P1 Flashcards
Is the process of creating a cross-sectional
tomographic plane of any part of the body.
CT-SCAN
The term “tomography” is derived from the Greek word “____”, meaning “____’’.
tomos, section
- refers to the old scanners that is only capable of displaying Axial Scans
CAT scan
- refers to the ability to define small objects.
Spatial Resolution
- refers to the ability of the system to differentiate small objects with
similar densities.
Low Contrast Resolution
- refers to the speed on which the data can be acquired. Increase in temporal resolution can decrease/eliminate artifacts
Temporal Resolution
The mathematical principles of CT were first developed by ___ in ____.
Radon, 1917
CT-scan was first demonstrated by ___ on ___
Godfrey Houndsfield, 1970
developed the mathematics used to reconstruct images
Allan Cormak
Allan Cormak (developed the mathematics
used to reconstruct images) and Godfrey
Houndsfield received the nobel prize award
on ____
1982
•Beam: Pencil-like x-ray beam
•EMI labs
•Detectors: One
•Tube-detector movements: Translate-rotate
•Number of slices per rotation: One
•Scan duration: 5 minutes
1ST GENERATION CT scanners
▪ Were also translate and rotate type.
▪ Fan beam
▪ Mutiple detector array
▪ 30 seconds imaging time
▪ Uses the bow tie filter to equalize the radiation intensity reaching the
detector
2nd Generation CT-scan
a CT imaging system consists of a finely collimated x-ray beam and a single detector, both of which move synchronously in a translate and rotate fashion. Each sweep of the source detector assembly results in a projection, which represents the attenuation pattern of the patient profile
1ST GENERATION CT scanners
operated in the translate and rotate mode with a multiple detector array intercepting a fan-shaped x-ray beam.
Second-generation computed tomography
imaging systems
▪rotate-only units, third generation imaging systems can now produce an image in less than 100 ms.
▪uses a curvilinear detector array and a fan beam
▪The curvilinear detector array produces a constant source-to-detector path length
▪seconds scanning time.
▪Still widely used today.
3rd Generation CT-scan
operate in the rotate-only mode with a fan x-ray beam and a multiple detector array revolving concentrically around the patient
Third-generation computed tomography imaging systems
▪Incorporates a rotate and stationary configuration
▪Radiation detection is accomplished through a fixed circular array of detectors
▪May contain as many as 4000 individual detectors.
▪Fan shape beam
▪Free of ring artifact
4th Generation CT-scan
operate with a rotating x-ray source and stationary detectors
Fourth-generation computed tomography imaging systems
▪Also known as Electron-Beam CT (EBCT)
▪Was pioneered for cardiac imaging
▪No moving parts in the gantry
5th Generation CT-scan
▪ Also known as the “Helical CT scanner”
• Data collection: Data is collected without pausing
• Speed: Helical CT scanners are faster than other CT scans
• Cost: These scanners are less expensive to produce and purchase
6th Generation CT-scan
▪ Multi-spiral CT makes use of multiple detector arrays to make use of a wider portion of the x-ray fan beam d
▪ Section thickness in multi-slice CT is determined by detector width rather than by collimation thickness, using data
acquisition.
▪ This technology makes better use of the x-ray tube output and decreases heat loading for a given coverage range.
▪ Was introduced in 1998
7th Generation CT-scan
CT Scan Modes
Step And Shoot
HELICAL (SPIRAL SCANNING)
Multi Detector Row scanning
-the x-ray tube rotated 360° around the patient to acquire data for a single slice
-the motion of the x-ray tube was halted while the patient was advanced on the CT table to the location appropriate to collect
data for the next slice
Step And Shoot
▪ When the examination begins, the x-ray tube rotates continuously. While the x-ray tube is rotating, the couch moves the patient through the plane of the rotating x-ray beam. Continuous data acquisition.
HELICAL (SPIRAL SCANNING)
▪ This technology was expanded on in 1992 when scanners were introduced that contained two rows of detectors, capturing data for two slices per gantry rotation. Multiple slices per rotation
Multi detector row scaning
- refers to the thickness of a cross sectional slice
Z axis
- Rows and colums of pixels that makes up the whole image.
-512 x 512 is the most common
Matrix
- the degree to which a beam’s energy is reduced.
