(Circulation. 1999;99:1-6.)
© 1999 American Heart Association, Inc.
Circulation Electronic Pages |
Dynamic Holographic Imaging of the Beating Human Heart
Presented in part at the 70th Scientific Sessions of the American Heart Association, Orlando, Fla, November 10–13, 1997, and published in abstract form (Circulation. 1997;96[suppl I]:I-698).
From the Cardiac Ultrasound Laboratory, Massachusetts General Hospital, Boston, and the Spatial Imaging Group (S.S., R.N., S.A.B.), Media Laboratory, Massachusetts Institute of Technology, Cambridge.
Correspondence to Michael H. Picard, MD, Cardiac Ultrasound Laboratory VBK 508, 55 Fruit St, Boston MA 02114. E-mail picard{at}olorin.mgh.harvard.edu
Abstract
Background—Currently, the reporting and archiving of echocardiographic data suffer from the difficulty of representing heart motion on printable 2-dimensional (2D) media.
Methods and Results—We studied the capability of holography to integrate motion into 2D echocardiographic prints. Images of normal human hearts and of a variety of mitral valve function abnormalities (mitral valve prolapse, systolic anterior motion of the mitral leaflets, and obstruction of the mitral valve by a myxoma) were acquired digitally on standard echocardiographic machines. Images were processed into a data format suitable for holographic printing. Angularly multiplexed holograms were then printed on a prototype holographic "laser" printer, with integration of time in vertical parallax, so that heart motion became visible when the hologram was tilted up and down. The resulting holograms displayed the anatomy with the same resolution as the original acquisition and allowed detailed study of valve motion with side-by-side comparison of normal and abnormal findings. Comparison of standard echocardiographic measurements in original echo frames and corresponding hologram views showed an excellent correlation of both methods (P<0.0001, r2=0.979, mean bias=2.76 mm). In this feasibility study, both 2D and 3D holographic images were produced. The equipment needed to view these holograms consists of only a simple point-light source.
Conclusions—Holographic representation of myocardial and valve motion from echocardiographic data is feasible and allows the printing on a 2D medium of the complete heart cycle. Combined with the recent development of online holographic printing, this novel technique has the potential to improve reporting, visualization, and archiving of echocardiographic imaging.
Key Words: echocardiography • imaging • heart diseases • mitral valve
Since prehistoric times, humans have been able to capture and display reality in the form of static 2-dimensional (2D) images. Only a century ago, however, it became possible to capture the motion of objects in the form of a series of single frames and to play them back as an animation. The ability to display motion and structure has stimulated the evolution of echocardiography from M-mode echocardiography,1 which displays only 1 spatial dimension over time, to 2D echocardiography, allowing improved understanding of cardiac structure in vivo and new insights into cardiac function. However, to view cardiac motion in a compact display is more cumbersome with 2D compared with M-mode echocardiography. Review of 2D echocardiograms requires a time-consuming search on videotapes and is restricted to the resources of the echocardiography laboratory. Digital imaging has not substantially relieved this situation, with the requirement for computers, networks, and databases demanding a huge storage capacity. In addition, the information content of echocardiographic data has markedly increased, especially with the introduction of 3D and 4D echocardiography.
The technique of holography offers the possibility of integrating additional data dimensions into a 2D print. Although this is typically used to convey information about the 3D structure of objects, we explored holography as a means of integrating the temporal dimension (cardiac motion) onto a 2D print medium.
Methods
Patients
Data sets from normal subjects and from patients referred to our
laboratory for echocardiograms for clinical indications were selected
with the aim of gathering a representative series of
mitral valve pathological conditions with sufficient image quality. We
chose these conditions because visual assessment of valve motion is
critical to the precise diagnosis of valve disease. For this
feasibility study, we chose image loops of (1) 2 patients with normal
mitral valves, (2) a patient with classic bileaflet mitral valve
prolapse, (3) a patient with isolated prolapse of the posterior
leaflet, (4) a patient with hypertrophic obstructive
cardiomyopathy and systolic anterior motion
of the mitral valve, and (5) a patient with diastolic
obstruction of the mitral inflow by a large left atrial
myxoma.
Image Acquisition
Image sequences of normal and abnormal hearts were acquired with
a standard echocardiograph (Hewlett Packard Sonos 2500). A
transthoracic 2.5-MHz transducer and a multiplane
transesophageal probe with 6.5/5.0-MHz transducer
frequencies were used. In 1 patient, a 3D echocardiogram was acquired
with the multiplane transesophageal probe with a
rotational acquisition in steps of 2°. Gated image acquisition was
done with the ECG R wave and expiration as acquisition triggers. Image
loops were stored in digital format on a magneto-optical disk in the
DSR file format.
