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62 Cards in this Set
- Front
- Back
Explain the dose gradient of a CT
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Figure 2. Dose gradient resulting from a full
360° exposure from a CT scan. The thicker lines represent the entrance skin dose, which is much larger than the dose at the inner radius, repre- sented by the thinner lines. This difference re- sults in a radially symmetric radiation dose gradi- ent within the patient. |
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What has a greater dosage of radiation inner or outer?
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Figure 3. Typical dose measurements in a 32-
cm-diameter (body) phantom from a single-de- tector CT scan. Values measured at the center and periphery (1 cm below the surface) positions within a polymethyl methacrylate circular dosim- etry phantom demonstrate a radial dose gradient with a 2:1 ratio from periphery to center. collimation. |
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What is a body phantom typically made out of
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polymethyl methacrylate
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How much radiation is obtained at the periphery compared to the center in a CT
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center are typically about 50% of the measured
value obtained at one of the peripheral positions. |
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What is a consideration if the there is a smaller body part or phantum used
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However, for a smaller-diameter
phantom—the 16-cm-diameter phantom referred to as the head phantom—measured under the identical exposure conditions, the center value reading climbs to approximately 40 mGy, as do peripheral values |
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Are the radiation levels similar in the periphery compared to the center in a smaller object such as a head
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yes
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Are the values of absorbed dose size dependent
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yes
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What are Z-axis radiation variations
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In addition to the variations within the scan
plane, there are variations along the length of the patient or phantom. |
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What is does the Z-axis variation of a CT look like
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Figure 5. Radiation profile of a full-rotation CT scan
measured at isocenter. This profile is the distribution of radiation dose along the axis of the patient (the z axis) and is known as D(z). |
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Explain why Z-axis variation occurs
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When multiple adjacent scans
are performed, the tails of the radiation profiles from adjacent scans can contribute to the ab- sorbed dose outside of the primary area being im- aged. If these tails are significant and are nonzero at some distance from the location of the originat- ing section, then these contributions can add up, creating additional absorbed dose in the primary area being imaged. |
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What influences the the z - axis radiation variation
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The size of the contributions from adjacent
sections is very directly related to the spacing of sections and the width and shape of the radiation profile. |
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What is the MSAD (Multiple Scan Average Dose) used to account for (during one CT)
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to account for the effect of multiple scans. This was used before CTDI
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Why was CTDI made
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Computed Tomogra-
phy Dose Index was created to computed the radiation dose, normalized to beam width, measured from 14 contiguous sections: |
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Why was CTDI not used
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only 14 sections could be measured
and one had to measure the radiation dose pro- file—typically done with thermoluminescent do- simeters (TLDs) or film, neither was very convienent |
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What was created to overcome the short fall of the CTDI
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CTDI 100
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What is different about the CDTI
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This index relaxed the constraint
on 14 sections and allowed calculation of the in- dex for 100 mm along the length of an entire pen- cil ionization chamber (pencil ionizing chambers are 100mm) |
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Does the amount of section size matter with the CTDI100
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no, but the width does matter
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Does there have to be a seperate calculation for the periphery and the center with CDTI 100
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yes, to describe variation in the scan plan
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Why was CTDIw created
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CTDIw was created to represent a dose index
that provides a weighted average of the center an peripheral contributions to dose within the scan plan |
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What was then created to acount for the pitch
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CTDIvol
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What is the pitch
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pitch is defined as table distance traveled in
one 360° rotation/total collimated width of the x-ray beam |
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What is DLP
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DLP = CTDIvol x scan length.
