Improved Attenuation Correction Research Articles

Thoracic aortic microcalcification activity in combined positron emission tomography and magnetic resonance imaging

IntroductionNon-invasive detection of pathological changes in thoracic aortic disease remains an unmet clinical need particularly for patients with congenital heart disease. Positron emission tomography combined with magnetic resonance imaging (PET-MRI) could provide a valuable low-radiation method of aortic surveillance in high-risk groups. Quantification of aortic microcalcification activity using sodium [18F]fluoride holds promise in the assessment of thoracic aortopathies. We sought to evaluate aortic sodium [18F]fluoride uptake in PET-MRI using three methods of attenuation correction compared to positron emission tomography computed tomography (PET-CT) in patients with bicuspid aortic valve,MethodsThirty asymptomatic patients under surveillance for bicuspid aortic valve disease underwent sodium [18F]fluoride PET-CT and PET-MRI of the ascending thoracic aorta during a single visit. PET-MRI data were reconstructed using three iterations of attenuation correction (Dixon, radial gradient recalled echo with two [RadialVIBE-2] or four [RadialVIBE-4] tissue segmentation). Images were qualitatively and quantitatively analysed for aortic sodium [18F]fluoride uptake on PET-CT and PET-MRI.ResultsAortic sodium [18F]fluoride uptake on PET-MRI was visually comparable with PET-CT using each reconstruction and total aortic standardised uptake values on PET-CT strongly correlated with each PET-MRI attenuation correction method (Dixon R = 0.70; RadialVIBE-2 R = 0.63; RadialVIBE-4 R = 0.64; p < 0.001 for all). Breathing related artefact between soft tissue and lung were detected using Dixon and RadialVIBE-4 but not RadialVIBE-2 reconstructions, with the presence of this artefact adjacent to the atria leading to variations in blood pool activity estimates. Consequently, quantitative agreements between radiotracer activity on PET-CT and PET-MRI were most consistent with RadialVIBE-2.ConclusionAscending aortic microcalcification analysis in PET-MRI is feasible with comparable findings to PET-CT. RadialVIBE-2 tissue attenuation correction correlates best with the reference standard of PET-CT and is less susceptible to artefact. There remain challenges in segmenting tissue types in PET-MRI reconstructions, and improved attenuation correction methods are required.

Influence of CT metal artifact reduction on SPECT/CT quantification of bone scintigraphy – Retrospective study for selected types of metal implants

Implanted metal prostheses can cause severe artifacts in reconstructed computed tomography (CT) images. To reduce the diagnostic impact of these artifacts and improve attenuation correction in single photon emission computed tomography (SPECT), an algorithm of iterative metal artifact reduction (iMAR) for SPECT/CT systems was developed. The aims of this study were (a) to assess the difference in visual image quality by comparing CT and SPECT images reconstructed with and without iMAR and (b) to determine the influence of iMAR on quantitative 99mTc-uptake in SPECT/CT. This retrospective study includes 21 patients with implanted metal prostheses who underwent SPECT/CT bone scintigraphy. CT data were reconstructed with iMAR and without (noMAR) and were used for attenuation correction of SPECT data for xSPECT Quant and xSPECT Bone reconstruction. The effect of iMAR on image quality was evaluated by visual analysis and the effect on quantitative SPECT/CT was assessed by measuring HU values and absolute uptake values (kBq/mL) in volumes of interest (VOIs). There was a significant reduction of visible metal artifacts with iMAR (p<0.01) in the CT images, but visual differences in the SPECT images were minor. The values of quantitative tracer uptake in VOIs near metal implants were lower for iMAR vs. noMAR xSPECT Quant (p<0.01). Only VOIs near metal showed significant differences in HU values, which were 14.6% lower for iMAR CT (p<0.01). The use of iMAR reduces metal artifacts in CT and improves the perceived image quality. Although in some cases a significant difference in the quantitative evaluation of SPECT/CT was observed, the influence of iMAR can be considered small in relation to other factors in the clinical setting.

MR-based truncation correction using an advanced HUGE method to improve attenuation correction in PET/MR imaging of obese patients.

