Dual Window Research Articles

Oversampled Gabor representation for transient signals

Considers the Gabor representation that uses a one-sided exponential window for detection and analysis of transient signals. Earlier results on the critically sampled case are extended to the more practically useful oversampled case. For oversampling by an integer factor, the authors derive an explicit analytical expression for the dual window (dual frame) function required for computing the Gabor representation. Based on this expression they develop an efficient procedure for computing the Gabor coefficients. Finally, they demonstrate the performance of the method by numerical examples.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Window selection for dual photopeak window scatter correction in Tc-99m imaging

The width and placement of the windows far the dual photopeak window (DPW) scatter subtraction method for Tc-99m imaging is investigated in order to obtain a method that is stable on a multihead detector system for single photon emission computed tomography (SPECT) and is capable of providing a good scatter estimate for extended objects. For various window pairs, stability and noise were examined with experiments using a SPECT system, while Monte Carlo simulations were used to predict the accuracy of scatter estimates for a variety of objects and to guide the development of regression relations for various window pairs. The DPW method that resulted from this study was implemented with a symmetric 20% photopeak window composed of a 15% asymmetric photopeak window and a 5% lower, window abutted at 7 keV below the peak. A power function regression was used to relate the scatter-to-total ratio to the lower window-to-total ratio at each pixel, from which an estimated scatter image was calculated. DPW demonstrated good stability, achieved by abutting the two windows away from the peak. Performance was assessed and compared with Compton window subtraction (CWS). For simulated extended objects, DPW generally produced a less biased scatter estimate than the commonly used CWS method with k=0.5. In acquisitions of a clinical SPECT phantom, contrast recovery was comparable for both DPW and CWS; however, DPW showed greater visual contrast in clinical SPECT bone studies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Distance-dependent restoration filtering of dual photopeak window scatter compensated SPECT images

A non-iterative SPECT reconstruction method is presented which consists of pre-processing the projection data for compensation of scatter, attenuation and the detector response prior to ramp filtered backprojection. Scatter is compensated for using the dual-photopeak window (DPW) method, attenuation is compensated for using Bellini's method, and compensation for the non-stationary detector response and noise suppression is performed with a frequency distance principle (FDP) Wiener filter. This approach was compared to a number of different processing methods using a Monte Carlo simulation study of an anthropomorphic digitized phantom of the liver and spleen. Compared to Butterworth smoothing, the DPW/FDP Wiener filtering method can provide a substantial increase in contrast with a noise increase ranging from minimal to moderate depending on the cut-off frequency of the Butterworth filter. >

Out-of-plane photon compensation for 3-D SPECT image reconstruction with generalized matrix inverses

A computationally efficient 3-D image reconstruction method which compensates for detected out-of-plane photons has been developed for SPECT image reconstruction with generalized matrix inverses (GMI). Fully 3-D image reconstruction is approximated by a series of coupled 2-D image reconstructions for projection data acquired with parallel hole collimators, significantly reducing computer memory requirements. In this method, projection data are compensated for detected scattered photons using dual energy window scatter subtraction (Step 1). An initial source activity estimate in each transverse plane is then made using line source response functions in the system matrix (Step 2). With these activity estimates the contributions of out-of-plane unscattered photons are modeled and subtracted from the projection data, and an updated source activity estimate is computed (Step 3). For noise-free projection data from a Monte Carlo simulated myocardial perfusion study, lesion contrast increases and activity spillover from the myocardium into the adjacent cardiac blood pool is reduced. For projection data with Poisson noise, activity estimates from Step 3 are inferior to those from Step 2. One reason is that the relative noise level of the projection data is substantially increased by subtracting the estimated out-of-plane contribution. The Step 3 activity estimation is also sensitive to the source activity distribution used for out-of-plane unscattered photon compensation. Though the out-of-plane compensation technique of Step 3 provides some benefit for noise-free projection data, this study suggests that it may not be well-suited for practical application to count-limited clinical SPECT studies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Addition of noise by scatter correction methods in PVI

