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Flashcards in CT-PPT Deck (111):
1

Computed Tomography (CT) is also known as

Computed Axial Tomography (or CAT scan)

2

The term “axial” describes

The place in which slices are acquired

3

How do you explain a CT scan?

Creating a cross‐sectional tomographic plane of any part of the body

4

“Tomography” is derived from two words:

- “tomos” meaning slice or section
- “graphia” meaning describing

5

9 Basic Process

1. Patient placed in CT scanner
2. X‐ray tube and detectors rotate around patient
3. Detector assembly Measures exiting radiation
4. ADC converts analog data to digital data
5. Digital data is sent to host computer
6. Data is calculated per a specific algorithm
7. Data is assembled into a matrix to form an axial image
8. DAC converts digital data back to analog data
9. Analog data (CT image) is displayed on a monitor

6

***Who received the Nobel Prize in medicine and physiology for their work in CT?

Godfey Hounsfield
Allan Cormack

7

***Who developed the first whole body scanner in 1974?

Dr. Robert Ledley

8

Hounsfield begins research CT in what year?

1969

9

When was the 1st brain CT scanner installed

1971

10

1st Scanner Generation

• Translate/rotate
• Pencil beam
(1 to 3 detectors)

11

2nd Scanner Generation

• Translate/rotate
• Fan beam
(30 detectors)

12

3rd Scanner Generation

• Rotate/Rotate
• Curved detector array
-Tube & detector array rotate simultaneously around patient

13

***4th Scanner Generation

• Rotate/Stationary
• Ring of detectors
-Tube rotates but detectors remain stationary

14

5th Scanner Generation

Electron Beam CT (EBCT
- Primarily used for cardiac studies
-High speed scanner
-Electron beam CT for cardiac studies

15

6th Scanner Generation

Dual-source CT (DSCT)
o-Two sets of x-ray tube and detectors set 90 apart

16

Multislice CT Scanners based on

3rd generation scanner geometry

17

How are CT scanners categorized and what is the difference between these categories?

-Generations
-Difference between generations related to number & arrangement of detectors

18

***The tube rotates around the patient and for every position of the tube, the detectors measure the transmitted x-ray values (known as …................) and converts them into an electrical signal.

-Projections
-Scan profile
-Raw data

19

Ray is

Part of x-ray beam that falls onto one detector

20

View is

a collection of rays for one translation across object

21

Profile is

a view generates a profile

22

Multislice detectors (MDCT or MSCT)

-Multiple detector rows
-As the number of detector rows increase, the fan-beam must
adapt
-2-slice up to 16-slice

23

Volume CT (VCT)

-64 detector rows or more
-Uses cone beam geometry

24

Flat-panel CT (FP-CT or CD-CT)

-Uses a digital detector plate
-Used for dedicated breast units or c-arm fluoroscopy

25

What happens to the electrical signal in CT?

-It is sent to the computer to be digitized where a number is assigned that is directly proportional to signal strength
-Those numbers are arranged in a grid (rows/columns) = Matrix

26

Matrix is

Composed of rows and columns of pixels
512 x 512 or 1024 x 1024

27

***Voxel is

Volume element
Slice thickness to pixel

28

***Pixel is

Single square or picture element within matrix

29

Each pixel represents

a voxel of tissue

30

2D representation of a

3D object

31

An ........................ in the matrix will cause the pixel to be smaller and allow ..................... details to be seen in the image (better resolution but increase noise in image).

Increase

Small

32

What is a Hounsfield unit?

Each pixel within image is assigned a number (CT number or Hounsfield unit) that is proportional to the degree of attenuation of that tissue

33

***What is the CT number of water?

0

34

What is the CT number of air?

-1,000

35

What is the CT number of bone?

1,000

36

CT Computer Link between

the technologist and other system components

37

4 basic functions of CT Computer are

-Control of data acquisition
-Image reconstruction
-Storage of image data
-Image display

38

***Gantry

Circular device that houses the x-ray tube, DAS, detector array, slip ring, and high voltage generator

39

***What is the central opening of the gantry?

Aperture
(Tilted ± 30 degrees)

40

CT X‐ray Tube Similar to

Conventional radiography

41

CT X‐ray Tube Able to handle excessive heat up to

4-5 MHU

42

X-ray tubes can handle about

2.1 MHU

43

Detectors

Measures the amount of radiation transmitted
through the body and coverts the measurement
into an electrical signal