Truncation artifacts in the periphery of the magnetic resonance (MR) field-of-view (FOV) and thus, in the MR-based attenuation correction (AC) map, may hamper accurate positron emission tomography (PET) quantification in whole-body PET/MR, which is especially problematic in patients with obesity with overall large body dimensions. Therefore, an advanced truncation correction (TC) method to extend the conventional MR FOV is needed. The extent of MR-based AC-map truncations in obese patients was determined in a dataset including n=10 patients that underwent whole-body PET/MR exams. Patient inclusion criteria were defined as BMI>30kg/m2 and body weight>100kg. Truncations in PET/MR patients with obesity were quantified comparing the MR-based AC-map volume to segmented non-AC PET data, serving as the reference body volume without truncations to demonstrate the need of improved TC. The new method implemented in this study, termed "advanced HUGE," was modified and extended from the original HUGE method by Blumhagen etal. in order to provide improved TC across the entire axial MR FOV and to unlock new clinical applications of PET/MR. Advanced HUGE was then systematically tested in PET/MR NEMA phantom measurements. Relative differences between computed tomography (CT) AC PET data of the phantom setup (reference) and MR-based Dixon AC, respectively Dixon + advanced HUGE AC, were calculated. The applicability of the method for advanced TC was then demonstrated in first MR-based measurements in healthy volunteers. It was found that the MR-based AC maps of obese patients often reveal truncations in the anterior-posterior direction. Especially, the abdominal region could benefit from improved TC, where maximal relative differences in the AC-map volume up to -17% were calculated. Applying advanced HUGE to improve the MR-based AC in PET/MR, PET quantification errors in the large-volume phantom setup could be considerably reduced from average -18.6% (Dixon AC) to 4.6% compared to the CT AC reference. Volunteer measurements demonstrate that formerly missing AC-map volume in the Dixon-VIBE AC-map could be added due to advanced HUGE in the anterior-posterior direction and thus, potentially improves AC in PET/MR. The advanced HUGE method for truncation correction considerably reduces truncations in the anterior-posterior direction demonstrated in phantom measurements and healthy volunteers and thus, further improves MR-based AC in PET/MR imaging.

Evaluation of improved attenuation correction in whole-body PET/MR on patients with bone metastasis using various radiotracers

PurposeThis study evaluates the quantitative effect of improved MR-based attenuation correction (AC), including bone segmentation and the HUGE method for truncation correction in PET/MR whole-body hybrid imaging specifically of oncologic patients with bone metastasis and using various radiotracers.MethodsTwenty-three patients that underwent altogether 28 whole-body PET/MR examinations with findings of bone metastasis were included in this study. Different radiotracers (18F-FDG, 68Ga-PSMA, 68Ga-DOTATOC, 124I–MIBG) were injected according to appropriate clinical indications. Each of the 28 whole-body PET datasets was reconstructed three times using AC with (1) standard four-compartment μ-maps (background air, lung, muscle, and soft tissue), (2) five-compartment μ-maps (adding bone), and (3) six-compartment μ-maps (adding bone and HUGE truncation correction). The SUVmax of each detected bone lesion was measured in each reconstruction to evaluate the quantitative impact of improved MR-based AC. Relative difference images between four- and six-compartment μ-maps were calculated. MR-based HUGE truncation correction was compared with the PET-based MLAA truncation correction method in all patients.ResultsOverall, 69 bone lesions were detected and evaluated. The mean increase in relative difference over all 69 lesions in SUVmax was 5.4 ± 6.4% when comparing the improved six-compartment AC with the standard four-compartment AC. Maximal relative difference of 28.4% was measured in one lesion. Truncation correction with HUGE worked robust and resulted in realistic body contouring in all 28 exams and for all 4 different radiotracers. Truncation correction with MLAA revealed overestimations of arm tissue volume in all PET/MR exams with 18F-FDG radiotracer and failed in all other exams with radiotracers 68Ga-PSMA, 68Ga-DOTATOC, and 124I- MIBG due to limitations in body contour detection.ConclusionImproved MR-based AC, including bone segmentation and HUGE truncation correction in whole-body PET/MR on patients with bone lesions and using various radiotracers, is important to ensure best possible diagnostic image quality and accurate PET quantification. The HUGE method for truncation correction based on MR worked robust and results in realistic body contouring, independent of the radiotracers used.

