FIGURE 10-48 Transverse limb-reduction defect. A. At 18 weeks' gestation, only a rudimentary hand was visible. B. By 24 weeks, the radius and ulna were normal in size and appearance, and small rudimentary digits were evident.
For 4-D imaging, one application known as spatiotemporal image co rrelatio n-S TIC improves visualization of cardiac anatomy. During an automated sweep over the heart, the STIC application acquires a volume that includes thousands of 2D images captured at a rate as high as 150 frames per second (Devore, 2003). These individual images are obtained at diferent locations in the heart but at the same point in time. These views are subsequently arranged according to their spatial and temporal domains. his permits display of an ordered sequence of volume sets in a continuous cine loop (or video clip) of the cardiac cycle (Yeo, 2016). For example, after obtaining a volume sweep over the cardiac apex, an application such as Fetal Intelligent Navigation Echocardiography (FINE) can be applied to
FIGURE 10-49 Fetal face. Surface rendered three-dimensional image of a normal fetal face and hand at 32 weeks.
display videos of each of the diferent cardiac views shown in
Fig. 10-28 (Garcia, 2016). It is hoped that such technology may eventually improve detection of fetal cardiac anomalies.
For selected anomalies, such as those of the face and skel eton, for tumors, and for some cases of neural-tube defects, 3-D sonography can add useful information (American College of
Obstetricians and Gynecologists, 2016; Goncalves, 2005). That said, comparisons of 3-D and conventional 2-D sonography for the diagnosis of most congenital anomalies have not demon strated better overall detection rates (Goncalves, 2006; Reddy, 2008). The American College of Obstetricians and Gynecolo gists (2016) concludes that proof of a clinical advantage of 3-D S sonography for prenatal diagnosis is generally lacking. D hen sound waves strike a moving target, the frequency of the waves reflected back is shifted in proportion to the velocity and direction of that moving target-a phenomenon known as the Doppler sht. Because the magnitude and direction of the frequency shift depend on the relative motion of the moving target, Doppler can help evaluate low within blood vessels. The Doppler equation is shown in Figure 10-50.
n important component of the equation is the angle of insonation, abbreviated as theta (8). This is the angle between the sound waves from the transducer and low within the vessel. Measurement error becomes large when 8 is not close to zero, in other words, when blood flow is not coming directy toward or away from the transducer. For this reason, ratios are often used to compare diferent waveform components, allowing cosine 8 to cancel out of the equation. Figure 10-51 is a schematic of the Doppler waveform and describes the three ratios commonly used. he simplest is the systolic-diastolic ratio (SID ratio), which compares the maximal (or peak) systolic low with end-diastolic flow to evaluate downstream impedance to flow. Currently, two types of Doppler modalities are available for clinical use.
Continuous-wave Doppler equipment has two separate types of crystals-one transmits high-frequency sound waves, and ,,,I ,
FIGURE 10-50 Doppler equation. Ultrasound emanating from the transducer with initial frequency fo strikes blood moving at velocity v. Reflected frequency fd is dependent on angle e between beam of sound and vessel.
FIGURE 10-51 Doppler systolic-diastolic waveform indices of blood flow velocity. S represents the peak systolic flow or velocity, and D indicates the end-diastolic flow or velocity. The mean, which is the time-average mean velocity, is calculated from computerdigitized waveforms.