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I Buvat

Publications and source records attributed to I Buvat.

At least 19 recordsLinked to original sources
PubMed2002

Monte Carlo simulations in SPET and PET.

Monte Carlo methods are extensively used in Nuclear Medicine to tackle a variety of problems that are difficult to study by an experimental or analytical approach. A review of the most recent tools allowing application of Monte Carlo methods in single photon emission tomography (SPET) and positron emission tomography (PET) is presented. To help potential Monte Carlo users choose a code, we present advantages and disadvantages of the different types of Monte Carlo codes currently available for SPET and PET, discuss common and specific features of the codes, classify the codes with respect to these features, comment key properties for a code to be appropriate for a given purpose and, at last, we consider the possibility of going towards a standardisation of the description of the codes which could facilitate their comparison.

Computer Simulation↗
PubMed2001

Importance of the choice of the collimator for the detection of small lesions in scintimammography: a phantom study.

99mTc methoxyisobutylisonitrile planar scintimammography (SMM) is mostly performed using low-energy high-resolution (LEHR) parallel collimators. We studied whether using a different collimator could improve the detection of small (< 1.5 cm) lesions for which SMM sensitivity is poor. Thirty four breast phantom configurations were considered, either with hot spheres simulating lesions or without any spheres. For each configuration, four planar acquisitions were performed using LEHR, low-energy ultra high-resolution (LEUHR), high-resolution fan-beam (HRFB) and ultra high-resolution fan-beam (UHRFB) collimators. Images corresponding to the 20% and 10% energy windows and to the Jaszczak subtraction were calculated. A database including 156 borderline images was derived. After training, 10 observers scored the images for the presence of a sphere. The performances in sphere detection were studied using receiver operating characteristic (ROC) analysis. For all types of image, the area under the ROC curve was highest with the UHRFB collimator and lowest with either the LEUHR or the HRFB collimator. For the 10% energy window images conventionally used in SMM, the detection sensitivities averaged 91%, 73%, 60% and 55% for the UHRFB, LEHR, HRFB and LEUHR collimators respectively, for the same specificity of 64%. We conclude that detection of small tumours in planar SMM might be significantly improved by using a UHRFB collimator instead of an LEHR collimator.

Breast Neoplasms↗
PubMed2000

Should scatter be corrected in both transmission and emission data for accurate quantitation in cardiac SPET?

Ideally, reliable quantitation in single-photon emission tomography (SPET) requires both emission and transmission data to be scatter free. Although scatter in emission data has been extensively studied, it is not well known how scatter in transmission data affects relative and absolute quantitation in reconstructed images. We studied SPET quantitative accuracy for different amounts of scatter in emission and transmission data using a Utah phantom and a cardiac Data Spectrum phantom including different attenuating media. Acquisitions over 180 degrees were considered and three projection sets were derived: 20% images and Jaszczak and triple-energy-window scatter-corrected projections. Transmission data were acquired using gadolinium-153 line sources in a 90-110 keV window using a narrow or wide scanning window. The transmission scans were performed either simultaneously with the emission acquisition or 24 h later. Transmission maps were reconstructed using filtered backprojection and mu values were linearly scaled from 100 to 140 keV. Attenuation-corrected images were reconstructed using a conjugate gradient minimal residual algorithm. The mu value underestimation varied between 4% with a narrow transmission window in soft tissue and 22% with a wide window in a material simulating bone. Scatter in the emission and transmission data had little effect on the uniformity of activity distribution in the left ventricle wall and in a uniformly hot compartment of the Utah phantom. Correcting the transmission data for scatter had no impact on contrast between a hot and a cold region or on signal-to-noise ratio (SNR) in regions with uniform activity distribution, while correcting the emission data for scatter improved contrast and reduced SNR. For absolute quantitation, the most accurate results (bias <4% in both phantoms) were obtained when reducing scatter in both emission and transmission data. In conclusion, trying to obtain the same amount of scatter in emission and transmission data, in addition to being impractical because of the difficulty in knowing the precise scatter components, did not yield such accurate absolute activity quantitation as when emission and transmission scatter were reduced.

Heart↗
PubMed2000

Relative impact of scatter, collimator response, attenuation, and finite spatial resolution corrections in cardiac SPECT.

