IR
 90 degrees to the plane of the film
 This point theoretically has no
 divergence, therefore the providing
 the point of least distortion
Factors that Control Distortion
 1. SID: Source image distance. Greater the SID,
 the less magnification; the shorter the SID, the
 more magnification. This is why chest x-rays are
 done at 72”- to prevent magnification of the heart.
 There must be a minimum of 40”.
 2. OID: Object image distance. The closer the
 object to the IR, the less magnification and shape
 distortion there will be. This is why doing
 extremities table top is advantageous for detail
 3. Object image alignment: plane of the object
 must be aligned to the plane of the IR
 4. CR alignment: the least possible distortion
 occurs at the CR; correct centering is essential to
 minimize distortion
In Review…….

 There are four quality factors:
 density, contrast, resolution/detail
 and distortion
 Density is controlled by mAs,
 contrast by kV, resolution by
 geometric factors, motion and screen
 speed and distortion by SID, OID,
 Object to IR and CR alignment
Image Quality in Digital
 Differs from film-screen in the method
 of image acquisition; other factors
 affecting x-ray production,
 attenuation, and geometry of the
 beam remain the same
 Digital involves the application of
 analog to digital conversion theory
 and computer software and hardware
Digital Images
 Digital images in x-ray are a numeric representation of x-ray
 intensities that are transmitted through the patient
 They are viewed on a computer monitor and are known as
 Each digital image is two-dimensional (just like screen-film)
 and is formed by a matrix of picture elements called pixels
 Pixel: represents one extremely small portion of the original
 Matrix: made up of many pixels
 Digital processing involves algorithms which are systematic
 application of highly complex mathematical formulas.
 Algorithms are applied by the computer to every dataset
 before the tech sees the image
Exposure Factors for Digital
 kV and mA have to still be selected for digital
 imaging; they do not have the same affect on the
 image quality as in film-screen; they do however
 still affect patient dose
 mA: mAs controls the number of x-rays times the
 duration; digital processing allows for correction of
 inaccurate mAs selection
 kV: kV controls the penetrating power. Compared
 with film screen imaging, changes in kV have less
 of a direct effect on final image contrast because
 the resultant contrast is also a function of the
 digital processing.
Wide Latitude

 There is a wide-latitude or wide
 acceptance of exposure factors to
 produce an acceptable image in
 digital technology as compared with
 the restricted range in film imaging.
 This equates to fewer repeats,
 increased efficiency and cost
Image Quality Factors in Digital

 Exposure Index
#1: Brightness
 The intensity of light that represents the
 individual pixels in the image on the
 The controlling factor of brightness is the
 processing software through the
 application of predetermined digital
 processing algorithms.
 Density cannot be altered post-processing
 in film imaging but the brightness can be
 adjusted in the digital image after exposure
#2: Contrast
 The difference between light and dark
 areas of an image
 Similar to film imaging in that contrast is
 the difference in density of adjacent areas
 on the film
 Contrast resolution: imaging system‟s
 ability to distinguish between similar
 Controlling factors: pixels and bit depth,
 pixel sizes and scatter radiation control
Explanation of Controlling
Factors of Contrast
 Pixels and bit depth: the greater the bit depth (or
 range of gray shades) the greater the contrast
      Bit depth is determined by equipment
 Most common bit depths are 10, 12 and 16.
 Pixel sizes: two pixel sizes in imaging-
 acquisition pixel size (inherent to the equipment)
 and display pixel size (the monitor display)
 Scatter radiation control: digital detectors are
 more sensitive to low energy radiation than film
 screen. Scatter can be controlled in much the
 same way as in film imaging- girds, close
 collimation and selection of optimal kV
#3: Resolution
 The recorded sharpness or detail of structures on
 the image (same as film imaging)
 Resolution in digital relies on the same factors in
 film imaging (focal spot size, geometric factors and
 motion) but also acquisition pixel size. Resolution
 size is measured in microns v. line pairs. Current
 acceptable range for digital is 100-200 microns
 Other than pixel size, another controlling factor for
 resolution is the display matrix. Perception of the
 image is dependent on the display capabilities.
#4: Distortion

 Again, this definition is the same as in
 film imaging: The misrepresentation
 of object size or shape as projected
 onto radiographic recording media.
 Factors that affect distortion also
 remain the same: SID, OID and CR
#5: Exposure Index
 Numeric value that is representative of the
 exposure the image receptor received.
 Also known as the S value, S number of
 sensitivity number
 Exposure index is reliant on the intensity of
 the radiation striking the detector
 It is a calculated value from the effect of
 mAs, kV, total detector area irradiated and
 objects exposed.
 Reading from book on page 52
#6: Noise
 Random disturbance that obscures or
 reduces clarity. This translates into a
 grainy or mottled appearance of the
 radiographic image.
 SNR: signal-to-noise ratio: the describing
 of noise in the digital image. High SNR is
 desirable, low is not. The number of x-ray
 photons that strike the detector can be
 considered the “signal” and other factors
 like scatter can be classified as “noise”.
 See examples of high and low SNR
Post-Processing of the Digital
 Post processing is the changing or enhancement
 of the electronic image in order to improve its
 diagnostic quality.
 There will be an acceptable exposure index range
 for each digital system. A technologist must
 determine if an image falls within this range
 There are a variety of post-processing options:
   Windowing: window width which controls contrast and
   window level which controls brightness
   Edge enhancement
   Subtraction: used in angiography
   Image reversal: light and dark pixel values are reversed
   Annotation: text can be added
Applications of Digital
 CT: computed tomography
 DF: digital fluroscopy (discuss two types currently
 CR: computed radiography which includes the
 use of IP (image plates), IP reader and the
 technologist workstation
 DR: direct digital radiography (no cassettes)
 PACS: picture archiving and communication
 systems (elimination of the hard copy; total soft
 copy storage) Know the acronyms DICOM, RIS
 and HIS. See the diagram on bottom of p.61
 Discuss telemedicine and teleradiology
 CAD: computer aided detection (used in
 IHE: integrated healthcare enterprise
Radiation Protection

   RAD: patient dose- radiation absorbed
   REM: radiation protection purposes-
   radiation equivalent man
   Roentgen: radiation exposure in air
Technologist Protection
 Annual dose limit for occupationally exposed
 workers is 5 rem of whole body ED (effective
 dose) per year
 The ED for general public is 0.1 rem for frequent
 and 0.5 rem for infrequent. Public dose is not
 technically monitored in any way.
 Cumulative dose limit is 1 rem times the years of
 age (if you are 21, your cumulative dose limit is 21
 Pregnant workers should not receive more than
 .05 during any one month or more than 0.5 rem for
 entire pregnancy
ALARA Principles

 Film badges
 Patient restraint
 Close collimation, filtration, optimum
 techniques, high speed screens/film
 and avoidance of repeats
 Cardinal rule: time, distance and
Patient Doses
 SEE: skin entrance exposure
 ED: effective dose
 Fluroscopy involve much higher
 doses than standard “overhead tube”
 Look over dose chart on page 67
 Discuss the sample exposure chart
 on p. 73 that is included in each
 procedure in the textbook
Patient Protection

 Minimum repeats
 Correct filtration
   0.5 mm Al equivalent (inherent)
   Added filtration is good
   Minimum total filtration (inherent +
   added) must be 2.5 mm Al equivalent
Patient Protection, con’t.

 Accurate collimation
 Specific area shielding
 Protection for pregnancy
 Optimum-speed film-screen
 combinations (not applicable in

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