This study evaluates new applications using a novel navigation system with electromagnetic (EM) tracking in clinical routine.
The navigation system (iGuide CAPPA, CAS innovations, Erlangen, Germany) consists of a PC with dedicated navigation software, the AURORA tracking system (NDI, Waterloo Ontario, Canada) and needles equipped with small coils in their tips for EM navigation. After patient positioning a 3D C-arm data set of the spine region of interest is acquired. The images are reconstructed and the 3D data set is directly transferred to the navigation system. Image
loading and image to patient registration are performed automatically by the navigation system. For image acquisition a
C-arm system with DynaCT option (AXIOM Artis, Siemens Healthcare, Forchheim, Germany) was used. As new clinical applications we performed kyphoplasty for reconstruction of collapsed vertebrae. All interventions were carried out without any complication. After a single planning scan the radiologists were able to place the needle in the designated vertebra. During needle driving 2D imaging was performed just in a few cases for control reasons. The time between planning and final needle positioning was reduced in all cases compared to conventional methods. Moreover, the number of control scans could be markedly reduced. The deviation of the needle to
the planned target was less than 2 mm. The use of DynaCT images in combination with electromagnetic tracking-based navigation systems allows a precise needle positioning for kyphoplasty.
Phase-contrast imaging using grating interferometers has been developed over the last few years for x-ray energies of up to 28 keV. We have now developed a grating interferometer for phase-contrast imaging that operates at 60 keV x-ray energy. Here, we show first phase-contrast projection and CT images recorded with this interferometer using an x-ray tube source operated at 100 kV acceleration voltage. By comparison of the CT data with theoretical values, we find that our measured phase images represent the refractive index decrement at 60 keV in good agreement with the theoretically expected values. The extension of phase-contrast imaging to this significantly higher x-ray energy opens up many new
applications of the technique in industry, medicine, and research.
To ensure precise needle placement in soft tissue of a patient for e.g. biopsies, the intervention is normally carried
out image-guided. Whereas there are several imaging modalities such as computed tomography, magnetic resonance tomography, ultrasound, or C-arm X-ray systems with CT-option, navigation systems for such minimally invasive interventions are still quite rare. However, prototypes and also first commercial products of optical and electromagnetic tracking systems demonstrated excellent clinical results. Such systems provide a reduction of control scans, a reduction of intervention time, and an improved needle positioning accuracy specially for deep and double oblique access. Our novel navigation system CAPPA IRAD EMT with electromagnetic tracking for minimally invasive needle applications is connected to a C-arm imaging system with CT-option. The navigation system was investigated in clinical interventions by different physicians and with different clinical applications. First clinical results demonstrated a high accuracy during needle placement and a reduction of control scans.
Integrated solutions for navigation systems with CT, MR or US systems become more and more popular for
medical products. Such solutions improve the medical workflow, reduce hardware, space and costs requirements.
The purpose of our project was to develop a new electromagnetic navigation system for interventional radiology
which is integrated into C-arm CT systems. The application is focused on minimally invasive percutaneous
interventions performed under local anaesthesia. Together with a vacuum-based patient immobilization device
and newly developed navigation tools (needles, panels) we developed a safe and fully automatic navigation
system. The radiologist can directly start with navigated interventions after loading images without any prior
user interaction. The complete system is adapted to the requirements of the radiologist and to the clinical
workflow. For evaluation of the navigation system we performed different phantom studies and achieved an
average accuracy of better than 2.0 mm.
Monochromatic X-rays have been proposed for medical imaging, especially in the mammographic energy range. Our previous investigations have shown that the contrast of objects such as lesions or contrast media can be enhanced considerably by using monochromatic X-rays instead of the common polychromatic spectra. Admittedly, only one specific polychromatic spectrum and one monochromatic energy have been compared so far. In this work, we investigated the contrast yielded by a series of different X-ray spectra obtained by varying tube voltage and beam filtering. This resulted in spectra of different mean energies and spectral widths. The objects under examination were aqueous solutions containing different chemical elements such as I, Gd, Dy, Yb, and Bi. A monoenergetic spectrum at 17.5 keV was obtained using a mammographic X-ray tube with a Mo anode and a monochromator equipped with a HOPG crystal. Moreover, we simulated quasi-monoenergetic spectra at different energies and with different widths. As a result, we demonstrated that in many cases spectra with an energetic width of some keV yield an equivalent contrast to monoenergetic radiation at the same energy. Therefore, the advantage in image contrast of monochromatic X-rays at 17.5 keV over narrow-band polychromatic X-ray spectra obtained by appropriate filtering is only slight. Thus, the additional expenditure on a mammography system with HOPG monochromator that can deliver only a small X-ray dose and the unfavorable slot-scan geometry can be avoided. Moreover, we carried out simulations of monochromatic versus polychromatic spectra throughout the whole radiographic energy range. We found advantages in using monochromatic X-rays at higher energies and thicker objects that will justify their application for diagnostic imaging in a number of specific cases.
Properties and performance of digital X-ray detectors for medical imaging can be studied by Monte Carlo simulations. Most simulations of such detectors simplify the setup by only taking the conversion layer into account neglecting everything behind. For hybrid detectors with Si as the conversion layer, such as the Medipix2 chip less photons are absorbed at higher photon energies in the conversion layer and thus may reach the detector ASIC including its bump bonds. For photon energies above the K-edges of the backscatter materials, fluorescence may occur. The fluorescence photons can have relatively long ranges and thus have a great impact on the MTF of the detector decreasing its spatial resolution. They also add noise to the detector decreasing the overall signal-difference-to-noise-ratio (SDNR). In our study we simulated the line spread functions (LSF) for photon-counting pixel detectors by Monte Carlo simulations, implementing the detectors in detail. We used the program ROSI (ROentgen SImulation) which is based on the well-established EGS4 algorithm. The appropriate MTFs were calculated by FFT. We show that internal backscattering, especially from Sn bump bonds, contributes to the so-called low-frequency drop of the MTF. For a 300 μm Si absorber on the Medipix2 chip, backscattering contributes up to 10% to the detected signal. This strongly decreases contrast by adding additional noise. Therefore, we also investigated the amount of noise added by internal backscattering.
