Monday, 28 May 2012


Acoustic Diode, Providing One-Way Transmission of Sound, Promises to Improve Ultrasound Imaging

An acoustic diode, enabling the one-way transmission of sound waves, could dramatically improve the quality of medical ultrasound imaging and lead to better sound dampening materials. Such a device has now been created by researchers at China's Nanjing University.

The team, led by professor Jian-chun Cheng, will describe its work at the Acoustics 2012 meeting in Hong Kong, May 13-18, a joint meeting of the Acoustical Society of America (ASA), Acoustical Society of China, Western Pacific Acoustics Conference, and the Hong Kong Institute of Acoustics.

Acoustic diodes are analogous to the electric diodes that produce unidirectional flow of current through electronic devices, protecting them from sudden and damaging reversals of flow. Electric diodes, which are akin to the check valves in car engines, work by providing nearly zero resistance to current flow in one direction and very high resistance in another. However, says associate professor and team member Bin Liang, "there is no analogous method to protect ultrasound sources from the disturbance of backtracking acoustic waves. Indeed, such unidirectional flow is far tougher to achieve with acoustic waves than with electric current because sound waves travel just as easily in both directions along any given path.

The acoustic diode consists of two parts. The first is an ultrasound contrast agent (UCA), made from a suspension of microbubbles. The UCA has a strong acoustic nonlinearity, which means it converts the acoustic energy of an incident wave into a wave with twice as many pulsations per second. Therefore, Liang says, "sound waves enter such a material at a particular frequency and leave with a frequency twice as great." The UCA microbubbles come in a broad range of sizes, so they can produce acoustic nonlinearity over a broad frequency range.
The second part of the acoustic diode is a superlattice consisting of thin alternating sandwich-like layers of water and glass. The superlattice acts like a filter that allows the sound waves with the doubled frequency to pass through the material but not the original sound waves.

"Hence," Liang says, "if the sound comes from the side of the nonlinear material, it will hit that material first, creating doubled frequency sound that passes through the filter, while any sound coming from the other side at the original frequency is blocked before it reaches the doubling layer."

In clinical medical imaging using ultrasound, acoustic waves are sent into the body and the reflected waves are received by the scanning instrument and the surrounding sensors to form the ultrasound images of the internal organs. "However, some of the reflecting waves interfere with the ingoing waves, which may lower the brightness and the resolution of the image. Therefore, preventing waves from coming back toward the ultrasound source would help to improve the quality of the ultrasound image," Liang says.
"In general," he adds, "we hope that the acoustic diode could apply to diverse situations where a special control of acoustic energy flux is required, for example, to improve the quality and effect of medical ultrasound diagnosis and therapy, or the design of unidirectional sound barriers."


30" Color Medical Diagnostic Monitor

Product Image 

30" Color 4-Megapixel Widescreen Medical Diagnostic Monitor

Designed for the demanding needs of medical imaging and PACS, the NEC MultiSync MD304MC, a 30" widescreen, 4-Megapixel color display, embodies the precision, high performance and intelligence you'd expect from a world leader in display technology.
  • Out-of-the-box calibration to the DICOM grayscale display function for luminance
  • ColorComp™ digital uniformity correction reduces screen uniformity errors and compensates for differences in color/grayscale and luminance across the entire screen
  • GammaComp™ MD software ensures consistent image quality, while providing a simple interface for conformance to the DICOM standard and an easy-to-use QA environment for medical imaging
  • 12-bit RGB lookup tables (LUTs) for gamma provide for more finely detailed, high-definition rendering of color images and crisper display of even the most delicate shadings and color differences
  • Auto Luminance control with X-Light™ Pro backlight/sensor design allows for consistent brightness and color
  • 4-way ergonomic design (height-adjust, pivot, tilt & swivel) enhances your viewing comfort
  • Wide-format screen increases productivity by expanding your working area to view multiple application windows
  • Auto Black Level Adjustment enables a greater ranges of black levels for better image performance


Thursday, 17 May 2012


Advancements in magnetic encoding technology have enabled the development of compact, low-cost encoders that are more tolerant of dirty and harsh operating environments where optical encoders could skip pulses.
The new magnetic encoder designs that use Hall effect technology to obtain high resolution from a durable magnetic target disc.Magnetic sensing does not need a clean transparent gap. Only some distance between the magnetic target and sensor is needed to operate properly.Dirt, dust, oil, condensation and other contaminants do not affect the reliability of a magnetic encoder.