UNLABELLED: We determined the relative effect of corrections for scatter, depth-dependent collimator response, attenuation, and finite spatial resolution on various image characteristics in cardiac SPECT. METHODS: Monte Carlo simulations and real acquisition of a 99mTc cardiac phantom were performed under comparable conditions. Simulated and acquired data were reconstructed using several correction schemes that combined different methods for scatter correction (3 methods), depth-dependent collimator response correction (frequency-distance principle), attenuation correction (nonuniform Chang correction or within an iterative reconstruction algorithm), and finite spatial resolution correction (use of recovery coefficients). Five criteia were considered to assess the effect of the processing schemes: bull's-eye map (BEM) uniformity, contrast between the left ventricle (LV) wall and the LV cavity, spatial resolution, signal-to-noise ratio (SNR), and percent errors with respect to the known LV wall and liver activities. RESULTS: Similar results were obtained for the simulated and acquired data. Scatter correction significantly improved contrast and absolute quantitation but did not have noticeable effects on BEM uniformity or on spatial resolution and reduced the SNR. Correction for the depth-dependent collimator response improved spatial resolution from 13.3 to 9.5 mm in the LV region, improved absolute quantitation and contrast, but reduced the SNR. Correcting for attenuation was essential for restoring BEM uniformity (78% and 89% without and with attenuation correction, respectively [ideal value being 100%]) and accurate absolute activity quantitation (errors in estimated LV wall and liver activity decreased from 90% without attenuation correction to approximately20% with attenuation correction only). Although accurate absolute activity quantitation was achieved in the liver using scatter and attenuation corrections only, correction for finite spatial resolution was needed to estimate LV wall activity within 10%. CONCLUSION: The respective effects of corrections for scatter, depth-dependent collimator response, attenuation, and finite spatial resolution on different image features in cardiac SPECT were quantified for a specific acquisition configuration. These results give indications regarding the improvements to be expected when using a specific processing scheme involving some or all corrections.

Heart↗
PubMed1999

Respective roles of scatter, attenuation, depth-dependent collimator response and finite spatial resolution in cardiac single-photon emission tomography quantitation: a Monte Carlo study.

The purpose of this study was to investigate the relative influence of scatter, attenuation, depth-dependent collimator response and finite spatial resolution upon the image characteristics in cardiac single-photon emission tomography (SPET). An acquisition of an anthropomorphic cardiac phantom was performed together with corresponding SPET Monte Carlo simulations. The cardiac phantom and the Monte Carlo simulations were designed so that the effect of scatter, attenuation, depth-dependent collimator response and finite spatial resolution could be studied individually and in combination. The impact of each physical effect and of combinations of effects was studied in terms of absolute and relative quantitative accuracy, spatial resolution and signal-to-noise ratio (SNR) in the resulting images. No corrections for these effects were assessed. Results obtained from Monte Carlo simulations and real acquisitions were in excellent agreement. Attenuation introduced about 90% activity underestimation in a 10-mm-thick left ventricle wall while finite spatial resolution alone introduced about 30% activity underestimation. Scatter had a negligible impact on quantitative accuracy in the recontructed slices when attenuation was present. Neither bull's eye map homogeneity nor contrast between a hot and a cold region were affected by depth-dependent collimator response or finite spatial resolution. Bull's eye map homogeneity was severely affected by attenuation but not by scatter. Attenuation and scatter reduced contrast by about 20% each. Both attenuation and scatter increased the full-width at half-maximum (FWHM) characterizing the spatial resolution of the imaging system by approximately 1 mm each but the main effect responsible for the observed 11-mm FWHM spatial resolution was the depth-dependent collimator response. SNR was reduced by a factor of approximately 2.5 because of attenuation, while scattered counts increased SNR by approximately 10%. In conclusion, the quantification of the relative influence of the different physical effects showed that attenuation is definitely the major phenomenon affecting cardiac SPET imaging accuracy, but that finite spatial resolution, scatter and depth-dependent collimator response also contribute significantly to the errors in absolute and relative quantitation and to the poor spatial resolution.

Computer Simulation↗
PubMed1999

Quantitation in planar renal scintigraphy: which mu value should be used?

The attenuation coefficient value mu used by different authors for quantitation in planar renal scintigraphy varies greatly, from the theoretical value of 0.153 cm-1 (appropriate for scatter-free data) down to 0.099 cm-1 (empirical value assumed to compensate for both scatter and attenuation). For a 6-cm-deep kidney, such variations introduce up to 30% differences in absolute measurement of kidney activity. Using technetium-99m phantom studies, we determined the mu values that would yield accurate kidney activity quantitation for different energy windows corresponding to different amounts of scatter, and when using different image analysis approaches similar to those used in renal quantitation. With the 20% energy window, it was found that the mu value was strongly dependent on the size of the region of interest (ROI) and on whether background subtraction was performed: the mu value thus varied from 0.119 cm-1 (loose ROI, no background subtraction) to 0.150 cm-1 (kidney ROI and background subtraction). When using data from an energy window that could be considered scatter-free, the mu value became almost independent of the image analysis scheme. It is concluded that: (1) when performing background subtraction, which implicitly reduces the effect of scatter, the mu value to be used for accurate quantitation is close to the theoretical mu value; (2) if the acquired data were initially corrected for scatter, the appropriate mu value would then be the theoretical mu value, whatever the image analysis scheme.

