Physics: Ct 2

Front 1

Explain the dose gradient of a CT

Back 1

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.

Front 2

What has a greater dosage of radiation inner or outer?

Back 2

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.

Front 3

What is a body phantom typically made out of

Back 3

polymethyl methacrylate

Front 4

How much radiation is obtained at the periphery compared to the center in a CT

Back 4

center are typically about 50% of the measured
value obtained at one of the peripheral positions.

Front 5

What is a consideration if the there is a smaller body part or phantum used

Back 5

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

Front 6

Are the radiation levels similar in the periphery compared to the center in a smaller object such as a head

Back 6

yes

Front 7

Are the values of absorbed dose size dependent

Back 7

yes

Front 8

What are Z-axis radiation variations

Back 8

In addition to the variations within the scan
plane, there are variations along the length of the
patient or phantom.

Front 9

What is does the Z-axis variation of a CT look like

Back 9

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).

Front 10

Explain why Z-axis variation occurs

Back 10

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.

Front 11

What influences the the z - axis radiation variation

Back 11

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.

Front 12

What is the MSAD (Multiple Scan Average Dose) used to account for (during one CT)

Back 12

to account for the effect of multiple scans. This was used before CTDI

Front 13

Why was CTDI made

Back 13

Computed Tomogra-
phy Dose Index was created to computed the radiation dose, normalized to beam width,
measured from 14 contiguous sections:

Front 14

Why was CTDI not used

Back 14

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

Front 15

What was created to overcome the short fall of the CTDI

Back 15

CTDI 100

Front 16

What is different about the CDTI

Back 16

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)

Front 17

Does the amount of section size matter with the CTDI100

Back 17

no, but the width does matter

Front 18

Does there have to be a seperate calculation for the periphery and the center with CDTI 100

Back 18

yes, to describe variation in the scan plan

Front 19

Why was CTDIw created

Back 19

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

Front 20

What was then created to acount for the pitch

Back 20

CTDIvol

Front 21

What is the pitch

Back 21

pitch is defined as table distance traveled in
one 360° rotation/total collimated width of the
x-ray beam

Front 22

What is DLP

Back 22

DLP
= CTDIvol x scan length.

Front 23

What is another way of saying x-ray beam energy

Back 23

kilovolt peak

Front 24

What are some direct influences on radiation dose

Back 24

beam energy
tube current
rotation or exposure time
section thickness
object thickness
pitch
dose reduction techniques
distance from x ray tube to isocenter

Front 25

What are ways of reducing dosage in CT

Back 25

tube current variation
modulation

Front 26

How is the beam xray energy choosen

Back 26

by the technologist who will choose a kilovolt peak

Front 27

What are examples of the absorbed dose in the head and body with changes in kVp (with everything else held constant)

Back 27

Front 28

Dose current time product have a direct influence on radiation dosage

Back 28

yes

Front 29

What are the units of current time products when discussing CT

Back 29

milliampere-second (mAs)

Front 30

Is the radiation dose directly proportional to the miliampere-seconds value

Back 30

yes

Front 31

What has a greater increase in radiation increasing the kVp from 120 to 140 or the mAs from 100 to 200

Back 31

the kVp

Front 32

If all factors are held constant what happens to the radiation dosage if the pitch is increased

Back 32

the radiation dose with decrease

Front 33

How much the pitch influence radiation dosage if everything else is held constant

Back 33

Front 34

How much does collimation effect radiation sosage if everything else is held constant

Back 34

Front 35

Does collimnation have a small effect on radiation dosage

Back 35

yes

Front 36

How much affect on radiation occurs if the current is adjusted with everything else held constant

Back 36

Front 37

What has the most effect on radiation dosage

Back 37

kVp

Front 38

What has the least effect on radiation dosage

Back 38

collimnation

Front 39

What is the pitch

Back 39

defined as
table distance traveled in one 360° rotation/total
collimated width of the x-ray beam

Front 40

What is the only CTDI descriptor that takes pitch into account

Back 40

CTDIvol (note that DLP also takes pitch into account)

Front 41

Does thinner colimnation result in a greater degree of overlap and therefore a higher CTDI value

Back 41

yes

Front 42

Is the degree of radiation dose by using thinner colimmators greatly enhanced when using mulitidector scanners

Back 42

yes (the previous table was a single detector scanner)

Front 43

What causes less radiation; thick or thin collimation

Back 43

thick

Front 44

How does changes in collimation change radiation dose in a MULTIDETECTOR CT

Back 44

Front 45

Who generally receives more radiation; a small or large person

Back 45

small person (important when it comes to pediatrics)

Front 46

Why do smaller people generally recieve a greater amount of radiation

Back 46

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

Front 47

How does the tube current modulation work

Back 47

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

Front 48

Can tube current modulation vary by length of the patient (ie less for the chest than for a fat mans stomach)

Back 48

yes

Front 49

What happens to the signal to noise if thinner reconstruction thicknesses are used

Back 49

decreased signal to noise

Front 50

How do you "offset" the increased noise created by thin slice reconstruction

Back 50

by increasing the kVp or mAs

Front 51

What percent does a 50% decrease in mAs increase noise

Back 51

41%

Front 52

What is the trade off in increasing pitch

Back 52

increasing the effective slice thickness which inturn will cause increased volume averaging (decreased signal)

Front 53

What happens to the signal to noise if thinner reconstruction thicknesses are used

Back 53

decreased signal to noise

Front 54

What is a technique for reducing radiation to smaller people

Back 54

size and weight based protocols

Front 55

How do you "offset" the increased noise created by thin slice reconstruction

Back 55

by increasing the kVp or mAs

Front 56

What percent does a 50% decrease in mAs increase noise

Back 56

41%

Front 57

What is the trade off in increasing pitch

Back 57

increasing the effective slice thickness which inturn will cause increased volume averaging (decreased signal)

Front 58

What is a technique for reducing radiation to smaller people

Back 58

size and weight based protocols

Front 59

What is the Monte Carlo simulations

Back 59

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)

Front 60

How is the monte carlo simulation used by the tech

Back 60

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.

Front 61

What is another type of simulation used to calculate effective dose

Back 61

weltforschung (GSF) performed simulations on
two different mathematical sex-specific phan-
toms, “Adam” and “Eva”

Front 62

Are there other methods

Back 62

yes, but not sure I have to know this shit so go back and continue if this comes up "CT-radiation dosage of...."