Biomedical subjects

B M Tsui

Publications and source records attributed to B M Tsui.

At least 19 recordsLinked to original sources

PubMed2000

Fast maximum entropy approximation in SPECT using the RBI-MAP algorithm.

In this work, we present a method for approximating constrained maximum entropy (ME) reconstructions of SPECT data with modifications to a block-iterative maximum a posteriori (MAP) algorithm. Maximum likelihood (ML)-based reconstruction algorithms require some form of noise smoothing. Constrained ME provides a more formal method of noise smoothing without requiring the user to select parameters. In the context of SPECT, constrained ME seeks the minimum-information image estimate among those whose projections are a given distance from the noisy measured data, with that distance determined by the magnitude of the Poisson noise. Images that meet the distance criterion are referred to as feasible images. We find that modeling of all principal degrading factors (attenuation, detector response, and scatter) in the reconstruction is critical because feasibility is not meaningful unless the projection model is as accurate as possible. Because the constrained ME solution is the same as a MAP solution for a particular value of the MAP weighting parameter, beta, the constrained ME solution can be found with a MAP algorithm if the correct value of beta is found. We show that the RBI-MAP algorithm, if used with a dynamic scheme for estimating beta, can approximate constrained ME solutions in 20 or fewer iterations. We compare results for various methods of achieving feasible images on a simulation of Tl-201 cardiac SPECT data. Results show that the RBI-MAP ME approximation provides images and quantitative estimates close to those from a slower algorithm that gives the true ME solution. Also, we find that the ME results have higher spatial resolution and greater high-frequency noise content than a feasibility-based stopping rule, feasibility-based low-pass filtering, and a quadratic Gibbs prior with beta selected according to the feasibility criterion. We conclude that fast ME approximation is possible using either RBI-MAP with the dynamic procedure or a feasibility-based stopping rule, and that such reconstructions may be particularly useful in applications where resolution is critical.

Algorithms↗

PubMed2000

Receiver operating characteristic evaluation of iterative reconstruction with attenuation correction in 99mTc-sestamibi myocardial SPECT images.

UNLABELLED: The purpose of this study was to evaluate differences in myocardial defect detection between 99mTc-sestamibi myocardial SPECT images reconstructed using conventional filtered backprojection (FBP) without attenuation correction (AC) and those reconstructed using maximum-likelihood expectation maximization with nonuniform attenuation correction (MLAC). METHODS: An observer study and receiver operating characteristic (ROC) curve analysis were performed using simulated 99mTc-sestamibi SPECT data from a population of 24 mathematic anthropomorphic torso phantoms, which realistically modeled a wide range of anatomic variations. The phantoms modeled male patients with a flat diaphragm, male patients with a diaphragm raised to the level of the heart, and female patients with large breasts. Transmural, cold defects with a contrast of 0.25 were simulated in the left ventricular wall for 6 locations. Noisy projection data were generated from the phantoms and included the effects of nonuniform attenuation, collimator-detector response, and scatter. The data were then reconstructed using FBP and MLAC. Images were displayed in the short- and long-axis formats, as in clinical practice. Eight observers viewed blocks of FBP and MLAC images and, for each image, indicated on a continuous rating scale the probability that a defect was present. From the rating data, FBP and MLAC ROC curves were generated, and their areas (Az) were estimated and compared. RESULTS: In general, the FBP and MLAC ROC curves did not cross and the MLAC curve showed a higher Az than did the corresponding FBP curve. For male phantoms with a flat diaphragm, the average difference in Az was 0.04 and was not statistically significant (at the P = 0.05 level) for 6 of 8 observers. For male phantoms with a raised diaphragm, the average difference in Az was 0.22 and was statistically significant for 6 of 8 observers. For female phantoms with large breasts, the average difference in Az was 0.19 and was statistically significant for all 8 observers. CONCLUSION: This study showed an improvement in defect detection in myocardial SPECT images using MLAC in comparison with images using FBP without AC, particularly for patients with large breasts or with a diaphragm raised to the level of the heart.

Breast↗

PubMed2000

Performance of ordered-subset reconstruction algorithms under conditions of extreme attenuation and truncation in myocardial SPECT.

