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Nuclear Science, IEEE Transactions on

Issue 6 • Date Dec. 1997

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Displaying Results 1 - 22 of 22
  • Conference Author Index

    Page(s): 0_8
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    Freely Available from IEEE
  • Optimization of transmission and emission scan duration in 3D whole-body PET

    Page(s): 2400 - 2407
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    Whole-body PET imaging is being increasingly used to identify and localize malignant disease remote from the site of the primary tumour. Patients are typically scanned at multiple contiguous bed positions over an axial length of 75-100 cm. Ideally, for oncology patients, the total scan duration should not exceed about an hour and therefore only 5-10 minutes of imaging can be performed at each bed position. To minimize the total scan duration, the transmission scan is often omitted and the emission scan reconstructed without attenuation correction. However, whole-body scans reconstructed without attenuation are non-quantitative and can lead to incorrect diagnoses, particularly for tumours located deep within the body. We have performed a series of torso phantom measurements on a 3D PET scanner (ECAT ART) to investigate the optimal partition of scan time between the emission and transmission scans for a fixed total scan duration. We find that 20%-40% of the total scan time should be dedicated to the transmission acquisition to minimize noise in the attenuation-corrected image. The optimal partition depends on the method of calculating the attenuation correction factors and on the reconstruction algorithm View full abstract»

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  • Development of a lead X-ray compensation method in simultaneous Tl-201 SPECT and Tc-99m TCT using a flood source

    Page(s): 2459 - 2464
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    The authors have developed an uncollimated sheet line source with a fluoroplastic tube for gamma ray transmission CT (TCT). This source filled with Tc-99m was attached to the collimator face on one head of a dual head single photon emission computed tomography (SPECT) system. In simultaneous SPECT/TCT scans using Tl-201 as the SPECT source and Tc-99m as the TCT source, scatter and spilldown contamination in the Tc-99m (141 keV) data and the Tl-201 (75 keV) data can be removed with the triple energy window (TEW) method. However, lead X-ray contamination in the Tl-201 data cannot be removed with the TEW method. The authors have developed an estimation method of the lead X-ray contamination in the Tl-201 data. This method depends on the hypothesis that the lead X-ray is proportional to the amount of attenuating media and primary photons along each projection path. The amount of attenuating media is given by a transmission image and that of primary photons is estimated by the TEW method. The coefficients needed for this estimation are experimentally determined by only Tc-99m TCT scan data The feasibility of the proposed method was demonstrated by a chest phantom study and a myocardial clinical study View full abstract»

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  • Measurement of absolute light yield and determination of a lower limit for the light attenuation length for YAP:Ce crystal

    Page(s): 2415 - 2418
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    In small animal PET applications, the YAP:Ce scintillator has given some promising results. Because of its somewhat lower efficiency with respect to other PET crystals at 511 keV the depth of the crystal is a crucial parameter. Past measurements of the light attenuation length for YAP:Ce scintillator have given rather different results with values as low as 3 cm. We have studied the pulse height spectra of full energy absorption events as a function of the interaction distance from the photomultiplier window for five match like 2×2×30 mm3 YAP:Ce crystals. Each crystal was covered on the four long sides and on one of the square section sides with a 5 μm reflective layer. The crystals were the middle row of a bundle of 5 by 5 such crystals. By using a transport model for light within the crystals, for this crystal configuration, we obtained a lower limit of 6.8±0.3 cm on the average `bulk' (not including the losses from reflection) light attenuation length. This lower limit is significantly higher than the attenuation length measured by other authors. From these measurements and calculations, we have also determined the average absolute light yield for the five crystals to be 24000±1400 photons/MeV View full abstract»

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  • Advantages of list-mode acquisition of dynamic cardiac data [ECG-gated nuclear medicine imaging]

