جستجوی مقالات مرتبط با کلیدواژه "attenuation correction" در نشریات گروه "پزشکی"

To develop the following three attenuation correction (AC) methods for brain 18F-fluorodeoxyglucose-positron emission tomography (PET), using deep learning, and to ascertain their precision levels: (i) indirect method; (ii) direct method; and (iii) direct and high-resolution correction (direct+HRC) method.

We included 53 patients who underwent cranial magnetic resonance imaging (MRI) and computed tomography (CT) and 27 patients who underwent cranial MRI, CT, and PET. After fusion of the magnetic resonance, CT, and PET images, resampling was performed to standardize the field of view and matrix size and prepare the data set. In the indirect method, synthetic CT (SCT) images were generated, whereas in the direct and direct+HRC methods, a U-net structure was used to generate AC images. In the indirect method, attenuation correction was performed using SCT images generated from MRI findings using U-net instead of CT images. In the direct and direct+HRC methods, AC images were generated directly from non-AC images using U-net, followed by image evaluation. The precision levels of AC images generated using the indirect and direct methods were compared based on the normalized mean squared error (NMSE) and structural similarity (SSIM).

Visual inspection revealed no difference between the AC images prepared using CT-based attenuation correction and those prepared using the three methods. The NMSE increased in the order indirect, direct, and direct+HRC methods, with values of 0.281×10-3, 4.62×10-3, and 12.7×10-3, respectively. Moreover, the SSIM of the direct+HRC method was 0.975.

The direct+HRC method enables accurate attenuation without CT exposure and high-resolution correction without dedicated correction programs.

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.

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.

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.

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.

Noninvasive diagnostic methods for coronary artery disease (CAD) are a health priority. Coronary angiography (CA) is currently the gold-standard method for diagnosing CAD. Myocardial perfusion imaging (MPI) is used to diagnose CAD as well. This study aimed to reevaluate the effects of computed tomography (CT)-based attenuation correction on MPI results compared with CA findings.
 
This cross-sectional study enrolled 293 patients referred to Rajaie Cardiovascular Medical and Research Center. The study population underwent MPI with CT-based attenuation correction and CA to diagnose CAD within 3 months.
 
In the right coronary artery (RCA) territory, CT-based attenuation correction led to a significant decrease in MPI sensitivity in men and an increase in specificity in all the patients except those with a normal body mass index. In the left circumflex coronary artery (LCX) territory, a significant reduction in sensitivity was noted just in overweight patients, while specificity improved merely in men. In the left anterior descending artery (LAD) territory, none of the diagnostic parameters changed significantly with attenuation correction.
 
Performing CT-based attenuation correction significantly enhanced diagnostic specificity and worsened sensitivity in the RCA and LCX territories. The advantage of CT-based attenuation correction is more pronounced in patients with a normal body mass index and women. (Iranian Heart Journal 2023; 24(3): 15-23)

Single photon emission computed tomography (SPECT)-alone imaging using the Tc-99m radiopharmaceutical labeled with methylene diphosphonate or similar analogs is usually employed to diagnose metastatic bone and is typically followed by complementary magnetic resonance (MR) imaging for support in clinical decision-making. In this study, two attenuation map generation approaches from MR and SPECT non-attenuation corrected (SPECT-nonAC) images were evaluated in the context of quantitative SPECT imaging.

The 2class-MR attenuation map was generated via segmenting an MR image into air and soft tissue. Likewise, SPECT-nonAC was segmented into background air and soft tissue to generate a 2class-SPECT attenuation map. The reference attenuation map was generated through manual bone segmentation from an MR image to develop a 3class-bone attenuation map. Standard uptake value (SUV) bias was calculated using the different attenuation maps on 50 vertebrae from normal patients and 16 vertebrae from metastatic patients.

The 2class-MR approach resulted in -16% and -8% SUV bias in normal and metastatic groups, respectively, while 2class-SPECT led to 33% and 26% SUV underestimation for the normal and metastatic patient groups, respectively.

The 2class-SPECT approach led to a significant underestimation of SUV due to the uncertainty of body contour delineation. However, the 2class-MR approach resulted in less than -9% SUV bias in metastatic patients, demonstrating its potential to support quantitative SPECT imaging.

