Doppler sonography (2. LF UK)

Introduction
Doppler sonography (ultrasonography) fits into the general diagnostic non-invasive imagery methods in medicine, that are used for assessing the movement velocity of blood flow in arteries. It is also famous as dopplers ultrasound (DUS). This article will talk about the theoretical principals of dopplers sonography and on the work procedure to be followed during biophysical practicals at the second faculty.

Theoretical part
Doppler sonography is about a modern, painless and today easily accessible diagnostical method, which uses two principals which are the Doppler effect and ultrasound. The inventor of this method was an Austrian mathematician, professor Christian Doppler (1803-1853). It is used daily to examine the blood vessels of the neck, the limbs and organs (for example, diagnosing varicose veins, thrombosis, arterial blockages in upper and lower limbs, and blood vessels mainly in the brain.), but also in  childbirth and neonatal medicine. Doctors consider the doppler sonography as a big step forward in the diagnosis of vascular diseases. There is still an open question, whether we can use this method as completely safe, especially while using ultrasound in pregnancies.

The advantage of this procedure is that the diagnosis doesnt require radiation of the patient using neither ionizing nor high energy radiation. Another advantage is the price of the diagnosis, which is much less than that of the diagnosis using comparable imaging methods, and also its wide usage.

The disadvantage of the method is seen in low permeability through areas with different acoustic impedances (eg subcutaneous fat, gas in the wood, or pulmonary parenchyma).

Ultrasound is a type of mechanical waves with a frequency higher than 16 kHz, which is inaudible to the human ear.

Doppler effect

The doppler effect is highlighted by using the siren height of the ambulance that is approaching and distancing. Doppler phenomenon involves changing the frequency and wavelength of the received signal versus the transmitted signal. The reason behind it is the non-zero reciprocal speed of the transmitter and the receiver (observer). Doppler effect can be observed in practice when changing the height of the tones emitted by the siren on the roof of a passing vehicle. If the vehicle is approaching, we perceive the sound as higher (shorter wavelength, higher frequency), if it is distancing, we perceive lower sound (longer wavelength, lower frequency).

This phenomenon is used, for example, when measuring the speed of a car using a radar. The Doppler effect affects both acoustic and electromagnetic waves, which are mainly used in astronomy for Earth's and universes` body movements. A shorter interval means more waves per second, and therefore a higher frequency. In the case of light, this means that the spectrum of the receding stars will be shifted toward the red color. Using the Doppler effect, a moving acoustic interface, such as the movement of the heart valves, can be easily monitored.

During doppler sonography, Doppler's ultrasound effect is used in the collision of ultrasound waves with basic reflecting structures in flowing blood- erythrocytes, which act as point sources of wave scattering and result in circular wavelengths spreading in all directions. To generate the doppler signal, the ultrasound wave energy that is directed back to the source is important, it is less than the transmitted energy by the ultrasound wave.. The difference between the transmitting frequency of the source of the wave and the received is called doppler frequency shift (Δf).

The goal of Doppler methods is to measure the speed of moving structures. Mainly concerning the measure of the velocity of blood flow rate with the combination of 2D imaging of noninvasive blood flow measurement. 1. Continuous doppler imaging - In this view, it is necessary to use separate transmitters and ultrasonic wave receivers suitably acoustically separated 2. Pulse Doppler methods - used in combination with echo-graphic methods. Doppler effect in medicine

The primary reflection structures in the flowing blood are erythrocytes. Since their magnitude is substantially smaller than the wavelength of the incident ultrasound waves, the erythrocytes act as scatter sources, which give rise to circular wavelengths propagating in all directions. These waves interfere with each other and their temporal and spatial summation occurs. For the formation of the doppler signal, the part of the ultrasound wave energy that is reflected back to the source is decisive. The amplitude of the reflected wave is proportional to the second power of the total number of elementary reflectors (erythrocytes). The frequency of reflected waves differs from the transmitted wave due to reflector movement. If the blood flows towards the source of the waves - to the ultrasonic probe, then the frequency of the reflected waves is higher than the transmitted frequency, if the blood flows from the source of the waves, the frequency is lower.

The actual blood flow rate depends on the Doppler frequency shift Δf and Doppler angle and (clustered in the Doppler beam direction and the movement of the moving structure, such as blood flow). The angle of doppler signal response relative to the direction of movement at the measurement site influences the shape of the spectral velocity curve. Underestimating the significance of the Doppler angle may lead to significant errors in the measurement of speeds that are critical at angles greater than 60 °.

