Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
63 Cards in this Set
- Front
- Back
What is the second type of detector
|
gas detector
|
|
How is a gas detector made
|
The gas detector is usually constructed utilizing a chamber made of a ceramic
material with long thin ionization plates usually made of Tungsten submersed in Xenon gas |
|
What is the function of the tungsten plate
|
to collect electrons
|
|
What happens when the photons react with the tungsten plate and xenon gas
|
ionization occurs which produces an electrical current
|
|
How do you increase the strength of signal from a gas detector
|
Utilizing more gas in a detector increases the
number of molecules that can be ionized therefore, the strength of the detector signal or response is increased. |
|
What generation of CTs was a xenon gas used
|
The xenon gas detectors are generally fixed
with the position of the x-ray tube which occurs with 3rd generation scanner geometry designs. |
|
Why must the xenon detector be fixed in position
|
the signal they relay is highly position dependent
|
|
What does the term detector array mean
|
The term detector array is used to describe the total number of
detectors that a CT system utilizes for collecting attenuated information. 3 rd generation CT imaging systems employ 800-1000 detectors while 4th generation scanners include 4000-5000 individual detectors in a detector array. |
|
What is a ray
|
The path that an x-ray beam travels from the tube to a single detector is referred
to as a ray. |
|
What is the ray sum
|
The attenuation measurement of each ray is
termed a ray sum. |
|
What is a view or projection
|
A complete set of ray sums is referred to as a view or
projection |
|
Does it take may views or projections to create a CT image
|
yes
|
|
Does obtaining a single view give the entire perspective of the object being
|
No it takes many
|
|
What is done after all the ray sums are collected
|
correlated with the position of each ray.
|
|
What determines the strength of the detector signal
|
The more photons collected,
the stronger and more accurate the detector signal. |
|
What is an attenuation profile
|
The more photons collected,
the stronger and more accurate the detector signal. The signal represents an absorption or attenuation profile. |
|
Is an attenuation profile
obtained for each view or projection. |
yes
|
|
Is each detector responsible for collecting an attenuation profile
|
yes
|
|
What are 2 techniques to lower the CT radiation dose (specificaly cardiace)
|
increase pitch and lower the tube current (during certain phases of the cardiac cycle
|
|
What direction are the CT rows added
|
the z-direction
|
|
Is it true that in previous conventional helical CT
scanners, a single connection between each indi- vidual detector provided a single set of projection data for each rotation of the gantry assembly. |
yes
|
|
Is it true that multiple rows of projection data along the z-axis are now given with each rotation of the gantry in MDCT
|
yes
|
|
What does the word 'channel' refer to (16 channel CT scanncer
|
row
|
|
What are 3 types of detectors
|
uniform (mosaic or matrix)
Non-uniform (variable) hybrid |
|
How does a mosaic (uniform, matrix) detector work
|
Mosaic detectors have elements that
are all a uniform size. The thickness of the sec- tions that can be generated from these detectors is a multiple value of the uniform size of the de- tector element. (In this case, sections can have a thickness of 1.25, 2.5, 3.75, 5, 7.5, or 10 mm.) |
|
What does a uniform detector look like
|
|
|
What does a non-uniform detector look like
|
|
|
What does a hybrid scanner look like
|
|
|
How does a detector that has detectors that r 1.25 measure obtain 2.5 mm slices
|
These channels are no longer fixed but are in-
stead variable, so that they can sample different detector elements and can even sample several detector elements simultaneously, effectively adding (or binning) their signals together |
|
What is the ability to add detectors together to form one detector called
|
variable detector sampling
|
|
What are the different rows or channels that are available
|
CT scanners can be purchased with various
numbers of channels: four-, six-, 10-, 16-, 32-, 40-, and 64-channel systems are now commer- cially available (Fig 4). Some manufacturers |
|
What is a good way to think of pitch
|
think of the CT x-ray beam as
