Ultrasound computed tomography in breast imaging: first clinical results of a custom-made scanner

PURPOSE

To test a system using ultrasound computed tomography (USCT) that superimposes ultrasound data acquired in one cross-sectional plane from multiple angles around the breast (Full Angle Spatial Compounding, FASC) and to reconstruct the distribution of the speed of sound in the breast (SoS reconstruction).

MATERIALS AND METHODS

We developed a system combining a conventional ultrasound scanner with a PC-controlled mechanical setup integrated in a custom-made examination couch. In a feasibility study, 3 volunteers (age 26 - 74 years) and one patient with breast cancer were studied. Subjects were placed in the prone position on this couch, with the breast hanging in a water tank. The ultrasound probe was moved in several planes around the breast. A curved reflector that followed the movement of the probe behind the breast was used to calculate the SoS within the breast tissue. Echo-data was processed offline by custom-made software to calculate both FASC and SoS images.

RESULTS

In FASC images a reduction of artifacts (i. e. shadowing of Cooper's ligaments and irregular edges of inhomogeneous lesions) and speckles as well as clear visualization of the inner architecture of the breast was achieved. SoS images delivered further diagnostic information and helped to compensate for geometric distortions in the computed images. Difficulties in the visualization of lesions near the thoracic wall and/or the axillary are limitations of this technique.

CONCLUSION

The first clinical results of USCT imaging have proven its feasibility as an automated and standardized technique for breast imaging.

Ultrasonic microscanning

A detailed review is given of the application of high-frequency ultrasound (HFUS) at frequencies of 20 MHz and above for high-resolution, cross-sectional imaging of biological soft tissue. The state of the art of HFUS imaging systems is discussed with respect to the underlying engineering concepts, system designs, and available transducer technology. Furthermore, the dependency of the spatial resolution on the system's parameters is analysed. Skin imaging, eye imaging, small animal imaging for preclinical research, and intravascular ultrasound in coronary arteries for arteriosclerotic disease diagnostics are presented as examples for the application of HFUS imaging in medical diagnostics. It is shown that, in the frame of the indicated applications, ultrasound in the frequency range 20-100MHz gives a good compromise between the contrary demands for a good spatial resolution and a sufficiently large penetration depth of ultrasound waves into the tissue. Scanning schemes for the imaging of tissue morphology are considered, including spatial compounding as a multidirectional imaging technique.

Sonohistology - ultrasonic tissue characterization for prostate cancer diagnostics

A computer-aided diagnostic system for imaging prostate cancer has been developed in order to supplement today's conventional methods for the early detection of prostate carcinoma. The system is based on analysis of the spectral content of radiofrequency ultrasonic echo data in combination with evaluations of textural, contextual, morphological and clinical features in a multiparameter approach. A state-of-the-art, non-linear classifier, the so-called adaptive network-based fuzzy inference system, is used for higher-order classification of the underlying tissue-describing parameters. The system has been evaluated on radio-frequency ultrasound data originating from 100 patients using histological specimens obtained after prostatectomy as the gold standard. Leave-one-out cross-validation over patient data sets results in areas under the ROC curve of 0.86 +/- 0.01 for hypoechoic and hyperechoic tumors and of 0.84 +/- 0.02 for isoechoic tumors, respectively.

Palpation imaging using a haptic system for virtual reality applications in medicine

In the field of medical diagnosis, there is a strong need to determine mechanical properties of biological tissue, which are of histological and pathological relevance. Malignant tumors are significantly stiffer than surrounding healthy tissue. One of the established diagnosis procedures is the palpation of body organs and tissue. Palpation is used to measure swelling, detect bone fracture, find and measure pulse, or to locate changes in the pathological state of tissue and organs. Current medical practice routinely uses sophisticated diagnostic tests through magnetic resonance imaging (MRI), computed tomography (CT) and ultrasound (US) imaging. However, they cannot provide direct measure of tissue elasticity. Last year we presented the concept of the first haptic sensor actuator system to visualize and reconstruct mechanical properties of tissue using ultrasonic elastography and a haptic display with electrorheological fluids. We developed a real time strain imaging system for tumor diagnosis. It allows biopsies simultaneously to conventional ultrasound B-Mode and strain imaging investigations. We deduce the relative mechanical properties by using finite element simulations and numerical solution models solving the inverse problem. Various modifications on the haptic sensor actuator system have been investigated. This haptic system has the potential of inducing real time substantial forces, using a compact lightweight mechanism which can be applied to numerous areas including intraoperative navigation, telemedicine, teaching and telecommunication.

Vibrography: first experimental results in swine brains

OBJECT

The aim of this study was to determine whether vibrography, an ultrasound-based real-time strain imaging method for registering the elastic properties of tissue, is superior to conventional ultrasound imaging techniques for detecting low-contrast space-occupying lesions in brain tissue and for delineating the boundaries between such lesions and the surrounding tissue.

METHODS

As our experimental model we used swine brains taken from freshly slaughtered pigs. After injecting agarose into these brains at different depths, we compared both the conventional ultrasonographic images and the elastographic images of the region of interest with the corresponding anatomical brain sections.