Data Processing
After transfer of the image files to a personal computer
workstation (MEDIS Imaging Systems), the data were converted to
standard tagged image file format (TIFF) using the DSR2TIFF software
supplied by Hewlett Packard. The images were cropped to achieve the
desired image size with GraphicsConverter 2.8 software (Lemke Software)
on a Macintosh PowerPC 7100 Computer. To optimize the images,
postprocessing was performed with the NIH Image software package
version 1.61.
To allow side-by-side viewing of normal and abnormal valves, 4 different image sequences were then assembled into 1 compound data set containing 1 loop in each image quadrant ("quad-screen format"), again with NIH Image software. For the 3D echocardiogram, the image data set was surface-rendered on a 3D echo workstation (Tomtek GmbH) to produce consecutive 3D perspective views before holographic printing.
Holographic Printing
To display motion instead of 3D anatomy by holography,
the conventionally used multiple perspective views in holography of 3D
objects were substituted with multiple time-consecutive views as shown
in Figure 1
. The data were transferred to
a compact prototype "one-step ultragram printer" described
elsewhere,2 and angularly multiplexed "vertical parallax
only" holographic stereograms were printed. In brief, a workstation
(Silicon Graphics Indigo) calculates the multiple required views from
the original data set and drives a liquid crystal display. Collimated
light from an Nd-YAG laser is modulated by the liquid crystal display,
then reshaped by a customized holographic optical element to interfere
with a reference beam. A light-sensitive holographic film (DuPont Corp)
records the resulting interference pattern. A flow diagram of the
process of hologram production is shown in Figure 2.
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Quantification and Statistical Analysis
To validate the accuracy of image representation on the
resulting hologram, we chose to test the accuracy of simple length
measurements, because the different holograms of the mitral valve were
acquired in a number of different views and by both the
transthoracic and the transesophageal
approaches and because virtually all anatomic quantification methods in
echocardiography are based on length measurements.
Therefore, the diameters and lengths of several structures visible in
each individual hologram, including ventricular, annular,
and atrial diameters; wall thickness; leaflet length; and leaflet
thickness, were measured by independent, blinded observers. Reader 1
(M.S.C.) measured the original digital image data sets, and reader 2
(P.R.H.) measured the resulting holograms. The results of measurements
on both media were then compared by correlation techniques and the
Bland-Altman method. A value of P<0.05 was considered
significant.
Results
After preliminary holographic prints were created to optimize
machine settings and data formatting, vertical parallax holograms were
made from image loops from 7 patients with normal and abnormal mitral
valve anatomy or function. The standard-size hologram was 5x8
cm. A frame of a parasternal long-axis view of a normal heart and a
video sequence of the resulting dynamic hologram are shown in Figure 3.
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For ease of comparison, images of 4 patients with different mitral
valve pathological conditions were integrated into 1 holographic print
of the same size. Frames from the original images and a video sequence
of the hologram resulting from these image loops are displayed in
Figure 4.
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The cross-sectional anatomy of the mitral valve can clearly be discerned in each loop. With tilting of the hologram (or the light source) up and down, mitral valve motion becomes apparent, and motion can be stopped in any desired phase of the heart cycle. In the quad-loop display, the viewer can easily see a slow-motion display for comparison of the normal and abnormal mitral valve motion patterns.
In the normal heart (upper left quadrant of each "quad print"), tilting of the hologram leads to perception first of mitral valve closure with onset of systole, followed by aortic valve opening. At the end of systole, the aortic valve closes and the mitral valve opens again.
In the lower left quadrant, the motion of a mitral valve with myxomatous degeneration is visible: With onset of systole, the thickened mitral valve moves across the mitral annular plane to form a convex shape toward the atrium, in contrast to the simultaneously visualized normal valve in the left upper quadrant, which does not cross the mitral plane.
In the upper right quadrant, which displays a heart with hypertrophic cardiomyopathy, the mitral valve closes normally in early systole. At this point, the redundancy of the mitral leaflet is clearly appreciated compared with the normal valve. Later in systole, it becomes apparent how the mitral valve moves toward the septum and finally obstructs the outflow tract. This anterior motion creates a visible gap in the mitral valve closure, thus demonstrating the mechanism of mitral regurgitation in this disease.
