Spiral CT

1
Spiral CT

• Bushong Chapter 5

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Spiral CT

• Continuous source rotation with the patient
  translation through x-ray beam
• Patient couch moves as x-ray tube rotates

3
Spiral CT

• High voltage supplied by slip rings or on board
  generator
• Slip ring replaces cables
• No interscan delay
• Interscan delay is a small delay between slices
  or volumes that is needed during standard axial
  scanning for the x-ray tube to stop and reverse
  direction. Interscan delay can also be used to
  allow extra time during a scan for tube cooling
• Volumetric imaging within one breath-hold, at
  least 25s

4
Spiral CT

• Hyperventilation may help patients extend
  breath-hold
• Tell the patient to breath in and out several
  times before final breath hold
• Contrast enhanced examination requires less
  contrast media
• Contrast amounts between 100-150ml are still
  common
• Some multislice/multidetector scanners are so
  fast that technologists need to be careful about
  not scanning faster than the contrast bolus moves
  through the vascular system
• Detector data transferred to computer by slip
  rings

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Spiral CT

• Examination time is greatly reduced
• Patient comfort is much improved
• Because the patient is moved through the gantry
  while the x-ray tube rotates, a spiral pattern
  results

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Spiral CT

• Z-axis resolution is slightly reduced with spiral
  CT
• Effective slice thickness increases with pitch

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Pitch

• Pitch is the patient couch movement per rotation
  divided by slice thickness
• Contiguous spiral
• Pitch 1, that is 10mm/10mm
• Extended spiral
• Pitch 2, that is 20mm/10mm
• Overlapping spiral
• ½, that is 5mm/10mm
• Page 78 Bushong shows diagram

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Pitch

• Low pitch results in better z-axis resolution
• Narrow collimation/high pitch results in better
  z-axis resolution than wide collimation/low pitch
• Narrow collimation/high pitch is recommended for
  high contrast, thin slice examination, for
  example, lung nodules

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Pitch

• Data is collected continuously but not from a
  transverse plane
• Instead a helical volume is created
• As spiral CT pitch increases, patient dose is
  reduced
• Patient dose is approximately proportional to
  1/pitch
• Patient dose is proportional to slice thickness
  divided by couch movement

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Pitch

• Pitch in excess of 2 is not recommended for any
  clinical examination
• Too much anatomy would be skipped
• Couch incrementation is usually set to equal
  collimation, pitch 1
• Couch speed (mm/s) should not exceed slice
  thickness (mm) in order to obtain best compromise
  between image quality and image volume

11
Pitch

• Couch speed will not normally exceed 10mm/s
• When pitch exceeds 1, 180 interpolation must be
  used to limit loss of z-axis resolution
• Longitudinal (z-axis) image coverage is the
  product of couch velocity (mm/s) and image time
  (s)
• If couch moves 10mm/s and the imaging time is 5s
  then 50mm (5cm) of axial coverage will result
• The larger the pitch, the more anatomy is covered
  per examination

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Interpolation

• Reconstruction of spiral CT images is the same as
  that for conventional CT except for interpolation
• A transverse planar image can be reconstructed at
  any position along the axis of the patient
  (z-axis)
• The transverse image is reconstructed from spiral
  data first by interpolation, then by filtered
  back projection

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Interpolation

• Either 360 degrees or 180 degrees interpolation
  may be employed
• Usually 180 degree interpolation is preferred
• Contiguous reconstruction can result in partial
  volume effect when object is contained in
  adjacent slices

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Interpolation

• Overlapping reconstruction may be necessary to
  ensure that object is fully contained within a
  slice
• Data acquisition is continuous along the z-axis
  therefore by interpolation, image reconstruction
  is at any z-axis position
• Regardless of z-axis position, slice thickness is
  determined by collimation

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Interpolation

• Volume averaging increases with increasing pitch
• By increasing the pitch the helix becomes more
  spread out and small structures can be missed
• This is similar to stretching a slinky toy

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Interpolation

• Image noise is higher with spiral CT versus
  conventional CT regardless of pitch
• Interpolation is the computation of an unknown
  value using known values on either side
• Z-axis resolution is improved with 180 degree
  interpolation compared to 360 degree
  interpolation

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Interpolation

• Extrapolation is the computation of an unknown
  value using known values on one side
• 180 degree interpolation results in a thinner
  slice than 360 degree interpolation
• 180 degree interpolation results in a noisier
  image than 360 degree interpolation

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Interpolation

• 180 degree interpolation results in approximately
  20 higher noise than conventional CT
• 360 degree interpolation results in approximately
  20 less noise than conventional CT
• 180 degree interpolation results in better z-axis
  resolution on reformatted longitudinal images
  than 360 interpolation

