Determination of weighting functions for energy-weighted acquisition

Energy-weighted acquisition (EWA) is an image filtering technique, with a different spatial filter (weighting function) for each energy. The imaging characteristics of EWA are governed by the weighting functions used during the acquisition of the image. The determination of weighting functions is more complicated than the determination of energy windows in conventional imaging because the number of degrees of freedom is much greater. A methodology by which weighting functions can be produced is described. The weighting function is determined by minimizing a generalized chi-square with variable contributions from coefficients quantifying key image characteristics, e.g., signal-to-noise ratio, spatial resolution, and scatter fraction. Varying the importance of these characteristics gives us a workable function-generation tool, able to address a variety of clinical needs. The resulting weighting functions exhibit good scatter reduction properties at various scatter depths, as demonstrated by measurements of line source response functions in a scattering medium at depths from 5 to 14 cm. Energy weighting can also be used to compensate for collimator penetration from high energy gamma rays. Weighting functions are tested in the laboratory using both planar and SPECT phantoms.     

Energy-weighted acquisition of scintigraphic images using finite spatial filters

Energy weighted acquisition (EWA) is a technique for improving image contrast by correcting for some of the blurring effects of Compton scattering within the patient. We outline image formation theory as it applies to energy weighting and present a pre-processing implementation that acquires images with real-number energy-dependent weighting functions of finite spatial extent. The effect of scattered radiation on quantitative accuracy, with and without EWA, is demonstrated with sheet and point sources at various depths. A planar phantom and a clinical 201TI study demonstrate enhanced contrast and edge definition. The performance of EWA in SPECT is shown by 99mTc and 123I phantom studies and a clinical 125I study.     

Initial evaluation of a Gaussian scatter correction technique

We have performed an initial evaluation of a new scatter correction algorithm, Gaussian Subtraction technique, for correction of SPECT images. This technique was evaluated by Monte Carlo simulations.The Gaussian Subtraction technique uses a non-scatter (standard 20% energy window) and high energy windows. The window on the high energy side of the non-scattered peak is used to fit a Gaussian to the spectral peak. This technique directly determines the number of unscattered photon events detected by a gamma camera.We find that this scatter correction technique can correct reasonably well (to within 10–15%) for scatter, for the simple phantom cases used here, using scatter windows < ∼ 20 keV wide, for ^99mTc. The Gaussian scatter technique does not require a priori knowledge of source distributions. Implementation of scatter correction would improve image contrast and quantitation in SPECT studies.     

Correction of scattered photons in Tc-99m imaging by means of a photopeak dual-energy window acquisition

We are proposing a new method for correcting of scattered photons in technetium-99m (99mTc) imaging by means of photopeak dual-energy window acquisition. This method consists of the simultaneous acquisition of two images and estimation of a scatter image included in the symmetric energy window (SW) image by the difference between these images. The scatter corrected image is obtained by subtracting the scatter image from the SW image. In order to evaluate this method, we imaged a planar and a SPECT phantom with cold lesions and calculated the contrast value with and without the scatter correction. In addition, we performed asymmetric energy window (ASW) imaging to compare with this scatter correction method for planar images. In the planar image with the tissue-equivalent material of 10 cm, the scatter correction method removed 32% of the counting rate of the SW image and improved from 0.81 to 0.94 of the contrast value for a 4 cm-diameter cold lesion, while the contrast value with the ASW was 0.87 for such a cold lesion. The scatter corrected SPECT image had a reduction of 18% of the counting rate of the SW SPECT image and improvement of approximately 11% in contrast for cold spot sizes larger than a 3 cm-diameter, compared with the SW SPECT image. In addition, a perfusion defect could be well visualized by this scatter correction method on 99mTc-HMPAO regional cerebral blood flow SPECT of a patient. Our proposed scatter correction method can improve both planar and SPECT images qualitatively and quantitatively.     

An evaluation of reconstruction techniques and scatter correction in bone SPECT of the spine

A selection of commonly used reconstruction and filter techniques in the processing of 99mTc oxidronate (i.e., 99mTc hydroxymethane diphosphonate) single photon emission computed tomography (SPECT) of the spine was compared. The possible additional value of scatter correction on image contrast was also evaluated. Twenty-eight bone SPECT examinations of consecutive patients were studied retrospectively. The reconstruction techniques used were filtered back-projection and iterative reconstruction with the use of ordered subsets estimation maximization. Three-dimensional post-filtering with a Metz filter and a Butterworth filter was used. Each combination was evaluated with or without scatter correction. Each study was also processed with the department's standard technique of two-dimensional pre-filtering with a Metz filter followed by filtered back-projection (without scatter correction). Five observers evaluated the image quality of reconstructed coronal and sagittal slices, with special reference to the resolution of vertebrae, vertebral processes, the spinal canal and suspected abnormal uptakes. A grading scale from -2 to +2 was used with the standard technique as the reference. The best image quality was found with iterative reconstruction in combination with a contrast enhancing Metz filter or a noise reducing Butterworth filter. Scatter correction did not improve image quality.     

