Wolfgang Knüpfer

New x-ray tube performance in computed tomography by introducing the rotating envelope tube technology.

The future demands of computed tomography imaging regarding the x-ray source can be summarized with higher scan power, shorter rotation times, shorter cool down times and smaller focal spots. We report on a new tube technology satisfying all these demands by making use of a novel cooling principle on one hand and of a novel beam control system on the other hand. Nowadays tubes use a rotating anode disk mainly cooled via radiation. The Straton x-ray tube is the first tube available for clinical routine utilizing convective cooling exclusively. It is demonstrated that this cooling principle makes large heat storage capacities of the anode disk obsolete. The unprecedented cooling rate of 4.8 MHU/min eliminates the need for waiting times due to anode cooling in clinical workflow. Moreover, an electronic beam deflection system for focal spot position and size control opens the door to advanced applications. The physical backgrounds are discussed and the technical realization is presented. From this discussion the superior suitability of this tube to withstand g-forces well above 20 g created by fast rotating gantries will become evident. Experience from a large clinical trial is reported and possible ways for future developments are discussed.

Perspectives of medical X-ray imaging

Abstract While X-ray image intensifiers (XII), storage phosphor screens and film-screen systems are still the work horses of medical imaging, large flat panel solid state detectors using either scintillators and amorphous silicon photo diode arrays (FD-Si), or direct X-ray conversion in amorphous selenium are reaching maturity. The main advantage with respect to image quality and low patient dose of the XII and FD-Si systems is caused by the rise of the Detector Quantum Efficiency originating from the application of thick needle-structured phosphor X-ray absorbers. With the detectors getting closer to an optimal state, further progress in medical X-ray imaging requires an improvement of the usable source characteristics. The development of clinical monochromatic X-ray sources of high power would not only allow an improved contrast-to-dose ratio by allowing smaller average photon energies in applications but would also lead to new imaging techniques.

Novel X-ray detectors for medical imaging

Abstract A number of different imaging systems are in use in X-ray medical diagnostics (e.g. digital radiography or computer tomography). The design goal of these imaging systems is to optimally use the information contained in X-ray quanta that have passed through the patient. The best image quality, as well as the minimisation of the X-ray dose applied to the patient are of prime importance. We report about innovations for novel detectors which reduce the X-ray dose and improve the image quality simultaneously. Advances in thin film electronics have permitted the development of large a -Si:H imaging arrays to design fat panel solid state detectors (short FD, up to 45 × 45 cm 2 ) for both digital radiography and fluoroscopy. The proposed detector consists of a CsI:T1 needle shaped scintillation crystal layer (thickness: 450 μm, needle diameter ∼ 10 μm) in front of an a -Si:H-panel. The Detective Quantum Efficiency (DQE) is about 65% (at spatial frequency zero) and spatial resolution is 2.8 lp/mm at 20% of the MTF and 6.2 lp/mm at 4%. In computed tomography (CT), a new geenration of linear detector array consists of Gadolinium Oxysulfid (GOS) ceramic scintillator elements, glued onto photodiodes. Important criteria for the selection of the detector material are good absorption of the incident X-rays (α > 95%) and high efficiency of conversion of the absorbed radiation energy to an electrical signal. A very short decay time to extremely low levels of afterglow is an advantage for the very short scanning times in CT. One gets a DQE (at 0 mm −1 ) about 80%. The next step toward dose reduction could be implemented by the application of monochromatic instead of polychromatic X-rays. This would additionally improve the DQE and thus enhance image quality. In addition, with the application of monochromatic X-rays, scattered radiation could be suppressed to a large extent by energy-selective single photon measurement, without loss of unscattered photons. At present, large area detectors in particular suffer from image quality losses, if no scattered radiation (multiline) grid is used.

Imaging characteristics of different mammographic screens

A study of mammography systems with green-emitting screens was conducted to determine how the image quality parameters (apart from dose requirement), such as modulation transfer function (MTF) and Wiener spectrum (WS), depend on the dye content of the compound and coating weight of the screen. In addition, the contribution to total noise of the individual components, i.e., film, screen, and quantum noise, was studied. The quantities derived from MTF and WS, namely detective quantum efficiency (DQE) and noise equivalent quanta (NEQ), were also investigated in regard to their dose dependency. It can be demonstrated that the MTF of the screens becomes more favorable when the dye content is increased, while noise is not significantly affected. This suggests the use of a mammography screen capable of greater detail recognition, requiring at least double the dose of todays conventional systems with approximately 80 microGy system dose. On the other hand, the manufacture of a screen with about 60% of the dose of the conventional system is possible with very little loss in image quality. For the systems in common use today (80 microGy), quantum noise represents a considerable share of the total noise at low spatial frequencies, whereas in high spatial frequencies, the graininess of the film dominates quantum noise and screen structure.