The conventional medical ultrasound (US) image is a remarkable
view of a 2-dimensional (2D) slice of a patient's anatomy. While
sonographers have become very proficient in manipulating these
images in 3 dimensions in their minds, these findings can be
difficult to reproduce and impossible to quantify.
Three-dimensional (3D) US technology can expand this viewpoint to a
registered volume that contains every conceivable plane within the
region of interest.
While there is no doubt that there are advantages to volume
imaging, does it deliver enough added clinical benefit to justify
the increased time, effort, and computational power required to
reconstruct the full data set? A review of the basic concepts in US
and options for volume data acquisition may aid in answering this
Review of US technology
How the image is generated
Ultrasound systems use the transmission and reflection of
high-frequency (ultra; >20,000 Hz) vibratory pressure waves
(sound) in tissue to form diagnostic and therapeutic images.
The mechanical properties of tissues as well as their distance from
the source of the sound differentially affect the energy of the
sound waves, and this information is used to generate image pixels
that vary according to the amplitude of the reflected wave
(brightness) and the depth from which the returning echo is
received (position). Strong reflections from denser tissues, such
as bone or gallstones, image as very bright (white) dots on a
monitor screen, weaker reflections from soft tissue solid organs
appear as gray dots, and fluids (such as within cysts, blood, or
urine) do not reflect sound waves and show as black dots. The
higher the frequency of the transducer scan-head, the greater the
resolution, but the lower the penetration into body tissues.
The sound waves are generated by the conversion of electric
energy into mechanical energy by piezoelectric crystals, which
change shape in an electric field. The dimensional change causes
pressure waves that resonate at a desired frequency. These crystals
also produce electric signals when deformed by pressure from sound
waves, enabling the transducer elements to serve as both the source
and receiver for the sound waves in pulsed-wave scanning. In
continuous wave scanning, different transducer crystals are used
for sending and receiving. Transducers can be arranged into arrays
that can be shaped to allow for focusing and can be sequentially
timed to "steer" the ultrasonic beam.
A beamformer is used to control the transducer elements and
determine the delay pattern and the pulse train that create the
Several focal points may be used to reach deeper areas in the body
(the far field), which require greater transmit energy. The
beamformer outputs are usually amplified before being sent to the
transducers. The returning waves also are usually amplified, and
the output from each element is stored, aligned in time, and
coherently summed in another beamformer before conversion to
digital signals for display on a monitor (digital beamformers
sample the analog signals, store them, and then sum them
How volume image data are acquired
To acquire 3D (or volume) US data, one must devise a method for
adding position and orientation information to the 2D image to
accurately determine spatial location. This can be done by using
mechanical scanning systems that move in a fixed coordinate system,
freehand methods with or without position-sensing devices, and 2D
Mechanical scanning methods use a motorized assembly to rotate or
translate the transducer within a fixed geometric grid. This allows
the 2D US images to be acquired at pre-defined spatial or angular
intervals. Al-though mechanical scanning methods are cumbersome for
the user, the precise localization they provide allows very
accurate 3D reconstructions that are available immediately after
Mechanical transducer assemblies may be integrated or external.
Integrated systems place a specialized transducer within the
motorized assembly housing. Such systems are usually smaller than
those using external assemblies but require a dedicated US system.
Integrated systems are popular in obstetrics.
External systems can be attached to any conventional transducer and
work with a standard US ma-chine. These systems are popular in vas-
cular, breast, and prostate imaging.
Three common types of mechanical scanning motion are linear,
tilt, and rotational (Figure 1). Linear scanning moves the
transducer along a set of tracks at a constant rate and can allow
adjustment of the interval between parallel images for proper
sampling. If the transducer is tilted, 3D color and power Doppler
scans are possible. In tilt scanning, the transducer is rocked
side-to-side at a constant rate in a fan-shaped pattern, producing
a volume image. This technique is appropriate for image acquisition
through a small acoustic window.
However, the resultant wedge shape produces images in the far field
that are farther apart than those in the near field, with a
resultant decrease in resolution with increasing depth.
Rotational scanning turns the transducer along its central axis,
and produces a cone-shaped imaging volume. This method may have a
similar loss of resolution in the far field, as does tilt scanning,
and because the images intersect at the axis of rotation, any
motion can cause artifacts at the center of the 3D image.
Freehand systems may be untracked or tracked with a position sensor
attached to the transducer. In untracked scanning, the operator
moves the transducer at a preselected linear or angular velocity.
Although this method may be a convenient way to produce a 3D image
for visualization of anatomy, geometric distance or volume
measurements are not accurate and should not be attempted.