Principle of Operation 

The AS5040 is a system-on-chip, combining integrated hall elements, analog front end and digital signal processing in a single device. It provides incremental output signals and the absolute angular position of a magnet that is placed above or below the device. This device is manufactured by austriamicrosystems.

The AS5040 can be configured to specific customer requirements by programming the integrated OTP (one time programmable) register. An internal voltage regulator allows operating the AS5040 device at either 3.3 V or 5 V supplies.

The AS5040 chip consists of a ring of hall elements placed at the center of the IC in a circle of 2.2mm diameter. The hall elements pick up the field of a magnet placed above this  hall array. This information is digitized and fed into a digital signal processor which calculates the angle of the magnet with a resolution of 0.35 degrees or 1024 positions per revolution at a sampling rate of 10 kHz.
The digital angle information is available in several formats; as a serial 10 bit data stream, as a pulse-width modulated (PWM) signal or as a quadrature incremental signal.

Physical Details
The chip itself is very small, measuring approx. 7.8 mm x 5 mm and has 16 pins and  can be built up to replace the potentiometers in many types of antenna rotators giving a far more accurate and consistent readout than the original setup as there aren't any  mechanical  contacts to wear. The only moving part is a small rare-earth magnet 6 mm in diam. that needs to be attached to a shaft and placed ~ 0.5 -5 mm above the AS5040.


  • Complete system-on-chip
  • Flexible system solution provides absolute, PWM and incremental outputs simultaneously
  • Ideal for applications in harsh environments due to contactless position sensing
  • Tolerant to magnet misalignment and air gap variations
  • No temperature compensation necessary
  • No calibration required 

Key Features

  • Contactless high resolution rotational position encoding over a full turn of 360 degrees
  • Two digital 10bit absolute outputs: - Serial interface and - Pulse width modulated (PWM) output
  • Three incremental output modes: - Quadrature A/B and Index output signal  - Step / Direction and Index output signal - 3-phase commutation for brushless DC motors- 10, 9, 8 or 7 bit user programmable resolution
  • User programmable zero / index position
  •  Failure detection mode for magnet placement monitoring and loss of power supply
  • Rotational speeds up to 30,000 rpm
  • Push button functionality detects movement of magnet in Z-axis
  •  Serial read-out of multiple interconnected AS5040 devices using Daisy Chain mode
  • Wide temperature range: - 40°C to + 125°C
  • Fully automotive qualified to AEC-Q100, grade 1
  • Small Pb-free package: SSOP 16 (5.3mm x 6.2mm)


Functional Description

The AS5040 is manufactured in a CMOS standard process and uses a spinning current Hall technology for sensing the magnetic field distribution across the surface of the chip.The integrated Hall elements are placed around the center of the device and deliver a voltage representation of the magnetic field at the surface of the IC.Through Sigma-Delta Analog / Digital Conversion and Digital Signal-Processing (DSP) algorithms, the AS5040 provides accurate high-resolution absolute angular position information. For this purpose a Coordinate Rotation Digital Computer (CORDIC) calculates the angle and the magnitude of the Hall array signals.The DSP is also used to provide digital information at the outputs MagINCn and MagDECn that indicate movements of the used magnet towards or away from the device’s surface.A small low cost diametrically magnetized (two-pole) standard magnet provides the angular position information.The AS5040 senses the orientation of the magnetic field and calculates a 10-bit binary code. This code can be accessed via a Synchronous Serial Interface (SSI). In addition, an absolute angular representation is given by a Pulse Width Modulated signal at pin 12 (PWM). Besides the absolute angular position information the device simultaneously provides incremental output signals. The various incremental output modes can be selected by programming the OTP mode register bits As long as no programming voltage is applied to pin Prog, the new setting may be overwritten at any time and will be reset to default when power is turned off. To make the setting permanent, the OTP register must be programmed The default setting is a quadrature A/B mode including the Index signal with a pulse width of 1 LSB. The Index signal is logic high at the user programmable zero position. The AS5040 is tolerant to magnet misalignment and magnetic stray fields due to differential measurement technique and Hall sensor conditioning circuitry.

