Walter Ruetten

Counting and integrating readout for direct conversion X-ray imaging concept, realization and first prototype measurements

A novel signal processing concept for X-ray imaging with directly converting pixelated semiconductor sensors is presented. The novelty of this approach compared to existing concepts is the combination of charge integration and single photon counting in every single pixel. Simultaneous operation of both signal processing chains extends the dynamic range beyond the limits of the individual schemes and allows determination of the mean photon energy. Medical applications such as X-ray computed tomography can benefit from this additional spectral information through improved contrast and the ability to determine the hardening of the tube spectrum due to attenuation by the scanned object. A prototype chip in 0.35-micrometer technology was successfully tested. The pixel electronics are designed using a low-noise differential current mode logic and provide configurable feedback modes, leakage current compensation and various test circuits. This paper will discuss measurement results of the prototype structures and give details on the circuit design

PET/MRI insert using digital SiPMs: Investigation of MR-compatibility.

In this work, we present an initial MR-compatibility study performed with the worlds first preclinical PET/MR insert based on fully digital silicon photo multipliers (dSiPM). The PET insert allows simultaneous data acquisition of both imaging modalities and thus enables the true potential of hybrid PET/MRI. Since the PET insert has the potential to interfere with all of the MRIs subsystems (strong magnet, gradients system, radio frequency (RF) system) and vice versa, interference studies on both imaging systems are of great importance to ensure an undisturbed operation. As a starting point to understand the interference, we performed signal-to-noise ratio (SNR) measurements as well as dedicated noise scans on the MRI side to characterize the influence of the PET electronics on the MR receive chain. Furthermore, improvements of sub-components’ shielding of the PET system are implemented and tested inside the MRI. To study the influence of the MRI on the PET performance, we conducted highly demanding stress tests with gradient and RF dominated MR sequences. These stress tests unveil a sensitivity of the PETs electronics to gradient switching.

Dynamic X-ray imaging system based on an amorphous silicon thin-film array

In this paper we address design concepts and performance characterization with our laboratory x-ray detector system. Key component is a 1k2 pixel TFT switched photodiode array with a pixel pitch of 200 micrometer. It is built of a-Si:H with a CsI:Tl scintillator layer. The detector system can be used for radiography and fluoroscopy applications with up to 30 images/s. It shows a S/N-ratio better than 23dB at a dose of 10nGy/frame and reaches a value for DQE of more than 60% at low spatial frequencies. We have developed a new evaporation process for CsI:Tl deposition directly on the array. It yields an x-ray sensitivity close to the theoretical limit and a spatial resolution on a sufficiently high level. An optimized plate design in combination with a dedicated charge sensitive readout amplifier chip lead to a very low level of electronic noise. In particular sources and properties of electronic noise and signal crosstalk have shown to be crucial for the clinical use of the new technology. The visual impression of the remaining noise in the images from our system is isotropic. This means especially that synchronous noise has been reduced to the edge of visibility.

X-Ray Detector-on-Plastic With High Sensitivity Using Low Cost, Solution-Processed Organic Photodiodes

We made and characterized an X-ray detector on a 25-μm-thick plastic substrate that is capable of medicalgrade performance. As an indirect conversion flat panel detector, it combined a standard scintillator with an organic photodetector (OPD) layer and oxide thin-film transistor backplane. Using solution-processed organic bulk heterojunction photodiode rather than the usual amorphous silicon, process temperature is reduced to be compatible with plastic film substrates, and a number of costly lithography steps are eliminated, opening the door to lower production costs. With dark currents as low as 1 pA/mm2 and sensitivity of 0.2 A/W the OPD also meets functional requirements: the proof-of-concept detector delivers high-resolution, dynamic images at 10 frames/s, and 200 pixels/in using X-ray doses as low as 3 μGy/frame.

Technical and clinical assessments of an experimental flat dynamic x-ray image detector system

Advanced technical investigations, including DQE measurements and threshold contrast detail-detectability experiments, have been performed in order to demonstrate the superior image quality of an experimental flat dynamic X-ray image detector (FDXD) system. The dose efficiencies throughout a range of dose levels used in fluoroscopic and radiographic applications have been measured and are presented. Together with the results of a range of clinical patient examinations, the results of the technical investigations fully confirm earlier expectations in terms of increased image quality and improved dose efficiency with respect to current imaging modalities. Several mixed applications performed with the FDXD system are presented including those where subtraction techniques were used. The dynamic aspects of the FDXD system are discussed in detail. In the fluoroscopic mode, images have been acquired with a dose-rate as low as 5 nGy per image using a frame rate of approx. 25 fps. Low dose fluoroscopic images will be presented and it will be confirmed that low readout noise of the detection system facilitates the clinical acceptability of the images, even without applying any noise reduction algorithms. Post-processing algorithms for exposures will also be discussed. It can be concluded that the results of the technical measurements, together with the clinical examinations, prove that in areas regarding dose efficiency and image quality, this new detector technology is superior to the current X-ray modalities in many aspects.