UNLABELLED: We studied the bias and variance characteristics of the ordered-subset expectation maximization (OSEM) and rescaled block-iterative EM (RBIEM) iterative reconstruction algorithms in myocardial SPECT under extreme, but realistic, conditions. METHOD: We used the 2-dimensional mathematic cardiac torso phantom to simulate 2 patient anatomies: a large male with a raised diaphragm and a female with large breast size, approximating extreme cases of attenuation conditions found in the clinic. For each anatomy, realistic 201Tl projection data were simulated for a 180 degrees acquisition arc. Three cases of truncation for a 90 degrees-configured dual detector system were simulated: no truncation, moderate truncation, and extreme truncation. For each case, an ensemble of 250 noise simulations was generated, and each noisy dataset was reconstructed with the OSEM and RBIEM algorithms. The reconstructions modeled only the effects of nonuniform attenuation and used a range of subset configurations. Over the ensemble, we computed means and variances of activity in 8 regions of interest (ROIs) in the heart as a function of iteration. RESULTS: Under conditions of no truncation and moderate truncation, the results from OSEM and RBIEM were very close to those from maximum-likelihood EM (MLEM); in all cases, the difference in ROI means was <2.5%. For extreme truncation, the errors increased to as much as 11% with OSEM, but these were no greater than the errors for MLEM under the same conditions. The OSEM algorithm with 2 views per subset was found to result in much higher variance of ROI estimates for the same bias as compared with RBIEM or OSEM with 4 or more views per subset. CONCLUSION: The OSEM and RBIEM algorithms are at least as robust to highly attenuating patients and truncation as MLEM algorithm and can be adequate substitutes for MLEM, even in extreme cases. Clinical users should apply the smallest number of subsets that can be accommodated by allowable processing time to reduce image noise and variance in quantitative estimates.

Algorithms↗

PubMed1999

Simultaneous technetium-99m/thallium-201 SPECT imaging with model-based compensation for cross-contaminating effects.

Simultaneous acquisition of dual-isotope SPECT data offers a number of advantages over separately acquired data; however, simultaneous acquisition can result in cross-contamination between isotopes. In this work we propose and evaluate two frameworks for iterative model-based compensation of cross-contamination in dual-isotope SPECT. The methods were applied to cardiac imaging with technetium-99m-sestamibi and thallium-201, and they were compared with a subtraction-based compensation method using a cross-talk estimate obtained from an auxiliary energy window. Monte Carlo simulations were performed to carefully study aspects of bias and noise for the methods, and a torso phantom with cardiac insert was used to evaluate the performance of the methods for experimentally acquired data. The cross-talk compensation methods substantially improved lesion contrast and significantly reduced quantitative errors for simultaneously acquired data. Thallium image normalized mean square error (NMSE) was reduced from 0.522 without cross-talk compensation to as low as 0.052 with model-based cross-talk compensation. This is compared with a NMSE of 0.091 for the subtraction-based compensation method. The application of a preliminary model for cross-talk arising from lead fluorescence x-rays and collimator scatter gave promising results, and the future development of a more accurate model for collimator interactions would probably benefit simultaneous Tc/Tl imaging. Model-based compensation methods provide feasible cross-talk compensation in clinically acceptable times, and they may ultimately make simultaneous dual-isotope protocols an effective alternative for many imaging procedures.

Drug Interactions↗

PubMed1999

A mathematical model of motion of the heart for use in generating source and attenuation maps for simulating emission imaging.

