Signals returning from the side lobe beams are interpreted by the echo machine as having arisen from the central beam, and can be displayed some distance away from the true location of the structure in question. Storage of echo studies can be on videotape, which is relatively inexpensive, although it rapidly becomes cumbersome to store and to review studies when large numbers are archived in this way. Digital archiving is now more commonly used, with storage on hard drives or optical disks.
This makes accessing studies easier and allows greater flexibility in image processing after the study has been completed.
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There are two main biological effects of exposure to ultrasound energy: thermal heating and mechanical e. Thermal effects are caused by conversion of the mechanical energy of the ultrasound into heat energy as it passes though the tissues. The amount of heating is hard to predict but relates to several factors including transducer frequency, transmit power, focus and depth.
Thermal effects are most relevant to TOE where the probe may remain stationary in the oesophagus for long periods, particularly during intraoperative studies.
Making Sense of Echocardiography
Heat may be generated not just by the ultrasound but also directly by the probe itself. It is prudent to keep imaging time to a minimum and to ensure that the TOE probe is repositioned regularly, and to monitor the temperature of the probe. Mechanical effects include cavitation, in which gas bubbles are created as ultrasound passes through the tissues. It is not thought to be a problem during standard transthoracic studies, but is important when bubble contrast agents are used as it can cause resonance and even disruption of the bubbles p.
Mechanical effects of ultrasound can also be measured by mechanical index MI , which is the peak negative rarefactional pressure divided by the square root of the transducer frequency. M-mode and 2-D echo have the lowest ultrasound intensity, and pulsed- wave Doppler has the highest intensity with colour Doppler having an intermediate value. Echo departments should have appropriate risk assessment tools and protocols in place to minimize risks to patients and staff.
Accessible from the BSE website www. Monaghan MJ. Second harmonic imaging: a new tune for an old fiddle? Heart ; —2. Reynolds T. This in turn provides valuable information about valvular function, intracardiac shunts and so forth. As well as allowing the assessment of blood flow, the Doppler principle has also, more recently, been applied to the study of myocardial function tissue Doppler imaging. A classic example is that of a moving ambulance sounding its siren — as the ambulance approaches an observer, its siren sounds higher pitched than when it is moving away.
Figure 4. The same phenomenon occurs with ultrasound waves when they are reflected from moving red blood cells. The frequency of the returning ultrasound is increased if the red blood cells are moving towards the ultrasound transducer, or decreased if they are moving away.
Making Sense of Echocardiography: A Hands-On Guide Andrew R. Houghton NEW
For this reason, when undertaking echo Doppler studies it is important to align the ultrasound beam with the direction of blood flow as closely as possible. This is a complex process as the returning signal contains a spectrum of frequencies, and a mathematical technique called a fast Fourier transform is used to undertake the necessary spectral analysis. A spectral Doppler display can then be produced Fig.
These displays conventionally plot frequency shifts shown as velocities on the vertical axis against time on the horizontal axis. A zero line is shown, and flow towards the transducer is plotted above the line and flow away from the transducer, as in Fig. For each time point the grey pixels show the blood flow velocity detected, and the density of the signal i.
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Such spectral displays form the basis of Doppler physics continuous wave CW and pulsed-wave PW Doppler techniques see below. Time Velocity Mitral regurgitation Fig. Gain, which amplifies the received signal to increase the brightness of the displayed spectral trace. High gain settings amplify weaker signals that might otherwise not be visible, but increase noise.
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Velocity range, which alters the vertical velocity scale to a higher or lower range. Two crystals in the transducer are used, one to transmit an ultrasound signal and the other to receive the returning signal. A typical CW Doppler display, obtained by interrogating flow across the mitral valve in the apical 4-chamber view, is shown in Figure 4. This shows a positive spectral trace above the zero line which corresponds to forward flow across the valve during diastole, and a negative trace below the line corresponding to regurgitant flow during systole.
The spectral display reflects the full range of red blood cell velocities detected along the beam at any particular time point, usually ranging from zero up to the peak velocity demarcated by the edge of the spectral trace. Pulsed-wave Doppler PW Doppler measures blood flow velocity at a specific location, which the sonographer chooses by placing a sample volume indicated by two parallel lines perpendicular to the main cursor line at the point of interest Fig.
The length of the sample volume can be adjusted by the sonographer — typically a length of 3 mm is used.
This gives rise to the phenomenon known as aliasing, which is one of the main limitations to the usefulness of PW Doppler. When the film is played back, the wheel will look as though it is stationary. Shifting the baseline can help reduce the problem of aliasing to some extent, but the phenomenon nevertheless places a significant limitation on the maximum velocity that can be assessed with PW Doppler. Ultimately it may prove necessary to switch to CW Doppler instead, where possible.
One further alternative is to use high-PRF PW Doppler, in which a higher PRF is used, which means that sampling now occurs at two or more distinct sites along the ultrasound beam but a higher velocity can be measured before aliasing occurs. Careful placement of the sample volumes so that one lies in the region of interest and all the others lie in low- velocity regions means that high-PRF PW Doppler can sometimes be a useful way round the aliasing problem.