RESULTS

In 83.6 % of the experiments, it was possible to detect the polymerized agarose in the brain tissue with vibrographic techniques. In 17 experiments agarose lesions which were not detectable by ultrasound were visualized via vibrography. Furthermore, statistical analysis revealed that elastography is a more precise tool than conventional ultrasound for determining lesion size.

CONCLUSION

These findings indicate that vibrography is a promising real-time imaging method with numerous potential applications in the field of neurosurgery. Visualization of the elastic properties provides the neurosurgeon with additional data on the lesion and the boundary between the lesion and the surrounding tissue.

Geometrical optimization of a phased array coil for high-resolution MR imaging of the carotid arteries

The geometry of an RF phased-array receiving coil for high-resolution MRI of the carotid artery, particularly the bifurcation, was optimized with respect to signal-to-noise ratio (SNR). A simulation tool was developed to determine homogeneity, sensitivity, and SNR for a given imaging situation. The algorithm takes into account the coil geometry, the parameters of the measured object, and the imaging parameters of the pulse sequence. The coil with the optimum geometry was implemented as a receive-only coil for 1.5 T and comparative SNR measurements with different coils were performed. The experimental SNR measurements verified the simulations. The optimized carotid artery phased array offered the best SNR over the desired field of view. In vivo high-resolution MRI of the carotid arteries of healthy volunteers and patients with known stenosis was conducted with the optimized phased array coil. The capability of the phased array coil for resolving components within the carotid artery walls is demonstrated. Magn Reson Med 50:439-443, 2003.

[Prostate cancer diagnosis using ultrasound elastography. Introduction of a novel technique and first clinical results]

During the last decade screening has improved prostate cancer detection. The main reason for this development is a better understanding of the margins of prostate-specific antigen (PSA) serum levels and the classification of PSA subtypes. In contrast, the introduction of transrectal ultrasound has not led to a measurable change in the prostate cancer detection rate. Our aim was to develop a novel ultrasound system for the acquisition of elastographic images of the prostate and evaluate the system regarding its clinical applicability. We used a technically modified conventional ultrasound system and analyzed the high-frequency ultrasonic data with a computer program. The first patient-based results suggest that elastography allows an accurate measurement of tumor size and localization in contrast to conventional transrectal ultrasound. Elastography visualizes different tissue elasticities to distinguish benign and cancerous tissue. Thus, we were able to even correctly classify prostate cancer lesions which are iso- or hyperechoic in B-mode sonography.

Strain imaging with intravascular ultrasound array scanners: validation with phantom experiments

Intravascular Ultrasound (IVUS) is routinely used in interventional cardiology for imaging coronary plaque morphology. However, the use of B-mode images for tissue characterization and detection of vulnerable coronary plaques is limited. Strain imaging with ultrasound is a new modality that provides additional information for tissue characterization by imaging differences in tissue stiffness. The aim is to differentiate between vulnerable (soft) plaques and less dangerous calcified (hard) plaques. In this work, the applicability of a time efficient strain imaging algorithm in conjunction with data from IVUS array transducers is evaluated. Unfocused radiofrequency (rf) data from the transducer array is acquired using custom made hardware. Rf line reconstruction is performed offline by synthetic aperture focusing techniques. Vessel mimicking phantoms of different geometries and material stiffness are made from agar and Polyvinyl Alcohol Cryogel (PVA). Experiments are conducted in a water tank and a water column is used for applying intraluminal pressure differences required for strain imaging. The results show that strain images can be calculated with A-lines reconstructed from unfocused rf raw data. Regions of different stiffness can be identified qualitatively by local strain variations. With the used algorithm strains of up to 2% can be imaged without significant decor-relation.

Real-time detection of vessel diameters with ultrasound

Transcutaneous vessel imaging is a frequently used ultrasound imaging modality in medicine. The measurement of vessel diameters can be done with conventional B-mode imaging systems, which work at frame rates up to 100 Hz. Furthermore, there are special systems available, which can track vessel walls very precisely using the phase of signals that are sent at frame rates up to several thousand Hz. Though, such systems are usually not able to provide the examiner with 2D images of the object. With respect to brachial artery flow-mediated vasodilatation (FMD), which is frequently used as a measure of endothelial function, it is necessary to observe diameter changes of small arterial vessels noninvasively for several minutes at a high resolution. In the past, the diameter had to be measured manually in tedious postprocessing of ECG-gated image sequences. We developed a system composed of a Siemens Omnia ultrasound system with a VF13-5 transducer (9 MHz center frequency) and a personal computer, that is capable of calculating vessel diameter changes with an accuracy below the wavelength of the ultrasound system in real-time at a frame rate of 27 Hz. We implemented a two-dimensional active contour model using the Viter-bi-algorithm and a phase-sensitive vessel wall tracking algorithm, in order to guarantee both, geometric information and accuracy. Results from carotid and brachial arteries show that arterial pulsations below 0.1 mm can be visualized reliably over several minutes. With this system we want to find out, if FMD is suitable for an individual assessment of the risk for cardiovascular diseases.

