Radiation Dosimetry in CT-Unit 6 Flashcards
Read over the following CT parameters that affect radiation dose:
Make a chart
- Tube Current (mA)
- Exposure Time
- Tube Current Modulation
- Tube voltage
- Pitch
- Effective tube-current time
- Slice Thickness
- Reconstruction Algorithm
- Filtered Back Projection
What happens to dose when mA increases
Dose increases
What is another name for exposure time in CT?
tube rotation time
What does tube rotation/exposure time mean?
Time for one complete rotation of the CT gantry
As the speed of rotation increases, what happens to dose?
Dose decreases
What is the relationship between x ray tube rotation time and dose?
They are directly proportional to each other
What is the relationship between radiation dose and rotation speed?
They are inversely proportional
What is the primary use of tube current modulation (TCM)?
To maintain image quality (SNR) by providing proper tube current settings for variable patient sizes and exam indications.
What are the three ways in which tube current is modulated in a CT scanner?
- Tube current is modulated in the x, y and z axis based on oval ratio, attenuation values from the CT localizer
- Modulated from attenuation values obtained in the first 180 degrees of tube rotation during image acquisition
- (or both).
As kVp increases, what happens to dose?
Dose increases
What is the relationship between kVp and dose?
kVp is directly (non-linearly) related to dose
For example, a 14% decrease in tube voltage from 140 to 120 kV will reduce patient exposure and decrease radiation dose by up to 30%–35%.
What is the disadvantage of decreasing kVp to try to reduce dose? How can this disadvantage be compensated?
-Disadvantage is Increased image noise
-To compensate for the increased noise, one can increase the mAs setting (if TCM is not utilized).
T/F
The low-tube-voltage, high-tube-current CT technique decreases dose while maintaining image quality.
True
In what types of patients is the low-tube-voltage, high-tube-current CT technique efficient for?
Small and average-sized patients
T/F
For large patients, lowering the kVp results in increased noise that may not be overcome by increasing the tube current.
True
In large patients, why is a higher tube voltage the most dose efficient strategy?
There is better penetration through the organs of interest.
Why is lowering the kVp for smaller/average size patients beneficial for attenuation produced by iodine based contrast material?
Attenuation produced by iodine-based contrast material increases up to 100% if tube voltage is decreased from 140 to 80 kVp because of the higher probability of photoelectric interactions at the lower tube voltage.
(dont do this if its a large patient)
VERY BENEFICIAL FOR angiographic studies!
T/F
Tube volatage cannot be modulated in real time.
True
(mA can be in real time)
What is automated tube voltage selection software?
This software automates/aids tube voltage selection based of the patien’ts attenuation profile from the scout and user’s chosen examination.
Does Tube voltage modification provide diagnostically acceptable image quality?
Yes
Initial reports show that when compared with fixed 120-kV protocols, automated tube voltage software frequently suggests a tube voltage of 100 or 80 kV in more than 50% of contrast-enhanced abdominal CT examinations.
Importantly, the reduced tube voltage values were found to provide diagnostically acceptable image quality.
What is the relationship between Pitch and dose?
They are inversely proportional to each other
If pitch is increased, what factor compensates for the change to maintain image quality?
Tube current automatically increases
(only in systems that use an effective mAs setting)
What is the relationship between patient dose and slice thickness?
Inversely proportional
As slice thickness decreases, what happens to dose?
It needs to increase
T/F
2.5-mm slice thickness will require about twice the dose of 5-mm slice thickness.
True
T/F
Reconstruction algorithms do not directly affect dose
True; they indirectly affect dose
How does the Reconstruction Algorithm indirectly affect dose?
Most newer algorithms have the goal of reducing image noise, which consequently allows use of lower dose CT techniques without sacrificing image quality.
What are the 2 reconstruction algorithms?
- Filtered Back Projection
- Iterative Reconstruction Algorithms
What type of reconstruction algorithm is used now days?
Iterative reconstruction algorithm
What are the pros and cons of filtered back projection?
Pros: Can obtain 20 images per second
Cons: Cannot discriminate between removing noise and signal.
