A nonparametric measure of contrast in x-ray images.

Objective.We propose a nonparametric figure of merit, the contrast equivalent distance CED, to measure contrast directly from clinical images.Approach.A relative brightness distanceδis calculated by making use of the order statistic of the pixel values. By multiplyingδwith the grey value rangeR, the mean brightness distance MBD is obtained. From the MBD, the CED and the distance-to-noise ratio DNR can be derived. The latter is the ratio of the MBD and a previously suggested nonparametric measureτfor the noise. Since the order statistic is independent of the spatial arrangement of the pixel values, the measures can be obtained directly from clinical images. We apply the new measures to mammography images of an anthropomorphic phantom and of a phantom with a step wedge as well as to CT images of a head phantom.Main results.For low-noise images of a step wedge, the MBD is equivalent to the conventional grey value distance. While this measure permits the evaluation of clinical images, it is sensitive to noise. Therefore, noise has to be quantified at the same time. When the ratioσ/τof the noise standard deviationσtoτis available, validity limits for the CED as a measure of contrast can be established. The new figures of merit can be calculated for entire images as well as on regions of interest (ROI) with an edge length not smaller than 32 px.Significance.The new figures of merit are suited to quantify the quality of clinical images without relying on the assumption of a linear, shift-invariant system. They can be used for any kind of greyscale image, provided the ratioσ/τcan be estimated. This will hopefully help to achieve the optimisation of image quality vs dose required by radioprotection laws.

A nonparametric measure of noise in x-ray diagnostic images-mammography.

Objective.In x-ray diagnostics, modern image reconstruction or image processing methods may render established methods of image quality assessment inadequate. Task specific quality assessment by using model observers has the disadvantage of being very labour-intensive. Therefore, it appears highly desirable to develop novel image quality parameters that neither rely on the linearity and the shift-invariace of the imaging system nor require the acquisition of hundreds of images as is necessary for the application of model observers, and which can be derived directly from diagnostic images.Approach.A new measure for the noise based on non-maximum-suppression images is defined and its properties are explored using simulated images before it is applied to an exposure series of mammograms of a homogeneous phantom and a 3D-printed breast phantom to demonstrate its usefulness under realistic conditions.Main results.The new noise parameter cannot only be derived from images with a homogeneous background but it can be extracted directly from images containing anatomic structures and is proportional to the standard deviation of the noise. At present, the applicability is restricted to mammography, which satisfies the assumption of short covariance length of the noise.Significance.The new measure of the noise is but a first step of the development of a set of parameters that are required to quantify image quality directly from diagnostic images without relying on the assumption of a linear, shift-invariant system, e.g. by providing measures of sharpness, contrast and structural complexity, in addition to the noise measure. For mammography, a convenient method is now available to quantify noise in processed diagnostic images.

The regression detectability index RDI for mammography images of breast phantoms with calcification-like objects and anatomical background.

Currently, quality assurance measurements in mammography are performed on unprocessed images. For diagnosis, however, radiologists are provided with processed images. This image processing is optimised for images of human anatomy and therefore does not always perform satisfactorily with technical phantoms. To overcome this problem, it may be possible to use anthropomorphic phantoms reflecting the anatomic structure of the human breast in place of technical phantoms when carrying out task-specific quality assessment using model observers. However, the use of model observers is hampered by the fact that a large number of images needs to be acquired. A recently published novel observer called the regression detectability index (RDI) needs significantly fewer images, but requires the background of the images to be flat. Therefore, to be able to apply the RDI to images of anthropomorphic phantoms, the anatomic background needs to be removed. For this, a procedure in which the anatomical structures are fitted by thin plate spline (TPS) interpolation has been developed. When the object to be detected is small, such as a calcification-like lesion, it is shown that the anatomic background can be removed successfully by subtracting the TPS interpolation, which makes the background-free image accessible to the RDI. We have compared the detectability obtained by the RDI with TPS background subtraction to results of the channelized Hotelling observer (CHO) and human observers. With the RDI, results for the detectabilityd'can be obtained using 75% fewer images compared to the CHO, while the same uncertainty ofd'is achieved. Furthermore, the correlation ofd'(RDI) with the results of human observers is at least as good as that ofd'(CHO) with human observers.

