[Model construction for reperfusion of the isolated pig liver]

In recent years thermal ablation of liver tumors as a minimally invasive method became a promising alternative to conventional strategies such as chemotherapy or resection of liver tissue. Thereby an electrode is placed inside the tumor delivering energy in the form of high frequency current into the target volume to achieve and maintain a tissue temperature between 60 and 100 degrees C. Cells exposed to this thermic stress undergo coagulation necrosis and are irreversibly damaged. To protect vital liver structures from heat, it is necessary to develop an online temperature monitoring system. An experimental setup perfusing isolated pig livers under physiological conditions with 0.9% NaCl solution was established to develop and evaluate the measuring technique.

The Ruhr Center of Competence for Medical Enginnering (Kompetenzzentrum Medizintechnik Ruhr KMR, Bochum)

The profile and the projects of the Ruhr-Center of Competence for Medical Engineering at the Ruhr-University Bochum (Germany) will be described. Main topic of the KMR is medical ultrasound with emphasis on image based tissue characterization including elastography and multimodality concepts, mainly in combination with other non-ionizing imaging modalities. Project aims are early detection of cancer (skin, prostate), vessel and perfusion diagnostics (early detection of arteriosclerosis, cardiac arteries, stroke), and intraoperative navigation using ultrasound.

[Ultrasound-based imaging modalities for thermal therapy monitoring]

Thermal therapy has been established as an alternative and minimally invasive approach for the treatment of tumors. During a thermal therapy tissue is heated locally up to above 60 degrees C. Cancerous tissue can thus be destroyed by coagulation. At present there are no suitable imaging modalities available for an accurate real-time monitoring of the coagulation process. A subproject of the Ruhr Center of Competence for Medical Engineering (KMR Bochum) aims at developing an ultrasound-based, real-time capable monitoring system for thermal therapy. Therefore several tissue characterizing imaging modalities will be combined in a new multimodal concept. Initial experiments with porcine liver in vitro have shown that real-time monitoring of a thermal therapy using various imaging methods simultaneously will be possible.

Registration of bone surfaces, extracted from CT-datasets, with 3D ultrasound

An essential task of computer assisted surgery is the registration of preoperative image data with the coordinate system of the operating room. This can be reached by using intraoperative imaging and registrating preoperative and intraoperative datasets. For intraoperative imaging ultrasound is a powerful tool due to the lack of ionizing radiation and because of its fast, inexpensive and easy data acquisition. We propose a surface volume matching algorithm for the registration of bone surfaces and ultrasound volume data. The bone surface is estimated from the preoperative CT data by taking into account that ultrasound only shows parts of the bone surface. By our method reliable matching results are obtained. They are shown with data of the lumbar spine.

A haptic sensor-actor-system based on ultrasound elastography and electrorheological fluids for virtual reality applications in medicine

Mechanical properties of biological tissue represent important diagnostic information and are of histological relevance (hard lesions, "nodes" in organs: tumors; calcifications in vessels: arteriosclerosis). The problem is, that such information is usually obtained by digital palpation only, which is limited with respect to sensitivity. It requires intuitive assessment and does not allow quantitative documentation. A suitable sensor is required for quantitative detection of mechanical tissue properties. On the other hand, there is also some need for a realistic mechanical display of those tissue properties. Suitable actuator arrays with high spatial resolution and real-time capabilities are required operating in a haptic sensor actuator system with different applications. The sensor system uses real time ultrasonic elastography whereas the tactile actuator is based on electrorheological fluids. Due to their small size the actuator array elements have to be manufactured by micro-mechanical production methods. In order to supply the actuator elements with individual high voltages a sophisticated switching and control concept have been designed. 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, space and telecommunication.

Registration of 3D CT and ultrasound datasets of the spine using bone structures

OBJECTIVE

In navigated orthopedic surgery, accurate registration of bones is of major interest. Usually, this registration is performed using landmarks positioned directly on the bone surface. These landmarks must be exposed during surgery. Our goal is to avoid the exposure of bone surface for the sole purpose of registration by using an intraoperative ultrasound device that can localize the bone through tissue.

METHOD

We propose an algorithm for the registration of CT and ultrasound datasets that takes into account the fact that ultrasound produces very noisy images (speckle) and shows only parts of the bone surface. This part is made from the CT dataset. Next, a surface volume registration is performed by searching for a position of the estimated surface that maximizes the average gray value of the voxels in the ultrasound dataset covered by the surface.

