Novel iterative reconstruction method with optimal dose usage for partially redundant CT-acquisition.

In CT imaging, a variety of applications exist which are strongly SNR limited. However, in some cases redundant data of the same body region provide additional quanta. Examples in dual energy CT, the spatial resolution has to be compromised to provide good SNR for material decomposition. However, the respective spectral dataset of the same body region provides additional quanta which might be utilized to improve SNR of each spectral component. Perfusion CT is a high dose application, and dose reduction is highly desirable. However, a meaningful evaluation of perfusion parameters might be impaired by noisy time frames. On the other hand, the SNR of the average of all time frames is extremely high.In redundant CT acquisitions, multiple image datasets can be reconstructed and averaged to composite image data. These composite image data, however, might be compromised with respect to contrast resolution and/or spatial resolution and/or temporal resolution. These observations bring us to the idea of transferring high SNR of composite image data to low SNR 'source' image data, while maintaining their resolution.It has been shown that the noise characteristics of CT image data can be improved by iterative reconstruction (Popescu et al 2012 Book of Abstracts, 2nd CT Meeting (Salt Lake City, UT) p 148). In case of data dependent Gaussian noise it can be modelled with image-based iterative reconstruction at least in an approximate manner (Bruder et al 2011 Proc. SPIE 7961 79610J). We present a generalized update equation in image space, consisting of a linear combination of the previous update, a correction term which is constrained by the source image data, and a regularization prior, which is initialized by the composite image data. This iterative reconstruction approach we call bimodal reconstruction (BMR). Based on simulation data it is shown that BMR can improve low contrast detectability, substantially reduces the noise power and has the potential to recover spatial resolution of the source image data.For different CT applications: dual energy imaging, liver imaging, spiral imaging, cardiac imaging, we show that SNR can efficiently be transferred from the composite image to the source image data at constant patient dose, while maintaining resolution properties of the source data.

[Detection of intraorbital foreign material using MDCT]. / Nachweismöglichkeit intraorbitaler Fremdkörper durch MDCT.

AIM: To judge the possibilities of detection of orbital foreign bodies in multidetector CT (MDCT) with a focus on glass slivers. MATERIALS AND METHODS: Experimental systematic measuring of Hounsfield Units (HU) of 20 different materials, containing 16 different types of glass with 4 different types of ophthalmic lenses among them. The measurements were performed using a standardized protocol with an orbita phantom being scanned with 16-slice MDCT. Using the resulting density values, the smallest detectable volume was calculated. Using this data we produced slivers of 5 different glass types in the sub-millimeter range and calculated their volume. Those micro-slivers underwent another CT scan using the same protocol as mentioned above to experimentally discern and confirm the detection limit for micro-slivers made of different materials. RESULTS: Glass has comparatively high density values of at least 2000 HU. The density of glasses with strong refraction is significantly higher and reaches up to 12 400 HU. We calculated a minimum detectable volume of 0.07 mm (3) for glass with a density of 2000 HU. Only glass slivers with a density higher than 8300 HU were experimentally detectable in the sub-millimeter range up to a volume as small as 0.01 mm (3). Less dense glass slivers could not be seen, even though their volume was above the theoretically calculated threshold for detection. CONCLUSION: Due to its high density of at least 2000 HU, glass is usually easily recognizable as an orbital foreign body. The detection threshold depends on the object's density and size and can be as low as 0.01 mm (3) in the case of glass with strong refraction and thus high density. The detection of glass as an orbital foreign body seems to be secure for slivers with a volume of at least 0.2 mm (3) for all types of glass.

Strategies for scatter correction in dual source CT.

