Bernhard Renger

Initial performance characterization of a clinical noise-suppressing reconstruction algorithm for MDCT.

OBJECTIVE The number of CT examinations is increasing relatively dramatically, hence the radiation dose of the associated population. Thus, there is a need for efficient reconstruction methods with dose reduction potential that also maintain the image quality. In this article, we present the initial performance evaluation of such a reconstruction algorithm (iDose, Philips Healthcare). MATERIALS AND METHODS iDose is a hybrid iterative reconstruction algorithm that provides enhanced image quality while reducing the radiation dose compared with the current clinical standard reconstruction. To quantify the advantages of this algorithm in image quality and dose reduction, we compared iDose with the conventional filtered back projection algorithm. Furthermore, we describe the performance of iDose with respect to several image quality metrics. RESULTS The HU values remain stable while employing iDose. With iDose, the noise is significantly reduced. This is reflected by an improvement in the contrast-to-noise ratio and in the noise power spectrum compared with a standard reconstruction. The measurements of the modulation transfer function confirm that, with iDose, there is no decline in spatial resolution. CONCLUSION We conclude that iDose is an important tool in the reduction of radiation dose in CT. However, continuous efforts to reduce radiation dose should be pursued.

Does iterative reconstruction lower CT radiation dose: evaluation of 15,000 examinations.

Purpose Evaluation of 15,000 computed tomography (CT) examinations to investigate if iterative reconstruction (IR) reduces sustainably radiation exposure. Method and Materials Information from 15,000 CT examinations was collected, including all aspects of the exams such as scan parameter, patient information, and reconstruction instructions. The examinations were acquired between January 2010 and December 2012, while after 15 months a first generation IR algorithm was installed. To collect the necessary information from PACS, RIS, MPPS and structured reports a Dose Monitoring System was developed. To harvest all possible information an optical character recognition system was integrated, for example to collect information from the screenshot CT-dose report. The tool transfers all data to a database for further processing such as the calculation of effective dose and organ doses. To evaluate if IR provides a sustainable dose reduction, the effective dose values were statistically analyzed with respect to protocol type, diagnostic indication, and patient population. Results IR has the potential to reduce radiation dose significantly. Before clinical introduction of IR the average effective dose was 10.1±7.8mSv and with IR 8.9±7.1mSv (p*=0.01). Especially in CTA, with the possibility to use kV reduction protocols, such as in aortic CTAs (before IR: average14.2±7.8mSv; median11.4mSv /with IR:average9.9±7.4mSv; median7.4mSv), or pulmonary CTAs (before IR: average9.7±6.2mSV; median7.7mSv /with IR: average6.4±4.7mSv; median4.8mSv) the dose reduction effect is significant(p*=0.01). On the contrary for unenhanced low-dose scans of the cranial (for example sinuses) the reduction is not significant (before IR:average6.6±5.8mSv; median3.9mSv/with IR:average6.0±3.1mSV; median3.2mSv). Conclusion The dose aspect remains a priority in CT research. Iterative reconstruction algorithms reduce sustainably and significantly radiation dose in the clinical routine. Our results illustrate that not only in studies with a limited number of patients but also in the clinical routine, IRs provide long-term dose saving.

Automatic bone segmentation technique for CT angiographic studies

PURPOSE The purpose of this work was to develop and evaluate an automatic bone segmentation technique for CT angiographic studies. METHOD An automatic bone segmentation scheme was developed and applied to 40 CT examinations. The results of the segmentation were evaluated subjectively by two radiologists. RESULTS The bone segmentation was, on average, rated between excellent and good. Automatic segmentation required approximately 25 s/case. CONCLUSION With this high quality technique, bone can be segmented easily and accurately and subsequently can be removed from CT data sets for further 3D visualization and analysis of various organs.

In-vivo X-ray Dark-Field Chest Radiography of a Pig

X-ray chest radiography is an inexpensive and broadly available tool for initial assessment of the lung in clinical routine, but typically lacks diagnostic sensitivity for detection of pulmonary diseases in their early stages. Recent X-ray dark-field (XDF) imaging studies on mice have shown significant improvements in imaging-based lung diagnostics. Especially in the case of early diagnosis of chronic obstructive pulmonary disease (COPD), XDF imaging clearly outperforms conventional radiography. However, a translation of this technique towards the investigation of larger mammals and finally humans has not yet been achieved. In this letter, we present the first in-vivo XDF full-field chest radiographs (32 × 35 cm2) of a living pig, acquired with clinically compatible parameters (40 s scan time, approx. 80 µSv dose). For imaging, we developed a novel high-energy XDF system that overcomes the limitations of currently established setups. Our XDF radiographs yield sufficiently high image quality to enable radiographic evaluation of the lungs. We consider this a milestone in the bench-to-bedside translation of XDF imaging and expect XDF imaging to become an invaluable tool in clinical practice, both as a general chest X-ray modality and as a dedicated tool for high-risk patients affected by smoking, industrial work and indoor cooking.

