CT-based attenuation correction in 82Rb-myocardial perfusion PET–CT: incidence of misalignment and effect on regional tracer distribution

Effect of PET-CT misalignment on the quantitative accuracy of cardiac 15O-water PET

BACKGROUND

Quantification of myocardial blood flow (MBF) with PET requires accurate attenuation correction, which is performed using a separate CT. Misalignment between PET and CT scans has been reported to be a common problem. The purpose of the present study was to assess the effect of PET CT misalignment on the quantitative accuracy of cardiac 15O-water PET.

METHODS

Ten clinical patients referred for evaluation of ischemia and assessment of MBF with 15O-water were included in the study. Eleven different misalignments between PET and CT were induced in 6 different directions with 10 and 20 mm amplitudes: caudal (+Z), cranial (- Z), lateral (±X), anterior (+Y), and anterior combined with cranial (+ Y and - Z). Blood flow was quantified from rates of washout (MBF) and uptake (transmural MBF, MBFt) for the whole left ventricle and the three coronary territories. The results from all misalignments were compared to the original scan without misalignment.

RESULTS

MBF was only minorly affected by misalignments, but larger effects were seen in MBFt. On the global level, average absolute deviation across all misalignments for MBF was 1.7% ± 1.4% and for MBFt 5.4% ± 3.2 Largest deviation for MBF was - 4.8% ± 5.8% (LCX, X + 20) and for MBFt - 19.3% ± 9.6% (LCX, X + 20). In general, larger effects were seen in LAD and LCX compared to in RCA.

CONCLUSION

The quantitative accuracy of MBF from 15O-water PET, based on the washout of the tracer, is only to a minor extent affected by misalignment between PET and CT.

Motion in nuclear cardiology imaging: types, artifacts, detection and correction techniques

In this paper, the authors review the field of motion detection and correction in nuclear cardiology with single photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging systems. We start with a brief overview of nuclear cardiology applications and description of SPECT and PET imaging systems, then explaining the different types of motion and their related artefacts. Moreover, we classify and describe various techniques for motion detection and correction, discussing their potential advantages including reference to metrics and tasks, particularly towards improvements in image quality and diagnostic performance. In addition, we emphasize limitations encountered in different motion detection and correction methods that may challenge routine clinical applications and diagnostic performance.

Simulation of Low-Dose Protocols for Myocardial Perfusion 82Rb Imaging

Quantification of myocardial perfusion and myocardial blood flow using ⁸²Rb PET is increasingly used for assessment of coronary artery disease. Current guidelines suggest injections of 1,100-1,500 MBq for both stress and rest. Reducing the injected dose avoids PET system saturation in first-pass flow images and reduces radiation exposure, but the impact on myocardial perfusion quantification of static perfusion images is not fully understood. In this study, we aimed to evaluate the feasibility of performing myocardial perfusion scans using either a half-dose (HfD) or quarter-dose (QD) protocol using reconstructions from acquired full-dose (FD) scans. Methods: This study comprised 171 patients who underwent rest/stress ⁸²Rb PET with a 3-dimensional 4-ring PET/CT scanner using a FD protocol and invasive coronary angiography within 6 mo of the PET emission scan. HfD and QD reconstructions were obtained using the prescribed percentage of events from the FD list-mode files. The total perfusion deficit was quantified for rest (rTPD), stress (sTPD), and ischemia (ITPD = sTPD - rTPD). Diagnostic accuracy for obstructive coronary artery disease, defined as at least 70% stenosis in any of the 3 major coronary arteries, was compared with area under the receiver-operating-characteristic curve (AUC). Results: Patients with a median body mass index of 28.0 (interquartile range, 23.9-31.7) were injected with doses of 1,165 ± 189 MBq of ⁸²Rb. For sTPD, FD and HfD protocols had similar AUCs (FD, 0.807; HfD, 0.802; P = 0.108), whereas QD had a reduced AUC (0.786, P = 0.037). There was no difference in the AUC obtained for ITPD among the 3 protocols (FD, 0.831; HfD, 0.835; QD, 0.831; all P ≥ 0.805). Conclusion: HfD imaging does not affect the quantitative diagnostic accuracy of ⁸²Rb PET on 3-dimensional PET/CT systems and could be used clinically.

