What To Know About Calibrating A CT Scanner

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What To Know About Calibrating A CT Scanner
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By Vikki Harmonay
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CT scanners are the workhorses of imaging
departments around the globe, being used in
therapeutic, diagnostic and interventional
procedures. In fact, there are more than 75
million CT scans per year in the U.S. The most
common CT scans are done on the neck, head and
facial regions, although there has been an
increase in the number of cardiac, angioplasty
and neuro CT studies. Because there are so many
CT scans performed, it is important to calibrate
your CT unit on a routine basis. This will
guarantee that your scans will have accurate
details in the high resolution images created of
arteries, nerves and other vascular structures.
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What Is CT Calibration? To calibrate a CT
scanner, you need to scan an object or phantom
with a known radio density in order to check
whether the measurements are giving the
appropriate HU or Hounsfield units. While there
are no hard and fast rules about how often you
need to check your scanners calibration, it is
important to execute them regularly.
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What Is A HU Value? The HU value measures the
absorption/attenuation of radiation within
tissue. A grayscale is produced during the CT
image reconstruction phase. It reflects the
appropriate HUs for the study thats done. More
than 200 shades of gray can be rendered by a CT
scanner, ranging from the lightest whites to the
darkest grays. The slightest difference within 2
in tissue density can be detected with this kind
of contrast. In order to maintain this level of
accuracy without image distortion or loss of
value, a regular CT calibration must be
performed. Without it, image distortion or lack
of proper contrast could lead to a misdiagnosis
or delay in treatments for patients that are
critically ill.
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• Calibration is also important if you are testing
  a CT scanner for purchase. This allows the buyer
  to test the accuracy of the scanner for studies
  at their practice.
• There are three things that affect the
  calibration
• Exposure factors used
• Technological characteristics
• Image viewing conditions

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• Parameters Tested During A CT Scanner Calibration
• The calibration will measure the homogeneity of
  an image. Its essential that hardening artifacts
  are avoided. Plus, the tissue thats being imaged
  should have a uniform appearance thats free from
  artifacts
• It will calculate the relevant number of
  different tissues in a body
• It will measure noise, or the unwanted densities
  or values in an otherwise homogenous image
• Linearity is measured with a phantom with several
  materials that mimic different tissues, then
  those images are compared with the actual HU of
  the tissue

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• Spatial resolution will be measured. A scan must
  be able to distinguish the differences between
  two structures with different densities, like fat
  cells versus muscle tissue
• Low contrast resolution will be tested, so CT
  imaging can distinguish between more than 200
  different shades of gray
• Slice thickness and the positioning of the couch
  (the movement of the table during the scan will
  also be tested, in 1mm to 4mm increments
• At this time, scientists are still searching for
  a universal standard for calibrate scanners.
  Until then, your bio team should work closely
  with department imaging directors and
  radiologists to determine the correct protocol
  during calibration.

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Talk To An Expert Its always wise to talk to an
expert about how often you should calibrate your
CT scanner. The team at Atlantis Worldwide is
happy to help you with this and any other
questions about medical imaging devices,
protocols and testing. Contact Us Today!
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