Computed Tomography Flashcards
what are the limitations of conventional radiography?
- when the abdomen is imaged in conventional radiography, the image is degraded
- low contrast
- superimposition
What is the fulcrum?
the pivot point
what is the object plane?
focal plane - this is what you will see in detail
what is the tomographic angle?
the angle that the x-ray tube moves during the procedure
- larger the angle, more things will be blurred out on either side of the object, thinner slice
what is the exposure angle?
the angle when the tube is energized
what is conventional tomography?
the X-ray tube moves across the body angled, to remove some superimposition from the image, while exposing throughout the whole time of movement
what plane does conventional tomography image?
axial tomography
- parallel to the long axis of the body
- sagittal and coronal images
What plane does computed tomography image?
trans axial or transverse
- perpendicular to the long axis of the body
when was the first Ct machine developed and by who?
Godfrey Hounsfield
1971
if the angle of the tomogram is larger is the slice larger or smaller?
smaller
First gen CT scanners
- images acquired in axial slices
- narrow beam
- beam width = slice thinkness
how did the first gen detector work?
- x-ray tube linked to detector
- tube and detector scan across the subject to create a slice
- x-ray tube and detector rotate 1 degrees and scan again in opposite direction
**what type of motion did the first gen CT scanner have?
translate - rotate motion
how long did the first gen CT scanner take?
5 minute imaging time
what was the detector made of In the first gen CT scanner?
NaI scintillation detector
- 2 for 2 slices at a time
How many views were collected per image in the first gen CT scanner?
- collected 180 views over 180 degrees
2nd gen CT scanner beam/detector
- fan beam
- detector array (5-30 detector cells)
- translate-rotate (5-10 degree increments)
pros of the 2nd gen CT scanner?
shorter imaging time
- scans could be preformed within a breath hold - 20-30 seconds
cons of the 2nd gen CT scanner?
- heavier x-ray tube and electronics could cause significant vibration
- scan time still too long
- unequal attenuation
**what is the movement of 2nd gen scanners?
translate-rotate
what is the movement of the 3rd gen scanners?
rotate-rotate
3rd gen scanner
- fan beam covered entire patient width
- curvilinear detector array
- subsection imaging time
3rd gen scanner collimation
- pre-patient collimator
- pre-detector collimator
3rd gen SSCT scanners
- SSCT - single slice per revolution
- major disadvantages - ring artifact
disadvantage of 3rd gen CT scanners
- if there is a faulty detector, the acquired consistent erroneous signal, or lack thereof results in a ring on the reconstructed image
ring artifact?
appears as bright or dark circular bands
- can be complete or partial
regular calibration required
Computed axial tomography (CAT)
- Ct table/couch moves and remains stationary while the x-ray tube rotates within the gantry
- early systems had a single row of detectors, obtained data for one slice with rotation
- interscan delay was table moves to the next location
How is conventional tomography performed? How do you change slice location and thickness?
- Conventional tomography is performed by equipment taking multiple exposures while moving through a tomographic angle
- You can change the slice location by altering the pivot point (fulcrum)
- You can change the thickness by altering the tomographic angle, the larger the angle the smaller the slice
Compare the different generations of CT units?
1st gen - axial slices, narrow beam - translate rotate motion - 5 min imaging time
2nd gen - fan beam, translate rotate, 5-30 detector cells - 20-30 sec imaging time
3rd gen - rotate-rotate, fan beam covers entire patient width, sub second imaging time
Step and shoot scanning (axial scanning)
axial slices lie parallel to one another, the slice beginning matches exactly the slice end
3rd gen MDCT scanners
- Multi-slice/row detector systems increase coverage
- Improved temporal resolution
- Scanners often referred to by number of detector rows (Ex. 64-slice scanner, 256-slice scanner)
why is there a need for slip ring technology?
after each 360 rotation, cables need to respool
what issues does slip ring technology resolve?
- increased interscan delays
- long procedure times
- poor temporal resolution
- increased patient motion
What is slip ring technology?
- set of parallel, stationary, circular, electrically conductive rings in contact with electrically conductive brushes or blocks
- generator supplies voltage to the ring, which transfer it to rotating components via brushes that glide in the contact 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
What is helical scanning with slip ring technology?