Block diagram for AS5040 Rotary Encoder IC

AS5040 10-bit Programmable Magnetic Rotary Encoder IC Block Diagram



  •  Industrial applications:
- Contactless rotary position sensing
- Robotics
- Brushless DC motor commutation
- Power tools
  • Automotive applications:
- Steering wheel position sensing
- Gas pedal position sensing
- Transmission gearbox encoder
- Headlight position control
- Power seat position indicator
  •  Office equipment: printers, scanners, copiers
  •  Replacement of optical encoders
  •  Front panel rotary switches
  •  Replacement of potentiometers

Wednesday, 16 May 2012


The Components of an Ultrasound Machine The Components of an Ultrasound Machine
Fig. 25: The Components of an Ultrasound Machine
The parts of an ultrasound machine
Central Processing Unit (CPU)
The CPU is the hub of an ultrasound machine. The CPU is like a computer that contains the microprocessor, memory, amplifiers and power supplies for the microprocessor and transducer probe. The transducer receives electrical currents from the CPU and sends electrical pulses that are created by returning echoes. The CPU does all of the calculations to produce an image on the monitor, and also stores the processed information on a disk.
Transducer Pulse Controls
The operator, called the ultrasonographer, uses the transducer pulse controls to set and change the frequency and duration of the ultrasound pulses. The transducer pulse controls also allow for scanning the mode of the machine. Commands from the operator are changed into fluctuating electrical currents that are applied to the piezoelectric (PZ) crystals in the transducer probe.
The display turns processed data from the CPU into an image. This image can be either in black-and-white or color, depending upon the model of the ultrasound machine.
Ultrasound machines have a keyboard and a cursor. The keyboard allows the operator to add notes and to take measurements of the image.
Disk Storage
The processed data and/or images can be stored on disks. These disks can be hard disks, floppy disks, compact disks (CDs), or digital video disks (DVDs). Most of the time, ultrasound scans are filled on floppy disks and stored with the patient's medical records.
Most ultrasound machines have printers which are thermal. These can be used to capture a printed picture of the image from the monitor.

Thursday, 10 May 2012


4D Real Time

 4D is shorthand for “four-dimensional” -- the fourth dimension being time. As far as ultrasound is concerned, 4D Ultrasound is the latest ultrasound technology. 4D Ultrasound takes three-dimensional ultrasound images and adds the element of time to the process. The result -- live-action images of your unborn child.

VOCAL II VCI (Volume Contrast Imaging)

A very thin volume 4D scan with some minor differences in mechanics of acquisition is called as VCI (volume contrast imaging)  VCI increases the inherent contrast in the image and delineates borders in a better manner.


SonoVCAD labor

Sonography-based Volume Computer Aided Display labor (SonoVCAD™ labor) is a proprietary 3D automated tool that allows you to confidently measure fetal head progression, rotation and direction while automatically documenting the labor procedure with objective ultrasound and manual data in one easy report.



DICOM (Digital Imaging and Communication in Medicine) is a standard that describe the way medical information are stored/exchanged in the radiological department. The purpose of DICOM is to facilitate interaction between medical devices.

Digital technology has in the last few decades entered almost every aspect of medicine. There has
been a huge development in noninvasive medical imaging equipment. Because there are many medical
equipment manufacturers, a standard for storage and exchange of medical images needed to be developed.
DICOM (Digital Imaging and Communication in Medicine) makes medical image exchange more easy and
independent of the imaging equipment manufacturer. Besides the image data, DICOM file format supports other information useful to describe the image.


STIC [Spatio-Temporal Image Correlation] 


In STIC technology, an automated device incorporated into the ultrasound probe performs a slow sweep acquiring a single threedimensional (3D) volume. This volume is composed of a great number of two-dimensional (2D) frames. As the volume of the fetal heart is small, appropriately regulating the region of interest allows a very high frame rate (in the order of 150 frames per second) during 3D volume acquisition. For example, an acquisition time of 10 seconds and a sweep angle of 25° would lead to the
recording of 1500 2D frames. During the acquisition time there will be approximately 20-25 cardiac cycles. Therefore, among the 1500 frames, 20-25 will show a systolic peak 

Power Doppler Mode


CFM Doppler Mode

HD-Flow Mode

CrossXBeam CRI

T.U.I [Tomographic Ultrasound Imaging]