Tracked freehand methods use a sensor attachment to provide
information on the exact orientation and angulation of the
transducer. There are at least 4 methods to track the freehand
and articulated arm.
Tracking with magnetism involves detecting changes in a spatially
varying magnetic field that is produced by a transmitter attached
to the transducer. The detector contains 3 orthogonal coils that
measure magnetic field strength. The relative position of the
transducer can be determined by measuring the local magnetic field.
However, the detector must be positioned close to the transducer to
minimize electromagnetic interference, and there should be no
ferrous or conductive metals, including pacemakers, in the
Acoustic systems use timed, pulsed sound waves from spatially fixed
emitters that are mounted on the transducer. The relative distance
and timing of the emitted sound waves that are received by
microphones located above the patient are used to calculate the
position of the transducer. Optical systems use a tracking method
similar to that of the acoustic systems. A set of
infrared-light-emitting diodes (LEDs) is mounted on the transducer,
and the emitted light is detected by cameras fixed above the field
of interest. Although this method is more accurate than the
magnetic tracking method, it requires a line of sight from the
transducer-mounted LEDs to the camera.
An optical tracking system has been used to coregister the US
transducer and the thermal applicator in the thermal ablation of
focal liver lesions.
Articulated arm systems use potentiometers located in the joints of
a mechanical support structure to record the position of the
2D transducer matrix arrays--
Mechanical scanning and freehand scanning methods move the
transducer crystal to produce 2D images that are reconstructed in
3D volume. Matrix, or 2D, transducer arrays use a stationary 2D
transducer with a built-in beamformer to electronically sweep the
pyramidal US beam over the area of interest to create real-time 3D
images, also called 4-dimensional (4D) imaging.
Although the transducer remains stationary, sophisticated software
controls the firing of many thousands of equal-sized, piezoelectric
crystal elements to steer the ultrasonic beam in a phased-array
Matrix transducers may be added to some US units as an upgrade, but
the machine would have to have the hardware design to do 3D imaging
(personal communication, Dr. Gerald Marx, December 2005).
Image processing and display
Data reconstruction methods--
The final 3D image must be reconstructed from the acquired 2D
planar data in a series of steps involving the following:
conversion of digital data into a 3D coordinate system;
interpolation, or the assigning of gray-scale values for any data
points not sampled (ie, located between the acquired planes);
segmentation, or delineating the region of interest for
reconstruction; and image enhancement.
Image reconstruction can be by feature-based methods, such as
surface rendering or multiplanar reformatting, or by volume-based
methods, such as volume rendering, which is voxel (volume
pixel)-based. Feature-based methods use segmentation of surfaces or
volumes to outline areas of interest that are to be distinguished
from the surrounding anatomy and displayed as 2D images with 3D
clues. This method requires less 3D data, produces fast
reconstruction times, and can have artificially enhanced contrast
between structures. However, subtle differences in tissue can be
lost or distorted. In addition, manually assigning boundary
descriptions can be tedious. Voxel-based methods assign each pixel
to a volume grid, retaining all original image information. This
method creates very large data sets (which require vast
computational power) but allows for different rendering and image
In surface-rendering techniques, the operator marks the boundaries
of the region of interest.
This can be a manual process or can be computer assisted through
the use of special algorithms. The computer next uses points and
lines, or vectors, to draw wire-frame representations of the area.
The resulting boundaries are shaded and illuminated to show
surfaces, which are viewed as opaque structures. Surface rendering
is well suited to displaying the physical features of fetuses
(Figure 2) and the sizes and spatial relationships between objects
(such as tumors) but does not show internal structures. Because it
manipulates less data, this vector-based method produces images
quickly and with less computational demands compared with
voxel-based imaging techniques, such as volume rendering.
In multiplanar reformatting, various intersecting or consecutive 2D
planar surfaces are generated for viewing. If 3 orthogonal planes
are chosen, the anatomy will be displayed in correct relative
orientation. In a variation called texture mapping,
planar surfaces are displayed on a geometric form, such as a
polyhedron, to provide a 3D effect. The model can be rotated or
"sectioned" as desired by moving any face of the polyhedron to
provide views throughout the region of interest. In addition, the
extracted planes can be shown side-by-side with intersecting lines
that are used to retain relative 3D orientation.
In volume rendering, the entire volume of data, and not just
selected planes, can be viewed as desired. This is done by using a
technique called ray casting.
