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Transcranial Doppler

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Introduction

Transcranial Doppler (TCD) is a non-invasive pulsed ultrasonic investigation that is very easy to learn and apply. This test is used in the diagnosis and investigation of cerebrovascular disease. The TCD would give reliable results in the investigation of the extra and intracranial circulation with high index of sensitivity and specifity. In this page you will learn the most applications used with TCD


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Principle of TCD

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Anatomical considerations

It is important to understand the anatomy and the anastamotic blood flow pattern in the extra and intracranial vessels in order to get a correct interpretation of a TCD study findings

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Normal TCD

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Normal TCD Values from Saudi Population

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Carotid vessel abnormalities

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Intracranial vessels abnormalities

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Vertebral vessel abnormalities

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Vasomotor reactivity

With Acetazolamide or Breath Holding

Cerebrovascular (Vasomotor) Reactivity Test using Transcranial Doppler (TCD) with Acetazolamide

This method is done to assess the cerebrovascular (vasomotor) reserve in patients with high risk for stroke.

These include:

  • Internal carotid artery stenosis (> 70 %).
  • MELAS and other mitochondrial cytopathy.
  • AVM
  • Migraine
  • Sickle cell disease

Procedure:

1.     Continues monitoring of target vessel(s) at best depth resolution by a 2 MHz TCD probe fixed with a head band, unilaterally or bilaterally; according to nature of study.

2.     A capnometer is connected to measure end-expiratory CO2 concentration continuously to assure hypercapnia.

3.     Patient should be relaxed and breathing normally before starting the test.

4.     A steady state of mean blood velocity (Vmean) and CO2 concentration should be achieved for 4 minutes prior to start of test.

5.     Then 1g of Acetazolamide is injected through an intravenous route.

6.     10 minutes after injection, Vmean is measured again.

7.     The vasomotor reactivity is calculated using the following formula:

% Change in Vmean = 100X (V1-V2)/V1   (Normal 36-42%)

Absolute Vmean increase = V1-V2   (Normal 28±12cm/s)

V1 is the Vmean10 minutes after acetazolamide injection

V2 is the Vmean at rest

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Cerebrovascular Reactivity Test using Transcranial Doppler (TCD) with Breath Holding Method

This method is done to assess the cerebrovascular (vasomotor) reserve in patients with high risk for stroke. 

These include:

  • Internal carotid artery stenosis (> 70 %).
  • MELAS and other mitochondrial cytopathy.
  • AVM
  • Migraine
  • Sickle cell disease

Method 

  1. Continues monitoring of target vessel(s) at best depth resolution by a 2 MHz TCD probe fixed with a head band is applied; unilaterally or bilaterally according to nature of study.
  2. A capnometer is connected to measure end-expiratory CO2 concentration continuously to assure Hypercapnia
  3. Patient should be relaxed and breathing normally before starting the test. 
  4. A steady state of Vmean and CO2 concentration should be achieved for 4 minutes prior to start of test. 
  5. Then the patient is asked to hold breath as long as he can. The breath holding time and blood velocities are noted on line, as well as recorded. 
  6. The test then is repeated, after a 2-3 minutes of normal breathing.
  7. The vasomotor reactivity is calculated using the following formula:

% change in Vmean =100X (V1-V2)/V1   (normal 25 ± 10%)

Breath holding index = (V1-V2)/T  (normal 1.42 ± 0.13)

T is the period of breath holding in seconds.

V1 is the Vmean exactly at the end of the breath holding period (which is mean value of both tests).

V2 is the Vmean-pretest (normal breathing)  

Another standard  measurement is the full range change of Vmean from hyperventilation to hypercapnea. This is achieved by the following:

Ask the patient to hyperventilate till a steady state of Vmean (and preferably a steady end expiratory CO2 concentration) is reached. This is usually accomplished with 2 minutes of hyperventilation.

  1. Observe and record the Vmean at the end of hyperventilation (V3). 
  2. After a period of normal breathing (4 minutes minimum), do the breath holding test as described above. 
  3. Observe and record the Vmean at the end of breath holding period (V2
  4. Calculate the full range Vmean change using the simple formula:

Full Vmean range = V2-V3   (Normal 101 ± 24 cm/s)

References

  1. Markus and Harrison, Stroke, Vol 23, No 5, May 1992, 668-673
  2. M. Muller, Stroke, Vol 26, No 1, January 1995, 96-100

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Bubble test

Embolic Detection in Patients with Right to Left Shunt

The test is done as demonstrated in the drawing after fixing the 2MHz Transcranial Doppler probe at the best depth identifying the MCA: 

  1. Mix the 2 ml of patient blood with 4 ml of saline and 0.5 ml of air well until a foamy liquid is formed. 
  2. Immediately inject the formed liquid through the 3 way valve into a well size canula inserted in a vein in the cubital fossa
  3. The start of injection is time 0 second for observing the Doppler trace. 
  4. The first injection is done with out valsalva maneuver(spontaneous).