The lower right quadrant displays mitral valve function in a patient with left atrial myxoma: During diastole, the large myxoma protrudes into the mitral valve orifice, leading to functional mitral stenosis.
Figure 5 shows 1 frame of a 3D
surface-rendered echocardiogram in a patient with a localized prolapse
of the posterior leaflet and a video recording of the resulting
hologram. The mitral valve is viewed en face from the left atrium. In
early systole, the mitral valve closes, and a localized prolapse of the
posterior mitral leaflet (with a torn chorda tendinea at its anterior
edge) becomes visible. With time progression during systole, it is easy
to see how the extent of the prolapse increases to reach its maximum
late in systole.
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With the chosen resolution of 640x480 pixels, sharpness and noise in the holograms correspond to sharpness and noise present in the original frames. (However, the video recording used here for demonstration caused a reduction in resolution and sharpness not observed in the original holograms.) Printing in higher resolution is technically possible but requires future echocardiographs to deliver more highly resolved images. Contrast in the holograms is dependent primarily on the light source used for viewing; when a strong point-light source is used, contrast is excellent, but diffuse ambient light causes significant deterioration of image contrast.
All echocardiographic measurements obtained are
shown as a composite in Figure 6A. A
total of 41 linear measurements was obtained. Measurements on the
holograms correlate well with the same measurements performed on the
original digital echo frames across the whole range of clinically
important dimensions (P<0.0001,
r2=0.979, SEE=0.514 cm). As seen in
the Bland-Altman plot (Figure 6B), the measurement bias was
minimal (mean bias, 0.276 cm).
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After development of the method, the time required to create a typical
hologram after the image acquisition was 
5 to 10 minutes for data
conversion, 5 to 10 minutes for postprocessing, and after electronic
transfer of the data, 10 minutes for hologram printing.
Discussion
Conventional holography allows representation of a static 3D object. A change in viewing angle results in a change in perspective. Dynamic holography allows the representation of motion of a 2D or 3D object in a unified and convenient format.
This is the first study demonstrating the feasibility of dynamic holographic imaging of the beating human heart. This novel technique is based on conventional display holography. With this approach, a change in the viewing angle of the printed hologram leads to the perception of time progression (cardiac motion). With this new technique, it is possible to reduce a series of images of the beating human heart to a single printable image that can be used for storage, documentation, and reporting in echocardiography. There was no alteration of image size or dimensions with this technique.
The present report of the use of dynamic holographic imaging in patients with normal and abnormal mitral valve function demonstrates the feasibility, applicability, and usefulness of this technology in clinical cardiology, particularly in valve disease, for which the visual perception of motion is fundamental in making the correct diagnosis.
Holographic Imaging
Display holography uses optical interference and diffraction to
record and display multidimensional images.3 In
optical holography, an object is illuminated with laser light, and the
scattered light is combined with a nonscattered reference laser
beam.4 The resulting interference pattern contains the
information regarding the 3D nature of the object. A photosensitive
substrate is then exposed to the interference pattern. In contrast, we
used a holographic stereogram,5 which records the
interference pattern of multiple perspective views. Thus, holograms can
be used with digital information without the necessity to illuminate a
real object with laser light,6 because the heart is
obviously not accessible for laser illumination in vivo.
Holographic images allow viewing of different
perspectives of an object with different viewing angles; this is most
commonly used for representation of 3 spatial dimensions. A
change of perspective with change of the viewing angle in the lateral
direction is encoded in the horizontal parallax of these holograms, and
a change of perspective with a vertical change in viewing angle has to
be encoded in the vertical parallax. For reasons of computational
efficiency, holographic stereograms often use horizontal parallax only
to create the illusion of depth. In our work, a different approach was
chosen: The images were limited to 2D frames readily obtained by
standard 2D echocardiography, and the vertical
parallax was used to represent time. The vertical instead of
the horizontal parallax was chosen for encoding time because otherwise
the 2 eyes, with their slightly different viewing angles, would have
seen 2 different time points, leading to confusion. Combining temporal
information on the vertical parallax and spatial information on the
horizontal parallax is feasible but increases computational
requirements by 2 orders of magnitude. As an intermediate step, we
created a dynamic hologram of a surface-rendered 3D echo, which conveys
information about 3D structure, although one is not able to view the
object from all perspectives.
