Quiz Flashcards
- Also called as trans-axial or transverse tomography
- produces a digital tomographic image from
diagnostic x-rays. - It uses a computer to process information collected from the passage of x-ray beams through an area of anatomy and display it in the monitor.
- The word tomography has as its root “tomo”, meaning “to cut, section, or layer” from the Greek “tomos” (a cutting).
- In the case of __, a sophisticated computerized method is used to obtain data and transform them into “cuts,” or cross-sectional slices of the human body.
Computed tomography
- Also called as axial tomography
- The plane of the image is parallel with the long axis of the body.
- It results in coronal and sagittal images.
- Uses blurring technique
Conventional tomography
We use computed tomography for?
Any region of the body,
staging primary tumors,metastasis, determine operability and baseline for chemotherapy
Radiotherapy planning
Advantages of ct scan
Excellent contrast resolution
Spatial resolution is average
Precise anatomical detail
Rapid examination technique
Helpful in separating organs – prevents superimposition
Disadvantage of ct scan
High examination and maintenance cause
High dose of ionizing radiation
Old names of computed tomography
Body Section Roentgenography
Computed Axial Tomography
Computerized Transaxial Tomography
Computerized Reconstruction Tomography
Digital Axial Tomography
History, Origin and Developments of CT
Old names of CT
1922:
1929:
1930:
1931:
1935:
1962:
Dr. Andre Edmund Marie Bocage (Roentgenograms)
Jean Keiffer (Laminagraph)
Allesandro Vallebona (Stratiography)
Ziedes des Plantes (Tomosynthesis)
Grossman (Grossman Principle or Tomograph)
ICRU (Tomography)
Father of CT, developed the 1st ct scan machine called EMI scanner
Sir Godfrey Newboid Hounsfield
Developed the CT reconstruction mathematics
Allan Mcleod Cormack
Proposed a single slice of body on the radiographic film and method to represent a single slice of body on radiographic film called topography
Allesandro vallebona
Developed the 1st whole body scanner
Dr. Robert S. Ledley
Developed the CT mathematical principle
Johann radon
First-scale commercial unit of CT “Brain tissue scanner” was installed at Atkinsons Morley Hospital
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First Ct scan in US was installed at Mayo Clinic
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First whole body scanner was developed at Georgetown University Medical Center.
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First clinical CT scanners were installed
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Major development and CT became widely available
1-3 seconds scan time with 1024x1024 matrix size
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EBCT
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Toshiba introduced Helical CT
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Elscint developed twin-slice CT
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General electric introduced multislice CT
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16-slice helical CT introduced
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64 slice helical CT introduced
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Siemens announced the dual source CT
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Toshiba introduced 320 slice helical CT
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AI contributed in CT
The name of AI is Watson
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2013
are acquired when x-rays pass through a patient to strike a detector and are recorded.
The major components that are involved in this phase of image creation are the gantry and the patient table.
Data
A process whereby a computer manipulates data collected from the detectors to create a CT image.
This includes algorithms, Fourier transformation, BP or FBP and interpolation.
Image reconstruction
The last phase in the creation of the CT image is that of display.
__ includes all of the system components necessary to convert the digital data created from the reconstruction process to electrical signals needed by the CT display monitor.
The display system also includes the ability to display patient information and scan protocol data, and provides many graphic aids
designed to assist in image interpretation.
Image display
Ct beam projections
- all rays in projection are parallel to each other
- rays at a given projection angle diverge and have the appearance of a fan
Parallel beam geometry
Fan beam geometry
Translate-rotate
Pencil xray beam
For neurologic examinations only
Sodium iodide detector
5 mins scan time
1st generation ct scan
Translate rotate
Narrow fan x-ray beam
Permits whole body examination
5-30 detectors
30 secs scan time
Uses bow tie filter
Second generation ct scan
Rotate rotate
Wide fan xray beam
Allows dynamic scanning
Curvilinear detector array
Less than <100 ms
Prone to ring artifacts
Slip ring widely available
3rd generation ct
Rotate-stationary
Wide fan xray beam
Closer sid
4800 detectors
Sub-second scan time
Free of ring artifact
Increased px dose
4th generation ct scan
No moving parts inside the gantry
Uses huge electron gun and detection coil
Consist of 4 tungsten target
For cardiac imaging
50 ms scan time
5th generation ct scan
Helical or spiral scanners
Pitch
Have dual source and energy cr
Excels in 3D multi-planar reformation
Uses latest slip ring
6th generation ct
Multi spiral ct
8 rows of detector
Improvement of detal
Fast scan times even large volume of FOV
7th generation ct
Scan modes
__ -Referred as axial, conventional or serial scanning. 360 degrees tube rotation and acquire data for the single slice and it was halted and patient moves in the desired location and the steps were repeated.
