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TELECONFRENCES
2004
The Changing Left Ventricle

2003
Aortic Valve Disease: New Dimensions in Evaluation and Management

2002
Heart Failure: Echo's Role in and Emerging Health Crisis

2001
Chest Pain in Children & Adults: The Role of Echo

2000
Mitral Regurgitation: New Concept

1998
The Falling Left Ventricle: Diastolic & Systolic Function

1997
Changing the Outcome of Coronary Artery Disease
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Chest Pain in Children and Adults

Mitral Regurgitation: New Concepts

Diastolic and Systolic Function

Changing the Outcome of CAD

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2000 MV
2001 Chest Pain
2002 Heart Failure


Pulsed and Continuous Wave Doppler
Pulsed Wave Doppler
Fig.1.18

Pulsed wave (PW) Doppler systems use a transducer that alternates transmission and reception of ultrasound in a way similar to the M-mode transducer (Fig. 1.18). One main advantage of pulsed Doppler is its ability to provide Doppler shift data selectively from a small segment along the ultrasound beam, referred to as the "sample volume". The location of the sample volume is operator controlled. An ultrasound pulse is transmitted into the tissues travels for a given time (time X) until it is reflected back by a moving red cell. It then returns to the transducer over the same time interval but at a shifted frequency. The total transit time to and from the area is 2X. Since the speed of ultrasound in the tissues is constant, there is a simple relationship between roundtrip travel time and the location of the sample volume relative to the transducer face (i.e., distance to sample volume equals ultrasound speed divided by round trip travel time). This process is alternately repeated through many transmit-receive cycles each second.

This range gating is therefore dependent on a timing mechanism that only samples the returning Doppler shift data from a given region. It is calibrated so that as the operator chooses a particular location for the sample volume, the range gate circuit will permit only Doppler shift data from inside that area to be displayed as output. All other returning ultrasound information is essentially "ignored".

Another main advantage of PW Doppler is the fact that some imaging may be carried on alternately with the Doppler and thus the sample volume may be shown on the actual two-dimensional display for guidance. PW Doppler capability is possible in combination with imaging from a mechanical or phased array imaging system. It is also generally steerable through the two-dimensional field of view, although not all systems have this capability.

Fig.1.19

In reality, since the speed of sound in body tissues is constant, it is not possible to simultaneously carry on both imaging and Doppler functions at full capability in the same ultrasound system. In mechanical systems, the cursor and sample volume are positioned during real-time imaging, and the two-dimensional image is then frozen when the Doppler is activated. With most phased array imaging systems the Doppler is variably programmed to allow periodic update of a single frame two-dimensional image every few beats (Fig. 1.19). In other phased arrays, two-dimensional frame rate and line density are significantly decreased to allow enough time for the PW Doppler to sample effectively. This latter arrangement gives the appearance of near simultaneity.

Fig.1.20

The sample volume is really a three-dimensional, teardrop shaped portion of the ultrasound beam (Fig. 1.20). Its volume varies with different Doppler machines, different size and frequency transducers and different depths into the tissue. Its width is determined by the width of the ultrasound beam at the selected depth. Its length is determined by the length of each transmitted ultrasound pulse.

Therefore, the farther into the heart the sample volume is moved, the larger it effectively becomes. This happens because the ultrasound beam diverges as it gets farther away from the transducer.

Fig.1.21

The main disadvantage of PW Doppler is its inability to accurately measure high blood flow velocities, such as may be encountered in certain types of valvular and congenital heart disease. This limitation is technically known as "aliasing" and results in an inability of pulsed Doppler to faithfully record velocities above 1.5 to 2 m/sec when the sample volume is located at standard ranges in the heart (Fig. 1.21). Aliasing is represented on the spectral trace as a cut-off of a given velocity with placement of the cut section in the opposite channel or reverse flow direction. Because aliasing is so common in disease states, it will be considered in more detail in the next section.

Fig.1.22

The spectral outputs from PW and CW appear differently
(Fig. 1.22). When there is no turbulence, PW will generally show a laminar (narrow band) spectral output. CW, on the other hand, rarely displays such a neat narrow band of flow velocities even with laminar flow because all the various velocities encountered by the ultrasound beams are detected by CW.

Fig.1.23

It can usually be said that when an operator wants to know where a specific area of abnormal flow is located that pulsed wave Doppler is indicated. When accurate measurement of elevated flow velocity is required, then CW Doppler should be used. The various differences between pulsed and continuous wave Doppler are summarized in Figure 1.23.

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