Noise reduction methods for hybrid subtraction

In digital subtraction angiography, hybrid subtraction provides selective vessel images free of soft-tissue motion artifacts but with a lower signal-to-noise ratio (SNR) than temporal subtraction images. An image processing method called measurement-dependent filtering has been developed to enhance the SNR of hybrid images without losing resolution or selectivity. Linear combinations of four images consisting of a pre- and postcontrast dual-energy measurement pair form both the hybrid image and a lower noise but less selective vessel image. The noise-reduced image is derived by combining the low-frequency components of the hybrid image with the high-frequency components of the lower noise image in a variety of ways. The results of the filtering method, when tested on both phantom and clinical data, display images with about the same degree of conspicuity as the hybrid image and a SNR approaching that of the temporal image.     

Tracking transplanted cells using dual-radionuclide SPECT

The purpose of this study was to characterize the performance of single photon emission computed tomography (SPECT) in tasks associated with tracking transplanted cells. Previous studies identified matters of hardware design, whereas we focus on biological variables impacting system performance, such as cell colony growth and non-specific radiolabelling. Using experimental data, a digital phantom was developed of in vitro 111In-radiolabelled stem cells, transfected with a reporter gene, transplanted into canine infarcted myocardium and interrogated using a peripherally injected 131I-radiolabelled reporter probe. Single- and dual-head SPECT acquisition was simulated. Performance was characterized using an estimation task, where the precision of parameter estimates (111In and 131I radiolabel quantity, cell colony size and location, and background) was tracked as the phantom evolved to simulate 111In-label efflux, cell colony growth and improved reporter probe specificity. In vitro pre-labelling of transplanted cells improved precision of parameter estimates via a priori size and location information. Precision of radiolabel quantity estimates improved with cell colony growth, despite 111In radiolabel dilution; size and location parameters were influenced little. Precision of radiolabel quantity estimates improved with reduced reporter probe non-specific uptake. The performance of SPECT in cell tracking is influenced strongly by biological variables. These should be considered when planning experiments or developing SPECT technology for cell tracking.     

Motion Artefact Reduction in Dual-Energy Radiography

Biomedical Engineering - Dual-energy radiography is a relatively simple but powerful tool of X-ray diagnosis. It increases the diagnostic value of radiographic examination by generating separate...     

A correlated noise reduction algorithm for dual-energy digital subtraction angiography

It has long been recognized that the problems of motion artifacts in conventional time subtraction digital subtraction angiography (DSA) may be overcome using energy subtraction techniques. Of the variety of energy subtraction techniques investigated, non-k-edge dual-energy subtraction offers the best signal-to-noise ratio (SNR). However, this technique achieves only 55% of the temporal DSA SNR. Noise reduction techniques that average the noisier high-energy image produce various degrees of noise improvement while minimally affecting iodine contrast and resolution. A more significant improvement in dual-energy DSA iodine SNR, however, results when the correlated noise that exists in material specific images is appropriately cancelled. The correlated noise reduction (CNR) algorithm presented here follows directly from the dual-energy computed tomography work of Kalender who made explicit use of noise correlations in material specific images to reduce noise. The results are identical to those achieved using a linear version of the two-stage filtering process described by Macovski in which the selective image is filtered to reduce high-frequency noise and added to a weighted, high SNR, nonselective image which has been processed with a high-frequency bandpass filter. The dual-energy DSA CNR algorithm presented here combines selective tissue and iodine images to produce a significant increase in the iodine SNR while fully preserving iodine spatial resolution. Theoretical calculations predict a factor of 2-4 improvement in SNR compared to conventional dual-energy images. The improvement factor achieved is dependent upon the x-ray beam spectra and the size of blurring kernel used in the algorithm.(ABSTRACT TRUNCATED AT 250 WORDS)     

An iterative computer algorithm for optimization of radionuclide subtraction studies.

A new algorithm for off-line computer optimization of radionuclide subtraction studies is presented. Within the digitized image, any region of interest may be chosen for study. No prior knowledge is required concerning the position and extent of the principal and interfering objects, nor concerning their image intensities. Tests show that even when statistical fluctuations are relatively large, the intensity quotient calculated by this algorithm may be used to estimate a uniform background and correct for its contribution. The effect of smoothing the images prior to the use of the algorithm has been investigated. The method has also been used for pancreas imaging and one example of this application is presented.     