    Page(s): 2431 - 2438
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    Conventional frame-mode acquisition of multigated blood-pool (MUGA) studies is an imprecise means to study cardiac dynamics in part because the energy and ECG waveform information is not retained Early methods for list-mode acquisition were also somewhat limited in that only the (x,y) coordinate, a timing pulse and an R-wave detect pulse were stored. The authors have extended list-mode capability to include full storage of the ECG waveform and the energy value associated with each spatial coordinate. The spatial coordinates (x and y), energy value (E), and an encoded physiologic/timing signal (p/t) are stored on an event-by-event basis as 8-bit values. These four bytes are streamed onto a hard disk storage device allowing even stress gated blood pool studies to be recorded. List-mode acquisition gives greater flexibility than frame-mode. Among the demonstrable advantages this flexibility provides are the ability to: (1) Measure and correct respirational displacement of the ventricle to minimize motion blurring; (2) Form gated sequences representing only a specific aberrancy for detection of ectopic ventricular foci; and (3) Use the stored multispectral energy information to perform scatter correction on the dynamic images. To conclusively demonstrate the improved resolution of the list-mode methodology, a dynamic phantom was used to simulate blurring due to respiration. The advantages of list-mode warrant its further use, and it may be especially valuable for wall thickness measurements View full abstract»

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  • Effects of detector sensitivity on image quality in multidetector SPECT

    Page(s): 2483 - 2488
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    The common imaging practice of each detector on a multidetector SPECT imaging system to acquire only a fraction of the total number of projections in a 360° projection set introduces the possibility of imbalances in system response which can cause artifacts and changes in image quality. Here we evaluate the effect of variations in detector sensitivity caused by differences in the position of the photopeak window, window width, and different collimators on reconstructed image quality. Analytical simulations of cardiac phantom geometries and experimental measurements with Data Spectrum's Cardiac and Deluxe SPECT phantoms on a three detector SPECT imaging system were used in the evaluation. The relative efficiency of one of three detectors was changed over a series of values between 100 and 0%, while the efficiency of the remaining two detectors was maintained at 100%. The results show that decreased activity and geometric distortions occur along directions perpendicular to the projections with altered sensitivity. The degree of distortion and decrease of activity depends on the geometry of the activity distribution and the reduction in sensitivity. The relative count ratios in the regions with decreased activity varied over a range of 1.0 to 0.59 as the detector sensitivity is decreased from 100 to 0% of the full energy window. Variations in detector sensitivity can degrade spatial resolution and uniformity and create artifacts in reconstructed tomographic slices View full abstract»

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  • A modeling-based factor extraction method for determining spatial heterogeneity of Ga-68 EDTA kinetics in brain tumors

    Page(s): 2522 - 2526
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    The ROI method applied to Ga-68 EDTA PET dynamic study data for the quantitative determination of brain tumor BBB permeability assumes that the tumor is homogeneous in terms of Ga-68 EDTA kinetics, even though it is known that brain tumors are highly heterogeneous in structure. The relatively high image noise of Ga-68 PET studies have prevented the examination of Ga-68 EDTA kinetics by nonlinear regression on a single pixel basis. In this study, we have developed an efficient and effective method to separate brain tumor tissue into sub-regions with different Ga-68 EDTA kinetics on a pixel-by-pixel basis. Computer simulation and ten Ga-68 EDTA PET patient studies were used to evaluate the performance of the new method. During a PET dynamic study (total 64 min), 20-25 arterial samples were taken for the input function. Whole-tumor ROIs were defined on T1-weighted MRI images and then copied to the registered PET dynamic images to measure whole-tumor time activity curves. The method uses a two-compartment model to extract three component factors (vascular component, fast and slow component factors) from whole-tumor kinetics by model fitting. The kinetics in each pixel were expressed as a linear combination of the three factors. The three coefficients in the expression can be estimated by the linear least-square method and produce three factor images corresponding, respectively, to the permeability of the fast and the slow component factors and the plasma volume. Whole-tumor regions were separated into two regions-one with mainly fast kinetics that was evident in the fast factor images and one with slow kinetics that was evident in slow factor images. The two regions have markedly different uptake (0.036±0.015 ml/min/g and 0.009±0.006 ml/min/g for fast and slow kinetic sub-regions, respectively) and clearance rates (0.22±0.15 /min and 0.023±0.021 /min for fast and slow sub-regions, respectively). The overlap of the two resulting sub-regions is small (3.5±1.7% of the two regions). Computer simulation and patient studies show that the method is robust for a wide range of noise levels. This method has combined the advantages of statistical factor analysis and the modeling approach View full abstract»