Pitfalls on PET/CT Due to Artifacts and Instrumentation

PET/CT has become a preferred imaging modality over PET-only scanners in clinical practice. However, along with the significant improvement in diagnostic accuracy and patient throughput, pitfalls on PET/CT are reported as well. This review provides a general overview on the potential influence of the limitations with respect to PET/CT instrumentation and artifacts associated with the modality integration on the image appearance and quantitative accuracy of PET. Approaches proposed in literature to address the limitations or minimize the artifacts are discussed as well as their current challenges for clinical applications. Although the CT component can play an important role in assisting clinical diagnosis, we concentrate on the imaging scenarios where CT is used to provide auxiliary information for attenuation compensation and scatter correction in PET.

3D PET/CT 82Rb PET myocardial blood flow quantification: comparison of half-dose and full-dose protocols

PURPOSE

Quantification of myocardial blood flow (MBF) has become central in the clinical application of Rubidium-82 (⁸²Rb) PET myocardial perfusion scans. Current recommendations suggest injections of 1100-1500 MBq of ⁸²Rb in bolus form, which poses a potential risk of PET system saturation on most 3D PET/CT systems currently being used. We aimed to evaluate the frequency and impact of PET system saturation and to test the potential use of a half-dose acquisition protocol.

METHODS

This study comprised 20 patients who underwent repeated rest scans in a single imaging session, one employing a full-dose (FD), and the other scan a half-dose (HfD) protocol. Datasets were evaluated for saturation based on visual assessments of input functions and sinograms. We compared FD and HfD MBF measurements using Bland-Altman plots, coefficients of variation (CV), and paired t tests. A correction factor permitting serial analyses using FD/HfD imaging protocols was obtained using only the datasets without saturation.

RESULTS

A dose reduction of 47% was reported for the HfD protocol (FD, 1247 ± 196 MBq; HfD, 662 ± 115 MBq). Saturation effects were observed in 4/20 (20%) FD scans, with none observed in the 20 HfD scans. Assessment of MBFs for FD and HfD protocols revealed bias in the MBF assessments of 0.09 ml/g/min (global MBF, FD = 1.03 ± 0.29 vs HfD = 0.94 ± 0.22 ml/g/min (p = 0.001)). Exclusion of patients with visually identified saturation effects (N = 4) reduced the bias to 0.05 ml/g/min (global MBF, FD = 0.97 ± 0.28 vs HfD = 0.92 ± 0.23 ml/g/min (p = 0.02)). From the datasets without saturation effect, it was possible to generate a bias-correction: Corrected MBF_HfD = 1.09*MBF_HfD-0.03 ml/g/min. MBF_FD and MBF_HfD did not differ following the bias correction (MBF_FD = 0.97 ± 0.28, MBF_{HfD,corrected} = 0.98 ± 0.25 ml/g/min, p = 0.77).

CONCLUSION

Saturation effects can be problematic in ⁸²Rb MBF studies using the recommended FD protocols for 3D PET/CT scanners. The use of HfD protocol eliminates the risks of saturation and should be used instead of clinical protocols to avoid erroneous results.

CT protocol optimisation in PET/CT: a systematic review

Purpose

Currently, no consistent guidelines for CT scans used within PET/CT examinations are available. This systematic review provides an up-to-date overview of studies to answer the following questions: What are the specific CT protocols used in PET/CT? What are the possible purposes of requiring a CT study within a PET/CT scan? Is the CT protocol obtained from a dosimetric optimisation study?

Materials and method

PubMed/MEDLINE, Cochrane Library, Embase and Scopus were systematically searched for relevant studies in accordance with the PRISMA statement. The literature search was conducted from January 2007 until June 2019. Data derived from studies were standardized in order to reduce possible biases, and they were divided into clinically homogeneous subgroups (adult, child or phantom). Subsequently, we divided the CT protocol intents into 3 types (anatomic localization only, attenuation correction only and diagnostic purpose). A narrative approach was used to summarise datasets and to investigate their heterogeneity (due to medical prescription methodology) and their combination in multiseries CT protocols. When weighted computed tomography dose index (CTDI_w) was available, we calculated the volumetric computed tomography dose index (CTDI_vol) using the pitch value to make the results uniform. Eventually, the correlation between protocol intents and CTDI_vol values was obtained using a Kruskal–Wallis one-way ANOVA statistical test.

