Transcranial doppler ultrasound analyzer of blood velocity
Transcranial Doppler (TCD) and transcranial color Doppler (TCCD) are types of Doppler ultrasonography that measure the velocity of blood flow through the brain's blood vessels by measuring the echoes of ultrasound waves moving transcranially (through the cranium). These modes of medical imaging conduct a spectral analysis of the acoustic signals they receive and can therefore be classified as methods of active acoustocerebrography. They are used as tests to help diagnose emboli, stenosis, vasospasm from a subarachnoid hemorrhage (bleeding from a ruptured aneurysm), and other problems. These relatively quick and inexpensive tests are growing in popularity. The tests are effective for detecting sickle cell disease, ischemic cerebrovascular disease, subarachnoid hemorrhage, arteriovenous malformations, and cerebral circulatory arrest. The tests are possibly useful for perioperative monitoring and meningeal infection.
The equipment used for these tests is becoming increasingly portable,
making it possible for a clinician to travel to a hospital, to a
doctor's office, or to a nursing home for both inpatient and outpatient
studies. The tests are often used in conjunction with other tests such
as MRI, MRA, carotid duplex ultrasound and CT scans. The tests are also used for research in cognitive neuroscience (see Functional transcranial Doppler, below).
Methods
Two methods of recording may be used for this procedure. The first uses "B-mode" imaging, which displays a 2-dimensional image of the skull, brain, and blood vessels as seen by the ultrasound probe. Once the desired blood vessel is found, blood flow velocities may be measured with a pulsed Doppler effect probe, which graphs velocities over time. Together, these make a duplex test.
The second method of recording uses only the second probe function,
relying instead on the training and experience of the clinician in
finding the correct vessels. Current TCD machines always allow both
methods.
How it works
The ultrasound probe emits a high-frequency sound wave (usually a multiple of 2 MHz) that bounces off various substances in the body. These echoes are detected by a sensor in the probe. In the case of blood in an artery, the echoes have different frequencies depending on the direction and speed of the blood because of the Doppler effect.
If the blood is moving away from the probe, then the frequency of the
echo is lower than the emitted frequency; if the blood is moving towards
the probe, then the frequency of the echo is higher than the emitted
frequency. The echoes are analysed and converted into velocities that
are displayed on the unit's computer monitor. In fact, because the probe
is pulsed at a rate of up to 10 kHz, the frequency information is
discarded from each pulse and reconstructed from phase changes from one
pulse to the next.
Because the bones of the skull
block most of the transmission of ultrasound, regions with thinner
walls (called insonation windows), which offer the least distortion to
the sound waves, must be used for analyzing. For this reason, recording
is performed in the temporal region above the cheekbone/zygomatic arch,
through the eyes, below the jaw, and from the back of the head. Patient
age, sex, race, and other factors affect bone thickness and porosity,
making some examinations more difficult or even impossible. Most can
still be performed to obtain acceptable responses, sometimes requiring
using alternative sites from which to view the vessels.
Implantable transcranial Doppler
Sometimes
a patient's history and clinical signs suggest a very high risk of
stroke. Occlusive stroke causes permanent tissue damage over the
following three hours (maybe even 4.5 hours),
but not instantly. Various drugs (e.g. aspirin, streptokinase, and
tissue plasminogen activator (TPA) in ascending order of effectiveness
and cost)
can reverse the stroke process. The problem is how to know immediately
that a stroke is happening. One possible way is the use of an
implantable transcranial Doppler device "operatively connected to a drug
delivery system".
Battery-powered, it would use an RF link to a portable computer running
a spectral analysis routine together with input from an oximeter
(monitoring the degree of blood oxygenation, which a stroke might
impair) to make the automatic decision to administer the drug.
Functional transcranial Doppler (fTCD)
Functional
transcranial Doppler sonography (fTCD) is a neuroimaging tool for
measuring cerebral blood flow velocity changes due to neural activation
during cognitive tasks.
Functional TCD uses pulse-wave Doppler technology to record blood flow
velocities in the anterior, middle, and posterior cerebral arteries.
