Axial VS Helical Scanning Flashcards
Scanning mode where the table stops at the scanning position and the tube rotates around the patient?
Conventional / Axial / Regular
Scanning mode where the patient is continuously moving in the z-axis direction while the tube rotates around?
Spiral / Helical Mode
Earliest scanning mode?
Step-and-shoot method
Alternative name for Step-and-shoot scanning?
Axial Scanning
Conventional / Regular Scanning
Serial Scanning
Sequence Scanning
CT table moves to the desired location and remains stationary while the x-ray tube rotates within the gantry, collecting data
STEP-AND-SHOOT SCANNING
Axial slices lie ___ to one another
parallel
The slice beginning matches
exactly the slice end
Perfect circles are formed that are ____ to z axis
perpendicular
Used by both third- and fourth-generation scanners
STEP-AND-SHOOT SCANNING
obtained data for one slice with each rotation (for early systems)
STEP-AND-SHOOT SCANNING
How many degrees rotation did the third generation systems use for each acquisition for step and shoot scanning?
360 degrees
How many degrees rotation did the fourth generation systems use for each acquisition for step and shoot scanning?
400 degrees
In all types of scanners using this method there is a slight pause in scanning between data acquisitions as the table moves to the next location. , referred to as the
interscan delay
In early scanners the time for a complete cycle allowed only a single scan to be acquired
each time the patient held her breath.
Newer scanners shortened the cycle time dramatically, allowing axial scans to be
“clustered”
Grouping scans in clusters reduces what problem
Slice misregistration
Advantages of axial scanning mode?
Highest Image quality, superior to helical methods
Why does step-and-shoot methods produces highest image quality
axial nature and the fact that the patient table remains stationary during data acquisition
Disadvantages of axial scanning?
Adds to the total examination time Less than 30 seconds Not useful in contrast studies Breathing creates motion artifact Data reconstruction and reformation limited Induce motion in patients organs Misregistration
Delay inherent in axial scan sequences which is caused by the differences in patients breathes with each data acquisition?
Misregistration
When was helical scanning introduced?
late 1980s
Alternative name for helical scanning?
Spiral or Continuous acquisition Scanning
What did helical scanning eliminate?
Interscan delay
three basic ingredients that define a helical scan process
- a continually rotating x-ray tube,
- constant x-ray output, and
- uninterrupted table movement.
Helical scanning is often referred to as
volumetric scanning
Refers to the fact that the end result of such a scanning method is a block of data, not separate slices, as occurs in traditional axial scanning?
Volumetric scanning
Increasing the scan speed results in improved image resolution owing to the ability to obtain images with:
Improved iodinated contrast concentration.
Decreased respiratory and cardiac motion artifact.
Superior multiplanar and three-dimensional (3-D) reformation capabilities.
Other advantages of helical scanning
Improving diagnostic accuracy.
Patient comfort.
Department productivity.
What major improvements lead to the development of the helical scan?
Slip ring design More Efficient Tube cooling Higher x-ray output More efficient detectors Smoother table movements Software improvements for table motion Improved raw data management
allow the tube to move continually in the same direction
Newer systems slip rings
eliminates cumbersome electrical cables and makes possible a data-gathering system using a continuous rotation of the x-ray source
Slip ring technology
There is constant x-ray output throughout a helical scan acquisition that Can last more than 60 seconds without a pause for cooling.
More-Efficient Tube Cooling
The tube makes a complete rotation in 1 second or less.
Overal decrease in mAs to one third (or less) compared to axial scan if the mA remained the same.
Higher X-ray Output
allow the total radiation dose to be lower for both helical and axial scans done on the newer systems.
more-efficient detectors
Motion artifact degrades images, and images are created while the table is in motion, helical scanning requires the table to move very smoothly.
Smoother Table Movement
Manufacturers then since have created special software that adjusts for the smooth table motion
Special Software
take the slant and blur out of are statistical methods that create images that closely resemble those acquired in a traditional axial mode
Interpolation
Two most common schemes in interpolation
360° and the 180° linear interpolations (abbreviated 360LI and 180LI)
Huge amounts of data are generated in a very short time
Improved Raw Data Management
a parameter that is commonly used to describe the CT table movement
Pitch
It is most commonly defined as the travel distance of the CT scan table per 360° rotation of the x-ray tube, divided by the x-ray beam collimation width
Pitch
When the table feed and beam collimation are identical how much is the pitch?
1
When the table feed is less than the beam collimation, pitch is?
Less than 1 and scan overlaps occur
To maintain a pitch of 1, the table speed will vary according to
the slice thickness selected
describes the relationship of the table speed to the slice thickness
Pitch
As the pitch increases, so does the
slice angle
is required to straighten the image and the effects of interpolation become more pronounced
More interpolation
More interpolation effects to
image unsharpness and effective slice thickness blooming
Increasing the pitch will result to
- a scan covering more anatomy lengthwise for a given total acquisition time
- reduce the radiation dose to the patient
A decrease in pitch will
slows down the table speed
will result in overlapping slices and will decrease the amount of anatomy covered per unit time and increase the radiation dose to the patient
A pitch of less than 1
A pitch that is not commonly used
pitch of less than 1
A compromise is necessary when the pitch is
extended beyond 1
exchange for the shortened examination time and the reduced patient radiation dose comes
comes the loss of image sharpness and a decrease in the SSP