This is because the limited sample volume of PW Doppler means that the red blood cells sampled have a narrower range of velocities than those sampled along the whole length of the ultrasound beam with CW Doppler. Turbulent flow occurs when this breaks down, for instance when passing through an area of stenosis, causing blood to flow in multiple directions and at different velocities Fig.
Stenosis Laminar Turbulent flow flow Fig. Blood flow is pulsatile, increasing and then decreasing in velocity with time during each cardiac cycle. A number of velocity measurements can be made from a spectral Doppler display. The outermost edge of the spectral trace represents the peak velocity at any particular time point. The brightest portion of the spectral display represents the velocity of the majority of the red blood cells modal velocity. The average velocity of the red blood cells is expressed as the mean velocity.
Stroke distance and volume Measurement of flow volume in a tube can, for a constant flow rate, be calculated simply by multiplying the cross-sectional area of the tube by the flow velocity. However, blood flow is pulsatile, not constant, so to calculate flow volume mL per heartbeat it is necessary to measure the cross-sectional area of the region of interest and to measure the velocity time integral VTI of flow in that region. VTI is measured by integrating the area under the spectral envelope — this can easily be achieved by tracing the outline of the spectral Doppler envelope and allowing the echo machine software to calculate the VTI.
VTI is measured in cm and represents the stroke distance — the distance travelled by a column of blood in the region of interest during one flow period Fig. Continuity equation The law of conservation of mass states that volume flow through the cardiovascular system is constant assuming that blood is incompressible and that the chamber or vessel carrying the blood is not elastic. Thus the flow rate in one area is equal to the flow rate in another, assuming a closed circuit i. If a particular cross-sectional area e.
However, rather than measuring blood flow at just a single sample volume, in colour Doppler the blood flow is assessed at multiple points within a pre-selected area. The size and position of this box can be adjusted so that it covers the region of interest Fig. At the edge of the display is a velocity scale, correlating the shade of colour with the measured flow velocity. Colour Doppler M-mode Colour Doppler M-mode uses the same principles as colour Doppler, but instead of overlaying the colour data on a 2-D display it instead overlays it on an M-mode display Fig.
It can be useful for precisely timing the occurrence of colour jets, and is commonly used for measuring the width of a jet of aortic regurgitation in relation to the diameter of the LVOT p. Although other structures, including the myocardium, move as well, filtering techniques were used to remove the Doppler signals returning from myocardium in order to optimize the signals relating to blood flow. However, since the s there has been growing interest in Doppler assessment of the myocardium tissue Doppler imaging, TDI. The Doppler signals returning from myocardium are distinct from signals from blood myocardial motion generates a stronger but lower-velocity signal and so can be selected with appropriate filtering.
The resulting signals can be displayed as colour Doppler images to show myocardial motion Fig. Available at: www. Technological advances in tissue Doppler imaging echocardiography. This chapter looks at service provision in terms of the departmental and staffing issues involved, and also examines the question of quality control.
Sonographers who undertake and report echo studies unsupervised should have appropriate accreditation.
Undertaking a recognised accreditation programme provides the sonographer with a structured means of attaining a minimum standard in echo. The process of learning about echo does not end with accreditation, but must continue to develop with continuing professional education and ongoing experience in performing echo studies, and by seeking reaccreditation at regular intervals. Several national societies provide accreditation programmes. Echo departments need to pay careful attention to health and safety, particularly with regard to musculoskeletal and eye problems, liaising with local occupational health and risk management departments as appropriate.
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Sonographers should not perform more than echo studies per year. Audit is one of the cornerstones of clinical governance, the process by which healthcare organizations are accountable for continually improving the quality and safety of their services. A method of collecting the data is then chosen, and then the necessary data are collected and analysed.
Any deficiencies should be identified while at the same time recognizing areas of good performance , and a mechanism agreed by which improvements can be made. Any changes should then be implemented and, after an appropriate timescale, the audit cycle should be repeated to see whether the changes have led to the expected improvements. The American College of Cardiology has also produced guidance jointly with a number of other societies. Clinicians requesting echo studies should be encouraged to include specific questions with their request e.
However, as with virtually every clinical test, echo has its limitations and can sometimes produce an erroneous result. Screening a large number of normal individuals for a rare disease using a test with imperfect specificity will produce a relatively large number of false positive results. The technique of Bayesian analysis takes this into account by considering how likely it is that the patient has the disease in question the pretest probability in order to predict how likely it is that a positive or negative test result is genuinely positive or negative.
Triage of inpatient echo requests Echo should be performed immediately if acute cardiac tamponade is suspected following an interventional procedure or if a patient presents with massive pulmonary embolism and echo is likely to help in deciding whether to administer thrombolysis.
Other inpatient echo requests can be performed more routinely, but should nevertheless be undertaken on an inpatient basis as soon as practicable. If circumstances do not permit this, the request should be discussed with the referring clinician to see if the study can be deferred and performed on an outpatient basis once the patient is discharged from hospital.