Similar to other neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography
(PET), fTCD is based on a close coupling between regional cerebral
blood flow changes and neural activation. Due to a continuous monitoring
of blood flow velocity, TCD offers better temporal resolution than fMRI
and PET. The technique is noninvasive and easy to apply. Blood flow
velocity measurements are robust against movement artifacts. Since its
introduction the technique has contributed substantially to the
elucidation of the hemispheric organization of cognitive, motor, and
sensory functions in adults and children.
fTCD has been used to study cerebral lateralization of major brain functions such as language,
face processing,
color processing,
and intelligence.
Moreover, most established neuroanatomical substrates for brain function
are perfused by the major cerebral arteries that could be directly
insonated. Lastly, fTCD has been used as a brain-computer interface
modality.
Functional transcranial Doppler spectroscopy (fTCDS)
Spectral density plots right and left middle cerebral arteries cross-amplitude plots in men.
Facial Paradigms
Conventional FTCD has limitations for the study of cerebral
lateralization. For example, it may not differentiate the lateralising
effects due to stimulus characteristics from those due to light
responsiveness, and does not distinguish between flow signals emanating
from cortical and subcortical branches of the cerebral arteries of the
circle of Willis. Each basal cerebral artery of the circle of Willis
gives origin to two different systems of secondary vessels. The shorter
of these two is called the ganglionic system, and the vessels belonging
to it supply the thalami and corpora striata; the longer is the cortical
system, and its vessels ramify in the pia mater and supply the cortex
and subjacent brain substance. Furthermore, the cortical branches are
divisible into two classes: long and short. The long or medullary
arteries pass through the grey substance and penetrate the subjacent
white substance to the depth of 3–4 cm. The short vessels are confined
to the cortex. Both cortical and ganglionic systems do not communicate
at any point in their peripheral distribution, but are entirely
independent of each other, having between the parts supplied by the two
systems, a borderline of diminished nutritive activity.
While, the vessels of the ganglionic system are terminal vessels, the
vessels of the cortical arterial system are not so strictly "terminal".
Blood flow in these two systems in the middle cerebral artery (MCA)
territory supplies 80% of both hemispheres,
including most neural substrates implicated in facial processing,
language processing and intelligence processing at cortical and
subcortical structures. The measurements of mean blood flow velocity
(MFV) in the MCA main stem could potentially provide information about
downstream changes at cortical and subcortical sites within the MCA
territory. Each distal arm of the MCA vascular system could be separated
into "near" and "far" distal reflection sites for the cortical and
ganglionic (subcortical) systems, respectively. To accomplish this
objective, one method is to apply Fourier analysis to the periodic time series of MFV acquired during cognitive stimulations. Fourier analysis would yield peaks
representing pulsatile energy from reflection sites at various harmonics, which are multiples of the fundamental frequency.
McDonald in 1974 showed that the first five harmonics usually contain
90% of the entire pulsatile energy within the system of pressure/flow
oscillations in the peripheral circulation. It could be presumed that
each arm of the vascular system represents a single viscoelastic tube
terminated by impedance, creating a single reflection site.
Psychophysiologic stimulation induced vasomotor activity at each
terminal site sets up a standing sinusoidal wave oscillation, comprising
a summation of waves due to effects of incident, reflected, and
re-reflected waves from distal to proximal point of measurement. fTCDS
studies are performed with the participant placed in a supine posture
with their head up at about 30 degrees. The probe holder headgear (e.g.
LAM-RAK, DWL, Sipplingen, Germany) are used with a base support on two
earplugs and on the nasal ridge. Two 2-MHz probes are affixed in the
probe holder and insonation performed to determine the optimal position
for continuous monitoring of both MCA main stems at 50 mm depth from the
surface of the probe. A serial recording of MFV for each stimulus is
acquired and latter used for Fourier analysis. Fourier transform algorithm uses standard software (for example, Time series and forecasting module, STATISTICA, StatSoft, Inc.).
The most efficient standard Fourier algorithm requires that the length
of the input series is equal to a power of 2. If this is not the case,
additional computations have to be performed. To derive the required
time series, the data were averaged in 10-second segments for 1-minute
duration or each stimulus, yielding 6 data points for each participant
and a total of 48 data points for all eight men and women, respectively.
Smoothing the periodogram values was accomplished using a weighted
moving average transformation. Hamming window was applied as a smoother.
