Hemodynamics & Exam Techniques

DOPPLER FLOW EVALUATION TECHNIQUES

Doppler Ultrasound Physics

Johann Christian Doppler first identified the physical principles involved in Doppler evaluation
Doppler shift: the mathematical difference between the transmitted and received frequencies, occurs when structures are moving toward or away from the "listener"
Train whistle blows as it moves toward you at the station, as it gets closer to the station the approaching train whistle has a higher pitch (frequency) whereas departing train whistle has a different, lower pitch (frequency)
Tissues and other non-mobile structures do not exhibit a Doppler shift
Reflected sound has the same frequency as the transmitted sound if the blood is stationary
If the blood is moving away from the transducer, the reflected sound has a lower frequency
If the blood is moving toward the transducer, the reflected sound has a higher frequency

Doppler calculates VELOCITY, not SPEED!

Speed – measure of magnitude, any distance divided by unit time, 30 m/s

Velocity – measure of magnitude & direction, positive or negative direction related to starting point, + 30m/s or - 30m/s

Quadrature Phase Detection:

Methods used to determine the direction of blood flow for bidirectional Doppler systems
Requires two speakers to allow one for flow toward and one for flow away

Blood Evaluation Techniques:

B Mode Flow
Color Doppler
Power Doppler
Spectral Doppler; Pulsed Wave Doppler and Continuous Wave Doppler
Analog Doppler
Tissue Doppler

Color Doppler:

Detecting Doppler shifts from hundreds of locations
Colors are encoded to display varying hues
Superimposed over 2D image; threshold control determines which shades of grey are covered by color pixels
Indicates the direction and the average velocity of blood flow
Thousands of samples to process
Autocorrelation is used to manipulate data

Velocity vs Variance Mode:

Velocity – colors on top of the stripe indicate blood flow toward the probe and colors on the bottom of the stripe indicate blood flow away from the probe; color change with velocity variation always seen up and down the stripe

Variance – used to determine turbulence and represent velocity; colors on top of the stripe indicate blood flow toward the probe and colors on the bottom of the stripe indicate blood flow away from the probe AND colors on the left side of the stripe represent laminar flow and colors on the right side of the stripe indicate turbulent flow; color change with velocity variation always seen up and down the stripe AND from left to right

Autocorrelation:

Used to process Doppler shift information for Color Doppler
Automatically correlates data from multiple sampling sites
Displays average Doppler shift frequencies
Velocity measurement accuracy is limited because angle correction is not utilized

Advantages of Color Doppler:

“Triplex” capabilities
Permits instantaneous identification of areas of abnormal blood flow.
Easier placement of the single line cursor that is used for spectral Doppler

Potential Errors:

Box position
Box size
Gain too low/high
Filter too high
Scale set too low/high

Color Doppler Hints:

Adjust scale, gain, and baseline
Decrease size of sample for improved flow visualization
Create angle of vessel across the screen, NO 90º

Reynold’s Number:

# expressing the balance of inertial and viscous forces acting on the blood flowing
Used to discuss Color Doppler turbulence
Average flow speed X vessel diameter X density
< 1500 for laminar flow
> 2300 abnormal

Power Doppler Imaging:

AKA Color Angio or Energy Mode
Displays an approximation of lower echo amplitudes
More sensitive to lower Doppler shifts than normal color-flow Doppler
Demonstrates very low velocity blood flow with no aliasing
Disadvantages: Low frame rates and more susceptible to tissue-motion artifacts

Spectral Doppler:

Doppler shifts are detected along a single line and used to produce a graphic representation of the blood velocity
Spectral Doppler systems contain spectrum analyzers to produce the graphic displays.