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What is another way of saying x-ray beam energy
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kilovolt peak
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What are some direct influences on radiation dose
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beam energy
tube current rotation or exposure time section thickness object thickness pitch dose reduction techniques distance from x ray tube to isocenter |
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What are ways of reducing dosage in CT
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tube current variation
modulation |
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How is the beam xray energy choosen
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by the technologist who will choose a kilovolt peak
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What are examples of the absorbed dose in the head and body with changes in kVp (with everything else held constant)
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Dose current time product have a direct influence on radiation dosage
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yes
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What are the units of current time products when discussing CT
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milliampere-second (mAs)
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Is the radiation dose directly proportional to the miliampere-seconds value
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yes
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What has a greater increase in radiation increasing the kVp from 120 to 140 or the mAs from 100 to 200
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the kVp
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If all factors are held constant what happens to the radiation dosage if the pitch is increased
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the radiation dose with decrease
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How much the pitch influence radiation dosage if everything else is held constant
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How much does collimation effect radiation sosage if everything else is held constant
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Does collimnation have a small effect on radiation dosage
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yes
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How much affect on radiation occurs if the current is adjusted with everything else held constant
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What has the most effect on radiation dosage
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kVp
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What has the least effect on radiation dosage
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collimnation
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What is the pitch
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defined as
table distance traveled in one 360° rotation/total collimated width of the x-ray beam |
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What is the only CTDI descriptor that takes pitch into account
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CTDIvol (note that DLP also takes pitch into account)
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Does thinner colimnation result in a greater degree of overlap and therefore a higher CTDI value
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yes
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Is the degree of radiation dose by using thinner colimmators greatly enhanced when using mulitidector scanners
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yes (the previous table was a single detector scanner)
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What causes less radiation; thick or thin collimation
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thick
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How does changes in collimation change radiation dose in a MULTIDETECTOR CT
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Who generally receives more radiation; a small or large person
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small person (important when it comes to pediatrics)
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Why do smaller people generally recieve a greater amount of radiation
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as the source moves around the patient.
For smaller patients, the exit radiation has been attenuated by less tissue and therefore is closer to the entrance radiation in its intensity, resulting in a much more uniform dose distribution |
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How does the tube current modulation work
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changes in tube cur-
rent based on the estimated attenuation of the patient at a specific location. Thus, the tube cur- rent will be programmed to a maximum value and can be reduced when there is information that a location along the patient is expected to be less attenuating than the most attenuating location to be imaged |
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Can tube current modulation vary by length of the patient (ie less for the chest than for a fat mans stomach)
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yes
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What happens to the signal to noise if thinner reconstruction thicknesses are used
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decreased signal to noise
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How do you "offset" the increased noise created by thin slice reconstruction
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by increasing the kVp or mAs
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What percent does a 50% decrease in mAs increase noise
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41%
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What is the trade off in increasing pitch
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increasing the effective slice thickness which inturn will cause increased volume averaging (decreased signal)
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What happens to the signal to noise if thinner reconstruction thicknesses are used
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decreased signal to noise
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What is a technique for reducing radiation to smaller people
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size and weight based protocols
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How do you "offset" the increased noise created by thin slice reconstruction
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by increasing the kVp or mAs
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What percent does a 50% decrease in mAs increase noise
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41%
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What is the trade off in increasing pitch
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increasing the effective slice thickness which inturn will cause increased volume averaging (decreased signal)
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What is a technique for reducing radiation to smaller people
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size and weight based protocols
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What is the Monte Carlo simulations
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This hermaphro-
dite patient model uses geometric shapes (cylin- ders, spheres, cones) to approximate the shape and location of all radiosensitive organs in the body;From these simulations, radiation doses for each organ in each imaged section were calculated and could be tallied to estimate the organ dose for each organ. These organ doses could then be combined with appropriate weighting factors to estimate effective dose (or effective dose equiva- lent) |
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How is the monte carlo simulation used by the tech
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This work formed the basis for several software
programs that have taken the results of these monte carlo simulations and put an interface on them to allow users to input some technical parameters and cal- culate an effective dose. |
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What is another type of simulation used to calculate effective dose
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weltforschung (GSF) performed simulations on
two different mathematical sex-specific phan- toms, “Adam” and “Eva” |
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Are there other methods
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yes, but not sure I have to know this shit so go back and continue if this comes up "CT-radiation dosage of...."
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