Rationale and Objectives
Ultrasound image quality can be improved by imaging an object (here: the female breast) from different viewing angles in one image plane. With this technique, which is commonly referred to as spatial compounding, a more isotropic resolution is achieved while speckle noise and further artifacts are reduced. We present results obtained from a combination of spatial compounding with contrast-enhanced ultrasound imaging in three dimensions to reduce contrast specific artifacts (depth dependency, shadowing, speckle) and reconstruct vascular structures.
Materials and Methods
We used a conventional ultrasound scanner and a custom made mechanical system to rotate an ultrasound curved array probe around an object (360°, 36 transducer positions). For 10 parallel image planes, ultrasound compound images were generated of a flow-mimicking phantom consecutively supplied with water and contrast agent. These compound images were combined to form a volume dataset and postprocessed to obtain a sonographic subtraction angiography.
Results
Image quality was significantly improved by spatial compounding for the native (ie, without contrast agent), and, in particular, for the contrast-enhanced case. After subtracting the native images from the contrast-enhanced ones, only structures supplied with contrast agent remain. This technique yields much better results for compound images than for conventional ultrasound images because speckle noise and an anisotropic resolution affect the latter.
Conclusions
With the presented approach contrast specific artifacts can be eliminated efficiently, and a subtraction angiography can be computed. A speckle reduced three-dimensional reconstruction of submillimeter vessel structures was achieved for the first time. In the future, this technique can be applied in vivo to image the vascularity of cancer in the female breast.
The early detection and staging of breast masses is one of the most challenging diagnostic problems in medical imaging. Because magnetic resonance imaging is too expensive to be applied routinely for breast screening, mammography is considered the gold standard. However, because of radiation exposure, this technique is generally not unobjectionable and limited (eg, in pregnant patients). Furthermore, the specificity in mammography screening is rather low, featuring high false-positive rates ( ). In particular, for dense breast tissue or diffuse lesions, mammography is inconclusive. As a second modality before biopsies or surgical inventions, sonographic breast imaging is routinely used for differential diagnosis. Primarily, high-frequency ultrasound of center frequencies above 10 MHz is capable to assess the size, texture, and morphology of the tumor with high spatial resolution. In the case that the tumor is sonographically visible, these features can be helpful to differentiate between benign and malignant lesions ( ). Moreover, many different kinds of tumors induce intense angiogenesis in the affected tissue. Because of a nonphysiologic and rapid genesis of tumor vascularity, structural abnormalities are typical, such as irregular and variable vessel diameters, elongated and coiled vessels, anarchic vascular networks with tortuous vessel courses, rings, sinusoids, arteriovenous shunts, vessel loops, and incomplete vascular walls ( ). Thus, vascularity and tissue perfusion are highly significant parameters for the assessment of breast cancer. Ultrasound Doppler techniques (eg, color Doppler, power Doppler, spectral Doppler) are capable of imaging blood flow in vessels and provide direct, absolute measurements of flow velocity. Much work has been published on Doppler two-dimensional (2D) and three-dimensional (3D) imaging of breast masses ( ). However, these techniques fail for low flow rates in small vessels, because tissue motion creates higher velocities than the flow, and the two phenomena cannot be differentiated ( ). Contrast-enhanced ultrasound imaging assesses both the macrovascular blood supply and the microvascular perfusion of tissue ( ). Ultrasound contrast agents (CA) consist of gas-filled microbubbles stabilized by a shell. By enhancing the weak echo signals from blood by strong linear and nonlinear scattering, they can not only be used to enhance Doppler signals but also directly for a contrast specific imaging. For this, modalities have been developed to detect CA using their nonlinear response to ultrasound ( ). In particular, different implementations of harmonic imaging, such as phase inversion technique ( ) or contrast pulse sequences ( ), are widely used. Unlike methods known from computed tomography or magnetic resonance imaging, the imaging process itself (ie, ultrasound insonification) can cause changes in the microbubble concentration (eg, ultrasound induced destruction of CA). Thus, ultrasound transmit power is usually kept low to achieve a nonlinear response of the CA and, at the same time, avoid bubble rupture. Having a diameter of less then 10 μm, the geometric dimension of the microbubbles is too small to be resolved by ultrasound with clinically used frequencies. Considering typical CA concentrations in the blood pool, multiple microbubbles contribute to the intensity of a pixel, and image intensity is a measure of CA concentration in a resolution cell. Furthermore, it depends on the type of vascularity whether vessel structures can be visualized: if a vessel is big enough to be spatially resolved with ultrasound or if it is too small but spatially separated from others, its structure and course can be seen from contrast enhancement. Here, a quasi-angiographic view of the macrovascular network can be realized ( ). If the vessel, however, is too small to be resolved (<1–3 mm), flow velocities are slow (<1–5 cm/s) and other adjacent vessels are too close, the vascular network cannot be identified. In such cases, the intensity of the imaged tissue increases with CA concentration and tissue types can be differentiated by their microvascular perfusion. Here, the perfusion can either be analyzed qualitatively from the image intensity of a single image or semiquantitatively by evaluating perfusion kinetics in an image series using, for example, the bolus, depletion, or replenishment method ( ). Contrast-enhanced ultrasound imaging has already been reported to provide potential for the detection of breast cancer ( ).
Although the application of ultrasound is generally promising in the context of breast imaging, there are restrictions due to the underlying physical principles of the imaging modality.
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Figure 1
Experimental setup: (a) principle of spatial compounding. (b) Mechanical applicator installed in a bed, ultrasound scanner, and PC.
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Materials and methods
Experimental Setup
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Flow Mimicking Phantom
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Data Processing
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Results
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Discussion
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Acknowledgment
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