Doppler sonography is used for other examinations, for example: to detect silent bubbles that may be a symptom of a decompression illness resulting from poor procedures during instrumental diving.

Dopplers systems

Doppler systems are currently designed as directional. For them, the flow velocity towards the probe is called forward, away from the probe as backward. The blood vessels and their branches pose a certain mechanical resistance on the bloodstream, which is analogous to the resistance that puts the electrical current on the conductors. Peripheral vascular resistance is inversely proportional to the 4th radius of the vessel. The geometry of the vessel determines not only the magnitude of the peripheral resistance, but also influences the character of the flow in a steady flow. If the velocity at a narrowing point exceeds a certain critical value, the laminar flow changes into turbulent - swirling. The velocity profile is rotated at the turbulence site to create a large velocity gradient adjacent to the vessel wall. In the color image, the turbulent flow is reflected by a mosaic of image structural units of different colors, indicating the various flows. Unlike the stationary flow, in which the fluid is moved with the same force, the blood flow in the vascular system is pulsating. In the period of the systole, acceleration of the flow occurs at the maximum speed, during the diastole there is a deceleration with a minimum speed at the end of the diastole. Continuous pulse flow is maintained due to the spring properties of the aorta and some other large vessels. These arteries constitute a blood reservoir and, due to the elasticity of their walls, they temporarily transform part of the kinetic energy flowing through the systole into their elastic tension, and in diastole this energy returns again to the blood. Another factor influencing the pulsating nature of blood flow and thus Doppler spectrum is peripheral vascular resistance. Accordingly, we distinguish between low-resistance curves (arteries supplying the brain and parenchymatous organs) and high-resistance (arteries supplying skeletal muscles).

We usually do not get real speeds, but only velocity components in the direction to the probe or from the probe. Therefore, if the blood flow measuring probe is located perpendicular to the vessel, it measures zero speed. Reflection occurs on the wall of the blood vessel, and in the passage through blood (a blood suspension) there is a phenomenon of scattering, especially on erythrocytes - the amount of waves that gets back to the probe is small (the blood is almost anechoic), but it is enough to determine the frequency shift; and from that it is possible to derive the flow rate of the blood as well as the nature of the flow (laminar, turbulent).

Spectral Doppler curve allows semi-quantitative blood flow analysis and hemodynamic assessment. For the responsible assessment of hemodynamic changes, it is necessary to determine some parameters of doppler spectral curves. These include the maximum systolic velocity S, the minimum diastolic velocity D, the S / D ratio (systolic/diastolic ratio), the acceleration index AI, the acceleration time AT, the PI (pulsatility index) and the resistance index RI (resistivity index, resistance index). These parameters have a large diagnostic cost, provided the measurement is accurate enough. Very important is the correct setting of the so-called Doppler angle, ie the angle between the flow direction and the doppler beam. The significance of these quantitative parameters will be highlighted in their comprehensive assessment, taking into account the history and current clinical status of the patientz However, the isolated use of only one parameter can lead to a diagnostic error. After marking the speed curve, the instrument calculates the values ​​automatically.

Definitions of some parameters

Maximum systolic velocity S- maximum velocity in systole. Most of the time it corresponds to the early systolic peak (ESP)

Minimum diastolic velocity S-m(telediastolic velocity)- velocity of the flow at the end of diastole

Mean velocity Vmean- instantaneous or timed. Determination of the instantaneous value is mostly based on the Doppler spectrum analysis - the velocities are weighted by the amplitude echoes, which are determined by the number of red blood cells involved in the generation of echoes. The average rate over time is then an average of instantaneous velocity over a period of time, usually at least one heart cycle.

S/D ratio(systolic / diastolic ratio) - ratio of maximum systolic velocity S and flow rate at the end of diastole D.

Resistive index RI (resistance index)- difference of maximum systolic velocity S and flow rate at the end of diastole D, divided by maximum systolic velocity S. Increasing the peripheral resistance leads to a reduction of the diastolic velocity and the value of the resistance index increases. The resistance index may provide information on peripheral resistance even in parts of the river basin being investigated which are not accessible to direct observation.

Pulse index PI (pulsatility index),pulse index - difference of maximum systolic velocity S and flow rate at the end of diastole D, divided by average velocity (Vmean); expresses the energy of pulsating blood. It has somewhat different values ​​for individual arteries and its diagnostic significance is not yet fully appreciated.

