Oliver Schuetz

Enhanced 3-D-reconstruction algorithm for C-arm systems suitable for interventional procedures

Increasingly, three dimensional (3-D) imaging technologies are used in medical diagnosis, for therapy planning, and during interventional procedures. The authors describe the possibilities of fast 3-D-reconstruction of high-contrast objects with high spatial resolution from only a small series of two-dimensional (2-D) planar radiographs. The special problems arising from the intended use of an open, mechanically unstable C-arm system are discussed. For the description of the irregular sampling geometry, homogeneous coordinates are used thoroughly. The well-known Feldkamp algorithm is modified to incorporate corresponding projection matrices without any decomposition into intrinsic and extrinsic parameters. Some approximations to speed up the whole reconstruction procedure and the tradeoff between image quality and computation time are also considered. Using standard hardware the reconstruction of a 256/sup 3/ cube is now possible within a few minutes, a time that is acceptable during interventions. Examples for cranial vessel imaging from some clinical test installations will be shown as well as promising results for bone imaging with a laboratory C-arm system.

Improving 3D image quality of x-ray C-arm imaging systems by using properly designed pose determination systems for calibrating the projection geometry

C-arm volume reconstruction has become increasingly popular over the last years. These imaging systems generate 3D data sets for various interventional procedures such as endovascular treatment of aneurysms or orthopedic applications. Due to their open design and mechanical instability, C-arm imaging systems acquire projections along non-ideal scan trajectories. Volume reconstruction from filtered 2D X-ray projections requires a very precise knowledge of the imaging geometry. We show that the 3D image quality of C-arm cone beam imaging devices can be improved by proper design of the calibration phantom.

Clinical applications of frequency-domain optical mammography

We present clinical results obtained with a frequency-domain (70 MHz), four-wavelength (690, 750, 788, 856 nm) prototype for optical mammography. The 2D projection images are taken on the slightly compressed breast in craniocaudal and oblique projections, similar to what is done in x-ray mammography. The amplitude and phase images are combined to enhance the contrast and the tumor detectability by reducing the edge effects caused by the breast thickness variations within the scanned area. The analysis of the first set of clinical data (63 patients) has yielded encouraging results. The success rate in the detection of breast cancer was 73%, and specificity was 49%. A comparison of the optical mammograms at the four wavelengths in the range 690 - 856 nm suggests that spectral information may allow for the discrimination of benign and malignant breast lesions, thereby enhancing specificity.

Online geometrical calibration of a mobile C-arm using external sensors

3D tomographic reconstruction of high contrast objects such as contrast agent enhanced blood vessels or bones from x-ray images acquired by isocentric C-arm systems recently gained interest. For tomographic reconstruction, a sequence of images is captured during the C-arm rotation around the patient and the precise projection geometry has to be determined for each image. This is a difficult task, as C- arms usually do not provide accurate information about their projection geometry. Standard methods propose the use of an x-ray calibration phantom and an offline calibration, when the motion of the C-arm is supposed to be reproducible between calibration and patient run. However, mobile C-arms usually do not have this desirable property. Therefore, an online recovery of projection geometry is necessary. Here, we study the use of external tracking systems such as Polaris or Optotrak from Northern Digital, Inc., for online calibration. In order to use the external tracking system for recovery of x-ray projection geometry two unknown transformations have to be estimated. We describe our attempt to solve this calibration problem. These are the relations between x-ray imaging system and marker plate of the tracking system as well as worked and sensor coordinate system. Experimental result son anatomical data are presented and visually compared with the results of estimating the projection geometry with an x-ray calibration phantom.

Monte Carlo modeling of time-resolved near-infrared transillumination of human breast tissue

A Monte Carlo method is used to model the propagation of near infrared light in human breast tissue. The corresponding scattering coefficient, g-factor, and absorption coefficient of the model are adjusted to fit the temporal point spread function. With this model the dependence of the spatial resolution as a function of absorption, thickness of tissue, and integration time of time resolved measurements has been studied. We find that light absorption within the object already acts as a natural filter to suppress long pathlengths of strongly scattered photons. Compared to the case of no absorption, we observe a considerable improvement of the spatial resolution for realistic values of the absorption length and of the tissue thickness. We conclude that by time of flight methods, the spatial resolution in breast tissue with thickness of about 3 - 4 cm may be improved by at most a factor of two, once absorption is taken into account properly.

Optical mammography with single-wavelength contrast enhancement and multiwavelength oxygenation assessment

We present a combination of (1) single-wavelength optical mammograms representing either the optical absorbance (N-images) or its spatial second-derivative (N’’-images), and (2) oxygenation-index images based on optical data at four wavelengths (690, 750, 788, and 856 nm). The enhanced contrast of the second-derivative images with respect to the absorbance images is complemented by the functional information provided by the oxygenation-index images. In a number of clinical cases, we have found that breast cancer is often associated with relatively low oxygenation indices, while benign breast lesions correspond to areas with relatively high oxygenation indices. However, the relative nature of the oxygenation-index presented in this work does not allow for a comparison of oxygenation values in different images, for which absolute oxygenation measurements would be needed.