This manuscript documents the alteration of the heart model of the three-dimensional (3D) mathematical cardiac torso (MCAT) phantom to represent cardiac motion. The objective of the inclusion of motion was to develop a digital simulation of the heart such that the impact of cardiac motion on single-photon emission computed tomography (SPECT) imaging could be assessed and methods of quantitating cardiac function could be investigated. The motion of the gated 3D MCAT's (gMCAT) heart is modeled using 128 separate and evenly spaced time samples from a blood volume curve approximating an average heart cycle. Sets of adjacent time samples can be grouped together to represent a single time interval within the heart cycle. Maximum and minimum chamber volumes were selected to be similar to those of a normal healthy person while the total heart volume stayed constant during the cardiac cycle. Myocardial mass was conserved during the cardiac cycle and the bases of the ventricles were modeled as moving towards the static apex. The orientation of the 3D MCAT heart was changed during contraction to rotate back and forth around the long axis through the center of the left ventricle (LV) using the end systolic time interval as the time point at which to reverse direction. Simple respiratory motion was also introduced by changing the orientation of the long axis of the heart to represent its variation with respiration. Heart models for 24 such orientations spanning the range of motion during the respiratory cycle were averaged together for each time sample to represent the blurring of the heart during the acquisition of multiple cardiac cycles. Finally, an option to model apical thinning of the myocardium was included. As an illustration of the application of the gMCAT phantom, the gated heart model was evaluated by measuring myocardial wall thickening. A linear relationship was obtained between maximum myocardial counts and myocardial thickness, similar to published results. Similar results were obtained for full width at half maximum (FWHM) measurements. With the presence of apical thinning, an apparent increase in counts in the apical region compared to the other heart walls in the absence of attenuation compensation turns into an apparent decrease in counts with attenuation compensation. The apical decrease was more prominent in end systole (ES) than end diastole (ED) due to the change in the partial volume effect. These observations agree with clinical trends. It is concluded that the gMCAT phantom can be used to study the influence of various physical parameters on radionuclide perfusion imaging.

Artifacts↗

PubMed1999

Effects of scatter substraction on detection and quantitation in hepatic SPECT.

UNLABELLED: The purpose of this investigation was to examine the effects of subtractive scatter compensation methods on lesion detection and quantitation. METHODS: Receiver operating characteristic (ROC) methodology was used to measure human observer detection accuracy for tumors in the liver using synthetic images. Furthermore, ROC results were compared with mathematical models for detection and activity quantitation to examine (a) the potential for predicting human performance and (b) the relationship between the detection and quantitation tasks. Images with both low and high amounts of scatter were compared with the ideal case of images of primary photons only (i.e., perfect scatter rejection) and with images corrected by subtracting a scatter image estimated by the dual photopeak window method. RESULTS: With low contrast tumors in a low count background, the results showed that scatter subtraction improved quantitation but did not produce statistically significant increases in detection accuracy. However, primary images did produce some statistically significant improvements in detection accuracy when compared with uncorrected images, particularly for high levels of scatter. CONCLUSION: Although scatter subtraction methods may provide improved activity quantitation, they may not significantly improve detection for liver SPECT. The results imply that significant improvement in detection accuracy for the conditions tested may depend on the development of gamma cameras with better scatter rejection.

Analysis of Variance↗

PubMed1998

Velocity sensitivity of slice-selective excitation.

The purpose of this study was to investigate how flow affects slice-selective excitation, particularly for radiofrequency (rf) pulses optimized for slice-selective excitation of stationary material. Simulation methods were used to calculate the slice profiles for material flowing at different velocities, using optimal flow compensation when appropriate. Four rf pulses of very different shapes were used in the simulation study: a 90 degrees linear-phase Shinnar-LeRoux pulse; a 90 degrees self-refocusing pulse; a minimum-phase Shinnar-LeRoux inversion pulse; and a SPINCALC inversion pulse. Slice profiles from simulations with a laminar flow model were compared with experimental studies for two different rf pulses using a clinical magnetic resonance imaging (MRI) system. We found that, for a given rf pulse, the effect of flow on slice-selective excitation depends on the product of the selection gradient amplitude, the component of velocity in the slice selection direction, and the square of the rf pulse duration. The shapes of the slice profiles from the Shinnar-LeRoux pulses were relatively insensitive to velocity. However, the slice profiles from the self-refocusing pulse and the SPINCALC pulse were significantly degraded by velocity. Experimental slice profiles showed excellent agreement with simulation. In conclusion, our study demonstrates that slice-selective excitation can be significantly degraded by flow depending on the velocity, the gradient amplitude, and characteristics of the rf excitation pulse used. The results can aid in the design of rf pulses for slice-selective excitation of flowing material.

Computer Simulation↗

PubMed1998

Fast implementations of reconstruction-based scatter compensation in fully 3D SPECT image reconstruction.