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  • RITM: a mini γ camera for pre and per-operative radio guided cancer surgery evaluation for bone tumor localization in theater blocks

    Page(s): 2445 - 2449
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    The authors have developed a multi-functional portable γ-radio-imager (RITM) based on a position sensitive photo-multiplier tube (PSPMT) in order to evaluate the potential of such a mini γ-camera concept in radio-pharmacology and nuclear medicine. The authors report here an evaluation of their RITM device for bone surgery. It concerns localization of the Osteoid Osteoma (benign bone tumor) performed in the theater block before skin incision and during the surgical lesion extraction. In the over 13 cases the authors studied, the diagnosis furnished by RITM was always confirmed by post-operation anatomopathological analysis. This shows how RITM can be used as an additional indicator to monitor the operation. Following RITM experience, the authors are developing a new small field of view γ-per-operative compact imager (POCI) with submillimeter spatial resolution performance. It consists of a high resolution collimator and a YAP:Ce crystal assembly coupled to an intensified position sensitive diode (IPSD). This hand-held imaging probe is first dedicated to intra-operative monitoring for thyroid tumor and neuroblastoma removal. Characteristics of the POCI device and preliminary results are briefly presented View full abstract»

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  • Investigation of angular smoothing of PET data

    Page(s): 2494 - 2499
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    Radial filtering of emission and transmission data is routinely performed in PET during reconstruction in order to reduce image noise. Angular smoothing is not typically done, due to the introduction of a non-uniform resolution loss. The goal of this paper was to assess the effects of angular smoothing on noise and resolution. Angular smoothing was incorporated into the reconstruction process on the Scanditronix PC2048-15B brain PET scanner. In-plane spatial resolution and noise reduction were measured for different amounts of radial and angular smoothing. For radial positions away from the center of the scanner, noise reduction and degraded tangential resolution with negligible loss of radial resolution were seen. Near the center, no resolution loss was observed, but there was also no reduction in noise even for large amounts of angular filtering. These results can be understood by recognizing that angular filtering is equivalent to a weighted sum of rotated images. We conclude that angular smoothing is not optimal due to its anisotropic noise reduction and resolution degradation properties View full abstract»

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  • Simulating the performances of an LSO based position encoding detector for PET

    Page(s): 2450 - 2458
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    The authors investigated the impact of replacing BGO by lutetium oxyorthosilicate (LSO) in the fabrication of the EXACT HR PLUS position encoding detector for PET. A detailed Monte Carlo simulation was used to track the interactions of energetic photons in the volume of the block as well as to treat the generation and propagation of scintillation light through its geometry. The simulation also accounts for LSO's non-proportional scintillation response, the bulk attenuation to its own scintillation and the noise contribution from the amplification of photoelectrons in the readout units. The model predicts that the increased photostatistics available in LSO compared to BGO leads to improvements by up to a factor five in the peak-to-valley ratios of the position response of the block to a uniform flood of 511 keV photons. For the crystals located along the diagonal, and considering events in a window of 350 to 650 keV, the position encoding accuracy is found to vary from 69% to 88% representing an absolute gain of at most 8% over its BGO precursor. With peak photoelectron counts of 678 to 2024, the energy resolution of the same crystals is found to vary from 14% to 9% View full abstract»

