Transcript Single-slice Spiral CT

CT
Seeram Chapter 13:
Single Slice Spiral Helical CT
Oh no, not
more
physics…
Spiral CT
 Incentives for development
 Shorter study times
 Improved 3D imaging
 New technology required
 Slip ring
 Allows continuous gantry rotation
Conventional (Non-spiral) CT
 Tube rotates once around patient
 Table stationary
 data for one slice collected
 Table increments one slice thickness
 Repeat
 Tube rotates opposite direction
Conventional Tube Rotation


Cables only allow ~ 360o rotation
Sequential scanning steps
Gantry must accelerate from full stop to constant
operating speed required for data acquisition
2. Data acquired during constant speed rotation
3. Gantry decelerated from constant operating speed to
full stop
4. Table & Patient indexed to next scanning position
1.

Interscan Delay

cycle time above which is not constant scanning
Non-spiral Intergroup Delay
 Scans grouped for single breath hold
 Inter-scan delay causes long study
 Because of delay, studies may require >1 group
 Reduced scanner throughput
Limitations of Conventional (nonspiral) Scanning
 Long exam times
 Inter-scan delays
 Table motion
 Inter-group delays
 Breathing
 Limitations for angiography
 Few scans made during maximum
contrast enhancement
Faked Image
 Respiration variations from
group to group can cause
 Anatomy omissions

Slice-to-slice misregistration
 Inaccurate 3D images

Step-line contours
Volume Scanning
 Also called
 Spiral Volume CT (SVCT)
 Spiral-helical scanning
 Data collected continuously
 Table moves continuously
 Tube traces spiral path with respect to
patient
Requirements for Volume
data Acquisition
 Continuous tube rotation
 requires slip ring technology
 Provides electricity to rotating components
 Continuous couch movement
 Increase in tube heat capacity & cooling rate
requirements
 No inter-scan tube cooling
Helical Reconstruction
Complication
 Patient moves as gantry rotates
 No two fan beams at same z coordinate
“z” direction
As Gantry Rotates,
Fan Angles Repeat
 Distance between repetitions is movement of table
during one rotation
“z” direction
Data Acquisition Challenges
 Projection data not confined to single slice
 Streak artifacts
 appear with “standard” or “conventional” (non-spiral)
reconstruction
 caused by motion
 special algorithms required
Position at
start of
rotation
Position at
start of
rotation
Position
of interest
Reconstruction Performed for Single Location
 Fan beam only at one orientation at slice location
 But other orientations needed for reconstruction
“z” direction
Calculating Fan Beams at Odd Locations using
Interpolation
 Use 2 beams in correct direction closest to slice location
 Calculate beam attenuation by interpolating between adjacent beams
“z” direction
Spiral Reconstruction Algorithms
 Uses interpolation for
 input projection data
 output slice attenuation data
Interpolated
data
 Slice can be calculated at any
position from raw projection
data
= real data point
coordinate
of interest
Interpolation
 Estimates value of function using known values on
either side
When x = 50, y = 311
When x = 80, y = 500
500
What will be the
value of y when
x=58?
?
311
y = x1+ (y2 – y1)* (x – x1) / (x2 – x1)
y = 311 + (500-311)* (58-50) / (80-50)
(x1,y1)
(x2,y2)
(x,y)
50
80
58
Disadvantage of Interpolation
 Can increase effective slice thickness
 Calculation averages data measured at many z values
“z” direction
Redundant Data
 All rays sampled twice in 360o of rotation
 Duplicate data called “Complimentary”
*
Redundant
Data
 All rays actually measured in 180o of rotation
 360o compared to 180o covers 2X thickness (“z”)
Distance moved during single
360o rotation
*
Redundant Data
 Can reduce slice thickness averaging substantially
by using only 180o worth of data
180o rotation
360o rotation
180o Reconstruction
for Spiral Scanning
 Substantially reduces effective slice thickness
 Better z-axis resolution
 Increases image noise
 Image based on only 180o instead of 360o of data
 Redundant data reduces noise
Spiral CT Challenges
 Requires special interpolation reconstruction
 More computing-intensive
Data Acquisition Challenges
 No single defined slice
 slice localization more difficult
 Different slice volume geometry
 conventional: cylinder
 spiral: wafer with radial crack
 Slight increase in effective slice thickness
 slice thickness influenced by


fan beam thickness
speed of table motion
Requirements for Volume data
Acquisition
 New reconstruction algorithms
required for spiral weighting
 Larger detector data memory
requirements
 larger buffer required if data acquired
faster than can be sent to computer
Spiral CT Advantages
 Shorter acquisition times
 no inter-scan delays
 shorter study times
 entire organs / volumes scanned
together
 Better throughput
 BUT: Larger demands on tube

Much less cooling time
Spiral CT Advantages
 No gaps in data
acquisition
 slice can be
reconstructed for any
axial position
 Patient motion artifacts
reduced
Spiral CT = Faster Scanning:
Advantages
 Less potential for motion
 Less effect of varying respiration
 spiral scan done in single breath hold
 Less effect of shifting anatomy between slices
 Improved contrast protocols possible
 faster scanning; less dilution
 more uniform contrast concentration
 Greater accuracy for multiplanar & 3D images
Table Moves During Helical
Scanning
table increment during one rotation
Slice Pitch = --------------------------------------slice thickness
Slice
thickness
Table
Increment
Table Moves During Helical Scanning



Slice thickness determined by collimation
Table motion per revolution determined by table speed
Coverage = table increment X # rotations
table motion during one rotation
Slice Pitch = --------------------------------------slice thickness
Slice
thickness
Table
Increment
Single-Slice Detectors
 Many detectors rotate around
patient
 Single row in z-direction
 Slice thickness determined by
collimation
Z-Axis
Single Slice CT: Changing Slice Thickness
Thin Slice
Thick Slice
Z-Axis
Z-Axis
Pitch = 1

Pitch = 1 means slices abut one another
table motion during one rotation
Slice Pitch = --------------------------------------slice thickness
Pitch >1

Pitch > 1 means gap in slices
table motion during one rotation
Slice Pitch = --------------------------------------slice thickness
Pitch <1

Pitch < 1 means overlap in slices
 Can improve visualization of objects
table motion during one rotation
Slice Pitch = --------------------------------------slice thickness
Spiral vs. Conventional CT &
Patient Dose
 Dose is strongly dependent on pitch
Please explain.
Inquiring minds wanna
know
Pitch = 1
 equivalent dose to non-spiral
Pitch >1
 lower dose for spiral if table increment per rotation >
one slice thickness
Pitch <1
 higher dose for spiral if table increment per rotation <
one slice thickness
Spiral vs. Conventional CT & Other
Observations
 Non-spiral phantoms may not be sufficient to test spiral
performance
 Performance characteristics compared







Spatial resolution
Image uniformity
Contrast
Noise
Slice sensitivity
Dose
artifacts
 Study showed subtle decrease in abdominal axial
resolution (not clinically significant)
Developments
 Multi-slice CT
 Real-time CT fluoro
 Better 3D imaging
 CT Angiography
 CT Endoscopy