Result

Starting from a total of 1440 retrieved records, twenty-four studies were eligible for inclusion in addition to two large multicentric works that we used to compare the results. We analyzed 87 CT protocols. There was a considerable range of variation in the acquisition parameters: tube current–time product revealed to have the most variable range, which was 10–300 mAs for adults and 10–80 mAs for paediatric patients. Seventy percent of datasets presented scans acquired with tube current modulation, 9% used fixed tube current and in 21% of them, this information was not available. Dependence between mean CTDI_vol values and protocol intent was statistically significant (p = 0.002). As expected, in diagnostic protocols, there was a statistically significant difference between CTDI_vol values of with and without contrast acquisitions (11.68 mGy vs 7.99 mGy, p = 0.009). In 13 out of 87 studies, the optimisation aim was not reported; in 2 papers, a clinical protocol was used; and in 11 works, a dose optimisation protocol was applied.

Conclusions

According to this review, the dose optimisation in PET/CT exams depends heavily on the correct implementation of the CT protocol. In addition to this, considering the latest technology advances (i.e. iterative algorithms development), we suggest a periodic quality control audit to stay updated on new clinical utility modalities and to achieve a shared standardisation of clinical protocols. In conclusion, this study pointed out the necessity to better identify the specific CT protocol use within PET/CT scans, taking into account the continuous development of new technologies.

Assessment of attenuation correction for myocardial PET imaging using combined PET/MRI

OBJECTIVE

To evaluate the frequency of artifacts in MR-based attenuation correction (AC) maps and their impact on the quantitative accuracy of PET-based flow and metabolism measurements in a cohort of consecutive heart failure patients undergoing combined PET/MR imaging.

METHODS

Myocardial viability studies were performed in 20 patients following a dual-tracer protocol involving the assessment of myocardial perfusion (13N-NH3: 813 ± 86 MBq) and metabolism (18F-FDG: 335 ± 38 MBq). All acquisitions were performed using a fully-integrated PET/MR system, with standard DIXON-attenuation correction (DIXON-AC) mapping for each PET scan. All AC maps were examined for spatial misalignment with the emission data, total lung volume, susceptibility artifacts, and tissue inversion (TI). Misalignment and susceptibility artifacts were corrected using rigid co-registration and retrospective filling of the susceptibility-induced gaps, respectively. The effects of the AC artifacts were evaluated by relative difference measures and perceived changes in clinical interpretations.

RESULTS

Average respiratory misalignment of (7 ± 4) mm of the PET-emission data and the AC maps was observed in 18 (90%) patients. Substantial changes in the lung volumes of the AC maps were observed in the test-retest analysis (ratio: 1.0 ± 0.2, range: 0.8-1.4). Susceptibility artifacts were observed in 10 (50%) patients, while six (30%) patients had TI artifacts. Average differences of 14 ± 10% were observed for PET images reconstructed with the artifactual AC maps. The combined artifact effects caused false-positive findings in three (15%) patients.

CONCLUSION

Standard DIXON-AC maps must be examined carefully for artifacts and misalignment effects prior to AC correction of cardiac PET/MRI studies in order to avoid misinterpretation of biased perfusion and metabolism readings from the PET data.

Single CT for attenuation correction of rest/stress cardiac SPECT perfusion imaging

Common practice is to use separate CT scans acquired during rest and stress for attenuation correction of SPECT myocardial perfusion imaging (MPI). We evaluated using a single CT scan to correct both rest and stress SPECT scans. Studies from 154 patients were reprocessed using one CT acquired at stress to correct both rest and stress scans (1CT) and compared to correction of each scan with its own CT (2CT). Two expert readers independently read the images and determined summed stress (SSS), rest (SRS), and difference (SDS) scores. The correlation in SRS between 2CT and 1CT was r ≥ 0.88. The concordance in SDS was ≥0.84 (kappa ≥ 0.62). The mean SDS difference between 2CT and 1CT for the averaged observer was not significantly different from zero (p > 0.31). 1CT images had a small but significant increase in SRS and an increase in SDS variability. However, the mean SDS difference was similar to the mean inter-observer SDS difference for the 2CT approach (-0.08 vs -0.23, p = 0.46) and had less uncertainty (1.02 vs 2.05, p < 0.001). Thus, the differences between 1CT and 2CT are unlikely to be clinically significant, and the 1CT approach is feasible for SPECT MPI.

Simultaneous dual-isotope solid-state detector SPECT for improved tracking of white blood cells in suspected endocarditis

Aims

High-energy resolution and sensitivity of novel cadmium-zinc-telluride (CZT) detector equipped SPECT systems facilitate simultaneous imaging of multiple isotopes and may enhance the detection of molecular/cellular signals. This may refine the detection of endocarditis. This study was designed to determine the feasibility and diagnostic accuracy of simultaneous imaging of inflammation with 111In-labeled white blood cells (WBCs) and myocardial perfusion with 99mTc-sestamibi, for localization of WBCs relative to the valve plane in suspected endocarditis.

