Tom J. C. Bruijns

Technical and clinical results of an experimental flat dynamic (digital) x-ray image detector (FDXD) system with real-time corrections

A clinical imaging system based upon an amorphous-Silicon (a- Si) flat dynamic (digital) X-ray image detector (FDXD) has been developed. The objectives of this experimental set-up were to determine the physical image quality and to establish the clinical feasibility of a flat-panel x-ray detector for radiography and fluoroscopy (R&F) applications. The FDXD acquires dynamic X-ray images at high frame rates in both continuous and pulsed fluoroscopic modes, lower frame rate exposures and single shots. The system has been installed in a clinical research room at The General Infirmary, Leeds (UK) and is being evaluated in a variety of universal R&F contrast medium aided examinations, including barium swallows, meals and enema examinations. In addition, general radiographic examinations have been performed. Both the established benefits and possible drawbacks of this type of system, together with the potential solutions, are discussed in this paper. Approach, design and set-up of the system are presented, and the dose efficiency and image quality achieved in clinical operation are explained. The technical and medical phantom images have been evaluated and analyzed. The results of the clinical examinations in mixed applications are discussed. The results of the measurements and examinations performed to date on this experimental FDXD system confirm the potential of this new type of digital X-ray image detector.

Threshold contrast detail detectability measurement of the fluoroscopic image quality of a dynamic solid-state digital x-ray image detector.

Solid-state digital x-ray imaging detectors of flat-panel construction will play an increasingly important role in future medical imaging facilities. Solid-state detectors that will support both dynamic (including fluoroscopic) and radiographic image recording are under active development. The image quality of an experimental solid-state digital x-ray image detector operating in a continuous fluoroscopy mode has been investigated. The threshold contrast detail detectability (TCDD) technique was used to compare the fluoroscopic imaging performance of an experimental dynamic solid-state digital x-ray image detector with that of a reference image intensifier television (IITV) fluoroscopy system. The reference system incorporated Plumbicon TV. Results were presented as a threshold detection index, or H(T)(A), curves. Measurements were made over a range of mean entrance air kerma (EAK) rates typically used in conventional IITV fluoroscopy. At the upper and mid EAK rate range (440 and 220 nGy/s) the solid-state detector outperformed the reference IITV fluoroscopy system as measured by TCDD performance. At the lowest measured EAK rate (104 nGy/s), the solid-state detector produces slightly inferior TCDD performance compared with the reference system. Although not statistically significant at this EAK rate, the difference will increase as EAK is lowered further. Overall the TCDD results and early clinical experiences support the proposition that a current design of dynamic solid-state detector produces image quality competitive with that of modern IITV fluoroscopy systems. These findings encourage the development of compact and versatile universal x-ray imaging systems based upon solid-state detector technology to support R & F and vascular/interventional applications.

Phantom-based experimental validation of computational fluid dynamics simulations on cerebral aneurysms.

PURPOSE Recently, image-based computational fluid dynamics (CFD) simulation has been applied to investigate the hemodynamics inside human cerebral aneurysms. The knowledge of the computed three-dimensional flow fields is used for clinical risk assessment and treatment decision making. However, the reliability of the application specific CFD results has not been thoroughly validated yet. METHODS In this work, by exploiting a phantom aneurysm model, the authors therefore aim to prove the reliability of the CFD results obtained from simulations with sufficiently accurate input boundary conditions. To confirm the correlation between the CFD results and the reality, virtual angiograms are generated by the simulation pipeline and are quantitatively compared to the experimentally acquired angiograms. In addition, a parametric study has been carried out to systematically investigate the influence of the input parameters associated with the current measuring techniques on the flow patterns. RESULTS Qualitative and quantitative evaluations demonstrate good agreement between the simulated and the real flow dynamics. Discrepancies of less than 15% are found for the relative root mean square errors of time intensity curve comparisons from each selected characteristic position. The investigated input parameters show different influences on the simulation results, indicating the desired accuracy in the measurements. CONCLUSIONS This study provides a comprehensive validation method of CFD simulation for reproducing the real flow field in the cerebral aneurysm phantom under well controlled conditions. The reliability of the CFD is well confirmed. Through the parametric study, it is possible to assess the degree of validity of the associated CFD model based on the parameter values and their estimated accuracy range.

Image quality of a large-area dynamic flat detector: comparison with a state-of-the-art II/TV system

The purpose of this research is to establish the technical and clinical image quality of a 30 X 40 cm2 dynamic flat detector (FD) compared to state-of-the-art IITV technology. A Trixell detector for vascular and RF applications is designed for a mixed use of fluoroscopy as well as exposure series and a range of radiographic applications. An RF system has been built which comprises both the FD as well as an IITV detector. This system enables a direct comparison of technical image quality measurements and patient images under exactly the same X-ray conditions. Image quality measurements comprise Detective Quantum Efficiency including transfer characteristics, Modulation Transfer Function, Noise Power Spectrum, lag, Low Frequency Drop and residual signals. Observation tests, using Threshold Contrast Detail Detectability (TCDD) techniques, are performed in order to confirm the results of the technical measurements. Results show a DQE (f) of the flat detector that is higher compared to IITV and above all constant over a wide dose range, the IITV DQE (f) drops at higher dose range due to fixed structure. Furthermore the Low Frequency Drop is substantially smaller in the FD-based system. The TCDD subjective tests show improved system performance in favor of the FD system.

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.

