non-AC Images Research Articles

Feasibility of direct brain 18F-fluorodeoxyglucose-positron emission tomography attenuation and high-resolution correction methods using deep learning.

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.

Deep Learning-Based Attenuation Correction Improves Diagnostic Accuracy of Cardiac SPECT.

To improve diagnostic accuracy, myocardial perfusion imaging (MPI) SPECT studies can use CT-based attenuation correction (AC). However, CT-based AC is not available for most SPECT systems in clinical use, increases radiation exposure, and is impacted by misregistration. We developed and externally validated a deep-learning model to generate simulated AC images directly from non-AC (NC) SPECT, without the need for CT. Methods: SPECT myocardial perfusion imaging was performed using 99mTc-sestamibi or 99mTc-tetrofosmin on contemporary scanners with solid-state detectors. We developed a conditional generative adversarial neural network that applies a deep learning model (DeepAC) to generate simulated AC SPECT images. The model was trained with short-axis NC and AC images performed at 1 site (n = 4,886) and was tested on patients from 2 separate external sites (n = 604). We assessed the diagnostic accuracy of the stress total perfusion deficit (TPD) obtained from NC, AC, and DeepAC images for obstructive coronary artery disease (CAD) with area under the receiver-operating-characteristic curve. We also quantified the direct count change among AC, NC, and DeepAC images on a per-voxel basis. Results: DeepAC could be obtained in less than 1 s from NC images; area under the receiver-operating-characteristic curve for obstructive CAD was higher for DeepAC TPD (0.79; 95% CI, 0.72-0.85) than for NC TPD (0.70; 95% CI, 0.63-0.78; P < 0.001) and similar to AC TPD (0.81; 95% CI, 0.75-0.87; P = 0.196). The normalcy rate in the low-likelihood-of-coronary-disease population was higher for DeepAC TPD (70.4%) and AC TPD (75.0%) than for NC TPD (54.6%, P < 0.001 for both). The positive count change (increase in counts) was significantly higher for AC versus NC (median, 9.4; interquartile range, 6.0-14.2; P < 0.001) than for AC versus DeepAC (median, 2.4; interquartile range, 1.3-4.2). Conclusion: In an independent external dataset, DeepAC provided improved diagnostic accuracy for obstructive CAD, as compared with NC images, and this accuracy was similar to that of actual AC. DeepAC simplifies the task of artifact identification for physicians, avoids misregistration artifacts, and can be performed rapidly without the need for CT hardware and additional acquisitions.

Long-term prognostic value of computed tomography-based attenuation correction on thallium-201 myocardial perfusion imaging: A cohort study.

BackgroundMyocardial perfusion imaging (MPI) is a well-established diagnostic tool to evaluate coronary artery disease (CAD) and also an effective prognostic tool for patients with CAD. However, few studies investigated the prognostic value of attenuation correction (AC) in MPI, and the results were controversial.ObjectivesTo investigate the prognostic value of computed tomography (CT)-based AC thallium-201 (Tl-201) MPI.MethodsA total of 108 consecutive patients who underwent Tl-201 MPI and received coronary angiography within 90 days were included. Medical records were reviewed and missing information was completed after telephone contact. The prognostic value was evaluated by Kaplan-Meier analysis, univariable and multivariable Cox proportional hazards model.ResultsAfter a mean follow-up of 7.72 ± 3.72 years, 27 patients had died, 41 had been readmitted for cardiovascular (CV)-related events and 44 had reached the composite of death plus CV-related re-admission. Kaplan-Meier curves for all-cause mortality for SSS with a cutoff value of 13 for AC and 16 for non-AC (NAC) images showed a significant difference between the two curves for both AC and NAC images (p = 0.011 for AC and p = 0.021 for NAC). In the multivariable model, SSS and SRS showed similar independent predictive values in predicting all-cause mortality and composite of all-cause mortality plus CV-related re-admission, in both AC and NAC images. Subgroup analysis implicated that AC MPI possibly provided better risk stratification in obese patients.ConclusionCT-based AC and NAC MPI showed similar value and were the only significant predictors for the composite of mortality and CV events.

Deep learning-guided estimation of attenuation correction factors from time-of-flight PET emission data.

