Dosimetry Flashcards
Diagnostic Reference Levels purpose
Maintains standards used to control amount of radiation exposure used for patient imaging
Maximum dose limits shouldn’t be used
Doses for procedures fluctuate due to variations in equipment and facility procedure
Is not considered for extreme body habitus, only average pt’s
Importance of Dosimetry
Determines effectiveness of the implementation of their radiation protection methods Regulatory bodies (Health Canada) have an anonymous data base that compare national doses Help maintain techniques that comply with ALARA
Purpose, advantages, and disadvantages of a Film Dosimeter
- personal dosimeter
- double emulsion technology advancement enabled detection of low and high energy photons
- least accurate form of measuring dose
- has light, heat and humidity sensitivity
- one time use only not used anymore
Purpose, advantages, and disadvantages of thermoluminescent Dosimeter
- personal dosimeter
- active crystal component that detects radiation exposure and then 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 and measured
Purpose, advantages and disadvantages of ionization chambers
- measures dose from radiation scans
- pencil ionization chambers used for ct dose measurements
- current method for measuring CTDI
- easiest method of recording exposure
- most accurate method of quantifying radiation exposure
How an pencil ionization chamber works
Small air filled container with thin walls that allows radiation to pass through
- x-rays collide with air molecules w/in the chamber
- some of the molecules are ionized
- ionized e- are collected on a conducting wire/plate and measured as an electric charge
- the collected charge is proportional to the amount of radiation
- charge is then removed from the chamber and measured by an electrometer measured in Coulumbs and reps Q
CT Dosimetry Phantoms characteristics
- phantoms mimic pt geometry
- large phantom is 32cm mimics body
- small phantom is 16cm mimics head
- homogeneous (made of acrylic)
- contain holes for placement of the pencil ionization chamber, must be plugged if they are not being used
- enables dose measurements in different locations
CT Dosimetry Phantoms Purpose
- help standardize dose measurements for various CT exams
- the dose is measured in the phantom using a consistent technique
- radiation will vary on location due to absorption and partial shielding effect on dose uniformity
- measures the CTDI
- Does not accurately estimate actual pt radiation dose*
What is Computed Tomography Dose Index (CTDI)
- Standardized measurement of radiation dose
- measures the multiple scan average dose by calculating the dose of total exposure to radiation of a single slice
- accounts for the scatter in each slice or dose will would be underestimated
- single slice(primary rad dose) + amount of scatter = total exposure
- provides a avg estimated measurement of the exposure per slice of tissue
What is the CTDI used for
- allows dose comparison b/w scanners
- Estimates are used to calculate helical scan dose (due to limitation of pencil ionization chambers)
- does not account for tissue density differences w/in the pt
- dose must be measured at several locations (increases accuracy)
- dose index and pt dose are not the same thing*
what is CDTIfda
Mean absorbed dose in the scanned object volume; fixed slice width measurements
Dose index as it relates to # of slices and slice widths
What is CTDI100
Measures a variety of slice widths
Accommodate smaller slice widths
What is CTDIw
Calculates the average some in the z - axis
Accommodates dose uniformity
What is CTDIvolume and formula
Calculates the average dose in the z-axis CTDIvolume = CTDIw/pitch Has no relationship to the length of the exam # of detectors and pitch must be considered
What is Dose Length Product and formula
- dose measurement of the total amount of exposure received in a scan series
- directly proportional to the length of the exam
- longitudinal anatomy coverage z-axis (length of anatomy scanned)
- DPL= CTDIvolume x Scan Length
What is the Effective Dose and the formula for it
- measurement that attempts to correlate the amount of dose absorbed by the pt’s tissues during a CT scan to the probability of developing biological effects
- Risk assessment of organs (gonads vs brain)
- compares doses produced in CT scanning to those of natural background exposures
- measured in sieverts formerly was Rem
ED = DLP x K
Exposure
The amount of radiation the pt is exposed to
Absorbed Dose
Amount of radiation that is absorbed by the pt
Measured in Gy
Used to determine what threshold deterministic effects will occur