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Interpolation

• 180 degree interpolation allows scanning at a
  higher pitch than 360 degree
• 360 interpolation broadens sensitivity profile
  more than 180 degree interpolation
• In general, image noise is less for 360 degree
  interpolation, spiral CT than for conventional CT
• In general, image noise is much higher for 180
  degree interpolation, spiral CT than for
  conventional CT

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Interpolation

• Whether 180 degree or 360 degree interpolation,
  there are linear and higher order reconstruction
  algorithms
• Two characteristic spiral CT artifacts have been
  identified as breakup and stair step
• Both the breakup artifact and the stair step
  artifact occur as a consequence of reformatting
  interpolation transverse images to the
  longitudinal plane coronal or sagittal

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Sensitivity Profile

• Generally when covering a given length of
  anatomy, thinner collimation and higher pitch are
  preferred because the result is better spatial
  resolution
• Pitch greater than 21 is not clinically useful
  because of a broadened sensitivity profile and
  reduced z-axis resolution
• Generally, higher pitch results in thinner slice
  thickness and less partial volume artifact

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Sensitivity Profile

• During spiral CT with pitch gt1, the sensitivity
  profile (z-axis resolution) is wider than that of
  conventional CT
• Spiral CT sensitivity is described by the full
  width at tenth maximum (FWTM) rather than the
  conventional full width at half maximum (FWHM)
• The higher the pitch, the wider will be the
  sensitivity profile

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Design Features Slip Ring Technology

• Slip ring technology made spiral CT possible
• Normal spiral CT gantry rotation is 1 revolution
  per second
• Although .5s revolution is possible, the
  engineering required by the stress of centrifugal
  force is formidable

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Slip Ring Technology

• There may be multiple slip rings, both high
  voltage and low voltage
• The design of spiral CT imagers is based on both
  third and fourth generation with no clear
  advantage to either
• The slip ring contacts or brushes wear and are
  designed to be replaced during preventive
  maintenance

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X-Ray Tube

• Spiral CT requires less than 1s 360 degree
  rotation time and at least 5 MHU x-ray tubes
• For very long scan times, mA must be reduced so
  that x-ray tube loading will not be exceeded
• Regardless of heat capacity (MHU) and anode
  cooling (kHU/min), spiral CT is usually limited
  by the heat capacity of the focal track
• High anode heat capacity (6-8 MHU) and rapid
  cooling (1 MHU/min) are required

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X-Ray Tube

• In spite of the high heat load, tube life is
  comparable to conventional CT at about 50,000
  exposures

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Technique SelectionReconstruction

• Index is the interval at which images are
  reconstructed
• Index is reconstruction distance divided by
  collimation
• An index of less than one indicates image overlap
• An index of greater than one indicates a gap

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Reconstruction

• An index of less than one should be employed to
  visualize suspected lung nodules
• Spiral CT significantly improves coronal and
  sagittal slice reconstruction
• High quality two-dimensional and
  three-dimensional image reformation are made from
  overlapping transverse images

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Reconstruction

• Spiral CT operation requires the following unique
  technique selections
• Scan time
• Beam collimation
• Couch feed velocity
• Z-axis spacing for image reconstruction
• Spiral images cannot be reconstructed as rapidly
  as they are acquired. Hence computer memory must
  be excessive

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Reconstruction

• Scan time cannot exceed the patients breath-hold
  capacity, usually about 25s
• Collimation and couch velocity can be selected as
  pitch

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Z-Axis Resolution

• Z-axis resolution is compromised in spiral CT but
  not significantly
• The ability to reconstruct images at any z-axis
  location improves small lesion detection by
  reducing partial volume effects

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Advantages and Limitations

• Image noise is usually less with spiral CT
• More data is acquired in spiral CT therefore,
  image reconstruction takes a little longer
• Spiral CT replaces single scan techniques with
  volume acquisition techniques
• Spiral CT misses no anatomy in the scanned volume

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Advantages and Limitations

• Spiral CT images can be reconstructed at any
  z-axis position
• Multiple overlapping transverse images are
  possible in a single breath-hold with no
  additional patient dose
• Overall scan time is less with spiral CT
  resulting in improved patient throughput
• Spiral CT takes a bit longer for image processing
  because of the required interpolation before
  planar image reconstruction

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Advantages

• Faster image acquisition
• Contrast can be followed quicker
• Reduced patient dose at pitch gt 1
• Physiologic imaging
• Improved 3d imaging
• Angiographic imaging
• Fewer partial volume artifacts
• Freeze breathing
• Fewer motion artifacts
• No misregistration
• Increased throughput
• Improved patient comfort
• Unlimited z-axis resolution
• Real time CT biopsy