What is this describing?
This algorithm improves on the initial estimate of the attenuation value by comparing a theoretical model with the acquired projection data and making an incremental changes to the previous “guess”.
Iterative reconstruction algorithms
Which reconstruction algorithm allows for use of a lower mAs?
iterative reconstruction
What are hybrid reconstruction algorithms?
A combination of filtered back projection which is then cleaned up with iterative reconstruction algorithms
What are 2 reasons that the dose is more uniform in CT than in general radiography?
- The beam is heavily filtered as it exits the tube.
- CT exposure comes from all directions, creating a more uniform exposure.
T/F
In CT, the difference between the dose at the center and the dose at the periphery is not nearly as great as that of conventional radiography.
True
What causes a decrease in the uniformity of the dose in CT?
An increase in patient thickness
What is more uniform in dose; a body scan or a head scan?
A head scan
(less increase in thickness for head)
What is the central dose for a body scan compared to the peripheral dose in percentages?
The central dose for a body scan is approximately one-third to one-half of the peripheral dose
Why is the organ dose in children larger than those for adults?
Because children don;t have a lot of tissue
How thick are phantoms used for CT Dose Measurements in the z direction?
15 cm
T/F
Doubling pitch results in 1/2 of the dose
True
What is the purpose of the 1-cm-diameter holes in the QC phantoms? Where are they located on the phantom?
-Purpose is for the insertion of dosimeters
-The holes are at the center of the phantom and at a 1cm-depth at the 3-, 6-, 9-, and 12-o’clock positions.
Some models have holes at other locations.
T/F
There is some scatter inherent in CT.
The overall amount is low, and the distance it travels is quite short.
True
What are the areas of scatter into adjacent tissue called?
Tails
How much additional dose do the “tails” resultant from scatter contribute to the entire study?
Tails will contribute approximately 25-40% additional dose to the entire study.
Therefore, to accurately assess the z axis dose distribution, the radiation that scatters into adjacent slices must be added to the dose from a single slice.
As the number of slices increase, what happens to scatter and dose? What is the relationship between these two factors
Scatter and dose increase directly proportionally
What is the multiple scan average dose? (MSAD)
The average cumulative dose from a series of slices with constant spacing is referred to as the multiple-slice average dose
How much more is the MSAD compared to the the single slice dose?
The MSAD may be 1.25 – 1.4 times the single slice dose
T/F
The MSAD will decreasse if slices overlap and increase if there are gaps between slices.
False; The MSAD will increase if slices overlap and decrease if there are gaps between slices.
Why is the cumulative dose to the end slices is somewhat lower than the central slices?
Because of a lack of contribution of tails from one side.
How is CTDI(100) measured?
Measured by using a pencil ionization chamber placed in a standardized circular acrylic (polymethyl methacrylate) phantom. Measured at the center of the phantom
What is this describing?
An estimate of the average radiation dose within a volume of tissue – should that tissue be the same size and have the same attenuation as the plastic phantom.
CTDI (100)
What are the measurement units for CTDI (100)?
mGy, mrad
What does CTDI(100) stand for?
Computed Tomography Dose Index collected by a 100mm long ionization chamber in a single CT scan.
What is the diameter of the adult body phantom for QC testing?
32 cm diameter
What is the diameter of the adult head/pediatric body phantom for QC testing?
16 cm diameter
What is the diamter of the pediatric head phantom for QC testing?
10 cm diameter
In an adult head/pediatic body phantom, what is the Typical Dose Distribution?
Uniform throughout
In an adult body phantom, what is the Typical Dose Distribution?
Uniform on the peripheries, lower dose in the center of the phantom
What does CTDI(w) compensate for?
Compensates for the non uniform dose distrubution by providing a weighted average of the center and peripheral doses.
How many slices does the the CTDIw calculate for?
Calculates the dose from 1 axial slice
What does CTDI volume (CTDIvol) compensate for?
Compensates for the fact that CTDIw only calculates dose for one single axial slice, and accounts for the full volume of tissue
T/F
The method for calculating CTDIvol is standardized across all manufacturers and models using the two reference phantoms.