K-edge digital subtraction imaging with dichromatic x-ray sources: SNR and dose studies

The aim of the present work is to analytically evaluate the signal to noise ratio (SNR) and the delivered dose in K-edge digital subtraction imaging (KES) using two types of x-ray sources: a monochromatic x-ray source (available at synchrotron radiation facilities and considered as gold standard) and a quasi-monochromatic compact source. The energy separation DeltaE between the two monochromatic beams is 1 keV and 4 keV for the two sources, respectively. The evaluation has been performed for both radiography and computed tomography. Different geometries have been studied to mimic clinical situations. In mammography, a pathology perfused by a contrast agent has been modelled; in angiography, a vessel superimposed to a ventricle or a stand-alone artery stenosis has been studied. The SNR and the skin dose have been calculated as a function of the detail diameter, the contrast agent (iodine and gadolinium), and its concentration in the tissues. Results show that for DeltaE = 4 keV a slightly higher delivered dose is required to obtain the same SNR with respect to DeltaE < 1 keV. A similar study has been performed for KES-CT. Computer simulations of CT images performed with Snark software are shown to validate the analytical calculations.     

An Artefact in Quantitative Immunoelectrophoresis of Spectrin Caused by Proteolytic Activity in Antibody Preparations

Many antibody preparations exhibit proteolytic activity due to the presence of plasmin. In crossed immunoelectrophoresis at pH 8.6 this enzyme can degrade certain proteins during electrophoresis in the antibody-containing gel, resulting in artefacts in the form of extra precipitation arcs of congruent shape. The degradation behaviour of spectrin, a major protein of human erythrocyte membranes, was investigated. The artefact could be completely abolished by the addition of protease inhibitors, e.g. aprotinin and soya bean trypsin inhibitor, to the antibody preparations.     

Dual-energy subtraction mammography

Dual-energy subtraction mammography was performed for breast examinations. To obtain a dual-energy subtraction image with a digital radiography unit, high- and low-energy images were obtained at an appropriate time interval under different x-ray exposure conditions. In about 50% of the patients with breast cancer included in this study, we obtained better diagnostic accuracy with dual-energy subtraction images than with conventional mammography. In some cases of breast cancer, it was possible to diagnose intraductal spread of this lesion on the subtracted images. Furthermore, abnormal lesions commonly observed on mammography in cases of fibrocystic disease tended to be erased on subtracted images. Thus, dual-energy subtraction mammography provided useful information for diagnosing breast diseases. However, there were several cases in which the subtracted images lacked sufficient image quality, and several technical problems with subtraction are thought to remain.     

Fc-Dependent IgG-Mediated Suppression of the Antibody Response: Fact or Artefact?

IgG antibodies have been shown to suppress the antibody response to all epitopes of their specific antigen as well as those the IgG do not bind to, so-called 'non-epitope-specific suppression'. The present study was undertaken to clarify whether there is a true IgG-mediated Fc-dependent suppression of the antibody response. This question is of fundamental importance to the understanding of the mechanism behind this phenomenon. It is demonstrated that F(ab')2 fragments of a monoclonal TNP (trinitrophenyl)-specific IgG2a antibody are unable to suppress the murine in vitro non-epitope-specific plaque-forming cell response against SRBC (sheep erythrocytes) when SRBC-TNP is used as antigen. The same monoclonal IgG antibody, when administered in intact form, is able to induce up to 98% suppression of the SRBC-specific antibody response. The lack of suppression is not due to mitogenic effects of pepsin in the F(ab')2 fractions or increased breakdown of F(ab')2 fragments, as compared with intact antibody, in the cultures. These data clearly demonstrate that there is indeed a highly efficient, Fc-dependent, non-epitope-specific suppressive mechanism mediated by IgG antibodies and support a hypothesis involving binding of the antigen-antibody complexes to Fc receptors as a step in the effector mechanism.     

Artefact cancellation in motor-sensory evoked potentials: two approaches using adaptive filtration and exponential approximation

Adaptive filtering for artefact cancellation in motor-sensory evoked potentials using signals obtained by subtraction methods (double-stimulus, off-nerve and subthreshold) is proposed. This is advantageous as inherent non-linear distortions can be overcome in an easier way by adaptive filtering. Efficiency is assessed with reference signals synthesised by varying the shape and reducing the amplitude of a ‘pure’ evoked potential in the range from 10% to 50%. The experiments show virtually identical shapes of the ‘pure’ and the filtered signal. The time shift between them is insignificant if a causal filter and small number of Widrow coefficients, e.g. N=8, are used. Further, two-exponential artefact approximation is applied with subsequent direct subtraction from the contaminated signal by a specially designed PC-controlled system for data acquisition and processing. For a fast procedure convergence, one-parametric optimisation of the time-constant τ is used, starting with τ=0.5 ms. The results obtained with artefact-corrupted evoked potentials from several subjects prove the efficiency of the approach. It has the substantial advantage of avoiding the need for reference signals. Both methods have advantages compared with other known software techniques.     