44

2 Types of Detectors

Scintillation (solid state)
Ionization (gas)

45

Scintillation (solid state detector)

Convert energy into light, then light is converted into electrical energy
Composed of photodiodes coupled with scintillation crystal material that converts transmitted x-ray energy into light which is then converted into electrical energy and then to a digital signal

46

Ionization (gas)

Convert x-ray energy directly to electrical energy
• Uses xenon gas

47

***Slip Ring Allows for continuous rotation of

the x-ray tube/detector assembly

48

Slip Ring Consists of

circular electrical conductive rings and brushes that transmit electrical energy across a rotating interface

49

Slip Ring Provides electrical power to

operate the x-ray tube and transfers signals from the detectors for input into the image reconstruction computer

50

CT scanners use

High-frequency generators
• Power ratings range from 20 – 100 kW

51

Filtration

• Removes long-wavelength x-ray photons from the xray beam
• Shapes the energy distribution across the x-ray beam

52

Filters are positioned between

the x-ray tube and the patient to shape the beam and provide more uniformity at the detectors

53

Collimation

Reduces patient dose
Improves image quality

54

Pre-patient collimator

-@ X-ray tube (Slice thickness)
-Reduces patient dose
-Ensures a constant beam width

55

Post-patient collimator Also known as

Pre-detector collimator or Detector collimator

56

Post-patient collimator

-@ Detector
-Refines the beam
-Reduces scatter radiation
-Helps define the thickness of the slice to be imaged

57

What is Data Acquisition System (DAS)

The detector electronics
Positioned between the detectors array and the
computer

58

3 main functions of Data Acquisition System

1. Measures the transmitted radiation beam
2. Encodes these measurements into binary (digital) data
3. Transmits the binary (digital) data to the computer

59

DAS consist of

the method of scanning (conventional/helical) and image reconstruction (array processor)

60

Patient Table is

Automated device linked to the computer and gantry

61

Patient Table Designed to

-move in move in increments or indexing
-Accessory devices (i.e. cradle for head procedures)

62

Patient Table Weight limits range from

300 lbs to 600 lbs

63

***What is another name for the CT table?

Patient Couch

64

What are the components on the operator console?

Keyboard
Graphic monitor
Touch display screen mouse

65

Data Acquisition definition

The systematic collection of information from the
patient to produce the CT image

66

2 Methods of Data Acquisition

a. Slice-by-slice data acquisition
b. Volume data acquisition

67

Slice-by-slice data acquisition also known as

Conventional scanning

68

Conventional (Slice-by-slice) Scanning

X-ray tube rotates around the patient and collects data from the first slice
-The tube then stops, and the patient move into position to scan the next slice
-The x-ray tube then rotates around the patient again to collect data from the next slice
• This process is repeated until the desired anatomy
has been imaged
• Takes longer than volume data acquisition

69

Volume data acquisition also known as

Spiral or helical scanning

70

***Volume (Spiral / Helical) Scanning

A volume of tissue is scanned at a time
o X-ray tube continuously rotates around the patient and traces a spiral/helical path to scan an entire volume of tissue in a single breath-hold
-SSCT
-MSCT

71

SSCT produces

1 slice per revolution of the x-ray tube

72

MSCT produces

multiple slices per revolution of the x-ray tube
-Scanners now are capable of 4, 8, 16, 32, 40, 64, 256, & 320 slices per revolution

73

***CT numbers correspond to different shades of ____ in the image.

gray

74

What is window width?

-The range of CT #s that are displayed as shades of gray
-Affects the contrast of the image

75

What is window level or Center (WL or WC)?

- Midpoint of range of gray levels
- The center of the range of CT numbers
- Affects the brightness of the image

76

What is Windowing?

-The process of changing the CT image gray scale by manipulating the WW and WL
-Adjustment of the gray scale to alter the
appearance of a digital image

77

Windowing is used to

Optimize visualization of specific tissues or
lesions under study

78

Technologist chooses

- Matrix size (512 x 512 or 1024 x 1024)
- Scan FOV
- Display FOV

79

What is Scan FOV?

