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Imaging Applications and Quality > Unit 4: CT Fundamentals > Flashcards

Unit 4: CT Fundamentals Flashcards

1
Q

What generation scanners are used for medical imaging purposes in Canada today?

A

3rd generation

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2
Q

T/F

3rd generation CT scanners have a rotate/rotate motion

A

True

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3
Q

T/F

3rd generation CT scanners have pencil beam geometry

A

False; they have fan beam geometry

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4
Q

What type of detector array does 3rd generation CT scanners have?

A

A curvilinear detector array

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5
Q

What is the primary difference between SDCT and MDCT

A

SDCT only takes an image of one slice per rotation, where as MDCT can take up to 64 slices in 1 rotation.

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6
Q

T/F

In SDCT the largest allowable slice thickness is less than the detector width.

A

True

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7
Q

T/F

SDCT is the most widely used compared to MDCT.

A

False; MDCT

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8
Q

Is it possible to change slice thickness with MDCT?

A

Yes; depends to the manfacterer

(can also change thickness so that thinner rows are ceen centrally and wider rows peripherally)

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9
Q

What is slice thickness determed by in MDCT?

A

Determined by a combination of the x ray beam width and the detector configuration

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10
Q

What is the smallest slice thickness seen in MDCT?

A

.5mm

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11
Q

What the 2 types of scan modes in CT?

A
  1. Axial
  2. Helical
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12
Q

What is the most common scan mode used?

A

Helical

(axial not really used anymore)

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13
Q

What is the axial scan mode?

A

Refers to when the CT table moves to the desired location and remains stationary while the x ray tube rotates within the gantry, collecting data. The CT table is then moved further after data is collected.

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14
Q

What is interscan delay?

A

The slight pause in between data acquisition as the table moves to the next location in axial scan mode

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15
Q

When would axial scan mode be used?

A

Used in studies where acquisition speed is not of a major concern.

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16
Q

What are the two ways in which axial slices can be obtained?

A
  1. Contiguous
  2. Non Contiguous
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17
Q

What is Cine mode?

A

With Axial slicing, scanners can be programmed to repeat scans at the same slice location (i.e. no table incremation)

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18
Q

Why are axial protocols that use overlapping slices rare?

A

They increase the radiation dose to the patient and do not provide additional diagnostic information

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19
Q

What is the primary disadvantage of axial scanning?

A

The cumulative effect of the pauses (interscan delay) between each data acquisition adds to the total examination time

(time is the disadvantage)

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20
Q

T/F

Data acquired in axial mode are more limited in how they can be reformatted or reconstructed.

A

True

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21
Q

What is the purpose of interpolation in helical scan mode?

A

To take the slant and blur out of the helical image

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22
Q

What is pitch?

A

The ratio of the patient couch movement (during one x ray tube revolution) to the width of the x-ray beam.

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23
Q

T/F

A pitch less than 1 results in overlapping images and higher patient radiation dose.

A

True

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24
Q

If the distance translated by the table equals the slice thickness, what is the pitch ratio?

A

1:1

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25
Q

If the table feed is 6 mm per roation, and the beam width is 4 x 1.25, what is the pitch?

A

1.2

26
Q

How do you calculate pitch?

A

Pitch=Couch movement per x ray tube revolution (mm)/beam width (mm)

27
Q

How do you calculate the amount of anatomy covered in MDCT?

A

Amount of anatomy covered=Pitch x acquistion time x beam width/X ray tube rotation time

28
Q

Calculate the anatomical coverage of a helical scan with a 20 second totatl acquisitioin time, a 0.5 rotation time, a 2.5 mm slice thickness and a pitch of 1.2 if 4 slices are acquired with each tube rotation.

A

480 mm

29
Q

T/F

An increase in anatomy coverage in helical scanning=increase in time

A

True

30
Q

Label 1-3

A
  1. Y axis
  2. X axis
  3. Z axis
31
Q

Label 1-4

A
  1. Z axis
  2. x axis
  3. Y axis
  4. X axis
32
Q

Why is it important that the patient be centered at the isocenter?

A

To ensure proper functioning of the bow tie filters.

33
Q

What is the purpose of a bow tie filter?

A

To convey maximum x-ray photons to the thicker part of the body and to reduce x-ray intensity to the thinner part of the body.

34
Q

T/F

In practice, off-center positioning occurs even though laser indicators identify the isocenter of a CT scanner

A

True

(thicker part of the filter aligns with the thinner anatomy)

35
Q

What does each slice of a patient in a CT scan represent?

A

Each slice represents a volume in the patient.

36
Q

What is each segmented portion of volume of a slice in CT called?

A

Voxel

37
Q

What is the voxel projected on the display screen as?

A

A pixel

38
Q

How is tissue volume calculated?

A

Voxel size (mm3) = Pixel size (mm2) x Slice thickness (mm)

39
Q

Calculate the tissue volume contained within the shown voxel:

A

117mm^3

40
Q

What is volume averaging?

A

All the various tissue attenuation values within a voxel are averaged and represented as a shade of grey (or brightness).

41
Q

What projections a?re taken in a CT Reference Image?

A

AP & Lateral

42
Q

How do you calculate pixel size?

A

Pixel Size = FOV ÷ Matrix Size

43
Q

T/F

The center point in CT is not at the center of the FOV.

A

True

44
Q

What is Image Reconstruction (Prospective)?

A

Process whereby the computer manipulates data collected from the detectors to create a CT image, based on scan parameters.

45
Q

Label 1

A

Aperture

46
Q

What are the 3 selections a technologist can make when selecting Scan Field of View?

A

Small
Medium
Large

47
Q

What does SFOV determine?

A

SFOV determines the number of detector cells collecting data of a certain diameter through the icocenter of the gantry

48
Q

T/F

Anything outside the SFOV is not imaged, as data is not collected outside this circle.

A

True

49
Q

T/F

To produce the highest quality image, the operator should select the SFOV that just encompasses the anatomy of interest.

A

True

50
Q

What is DFOV?

A

DFOV is the reconstructed area seen on the display monitor, following the scan.

51
Q

What does DFOV determine?

A

Determines how much of the acquired raw data is used to create the image

CT image acquisition:
1. Scout of chest (SFOV)
2. Select DFOV
3. Scan anatomy slice

52
Q

T/F

DFOV can be equal to or less than the SFOV.

A

True

53
Q

What is the benefit of using DFOV?

A

You dont loose quality compared to zooming into an image and the image is more localized to the anatomy of interest

54
Q

As pixel size goes down, what happens to SR?

A

SR increases

55
Q

T/F

Using DFOV decreases dose

A

FALSE; still exposes the anatomy in the SFOV

56
Q

What is the range of HU?

A

-1000 to +3000

57
Q

What is the HU for water

A

0

58
Q

What is the HU for fat?

A

-100

59
Q

What is the HU for bone/metal?

A

+1000

60
Q

What is the HU of air?

A

-1000

Imaging Applications and Quality (9 decks)

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