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

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

Scatter correction on its own increases image contrast in TI-201 myocardium perfusion scintigraphy,but does it also improve diagnostic accuracy?

Poor and variable spatial resolution of the gamma camera, the movement of the heart and, above all, the inclusion of scattered photons in the acquisition data contribute to the deterioration of image contrast in 201Tl myocardium perfusion studies. Scatter correction algorithms may correct for the latter factor by removing (most of) the scattered photons from the acquisition data. METHODS: In this study we investigated the contrast changes induced by the Triple Energy Window scatter correction method (TEW) applied to clinical 201Tl myocardium perfusion studies and its influence on the reading of the images. Stress and rest studies of 30 consecutive patients were used for this study. Maximum image contrasts were measured between the myocardium and the left ventricular cavity in four mid-ventricular short axis slices, as well as between normally and abnormally perfused myocardium using bull's-eye displays of the activity within the myocardium. To assess image quality and perfusion abnormalities, an experienced nuclear medicine physician, blind to patient characteristics, visually reviewed all studies. RESULTS: In all individual measurements, the maximum contrast after scatter correction was higher than without correction (p < 0.001). The average increase in contrast between the myocardium and the left ventricular cavity was 43% and 48% for stress and rest studies respectively. The contrast within the myocardium increased by 25% and 32% respectively. After TEW, image quality was rated lower in almost half of the studies, while in only one study the quality was rated higher. In stress studies 11 additional perfusion defects were observed, with rest studies revealing 15 more defects after TEW, but this difference was not significant. Cohen's kappa indicated a moderate agreement of the image reading between studies with and without scatter correction. CONCLUSION: We conclude that image contrast improves significantly by scatter correction. However, image quality decreased as a result of an unfavorable signal-to-noise ratio. As an overall result, no significant change in the clinical outcome of the studies could be shown. Additional training of the readers may be required to obtain optimal results.     

Simple techniques to reduce bowel activity in cardiac SPECT imaging

BACKGROUND AND AIM: Scatter from the bowel degrades image quality in 99mTc sestamibi myocardial perfusion imaging (MPI). Iodinated oral contrast, which has been used to outline bowel in medical imaging, absorbs X-rays as well as gamma rays. The purpose of this study was to test our hypothesis that iodinated oral contrast during MPI would absorb gamma rays emitted from 99mTc sestamibi in the bowel, thereby reducing scatter and improving cardiac SPECT images. METHODS AND RESULTS: Thirty subjects undergoing adenosine stress 99mTc sestamibi cardiac SPECT were randomized to receive either iodinated oral contrast (IOC), water or no intervention (controls). Subjects had 1 day rest-stress MPI using the adenosine stress protocol. Images were analysed using infra-cardiac counts, image variability, image contrast and the ratios of anterior to inferior and septal to lateral walls. The improvement in image contrast and variability between first and second images were significant in both the IOC and water groups. The IOC group had a more significant improvement in variability than did the water group. The reduction in infra-cardiac counts was also more significant in the IOC group. CONCLUSION: The use of oral contrast and water improved the image variability and contrast by decreasing the infra-cardiac scatter. The improvement was even more significant in the oral contrast group.     

Improvement of brain perfusion SPET using iterative reconstruction with scatter and non-uniform attenuation correction