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  • 1996 Medical Imaging Conference (MIC)

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    The following topics were discussed: mammography and tumor imaging; instrumentation-data quantitation and data processing; and image reconstruction, image processing and modeling View full abstract»

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  • High pressure xenon electronically collimated camera for low energy gamma ray imaging

    Page(s): 2408 - 2414
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    A high pressure xenon scintillation drift chamber with 30 cm diameter field of view has been built and tested as a two-dimensional imaging camera for low energy γ-rays. Experiments demonstrate that the camera can be also operated in an electronically collimated mode as a three-dimensional imaging Compton camera. A spatial resolution of 25 mm is measured for a 140 keV point-like source placed 10.5 cm in front of the camera without any collimator. The resolution is limited by the intrinsic resolution of the 19 channel readout system which is based on 80 mm diameter phototubes. A way to improve the imaging performance is discussed View full abstract»

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  • Direct estimation of dynamic parameters in SPECT tomography

    Page(s): 2425 - 2430
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    This paper solves the problem of estimating dynamic parameters of SPECT reconstruction directly from the projections. This is done by spanning the dynamic parameters of each pixel with a basis of exponentials. Any time decay system can be modeled in this way. This process leads to a linear Fredholm integral equation of the first kind which is discretized and then solved with conjugate gradient methods. We tested our algorithm by applying it to a numerical phantom and to a simple real phantom View full abstract»

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  • Computer-aided lesion detection with statistical model-based features in PET images

    Page(s): 2509 - 2521
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    Positron emission tomography (PET) with the glucose analog [18F] fluorodeoxyglucose is proving to be useful in cancer diagnosis and treatment. However, as in all nuclear medicine imaging technologies, lesion detection with PET is often hindered by limited spatial resolution and low signal-to-noise ratios. Under such conditions, conventional diagnosis by visual inspection usually becomes difficult and potentially inaccurate. In this paper, we propose use of computer-aided lesion detection methods for PET imaging by applying a maximum likelihood ratio test and a composite hypothesis test, assuming that the mean positron emission rate is deterministic or random, respectively. In our approach, different statistical models characterizing the mean positron emission rate, the raw sinogram data and the filtered backprojection (FBP) reconstructed image are used to derive the test criteria. Three methods to estimate the unknown parameters of the test functions from observations are presented. The performance of one of the proposed methods is evaluated and compared with both simulated and experimental phantom data. In the preliminary trials, the methods detect correctly (with a high probability >0.9) lesions of diameter⩾15 mm with lesion-to-background contrast 1.1:1. Under the same conditions, the test lesion could not be detected by visual inspection alone in the images reconstructed by either the FBP or the maximum likelihood iterative algorithms. The methods may also be used for the objective assessment of the quality of images reconstructed from different algorithms View full abstract»

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  • Automated 3-D segmentation of respiratory-gated PET transmission images

    Page(s): 2473 - 2476
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    As a preliminary step toward performing respiration compensated attenuation correction of respiratory-gated cardiac PET data, we acquired and automatically segmented respiratory-gated transmission data for a dog breathing on its own under gas anesthesia. Transmission data were acquired for 20 min on a CTI/Siemens ECAT EXACT HR (47-slice) scanner. Two respiratory gates were obtained using data from a pneumatic bellows placed around the dog's chest. For each respiratory gate, torso and lung surfaces were segmented automatically using a differential 3-D image edge detection algorithm. Three-dimensional visualizations showed that during inspiration the heart translated about 4 mm transversely and the diaphragm translated about 9 mm inferiorly. The observed respiratory motion of the canine heart and diaphragm suggests that respiration compensated attenuation correction may be necessary for accurate quantitation of high-resolution respiratory-gated human cardiac PET data. Our automated image segmentation results suggest that respiration compensated segmented attenuation correction may be possible using respiratory-gated transmission data obtained with as little as 3 min of acquisition time per gate View full abstract»