- The circular region from which the transmission measurements are recorded during scanning
- Includes entire cross section of body part or only a specified region within body part

80

***What is the display field of view?

- Refers to the anatomy that is displayed on the monitor
- Can be smaller than the scan field of view but not larger

81

Collimation of SSCT & MSCT

- SSCT: Width of the pre‐collimator defines the slice thickness
- MSCT: Width of the pre‐collimator divided among the number of detector rows defines the slice thickness

82

Pitch of SSCT & MSCT

- SSCT: The ratio of the distance the table translates per gantry rotation to the beam width or
pre‐collimator width
- MSCT: The ratio of the distance the table travels per gantry rotation to the total collimation

83

Slice Thickness of SSCT & MSCT

- SSCT: Determined by the pitch & the width of the pre‐collimator
- MSCT: Determined by the beam width, the pitch, & the shape & width of the reconstruction filter in the zaxis

84

Basic Scan Parameters kVp – ranges from

80 to140

85

Basic Scan Parameters mA – ranges from

10 to 500 mA (adjusted in increments of 10)

86

What is Scan Time?

- The duration of the scan
- Choice depends on the patient’s respiratory condition

87

Reconstruction Increment (RI) also called

Reconstruction Interval or Reconstruction Spacing

88

Reconstruction Increment (RI) Determines

the degree of overlap of the slices

89

How do you define slice thickness?

- Number of millimeters intersecting thickness of x-ray beam
- Partially defined by size of focal spot
- Further refine the thickness by the collimators

90

What is spatial resolution?

• Degree of blur
• Ability to see the difference between two objects close together
• Measured size of the smallest object in line pairs per cm (lp/cm)

91

***What is contrast resolution?

• Ability to see small differences in density
• Tissue density differences of less than 0.5% can be distinguished on CT

92

What is Temporal Resolution?
(how fast?)

• The ability of the CT system to freeze any motion of a scanned object
• Shortest amount of time needed to complete scan
• High temporal resolution needed for cardiac CT

93

***What is noise?
(scatter)

• Quantum noise arises from random variations in photon detection
- A fluctuation in the CT number within a uniform material making the image appear grainy, speckled, or mottled
- Difficult to see subtle contrast differences or fine details

94

***What is an artifact?

• Obscure diagnostic information
• Caused by metal, x‐ray barium, beam hardening, motion, etc.
- Beam hardening or streak artifacts
- Metallic “spray” artifact
dental fillings
Pacemakers
Prosthesis
Bone-soft tissue interface (skull)

95

ALARA still applies

- Use lowest mA possible (balance between high image quality & lowest radiation dose possible)
- Adjust for pediatric patients

96

What is Multiple Scan Average Dose (MSAD)?

Average dose resulting from scans over an interval length on the patient

97

What is Computed Tomography Dose Index (CTDI)?

CTDIvol - used for volume scanning

98

What is Dose Length Product (DLP)?

- The method used to report dose
- CTDIvol multiplied by the scan length (in cm)

99

Applications of CT

• CT Angiography (CTA) - Imaging of blood vessels
• Cardiac CT
• 3D CT
• Virtual imaging
• Perfusion imaging
• Biopsy/drainageguidance
• Radiation treatment planning
• PET/CT

100

***What is MPR?

- Reconstructing axial images into coronal, sagittal or oblique planes without the need for additional radiation
- Postprocesing technique

101

What is an algorithm or kernel?

- Mathematical formula designed for computers to carry out complex calculations
- For enhancement of soft tissue, bone and edge resolution

102

What is a region of interest or ROI?

Measurement of CT numbers within a specified area for evaluation of average tissue density

103

***What are the names used for the preliminary image?

- Scanogram
- Scout
- Topogram

104

***What is the barium concentration used for CT procedures?

2% - low concentration to prevent streak artifacts

105

Intravenous Iodinated Contrast

Opacifies the vasculature
- Non-ionic contrast
• Higher cost that ionic contrast
• Lower incidence of reaction

106

Oral Contrast

Opacifies the GI tract
- 2% barium sulfate suspension
- Iodinated contrast (Hypaque or Gastrografin)
• Must be mixed in low concentration to prevent artifacts
- Omnipaque is now being used orally (diluted with other liquid)

107

Rectal Contrast

Opacifies distal colon

108

What is dynamic scanning?

Rapid sequential scanning @ the same level to observe contrast filling or incremental scanning of rapid series scanning during bolus injection of contrast

109

***What is spiral or helical scanning?

Gantry continuously rotates while table moves through aperture at all the same time

110

What special technology does it utilize?

Slip ring technology replaces high tension cables of earlier scanners

111

What type of data set is acquired with spiral/helical scanning?

- Volume of data
- Forms a spiral path
- Scans volume rather than group of individual slices