Coded Contrast Imaging


In recent years, the capabilities of ultrasound flow imaging have increased enormously. Color flow imaging is now commonplace and facilities such as ‘power’ or ‘energy’ Doppler provide new ways of imaging flow. With such versatility, it is tempting to employ the technique for ever more demanding applications and to try to measure increasingly subtle changes in the maternal and fetal circulations. To avoid misinterpretation of results, however, it is essential for the user of Doppler ultrasound to be aware of the factors that affect the Doppler signal, be it a color flow image or a Doppler sonogram.
Competent use of Doppler ultrasound techniques requires an understanding of three key components:
(1) The capabilities and limitations of Doppler ultrasound;
(2) The different parameters which contribute to the flow display;
(3) Blood flow in arteries and veins.
This chapter describes how these components contribute to the quality of Doppler ultrasound images. Guidelines are given on how to obtain good images in all flow imaging modes. For further reading on the subject, there are texts available covering Doppler ultrasound and blood flow theory in more detail 1-3 .
Ultrasound images of flow, whether color flow or spectral Doppler, are essentially obtained from measurements of movement. In ultrasound scanners, a series of pulses is transmitted to detect movement of blood. Echoes from stationary tissue are the same from pulse to pulse. Echoes from moving scatterers exhibit slight differences in the time for the signal to be returned to the receiver (Figure 1 ). These differences can be measured as a direct time difference or, more usually, in terms of a phase shift from which the ‘Doppler frequency’ is obtained (Figure 2). They are then processed to produce either a color flow display or a Doppler sonogram.
Figure 1 Ultrasound velocity measurement. The diagram shows a scatterer S moving at velocity V with a beam/flow angle q.
The velocity can be calculated by the difference in transmit-to-receive time from the first pulse to the second (t2), as the scatterer moves through the beam
Figure 2: Doppler ultrasound. Doppler ultrasound measures the movement of the scatterers through the beam as a phase change in the received signal. The resulting Doppler frequency can be used to measure velocity if the beam/flow angle is known.
As can be seen from Figures 1 and 2, there has to be motion in the direction of the beam; if the flow is perpendicular to the beam, there is no relative motion from pulse to pulse. The size of the Doppler signal is dependent on:
(1) Blood velocity: as velocity increases, so does the Doppler frequency;
(2) Ultrasound frequency: higher ultrasound frequencies give increased Doppler frequency. As in B-mode, lower ultrasound frequencies have better penetration.
(3) The choice of frequency is a compromise between better sensitivity to flow or better penetration;
(4 The angle of insonation: the Doppler frequency increases as the Doppler ultrasound beam becomes more aligned to the flow direction (the angle q between the beam and the direction of flow becomes smaller). This is of the utmost importance in the use of Doppler ultrasound. The implications are illustrated schematically in Figure 3.