- as the tube rotates continuously, the patient is translated through the gantry opening
- scanning in spiral/helical geometry
what is helical scanning?
- slices beginning and end at different points on the same z axis
- creates slices that are at a slight tilt
- interpolation takes the slant and blur out of the helical image
4th gen scanners?
- rotating x-ray tube placed within a stationary circular detector array
- not clinically used
how does CT works?
- images of the human body reconstructed by using many projections from different locations
- radiation passes through each cross section in a specific way
the formation of CT images by a CT scanner involves what 3 major steps?
- data acquisition
- image reconstruction
- image display
what is data acquisition?
- x-ray transmission measurements collected from the patient
- special electronic detectors measure the attenuation values - data converted into digital (binary) for input into the computer
What is the data acquisition system?
- electronics positioned between detector array and computer
- measures the transmitted radiation beam, encodes this into binary data and transmits these data to the computer
what is raw data?
- digitized raw data is stored in the RAM of the computer attached to the CT system - storage is limited
- raw data undergoes preprocessing - which allows for certain corrections to be made to the data
- raw data is then used by the computer for image reconstruction
what is pitch?
relates to the speed at which the patient is moving through the detector
what is image reconstruction?
- the computer then preforms the image (prospective) reconstruction process using this raw data
- the reconstructed image is in numerical form and must be converted into electrical signals for viewing
what is viewing?
- images and related data (dose reports, technical parameters used) are then sent to PACS
- images can also be stored onto optical discs
slide 45
understand where things are in the cycle
CT detector characteristics
- efficiency refers to the ability to capture, absorb and convert x-ray photons to electrical signals
- CT detectors must process high capture efficiency, absorption efficiency and conversion efficiency
what is capture efficiency?
- ability to capture photons transmitted from the patient
- the size of the detector area facing the beam and distance determines capture efficiency
What is absorption efficiency?
- number of photons absorbed by th detector
- depends on the atomic number, physical density, size and thickness of the detector face
How does AE affect SNR and patient dose?
higher AE the higher the SNR and lower patient dose
what is prospective
set up the slices, etc on the computer before hand
what is reconstruction
redo the slice thicknesses
What is conversion efficiency?
- how well the detector converts the incoming x-ray signal to a digital signal
what is response time (resolving time)?
- recovery time between detecting x-ray events
- should be very short to avoid problems such as afterglow and detector “pile up”
what is afterglow?
-persistence of the image after the radiation is turned off
- CT detectors should have low afterglow values (100 ms after termination)
what is dynamic range?
- ratio of the largest to smallest signal that can be measured
- the dynamic range for most CT scanners is about 1 million to 1
What is total detector efficiency?
- product of the capture efficiency, absorption efficiency and conversion efficiency
What is detector “pile-up”?
too many x-rays hitting the detector can’t read them and just misses some that “pile up”
Solid State Scintillation detectors
- individual detector elements are affixed to a circuit board
- crystals are optically bonded to the photodiodes
what are the scintillation materials currently used with photodiodes
- cadmium tungstate and ceramic material made of high-purity, rare earth oxides
- gadolinium oxysulfide with ultrafast ceramic - helps to remove that afterglow
slide 56 diagram
understand scintillation crystal detection and conversion
what are the 2 categories of multi row CT detectors?
- matrix array detectors
- adaptive array detectors
matrix array detectors
- fixed array detectors
- cells have equal dimensions
- isotropic
adaptive array detectors
- cells have unequal dimensions
- anisotropic
what is the CT gantry?
houses the rotating course and detector, generator
aperture is 70-90cm
how does the CT gantry interact with air?
- draws in air through filters
- warm air expelled through filters
- filters must be cleaned on a regular basis
CT gantry controls
- gantry controls located on the gantry and operator console
gantry angulation
- on scanners that allow gentry angulation, the tilt may be up to +/- 30 degrees
CT Table/Couch
- must be low Z material, but strong and rigid
- weight limit is approximately 200-300kg
CT computers
- microprocessors and primary memory determines image reconstruction time
- most RAM is for pre-processing and reconstructing
What is the Z-axis in terms of detector?
thickness/length of slice
What is the isocentre?
point in gantry where all 3 axes intersect
what is a voxel?
