Test 2 Flashcards
CT History
I THINK MOST IMPORTANT:
1968-70: Hounsfield constructs a CT prototype at EMI
1971: CT brain scanner installed at Atkinson-Morley’s Hospital in London
1972: CT debuts at Radiologic Society of North America
1974: DR. Robert Ledley made 1st Whole body scanner at Georgetown
1st generation CT
single detector
translate-rotate mode
pencil beam
180 translations separated by one degree
2nd generation CT
Mechanical gantry motion
multiple detector assembly
fan shaped beam
translate-rotate mode
18 translations separated by 10 degree increments
scan time reduced to 20 seconds per slice
3rd generation CT
Curved detector assembly
fan shaped beam
rotate mode
scan times reduced to one second
4th generation
fixed circular detector array
rotate mode
scan times reduced to one second
x axis
sagittal plane left to right
width
y axis
coronal plane, anterior to posterior
height
z axis
axial head to feet
thickness of the slice (depth)
Image Matrix
Layout of cells in rows and columns
Pixel
2d cell
Pixel size = Field of view/Image matrix
Voxel
3D cell
Spatial Resolution
Image matrix increases
Resolution increases
The ability to differentiate small forms that are close together as separate objects
AKA: image detail, line pairs per centimeter
FOV
FOV determines area within gantry from which raw data is acquired
As FOV decreases, Resolution INCREAES
Contrast Resolution
The ability to distinguish between small differences in densities
As slice thickness decreases, resolution INCREASES
Absorption Profile
The degree to which a beam is reduced by an object is attenuation
beam attenuation is quantified by attenuation profile and absorption profile
Hounsfield Unit
CT numbers
attenuation value of water
CT Components
Gantry: x ray tube, detector array, collimator assembly
Computer: two types of software, operating system and applications
Window width
determines the range of HU
Values higher will be white and values lower will be black
Window Level
Selects the CENTER CT VALUE of window width
AKA window center
Window width and level for bone
Window level: 350
Window width: 2500
Window width and level for Lung
Window level: -500
Window width: 1500
Window width and level for Soft tissue
Window level: 40
Window width: 400
Isotropic
Window width, length and depth is the same
Slip rings
permit gantry frame to rotate continuously
Make helical scan modes possible
Generator
Mounted on the slip rings
External to the gantry
Filtration
Reduces pt dose
shapes the beam
creates beam uniformity
Collimation
Pre patient: restricts the xray beam to a specific area and decreases patient dose
Post patient: reduces scatter radiation and improves contrast resolution
Algorithms
a finite set of unambiguous steps performed in a prescribed sequence to solve a problem
Fourier transform
an important image processing tool that decomposes an image into components
Interpolation
A mathematical method of estimating the value of an unknown function using the known value on either side of the function
Adding two densities into one
Image reconstruction
the process of using raw data to create an image
Prospective reconstruction
What is automatically produced during scanning
Retrospective reconstruction
using the raw data later to create a new image
Back projection
The process of converting the data from the attenuation profile to a matrix
Smooth Filter
Reducing the difference between adjacent pixels
reduces artifacts but also reduces spatial resolution
Improves contrast resolution
We want this for soft tissue
Sharp Filter
Enhances contrast by accentuating the difference between neighboring pixels
Improves spatial resolution and reduces low contrast resolution
Reconstruction Algorithm vs Window setting
Changing the algorithm changes the way the raw data are manipulated to reconstruct the image
This contrasts with changing the window setting, which merely changes the way the image is viewed
Adaptive Statistical Iterative Reconstruction
Newer method of construction
Complex method that computes projections from the image, compares it with the original projection data, and updates the image based on the difference
can reduce image noise
shown to reduce the radiation dose to the patient by as much as 50%
LIST TWO ADVANTAGES OF ADAPTIVE STATISTICAL ITERATIVE RECONSTRUCTION
REDUCES IMAGE NOISE
REDUCES RADIATION DOSE TO PATIENT
Scan field of view
determines the area, within the gantry, from which the raw data is acquired
in the isocenter of the gantry
SFOV selection determines the number of detector cells collecting data
Display field of view
determines how much of the collected raw data is used to create an image
cannot be larger than SFOV
AKA ZOOM
Changing DFOV changes the pixel size
Spatial Resolution
Pixel size = Field of view/Image matrix
2 reasons why we do AP localizer
Start-Stop
Right left centering
Lateral Localizer
Start-stop
Anterior posterior centering
!!!!! Disadvantages of slice by slice scanning?!!!!!!
The cumulative effect of the pauses between each data acquisition adds to the total examination time
Inefficient use of contrast
Slice misregistration
Poor 3D an/or MPR
!!!!Requirements of Helical scanning
Continually rotating x ray tube/detector assembly
high heat capability/dissipation x ray tube
Continuous table movement
Increased data processing system
!!!!!!!Advantages of helical scanning!!