Myocardial perfusion imaging is a non-invasive procedure that plays an integral role in the diagnosis and management of coronary artery disease. With the routine use of computerised tomography attenuation correction (CTAC) in myocardial perfusion imaging still under debate, the aim of this review was to determine the impact of CTAC on image quality in myocardial perfusion imaging. Medline, Embase and CINAHL were searched from the earliest available time until August 2019. Methodological quality was assessed using the Quality Assessment of Diagnostic Accuracy Studies version 2. Details pertaining to image quality and diagnostic accuracy were analysed and results summarised descriptively. Three studies with ‘unclear’ risk of bias and low applicability concerns (1002 participants) from a yield of 2725 articles were identified. Two studies demonstrated an increase in image quality, and one study found no difference in image quality when using CTAC compared to no attenuation correction. Benefits of CTAC for improving image quality remain unclear. Given the potential exposure risk with the addition of CTAC, patient and clinician factors should inform decision making for using of CTAC in myocardial perfusion imaging for coronary artery disease.

Integration of single photon emission computed tomography (SPECT) and computed tomography (CT) scanners into SPECT/CT hybrid systems permit detection of coronary artery disease in myocardial perfusion imaging (MPI). Misregistration between CT and emission data can produce some errors in uptake value of SPECT images. The aim of this study was evaluate the influence of attenuation correction (AC) versus non-attenuation correction (NC) images and the effect of misregistration on all segments of SPECT images for quantitative and qualitative analysis. 99 patients (45 males, 54 females) underwent stress/rest myocardial perfusion imaging (MPI) using 99mTc-MIBI were used in this study. We also utilized cardiac insert and lung insert in cylinder phantom. Phantom studies were performed with and without defect. The misregistration of all patient data was measured and variation in misregistration of our population was recorded. The effect of attenuation correction (AC) and non-attenuation correction (NC) images were also evaluated in both phantom and patient data. The CT images were shifted by ±1, ±2, ±3 pixels along X-, Y- and Z-axis (Left/right, dorsal/ventral, cephalic/caudal) for both phantom and patient studies. Differences between misalignment data and misregistration correction images were also measured. Results displayed with 20 segments polar map analysis and illustration in standard orientations for cardiac tomographic images. In the patient population data, 1.5% were perfectly registered, 17% and 73% misaligned under 1 pixel and more than 1 pixel, respectively. AC of SPECT images showed increased uptake value in normal phantom and false positives findings were disappeared versus to NC images. In patient data, statistically significant variation were shown for the most segments before and after AC (P-value<=0.004) and also between AC of SPECT image and misregistration correction images (P-value<=0.048). Along X-axis, in 3 pixel shift in right direction, the percent of relative difference in lateral wall were 11.94% for mid anterolateral. Along Y-axis, the Ventral shift caused -15.9% changes in basal inferolateral and along Z-axis -8.59 % changes in apical anterolateral were also observed in caudal direction when 3 pixel shifts were used. This study showed that CT-based attenuation correction of cardiac images in hybrid SPECT/CT is important to improve image quality. Misalignment in caudal, cephalad, ventral and right direction introduced significant variation even in 1 pixel shift. It is important to apply misregistration correction even in small misalignment routinely in clinical myocardial perfusion imaging.

One of the challenges of PET/MRI combined systems is to derive an attenuation map to correct the PET image. For that, the pseudo-CT image could be used to correct the attenuation. Until now, most existing scientific researches construct this pseudo-CT image using the registration techniques. However, these techniques suffer from the local minima of the non-rigid deformation energy function which leads to unsatisfactory results.
We propose in this paper a new approach for the generation of a pseudo-CT image from an MR image.
This approach is based on a dense stereo matching concept, for that, we encode each pixel according to a shape related coordinates method, and we apply a local texture descriptor to put into correspondence pixels between MRI patient and MRI atlas images. The proposed approach was tested on a real MRI data, and in order to show the effectiveness of the proposed local descriptor, it has been compared to three other local descriptors: SIFT, SURF and DAISY. Also it was compared to registration method.
The calculation of structural similarity (SSIM) index and DICE coefficients, between the pseudo-CT image and the corresponding real CT image show that the proposed stereo matching approach outperforms a registration one.
The use of dense matching with atlas promises good results in the creation of pseudo-CT. The proposed approach can be recommended as an alternative to registration techniques.