Filtered back-projection (FBP) is generally used as the reconstruction method for single-photon emission tomography although it produces noisy images with apparent streak artefacts. It is possible to improve the image quality by using an algorithm with iterative correction steps. The iterative reconstruction technique also has an additional benefit in that computation of attenuation correction can be included in the process. A commonly used iterative method, maximum-likelihood expectation maximisation (ML-EM), can be accelerated using ordered subsets (OS-EM). We have applied to the OS-EM algorithm a Bayesian one-step late correction method utilising median root prior (MRP). Methodological comparison was performed by means of measurements obtained with a brain perfusion phantom and using patient data. The aim of this work was to quantitate the accuracy of iterative reconstruction with scatter and non-uniform attenuation corrections and post-filtering in SPET brain perfusion imaging. SPET imaging was performed using a triple-head gamma camera with fan-beam collimators. Transmission and emission scans were acquired simultaneously. The brain phantom used was a high-resolution three-dimensional anthropomorphic JB003 phantom. Patient studies were performed in ten chronic pain syndrome patients. The images were reconstructed using conventional FBP and iterative OS-EM and MRP techniques including scatter and non-uniform attenuation corrections. Iterative reconstructions were individually post-filtered. The quantitative results obtained with the brain perfusion phantom were compared with the known actual contrast ratios. The calculated difference from the true values was largest with the FBP method; iteratively reconstructed images proved closer to the reality. Similar findings were obtained in the patient studies. The plain OS-EM method improved the contrast whereas in the case of the MRP technique the improvement in contrast was not so evident with post-filtering.     

Improvement of brain perfusion SPET using iterative reconstruction with scatter and non-uniform attenuation correction

Filtered back-projection (FBP) is generally used as the reconstruction method for single-photon emission tomography although it produces noisy images with apparent streak artefacts. It is possible to improve the image quality by using an algorithm with iterative correction steps. The iterative reconstruction technique also has an additional benefit in that computation of attenuation correction can be included in the process. A commonly used iterative method, maximum-likelihood expectation maximisation (ML-EM), can be accelerated using ordered subsets (OS-EM). We have applied to the OS-EM algorithm a Bayesian one-step late correction method utilising median root prior (MRP). Methodological comparison was performed by means of measurements obtained with a brain perfusion phantom and using patient data. The aim of this work was to quantitate the accuracy of iterative reconstruction with scatter and non-uniform attenuation corrections and post-filtering in SPET brain perfusion imaging. SPET imaging was performed using a triple-head gamma camera with fan-beam collimators. Transmission and emission scans were acquired simultaneously. The brain phantom used was a high-resolution three-dimensional anthropomorphic JB003 phantom. Patient studies were performed in ten chronic pain syndrome patients. The images were reconstructed using conventional FBP and iterative OS-EM and MRP techniques including scatter and nonuniform attenuation corrections. Iterative reconstructions were individually post-filtered. The quantitative results obtained with the brain perfusion phantom were compared with the known actual contrast ratios. The calculated difference from the true values was largest with the FBP method; iteratively reconstructed images proved closer to the reality. Similar findings were obtained in the patient studies. The plain OS-EM method improved the contrast whereas in the case of the MRP technique the improvement in contrast was not so evident with post-filtering.     

Comparative evaluation of scatter correction in 3D PET using different scatter-level approximations

Objective

In 3D PET, scatter of the gamma photons is one of the most significant physical factors which degrades not only image quality but also quantification. The currently most used scatter estimation method is the analytic single scatter simulation (SSS) which usually accommodates for multiple scattering by scaling the single scatter estimation. However, it has not been clear yet how accurate this approximation is for cases where multiple scatter is significant, raising the question: “How important is correction for multiple scattered photons, and how accurately do we need to simulate all scattered events by appropriate scaling?” This study answers these questions and evaluates the accuracy of SSS implementation in the open-source library STIR.

Methods

Different scatter orders approximations are evaluated including different levels of scattering and different scaling approaches using Monte Carlo (i.e. SimSET) data. SimSET simulations of a large anthropomorphic phantom were reconstructed with iterative reconstruction algorithms. Images reconstructed with 3D filtered back-projection reprojection algorithm have been compared quantitatively in order to clarify the errors due to different scatter order approximations.

Results

Quantification in regions has improved by scatter correction. For example, in the heart the ideal value was 3, whereas before scatter correction the standard uptake value (SUV) was 4.0, after single scatter correction was 3.3 and after single and double scatter correction was 3.0. After correction by scaling single scatter with tail-fit, the SUV was 3.1, whereas with total-fit it was 3.0. Similarly, for the SSS correction methodology implemented in STIR using tail-fit the heart SUV was 3.1 whereas using total-fit it was 3.0.

Conclusions

The results demonstrate that correction for double scatter improves image contrast and therefore it is required for the accurate estimation of activity distribution in PET imaging. However, it has been also shown that scaling the single scatter distribution is a reasonable approximation to compensate for total scatter. Finally, scatter correction with STIR has shown excellent agreement with Monte Carlo simulations.