Accurate scatter compensation in SPECT can be performed by modelling the scatter response function during the reconstruction process. This method is called reconstruction-based scatter compensation (RBSC). It has been shown that RBSC has a number of advantages over other methods of compensating for scatter, but using RBSC for fully 3D compensation has resulted in prohibitively long reconstruction times. In this work we propose two new methods that can be used in conjunction with existing methods to achieve marked reductions in RBSC reconstruction times. The first method, coarse-grid scatter modelling, significantly accelerates the scatter model by exploiting the fact that scatter is dominated by low-frequency information. The second method, intermittent RBSC, further accelerates the reconstruction process by limiting the number of iterations during which scatter is modelled. The fast implementations were evaluated using a Monte Carlo simulated experiment of the 3D MCAT phantom with 99mTc tracer, and also using experimentally acquired data with 201Tl tracer. Results indicated that these fast methods can reconstruct, with fully 3D compensation, images very similar to those obtained using standard RBSC methods, and in reconstruction times that are an order of magnitude shorter. Using these methods, fully 3D iterative reconstruction with RBSC can be performed well within the realm of clinically realistic times (under 10 minutes for 64 x 64 x 24 image reconstruction).

Humans↗

PubMed1998

Block-iterative techniques for fast 4D reconstruction using a priori motion models in gated cardiac SPECT.

We introduce a fast block-iterative maximum a posteriori (MAP) reconstruction algorithm and apply it to four-dimensional reconstruction of gated SPECT perfusion studies. The new algorithm, called RBI-MAP, is based on the rescaled block iterative EM (RBI-EM) algorithm. We develop RBI-MAP based on similarities between the RBI-EM, ML-EM and MAP-EM algorithms. RBI-MAP requires far fewer iterations than MAP-EM, and so should result in acceleration similar to that obtained from using RBI-EM or OS-EM as opposed to ML-EM. When complex four-dimensional clique structures are used in the prior, however, evaluation of the smoothing prior dominates the processing time. We show that a simple scheme for updating the prior term in the heart region only for RBI-MAP results in savings in processing time of a factor of six over MAP-EM. The RBI-MAP algorithm incorporating 3D collimator-detector response compensation is demonstrated on a simulated 99mTc gated perfusion study. Results of RBI-MAP are compared with RBI-EM followed by a 4D linear filter. For the simulated study, we find that RBI-MAP provides consistently higher defect contrast for a given degree of noise smoothing than does filtered RBI-EM. This is an indication that RBI-MAP smoothing does less to degrade resolution gained from 3D detector response compensation than does a linear filter. We conclude that RBI-MAP can provide smooth four-dimensional reconstructions with good visualization of heart structures in clinically realistic processing times.

Algorithms↗

PubMed1998

Improved estimation of the detector response function for converging beam collimators.

Converging beam collimator geometries offer improved tradeoffs between resolution and noise for single photon emission computed tomography (SPECT). The major factor limiting the resolution in SPECT is the collimator-detector response blurring. In order to compensate for this blurring it is useful to be able to calculate the collimator response function. A previous formulation presented a method for calculating the response for parallel and converging beam collimators that assumed that the shape of the holes did not change over the face of the collimator. However, cast collimators are fabricated using pins with a constant cross-section (shape perpendicular to the pin axis). As a result, due to the angulation of the pins, the holes made by these pins have shapes on the front and back faces of the collimator that change with position. This change in hole shape is especially pronounced when the angle between the collimator hole and the collimator normal is large, as is the case for half-fan-beam or short-focal-length collimators. This paper presents a derivation of a modification to the original method that accounts for the change in shape of the collimator holes. The method has been verified by comparing predicted line spread functions to experimentally measured ones for a collimator with a maximum hole angle of 35 degrees with respect to the normal. This formulation is useful for predicting the response of fan-beam collimators in the design process and for use in detector response compensation algorithms.

Equipment Design↗

PubMed1998

Application of reconstruction-based scatter compensation to thallium-201 SPECT: implementations for reduced reconstructed image noise.