PURPOSE: Dual source CT (DSCT) systems utilize two measurement systems (A) and (B) offset by about 90 degrees. A special challenge in DSCT is cross-scattered radiation, i.e., scattered radiation from x-ray tube (B) detected in detector (A) and vice versa. Cross-scattered radiation can produce artifacts and degrade the contrast-to-noise ratio (CNR) of the images. Correction algorithms are mandatory to mitigate the negative effects of cross-scattered radiation. The purpose of this work is to describe and evaluate different methods for cross-scatter correction in DSCT. METHODS: The authors present two techniques for cross-scatter correction in DSCT. The first technique (1) is model-based. Assuming that cross-scatter is predominantly surface scatter, adequate knowledge about the surface of the scattering object is sufficient to describe the magnitude and distribution of cross-scatter. The relevant surface information is derived from an analysis of the raw-data sinogram during the CT-scan. The correction is performed by a table look-up into previously measured and stored cross-scatter distributions for a variety of objects with different surface characteristics. The second technique (2) is measurement-based. Dedicated sensors outside the penumbra of the fan beam in the z direction on both detectors (A) and (B) are used for an online measurement of both cross-scattered and forward scattered radiation during the CT-scan. In addition to the two scatter-correction techniques, the authors describe a low-pass filter method for the scatter-correction term with the goal to improve the CNR of the corrected images. This filter can be applied to both model-based (1) and measurement-based (2) scatter correction. Both scatter-correction techniques (1) and (2) are quantitatively assessed and the performance of the low-pass filter method is evaluated using DSCT data of phantoms (water cylinders and anthropomorphic phantoms) and DSCT patient scan data. RESULTS: Both scatter-correction techniques restore image contrasts and reduce cross-scatter induced artifacts in DSCT images. The measurement-based technique results in higher CNR than the model-based technique if the proposed low-pass filtering of the scatter-correction term is applied. Low-pass filtering improves the CNR of cross-scatter-correction approaches beyond the limits published in the literature [Engel et al., "X-ray scattering in single- and dual-source CT," Med. Phys. 35(1), 318-332 (2008)]. CONCLUSIONS: Both model-based and measurement-based scatter correction can mitigate the negative effects of cross-scatter in DSCT. The application of low-pass filtering to the scatter-correction term improves the CNR whenever the ratio of scattered radiation to total signal is high, as in larger patients.

Image reconstruction and image quality evaluation for a dual source CT scanner.

The authors present and evaluate concepts for image reconstruction in dual source CT (DSCT). They describe both standard spiral (helical) DSCT image reconstruction and electrocardiogram (ECG)-synchronized image reconstruction. For a compact mechanical design of the DSCT, one detector (A) can cover the full scan field of view, while the other detector (B) has to be restricted to a smaller, central field of view. The authors develop an algorithm for scan data completion, extrapolating truncated data of detector (B) by using data of detector (A). They propose a unified framework for convolution and simultaneous 3D backprojection of both (A) and (B) data, with similar treatment of standard spiral, ECG-gated spiral, and sequential (axial) scan data. In ECG-synchronized image reconstruction, a flexible scan data range per measurement system can be used to trade off temporal resolution for reduced image noise. Both data extrapolation and image reconstruction are evaluated by means of computer simulated data of anthropomorphic phantoms, by phantom measurements and patient studies. The authors show that a consistent filter direction along the spiral tangent on both detectors is essential to reduce cone-beam artifacts, requiring truncation of the extrapolated (B) data after convolution in standard spiral scans. Reconstructions of an anthropomorphic thorax phantom demonstrate good image quality and dose accumulation as theoretically expected for simultaneous 3D backprojection of the filtered (A) data and the truncated filtered (B) data into the same 3D image volume. In ECG-gated spiral modes, spiral slice sensitivity profiles (SSPs) show only minor dependence on the patient's heart rate if the spiral pitch is properly adapted. Measurements with a thin gold plate phantom result in effective slice widths (full width at half maximum of the SSP) of 0.63-0.69 mm for the nominal 0.6 mm slice and 0.82-0.87 mm for the nominal 0.75 mm slice. The visually determined through-plane (z axis) spatial resolution in a bar pattern phantom is 0.33-0.36 mm for the nominal 0.6 mm slice and 0.45 mm for the nominal 0.75 mm slice, again almost independent of the patient's heart rate. The authors verify the theoretically expected temporal resolution of 83 ms at 330 ms gantry rotation time by blur free images of a moving coronary artery phantom with 90 ms rest phase and demonstrate image noise reduction as predicted for increased reconstruction data ranges per measurement system. Finally, they show that the smoothness of the transition between image stacks acquired in different cardiac cycles can be efficiently controlled with the proposed approach for ECG-synchronized image reconstruction.

Novel ultrahigh resolution data acquisition and image reconstruction for multi-detector row CT.