     

Combination of multiple pencil-beam imaging to computed storage phosphor radiography: a new method

Computed radiography (CR) with storage phosphors offers a wide dynamic range and improved sensitivity compared to film-screen technology. CR was combined in this study with a prototype multiple pencil-beam (MPB) imaging device which has been shown to be very effective in scatter reduction. The combination was analyzed and compared to the standard technique of grid screening in two ways: a free-response ROC (FROC) analysis was first performed followed by a blinded test arrangement for visual analysis of image quality in a series of computed radiography of the lumbar spine by both the MPB and grid modalities. The results of the FROC study showed a statistically significant (P less than or equal to 0.01) improvement in signal detection. The MPB-CR images of the lumbar spine had more contrast but also a slightly mottled or grainy appearance. Image quality was found good but contrast processing was criticized because it seemed to result in a too steep display of contrast in MPB imaging. This should be avoidable by changing the image processing parameters.     

Does scatter correction of cardiac spect improve image quality in the presence of high extracardiac activity?

BACKGROUND: Extracardiac activity confounds conventional cardiac single photon emission computed tomography (SPECT) image reconstruction. It has been proposed that applying scatter correction (SC) may improve image quality. This study was done to test whether SC improves several quantitative measures of cardiac imaging in the presence of high extracardiac activity. METHODS AND RESULTS: An anatomic anthropomorphic phantom with a cardiac insert filled with technetium 99m was used. We obtained acquisitions using a dual-headed SPECT camera at 13 different levels of liver-to-heart activity. Each acquisition was reconstructed by use of each of 6 different methods: filtered backprojection with or without SC, maximum likelihood with or without SC, and maximum likelihood with attenuation correction (AC) and with or without SC. Three different parameters were used to assess the effect of the processing methods on image quality: image variability, contrast, and signal-to-noise ratio. Only image contrast improved significantly with SC. By adding SC to filtered backprojection, image contrast improved by 13% (P <.01). Maximum likelihood reconstruction with AC resulted in further improvement in contrast (increase of 17%), variability (decrease of 5%), and signal-to-noise ratio (increase of 6%) over filtered backprojection (all P <.01). CONCLUSION: Image quality improved significantly when SC was applied, especially when combined with maximum likelihood reconstruction with AC. This improvement was present despite increased extracardiac activity in close proximity to the heart.     

Evaluation of optical unsharp masking and contrast enhancement of low-scatter chest radiographs

Conventional chest radiography poses a challenging technical problem because of its requirement for simultaneous high-contrast display and wide-latitude recording across the entire image. We developed and evaluated a method of producing chest radiographs by using a tantalum air-interspace grid for highly efficient scatter rejection, wide-latitude X-ray film for recording the low-scatter image, and a LogEtronics printer for optical unsharp masking and contrast enhancement of the recorded image (TWL technique). TWL images can be readily obtained and have excellent contrast and detail across the entire image. In comparison with a conventional technique, the TWL technique provides about a 15% improvement in image contrast in well-penetrated areas and a threefold to tenfold improvement in poorly penetrated areas. A detection study using simulated lung nodules and a chest phantom showed about 10% overall improvement in nodule detection with the TWL technique (51% vs 42%), most of which was due to improvement in detection rates in poorly penetrated areas of the chest (62% vs 26%). In well-penetrated areas, there was a decrease in detection rates (52% vs 44%) using TWL images despite measured improvements in image contrast in these areas. Possibly this was due to the observers' unfamiliarity with the reversed-contrast TWL images. Our results show the TWL technique to be valuable for improving image quality and diagnostic accuracy in chest radiography.     

Posterior beam-stop method for scatter fraction measurement in digital radiography.

The authors presented a new posterior beam-stop (PBS) technique for measuring the ratio of scattered to total-detected photon flux (scatter fraction) in a radiographic examination while preserving the diagnostic quality of the image. The scatter measurement was made using a standard imaging geometry with both beam stops and an additional x-ray detector placed behind the standard imaging detector. This PBS geometry differs from the standard beam-stop (SBS) technique for scatter measurement. With SBS, a beam-stop shadow appears on the image. To evaluate the PBS technique, scatter fraction measurements were performed on an anatomic phantom using both the PBS and SBS techniques. When compared with the standard technique, PBS provided accurate estimation of scatter fractions. Since the measurement can be performed without degrading a standard clinical radiographic examination, the PBS technique allows simultaneous acquisition of scatter measurements from human patients in combination with a standard radiographic examination.     

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

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.     

