-
slip ring technology's purpose
- (found in 3rd gen ct)
- allows xray tube & detectors to rotate continuously so that a volume of the pt (rather than one slice) can be scanned very quickly in a single breath hold
-
define Pitch in SSCT
- distance between turns and fastener
- distances in mm that the ct table moves during one revolution of xray tube
-
pitch ratio in SSCT
distance table travels during 360 deg revolution divided by slice thickness (or beam collimation)
-
When the distance the table travels in 1 complete revolution of the xray tube = the ___________ then the pitch ratio is 1:1
slice thickness or beam collimation
-
Pitch affects what (SSCT)
image quality & pt dose
-
(SSCT) volume coverage = _________x slice thickness or
- pitch x slice thickness
- (beam cvollimation x scan time)
-
(SSCT) collimation determines
the slice thickness & in most cases equals the table increments (pitch of 1)
smaller structures require smaller/narrower collimation (whereas larger structures require wider collimation)
-
Table increments is aka
table speed
-
(SSCT) as table increment increases, pitch ____(which results in ____ image quality.
pitch increases (decrease or loss of image detail)
-
SSCT-compared to conventional for resolution
- when using same parameters no change
- however in some studies showed SSCT better at imaging smaller objects
-
SSCT-Noise is affected by
beam intensity, beam quality, slice thickness & matrix size & interpolation algorithms
-
SSCT compared to conventional CT
Noise 360 deg LI algorithm produces ____ noise
less
-
SSCT compared to conventional CT
Noise 180 deg LI algorithm produces ____ noise
more (degrades image quality)
-
SSCT dose compared to conventional CT
about equal
-
helical/spiral CT scan uses a pitch of greater than 1 (which ______ pt dose.)
reduces
-
Limitations of SSCT
volume coverage & speed can be increases by using a higher pitch, but image quality is degraded
-
Multislice CT scanners aka
- multisection
- multidetector
- multichannel
-
MSCT 2 major components
- radiation sensor
- suitable electronic device
-
Dual slice CT scanner became available in
1972
-
MSCT became available in
1998
-
overal goal of MSCT
improve volume coverage speed performance of both single & dual slice ct scanners
-
SSCT
collimation defines the
slice thickness & affects volume coverage speed performance
a thin collimation results in better resolution but takes longer to scan
-
SSCT beam geometry
fan beam
-
SSCT-slice thickness is determined by
pitch and width of precollimator
-
(MSCT) precollimator widthe divided by the number of rows detector arrays =
slice thickness
-
(MSCT) beam geometry-as teh #of rows in a multirow detector array increases, the beam
becomes wider to cover the 2D detector array
-
MSCT Beam geometry
cone beam
-
MSCT--with a cone beam there is more pt dose, with less radiation how
more scatter radiation in all directions
-
MSCT definition of PITCH
distance the table travels per rotation divided by the total collimation
-
MSCT
a pitch of less than 1 is effectively overlaping slices & _____ dose
increases
-
MSCT
a pitch greater than 1 results in ______ pt dose
reduced
because could potentionally be missing slices
-
MSCT
# of detector rows can vary from __ to ___
4-320
-
SSCT z-gap
distance between 2 points used for interpolation
smaller the Z-gap, better the image quality
-
MSCT z-gap
same as SSCT-distance between 2 points used for interpolation (can increase overlap between helices)
however determined by pitch & detector row spacing
-
MSCT
pitch can be increased with a shorter Zgap =
better quality images
-
algorithms based on MSCT 4detector rows
spiral/helical scanning, longitudinal interpolation & fan beam reconstruction
-
in MSCT
as the # of detector rows increase the cone beam becomes ____
larger, artifacts become more pronounced
-increases dose
-
MSCT--Cone beam algorithms
2 classes
Exact & Approximate
-
exact algorithm is
- a cone beam algorithm (MSCT)
- not successful
- computionally complex & difficult to implement
-
Approximate algorithm is
- a cone beam algorithm (MSCT)
- 3D-feldkamp-Davis-Kress (FDK)
- 2D-advanced single slice rebinning (ASSR)
-
types of detectors in MSCT
- 1. uniform
- 2. nonuniform
- 3. hybrid
-
Uniform detectors aka
(MSCT)
matrix detector, or fixed-array or linear array detector
-
Nonuniform detectors aka
(MSCT)
variable, adaptive-array detectors
-
matrix detector
- (type of MSCT detector)
- aka uniform, fixed array or linear array detector
-
Variable detector
- type of MSCT detector
- aka non uniform, adaptive-array detector
-
Detector materials
- Gas ionization detectors
- Solid-State detectors
-
gas ionization for detector material why?
