Computed Tomography Flashcards
Can we take tomograms in general radiography?
Yes
What is the relationship between the angle used in conventional linear tomography, and the slice thickness?
Conventional linear tomography is inversely proportional to slice thickness
To see another area of the body in conventional tomography, what do we have to do?
Change the falcrum (either the height of the tube or the patient)
With a smaller angle used, will that produce a thinner, or a thicker slice?
Thicker slice
With a larger angle, would you have a thinner or thicker slice?
A thinner slice
Where is the pivot point/fulcrum in conventional tomography?
It is a point on the body
What is the focal/object plane?
The area that we will see in best resolution
What is the exposure angle?
The angles when the tube actually starts and stops exposing
Which is larger, the tomographic angle or the exposure angle?
The tomographic angle
What does axial tomography refer to?
Conventional tomography
Which way is the tube positioned in conventional tomorgraphy?
Parallel to the long axis of the body
What type of images can you aquire in conventional tomography?
Saggital or coronal imags
In what position would the patient have to be placed ot acquire sagittal images in conventional tomography?
In a lateral position
Which way is the tube directed in computed tomography?
Perpendicular to the long axis of the body
What images can computed tomography acquire?
Trans axial or transverse images
Who developed Computed tomography?
Developed by Gondfrey Housfield
In which way does first generation CT scanners acquire images?
Images aquired in axial slices
What type of beam is used in first gen CT scanner?
Narrow beam
What is the slice thickness determined by in first generation CT scanners?
Determined by beam width
Describe the process of how an image is obtained in first generation scanners:
- The tube at detector scan across the subject to create a slice (Translation)
- The x ray tube and detector rotate 1 degree and scan again in the opposite direction (Rotation)
Where are the transmission measurements collected in first generation CT scanners? What is this set of measurements called?
-Transmission measurements are collected at the detector at many locations as is more across the patient
-This set of measurements is called a view
How many rays per view is each translation measured?
160 rays per view
How many views are collected and over how many degrees?
180 views aer collected over 180 degrees
What creates translate-rotate motion?
Linear translation+incremental rotation
How long does it take to create a single image in first generation CT scanners?
5-6 minutes
What element makes up the detector in first generation CT scanners?
NaI (sodium iodide)
What is one modification that was made to first generation scanners to reduce time?
-Two adjacent detectors and a 26 mm wide x ray beam (in the slice thickness directon)
-This allowed for simultaneously collected data for 2 slices
What type of beam is used in 2nd generation scanners?
Fan beam
T/F
The 2nd generation scanners have a detector array of 5-30 detector cells
True
T/F
The second generation scanners use a translate and rotate method at 50-100 degree increments
The second generation scanners use a translate and rotate method at 5-10 degree increments
What are the pros of 2nd generation scanners?
-Increase in speed and imaging time
-Are able to image the rest of the body as opposed to first gen
-Scans could be performed within a breath hold for most patients (reduced motion artifacts)
What are the cons of 2nd generation CT scanners?
- Heavy x ray tube and associated electronics needed to be moved without causing significant vibration
- Scan times of 20 seconds or longer
- Expensive
What type of beam is used in 3rd generation scanners?
Fan beam covering entire patient width
What type of acquesition is used in 3rd generation scanners?
Rotate and rotate acquesition
What type of detector array is used in 3rd generation scanners?
Curvilinear detector array
What are the major disadvantages of 3rd generation CT scanners?
- Lenth of time needed to acquire image data
- Ring artifact
T/F
2nd generation scanners have pre-patient collimators
False; 3rd generation scanners have prepatient collimators
What are the types of rings that can appear in 3rd generation CT images?
- Complete
- Partial
What does the ring artifact appear as in an image?
Appears as bright or dark circular bands
What is the ring artifact a result of?
A faulty detector
Describe ring artifact:
If one detector is not working, as it goes around the patient, it creates a ring of either dark or bright lines due to a lack of signal in that area
What is used to reduce scatter incident on the detector array?
Pre-detector grids reduce scatter
Does CT imaging use sheilding?
No
What is the downside of computed axial tomography?
It is not great for imaging the chest
T/F
In CAT scans, the images are obtained in uniform slices.
True
What is the type of acquesition does CAT use to acquire images?