In vivo biomicroscopy of the skin with high-resolution magnetic resonance imaging and high frequency ultrasound

Noninvasive imaging and characterization of the skin is of great interest in dermatology. In order to get relevant diagnostic information, high-resolution imaging techniques have to be applied. Ultrasonic imaging is a potential method for this purpose where the special requirements concerning the spatial resolution make it essential to apply high frequency ultrasound (HFUS). Alternatively, magnetic resonance imaging (MRI), being a very promising imaging modality, also shows the perspective of becoming a valuable diagnostic tool in dermatology. However, to account for the small dimensions of the structures under observation, very specialized system designs have to be developed. In this paper, a HFUS imaging system working in the 50 MHz and 100 MHz range is applied for high-resolution skin imaging. Furthermore, a commercial MRI-system was equipped with specially designed low noise rf (radio frequency) coils with minimized volume, and customized imaging sequences were applied to optimize the signal-to-noise ratio. With HFUS and high-resolution magnetic resonance (HR-MR) imaging complementary imaging techniques for in vivo biomicroscopy of the skin are available.

A nonuniform sampling approach for fast ultrasonic flow imaging

Conventional Pulsed Wave Doppler (PWD) systems acquire an ensemble of N echoes per beam line at a constant pulse repetition frequency fprf, so that the pulse repetition interval equals Tpri = 1/fpn. The total time span determines the velocity resolution, and Tpri the unambiguous velocity range. The ensemble size N is by approximation inversely proportional to the frame rate, assuming that the system performs interleaving. For a given frame rate, a tradeoff can only be made between velocity resolution and velocity range. We propose an approach that allows increasing velocity resolution or range while keeping the frame rate constant. The approach is based on nonuniform sampling, i.e. sampling with varying sampling intervals. Thus, for a given ensemble size N a larger total time span, which would increase velocity resolution, or a shorter minimal Tpn, which would increase the velocity range, may be chosen. The conventional Doppler signal processing techniques are not compatible with nonuniform sampling. We, therefore, developed a velocity estimation algorithm for arbitrary sampling that is based on cross correlation. Furthermore, an adaptive wall filter was implemented that differentiates between tissue motion and blood flow. The new approach was successfully tested with in vitro and in vivo data.

Comparison of high frequency ultrasound and optical coherence tomography as modalities for high resolution and non invasive skin imaging

High frequency ultrasound (HFUS) and optical coherence tomography (OCT) are techniques for high resolution imaging of tissues. The penetration depth of these modalities is limited, but it is sufficiently large enough for non invasive skin imaging. HFUS and OCT are based on the same concept. Waves (ultrasonic waves, respectively light waves) propagate along a narrow beam, are backscattered at tissue inhomogeneities and analyzed over time of flight to obtain spatially resolved morphological information. The objective of this paper is to compare HFUS and OCT in terms of resolution, dynamic range and contrast and to assess their value as tools for high resolution skin imaging. Measurements on phantoms and in vivo have been performed with a 100 MHz ultrasound system and an OCT-scanner working in the near infrared spectrum at 1300 nm wave-length. From the measurements, it can be concluded that OCT delivers an almost isotropic resolution (axial resolution about 5.8 microns, lateral resolution about 4.1 microns), whereas the resolution of the investigated HFUS system is more anisotropic (axial resolution about 9.3 microns, lateral resolution about 60 microns). HFUS and OCT show different penetration depths and a different contrast. Both techniques can, therefore, be combined advantageously in a multimodality approach to account for their individual characteristics.

Ultrasonic tissue characterization for prostate diagnostics: spectral parameters vs. texture parameters

An ultrasonic multi-feature tissue characterizing system for the detection of prostate cancer is presented. The system is based on the processing of radio frequency (RF) ultrasonic echo data. Data from 100 patients was acquired in a clinical study. Parameters are extracted from the RF echo data and classified using two adaptive network-based fuzzy inference systems (FIS) working in parallel as a nonlinear classifier. Next to spectral parameters, conventional texture parameters are calculated using demodulated and log-compressed echo data. In the first approach, the classifier is trained on both, spectral and texture parameters. In the second approach, the classifier is only trained on texture parameters. Classification results of both approaches are compared and it is demonstrated, that only the use of spectral parameters yields satisfying classification results. Results of a minimum distance classifier (MDC) are presented for comparison with the fuzzy inference system. For the final fuzzy inference systems used in this approach, the area under the ROC curve is between 84% and 86% for the combined approach and between 70% and 74% for the approach based on texture parameters only.

[Measurement of flow mediated vasodilation (FMD) using kalman-filtering]

Brachial artery flow-mediated vasodilation is increasingly used as a measure of endothelial function. High resolution ultrasound provides a noninvasive method to observe this flow-mediated vasodilation by monitoring the diameter of the artery over time. In the past, the diameter had to be measured in tedious postprocessing routines, usually by the examiner himself. We present a system, which is able to process ultrasound rf-data in realtime. On that system, we implemented a kalman filter, which makes the tracking of both vessel walls possible. The diameter can be calculated accurately, taking into account process noise as well as measurement noise.