Spectrum Analyzer:

Contained within the Doppler system
Produces the graphic displays
Blood velocity is calculated, NOT measured
Calculates of peak and mean Doppler shift frequencies
Uses the mathematical process Fast Fourier Transform (FFT) to process data and provide it for graphic display
Displays positive Doppler shifts above the zero baseline
Displays negative Doppler shifts below the zero base line
Magnitude of the shift directly related to the amplitude of the tracing

PW Doppler:

AKA gated Doppler
Uses controlled time intervals between transmitted and received sounds to collect information using a single PZT element
Depth selected by technologist
Gate size or sample volume used to select location and amount of blood sampled (range resolution or free of range ambiguity)
Aliasing is the biggest disadvantage, limiting velocities that can be evaluated
Nyquist Limit indicates the highest velocity that can be displayed without aliasing
Aliasing can be corrected by using a lower frequency transducer, increasing PRF or scale and adjusting baseline
Proper setting of the angle correction cursor = accurate velocity calculations
Increased transmitted frequency will cause an increase in the Doppler shift detected; Incident frequency is directly related to the Doppler shift
As the Doppler angle increases, the Doppler shift decreases (gets closer to 90 degrees and 0cm/s)

Potential Errors:

Improper sample location #1 reason missing a stenosis
Improper probe position
Improper angle of incidence
Large sample size
Excessive probe pressure
Improper filter settings
Insufficient gel
Aliasing due to structure depth

PW Doppler Hints:

Always sample the center of the vessel EXCEPT when stenosis is present
Take multiple samples in the area of the stenosis in case an eccentric jet is present
Always use <60° angle on cursor with arterial exams
Adjust scale, baseline, and gain to properly demonstrate waveforms
Smaller sample sizes give more accurate waveforms at specific depths
Cursor angle must be as parallel to flow as possible, can be less than 60° in order to stay parallel

CW Doppler:

Dedicated CW probes do not use B-mode imaging
Separate transmit and receive PE elements
Biggest advantage is can be used to calculate blood flow at HIGH velocities
Dedicated CW Doppler probes have increased sensitivity to small Doppler shifts
Cannot determine location of highest velocity which is called Range Ambiguity
Provides Doppler shift information for all vessels in its path, no depth control
Insonation of multiple vessels is a significant limitation

Analog Doppler:

Graphical recording of pulsatile Doppler in a non-spectral (in the form of compressed, thin-line analog tracings) or a strip chart recording
Uses pencil style transducer

Acceleration Time :

Time from onset of systole to point of maximum systolic peak
Increases in extremities as stenosis increases
Decreased cardiac output can decrease the AT

Resistive Index:

Measurement of vascular resistance
Compares the difference between the systolic and diastolic velocities to the maximum velocity of the vessel segment
Used to evaluate arterial stenosis

Pulsatility Index:

Compares the difference between the systolic and diastolic velocities to the average velocity of the vessel segment
Used to distinguish inflow disease from outflow disease
Values increase as you evaluate vessels further from the aorta
Higher in extremities than organs, normally

Energy Types:

Kinetic: energy of motion, varies with mass and velocity squared, greater the mass = more kinetic energy = greater velocity
Potential: stored energy of motion, hair spray can; primary form of energy driving blood flow; related to the pressure distending the vessels
Gravitational: stored energy related to elevated position, roller coaster (Hydrostatic pressure)

Kinetic energy reduced in blood vessels due to :

Viscosity – thicker blood flows more slowly, increased hematocrit = decreased flow
Friction – layers of blood sliding against each other; energy lost due to friction is lost in the form of heat
Inertia – energy is lost when the speed of blood changes with systole and diastole of the heart

Arterial Blood Flow:

Blood flow is related to kinetic and potential energy
The moving blood represents kinetic energy
The pressure build up caused by cardiac contraction represents the potential energy
Total fluid energy is the sum of the kinetic and potential components

Factors Affecting Blood Flow:

Cardiac Function: decreased function = decreased flow
Peripheral Resistance: determines the rate of flow in the arteries and is regulated by the arterioles
Vessel Compliance: stiffer or more calcific vessels = higher resistance flow
Tone of vascular musculature: more muscular the patient = less compliant vessels = higher resistance flow
Pattern of branches or collaterals
Vasoconstriction: tightening of vessel walls due to stimuli; cold, anxiety, smoking
Vasodilation: expansion of vessel walls due to stimuli; heat, HTN medication, proximal stenosis
Viscosity: refers to internal friction between the adjacent layers of a fluid; energy lost in the form of heat; The greater the viscosity, the greater the resistance; Directly related to hematocrit levels in blood
Exercise: induces vasodilation in the vessels supplying the skeletal muscles
Autoregulation: vascular beds alter resistance to flow to maintain the levels needed for normal function

