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Instrumentation & methods used to measure patient dose from a CT scanner (calculation and assessment of the dose recieved by the patient during CT imaging)

  • Measurements used for:

-Risk assessment (benefits outweigh the risks)

-Radiation protection guidelines (DRLs)

-Dose optimization

**DRL (diagnostic reference level)

measurements are used to develop dose optimization protocols

-ALARA principles while maintaining imag quality



Diagnostic Reference Levels (better to use than max dose levels)

  • Purpose:

-Maintains standards used to control amount of radiation exposure used for patient imaging

  • Considerations:

-Maximum dose limits should not be used

     -Will not regulate safe dose limits for patient exposures (Dose limit values would be too high)

 -Doses for procedures fluctuate due to variances in:


      -Facility protocols

  • based on average patient popukation (extreme body habitus not covered in DRLs)


Importance of Dosimetry

Dosimetry enables technologists the ability to:

  • Compare their doses to national averages

           -Regulatory bodies (Health Canada) have an anonymous database for comparison

           -Determines effectiveness of the implementation of their radiation protection methods

  • Inform the public & other personnel about CT doses
  • Perform dose measurements (When medical physicist is unavailable)


Dosimetry Concepts

In order to appreciate the data collected from dosimetry measurements technologists should understand:

  1. Types of dosimeters used to measure CT doses
  2. CT dosimetry phantoms
  3. Dose descriptors specific to CT

-Including their units of measurement


1. Radiation Dosimeters

Dosimeters are devices used for measuring exposure to ionizing radiation

Two main uses:

  1. Human radiation protection
  • Personal dosimeters

    2. Measurements of dose in medical processes

  • Measures radiation emitted from CT scans


Types of Dosimeters

Types of Dosimeters:

  1. Film
  2. TLD
  3. Specially designed Ionization Chambers (preferred and currently most used in CT)


Film Dosimeters

Film badge dosimeter:

  • A type of Personal Dosimeter
  • Has two parts: (Film & Holder)
  • Double emulsion technology advancement

             -Enabled detection of low & high energy photons

             -one detects low energy photons

             -other detects high energy photons

  • Least accurate form of measuring CT dose

            -Light, heat & humidity sensitivities

            -One time use only



Thermoluminescent Dosimeter:

  • A type of Personal Dosimeter
  • Active crystal component:

          -Detects radiation exposure

          -Emits light when heated

               -Light energy is proportional to the amount of radiation absorbed by the crystal

  • Reusable

           -Can be worn for up to 3 months

           -No record of previous exposure once heated & measured

How it works:

  1. measures the amount of visible light emitted from a crystal
  2. light energy is proportional to the amount of radiation absorbed by the crystal
  3. the crystal must be heated to release the light energy and get a reading of radiation exposure



Ionization Chambers

Used to measure radiation doses from CT scans

  • Specially designed Ionization Chambers

         -Pencil ionization chambers used for CT dose measurements (Current method used for measuring CTDI)

        -Easiest method of recording exposure

        -Most accurate method of quantifying radiation exposure

there are two versions:

  1. self-reading: produces an instant readout and can be reused immediately
  2. non-self reading: requires an electrometer to read exposure


Pencil Ionization Chamber

Small air-filled container with thin walls that allows radiation to pass through

  • X-rays collide with air molecules within the chamber
  • Some of these molecules are ionized
  • The ionized electrons are collected on a conducting wire or plate and measured as an electric charge
  • The collected charge is proportional to the amount of ionization (Which is proportional to the amount of radiation)
  • The charge is removed from the chamber and measured by an electrometer
  • represents Q and measured in Columbs

***** Q=rad weighting factor


2. Dosimetry Phantoms

Standardize dose measurements for various CT exams

                 -Phantoms mimic patient geometry

  • Phantom Characteristics:

            -Homogenous (Made of acrylic)

            -Contain holes

                -For placement of the pencil ionization chamber

                -Unused holes need to be “plugged”

                -Enables dose measurements at different locations

             -Come in two diameters with the same length:

  • 16cm
  • 32cm
  • round to mimic pt geometry


Phantom Considerations

The dose measured from the phantoms using a consistent technique:

  • Varies (among locations in the phantom)

           -Dependent on location of measurement

                 -Partial shielding (effect on dose uniformity)

  • Measures CTDI

           -Index of CT radiation dose

  •  (phantoms) Does not accurately estimate actual patient radiation dose (because phantom is uniform and a human is not ex organs and tissues)