We present and evaluate a special ultrahigh resolution mode providing considerably enhanced spatial resolution both in the scan plane and in the z-axis direction for a routine medical multi-detector row computed tomography (CT) system. Data acquisition is performed by using a flying focal spot both in the scan plane and in the z-axis direction in combination with tantalum grids that are inserted in front of the multi-row detector to reduce the aperture of the detector elements both in-plane and in the z-axis direction. The dose utilization of the system for standard applications is not affected, since the grids are moved into place only when needed and are removed for standard scanning. By means of this technique, image slices with a nominal section width of 0.4 mm (measured full width at half maximum=0.45 mm) can be reconstructed in spiral mode on a CT system with a detector configuration of 32 x 0.6 mm. The measured 2% value of the in-plane modulation transfer function (MTF) is 20.4 lp/cm, the measured 2% value of the longitudinal (z axis) MTF is 21.5 lp/cm. In a resolution phantom with metal line pair test patterns, spatial resolution of 20 lp/cm can be demonstrated both in the scan plane and along the z axis. This corresponds to an object size of 0.25 mm that can be resolved. The new mode is intended for ultrahigh resolution bone imaging, in particular for wrists, joints, and inner ear studies, where a higher level of image noise due to the reduced aperture is an acceptable trade-off for the clinical benefit brought about by the improved spatial resolution.

Image reconstruction and image quality evaluation for a 64-slice CT scanner with z-flying focal spot.

We present a theoretical overview and a performance evaluation of a novel z-sampling technique for multidetector row CT (MDCT), relying on a periodic motion of the focal spot in the longitudinal direction (z-flying focal spot) to double the number of simultaneously acquired slices. The z-flying focal spot technique has been implemented in a recently introduced MDCT scanner. Using 32 x 0.6 mm collimation, this scanner acquires 64 overlapping 0.6 mm slices per rotation in its spiral (helical) mode of operation, with the goal of improved longitudinal resolution and reduction of spiral artifacts. The longitudinal sampling distance at isocenter is 0.3 mm. We discuss in detail the impact of the z-flying focal spot technique on image reconstruction. We present measurements of spiral slice sensitivity profiles (SSPs) and of longitudinal resolution, both in the isocenter and off-center. We evaluate the pitch dependence of the image noise measured in a centered 20 cm water phantom. To investigate spiral image quality we present images of an anthropomorphic thorax phantom and patient scans. The full width at half maximum (FWHM) of the spiral SSPs shows only minor variations as a function of the pitch, measured values differ by less than 0.15 mm from the nominal values 0.6, 0.75, 1, 1.5, and 2 mm. The measured FWHM of the smallest slice ranges between 0.66 and 0.68 mm at isocenter, except for pitch 0.55 (0.72 mm). In a centered z-resolution phantom, bar patterns up to 15 lp/cm can be visualized independent of the pitch, corresponding to 0.33 mm longitudinal resolution. 100 mm off-center, bar patterns up to 14 lp/cm are visible, corresponding to an object size of 0.36 mm that can be resolved in the z direction. Image noise for constant effective mAs is almost independent of the pitch. Measured values show a variation of less than 7% as a function of the pitch, which demonstrates correct utilization of the applied radiation dose at any pitch. The product of image noise and square root of the slice width (FWHM of the respective SSP) is the same constant for all slices except 0.6 mm. For the thinnest slice, relative image noise is increased by 17%. Spiral windmill-type artifacts are effectively suppressed with the z-flying focal spot technique, which has the potential to maintain a low artifact level up to pitch 1.5, in this way increasing the maximum volume coverage speed that can be clinically used.

[On the way to isotopic spatial resolution: technical principles and applications of 16-slice CT]. / Auf dem Weg zur isotropen räumlichen Auflösung: Technische Grundlagen und Anwendungen der 16-Schicht-CT.

The broad introduction of multi-slice CT by all major vendors in 1998 was a milestone with regard to extended volume coverage, improved axial resolution and better utilization of the tube output. New clinical applications such as CT-examinations of the heart and the coronary arteries became possible. Despite all promising advances, some limitations remain for 4-slice CT systems. They come close to isotropic resolution, but do not fully reach it in routine clinical applications. Cardiac CT-examinations require careful patient selection. The new generation of multi-slice CT-systems offer simultaneous acquisition of up to 16 sub-millimeter slices and improved temporal resolution for cardiac examinations by means of reduced gantry rotation time (0.4 s). In this overview article we present the basic technical principles and potential applications of 16-slice technology for the example of a 16-slice CT-system (SOMATOM Sensation 16, Siemens AG, Forchheim). We discuss detector design and dose efficiency as well as spiral scan- and reconstruction techniques. At comparable slice thickness, 16-slice CT-systems have a better dose efficiency than 4-slice CT-systems. The cone-beam geometry of the measurement rays requires new reconstruction approaches, an example is the adaptive multiple plane reconstruction, AMPR. First clinical experience indicates that sub-millimeter slice width in combination with reduced gantry rotation-time improves the clinical stability of cardiac examinations and expands the spectrum of patients accessible to cardiac CT. 16-slice CT-systems have the potential to cover even large scan ranges with sub-millimeter slices at considerably reduced examination times, thus approaching the goal of routine isotropic imaging.