The test is then repeated but this time with the valsalva maneuver as follows: 

  • The patient is asked to take deep breath, hold it and strain for 1-2 seconds then exhale. This maneuver is done at the 4th second after start of injection. If the test is negative or doubtful; it can be repeated up to 3-4 times, but the only change is that of increasing the time span of starting the valsalva maneuver in a step wise from 4 to 6, 8 and 10 seconds as the test is repeated.

Observe the Doppler trace  through out the test to identify and count any emboli (high intensity signals-HITS), if any, with particular attention to the earliest and last emboli.(better to memorize their onset to the injection and valsalva).

Note the following:

  1. Earlier HITS are the most significant  to indicate the presence of a Rt. to Lt. shunt; either at cardiac (PFO) or lung (fistula) levels.
  2. The number of HITS correlates with the severity of the shunt.
  3. If HITS occur with the spontaneous method, they indicate a severe Rt. to Lt. shunt.
  4. If HITS occur at or after the 20th seconds to the onset of injection, they are insignificant. 
  5. In case of a ventilated patient, the valsalva is replaced with manual maneuver, where after an inspiratory phase; the ventilator is held for 4-6 seconds in order to maintain an increased intrathoracic pressure temporarily then release for exhalation or simply by applying pressure to the abdomen

 

Click on picture to see the videos for demonstrations, or click download

 

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Subclavian steel

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HITS

Basic Criteria for Doppler High Intensity Signal Transients (HITS)

·        HITS are transient, usually lasting less than 300 milliseconds, signals that occurs at any time during the cardiac cycle.

·        Its amplitude is usually at least 3 dB higher than that of the back ground blood flow signal (or > 12 dB of the screen back ground).

·        It is unidirectional within the Doppler velocity spectrum.

·        It is accompanied by a “snap,” “chip,” or “moan” on the audible output.

·        Solid HITS are difficult to differentiate from gaseous HITS which occurs in patients having prosthetic valves, except by using a spectral analysis. 

·        Gaseous emboli has a maximum amplitude of  > 110 and a sum of amplitudes > 600

·        A soft-ware using bigated TCD with a 2-5 mm depth difference can be applied to differentiate true from artifact signals.

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TCD and AVM

Blood Flow Velocities and Cerebrovascular (Vasomotor) Reactivity in Arteriovenous Malformation (AVM) using TCD

AVM’s are classified to small (£ 2.5 cm), medium (> 2.5 and £ 5.0 cm) and  large (>5.0 cm) according to their diameter angiographically.

TCD findings in an AVM are:

  1. Increased blood flow velocity in the ipsilateral extracranial ICA and/or ECA  (this is not a common finding though). 
  2. Increased peak and mean blood flow velocities of the ipsilateral vessels, more higher in feeder vessels. Less commonly; velocities are high in the contralateral vessels, but are less compared to the ipsilateral, particularly the feeder vessel(s). 
  3. Pulsatility index (PI) is reduced in feeder vessel(s). 
  4. Decreased vasomotor reactivity (VMR) in feeder vessel(s) and to a lesser extent in other non feeders. 

Points to consider:

1.               Blood flow velocity of an AVM feeder correlates with its size 

2.               Small AVM (£ 2.5 cm) is not detected easily 

3.               Slightly reduced VMR of an AVM feeder vessel indicates a low-flow and a high-pressure in that vessel which is a risk for a hemorrhage. 

4.               Markedly reduced VMR of an AVM feeder vessel indicates a high-flow and a low-pressure in that vessel which is a risk for a progressive neurological deficit (steel). 

5.               Neither the blood flow velocity nor the PI can predict the risk of hemorrhage. 

6.               The feeding artery pressure does not correlate with the AVM size. 

7.               TCD can plan the staging of surgery by studying the hemodynamics of the AVM. 

Characteristic high risk factors for an AVM to bleed are:

  1. Small AVM 
  2. Presence of a deep venous drainage 
  3. High feeding artery pressure 
  4. Slightly reduced VMR (low-flow and high-pressure) of the feeder vessel 

References 

  1. Rolf R. Diehl et al, Stroke, Vol 25, No 8, August 1994, 1574-80 
  2. Kader et al, Neurosurgery, Vol 34, No 5, May 1994, 801-6 
  3. Kader et al, Surgical Neurology 1993;39;392-98 
  4. H. Mast et al, Stroke, Vol 26, No 6, June 1995, 1024-26

 


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Transcranial Duplex

Transcranial color duplex (TCCD) is a rapidly growing field in Neurosonology. The use of this technique led to a more understanding of different pathological disease processes.

The applications of transcranial duplex are introduced recently and they include:

1.     Visualization of the circle of Willis arteries

2.     Identifying midline structures and measuring hemispheric diameters

3.     Identifying third ventricle and its width

4.     Identifying cerebral venous sinuses

5.     Studying the dynamics of AVM and large aneurysms

TCCD showing the circle of Willis

TCCD showing flow in MCA and ACA

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2 mega Hz probe at temporal window to visualize mid brain

2 mega Hz probe at temporal window to visualize third ventricle

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