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2004
The Changing Left Ventricle

2003
Aortic Valve Disease: New Dimensions in Evaluation and Management

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Heart Failure: Echo's Role in and Emerging Health Crisis

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Chest Pain in Children & Adults: The Role of Echo

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Mitral Regurgitation: New Concept

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The Falling Left Ventricle: Diastolic & Systolic Function

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Pulsed and Continuous Wave Doppler

There are two main types of Doppler echocardiographic systems in common use today, continuous wave and pulsed wave. They differ in transducer design and operating features, signal processing procedures and in the types of information provided. Each has important advantages and disadvantages and, in our opinion, the current practice of Doppler echocardiography requires some capability for both forms.

Continuous Wave Doppler

Fig.1.17

Continuous wave (CW) Doppler is the older and electronically more simple of the two kinds. As the name implies, CW Doppler involves continuous generation of ultrasound waves coupled with continuous ultrasound reception. A two crystal transducer accomplishes this dual function with one crystal devoted to each function (Fig. 1.17).

The main advantage of CW Doppler is its ability to measure high blood velocities accurately. Indeed, CW Doppler can accurately record the highest velocities in any valvular and congenital heart disease. Since velocities exceeding 1.5 m/sec are frequently seen in such disorders, accurate high velocity measurement is of particular importance for allowing the recognition of the full abnormal flow profile. It is also of considerable importance for the quantitative evaluation of abnormal flows, as will be seen later.

The main disadvantage of CW Doppler is its lack of selectivity or depth discrimination. Since CW Doppler is constantly transmitting and receiving from two different transducer heads (crystals) there is no provision for imaging or range gating to allow selective placing of a given Doppler sample volume in space. As a consequence, the output from a CW examination contains Doppler shift data from every red cell reflecting ultrasound back to the transducer along the course of the ultrasound beam.

Thus, true CW Doppler is functionally a stand-alone technique whether or not the capability is housed within a two-dimensional imaging transducer. The absence of anatomic information during CW examination may lead to interpretive difficulties, particularly if more than one heart chamber or blood vessel lies in the path of the ultrasound beam.

It is possible , however, to program a phased array system to perform both two-dimensional and CW Doppler functions almost simultaneously. The quasi-simultaneous CW-imaging uses a time sharing arrangement in which the transducer rapidly switches back and forth from one type of examination to the other. Because this switching is done at very high speeds, the operator gets the impression that both studies are being done continuously and in real-time. During the imaging period, no Doppler data is being collected, so an estimate is generated, usually from the preceding data. During the Doppler collection period, previously stored image data is displayed. This arrangement usually degrades the quality of both the image and Doppler data.

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