Performance evaluation of a 64-slice CT system with z-flying focal spot.

The meanwhile established generation of 16-slice CT systems enables routine sub-millimeter imaging at short breath-hold times. Clinical progress in the development of multidetector row CT (MDCT) technology beyond 16 slices can more likely be expected from further improvement in spatial and temporal resolution rather than from a mere increase in the speed of volume coverage. We present an evaluation of a recently introduced 64-slice CT system (SOMATOM Sensation 64, Siemens AG, Forchheim, Germany), which uses a periodic motion of the focal spot in longitudinal direction (z-flying focal spot) to double the number of simultaneously acquired slices. This technique acquires 64 overlapping 0.6 mm slices per rotation. The sampling scheme corresponds to that of a 64 x 0.3 mm detector, with the goal of improved longitudinal resolution and reduced spiral artifacts. After an introduction to the detector design, we discuss the basics of z-flying focal spot technology (z-Sharp). We present phantom and specimen scans for performance evaluation. The measured full width at half maximum (FWHM) of the thinnest spiral slice is 0.65 mm. All spiral slice widths are almost independent of the pitch, with deviations of less than 0.1 mm from the nominal value. Using a high-resolution bar pattern phantom (CATPHAN, Phantom Laboratories, Salem, NY), the longitudinal resolution can be demonstrated to be up to 15 lp/cm at the isocenter independent of the pitch, corresponding to a bar diameter of 0.33 mm. Longitudinal resolution is only slightly degraded for off-center locations. At a distance of 100 mm from the isocenter, 14 lp/cm can be resolved in the z-direction, corresponding to a bar diameter of 0.36 mm. Spiral "windmill" artifacts presenting as hyper- and hypodense structures around osseous edges are effectively reduced by the z-flying focal spot technique. Cardiac scanning benefits from the short gantry rotation time of 0.33 s, providing up to 83 ms temporal resolution with 2-segment ECG-gated reconstruction.

Image reconstruction and performance evaluation for ECG-gated spiral scanning with a 16-slice CT system.

We present an image reconstruction approach and a performance evaluation for ECG-gate cardiac spiral scanning with recently introduced 16-slice CT equipment. We present an extension of the Adaptive Cardio Volume (ACV) reconstruction approach for ECG-gated multislice spiral scanning. We discuss the image z reformation introduced to control the spiral slice width of the final images and give an overview of the reformation functions chosen. We investigate image quality and discuss the maximum number of slices that can be reconstructed without severe cone-beam artifacts. Slice sensitivity profiles (SSPs) and transverse resolution are evaluated as a function of the patient's heart rate. We demonstrate the influence of slice width on the visualization of stents and plaques and show the impact of reduced gantry rotation time (0.42 s) on temporal resolution. Deviating from general purpose spiral scanning cone-beam reconstruction is not required for ECG-gated cardiac CT with up to 16 slices. Using the ACV approach with image reformation, SSPs are well defined and independent of the patient's heart rate. With 0.75 mm collimated slice width, the measured full width at half-maximum (FWHM) of the smallest reconstructed slice is about 0.83 mm. Using this slice width and overlapping image reconstruction, cylindrical holes 0.6-0.7 mm in diameter can be resolved in a z-resolution phantom. Adequate visualization of the coronary arteries requires reconstruction slice widths not larger than 1.5 mm. Visualization of stents and severe calcifications is significantly improved with sub-mm slice width. Experimental evidence for the theoretically predicted temporal resolution and for the variation of temporal resolution depending on the position in the field of measurement (FOM) is presented. With 0.42 s gantry rotation temporal resolution reaches its optimum of 105 ms in the center of the FOM at 81 bpm. First scans on human subjects demonstrate the potential to expand the range of heart rates accessible to routine clinical examinations. A 16-slice platform can cover the heart with sub-mm slices within short breath-hold times, allowing for improved cardiac imaging due to isotropic sub-mm spatial resolution.

