Special Imaging Techniques

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Special Imaging Techniques

• Chapter 6
• Bushong

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Dynamic Computed Tomography(DCT)

• Dynamic scanning implies 15 or more scans in
  rapid sequence within one minute
• Dynamic scanning us used for trauma, cardiac, and
  vascular imaging
• Dynamic scanning allows imaging in the arterial
  phase following a bolus injection

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DCT

• Typical dynamic scanning would be 15 one second
  scans, separated by a two-second interscan time
  during which the couch would increment a distance
  equal to the slice thickness
• Tube cooling will limit the extent of dynamic
  scanning
• Dynamic scanning has largely been replaced by
  spiral CT

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DCT

• Spiral CT permits imaging of the entire liver and
  pancreas at a specified time phase such as
  arterial, portal, or venous
• CT guided biopsy is performed to localize using a
  region of interest feature
• Radiopaque markers are used to help guide a
  needle to the area of interest

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DCT

• Wide window width (WW 500 to 1000), low window
  level (WL 40 to 60) for bone imaging
• Wide WW (700 900), high WL (800) for lung
  imaging
• Narrow WW (200 400), low WL (40 60) for
  abdomen and pelvis imaging
• Computed tomography fluoroscopy provided 8
  images/second for near real time imaging

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DCT

• Computed tomography fluoroscopy is most useful
  for angiointervention in the abdomen and chest
• Computed tomography fluoroscopy results in high
  patient and personnel dose
• CT fluoroscopy is particularly helpful for
  CT-guided biopsy
• Patient dose during CT fluoro is kept low by
  using low mA (30)

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Quantitative CT

• Quantitative CT (QCT) employs an ROI to determine
  the average CT number of a tissue as an aid to
  diagnosis
• QCT can be helpful in characterizing a tumor
• QCT can distinguish between cystic and solid
  lesions

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QCT

• With spirometric control and breath-triggered
  imaging, QCT can measure the density and
  characterize the structure of lung tissue
• QCT is very helpful for measuring tissue
  perfusion following bolus injection of an
  iodinated contrast media
• QCT allow measurement of cerebral blood flow
  following xenon inhalation

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Heart Scan

• Electron Beam CT (EBCT) of the heart can reveal
  plaque volume and calcium content in coronary
  arteries
• EBCT of the heart is a cost effective screening
  for coronary artery disease with no patient
  discomfort
• A typical heart scan consists of 40 ECG triggered
  3 mm images acquired at 100 ms each

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Heart Scan

• Calcium is a natural marker in atherosclerotic
  plaques making EBCT an effective screening device

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CT Angiography (CTA)

• CTA allows maximum visualization of the pulmonary
  artery and its segmental branches
• Stroke is the highest cause of morbidity and the
  third highest cause of mortality
• CTA requires low kVp and mA (90kVp, 100mA)
• CTA is best performed with spiral CT

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CTA

• Unlike MRA, CTA is not susceptible to motion or
  flow artifacts
• CTA is much less invasive and lower in dose than
  conventional angiograms
• Bone is removed from CTA maximum intensity
  projection images by an editing procedure called
  image segmentation

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CTA

• CTA requires less film than conventional
  angiography
• CTA requires less staff than conventional
  angiography
• CTA requires less contrast media than
  conventional angiography
• CTA is at least a factor of two less expensive
  than conventional angiography

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CTA

• CTA employs MIP and multiplanar reconstruction to
  maximum advantage
• CTA success depends on collimation, pitch, vessel
  orientation, and reconstruction interval

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Multislice Imaging (MI)

• This development was first produced by Elscint
  and is now available from all CT manufacturers
• Multislice imaging incorporates two or more
  contiguous detector arrays
• Multislice imaging produces two or more section
  images simultaneously
• Multislice imaging is a spiral CT technique

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MI

• Multislice imaging greatly reduces imaging time,
  from approximately 3 minutes with conventional CT
  to less than 30 seconds
• The main advantage to MI is faster imaging with
  better spatial resolution
• Complete x-ray tube/detector array rotation in
  less than 1s
• Partial scan images can be obtained in
  approximately 100ms

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MI

• Best imaging requires a pitch of 31 to 61
• Multislice imaging requires exceptional
  engineering because of the mechanical forced
  produced by gantry rotation
• Image reconstruction uses 360 degree
  interpolation rather than 180 degree
  interpolation

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MI

• 360 degree interpolation of multislice images
  allows faster imaging with improved spatial and
  temporal resolution and reduced noise
• Misregistration of anatomy is reduced because of
  the faster couch speed
• Motion artifacts are greatly reduced
• Patient breath-hold is much less demanding

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MI

• Imaging larger z-axis volume in less time is
  possible with MI
• Less contrast media is required
• Patient throughput is increased with MI
• Variable slice thickness can be produced with
  postprocessing. In many cases, rescan is
  unnecessary because of postprocessing

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MI

• CTA is greatly improved with MI
• Because of imaging speed, coronary artery
  calcification assay with MI is a challenge to EBCT