Scatter compensation in Tl-201 single photon emission computed tomography (SPECT) presents an interesting challenge because of the multiple emission energies and relatively large proportion of scattered photons. In this paper, we present a simulation study investigating reconstructed image noise levels arising from various implementations of iterative reconstruction-based scatter compensation (RBSC) in Tl-201 SPECT. A two-stage analysis was used to study single and multiple energy window implementations of reconstruction-based scatter compensation, and RBSC was compared to the upper limits on performance for other approaches to handling scatter. In the first stage, singular value decomposition of the system transfer matrix was used to analyze noise levels in a manner independent of the choice of reconstruction algorithm, providing results valid across a wide range of regularizations. In the second stage, the data were reconstructed using maximum-likelihood expectation-maximization, and the noise properties of the resultant images were analyzed. The best RBSC performance was obtained using multiple energy windows, one for each emission photopeak, and RBSC outperformed the upper limit on subtraction-based compensation methods. Implementing RBSC with the correct choice of energy window acquisition scheme is a promising method for performing scatter compensation for Tl-201 SPECT.

Phantoms, Imaging↗

PubMed1998

Effect of truncated projections on defect detection in attenuation-compensated fanbeam cardiac SPECT.

UNLABELLED: For some camera systems used in cardiac SPECT, the limited field of view of fanbeam-collimated detectors produces truncation in the projection data. This truncation may generate artifacts and distortions in the transmission CT images and in the attenuation-corrected myocardial SPECT images (that use the transmission CT images as attenuation maps), thus affecting clinical diagnosis. Concern over this problem stimulated us to evaluate the effect of truncation with human observer performance studies. METHODS: A three-dimensional mathematical cardiac-torso phantom that realistically models the attenuation and 201Tl uptake distributions in different organs of the upper torso was used for the investigation. Five degrees of truncation (from 0% to 40% truncation) in the projection data were simulated by the use of five different detector sizes collimated by fanbeam collimators. The attenuation maps were obtained by reconstructing the truncated transmission data generated from these fanbeam geometries. The emission data obtained with the same fanbeam geometries were reconstructed by using the maximum likelihood-expectation maximization algorithm and were corrected for attenuation using the reconstructed attenuation maps. Two observer performance studies were performed. In Study 1, the images were reconstructed without using the body contour as support, whereas in Study 2, the exact body contour was used in reconstructing both the attenuation maps and the attenuation-corrected SPECT images. RESULTS: The results of the receiver operating characteristics analysis indicate that there is very little difference between detection for the various degrees of truncation, and this difference only becomes noticeable for severe truncation of greater than 40%. CONCLUSION: Using the acquisition and processing methods, we found the use of the body contour as support in reconstructing truncated transmission CT and SPECT data reduces the loss of defect detectability in attenuation-compensated myocardial SPECT images due to truncated data.

Algorithms↗

PubMed1998

A comparison of 180 degrees and 360 degrees acquisition for attenuation-compensated thallium-201 SPECT images.

UNLABELLED: This study compared attenuation compensated, myocardial SPECT images reconstructed from 180 degrees and 360 degrees data to determine if either data acquisition method might yield improved image quality. Specifically, this study analyzed how the use of either 180 degrees or 360 degrees data affects: (a) the relative count density distribution, (b) defect contrast and (c) level of statistical noise in the left ventricular (LV) wall in the reconstructed SPECT images. METHODS: Using the three-dimensional MCAT phantom simulating 201Tl uptake in the upper torso and the SIMSET Monte Carlo code, noise-free projection datasets for both 180 degrees (45 degrees LPO to 45 degrees RAO) and 360 degrees acquisition were generated with the effects of nonuniform attenuation, collimator-detector response and scatter. In addition, low-noise experimental phantom data were acquired over 180 degrees and 360 degrees. Assuming the same total acquisition time, four sets of noisy projection data were simulated from scaled noise-free, simulated data for the following acquisitions: (a) 180 degrees and (b) 360 degrees data acquired on a 90 degrees dual-detector system and (c) 180 degrees and (d) 360 degrees data acquired on a 120 degrees triple-detector system. For each of the four acquisition schemes, 400 realizations of noisy projection data were generated, and the normalized s.d. in the reconstructed images was calculated for five ROIs in the LV wall. Images were reconstructed with nonuniform attenuation compensation using ML-EM algorithm for 25, 50 and 75 iterations. RESULTS: Both the simulated noise-free and experimental low-noise images reconstructed from 180 degrees and 360 degrees data showed nearly identical count densities and defect contrasts in the LV wall. For the 90 degrees dual-detector system, 180 degrees images showed less noise, while for the 120 degrees triple-detector system, 360 degrees showed less noise; however, these differences in noise level were extremely small after a smoothing filter was applied. The 180 degrees images acquired with the 90 degrees dual-detector system showed the same noise level as the 360 degrees images acquired with the 120 degrees triple-detector system, so neither system geometry had an advantage with respect to reduced noise in the SPECT images. CONCLUSION: When nonuniform attenuation compensation is included in the reconstruction, the count density in the LV wall is nearly identical for 180 degrees and 360 degrees SPECT images, and the 90 degrees dual-detector and 120 degrees triple-detector SPECT systems produced similar SPECT images for the same total acquisition time.