spray paint coming out of the can. If the table were to advance exactly in concert with the gantry spin, that is, the width of the paint spray, the pa- tient would be painted uniformly. were to move a bit faster than the gantry spin, the patient would have a candy cane–striped appear- ance with unmarked flesh between ribbons of paint. If the table were to move a bit slower than the gantry spin, the patient would be coated with ribbons of overlapping paint, and the color would be darker on the overlapping areas than elsewhere. |
|
What is the beam width of a detector with 64 channels and a 0.625-mm channel width
|
requires a 40-mm beam
width (however keep in mind that all 64 channels may not be in use) |
|
What is the ability to ionize air called
|
exposure
|
|
What is exposure measured in
|
roentgens (coulombs/kg)
|
|
Does an exposure indicate how much radiation is absorbed
|
no only how much is present
|
|
What are the units of absorbed dose
|
rads (1
rad 100 erg/g) or grays (1 Gy 1 J/kg) |
|
What device is used to measure absorbed dose
|
thermoluceminescece dosimeter
|
|
What determines the effective dose
|
The effective
dose depends on two factors; one is the tissue type being irradiated (relatively sensitive or insen- sitive to radiation damage), and the other is the type of radiation being delivered (the radiation weighting coefficient, currently equal to unity for x-rays). |
|
What are the units of effective dose
|
rem or Sv
|
|
What is the annual background dose measured in
|
effective dose
|
|
What is used to measure CTDI
|
phantom
|
|
What does weighted CTDI take account of
|
the difference of dosage in the center and the periphery
|
|
What does the CTDI 100 take into account
|
the dosage changes along the Z-axis
|
|
What takes into account the pitch
|
CTDI vol and DLP
|
|
What is the order of development of CTDI
|
CTDI 100
CTDI w CTDI vol DLP |
|
What is going to be more CTDI vol or CTDI w
|
A scan with a pitch
less than unity would therefore have a CTDIvol value larger than the CTDIw value, and scans with a pitch greater than unity would have a CTDIvol value smaller than the CTDIw value. |
|
How is the DLP different from the CTDI vol
|
The DLP is calculated by multiplying CTDIvol in
grays by the scan extent in centimeters. The DLP essentially provides a value that allows us to com pute the desired quantity, the effective dose. |
|
Is effective dose measured automaticall by the CT Scanner
|
no,
|
|
What can be compared with effective dose
|
with annual background dose
|
|
Why is effective dose very difficult to calculate and not given with each standard CT
|
To assess the risk of radiation for an individual
patient, we need to know exactly which organs have been irradiated. Sometimes this information is available, but it can often be difficult to deter- mine how much of the thyroid, for example, was in the x-ray beam for a typical chest examination. |
|
What are some factors which may cause differences in effective dose
|
organs scanned
depth of organs (varies by person) if the person got oral contrast |
|
How do you get a very rough estimate of effective dose following CT
|
the CTDIvol by a constant value (called the k fac
tor) that generally accounts for the sensitive or- gans that may be affected by the x-ray beam dur- ing CT (2). This constant is expressed in units o millisieverts per milligray-centimeters, yielding an effective dose estimate in millisieverts. |
|
What is the radiation that passes through the patient on the sides and does not influence the image
|
the pneumbra
|
|
What does an imaging pneumbra cause
|
decreased dose efficiencey
|
|
What is the pneumbra in a 5mm slice
|
If thin sections are required and
the overall x-ray beam width is small—5 mm, for example—then the proportion of wasted x-rays could be 20%–60% (resulting in a dose efficiency of 40%–80%). |
|
When is there is greater imaging pneumbra in small or large beams
|
small
|
|
Show the effect of a medium sized beam
|
|
|
What is the main concern in dose efficiency
|
the size of the imaging pneumbra
|
|
What causes a larger pneumbra
|
small beam imaging
|
|
What type of MDCT are most efficient
|
the more dectors (64 and up) because they can use a very large beam and this results in a small pneumbra
|
|
What is the result of the newer scanners that have two xray tubes and two sets of detectors
|
better temporal resolution
|
|
What is the difference in protocol between a standard CT study of the chest and the heart
|
faster gantry rotation
slower pitch |