Image reconstruction and image quality evaluation for a 16-slice CT scanner.

We present a theoretical overview and a performance evaluation of a novel approximate reconstruction algorithm for cone-beam spiral CT, the adaptive multiple plane reconstruction (AMPR), which has been introduced by Schaller, Flohr et al. [Proc. SPIE Int. Symp. Med. Imag. 4322, 113-127 (2001)] AMPR has been implemented in a recently introduced 16-slice CT scanner. We present a detailed algorithmic description of AMPR which allows for a free selection of the spiral pitch. We show that dose utilization is better than 90% independent of the pitch. We give an overview on the z-reformation functions chosen to allow for a variable selection of the spiral slice width at arbitrary pitch values. To investigate AMPR image quality we present images of anthropomorphic phantoms and initial patient results. We present measurements of spiral slice sensitivity profiles (SSPs) and measurements of the maximum achievable transverse resolution, both in the isocenter and off-center. We discuss the pitch dependence of image noise measured in a centered 20 cm water phantom. Using the AMPR approach, cone-beam artifacts are considerably reduced for the 16-slice scanner investigated. Image quality in MPRs is independent of the pitch and equivalent to a single-slice CT system at pitch p approximately 1.5. The full width at half-maximum (FWHM) of the spiral SSPs shows only minor variations as a function of the pitch, nominal, and measured values differ by less than 0.2 mm. With 16 x 0.75 mm collimation, the measured FWHM of the smallest reconstructed slice is about 0.9 mm. Using this slice width and overlapping image reconstruction, cylindrical holes with 0.6 mm diameter can be resolved in a z-resolution phantom. Image noise for constant effective mAs is nearly independent of the pitch. Measured and theoretically expected dose utilization are in good agreement. Meanwhile, clinical practice has demonstrated the excellent image quality and the increased diagnostic capability that is obtained with the new generation of multislice CT systems.

[CT-angiography of the carotid artery: First results with a novel 16-slice-spiral-CT scanner]. / CT-Angiographie der A. carotis: Erste Erfahrungen mit einem 16-Schicht-Spiral-CT.

PURPOSE: To evaluate a novel multislice CT system (16-slice-spiral-CT scanner) for the diagnosis of carotid artery stenosis. MATERIAL AND METHODS: Five patients with symptomatic atherosclerotic disease of the carotid arteries were examined with a 16- slice-spiral-CT scanner. Collimation was 16 x 0.75 mm, table speed 36 mm/s (pitch of 1.5), rotation time 0.5 s, tube current was 160 eff.mAs at 120 kV. 60 ml of contrast material were injected with a power injector followed by a saline flush. The start delay was measured with test bolus method (20 ml CM). Interactive multiplanar reformation (iMPR) and thin slab MIP as well as volume rendering were used for image evaluation and presentation. RESULTS: Scan time was 9 s for a range of 300 mm. This allowed imaging the whole length of the carotid artery (aortic arch to circle of Willis) in a true arterial phase. Pulsation artefacts did not impair the evaluation of the vessels at the level of the aortic arch. Overall image quality of both "source images" and 3D-reconstructions was excellent, due to a reduced voxel size of 0.03 mm (3). Image evaluation and postprocessing (iMPR, MIP) was done within 15 min. iMPR was highly accurate for demonstrating plaque morphology and determining the percentage of the stenosis. CONCLUSION: For the first time, true arterial phase images of the entire carotid artery with high spatial resolution could be acquired using a 16-slice-spiral-CT scanner. This method offers the potential to replace catheter angiography in the evaluation of carotid artery stenosis.

New technical developments in multislice CT--Part 1: Approaching isotropic resolution with sub-millimeter 16-slice scanning.

