INVESTIGATION

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INVESTIGATION

• Non-invasive Vascular Assessment
• (NIVA)
• The Carotid Duplex Ultrasonography

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What is Non-invasive Vascular Assessment?

• NIVA uses advanced imaging techniques to
  painlessly evaluate the circulatory system
  without the use of needles, dye or radiation
• NIVA includes ultrasound, which uses sound waves
  far above the range of human hearing to look
  inside the veins and arteries or listen to the
  sound of blood flow
• Complementary information can be obtained for
  diagnosis of cerebrovascular disease in patients
  with acute, subacute, chronic, or asymptomatic
  conditions. These techniques are also used for
  monitoring purposes, followup studies, clinical
  trials evaluating the benefit of medical and
  surgical procedures and for treatment and
  prevention of stroke

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Carotid duplex ultrasonography

• It combines high-resolution gray scale imaging
  and pulsed Doppler spectral analysis to yield
  excellent anatomic and physiologic data
• In addition, color-flow imaging helps the
  examiner and interpreter confirm vessel
  orientation, anatomic variants and various
  pathologies
• Carotid duplex ultrasound can reliably detect
  stenotic disease that may be the source of an
  embolus in a stroke patient
• Also, ultrasonography can be used to confirm
  occlusion of the internal carotid artery, which
  may also produce a stroke.

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What will happen during a Carotid NIVA Exam?

• GEL - neck area
• TRANSDUCER - converts electrical energy into
  sound waves - placed over each side of the neck -
  Sound waves bounce off the organs and tissue in
  the body and the blood moving in the arteries AND
  creates echoes that are reflected back to the
  transducer
• TELEVISION MONITOR - shows images as the
  transducer converts the echoes to electronic
  signals
• These IMAGES may be viewed immediately or
  photographed for further study

This exam takes 45 to 60 minutes
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The ultrasound demonstrates grey scale imaging in
the upper portion with spectral Doppler graphed
on the bottom.  The velocity measured at 3
meters/second is elevated and signifies an area
of stenosis.  Also the waveform is broadened
consistent with flow disturbance which is seen in
regions of stenosis.
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• The procedure is done in the CCA, internal
  carotid artery (ICA), and external carotid artery
  (ECA) at least 2 or 3 spectral analyses of each
  vessel should be obtained
• Color imaging and Power Doppler may be used but
  may not necessarily provide additional
  information.
• After assessment of the anterior circulation, the
  vertebral circulation is assessed. Usually, the
  C4-C6 segment is accessible

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• Power Doppler
• Color imaging that is independent of direction
  or velocity of flow
• Gives angiographic-like picture of artery
• Power Doppler Observation of flow is independent
  of velocity or direction.

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Plaque Characteristics

• The primary purpose of carotid scanning is the
  detection and assessment of carotid stenosis
• Ultrasound has been used to image and
  characterize plaque within the carotids. It is
  characterized as low, medium, or high in
  echogenicity and as homogenous or heterogenous.
• Low echo plaque contains a large amount of lipid
  material and is difficult to image. Moderate echo
  plaque is fibrous plaque made up of collagen and
  lipids. Strong echo plaque has strong reflections
  caused by vessel calcification.
• Calcified plaque can also be termed as
  heterogenous. Homogenous plaque is more uniform
  in texture.

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• When disease is detected in the carotid arteries,
  it is important to measure prestenotic, stenosic,
  and post stenotic velocities. It is very
  important that velocities are taken at the
  highest velocity.

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Degree of Stenosis

• The most commonly used methods of acoustic
  estimation of the degree of stenosis include the
  following
• Measurement of peak systolic velocities (PSV)
  and peak diastolic velocities (PDV)
• Measurement of peak systolic frequencies (PSF)
  and peak diastolic frequencies (PDF)
• Measurement of ratios (eg, ICA systolic
  frequency/CCA systolic frequency)

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General rules of thumb

• PSVs over 200 cm/s or PSFs over 5 kHz correspond
  to stenosis greater than 50
• As stenosis approaches 80-99, PSVs and PSFs tend
  to rise (as high as 400 cm/sec and 10 kHz,
  respectively), and the PDVs and PDFs tend to rise
  as well (often over 150 cm/s and 2.5 kHz,
  respectively)
• With stenosis over 90 (near occlusion),
  velocities and frequencies may actually drop as
  mechanisms to maintain flow fail
• Ratios may be particularly helpful in situations
  in which cardiovascular factors (eg, poor
  ejection fraction) limit the increase in velocity
  and frequency. In such cases, ICA/CCA ratios
  above 3 may signify significant stenosis

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Vessel Identification

• Common carotid artery (CCA)
• Pulsatile walls
• Smaller caliber than jugular vein
• Systolic peak and diastolic endpoints in between
  that of external and internal carotid arteries on
  spectral analysis

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Distinguishing internal and external carotid
arteries

• Differentiating between the ICA and ECA is
  probably the most difficult part of a carotid
  exam
• Anatomically, the ICA usually runs more posterior
  and lateral, while the ECA runs more anterior and
  medial. The ICA is normally larger than the ECA
  and slowly tapers as it travels toward the head.
  The extracranial ICA has no branches
• The ECA has many branches to feed the neck and
  face
• Vessel identity can be aided by spectral and
  color Doppler. Differences in pulsatility between
  the ICA and ECA are also visible with color flow
  Doppler

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Internal Carotid artery as seen on spectral
analysis

• Note that flow is continuous through diastole.
• The ICA has a low resistance waveform with a
  blunted systolic upstroke and more diastolic flow
• Blood flow continues throughout the entire
  cardiac cycle in the ICA resulting in continuous
  color fill

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External Carotid Artery as seen on Spectral
analysis

• Note the absence of flow in diastole and the
  sharp upstroke in systole
• The ECA waveform is more high resistance with a
  steeper systolic upstroke, and little to no
  diastolic flow
• In the ECA, color flickers or flashes because
  flow is diminished or absent in diastole

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ICA ECA
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Advantages

• Carotid duplex ultrasonography (CUS) is a useful
  diagnostic tool for assessing cervical carotid
  artery disease and examine the extracranial
  cerebrovascular system.
• Duplex scanning can localize arterial disease in
  the extracranial carotid arteries
• It can differentiate between a tight stenosis and
  occlusion
• This exam provides information about the surface
  character of plaque and can also evaluate
  pulsatile masses in the carotid.
• It is noninvasive, safe and relatively
  inexpensive. 
• It does not involve use of radiation or contrast
  dye

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Limitations

• Patients may have dressings, skin staples, or
  sutures that interfere with the exam
• The size and contour of the neck can also make
  the exam difficult
• Acoustic shadowing from calcified vessels may
  impair visualization
• Disease can be over estimated when an artifact is
  mistaken for plaque
• Disease may also be underestimated by failing to
  appreciate the low level echoes of soft plaque
• Using the wrong Doppler angle can either over or
  under estimate the severity and extent of the
  disease.