Dosimetry
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
DRL
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)
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:
- Types of dosimeters used to measure CT doses
- CT dosimetry phantoms
- Dose descriptors specific to CT
-Including their units of measurement
- Radiation Dosimeters
Dosimeters are devices used for measuring exposure to ionizing radiation
Two main uses:
- Human radiation protection
* Personal dosimeters - Measurements of dose in medical processes
* Measures radiation emitted from CT scans
Types of Dosimeters
Types of Dosimeters:
- Film
- TLD
- 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
TLD
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:
- measures the amount of visible light emitted from a crystal
- light energy is proportional to the amount of radiation absorbed by the crystal
- 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:
- self-reading: produces an instant readout and can be reused immediately
- 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
- 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
- CT Dose Descriptors
Three primary dose measurements for CT:
- Computed Tomography Dose Index (CTDI) (a slice)
* Units of measure = Grays (Gy) - Dose Length Product (DLP) (entire scan series aka all slices)
* Units of measure = mGy/cm (milligrays per centimeter) - Effective Dose (ED)
* Units of measure = Sieverts (Sv)
I. CTDI
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
II. DLP
(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
III. ED
(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
DLP
- 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
CTDI
the quantity measured by the pencil ionization chamber technique is the:
CTDI