Heart rate HR (heart rate) is the number of heart rate per unit time, most often per minute. HR = 1 / T where T is the length of the heart period.

Systolic acceleration is characterized by a change in the rate of blood flow at a given artery site from

the onset of the systole to achieve a systolic peak. It is expressed either as the acceleration index (AI), which characterizes the line slope from the onset of the early systole to the systolic peak, or the acceleration time (AT), the time between the systole's start and the systolic peak.

In general, doppler measurements can be performed in two modes.

Systems with unmodulated carrier wave

Another name of continuous Doppler imaging - (CW; continuous wave). They have a probe with two electroacoustic transducers, one of which functions permanently as a transmitter, the other as a receiver. Both inverters are inclined at each other at a very obtuse angle so that both bundles, both transmitted and received, overlap in a so-called sensitive area that is relatively long (even a few centimeters). A single-transducer continuously generates an acoustic signal, so it can not be switched to receiver mode. For blood flow rates there is a frequency shift in the audible area, the Doppler flowmeters are equipped with an acoustic output. Because flow signals from different depths are captured, it is not possible to distinguish the flow velocities in individual vessels. The devices are simple and affordable, but you can not view the layout and location of the vessels being monitored, their overlaps, etc. The method is mainly used to monitor the flow of blood in the limbs.

Systems with impulse modulated carrier wave

The short for these systems is PW, pulsed wave. The transmitter transmits in pulses. Modulated carrier wave systems are a combination of a pulsed ultrasound signal and a directional detection of its reflections from the flowing blood that occurs in the section between transmitted pulses. Unlike ultrasonic imaging pulses, the doppler pulses have a slightly longer length and are transmitted with greater repetition rate. The mode allows you to measure not only the frequency change between the transmitted and received signals, but also the time the reflected signal returned to the probe. The time lag between the impulse sending and capturing its reflection determines the depth at which the flow rate can be measured. This allows you to determine not only the flow rate but also the depth at which the reflection occurred. The devices allow lines to show the direction of the ultrasound wave propagation as well as to indicate the direction of blood flow; this leads to automatic subtraction of the angle clamped by these lines. It is also possible to define the area in which the velocity is measured - so-called sampling volume. This also allows you to track the speed distribution. The size of the sampling volume (gate) and its location in the vessel affects the result of flow rate measurement. The narrow sample volume, located in the center of the artery, measures the maximum speed, the width, including the entire vessel diameter, average speed. Doppler measurement in PW mode is possible on most commonly used devices, the result is displayed as a two-dimensional image of measured speeds. The advantage of this method is the possibility of measuring the speed parameters at the chosen depth, without this measurement being adversely affected by the flows in the other vessels lying between the probe and the sampling volume.

The Equipment
- Doppler ultrasonic measuring device: Bidop Hadeco (ES-100V3)

- Probe & probe cable

- Ultrasound exam gel

- PC software: Smart-V-Link 4.1 & Gimp 8.2

Protocol
During this practical, you will be working with the Bidop Hadeco (ES-100V3) portable doppler ultrasonic measuring device.
 * 1) If it is not already connected, connect the Bidop device to your computer using a USB cable. There are only two ports on the device with a directional label. To connect your device to the computer, use the port with the output symbol.
 * 2) Run the Smart-V-Link software using the desktop icon. Fill out the data on the home screen. Disregard the automatically computed data, fill in all information manually (basic information is sufficient), and save. Continue to the program home page.
 * 3) Remove the black cap from the probe.
 * 4) Hold the small button on the probe to turn it on.
 * 5) In Settings, set the device to communicate with the computer (COM). The access can be connected via any USB port on the computer. (In order for this option to be available, it is necessary to switch on the measuring device.) If the option does not appear even when the device is turned on, press Search Comm. Once found, return to the main screen.
 * 6) Apply a thin layer of gel to the probe (approx. 3 mm thick).
 * 7) For the purpose of this practical, the Individual Waveform is the most appropriate measurement setting. However, in practice there are alternative methods for data collection. (For example, with Venous Doppler is is possible to measure and compare venous flow in both limbs.)
 * 8) Double-click on the gray field on the screen and device will start to measure . The values ​​will appear on the screen as a moving curve.
 * 9) Place the probe at an angle of approximately 45° to either are against the flow of blood and try to find the radial artery (ateria radialis). (Near the elbow is the most suitable place.)
 * 10) Turn the dial on the device to adjust the volume so that you can orient yourselves aurally as well. When you find the location of an artery, the probe will emit a whistle-like sound.
 * 11) Hold the probe steadily in place and wait until the field on the screen is fully filled with a periodic waveform. Continue measuring until you have at least 5 QRS complexes ("peaked" graphs), then end the measurement by briefly pressing the small button on the probe.
 * 12) Press Decision. You will see the values ​​at the top of the screen that you will later use to fill in your log. Save the graph via PrintScreen on the keyboard. This file can be printed directly.
 * 13) Be sure to clean off any residual gel from the probe before leaving your station.

