Harmonic imaging in fetal echocardiography

An Automated ALARA Method for Ultrasound: An Obstetric Ultrasound Feasibility Study

OBJECTIVES

Ultrasound users are advised to observe the ALARA (as low as reasonably achievable) principle, but studies have shown that most do not monitor acoustic output metrics. We developed an adaptive ultrasound method that could suggest acoustic output levels based on real-time image quality feedback using lag-one coherence (LOC).

METHODS

Lag-one coherence as a function of the mechanical index (MI) was assessed in 35 healthy volunteers in their second trimester of pregnancy. While imaging the placenta or the fetal abdomen, the system swept through 16 MI values ranging from 0.15 to 1.20. The LOC-versus-MI data were fit with a sigmoid curve, and the ALARA MI was selected as the point at which the fit reached 98% of its maximum.

RESULTS

In this study, the ALARA MI values were between 0.35 and 1.03, depending on the acoustic window. Compared to a default MI of 0.8, the pilot acquisitions suggested a lower ALARA MI 80% of the time. The contrast, contrast-to-noise ratio, generalized contrast-to-noise ratio, and LOC all followed sigmoidal trends with an increasing MI. The R2 of the fit was statistically significantly greater for LOC than the other metrics (P < .017).

CONCLUSIONS

These results suggest that maximum image quality can be achieved with acoustic output levels lower than the US Food and Drug Administration limits in many cases, and an automated tool could be used in real time to find the ALARA MI for specific imaging conditions. Our results support the feasibility of an automated, LOC-based implementation of the ALARA principle for obstetric ultrasound.

Impact of changing indications and increased utilization of fetal echocardiography on prenatal detection of congenital heart disease

BACKGROUND

Antenatal diagnosis of congenital heart defects (CHD) can impact outcomes in neonates with severe CHD. Obstetric screening guidelines and the indications for fetal echocardiography (FE) have evolved in an attempt to improve the early prenatal detection of CHD. Analyzing yield for specific indications will help clinicians better stratify at-risk pregnancies.

METHODS

Retrospective cohort study of all FE performed between 2000 and 2010 at a single tertiary care academic medical center in New York City. A total of 9878 FE met inclusion criteria for our study. In cases of multiple gestations (MG), each fetus was counted as a separate study.

RESULTS

The number of new diagnosis of fetal CHD by FEs increased 200%. There was a statistically significant increase in those referred for suspected CHD, increased nuchal translucency (NT), MG, and suboptimal imaging (P < .001). The indication of "suboptimal imaging" (SO) not only accounted for 5.23% of all referrals from 2000 to 2002, compared to 22.26% of all referrals from 2008 to 2010 (P < .0001), but also had the lowest yield for diagnoses of CHD (P < .02).

CONCLUSIONS

Over the past decade, there has been an increase in utilization of FE with a proportional increase in prenatally diagnosed CHD. For indications such as suspected CHD, NT and MG increases in referrals have led to a proportionate increase in fetal diagnosis of CHD. SO as an indication has the lowest yield of fetal diagnosis of CHD. Antenatal detection of CHD may be improved by a change in obstetric imaging protocols to ensure appropriate referrals.

Obstetrical sonography in obese women: a review

Obstetric sonographic imaging in pregnant women is adversely affected by obesity with a negative impact on the detection rate of congenital anomalies. This review aims to analyze relevant data regarding this issue and to discuss clinical and technical problems associated with sonographic examination of obese pregnant women.

Impact of tissue harmonic imaging on measurement of nuchal translucency thickness

OBJECTIVES

To evaluate the effect of tissue harmonic imaging (THI) on the measurement of fetal nuchal translucency thickness (NT).

METHODS

One-hundred and three pregnant women underwent first-trimester NT measurement according to The Fetal Medicine Foundation criteria. NT was evaluated using conventional ultrasonography (CUS) and THI without modifying any other ultrasound parameter (e.g. gain). For each patient three images with CUS and three images with THI were stored and then measured independently on the ultrasound system by two different operators. The maximum measurements using CUS and THI were compared. Differences between the values of CUS and THI NT measurements were tested using the Wilcoxon signed-rank test. Bland-Altman plots were constructed, and intraobserver and interobserver variabilities were assessed by calculation of the intraclass correlation coefficient. Probability values of < 0.05 were considered significant.

RESULTS

Mean maternal age was 32.8 (range, 20-42) years, mean gestational age at examination was 12 + 1 (11 + 0 to 13 + 6) weeks and mean crown-rump length (CRL) was 55.8 (SD, 7.2) mm. Median fetal NT was 1.4 (0.8-3.5) mm using CUS and 1.2 (0.6-3.3) mm using THI (P < 0.001) for the first operator. A second operator remeasured the first 51 cases: median fetal NT was 1.4 (0.8-3.8) mm using CUS and 1.1 (0.6-3.1) mm using THI (P < 0.001). Fetal NT measurements were above the expected median value according to CRL in 49 cases (47.6%) using CUS and in only 24 cases (23.3%) using THI.

CONCLUSIONS

THI leads to a small, but significant, reduction of the NT measurement and this could reduce the sensitivity of screening for Down syndrome.

The role of tissue harmonic imaging in fetal echocardiography

OBJECTIVE

To define the role of tissue harmonic imaging (THI) in fetal echocardiography.

METHOD

Three trials were conducted in a tertiary referral center: Study A was a prospective randomized trial including 50 women referred for fetal echocardiography. Those allocated to Arm 1 underwent conventional fundamental frequency ultrasound (FFU) and those allocated to Arm 2 underwent THI. Study B was conducted in 21 patients who were obese or overweight with significant weight gain in pregnancy. In these patients, both THI and FFU echocardiography were performed and compared. In Study C, THI was employed as a second-line rescue technique in cases of inadequate or incomplete examination by FFU. A subjective scoring system was used by a reviewer who had not performed the examinations to assess the comprehensiveness of the examination and the image resolution. The reviewer was blinded to the image modality.

RESULTS

In Study A, no difference was found in the diagnostic accuracy between THI and FFU echocardiography but the resolution was significantly poorer in the THI arm. In studies B and C, THI performed significantly better than FFU (P < 0.001).

CONCLUSIONS

THI echocardiography seems to be the best technique to employ in obese women and in those in whom FFU fails to provide diagnostic information. However, due its poorer resolution in women of average weight, FFU echocardiography should remain the technique of choice.

New fetal cardiac imaging techniques

Rapid advances in graphics computing and micro-engineering have offered new techniques for prenatal cardiac imaging. Some of them can be non-invasively applied to both clinical and laboratory settings, including dynamic three-dimensional echocardiography, myocardial Doppler imaging, harmonic ultrasound imaging, and B-flow sonography. With clinical constraints, a few others have been mainly used in laboratories, such as endoscopic ultrasound, magnetic resonance imaging and biomicroscopy. Appropriate use and co-use of these new tools will not only provide unique information for better clinical assessment of fetal cardiac disease but also offer new ways to improved understanding of cardiovascular development and pathogenesis.