In this method, an algorithm produces a projection, or ray, that is
directed through a row of vox-els in a set of image data. As the
ray encounters each voxel, it assigns it a value according to a
specific algorithm that first weights the volume elements (for
example, by using a multiplication factor) and then sums them to
produce varying degrees of translucency. This method allows for the
display of many different effects such as maximum intensity
projection images wherein only the voxel with the greatest
intensity along each ray is displayed.
In this way, internal features can be visually explored throughout
the entire data volume (Figure 3).
3D compared with 2D US:Advantages and limitations
Many physicians would agree with Dr. Beryl Benacerraf, Clinical
Professor of Obstetrics, Gynecology, and Radiology at Harvard
Medical School, Boston, MA, who believes that "the difference
between volume 3D and just taking one slice of image as you see it
is mind boggling." A lot of Dr. Benacerraf's research is based on
the concept of making 3D US a standardized screening tool that
could do body imaging similar to computed tomography (CT) (Figure
4). "Up until last year, we were making pretty baby face pictures,
and nobody really had a handle on what 3D was going to be useful
for." Ultrasound has been losing market share to the more glamorous
imaging modalities, such as CT or magnetic resonance imaging (MRI).
"If you look at the displays that CT and MRI are coming up with,
they are incredibly innovative and amazing. It leaves ultrasound in
the dust. No wonder that the people coming out of residency right
now want to work with the new and sexy stuff." Dr. Benacerraf
believes the new ability to do 3D imaging has put US back into the
competitive arena. "It's faster, it's cheaper, and it has a lot of
One of the advantages of 3D US is that a surgeon can visualize
the anatomy in a manner similar to the actual views encountered
during surgery, which facilitates surgical planning. "3D will
present to the surgeon how the heart would look at the time that
the chest is opened," notes Dr. Lang, Director of Noninvasive
Cardiac Imaging Laboratories at the University of Chicago
Hospitals. Three-dimensional US can also be used during the surgery
to monitor the procedure and its results. Three-dimensional color
Doppler, for example, can show the jet in 3 dimensions to
characterize mitral valve regurgitation (Figure 5) or leakage of
valve repair procedures.
Three-dimensional US examinations spare the patient the risks of
radiation exposure. According to Dr. Gerald Marx, Associate
Professor of Pediatrics, Harvard School of Medicine, and Senior
Associate in Cardiology at Boston Children's Hospital, "I think we
have moved into an arena in which we clearly rarely do
catheterization for diagnostic purposes. But I think, even further,
with 3D analysis and 3D volume analysis, we will be able to obviate
that radiation exposure." He notes that more and more angiography
is done with MRI. "The difference is that with MRI, although the
acquisition time is getting much shorter, infants and even young
children have to receive general anesthesia, whereas we hope, for
an infant or a young child, we can make those acquisitions in only
4 heartbeats and do not need general anesthesia to do a 3D
echocardiogram." Unlike MRI, with US imaging there is no strong
magnetic field and no contraindications for ferromagnetic
instruments or equipment. In addition, contrast agents are not
required for 3D US.
Dr. Eric Berthelet, Radiation Oncologist at British Colombia
Cancer Agency, Vancouver Island Center, and Clinical Associate
Professor, Faculty of Medicine, University of British Colombia, BC,
Canada, is studying 3D US with optical sensors on the transducer to
track the movement of the prostate gland during radiotherapy and
compare accuracy with the commonly accepted method of adjusting
treatment by the use of gold fiducial markers (Figure 6A). He
thinks that one of the advantages compared with 2D US images, which
have to be registered with the pretreatment CT scan, is that the 3D
US images that are obtained daily can be registered to the
reference US (Figure 6B). "It takes away a little bit of the
variability that is associated with registering different image
modalities-registering, for example, US and CT, or US and MRI." The
current use of gold markers requires an extra appointment, is an
invasive procedure associated with discomfort to the patient, and
can cause bleeding and infection. Dr. Berthelet hopes that using US
in this application will spare patients these risks. He noted that
another application is to assess response to treatment of patients
with head and neck cancer. Volume US imaging can track the response
of the lymph nodes in the neck region and help decision making for
treatment field reduction or dose modifications.
The main disadvantage of 3D US appears to be the extra time
required for image reconstruction and rendering. Regarding
real-time 3D images, Dr. Marx notes that "right now, the biggest
limitation is that it does take additional time to analyze and
reconstruct the pictures-maybe a couple of minutes to 15 to 20
minutes." Currently, a similar analysis done in an MRI suite or a
CT suite is considered to be part of the study. But this is not the
case for ultrasonography. "In our laboratory, we do anywhere from
50 to 70 studies a day, and it's very hard to have that additional
time to do the offline analysis." Dr. Marx hopes that medical
insurance payers will see the importance of volume echocardiography
and reimburse for these analyses.