Laminar Flow:

Concentric layers of flow each w/ a slight difference in velocity
Center layers have the highest velocity
Outer layers are slower due energy loss from friction
Plug flow and parabolic flow are types of laminar flow

Resistance:

Determines the rate of flow in the arteries and is regulated by the arterioles
"Push-back" from vascular beds against forward flow of blood during the cardiac cycle
Extremities/muscles have much higher resistance to blood flow than the organs
As resistance increases, blood flow decreases with no change on pressure
If resistance increases, pressure must increase to maintain constant blood flow
Stenosis causes resistance to increase proximal to a stenosis
Resistance decreases distal to a stenosis

Pressure:

Increases with systole
As peripheral pressure meets and exceeds arterial pressure, diastole occurs
Average pressure in the veins 2 mm Hg, average arterial pressure 100 mm Hg
Arterial stenosis leads to decreased pressure as the velocity increases

Volume:

Always proportional to blood pressure
Increased volume = increased blood pressure

Bernoulli Effect:

In order to maintain flow energy there is a reduction in pressure seen with an increased blood velocity at a stenosis; decreased pressure = increased velocity
After the stenosis, the velocity decreases causing an increase in pressure

Blood Flow Terminology:

Antegrade: moving forward in the normal direction of blood flow
Retrograde: backward flow or filling, or against the normal direction of flow
Turbulence: disrupted flow cause by a stenosis, tortuosity or bifurcation, appears at the exit point of a stenosis
Murmur – abnormal blood flow sound in the heart, usually valvular regurgitation
Bruit – abnormal blood flow sound in a blood vessel, can be due to stenosis; also seen with vessel branching or tortuosity
Thrill – abnormal blood flow sensation in a blood vessel (vibration), can be due to stenosis; also seen with pseudoaneurysm and hemodialysis grafts

Arterial Blood Flow:

Arterial waveforms are determined by where the artery arises and what it is feeding

Systole: Cardiac contraction pushes blood forward through the arterial system

Diastole: The heart relaxes and refills with blood from the lungs, forward flow continues in low resistance arteries

Pulsatility: Continuous throbbing or beating, flow in the arteries pulsates due to continuous cardiac contractions, venous flow is not considered pulsatile because it is not moved by cardiac contraction

Arterial Pulsatility: related to the number of changes in flow direction during one cardiac cycle

Monophasic - all flow is antegrade during the entire cardiac cycle, vessels that feed low resistance vascular beds

Biphasic - Flow during systole is antegrade and some flow is moving in a retrograde fashion during dyastole, vessels feeding medium to high resistance vascular beds

Triphasic - Flow during systole is antegrade and during diastole initially some flow moves in a retrograde direction followed by a small amount of forward flow in end diastole

Stenosis:

Narrowing of a vessel lumen
50% diameter and 75% area stenosis considered hemodynamically significant in most arteries
Leads to increased resistance proximal to site
Causes increased peak systolic and end diastolic velocities at the site due to body trying to maintain blood volume
Distal to the stenosis lower velocity, low resistance flow with an increase in diastolic flow will occur as the vascular beds try to "encourage" more flow distally
Collaterals usually have higher resistance flow than their native vessels due to decreased vessel diameter and increased length

Leads To:

Increased velocity
Post Stenotic Turbulence (eddy currents)
Drop in pressure at the stenosis - greater the pressure gradient, the greater the velocity at the stenosis
Decreased resistance and velocity distal to stenosis
Leads to changes in the pulsatility and resistance of the waveform/flow pattern

VENOUS HEMODYNAMICS

Hydrostatic Pressure:

Weight of the column of blood inside the vessels
Equal to zero at the level of the heart
Supine: HP = 0mmHg
Standing:

HP = 100mmHg at ankle

HP = 0mmHg at heart

HP = -30mmHg at head

Transmural Pressure:

Pressure within the veins pushing outward
Normally low
Increases with venous volume

Calf Muscle Pump:

Veins act as reservoirs
Muscle contractions move flow through venous system toward heart
Competent valves (unidirectional flow)
Effective calf pump reduces blood stasis/pooling

Doppler Flow Characteristics of Veins:

