Attempts
to record and display ultrasound images in 3D format were first reported in the
1960s. One of the earliest studies described the acquisition of a series of
parallel scans of a human orbit to reconstruct 3D anatomy. Despite the limited technology
of the day, these initial studies demonstrated that complex anatomic structures
were ideally displayed using 3D techniques. Concerns about image quality and
the computational power needed for storage and reconstruction greatly limited
the early application of this methodology.
More
than a decade later, investigators began to obtain 3D ultrasound images of the
heart. Through the careful
tracking of a transducer, a sequence of 2-dimensional (2D) echocardiograms
could be recorded, aligned, and reconstructed into a 3D data set. This
methodology was limited by the need for offline data processing to create and
display the 3D images. In the early 1990s, von Ramm and colleagues developed the first
real-time 3D (RT3D) echocardiographic scanner, capable of acquiring volumetric
data at frame rates sufficient to depict cardiac motion. More recently, further
improvements in design and engineering have led to the commercialization of
RT3D echocardiography. This methodology has evolved quickly, and different versions
of RT3D imaging are currently available on several platforms.
Reconstruction Techniques
Early
approaches to 3D echocardiography were based on the principle that a 3D data
set could be reconstructed from a series of 2D images. In this method, serial
2D images are obtained using either freehand scanning or a mechanically driven
transducer that sequentially recorded images at predefined intervals. With freehand scanning, a
series of images is obtained by manually tilting the transducer along a fixed
plane, and a spatial locator attached to the transducer translates the 3D
spatial location onto a Cartesian coordinate system. This approach has several
practical limitations, including the relative bulk of the acoustic spatial
locators, which makes transducer manipulation difficult, and the need for a
clear and direct path between the acoustic locators and the transmitter. For
electromagnetic spatial locator systems, an additional problem is the potential
for interference of the electromagnetic field by ferromagnetic material in
close proximity to the transducer (eg, material in hospital beds and medical
equipment).
An
alternative to freehand scanning is the use of a mechanized transducer to
obtain serial images at set intervals in a parallel fashion or by pivoting
around a fixed axis in a rotational, fanlike manner. Because the intervals and
angles between the 2D images are defined, a 3D coordinate system can be derived
from the 2D images in which the volume is more uniformly sampled than with the freehand scanning
approach.
More
recently, the use of a transesophageal or transthoracic multiplane probe has
emerged as a readily available method to obtain rotational images at defined
interval angles around a fixed axis. Typically, images are collected over a 180-degree rotation at set
intervals. To minimize reconstruction artifacts, sequential images are gated to
both electrocardiography (ECG) and respiration. Acquisition of a complete data
set typically takes 1 to 5 minutes, depending on respiratory and heart rates
and the predefined spatial intervals. During cardiac surgery, respiration can
be suspended during acquisition to minimize the effects of respiratory motion.
The
quality of 3D reconstructions from 2D images depends on a number of factors,
including the intrinsic quality of the ultrasound images, the number (or
density) of the 2D images used to reconstruct the 3D image, the ability to
limit motion artifact, and adequate ECG and respiratory gating. In general, the
greater the number of images obtained (ie, the smaller the space intervals between
images), the better the 3D reconstruction. However, increasing the number of images
also lengthens the acquisition time, which can potentially introduce motion
artifact. Consequently, the optimal number of images necessary for 3D reconstruction
depends on the cardiac structure being examined and the resolution required.
For example, 4 to 6 serial images are usually adequate for volume
reconstructions of the left ventricle (LV), whereas more images are often
needed to visualize more complex, rapidly moving structures, such as mitral and
aortic valves. Once the 2D images have been obtained, they are processed
offline with customized or commercially available software. The cardiac
structures are manually or semiautomatically traced to the 3D spatial coordinates
to reconstruct a 3D image.
Protocols
A
complete 3D echocardiographic study includes an assessment of ventricular
function, valvular morphology, and hemodynamic status. Unlike 2D
echocardiography, in which standard views are described based on the plane through
which they pass, 3D echocardiography is inherently volumetric. As such, it
permits both an external view of the heart and multiple internal perspectives
(through cropping).
As
an alternative to a complete 3D study, 3D echocardiography can be performed
selectively as a complement to a 2D study. Instead of a complete 3D echocardiogram,
a more focused 3D imaging study may be appropriate in some cases. For example,
in a patient with mitral stenosis, the 3D portion of the study may be limited
to visualization and quantification of the mitral orifice. Focused 3D imaging
for LV volume calculation, typically performed with an apical 4 chamber
wide-angle acquisition, also can be used to complement standard 2D imaging.