          -Uses estimates to calculate Helical scan doses (for an average pt)

CTDI: computed tomography dose index


3. CT Dose Descriptors

Three primary dose measurements for CT:

1. Computed Tomography Dose Index (CTDI)  (a slice)

  • Units of measure = Grays (Gy)

2. Dose Length Product (DLP)  (entire scan series aka all slices)

  • Units of measure = mGy/cm (milligrays per centimeter)

3. Effective Dose (ED)

  • Units of measure = Sieverts (Sv)



Dose is usually calculated from multiple scans

      -CT scans require multiple slices (Multiple scans are acquired in a scan series)

  • CTDI measures the MSAD (multiple scan average dose)

       -Calculates the dose (of total exposure to radiation) of a single slice (measured by center of slice and several points @ periphery using acrylic phantoms)

                 -Accounts for scatter within each slice

        -Calculated using the primary radiation dose from each slice along with the amount of scatter created by each slice

  • Provides an average estimated measurement of the exposure per slice of tissue

Increase dose when slice overlap

decrease dose when gas

(don't want either)

  • measures total amount of dose a single slice is recieving


I. CTDI (cont’d)

CTDI is a standardized measurement of radiation dose

  • Allows dose comparison between scanners
  • Only measures contiguous slices

         -Estimates are used to calculate Helical scan doses

               -Due to limitations caused by the geometry of pencil ionization chambers

  • Does not account for differences in tissue densities within a patient
  • not an accurate measurement/estimate of radiation dose
  • units of measurements are Gray (Gy)
  • dose index and pt dose are NOT the same thing!


CTDI Measurements

Dose must be measured at several locations

-Increases accuracy of average dose estimates

  • Difference between CTDI measurements:

CTDI(FDA) = mean absorbed dose in the scanned

object volume; fixed slice width measurements (# of slices and slice widths used)

CTDI(100) = measures a variety of slice widths (smaller slice widths)

CTDI(W) = calculates average dose in the x-y axis (accomodate dose uniformity)

CTDI(Volume) = calculates average dose in the z-axis (slice thickness and where radiation source is coming from)

To calculate average dose of a scan slice in Helical CT imaging:

CTDI(Volume)= CTDI(W)/ Pitch

*doses change with pt thickness

*W= weighting factor




(dose line product)

DLP is a dose measurement of the total amount of exposure received in a scan series

  • Directly proportional to the length of the scan

           -Longitudinal anatomy coverage

  • Must calculate the scan length as well as the CTDI(volume)

To calculate the average dose received in a series:

DLP = CTDI(volume) x Scan Length



(Effective Dose)

A measurement that attempts to correlate the amount of dose absorbed by the patients tissues during a CT scan to the probability of developing biologic effects (of body parts exposed during scan measured in Sv)

  • Risk assessment

         -Radiosensitivity of organs varies

  • Compares doses produced during CT scanning to those of natural background exposures (BERT)
  • ED=DLPxK



  • directly proportional to scan length
  • scan length and slice thickness must be known
  • DLP= dose recieved by all slices used in scan series


CTDI volume

  • unaffected by scan length
  • CTDI= average of all slices to determine an individual slice average dose
  • # of detectors and pitch must be considered


the easiest and probably most accurate method of measuring the dose in CT is:

pencil ionization method


the average dose to the patient from a scan series is the:

DLP (dose lined product)


the area under the typical dose distribution profile for a single scan divided by slice width is the



the quantity measured by the pencil ionization chamber technique is the:



the dose to the patient in CT is affected by:

  1. noise
  2. slice thickness
  3. kVp and mAs
  4. linear attenuation coefficient

all of the above!


pitch is used to calculate the following dose descriptor for helical scans:

CTDI (volume)


which dose descriptor provides a way to compare risk with other types of radiation exposures?



which is not a consideration when imaging a dosimetry phantom to measure dose?

  1. unused holes must be plugged
  2. must scan 2 phantoms of different diameters
  3. should scan the phantoms using two different techniques
  4. measurements should be made at different locations

3. should scan the phantoms using two different techniques


which of the following allows dose comparison between scanners?

  1. TLD
  2. ED
  3. DLP
  4. CTDI



the type of dosimeter which detects radiation exposure via ionization is a:

  1. film badge dosimeter
  2. TLD
  3. ionization chamber

3. ionization chamber