Figure 3 - Effect of the Doppler angle in the sonogram. (A) higher-frequency Doppler signal is obtained if the beam is aligned more to the direction of flow. In the diagram, beam (A) is more ali)gned than (B) and produces higher-frequency Doppler signals. The beam/flow angle at (C) is almost 90° and there is a very poor Doppler signal. The flow at (D) is away from the beam and there is a negative signal.
All types of Doppler ultrasound equipment employ filters to cut out the high amplitude, low-frequency Doppler signals resulting from tissue movement, for instance due to vessel wall motion. Filter frequency can usually be altered by the user, for example, to exclude frequencies below 50, 100 or 200 Hz. This filter frequency limits the minimum flow velocities that can be measured.
As the name suggests, continuous wave systems use continuous transmission and reception of ultrasound. Doppler signals are obtained from all vessels in the path of the ultrasound beam (until the ultrasound beam becomes sufficiently attenuated due to depth). Continuous wave Doppler ultrasound is unable to determine the specific location of velocities within the beam and cannot be used to produce color flow images. Relatively inexpensive Doppler ultrasound systems are available which employ continuous wave probes to give Doppler output without the addition of B-mode images. Continuous wave Doppler is also used in adult cardiac scanners to investigate the high velocities in the aorta.
Continuous-wave doppler transducer
Pulsed-wave doppler transducer
Doppler ultrasound in general and obstetric ultrasound scanners uses pulsed wave ultrasound. This allows measurement of the depth (or range) of the flow site. Additionally, the size of the sample volume (or range gate) can be changed. Pulsed wave ultrasound is used to provide data for Doppler sonograms and color flow images.
Pulsed wave systems suffer from a fundamental limitation. When pulses are transmitted at a given sampling frequency (known as the pulse repetition frequency), the maximum Doppler frequency fd that can be measured unambiguously is half the pulse repetition frequency. If the blood velocity and beam/flow angle being measured combine to give a fd value greater than half of the pulse repetition frequency, ambiguity in the Doppler signal occurs. This ambiguity is known as aliasing. A similar effect is seen in films where wagon wheels can appear to be going backwards due to the low frame rate of the film causing misinterpretation of the movement of the wheel spokes.
Figure 4 : Aliasing of color doppler imaging and artefacts of color. Color image shows regions of aliased flow (yellow arrows).
Figure 5 : Reduce color gain and increase pulse repetition frequency.
Figure 6 (a,b): Example of aliasing and correction of the aliasing. (a) Waveforms with aliasing, with abrupt termination of the peak systolic and display this peaks bellow the baseleineSonogram clear without aliasing. (b) Correction: increased the pulse repetition frequency and adjust baseline (move down)
The pulse repetition frequency is itself constrained by the range of the sample volume. The time interval between sampling pulses must be sufficient for a pulse to make the return journey from the transducer to the reflector and back. If a second pulse is sent before the first is received, the receiver cannot discriminate between the reflected signal from both pulses and ambiguity in the range of the sample volume ensues. As the depth of investigation increases, the journey time of the pulse to and from the reflector is increased, reducing the pulse repetition frequency for unambiguous ranging. The result is that the maximum fd measurable decreases with depth.
Low pulse repetition frequencies are employed to examine low velocities (e.g. venous flow). The longer interval between pulses allows the scanner a better chance of identifying slow flow. Aliasing will occur if low pulse repetition frequencies or velocity scales are used and high velocities are encountered (Figure 4,5 and 6). Conversely, if a high pulse repetition frequency is used to examine high velocities, low velocities may not be identified.
Figure 7 (a,b): Color flow imaging: effects of pulse repetition frequency or scale. (above) The pulse repetition frequency or scale is set low (yellow arrow). The color image shows ambiguity within the umbilical artery and vein and there is extraneous noise. (b) The pulse repetition frequency or scale is set appropriately for the flow velocities (bottom). The color image shows the arteries and vein clearly and unambiguously.

Since color flow imaging provides a limited amount of information over a large region, and spectral Doppler provides more detailed information about a small region, the two modes are complementary and, in practice, are used as such.
Color flow imaging can be used to identify vessels requiring examination, to identify the presence and direction of flow, to highlight gross circulation anomalies, throughout the entire color flow image, and to provide beam/vessel angle correction for velocity measurements. Pulsed wave Doppler is used to provide analysis of the flow at specific sites in the vessel under investigation. When using color flow imaging with pulsed wave Doppler, the color flow/B-mode image is frozen while the pulsed wave Doppler is activated. Recently, some manufacturers have produced concurrent color flow imaging and pulsed wave Doppler, sometimes referred to as triplex scanning.
When these modes are used simultaneously, the performance of each is decreased. Because transducer elements are employed in three modes (B-mode, color flow and pulsed wave Doppler), the frame rate is decreased, the color flow box is reduced in size and the available pulse repetition frequency is reduced, leading to increased susceptibility to aliasing.

