Volumetric Imaging

Volumetric imaging may be a staple concept in science fiction but this field of imaging, which first appeared in the early part of the twentieth century, is still in its infancy and is only just becoming accessible. Volumetric display systems were, for a long time, confined to use in a small number of academic research laboratories and corporate facilities, and by some military scientists. Significant improvements in volumetric imaging in recent years have led to wider use of the technology, particularly in medicine. There are significant advantages to the use of volumetric imaging devices in this field, including improved visualisation of anatomical structures for surgical applications, research, diagnostic and educational purposes. Air traffic control and military operations also benefit from the volumetric imaging systems already available.

What Are Volumetric Images?

Volumetric imaging refers to the production of images with height, depth, and length, in contrast to the majority of images produced artificially as two dimensional representations. Viewers of volumetric images are able to view them from all angles and may even be able to interact with the image, depending on its characteristics. There is no clear consensus on taxonomy within the field of volumetric imaging displays, although attempts have been made to classify devices according to whether they produce an image viewable by the naked eye and whether an intermediate surface is required to house the image. When discussing volumetric imaging it is important to differentiate these types of displays from holograms.

Holograms and Volumetric Imaging

Holograms are stereoscopic, and may appear three-dimensional but are viewed on a two-dimensional surface such as glass or film. ‘True’ volumetric images have no need of this two-dimensional plane and can be projected, instead, into the air itself. Perhaps the most easily recognizable type of volumetric imaging is that used in Star Trek, where the ‘Holo Deck’ allowed crew members to enter into a virtual reality viewable from all angles and with which they could interact. Such images are not yet able to be produced, however, and the majority of existing volumetric imaging systems continue to use holographic technology.

‘True’ Volumetric Imaging

‘True’ volumetric images appear to float in the air but remain the purview of a small number of scientists and engineers. One device that is capable of producing these types of images is the fluorescent vapor imaging device. This volumetric imaging device uses a combination of mercury vapors and infrared light beams bounced off chemically-coated surfaces to alter their wavelengths. At the crossing point of two beams of light, the mercury vapors glow, making the image visible in the darkness. Clearly, there are some drawbacks with such a complex device designed to be used in the dark but other researchers are working on developing less problematic volumetric imaging devices.

Static and Swept Volume Displays

Swept volume displays are particularly useful in oncology imaging as they change with the movement of the surface on which they are formed. Such images are produced through the projection of computationally decomposed slices onto a spinning surface, creating a 3D image on a 2D surface, through embedded light-emitting diodes (LEDs), or by using other techniques.

Static volume 3D displays are also increasingly being used in the field of volumetric imaging as these create volume images without any macroscopic moving parts. Instead, most static volume imaging systems use intersecting beams of laser light combined with a solid, liquid, or gas to create visible radiation. Newer devices are able to create the floating types of images mentioned above by using a rapidly pulsing infrared laser to create glowing focal points in the air, removing the need for a projection surface.

Tissue-Volume Images – Volumetric Imaging in Action

Developments in volumetric imaging have also resulted in three-dimensional (3D) fluorescence images of organic tissue as an alternative to the ‘stacked’ two-dimensional images normally acquired from confocal or light-sheet microscopy. These imaging techniques are costly and take a long time to produce, making them unsuitable for visualising brief biological occurrences. The development of scanless volumetric imaging systems means that a single shot can capture an entire three-dimensional object using an adapted epifluorescence microscope. A key device in this field is the light-field microscope, developed at Stanford University, which produces 3D videos by recording the different points at which light rays pass through a microlens and the main lens of the sensor plane of the optical microscope.
A secondary approach to volumetric imaging involves an adapted multifocus fluorescence microscope that captures the full focal stack all at once on a single camera. This type of volumetric imaging system is fast, similar in quality to that of a wide-field microscope, and can be used for imaging of small organisms and single molecules, giving it a wide range of applications.

Freehand Volumetric Imaging Systems

Freehand volumetric imaging systems are particularly helpful in visualising musculoskeletal, gynaecological, and cardiac structures and tissues and include the xMATRIC electronic array, manufactured by Philips. This is an ultrasound system that provides a clear, live image while capturing quantifiable data, making it well-suited to endometrial evaluation, fetal cardiac examination and so forth. Another device on the market, the iSlice, automatically updates the given 2D projection as the volume view is rotated and allows physicians to select 4, 9, 16 or 25 2D image slices from the volume set for improved decision-making during diagnosis.

4D Volumetric Imaging

Volumetric imaging tools can allow physicians to visualize directional blood flow, measure specific hypoechoic structures such as the bladder, follicles, and gallbladder, and even create a 4D representation of fetal heart movement. This is facilitated by Spatio-Temporal Image Correlation (STIC) which pairs a calculated heart rate with the captured volumes in order to display and examine the image in real time, allowing for the detection of abnormalities in fetal heart rhythm.

Advantages of Volumetric Imaging

Advanced volumetric imaging systems provide a significant amount of information in a short space of time that is able to be interrogated for rapid diagnosis and patient management. Volumetric imaging displays rely on the emission, scattering, or relay of light in clearly described regions of space in order to create these incredibly useful images, with a variety of devices now available and an increasing appreciation of their value in numerous fields, especially medicine.

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