A clinical comparison study of a novel statistical iterative and filtered backprojection reconstruction

The conventional filtered backprojection (FBP) algorithm employed in reduced dose MDCT acquisitions provides low reconstruction quality, e.g. high noise level, and many artifacts. Thus, there is the need for efficient reconstruction methods that have dose reduction potential while providing high reconstruction quality. In this work we present a comparison study between a statistical iterative reconstruction algorithm called iDose and the FBP algorithm. iDose is a hybrid iterative reconstruction algorithm which provides enhanced image quality while reducing the radiation dose compared to conventional algorithms. We report on the performance of the two algorithms with respect to uniformity, noise characteristics, spatial resolution, and patient studies. With respect to the uniformity of the Hounsfield Units (HU), we found that the mean HU value remains stable while employing iDose. With iDose the noise is significantly reduced. This is reflected by an improvement in the contrast-to-noise ratio and in the noise-power-spectrum compared to the FBP. The measurements of the modulation-transfer-function confirm that with iDose there is no decline in spatial resolution. In clinical studies, slices reconstructed with the iDose algorithm showed significantly lower mean noise. Inspired by our phantom and clinical results, we come to the conclusion that iDose is an important tool when considering the reduction of radiation dose in CT. However, continuous efforts to reduce radiation dose should be further proceeded.

Assessment of quantification accuracy and image quality of a full‐body dual‐layer spectral CT system

Abstract The performance of a recently introduced spectral computed tomography system based on a dual‐layer detector has been investigated. A semi‐anthropomorphic abdomen phantom for CT performance evaluation was imaged on the dual‐layer spectral CT at different radiation exposure levels (CTDI vol of 10 mGy, 20 mGy and 30 mGy). The phantom was equipped with specific low‐contrast and tissue‐equivalent inserts including water‐, adipose‐, muscle‐, liver‐, bone‐like materials and a variation in iodine concentrations. Additionally, the phantom size was varied using different extension rings to simulate different patient sizes. Contrast‐to‐noise (CNR) ratio over the range of available virtual mono‐energetic images (VMI) and the quantitative accuracy of VMI Hounsfield Units (HU), effective‐Z maps and iodine concentrations have been evaluated. Central and peripheral locations in the field‐of‐view have been examined. For all evaluated imaging tasks the results are within the calculated theoretical range of the tissue‐equivalent inserts. Especially at low energies, the CNR in VMIs could be boosted by up to 330% with respect to conventional images using iDose/spectral reconstructions at level 0. The mean bias found in effective‐Z maps and iodine concentrations averaged over all exposure levels and phantom sizes was 1.9% (eff. Z) and 3.4% (iodine). Only small variations were observed with increasing phantom size (+3%) while the bias was nearly independent of the exposure level (±0.2%). Therefore, dual‐layer detector based CT offers high quantitative accuracy of spectral images over the complete field‐of‐view without any compromise in radiation dose or diagnostic image quality.

Comparison of different radiography systems in an experimental study for detection of forearm fractures and evaluation of the Müller-AO and Frykman classification for distal radius fractures

Objectives:We sought to compare the diagnostic performance of screen-film radiography, storage-phosphor radiography, and a flat-panel detector system in detecting forearm fractures and to classify distal radius fractures according to the Müller-AO and Frykman classifications compared with the true extent, depicted by anatomic preparation. Materials and Methods:A total of 71 cadaver arms were fractured in a material testing machine creating different fractures of the radius and ulna as well as of the carpal bones. Radiographs of the complete forearm were evaluated by 3 radiologists, and anatomic preparation was used as standard of reference in a receiver operating curve analysis. Results:The highest diagnostic performance was obtained for the detection of distal radius fractures with area under the receiver operating curve (AUC) values of 0.959 for screen-film radiography, 0.966 for storage-phosphor radiography, and 0.971 for the flat-panel detector system (P > 0.05). Exact classification was slightly better for the Frykman (kappa values of 0.457–0.478) compared with the Müller-AO classification (kappa values of 0.404–0.447), but agreement can be considered as moderate for both classifications. Conclusions:The 3 imaging systems showed a comparable diagnostic performance in detecting forearm fractures. A high diagnostic performance was demonstrated for distal radius fractures and conventional radiography can be routinely performed for fracture detection. However, compared with anatomic preparation, depiction of the true extent of distal radius fractures was limited and the severity of distal radius fractures tends to be underestimated.