Algorithms↗

PubMed1997

Analysis of the reconstructibility and noise properties of scattered photons in 99mTc SPECT.

Since scattered photons carry degraded spatial information, scatter is typically considered a source of contamination in SPECT. However, with the advent of scatter modelling methods and reconstruction-based scatter compensation (RBSC), it may be possible to utilize scattered data in a productive manner. In this work we analyse the reconstructibility of scattered photon projection data and investigate the potential for using scattered photons to reduce the noise levels of SPECT images. We have simulated projection data for an elliptical phantom containing three cold rods in a uniform background of 99mTc activity. A variety of photopeak and scatter energy windows were formed, as well as corresponding RBSC transfer matrices. Each statistically weighted matrix was decomposed using SVD and analysed in terms of reconstructibility and noise properties. Results indicate that scattered photons contain sufficient information to reconstruct the source activity, but the scatter-only matrices are very poorly conditioned. We have also evaluated several methods of utilizing scattered events via RBSC, and compared them with other, idealized methods of handling scatter. It was found that scattered photons can be used productively when photopeak and non-photopeak data are separated through the use of multiple energy windows. The RBSC methods outperformed ideal scatter subtraction, but fell short of methods which assume perfect discrimination between scattered and primary events. The knowledge gained by this study may help guide future research and lead to better approaches to handling scatter in SPECT.

Computer Simulation↗

PubMed1997

Choice of initial conditions in the ML reconstruction of fan-beam transmission with truncated projection data.

We investigate the effects of initial conditions in the iterative maximum-likelihood (ML) reconstruction of fan-beam transmission projection data with truncation. In an iterative ML reconstruction, the estimate of the transmission reconstructed image in the previous iteration is multiplied by some factors to obtain the current estimate. Normally, a flat initial condition (FIC) or an image with equal positive pixel values is used as initial condition for an ML reconstruction. Usage of FIC has also been perceived as a way of preventing any bias on the reconstruction which may have come from the initial condition. When projection data have truncation, we show that using an FIC in an ML iterative reconstruction can introduce a bias to the reconstruction inside the densely sampled region (DSR), whose projection data have no truncation at any angle. To reduce this bias, we propose to use the largest right singular vector (LRSV) of the system matrix as the initial condition, and demonstrate that the bias can be reduced with the LRSV. When data truncation is reduced, the LRSV approaches the FIC. This result does not contradict to the use of FIC when projection data are not truncated. We also demonstrate that the reconstructed transmission image using LRSV as initial condition provides a more accurate attenuation coefficient distribution than that using FIC. However, the improvement is mostly in the area outside the DSR.

Humans↗

PubMed1997

Evaluation of right and left ventricular volume and ejection fraction using a mathematical cardiac torso phantom.