Methods and results

A dedicated cardiac CZT camera (Discovery 530c, GE Healthcare) was employed. Anthropomorphic thorax phantom studies were followed by clinical studies in 34 patients with suspected infection of native valves (n = 12) or implants (n = 22). Simultaneous 111In-WBC/99mTc perfusion imaging was performed, and compared with standard 111In-WBC planar scintigraphy and SPECT-CT. Phantom studies ruled out significant radioisotope crosstalk. Downscatter on 99mTc images was not observed for 111In activity as high as 2.5*99mTc activity. In patients, image quality was superior for CZT imaging vs. conventional SPECT-CT and planar scintigraphy (P < 0.01). Cadmium-zinc-telluride dual isotope imaging improved reader confidence for detection of inflammatory foci. Diagnostic accuracy based on surgery or Duke Criteria during follow-up was highest for CZT imaging (P < 0.001).

Conclusion

Novel CZT SPECT technology improves the accuracy of molecular/cellular cardiac imaging. Simultaneous multi-isotope imaging with 111In and 99mTc is feasible and aids in the workup of suspected endocarditis.

Targeting Cardiovascular Implant Infection: Multimodality and Molecular Imaging

Owing to their success in clinical practice, the prevalence of cardiovascular implants is continuously increasing. Implant infection is a relevant complication but remains a diagnostic challenge because echocardiography as a first-line test may be limited. Accordingly, a multimodality approach is increasingly used for diagnostic workup and supported by recent guidelines. As reviewed here, computed tomography and nuclear imaging provide incremental diagnostic value and may be combined in a single hybrid imaging session using positron emission tomography/computed tomography or single photon emission computed tomography/computed tomography. Molecular or cellular imaging helps to overcome the limitations of morphological imaging in implants. Larger-scale clinical studies, earlier application in the time course of diagnosis, monitoring of therapy success, technical advances, and novel radiopharmaceuticals will all contribute to sustained growth of advanced infection-targeted imaging in cardiovascular medicine.

Enhancing Cardiac PET by Motion Correction Techniques

PURPOSE OF REVIEW

Cardiac positron emission tomography (PET) images often contain errors due to cardiac, respiratory, and patient motion during relatively long image acquisition. Advanced motion compensation techniques may improve PET spatial resolution, eliminate potential artifacts, and ultimately improve the research and clinical capabilities of PET.

RECENT FINDINGS

Combined cardiac and respiratory gating has only recently been implemented in clinical PET systems. Considering that the gated image bins contain much lower counts than the original PET data, they need to be summed after correcting for motion, forming motion-corrected, high-count image volume. Furthermore, automated image registration techniques can be used to correct for motion between CT attenuation scan and PET acquisition. While motion correction methods are not yet widely used in clinical practice, approaches including dual-gated non-rigid motion correction and the incorporation of motion correction information into the reconstruction process have the potential to markedly improve cardiac PET imaging.

PET/CT scanning with 3D acquisition is feasible for quantifying myocardial blood flow when diagnosing coronary artery disease

Background

The quantification of myocardial blood flow (MBF) and coronary flow reserve (CFR) are useful approaches for evaluating the functional severity of coronary artery disease (CAD). ¹⁵O-water positron emission tomography (PET) is considered the gold standard method for MBF quantification. However, MBF measurements in ¹⁵O-water PET with three-dimensional (3D) data acquisition, attenuation correction using computed tomography (CT), and time of flight have not been investigated in detail or validated. We conducted this study to evaluate the diagnostic potential of MBF measurements using PET/CT for a comparison of a control group and patients suspected of having CAD.

