Physics: Ct 5

Front 1

What is the second type of detector

Back 1

gas detector

Front 2

How is a gas detector made

Back 2

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

Front 3

What is the function of the tungsten plate

Back 3

to collect electrons

Front 4

What happens when the photons react with the tungsten plate and xenon gas

Back 4

ionization occurs which produces an electrical current

Front 5

How do you increase the strength of signal from a gas detector

Back 5

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.

Front 6

What generation of CTs was a xenon gas used

Back 6

The xenon gas detectors are generally fixed
with the position of the x-ray tube which occurs with 3rd generation scanner
geometry designs.

Front 7

Why must the xenon detector be fixed in position

Back 7

the signal they relay is highly position dependent

Front 8

What does the term detector array mean

Back 8

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.

Front 9

What is a ray

Back 9

The path that an x-ray beam travels from the tube to a single detector is referred
to as a ray.

Front 10

What is the ray sum

Back 10

The attenuation measurement of each ray is
termed a ray sum.

Front 11

What is a view or projection

Back 11

A complete set of ray sums is referred to as a view or
projection

Front 12

Does it take may views or projections to create a CT image

Back 12

yes

Front 13

Does obtaining a single view give the entire perspective of the object being

Back 13

No it takes many

Front 14

What is done after all the ray sums are collected

Back 14

correlated with the position of each ray.

Front 15

What determines the strength of the detector signal

Back 15

The more photons collected,
the stronger and more accurate the detector signal.

Front 16

What is an attenuation profile

Back 16

The more photons collected,
the stronger and more accurate the detector signal. The signal
represents an absorption or attenuation profile.

Front 17

Is an attenuation profile
obtained for each view or projection.

Back 17

yes

Front 18

Is each detector responsible for collecting an attenuation profile

Back 18

yes

Front 19

What are 2 techniques to lower the CT radiation dose (specificaly cardiace)

Back 19

increase pitch and lower the tube current (during certain phases of the cardiac cycle

Front 20

What direction are the CT rows added

Back 20

the z-direction

Front 21

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.

Back 21

yes

Front 22

Is it true that multiple rows of projection data along the z-axis are now given with each rotation of the gantry in MDCT

Back 22

yes

Front 23

What does the word 'channel' refer to (16 channel CT scanncer

Back 23

row

Front 24

What are 3 types of detectors

Back 24

uniform (mosaic or matrix)
Non-uniform (variable)
hybrid

Front 25

How does a mosaic (uniform, matrix) detector work

Back 25

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.)

Front 26

What does a uniform detector look like

Back 26

Front 27

What does a non-uniform detector look like

Back 27

Front 28

What does a hybrid scanner look like

Back 28

Front 29

How does a detector that has detectors that r 1.25 measure obtain 2.5 mm slices

Back 29

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

Front 30

What is the ability to add detectors together to form one detector called

Back 30

variable detector sampling

Front 31

What are the different rows or channels that are available

Back 31

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

Front 32

What is a good way to think of pitch

Back 32

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.

Front 33

What is the beam width of a detector with 64 channels and a 0.625-mm channel width

Back 33

requires a 40-mm beam
width

(however keep in mind that all 64 channels may not be in use)

Front 34

What is the ability to ionize air called

Back 34

exposure

Front 35

What is exposure measured in

Back 35

roentgens (coulombs/kg)

Front 36

Does an exposure indicate how much radiation is absorbed

Back 36

no only how much is present

Front 37

What are the units of absorbed dose

Back 37

rads (1
rad
100 erg/g) or grays (1 Gy
1 J/kg)

Front 38

What device is used to measure absorbed dose

Back 38

thermoluceminescece dosimeter

Front 39

What determines the effective dose

Back 39

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).

Front 40

What are the units of effective dose

Back 40

rem or Sv

Front 41

What is the annual background dose measured in

Back 41

effective dose

Front 42

What is used to measure CTDI

Back 42

phantom

Front 43

What does weighted CTDI take account of

Back 43

the difference of dosage in the center and the periphery

Front 44

What does the CTDI 100 take into account

Back 44

the dosage changes along the Z-axis

Front 45

What takes into account the pitch

Back 45

CTDI vol and DLP

Front 46

What is the order of development of CTDI

Back 46

CTDI 100
CTDI w
CTDI vol
DLP

Front 47

What is going to be more CTDI vol or CTDI w

Back 47

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.

Front 48

How is the DLP different from the CTDI vol

Back 48

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.

Front 49

Is effective dose measured automaticall by the CT Scanner

Back 49

no,

Front 50

What can be compared with effective dose

Back 50

with annual background dose

Front 51

Why is effective dose very difficult to calculate and not given with each standard CT

Back 51

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.

Front 52

What are some factors which may cause differences in effective dose

Back 52

organs scanned
depth of organs (varies by person)
if the person got oral contrast

Front 53

How do you get a very rough estimate of effective dose following CT

Back 53

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.

Front 54

What is the radiation that passes through the patient on the sides and does not influence the image

Back 54

the pneumbra

Front 55

What does an imaging pneumbra cause

Back 55

decreased dose efficiencey

Front 56

What is the pneumbra in a 5mm slice

Back 56

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%).

Front 57

When is there is greater imaging pneumbra in small or large beams

Back 57

small

Front 58

Show the effect of a medium sized beam

Back 58

Front 59

What is the main concern in dose efficiency

Back 59

the size of the imaging pneumbra

Front 60

What causes a larger pneumbra

Back 60

small beam imaging

Front 61

What type of MDCT are most efficient

Back 61

the more dectors (64 and up) because they can use a very large beam and this results in a small pneumbra

Front 62

What is the result of the newer scanners that have two xray tubes and two sets of detectors

Back 62

better temporal resolution

Front 63

What is the difference in protocol between a standard CT study of the chest and the heart

Back 63

faster gantry rotation
slower pitch