The aim of this study was to determine the optimal reconstruction parameters for iterative reconstruction in different devices and collimators for dopamine transporter (DaT) single-photon emission computed tomography (SPECT). The results were compared between filtered back projection (FBP) and different attenuation correction (AC) methods.
An anthropomorphic striatal phantom was filled with 123I solutions at different striatum-to-background radioactivity ratios. Data were acquired using two SPECT/CT devices, equipped with a low-to-medium-energy general-purpose collimator (cameras A-1 and B-1) and a low-energy high-resolution (LEHR) collimator (cameras A-2 and B-2).
The SPECT images were once reconstructed by FBP using Chang’s AC and once by ordered subset expectation maximization (OSEM) using both CTAC and Chang’s AC; moreover, scatter correction was performed. OSEM on cameras A-1 and A-2 included resolution recovery (RR). The images were analyzed, using the specific binding ratio (SBR). Regions of interest for the background were placed on both frontal and occipital regions.
The optimal number of iterations and subsets was 10i10s on camera A-1, 10i5s on camera A-2, and 7i6s on cameras B-1 and B-2. The optimal full width at half maximum of the Gaussian filter was 2.5 times the pixel size. In the comparison between FBP and OSEM, the quality was superior on OSEM-reconstructed images, although edge artifacts were observed in cameras A-1 and A-2. The SBR recovery of OSEM was higher than that of FBP on cameras A-1 and A-2, while no significant difference was detected on cameras B-1 and B-2. Good linearity of SBR was observed in all cameras. In the comparison between Chang’s AC and CTAC, a significant correlation was observed on all cameras. The difference in the background region influenced SBR differently in Chang’s AC and CTAC on cameras A-1 and B-1.
Iterative reconstruction improved image quality on all cameras, although edge artifacts were observed in images captured by cameras with RR. The SBR of OSEM with RR was higher than that of FBP, while the SBR of OSEM without RR was equal to that of FBP. Also, the SBR of Chang’s AC varied with different background regions in cameras A-1 and B-1.

Potential causes of misalignment between anatomical and functional images incardiac PET/CT imaging include respiratory and cardiac motion as well as bulk motion. In this study we evaluated the impact of respiratory and cardiac motion between CT and corresponding CT-based attenuation corrected (CTAC) PET images on apparent myocardial uptake. PET projection data of the 4D XCAT phantom were analytically generated using an analytic simulator considering the effect of photon attenuation and Poisson noise. Theassessment of PET images was performed through qualitative interpretation by an experiencednuclear medicine physician and a volume of interest based quantitative analysis. Moreover,Box and Whisker plots were calculated and bull’s eye view analysis performed. PET imageswere also reoriented along the short, horizontal and vertical long axis views for a betterqualitative interpretation. The simulation study showed that using the attenuation map at end-exhalation of therespiratory phase consistently overestimated the activity concentration in all segments of themyocardial wall as opposed to using the end-inhalation attenuation map image which resulted inunderestimation CT images acquired at end-exhalation could introduce larger errors compared toend-inhalation. These errors decrease significantly when the attenuation map was acquired atmid-inhalation or mid-exhalation phases of the respiratory cycle.

Non simultaneous acquisition between CT and PET module can introduce misalignment artefact in cardiac PET/CT imaging due to patient motion. We assessed the clinical impact of patient motion and the resulting mismatch between CT and corresponding CT-based attenuation corrected (CTAC) PET images on apparent myocardial uptake values in cardiac PET/CT imaging. The evaluation of patient motion was performed using clinical and experimental phantom studies acquired on the Biograph TP 64 PET/CT scanner. In order to simulate patient motion, CT images were manually shifted from 0 to 20 mm in steps of 5-mm in six different directions. The reconstructed PET images using shifted CT were compared with the original PET images. The assessment of PET images was performed through qualitative interpretation by an experienced nuclear medicine physician and through quantitative analysis using volume of interest based analysis. Moreover, Box and Whisker plots were calculated and bull’s eye view analysis performed. PET images were also reoriented along the short, horizontal and vertical long axis views for a better qualitative interpretation. A 20-mm shift in the right direction between attenuation and PET emission scans produced mean absolute percentage difference in uptake values in the lateroanterior (33.42±9.07) and lateroinferior (27.39±10.43) segments of the myocardium. Misalignment could introduce artifactual nonuniformities in apparent myocardial uptake value and the variations were more significant for the misalignment toward the right, feet and head directions, in such a way that even with a 5-mm shift in the CT image, errors in interpretation of PET images could occur. Furthermore, errors in PET uptake estimates were observed for movements as large as 10-mm in the left, posterior and anterior directions.

