Docstoc

Beam-restricting Devices _ Grids - El Camino College

Document Sample
Beam-restricting Devices _ Grids - El Camino College Powered By Docstoc
					Control of Scatter
   Radiation
    Bushong Ch. 14
                  Objectives
   Begin discussing factors that influence image
    detail or visibility of detail
   Spatial & Contrast resolution
   Radiographic Noise
   Scatter Radiation
   Ways to reduce scatter & improve image quality
   Primary beam restriction & Grids
   Technique adjustments when using grids
What are some factors that increase
                 scatter radiation?
3 factors contribute to an increase in
                scatter
   Increased kVp

   Increased x-ray field size

   Increased patient thickness
    2 principal characteristics of any image
       are Spatial & Contrast Resolution
   Spatial resolution
      Resolution is the ability to image two separate
       objects and visually distinguish one from the other
      Spatial resolution is the ability to image small objects
       that have high subject contrast (eg. bone-soft tissue
       interface, calcified lung nodules)
      Determined by focal-spot size and other factors that
       contribute to blur
      Diagnostic x-ray has excellent spatial resolution. It is
       measured in line pairs per mm. (CT measured in cm)
SMPTE Test Pattern
           Contrast Resolution
 Determined by scatter radiation and other
  sources of radiographic noise
 Radiographic noise (image fog) =
    A   uniform signal produced by scattered x-rays
     Digital imaging the grainy or uneven
      appearance of an image caused by an
      insufficient number of primary x-rays
MRI   CT
           Image-forming x-rays
   Two kinds of photons are responsible for the
    OD and contrast on an image: Photons that pass
    through without interacting and those that are
    scattered though Compton.

   X-rays that exit the patient are remnant and
    those that interact with the IR are image-
    forming.
Ideally, only those x-rays that do not interact
  with the patient should reach the IR….

   However, scatter radiation is a factor that
    must be managed
   Proper collimation has the PRIMARY effect of
    reducing patient dose by _________ ?
   Proper collimation also improved image
    contrast by reducing radiographic noise or
    fog caused by scatter
Fog or Noise
Contrast changes with the use of a grid
Less scatter radiation & less radiographic
noise – shorter scale = “better contrast”




      With Grid                 No Grid
                        kVp
   As x-ray energy increases Photoelectric and
    Compton interactions decrease. Explain?

   At 50 kVp 79% photoelectric, 21% Compton &
    less than 1% transmission

   At 80 kVp 46% photoelectric, 52% Compton &
    2% transmission
                                                 Pg. 225
How does increasing kVp affect
       patient dose?
              Patient Thickness
   Imaging thick parts of the body results in more
    scatter radiation than thin parts




    IMAGE TEST TOOL
Is patient thickness something the
     radiographer can control?
              Patient thickness
   Normally, No

   Compression devices improves spatial resolution
    by reducing patient thickness and bringing the
    object closer to the IR. Compression also
    reduces patient dose and contrast resolution
Compression
         Improves spatial
          resolution
         Reduces OID
         Reduces patient dose
         Improves contrast
          resolution (reducing
          fog or noise)
Compression
                    Field Size
   As field size increases, intensity of scatter
    radiation also increases rapidly. Especially during
    fluoroscopy
Compare images: What do you think about
  radiographic contrast & image noise?
      Control of Scatter Radiation
   Technologists routinely use two types of devices
    to reduce the amount of scatter radiation
    reaching the IR

   Beam restrictors

   Grids
3 Types of beam-restricting devices

   Aperture
    Diaphragm

   Cones or Cylinders

   Variable aperture
    collimator
           Aperture Diaphragm
   The simplest of all beam-restricting devices

   Lead or lead-lined metal diaphragm attached to
    the x-ray tube head

   The opening in the diaphragm is usually
    designed to cover just less than the IR used
                 Diaphragm
   Fixed lead opening

   Fixed image receptor
    size
   Constant SID

   Source-to-diaphragm
    distance = SDD
           Cones & Cylinders
  Are modifications of the aperture diaphragm
 Alignment
is one difficulty
when using
cones
Now mostly
used with spines
teeth & heads
Improved contrast resolution of the
         frontal sinuses
     Variable Aperture Collimator
   The most common beam-restricting device is
    the light-localizing variable aperture collimator