Spontaneous Flow:

Flow is present in the vein without flow augmentation

Respiratory Phasicity:

Flow moves from areas of high pressure to low pressure

Inspiration:

Decreased intrathoracic pressure with increased intra-abdominal pressure
Increased flow in upper extremities toward the heart
Decreased flow in lower extremities

Expiration:

Increase in intrathoracic pressure with decreased intra-abdominal pressure
Decreased flow in upper extremities
Increased flow in lower extremities toward the heart

Cardiac Pulsatility:

"Ripples" of motion caused by adjacent cardiac contraction
Causes turbulent flow in the upper vena cava, upper arms, and hepatic veins
Abnormal if seen in lower extremities = CHF

Arterial waveforms are determined by where the artery arises and the structure it feeds!

Distal Augmentation:

Lightly squeeze muscular portion of patient's calf/forearm or have patient flex their foot/hand quickly
Normally increases flow toward heart followed by a lack of flow reversal and flow normalization toward heart
Decreased augment = obstruction between probe and augmentation location
Valvular insufficiency will demonstrate an increase in flow toward the heart followed by retrograde flow toward distal extremity
Length of time reflux occurs is directly related to the severity of valvular insufficiency

Valsalva Maneuver:

Increase intrabdominal pressure; simulate stressing during bowel movement
Evaluates Extremity veins, Iliac veins and IVC
Normally the maneuver performed, flow toward heart should stop with no flow reversal
Augmentation of signal toward the heart upon release
Augmented flow reversal at onset of Valsalva = incompetence

DOPPLER ARTIFACTS:

Clutter:

Unwanted Doppler display patterns typically caused by a vessel’s wall motion
Called ghosting for color Doppler
Filters used to correct the issue

Filter: an electronic circuit designed to allow signals to pass while stopping signals of other frequencies
High-pass: a filter that allows high, but not low, frequencies to pass through
Low-pass: a filter that allows low, but not high, frequencies to pass through

Aliasing:

Misrepresentation of the Doppler signal due to PRF limitations
Causes colors/waveform to “wrap around” from top to bottom on the baseline

SOLUTIONS

Increase scale
Lower freq. probe
Change angle°
Adjust approach to decrease distance to vessel
Switch to CW

Cross Talk:

Type of mirror imaging
Caused by receiver gain too high or incident angle near 90°

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Reference List

Electrical Interference:

Appears as repetitive, consistent artifactual echoes demonstrated on the Doppler tracing
The frequency of the electrical signal is directly related to the number of artifact echoes demonstrated in one second

Refers to the study of blood and how/why it moves
Arterial and Venous Hemodynamics very different due to structure and purpose

HEMODYNAMICS

Systemic VS Pulmonary Circulation:

Systemic:

Heart – aorta – arteries – arterioles – capillaries – venules – veins – Heart

Pulmonary:

Heart – pulmonary artery – lungs – pulmonary veins – Heart

Note: Click any image to enlarge.

Cardiac Pump:

Pumps 70 ml of blood into aorta with each contraction
Increased heart rate leads to increased cardiac output
Systemic pressure is greatest at the heart and it gradually decreases as it moves distally
Systole - LV pressures increases rapidly to exceed aortic pressure and push a bolus of blood flow into the arteries
Diastole - LV pressure decreases rapidly and it refills with blood from the LA

Hemodynamics, Exam Techniques and General Considerations

B Mode Flow Imaging:

Recent technology
Maintains the available details in the echoes from the RBCs
Main benefit is this technique provides more hemodynamic information and increased visual differentiation between true blood flow and wall motion artifacts when compared to color Doppler

Poiseuille’s Equation:

Small changes in vessel radius cause most significant changes in flow
Vessel length and viscosity are relatively constant; increases in either one will caused increased resistance to flow

Long Version Poiseuille's Equation:

P = pressure P1 - P2 = 8mLQ

m = viscosity pr

L = vessel length

Q = volume flow

r = radius

Simplified Poiseuille's Equation:

R= resistance

Q= flow P1 - P2 = R x Q

P1 - P2 = pressure difference

A high resistance waveform demonstrates a sharp systolic peak with minimal flow in diastole.

A low resistance waveform demonstrates a rounded peak with significant diastolic flow.

