Technical Factors

Technical Factors

The technical aspects of acquiring a high-quality,

diagnostic 3D echocardiogram are similar to those

for 2D echocardiography. As with any new imaging

technique, a learning curve exists, and recognizing

and avoiding potential artifacts is critical.

Many of the artifacts are related to respiratory or

ECG gating and/or incorrect gain settings. The use

of optimal gain settings before acquisition is essential

for accurate diagnosis. Low gain settings

can artificially eliminate certain structures that

then will not be viewable during postprocessing.

Alternatively, using high gain settings can mask

structures and lead to significant misdiagnoses.

Therefore, overcompensating for the brightness

of the image using the time-gain compensations is

recommended, to allow the overall gain to be set

at midrange values. This maneuver will allow

maximum flexibility with postprocessing settings.

Most 3D echocardiographic systems use some

form of gating to obtain volumetric data. Gated

data sets are most challenging in patients with

arrhythmias or respiratory difficulties. The confounding

effects of the gating artifacts can be

minimized in different ways. For example, if the

gated system acquires sector slices in a sweeping

motion parallel to the reference image, then every

image viewed parallel to the reference image will

appear normal, whereas the gated artifacts will be

most noticeable when viewed from a plane orthogonal

to the reference image.

Segmentation is the process by which anatomic

features are extracted from the raw ultrasound

data. Segmentation can be accomplished using

low-level techniques, such as edge detection (in

2D) and surface detection (in 3D) based on local

features, such as the spatial gradient in ultrasound

intensity. More sophisticated techniques attempt

to extract entire boundaries or surfaces at once

based on local features and the anticipated shape

of the overall structure. Compression is a mathematical

technique that can be applied to the

original digital image file to reduce the amount of

data, thereby decreasing storage requirements and

improving retrieval rates. A single-volume data set

from a typical RT3D echocardiographic system

consists of 64 64 512 bytes (approximately 2

MB), or more than 50 MB for a 1-second loop, a

load that can overwhelm storage systems. Compression

of the digital data files can reduce this

load by about 3:1. The motion-JPEG algorithm

currently used by DICOM (Digital Imaging and

Communications in Medicine) and applied to individual

2D slices could be expected to achieve an

approximate 20:1 compression. More advanced

algorithms, such as wavelets (JPEG-2000), potentially

can yield better results. The use of compression

algorithms can decrease the size of data files,

optimizing storage efficiency without sacrificing

image quality.