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  • The spatially-variant backprojection point kernel function of an energy-subtraction Compton scatter camera for medical imaging

    Page(s): 2477 - 2482
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    An energy-subtraction Compton scatter camera (ESCSC) was previously proposed for in-vivo imaging of radiopharmaceuticals used as bio-tracers in nuclear medicine. To further evaluate the usefulness of this ESCSC design, studies pertaining to image reconstruction are explored and presented. Generally speaking, a Compton scatter camera works on the principle that an emitted gamma ray undergoes a Compton scatter interaction in a primary detector system and then is subsequently absorbed by a secondary detector system. Using the measured interaction energies and positions, a cone surface can be backprojected which intercepts the emission space near the point of the gamma-ray emission (proximity depends on resolution). When backprojecting and linearly superposing multiple cones into a source space, calculations should include normalizing the total weight contributed by each cone as well as how the differentially intercepted area increases as you move farther away from the vertex of the cone (i.e., intercepted voxels farther away from the vertex are given less weight). Backprojected “point kernel profiles”, based upon simulated data, are presented corresponding to point sources located at several positions (revealing the degree of spatial variance) within the ESCSC camera geometry. From these results the spatially variant point kernel function may be deduced for future use in image reconstruction. Additionally, two different algorithms for backprojection are compared View full abstract»

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  • Correction of energy-dependent systematic errors in dual-energy X-ray CT using a basis material coefficients transformation method

    Page(s): 2419 - 2424
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    Computer simulation results from our previous studies showed that energy dependent systematic errors exist in the values of attenuation coefficient synthesized using the basis material decomposition technique with acrylic and aluminum as the basis materials, especially when a high atomic number element (e.g., iodine from radiographic contrast media) was present in the body. The errors were reduced when a basis set was chosen from materials mimicking those found in the phantom. In the present study, we employed a basis material coefficients transformation method to correct for the energy-dependent systematic errors. In this method, the basis material coefficients were first reconstructed using the conventional basis materials (acrylic and aluminum) as the calibration basis set. The coefficients were then numerically transformed to those for a more desirable set materials. The transformation was done at the energies of the low and high energy windows of the X-ray spectrum. With this correction method using acrylic and an iodine-water mixture as our desired basis set, computer simulation results showed that accuracy of better than 2% could be achieved even when iodine was present in the body at a concentration as high as 10% by mass. Simulation work had also been carried out on a more inhomogeneous 2D thorax phantom of the 3D MCAT phantom. The results of the accuracy of quantitation were presented here View full abstract»

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  • A technique for measuring the energy response of a PET tomograph using a compact scattering source

    Page(s): 2500 - 2508
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    A technique is presented for verifying the energy calibration of a positron emission tomography (PET) scanner by measuring the energy response spectrum of its detectors. Singly scattered annihilation radiation provides a continuous energy reference source of coincident photons. The response of the scanner vs. scattering angle, and hence vs. energy is recorded within the sinogram. From these data, the position and resolution of discriminator levels which are set below 511 keV can be extracted. This is shown to be a useful diagnostic of the scanner's energy calibration, and of its response to scatter events in three dimensional (3-D) acquisitions View full abstract»

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  • The digital mammography program at the SR light source in Trieste

    Page(s): 2395 - 2399
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    A synchrotron radiation beamline devoted to medical imaging has been built by the SYRMEP collaboration at Elettra in Trieste, Italy, and is now in operation. The SYRMEP imaging system is based on the combination of a monochromatic, laminar X-ray beam, with a laminar, high-efficiency pixel silicon detector. To image soft tissue, including small, low-contrast details, as in mammography, beam energies should be chosen in the range from 15 keV to 30 keV and the detector should be capable of single-photon counting. The SYRMEP beamline originates from a bending magnet light port and consists basically of a Be window, a slit system and a Si(1,1,1) crystal monochromator. The experimental area is equipped with a radiation flux monitor, a sample movement stage, and a positioning system holding the silicon detector. Images are obtained by keeping the detector stationary with respect to the beam and by scanning the sample through the beam itself. The current detector is a linear array of 200×300 μm2 pixels, each coupled to its electronic counting chain on a custom VLSI read-out chip. We present a summary of beam studies, and digital images of standard RMI 160 and RMI 180 Ackermann Mammographic phantoms. Results show the high contrast resolution and the large dynamic range which constitute the defining characteristics of the SYRMEP imaging system View full abstract»