Beam Attenuation
- the amount of xray photons absorbed/scattered per unit thickness
Linear Attenuation Coefficient
quantifies the degree that a structure
attenuates an x-ray beam
Density
CT numbers or Density Value
Houndsfield Units
- is an artifact that occurs when multiple tissues with different densities occupy the same voxel
Volume Averaging
-composed of low and high intensity beams
-causes artifacts due to scattered radiation
-filtering with a substance such as Teflon or
aluminum eliminates the photons with weaker energies, thus reducing patient dose and improves image quality
Polychromatic X-rays
-all the thousands of bits of data acquired with each scan.
-These are the data that is waiting to be
processed in order to create the “Image Data”
RAW data
-the process of converting Raw data to Image Data
Reconstruction
-processed Raw Data
Image Data
-then ring shape part of the CT-scan that houses components necessary to produce and detect Xrays.
The Gantry
-permits the gantry frame to rotate continuously making helical scanning possible
Slip Rings
-located within the gantry
-prevents fluctuating temperatures within the gantry.
Cooling System
-CT-tubes made of tungsten anode, has a higher intensity beam. Higher heat capacity.
Xray Source
-improves image quality and reduces patient dose
Filtration
-restricts the x-ray beam to a specific area
-decreases scattered radiation, improves image quality and reduces patient dose
Collimator
2 types of collimators
Source Collimator
Pre detector Collimator
-affects the patients dose and determines how the dose is distributed across the slice thickness.
Source Collimator
-ensures the beam coming from the patient is the exact width and prevents scatter radiation reading.
Pre Detector Collimator
-collects the information regarding the degree to which each anatomic structure is attenuated by the beam
Detectors
-absorption rate of 60% to 87%
-less expensive
-still being manufactured but has become
obsolete
-due to its design prevents them for MDCT
Xenon Gas Detectors
-uses crystals that fluoreces when struck with and xray photon
Solid State Crystal Detectors/Scintillation Detectors
- transforms light to energy
Photo iodide
-measures the number of photons that strikes the detector, converts the analog signal to digital (ADS - Analog to Digital Converter) and then sends it to the computer
DAS (Data Acquisition System)
- is a finite set of unambiguous steps performed in a sequence to solve a problem
- they are a precise set of steps to be performed in a specific to solve a problem..
Algorithm
- a mathematical tool that allows for the efficient reconstruction
Fourier Transform
- Fourier Transform is developed by the 17th century mathematician ____
Baron Jean- Baptiste-Joseph Fourier.
-is a mathematical method of creating missing data
Interpolation
- portion of the computer that can be physically touched.
Hardware
- is the instructions that tell the computer what to do to do it.
Software
- essential component of the CT system. Determines the number of images the CT system can store.
Hard Disk
- interprets computer program instructions and sequences task
- Brain of the CT system.
CPU (central processing unit)
- imprinted in the factory. Used to store frequently used instructions. Required to start up the system.
ROM (read only memory)
- includes instructions that are frequently changed. Data that are used to reconstruct images.
RAM (random access memory)
type of data storage device that reads date sequentially
SAM (sequential access memory)
- a computer storage device that can be written once, but read many timed (CD-R, DVD-r, DVD+R, rewritable disk)
WORM (write once read many)
- includes all measurements obtained from the detector array. Requires larger computer storage space than image data.
RAW DATA (Scan Data)
-are those which result once the computer has processed the raw data. One Houndsfield unit is assigned in every pixel
IMAGE DATA
- compiles the information from all of the attenuation profiles to create an image
Back Projection
- helps reduce the appearance of artifacts.
Filter Functions
- determines the area, within the gantry, from which the raw data are required.
-determines the number of detector cells collecting data.
SFOV (Scan Field of View)
- also called zoom or target. Determines how much of the raw data
is to be used to create and image. Affects the image quality by changing
the pixel size
DFOV (Display Field of View)
-used in mammography
⚬ uses an area x-ray beam to produce
multiple digital images.
⚬ form a three-dimensional data set from
which any anatomical plane can be
reconstructed.
⚬ even better image contrast
Digital Radiographic Tomosynthesis.