Effective scatter correction techniques are required to account for errors due the high scatter fraction seen in positron volume imaging (PVI). To be effective, the correction techniques must be accurate and practical, but they also must not add excessively to the statistical noise in the image. The authors have investigated the noise added by three correction methods: a convolution/subtraction method; a method that interpolates the scatter from the events outside the object; and a dual energy window method with and without smoothing of the scatter estimate. The methods were applied to data generated by Monte Carlo simulation to determine their effect on the variance of the corrected projections. The convolution and interpolation methods did not add significantly to the variance. The dual energy window subtraction method without smoothing increased the variance by a factor of more than twelve, but this factor was improved to 1.2 by smoothing the scatter estimate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Simultaneously constraining SPECT activity estimates with primary and secondary energy window projection data

Single photon emission computed tomography (SPECT) projection data acquired in primary (photopeak) and secondary energy windows provide independent constraints on the source activity distribution. These constraints are used simultaneously and a single matrix equation is solved to estimate source activity. This simultaneous constraint reconstruction method (I) is evaluated in a Monte Carlo experiment and compared with activity estimation using only primary energy window projection data with (m) and without (II) modeling the detection of scattered photons in the system matrix, and with dual energy window scatter subtraction (TV). The transport and detection of photons emitted by Tc-99m within a water-filled cylinder is simulated. Images are reconstructed using generalized matrix inverses, which are designed so that the estimated variance of the reconstructed source voxel activities is approximately constant. For 40 projection data sets with about one million counts/slice in the primary energy window, the mean activity in regions of interest is within 2.2%, 28.2%, 2.2% and 3.5% of the source model activity for reconstruction methods I-IV, respectively. The full-widths at half-maximum (FWHM) of resolution kernels from methods I and III differ on average by less than 1%; the FWHM of kernels from methods II and IV average 8% larger. In this experiment, the simultaneous constraint method yields no appreciable improvement in quantitative accuracy or resolution over the use of primary energy window data alone when the detection of scattered photons is modeled. These two methods are only slightly superior to activity estimation using dual energy window scatter subtraction.

Influence of phantom size, shape, and density, and collimator selection on the dual photopeak window scatter correction method /spl lsqb/SPECT/spl rsqb/

Results from a Monte Carlo investigation are presented for the application of the dual photopeak window (DPW) scatter correction method to phantom geometries of varying shapes, sizes, and densities as well as to several collimators. A Monte Carlo program was used to simulate a Tc-99m point source at locations of 0.5 cm from the phantom surface, at the center, 0.5 cm from the bottom, and 4/5 the distance lateral from the center for each case of variation. DPW was then used to correct for scatter using a regression relation determined for a 30.5 cm/spl times/23.0 cm water-filled elliptical tub phantom using an ultrahigh resolution collimator. The application of DPW in all cases accounted for an approximate twelve-fold reduction in scatter fraction with an excellent agreement between the true scatter and scatter estimate distributions both at the tails and under the peak of the point spread functions. >

Quantitative imaging of holmium-166 with an Anger camera

The objective of this study was to develop a quantitative Anger camera imaging approach for 166Ho in the skeletal system of patients. A dual energy window method was designed to subtract the interference from septal penetration and bremsstrahlung events in Anger camera images acquired with the 80 keV X-rays emitted by 166Ho. The validity of this scatter subtraction method for 166Ho images was demonstrated as improvements of the line spread function and modulation transfer function. Camera sensitivity was found to be nearly independent of source-to-collimator distance only for images acquired with a high-energy collimator. Studies in an Alderson abdominal water phantom demonstrated scatter subtraction can provide quantitative Anger camera images of 166Ho with a scatter multiplier of k=1.0 and a correction for attenuation. Attenuation correction factors derived from a transmission image were measured for the phantom and verified with water-equivalent blocks of known thickness. Whole-body scan images of 166Ho localized in the skeletal system of patients were significantly improved with this simple scatter subtraction method, and when used to estimate the activity distribution within separate bone regions of the skeleton.

