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CT Flashcards

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AP® Physics flashcards Physics 101 flashcards
1
Q

Moving part of the scanner apparatus

A

Gantry

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2
Q

Refers to the 2D geometry where there is only minimal divergence of the xray beam trajectory in and out of the plane of the figure

A

True fan beam

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3
Q

Power rating of xray tubes in ct

A

5-7 megaJoule

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4
Q

Rotation of gantry

A

Clockwise

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5
Q

Electron beam striking anode is steered

A

Counterclockwise

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6
Q

HU of fat

A

-80 to -30

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7
Q

Most organ parenchyma HU

A

+30 to +220

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8
Q

Whole body CT scanners use this type of filter to attenuate more toward the periphery of the field

A

Bow tie filter

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9
Q

All modern multidetector array CT scanners use

A

Indirect (scintillating) solid-state detectors

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10
Q

Scintillating crystals

A

Gadolinium oxysulfide

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11
Q

Involves hearing up the phosphor crystals to just below their melting point for relatively long periods of time

A

Process of sintering

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12
Q

Penumbra is positioned outside of the active detector arrays— those xrays essentially strike lead shielding on the sides of the detector assemblu or inactive detectors. This is called

A

Overbeaming

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13
Q

Basic step and shoot mode of the CT scanner—— the xray beam is not “on” while the patient is being translated between the acquisition cycles

A

Axial or sequential CT acquisition

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14
Q

The xray tube has a helical trajectory around the patient’s body

A

Helical or spiral CT acquisition

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15
Q

Important relationship between the width of the active detector arrays used during the scan and the table feed per rotation of the gantry

A

Pitch

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16
Q

Main advantage of helical CT versus axial ct

A

Speed

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17
Q

For most CT scanning, the pitch can range between

A

0.75 and 1.5

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18
Q

A pitch lower than 1.0 results in

A

Overscanning and higher radiation

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19
Q

Pitch greater than 1.0 results to

A

Underscanning and lower radiation

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20
Q

Pitch settings near 1.5 allow for faster scanning and are used for______ where speed is important

A

Thoracic or pediatric scanning

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21
Q

In a low pitch study, the helix appears

A

Tight

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22
Q

In a high pitch helical ct, the helix appears

A

Loose

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23
Q

Reconstruction algorithm used for converting the raw data into CT images takes into account the

A

Cone angle

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24
Q

To freeze cardiac motion, an image acquisition time window of _____ ms or less is required

A

100 ms

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25
Q

Fan angle jn cardiac CT is

A

180-190

26
Q

Used to evaluate vascular perfusion and other physiological parameters related to blood flow to a specific organ

A

CT perfusion

27
Q

Allows a CT scanner to repeatedly image a volume of tissue that is wider than the detector array

A

Shuttle mode CT

28
Q

Angular modulation as a function of gantry angle is ________ in elliptical body parts and _____ in round body regions

A

Quasi-sinusoidal in eliptical body parts

Constant in round body parts

29
Q

Reconstruction that is used in commercial scanners more often than the convolution approach

A

Fourier-based reconstruction

30
Q

Reconstruction that keeps track of the beam divergence in the z-direction

A

Cone beam reconstruction

31
Q

Primary method of reconstruction for many decades

A

Filtered backprojection

32
Q

Type of reconstruction that starts with an initial estimate of what the object may look like. This guess is then used to compute forward projection data sets, which are then compared with the measured projection data sets

A

Iterative reconstruction

33
Q

Factors that determine the ultimate resolution in ct include

A

Focal spot size and distribution, the detector dimensions, the magnification factor, whether or not gantry motion is compensated for and patient motion

34
Q

In CT literature, the resolution is quoted in line pairs per ____ instead of line pairs per mm as is typical in the rest of the xray imaging

A

Per cm

35
Q

A more scientific measure of the spatial resolution characteristics of an imaging system. It can be measures using a wire or plane of metal foil scanned on a CT scanner is computed from the measured line spread function

A

Modulation transfer function

36
Q

Shape of the system response to a point input in the z-dimension

A

Slice sensitivity profile

37
Q

Describes how the noise propagates thru an imaging system. It describes both the overall noise level and the noise texture

A

Noise power spectrum

38
Q

Fundamental determinants of dose levels used for CT scanning, and they fundamentally impact the noise levels in the CT images

A

kV, mA, time and pitch

39
Q

Relationship of mA and time with dose

A

Linear relationship

40
Q

Thicker images combine signals from more detected xray quanta and therefore are ____ noisy than thinner images acquired at the same technique levels

A

Less noisy

41
Q

Artifacts that occur when the attenuation levels of a region in the patient are excessive, which can exceed the dynamic range of the detector systems or the effective linear range of detectors

A

Streak artifacts

42
Q

High degree of attenuation causes the xray spectrum to become hardened, hence this artifact

A

Beam hardening

43
Q

Refers to use of too few projection images acquired to reconstruct high-frequency objects in the image

A

View aliasing

44
Q

Occurs when the CT voxels are large enough to encompass several types of tissue, such as bone and tissue or tissues from different organs

A

Partial volume artifacts

45
Q

Artifact caused by undersampling in the cone angle dimension, and accentuated by using the defrise phantom

A

Cone beam artifact

46
Q

Rotate-translate geometry using pencil beam, is what generation of CT

A

First

47
Q

Used the same rotate-translate motion, however, more detectors were added and so the initial pencil beam geometry became a narrow fan beam—- what generation

A

Second generation

48
Q

What generation of CT uses rotate-rotate geometry and is still the most widely used geometry on modern scanners today

A

3rd generation

49
Q

Generation of CT that used an entire 360 degree ring of detectors mounting in the stationary frame, with a rotating xray tube resulting in rotate-stationary acquisition

A

4th generation

50
Q

3rd generation CT geometry leads to potential for what artifact

A

Ring artifact production

51
Q

How many degrees is the detector of 3rd gen CT

A

60 degrees

52
Q

What CT generation uses a detector fan geometry

A

4th generation

53
Q

Value that most closely matches the attenuation profile of a polyenergetic xray spectrum

A

Effective linear attenuation coefficient

54
Q

Xray beams are attenuated

A

Exponentially

55
Q

Necessary for creating a projection image

A

Attenuation

56
Q

Source to skin distance can be estimated by knowing the following

A

Source to skin distance (SID)
Thickness of the patient
Dead space between the cover of the detector and the actual detector surface

57
Q

Function of mA and kV, and the mA fluctuates during the procedure depending on the positioning of the fluoroscopic system relative to the patient’s body and on the mode settings of the fluoroscopy system

A

Kerma rate

58
Q

Most modern xray dosimetry is based upon what calculation, which used computer stimulation to study the dose deposition during an xray procedure

A

Monte Carlo

59
Q

Refers to the beam width as reported by the scanner, not the actual measured beam width, which is generally slightly wider

A

Nominal beam width

60
Q

In helical CT scanning, the CT dose is _____ related to the helical pitch

A

Inversely

61
Q

Dose length product is approximately _____ to effective dose

A

Proportional

62
Q

Measured using either a 16 cm or 32 cm diameter polymethyl methacrylate phantom. The dosimeter is placed serially in the center hole and the peripheral hole, and the measurements are combined

A

Computed tomography dose index

Physics (7 decks)

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