During cardiac systole, the pressure in the arteries steadily increases until the start of diastole. Pressures in the arterial system will decrease to a presystolic pressure level with cardiac diastole. The normal flow volume that is moved through the arteries during the cardiac cycle varies with the resistance of the vascular bed it is supplying.

The higher the resistance in the vessel, the more pulsatile the flow. A triphasic Doppler waveform is normally seen in a high resistance vessel (extremities). A monophasic Doppler waveform is normally seen in a low resistance vessel (organs).

Muscles and extremities do not require a significant amount of arterial flow when at rest, which leads to an increase in resistance in the vessels supplying these tissues. Organs, such as the kidneys and brain, require a continuous amount of arterial inflow, which leads to a significant decrease in the resistance to flow in these vessels.

Arterial stenosis can be estimated using 2D imaging. By measuring the actual lumen and comparing this to the vessel size, the amount of narrowing can be calculated. 50% diameter stenosis equates to 75% area stenosis. PW Doppler evaluation provides a more accurate estimate of stenosis.

Arterial stenosis causes several key changes to the PW Doppler waveform. The example demonstrates a "normal" extremity waveform with high resistance flow proximal to the stenosis. At the area of the stenosis, the velocity of the flow increases significantly and spectral broadening occurs due to turbulent flow. Blood flow distal to the stenosis will demonstrate a monophasic, low resistance waveform with increased diastolic flow.

Distal to a stenosis, the arterial resistance will become very low to aid in compensating the distal vascular bed for the loss of volume due to the stenosis.

The image demonstrates an ICA stenosis with significant post stenotis turbulence. Note the increased peak velocity, spectral broadening and increased diastolic flow levels consistent with stenosis.

Transmural pressure within the venous system is normally very low which allows for a flexible shape and increased compressibility. When the pressure in the venous system increases to an abnormal level, the veins become engorged with blood and demonstrate diminished flexibility.

Normal venous flow is spontaneous, demonstrates respiratory phasicity and responds to augmentation maneuvers properly.

The respiratory response to venous flow in the deep veins in the lower extremities is the opposite of the response that occurs in the upper extrtemity deep veins. As the patient takes in a breath, the diaphragm moves inferiorly to increase the pressure in the abdomen while decreasing the pressure in the chest cavity. Flow in the lower extremities will decrease as the diaphragm moves inferiorly and flow in the arms will increase toward the heart. When the patient lets the breath out, the diaphragm moves superiorly to decrease the intra-abdominal pressure while increasing the intra-thoracic pressure. Expiration will allow an increase in lower extremity flow while the upper extremities will decrease their flow volume toward the heart.

Distal augmentation can also be used to evaluate the presence/absence of valvular insufficiency. The flow will increase toward the heart with distal augmentation but will reverse in direction after the initial increase.

Normal venous response to the Valsalva maneuver is cessation of venous flow toward the heart. Flow reversal at the onset of the Valsalva maneuver indicates the presence of reflux.

Blood flow toward the probe will demonstrate a positive change in frequency which will be displayed on the top side of the Doppler baseline. Blood flow away from the probe will demonstrate a negative change in frequency which will be displayed on the bottom side of the Doppler baseline.

The smaller the area of color evaluation, the better the color display will be on the image. A smaller color Doppler FOV offers a faster frame rate for improved flow evaluation and display.

The transducer will detect blood from that is moving toward or away from the probe. Steering the color box will make the flow in the vessel less perpendicular to the US beam, therefore improving the detection and display of the flow.

Power Doppler is much more sensitive to low velocity flow movement than color Doppler but in most US systems the direction of flow is not displayed with Power Doppler.

