CT-PPT Flashcards
1
Q
Computed Tomography (CT) is also known as
A
Computed Axial Tomography (or CAT scan)
2
Q
The term “axial” describes
A
The place in which slices are acquired
3
Q
How do you explain a CT scan?
A
Creating a cross‐sectional tomographic plane of any part of the body
4
Q
“Tomography” is derived from two words:
A
- “tomos” meaning slice or section
- “graphia” meaning describing
5
Q
9 Basic Process
A
- Patient placed in CT scanner
- X‐ray tube and detectors rotate around patient
- Detector assembly Measures exiting radiation
- ADC converts analog data to digital data
- Digital data is sent to host computer
- Data is calculated per a specific algorithm
- Data is assembled into a matrix to form an axial image
- DAC converts digital data back to analog data
- Analog data (CT image) is displayed on a monitor
6
Q
***Who received the Nobel Prize in medicine and physiology for their work in CT?
A
Godfey Hounsfield
Allan Cormack
7
Q
***Who developed the first whole body scanner in 1974?
A
Dr. Robert Ledley
8
Q
Hounsfield begins research CT in what year?
A
1969
9
Q
When was the 1st brain CT scanner installed
A
1971
10
Q
1st Scanner Generation
A
• Translate/rotate
• Pencil beam
(1 to 3 detectors)
11
Q
2nd Scanner Generation
A
• Translate/rotate
• Fan beam
(30 detectors)
12
Q
3rd Scanner Generation
A
• Rotate/Rotate
• Curved detector array
-Tube & detector array rotate simultaneously around patient
13
Q
***4th Scanner Generation
A
• Rotate/Stationary
• Ring of detectors
-Tube rotates but detectors remain stationary
14
Q
5th Scanner Generation
A
Electron Beam CT (EBCT
- Primarily used for cardiac studies
- High speed scanner
- Electron beam CT for cardiac studies
15
Q
6th Scanner Generation
A
Dual-source CT (DSCT)
o-Two sets of x-ray tube and detectors set 90 apart
16
Q
Multislice CT Scanners based on
A
3rd generation scanner geometry
17
Q
How are CT scanners categorized and what is the difference between these categories?
A
- Generations
- Difference between generations related to number & arrangement of detectors
18
Q
***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.
A
- Projections
- Scan profile
- Raw data
19
Q
Ray is
A
Part of x-ray beam that falls onto one detector
20
Q
View is
A
a collection of rays for one translation across object
21
Q
Profile is
A
a view generates a profile
22
Q
Multislice detectors (MDCT or MSCT)
A
-Multiple detector rows
-As the number of detector rows increase, the fan-beam must
adapt
-2-slice up to 16-slice
23
Q
Volume CT (VCT)
A
- 64 detector rows or more
- Uses cone beam geometry
24
Q
Flat-panel CT (FP-CT or CD-CT)
A
- 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
