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H Zaidi

Publications and source records attributed to H Zaidi.

8 recordsLinked to original sources
PubMed2002

Fuzzy clustering-based segmented attenuation correction in whole-body PET imaging.

Segmented attenuation correction is now a widely accepted technique to reduce noise propagation from transmission scanning in positron emission tomography (PET). In this paper, we present a new method for segmenting transmission images in whole-body scanning. This reduces the noise in the correction maps while still correcting for differing attenuation coefficients of specific tissues. Based on the fuzzy C-means (FCM) algorithm, the method segments the PET transmission images into a given number of clusters to extract specific areas of differing attenuation such as air, the lungs and soft tissue, preceded by a median filtering procedure. The reconstructed transmission image voxels are, therefore, segmented into populations of uniform attenuation based on knowledge of the human anatomy. The clustering procedure starts with an overspecified number of clusters followed by a merging process to group clusters with similar properties (redundant clusters) and removal of some undesired substructures using anatomical knowledge. The method is unsupervised, adaptive and allows the classification of both pre- or post-injection transmission images obtained using either coincident 68Ge or single-photon 137Cs sources into main tissue components in terms of attenuation coefficients. A high-quality transmission image of the scanner bed is obtained from a high statistics scan and added to the transmission image. The segmented transmission images are then forward projected to generate attenuation correction factors to be used for the reconstruction of the corresponding emission scan. The technique has been tested on a chest phantom simulating the lungs, heart cavity and the spine, the Rando-Alderson phantom, and whole-body clinical PET studies showing a remarkable improvement in image quality, a clear reduction of noise propagation from transmission into emission data allowing for reduction of transmission scan duration. There was very good correlation (R2 = 0.96) between maximum standardized uptake values (SUVs) in lung nodules measured on images reconstructed with measured and segmented attenuation correction with a statistically significant decrease in SUV (17.03% +/- 8.4%, P < 0.01) on the latter images, whereas no proof of statistically significant differences on the average SUVs was observed. Finally, the potential of the FCM algorithm as a segmentation method and its limitations as well as other prospective applications of the technique are discussed.

Algorithms↗
PubMed2001

Scatter modelling and correction strategies in fully 3-D PET.

PET offers the possibility of quantitative measurements of tracer concentration in vivo. However, there are several issues that must be considered in order to fully realise this potential. Whilst, a correction for a number of background and physical phenomena need to be performed, the two most significant effects are the photon attenuation in the patient and the contribution in the images of events arising from photons scattered in the patient and the gantry. The non-homogeneous distribution of attenuation within the thoracic cavity complicates the interpretation of PET images and precludes the application of simple scatter correction methods developed for homogeneous media. The development of more sophisticated techniques for quantification of PET images are still required. Recent progress in 3D PET instrumentation and image reconstructions has created a need for a concise review of the relevance and accuracy of scatter correction strategies. Improved quantification of PET images remains an area of considerable research interest and several research groups are concentrating their efforts towards the development of more accurate scatter modelling and correction algorithms.

Humans↗
PubMed2000

Comparative evaluation of scatter correction techniques in 3D positron emission tomography.