True
What is this describing?
A measure of the radiation output from the CT scanner as determined with either a 16- or 32-cm phantom.
CDTI(vol)
What is the unit for CDTI (vol)
mGy
What factors is CTDI (vol) afffected by?
- kVp
- mA
- Pitch
- Phantom specific
What type of affect does CDTI (vol) have on dose?
CDTI (vol) has a linear directly proportional affect on dose
T/F
CTDIvol is the patient dose
False; CTDIvol is NOT the patient dose
When is CDTI(vol) displayed?
CTDIvol on the operator’s console before the scan begins.
What is dose check in CT scanners?
A software upgrade, which allows CT users to set a maximum CTDIvol value for each CT protocol and to alert the user when any change in scan parameters are chosen that can lead to values higher than the limit
What is Dose Length Product/DLP?
DLP represents total dose in terms of the total scan length (# of slices x slice thickness). Therefore, DLP is a dose descriptor that considers the scan length.
Why do we need DLP calculations?
In CT, we are not limited to one helical rotation, and CTDI(vol) only considers a single rotation
What is the unit for dose lenth product?
mGy.cm
T/F
Linear changes in DLP would
result in linear changes
of exposure
True
What 3 factors is DLP most affected by?
- CTDIvol
- Scan length
- Number of acquisitions
(irradiation events)
What is the defenition of effective dose/DE?
The theoretical uniform whole-body dose that considers organ radiosensitivity
How is effective dose measured?
mSv
T/F
Linear change in DE
represents a proportional
change in absorbed dose
True
What 3 factors is effective dose most affected by?
- DLP
- Patient size
- Relative radiosensitivity of the
imaged organs
T/F
Effective dose also allows comparison of dose from medical examinations to other forms of radiation such as cosmic rays, radon, and occupational radiation exposure.
True
What is the measurement evolution of CT Dosimetry?
What is the defenition of Size Specific Dose Estimate (SSDE)?
A measure of dose that includes individual
patient size
What is the unit of measurement for Size Specific Dose Estimate (SSDE)?
mGy
What is Size Specific Dose Estimate (SSDE) most affected by?
Scouts: Patient positioning in the gantry
What is this describing?
Incorporates patient’s size as a modifying correction factor to better estimate the patient dose.
Size Specific Dose Estimate (SSDE)
What is the Size Specific Dose Estimate (SSDE) effect on Radiation Dose?
Improves accuracy of average
patient dose for all sized patients
How much does CT contribute to the total annual worldwide exposure from all sources in advanced countries?
Contributes to 40% of the total annual worldwide exposure
Mitigating this radiation exposure risk to patients requires the ? for the need of the CT exam and ? of associated radiation dose and ?
Mitigating this radiation exposure risk to patients requires the justification for the need of the CT exam and optimization of associated radiation dose and management
Who can order a CT?
Physician (MD)
Nurse Practitioner (NP)
Who protocools CT exams?
The radiologist
How can physicians tell what protocools need to be ordered?
By checking the webiste of the Canadian Assosiation of Radiologists (CAR)
Who is responsible for Dose Optimization and when?
The technologist during the exam
CT dose management is the organization and coordination of activities to ? and ? radiation doses to ensure patient safety
CT dose management is the organization and coordination of activities to review and optimize radiation doses to ensure patient safety
Who is responsible for dose managment and when?
Radiologists
Technologists
Management
Physicists
-After the scan
What are the 3 clinical indications which aid in minimizing dose?
- Organ attenuation/attenuation in area of interest
- Extent of scan
- Multiphasic studies
How does acessing organ attenuation in the area of interest reduce dose?
If we use a technique that has higher noise in certain areas, but reduces dose and allows the anatomy of interest to be viewed in clear detail, dose will be reduced.
What is “extent of scan” refering to in relation to the reduction of dose?
Limiting the extent of the scan along the z-axis to the area of interest.
What two questions should we consider when preforming multiphasic studies?
- Do we need multiple phases to get to an accurate diagnosis?
- If so, can we limit z-axis coverage for certain phases?