- have low quantom detection efficiency and they have low xray absorption
- used in SSCT, not MSCT
-
Solid state detectors
examples are
- scintilation crystal or ceramics
- rare earth crystals, cadmium tungstate
detectors are doped with dopants (ex europium) to decrease the afterglow below 0.1% at 100ms
-
The most significant difference between SSCT & MSCT is
detector technology
- SSCT=gas ionization detector
- MSCT=solid state detector
-
Detector configuration describes what
a term that describes the number of data collection channels & the effective section of thickness determined by the data acquisition system settings
-
MSCT computer system consists of
- preprocessing
- image reconstruction
- post processing
-
Isotropic imaging definition
- size of voxel used in volume data set
- when the slice thickness is equal to pixel size, all dimensions of the voxel are equal-or a perfect cube
-
spatial resolution in MSCT
- ability of scanner to improve fine detail
- measured in lp/cm or lp/mm
-
MSCT- contrast resolution
ability of the scanner to differentiate small differences in tissue contrast
-
Advantages of MSCT
- increase in speed & volume coverage
- improved spatial resolution
- efficient use of xray beam
- potential to reduce radiation exposure
- cardiac ct imaging
-
symptoms of a stroke include
headache, slurred speech, confusion, facial droop, tremors, vomiting, & tetany
-
Ischemic stroke appears
- normal brain tissue appears as shades of gray
- ventricles appear dark
- portions that are dark are areas lacking blood supply due to stroke
-
Hemorrhagic stroke appears
areas of dense white indicate a hemorrhage (bleed)
-
Appendicitis symptoms
- RLQ pain
- vomiting
- cramping
- fever
- abdominal pressure
-
Symptoms of Pulmonary Embolism
- chest pain
- back pain
- arm pain
- shortness of breath
- high D-dimer
-
AAA symptoms
- abdominal aortic aneurysm
- back pain
- flank pain
- weakness
- pallor
- ashen skin tone if ruptured
- & abdominal pressure
-
Aortic Disection symptoms
- same as AAA
- usually very painful
- back pain
- & painful to move in any way
-
Artifact appearance
- streaks
- shading
- rings & bands
-
Streak is what?
- an artifact
- intense straight lines across an image
-
streaks can be caused from
- improper sampling
- partial volume averaging
- motion
- metal
- beam hardening
- noise
- spiral/helical scanning
- mechanical failure
-
Rings or Bands are what
- an artifact
- produced when the projection readings of a single channel or a group of channels consistently deviate from the truth
-
Rings or Bands can be caused by
defective detector cells, deficiencies in system calibration or from suboptimal image-gen scanner
-
ring & bands are predominately from what type of scanner?