__ -Scanning is continuous. No cables to cause interruption during scanning. Uses slip ring.
__ -Multiple detector rows. Multiple slices. Scan time minimized. Quality images maximized
Step and shoot
Helical or soiral scanning
Multidetector row scanning
The movable frame of the CT unit and the most visible part of the unit.
The __ is the ring-shaped part of the CT scanner.
The __ includes a 50-85 cm (20”- 34”) (28” most scanners) aperture.
Most __ can be angled up to ± 15 to ±30 degrees to permit positioning for partial coronal images.
Gantry
The gantry houses the following:
_
_
_
_
_
_
_
X-ray tube
Detector array
High-voltage generator
Cooling system
Patient support couch
Mechanical support
Data acquisition system (DAS)
The source of x-rays
The power capacity must be high
Most CT tubes uses a rotating anode.
The anode heating capacity must be expressed in MHU (8 MHU).
2.1 MHU - most CT scanner
4-5 MHU – advanced CT scanner
The minimum anode heat capacity of a CT x-ray tube is 500,000 HU.
High-speed rotors are used for best heat dissipation.
__ failure is the principal cause of CT scanner malfunction and principal limitation on sequential scanning frequency.
CT scan uses high spatial resolution incorporating __ withbsmaller focal spot.
Xray tubes
Power capacity of the generator is listed in kilowatts, determines the range of exposure techniques
Ct generators produces high kV (generally 120-140 kV) to increase the intensity of beam which increase the penerability of xray beam thereby reduce px dose
Higher kV setting will help reduce the heat load on xray tube allowing a lower mA setting
Reduce heat load on xray tube will extend life of tube
All CT scanners operate on three-phase or high frequency power to accommodate the higher rotor speeds of the x-ray tube and power surge.
Most generators are built within the rotating wheel of the gantry to conserve space and winding and unwinding of power cable is unnecessary.
CT x-ray techniques use high x-ray tube voltages, tube currents of hundreds of milliamperes, and scan times between 0.5 and 2 seconds.
High-frequency power supplies are used in CT to provide a very stable tube current and voltage.
High voltage generator
Cooling mechanisms are included in the gantry.
They can take different forms, such as blowers, filters, or devices that perform oil-to-air heat exchange.
Cooling mechanisms are important because many imaging components can be affected by temperature fluctuation.
Cooling system
Measures the amount of radiation transmitted through the body and then converts the measurement into electrical signal proportional to the radiation intensity.
CT detectors should have a high x-ray detection efficiency, a fast response, and should operate over a wide dynamic range.
Detector size used in CT is measured in millimeters.
44 inches or 110 cm SID
Detector assembly
How well the detectors receive photons from the patient.
Determined by the detector size and the distance between detectors.
Capture efficiency
How well the detectors convert incoming x-ray photons.
The number of photons that have been absorbed.
Determined by the detector material.
Size and thickness of the detector.
Absorption efficiency
How well the detectors convert absorbed photon information to digital signal for the computer.
Conversion efficiency
The area of the detectors that is sensitive to radiation as the fraction of total exposed area.
Geometric efficiency
Controlled by how often the detectors must be calibrated to meet quality control standards.
Stability
The speed with which the detector can react to recognize an incoming photon and recover for the next input.
Response time
The ratio of the largest signal that can be measured to the smallest.
Typical modern scanners are capable of 1,000,000 – 1 dynamic range.
Dynamic range
Notes The product of those important efficiencies is the __ of the detector
Generally lies between?
Value more than 1 indicates a? Results in a required __ if imaged quality to be maintained
The term __ sometimes has been used to indicate the overall efficiency
Overall efficiency
0.45 and 0.85
Non-ideal detector system, increased px dose
Dose efficiency
They absorb nearly 100% of the photons that reach them.
50 % detector efficiency
The most common material used in solid-state detectors is cadmium
tungstate (CdW04), which is an efficient x-ray detector.
Only solid-state detectors are used for
fourth-generation scanners
Absorption efficiency is the chief advantage of solid-state detectors.
Solid-state detectors are more sensitive to fluctuation in temperature and moisture than the gas variety.
Executes after glow
Scintillation detector
Materials used in scintillation detector
__ (Nal)- Used in earliest scanners, has disadvantage of having phosphorescent afterglow or lag which makes it useless for rapid sequential exposure.
__ (BGO)
__ (Csl)
__ (CdWO4)- used by most modern scanners (current)
__- current ceramic crystal of choice
Sodium iodide
Bismuth germanate
Cesium iodide
Cadmium tungstate
Gadolinium/yttrium ceramics
Absorbs approximately 60% to 87% of the photons that reach them.
45% detector efficiency
Xenon gas is used because of its ability to remain stable under pressure.
Less expensive to produce, somewhat easier to calibrate, and are highly stable.
A __ channel consists of three tungsten plates.
A disadvantage of xenon gas is that it must be kept under pressure in an aluminum casing.
