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MRAD2230 > CT Review > Flashcards

CT Review Flashcards

(62 cards)

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1
Q

In what plane are CT images acquired

A

Axial plane (z-axis)

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2
Q

What axis runs from the left to right of the patient?

A

X-axis (sagittal plane)

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3
Q

What unit represents how much attenuation occurs in a certain material at a certain kV?

A

Hounsfield unit (HU) or CT number

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4
Q

What is the HU for water?

A

0

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5
Q

What body tissue has a Hounsfield unit of +1000

A

Bone

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6
Q

How does the computer average the attenuation measurements in a voxel, and create a pixel with a single HU?

A

Volume averaging

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7
Q

What is the partial volume effect?

A

When HU’s within a voxel vary significantly, and averaging them creates a very inaccurate pixel

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8
Q

Do HU’s change when we change WW or WL?

A

No

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9
Q

What kind of contrast would a wide WW demonstrate?

A

Low contrast (many shades of grey)

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10
Q

If a structure with an HU value falls above the WW, what shade will it appear on the image?

A

White

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11
Q

If a structure with an HU value falls below the WW, what shade will it appear on the image?

A

Black

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12
Q

If structures have very similar HU’s, what type of contrast would be best?

A

High contrast (Narrow WW)

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13
Q

What extra step needs to be taken to create an image in CT?

A

Raw data needs to be converted to image data

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14
Q

What innovation has allowed CT scanners to scan 360 degrees continuously around the patient?

A

Slip ring technology

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15
Q

What are some features that help CT x-ray tubes dissipate heat?

A

Pulsed x-ray
Tungsten and rhenium anode (layered)
Metal and ceramic tube housing
Blowers and fans
Oil to air heat exchange (oil in contact with anode)

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16
Q

What component of the CT machine collects x-ray photons and converts them to an electrical signal?

A

Detector array

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17
Q

Where is the filtration in a CT machine found?

A

After pre-patient collimators and before the patient

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18
Q

What are the 3 technical factors in CT?

A

mA - number of photons
kV - average energy of the beam
rotation time - how fast the gantry spins

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19
Q

How does a solid state detector in the detector array convert x-rays into an electrical signal

A

X-ray photons –> light by scintillator crystal –> electrical signal by photodiode

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20
Q

What features of a detector would improve scatter absorption?

A

Detectors placed close together
Deep, narrow detector
Small detector

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21
Q

What does the pre-patient collimators do?

A

Limits radiation exiting tube to desired beam shape
Determines beam width in an MDCT (z-axis)

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22
Q

Which detector array configuration provides the most anatomy coverage per rotation:
a) 16 rows x 1.25mm slices
b) 32 rows x 0.2mm slices
c) 8 rows x 3mm slices

A

C

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23
Q

What is the purpose of post-patient collimators?

A

Decrease scatter entering detectors
Ensure correct beam width at detector array

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24
Q

What component of CT machine converts the electrical signal from the detector array into a digital signal?