Accuracy of iodine quantification in dual-layer spectral CT: Influence of iterative reconstruction, patient habitus and tube parameters

PURPOSE Evaluation of the influence of iterative reconstruction, tube settings and patient habitus on the accuracy of iodine quantification with dual-layer spectral CT (DL-CT). MATERIAL AND METHODS A CT abdomen phantom with different extension rings and four iodine inserts (1, 2, 5 and 10 mg/ml) was scanned on a DL-CT. The phantom was scanned with tube-voltages of 120 and 140 kVp and CTDIvol of 2.5, 5, 10 and 20 mGy. Reconstructions were performed for eight levels of iterative reconstruction (i0-i7). Diagnostic dose levels are classified depending on patient-size and radiation dose. RESULTS Measurements of iodine concentration showed accurate and reliable results. Taking all CTDIvol-levels into account, the mean absolute percentage difference (MAPD) showed less accuracy for low CTDIvol-levels (2.5 mGy: 34.72%) than for high CTDIvol-levels (20 mGy: 5.89%). At diagnostic dose levels, accurate quantification of iodine was possible (MAPD 3.38%). Level of iterative reconstruction did not significantly influence iodine measurements. Iodine quantification worked more accurately at a tube voltage of 140 kVp. Phantom size had a considerable effect only at low-dose-levels; at diagnostic dose levels the effect of phantom size decreased (MAPD <5% for all phantom sizes). CONCLUSION With DL-CT, even low iodine concentrations can be accurately quantified. Accuracies are higher when diagnostic radiation doses are employed.

Investigation of a method to estimate the MTF and NPS of CT towards creating an international standard

The current IEC standard method for characterizing noise in CT scanners is based on the pixel standard deviation of the CT image of a water-equivalent uniform phantom. However, the standard deviation does not account for correlations in the noise, potentially generating misleading results about image quality. With this paper we investigate a method for estimating the Fourier based noise power spectrum (NPS) for the characterization of noise in CT, for CT scanners with linear, non-adaptive reconstruction algorithms. The IEC currently evaluates the deterministic properties of CT scanners with the Fourier based modulation transfer function (MTF). By accounting for the spatial correlations in both the stochastic and deterministic description of an imaging system, the system signal-to-noise ratio (SNR) can be determined more accurately. In this paper we investigate a method for estimating the MTF and the NPS of a CT scanner in the axial plane. Furthermore, we present examples of the Fourier SNR calculated from the MTF and the NPS in order to demonstrate that it gives more reasonable results than the pixel SNR. The MTF was estimated by following methods available in current literature. For the characterization of noise we used a standard water phantom, while for the point spread function (PSF) we used a tungsten wire phantom in air. Images were taken at four different source current settings and reconstructed with four different lters. We showed that the pixel SNR ranks the reconstruction lters differently from the Fourier SNR.

Experimental proof of an idea for a CT-scanner with dose reduction potential

Preliminary results for a new CT scanning device with dose-reduction potential were presented at the SPIE Medical Imaging conference 2007. The new device acquires the Radon data after the X-ray beam is collimated through a special mask. This mask is combined with a new and efficient data collection geometry; thus the device has the potential of reducing the dose by a factor of two. In this work, we report the first complete proof of the idea using the same simplified mask of 197 detectors as last year, and a clinical C-arm with a flat panel detector to simulate the gantry. This addition enables the acquisition of two independent and complementary data sets for reconstruction. Moreover, this clinical set-up enables the acquisition of data for clinically relevant phantoms. Phantom data were acquired using both detector sets and were reconstructed with the robust algorithm OPED. The independent sinograms were matched to a single one, and from this a diagnostic image was reconstructed successfully. This image has improved resolution, as well as less noise and artifacts compared to each single independent reconstruction. The results obtained are highly promising, even though the current device acquires only 197 views. Dose comparisons can be carried out in the future with a more precise prototype, comparable to current clinical devices with respect to imaging performance.