Humans↗
PubMed1999

A spline-regularized minimal residual algorithm for iterative attenuation correction in SPECT.

In SPECT, regularization is necessary to avoid divergence of the iterative algorithms used for non uniform attenuation compensation. In this paper, we propose a spline-based regularization method for the minimal residual algorithm. First, the acquisition noise is filtered using a statistical model involving spline smoothing so that the filtered projections belong to a Sobolev space with specific continuity and derivability properties. Then, during the iterative reconstruction procedure, the continuity of the inverse Radon transform between Sobolev spaces is used to design a spline-regularized filtered backprojection method, by which the known regularity properties of the projections determine those of the corresponding reconstructed slices. This ensures that the activity distributions estimated at each iteration present regularity properties, which avoids computational noise amplification, thus stabilizing the iterative process. Analytical and Monte Carlo simulations are used to show that the proposed spline-regularized minimal residual algorithm converges to a satisfactory stable solution in terms of restored activity and homogeneity, using at most 25 iterations, whereas the non regularized version of the algorithm diverges. Choosing the number of iterations is therefore no longer a critical issue for this reconstruction procedure.

Algorithms↗
PubMed1999

Impact of attenuation correction by simultaneous emission/transmission tomography on visual assessment of 201Tl myocardial perfusion images.

UNLABELLED: It has been shown in clinical studies that for subjects with a low likelihood of coronary artery disease (CAD), attenuation correction (AC) improves the specificity of defect detection in the inferior wall (right coronary artery [RCA] region). The aim of this study was to investigate the effect of AC on the visual interpretation of the RCA and anteroseptal (corresponding to the left anterior descending artery [LAD]) regions in CAD patients. METHODS: Fifty-six patients with suspected CAD underwent 20Tl stress/4 h-delayed imaging SPECT using a simultaneous 201Tl emission/99mTc transmission imaging protocol. Images were reconstructed using the maximum likelihood-expectation maximum algorithm without and with AC. The stress/4 h-delayed images were interpreted blindly for reversible or fixed defects in the RCA and LAD regions by three experienced physicians. Coronary angiography, electrocardiography and enzyme findings were used to establish diagnoses of ischemia or infarction, and receiver operating characteristic (ROC) analyses were performed. RESULTS: Statistical testing of ROC curve areas showed that defect detection performance improved with AC when compared with performance without AC in the RCA region. This was mainly the result of a systematic increase in specificity of 12% or more (for any observer and any type of defect) for a similar sensitivity (no definite change in sensitivity values). However, defect detection performance significantly decreased in the LAD territory with AC images (P < 0.05) because of a systematic decrease in sensitivity of 20% or more, with no consistent change in specificity. Similar trends were observed when reversible and fixed defects were considered separately. CONCLUSION: AC significantly affects the visual interpretation of 201Tl stress/4 h-delayed SPECT images. This study confirmed the increase in specificity obtained with AC in the RCA territory. However, in the population considered, the studied AC was deleterious for the LAD territory assessment.

Coronary Disease↗
PubMed1998

Two-dimensional statistical model for regularized backprojection in SPECT.

In SPECT, both the noise affecting the data and the discretization of the inverse Radon transform are responsible for the ill-posed nature of the reconstruction. To constrain the problem, we propose a regularized backprojection method (RBP) which takes advantage of the relationships existing between the continuity properties of the projections and those of the reconstructed object. The RBP method involves two stages: first, a statistical model (the fixed-effect model) is used to estimate the noise-free part of the projections. Then, the filtered projections are reconstructed using a backprojection algorithm (spline filtered backprojection) which ensures that the reconstructed object belongs to a space consistent with that containing the projections. The method is illustrated using analytical simulations, and the RBP approach is compared to the conventional filtered backprojection. The effect on the reconstructed slices of the parameters involved in RBP is studied in terms of spatial resolution, homogeneity in uniform regions and quantification. It is shown that appropriate combinations of these parameters yield a better compromise between homogeneity and spatial resolution than conventional FBP, with similar quantification performances.