Doppler Interpretation:

Arteries carrying blood to organs normally exhibit low resistance waveforms with antegrade flow throughout the cardiac cycle
Arteries carrying blood to extremities/muscles normally exhibit high resistance waveforms with retrograde flow during diastole
Anytime a monophasic waveform is seen in an extremity artery = ABNORMAL
Anytime a biphasic/triphasic waveform is seen in an artery supplying an organ = ABNORMAL

Spectral Broadening:

Widening of the spectral waveform with filling in of the window
Causes: stenosis, tortuosity, and bifurcations

Post Stenotic Turbulence:

Flow spreads out into the larger vessel area distal to stenotic area
Wide range of flow velocities
Causes aliasing on Color Doppler and spectral braodening on PW Doppler

GENERAL CONSIDERATIONS:

Considerations for ALL Exams:

Universal Precautions guidelines, handwashing, isolation techniques, AKA Standard Precautions
Personal protective equipment (gowns, goggles, masks etc)
CPR guidelines - 30 compressions/ 2 breaths adult and child single rescuer protocol
Choking - abdominal thrusts
Syncope - place patient in supine position and elevate legs
Be careful of other medical equipment connected to patient; oxygen, IV therapy, monitors; NEVER alter settings or alter other equipment in use.
Invasive procedures - sterile technique and informed consent, trays and instruments opened at the start of the procedure, NOT before.
Communication imperative
Probe sanization after each patient, Sani-wipes for most probes is sufficient except Cidex is used for TV probes.

Sonographer and Patient - explain exam, obtain pertinent history, when to expect results
Sonographer and Radiologist - findings, exam difficulties, prior exams

EVALUATING TEST RESULTS:

1. Sensitivity-The ability of a test to detect disease when it IS present. The number of truly positive tests performed in the current modality is compared to the positive tests diagnosed by the gold standard test

# True Positive Tests / Total # Positive Tests proven positive by the gold standard test

2. Specificity-The ability of a test to rule out disease when it IS NOT present. The number of truly negative tests performed in the current modality is compared to the negative tests diagnosed by the gold standard test

# True Negative Tests / Total # Negative Tests proven negative by the gold standard test

3. Positive Predictive Value-Calculates how often a positive study is correctly diagnosed; ability of an exam to predict the presence of disease

# True Positive Tests / Total # Positive Tests

4. Negative Predictive Value-Calculates how often a negative study is correctly diagnosed; ability of the exam to predict the absence of the disease

# True Negative Tests / Total # Negative Tests

5. Accuracy-ability of test to correctly identify disease or lack of disease; total number of correct diagnoses compared to the total number of tests;

# True Positive Tests + # True Negative Tests / Total # Tests Performed

Overall accuracy value is always between sensitivity and specificity AND between PPV and NPV

EX: sensitivity 80%, specificity 92%, PPV 94%, NPV 88%

Accuracy must be between 80 - 92% AND between 88 - 94%

To meet all requirements the accuracy must be between 89 -91%

89% is greater than 80% sensitivity and greater than 88% NPV

91% is less than 92% specificity and less than 94% PPV

Four Components of a QA Program:

Preventive Maintenance
Repairs
System assessment of all components
Record Keeping

Preventive Maintenance:

Performed by the manufacturer every 6 months, included in most service contracts
Vacuum filters, wipe down entire machine on a biweekly/monthly basis
Wipe screen, cords on a weekly basis
Clean keyboard, trackball, probe, and handles/levers used for equipment maneuvering between exams

Record Keeping:

Another important part of preventive maintenance/QA program
Current log of QA results
Warranty contract for equipment
System manual for equipment
Maintenance Log with downtime recorded

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Ergonomics of Sonography:

Work Related Muskuloskeletal Disorders (WRMSD) are caused by repetative motion, forced positioning, poor posture, excessive pressure and strain
Neck injuries most common for Sonographers

Commonly affected muscles in neck:

Levator scapulae
Sternocleidomastoid
Scalene

Commonly affected muscles in the shoulder:

Suprasinatus
Infraspinatus
Trapezius
Rhomboids
Teres Major/Minor

Common Injuries:

Rotator cuff tear - injury of the connective tissues of the shoulder causing pain, decreased mobility with limited range of motion
Carpal tunnel syndrome - numbness, weakness, pain and swelling caused by compression of the median nerve by the carpal ligament
Chronic neck pain
Chronic back pain
Paresthesia in arm/hand/fingers

How to Prevent Injury:

Limit amount of arm abduction (less than 30 degrees is optimum)
Adjust chair/table height to decrease angle of arm to body and wrist flexion
Position yourself as close to patient as possible
Use a machine with adjustable keyboard and monitor height
Vary the type of exam performed
Perform stretches daily