Power Doppler is also referred to as energy Doppler, amplitude Doppler and Doppler angiography. The magnitude of the color flow output is displayed rather than the Doppler frequency signal. Power Doppler does not display flow direction or different velocities. It is often used in conjunction with frame averaging to increase sensitivity to low flows and velocities. It complements the other two modes (Table 01). Hybrid color flow modes incorporating power and velocity data are also available from some manufacturers. These can also have improved sensitivity to low flow. A brief summary of factors influencing the displays in each mode is given in the following sections. Most of these factors are set up approximately for a particular mode when the application (e.g. fetal scan) is chosen, although the operator will usually alter many of the controls during the scan to optimize the image.
Table 1 - Flow imaging modes
Spectral Doppler
  • Examines flow at one site
  • Detailed analysis of distribution of flow
  • Good temporal resolution – can examine flow waveform
  • Allows calculations of velocity and indices
Color flow
  • Overall view of flow in a region
  • Limited flow information
  • Poor temporal resolution/flow dynamics (frame rate can be low when scanning deep)
  • color flow map (diferent color maps)
  • direction information
  • velocyty information (high velocity & low velocity)
  • turbulent flows
Power/energy/amplitude flow
  • Sensitive to low flows
  • No directional information in some modes
  • Very poor temporal resolution
  • Susceptible to noise
"Color Power Angio" of the Circle of Willis "Color Power Angio" of a submucosus fibroid, note the small vessels inside the tumor.
Color flow imaging
Color flow Doppler ultrasound produces a color-coded map of Doppler shifts superimposed onto a B-mode ultrasound image (Color Flow Maps). Although color flow imaging uses pulsed wave ultrasound, its processing differs from that used to provide the Doppler sonogram. Color flow imaging may have to produce several thousand color points of flow information for each frame superimposed on the B-mode image. Color flow imaging uses fewer, shorter pulses along each color scan line of the image to give a mean frequency shift and a variance at each small area of measurement. This frequency shift is displayed as a color pixel. The scanner then repeats this for several lines to build up the color image, which is superimposed onto the B-mode image. The transducer elements are switched rapidly between B-mode and color flow imaging to give an impression of a combined simultaneous image. The pulses used for color flow imaging are typically three to four times longer than those for the B-mode image, with a corresponding loss of axial resolution.
Assignment of color to frequency shifts is usually based on direction (for example, red for Doppler shifts towards the ultrasound beam and blue for shifts away from it) and magnitude (different color hues or lighter saturation for higher frequency shifts). The color Doppler image is dependent on general Doppler factors, particularly the need for a good beam/flow angle. Curvilinear and phased array transducers have a radiating pattern of ultrasound beams that can produce complex color flow images, depending on the orientation of the arteries and veins. In practice, the experienced operator alters the scanning approach to obtain good insonation angles so as to achieve unambiguous flow images.

Table 2 - Factors affecting color flow image

Main factors
Power: transmitted power into tissue*
Gain: overall sensitivity to flow signals
Frequency: trades penetration for sensitivity and resolution*
Pulse repetition frequency (also called scale): low pulse repetition frequency to look at low velocities, high pulse repetition frequency reduces aliasing*
Area of investigation: larger area reduces frame rate*
Focus: color flow image optimized at focal zone*
Other factors
Triplex color: pulse repetition frequency and frame rate reduced by need for B-mode/spectral pulses
Persistence: high persistence produces smoother image but reduces temporal resolution*
Pre-processing: trades resolution against frame rate*
Filter: high filter cuts out more noise but also more of flow signal*
Post-processing assigns color map/variance*
*Settings appropriate for specific examinations assigned by set-up/application keys
The controls that affect the appearance of the color flow image are summarized in Table 2. The main factors include:
(1) Power and gain:Color flow uses higher-intensity power than B-mode. Attention should be paid to safety indices. Power and gain should be set to obtain good signal for flow and to minimize the signals from surrounding tissue.
Figure 8 : Setting the color gain to minimize the signals (artefacts) from surrondng tissue, on left color gain = 71, then on right decreasing the color gain to 35.
(2) Frequency selection: Many scanner/transducer combinations permit changes of frequency. High frequencies give better sensitivity to low flow and have better spatial  resolution. Low frequencies have better penetration (Figure 5) and are less susceptible to aliasing at high velocities.
(3) Velocity scale/pulse repetition frequency: Low pulse repetition frequencies should be used to examine low velocities but aliasing may occur if high velocities are encountered (Figura 7a,b).
(4) Region of interest: Because more pulses are needed to look at flow than for the B-mode image, reducing the width and maximum depth of the color flow area under investigation will usually improve frame rate and may allow a higher color scan line density with improved spatial resolution (Figure 9).
(5) Focus: The focus should be at the level of the area of interest. This can make a significant difference to the appearance and accuracy of the image (Figure 7).
Figure 9 : Set the focus at the region of interest, and also could use more than one focal zone.

In practice, the operator will make many changes to the controls and will try different probe positions to optimize the image. Practical guidelines are given in Table 3.