Step and shoot method
Describe the process of how an image is obtained in CAT scan imaging:
-The CT table/couch moves and remains stationary while the x ray tube rotates within the gantry
-Obtains image through rows of detectors
T/F
With 5th generation CT scanners, there is interscan delay as the table moves to the next location.
False; With CAT scans this occurs
Why do 3rd generation MDCT scanners have improved temporal resolution?
Since the multi slice/row detector systems increase volume coverage
How are scanners refered to by?
By the number of detector rows
Why is slip ring technology needed?
-Since after each 360 degree rotation, cables connecting rotating components required that rotation stop, and reverse the direction (respolling)
-During this time, there is no x ray exposure
What is the result of scanners with no slip ring technology?
-Interscan delays
-Long proceedure times
-Increased patient motion
-Poor temporal resolution
What are slip rings?
They are a set of parallel, stationary ciruclar, electrically conductive rings in contact with multiple electrically conductive brushes or blocks
How do rotating components of CT scanners recieve electricity?
The generator supplies voltage to the ring, which transfers it to rotating components via brushes that glide in the contract grooves of the ring
What are the functions of slip ring technology?
- Provide the electrical power to operate tube and detector
- Provide scanning instructions to the gantry components
- Transfer detector signal into image reconstruction computer
What are the advantages of slip ring technology?
- Facilitates continuous rotation of the x-ray tube
- No interscan delay
- Less motion artifacts, quicker scans and improved temporal resolution
Describe how helical scanning works in computed tomography:
- As the tube rotates continuously, the patient is translated continuously throught the gantry opening
- This results in CT scanning in a spiral/helical geometry (slices at a slight tilt)
What axis are the beginning and end slices located at?
The same z axis at different points
In helical scanning, how do we get rid of the slant and blur in the image?
The computer uses interpolation to take out slant and blur
T/F
Third gen CT scanners use filtration of the x ray beam
True
What is the function of the filter in 3rd generation CT scanners?
- To make the image roughly uniform by compensating for the attenuation of the lateral edges of the beam
- To remove long wavelenth x rays as they only contribute to dose
What would occur if filtration was not present in 3rd generation CT scanners?
-Increase in dose
-The computer would give incorrect information about attenuation
T/F
Fourth generation scanners are routinely used clinically
False; they are not
How are images obtained in 4rth generation scanners?
By using a rotating x-ray tube placed within a stationary circular detector array
What are the 3 major steps of CT image formation?
- Data acquesition
- Image reconstruction
- Image display
Define data acquisition:
The collection of x ray transmission measurements from the patient
Define data aquesition system (DAS) with respect to CT scanners:
This refers to the detectors electronics positioned between the detector array and the computer
Once the image is obtained, how is the image reconstructed?
- the computer performs the image prospective reconstruction process using raw data and converts into numerical form
- Numerical data is converted into electrical signals for the technologist to view on a monitor
Where is the digitized raw data stored?
Stored in the RAM (random acess memory) of the computer attached to the CT sytstem
T/F
The storage capcity witin the CT computers is unlimited.
False; The storage capcity witin the CT computers is limited
Define efficiency in relation to CT detectors:
The ability to capture, absorb and convert x ray photons to electrical signals
What is capture efficiency in relation to CT detectors?
The ability to capture photons transmitted from the patient
What is absorption efficiency with respect to CT detectors?
The number of photons absorbed by the detector
What is conversion efficiency with respect to CT detectors?
How well the detector converts the incoming x ray signal to a digital signal
What are the determinants of capture efficiency in CT detectors?
- The size of the detector area facing the beam
- The distance between two detectors
What does absorption efficiency of CT detectors depend on?
- Atomic number
- Physical density
- Size
- Thickness of detector face
How does better AE affect SNR and patient dose?
- Increases SNR
- We can use less of a technique
What is response time with respect to CT detectors?
The spee with which the detector can detect an x ray event and recover to detect another event
Why is it important that we have a low response time in CT detectors?
-To avoid afterglow and detector pile-up
How long should the afterglow typically be in CT detectors?
0.01%-100ms after the radiation has been terminated
What is the dynamic range for msost CT scanners?
1 million to 1
What is total detector efficiency?
The product of capture efficiency, absorption efficiency and conversion efficiency
What is the scintilator crystal in detectors bonded to in CT scanners?