There is a learning curve involved for both physician and
sonographer who begin to perform 3D US imaging. Because there are
many options for viewing the region of interest, there are more
controls. Instead of acquiring a plane of data, for example,
real-time systems acquire a volume of data. This imaging method
requires vast computational power and data storage. In addition,
motion artifacts can be a problem. According to Dr. Benacerraf,
there is also some loss of image quality with any reconstruction,
but this is improving with new advances in imaging technology.
The medical community is attempting to create some guidelines
for standardization of 3D views and patient positions. Dr. Marx has
been working on a position paper in echocardiography on generalized
3D imaging. "This is something that is realized and
recog-nized-that we have to start developing a nomenclature or
language as to how this should be best performed, best understood,
and best communicated between people doing the procedures."
Sampling of clinical trials of 3D US
The advantages of 3D US have been exploited in a number of
clinical applications, from image-guided therapy, field placement
verification in radiotherapy, and intraoperative imaging to
measurement and quantification of organ function, tumor
vascularity, and anatomic volumes. Just a few of the many and
diverse clinical trials in which 3D US imaging has shown promise
are in the fields of cardiology and vascular medicine, obstetrics
and gynecology, and ophthalmology.
Cardiology and vascular medicine
Three-dimensional US excels at diagnostic imaging of the heart,
according to Dr. Roberto Lang: "The heart is a 3-dimensional
object, so it makes sense to image it as a 3-dimensional object."
Dr. Lang uses real-time 3D US to obtain dynamic volumetric
information on cardiac function. "When you image the heart in 2
dimensions, you make assumptions about the cardiac volumes based on
a geometric model, rather than take actual measurements." He and
his colleagues recently tested the feasibility of volumetric
quantification of global and regional ventricular function by using
In this study, global and regional left ventricular volume time and
wall motion curves were obtained for 30 patients and analyzed
according to 3 protocols that compared the results with 2D US and
MRI findings. They concluded that 3D US findings correlated well
with MRI data and could differentiate populations by left
ventricular function. Automated detection of regional wall motion
abnormalities agreed with 2D US findings in 86% of segments.
Dynamic 3D US imaging of the fetal heart has been done by using
tissue Doppler data to calculate a gating signal.
In this experimental study, B-mode cineloops were synchronized with
tissue Doppler gating signals. The dynamic cardiac reconstructions
thus produced were of high quality, and these, along with classical
2D views, were judged to be adequate for clinical use.
Three-dimensional power Doppler US has been used to quantify
arterial stenosis and may become an alternative to X-ray
A study comparing digital subtraction angiography (DSA), 2D color
Doppler sonography (CDS), and 3D reconstruction for assessment of
internal carotid artery stenosis in 49 patients found good
agreement between DSA and 3D CDS (mean sensitivity of CDS, 93%;
specificity, 82.5%; positive predictive value, 82%; negative
predicative value, 98%).
Gynecology and obstetrics
In addition to the well-known application of imaging fetal
development, 3D US can be used for imaging female reproductive
anatomy. After commenting that looking at the fallopian tubes is a
lot easier with reconstructed views, Dr. Benacerraf continued,
"With 2D images, all of the views of the pelvis are acquired
through the transvaginal portal of entry, whereas if volume data
are acquired, you can reconstruct the pelvis in whatever
orientation you want, making the portal of entry irrelevant."
Infertility specialists need to evaluate the shape of the uterus
for any abnormality that might lead to reproductive failure, but
uterine shape abnormalities are undetectable by regular 2D US. In
the past, an MRI study was required, but now abnormalities in
uterine shape can be diagnosed with 3D US (Figure 7).
The reproducibility of 3D US findings in gynecology have been
confirmed by several recent studies. Yaman and colleagues
found that measurement of endometrial volume was more reproducible
by 3D US than by 2D US in women with postmenopausal bleeding. Salim
found that interobserver and intraobserver variabilities were
within satisfactory limits of agreement in the diagnosis of
congenital uterine anomalies.
The structure and vascularity of the ovaries can be easily
assessed with 3D US. Kurjack and colleagues
compared 3D sonography and power Doppler imaging with standard 2D
transvaginal gray-scale and color/power Doppler modalities to
assess suspected ovarian stage I cancer in 43 patients in a
retrospective analysis. They found that combined 3D sonography and
power Doppler achieved a diagnostic accuracy of 97.7% compared with
only 69.8% and 86.1% for 2D gray-scale and combined 2D gray-scale
and color Doppler, respectively.
