Doppler Ultrasound: from basics to practice Poster No.: C-1643 Congress: ECR 2016 Type: Educational Exhibit Authors: J. A. Abreu, A. Vasquez, J. Romero, H. Rivera; Bogota/CO Keywords: Ultrasound physics, Ultrasound-Colour Doppler, UltrasoundPower Doppler, Ultrasound-Spectral Doppler, Education, Image registration DOI: 10.1594/ecr2016/C-1643 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. www.myesr.org Page 1 of 23
Learning objectives Present the physical principles of Doppler phenomenon. Establish the differences between pulsed and continuous Doppler. Recognize the components of the spectral analysis and the technical factors that influence it. Establish the relationship between Aliasing and Nyquist frequency. Present the properties of the color and power Doppler. Background Doppler ultrasound describes a frequency shift between an emitted ultrasound beam and the received echo. It has three components (Fig1): 1.Source / transmitter / emitter. 2.Receiver / observer / detector. 3.Physical propagation medium. Page 2 of 23
Fig. 1: Interaction between the source, receiver and the physical propagation medium. It is defined by Doppler equation (Fig2) which comprise the variables that influence it. Fig. 2: Doppler equation. Shift in frequency is directly related with the variation in frequency of the receptor and cocine of the angle of incidence of the ultrasound beam. It is inversely related with the velocity of sound in the tissue. In the dinamics of blood flow, the transducer is the source and erythrocytes are the receptors. Then, the echo is reflected by the erythrocytes, which now act as source, while the transducer acts as a receptor. The frequency of the source does not change, but the perceived frequency is increased due to the shortening of the distance to the receptor. Page 3 of 23
IMPORTANCE OF DOPPLER ANGLE In the B-mode, when we examinate structures with specular surfaces, an incidence of 90 is ideal to obtain a stronger echo. In Doppler measurement the opposite occurs. The beam incidence should be from 0 to 89 because at 90 there is no signal. (cos 90 = 0). Practically, 30 to 60 offer the best Doppler angle (Fig3). Fig. 3: Angle of Doppler insonation. Given a flow direction, ideally the transducer must be parallell to the blood vessel, but is impossible place the transducer into the vessel!!, so we have to obtain an angle of insonation placing the probe in the body surface. There are 2 types of Doppler signals: - Continuous: Contains a source and a separate beam receptor (Fig4). Page 4 of 23
Fig. 4: Example of continuous Doppler probe. The source and receptor (blue arrows) of the ultrasound beam (green dashed arrows) are separated. - Pulsed: System most widely used today. Sound is produced in a pulsed way rather than continuously (Fig5). It has the ability to emit and receive signals with the same device. The operator can control the position of the sample volume. Page 5 of 23
Fig. 5: Example of pulse Doppler. In this case the device can produce and receive ultrasound signal with the same detector (red and blue arrows). We can choose the vessel of interest no matter the depth. Page 6 of 23
SIGNAL PROCESSING Changes in frequency are expressed in a graphics mode with spectral representation wich comprise the feedback signal frequency (longitudinal axis) versus time (transverse axis). The analysis of the frequency is performed using the Fast Fourier transformation (Fig6). FAST FOURIER TRANSFORMATION Fig. 6: Fast Fourier Transformation. This physical phenomena allows that variations in the amplitude of the wave lenght obtained by the device (left) be displayed in range of frecuencies (right). Allows that changes in Doppler frequency measured by the instrument, be displayed in a range of frequencies present in the sample of interest. DOPPLER SPECTRAL ANALYSIS Horizontal axis represents time. Vertical axis represents the frequency of the beam - flow velocity (Fig7). Page 7 of 23
Fig. 7: An example of how spectral Doppler is displayed. On the top we see the vessel of interest with with the sample box in adequate position with a given angle of insonation (<60 ) (blue circle). On the bottom we see the representation of the change in fequencies of the sample, with a resultant spectrum. We can evaluate the morphology of the spectrum and measure blood flow parameters within it. Page 8 of 23
Images for this section: Page 9 of 23
Fig. 5: Example of pulse Doppler. In this case the device can produce and receive ultrasound signal with the same detector (red and blue arrows). We can choose the vessel of interest no matter the depth. Fundación SantaFe de Bogotá - Bogota/CO Fig. 7: An example of how spectral Doppler is displayed. On the top we see the vessel of interest with with the sample box in adequate position with a given angle of insonation (<60 ) (blue circle). On the bottom we see the representation of the change in fequencies of the sample, with a resultant spectrum. We can evaluate the morphology of the spectrum and measure blood flow parameters within it. Fundación SantaFe de Bogotá - Bogota/CO Page 10 of 23
Findings and procedure details In this section, we present schemes and cases from our institution which represent the way the ultrasound machine express changes in frequencies in a graphics mode. We also show how to identify and correct the parameters that influence the quality of the spectral analysis. SPECTRAL ANALYSIS It is influenced by different technical factors included by the fabricant (Fig8), that radiologist in training must to know and adjust. Fig. 8: Technical factors influencing quality of the spectrum. All of them contribute to success in Doppler examination. Page 11 of 23
PULSE REPETITION FREQUENCY Determines the number of pulses originated in the machine, they differ if we use B mode or Doppler at the examination (Fig9). Fig. 9: Graphic representation of pulse repetition frequency. On the top we see the number of pulses used in the b mode. On the bottom we notice that in Doppler examination there are a greater number of pulses used to obtain flow information. ALIASING EFECT Occurs when the Doppler frequency shift exceeds the repetition frequency (PRF), so is dependent of the Nyquist frequency. We can identify it in color Doppler (Fig10) or spectral Doppler (Fig11) examination. Is a Pulsed Doppler technology disadvantage. Page 12 of 23