This casing filters the x-ray beam to a certain extent:
1. Loss of x-ray photons in the casing window.
2. Space taken up by the plates are the major factors hampering detector efficiency.
3. __ are usually relatively deep and maintained at a high pressure to increase x-ray detection efficiency, and are used in third-generation scanners.
Gas-filled detectors
How does gas-filled detectors work?
They measure ionization in air by attracting electrons to an electrode the ions will be created by x-rays photons in air
Materials used in gas-filled detectors
Xenon
Xenon krypton
Types of gas-filled detectors
- for gamma, beta and xray
- for alpha beta and xray
Ion chamber
Proportional counter
Gm counter
Survey meter or cutie pie
High photon absorption
Sensitive to temperature and moisture
Solid material (crystal)
Have afterglow
No front window loss
Solid state detector
Moderate photon absorption
High stable (advantage)
Low density material (air)
No afterglow
Have front window loss and spaces taken up by plates
Gas-filled detectors
Limits the area of the patient that intercepts the useful beam.
Determines the dose profile and patient dose.
Mounted with the tube or adjacent to the tube.
Pre px collimator
Restricts the x-ray beam viewed by the detector.
Reduces the scatter radiation incident on the detector array and therefore improves image contrast.
Determines sensitivity profile and slice thickness.
Does not affect patient dose.
Pre detector or post px collimator
Must be constructed of material with low atomic number such as carbon
fiber.
Should be smooth and power driven to allow for precise patient
positioning.
Should be capable of automatic indexing.
Maximum patient limit of 300-600 lbs.
Exceeding the patient limit can cause:
1. Inaccurate indexing
2. Damage to the table motor
3. Breakage of the table top
Px couch
Data acquisition system. This controls the following:
Tube and detector collimation or pixel size
Matrix size
Gantry angle
Tabletop entrance into the gantry aperture
Section increment movements of tabletop
X-ray tube voltage and amperage
Scan speed and direction of detector signal
The unique subsystem of the CT scanner.
Uses binary system
Provides the link between the CT technologist and other components of
the imaging system.
Bits = 1 byte = 8 bits
= 2 byte = 16 bits
A total of 256 shades of gray can be coded for by 1 byte or 8 bits
A total of 4096 shades of gray can be coded for by 12 bits
Storage requirement: kilobytes, megabytes, gigabytes or terabytes
Computer
Often displayed as the first image of an examination.
It allows technologist to prescribe the location of cross-sectional slices
Various names depending on the manufacturer: Scout, Surview,
Topogram, Localizer Scan and Pilot.
Scanogram
Used to outline the area of interest
Cursor
Available to reverse the image from left to right and or reverse the density display
Reverse display
Designed to suppress the information from surgical clips. Clips can be outlined and deleted from the reconstructed data
Suppression
Added to images for descriptions or anatomical labeling
Annotations
A bar graph which may provide usedul diagnostic data in some instances
Histograms
Image can be rotated, tumbled, tilted to demonstrate structures that would otherwise be hidden.
3D imaging or reconstruction
Point from which the CT technologist control the scanner
Many CT scans are equipped with 2-3 consoles.
Console to operate the scanner
Console to process the image for filming.
Console for the physician to view the image and manipulate its contrast,
size and general appearance.
The console controls the technique factors: 120 kVp above, 100 mA
(usual tube current) and 1-5 seconds (scanning time).
The slice thickness can also be adjusted in the console panel.
Normal thickness are 1-10 mm (0.5 mm for high resolution)
Operating console
Current CT scanners store images on either magnetic tapes or disc.
Each tape generally accommodates 150 scans which is equivalent to 5-
10 patients.
CT scans are usually recorded on film (14 x 17) in a laser camera
Image storage
Commonly used filming device in CT
__- once wascthe standard imaging device
__- preferred current device for imaging
Matrix camera
Laser printer
CT image characteristics:
In CT-scan the x-rays form a stored electronic image that is displayed as
a __.
CT scan format are consist of many cells, each assigned a number and displayed as an __ on the video monitor.
The original EMI format is consisted of __ for a total of __ individual cells of information.
Current CT scan uses a __ matrix that results in __ cells of information.
matrix intensities.
optical density or brightness level
80x80 matrix, 6,400
512x512, 262,144
Picture element, two dimensional
Pixel
Volume element, three dimensional
Voxel
The diameter of image reconstruction is called the?
FOV and Pixel size is?
Matrix size and Pixel size is?
The tissue volume is known as a?
The voxel is determined by __ the square of the pixel size and by the thickness of the CT image slice.
Field of View (FOV).
directly proportional.
inversely proportional.
voxel (voxel element).
multiplying
In CT pixel size is displayed on the monitor as a level of brightness.
CT number of any given pixel is related to the x-ray attenuation coefficient
of the tissue contained in the voxel.
Determined by the average energy of the x-ray beam.
Effective atomic number of the absorber
CT numbers