Potential Clinical Applications of PET/MR

Since the introduction of simultaneous positron emission tomography magnetic resonance imaging (PET/MR) in 2010, technological advances led to improved image quality, improved attenuation correction, and reduced scan time, leading to an increasing number of clinical applications for this new technology. Lower radiation exposure and higher soft-tissue contrast are the main advantages of PET/MR over PET/CT. However, its real added value in clinical applications is still under investigation. This short review is focusing on three PET/MR applications that had an increasing clinical acceptance in our institution and where we believe that PET/MR will play an important role in the future: prostate cancer, head and neck cancer, and cardiac imaging.

An improved self-consistent approach to attenuation correction for C-band polarimetric radar measurements and its impact on quantitative precipitation estimation

Attenuation correction of measured reflectivity (ZH) and differential reflectivity (ZDR) is critical in short-wavelength radar applications, especially during extreme precipitation events such as heavy rain. This paper develops an improved self-consistent approach (improved ZPHI method) for attenuation correction in practical environment. In particular, a non-negative constraint is imposed on the specific attenuation, which is inferred from the monotonic increasing characteristic of differential propagation phase (ΦDP). The copolar correlation coefficient (ρHV) is used to partition the ΦDP profiles into independent range segments, which are featured by different hydrometeor phases such as liquid rain or mixed-phase precipitation. Additional minimization constraint is imposed on the cost function of the difference between preprocessed ΦDP and reconstructed ΦDPto ensure its appropriate convergence. In addition, an exponential ZDR − ZH relation derived from local raindrop size distribution (DSD) data is applied in the procedure of ZDR correction. Polarimetric measurements from a C-band radar (CPOL) in Hangzhou of China during two extreme precipitation events are used to demonstrate the improved attenuation correction method. DSD observations from disdrometers and radar data from a nearby system operating at non-attenuated frequency (i.e., S-band) at Huangshan Mountain are used to evaluate the attenuation correction performance. This paper also studies the impact of attenuation correction on the radar-derived quantitative precipitation estimation (QPE) product, through cross-comparison with rain gauge observations. Results show that the polarimetric observations from CPOL radar are effectively enhanced and more consistent with collocated S-band measurements and the simulated radar moments based on DSD data. Hourly rainfall products derived from R(ZH) and R(ZH,ZDR) are significantly improved, which are comparable to R(KDP) after attenuation correction.

Impact of improved attenuation correction featuring a bone atlas and truncation correction on PET quantification in whole-body PET/MR.

Recent studies have shown an excellent correlation between PET/MR and PET/CT hybrid imaging in detecting lesions. However, a systematic underestimation of PET quantification in PET/MR has been observed. This is attributable to two methodological challenges of MR-based attenuation correction (AC): (1) lack of bone information, and (2) truncation of the MR-based AC maps (μmaps) along the patient arms. The aim of this study was to evaluate the impact of improved AC featuring a bone atlas and truncation correction on PET quantification in whole-body PET/MR. The MR-based Dixon method provides four-compartment μmaps (background air, lungs, fat, soft tissue) which served as a reference for PET/MR AC in this study. A model-based bone atlas provided bone tissue as a fifth compartment, while the HUGE method provided truncation correction. The study population comprised 51 patients with oncological diseases, all of whom underwent a whole-body PET/MR examination. Each whole-body PET dataset was reconstructed four times using standard four-compartment μmaps, five-compartment μmaps, four-compartment μmaps+HUGE, and five-compartment μmaps+HUGE. The SUVmax for each lesion was measured to assess the impact of each μmap on PET quantification. All four μmaps in each patient provided robust results for reconstruction of the AC PET data. Overall, SUVmax was quantified in 99 tumours and lesions. Compared to the reference four-compartment μmap, the mean SUVmax of all 99 lesions increased by 1.4±2.5% when bone was added, by 2.1±3.5% when HUGE was added, and by 4.4±5.7% when bone+HUGE was added. Larger quantification bias of up to 35% was found for single lesions when bone and truncation correction were added to the μmaps, depending on their individual location in the body. The novel AC method, featuring a bone model and truncation correction, improved PET quantification in whole-body PET/MR imaging. Short reconstruction times, straightforward reconstruction workflow, and robust AC quality justify further routine clinical application of this method.

