Introduction: Attenuation correction is a useful process for improving myocardial perfusion SPECT and is dependent on activity and distribution of Attenuation coefficients in the body (Attenuation map). Attenuation artifacts are a common problem in myocardial perfusion SPECT. The aim of this study was to compare the effect of Attenuation correction using different Attenuation maps and different activities in a specially designed heart phantom.Methods: The SPECT imaging for different activities and different body contours were performed by a phantom using tissue-equivalent boluses for making different thicknesses. The activity was ranged from 0.3-2mCi and the images were acquired in 180 degree, 32 steps. The images were reconstructed by OSEM method in a PC computer using Matlab software. Attenuation map were derived from CT images of the phantom. Two quality and quantity indices, derived from universal image quality index have been used to investigate the effect of Attenuation correction in each SPECT image.Results: The result of our measurements showed that the quantity index of corrected image was in the range of 3.5 to 5.2 for minimum and maximum tissue thickness and was independent of activity. Comparing Attenuation corrected and uncorrected images, the quality index of corrected image improved by increasing body thickness and decreasing activity of the voxels.Conclusion: Attenuation correction was more effective for images with low activity or phantoms with more thickness. In our study, the location of the pixel relative to the associated attenuator tissues was another important factor in Attenuation correction. The more accurate the registration process (Attenuation map and SPECT) the better the result of Attenuation correction.

Background: Photon Attenuation as an inevitable physical phenomenon influences on the diagnostic information of SPECT images and results to errors in accuracy of quantitative measurements. This can be corrected via different physical or mathematical approaches. As the correction equation in mathematical approaches is nonlinear, in this study a new method of linearization called ‘Piece Wise Linearization’ (PWL) is introduced and to substantiate its validity for SPECT image reconstructions, a phantom study is performed. Material and Methods: A SPECT scan of a homemade heart phantom filled with 2 mCi 99mTc was acquired by dual head Siemens E.Cam gamma camera equipped with LEHR collimator. Row data of the scan were transferred in DICOM format to a pc computer for reconstruction of the images using MLEM iterative algorithm in Matlab software. Result: Attenuation map of the phantom m(x) were derived using PWL with linear optimization approach. Based on that, the Attenuation corrected SPECT image of the phantom were reconstructed and compared with non-corrected image, using MLEM iterative algorithm. Comparison of the corrected and non-corrected images confirmed with CT Attenuation correction method. Conclusion: Attenuation correction in SPECT image can be achieved successfully, using emission data and piecewise linearization with linear optimization approach. The corrected image of ¦(x) and Attenuation map m(x) of the heart phantom using this approach promise acceptable image quality for diagnostic clinical use.

Introduction: Photon Attenuation is one of the main causes of quantitative errors and artifacts in SPECT. CT based Attenuation map is necessary to correct for this effect accurately. A number of Attenuation related artifacts are described. A fast and memory efficient algorithm is proposed for Attenuation correction.Methods: In our CT based Attenuation correction algorithm, attenuated projections of an arbitrary estimation are calculated. These projections are compared with real projections. The error due to comparison updates the first arbitrary image. This iterative procedure continues till there is no difference between mathematical and real projections. To consider the effect of Poisson noise during reconstruction, EM statistical reconstruction is used. The effect of Attenuation is enrolled in both projection and backprojection steps. To accelerate the reconstruction algorithm, a rotating image in a fix grid with bilinear interpolation is used to simplify ray tracing for creating mathematical projections. OSEM with much faster convergence rate is also used instead of MLEM as iterative algorithm. Different phantom configurations with uniform and non-uniform Attenuation map are used to evaluate the accuracy of this algorithm. Projections free from the effect of Attenuation were also simulated. Reconstructed image from these Attenuation free projections is considered as reference image. Normalized mean square error between reference and corrected image and image contrast were used for quantitative evaluation of this algorithm.Results: contrast of central hot spot reduced to about half of its real contrast due to Attenuation. Non-uniformities, like hot skin, bright long related spots and colds bone related spots were also created in non-corrected images due to Attenuation. All Attenuation related artifacts removed after Attenuation correction. NMSE is reduced from 1 in non-corrected images to 0.1 in corrected images. Much better fit in line profile of corrected images and reference images were also observed.Conclusion: By using more suitable models for image formation it is possible to reconstruct images with much higher quantitative and qualitative accuracy which is essential in both diagnostic and image guided therapy procedures.