Assessment of photodiodes as a light detector for PET scanners

Current PET systems based on pixelated scintillator arrays coupled to photomultiplier tubes suffer from pile-up and electronics dead time at high count rates. With a pixelated readout, i.e. one-to-one coupling of a scintillator crystal to a photo detector, these effects can be strongly reduced. Recent developments of high light output scintillators like LYSO and LaBr3 in combination with very low noise amplifiers based on modern CMOS processes make it possible to use high quantum efficiency blue-sensitive PIN photodiodes as a light detector. To explore the potential of this approach, a model of the signal detection chain was implemented. It comprises the scintillation light pulse and its quantum noise, optical coupling, charge conversion in the diode, noise sources of the integrating amplifier, shaper circuits for the energy and timing channel, and the discriminator for the timing channel. The model is verified using off-the-shelf PIN photodiodes and a dedicated CMOS preamplifier excited by a picosecond laser as well as scintillator pulses. The model predicts that with high light output scintillators, high quantum efficiency photodiodes and optimized preamplifiers, a pixelated PET readout with very good energy resolution and sufficient timing resolution can be realized. To complement the study, an APD-based readout is also considered and the related signal to noise issues are discussed

Methodology to measure fundamental performance parameters of x-ray detectors

To judge the potential benefit of a new x-ray detector technology and to be able to compare different technologies, some standard performance measurements must be defined. In addition to technology-related parameters which may influence weight, shape, image distortions and readout speed, there are fundamental performance parameters which directly influence the achievable image quality and dose efficiency of x-ray detectors. A standardization activity for detective quantum efficiency (DQE) for static detectors is already in progress. In this paper we present a methodology for noise power spectrum (NPS), low frequency drop (LFD) and signal to electronic noise ratio (SENR), and the influence of these parameters on DQE. The individual measurement methods are described in detail with their theoretical background and experimental procedure. Corresponding technical phantoms have been developed. The design of the measurement methods and technical phantoms is tuned so that only minimum requirements are placed on the detector properties. The measurement methods can therefore be applied to both static and dynamic x-ray systems. Measurement results from flat panel imagers and II/TV systems are presented.

Flat detector with integrated dose sensing

Integrated dose sensing in Flat Detectors allows a during pulse control of the X-ray illumination without the need for external dose sensing devices. Standard designs of Flat Detectors do not allow during pulse dose sensing since the information is collected from the pixels only in the read-out phase after the X-ray illumination. This paper introduces a special detector plate design for obtaining dose sensing information directly from the X-ray detector while the X-ray pulse is being applied. This dose sensing information is read at a lower spatial resolution than the actual X-ray image but with a sub-millisecond temporal resolution. The dose sensing operates without any additional radiation burden on the patient and without attenuation of the image information. Experimental results from a small area (4x4 cm2) detector are presented, including an analysis of noise, linearity and cross-talk.

High performance x-ray imaging detectors on foil using solution-processed organic photodiodes with extremely low dark leakage current (Presentation Recording)

High performance X-ray imaging detectors on foil using solution-processed organic photodiodes with extremely low dark leakage current Abhishek Kumara, Date Moeta, Albert van Breemena, Santhosh Shanmugama, Jan-Laurens van der Steena, Jan Gilota, Ronn Andriessena, Matthias Simonb, Walter Ruettenb, Alexander U. Douglasb, Rob Raaijmakersc, Pawel E. Malinowskid, Kris Mynyd and Gerwin H. Gelincka,e a. Holst Centre/TNO, High Tech Campus 31, Eindhoven 5656 AE, The Netherlands b. Philips Research, High Tech Campus 34, 5656 AE Eindhoven, The Netherlands c. Philips Healthcare, Veenpluis 6-8, 5684 PC Best, The Netherlands d. Department of Large Area Electronics, imec vzw, Kapeldreef 75, Leuven B3001, Belgium e. Applied Physics Department, TU Eindhoven, Eindhoven, The Netherlands We demonstrate high performance X-ray imaging detectors on foil suitable for medical grade X-ray imaging applications. The detectors are based on solution-processed organic photodiodes forming bulk-heterojunctions from photovoltaic donor and acceptor blend. The organic photodiodes are deposited using an industrially compatible slot die coating technique with end of line processing temperature below 100°C. These photodiodes have extremely low dark leakage current density of 10-7 mA/cm2 at -2V bias with very high yield and have peak absorption around 550 nm wavelength. We combine these organic photodiodes with high mobility metal oxide semiconductor based thin film transistor arrays with high pixel resolution of 200ppi on thin plastic substrate. When combined with a typical CsI(TI) scintillator material on top, they are well suited for low dose X-ray imaging applications. The optical crosstalk is insignificant upto resolution of 200 ppi despite the fact that the photodiode layer is one continuous layer and is non-pixelated. Low processing temperatures are another key advantage since they can be fabricated on plastic substrate. This implies that we can make X-ray detectors on flexible foil. Those detectors can be mechanically more robust and light weight when compared to amorphous Si based detectors fabricated on glass substrate.