Noise transfer in screen-film subtraction radiography

The transfer of noise through the stages of a screen-film subtraction process was investigated. In particular the effect of measuring aperture size and film density on the measured noise granularity was studied. The relative contributions of quantum noise and film grain noise were found to depend on both aperture and density. Noise measurements are presented for a medium speed screen-film combination and also for a low noise alpha 16-Kodak Industrex C system. The effect of grain mottle and film blur on noise transfer through the multiple print process employed in subtraction radiography is demonstrated.     

Artefact as the pathologist's friend: peritumoral retraction in in situ and infiltrating duct carcinoma of the breast

Peritumoral retraction artefact appears in tissue sections as an empty space partially or completely encircling a nest of tumor cells, usually in conformity with the rounded or angular outline of that particular nest. The present study was designed to test this finding in a large series of cases and to quantify the appearance of peritumoral retraction artefact in, in situ and infiltrating duct carcinoma of the breast. We examined 199 cases of infiltrating duct carcinoma (IDC) and 188 cases of ductal carcinoma in situ (DCIS). Of the total of 387 cases, 111 were core needle biopsies, whereas the others were larger resections. In each specimen, retraction was evaluated on hematoxylin and eosin-stained slides as negative, 1+ (1% to 25% of tumor showing retraction), 2+ (26% to 50%), 3+ (51% to 75%), or 4+ (76% to 100%). Overall, peritumoral retraction was noted in 168 of 199 cases (84.4%) of IDC, versus 30 of 188 cases (16%) of DCIS (P < 0.0001). Peritumoral retraction scored as 2+ or greater (26% to 50%) was seen in only 1 of 188 DCIS specimens, compared with 77 of 199 IDC. Thus, peritumoral retraction artefact appears to be a significant finding seen during the evaluation of hematoxylin and eosin specimens for the diagnosis of carcinoma. We discuss the possibility that this phenomenon might represent true prelymphatic space involvement rather than a fixation artefact.     

Noise in stenosis measurement using digital subtraction angiography

This paper examines statistical errors in the measurement of arterial stenoses by digital videodensitometry. Images of vessel phantoms were acquired using digital subtraction angiographic techniques with low concentrations of an iodine contrast medium and low levels of x-ray exposure. Effects of the spatial and temporal averaging of image information on signal-to-noise ratios in the stenosis measurement were of primary interest. The influences of iodine concentration, x-ray scatter, veiling glare, x-ray energy spectrum, x-ray exposure, and detective quantum efficiency of the system were also included in the theoretical analysis. The agreement between theoretical calculation and experimental measurement of a simulated vessel was verified using measured values of the imaging system parameters. With a 14.2 mg/ml iodine concentration, using 20 mR per image at the entrance to a 13-cm water phantom, and averaging over a 6-mm length of a vessel 6.2 mm in diameter, the standard deviation in a measurement of a vessel's relative cross-sectional area was about 0.05. The extension of these results to practical applications in vivo is discussed.     

Dynamic powerline interference subtraction from biosignals

The performance of a previously developed and widely used method for powerline interference subtraction from the ECG is slightly reduced in the presence of continuous well-expressed EMG noise. Applying automatic adaptation of the 'linearity criterion' value in accordance to the ECG/noise ratio, the best conditions for application of the procedure can be obtained. The proposed method allows for reduction of possible distortions when applied on noise-free ECG and enhances its efficiency in the presence of non-powerline noise. Apart from ECG, it is applicable to impedance-cardiogram, plethysmogram, EEG and other biosignals.     

基于FPGA的实时数字血管减影系统

介绍了一种基于FPGA的实时数字血管减影系统.重点介绍了在数字血管减影过程中图像对数放大、减影、像素移位以及最大充盈等特殊要求的实现.该系统工作稳定,可靠地实现了X光图像1024×1024大小每秒30帧的实时减影处理,并可期望短期内应用于临床.