Martin Hoheisel

X-ray spectrum optimization of full-field digital mammography : Simulation and phantom study

In contrast to conventional analog screen-film mammography new flat detectors have a high dynamic range and a linear characteristic curve. Hence, the radiographic technique can be optimized independently of the receptor exposure. It can be exclusively focused on the improvement of the image quality and the reduction of the patient dose. In this paper we measure the image quality by a physical quantity, the signal difference-to-noise ratio (SDNR), and the patient risk by the average glandular dose (AGD). Using these quantities, we compare the following different setups through simulations and phantom studies regarding the detection of microcalcifications and tumors for different breast thicknesses and breast compositions: Monochromatic radiation, three different anode/filter combinations: Molybdenum/molybdenum (Mo/Mo), molybdenum/rhodium (Mo/Rh), and tungsten/rhodium (W/Rh), different filter thicknesses, use of anti-scatter grids, and different tube voltages. For a digital mammography system based on an amorphous selenium detector it turned out that, first, the W/Rh combination is the best choice for all detection tasks studied. Second, monochromatic radiation can further reduce the AGD by a factor of up to 2.3, maintaining the image quality in comparison with a real polychromatic spectrum of an x-ray tube. And, third, the use of an anti-scatter grid is only advantageous for breast thicknesses larger than approximately 5 cm.

Inverse geometry for grating-based x-ray phase-contrast imaging

Phase-contrast imaging using conventional polychromatic x-ray sources and grating interferometers has been developed and demonstrated for x-ray energies up to 60 keV. Here, we conduct an analysis of possible grating configurations for this technique and present further geometrical arrangements not considered so far. An inverse interferometer geometry is investigated that offers significant advantages for grating fabrication and for the application of the method in computed tomography (CT) scanners. We derive and measure the interferometer’s angular sensitivity for both the inverse and the conventional configuration as a function of the sample position. Thereby, we show that both arrangements are equally sensitive and that the highest sensitivity is obtained, when the investigated object is close to the interferometer’s phase grating. We also discuss the question whether the sample should be placed in front of or behind the phase grating. For CT applications, we propose an inverse geometry with the sample ...

Amorphous silicon x-ray image sensor

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 micrometers2. 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.

3D Imaging with Flat-Detector C-Arm Systems

Three-dimensional (3D) C-arm computed tomography is a new and innovative imaging technique. It uses two-dimensional (2D) X-ray projections acquired with a flat-panel detector C-arm angiography system to generate CT-like images. To this end, the C-arm system performs a sweep around the patient, acquiring up to several hundred 2D views. They serve as input for 3D cone-beam reconstruction. Resulting voxel data sets can be visualized either as cross-sectional images or as 3D data sets using different volume rendering techniques. Initially targeted at 3D high-contrast neurovascular applications, 3D C-arm imaging has been continuously improved over the years and is now capable of providing CT-like soft-tissue image quality. In combination with 2D fluoroscopic or radiographic imaging, information provided by 3D C-arm imaging can be valuable for therapy planning, guidance, and outcome assessment all in the interventional suite.

Phase-contrast imaging and tomography at 60 keV using a conventional x-ray tube source

Phase-contrast imaging at laboratory-based x-ray sources using grating interferometers has been developed over the last few years for x-ray energies of up to 28 keV. Here, we show first phase-contrast projection and tomographic images recorded at significantly higher x-ray energies, produced by an x-ray tube source operated at 100 kV acceleration voltage. We find our measured tomographic phase images in good agreement with tabulated data. The extension of phase-contrast imaging to this significantly higher x-ray energy opens up many applications of the technique in medicine and industrial nondestructive testing.

Needle and catheter navigation using electromagnetic tracking for computer-assisted C-arm CT interventions

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.

Requirements on amorphous semiconductors for medical X-ray detectors

Abstract Solid state X-ray detectors based on large-area amorphous semiconductors, such as amorphous silicon or selenium, have been developed. The requirements for various applications in medical diagnosis determine the boundary conditions for different detectors. Key parameters are image-receptor size, spatial resolution, image frequency, signal-to-noise ratio, and long-term stability. Conventional thorax radiographs are 43×35 cm 2 in size. Therefore, to replace film, large detectors are required. Mammograms need to display microcalcifications of some 100 μm in detail. Fluoroscopic images are taken at a rate of 30 images/s, or even faster. Moreover, all detectors need to deliver optimum image quality at the lowest tolerable dose. The lifetime should be at least 10 years. In this paper we discuss different types of detectors that could fulfill those needs. Two fundamental concepts are compared: a scintillator coupled to a photodiode and a direct converting semiconductor, both being arranged on an amorphous silicon-switching matrix.

Amorphous silicon X-ray detectors

Abstract Amorphous silicon (a-Si) has proven to be the most suitable semiconductor for large-area devices. Our detector prototype with a pixel pitch of 200 μ m and an active area of 20×20 cm 2 uses one PIN photodiode and one PIN switching diode per pixel for readout. Cesium iodide is used as scintillator. Evaluation of the detector was performed in the laboratory as well as in a clinical site where it was integrated in a C-arm for cardiological investigations. In this paper, modulation transfer function, dynamic behavior, noise figures, and quantum yield will be discussed. The performance of these detectors represents a first step towards the goal of replacing existing fluoroscopic or radiographic X-ray systems for medical diagnosis.

SCLC transients in a-Si:H — New features and possibilities

The new possibilities and advantages of SCLC limited TOF transients are demonstrated. First, the effects related with the realistic absorption profile, screening current and effective thickness are used to explain the shift of the “cusp” position with the increasing laser pulse intensity. The model of a full charge extraction is used to explain the observed “extraction” time (t*) and exemplified by c-Si diode. The first results on a-Si:H p-i-n junctions together with the corresponding theory are presented.

Electromagnetic tracking system for minimal invasive interventions using a C-arm system with CT option: First clinical results.

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.