The history of Doppler ultrasonography of the uterine arteries in the literature is confusing because many reports failed to differentiate between spectral Doppler and color flow Doppler imaging. Early studies tend to be based on spectral Doppler examinations, which are a means of evaluating the resistance to blood flow using calculations of the pulsatility index (PI), resistance index (RI), Vmax, or the systolic-to-diastolic ratio (S/D ratio). Color flow Doppler and power flow Doppler imaging are means of turning motion, either toward or away from the transducer in the case of color flow
Figure 4 Midsagittal view of the uterus. The cervix is to the right of the image and the fundus is to the left. The endometrium is well demarcated and shows a pronounced, thick "triple-line" pattern associated with a higher probability of implantation following embryo transfer.
Doppler, or motion in any direction in the case of power flow Doppler, into a visually detectable color overlay on the two-dimensional ultrasound image (Fig. 2) (89).
Initially, attempts to determine if evaluation of blood flow in the uterine arteries could be useful were based on RI calculations to look for differences in uterine receptivity, where in a small series of women, no differences were found between women who conceived and those who did not (90). When the PI of uterine arteries was examined and data were grouped into low, medium, and high categories, no differences were found between cycles where women either conceived or did not; however, no pregnancies were established in the women with high PI values (91). Elevated PI, as a measure of impedance to vascular flow in the uterine artery, was associated with a significantly lower pregnancy rate (92). However, no differences in uterine artery PI were observed between conception and non-conception cycles (93). A study assessing RI of the uterine arteries was inconclusive, except that absent or low diastolic flow was associated with failure to conceive (93). Uterine artery vascular impedance measured by RI was not found to be useful for predicting the probability of pregnancy, but if the PI values were found to be greater than 0.79 before hCG administration, poor uterine vascular perfusion was assumed (94). A subsequent study reported the PI and RI in the uterine arteries to be lower in conception cycles, and the authors suggested that a PI greater than 3.3 and an RI greater than 0.95 before embryo transfer were associated with a low probability of conception (95). Furthermore, a study of women undergoing cycles in which embryos were produced using ICSI reported no demonstrable differences in PI on the day of embryo transfer between conception and non-conception cycles (96).
In a critical review of the literature prior to 1996, some ultrasonographi-cally detectable criteria were observed to be associated with negative pregnancy outcomes; however, no prognostic value was observed in any measurement of vascular perfusion (97). A more recent study appears to confirm the results that women who conceived exhibited lower PI than those who did not (98).
Studies that evaluated endometrial perfusion on the day of hCG administration consistently reported that values for PI and Vmax were not different, irrespective of whether or not conception was established (95,97,99-101). However, when only the color flow data were examined, absence of detectable subendometrial vascular flow, indicative of poor vascular penetration, was associated with failure of implantation (99). Power flow Doppler ultrasonography was subsequently used to examine women whose endometrial thickness was >10 mm. Intra-endometrial flow calculations of the maximal area that showed evidence of motion indicative of vascular flow of < 5 mm2 were associated with a lower pregnancy rate (100). Subsequently, a high degree of endometrial perfusion visualized using three-dimensional ultrasonography was thought to indicate a more favorable endometrium (102). The most recent study in this line of inquiry concluded that spiral arterial flow and uterine artery flows were not different between pregnant and non-pregnant women; however, if spiral arterial flow could not be detected, there was no probability of conception (101).
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