Table 3: Color flow imaging: practical guidelines

(1) Select the appropriate applications/set-up key. This optimizes parameters for specific
(2) Set power to within fetal study limits. Adjust color gain. Ensure focus is at the region of interest and adjust gain to optimize color signal
(3) Use probe positioning/beam steering to obtain satisfactory beam/vessel angle
(4) Adjust pulse repetition frequency/scale to suit the flow conditions. Low pulse repetition frequencies are more sensitive to low flows/velocities but may produce aliasing. High pulse repetition frequencies reduce aliasing but are less sensitive to low velocities
(5) Set the color flow region to appropriate size. A smaller color flow ‘box’ may lead to a better frame rate and better color resolution/sensitivity

Pulsed wave Doppler ultrasound is used to provide a sonogram of the artery or vein under investigation (Figure 12). The sonogram provides a measure of the changing velocity throughout the cardiac cycle and the distribution of velocities in the sample volume (or gate) (Figure 11). If an accurate angle correction is made, then absolute velocities can be measured. The best resolution of the sonogram occurs when the B-mode image and color image are frozen, allowing all the time to be employed for spectral Doppler. If concurrent imaging is used (real-time duplex or triplex imaging), the temporal resolution of the sonogram is compromised.
Figure 10 (a,b): Doppler spectra of uterine artery flow. (a) The color flow image allows beam/flow angle visualization. The sonogram shows high velocities throughout the cardiac cycle, indicating low distal resistance. (b) The sonogram shows a pulsatile flow waveform with low diastolic velocities. This is indicative of high distal resistance
(b) - direction of the Doppler beam
(g) - gate or sample volume
(a) - angle correction
Sonogram of the descending aorta. With the angle correction the peak velocities could be measured.
Figure 11: Setting up the sample volume
The controls that affect the appearance of the sonogram are summarized in Table 4. The main factors include:
(1) Power and gain: Pulsed wave Doppler uses higher intensity power than B-mode. Attention should be paid to safety indices. Power and gain should be set so that clear signals are  obtained.
(2) Velocity scale/pulse repetition frequency: Low pulse repetition frequencies should be used to look at low velocities but aliasing may occur if high velocities are encountered.
(3) Gate size: If flow measurements are being attempted, the whole vessel should be insonated. A large gate may include signals from adjacent vessels (Figure 13).

Table 4 - Factors affecting the spectral Doppler image

Main factors
Power: transmitted power into tissue*
Gain: overall sensitivity to flow signals
Pulse repetition frequency (also called scale): low pulse repetition frequency to look at low velocities, high pulse repetition frequency reduces aliasing*
Gate size*
Beam steering can allow improved beam/flow angle for better accuracy of velocity
Live duplex/triplex spectral resolution constrained by need for B-mode/color pulses
Other factors
Gate: sharpness of resolution*
Filter: high filter cuts out more noise but more of flow signal*
Post-processing: assigns brightness to output*
*Settings appropriate for specific examinations assigned by set-up/application keys
Figure 12: Umbilical cord displaying umbilical artery (red) and umbilical vein (blue), the gate or sample volume include both signals (left). Sonogram of the umbilical artery and vein (right).
Figure 13Influence of gate size. The spectral Doppler gate insonates an artery and vein and the sonogram shows flow from both of these vessels. The calculation of mean velocity (arrow) is meaningless since velocities from one vessel subtract from those of the other
Guidelines for a practical approach to obtain good-quality spectral images are given in Table 5 .

Table 5: Spectral Doppler imaging: practical guidelines

(1) Set power to within fetal study limits
(2) Position the pulsed wave Doppler cursor on the vessel to be investigated
(3) Adjust gain so that the sonogram is clearly visible and free of noise
(4) Use probe positioning/beam steering to obtain a satisfactory beam/vessel angle. Angles close to 90° will give ambiguous/unclear values. The beam/vessel angle should be 60° or less if velocity measurements are to be made
(5) Adjust the pulse repetition frequency/scale and baseline to suit flow conditions. The sonogram should be clear and not aliased
(6) Set the sample volume to correct size. Correct the angle to obtain accurate velocities. Use the B-mode and color flow image of the vessel to make the angle correction

