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CT Theory > Dosimetry > Flashcards

Dosimetry Flashcards

1
Q

Dosimetry

A

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

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2
Q

DRL

A

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:
  • Equipment
  • Facility protocols
  • based on average patient popukation (extreme body habitus not covered in DRLs)
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3
Q

Importance of Dosimetry

A

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)
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4
Q

Dosimetry Concepts

A

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

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5
Q
  1. Radiation Dosimeters
A

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
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6
Q

Types of Dosimeters

A

Types of Dosimeters:

  1. Film
  2. TLD
  3. Specially designed Ionization Chambers (preferred and currently most used in CT)
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7
Q

Film Dosimeters

A

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
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8
Q

TLD

A

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
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9
Q

Ionization Chambers

A

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
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10
Q

Pencil Ionization Chamber

A

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

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11
Q
  1. Dosimetry Phantoms
A

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
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12
Q

Phantom Considerations

A

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

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13
Q
  1. CT Dose Descriptors
A

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)
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14
Q

I. CTDI

A

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
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15
Q

I. CTDI (cont’d)

A

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!
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16
Q

CTDI Measurements

A

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

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17
Q

II. DLP

A

(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

18
Q

III. ED

A

(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
19
Q

DLP

A
  • directly proportional to scan length
  • scan length and slice thickness must be known
  • DLP= dose recieved by all slices used in scan series
20
Q

CTDI volume

A
  • unaffected by scan length
  • CTDI= average of all slices to determine an individual slice average dose
  • # of detectors and pitch must be considered
21
Q

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

A

pencil ionization method

22
Q

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

A

DLP (dose lined product)

23
Q

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

A

CTDI

24
Q

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

A

CTDI

25
Q

the dose to the patient in CT is affected by:

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

all of the above!

26
Q

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

A

CTDI (volume)

27
Q

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

A

ED

28
Q

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
A
  1. should scan the phantoms using two different techniques
29
Q

which of the following allows dose comparison between scanners?

  1. TLD
  2. ED
  3. DLP
  4. CTDI
A
  1. CTDI
30
Q

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

  1. film badge dosimeter
  2. TLD
  3. ionization chamber
  4. MOSFET
A
  1. ionization chamber
31
Q

the unit of exposure in air is?

A

R

32
Q

the Gray is the unit of measurement for _________ and used to be measured in ____________

A

radiation absorbed dose; rad

33
Q

which factor accounts for the different biologic effects caused by ionizing radiation?

A

Q

34
Q

which unit represents the quality factor being applied to rad?

A

Rem

35
Q

CT exam for: sinuses has a dose of 0.6 mSv what is the BERT equivalent?

A

2 months

36
Q

CT exam for: head has a dose of 2.0 mSv what is the BERT equivalent?

A

8 months

37
Q

CT exam for: chest has a dose of 7.0 mSv what is the BERT equivalent?

A

2 years

38
Q

CT exam for: chest (PE) has a dose of 10.0 mSv what is the BERT equivalent?

A

3 years

39
Q

CT exam for: Abd/Pelvis has a dose of 10.0 mSv what is the BERT equivalent?

A

3 years

40
Q

CT exam for: multiphase abd/pel has a dose of 31.0 mSv what is the BERT equivalent?

A

10 years

41
Q

order the following from the most radiosensitive to the least:

  • Lung 0.12
  • gonads 0.20
  • thyroid 0.05
  • bone 0.01
  • (numbers are the weighting factors)
A
  1. gonads
  2. lungs
  3. thyroid
  4. bone

CT Theory (11 decks)

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