An optimal Doppler signal requires optimization of quite a large number of system parameters. Thus, for example, if the PRF selected is too low, high velocities will result in aliasing. If, on the other hand, the PRF selected is too high, the overall measurement time will be short so that the measurement resolution will be low. As also discussed above, tissue and vessel wall motion need to be filtered out. This again is quite tricky. Too much filtering will also eliminate ‘legitimate ’ low velocity blood flow signals. Color flow imaging still presents more problems. The ‘quality ’ of the measurement depends on the number of ultrasonic beams that are transmitted in a given direction, which defines at each gate the number of data points that are available for the calculation. Increasing the ‘quality ’ will improve the accuracy of the calculation, but will also adversely affect the frame-rate.
Table 6.7 below lists the various controls used in the operation of the various Doppler modes.
As seen from Table 6.7 , the number of controls that need to be optimized is quite large. In order to allow rapid optimization of the gray-scale image, Doppler, and color, most systems will use presets for all clinical applications.
Proper use of the presets saves time, reduces button push-ing, and ensures diagnostic results. Note, however, that presets are usually designed for average set of conditions in the given application. Fine adjustments are usually for the individual cases.
However, MRI is not widely available for clinical use because it is expensive and time-consuming. Other limi-tations include relatively low spatial ( ∼ 2 – 5 mm) and tem-poral resolution (at best ∼ 30 ms) of the magnetic tags, difficulties in analyzing the whole cardiac cycle due to the short persistence of the tagging, and its inability to analyze beat-to-beat variability.
General Electric introduced into the market a new diagnostic tool, similar in concept to MRI tagging, that allows objective analysis of the complete myocardial motion throughout the entire cardiac cycle. Similar in concept to MRI tagging, two-dimensional (2D) strain analyzes motion by tracking ‘tags ’ (natural acoustic mark-ers) in the ultrasonic image in two dimensions. These natural markers are used in a way similar to the magnetic tags in MRI. As with tagged MRI, the tags are short-lived;
one cannot expect the natural acoustic markers to persist throughout the entire cardiac cycle, mainly due to their movement in and out of the imaging plane. However, unlike in MRI, in which the entire tagging fades out and limits the analysis time to only part of the heart cycle, ultrasound ’ s new acoustic markers keep coming in as some of the previous markers fade out. This is illustrated in Figure 6.27 . Myocardial motion and velocities are then analyzed by calculating frame-to-frame changes. 2D strain is in a way a natural extension of one-dimensional pler motion analysis. Similar to one-dimensional Dop-pler, myocardial motion is characterized in terms of tissue velocity and tissue deformation parameters, such as strain and strain rate.
Indications for functional evaluation of the fetal heart include: intrauterine growth restriction, where cardiac fail-ure may be present, ischemia, placental dysfunction/insuf-ficiency, congenital heart defects such as hypertrophic cardiac myopathies, fetal anemia and other causes of hydrops fetalis, fetal arrhythmias, twin-to-twin transfusion
Figure 6.27
‘Natural acoustic tagging ’ . New features (blue circles) keep coming into the image as old ones (yellow circles) fade away.
Kremkau WF . Diagnostic Ultrasound: Principles, Instrumenta-tion and Exercises. Orlando : Grune & Stratton , 1984 . Leitman M , Lysyansky P , Sidenko S et al. Two-dimensional
strain – a novel software for real-time quantitative echocar-diographic assessment of myocardial function. J Am Soc Echocardiogr 2004 ; 17 : 1021 – 9 .
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Ultrasound Annual 1985. New York : Raven Press , 1985; 1–16 .
Moore CC , McVeigh ER , Zerhouni EA . Quantitative tagged magnetic resonance imaging of the normal human left ventricle. Top Magn Reson Imaging 2000 ; 11 : 359 – 71 . Scanlan KA . Sonographic artifacts and their origins. AJR Am J
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Table 6.7 The system controls available to the operator for optimizing Doppler measurements
Control Pulsed wave (spectral) Doppler Color flow imaging
Total gain Increases signal received from moving
objects of low reflectivity. Noise is also amplified
Increases signal received from moving objects of low reflectivity. Noise is also amplified Wall motion filter Decreases the signal from slowly moving
objects such as vessel walls or surrounding tissue (irrelevant in tissue Doppler). Effect will increase with higher setting, but signal from slowly moving blood particles will not be observed
Decreases the signal from slowly moving objects such as vessel walls or surrounding tissue (irrelevant in tissue Doppler). Effect will increase with higher setting, but signal from slowly moving blood particles will not be observed
Baseline Shifting the baseline will increase the measurement range of positive (negative) velocities and reduce the measurement range of negative (positive) velocities
Shifting the baseline will increase the measurement range of positive (negative) velocities and reduce the measurement range of negative (positive) velocities.
Not provided by some manufacturers. Irrelevant in power Doppler mode
Sample volume position
Defines the precise location on the anatomical gray-scale image from where Doppler data are sampled
NA
ROI position NA Defines the part of the image where blood
flow will be imaged Sample volume size Increasing the sample volume will
increase signal intensity, as long as the sample volume size does not exceed the boundaries of the vessel
NA
Size of the ROI NA The size and position of the area on the image
where flow will be demonstrated. Increasing the size of the ROI will reduce the frame rate Pulse repetition
frequency (PRF)
The PRF determines the maximum velocity that can be measured without causing aliasing. The maximum PRF is depth range dependent. Aliasing can sometimes be indicated by ‘cropped’ spectral displays
The PRF determines the maximum velocity that can be measured without causing aliasing. The maximum PRF is depth range dependent. Aliasing can sometimes be recognized when colors are mixed within the same vessel
Color ‘quality’ NA Defines the number of beams transmitted in each
direction, for computation of the velocities along the vector. Increasing the ‘quality’ may be required for demonstrating very low flow, but will result in reduced frame rates
ROI, region of interest; NA, not applicable.
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