RESULTS

The algorithm was implemented and validated using an ex vivo preparation of a human lumbar spine with surrounding muscle tissue. On the basis of this data, the method has a large radius of convergence and a repeatability of 0.5 mm for displacement and 0.5 degrees for rotation.

CONCLUSIONS

A robust algorithm for the registration of 3D CT and ultrasound datasets is presented. The computation time seems sufficiently short to permit intraoperative use.

Freehand ultrasound elastography of breast lesions: clinical results

We developed a freehand method for ultrasound elastography, which can be applied during a routine sonographic examination with off-line calculation of strain images (elastograms). Forty-eight patients with 53 breast lesions were examined and, after biopsy or operation, histologic reports were available for all lesions. The correlation coefficient of time delay estimates was used as a quality criterion for the subsequent calculation of elastograms. Beyond the qualitative evaluation of elastograms, we suggested a semiquantitative approach. For that purpose, the elastogram of each lesion was normalized to an overall strain of 1% (i.e., the average strain in the image was set to 1%). After normalization, we determined mean strain values inside and outside of each lesion, respectively. Defining solid lesions as benign and malignant lesions except for fibrous mastopathy, we found significant difference in strain between solid lesions and their surrounding tissue. However, that result must not be misunderstood to suggest that it was possible to distinguish benign from malignant lesions in general. Still, we address the potential of ultrasound elastography to improve the detection and localization of breast lesions as well as their differential diagnosis. Besides, we developed a freehand applicator for further studies, which guarantees a homogeneous axial compression regardless of the experience of the examiner.

Synthetic aperture-based beam compression for intravascular ultrasound imaging

In this paper, intravascular ultrasound (IVUS) images acquired with a 64-element array transducer using a multistatic acquisition scheme are presented. The images are reconstructed from a collection of pulse-echo measurements using a synthetic aperture array imaging technique. The main limitations of IVUS imaging are a poor lateral resolution and elevated grating lobes caused by the imaging geometry. We propose a Synthetic Aperture Focusing Technique (SAFT), which uses a limited number of A-scan signals. The focusing process, which is performed in the Fourier domain, requires far less computation time than conventional delay-and-sum methods. Two different reconstruction kernel functions have been derived and are compared for the processing of experimental data.

Contrast agent specific imaging modes for the ultrasonic assessment of parenchymal cerebral echo contrast enhancement

Previous work has demonstrated that cerebral echo contrast enhancement can be assessed by means of transcranial ultrasound using transient response second harmonic imaging (HI). The current study was designed to explore possible advantages of two new contrast agent specific imaging modes, contrast burst imaging (CBI) and time variance imaging (TVI), that are based on the detection of destruction or splitting of microbubbles caused by ultrasound in comparison with contrast harmonic imaging (CHI), which is a broadband phase-inversion-based implementation of HI. Nine healthy individuals with adequate acoustic temporal bone windows were included in the study. Contrast harmonic imaging, CBI, and TVI examinations were performed in an axial diencephalic plane of section after an intravenous bolus injection of 4 g galactose-based microbubble suspension in a concentration of 400 mg/mL. Using time-intensity curves, peak intensities and times-to peak-intensity (TPIs) were calculated off-line in anterior and posterior parts of the thalamus, in the region of the lentiform nucleus, and in the white matter. The potential of the different techniques to visualize cerebral contrast enhancement in different brain areas was compared. All techniques produced accurate cerebral contrast enhancement in the majority of investigated brain areas. Contrast harmonic imaging visualized signal increase in 28 of 36 regions of interest (ROIs). In comparison, TVI and CBI examinations were successful in 32 and 35 investigations, respectively. In CHI examinations, contrast enhancement was most difficult to visualize in posterior parts of the thalamus (6 of 9) and the lentiform nucleus (6 of 9). In TVI examinations, anterior parts of the thalamus showed signal increase in only 6 of 9 examinations. For all investigated imaging modes, PIs and TPIs in different ROIs did not differ significantly, except that TVI demonstrated significantly higher PIs in the lentiform nucleus as compared with the thalamus and the white matter (P < 0.05). The current study demonstrates for the first time that CBI and TVI represent new ultrasonic tools that allow noninvasive assessment of focal cerebral contrast enhancement and that CBI and TVI improve diagnostic sensitivity as compared with CHI.

