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How to Calculate the Multiple Scan Average Dose for a CAT Scan

Dr. Morin, a medical physicist at the Mayo Clinic in Jacksonville, Florida, and his research team define the multiple scan average dose or MSAD of a CT scan as "the average radiation dose over the central scan of a CT study consisting of multiple parallel scans." The preferred method for obtaining the MSAD in clinical practice is to first calculate the computed tomographic dose index, or CDTI, and then correct for the MSAD.

Things You'll Need

  • Multislice CT scanner
  • 16- or 32-cm-diameter Plexiglas (polymethylmethacrylate) phantom
  • 100-mm ionization chamber
  • Electrometer
  • Lucite acrylic rods
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Instructions

    • 1

      Place the 16- or 32-cm Plexiglas phantom on the CT scanner tabletop so that one dosimeter hole is at the point of maximum exposure (according to manufacture guidelines). Plug the empty holes with Lucite rods.

    • 2

      Perform one scan of the phantom per the usual clinical technique to ensure proper position. The central hole of the phantom should be within 5 mm of the center of the viewing field. If necessary, reposition the phantom.

    • 3

      Place the 100-mm ionization chamber into the central hole of the phantom.

    • 4

      Scan the phantom containing the ionizing chamber. According to Dr. Morin, the ionizing chamber "integrates the radiation exposure of a single axial scan over a length of 100 mm." Note the current measured by the electrometer.

    • 5

      Obtain electrometer measurements at the surface, or peripheral, holes. To do this, simply move the ionization chamber from the central hole of the phantom to the other holes and rescan. Remember to plug the empty holes with Lucite rods. Be sure to note the slice thickness during each scan.

    • 6

      Calculate the CTDI at each hole using the following formula: Xc = Xr · Cc · f · L/T, from the American Association of Physicists in Medicine. Xc is the calculated CDTI, Xr is the electrometer reading from each hole, Cc is the calibration correction factor (specific for each ionization chamber and electrometer) and is usually 1.0 but may be 2.0, f is the factor used to convert exposure to air to exposure to another material (such as acrylic) and is equipment and radiation exposure specific (usually 0.87 rad/R), L is the length of the ion chamber (100 mm) and T equals slice width.

    • 7

      Take the average of the measurements in Step 6 to get the CDTI.

    • 8

      Calculate MSAD for non-spiral or axial scans using this formula: MSAD = [ (N X T X CDTI) / I ] where N equals the number of scans, T equals scan thickness, CDTI is the number calculated in Step 7 and I equals the distance between scans.

    • 9

      Calculate MSAD for spiral or helical scans using this formula: MSAD = [ (1 X CDTI) / Pitch ] where CDTI is the number calculated in Step 7 and pitch =[ I / (N X T) ], where I equals the distance between scans, N equals the number of scans and T equals scan thickness.

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