Fig. 10: Color representation of the aliasing effect. Notice that velocity of the eritrocytes in the blood vessel exceed the given velocity of insonation (7,5 cm/s (left)). This resultls in heterogeneity of the color within the blood vessel (sample box). Fig. 11: Spectral representation of the aliasing effect. On the top we see the vessel of interest demonstrating color aliasing with a given angle of insonation (60 ). In the bottom we see how appears a register of the spectrum below the baseline ( red arrow) due to the velocity of the vessel exceed the velocity purposed in the examination. SIZE OF THE SAMPLE BOX Page 13 of 23
In this topic, the operator can adjust the volume of the sample. it modification produces effects on the spectrum. If the size is too big, you get signal record below the spectrum (Fig12). Fig. 12: Ultrasound spectral examitaion showing the influence of the size of sample box in the quality of the spectrum. On the top, notice that the size of the box exceeds the limits of the vessel. On the bottom we can see how this phenomena contributes to spectral broadening. GAIN CONTROL Can be controlled by the operator. If it is augmented, can increase noise in the spectrum background and overestimate the velocity (Fig13). Page 14 of 23
Fig. 13: Ultrasound spectral examination showing the influence of gains in the quality of the spectrum. On the top we see the vessel of interest. On the bottom we notice the increase in the noise of the spectrum background. This can overestimate the velocity. In the other hand, if it is reduced, the spectral record is not well demostrated and can understimate the velocity (Fig14). Page 15 of 23
Fig. 14: Spectral representation of decreased gains. On the top we see the vessel of interest with the sample box in adequate position. On the bottom, the spectrum is poorly delineated due to decreased gains. It can underestimate the velocity. ANGLE CONTROL As mentionated before, the angle of insonation is critical in estimating the correct velocity of the vessel of interest. When the angle is correctly adjusted under 60, the spectrum is better delineated and depicted. When the insonation includes an angle greater than 60, there is spectral broadening (Fig15). Page 16 of 23
Fig. 15: Ultrasound spectral examitaion showing the influence of the angle correction in the quality of the spectrum. On the left, we see a well depicted spectrum with an angle of insonation <60. On the right, with an angle of insonation of 72 there is spectral broadening due to inadecuate angle of insonation. WALL FILTER USE Wall filter annuls the signals on the wall or out of the blood vessels, Ignores frequencies below a threshold and can be controlled by the operator. When a low-speed flow is explored a very high filter, it could suppress the Doppler signal (Fig16). Page 17 of 23
Fig. 16: Spectral representation of wall filter use. On the left there is signal suppression next to de baseline. This can be corrected (right) decreasing the wall filter, obtaining then, a well delineated and depicted spectrum. COLOR DOPPLER ULTRASOUND It represents a variation of pulsed Doppler technology. It is based on the analysis of a pulse sequence in a region of interest. It identifies Doppler shift frequency and transforms it in pixels (Fig17). Page 18 of 23
Fig. 17: Gray scale and Color Doppler examitaion over a box of interest in the hepatic hilum. On the left we see the color scale determining that elements going to the trnasducer are of red-yellow color and elements getting away from the transducer are of blue - green color. We can say then, that the portal flow is hepatopetal (red) going to the transducer. POWER DOPPLER ULTRASOUND It is obtained as a representation of the total energy due to the motion of the erythrocytes regardless the speed which they move. It offers no information about flow direction (Fig18). Page 19 of 23
Fig. 18: Longitudinal gray scale and Power Doppler examination over a box of interest. In this case we only identify that there is flow within the vessel but we can not define its direction. It is useful when there are debts in presence of flow within a lesion or vessel, not identified in color Doppler examination. FLOW PARAMETERS They provide diagnostic thresholds and are derived from the spectral frequency. The mayor representatives are Resistive Index (RI) and Pulsatility Index (PI) (Fig 19). Page 20 of 23
Fig. 19: Resistive index (left) and pulsatility index (right) equations. They are directly related with the difference of maximum and minimum velocities and inversely related to maximum velocity in the RI and to the mean velocity in the PI. The RI and PI are measurements defined by the operator in the same spectrum, not influenced by the angle of insonation. Both are used to determine flow resistance in the vascular system. The operator can calculate them using the spectrum (Fig 20). Page 21 of 23
Fig. 20: Spectral representation showing how to calculate the resistive index. In this arterial spectral morphology, the operator should place the cursor at the level of maximum or peak velocity and at the level of the end diastolic (minimum) velocity. Page 22 of 23
Conclusion Doppler ultrasound is a useful tool in estimating physical flow phenomena. Knowledge of the physical principles guarantees a better understanding of ultrasound Doppler studies. The application of an adequate ultrasound technique requires time and expertise in order to obtain the best of information in each scan. Personal information References 1. 2. 3. 4. 5. Donald G. Mitchell, Color Doppler Imaging: Principles, Limitations, and Artifacts Radiology 1990; 177:1-10 Kenneth J. W. Taylor, Holland, Doppler US Part I. Basic Principles, Instrumentation, and Pitfalls Radiology 1990; 174:297-307. Christopher R. Doppler US: The Basics. RadioGraphics 1991; 11:109-119. Evan J. AAPM/RSNA Physics Tutorial for Residents: Topics in US Doppler US Techniques: Concepts of Blood Flow Detection and Flow Dynamics, RadioGraphics 2003; 23:1315-1327 Jonathan M, AAPM Tutorial Spectral Doppler RadioGraphics 1994; 14:139-150 Page 23 of 23