-also called axial tomography
-plane of the image is parallel to the long axis of the body
-results in sagittal and coronal images
Conventional tomography
-Trans-axial or transverse image
-perpendicular to the long axis
-Coronal and sagittal images can be reconstructed from the transverse image data set
Computed Tomography
In CT, the most common matrix
size is
512
This speed is particularly important to
reduce or eliminate artifacts that result
from object motion, such as those
commonly seen when imaging the heart
Temporal Resolution
The degree to which an X-ray beam is reduced by an object is referred to as
attenuation
If an x-ray passed into an object and is less attenuated, the shade will be _____.
black
If an x-ray will pass into a structure and there is more attenuation them the shade will be ____.
white
the quantity of matter per unit volume, specified in units of kilograms per cubic meter
Mass density
The amount of x-ray beam that is scattered
or absorbed per unit thickness of the
absorber is expressed by the ______, represented by the Greek letter μ
linear attenuation coefficient
In general, the attenuation coefficient
_____ with increasing photon energy and ____ with increasing atomic number and density
decreases, increases
These units are also referred to as CT numbers or density values
Hounsfield Unit
The purpose of the ____ is to identify if there is something wrong with a particular organ
Hounsfield unit
Using the system of _____, a measurement of an unknown structure that appears on an image is taken and compared with measurements of known structures
Hounsfield units
CT can be broken down into three segments
Data acquisition
Image reconstruction
Image display
It houses many of the components necessary to produce and detect x-rays
Gantry
The range of aperture size of gantry is typically
70 to 90 cm
The degree of tilt varies among systems of gantry but _____ is usual
15 to 30 degree
Current systems use electromechanical devices called
Slip rings
___ use a brushlike apparatus to provide continous electrical power and electronic communication across a rotating surface
Slip rings
They permit the gantry frame to rotate continuously
Slip rings
Generator that are currently used in in CT
High frequency generators
The power capacity of generator is listed in
Kilowatts
_____ produce the xray photons that create the Xray image
X ray tubes
Are used to shape the xray beam
Compensating filters
Used to describe the entire collection of detectors included in a CT system
detector array
The space between the grid
Interspace material
refers to the ability with which the detector obtains photons that have passed through the patient
Capture Efficiency
•refers to the number of photons absorbed by the detector and is dependent on the physical properties of the detector face (e.g., thickness, material).
Absorption Efficiency
•Is the time required for the signal from the detector to return to zero after stimulation of the detector by x-ray radiation so that it is ready to detect another x-ray event
Response Time
•Is the ratio of the maximum signal measured to the minimum signal the detectors can measure.
Dynamic Range
is used because of its ability to remain stable under pressure.
Xenon gas
A xenon detector channel consists of ____
tungsten plates
three tungsten plates
The process of moving the table by a specified measure us most commonly called
incrementation
The degree to which a table can move horizontally is called the
Scannable range
is a method to study waves
of many different sorts and also to solve several kinds of linear differential equations.
Fourier transform
is a mathematical method of
estimating the value of an unknown function using the known value on either side of the function
interpolation
is a mathematical method of
creating missing data
Interpolation
• is the simplest type and is frequently used in mathematics and science
Linear interpolation
• assumes that an unknown
point falls along a straight line between two
known points
Linear interpolation
are ancillary pieces of computer hardware
designed to feed data into the computer or accept processed data from the computer.
Input and output devices
include monitor, laser camera, printer, and archiving equipment such as optical disks or magnetic tape
Output devices
The ___ is the component that interprets
computer program instructions and sequences tasks.
CPU
It contains the microprocessor, the control
unit, and the primary memory
CPU
In the past the CPU design frequently used for CT image reconstruction was the ____ also called as
array processor, vector processor
, this design was able to run mathematical operations on multiple data elements simultaneously
vector processor
refers to the computers internal memory
Primary storage
-is used to store data that are likely to be in active use.
-is typically very fast
Primary memory
is very fast, but is also volatile, losing
the stored data in the case of a power loss
RAM
The opposite of RAM is ____, which stores data that can only be accessed sequentially (like a cassette tape)
serial access memory (SAM)
• The reconstruction that is automatically
produced during scanning
Prospective Reconstruction
• The same raw data may be used later to
generate a new image
Retrospective Reconstruction
- displayed in the monitor
Pixel
- contains value acquired by the
algorithm and it will be averaged to have one value to place it in front
as a HU and this is the pixel
Voxel
The path that the x-ray beam takes from the tube to the detector is referred to as a ___.
ray
: the detector senses each
arriving ray and measures how much
of the beam has been attenuated.
Ray Sum
: a complete set of ray sums
View
: the system accounts for the attenuation
properties of each ray sum and correlates it to the position of the ray.
Attenuation Profile
The process of applying a filter function
to an attenuation profile is called
convolution
⚬ common type of image reconstruction in CT scan
Filtered back-projection
is also called calibration field of view.
Scan Field of view