Triple energy window scatter correction technique in PET

A practical triple energy window technique (TEW) is proposed, which is based on using the information in two lower energy windows and one single calibration, to estimate the scatter within the photopeak window. The technique is basically a conventional dual-window technique plus a modification factor, which can partially compensate object-distribution dependent scatters. The modification factor is a function of two lower scatter windows of both the calibration phantom and the actual object. In order to evaluate the technique, a Monte Carlo simulation program, which simulates the PENN-PET scanner geometry, was used. Different phantom activity distributions and phantom sizes were tested to simulate brain studies, including uniform and nonuniform distributions. The results indicate that the TEW technique works well for a wide range of activity distributions and object sizes. The comparisons between the TEW and dual window techniques show better quantitative accuracy for the TEW, especially for different phantom sizes. The technique is also applied to experimental data from a PENN-PET scanner to test its practicality.

A scatter correction method for Tl-201 images: A Monte Carlo investigation

Results from the application of a modified dual photopeak window scatter correction method to Monte Carlo simulated Tl-201 emission images are presented. In the Monte Carlo investigation, individual simulations are performed for six radiation emissions of Tl-201. For each emission, point sources of Tl-201 are imaged at various locations in a water-filled elliptical tub phantom using three energy windows. The DPW method is applied to each point source image to estimate the scatter distribution. For point source images in both homogeneous and nonhomogeneous attenuating media, the application of this modified version of DPW results in an approximately sixfold reduction in the scatter fraction and an excellent agreement of the shape of the tails between the estimated scatter distribution and the Monte Carlo-simulated truth. This method is also applied to two views of an extended cardiac distribution within an anthropomorphic phantom, again resulting in at least a sixfold improvement between the scatter estimate and the Monte Carlo-simulated true scatter. >

Capillary Transit Times and Kinetics of Oxygenation in the Primary Respiratory Organ of Early Chick Embryo

In early avian embryos respiratory gas exchange between environment and blood takes place via the blood vessels of the yolk sac. To investigate the principles governing oxygen transfer we determined the RBC velocity, the length of the a-v channels, and the RBC transit time in the vitelline capillary network of 4-day-old chick embryos. Measurements were carried out using a digital video analyzing system for on-line velocity measurements (modified dual video window technique) and morphometric determinations. Velocity measurements were performed on fluorescent (FITC)-labeled erythrocytes. All measured parameters exhibited a large scatter. The mean RBC velocity was 193 ± 117 (±SD) μm/sec and the mean length of the a-v channels was 434 ± 291 μm. The transit time ranged between 0.1 and 17 sec (median value 2.5 sec). Using previously determined oxygen hemoglobin dissociation curves and blood oxygen partial pressure (Meuer and Baumann, 1988, Respir. Physiol 71, 331-342; Meuer et al., 1989, J. Dev. Physiol. 11, 354-359) the kinetics of the oxygen uptake by the blood was calculated. This gave an estimated transit time required for 99% oxygen saturation of 1.5 sec. The data show that in more than a third of all a-v channels the transit time is shorter than required for complete blood oxygenation. This is consistent with recent determinations of the blood oxygen saturation of mixed arterialized blood (89% at Day 4; Meuer and Baumann, 1988).

A Monte Carlo investigation of the dual photopeak window scatter correction method (SPECT)

Results from a Monte Carlo investigation of the dual photopeak window (DPW) scatter correction method are presented for point and extended sources of Tc-99m in both homogeneous and nonhomogeneous attenuating media. The DPW method uses the ratio of counts in two nonoverlapping energy windows within the photopeak region as input to a regression relation. A pixel-by-pixel estimate of the scatter in the summed windows is obtained and subtracted to yield an estimate of the primary. An approximately tenfold decrease in the scatter fraction and an excellent agreement with the shape of the true scatter distribution were observed. >