The spectral density estimates, derived from single series Fourier
analysis, were plotted, and the frequency regions with the highest
estimates were marked as peaks. The origins of the peaks are of interest
in order to determine the reliability of the present technique. The
fundamental (F), cortical (C) or memory (M), and subcortical (S) peaks
occurred at regular frequency intervals of 0.125, 0.25, and 0.375,
respectively. These frequencies could be converted to Hz, assuming that
the fundamental frequency of cardiac oscillation was the mean heart
rate. The fundamental frequency (F) of the first harmonic could be
determined from the mean heart rate per second. For example, a heart
rate of 74 bpm, suggests 74 cycles/60 or 1.23 Hz. In other words, the
F-, C-, and S-peaks occurred at multiples of the first harmonic, at
second and third harmonics, respectively. The distance of the reflection
site for F-peak could be presumed to emanate from a site at D1
= wavelength/4 = cf/4 = 6.15 (m/s)/(4×1.23 Hz) = 125 cm, where c is the
assumed wave propagation velocity of the peripheral arterial tree
according to McDonald, 1974.
Given the vascular tortuosity, the estimated distance approximates that
from the measurement site in the MCA main stem, to an imaginary site of
summed reflections from the upper extremities, close to the finger tips
when stretched
sideways. The C-peak occurred at the second
harmonic, such that the estimated arterial length (using common carotid
c = 5.5 m/s) was given by D2 = wavelength/8 = cf2/8
= 28 cm, and a frequency f of 2.46 Hz. The distance approximates the
visible arterial length from the main stem of the MCA, through vascular
tortuosity and around the cerebral convexity, to the end vessels at
distal cortical sites such as the occipito-temporal junction on
carotid angiograms of adults. The S-peak occurred at the third harmonic, and may have arisen from an estimated site at D3= wavelength/16 = cf3/16 = 9.3 cm and a frequency f3
of 3.69 Hz. The latter approximates the visible arterial length of the
lenticulostriate vessels from the main stem of the MCA on carotid
angiograms. Although not displayed, the fourth harmonic would be expected to arise from the MCA bifurcation in closest proximity to
the measurement site in the main stem of the MCA. The pre-bifurcation
length from the measurement point would be given by D4 = wavelength/32 = cf4/32 = 3.5 cm and a frequency f4
of 4.92 Hz. The calculated distance approximates that of the segment of
MCA main stem just after the carotid bifurcation, where probably the
ultrasound sample volume was placed, to the MCA bifurcation. Thus, these
estimates approximate actual lengths. However, it has been suggested
that the estimated distances may not correlate exactly with known
morphometric dimensions of the arterial tree according to Campbell et
al., 1989. The method was first described by Philip Njemanze in 2007,
and was referred to as functional transcranial Doppler spectroscopy
(fTCDS).
fTCDS examines spectral density estimates of periodic processes induced
during mental tasks, and hence offers a much more comprehensive picture
of changes related to effects of a given mental stimulus. The spectral
density estimates would be least affected by artefacts that lack
periodicity, and filtering would reduce the effect of noise.
The changes at the C-peak may show cortical long-term potential (CLTP)
or cortical long-term depression (CLTD), which has been proposed to be
suggest equivalents of cortical activity during learning
and cognitive processes. The flow velocity tracings are monitored
during paradigm 1 comprising a checkerboard square as object perception
are compared to whole face (paradigm 2) and facial element sorting task
(paradigm 3). Fast Fourier transform calculations are used to obtain the
spectral density and cross amplitude plots in the left and right middle
cerebral arteries. The C-peak also called memory (M-peak) cortical peak
could be seen arising during paradigm 3, a facial element sorting task
requiring iterative memory recall as a subject constantly spatially fits
the puzzle by matching each facial element in paradigm 3 to that stored
in memory (Paradigm 2) before proceeding to form the picture of the
whole face.
Accuracy
Although
TCD is not so accurate due to relative velocity of blood flow, but it
is still useful for diagnosis of arterial occlusions in patients with
acute ischemice stroke, especially for middle cerebral artery. A
research has been done to compare Power Motion Doppler of TCD (PMD-TCD)
with CT angiography
(CTA), both are valid, but PMD-TCD accuracy is not higher than 85
percent. The advantages of PMD-TCD is portable, so can be used in bed
side or in emergency room, no radiation as CTA, so can be repeated, if
necessary for monitoring and less expensive than CTA or Magnetic
Resonance Angiography.
