Flashcards in CT-PPT Deck (111)
Computed Tomography (CT) is also known as
Computed Axial Tomography (or CAT scan)
The term “axial” describes
The place in which slices are acquired
How do you explain a CT scan?
Creating a cross‐sectional tomographic plane of any part of the body
“Tomography” is derived from two words:
- “tomos” meaning slice or section
- “graphia” meaning describing
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
***Who received the Nobel Prize in medicine and physiology for their work in CT?
***Who developed the first whole body scanner in 1974?
Dr. Robert Ledley
Hounsfield begins research CT in what year?
When was the 1st brain CT scanner installed
1st Scanner Generation
• Pencil beam
(1 to 3 detectors)
2nd Scanner Generation
• Fan beam
3rd Scanner Generation
• Curved detector array
-Tube & detector array rotate simultaneously around patient
***4th Scanner Generation
• Ring of detectors
-Tube rotates but detectors remain stationary
5th Scanner Generation
Electron Beam CT (EBCT
- Primarily used for cardiac studies
-High speed scanner
-Electron beam CT for cardiac studies
6th Scanner Generation
Dual-source CT (DSCT)
o-Two sets of x-ray tube and detectors set 90 apart
Multislice CT Scanners based on
3rd generation scanner geometry
How are CT scanners categorized and what is the difference between these categories?
-Difference between generations related to number & arrangement of detectors
***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.
Part of x-ray beam that falls onto one detector
a collection of rays for one translation across object
a view generates a profile
Multislice detectors (MDCT or MSCT)
-Multiple detector rows
-As the number of detector rows increase, the fan-beam must
-2-slice up to 16-slice
Volume CT (VCT)
-64 detector rows or more
-Uses cone beam geometry
Flat-panel CT (FP-CT or CD-CT)
-Uses a digital detector plate
-Used for dedicated breast units or c-arm fluoroscopy
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
Composed of rows and columns of pixels
512 x 512 or 1024 x 1024
Slice thickness to pixel
Single square or picture element within matrix
Each pixel represents
a voxel of tissue
2D representation of a
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).
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
***What is the CT number of water?
What is the CT number of air?
What is the CT number of bone?
CT Computer Link between
the technologist and other system components
4 basic functions of CT Computer are
-Control of data acquisition
-Storage of image data
Circular device that houses the x-ray tube, DAS, detector array, slip ring, and high voltage generator
***What is the central opening of the gantry?
(Tilted ± 30 degrees)
CT X‐ray Tube Similar to
CT X‐ray Tube Able to handle excessive heat up to
X-ray tubes can handle about
Measures the amount of radiation transmitted
through the body and coverts the measurement
into an electrical signal
2 Types of Detectors
Scintillation (solid state)
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
Convert x-ray energy directly to electrical energy
• Uses xenon gas
***Slip Ring Allows for continuous rotation of
the x-ray tube/detector assembly
Slip Ring Consists of
circular electrical conductive rings and brushes that transmit electrical energy across a rotating interface
Slip Ring Provides electrical power to
operate the x-ray tube and transfers signals from the detectors for input into the image reconstruction computer
CT scanners use
• Power ratings range from 20 – 100 kW
• Removes long-wavelength x-ray photons from the xray beam
• Shapes the energy distribution across the x-ray beam
Filters are positioned between
the x-ray tube and the patient to shape the beam and provide more uniformity at the detectors
Reduces patient dose
Improves image quality
-@ X-ray tube (Slice thickness)
-Reduces patient dose
-Ensures a constant beam width
Post-patient collimator Also known as
Pre-detector collimator or Detector collimator
-Refines the beam
-Reduces scatter radiation
-Helps define the thickness of the slice to be imaged
What is Data Acquisition System (DAS)
The detector electronics
Positioned between the detectors array and the
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
DAS consist of
the method of scanning (conventional/helical) and image reconstruction (array processor)
Patient Table is
Automated device linked to the computer and gantry
Patient Table Designed to
-move in move in increments or indexing
-Accessory devices (i.e. cradle for head procedures)
Patient Table Weight limits range from
300 lbs to 600 lbs
***What is another name for the CT table?
What are the components on the operator console?
Touch display screen mouse
Data Acquisition definition
The systematic collection of information from the
patient to produce the CT image
2 Methods of Data Acquisition
a. Slice-by-slice data acquisition
b. Volume data acquisition
Slice-by-slice data acquisition also known as
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
Volume data acquisition also known as
Spiral or helical scanning
***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
1 slice per revolution of the x-ray tube
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
***CT numbers correspond to different shades of ____ in the image.
What is window width?
-The range of CT #s that are displayed as shades of gray
-Affects the contrast of the image
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
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
Windowing is used to
Optimize visualization of specific tissues or
lesions under study
- Matrix size (512 x 512 or 1024 x 1024)
- Scan FOV
- Display FOV
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
***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
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
Pitch of SSCT & MSCT
- SSCT: The ratio of the distance the table translates per gantry rotation to the beam width or
- MSCT: The ratio of the distance the table travels per gantry rotation to the total collimation
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
Basic Scan Parameters kVp – ranges from
Basic Scan Parameters mA – ranges from
10 to 500 mA (adjusted in increments of 10)
What is Scan Time?
- The duration of the scan
- Choice depends on the patient’s respiratory condition
Reconstruction Increment (RI) also called
Reconstruction Interval or Reconstruction Spacing
Reconstruction Increment (RI) Determines
the degree of overlap of the slices
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
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)
***What is contrast resolution?
• Ability to see small differences in density
• Tissue density differences of less than 0.5% can be distinguished on CT
What is Temporal Resolution?
• 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
***What is noise?
• 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
***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
Bone-soft tissue interface (skull)
ALARA still applies
- Use lowest mA possible (balance between high image quality & lowest radiation dose possible)
- Adjust for pediatric patients
What is Multiple Scan Average Dose (MSAD)?
Average dose resulting from scans over an interval length on the patient
What is Computed Tomography Dose Index (CTDI)?
CTDIvol - used for volume scanning
What is Dose Length Product (DLP)?
- The method used to report dose
- CTDIvol multiplied by the scan length (in cm)
Applications of CT
• CT Angiography (CTA) - Imaging of blood vessels
• Cardiac CT
• 3D CT
• Virtual imaging
• Perfusion imaging
• Radiation treatment planning
***What is MPR?
- Reconstructing axial images into coronal, sagittal or oblique planes without the need for additional radiation
- Postprocesing technique
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
What is a region of interest or ROI?
Measurement of CT numbers within a specified area for evaluation of average tissue density
***What are the names used for the preliminary image?
***What is the barium concentration used for CT procedures?
2% - low concentration to prevent streak artifacts
Intravenous Iodinated Contrast
Opacifies the vasculature
- Non-ionic contrast
• Higher cost that ionic contrast
• Lower incidence of reaction
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)
Opacifies distal colon
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
***What is spiral or helical scanning?
Gantry continuously rotates while table moves through aperture at all the same time
What special technology does it utilize?
Slip ring technology replaces high tension cables of earlier scanners