 |
| Fig.1.14 |
The Doppler equation also tells us that the angle the Doppler
beam is relative to the lines of flow being evaluated is very
important. This angle theta, (written as Ø in the Doppler equation),
is of crucial importance in the calculation of velocities from
Doppler shift data in Figure
1.14 where the effect of varying angle on the measurement
of peak velocity of an aortic stenotic jet is shown.
When the ultrasound beam is directed parallel to blood flow, angle
Ø (cosine 0° = 1) and measured velocity on the recording will be
true velocity. In contrast, with the ultrasound beam directed perpendicular
to flow, angle Ø = 90 degrees (cosine 90° = 0) and measured velocity
will be zero. Therefore, the smaller the angle, the closer angle
cosine Ø is to 1.0 and the more reliable is the recorded Doppler
velocity. A wider angle will result in a greater reduction in measured
velocity compared to true velocity.
Thus, the more parallel to flow the Doppler ultrasound beam is directed
the more faithfully the measured velocity will reflect true velocity.
For practical purposes, angles of greater than 25° between the ultrasound
beam and the blood flow being studied will generally yield clinically
unacceptable qualitative estimates of velocity.
 |
| Fig.1.15 |
A Doppler operator seeking the best quantitative estimates of flow
must, therefore, always attempt to orient the beam parallel to flow.
This concept is of fundamental importance in the clinical examination.
The actual effect of changing angle on a systolic aortic stenotic
jet toward a transducer in the suprasternal notch is shown in Figure
1.15. The first beat (open arrow) shows the only fully formed profile.
 |
| Fig.1.16 |
Such abnormal jets are often eccentric and their directions cannot
always be predicted. A Doppler examiner must therefore interrogate
the jet from a variety of angles. Note that the full jet is not
seen from the suprasternal area in this patient but is detected
from the apical approach. The great importance of this concept in
the clinical examination for aortic stenosis is demonstrated in
Figure 1.16. The need to be parallel to flow leads the Doppler examiner
to depend on some windows for examination that may sacrifice the
quality of the two-dimensional image. For example, the direction
of the ultrasound beam through either the mitral or tricuspid orifices
from the apical position offers an excellent Doppler window but
one which may allow significant echocardiographic "drop-out" since
the imaging beams are parallel to the endocardium.
2004