The contrast of X-ray imaging depends on the radiation energy and acquires its maximum value at a certain optimum energy typical for the object under investigation. Usually, higher energies result in reduced contrast, lower energies are absorbed in the object thus having a smaller probability of reaching the detector. Therefore, broad X-ray spectra contain non-optimal quanta to a large extent and deliver images with deteriorated contrast. Since investigations with monochromatic X-rays using synchrotrons are too complex and expensive for routine diagnostic imaging procedures, we propose a simpler approach. A conventional mammography system (Siemens Mammomat 300) with an X-ray tube with a molybdenum anode was supplemented with an X-ray HOPG monochromator (HOPG = Highly Oriented Pyrolytic Graphite) and an exit slit selecting those rays fulfilling Bragg’s condition. The detector is a CCD (Thales TH9570), 4092 x 200 pixels, 54 μm in size. At this slot-scan setup<sup>1</sup>, measurements have been carried out at 17.5 keV as well as with a polychromatic spectrum with 35 kV tube voltage. The modulation transfer function (MTF) and the detective quantum efficiency (DQE) have been determined from images of a lead bar pattern and flat-field images. Both MTF and DQE depend on orientation (scan or detector direction) for the 17.5 keV monochromatic case. Above 3 mm<sup>-1</sup> the DQE values are smaller than those for polychromatic radiation. The contrast yielded by foils of different materials (Al, Cu, Y, Ag) has been studied. In all cases the monochromatic X-rays give rise to about twice the contrast of a polychromatic spectrum.
A generalized, objective image quality measure can be defined for X-ray based medical projection imaging: the spatial frequency-dependent signal-to-noise ratio <i>SNR</i> = <i>SNR(u,v)</i>. This function includes the three main image quality parameters, i.e. spatial resolution, object contrast, and noise. The quantity is intimately related to the <i>DQE</i> concept, however its focus is not to characterize the detector, but rather the detectability of a certain object embedded into a defined background. So also effects from focus size and radiation scatter can be quantified by this method. The <i>SNR(u,v)</i> is independent of basic linear post-processing steps such as appropriate windowing or spatial filtering. The consideration of the human visual system is beyond the scope of this concept. By means of this quantity, different X-ray systems and setups can be compared with each other and with theoretical calculations. Moreover, X-ray systems (i.e. detector, beam quality, geometry, anti-scatter grid, basic linear post-processing steps etc.) can be optimized to deliver the best object detectability for a given patient dose. In this paper <i>SNR(u,v)</i> is defined using analytical formulas. Furthermore, we demonstrate how it can be applied with a test phantom to a typical flat panel detector system by a combination of analytical calculations and Monte Carlo simulations. Finally the way this function can be used to optimize an X-ray imaging device is demonstrated.
The requirements for medical X-ray detectors tend towards higher spatial resolution, especially for mammography. Therefore, we have investigated common absorber materials with respect to the possible intrinsic limitations of their spatial resolution.
Primary interaction of an incident X-ray quantum is followed by a series of processes: Rayleigh scattering, Compton effect, or the generation of fluorescence photons and subsequent electrons. Lateral diffusion of carriers relative to their drift towards the electrodes also broadens the point-spread function. One consequence is that the spatial resolution of the detector, expressed in terms of the modulation transfer function (MTF), is reduced.
Monte Carlo simulations have been carried out for spectra with tube voltages of 28-120 kV using the program ROSI (Roentgen Simulation) based on the well-established EGS4 algorithm. The lateral distribution of deposited energy has been calculated in typical materials such as Se, CdTe, HgI<sub>2</sub>, and PbI<sub>2</sub> and used to determine the line spread function.
The complex absorption process is found to determine the spatial resolution of the detector considerably. The spectrum at energies closely above the K-edge of the absorber material tends to result in a reduced MTF. At energies above 50 keV, electron energy loss increasingly reduces spatial resolution in the high frequency range. The influence of fluorescence is strongest in the 5-20 lp/mm range. If a very high spatial resolution is required, a well-adapted semiconductor should be applied.
The design and the performance of a 20 cm by 20 cm flat panel x-ray detector for digital radiography and fluoroscopy is described: Thin film amorphous silicon (aSi) technology has been used to build a 1024 by 1024 photodetector matrix, each pixel including both a photodiode and a switching diode; the pixel size is 196 by 196 micrometers<SUP>2</SUP>. A high resolution and high absorption CsI(Tl) scintillator layer covers the top of the photodetector matrix in order to provide for x ray to light conversion. For low electronic noise and 30 fr/s operating rate we developed a custom design charge readout integrated circuit. The detector delivers a 12 bit digital output. The image quality, signal to noise ratio, and DQE are presented and discussed. The flat panel detector provides a MTF in excess of 30% at 2 lp/mm and a high contrast ratio without any distortion on the whole imaging area. The x-ray absorption is 70% for 50 KeV photons. The readout amplifier is optimized to reduce the electronic noise down to 1000 e-. This low noise level, combined with high sensitivity (1150 e-/incident x-ray quantum) provides the capability for fluoroscopic applications. The digital flat panel detector has been integrated in a C-arm system for cardiology and has been used on a regular basis in a European hospital since February 1995. The results are discussed for several operating modes: radiography and fluoroscopy. Conclusions on present detector performances, as well as further improvements, are presented.