Velocity measurement
Theoretically, once the beam/flow angle is known, velocities can be calculated from the Doppler spectrum as shown in the Doppler equation. However, errors in the measured velocity may still occur 1,4. Sources of error can be broadly divided into three categories.
(1) Errors can arise in the formation of the Doppler spectrum due to:
(a) Use of multiple elements in array transducers;
(b) Non-uniform insonation of the vessel lumen;
(c) Insonation of more than one vessel;
(d) Use of filters removing low-velocity components.
(2) Errors can arise in the measurement of the ultrasound beam/flow velocity angle.
(a) Use of high angles (q > 60o) may give rise to error because of the comparatively large changes in the cosine of the angle which occur with small changes of angle (Figure 14).
(b) The velocity vector may not be in the direction of the vessel axis.
(3) Errors can arise in the calculation packages provided by the manufacturers for analysis of the Doppler spectrum (for instance, of intensity weighted mean velocity).
(a) While efforts can be made to minimize errors, the operator should be aware of their likely range. It is good practice to try to repeat velocity measurements, if possible using a different beam approach, to gain a feel for the variability of measurements in a particular application. However, even repeated measurements may not reveal systematic errors occurring in a particular machine.
(b) The effort applied to produce accurate velocity measurements should be balanced against the importance of absolute velocity measurements for an investigation.
(c) Changes in velocity and velocity waveform shape are often of more clinical relevance when making a diagnosis. In this and other cases, absolute values of velocity measurement may not be required.
Figure 14: Effect of high vessel/beam angles. (a) and (b) A scan of fetal aortic flow is undertaken at a high beam/vessel angle. Beam/flow angles should be kept to to 60° or less. A hudge discrepancy is observed when use unapropiate angles > 60°. If absolute velocities are to be measured, beam/flow angles should be kept to 60° or less.
Calculation of absolute flow
Total flow measurement using color or duplex Doppler ultrasound is fraught with difficulties, even under ideal conditions 5. Errors that may arise include:
(1) Those due to inaccurate measurement of vessel cross-sectional area, for example the cross-sectional area of arteries which pulsate during the cardiac cycle;
(2) Those originating in the derivation of velocity (see above).
These errors become particularly large when flow calculations are made in small vessels; errors in measurement of diameter are magnified when the diameter is used to derive cross-sectional area. As with velocity measurements, it is prudent to be aware of possible errors and to conduct repeatability tests.
Flow waveform analysis
Non-dimensional analysis of the flow waveform shape and spectrum has proved to be a useful technique in the investigation of many vascular beds. It has the advantage that derived indices are independent of the beam/flow angle.
Changes in flow waveform shape have been used to investigate both proximal disease (e.g. in the adult peripheral arterial circulation) and distal changes (in the fetal circulation and uterine arteries). While the breadth of possible uses shows the technique to be versatile, it also serves as a reminder of the range of factors which cause changes to the local Doppler spectrum. If waveform analysis is to be used to observe changes in one component of the proximal or distal vasculature, consideration must be given to what effects other components may have on the waveform.
Flow waveform shape: indices of measurement
Many different indices have been used to describe the shape of flow waveforms 1 . Techniques range from simple indices of systolic to diastolic flow to feature extraction methods such as principal component analysis. All are designed to describe the waveform in a quantitative way, usually as a guide to some kind of classification. In general, they are a compromise between simplicity and the amount of information obtained.
Figure 15: Arterial velocity sonogram (waveform).
The relative merits of indices used in uterine arteries have been discussed elsewhere. 6,7 .Commonly used indices available on most commercial scanners are:
(1) Resistance index (RI) (also called resistive index or Pourcelot’s index);
(2) Systolic/diastolic (S/D) ratio, sometimes called the A/B ratio;
(3) Pulsatility index (PI) 8.
These indices are all based on the maximum Doppler shift waveform and their calculation is described in Figure 12. The PI takes slightly longer to calculate than the RI or S/D ratio because of the need to measure the mean height of the waveform. It does, however, give a broader range of values, for instance in describing a range of waveform shapes when there is no end-diastolic flow.
Figure 16 - Flow velocity indices
In addition to these indices, the flow waveform may be described or categorized by the presence or absence of a particular feature, for example the absence of end-diastolic flow and the presence of a post-systolic notch.
Generally, a low pulsatility waveform is indicative of low distal resistance and high pulsatility waveforms occur in high-resistance vascular beds (Figure 8), although the presence of proximal stenosis, vascular steal or arteriovenous fistulas can modify waveform shape. Care should be taken when trying to interpret indices as absolute measurements of either upstream or downstream factors. For example, alterations in heart rate can alter the flow waveform shape and cause significant changes in the value of indices.