Respiratory average CT for attenuation correction in myocardial perfusion SPECT/CT.

Cine average CT (CACT) and interpolated average CT (IACT) have been proposed to improve attenuation correction (AC) for PET/CT in oncologic and cardiac studies. This study aims to evaluate their effectiveness on myocardial perfusion SPECT/CT using computer simulation and physical phantom experiments. We first simulated normal male with 99mTc-sestamibi distribution using digital XCAT phantom with respiratory motion amplitudes of 2, 3, and 4cm. Average activity and attenuation maps represented static SPECT and CACT, while the attenuation maps of end-inspiration and end-expiration represented two helical CTs (HCTs), respectively. Sixty noise-free and noisy projections were simulated over 180° using an analytical parallel-hole projector. We then filled 673MBq 99mTc into an anthropomorphic torso phantom with normal heart or heart with a defect which placed on a programmable respiratory platform to model various respiratory amplitudes. Sixty projections were acquired over 180° using a clinical SPECT/CT scanner. The CACT, standard HCT, and 2 HCTs at extreme phases were acquired. Interpolated CT phases were generated between them using affine plus b-spline registration, and IACT was obtained by averaging the interpolated phases and the 2 original extreme phases for both simulation and phantom experiments. Projections were reconstructed with AC using CACT, IACT, and HCTs, respectively. Polar and 17-segment plots were analyzed by relative difference (RD) of the uptake. Two regions-of-interest (ROI) were drawn on the defect and background area to obtain the intensity ratio (IR). No substantial difference was observed on the polar plots generated from different AC methods, while the quantitative RD measurements showed that SPECTCACT were most similar to the original phantom, followed by SPECTIACT, with RDmax<8 and<10% in the simulation study. The RD of SPECTHCTs deviated from the original phantom and SPECTCACT in various segments, with RDmax of 19.76 and 16.68% in the simulation and phantom experiment, respectively. The IR of SPECTHCTs fluctuated more from the truth for higher motion amplitude. Both CACT-AC and IACT-AC reduced respiratory artifacts and improved quantitation in myocardial perfusion SPECT as compared to HCT-AC. The use of IACT further reduced the radiation dose.

Flame-temperature, light-attenuation, and CO measurements by spontaneous Raman scattering in non-sooting diesel-like jets

Quantitative spatially resolved measurements of temperature and species are lacking particularly in the core of combusting diesel jets. Major problems are light attenuation and interfering light emissions. However, these factors are reduced in non-sooting diesel-like jets, as demonstrated in the present work, because light is not attenuated by soot and interfering LIF (laser-induced fluorescence) from PAHs (polycyclic aromatic hydrocarbons) is substantially lower. The current results show that thermometry by SRS (spontaneous Raman scattering) excited by a UV (ultraviolet) laser is therefore feasible even in the core of a non-sooting diesel-like jet in a combustion vessel. Two diagnostic approaches are assessed. The first one is based on the spectral band shape of the Stokes (red-shifted) ro-vibrational SRS from N2, whereas the ratio of integrated ro-vibrational Stokes to anti-Stokes (blue-shifted) N2-SRS bands is exploited in the second one. It turns out that the first method is advantageous in terms of light attenuation by molecular species, the influence of interfering emissions, and resulting single-shot capability. However, these investigations also show that the anti-Stokes N2-SRS signal can be used for quantification of light attenuation. This is particularly attractive because this SRS band at ∼235 nm nearly coincides with a LIF emission from NO at ∼237 nm, leading to improved attenuation correction of NO-LIF. Furthermore, the recorded spectra indicate that additional quantitative species measurements by SRS are feasible in the non-sooting jet. For instance, the mole fraction of CO is quantified in this work for the first time in the jet core.