Reconstruction methods for quantitative brain SPECT

The effects of various reconstruction algorithms and compensation techniques for brain SPECT (single-photon-emission computed tomography) reconstruction are investigated. The conventional FB (filtered backprojection) and iterative ML-EM (maximum-likelihood estimation-maximization) algorithms are used. Projection data were acquired via dual energy windows for use in scatter compensation. Assuming uniform attenuation, the Chang attenuation compensation algorithm is applied. Restoration filters are used with the FB algorithms to compensate for the average effects of collimator and scatter response functions. Two-dimensional (2-D) and three-dimensional (3-D) compensation methods are compared. Compensation for the one-dimensional spatially variant collimator response, using an iterative reconstruction method, is shown. The results show that conventional reconstruction and compensation methods provide substantial improvement in reconstructed image quality and quantitative accuracy compared with no compensation. 3-D processing gives better results than 2-D processing. Iterative reconstruction provided more accurate quantitation with a substantial increase in computational cost.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evaluation of projection pixel-dependent and pixel-independent scatter correction in SPECT

Dual energy window scatter subtraction methods in single photon emission computed tomography (SPECT) to compensate for the detection of Compton-scattered photons are based on the assumption that the number of scattered photons detected by each projection pixel in the primary (photopeak) energy window is proportional to the number of photons detected in a secondary (scatter) energy window. The reliability of this assumption is tested using Monte Carlo modeling of photon detection kernels for photon emission by Tc-99 m within a water-filled cylinder. The primary window scatter kernels and the secondary window kernels are correlated highly, and projection pixel-dependent and pixel-independent scatter correction factors are obtained from least squares fits of these kernels to each other. In a numerical experiment with noise-free projection data, average reconstructed intensities in regions of interest are nearly as well-estimated for both pixel-independent and pixel-dependent scatter-corrected projection data reconstructed using the approximate generalized matrix inverse (GMI) of the nonscatter photon detection matrix as for primary window projection data reconstructed with the approximate GMI of the photon matrix which includes scatter. >

Cell-free plasma layer in cerebral microvessels.

Two diameters of vessel and red cell column in cerebral microvessels (> 29.8 microns in diameter) of cat were measured together with red cell velocity, using a two fluorescent tracer method. A fluorescein isothiocyanate (FITC)-labeled red cell was adopted as a flow tracer to measure the cell velocity with a dual window technique. Based on the fluorescence image, the red cell column diameter was measured. Plasma was stained with rhodamine-B isothiocyanate (RITC)-labeled dextran to measure the vessel diameter. The thickness of the cell-free plasma layer could be determined from the difference of the two diameters. The obtained thickness of the cell-free layer was not described by a simple function of vessel diameter or red cell velocity; it was dependent on the pseudo shear rate defined by the ratio of cell velocity to vessel radius. The layer thickness increased with a decrease in the pseudo shear rate.

Air Movement

Here are examples of air movement patterns in three office buildings. Building 1 is a 1930s' office with natural ventilation and openable windows. Building 2 is 1984‐vintage with partial tint in the glazing, openable windows (but discouraged by management) and VAV air conditioning. Building 3 is late 1970s, with a dual air duct system and openable windows. Patterns vary considerably. In building 2, air speeds are below 0.1m/s at the work positions. This is below comfort criteria. In building 3 air speeds are above 0.3m/s which is higher than comfort levels.

Comparison of dual-window and convolution scatter correction techniques using the Monte Carlo method

In order to improve the accuracy of SPECT measurements, correction methods have to be used. Two different scatter correction techniques-the dual-window (DW) technique and the convolution (CV) technique-were compared using projection data, simulated by the Monte Carlo method. Comparison with measured data was also made to validate the accuracy of the Monte Carlo code. The main goal was to investigate how well the estimated scatter distributions, using these two scatter correction techniques, fit the simulated, true scatter distribution of the projection data obtained in the photo-peak window. The scatter distributions predicted by the CV technique were found to be consistently lower than those simulated by the Monte Carlo method in the part of the scatter distribution corresponding to the locations of the sources. The DW technique gives lower estimates of the scatter distribution. However, the scatter distribution estimated by the DW technique and the simulated scatter distribution bear a close resemblance to each other.

