RAD309

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RAD309

• Patient Dose

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CT

• Patients are exposed to higher radiation levels
  from the use of computed tomography compared to
  most imaging techniques

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Radiation Dose in CT

• How much dose is scanner delivering?
• Inter-scanner comparison of dose
• Estimate patient potential risk
• Weigh risk against benefit
• Patient exposure tables/ a requirement by
  regulatory agencies (JCAHO)

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CT Beam Geometry

• Most modern CT
• Emit fan shaped beam
• Narrow cross section
• Thin beam slice across patient

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How do we measure dose?

• Ionization chamber
• Quantifies radiation exposure
• Air filled container
• Amount of collected charge is proportional to
  amount of radiation passing through it
• Charge is removed and measured with electrometer
• Total Q measured in coulombs
• 1 C 1.6 x 109 e

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Multiple Scan Average Dose

• Ave dose delivered to patient when a series of
  scans are performed
• Graphical

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Multiple Scan Average Dose

• MSAD is dose from many slices

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BED Index

• distance bed is moved between adjacent scans (mm)
• each scans the patient is moved a bed index
  distance

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Computed Tomography Dose Index

• (CTDI) special quantity expresses radiation dose
  in CT
• Measured with a dosimeter inserted into a phantom
  that represents a patient
• Dose is measured by scanning one slice
• factors are applied to convert the measured
  phantom CTDI to an actual patient scan
• a reasonable estimation of the actual dose to the
  patient

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CTDI and MSAD

• Slice dose versus procedure dose
• CTDI ionization chamber used for measurement
• Area of dose cure for single slice dived by slice
  width
• MSAD use CTDI to calculate an ave dose in middle
  of series of scans
• Ratio of slice width to slice spacing multiplied
  by CTDI

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CTDI and MSAD Affect

• To increase CTDI area under curve
• intensity ( raises height of curve) OR
• Widening the curve ( open collimator)
• CTDI Patient
  Dose
• CTDI is the MSAD at canter of a series of 14
  contiguous slices

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MSAD and BI

• BI MSAD
• Large gap b/w slices (slices further apart)
• Radiation spread over a large area (ave. Dose)
• BUT relevant tissue can be missed out
• Limit to BI increase

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BRH Recommendation

• A measurement based on 2 concepts CTDI and MSAD
  (using pencil ion chamber)
• Dose from CT is commonly specified in terms of
  CTDI
• CTDI is derived from measurement dose from single
  slice

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to use pencil chamber methods
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CT DOSIMETRY
dosetools
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CTDI/MSAD method

• a pencil ionization chamber
• 2 sized phantoms (16cm and 32cm) made of acrylic
  to standardize CT dose measurements
• Both phantoms have holes drilled at specific
  locations to accommodate the ionization chamber
  during dose measurement
• The chamber is positioned in 1 hole at a time
  while the other holes are filled with acrylic
  plugs
• An exposure is made and recorded. This is done
  for all holes so that dose measurements can be
  obtained for a number of positions in the phantom
• It is critical to note at this point that the
  ionization chamber is measuring the exposure and
  not dose
• a factor is used to convert exposure to dose

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Reduction of Dose

• You ,the operator, must know dose
• You, the operator, how to keep it at minimum
• What can be done to reduce MSAD
• How does this affect image

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Factors Affecting Patient Dose

• KVp
• mAs
• Pitch
• Collimation
• Bed Index
• Beam Geometry
• Detector Setup
• Other repeats, shielding, alignment, patient
  size...etc..

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How does KV affects Dose

• Reducing x-ray tube voltage (kV) while keeping
• mAs constant, the patient dose is decreased
• Dropping KV from 120 to 80 kV at a
• constant current (mAs) typically reduces
• the patient dose by about 60

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Pitch and Dose

• Pitch The distance the patient couch travels
  during
• one 360 degree turn
• As pitch increases, the time spent in any one
  point in
• space is decreased
• Pitch lt1 Higher Radiation Dose

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• Pitch is 0.75
• Image shows 25 overlap

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• Shield thyroid, breast, eye lens, gonads
• to reduce organ dose by 3060

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Patient AlignmentPatient Alignment

• Correctly centering patient on CT gantry can
• reduce radiation dose by as much as 56
• When the patient is in the incorrect position the
  patient must be moved and the CT scan repeated

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Perspective

• When a CT scan is justified by medical need, the
  associated risk is small relative to the
  diagnostic information obtained
• CT scans save thousands of lives daily
• CT scans greatly reduce exploratory surgeries

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• Justification is the scan necessary for ongoing
  patient care? Can the examination be replaced by
  a low- or no-dose examination
• Equipment maintenance Having to repeat a slice
  or an entire examination due to equipment failure
  increases dose with no benefit to the patient.
  QA program
• Limit the scan boundaries to area of interest.
  careful positioning of the scan volume, source to
  object distance, limiting the scan coverage to
  the area of interest, or angling the gantry away
  from sensitive structures can be very effective
• Decrease the exposure The correct balance
  between dose and image quality .The guidelines
  also give reference values for patient dose for
  particular examinations.
• Customize the exposure to patient size adjust
  the exposure factors to compensate for changes in
  patient size (mAs for children examinations)

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• Customize scan parameters to examination type
  High resolution (fine detail) scans such as
  sinuses, inner ear and skeletal structures can be
  performed with lower exposure factors as spatial
  resolution is relatively independent of dose
  levels. for example, report dose levels as low as
  20mAs
• Education and research Radiographers review CT
  protocols at their institution and compare image
  quality and dose to the ICRP guidelines.
  education on dose reduction techniques in
  departmental CT training Programs
• Detector sensitivity When purchasing CT scanners
  ask questions about scanner performance and
  radiation dose. Don't assume that all
  manufacturers scanners will have similar detector
  sensitivity and performance