   The first part of the collimator serves to control
    off-focus radiation. What is off-focus radiation?
           Off - focus Radiation
   X-ray tubes are designed so that the projectile e-
    interacts with the target. However, some of the
    e- bounce off the target and land on other areas

   This caused x-rays to be produced out side the
    focal spot
            Extrafocal Radiation
   These rays can also be called off-focus radiation

   Extrafocal radiation is undesirable because it
    extends the size of the focal spot, increases
    patient skin dose & reduces image contrast
Off-focus radiation
    Fixed diaphragm in the tube
              housing
 Using a grid
does not reduce
extrafocal
radiation
    First-stage entrance shuttering device

   Has multiple
    collimator blades
    protruding from the
    top of the collimator
    into the tube housing
    The second-stage collimator shutters

   Pb leaves are at least
    3 mm thick
   They work in pairs
    and are independently
    controlled
         The collimator lamp & mirror

   Must be adjusted so
    that the projected
    light field coincides
    with the x-ray beam
   Misalignment of the
    light field and beam
    can result in
    collimator cutoff of
    anatomic structures
• Always keep the
  collimated area
  smaller than the
  size of the cassette

• What is a PBL
               Total Filtration
   Filtration review…
   Total Filtration = _______________ +
    _____________?

   The collimator assembly is usually equivalent to
    approximately _______ mm Al filtration.

   Minimum filtration for tubes that can operate
    about 70 kVp is _______ mm Al or equiv.
The Grid
    Contrast & Contrast Resolution
   Two devices are used to reduce Compton effect
    beam-restricting devices and radiographic grids
   Beam-restricting devices effects what reaches
    the patient. Grids effect the remnant beam
    Contrast & Contrast Resolution
   Contrast = the comparison of areas of light,
    dark and shades of gray on the image
   Contrast Resolution = the ability to image
    adjacent similar tissues
        Beam-restricting devices
   Are helpful to improve contrast resolution
    however the inherent problem is they are placed
    between the source and the patient. Even under
    the most favorable conditions, most if the
    remnant x-rays are scattered.


                       Table pg. 237
      Effects of Scatter Radiation on
              Image Contrast
   Contrast is the degree of difference in OD
    between areas of an image
   If you could only capture transmitted,
    unscattered x-rays, the image would be very
    sharp
   The corresponding bone-soft tissue interface,
    would be very abrupt, and therefore the image
    contrast would be high
                        Grids
   Are very effective device for reducing scatter
    radiation
   The grid is a series of sections of radiopaque
    material (grid strips) alternating with sections of
    radiolucent material (interspace material)
   The grid is designed to transmit only x-rays that
    are traveling in a straight line from the source to
    the IR
Grids “clean up” scatter radiation
  A high quality
grid can attenuate 80 –
90 percent of scatter
radiation
                  Grid Strips
   Should be very thin and have high photon
    absorption properties

   Lead is most common

   Tungsten, platinum, gold, and uranium have
    been tried but Pb is still most desirable
            Interspace Material
   Used to maintain precise separation between the
    delicate lead strips

   Aluminum or Plastic Fiber

   Grid Casing = covered completely by thin
    aluminum to provide rigidity and to seal out
    moisture. Yuck!
                   Grid Ratio
   3 important dimensions on a grid: The thickness
    of the grid strips, the width of the interspace
    material, and the height of the grid

   The grid ratio is the HEIGHT of the grid
    divided by the INTERSPACE WIDTH:
      Grid ratio = h
                   D
  h = height of the grid, T = thickness of the
grid strip, D = width of the interspace material
                   Grid Ratio
   High-ratio grids are more effective in cleaning
    up scatter radiation than low-ratio grids
   The angle of deviation is smaller for high-ratio
    grids. (the photon must be traveling in a
    straighter line to make it through the grid)
   However, the higher the ratio the more radiation
    exposure necessary to get a sufficient number of
    x-rays through the grid to the IR
    The higher the ratio the straighter the
     photon must travel to reach the IR
 Grid ratios range
from 5:1 to 16:1
 Most common

