Ultrasound computer tomography

Ultrasound computer tomography (USCT), sometimes also Ultrasound computed tomography, Ultrasound computerized tomography or just Ultrasound tomography, is a form of medical ultrasound tomography involving ultrasound as physical wave used for imaging. It is mostly in use for soft tissue medical imaging, especially breast imaging.

Description

Ultrasound computer tomographs use ultrasound waves for creating images. In the first measurement step a defined ultrasound wave is generated with typically Piezoelectric ultrasound transducers, transmitted in direction of the measurement object and received with other or the same ultrasound transducers. While traversing and interacting with the object the ultrasound wave is changed by the object and carries now information about the object. After being recorded the information from the modulated waves can be extracted and used to create an image of the object in a second step. Unlike X-ray or other physical properties, who provide typically one image information, ultrasound provides multiple information of the object: the attenuation the wave's sound pressure experiences indicate on the object's attenuation coefficient, the time-of-flight of the wave gives speed of sound information, and the received reflection distribution indicates on the reflectivity properties of the object (e.g. refraction index, surface morphology, etc). Unlike conventional ultrasound sonography most USCT systems don't use a phased array for beamforming but unfocused spherical waves for imaging. Most USCT systems aiming for 3D-imaging, either by synthesizing ("stacking") 2D images or by full 3D aperture setups.

The idea of Ultrasound computer tomography goes back to the 1950s with analogue compounding setups, in the mid 1970s the first "computed" USCT systems were built up, utilizing digital technology. The "computer" in the USCT concept indicates the heavy reliance on computational intensive advanced digital signal processing, image reconstruction and image processing algorithms for imaging. Successfully realization of USCT systems in the last decades was possible through the continuously growing availability of computing power and data bandwidth provided by the digital revolution.

Setup

USCT systems designed for medical imaging of soft tissue typically aim for resolution in order of centimeters to millimeters and require therefore ultrasound waves in the order of mega-hertz frequency. This requires typically water as low-attenuating transmission medium between ultrasound transducers and object to retain suitable sound pressures.

USCT systems share with the common Tomography the fundamental architectural similarity that the aperture, the active imaging elements, surround the object. For the distribution of ultrasound transducers around the measurement object, forming the aperture, multiple design approaches exist. There exist mono-, bi- and multistatic setups of transducer configurations. Common are 1D- or 2D- linear arrays of ultrasound transducers acting as emitters on one side of the object, on the opposing side of the object a similar array acting as receiver is placed, forming a parallel setup. Sometimes accompanied with the additional ability to be moved to gather more information from additional angles. While cost efficient to build the main disadvantage of such an setup is the limited ability (or inability) of gathering reflectivity information, as such an aperture is limited to only transmission information. Another aperture approach is a ring setup of transducers, sometimes with the degree of freedom of lifting, to be able to synthesize 3d images by gathering additional information over the height. Full 3D setups, with no inherent need for aperture movements, exist in form of apertures formed by semi-spherical distributed transducers. While the most expensive setup they offer the advantage of nearly-uniform data gathering from many directions and are also fast in data taking as they don't require the time-costly mechanical movements.

Imaging methods

Classical inverse radon transform and fourier slice theorem based methods are in use for transmission information based imaging with USCT systems, ART-based approaches are an advanced alternative also in use. For high resolution and speckle noise reduced reflectivity imaging synthetic aperture focusing techniques (SAFT), similar to radar's SAR and sonar's SAS, are widely used. Iterative wave equation inversion approaches for imaging are under academic research but currently a big challenge computational and memory constraint wise for real world applications.

See also

• Medical ultrasound
• Tomography
• Ultrasound transmission tomography
• Ultrasound-modulated optical tomography