Attenuation correction (AC) is essential for quantitative PET imaging. In the absence of concurrent CT scanning, for instance on hybrid PET/MRI systems or dedicated brain PET scanners, an accurate approach for synthetic CT generation is highly desired. In this work, a novel framework is proposed wherein attenuation correction factors (ACF) are estimated from time-of-flight (TOF) PET emission data using deep learning. In this approach, referred to as called DL-EM), the different TOF sinogram bins pertinent to the same slice are fed into a multi-input channel deep convolutional network to estimate a single ACF sinogram associated with the same slice. The clinical evaluation of the proposed DL-EM approach consisted of 68 clinical brain TOF PET/CT studies, where CT-based attenuation correction (CTAC) served as reference. A two-tissue class consisting of background-air and soft-tissue segmentation of the TOF PET non-AC images (SEG) as a proxy of the technique used in the clinic was also included in the comparative evaluation. Qualitative and quantitative PET analysis was performed through SUV bias maps quantification in 63 different brain regions. The DL-EM approach resulted in 6.1±9.7% relative mean absolute error (RMAE) in bony structures compared to SEG AC method with RMAE of 16.1±8.2% (p-value <0.001). Considering the entire head region, DL-EM led to a root mean square error (RMSE) of 0.3±0.01 outperforming the SEG method with RMSE of 0.8±0.02 SUV (p-value <0.001). The region-wise analysis of brain PET studies revealed less than 7% absolute SUV bias for the DL-EM approach, whereas the SEG method resulted in more than 14% absolute SUV bias (p-value <0.05). Qualitative assessment and quantitative PET analysis demonstrated the superior performance of the DL-EM approach over the segmentation-based technique with clinically acceptable SUV bias. The results obtained using the DL-EM approach are comparable to state-of-the-art MRI-guided AC methods. Yet, this approach enables the extraction of interesting features about patient-specific attenuation which could be employed not only as a stand-alone AC approach but also as complementary/prior information in other AC algorithms.

A study of false apical defects in myocardial perfusion imaging with SPECT/CT

An important problem of myocardial SPECT/CT imaging with attenuation correction (AC) is a reduction of radiotracer activity at the apex in reconstructed images. This problem is known as the false apical defect. False apical defects mask real defects and lead to interpretation uncertainties. The aim of the present work is to study this phenomenon to understand the cause of apical defects on AC images. Mathematical modeling and clinical studies are carried out. Special mathematical models were designed to study the effect of the myocardial left ventricle (LV) form and the effect of patient constitution on false apical defects severity. Computer simulations of the SPECT/CT myocardial perfusion imaging procedure with and without attenuation correction were performed. The dependence of apical false defects on the LV form and on the data acquisition scheme was studied. The results of mathematical modeling have shown that the cause of false apical defects is not associated with attenuation correction. Rather, it is related to the LV form, especially to the presence of anatomical apical thinning. The LV form with apical thinning in diastole phase can not be exactly reconstructed by using the standard data acquisition schemes with limited 32 and 60 views. The reconstruction error appears in the rounded apical region in reconstructions with and without attenuation correction. But, in non-AC images, this error is masked by the second error related to the lack of attenuation correction. Interpretation based on the results of two compensating errors cannot be reliable. Taking into account the results obtained, we suggest the following hypothesis: gated systolic AC images can be useful in the interpretations of findings at the apex. This hypothesis was confirmed in preliminary clinical study.

Integrated PET/MR breast cancer imaging: Attenuation correction and implementation of a 16-channel RF coil.

This study aims to develop, implement, and evaluate a 16-channel radiofrequency (RF) coil for integrated positron emission tomography/magnetic resonance (PET/MR) imaging of breast cancer. The RF coil is designed for optimized MR imaging performance and PET transparency and attenuation correction (AC) is applied for accurate PET quantification. A 16-channel breast array RF coil was designed for integrated PET/MR hybrid imaging of breast cancer lesions. The RF coil features a lightweight rigid design and is positioned with a spacer at a defined position on the patient table of an integrated PET/MR system. Attenuation correction is performed by generating and applying a dedicated 3D CT-based template attenuation map. Reposition accuracy of the RF coil on the system patient table while using the positioning frame was tested in repeated measurements using MR-visible markers. The MR, PET, and PET/MR imaging performances were systematically evaluated using modular breast phantoms. Attenuation correction of the RF coil was evaluated with difference measurements of the active breast phantoms filled with radiotracer in the PET detector with and without the RF coil in place, serving as a standard of reference measurement. The overall PET/MR imaging performance and PET quantification accuracy of the new 16-channel RF coil and its AC were then evaluated in first clinical examinations on ten patients with local breast cancer. The RF breast array coil provides excellent signal-to-noise ratio and signal homogeneity across the volume of the breast phantoms in MR imaging and visualizes small structures in the phantoms down to 0.4 mm in plane. Difference measurements with PET revealed a global loss and thus attenuation of counts by 13% (mean value across the whole phantom volume) when the RF coil is placed in the PET detector. Local attenuation ranging from 0% in the middle of the phantoms up to 24% was detected in the peripheral regions of the phantoms at positions closer to attenuating hardware structures of the RF coil. The position accuracy of the RF coil on the patient table when using the positioning frame was determined well below 1 mm for all three spatial dimensions. This ensures perfect position match between the RF coil and its three-dimensional attenuation template during the PET data reconstruction process. When applying the CT-based AC of the RF coil, the global attenuation bias was mostly compensated to ±0.5% across the entire breast imaging volume. The patient study revealed high quality MR, PET, and combined PET/MR imaging of breast cancer. Quantitative activity measurements in all 11 breast cancer lesions of the ten patients resulted in increased mean difference values of SUVmax 11.8% (minimum 3.2%; maximum 23.2%) between nonAC images and images when AC of the RF breast coil was applied. This supports the quantitative results of the phantom study as well as successful attenuation correction of the RF coil. A 16-channel breast RF coil was designed for optimized MR imaging performance and PET transparency and was successfully integrated with its dedicated attenuation correction template into a whole-body PET/MR system. Systematic PET/MR imaging evaluation with phantoms and an initial study on patients with breast cancer provided excellent MR and PET image quality and accurate PET quantification.