8:1 to 10:1
 A 5:1 grid will

clean up 85%
16:1 clean up 97%
               Grid Frequency
   The number of grid strips or grid lines per inch
    or centimeter
   The higher the frequency the more strips and
    less interspace material and the higher the grid
    ratio
   As grid frequency increases, patient does is
    increase because more scatter will be absorbed
               Grid Frequency
   Some grids reduce the thickness of the strips to
    reduce the exposure to the patient, this over all
    reduces the grid clean up

   Grids have frequencies in the range of 25 to 45
    lines per centimeter (60 to 110 lines per inch)
Higher frequency with the same interspace
 distance reduces the grid effectiveness
             Grid Performance
   The principal function of a grid is to improve
    image contrast
   Contrast Improvement Factor (k) = the ratio of
    the contrast of a radiograph made with a grid to
    the contrast of the radiograph made without a
    grid. A contrast improvement factor of 1
    indicates no improvements
   The higher the grid ratio & frequency the higher
    the k
                Bucky Factor
   Using grids require more patient dose. Why is
    this?

   When a grid is used technique must be increased
    to maintain OD

   The amount of increase is given by the Bucky
    factor (B) or grid factor
       Bucky Factor or grid factor
   The higher the grid ratio or frequency the higher
    the bucky factor

   The Bucky factor increases with increasing kVp

   Pg 235: We will use the average values for
    calculations.
Selectivity or ability to “clean up”
the heavier the grid the more Pb it contains
                   Grid Types
   Parallel Grid – simplest type of grid
   All the lead strips are parallel
   Only clean up scatter in one direction (along the
    axis of the grid)

   Easy to make, however can cause grid cutoff
    with short SID’s.
               Grid cutoff
 Distance to cutoff
     SID
  Grid ratio
 With decreasing
SID more potential
for grid cutoff
 IR size will also
Influence grid cutoff
Grid Cutoff – Parallel grid




 THE HIGHER-RATIO THE MORE CUTOFF POTENTIAL
                 Crossed Grid
   Have lead strips running along the long and
    short axes of the grid
   Made by placing two parallel grid on top of each
    other
                  Crossed Grid

   Have twice the grid ratio as linear
    grids

   However, CR vs grid placement is critical. The
    CR must align with the center of the grid and
    the grid and CR must be exactly parallel or grid
    cutoff will occur
                 Focused Grid
   Designed to minimize grid cutoff
   Lead strips are aligned with the divergence of
    the x-ray beam

   Each focused grid must be identified with the
    appropriate SID
   Wrong SID = Grid cutoff
Focused grid have a little SID latitude (eg.
100cm grid could be used at 90cm – 110cm)
                Moving Grids
   All stationary grids will give you grid lines on
    your radiograph. Thinner Pb strips will give you
    less noticeable lines. However, thinner strips
    have less Pb content not “cleaning up” as well

   Grid Lines are made when primary x-rays are
    absorbed in the grid strips.
    Focused grids are usually used as
             moving grids
   The grid is placed in a holding mechanism that
    begins moving just before the x-ray exposure
    and continues moving after the exposure ends

   2 types of movement Reciprocating &
    Oscillating
                 Grid Motion
   Reciprocating = moves several times about 2cm
    back and forth during the exposure

   Oscillating = moves several times about 2 – 3
    cm in a circular pattern

   Most grids are moving. Except for portable
    imaging
                Grid Problems
   Increased OID, especially with moving grids
   The biggest problem with grids is misalignment
                             GRID PROBLEMS
                             RESULT IN:
                             UNDEREXPOSED IMAGE
                             OR UNDEREXPOSED
                             EDGES OF IMAGE
Grid Problems – Off Level
       Grid Problems – Off Center




   A problem with focused & crossed grids
Grid Problems – Off Focus (wrong SID)
       Grid Problems – Upside-Down




   A problem with focused & crossed grids
                  Grid Selection
   Patient Dose
       Pg 241 – mAs changes
   Exam
   Detail required
   Part thickness
   Desired technique (kVp)
   Equipment availability
Questions….?
             Questions …?




• Technique adjustments
  problems

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:1
posted:3/30/2013
language:Unknown
pages:77