They are bonded to photodiodes
What 2 materials can photodiodes be made of in CT scanners?
- Cadminium tungstate and ceramic material made of high purity rare earth oxides
OR - Gadolinium oxysulfide with ultrafast ceramic
What does the photodiode convert light to in the PMT?
It converts light into electrons
T/F
CT scanners use a PMT
True
What are the 2 catagories of multirow CT detectros?
- Matrix array detectors
- Adaptive array detectors
What determines the number of slices and the thickness of each slice in CT scanners?
Determined by detector configuration
Which type of detector has better SR; matrix array detectors or adaptive array detectors? Why?
Adaptive array detectors because each slice is smaller
What type of multi-row detector has better signal; matrix array detectors or adaptive array detectors? Why?
Matrix array detector; because the detector elements are bigger
What is the design of adaptive array detectors?
They are anisotropic (the cells are not equal, they have different sizes)
What is the design of matrix array detectors?
They are isotropic (All cells are equal in all dimensions)
T/F
CT scanners that are used currently use a detector, and an x-ray source that rotates around the patient.
True
Where is the rotating detector and x ray tube located within current CT scanners?
They are located within the gantry
What is the average size of the gantry?
70-90 cm
What type of force has the frame of the gantry beeen designed to withstand?
The frame has been designed to withstand centrifugal forces
What is the maximum tilt that a CT gantry can be angled?
Gantry angulation tilt may be up to +/- 30 degrees
Where are the operator controls for the gantry located in a CT room?
1.On the gantry itself
2. In the control room
How does the cooling system work with CT scanners?
- Draws in air from the room through filters to cool electrical components
- Warm air is expelled through filters at the top gantry
What movements does the CT table/couch allow?
- Vertical movement
- Horizontal movement
- Indexing after every x-ray exposure or continiously at a steady pre-set rate
What is the weight limit of the CT table/couch
450 lbs
T/F
CT tables/couches must be made of a high atomic number material
False; must be made of a low atomic number material
What is the orgin in CT scanners? What is another name for the orgin?
-The orgin is the point at which all 3 axes intersect
-Also called the isocenter
What makes up the y-axis?
The patient’s body anterior to posterior
What makes up the x-axis?
The width of the patient
What makes up the Z axis?
The patients thickness from head to toe
What determines the image reconstruction time in CT scanners?
- Microprocessors
- Primary memory
What part of the computer is dedicated to pre-processing and image reconstruction?
The RAM (random acess memory)
What are the cartesian coordinates for 1 and 2? What type of slice is this?
- Y
- Z
-This is a sagittal slice
What are the cartesian coordinates for 3 and 4? What type of slice is this?
- X
- Z
-This is a coronal slice
What is slice thickness?
-Z axis
-It is the x ray beam thickness in SDCT
What does SDCT stand for?
Spectral detector computed tomography
Where should our isocenter be when positioning a patient that is either supine or prone?
Halfway at the midsaggital/midcoronal plane
How many pixels make up the matrix of a CT image?
512 x 512 pixels
What is a voxel?
It is the volume element of a matrix that results in a cube
What does each cube of a voxel represent?
Each cube represents a different slice of the image
What is the SI unit for voxel size?
mm^3
What determines the pixel size?
The X and Y coordinates
What happens if FOV is increased for a fixed matrix size?
Decreased SR
What is another name for a CT scout?
-Scantogram/topogram-scout
T/F
Scouts have a higher dose than normal x-ray proceedures.
True
How many images are taken in a CT scout?
2 images; AP and PA
When is a scout taken and what is the purpose of acquiring these images?
-These images are taken prior to the CT acquisition
-These images allows for the operator to plan the extend of the scan and determine the FOV
T/F
During a scout, the detector and the x-ray tube are moving.
False; they are stationary
What determines the attenuation of the x ray beam along a given ray path? What is this known as?
-The sum of the linear attenuation coefficient of tissues
-This is called a line integral
What determines how much attenuation occurs in each voxel?
Image reconstruction
What was the mathematical method that Hounsfield developed for the first CT scanner called?
Algebraic reconstruction technique
What algorithm does the algebraic reconstruction technique belong to?
It belongs to a class of algorithms referred to as iterative reconstruction algorithms
What algorithm is used now days for image reconstruction?