The introduction of multislice CT was a breakthrough with regard to increased scan speed, improved axial resolution and better utilization of the tube output. The new generation of multislice CT scanners offering simultaneous acquisition of up to 16 sub-millimeter slices represents an important leap on the way towards true isotropic scanning. We present an evaluation of a state-of-the-art 16-slice CT system (SOMATOM Sensation 16, Siemens AG, Forchheim, Germany). After an introduction to the detector design we discuss dose utilization and finally elaborate on multislice spiral scanning with 16 slices. Due to the increased number of slices dose utilization is improved compared to current 4-slice CT scanners, and sub-millimeter collimation needs no longer be restricted to special applications. For CT systems with 8 or more slices, the cone-beam geometry causes severe artifacts if not corrected for by a so-called cone-correction, which thus becomes mandatory in this case. With the Adaptive Multiple Plane Reconstruction AMPR, cone beam artifacts are effectively suppressed, while the benefits of Adaptive Axial Interpolation are maintained: free selection of the spiral pitch according to the clinical needs of an examination, slice width independent of the pitch, full dose utilization at all pitch values. Clinical practice will have to demonstrate the application spectrum that is opened with the new generation of multislice CT systems.

New technical developments in multislice CT, part 2: sub-millimeter 16-slice scanning and increased gantry rotation speed for cardiac imaging.

Despite all promising advances, some challenges remain for ECG-gated multislice CT examinations of the heart and the coronary arteries with current 4-slice detectors: adequate visualization of stents and severely calcified coronary arteries, examination of patients with higher heart rates and patients, who cannot adequately hold their breath for at least 30 sec. The new generation of multislice CT systems offering simultaneous acquisition of up to 16 sub-millimeter slices and gantry rotation times shorter than 0.5 sec has the potential to overcome these limitations. We describe the technical principles of cardiac scanning with a state-of-the-art 16-slice CT equipment (SOMATOM Sensation 16, Siemens AG, Forchheim, Germany). We discuss an extension of the Adaptive Cardio Volume (ACV) reconstruction approach for ECG-gated multislice spiral CT. We show the impact of reduced gantry rotation time (0.42 sec) on temporal resolution, and we demonstrate the influence of slice width on the visualization of stents and plaques. Deviating from general purpose applications a cone-correction is not required for cardiac scanning with 16-slice CT systems. In addition to the absolute improvement, the temporal resolution shows a different dependence on the patient's heart rate for 0.42 sec rotation time, reaching its optimum (105 msec) at 81 BPM. This has the potential to expand the range of heart rates accessible to routine clinical examinations. Owing to sub-millimeter slice width and optimized in-plane resolution characteristics, visualization of stents and severe calcifications in coronary arteries is significantly improved. Clinical experience will be needed to fully evaluate the potential of 16-slice technology for cardiac imaging.

Single-slice rebinning reconstruction in spiral cone-beam computed tomography.

At the advent of multislice computed tomography ICT) a variety of approximate cone-beam algorithms have been proposed suited for reconstruction of small cone-angle CT data in a spiral mode of operation. The goal of this study is to identify a practical and efficient approximate cone-beam method, extend its potential for medical use, and demonstrate its performance at medium cone-angles required for area detector CT. We will investigate two different approximate single-slice rebinning algorithms for cone-beam CT: the multirow Fourier reconstruction (MFR) and an extension of the advanced single-slice rebinning method (ASSR), which combines the idea of ASSR with a z-filtering approach. Thus, both algorithms, MFR and ASSR, are formulated in the framework of z-filtering using optimized spiral interpolation algorithms. In each view, X-ray samples to be used for reconstruction are identified, which describe an approximation to a virtual reconstruction plane. The performance of approximate reconstruction should improve as the virtual reconstruction plane better fits the spiral focus path. The image quality of the respective reconstruction will be assessed with respect to image artifacts, spatial resolution, contrast resolution, and image noise. It turns out that the ASSR method using tilted reconstruction planes is a practical and efficient algorithm, providing image quality comparable to that of a single-row scanning system even with a 46-row detector at a table feed of 64 mm. Both algorithms tolerate any table feed below the maximum value associated to the detector height. Due to the z-filter approach, all detector data sampled can be used for image reconstruction.

Self-normalizing method to measure the detective quantum efficiency of a wide range of x-ray detectors.

The detective quantum efficiency (DQE) is widely accepted as the most relevant parameter to characterize the image quality of medical x-ray systems. In this article we describe a solid method to measure the DQE. The strength of the method lies in the fact that it is self-normalizing so measurements at very low spatial frequencies are not needed. Furthermore, it works on any system with a response function which is linear in the small-signal approximation. We decompose the DQE into several easily accessible quantities and discuss in detail how they can be measured. At the end we lead the interested reader through an example. Noise equivalent quanta and normalized contrast values are tabulated for standard radiation qualities.