Assessment of patient organ dose in CT virtual colonoscopy for bowel cancer screening.

Justification and optimisation form the basic elements for the radiological protection of individuals for medical exposures. Justification includes the assessment of patient organ doses from which radiation risks are deduced. Medical radiation exposures are justified only in the case of a sufficient net benefit. For screening examinations, such as CT virtual colonoscopy, this implies that patient organ doses should be relatively low to minimise the radiation detriment. Image quality should be sufficient to maximise the potential diagnostic benefits. The Medical Exposures Directive places special attention on medical exposures as part of health screening programmes and examinations involving high individual doses to the patient, both of which apply to CT virtual colonoscopy. Technical factors were recorded for a series of patients having virtual colonoscopy on a CT scanner. In addition, the dose-length product was assessed. Patient organ doses were deduced using a CT dose calculation program. The typical effective dose was 7.5 mSv for male patients and 10.2 mSv for female patients. The effective dose is higher for female patients, as some gender-specific organs are irradiated during virtual colonoscopy. Each patient has two series of scans resulting in doses of 15 mSv for male patients and 20 mSv for female patients.

Average glandular dose in digital mammography and digital breast tomosynthesis: comparison of phantom and patient data.

For the evaluation of the average glandular dose (AGD) in digital mammography (DM) and digital breast tomosynthesis (DBT) phantoms simulating standard model breasts are used. These phantoms consist of slabs of polymethyl methacrylate (PMMA) or a combination of PMMA and polyethylene (PE). In the last decades the automatic exposure control (AEC) increased in complexity and became more sensitive to (local) differences in breast composition. The question is how well the AGD estimated using these simple dosimetry phantoms agrees with the average patient AGD. In this study the AGDs for both dosimetry phantoms and for patients have been evaluated for 5 different x-ray systems in DM and DBT modes. It was found that the ratios between patient and phantom AGD did not differ considerably using both dosimetry phantoms. These ratios averaged over all breast thicknesses were 1.14 and 1.15 for the PMMA and PMMA-PE dosimetry phantoms respectively in DM mode and 1.00 and 1.02 in the DBT mode. These ratios were deemed to be sufficiently close to unity to be suitable for dosimetry evaluation in quality control procedures. However care should be taken when comparing systems for DM and DBT since depending on the AEC operation, ratios for particular breast thicknesses may differ substantially (0.83-1.96). Although the predictions of both phantoms are similar we advise the use of PMMA + PE slabs for both DM and DBT to harmonize dosimetry protocols and avoid any potential issues with the use of spacers with the PMMA phantoms.

Guideline for determining the mean glandular dose according to DIN 6868-162 and threshold contrast visibility according to the quality assurance guideline for digital mammography systems.

As part of the physico-technical quality assurance of the German breast cancer screening program, the threshold contrast visibility and the average glandular dose of every digital mammography system have to fulfill the requirements of the "European guidelines for quality assurance in breast cancer screening and diagnosis" (4th Edition). To accomplish uniform measurements in all federal states of Germany, the physical board of the reference centers developed a special guideline in 2009. Due to recent changes in the guidelines and standards, a second version of the guideline was developed by the reference centers. This guideline describes the determination of the average glandular dose as well as the CDMAM image acquisition and the CDMAM image evaluation. The determination of the threshold contrast visibility can be performed visually or automatically. The determination of the average glandular dose is based on DIN 6868 - 162 and the threshold contrast visibility test is based on the German "Quality Assurance Guideline".

[Guideline for the additional test positions according to the EPQC 4th Edition for Digital Mammography Systems]. / Prüfanleitung für die Durchführung der ergänzenden Prüfpositionen nach EPQC 4th Edition für digitale Mammografie-Systeme.

Within the physical-technical quality assurance of the German breast cancer screening program all digital mammography systems have to perform the contrast resolution test and the determination of the average glandular dose based on the European guidelines for quality assurance in breast cancer screening and diagnosis (4th Edition). Since 1.1.2009 this applies to digital systems outside the screening program too. To accomplish uniform measurements in all federal states of Germany, the physical board of the reference centers developed together a special guideline for these test position. This Guideline describes the determination of the average glandular dose for different types of mammography systems, the CDMAM image acquisition and the CDMAM image evaluation as well. This guideline was verified by the German task group "Röntgenverordnung".