Analytical ersconstruction algorithms
1. Filtered back projection algorithms
2. Interpolation algorithms
What 4 angles are voxels collected at?
- 0 degrees
- 90 degrees
- 45 degres
- 135 degrees
What 2 factors does iterative reconstruction compare?
- Assumed vales
- Measured values
What method is ART an example of? Why?
ART is an example of iterative reconstruction method because it involves the repitition of a mathematical or comutational process
What are the disadvantages of the interative reconstruction method?
- Takes longer to calculate
- Need to have a lot of computer power
What are the advantages of the interative reconstruction method?
- Reduced image nose
- Allowance to use lower techniques (lower patient dose)
What is the standard method of reconstructing CT slices?
Filtered-Back projection
TF
The smearing back projection takes place across the image at the angle that it was acquired.
True
How is the blurring inherent in the simple back projection supressed?
Through using filtering techniques (i.e. sharpening filter)
What is another way to refer to a convolution filter?
-Kernel
T/F
FBP is also reffered to as a convolution method.
True
What does a smoothining filter do to SR and CR?
SR decreases, CR increases
What does a sharpening filter do to SR and CR?
SR increases, CR decreases
What is being shown here?
Ring artifact
What does the graph represent?
The integrated attenuation profile
T/F
Sharpening filters increase noise.
True
For what type of anatomy would we use a sharpening filter?
When imaging lungs or bones and looking for small, discrete nodules or fractures
For what type of anatomy would we use a smoothening filter?
For soft tissues such as the brain or abdomen to look for larger features with mild differences in attenuation
Which kernel would you use where the inherent contrast is very high?
A sharpening/noisier filter
Which kernel would you use where the inherent contrast is very low?
Softer kernels
What is the defenition of interpolation?
A mathematical method of estimating the value of an unknown function using the known values on either side of that function
How is interpolation performed?
Through a computer program called an interpolation algorithm that takes 2 known values, creates a line aand then chooses a value somwhere inbetween
What controls interpolation?
Pitch
What is pitch?
The ratio of the patient table movement (during one revolution) to the width of the x-ray beam
If the distance translated by the table equals the slice thickness, what is the pitch ratio?
1:1
(same as saying that the pitch is 1)
What does a pitch of 2:1 indicated?
Thtat the table will move twice the distanceof the slice thickness for each rotation of the gantry
What does SDCT stand for?
Single detector CT scans
In SDCT, what determines the beam width/slice thickness?
The width of the collimator
In SDCT, what is pitch?
The relationship of the table speed to the slice thickness
If the pitch is 2, and slice thickness is 5mm, what distance does the CT table travel during one gantry roation?
The table travels 10 mm every 5mm
For a 4 slice MDCT at a 1.25mm slice thickness and table feed of 6mm per rotation, what is the pitch?
1.2
What does a pitch of greater than 1 indicate when it comes to patient dose and imaging?
Results in extended imaging (missing anatomy) and reduced patient dose
What does a pitch of less than 1 indicate when it comes to patient dose and imaging?
Results in overlatpping images (less missing anatomy) and a hgih patient radiation dose
What is the amount of anatomy imaged in a helical scan with a 20 second acquisition time, a 1 second rotation time, and 2.5 mm slice thickness, 4 slices per rotation, with a pitch of 1.2? What if the rotation is .5 seconds?
- VI=240
- VI=480
What is the x ray attenuation coeffficient of the volume of the tissue (voxel) displayed as?
Displayed as a level of brightness in its pixel based off of the calculated u
What is the CT number of water?
0
What is the CT number of fat?
-30HU
What is the CT number of air?
-1000
What is the CT number of soft tissue?
+50
What is the CT number of bone/metal?
+1000
T/F
The CT number of any given pixel is related to the u of all the tissues contained in that voxel
True
If a tissue attenuates less than water, will it have a negative or a positive CT number?
Negative CT number
T/F
If a slice is thick, there is less tissue within the voxel.
False; If a slice is thin, there is less tissue within the voxel.
What is beam hardening due to?
The polychromatic beam
What is beam hardening?
The gradual increase in effective energy of the polychromatic beam as they penetaate deeper into attenuating tissue
What is cupping due to?
It is due to lower u values being reconstructed for deeper voxels, where the material appears darker in the center of the image
What is being shown here?