Interpolated average CT for cardiac PET/CT attenuation correction

BackgroundPreviously, we proposed interpolated averaged CT (IACT) for improved attenuation correction (AC) in thoracic PET/CT. This study aims to evaluate its feasibility and effectiveness on cardiac PET/CT. MethodsWe simulated 18F-FDG distribution using the XCAT phantom with normal and abnormal cardiac uptake. Average activity and attenuation maps represented static PET and respiration average CT (ACT), respectively, while the attenuation maps of end-inspiration/expiration represented 2 helical CTs (HCT). IACT was obtained by averaging the 2 extreme phases and the interpolated phases generated between them. Later, we recruited 4 patients who were scanned 1 hr post 315-428 MBq 18F-FDG injection. Simulated and clinical PET sinograms were reconstructed with AC using (1) HCT, (2) IACT, and (3) ACT. Polar plots and the 17-segment plots were analyzed. Two regions-of-interest were drawn on lesion and background area to obtain the intensity ratio (IR). ResultsPolar plots of PETIACT-AC were more similar to PETACT-AC in both simulation and clinical data. Artifacts were observed in various segments in PETHCT-AC. IR differences of HCT as compared to the phantom were up to ~20%. ConclusionsIACT-AC reduced respiratory artifacts and improved PET/CT matching similarly to ACT-AC. It is a promising low-dose alternate of ACT for cardiac PET/CT.

Feasibility of simultaneous whole-brain imaging on an integrated PET-MRI system using an enhanced 2-point Dixon attenuation correction method.

Purpose: To evaluate a potential approach for improved attenuation correction (AC) of PET in simultaneous PET and MRI brain imaging, a straightforward approach that adds bone information missing on Dixon AC was explored.Methods: Bone information derived from individual T1-weighted MRI data using segmentation tools in SPM8, were added to the standard Dixon AC map. Percent relative difference between PET reconstructed with Dixon+bone and with Dixon AC maps were compared across brain regions of 13 oncology patients. The clinical potential of the improved Dixon AC was investigated by comparing relative perfusion (rCBF) measured with arterial spin labeling to relative glucose uptake (rPETdxbone) measured simultaneously with 18F-flurodexoyglucose in several regions across the brain.Results: A gradual increase in PET signal from center to the edge of the brain was observed in PET reconstructed with Dixon+bone. A 5–20% reduction in regional PET signals were observed in data corrected with standard Dixon AC maps. These regional underestimations of PET were either reduced or removed when Dixon+bone AC was applied. The mean relative correlation coefficient between rCBF and rPETdxbone was r = 0.53 (p < 0.001). Marked regional variations in rCBF-to-rPET correlation were observed, with the highest associations in the caudate and cingulate and the lowest in limbic structures. All findings were well matched to observations from previous studies conducted with PET data reconstructed with computed tomography derived AC maps.Conclusion: Adding bone information derived from T1-weighted MRI to Dixon AC maps can improve underestimation of PET activity in hybrid PET-MRI neuroimaging.

Attenuation correction synthesis for hybrid PET-MR scanners: application to brain studies.

Attenuation correction is an essential requirement for quantification of positron emission tomography (PET) data. In PET/CT acquisition systems, attenuation maps are derived from computed tomography (CT) images. However, in hybrid PET/MR scanners, magnetic resonance imaging (MRI) images do not directly provide a patient-specific attenuation map. The aim of the proposed work is to improve attenuation correction for PET/MR scanners by generating synthetic CTs and attenuation maps. The synthetic images are generated through a multi-atlas information propagation scheme, locally matching the MRI-derived patient's morphology to a database of MRI/CT pairs, using a local image similarity measure. Results show significant improvements in CT synthesis and PET reconstruction accuracy when compared to a segmentation method using an ultrashort-echo-time MRI sequence and to a simplified atlas-based method.