X, Y (pixel size) and Z (thickness)
Voxel size = Pixel size * slice thickness
Pixel vs. Voxel?
2D - pixel
3D - voxel, takes into account thickness of each piece
what is the equation for pixel size?
FOV/Matrix size = pixel size
What happens if FOV is increased for a fixed matrix size?
Would increase the pixel size
What if matrix size in increased for a fixed FOV, for example, 512 x 512 to 1024 x 1024?
Pixel size would decrease
What is a CT scout?
- Aka scanogram/Topogram
- low dose x-ray taken prior to CT acquisition
- tube and detector are stationary
- use scout to choose FOV
Image reconstruction formula?
Ni=N0e^-ux
how does image reconstruction work?
- line integrals along all ray paths - sum of the linear attenuation coefficient of tissues
- system determines attenuation in each voxel - displayed as shades of grey
what was the first image reconstruction method?
algebraic reconstruction technique (ART)
- a type of iterative reconstruction algorithm
what algorithm of image reconstruction do most systems use?
analytical reconstruction algorithms
- filtered back-projection
- interpolation
what is iterative reconstruction?
- start with an assumed value, then compare with measured ones
- make corrections until the assumed and measured values are the same or within acceptable limits
Algebraic Reconstruction Technique (ART)
views are collected at 4 angles 0, 45, 90 and 135)
- each measurement = sum of attenuation values along each ray
- process begins by taking measurements of the first view and assuming that these attenuation values occurred uniformly along the rays, yielding the first image estimate
Advantages of iterative reconstruction advantages?
- reduced image noise and artifacts because you are getting true data for every pixel
- lower patient dose
What would be the biggest disadvantage of iterative reconstruction algorithms?
takes longer and takes a lot of resources - need computers capable of processing this data
What is filtered back projection?
- current standard method of reconstruction
- “smearing back” the projection across the image at the angle it was acquired
- by smearing back all the projections, you reconstruct an image
- image is blurry
FBP or Convolution method?
- blur is suppressed mathematically using filtering techniques
- attenuation profile is filtered to counteract the effect of sudden density changes
- convolution filter or a kernel
what does a sharpening filter do?
reduces blurring BUT accentuates noise
what does a smoothing filter do?
decreases noise BUT increases blur
when would you use a sharp kernel?
use noisier/sharper/harder kernels when inherent contrast is very high - high contrast overcomes noise
when would you use a smooth kernel?
use softer/smoother kernels when there is less contrast
what is interpolation?
method of estimating the value of an unknown function using known values on either side
Pitch
- ratio of patient table movement (during one revolution) to the width of the x-ray beam
what is the equation for pitch?
pitch = couch movement each 360/beam width
What does a pitch ratio of 1:1 indicate?
if distance translated by the table equals the slice thickness
what does a pitch ratio of 2:1 indicate?
this indicates that the table will move twice the distance of the slice thickness for each rotation of the gantry
In single detector CT pitch?
- In SDCT, the width of the collimator opening determines beam width = slice thickness
- In SDCT, pitch describes the relationship of the table speed to the slice thickness
What is beam width in SDCT?
slice thickness
What is beam width in MDCT?
slice thickness*number of slices
For a 4 slice MDCT at a 1.25mm slice thickness and table feed of 6mm per rotation, what is the pitch?
Pitch = movement of table for 360/beam width
Pitch = 6/5
Pitch = 1.2
If pitch = 2, and slice thickness = 5mm, what distance does the CT table travel during one gantry rotation?
Pitch*beam thickness = 10mm (table feed)
when the pitch is equal to 1
contiguous slices
When pitch is > 1
- extended imaging (missing anatomy)
- reduced patient dose
when pitch is <1
- overlapping images
- higher patient radiation dose
considerations for choosing pitch?