Much research and development has been concentrated on the scatter compensation required for quantitative 3D positron emission tomography (PET). Increasingly sophisticated scatter correction procedures are under investigation, particularly those based on accurate scatter models and iterative reconstruction-based scatter compensation approaches. The main difference among the correction methods is the way in which the scatter component in the selected energy window is estimated. Monte Carlo methods provide further insight and might in themselves offer a possible correction procedure. Five scatter correction methods were compared in this study where applicable: the dual-energy window (DEW) technique, the convolution-subtraction (CVS) method, two variants of the Monte Carlo-based scatter correction technique (MCBSCI and MCBSC2) and our newly developed statistical reconstruction-based scatter correction (SRBSC) method. These scatter correction techniques were evaluated using Monte Carlo simulation studies, experimental phantom measurements and clinical studies. Accurate Monte Carlo modelling is still the gold standard since it allows the separation of scattered and unscattered events and comparison of the estimated and true unscattered component. In this study, our modified version of Monte Carlo-based scatter correction (MCBSC2) provided a good contrast recovery on the simulated Utah phantom, while the DEW method was found to be clearly superior for the experimental phantom studies in terms of quantitative accuracy at the expense of a significant deterioration in the signal-to-noise ratio. On the other hand, the immunity to noise in emission data of statistical reconstruction-based scatter correction methods makes them particularly applicable to low-count emission studies. All scatter correction methods gave very good activity recovery values for the simulated 3D Hoffman brain phantom, which averaged within 3%. The CVS and MCBSC 1 techniques tended to overcorrect while SRBSC undercorrected for scatter in most regions of this phantom. It was concluded that all correction methods significantly improve the image quality and contrast compared to the case where no correction is applied. Generally, it was shown that the differences in the estimated scatter distributions did not have a significant impact on the final quantitative results. The DEW method showed the best compromise between ease of implementation and quantitative accuracy, but entailed a significant deterioration in the signal-to-noise ratio.

Algorithms↗
PubMed1999

An object-oriented Monte Carlo simulator for 3D cylindrical positron tomographs.

Monte Carlo simulation is a very powerful tool in understanding performances of positron tomographs as well as in assessing image reconstruction algorithms and their implementations. We present an object-oriented Monte Carlo simulator developed for 3D positron tomography. Results from phantom simulation studies including absorption and scattering of the photons in the field-of-view are presented. Scatter fractions determined from these studies are in good agreement with measured scatter fractions published in the literature. Limitations and future prospects are discussed.

Algorithms↗
PubMed1999

Relevance of accurate Monte Carlo modeling in nuclear medical imaging.

Monte Carlo techniques have become popular in different areas of medical physics with advantage of powerful computing systems. In particular, they have been extensively applied to simulate processes involving random behavior and to quantify physical parameters that are difficult or even impossible to calculate by experimental measurements. Recent nuclear medical imaging innovations such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), and multiple emission tomography (MET) are ideal for Monte Carlo modeling techniques because of the stochastic nature of radiation emission, transport and detection processes. Factors which have contributed to the wider use include improved models of radiation transport processes, the practicality of application with the development of acceleration schemes and the improved speed of computers. In this paper we present a derivation and methodological basis for this approach and critically review their areas of application in nuclear imaging. An overview of existing simulation programs is provided and illustrated with examples of some useful features of such sophisticated tools in connection with common computing facilities and more powerful multiple-processor parallel processing systems. Current and future trends in the field are also discussed.

Algorithms↗
PubMed1996

Comparative methods for quantifying thyroid volume using planar imaging and SPECT.

UNLABELLED: SPECT enables improved accuracy over planar imaging in the determination of the volume since it is derived from three-dimensional data rather than from a two-dimensional projection with a certain geometric assumption regarding the thyroid configuration. METHODS: By using the phantoms of known volume, it was possible to estimate the accuracy of three different methods of determining thyroid volume from planar imaging used in clinical routine: the standard method used at Malmö General Hospital; a modified version of this standard method; and the method used in Lund Hospital in combination with different ways of defining the regions of interest (ROIs), and to assess the accuracy of the adaptive threshold of gray level histogram method based on SPECT imaging which determines a threshold that maximizes the separability of two classes (object and surround). RESULTS: The correlation coefficient (r) and the regression equation between the true (x) and the calculated volume (v) were as follows: r = 0.99 and y = 0.98x + 3.6 using SPECT and the gray level histogram method for edge detection combined with attenuation and scatter corrections, while r = .97 and y = 0.67x + 3.2 using the standard method based on planar scintigraphy. The standard method as used in routine was found to produce large errors (24.8%). The error on the volume estimate was reduced to approximately 7% for volumes in the range 16-to-75 ml using SPECT. CONCLUSION: Compared with conventional scintigraphy, thyroid phantom volumes were most accurately determined with SPECT when attenuation and scatter corrections are performed, which allows accurate radiation dosimetry in humans without the need for assumptions on organ size or concentration.

Adult↗