Metal artifact reduction strategies for improved attenuation correction in hybrid PET/CT imaging

Metallic implants are known to generate bright and dark streaking artifacts in x-ray computed tomography (CT) images, which in turn propagate to corresponding functional positron emission tomography (PET) images during the CT-based attenuation correction procedure commonly used on hybrid clinical PET/CT scanners. Therefore, visual artifacts and overestimation and/or underestimation of the tracer uptake in regions adjacent to metallic implants are likely to occur and as such, inaccurate quantification of the tracer uptake and potential erroneous clinical interpretation of PET images is expected. Accurate quantification of PET data requires metal artifact reduction (MAR) of the CT images prior to the application of the CT-based attenuation correction procedure. In this review, the origins of metallic artifacts and their impact on clinical PET/CT imaging are discussed. Moreover, a brief overview of proposed MAR methods and their advantages and drawbacks is presented. Although most of the presented MAR methods are mainly developed for diagnostic CT imaging, their potential application in PET/CT imaging is highlighted. The challenges associated with comparative evaluation of these methods in a clinical environment in the absence of a gold standard are also discussed.

Radiotracers for SPECT imaging: current scenario and future prospects

Abstract Single photon emission computed tomography (SPECT) has been the cornerstone of nuclear medicine and today it is widely used to detect molecular changes in cardiovascular, neurological and oncological diseases. While SPECT has been available since the 1980s, advances in instrumentation hardware, software and the availability of new radiotracers that are creating a revival in SPECT imaging are reviewed in this paper. The biggest change in the last decade has been the fusion of CT with SPECT, which has improved attenuation correction and image quality. Advances in collimator design, replacement of sodium iodide crystals in the detectors with cadmium zinc telluride (CZT) detectors as well as advances in software and reconstruction algorithms have all helped to retain SPECT as a much needed and used technology. Today, a wide spectrum of radiotracers is available for use in cardiovascular, neurology and oncology applications. The development of several radiotracers for neurological disorders is briefly described in this review, including [123I]FP-CIT (DaTSCANTM) available for Parkinson's disease. In cardiology, while technetium-99m labeled tetrofosmin and technetium-99m labeled sestamibi have been well known for myocardial perfusion imaging, we describe a recently completed multicenter clinical study on the use of [123I]mIBG (AdreViewTM) for imaging in chronic heart failure patients. For oncology, while bone scanning has been prevalent, newer radiotracers that target cancer mechanisms are being developed. Technetium-99m labeled RGD peptides have been reported in the literature that can be used for imaging angiogenesis, while technetium-99m labeled duramycin has been used to image apoptosis. While PET/CT is considered to be the more advanced technology particularly for oncology applications, SPECT continues to be the modality of choice and the workhorse in many hospitals and nuclear medicine centers. The cost of SPECT instruments also makes them more attractive in developing countries where the cost of a scan is still prohibitive for many patients.

WE‐SAM‐304A‐02: SPECT/CT: Technology Updates, Quality Assurance and Applications

Hybrid SPECT/CT is rapidly becoming a mainstream imaging modality and creating a new paradigm for SPECT imaging. The ability to contemporaneously acquire electromechanically registered dual‐modality SPECT and CT scans improves the SPECT image quality due to CT‐based attenuation correction, and enhances the diagnostic confidence of SPECT by providing an anatomical overlay. The first generation SPECT/CT systems used single‐slice CT scanners and produced CT images of limited quality for anatomical overlay. The CT systems integrated into the latest generation of SPECT/CT systems are fully‐functional diagnostic scanner (scan parameters includes tube current, tube voltage, rotation speed, collimation, pitch, slice thickness, FOV, etc.). In addition to generating high‐quality attenuation maps these CT systems can produce CT scans with diagnostic image quality, capable of greatly improving both the localization and specificity of abnormalities detected on the corresponding SPECT scan. In some cases, these systems are also capable of performing diagnostic CT scans with contrast enhancement. The widest use of SPECT/CT is currently in oncology, with applications including: tumor localization, staging and response to treatment; pre‐surgical mapping (e.g., parathyroid adenomas, sentinel lymph nodes); differentiation of skeletal metastases from other disease processes; functional image‐based radiation therapy treatment planning (e.g., lung perfusion); and quantitative SPECT/CT‐based internal radionuclide therapy dosimetry/treatment planning. Cardiac SPECT/CT is currently focused primarily on improved attenuation correction of SPECT myocardial perfusion images. SPECT/CT is also being utilized for imaging bone and other non‐malignant diseases.This lecture will review the physics principles underlying SPECT/CT imaging, present several examples of the clinical application of SPECT/CT, and provide an overview of the currently available SPECT/CT scanner types and models.Learning Objectives:1. To understand the physical principles underlying SPECT/CT image acquisition, processing, and reconstruction.2. To understand current and future clinical applications of SPECT/CT imaging.3. To become familiar with the various commercially‐available SPECT/CT product offerings.