Choice of pitch is examination dependent, involving trade off between coverage, accuracy and patient dose
Volume imaging formula
volume imaged = (pitchbeam widthimaging time)/rotation time
What is the amount of anatomy imaged in a helical scan with a 20 second acquisition time, a 1 sec rotation time, and 2.5 mm slice thickness, 4 slices per rotation, with a pitch of 1.2?
volume imaged = (pitchbeam widthimaging time)/rotation time
volume imaged = (1.21020)/1
Volume imaged = 240
What is the amount of anatomy imaged in a helical scan with a 20 second acquisition time, a 0.5 sec rotation time, and 2.5 mm slice thickness, 4 slices per rotation, with a pitch of 1.2?
volume imaged = (pitchbeam widthimaging time)/rotation time
volume imaged = (1.21020)/0.5
Volume imaged = 480
CT Number/Hounsfield unit
- Voxel µ converted to a CT number (HU) and displayed as a level of brightness in the pixel
- CT number = µ-µw/µwx1000
- µw = attenuation coefficient of water (changes based on keV)
- CT number affected by all tissues in voxel
CT number for soft tissue?
50
CT number for water
0
CT number for fat
-100
CT number for air
-1000
CT number for bone/metal
+1000
What is beam hardening?
- polychromatic beams increase in quality as they penetrate
- lower µ values for deeper voxels
Slide 130
cupping artifact - due to beam hardening
CT stimulation: radiation therapy
- radiation therapy delivers maximum radiation dose to cancerous cells while protecting surrounding healthy tissues
- CT helps with treatment planning; localize and provide info about tumour density and size - create 3D images
Steps of CT stimulation
- scan the patient in the CT scanner
- virtual simulation
- treatment setup
scanning the patient for CT stimulation?
- The patient is positioned, immobilized, and scanned in the same position that they would be on the treatment machine
- The obtained electron densities are used to compute dose distributions
Virtual simulation
- Takes the patient’s CT dataset and creates a virtual representation in the radiation treatment machine
- Contours target and normal structure and plans treatment isocenter and beams used
Treatment setup
CT simulation results are used to set up the patient in the treatment machine
Quantitative CT (QCT)
- most sensitive x-ray techniques for measurement of BMC in osteoporosis
Dual Energy CT (DECT)
- use 2 energies to differentiate similar attenuating materials
What does DECT use?
- Subsequential acquisition of 2 different scans
- Rapid tube potential switching
- multilayer detectors
- dual x-ray sources
Temporally Sequential Scans (through entire volume)
- two temporally sequential scans performed to acquire data at two tube potentials
pro of temporally sequential scans (through entire volume)?
can be done on any CT scanner
con of temporally sequential scans (through entire volume)?
risk of patient motion
Temporally sequential scans (through each tube rotation)?
- reduces delay between consecutive scans of the anatomy of interest
- one tube rotation performed at each tube potential prior to the stable incrementation
Pros of temporally sequential scans (through each tube rotation)
- reduces interscan delay between low and high energy acquisitions
- can be done on any scanner
Cons of temporally sequential scans (through each tube rotation)
- vascular anatomy or organs prone to motion can still misregister
- Best temporal resolution is obtained with partial-scan reconstructions
rapid kVp switching
- requires transition times between tube potentials of less than a millisecond
pros of rapid kVp switching?
near-simultaneous data acquisition of low and high energy data sets
cons of rapid kVp switching
- transitions must be abrupt to maximize the energy separation
- requires rapidly modulating tube current (mA) - would result in increased noise at low kV and excessive radiation at high kV
- solved by asymmetric sampling of the high and low energy projections - longer sampling interval(s) for the low energy data
- difficult to optimize filtration
- requires very fast detector materials and electronics
- generator must be capable of very rapid transitions between the low and high tube potentials
multilayer detector
- single high tube potential
- layered scintillation detectors
- low energy data is collected by the front or innermost detector layer
- high energy data is collected by the back or outermost layer
pro of multilayer detector
- low and high energy data sets are acquired simultaneously
con of multilayer detector
- to achieve comparable noise in the low and high energy images, different detector thickness are used
Dual X-ray Source
- 2 x-ray sources and 2 detectors, both perpendicular to one another
- each tube source has its own high voltage generator - independent control of kV and mA (noise levels are adjustable)
Pros of dual x-ray source?
- tube current (and noise) can be optimized for each tube potential
- filtration can be optimized for each tube-detector pair
cons of dual x-ray source?
- scatter from one tube may be detected by the other detector
- 90 degree phase shift between low and high energy data
types of images in DECT?
- mono energetic image
- material decomposition images
material decomposition images
- virtual non-contrast image (iodine removed)
- iodine concentration (iodine maps)
- calcium suppression (calcium removed)
- uric acid suppression