3rd generation
-
Shading appears
- an artifact
- near an object of high densities
-
shading can be caused by
- beam hardening
- partial volume averaging
- spiral/helical scanning
- scatter
- off-focal radiation
- incomplete projection
-
Patient motion can be from
- voluntary-breathing, swallowing
- involuntary-head motion with injury, peristalsis, cardiac motion
-
Rationale means
purpose of 3D imaging is to use the vast amounts of data collected from the patient by volume ct scaning to provide both qualitative & quantitative info in a wide range of clinical applications
-
Qualitative-
- type of rationale
- used to compare how observers perform on a specific vast to demonstrate the diagnostic value of 3D imaging
-
Quantitatve-
- type of rationale
- used to assess three elements of the technique: precision, accuracy & efficiency of 3D imaging procedure
-
Most common coordinate system used on 3D scanners
-
(3D imaging)
Scene
multidimensional image; rectangular array of voxels with assigned values
-
(3D imaging)
scene domain
Anatomical region represented by the scene
-
(3D Imaging)
scene Intensity
values assigned to the voxels in a scene
-
(3D Imaging)
Pixel size
length of a side of the square cross-section of a voxel
-
(3D Imaging)
scanner coordinate system
origin & orthogonal axes system affixed to the imaging device
-
(3D Imaging)
scene coordinate system
origin & orthogonal axes system affixed to the scene
-
(3D Imaging)
object coordinate system
origin & orthogonal axes system affixed to the object or object system
-
(3D Imaging)
Display Coordinate System Rendition
2D image depicting the object info captured in a scene or object system
-
(3D Imaging)
Image space
translate, rotate, or scale scenes, objects or surfaces
-
(3D Imaging)
Object Space
extract structural information about the object from the 3Dspace
-
(3D Imaging)
Parameter Space
take measurements from the image's view space on the computer screen
-
(3D Imaging)
View space
view the 2D screen of the computer monitor
-
(3D Imaging)
Modeling is what
- generation of a 3D object using computer software
- Uses mathematics to describe physical properties of an object
- used to transform a 2D profile into a 3D object
-
how are shading & lighting used in 3D imaging
- several shading algorithms
- Shading determines the final appearance of surfaces of the 3D object
- Lighting helps us to see the shape & texture of the object
-
Rendering is what
- final step in process of generating a 3D object
- a computer program that converts the anatomical data collected from the patient into the 3D image seen on the computer screen
-
types of rendering (3D imaging)
Surface & Volume
-
Surface Rendering
- type of 3D rendering
- uses only the surface contour data
-
Volume Rendering
- type of 3D rendering
- uses the entire data set
-
slice imaging is waht (3D)
- simplest method of 3D imaging
- Like MPR (sag, coronal, paraaxial)
- it is a 2D image displayed on a flat screen
-
Projective imaging (3D)
- Most popular 3D imaging approaph
- Techniques for extracting multidimensional info from the given image data & for depicting such info in the 2D view space by a process of projection
- (surface & volume redering)
-
Projective imaging is technicall a ____ image
2D
-
Volume Imaging is what
- produces a true 3D visualization mode
- Must not be confused with volume rendering
-
A generic 3D imaging systems consists of 4 major elements and they are
- Input-refers to device that acquired data
- Workstation-a powerful computer that can hand 3D imaging operations
- Output-displays the results
- User
-
Definition of 3D imaging
process that starts with a stack of sectional slices collected by some medical imaging device and results in computer synthesized displays that facilitate the visulazation of underlying spatial relationships
-
4 steps are needed to create a 3D image
- 1. data acquisition
- 2. creation of 3D space or scene space
- 3. processing for 3D image display
- 4. 3D image display
-
Surface Rendering is what
- aka surface display
- computer creates an internal representation of surfaces that will be visible in the displayed image...it then lights them according to a standard protocol & displays the image according to its calculation of how the light rays would be reflected to viewers eye
-
2 steps of surface rendering are
- surface formation
- depiction on a computer screen
-
Advantage of surface rendering
- much less storage space
- (but not as much info as volume rendering)
-
Volume Rendering is what
- 3D images have better image quality & provide more info compared with SR techniques
- Uses the entire data set
- & requires more computer power
-
2 stages of Volume rendering are
-
Intensity Projection Renderings are what
- used in 3D imaging
- extension of MPR techniques
- consist of generating arbitrary thick slices from thin slices, reducing the noise level and possibly improving the visualization of structures
-
3 types of Intensity projection renderings are
- average intensity projection
- maximum intensity projection
- minimum intensity projection
-
Average intensity projection is what
- type of Intensity projection rendering used in 3D CT
- algorithm intended to create a thick MPR image by using the average of the attenuation through the tissues of interest to calculate the pixel viewd on the computer
-
Maximum Intensity Projection
- type of Intensity projection rendering used in 3D ct
- Volume Rendering technique
- Algorithm where only tissues with greatest attenuation will be displayed for viewing by observer
-
What is the most popular type of intensity projection used in CTA and why
- Maximum intensity projection
- vessels containing contrast are clearly seen
-
Minimum Intensity Projection is
- type of Intensity Projection Rendering used in 3D CT
- ensures only the tissues with the minimum or lowest attenuation will be displayed for viewing by the observer
- Least used projection
-
Minimum Intensity Projection is the least used but it is useful in which way
provides a valuable perspective in defining lesions for surgical planning or detecting subtle small airway diseases
|
|