Segmentation-based MR attenuation correction including bones also affects quantitation in brain studies: an initial result of 18F-FP-CIT PET/MR for patients with parkinsonism.

Attenuation correction (AC) with an ultrashort echo time (UTE) sequence has recently been used in combination with segmentation for cortical bone identification for brain PET/MR studies. The purpose of this study was to evaluate the quantification of (18)F-fluoropropyl-carbomethoxyiodophenylnortropane ((18)F-FP-CIT) binding in brain PET/MR, particularly focusing on effects of UTE-based AC including bone segmentation. Sixteen patients with initially suspected parkinsonism were prospectively enrolled. An emission scan was acquired 110 min after (18)F-FP-CIT injection on a dedicated PET/MR scanner, immediately followed by another emission scan using a PET/CT scanner 120 min after the injection. A UTE-based attenuation map was used to classify the voxels into 3 tissues: bone, soft tissue, and air. All PET images were spatially normalized, and a specific-to-nonspecific dopamine transporter (DAT) binding ratio (BR) was calculated using statistical probabilistic anatomic mapping. The level of agreement was assessed with intraclass correlation coefficients (ICCs). Voxelwise comparison between PET images acquired from PET/MR and PET/CT was performed. We compared non-attenuation-corrected images to analyze UTE-based AC effects on DAT quantification. BR in the putamen obtained from PET/MR and PET/CT showed low interequipment variability, whereas BR in the caudate nucleus showed significant variability (ICC = 0.967 and 0.682 for putamen and caudate nucleus, respectively). BR in the caudate nucleus was significantly underestimated by PET/MR, compared with PET/CT (mean difference of BR = 0.66, P < 0.0001). Voxelwise analysis revealed that PET/MR showed significantly low BR in the periventricular regions, which was caused by a misclassification of the ventricle as air on the attenuation map. We also compared non-AC images, revealing low interequipment variability even in the caudate nucleus (ICC = 0.937 and 0.832 for putamen and caudate nucleus, respectively). Our data demonstrate spatial bias of the DAT BR on (18)F-FP-CIT PET/MR. Voxelwise analysis and comparison to non-AC images identified the misclassification of ventricle as air to be the cause of bias. To obtain reliable quantification for brain PET/MR studies including (18)F-FP-CIT PET, alternative and more reliable segmentation strategies are required.

Clinical value of stress-only Tc-99m SPECT imaging: Importance of attenuation correction

BackgroundIn selected patients, stress-only SPECT imaging has been proposed as an alternative to rest-stress SPECT imaging to improve laboratory efficiency and reduce radiation exposure. The impact of attenuation correction (AC) upon interpretation, post-test patient management and cardiac risk stratification in relation to stress-only imaging is not well understood. ObjectivesThe purpose of this study was to determine the clinical value for laboratory throughput and predicting outcomes of normal and abnormal stress-only SPECT imaging with AC in a consecutive series of clinically referred patients. MethodsA retrospective analysis of 1,383 consecutive patients who were scheduled for stress-only SPECT imaging for symptom assessment of suspected myocardial ischemia was performed. All images had been interpreted and categorized using the standard 17-segment model without AC followed by AC. Follow-up data for 2.1 ± 1.3 years after SPECT imaging for the occurrence of cardiac events (non-fatal MI, cardiac death, and cardiac revascularization) previously collected by routine methods were reviewed. ResultsNon-AC SPECT image interpretation revealed that 58% (802/1383) of patients had abnormal stress images. AC image interpretation of the abnormal non-AC images re-classified 83% (666/802) of these as normal. Among patients with abnormal stress images after AC (136/1383), 63% (86/136) returned for additional rest scans, while the remaining 37% (50/136) were clinically managed without further rest images. The incidence of cardiac death or non-fatal MI was very low in patients with normal stress-only scans (0.7%). ConclusionA strategy of stress-only imaging with AC in symptomatic patients is an efficient method which appropriately identifies at risk and low-risk patients yielding a low percentage requiring rest imaging. Clinical decisions can be made based on abnormal stress-only imaging without further rest imaging if clinically appropriate.