What are the 4 phases of a multiphasic study of the kidneys?
- Non-contrast
- Cortico-medullary
- Nephrographic
- Excretory
Read over the 9 following ways to reduce dose:
(don’t memorize, understand how they reduce dose)
- Consider employing manufacturer-specific CT protocools developed by subspecialty societies
- Tailor protocool to indiciations
- Use TCM
- Use kVp modification
- Accurate patient centering
- Limit image creep
- Limit phase creep
- Review the need for thinner slices in protocols
- Use iterative reconstruction algorithms
In pediatric studies, what is kVp based on?
Patient size
Fill in the blank.
Studies have found that miscentering of only ? cm from isocenter can increase CTDIvol by an average of ? due to inaccurate automated tube current modulation
Studies have found that miscentering of only 2.2 cm from isocenter can increase CTDIvol by an average of 23% due to inaccurate automated tube current modulation
Read over the following tips for improving centering accuracy?
- Position the tube at the top of the gantry rather than below the table since most patients are centered too low.
- Use both anterior-posterior and lateral scouts to assess centering.
- Repeat the scout image if a patient is recentered, otherwise the tube current modulation will wrongly operate using the non-centered initial scout image.
- In some scanners, only the last scout image may be used for tube current modulation, so because the anterior-posterior scout tends to be wider than the lateral scout, it should be performed last
What are the 2 ways in which image creep can be limited?
- Only scan the region that is required.
- Limit z-axis coverage to area of interest.
What is DECT application?
Virtual non–contrast-enhanced images that can be created by subtracting out the iodine
How can phase creep be limited?
Use DECT application
Insetead of taking two images before and after contrast, only take one with contrast and then use subtracton method
How do thinner slices affect the isotropic voxels?
Thinner slices also create isotropic voxels which produce high resolution coronal and sagittal reformations.
As slice thickness decreases, spatial resolution (increases/decreases?) and contrast resolution (increases/decreases?).
As slice thickness decreases, spatial resolution (increases/decreases?) and contrast resolution (increases/decreases?).
Advantage of acquiring data in thinner slices is that it can be reconstructed into thicker slices & preserves high spatial resolution, but also improves SNR (because mA modulates upwards) and therefore, reduces noise and increases contrast resolution.
T/F
All scanners now days use Iterative Reconstruction Algorithms
True
Why is sheilding less beneficial in CT?
Because of narrow collimation, radiation to areas outside that of the selected scan area is minimal.
Shielding of the breast and thyroid may be of value in terms of dose reduction to these areas.
What is organ dose modulation?
Reduces dose via tube current modulation for superficial tissue and further reduces mA on the anterior surface where radiosensitive organs exists.
What is Dose Check/Alert?
A value shown on the display computer before the exams that may indicate the dose that you need
What values are shown on a dose report?
CTDI(vol) and DLP
What kind of software monitors large amounts of CT Radiation Dose?
Dose tracking software
T/F
DRLs (Diagnostic Reference Levels) are not dose limits
True
What are Diagnostic Reference Levels (DRLs)?
They serve as levels to identify facilities with unusually low or high doses and optimize doses during proceedures
It is not expected that all patients should receive these dose levels but that the average of the patient population should.
What are Diagnostic Reference Levels (DRLs) based off of?
Based on typical examinations of standardized patients or phantom sizes.
When patients are used to establish DRLs, measurements should be done only on patients whose individual weight is 70 ± 20 kg, and the average weight measurement of the patients should be ? kg.
70 ± 5 kg
It is recommended that the minimum sample size for a specific procedures or equipment be ? patients.
10 patients
T/F
The use of DRLs is endorsed by professional, advisory and regulatory bodies worldwide.
True
How are DRLs are typically established?
Typically established based on available survey data.
What happens after the surveys for DRLs are submitted?
Once surveys are submitted, the regulatory body calculates facility reference levels (FRLs) for participants
How are DRLs finally decided after the survey is submitted?
DRLs are based on the 75th percentile (third quartile) of this calculated FRL distribution from all surveyed facilities.