Improved attenuation correction via appropriate selection of respiratory-correlated PET data

Purpose We propose a respiratory-correlated PET data processing method (called “BH-CT-based”) based on breath-hold CT acquisition to reduce the smearing effect and improve the attenuation correction. The resulting images are compared with the ungated PET images acquired using a standard, free-breathing clinical protocol. Methods The BH-CT-based method consisted of a list-mode acquisition with simultaneous respiratory signal recording. An additional breath-hold CT acquisition was also performed in order to define a tissue position from which PET events can be selected. A phantom study featured a 0.5-ml sphere (filled with 18F-fluorodeoxyglucose ( 18F-FDG) solution) pushed onto a rubber balloon (filled with 18F-FDG solution and iodinated contrast agent). The feasibility of the BH-CT-based method was also assessed in two patients. Results In the phantom study, the contrast-to-noise ratios (CNRs) were −1.6 for the Ungated volume and 5.1 for the BH-CT-based volume. For patients, CNRs were higher for BH-CT-based volumes than those for Ungated volumes (17.3 vs. 6.3 and 7.3 vs. 3.8, for patients 1 and 2, respectively). Bias–variance measurements were performed and yielded bias reduction of 40% with BH-CT-based. Conclusion The application of a BH-CT-based method decreases motion bias in PET images. This method resolves issues related to both PET-to-CT misregistration and erroneous attenuation correction and increases lesion detectability.

WE-SAMS-332-02: SPECT/CT Instrumentation and Clinical Applications

Hybrid SPECT/CT is rapidly becoming a mainstream imaging modality, with the recent commercial introduction of systems that incorporate state‐of‐the‐art multi‐slice, diagnostic CTscanners. The ability to acquire contemporaneous, electromechanically registered dual‐modality scans has created a new paradigm for SPECTimaging. The first generation, single‐slice hybrid scanner produced acceptable attenuation coefficient (mu) maps for SPECT attenuation correction, however the quality of its anatomical overlay CT for SPECT/CT fusion imaging was limited. The CT integrated into the latest generation of hybrid systems is a fully‐functional diagnostic scanner. In addition to generating high‐quality mu maps, these systems can produce diagnostic image quality CT scans with variable scan parameters (speed, collimation, pitch, mA, slice thickness and FOV), that are capable of greatly improving both the localization and specificity of abnormalities detected on the corresponding SPECT scan. In some cases, these systems are also capable of performing billable diagnostic CT scans with contrast enhancement. The widest use of SPECT/CT is currently in oncology, with applications including: tumor localization, staging and response to treatment; presurgical mapping (e.g., parathyroid adenomas, sentinel lymph nodes); differentiation of skeletal metastases from other disease processes; functional image‐based radiation therapy treatment planning (e.g., lung perfusion); and quantitative SPECT/CT‐based internal radionuclide therapy dosimetry/treatment planning. Cardiac SPECT/CT is currently focused primarily on improved attenuation correction of SPECT myocardial perfusion images; although the newest, 16‐to‐64‐slice integrated scanners offer the potential for contemporaneous CTcardiacimaging (e.g., coronary angiography, calcium scoring). SPECT/CT is also being utilized for imaging bone and other non‐malignant diseases. This lecture will review the underlying physics of SPECT/CT imaging, present several examples of the clinical application of SPECT/CT, and provide an overview of the currently available SPECT/CT scanner types and models. Educational Objectives: 1. To understand the underlying physical principles of SPECT/CT image acquisition, processing and reconstruction. 2. To understand current and future clinical applications of SPECT/CT imaging. 3. To become familiar with the various commercially‐available SPECT/CT product offerings.