An investigation into truncation artefacts experienced in cardiac imaging using a dedicated cardiac SPECT gamma camera with transmission attenuation correction

In SPECT images truncation artefacts may occur when the object does not remain in the field of view (FOV). This truncation may lead to distortions in the transmission map (μ map), which would affect the attenuation correction (AC) and possibly the final image. Our aim was to investigate this distortion on our dedicated small FOV cardiac camera and evaluate the efficacy of the truncation correction. Using a phantom, transmission data were acquired with our cardiac camera using arrays of (153)Gd sources. The width of the phantom was varied by adding attenuating material. AC and non-AC images were analysed. The results showed that distorted µ maps were produced, which remained distorted even after the application of correction algorithms for chest width measurements greater than 43 ± 1 cm. This distortion was observed to worsen with increasing chest width measurements. Artefacts were not seen for chest width measurements smaller than 43 cm, even when significant anterior attenuation was modelled. Our findings demonstrate that AC-corrected images acquired with our dedicated small FOV cardiac camera are not suitable for patients with chest width measurements greater than 43 cm.

Synergistic impact of attenuation correction and gating in routine myocardial SPECT reporting: 2 year follow-up study

To look at the combined impact of non-uniform attenuation correction (AC) and gated SPECT in the visual interpretation of myocardial SPECT imaging. This was compared to the individual benefit obtained by adding AC information and gated SPECT information to non-AC image information. We retrospectively studied a group of 141 patients with a 22-26 month follow-up who underwent myocardial perfusion scintigraphy imaging. All the studies were corrected for attenuation with Gd line source transmission data and were ECG gated. In patients who had abnormal studies, follow-up coronary angiography information was also obtained in addition to medical follow-up information. Two experienced nuclear medicine physicians interpreted the images independently and were blinded to the other person's report. Non-attenuation corrected data was first evaluated followed by attenuation corrected data and gated SPECT data. Four approaches to interpretation of images were undertaken: (1) non-AC images only, (2) non-AC+AC images, (3) non-AC+gated images, and (4) non-AC+AC+gated images. Study results were divided into four categories based on how confident the observers were of the diagnosis: (1) normal, (2) borderline normal, (3) borderline abnormal, and (4) abnormal. When results for sensitivity and specificity using the four different interpretation techniques were compared there was a statistically significant improvement in the specificity compared to non-AC image (48%) with the addition of AC (77%) and gating (82%) information (P<0.001). The best improvement in the specificity was noted when both AC and gated information (91%) was used along with non-AC information. The normalcy rates almost doubled following the addition of AC and gated data. There was also a decrease in the number of borderline results, showing an improvement in the reporter confidence in interpreting myocardial SPECT studies. Sensitivity, however, did not show a significant change between the four different approaches to interpretation of the study. Attenuation correction and gating when combined have a synergistic impact upon improving the specificity of myocardial SPECT reporting when compared to the use of individual techniques alone to improve the specificity.

Quantitative assessment of the effect of x-ray CT attenuation correction on myocardial perfusion in patients with single vessel coronary artery disease