A

Data Acquisition System (DAS) or ADC

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25
Why is it important to place anatomy of interest at the isocenter of the scanning field?
Dose reduction Accuracy in scanning
26
In what axis does the SFOV determine the number of active detector cells collecting data?
X-axis
27
What are the advantages of thinner acquisition slices?
Higher spatial resolution
28
What are the advantages of a thicker acquisition slice?
Lower patient dose Faster scan time
29
What scan mode provides increased spatial resolution, but a very long scan time, and higher potential for misregistration?
Axial/Step-and-shoot
30
What scan mode would be best when scanning pediatric patients, or cardiac studies?
Volume scanning
31
What are 3 ways a spectral CT uses dual-energy imaging to improve material differentiation on images?
Dual source Dual energy Dual layer detectors
32
What does a pitch of <1 mean? What are the advantages and disadvantages?
Means the table is moving less/slower than the beam is wide You will get overlap of data = oversampling = better spatial resolution Scan takes longer
33
What does a pitch of >1 mean? What are the advantages and disadvantages?
Means the table is moving more/faster than the beam is wide There will be gaps between each rotation of the tube = decreased spatial resolution (more need to interpolate) Faster scan and less dose to the patient
34
If you decrease rotation time, but keep table movement and beam width constant, what would happen to your pitch?
Pitch <1 - more anatomy coverage per rotation of the tube
35
What process occurs when we revisit raw data and manipulate it to create new image data?
Retrospective reconstruction
36
What process occurs when we use image data to create new images?
Reformatting
37
What step in prospective reconstruction involves filling in missing data?
Interpolation
38
What process converts data from a scan to a matrix from attenuation profiles, and uses a Fourier transformation to reduce noise?
Filtered back projection
39
What are the advantages of iterative reconstruction?
Reduced image noise (better low contrast resolution) Reduced radiation dose to the patient
40
What type of reconstruction algorithm/kernel would be best to view bony anatomy?
Sharpening/ edge-enhancement filter
41
What factors can be manipulated in retrospective reconstructions?
DFOV Image center Reconstruction kernel/algorithm Slice thickness (can't be thinner than acquisition thickness and also limited by detector element size) Slice intervals (overlapping = better spatial resolution)
42
What type of reformatting displays voxels with minimum values? Is volume averaging used?
MinIP (Minimum intensity projection) - to view bronchial tree for example MinIP and MIPs do not use volume averaging!
43
What image quality factor describes the ability to resolve small objects that are very close together as separate objects?
Spatial resolution
44
What does an MTF of 0 mean for spatial resolution?
The system was not able to reproduce an object at all (image is nothing like original object)
45
What factors would improve spatial resolution?
Thin slices Increased rotation time (decreased speed) Pitch <1 Overlapping slices in retrospective recons Large matrix and small pixels Small DFOV Sharpening/edge-enhancement filter
46
What factors decrease low contrast resolution?
NOISE! Decrease technical factors Thicker slices Increased patient thickness No contrast media Sharpening, edge-enhancement reconstruction algorithm Wide WW
47
What is temporal resolution? What factors control it?
How rapidly data acquired Faster scan time = better temporal resolution Gantry rotation speed - faster better # of detector channels - more detector channels active is better Pitch - >1 better
48
What are out of field artifacts?
When there is anatomy outside of the SFOV - the beam is attenuated or hardened by it and confuses the image reconstruction process
49
What artifact occurs when only part of a structure is scanned during one part of rotation around patient?
Partial volume artifact
50
When do ring artifacts occur?
When 1 or more detectors is out of alignment/calibration
51
What artifact occurs when MPRs are created from thick helical sections? How can it be prevented?
Stair-step artifact Use thinner slices
52
What CT dose measurement gives us the average dose a patient gets for a scan, and includes central slice and "tail" dose readings?
Multiple scan average dose (MSAD)
53
What dose measurement is an estimate of MSAD based on the manufacturer?
CTDI (CT dose index)
54
What dose measurement tells us the exposure per slice, independent of the scan length?
CTDIvol
55
What does dose length product (DLP) measure?
total amount of radiation incident on patient - a produce of CTDIvol and scan length
56
What is tube current modulation?
The CT machine adjusts mA throughout the scan based upon localizer scan values
57
How does increase in rotation time affect dose?
Increased dose
58
What pitch will improve patient dose, but decrease spatial resolution?
Pitch >1
59
What is the purpose of CT Tube warm-up and air calibrations?
Warm up - brings focal spot/anode up to appropriate heat so it doesn't crack Air calibrations - takes exposures at various settings to ensure HU for air is accurate
60
What is the CT number accuracy QC test for? How often should it performed?
Determines accuracy for CT number for water (0) Weekly
61
What CT QC test determines the standard deviation of CT#'s across an image (using ROI's), and is done weekly?
CT noise
62
How often is the CT spatial resolution QC test done?
Quarterly

MRAD2230 (2 decks)

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