Eliminates the interscan delay
Ability to optimize iodinated contrast agent administration
Reduces respiratory misregistration
Reduces motion artifacts from organs
Improve 3D and MPR
Pitch
Ratio between table speed and slice thickness
Calculates amount of anatomy examined during a particular multislice scan
Pitch= bed increment x rotation
Divided by Slice thickness
Contiguous= 1 and all next to each other. table and beam are identical
Non contiguous is above 1 and there are gaps. Table is more than beam collimation
Over lapping is below 1 and there is overlap. table is less than beam colimation
!!!MDCT
Thin slices (0.625) can be added together to create thick (2.5) reconstructed slice for viewing
!!!!!!!! Two main features are used to measure image quality!!!!!!!!!
Spatial resolution: ability to resolve (as separate objects) small, high contrast objects
Contrast resolution: The ability to differentiate between objects with very similar densities as their background
!!!!!!!Factors affecting spatial resolution!!!!!!
matrix size display field of view pixel size slice thickness reconstruction algorithm Pitch Patient motion lp/mm
Contrast Resolution
AKA low contrast detectability or system sensitivity
CT is superior to all other modalities in its contrast resolution
objects with a 0.5% contrast variation can be distinguished
Factors affecting contrast resolution
mAs/dose pixel size slice thickness reconstruction algorithm patient size
Temporal resolution
how rapidly data is acquired
Controlled by: gantry rotation speed, number of detector channels in the system, speed with which the system can record changing signals
Reported in milliseconds
Image noise
Noise is measured by obtaining the standard deviation of the CT numbers within an ROI
Image Uniformity
Ability of CT scanner to yield the same CT number regardless of location in ROI
Center CT number must be between -7 and +7 HU (+/-5 preferred)
Quality Control
Window width and level is 100
5 lp/mm bar clearly resolved for adult abdomen protocol (smooth filter)
6 lp/mm bar must be resolved for high resolution adult chest protocol (Sharp filter)
Linearity
Relationship between CT numbers and linear attenuation values
Requires phantom with standard materials
Image artifacts
Classifications: physics based, patient based and equipment induced
Beam hardening artifacts
Caused by the polychromatic nature of the xray beam
As an xray beam passes through an object, lower energy photons are preferentially absorbed creating a “harder” beam that can’t be adjusted for by the system
Systems minimize this in 3 ways: filtration, calibration correction, beam hardening correction software
looks like dark bands or streaks between dense objects in the image
Partial volume artifacts
occurs when more than one type of tissue is contained within a voxel
minimized by thinner slices
Edge Gradient Effect artifacts
results in streak artifact or shading arising from irregularly shaped objects that have a pronounced difference in density from surrounding structures- bone and soft tissue (skull and brain) or soft tissue and lung (chest and abdomen)
largely unavoidable but can be reduced by thinner slices or using a low HU oral contrast instead of barium
Motion artifacts
artifacts from patient motion appear as shading, streaking, blurring or ghosting
Metallic artifacts
metal objects in SFOV creates streaks
best reduced by removing metal
non removable objects avoided by angling the gantry (dental fillings)
Out of field artifacts
caused by anatomy that extends outside the selected SFOV
appear as streaks and shading
Ring artifacts
Caused by imperfect detector elements
appear on the image as a ring or concentric rings
can sometimes be eliminated by recalibrating the scanner
Reconstructions
When RAW data are manipulated to create pixels that are then used to create an image
Reformation
when IMAGE data are assembled to produce images in different planes or to produce 3D images
3D reformation
represents the entire scan volume in only one image
manipulate/combine CT values to display an image
3 types: surface rendering shaded surface display, maximum intensity projection and volume rendering
Surface rendering or Shaded surface display
Only voxels on the surface of the structure are used
Threshold CT value is selected
Maximum Intensity Projection
Selects voxels with the highest value to display
Displays 3D data on a 2D image
Volume Rendering
3D semitransparent representation of the imaged structure
favored 3D image technique
an advantage is that all voxels contribute to the image
Allows image to display multiple tissues and their relationship to one another
Endoluminal Imaging
Virtual endoscopy
Unit of x ray exposure in air
Roentgen (R)
SI unit is C/kg
Unit of absorbed dose
Radiation absorbed dose (rad)
Gray (Gy) is the SI unit
100 rad=1Gy
1 rad= 1cGy
Equivalent dose
absorbed dose x radiation weighting factor
Effective dose
Absorbed dose x radiation weighting factor x tissue weighting factor
CT dose parameters
CT dose index
Dose length product
multiple scan average dose
effective dose
CTDI
CT Dose Index
Measure of dose (mGy) per slice
One revolution around
obtained using standard head and body CTDI phantoms and a pencil ionization chamber
Dose Length Product
DLP (mGy)= CTDI x scan length
contiguous slice by slice
Multiple Scan Average Dose
MSAD (mGy) = DLP + overlap
helical/spiral
!!!!!!! Effective Dose Efd
EfD (mSv)= MSAD x tissue weighting factor
Dose Comparisons
Avg background radiation for Americans is about 3.6 mSv
exposure from a chest xray is baout 0.1 mSv
Exposure from a chest, abdomen or pelvis CT is about 10 mSv
Strategies for Reducing dose
automatic bolus tracking
Decrease mA to suit individual pt
Automatic tube current modulation
increase pitch (table speed)
Limit use of thin slices
prevent repeat exams
shielding