The
ability to extract hemodynamic information derived from 3D color Doppler
ultrasonography is currently being investigated. To capture and analyze color
flow imaging in 3 dimensions, the area of interest should be obtained within
the 3D data set, with the angle of the ultrasound beam aligned as parallel as possible
to the direction of blood flow. Depth and sector settings should be optimized
for color Doppler resolution. Extraneous flows can be cropped so that only the
area of interest is displayed. The color Doppler flow patterns can be analyzed
in multiple views to provide a complete assessment of the color Doppler data.
Clinical Application: Chamber Quantification
Left Ventricle
LV
chamber and mass quantification have been studied extensively using 3D
echocardiography. Initial 3D methods to measure LV volumes used reconstruction
techniques that, although more accurate and reproducible than 2D methods,
required long acquisition and post processing times. Moreover, the accuracy of the
volume calculations was highly dependent on image quality. The introduction of
real-time imaging systems that use matrix phased-array transducers with more
processing elements has significantly improved image quality. In addition, LV
quantification algorithms that can interface with 3D data sets obtained with matrix
phased-array transducers are now widely available and are increasingly robust.
The
wideangle acquisition mode is often used to acquire the entire LV volume, from
which a detailed analysis of global and regional wall motion can be done.
Images may be displayed with either orthogonal long-axis views or multiple
short-axis views. Currently, data analysis is performed offline on a personal
computer with dedicated 3D software. Data also can be analyzed online with
software intrinsic to the ultrasound machine. Because a data set comprises the entire
LV volume, multiple slices from different orientations can be obtained from
base to apex to evaluate wall motion. If image quality is limited, then acquisition
can be combined with infusion of contrast to improve the delineation of the
endocardial border. An advantage of a 3D data set over 2D is the ability to manipulate
the plane to align the true long axis and minor axis of the LV, hence avoiding
foreshortening and oblique imaging planes. Once the LV axes are appropriately
aligned, LV volumes can be calculated with a centroid-based algorithm that typically
uses 2 or 3 planes, thereby shortening processing time. In addition, the LV
volumes can be segmented, which allows for regional LV function assessment.
LV
volume assessment by RT3D has been demonstrated to be rapid, accurate, reproducible,
and superior to conventional 2D methods. The superiority of the RT3D approach
has been demonstrated in various clinical situations, but its use is limited in
patients with a poor acoustic window. An alternative method of calculating
ventricular volumes from an RT3D cardiac volume data set uses the disc
summation method. This technique may be advantageous in patients with
asymmetrical ventricles. LV volume and mass obtained by RT3D echocardiography compare
favorably with those obtained with cardiac magnetic resonance imaging (MRI) or
radionuclide volumes. In addition, RT3D echocardiography has demonstrated efficacy and
accuracy in assessing LV volumes in remodeled ventricles after myocardial
infarction and in assessing global LV dyssynchrony.
Preliminary
clinical studies on the use of RT3D in stress echocardiography confirm the
feasibility of this technique and report sensitivity and specificity comparable
to 2D stress imaging. An advantage of RT3D stress imaging is the decreased imaging time;
standard views can be obtained with only 1 or 2 image acquisitions. In
preliminary clinical studies, average acquisition times decreased from 65 to 28
seconds with RT3D imaging.
Right Ventricle
Assessment of right
ventricle (RV) function by 2D echocardiography is limited because of the
asymmetrical, pyramidal shape of the RV, which does not conform to simple
geometric assumptions. In theory, direct visualization of the entire chamber should
be possible with 3D techniques, thereby overcoming the inherent limitations of
tomographic methods. To date, most studies that have applied 3D echocardiographic
techniques to the RV have involved primarily rotational or freehand scanning
methods, most of these series
demonstrated improved accuracy of RV function assessment. The recent development
and availability of RT3D echocardiography has the potential to further improve
the ability to assess RV chamber size, volume, and function.
Left Atrium
In
a limited number of studies, left atrial volume has been accurately quantified
by 3D echocardiography using both reconstructive and realtime techniques. The
3D echocardiographic methods correlate well with MRI and appear to have accuracy comparable
to 2D left atrial volume methods.
Clinical Application : Valvular Heart Disease
The
recent widespread availability of RT3D echocardiography obviates many of the
practical limitations of reconstructive 3D techniques and offers the potential for
greater clinical application for valvular heart disease both in standard
diagnostic evaluation and in real-time guidance during surgical valve repair.
This technique is ideally suited for assessing valve function given the
nonplanar anatomy of the cardiac valves and the associated anatomic and spatial
alterations associated with valvular heart disease.
