Martin Šámal

Rotation to simple structure in factor analysis of dynamic radionuclide studies

The physiological interpretation of factors in the factor analysis of dynamic radionuclide studies is dependent on the proper solution of the problem of factor rotation. A new solution suitable for scintigraphic data is suggested using a generalised concept of simple structure in data configurations. The method is described in detail, its efficiency is demonstrated on a phantom and its relationship to existing methods is discussed.

Comparison of Advanced Iterative Reconstruction Methods for SPECT/CT

AIM Corrective image reconstruction methods which produce reconstructed images with improved spatial resolution and decreased noise level became recently commercially available. In this work, we tested the performance of three new software packages with reconstruction schemes recommended by the manufacturers using physical phantoms simulating realistic clinical settings. METHODS A specially designed resolution phantom containing three (99m)Tc lines sources and the NEMA NU-2 image quality phantom were acquired on three different SPECT/CT systems (General Electrics Infinia, Philips BrightView and Siemens Symbia T6). Measurement of both phantoms was done with the trunk filled with a (99m)Tc-water solution. The projection data were reconstructed using the GEs Evolution for Bone(®), Philips Astonish(®) and Siemens Flash3D(®) software. The reconstruction parameters employed (number of iterations and subsets, the choice of post-filtering) followed theses recommendations of each vendor. These results were compared with reference reconstructions using the ordered subset expectation maximization (OSEM) reconstruction scheme. RESULTS The best results (smallest value for resolution, highest percent contrast values) for all three packages were found for the scatter corrected data without applying any post-filtering. The advanced reconstruction methods improve the full width at half maximum (FWHM) of the line sources from 11.4 to 9.5mm (GE), from 9.1 to 6.4mm (Philips), and from 12.1 to 8.9 mm (Siemens) if no additional post filter was applied. The total image quality control index measured for a concentration ratio of 8:1 improves for GE from 147 to 189, from 179. to 325 for Philips and from 217 to 320 for Siemens using the reference method for comparison. The same trends can be observed for the 4:1 concentration ratio. The use of a post-filter reduces the background variability approximately by a factor of two, but deteriorates significantly the spatial resolution. CONCLUSIONS Using advanced reconstruction algorithms the largest improvement in image resolution and contrast is found for the scatter corrected slices without applying post-filtering. The user has to choose whether noise reduction by post-filtering or improved image resolution fits better a particular imaging procedure.

On the existence of an unambiguous solution in factor analysis of dynamic studies

Achievement of an unambiguous solution in factor analysis of dynamic radionuclide studies depends on constraints reflecting the known properties of factors. The constraints should be tight enough to prevent ambiguity but sufficiently general in order to ensure the data-based derivation of factors. In dynamic scintigraphy, the non-negativity of factors is their essential property which is implied by the physical nature of measured quantities. Considering factors as the images of compartments in the distribution space of a radiopharmaceutical (i.e. performing the factor analysis in the spatial domain), a powerful additional constraint can be applied. This constraint is based on the presence of segments in the image matrix where the subtotal number of compartments is projected. Using this constraint, the existence of physiologically related unique solution in factor analysis can be proved providing the number of factors is chosen properly.

Enhancement of physiological factors in factor analysis of dynamic studies

Factor analysis of dynamic radionuclide studies provides their decomposition into the images and time-activity curves corresponding to the underlying dynamic structures. The method is based on the analysis of study variance and on the subsequent differential imaging of its principal components into a simplified factor space. By changing the amount and the composition of the variance processed in the analysis it is possible to enhance the factors that are important for diagnosis while the less important factors can be suppressed. In our report, a short theoretical review of the problem is given and illustrated by the analysis of dynamic cholescintigraphy. It is shown that a suitable choice of region and/or the temporal interval of interest anables the differential evaluation of such intrahepatic compartments, which could not be observed without enhancement.

A simple method for determining split renal function from dynamic (99m)Tc-MAG3 scintigraphic data.

PurposeCommonly used methods for determining split renal function (SRF) from dynamic scintigraphic data require extrarenal background subtraction and additional correction for intrarenal vascular activity. The use of these additional regions of interest (ROIs) can produce inaccurate results and be challenging, e.g. if the heart is out of the camera field of view. The purpose of this study was to evaluate a new method for determining SRF called the blood pool compensation (BPC) technique, which is simple to implement, does not require extrarenal background correction and intrinsically corrects for intrarenal vascular activity.MethodsIn the BPC method SRF is derived from a parametric plot of the curves generated by one blood-pool and two renal ROIs. Data from 107 patients who underwent 99mTc-MAG3 scintigraphy were used to determine SRF values. Values calculated using the BPC method were compared to those obtained with the integral (IN) and Patlak-Rutland (PR) techniques using Bland-Altman plotting and Passing-Bablok regression. The interobserver variability of the BPC technique was also assessed for two observers.ResultsThe SRF values obtained with the BPC method did not differ significantly from those obtained with the PR method and showed no consistent bias, while SRF values obtained with the IN method showed significant differences with some bias in comparison to those obtained with either the PR or BPC method. No significant interobserver variability was found between two observers calculating SRF using the BPC method.ConclusionThe BPC method requires only three ROIs to produce reliable estimates of SRF, was simple to implement, and in this study yielded statistically equivalent results to the PR method with appreciable interobserver agreement. As such, it adds a new reliable method for quality control of monitoring relative kidney function.