Background: Attenuation artifacts diminish the diagnostic accuracy of myocardial perfusion imaging (MPI). Little quantitative data exist regarding the effect of x-ray CT based attenuation correction (AC) on regional tracer distribution. The purpose of this study is to quantitatively evaluate the effect of AC on MPI in individual vascular territories. Methods: In this study we selected patients with known single vessel disease who had stress technetium-99m sestamibi SPECT testing with and without AC and cardiac catheterization within 6 months. Coronary stenoses were considered significant if ≥ 70% of the mean luminal diameter. Patients with previous coronary bypass or acute myocardial infarction were excluded. Perfusion stress images were analyzed using automated bulls-eye 20 segment quantitative software. Segments below the 75% maximum threshold were considered abnormal. Specific vascular territories were evaluated. The basal anteroseptum and inferoseptum were excluded due to membranous septum variability. Results: Twenty nine patients were selected (8 LAD, 10LCX, 11RCA). Stress imaging showed more abnormal segments in the non-AC compared to the AC images (mean 13.45 vs. 11.41 p=0.0005). When considering specific vascular territories, there were more false positive segments in the RCA (inferior) territory in the non-AC images in all CAD patients (see table) likely representing diaphragmatic attenuation artifact. Abnormally Perfused Regions During Stress (mean) Patient's known CAD LAD AC/Non-AC LCX AC/Non-AC RCA AC/Non-AC LAD 5.3/4.8 p = NS 1.5/2.0 p = NS 3.6/5.3 p = 0.01 LCX 2.9/2.4 NS 3.2/3.1 NS 3.3/5.0 0.008 RCA 3.2/3.0 NS 2.2/3.3 p = 0.02 3.8/5.7 0.0004 Conclusions: Myocardial perfusion imaging using x-ray CT based attenuation correction is an accurate means of detecting single vessel CAD with improved visualization of the inferior wall and less diaphragmatic artifact compared to non-corrected imaging.

Positron emission tomography imaging in nonmalignant thoracic disorders

The role of the fluorodeoxyglucose (FDG) technique positron emission tomography (PET) is well established in the management of patients with lung cancer. Increasingly, it is becoming evident that FDG-PET can be effectively employed to diagnose a variety of benign pulmonary disorders. Knowledge of such applications further expands the domain of this powerful modality and further improves the ability to differentiate benign from malignant diseases of the chest. We describe pertinent technical factors that substantially contribute to optimal imaging of the thoracic structures. Particularly, the complementary role of attenuation correction (AC) to that of non-AC images is emphasized. We further outline the need for and the state of the art for co-registration of PET and anatomic images for diagnostic and therapeutic purposes. We then review patterns of physiologic uptake of FDG in thoracic structures, including the lung, the heart, the aorta and large arteries, esophagus, thymus, trachea, thoracic muscles, bone marrow, and joints and alterations following radiation therapy to the thorax. A great deal of information is provided with regard to differentiating benign from malignant nodules and in particular, we emphasize the role of dual time point imaging and partial volume correction for accurate assessment of such lesions. Following a brief review of the diagnostic issues related to the assessment of mediastinal adenopathies, the role of FDG-PET imaging in environment-induced lung diseases, including pneumoconiosis, smoking, and asthma are described. A large body of information is provided about the role of this technology in the management of patients with suspected infection and inflammation of the lungs such as acquired immunodeficiency syndrome, fever of unknown origin, sarcoidosis, chronic granulomatous disease and monitoring the disease process and response to therapy. Finally, the value of FDG-PET in differentiating benign from malignant diseases of the pleura including asbestosis-related disorders is described at the conclusion of this comprehensive review.

Low event rate for stress-only perfusion imaging in patients evaluated for chest pain

ObjectivesWe sought to demonstrate the safety of stress-only perfusion imaging among patients with low to medium probability of coronary disease being evaluated for chest pain. BackgroundThe need for performing rest in addition to stress perfusion imaging to confirm normalcy is due largely to defects created on the stress images by attenuation artifacts. A low cardiac event rate among patients undergoing stress-only imaging with attenuation correction (AC) would validate the safety of stress-only imaging. MethodsPatients with low to medium pretest probability for coronary artery disease (CAD) referred for chest pain evaluation from July 1, 1997 to July 1, 1999, were scheduled for a two-day stress/rest tomographic (single photon emission computerized tomography, or SPECT) perfusion imaging study with Tc-99m sestamibi. Patients were imaged on a variable angle camera with AC using Gd-153 scanning line sources (Vantage ADAC, Milpitas, California). If the stress scan was normal without AC or corrected with AC, the patient did not return for rest scan and was followed. ResultsSeven hundred twenty-nine patients underwent stress-only imaging, and follow-up was obtained on 652 (89%) of those patients. There were 224 males and 428 females with mean age of 52 ± 13 years. Mean follow-up was 22.3 ± 6.4 months. The mean pretest probability was 37 ± 24%. The non-AC images showed breast and/or diaphragmatic attenuation artifacts severe enough to have required the patient to return for rest imaging in 37% of patients, and all corrected completely with AC. During follow-up, there were two noncardiac deaths and no cardiac deaths. There was one myocardial infarction; three patients with progressive unstable angina underwent diagnostic coronary angiography showing significant CAD. The overall cardiac event rate was 0.6%. ConclusionsThese results support stress-only imaging in patients with low to medium probability for CAD as a safe, time- and cost-efficient imaging modality.