CT Radiation Protection Quiz Flashcards
Why dose CT have a considerably higher dose than general x-ray?
Quantum mottle with small objects/slices and low contrast resolution requirements
The level of risk associated with a CT exam is considered acceptable if?
- the patient is aware of the risk
- the patient receives some type of benefit
- ALARA is maintained to reduce risk
2 general components that will help us determine appropriate exposure levels
- Appropriate patient selection
- ALARA without compromising the diagnostic quality of the image
Who provides a dose table that identifies doses delivered to patient?
The manufacturer
What is exposure dose?
Refers to the concentration (quantity) of radiation at a particular point on the patient
- roentgen or Coulombs per kg
- replaced by air kerma in grays (Gy)
What is absorbed dose?
Amount of energy absorbed per unit mass of material (patient)
- risks associated with radiation exposure are related to the amount of energy absorbed
- grays (Gy)
- rads (rad)
What is the effective dose?
Attempts to account for the effects specific to the patient’s tissues that absorbed the radiation dose
- Quantifies the risk from partial-body exposure to that of an equivalent whole body dose
- weighted average organ dose
- related exposure to risk
- considered type of radiation used (radiation weighting factor) and radiosensitivity of the tissue (tissue weighting factor)
- sievert (Sv)
- rem (rem)
2 main classifications of radiation effects?
- Stochastic (non-threshold): any dose no matter how small is a risk, probability depends on dose, measured using linear non-threshold dose response model, late effects: cancer, leukemia, hereditary effects
- Deterministic (threshold): severity increased with increased dose, dose below threshold = no effect, can kill cells and cause degenerative changes, early effects: skin erythema, epilation, pericarditis, occur with high exposures
How is the dose distributed in CT?
The entrance skin dose is greater than the dose at the center of the patient
Uniformity of dose __________ as the SFOV and patient size ___________?
Decreases, increase
Do small body parts or large body parts have a more uniform dose distribution? Does this mean a higher or lower total dose?
Smaller body parts have a more uniform dose distribution
Higher total dose
What is partial shielding?
Some body parts block the radiation from reaching other body parts
2 categories of factors affecting dose?
- Direct: we control them e. kVp
- Indirect: indirect effect on dose, but a direct effect on image quality ex. Noise, spatial resolution, contrast resolution
Factors affecting dose?
- Technique
- Collimation
- Pitch
- Centering
- Over-ranging
- Beam geometry
- Repeat scans
- Filtration
- Detector efficiency
- Scan field diameter
- Localizers
- Slice width and spacing
- Patient size and part thickness
- ATCM
Regarding collimations, which produces a higher patient dose: MDCT or SDCT? Why?
MDCT increased patient dose because the beam width must be increased so that the penumbra extends beyond the active detectors so that the detectors all receive the same amount of photons
Miscentering ________ image quality and __________ patient surface dose?
Decreases, increases
Improper side to side centering equals what percentage of dose increase?
18-41%
Improper up and down centering equals what percentage of dose increase?
140%
What is over-ranging and how does it affect patient dose?
Exposed scan length is greater than planned scan length (helical scans) = increased dose
Affected by reconstruction algorithms (iterative reconstruction algorithms are best)
What beam geometry do we use to decreased dose? What about to increase image quality?
180 deg to decrease dose
360 deg to increase image quality
Decreased SFOV = __________ dose? Why?
Increased
-smaller patient have more uniform dose distribution = increased total dose
How do localizer scans affect dose?
Decrease, used for planning purposes and assist in lowering total scan dose
Decreased slice thickness = ___________ patient dose?
Increased due to increased technique requirements (or else too much noise)
“Tails” of scatter create ________ % more dose?
25-40%
What new technologies decrease awareness of dose effects?
- ATCM
- Adult technique on pediatric patients
- Anatomy (lower technique should be used for chest vs. abdomen
3 cardinal rules of radiation protection?
- Time
- Distance
- Shielding
Types of shielding?
- In-plane (bismuth): don’t use during scout image, but used during scanning
- Out-of-plane (lead): allows significant organ and effective dose reduction due to reduction of external scatter, must wrap patient 360 deg
What is secondary shielding required for?
Room walls due to scatter
How to reduce dose in CT?
- Appropriate patient selection
- Adjust scan protocol to meet the clinical need: customize exam (slice thickness, pitch, reconstruction methods)
- Adjust technical factors based on: patient size, region being scanned, indication for exam
- Check for alternate modality
Why is there an increased likelihood of radiation induced cancer in children?
- exposure in cumulative
- latency = 10-30yrs
- cells divide more rapidly
- high effective doses
- increasing CT use
At what point are fetuses at the greatest risk if exposed?
Conception to 3 months because of organ development
What is dosimetry? What are the measurements used for?
The calculation and assessment of the dose received by the patient during a CT image
The measurements are used for
-risk assessment
-radiation protection guidelines
What is the purpose and considerations of DRLs?
Purpose:
-promote better control of patient exposure to x-rays, identified acceptable limits for a specific exam
Considerations:
-doses for procedures vary widely between equipment and facilities
-based on average patient population
Why is dosimetry important?
Allows techs to compare their doses with a national average
Helps maintain techniques that comply with ALARA
What should techs understand about dosimetry?
- types of dosimeters used to measure doses
- CT dosimetry phantoms
- Dose descriptors specific to CT and their units to measure
2 main uses of dosimeters?
- Human radiation protection: personal dosimeters
2. Measurements of dose in medical processes: measure radiation emitted from CT scans
Types of dosimeters?
- Film: film and holder, 2 emulsions to detect low and high energy photons, film developed to determine exposure, heat, light, and moisture sensitive, one time use only
- TLD: measured amount of visible light emitted from a crystal after it has been heated to released the light, light proportional to radiation dose, can be worn up to 3 months, reusable, no record of previous exposure
- Ionization Chambers: radiation interacts with air in container, air molecules become ionized, free electrons collected as an electric charge for measurement, Q measured in Coulombs
What is Q?
Quality factors, radiation weighting factor
Characteristics of dosimetry phantoms
- 2 sizes: small for head, large for body, same length different diameters
- Homogenous acrylic
- Round
- have holes for placement of ionization chamber
What do dosimetry phantoms measure? Will the dose vary among locations on the phantom, why/why not?
CTDI
-yes because of partial shielding
3 dose measurements we should be aware of?
- CTDI: computed tomography dose index: measured the dose of 1 slice, Grays (Gy)
- Dose Length Product: measured dose of an entire scan series (mGy/cm)
- Effective Dose: sieverts (Sv)
CTDI
- measured multiple scan average dose (dose per slice)
- accounts for scatter
- sliced must be contiguous to perform test
- not meant to be an accurate estimate of the radiation dose received by a specific patient
- standardized measurement of radiation across scanners
- grays (Gy)
CTDI measurements
- CTDI(fda): dose index as it relates to number of sliced and slice width used
- CTDI(100): modification to accommodate smaller slice widths
- CTDI(W): calculates dose in the x-y axis to accommodate dose uniformity
- CTDI(volume): calculates dose in the z-axis (slice thickness)
Helical imaging dose calculation formula
CTDI(volume) = CTDI(w) / Pitch
DLP
- Measurement of the total amount of exposure received in a scan series
- directly proportional to the length of the scan
- must calculate scan length as well as CTDI(volume)
Formula to calculate the average dose received in a series?
DLP = CTDI(volume) x scan length
ED
- takes the dose measurements and puts the data into terms we can relate to
- risk
- tissue radiosensitivity
- Sieverts (Sv) (formerly rem)
Formula to calculate ED?
Effective dose = DLP x k(factor always given)
