Fetal Three-Dimensional Echocardiography

Published on 07/06/2015 by admin

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28 Fetal Three-Dimensional Echocardiography

Rima S. Bader, David J. Sahn

I. WHY A FETAL SCAN IS A CHALLENGE

A. Obstacles

1. In the pediatric or adult patient, predetermined windows are used to obtain standardized views of the heart.

2. Obstacles to views in the fetus.

a. The size of the heart varies as a function of gestational age.

b. The fetal position can vary during the examination.

    (1) This makes imaging of structures more difficult.
    (2) It prevents consistent acquisition of any specific view.

c. Fetal movement during the examination can result in not acquiring all of the diagnostic images for analysis.

d. Oligohydramnios can make imaging more difficult.

e. Acquiring the images of the heart can be time consuming.

B. Evaluation

1. For effective conversation with the parents, interpretation must be done in real time, either during the examination or quickly after reviewing digital records.

2. Given these potential obstacles, evaluation of the fetal heart is one of the most challenging diagnostic tasks the fetal examiner encounters during an early second- or third-trimester ultrasound examination.

II. OVERVIEW

A. Superiority to two-dimensional echocardiography

1. Accurate quantification of:

a. Volumes of the right ventricle (RV) and left ventricle (LV).

b. LV mass.

c. LV ejection fraction without the need for geometric assumptions.

2. Improved visualization of:

a. Spatial relations between cardiac structures, which is important when assessing complex congenital heart disease.

b. Views of valve structures.

    (1) Three-dimensional (3-D) views are not readily obtained from conventional two-dimensional (2-D) images, regardless of fetal position.
    (2) For example, en face views of the mitral valve apparatus from either an atrial or ventricular perspective can offer a unique insight into the physiology of valve mechanics.

B. Disadvantages

1. In its current format, 3-D echocardiography is difficult to acquire.

2. Data analysis is cumbersome.

3. Particularly in the fetal arena, there are ongoing limitations to image resolution of more diminutive cardiac structures.

C. Scanning methods

1. Free-hand scanning using mechanical or nonmechanical positional locators.

2. Gated or spatiotemporal realigned acquisition.

3. Matrix array–based volume imaging transducers to obtain real-time 3-D or gated volume data sets.

III. THREE-DIMENSIONAL TERMINOLOGY AND PHYSICS

The most important developments for accomplishing the transition to fetal echocardiography from 2-D to 3-D echocardiography are real-time 3-D imaging and spatiotemporal realignment motion gating. Real-time means there is no significant delay between acquisition and display.

A. Terminology

1. The most commonly used term is real-time.

a. Real-time 3-D imaging.

b. Real-time four-dimensional (4-D) imaging.

c. Real-time volumetric imaging.

2. The actual volume rates to which they refer vary from 8 Hz to 24 Hz.

B. Physics

1. Overview.

a. It is generally stated that the human eye and brain retain a visual impression for about 1/10 to 1/30 of a second, equivalent to a frame or volume rate of 10 to 30 Hz.

b. The exact time depends on the brightness and clarity of the image and pattern recognition.

c. When referring to real-time imaging, we are actually talking about whether an imaging system can achieve the visual continuity that will satisfy the human eye and brain.

2. Built-in, spatial, and temporal realignment of 4-D data.

a. It is hard to consistently find a stationary volume of interest in utero due to unwanted patient movements (e.g., fetal activity, maternal respiration) and/or environmental movements such as abdominal deformation from probe movement when using conventional approaches.

b. Real-time 3-D systems can acquire a volume data set without manual displacement of the transducer.

    (1) This is achieved by using a built-in mechanism for registering spatial and temporal information during a volume scan.
    (2) Example systems include:
       (a) Philips Live 3D Echo (Koninklijke Philips Electronics NV, Eindhoven, Netherlands), based on matrix-array transducer technology.
       (b) Philips and Siemens volume scanners. The new Siemens S class scanner also offers real-time full volume 4D “Echo in a Heartbeat.”
       (c) The Philips (iU22 and iE33) and Siemens Antares products.
       (d) GE Voluson 730, which is based on rapidly oscillating cross-sectional transducer technology of spatiotemporal realignment.

3. Sufficient temporal resolution for the time scale of interest.

a. For general analysis of dynamic anatomy of the fetal heart, a minimum volume rate of about 15 to 25 Hz is required with fetal heart rates of 150 bpm or more. This is equivalent to a maximum of 40 ms for each imaging volume sampling period.

b. Except for isovolumetric contraction and relaxation, other cardiac phases each rarely last longer than 80 ms (rounded numbers are used in this discussion for mathematical simplicity) and can be fully sampled at least once in one imaging volume per cardiac cycle.

c. To achieve 90% accuracy for the measurement, a 500-Hz volume rate may be necessary.

    (1) This is far beyond the capability of any of the current real-time 3-D systems.
    (2) Therefore, none of the existing systems is capable of direct-volume scanning for analyzing this important phase.

IV. THREE-DIMENSIONAL ECHOCARDIOGRAPHY SYSTEMS

A. Rapidly oscillating cross-sectional transducer

1. Volumetric scanning.

a. Volumetric scanning is less dependent on the angle of acquisition, and thus less dependent on fetal lie or operator expertise, than 2-D scanning.

b. The most promising aspect is the possibility of storing and compressing a volume of 4-D information for later offline evaluation by an expert.

c. Because the heart is beating, nongated acquisition produces artifacts in the reconstructed volume data.

2. Spatiotemporal image correlation (STIC).

a. Description.

    (1) One new technology available on the Voluson 730 Expert series and Voluson E8 (GE Medical Systems, Kretz Ultrasound, Zipf, Austria), as well as the Philips iU22 and Philips iE33, is spatiotemporal image correlation (STIC).
    (2) STIC is automatic volume acquisition through the fetal heart, resulting in a complete fetal cardiac cycle displayed in motion in an endless 3-D cineloop sequence.
    (3) The array inside the transducer housing performs a slow, single sweep, recording one single 3-D data set over an area of 15 to 40 degrees, offering information on the entire fetal heart, including surrounding structures, with the heart in motion.
    (4) If the acquisition begins at the level of the four-chamber view, a real-time motion sequence of a sweep from 15 to 40 degrees cranial and caudal to the four-chamber view is obtained, also giving the possibility of a multiplanar view.

b. Method.

    (1) The acquisition is performed in two steps.
       (a) First, images are acquired by a single, automatic volume sweep.
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