Mitral Valve
The
3D echocardiography technique has contributed significantly to our
understanding of mitral valve function and anatomy. The mitral valve is particularly
suited to 3D assessment because of the complex interrelationships among the valve,
chordae, papillary muscles, and myocardial walls. This technique can provide
important insight into mitral valve structure, demonstrating the saddle shape
of the mitral annulus, with high points located anteriorly and low points
oriented in a mediolateral direction. This has helped clarify the appropriate
diagnostic imaging planes from which mitral valve prolapse should be diagnosed,
thereby avoiding false positive interpretations.
In
addition, 3D echocardiography has provided important mechanistic insights into
functional and ischemic mitral regurgitation resulting from derangements of the
normal spatial relationships of the mitral valve leaflets to its chordal
attachments, papillary muscles, and the LV. Distortion of the normal spatial relationship between the LV and
mitral valve apparatus results in papillary muscle displacement and tethering of
the mitral leaflets, leading to incomplete closure of the leaflets and mitral
regurgitation . The 3D echocardiography technique has identified changes in
annular shape occurring with functional mitral regurgitation. These mechanistic and
anatomic insights based on 3D analysis have provided the basis for the
development of new approaches to treating ischemic mitral regurgitation.
Three-dimensional
echocardiography has been used to define and localize mitral leaflet lesions in
mitral valve prolapse, endocarditis, and congenital mitral abnormalities. This application has been particularly important in guiding
surgical repair.
The
RT3D approach has also demonstrated efficacy in quantifying mitral regurgitation
by using 3D guidance to directly measure the proximal flow convergence region. It has provided insight
into how premitral orifice geometry affects the calculation of mitral valve
area in mitral stenosis. Calculation of mitral valve area by 3D echocardiography has been demonstrated
to be accurate, reproducible, and less variable than conventional 2D methods
and thus has been recommended as the first line method. In addition, 3D
echocardiography has been used for guidance during percutaneous mitral valvuloplasty.
Aortic Valve
Three-dimensional
echocardiography has been applied for anatomic assessment of the aortic valve
and root morphology and to calculate the valve area in aortic stenosis. The technique has been used
to delineate aortic flow patterns and has demonstrated feasibility and accuracy
in quantifying aortic regurgitation. Other applications have included the detection and localization of
aortic valve vegetations, assessment of congenital outflow obstruction abnormalities,
and demonstration of morphological changes in the valve after balloon dilation.
Tricuspid and Pulmonary Valves
Compared
with the aortic and mitral valves, the tricuspid and pulmonary valves have been
less widely studied with 3D echocardiography. This technique has demonstrated anatomic
changes with rheumatic and degenerative tricuspid valve disease and has accurately
reconstructed congenital tricuspid valve abnormalities, such as
atrioventricular canal defects. For the pulmonary valve, 3D assessment has been limited to
descriptive case reports defining anatomic abnormalities associated with
pulmonary valve stenosis and endocarditis.
Clinical
Application : Congenital Heart Disease
Clinical
investigations examining the role of 3D echocardiography in patients with
congenital heart disease have emphasized the unique perspective provided by 3D
imaging and the versatility of the technique in patients
with simple defects or complex conditions and
in the postoperative state. Three-dimensional echocardiography,
using both reconstruction methods and RT3D,
has been used to detect several forms of
congenital heart disease. The ability to record and
analyze the entire cardiac structure and the ability to display complex spatial relationships are potential
advantages of 3D imaging over 2D echocardiography.
In addition, the decreased examination time afforded
by RT3D echocardiography may reduce the need for
sedation in some children.
In
patients with atrial septal defects, 3D echocardiography can record the size
and shape of the defect. It also can show the precise location of the defect
and the extent of residual surrounding tissue. In patients with secundum atrial
septal defects, the extent of the retroaortic rim often determines the
feasibility of repair with percutaneous closure devices. Three-dimensional
echocardiography also has been used after atrial septal defect closure to evaluate
the success of the procedure and identify the origin of residual shunting. In patients with ventricular
septal defects, the ability to interrogate the entire septum is frequently
cited as an advantage of the 3D technique. A novel application of 3D imaging in patients with ventricular
septal defects involves using offline reconstruction to measure the shape and
size of the color flow jet, which allows for accurate measurement of the
magnitude of shunting in patients with isolated ventricular septal defects.