New real-time strain imaging concepts using diagnostic ultrasound

Two real-time strain imaging concepts and systems are presented. Both systems are based on a conventional ultrasound scanner that is connected to a PC with an A/D converter card for real-time data acquisition of rf data. Differential strain between successively acquired rf frames are estimated using phase root seeking. The first concept uses a special real-time implementation of manual elastography. In the second concept, denoted 'vibrography', the static compression is replaced by low-frequency axial vibration of the probe, still operating in quasistatic acquisition mode. The properties of both concepts are discussed with regard to noise and motion artefacts, and it is shown, using simulations and phantom experiments, that both imaging concepts yield the same kind of strain images. Vibrography has the advantage that no manual compression has to be applied, total compression can be very low and some motion artefacts are better suppressed.

Axial strain imaging using a local estimation of the scaling factor from RF ultrasound signals

The main signal-processing techniques used in elastography compute strains as the displacement derivative. They perform well for very low deformations, but suffer rapidly from decorrelation noise. Aiming to increase the range of accurate strain measurements, we developed an adaptive method based on the estimation of strains as local scaling factors. Its adaptability makes this method appropriate for computing scaling factors resulting from larger strains or a wide spread of strain variations. First, segments corresponding to the same part of tissue are adaptively selected in the rest and stressed state echo signals. Then, local scaling factors are estimated by iteratively varying their values until reaching the zero of the phase of the complex cross-correlation function. Results from simulation and from experimental data are presented. They show how this adaptive method can track various local deformations and its accuracy for strain up to 7%.

Segmentation of 3D intravascular ultrasonic images based on a random field model

Segmentation of intravascular ultrasound images provides important information about the degree of vessel obstruction as well as about the shape and size of plaques. To address the problems of inter- and intra-observer variances associated with conventional manual tracing, a fully automated segmentation was developed. The algorithm is based on the optimisation of a maximum a posteriori estimator, implementing the Rayleigh distribution of speckle and a priori information about the contours. Within 3D image sets, additional information by the blood flow resulting in a decorrelation of the pixels within the luminal boundary is used to initialise the segmentation. To accelerate the estimation, dynamic programming was used. The segmentation algorithm was realised as a Windows 95 application on a Pentium II/233 MHz and delivered reliable and reproducible results independent of the catheter position and the total image brightness (except overflow). In contrast, contours drawn by two physicians for an evaluation of 29 clinical cases showed large intra- and inter-observer variances. In vivo images were acquired with a 20 MHz transducer array (EndoSonics InVision). Comparison with the contours drawn by the physicians and histology demonstrates the potential of the segmentation algorithm.

[Ultrasound elastography of the prostate. A new technique for tumor detection]

AIM

Prostate tumours are often of harder consistency than the surrounding tissue. During digital rectal examination, this fact can be used not only to detect hypertrophy but also localized hardenings. The examination by digital palpation is inaccurate and, even in combination with PSA-value and a transrectal ultrasonic examination, the result is often not reliable. Ultrasound elastography enables us to measure and visualize the elastic properties of a tissue region and is a useful supplement to the examination by digital palpation. Ultrasound elastography is able to measure and visualize the elastic properties of a tissue region, therefore it is a useful supplement to commonly used diagnostic procedures.

METHOD

We have developed a new system for elastographic prostate diagnosis which can be used during the transrectal ultrasonic examination. During the examination a sequence of ultrasonic images is acquired while the organ is slightly compressed by the ultrasound probe. Using numerical analysis of image pairs for the acquired sequence we calculate the tissue strain which represents the spatial elasticity distribution of a specific cross-section of the organ. This enables us to distinguish hard areas in the tissue. Image artifacts resulting from lateral motion components, i.e., orthogonal to the direction of the applied force, are compensated for by a special approach.

RESULTS

We present results obtained from a typical elastography phantom and also the first in vivo images from patients who were undergoing radical prostatectomy. Our images prove that ultrasound elastography has the potential to detect malignant tissue areas, which are inconspicuous in the B-mode image. Our findings are confirmed by the corresponding histological specimens.