Algorithms↗
PubMed1998

Statistical distribution of factors and factor images in factor analysis of medical image sequences.

From a time or energy image sequence, factor analysis of medical image sequences (FAMIS) estimates factors, representing kinetics or spectra in a given physiological compartment, and associated factor images, showing the compartments corresponding to each curve. In this paper, we show that the statistical properties of factor images and associated factors can be determined using a well known result from elementary probability theory. Numerical experiments are conducted to demonstrate that the variance observed in factor images can be predicted when the statistical properties of the original data are known. It is shown how these theoretical results can be used to relax the non-negativity constraints during FAMIS oblique analysis and to improve the quantitative interpretation of the factor images by associating a confidence interval with each pixel value.

Biophysical Phenomena↗
PubMed1998

Impact of scatter correction in planar scintimammography: a phantom study.

UNLABELLED: This study examines how scatter correction might affect lesion detection and quantitation of tumor-to-normal breast tissue activity ratio in planar scintimammography. METHODS: Forty-one phantom acquisitions were performed to mimic a wide variety of scintimammographic imaging conditions in which lesions would be close to the chest wall. For each acquisition, the images corresponding to a 10% energy window (110) and two scatter correction methods [the Jaszczak (JA) method and a factor analysis (FA)-based method] were obtained in addition to the conventional 20% image (120). A total of 368 images in which detection of the "tumor" was judged borderline were selected, and 10 independent observers were asked to detect lesions in these images. Receiver operating curve analyses were performed to assess detection performance. Tumor-to-normal tissue activity ratios were calculated for quantitative analysis. RESULTS: Detection performance significantly improved for the I10, JA and FA images compared to the 120 images, with an increase in sensitivity up to 8% for FA images. Sensitivity was especially increased for small lesions (13- and 16-mm3 spheres) and true heart-to-normal tissue activity ratios of > 12. Scatter correction also increased the certainty with which the readers gave their judgment. The tumor-to-normal tissue activity ratio was approximately 8% larger on JA or FA images and 1% larger on the I10 images compared to the 120 images. For a given image, the variability with which this ratio was estimated was reduced by approximately 4% on JA and FA images. CONCLUSION: Based on these phantom results, scatter correction might be used with benefit in scintimammography.

Breast↗
PubMed1997

Implications of dual-energy-window (DEW) scatter correction inaccuracies for 111In quantitative geometric mean imaging.

There is increasing clinical interest in the use of quantitative imaging for radiopharmaceuticals labelled with 111In. Dual-energy-window (DEW) scatter correction is a frequently used component of planar geometric mean quantitative imaging, but it is known that the scatter multiplier k suffers from significant dependence on the characteristics of the scatter medium. Phantom studies with a variety of source geometries were carried out to determine the clinical impact of this dependence on the quantitative accuracy of tumour imaging carried out in conjunction with attenuation correction. Spheres of various sizes (5-20 ml volumes) containing approximately 3.7 MBq (100 microCi) 111In were imaged at a variety of depths (4.8-10.5 cm) within an elliptical water-filled phantom, as well as in air. Geometric mean emission images were acquired using a 20% photopeak window at 247 keV and a 10% scatter window at 205 keV. These emission images were corrected for attenuation using measured 99Tcm transmission data that were scaled to 111In photon energies. Scatter correction was performed in two ways: (1) using the standard DEW method and (2) using a modified DEW method that takes into account benign scatter in the detector crystal. Errors in the activity estimates ranged from -4% to +3% for method 1 in water, and -5% to +3% for method 2 in water. In air, method 1 ranged from -13% to -5%, and method 2 ranged from -10% to -1%. Method 1 was found to yield an accuracy equivalent to that of method 2, except in conditions of very low patient scatter, when the modified method behaved significantly better. We conclude that in a variety of realistic geometries, variations in scatter fraction as determined by the DEW scatter correction method combined with appropriate attenuation correction need not inhibit accurate absolute quantitation of spherical 'tumours' labelled with 111In when using planar imaging.

Gamma Cameras↗
PubMed1997

A "hybrid" method for measuring myocardial wall thickening from gated PET/SPECT images.