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  • Analytical modeling of PET imaging with correlated functional and structural images

    Page(s): 2439 - 2444
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    Objective evaluation of dynamic imaging protocols needs a realistic simulation tool to model the data acquisition and image reconstruction of a PET system. Availability of the correlated functional and anatomical images in many centers allows the creation of highly realistic objects to represent brain activity and attenuation distribution for each study. The authors have developed a fast analytical model that incorporates key physical factors inherent in coincidence detection along with spatially variant 3-D detection efficiency and deadtime. Here, they use MR and PET data of a 3-D Hoffman brain phantom to demonstrate and validate their computational methods. The simulated total projection, attenuation factor: and scatter profiles are in very good agreement with the experimental data. The corresponding regional activity values in the gray matter and white matter have a discrepancy of ⩽8.5%. The authors also show typical emission and transmission images generated from MR brain scans. This tool is particularly useful in evaluating projection data processing and image reconstruction algorithms View full abstract»

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  • Study of multispectral frame-by-frame convolution scatter correction in high resolution PET

    Page(s): 2489 - 2493
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    High resolution PET scanners based on small individual detectors have a low sensitivity resulting from increased inter-detector scatter. It has been postulated that this limitation can be overcome by using enlarged discrimination windows to include more low-energy events and by developing more efficient energy-dependent methods to correct for scatter. In this work, we investigate one such method based on the frame-by-frame scatter correction of multispectral data. Images acquired in the conventional (344-658 keV), broad (129-658 keV) and multispectral (129-658 keV) window modes were processed by stationary and nonstationary convolution-restoration scatter correction methods. Broad and multispectral window acquisition improved counting efficiency relative to the conventional window. The degradation of image quality due to added scatter can be mostly recovered by the subtraction-restoration scatter correction. However, the multispectral frame-by-frame method was found to be more sensitive to the effects of the nonstationary scatter response functions and its performance was not as good as that of the broad window. It is concluded that more sophisticated scatter degradation models and correction methods need to be established to take full advantage of multispectral data View full abstract»

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  • Monte Carlo evaluation of accuracy and noise properties of two scatter correction methods for 201Tl cardiac SPECT

    Page(s): 2465 - 2472
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    Two independent scatter correction techniques, transmission dependent convolution subtraction (TDCS) and triple-energy window (TEW) method, were evaluated in terms of quantitative accuracy and noise properties using Monte Carlo simulation (EGS4). Emission projections (primary, scatter and scatter plus primary) were simulated for three numerical phantoms for 201Tl. Data were reconstructed with ordered-subset EM algorithm including noise-less transmission data based attenuation correction. Accuracy of TDCS and TEW scatter corrections were assessed by comparison with simulated true primary data. The uniform cylindrical phantom simulation demonstrated better quantitative accuracy with TDCS than with TEW (-2.0% vs. 16.7%) and better S/N (6.48 vs. 5.05). A uniform ring myocardial phantom simulation demonstrated better homogeneity with TDCS than TEW in the myocardium; i.e., anterior-to-posterior wall count ratios were 0.99 and 0.76 with TDCS and TEW, respectively. For the MCAT phantom, TDCS provided good visual and quantitative agreement with simulated true primary image without noticeably increasing the noise after scatter correction. Overall TDCS proved to be more accurate and less noisy than TEW, facilitating quantitative assessment of physiological functions with SPECT View full abstract»

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