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Coronary angiography is the primary technique for diagnosing coronary abnormalities as it is able to locate precisely the coronary artery lesions. However, the clinical relevance of an appearing stenosis is not that easy to assess. In previous work we have analyzed the myocardial perfusion by comparing basal and hyperemic coronary flow. This comparison is the basis of a Relative Coronary Flow Reserve (RCFR) measure. In a Region-of-Interest (ROI) on the angiogram the contrast is measured as a function of time (the so-called time-density curve). The required hyperemic state of exercise is induced artificially by the injection of a vasodilator drug e.g. papaverine. In previous work we have presented the results of a small study of 20 patients. In this paper we present an analysis of the sensitivity of the method for variations in Xray exposure between the two runs due to the Automatic Exposure Control (AEC) unit. The AEC is a system unit with the task to ensure a constant dose rate at the entrance of the detector by making the appropriate adaptations in X-ray factor settings for patients which range from slim to more obese. We have setup a phantom study to reveal the expected exposure variations. We present several of the developed phantoms together with a compensation strategy.

This work aims at defining an information-theoretic quality assessment technique for cardiovascular X-ray images, using a full-reference scheme (relying on averaging a sequence to obtain a noiseless reference). With the growth of advanced signal processing in medical imaging, such an approach will enable objective comparisons of the quality of processed images. A concept for describing the quality of an image is to express it in terms of its information capacity. Shannon has derived this capacity for noisy channel coding. However, for X-ray images, the noise is signal-dependent and non-additive, so that Shannon's theorem is not directly applicable. To overcome this complication, we exploit the fact that any invertible mapping on a signal does not change its information content. We show that it is possible to transform the images in such a way that the Shannon theorem can be applied. A general method for calculating such a transformation is used, given a known relation between signal mean and noise standard deviation. After making the noise signal-independent, it is possible to assess the information content of an image and to calculate an overall quality metric (e.g. information capacity) which includes the effects of sharpness, contrast and noise. We have applied this method on phantom images under different acquisition conditions and computed the information capacity for those images. We aim to show that the results of this assessment are consistent with variations in noise, contrast and sharpness, introduced by system settings and image processing.

Radiation exposure due to medical imaging is a topic of emerging importance. In Europe this topic has been dealt with for a long time and in other countries it is getting more and more important and it gets an aspect of public interest in the latest years. This is mainly true due to the fact that the average dose per person in developed countries is increasing rapidly since threedimensional imaging is getting more and more available and useful for diagnosis. This paper introduces the most common dose quantities used in medical radiation exposure characterization, discusses usual ways for determination of such quantities as well as some considerations how these values are linked to radiation risk estimation. For this last aspect the paper will refer to the linear non threshold theory for an imaging application.

The international standard IEC 62220-1-2 defines the measurement procedure for determination of the detective quantum efficiency (DQE) of digital x-ray imaging devices used in mammography. A mobile setup complying to this standard and adaptable to most current systems was constructed in the Helmholtz Zentrum München to allow for an objective technical comparison of current full field digital mammography units employed in mammography screening in Germany. This article demonstrates the setup's capabilities with a focus on the measurement uncertainties of all quantities contributing to DQE measurements. Evaluation of uncertainties encompasses results from measurements on a Sectra Microdose Mammography in clinical use, as well as on a prototype of a Fujifilm Amulet system at various radiation qualities. Both systems have a high spatial resolution of 50 μm × 50 μm. The modulation transfer function (MTF), noise power spectrum (NPS) and DQE of the Sectra MDM are presented in comparison to results previously published by other authors.

An angular parameterization of parallel Radon projections referred to in this paper as ψ-parameterization is discussed in relevance to the efficiency of reconstruction from fan data. The fact that the ψ-parameterization coincides with the equiangular fan beam parameterization allows us to develop a simple and efficient approach useful for the reconstruction from fan data. Within this approach parallel projections are approximated by groups of semi-parallel rays. The reconstruction is carried out directly, i.e. without any modification of original data, at the speed which is comparable or even higher than that of the parallel Filtered Back Projection (FBP) algorithm.

In order to compare different imaging systems, it is necessary to obtain detailed information about the system noise, its deterministic properties and task specic signal-to-noise ratio (SNR). The current standard method for characterizing noise in CT scanners is based on the pixel standard deviation of the image of a water-equivalent uniform phantom. The Fourier-based noise power spectrum (NPS)improves on the limitations of the pixel standard deviation by accounting for noise correlations. However, it has been shown that the Fourier-methods used to describe the system performance result in systematic errors as they make some limiting assumptions such as shift invariance and wide sense stationarity, which are not satised by real CT systems. For a more general characterization of the imaging system noise, a covariance matrix eigenanalysis can be performed. In this paper we present the experimental methodology for the evaluation of the noise of computed tomography systems. We used a bench-top at-panel-based cone-beam CT scanner and a cylindrical water-lled PMMA phantom. For the 3-dimensional reconstructed volume, we calculated the covariance matrix, its eigenvectors and eigenvalues for the xy-plane as well as for the yz-plane, and compared the results with the NPS. Furthermore, we analyzed the location-specic noise in the images. The evaluation of the noise is a rst step toward determining the task-specic SNR.

Novel geometrical designs of computed tomography (CT) scanners in combination with novel image reconstruction algorithms promise to reduce ionizing radiation exposure to the patient in CT scans. While the sampling density of the Field Of View (FOV) is retained, the image quality can even be increased in contrast to conventional CT scanners. In this study, we present first images obtained with a novel CT scanner that we developed in our working group. In this open CT system with irradiation within a fan beam, parallel Radon data are directly obtained for image reconstruction using the OPED (Orthogonal Polynomial Expansion on the Disk) algorithm. This algorithm uses Radon data directly, i.e., without any further data processing such as rebinning and interpolation. We experimentally test theoretical predictions for this system by quantifying image quality parameters in comparison with corresponding parameters that are derived from the images of a conventional scanner of the 3rd generation. The modulation transfer function (MTF) and noise power spectrum (NPS) are determined using a test phantom. The novel CT system quantitatively shows the same noise property as the conventional scanner. The resolution that is reached in the center of a reconstructed image is nearly identical for both scanner types. But we found that the resolution that is achieved in the novel CT system does not depend on the image position while the MTF of the conventional scanner decreases for radially outer regions of the image.