Development of a practical image-based scatter correction method for brain perfusion SPECT: comparison with the TEW method

Purpose An image-based scatter correction (IBSC) method was developed to convert scatter-uncorrected into scatter-corrected SPECT images. The purpose of this study was to validate this method by means of phantom simulations and human studies with ^99mTc-labeled tracers, based on comparison with the conventional triple energy window (TEW) method.Methods The IBSC method corrects scatter on the reconstructed image with Changs attenuation correction factor. The scatter component image is estimated by convolving with a scatter function followed by multiplication with an image-based scatter fraction function. The IBSC method was evaluated with Monte Carlo simulations and ^99mTc-ethyl cysteinate dimer SPECT human brain perfusion studies obtained from five volunteers. The image counts and contrast of the scatter-corrected images obtained by the IBSC and TEW methods were compared.Results Using data obtained from the simulations, the image counts and contrast of the scatter-corrected images obtained by the IBSC and TEW methods were found to be nearly identical for both gray and white matter. In human brain images, no significant differences in image contrast were observed between the IBSC and TEW methods.Conclusion The IBSC method is a simple scatter correction technique feasible for use in clinical routine.     

Scatter compensation in digital chest radiography using Fourier deconvolution

The authors present a numerical deconvolution technique to compensate for image degrading effects caused by scattered photons in radiographic chest images. Fourier transform techniques are used to deconvolve a shift invariant model of the two dimensional point spread response functions of the scattered radiation. This approach uses a digitized radiograph acquired with a standard chest imaging protocol, so no specialized imaging equipment is required. While the shift variant shape of the scatter model is optimized for the lung field, effective compensation is provided when this model shape is applied to other chest regions. Preliminary evaluation suggests that this technique can provide improved image contrast over the entire chest region.     

The scatter-to-primary ratio as a function of varying X-ray absorption measured by computed radiography.

Some scatter studies have previously been conducted using film as a detector. The serious limitations caused by the narrow latitude, the non-linear density response, and the required optical densitometric measurements of film can be avoided by computed radiography (CR) which provides linear numeric data over a wide dynamic range. The imaging plate is used as a large-area detector and the data is analyzed from the computer memory. Variation in the scatter-to-primary ratio within an image caused by absorption differences was simulated in a water-aluminum phantom. The measurement technique showed repeatable results, being comparable to the values expected on the basis of previous studies. A multiple pencil-beam (MPB) imaging device was also compared to a standard 1:12 grid by this technique. The maximal scatter-to-primary ratio in our model was up to 7.9 with no scatter reduction, 1.5 with grid, and 0.4 with the MPB device. The variation caused by the absorption of primary radiation was much less in the MPB modality, and the MPB system was also less sensitive to an increase in the used tube voltage from 60 to 120 kVp. The benefits of multiple pencil-beam imaging in scatter reduction are briefly discussed.     

Three-dimensional reconstruction of fine vascularity in ultrasound breast imaging using contrast-enhanced spatial compounding: in vitro analyses

RATIONALE AND OBJECTIVES: Ultrasound image quality can be improved by imaging an object (here: the female breast) from different viewing angles in one image plane. With this technique, which is commonly referred to as spatial compounding, a more isotropic resolution is achieved while speckle noise and further artifacts are reduced. We present results obtained from a combination of spatial compounding with contrast-enhanced ultrasound imaging in three dimensions to reduce contrast specific artifacts (depth dependency, shadowing, speckle) and reconstruct vascular structures. MATERIALS AND METHODS: We used a conventional ultrasound scanner and a custom made mechanical system to rotate an ultrasound curved array probe around an object (360 degrees , 36 transducer positions). For 10 parallel image planes, ultrasound compound images were generated of a flow-mimicking phantom consecutively supplied with water and contrast agent. These compound images were combined to form a volume dataset and postprocessed to obtain a sonographic subtraction angiography. RESULTS: Image quality was significantly improved by spatial compounding for the native (ie, without contrast agent), and, in particular, for the contrast-enhanced case. After subtracting the native images from the contrast-enhanced ones, only structures supplied with contrast agent remain. This technique yields much better results for compound images than for conventional ultrasound images because speckle noise and an anisotropic resolution affect the latter. CONCLUSIONS: With the presented approach contrast specific artifacts can be eliminated efficiently, and a subtraction angiography can be computed. A speckle reduced three-dimensional reconstruction of submillimeter vessel structures was achieved for the first time. In the future, this technique can be applied in vivo to image the vascularity of cancer in the female breast.