WE‐A‐I‐617‐01: PET/CT Attenuation Correction and Image Fusion

PET/CT scanners provide intrinsically co‐registered CT and PET images in a single scan session, which can then be viewed together as a fused image showing both anatomy and function. This in particular allows for excellent localization of features in the PET image. Furthermore, with the CT scan performed prior to the PET scan, routine attenuation correction of the acquired PET data is possible. The use of CT images for this purpose in PET/CT scanners has the advantages of shorter scan times and effectively noiseless transmission images as compared to the transmission scans that are used in standalone PET cameras. This does however require the non‐trivial step of transforming the CT images to 511 keV attenuation images that are reprojected to obtain the desired attenuation correction factors. This lecture will describe the use of CT images for attenuation correction in PET/CT scanners and address the advantages and challenges of this procedure and the use of fused images in the clinical environment. Improved attenuation correction procedures to take account of different kVp settings in the CT protocol, as well as the particular challenges in emerging applications such as cardiac PET/CT will be addressed.Educational Objectives:1. Review the principles of attenuation correction in PET and the implementation of this procedure using CT images in PET/CT imaging.2. Understand the particular challenges of CT‐based attenuation correction, including the choice of respiration protocols and other problems of patient motion, the use of CT contrast agent, and the presence of foreign objects in the body.3. Review the advantages of image fusion in PET/CT, and the clinical challenges of registering the CT and PET imaging modalities that remain, such as respiration.

Cardiac positron emission tomography and the role of adenosine pharmacologic stress

Although single-photon emission computed tomography (SPECT) provides excellent diagnostic and prognostic value in the evaluation of coronary artery disease, its progress has slowed relative to emerging modalities, such as cardiac positron emission tomography (PET). PET imaging provides certain advantages versus SPECT, including higher spatial resolution, improved attenuation correction, and the capability to perform quantitative measurements at the peak of stress. Cardiac PET scanning is a well-validated, reimbursable means to noninvasively assess myocardial perfusion, left ventricular function, and viability by dynamically imaging positron-labeled radiopharmaceuticals in vivo. For the stress portion of rubidium-82 (Rb-82) PET protocols, pharmacologic agents are commonly used because of the short half-life of Rb-82. In light of recent advances in cardiac PET equipment, the expansion of PET/computed tomography scanners, and the resulting potential for streamlined protocols, adenosine may become widely used in electrocardiographic-gated rest–stress acquisition protocols with Rb-82 cardiac PET. With the resolution of technical issues, cardiac PET has an opportunity to become the standard for evaluation of myocardial perfusion in the coming years.

Fast-channel LSO detectors and fiber-optic encoding for excellent dual photon transmission measurements in PET

Improved attenuation correction remains critical to PET. Currently with dual photon rotating rod sources, benefits of windowing are limited by counting losses of detectors nearest the rods, the near detectors. With single photon sources, improved statistics are offset by a greater need for collimation and more complex emission background correction. Now, a dual photon point source array with fast-channel, near detectors improves on these earlier techniques-here, adding transmission measurement to dual-head rotating PET. Arrays of collimated point sources (/sup 68/Ge/Ga-3 to 21 per head) are aligned axially and orbit the FOV. With each source is a dedicated near detector (LSO crystal). Crystals couple to photomultipliers (PMTs). As the crystals are not block encoded, pulse processing time is reduced (to 120 ns). Reduced processing time lowers dead time and permits hotter sources. For improved axial sampling, larger arrays (21 sources/head) may be configured. To reduce cost, crystals couple fiber-optically into unique PMT pairs-decreasing the total number of near-detector PMTs by 71%.