Processing of Individual Measurement Results
 * 1) Once you have finished the measurements with Smart-V-Link, open the print screen by pressing the PrtSc key.
 * 2) Import the PrintScreen graph into GIMP (Ctrl+v). From here, you will transfer your values ​​into the Excel sheet.
 * 3) Cut the image either by selecting the Trim tool (Ctrl+Shift+C) directly on the keyboard or by the path: Tools → Transformation Tools → Trimming and Dimensions.
 * 4) Copy the “Doppler new” Excel spreadsheet to the desktop and rename it using the name of your group. Then, open it and write the measurement results from the in the yellow fields.
 * 5) First calibrate. Place the cursor at point 0, where the pixel coordinates {X:Y} will appear in the lower left bar. Copy these coordinates to the first table. In a similar fashion, fill in all subsequent calibration values. (Note: the end of diastole is equal to the onset of systole.)
 * 6) At the bottom right of the Excel sheet will be the manually retrieved values, which are calculated from the table macros. Fill in the yellow boxes next to the manual values with the automatic values recorded ​​in the Smart-V-Link program. You can find these values at the top of your graph when you clicked the Decision tab.
 * 7) Compare the results.
 * 8) Save the file (File->Save as) in .bmp format on the desktop (for easier access). This makes the file ready for printing.
 * 9) Print both the graph and the Excel table. Drag all saved files into the desktop folder labeled: “Results.”

 Information for Automatic and Manually Measured Values 

Measured Pulse Wave Parameters:
 * HR (Heart Rate)
 * RI (Resistance Index)
 * SD (Systolic / Diastolic ratio = S / D ratio)
 * MEAN (Mean Flow = average velocity [Vmean])
 * PI (Pulsatility Index = pulse index)

Important Notes for the Protocol
 * Do not use this device on the chest, abdomen, head, or neck area.
 * To prevent any unwanted reflection of ultrasound waves, apply a layer of gel to the contact tip of the probe about 3 mm thick. Once the gel begins to dry, apply a new layer. However, if the gel covers too much dermal surface area, the probe may not record properly.


 * While using the probe, carefully and patiently locate the signal. The place for examination is good in advance, for example, as a place where you can feel the pulse with the attached fingers.
 * Do not move the probe after finding the appropriate probe location (unlike conventional sonography).
 * The program may unexpectedly stop (program not responding). Typically, you can resolve the issue by restarting the program without further problems.

Protocol Preparation
 * To prepare for the protocol, use the form found under P8 - Doppler Practical on Moodle. Just before the start of taking measurements, fill in the boxes noting the order of the examiner and patient, the team number, the date, the start time of the examination, the investigated artery, and the investigated side (i.e.- sin means left, dx right). At the end of the experiment, record the end-time of the examination.
 * Write down the measured values ​​from the experiment straight into the data table.
 * In the discussion, compare your calculated values ​​with the automatically calculated and any theoretical assumptions you had about your measurements. Also, mention the fact that affects the outcome of the examination.
 * In the conclusion, briefly state whether the result of your measurement corresponds to a normal finding and whether a low-resolution or high-resolution curve was measured.

Conclusion
Over the course of the last half decade the development on the field of diagnostic ultrasound has been rapid, the newest technology being 3D-imaging.

The development has not come to an end, au contraire; new techniques for volume blood flow estimation have emerged. Based on Gauss’s theorem, an angle-independent measurement is now possible; the theorem is based on the relation of the integrated flux of a vector field through a surface, and the divergence of the vector field in the closed surface, i.e the inside of the surface.

Doppler sonography improves diagnosis due to provision of immediate clinical information through imagery, hence the reduction of wrong diagnosis consequently a reduction of harm to the patient which could be caused by wrong treatments. On the financial side the immediate result reduce overall healthcare costs.