UNLABELLED: We introduce a hybrid index, HYB, which combines counts with geometric information to measure wall thickening from PET/SPECT gated images. Its accuracy is compared with that of a count-based index (MAX) and a geometric index (FWHM). METHODS: For each index, the index values versus thickness and the estimated thickening values versus true thickening were investigated using theoretical analyses, realistic simulated data obtained from clinical gated MR scans, phantom measurements and preliminary gated MRI and PET patient studies. Each index was studied for different spatial resolutions and noise and background conditions. The performance of each index was quantified using a parameter "Q" reflecting bias and variability of thickening estimates. RESULTS: HYB varied more linearly with thickness than MAX and FWHM, resulting in a better Q value than with MAX and FWHM for all noise, background and spatial resolutions. ROC analysis confirmed that HYB significantly increases the sensitivity and specificity for detection of wall thickening abnormalities (sensitivity = 100%; specificity = 85% for HYB, 95% and 50% for MAX and 100% and 0% for FWHM, respectively). CONCLUSION: Use of the hybrid index instead of conventional count-based or geometric indices should improve the classification of normal/abnormal wall thickening values in gated SPECT and PET.

Heart↗
PubMed1995

A new correction method for gamma camera non-uniformity due to energy response variability.

We present a new uniformity correction (Fourier energy correction) which is designed to correct for gamma camera non-uniformity caused by variations of the energy response function within a wide spectral range. A convolution model is used to describe the spatial distortions of the energy response function. The model is solved in Fourier space. A preliminary flood acquisition is required to obtain energy-dependent Fourier weights which are used to correct subsequent acquisitions. The influence of the parameters involved in the correction procedure is studied and the Fourier energy correction is compared to a conventional multiplicative energy correction for different acquisition geometries. The Fourier energy correction appears especially useful when the energy information associated with each detected photon is analysed using a fine sampling, or when windows different from the photopeak window are used.

Biophysical Phenomena↗
PubMed1995

Comparative assessment of nine scatter correction methods based on spectral analysis using Monte Carlo simulations.

UNLABELLED: We compared nine scatter correction methods based on spectral analysis which process SPECT projections. METHODS: Monte Carlo simulation was used to generate histories of photons emitted from a realistic 99mTc phantom. A particular projection was considered. Information regarding the history, location and energy of the photons detected in this projection was analyzed to test the assumptions underlying each scatter correction method. Relative and absolute quantification and signal-to-noise ratio were assessed for each scatter corrected image. RESULTS: For the simulated data, two methods do not enable activity quantification. Among the methods requiring some parameters to be calibrated, the dual-energy window method shows the best compromise between accuracy and ease of implementation but introduces a bias in relative quantification. In this respect, a triple-energy window technique is more accurate than the dual-window method. A factor analysis approach results in more stable quantitative accuracy (error approximately 10%) for a wide range of activity but requires a more sophisticated acquisition mode (30 energy windows). CONCLUSION: These results show that a scatter correction method using spectral analysis can be used to substantially improve accurate quantification.

Humans↗
PubMed1994

A comparative study of scatter correction methods for scintigraphic images.

Phantom studies have demonstrated that factor analysis of medical image sequences using target apex-seeking (FAMIS-TAS) applied to spectral scintigraphic image sequences is an efficient adaptive scatter correction method. We assessed the improvement in quality of clinical images using FAMIS-TAS as compared with two other scatter correction techniques: conventional 20% photopeak window (PW) and scatter window subtraction (SWS). Thirty normal technetium-99m hydroxymethylene diphosphonate bone scans were processed. Bone to soft tissue contrasts and signal-to-noise and contrast-to-noise ratios were measured. The overall image quality was evaluated using an observer testing questionnaire submitted to four physicians. Quantitative parameters showed that FAMIS-TAS images displayed the best bone to soft tissue contrasts and contrast-to-noise ratios, but the lowest signal-to-noise ratios. PW images presented the lowest contrasts and contrast-to-noise ratios, and the highest signal-to-noise ratios. SWS gave intermediate results. According to the observer testing results, PW images showed the lowest bone to soft tissue contrasts and the highest signal-to-noise ratios. FAMIS-TAS images showed the lowest signal-to-noise ratios. The images processed by the three methods displayed the same anatomical information.

Adolescent↗
PubMed1994

Scatter correction in scintigraphy: the state of the art.

In scintigraphy, the detection of scattered photons degrades both visual image analysis and quantitative accuracy. Many methods have been proposed and are still under investigation to cope with scattered photons. The main features of the problem of scattering in radionuclide imaging are presented first, to provide a sound foundation for a critical review of the existing scatter correction techniques. These are described using a classification relating to their aims and principles. Their theoretical potentials are analysed, as well as the difficulties of their practical implementation. Finally, the problems of their evaluation and comparison are discussed.

Humans↗