Holography in Biomedical Imaging
Different holographic techniques have been applied in medical
imaging for 30 years: An early report describing holography to
produce 3D representation of x-ray information appeared in
1968.7 By 1969, the combination of holographic techniques
with static ultrasound information was used in an attempt to detect
soft tissue tumors,8 but neither the display of motion nor
holographic printing was possible at that time. In addition to
applications in microscopy, holography has since been applied to
produce static 3D images of the pelvis,9 the
spine,10 and the brain11 by use of
CT,12 MRI,13 and sonographic
techniques14 to acquire image information. In these
applications, holography was limited to display of static 3D
anatomy but not motion.
For cardiac applications, the use of holographic interferometry for examination of explanted heart valves15 and for nondestructive testing of heart valve prostheses16 has been described. Our report expands these techniques by allowing holographic representation of the entire heart during the whole cardiac cycle in vivo.
Vannan et al17 applied optical, volumetric multiplexed transmission holography to excised animal hearts imaged by ultrasound and created images representing the static 3D anatomy of these hearts. Viewing of the resulting hologram necessitated a special viewing box incorporating Fresnell lenses and diffraction gratings. Because a "virtual image" (as opposed to a "real image" in optical terms) was produced, no measurements could be performed with images produced by this technique.
In contrast to all prior reports, the technique developed at our laboratories allows motion display in addition to 2D or 3D anatomy, is applicable to patients in vivo, yet does not require any special viewing devices; quantification is easily performed directly on the holographic print.
To date, due to the complexity of the production of holograms together with the difficulty of obtaining multidimensional digital data, holographic image representation has not found widespread clinical use. The technique described here is feasible because of both the advances in digital echocardiographic imaging, which is available in most current echocardiographic equipment, and the recent progress in 1-step printers for holography.
Limitations
A major limitation for broad application of the described
technique at the present time is the availability of a holographic
printing device. We used a prototype laser printer developed at
Massachusetts Institute of Technology.2 This device allows
rapid hologram printing. It is expected that similar devices will be
commercially available in the near future and can be integrated into a
digital echocardiography laboratory. Although even
at the present time, commercially available conventional, optical
hologram production techniques might be used for our purposes,
they are very time-consuming and expensive; in contrast, the technique
described here is very fast, and after the introduction of desktop
holographic printers, it is anticipated that the cost for printing a
hologram will decrease, as in the development in current office laser
printers.
The time requirement to generate reports is a critical factor for the
use of a new modality in a busy echocardiography
laboratory: in our experience, data conversion, the change of the
computer platform, and the transfer of the image data from one to the
other institution were the main determinants of time consumption.
Holographic printing itself was done in a matter of minutes. Direct
connection of a holographic printing device to an
echocardiography machine delivering digital output
(eventually over a local network) will eliminate the time-consuming
data transfer between different machines and allow online printing
similar to that by current office laser printers within 
10 minutes
after image acquisition if technology available today is applied.
All selected data sets in our study could be transformed into a readable hologram. For optimal results, a high signal-to-noise level was useful. Postprocessing of the images to optimize overall gray level and image contrast was useful, but overall, these changes were minor and could be done as automatic preprocessing in a dedicated device in the future within seconds.
Most of the examples shown were acquired by transesophageal echocardiography to achieve optimal resolution at the level of the mitral valve. Although the quality of the holographic result certainly depends on the quality of the acquired loops, we have also obtained successful results from transthoracic acquisitions of valvular and ventricular motion.
For the described holograms, image resolution of 640x480 pixels was chosen according to current echo screens after the borders not containing image information were cropped, but printing in much higher resolution is technically possible as soon as more higher resolved ultrasound images can be delivered by clinical echocardiographs.
Holographic printing is at present essentially a 1-way method: Although the printed holograms contain the exact quantitative data in terms of anatomy, brightness, and other image information, there is no easy way to reenter the printed image data into a computer for further measurements or postprocessing. However, accurate measurements of standardized heart dimensions can be performed directly on the holographic print.
Clinical Implications
The ability to print representations of cardiac structure
and motion on a 2D medium may facilitate visualization and reporting of
echocardiographic studies. This method also offers
advantages for storing of images: in contrast to conventional
videotapes, the space needed to store a series of loops of standard
echocardiographic views is decreased to a few pages.
Likewise, the technique offers advantages for comparison with old data
by reducing the time-consuming searches of previously recorded data
on videotapes.
Conclusions
Dynamic holographic imaging of the beating human heart is
feasible. Combined with the recent development of online holographic
printing, it has the potential to improve display, storage, and
reporting of image loops of the beating human heart acquired by
echocardiography.
Acknowledgments
This work was supported by a grant from the Swiss National Research Foundation (Dr Hunziker).
Received July 17, 1998; revision received November 17, 1998; accepted December 4, 1998.
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