Effective Dose
Used to measure the risk of partial body exposure from the equivalent whole body dose
Takes into account the type of radiation and the exposures tissues
Measured in Sieverts
Used when considering the probability that a stochastic effect may occur
Stochastic Effect
Effects occur by chance There is no dose threshold The severity of the adverse effect is not dependent on dose Usually appears years later Cancer and genetic effects Non linear threshold dose model
Deterministic Effect
There is a dose threshold after which an adverse effect will happen
Severity of the adverse effect is directly proportional to the amount of radiation received (increase dose increase adverse effect)
Skin erythema, necrosis, epilation, cataracts
Dose distribution in CT
Is more uniform
Dose spread becomes less uniform as the pt thickness increases and SFOV
Entrance skin dose if greater than the dose at the center of the pt
Caused by the partial shielding effect
Partial shielding effect
Body acts a kind of shield
Some body parts will block the radiation from reaching other body parts
Why organ doses are higher in kids
Collimation and effect on dose
Beam width increases so that the pneumbra extends beyond the active detectors and increase pt dose
Over ranging and its effect dose
Applies to helical scans and increases pt dose
Exposed scan length > planned scan length
Localizer and its effect on dose
Are very low dose and used to assist in decreasing total scan dose
Scout image
Radiation Beam Geometry and its effect on dose
A 180 degree arc would decrease pt dose however a 360 degree arc is typically used for increased image quality
Filtration and its effect on dose
Designed to absorbed low energy photons which increase absorbed dose and decrease image quality
Repeat scans effect on dose
Multiphase enhancement studies fall under this category and increase pt dose
Iterative Reconstruction effect on dose
Can decrease pt dose by 50% compared to other back projections methods
Detector Efficiency effect on dose
Scintillation system have increased absorption efficiency which decrease pt dose and improve image quality
Pitch effect on dose
Moves the pt through the gantry faster and decrease pt dose by 33%
Decreased SFOV effect on dose
More uniform beam distribution decreases dose
MA effect on dose
Should be decreased for smaller pt’s
KVp effect on dose
Will increase dose to paediatrics if not decreased from adult protocol
Centring effect on dose
Important when using ACTM
If not perfect image quality is compromised due to increased image noise
Decreased slice width and spacing effect on dose
If the same volume of tissue is scanned
- taking more slices increases pt dose
- overlapping slices increases pt dose
Factors that have and Indirect effect on dose but direct effect on image quality
- spatial resolution
- contrast resolution
- noise
Use ALARA to maintain diagnostic image quality
Automatic Tube Current Modulation effect on dose
- AEC for scanners
- Optimizes the dose to the pt while maintaining image quality regardless of pt size or tissue attenuation
- uses info from the scout image to determine the appropriate Ma during the scan
- a predefined level of IQ is defined by the vendor and protocolled by the users
3 Cardinal rules of radiation protection
Time - limit # of scans, adjust pitch
Distance - distance yourself from source of radiation
Shielding - less beneficial in CT due to the amount of scatter
Bismuth Shielding
Used for anterior dose reduction
In-plane shielding
Must be outside the SFOV during scout acquisition
Cause problems with ATCM
Lead Shielding
Used to enhance the precaution of safety Out of plane shielding Must not be in the SFOV, covers anatomical parts of the pt that aren't of interest No effect on IQ Must be 360 degrees around the pt
shielding the public and personnel
Secondary shielding is required - room walls (protect public)
Based on equipment
Lead shielding is required for any person who must remain in the room during the scan acquisition
areas around the gantry are where radiation exposure is the highest
Limit dose by
Appropriate pt selection Limit multiphase scans Develop protocols based on pt's body habitus - uses lowest mA possible - adjust pitch when appropriate
Peads considerations and how to limit dose
- Law of Bergonie and tribondeau states that the radio sensitivity of a tissue is dependent on the metabolic state
- why kids are more radiosensitive than adults
Have separate protocols for kids
Use CT when only clinically necessary - use other modalities
Limit multiphase scans
Consider pt shielding
Apply ALARA