Various
3D echocardiographic techniques have been used to evaluate RV and LV size and function
in patients with congenital heart disease. The approach to the LV is similar to
that described previously and permits quantification of dimension, volume,
mass, and ejection fraction. Owing to the ellipsoidal shape of the LV, the advantages of 3D over
2D echocardiographic techniques are limited, because simple geometric assumptions
can be used to calculate LV volumes; however, the RV’s asymmetrical shape invalidates
the simple geometric assumptions used for LV volume calculations. In this case,
the ability to record and analyze the entire chamber rather than relying on simplifying
assumptions has proven superior. In patients with congenital heart diseases that involve RV
pathology, 3D echocardiography correlates well with MRI for the measurement of RV
volume. Three-dimensional
echocardiography has been successfully applied
to the detection and assessment of several
anatomic defects. For example, the
circumferential extent and severity of discrete subaortic
membranes have been successfully visualized with 3D echocardiography. With the apical view, a
unique en face image of the membrane can be recorded, which permits analysis of
the effective orifice area and the dynamic nature of the defect. Congenital
malformations of the mitral valve also have been assessed with 3D echocardiography. The complex nature of
these defects can make a thorough anatomic evaluation difficult. In such cases,
the perspective provided by 3D echocardiography can provide a complete preoperative
assessment of the extent and severity of the valvular abnormality.
Clinical Application : Intraoperative Applications
The
accuracy, feasibility, and value of 3D echocardiography also have been
demonstrated in the intraoperative environment. Intraoperative 3D echocardiography
provides accurate and often additional anatomic information compared with 2D
transesophageal (TEE) imaging. In limited studies examining 2D and 3D TEE intraoperative evaluation
of mitral valve prolapse anatomy, 3D TEE evaluation provided complementary and
additional information compared with 2D TEE for localization of prolapsed scallops. Intraoperative 3D TEE also
has been used to identify distortion and folding of the mitral annulus as a
cause of functional mitral stenosis or worsening mitral regurgitation during
beating heart surgery. Finally, intraoperative 3D TEE has proven valuable in patients
undergoing surgery for congenital heart lesions. For example, the superiority
of intraoperative 3D TEE compared with 2D has been demonstrated by its ability
to provide en face and oblique views of left atrioventricular valve
malformations in patients undergoing reoperation for persistent regurgitant
lesions after previous repair of atrioventricular septal defects.
Intraoperative
epicardial RT3D echocardiography has been used to improve spatial orientation and
assess the extent of septal thickening, mitral valve systolic anterior motion, and
postsurgical LV outflow tract patency in a patient with hypertrophic cardiomyopathy
undergoing septal myectomy. It also has been used to guide and monitor off-pump atrial septal
defect closure in a beatingheart animal model. Finally, intraoperative epicardial and postoperative transthoracic
RT3D echocardiography has been used to evaluate changes in LV volume and
function during cardiac surgery in patients undergoing infarct exclusion surgery
for ischemic cardiomyopathy. In contrast to 3D echocardiographic imaging, conventional 2D
methods may not accurately quantify LV volumes in patients with severe ischemic
cardiomyopathy,
especially in the presence of significant geometric
changes due to LV aneurysm.
Clinical Application : Contrast Echocardiography
The
use of contrast with 3D echocardiography to improve quantification of LV
volumes offers several advantages. The RT3D technique (single or full volume)
provides the most practical approach. Triggering, although not essential,
increases the signal-to-noise ratio and thus is superior to non triggered
imaging. Preliminary clinical
studies have shown promise with regard to improved LV surface identification
and volume and ejection fraction measurement. Another evolving application of
contrast 3D echocardiography is in the evaluation of myocardial perfusion. The ability
to record the entire LV and to quantify the full extent of hypoperfused
myocardium is a potential advantage of this approach. However, the problem of
microbubble destruction, even with triggered imaging, remains a challenge. This
is especially true when matrix array transducers are used, which results in suboptimal
myocardial opacification due to high acoustic power. Further technological
developments should lead to improvements in all of these areas and will
contribute to more practical applications of contrast 3D echocardiography.
Future
Direction
Ongoing
developments in 3D echocardiography include technological innovations and
expanding clinical applications. Automated surface extraction and quantification,
single-heartbeat full-volume acquisition, transesophageal RT3D imaging, the
ability to navigate within the 3D volume, and stereoscopic visualization of 3D
images are some of the technological advances that can be expected over the
next several years. These will further enhance the quality and clinical
applications of 3D echocardiography. In addition, standardized and focused 3D
protocols will be developed and refined to optimize clinical application of
this technique. Tagging and/or tracking the LV surface in real time may provide
new approaches to quantifying myocardial mechanics, such as regional shape and strain.
This approach has great potential and will complement and likely compare
favorably with the quantitative ability of cardiac MRI. The superior temporal
resolution of echocardiography should offer unique advantages for this purpose.
In the future, combining the greater temporal resolution of 3D echocardiography
with the excellent spatial resolution of MRI (or computed tomography) may yield
an imaging data set with unsurpassed anatomic and physiological infrmation, an
approach called “fusion imaging”.
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