Three-dimensional US reconstruction of eyes with retinoblastoma
were analyzed as to provide unique planar views that were
previously unavailable with 2D instruments (Figure 8). Oblique and
coronal images have been shown to be indispensable in evaluation
and follow-up after treatment. In an initial experience reported by
Finger and coworkers,
volume US imaging of 5 eyes of 3 children allowed the discovery of
new oblique and coronal views that could be used to determine
relative distances between tumors; simultaneous viewing of these
tumors was not possible with ophthalmoscopy. In addition,
interactive sectioning of the ocular volume, which is not possible
with 2D US, allowed differentiation of the optic nerve from orbital
shadowing caused by intratumor calcification.
In another report by Finger and colleagues,
3D US was used to measure tumor volumes of choroidal melanomas
(Figure 9). They evaluated 50 3D US images and found that derived
measurements of tumor volume correlated well with standard tumor
measurement techniques. They believed 3D US volume measurements
better accounted for tumor geometry than did calculated estimates
of tumor volume based on basal area and height, and they concluded
that volume US could be used for measurement of choroidal
Recently, this same group explored the use of coronal C-scan
images to measure optic nerve sheath diameter in healthy
volunteers, patients with optic nerve sheath meningiomas, and in 1
patient with retinoblastoma.
The automated measurements were similar to those obtained with MRI
and CT, and the authors conclude that 3D US may have a role as a
screening tool for this application.
Although 3D US imaging is gaining popularity, the extent to
which it will become integrated into routine US examinations may be
related to the time required for volume calculations and various
data set reconstructions. Dr. Marx believes that automation is the
key to overcoming these problems. He would also like to see
miniaturization of the matrix array transducer to allow 3D imaging
by use of the transesophageal echocardiography probe, or in
intravascular probes that could image inside the heart itself. In
Dr. Marx's ideal future, wireless technology would accompany this
new intraorgan scanning. "Certainly, having a miniaturized
transducer that could both send and receive electronically and
communicate with the US machine via wireless technology would be
Once surgical instrumentation is also miniaturized, robotic
assistance could be exploited along with 3D ultrasonography to
assist with the repair of difficult-to-access anatomic
abnormalities. For example, surgery on the beating heart would
prevent the need for heart-lung bypass machines. A global "road
map" could be obtained of the heart and its great vessels and
extracardiac structures with CT or MRI, and real-time monitoring
could be provided by 3D echocardiography.
Lastly, fine molecular details of living subcellular structures
at nanometer resolution may become possible when US waves are
substituted for laser beams in futuristic holographic applications
(see sidebar, "3D ultrasound holography").
New 3D ultrasound technology brings the ability to accurately
reproduce almost any view of the anatomy with precision. The
resultant image display capabilities allow physicians to plan and
monitor invasive therapeutic procedures. Because anatomic
orientation is retained, accurate volume measurements can be made.
Patients can be spared the radiation exposure associated with CT,
the discomfort, expense, and strong magnetic fields of MRI, and the
morbidity associated with more invasive diagnostic procedures.
Because it is possible to view any plane in the image set, even
views that would be difficult or impossible with 2D US can be
obtained with subsequent 3D image reconstruction. Upgrading
conventional 2D machines to 3D imaging may be possible with the
addition of 3D transducers and additional software. It would appear
that this new technology is well worth the increased time, effort,
and computational power required to bring its full capabilities to
bear on modern clinical imaging.
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Vendors of 3-dimensional ultrasound products
3D US Software Vendors
Able Software Corp.
Creative Biomedical Computing Ltd.
Fax: (++972-2) 679-6358
Medge Platforms, Inc.
Philips Medical Systems
QLAB 3D/4D quantification software
Fax: 31-40-27-64-887 email@example.com
Resonant Medical Inc.
Fax: 514-985-2662 firstname.lastname@example.org
3D US System Vendors
LOGIQ 9, LOGIQ 7, Voluson 730, Vivid 7
Hitachi Medical Systems America, Inc.
HI VISION Ultrasound Systems
Philips Medical Systems
iU22, iE33, HD11, HDI 4000, EnVisor, HD3
Resonant Medical Inc.
Shimadzu Medical Systems
310-217-8855, ext. 101
Siemens Medical Solutions
ACUSON Antares ACUSON Sequoia
Terason Ultrasound Systems
Toshiba America Medical Systems, Inc.
Aplio, Xario, Nemio
Ultrasonix Medical Corporation
Sonix Diagnostic Ultrasound System
3D US Workstation Vendors
LOGIQWorks, EchoPack 888-202-5528 www.gehealthcare.com/usen/
Resonant Medical Inc.