Cupping artifact
What is streaking due to?
High atomic number artifacts within the image
What is CT simulation?
The use fo the CT scanner in radiation treatment planning
What are the 3 main steps in CT simulation?
- Scan the patient in the CT scanner
- Virtual simulation
- Treatment setup
What occurs during the step 1 of CT simulation?
-Patient positioned and scanned in the same position that would be on the treatment machine
-Image dataset computes dose distributions
What occurs in stage 2 of CT simulation?
-Virtual simulation software takes the patient’s CT dataset and provides a virtual representation of that dataset in the radiation treatment machine
This software is responsible for the contouring of the target and normal structures, placement of the treatment isocenter and the beams. What software is this?
Virtual simulation software
T/F
Virtual simulation takes place within the time that the patient is present in the CT room.
False; takes place after the patient has already been scanned and left the room
What occurs during the third stage of CT simulation?
The CT simulation software results are used to set up the patient in the treatment machine.
What is quantitative CT (QCT)?
The most sensitive of all x-ray techniques for the measurement of the mineral content of trabecular bone in osteoperosis.
Pic for understanding
What are the pros of temporally sequential scans? (through entire volume)
Can be done on any CT scanner
What are the cons of temporally sequential scans (through entire volume)?
Risk of patient motion
What are the pros of temporally sequential scans (through each tube rotation)?
-Reduction in interscan delay between the low and high energy aqueisitions
-Can be done on any scanner
What are the cons of temporally sequential scans (through each tube rotation)?
-Suseptibility to motion misregistration between the low and high energy scans may limit usefulness for imaging vascular anatomy
What reconstruction method has the best temporal resolution?
Partial scan reconstructions
What is temporally sequential scans through each tube rotation?
One tube rotation is performed at each tube potential prior to the table incrementation to minimize the time delay between scans
What is temporally sequential scans through the entire volume?
Two temporally sequential scans performed to aquire data at 2 different tube potentials
T/F
Materials having different elemental compoistions can be represented by the same CT numbers
True
(this makes classification of diff tissues difficult)
What are the different methods for acquiring dual energy data?:
- Sequential acquisition of 2 different scans
- Ra[id tube potential sqitching
- Multilayer detectors
- Dual x-ray sources
What are the pros of rapid kVp switching?
Near simultaneous data acquistion of low and high energy data sets
What are the pros of multilayer detectors?
Low and high energy data sets are acquired simultaneously
What are the cons to multilayer detectors?
To achieve noise in the low and high energy images, differnet detector thicknesses are used to keep the signal, and SR decreases
What are the pros to dual energy sources?
-Tube current (and noise) can be optimized for each tube potential independently
-Filtration can be independently optimized for each tube-detector pair
What are the cons to dual x-ray sources?
-90 degree phase shift between low and high energy data
-Scatter resulting from one tube may be detected by the detector of the other tube because both tubes are simultaneously energized
What transition time is requrired for rapid kVp switching?
Less than a millisecond
What are 3 cons to rapid kVp switching?
- Requires rapidly modulating tube current (mA)
- Lack of rapidly modulating mA would result in increased noise in the low KV data and excessive radiation dose levels from the high energy projections
- Difficult to optimize filtration
(read the rest of the cons)
How do multilayer detectors work?
-One single high tube potential is used
-Scintillation detectors are layered, which causes the low E data to obe collected by the front/innermost layer
-High E data is collected by the back/outermost layer
How do dual x-ray sources work?
-2 x ray sources and 2 detectors are mounted on the same gantry positioned 90 degrees from one another
T/F
For dual x-ray sources, each x ray source is equipped with its own high V generator. Why or why not?
True; so that you can have independant control of kV, mA and be able to adujst the noise
What is another way of saying that two objects are placed apart at 90 degrees?
Orthogonally positioned
What are the 2 types of images that can be acquired from DECT?
- Virtual monoenergetic images
- Material decomposition images
What are material decomposition images?
Mapping or removing substances of known attenuation characteristics, such as iodine, calcium & uric acid
What is being shown here?
A portable CT machine
Virtual non-contrast images (iodine removed, iodine concentration (iodine maps) calcium suppression (calcium removed) and uric acid suppression are all part of what type of CT imaging?
DECT