Bayesian Identification of a Physiological Model in Dynamic Scintigraphic Data

The report summarizes theoretical background and demonstrates experimental results of an objective procedure for a quantitative evaluation of dynamic scintigraphic data. The method is based on a Bayesian interpretation of a linear regression model and used for an automatic identification of a compartment structure in image data, objective derivation of regional time-activity curves, and subsequent estimation of both the structure and function of an underlying compartmental model.

Model-based extraction of input and organ functions in dynamic scintigraphic imaging

Image-based definition of input function (IF) and organ function is a prerequisite for kinetic analysis of dynamic scintigraphy or positron emission tomography. This task is typically done manually by a human operator and suffers from low accuracy and reproducibility. We propose a probabilistic model based on physiological assumption that time–activity curves (TACs) arise as a convolution of an IF and tissue-specific kernels. The model is solved via the Variational Bayes estimation procedure and provides estimates of the IF, tissue-specific TACs and their related spatial distributions (images) as its results. The algorithm was tested with data of dynamic renal scintigraphy. The method was applied to the problem of differential renal function estimation and the IF estimation and the results are compared with competing techniques on data-sets with 99 and 19 patients. The MATLAB implementation of the algorithm is available for download.

Confirmatory Aspects in Factor Analysis of Image Sequences

Confirmatory approach in factor analysis of image sequences is specified by an employment of considerable initial information in the processes of factor extraction and rotation and by the possibility to verify hypotheses assumed in advance. Confidence interval for factor contribution is introduced and its utility in an assessment of factor significance demonstrated. Based on a partial apriori knowledge of resulting factor image, the method for a multiple subtraction of images is derived and its noise-rejection properties demonstrated. Quantitative transformation of factor curves into the compartmental scheme is described and the method is applied to a dynamic radionuclide study of renal function.

Probable range for whole kidney mean transit time values determined by reexamination of Uk audit studies

ObjectivesInconsistency in the intercentre measurement of whole kidney mean transit time (MTT) has been reported in a previously published UK audit. The main objectives of this study were to identify a probable value of MTT for each kidney in the UK audit data and to find likely reasons for the reported variations. MethodsDatasets of MTT values were obtained by an independent review of the audit data by four experienced practitioners of deconvolution techniques. The deconvolution techniques used included the matrix method, a constrained least squares method as well as a residence time technique. The datasets were compared using t-test, linear regression, and mean difference analysis. ResultsTwelve of a total of 13 datasets showed nonsignificant differences using a paired t-test (P>0.05). For each kidney (x), a collective mean and standard deviation, Mx and SDx, respectively, were calculated from these 12 datasets and a probable range was defined as Mx±3SDx. Average SDx/Mx was 3.6% (range 1.5–7.7%). For five kidneys, Mx exceeded the median of the audit results by 3.5–15.3 SDx (P<0.001). ConclusionProbable ranges for whole kidney MTT have been estimated with good precision. Underestimation of the area under the plateau of the renal retention function as well as overestimation of the plateau height might have contributed to an underestimation of MTT apparent in some audit results. Visual display of both the renal retention function and the reconvolution curve are suggested as simple quality control measures for analysis software.

Improved scatter correction with factor analysis for planar and SPECT imaging

Quantitative nuclear medicine imaging is an increasingly important frontier. In order to achieve quantitative imaging, various interactions of photons with matter have to be modeled and compensated. Although correction for photon attenuation has been addressed by including x-ray CT scans (accurate), correction for Compton scatter remains an open issue. The inclusion of scattered photons within the energy window used for planar or SPECT data acquisition decreases the contrast of the image. While a number of methods for scatter correction have been proposed in the past, in this work, we propose and assess a novel, user-independent framework applying factor analysis (FA). Extensive Monte Carlo simulations for planar and tomographic imaging were performed using the SIMIND software. Furthermore, planar acquisition of two Petri dishes filled with 99mTc solutions and a Jaszczak phantom study (Data Spectrum Corporation, Durham, NC, USA) using a dual head gamma camera were performed. In order to use FA for scatter correction, we subdivided the applied energy window into a number of sub-windows, serving as input data. FA results in two factor images (photo-peak, scatter) and two corresponding factor curves (energy spectra). Planar and tomographic Jaszczak phantom gamma camera measurements were recorded. The tomographic data (simulations and measurements) were processed for each angular position resulting in a photo-peak and a scatter data set. The reconstructed transaxial slices of the Jaszczak phantom were quantified using an ImageJ plugin. The data obtained by FA showed good agreement with the energy spectra, photo-peak, and scatter images obtained in all Monte Carlo simulated data sets. For comparison, the standard dual-energy window (DEW) approach was additionally applied for scatter correction. FA in comparison with the DEW method results in significant improvements in image accuracy for both planar and tomographic data sets. FA can be used as a user-independent approach for scatter correction in nuclear medicine.