Four window differential capillary velocimetry

A video red blood cell velocimeter was implemented with four photometric windows in such a fashion that the upstream and downstream signals are the difference between spatially separated window pairs. The performance of this system was compared with that of a conventional dual window photometric video velocimeter. Tests were made with artificial patterns of red blood cells that simulated long trains of contiguous cells, or large plasma gaps. It was found that the four window system produces a correlogram that is better suited for delay to maximum cross-correlation detection. Similarly, when the responses of the two methods were compared in terms of ability to detect changes of velocity, the time constant for a test step velocity change was found to be 1.2 ± 0.7 sec for the four window system vs 2.3 ± 0.6 sec for the two window system. It is concluded that this modification of the capillary red blood cell velocimetry methodology is better suited for detecting the spontaneous flow variations due to vasomotion.

Dual isotope thallium and indium antimyosin SPECT imaging to identify acute infarct patients at further ischemic risk.

Forty-two patients (28 men and 14 women) with acute myocardial infarction (35 Q, seven non-Q wave) were injected with 2.0 mCi indium 111-labeled antimyosin (AM) monoclonal antibody (111In AM) within 48 hours of the onset of chest pain. Forty-eight hours later (72-96 hours after onset of chest pain), patients were injected with 2.2 mCi thallium 201, and two sets of single-photon emission computed tomography (SPECT) images were obtained simultaneously using dual energy windows set for the 247 keV indium photopeak and the 70 keV thallium peak. Seventeen patients had repeat scans at 4 hours. 111In AM uptake and 201Tl defects were localized to one or more of 24 coronal and sagittal segments. Scans with only 201Tl defects and corresponding 111In AM uptake were classified as matches; scans with unmatched 201Tl defects in addition to matching regions corresponding to electrocardiographic infarct location were classified as mismatches; and scans with 201Tl and 111In AM uptake in the same segments were classified as overlap. Scan patterns were correlated with clinical evidence for residual ischemia occurring within 6 weeks of infarct and including infarct extension, recurrent angina, and positive predischarge low-level or 6-week symptom-limited stress tests and with coronary anatomy. Fourteen patients had only matching patterns (group 1), 23 had mismatches (group 2), and five had 201Tl-111In overlap as the predominant pattern. None of the patients in group 1 had previous myocardial infarction; in each, the matched area corresponded to the Q wave location on electrocardiogram, and none had further in-hospital ischemic events or positive stress tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of acute Q-wave myocardial infarct size with single photon emission computed tomography imaging of indium-111 antimyosin

Myocardial infarct size was measured by single photon emission computed tomography (SPECT) following injection of indium-111 antimyosin in 27 patients (18 male and 9 female; mean age 57.4 ± 10.5 years, range 37 to 75) who had acute transmural myocardial infarction (MI). These 27 patients represent 27 of 35 (77%) consecutive patients with acute Q-wave infarctions who were injected with indium-111 antimyosin. In the remaining 8 patients either tracer uptake was too faint or the scans were technically inadequate to permit infarct sizing from SPECT reconstructions. In the 27 patients studied, infarct location by electrocardiogram was anterior in 15 and inferoposterior in 12. Nine patients had a history of prior infarction. Each patient received 2 mCi of indium-111 antimyosin followed by SPECT imaging 48 hours later. Infarct mass was determined from coronal slices using a threshold value obtained from a human torso/cardiac phantom. Infarct size ranged from 11 to 87 g (mean 48.5 ± 24). Anterior infarcts were significantly (p < 0.01) larger (60 ± 20 g) than inferoposterior infarcts (34 ± 21 g). For patients without prior MI, there were significant inverse correlations between infarct size and ejection fraction (r = 0.71, p < 0.01) and wall motion score (r = 0.58, p < 0.01) obtained from predischarge gated blood pool scans. Peak creatine kinase-MB correlated significantly with infarct size for patients without either reperfusion or right ventricular infarction (r = 0.66). Seven patients without prior infarcts had additional simultaneous indium-111/thallium-201 SPECT studies using dual energy windows. Size of thallium-201 perfusion defect correlated with indium-111 antimyosin infarct size (r = 0.89, p < 0.01). Infarct size as percentage of left ventricle infarcted ranged from 11 to 33%. These data suggest that acute transmural infarct size can be accurately measured from SPECT indium-111 antimyosin imaging. This technique may be useful when acute myocardial damage is not related to creatine kinase values (right ventricular infarct and early reperfusion) or to ejection fraction (prior infarcts).