UNLABELLED: The availability of gated SPECT has increased the interest in the determination of volume and ejection fraction of the left ventricle (LV) for clinical diagnosis. However, the same indices for the right ventricle (RV) have been neglected. The objective of this investigation was to use a mathematical model of the anatomical distribution of activity in gated blood-pool imaging to evaluate the accuracy of two ventricular volume and ejection fraction determination methods. In this investigation, measurements from the RV were emphasized. METHODS: The mathematical cardiac torso phantom, developed to study LV myocardium perfusion, was modified to simulate the radioactivity distribution of a 99mTc-gated blood-pool study. Twenty mathematical cardiac torso phantom models of the normal heart with different LV volumes (122.3 +/- 11.0 ml), RV volumes (174.6 +/- 22.3 ml) and stroke volumes (75.7 +/- 3.3 ml) were randomly generated to simulate variations among patients. An analytical three-dimensional projector with attenuation and system response was used to generate SPECT projection sets, after which noise was added. The projections were simulated for 128 equidistant views in a 360 degrees rotation mode. RESULTS: The radius of rotation was varied between 24 and 28 cm to mimic such variation in patient acquisitions. The 180 degrees and 360 degrees projection sets were reconstructed using the filtered backprojection reconstruction algorithm with Butter-worth filtering. Comparison was made with and without application of the iterative Chang attenuation correction algorithm. Volumes were calculated using a modified threshold and edge detection method (hybrid threshold), as well as a count-based method. A simple background correction procedure was used with both methods. CONCLUSION: Results indicate that cardiac functional parameters can be measured with reasonable accuracy using both methods. However, the count-based method had a larger bias than the hybrid threshold method when RV parameters were determined for 180 degrees reconstruction without attenuation correction. This bias improved after attenuation correction. The count-based method also tended to overestimate the end systolic volume slightly. An improved background correction could possibly alleviate this bias.

Female↗

PubMed1996

The AAPM/RSNA physics tutorial for residents. Physics of SPECT.

Single-photon emission computed tomography (SPECT) provides three-dimensional (3D) image information about the distribution of a radiopharmaceutical injected into the patient for diagnostic purposes. By combining conventional scintigraphic and computed tomographic methods, SPECT images present 3D functional information about the patient in more detail and higher contrast than found in planar scintigrams. A typical SPECT system consists of one or more scintillation cameras that acquire multiple two-dimensional planar projection images around the patient. The projection data are reconstructed into 3D images. The collimator of the scintillation camera has substantial effects on the spatial resolution and detection efficiency of the SPECT system. Physical factors such as photon attenuation and scatter affect the quantitative accuracy and quality of SPECT images, and various methods have been developed to compensate for these image-degrading effects. In myocardial SPECT, an important application of SPECT, recent use of attenuation compensation methods has provided images with reduced artifacts and distortions caused by the non-uniform attenuation in the chest region and by the diaphragmatic and breast attenuation. Attenuation-compensated myocardial SPECT images have the potential to improve clinical diagnosis by reducing the false-positive and false-negative detection of myocardial defects. In the future, further improvement in SPECT images will be realized from the continuous development of new radio-pharmaceuticals for new clinical applications, instrumentation with high spatial resolution and detection efficiency, and image reconstruction algorithms and compensation methods that reduce the image-degrading effects of the collimator-detector, attenuation, and scatter.

Humans↗

PubMed1995

A fast and stable maximum a posteriori conjugate gradient reconstruction algorithm.

We have derived a maximum a posteriori (MAP) approach for iterative reconstruction based on a weighted least-squares conjugate gradient (WLS-CG) algorithm. The WLS-CG algorithm has been shown to have initial convergence rates up to 10x faster than the maximum-likelihood expectation maximization (ML-EM) algorithm, but WLS-CG suffers from rapidly increasing image noise at higher iteration numbers. In our MAP-CG algorithm, the increasing noise is controlled by a Gibbs smoothing prior, resulting in stable, convergent solutions. Our formulation assumes a Gaussian noise model for the likelihood function. When a linear transformation of the pixel space is performed (the "relaxation" acceleration method), the MAP-CG algorithm obtains a low-noise, stable solution (one that does not change with further iterations) in 10-30 iterations, compared to 100-200 iterations for MAP-EM. Each iteration of MAP-CG requires approximately the same amount of processing time as one iteration of ML-EM or MAP-EM. We show that the use of an initial image estimate obtained from a single iteration of the Chang method helps the algorithm to converge faster when acceleration is not used, but does not help when acceleration is applied. While both the WLS-CG and MAP-CG methods suffer from the potential for obtaining negative pixel values in the iterated image estimates, the use of the Gibbs prior substantially reduces the number of pixels with negative values and restricts them to regions of little or no activity. We use SPECT data from simulated hot-sphere phantoms and from patient studies to demonstrate the advantages of the MAP-CG algorithm. We conclude that the MAP-CG algorithm requires 10%-25% of the processing time of EM techniques, and provides images of comparable or superior quality.

Algorithms↗