Clinical study of model-based blood flow quantification on cerebrovascular data

Diagnosis and treatment decisions of cerebrovascular diseases are currently based on structural information like the endovascular lumen. In future, clinical diagnosis will increasingly be based on functional information which gives direct information about the physiological parameters and, hence, is a direct measure for the severity of the pathology. In this context, an important functional quantity is the volumetric blood flow over time. The proposed flow quantification method uses contrasted X-ray images from cerebrovascular interventions and a model of contrast agent dispersion to estimate the flow parameters from the spatial and temporal development of the contrast agent concentration through the vascular system. To evaluate the model-based blood flow quantification under realistic circumstances, dedicated cerebrovascular data has been acquired during clinical interventions. To this aim, a clinical protocol for this novel procedure has been defined and optimized. For the verification of the measured flow results ultrasound Doppler measurements have been performed acting as reference measurements. The clinical data available so far indicates the ability of the proposed flow model to explain the in-vivo transport of contrast agent in blood. The flow quantification results show good correspondence of flow waveform and mean volumetric flow rate with the accomplished ultrasound measurements before or after angiography.

Experimental validation and sensitivity analysis for CFD simulations of cerebral aneurysms

In this work, by exploiting a phantom aneurysm model, we illustrate the correlation between experimental data and computational fluid dynamics (CFD) simulation results under well controlled conditions. This is difficult to achieve with clinical patient cases where several uncertainties are present. Quantitative measures are defined for CFD validation by virtual angiography. In addition, a parametric study has been carried out to systematically investigate the sensitivity of current measuring technique on the flow pattern.

Simulation of the image quality of an a-Si flat x-ray detector system in low-dose fluoroscopic applications

One of the issues in (alpha) -Si:H X-ray detectors is signal to noise ratio for low dose fluoroscopic applications. An optimized sensitivity of the X-ray detection system together with low and isotropic system noise characteristics are primary pre-conditions needed for maximum image quality. However, in spite of high DQE numbers of this Flat Detector technology in radiological and fluoroscopic application areas, a SNR for low dose fluoroscopy is found, which is inferior to that found with Image Intensifier-TV based systems. The problem area is a small dose range, producing gray levels just above absolute dark. Except for the dark level, these levels can (dependent on the application area) contain clinically relevant information. Since scatter affects the darker parts of the relevant image areas there will be noise in those areas, caused by X-ray quantum statistics and readout noise. The objective of the simulations is to investigate the influence of the various system noise components on the image quality. A level of system noise can be found where the subjective image quality is mainly determined by the X-ray quantum statistics and where the readout noise does not necessarily have to be invisible in totally dark parts. The simulation concerns a threshold contrast detail detectability (TCDD) observation test, where observers score discs of various diameter and absorption in an image sequence (being a fixed scene of the test object with (temporal) X-ray noise and system noise). The dynamic sequence is based upon total simulation, i.e. the test object as well as the X-ray noise and the system noise components were simulated. To verify the simulations also an image sequence was acquired on a Flat Detector system. The observations are done at various dose levels, with and without post processing to obtain noise reduction like it is used in clinical practice for this kind of system. The sequences are observed on a medical CRT display.

Image quality of digital radiography using flat detector technology

One of the most demanding applications in dynamic X-Ray imaging is Digital Subtraction Angiography (DSA). As opposed to other applications such as Radiography or Fluoroscopy, there has been so far limited attempts to introduce DSA with flat detector (FD) technology: Up to now, only part of the very demanding requirements could be taken into account. In order to enable an introduction of FD technology also in this area, a complete understanding of all physical phenomena related to the use of this technology in DSA is necessary. This knowledge can be used for detector design and performance optimization. Areas of research include fast switching between several detector operating modes (e.g. switching between fluoroscopy and high dose exposure modes and vice versa) and non stability during the DSA run e.g. due to differences in gain between subsequent images. Furthermore, effects of local and global X-Ray overexposure (due to direct radiation), which can cause temporal artifacts such as ghosting, may have a negative impact on the image quality. Pixel shift operations and image subtraction enhance the visibility of any artifact. The use of a refresh light plays an important role in the optimization process. Both an 18x18 cm2 as well as a large area 30x40 cm2 flat panel detector are used for studying the various phenomena. Technical measurements were obtained using complex imaging sequences representing the most demanding application conditions. Studies on subtraction test objects were performed and vascular applications have been carried out in order to confirm earlier findings. The basis for comparison of DSA is, still, the existing and mature IITV technology. The results of this investigation show that the latest generation of dynamic flat detectors is capable of handling this kind of demanding application. Not only the risk areas and their solutions and points of attention will be addressed, but also the benefits of present FD technology with respect to state-of-the-art IITV technology regarding DSA will be discussed.

Digital high-resolution high-performance CCD camera with 2048 x 2048 pixels for dynamic applications

A modular, multi-channel digital camera system has been designed.It provides a platform for the investigation of high-speed, high-resolution CCD sensors that are suited for dynamic medical x-ray imaging. The system supports frame rates from 30 frames per second (fps) for images with 1024 X 1024 pixels and 6-8 fps for images with 2048 X 2048 pixels. The complete system comprises camera head and correction circuits and yields a consecutive output stream of data. The camera electronics permits the testing of sensors with up to four parallel readout channels. It offers a variable bandwidth of up to 25 MHz per channel with a digitization resolution of 12 bits. The modulatory and programmability allow the adaptation of the camera system to a wide range of interesting applications. The camera electronics consists of a majority of sensor independent and a relatively small section of sensor dependent components. A detailed description of the camera, methods for performance analysis, measurement results, images, and a noise analysis are presented and discussed. As an example, linearity, MTF and noise of the camera system equipped with the FTF2020, a 2k X 2k CCD sensor from Phillips Professional Imaging have ben measured. The noise contains fixed pattern and random components. The developed camera architecture is capable of fulfilling the high requirements of dynamic, high-resolution medical imaging.