In modern PET/CT systems, the CT provides a fast and relatively noise-free Attenuation map and improving lesions localization and the possibility of accurate quantitative analysis. In cardiac imaging, there is a strong Attenuation gradient along the myocardial free wall, with muscle next to the air of the lung space and heart has movement and located in the place that move due to breathing cause misalignment increases in this area. If misalignments occur along these boundaries, the Attenuation correction factors are potentially inaccurate, causing as much as a 60% error in the PET tracer emission image in the critical regions of diagnostic interest. Artifacts caused by misalignment are particularly disconcerting in cardiac imaging because they can present themselves as perfusion abnormalities or erroneous information on myocardial viability. In This paper the accuracy of some CT protocols such as gated, normal (a high-pitch helical CT), slow ct, low-temporalresolution helical CT, time-averaged CT (ACT), ultra low dose CT that normally used for Attenuationcorrection in PET/CT were compared, moreover the image quality and dose that induced to patient from each protocols. Acquiring a slow CT improved registration between the transmission and emission. Potential for a heightened radiation dose delivered by the slow CT was compensated by doubling the default noise index and increasing the slice thickness to 5 mm. In the low-dose average cine CT, Further reduction in dose is possible by lowering the upper threshold of the auto-mA settings or modulation of the CT tube current based on anatomy. ACT protocols consist of multiple images acquired sequentially (also referred to as cine or axial) along the bed length over the span of one or more respiratory cycles. 2% average increase in ACT-PET rest reconstruction values compared the HCT-PET rest reconstruction values was slightly higher than the bias calculated. Contrary to the HCT protocol, the ACT protocol provides more flexibility in addressing artifacts such as varying the respiratory phases used to create the time-averaged CT to suppress respiratory artifacts. In addition, photon starvation can be addressed by optimizing the acquisition parameters, such as increasing the tube voltage and current in patients with high BMI values. Ultra-low-dose CT’s shorter duration and the lower radiation and revealed no severe shift of the myocardium between the CT-based transmission and the emission in the patients.

Purpose: The main goal of this study was to determine the optimal collimator in the absence of medium energy collimators along with the impact of Attenuation correction (AC) and different iterative reconstruction protocols on the quantitative evaluation of Gallium-67 (67Ga) SPECT/CT imaging. Materials and Methods: A GE Discovery 670 dual-head SPECT/CT scanner and a NEMA phantom filled with 67Ga solution were used to scan the patients. The projections were acquired with both Low Energy High Resolution (LEHR) and High Energy General Purpose (HEGP) collimators, and CT images were acquired to evaluate the effect of Attenuation correction. SPECT data were reconstructed using the ordered subset expectation maximization (OSEM) method with various combinations of iterations and subsets. The performance was quantified, and a clinical study validated the phantom study. Results: Acquired images by the HEGP collimator yielded higher Contrast Recovery (CR) and Contrast to Noise Ratio (CNR) in images with AC than those without non-AC (41.6% and 74.2%, respectively). The CNR in all spheres after AC was increased by 80.4% (82.1%) for the HEGP collimator against the LEHR collimator. Also, an increase in iterations × subsets from 16 to 48 led to the Coefficient of Variation (COV) increasing by 17.2%, 16.67%, 15.50%, 14.4%, 14.2%, and 14.1% for 10 mm to 37 mm sphere diameter, respectively. Conclusion: CT-based AC and HEGP collimators can yield improved 67Ga SPECT quantification compared to Non-AC and LEHR collimators. The choice of the optimal collimator with the reconstruction protocol led to changes in the image quality and quantitative accuracy, emphasizing the need to carefully select the appropriate combination of data acquisition factors.

The advent of dual-modality PET/CT imaging has revolutionized the practice of clinical oncology, cardiology and neurology by improving lesions localization and the possibility of accurate quantitative analysis. In addition, the use of CT images for CT-based Attenuation correction (CTAC) allows to decrease the overall scanning time and to create a noise-free Attenuation map (mmap). The near simultaneous data acquisition in a fixed combination of a PET and a CT scanner in a hybrid PET/CT imaging system with a common patent table minimizes spatial and temporal mismatches between the modalities by elimination the need to move the patient in between exams. As PET/CT technology becomes more widely available, studies are beginning to appear in the literature that document the use of PET/CT in a variety of different cancers, including lung, thyroid, ovarian, lymphoma, and unknown primary cancers, and for general oncology, cardiology and neurology applications. Specific applications of PET/CT, such as those for radiation therapy planning, are also being explored. The purpose of this review paper is to introduce the principles of PET/CT imaging systems and describe the sources of error and artifact in CT-based Attenuation correction algorithm. This paper also focuses on recent developments and future trends in hybrid imaging and their areas of application. It should be noted that due to limited space, the references contained herein are for illustrative purposes and are not inclusive; no implication that those chosen are better than others not mentioned is intended.