The main purpose of this study was to compare transient ischemic dilation (TID) ratios in SPECT-low dose CT and SPECT Myocardial Perfusion Imaging (MPI) by application of different quantitative programs and quantify the possible shift in the upper normal limits of TID ratio in the SPECT-CT MPI. 149 Patients with low pre-test probability for coronary artery disease (CAD), based upon Diamond and Forrester method entered the study. Each patient underwent both attenuation correction (AC) SPECT-CT MPI and non attenuation correction (NAC) SPECT MPI (two day Tc-99m sestamibi stress-rest protocol). Normalcy rates were also calculated and compared. The comparison was based on both visual interpretation and quantitative analysis. In the low pre-test probability group visual interpretations lead to a statistically significant improvement in normalcy rate in the SPECT-CT acquisition compared to the SPECT MPI. Regardless of the stress type and software programs used, no significant difference was noted in the upper normal limits of the TID ratios between the AC and NAC acquisitions. The study showed superiority of SPECT-CT MPI to SPECT MPI in terms of normalcy rate. We also propose new upper normal limits of TID ratios for different sets of acquisition-gender-stress modality-software programs.

In this study, Quantitative 32P bremsstrahlung planar and SPECT imaging and consequent dose assessment were carried out as a comprehensive phantom study to define an appropriate method for accurate Dosimetry in clinical practice. CT, planar and SPECT bremsstrahlung images of Jaszczak phantom containing a known activity of 32P were acquired. In addition, Phantom contour was determined for attenuation correction and image registration. Reconstructed SPECT slices were corrected for attenuation effect using two different conventional Chang`s method and an expectation maximization algorithm followed by CT and SPECT image registration. Cumulated activity was calculated by a predefined calibration factor. Both attenuation correction algorithms were quantitatively assessed by the Monte Carlo SIMIND program. Acquired planar Bremsstrahlung images were quantified by the Conjugate View Method, as well. Calculated activities were statistically different among various quantification methods (P= 0.0001). When iterative expectation maximization algorithm and applied methods were used, mean calculated activity had the least difference with real activity of ±3%. Quantitative 32P Bremsstrahlung SPECT imaging could accurately determine administered activity and assess radiation dose if precise attenuation correction and appropriate registration with CT were done even without sophisticated scatter correction or when SPECT/CT machines are not available. Therefore, it has the potential of specific tumor/organ dosimetry in clinical practice. The best method for calculating activity is quantitative SPECT using iterative expectation maximization algorithm. Additionally, applied method for determining phantom contour was practical for attenuation correction and image registration.

Photon attenuation in tissues is the primary physical degrading factor limiting both visual qualitative interpretation and quantitative analysis capabilities of reconstructed Single Photon Emission Computed Tomography (SPECT) images. The aim of present study was to investigate the effect of attenuation correction on the detection of activation foci following statistical analysis with SPM. The study population consisted of twenty normal subjects (11 male, 9 female, and age 30-40 years). SPECT images were reconstructed using filter back projection and attenuation correction was done by the Chang method. The SPECT imagings was obtained 20 min after intravenous injection of 740-1110 MBq (20-30 mCi) of Tc99m-ECD and were acquired on 128×128 matrices with a 20% symmetric energy window at 140 keV. These data publicly distributed by the Society of Nuclear Medicine of Toronto Hospital. The data was standardized with respect to the Montreal Neurological Institute (MNI) atlas with a 12 parameter affine transformations. Images were then smoothed by a Gaussian filter of 10 mm FWHM. Significance differences between SPECT images were estimated at every voxel using statistical t-test and p-value as the significant criteria was set at 0.05. The contrast comparing non attenuation corrected to attenuation corrected images suggest that regional brain perfusion activity increase in the cerebrum, frontal (T-value 12.06), temporal (T-value 10.63) and occipital (T-value 9.31) lobe and decrease in the sub-lobar, extra-nuclear (T-value 17.46) and limbic lobe, posterior cingulate (T-value 17.46) before attenuation correction compare with attenuation correction. It can be concluded that applying attenuation correction in brain SPECT can effectively improve the accuracy of the detection of activation area (p<0.05).