Results

Twenty-four patients with known or suspected CAD and eight age-matched healthy volunteers underwent rest and pharmacological stress perfusion studies with ¹⁵O-water PET/CT. The whole and three regional (left anterior descending (LAD), left circumflex (LCX), and right coronary artery (RCA) territory) MBF values were estimated. The CFR was computed as the ratio of the MBF during adenosine triphosphate-induced stress to the MBF at rest. The inter-observer variability was assessed by two independent observers. PET/CT using a ¹⁵O-water dose of 500 MBq and 3D data acquisition showed good image quality. A strong inter-observer correlation was detected in both the whole MBF analysis and the regional analysis with high intra-class correlation coefficients (r > 0.90, p < 0.001). Regional MBF at rest (LAD, 0.82 ± 0.15 ml/min/g; LCX, 0.83 ± 0.17 ml/min/g; RCA, 0.71 ± 0.20 ml/min/g; p = 0.74), MBF at stress (LAD, 3.77 ± 1.00 ml/min/g; LCX, 3.56 ± 1.01 ml/min/g; RCA, 3.27 ± 1.04 ml/min/g; p = 0.62), and CFR (LAD, 4.64 ± 0.90; LCX, 4.30 ± 0.64; RCA, 4.64 ± 0.96; p = 0.66) of the healthy volunteers showed no significant difference among the three regions. The global CFR of the patients was significantly lower than that of the volunteers (2.75 ± 0.81 vs. 4.54 ± 0.66, p = 0.0002). The regional analysis of the patients demonstrated that the CFR tended to be lower in the stenotic region compared to the non-stenotic region (2.43 ± 0.81 vs. 2.95 ± 0.92, p = 0.052).

Conclusions

¹⁵O-water PET/CT with 3D data acquisition can be reliably used for the quantification of functional MBF and CFR in CAD patients.

The Effects of Misalignment between PET and CT Scans on Brain PET Study Using CT-based Attenuation Correction: A Phantom Study

PURPOSE

The aim of this study was to evaluate the effects of inaccurate attenuation correction due to the misalignment between the computed tomography (CT)-based μ-map and the positron emission tomography (PET) data on a brain PET.

METHODS

CT and PET scans were performed on a 3-dimension (3D) brain phantom, in which the grey matter region was filled with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG), and the skull region was filled with/without the bone-equivalent solution. The shifted PET images relative to the CT image were generated by the software-based translation of PET data in the cephalad/caudal and right directions, with a magnitude of the shift up to 30 mm and a step size of 5 mm. The regions of interest (ROIs) were drawn on the areas of the temporal lobes, parietal lobes, thalami, and cerebellums in the no-shifted image (reference). For each ROI, the radioactivity concentrations in the shifted images were compared with those of the reference.

RESULTS

The errors in the radioactivity concentrations were increased with the increasing magnitude of the shift in all brain regions except for thalamus. For a 5 mm shift in the right direction, ± 10% errors were observed in the left/right temporal lobes. The accuracy of the radioactivity concentration in the temporal lobe was very sensitive to misalignment in the right directions.

CONCLUSION

The misalignment between CT-based μ-map and PET data had larger effects on the surface regions of the brain rather than on deep brain structures.

Clinical use of quantitative cardiac perfusion PET: rationale, modalities and possible indications. Position paper of the Cardiovascular Committee of the European Association of Nuclear Medicine (EANM)

Until recently, PET was regarded as a luxurious way of performing myocardial perfusion scintigraphy, with excellent image quality and diagnostic capabilities that hardly justified the additional cost and procedural effort. Quantitative perfusion PET was considered a major improvement over standard qualitative imaging, because it allows the measurement of parameters not otherwise available, but for many years its use was confined to academic and research settings. In recent years, however, several factors have contributed to the renewal of interest in quantitative perfusion PET, which has become a much more readily accessible technique due to progress in hardware and the availability of dedicated and user-friendly platforms and programs. In spite of this evolution and of the growing evidence that quantitative perfusion PET can play a role in the clinical setting, there are not yet clear indications for its clinical use. Therefore, the Cardiovascular Committee of the European Association of Nuclear Medicine, starting from the experience of its members, decided to examine the current literature on quantitative perfusion PET to (1) evaluate the rationale for its clinical use, (2) identify the main methodological requirements, (3) identify the remaining technical difficulties, (4) define the most reliable interpretation criteria, and finally (5) tentatively delineate currently acceptable and possibly appropriate clinical indications. The present position paper must be considered as a starting point aiming to promote a wider use of quantitative perfusion PET and to encourage the conception and execution of the studies needed to definitely establish its role in clinical practice.

Automatic registration of misaligned CT attenuation correction maps in Rb-82 PET/CT improves detection of angiographically significant coronary artery disease

BACKGROUND

We aimed to evaluate the utility of fully automated software registration intended to improve CT attenuation correction (CTAC) map misalignments during cardiac (82)Rb PET/CT myocardial perfusion imaging (MPI).

METHODS

171 consecutive patients (108 males, mean age 69 years), undergoing both rest-stress (82)Rb PET/CT MPI and invasive coronary angiography within 6 months (mean 14 days, range 0-170), were studied. List mode data were automatically processed in batch mode to generate transaxial attenuation corrected slices with four different CTAC alignment correction strategies: (i) no alignment correction (NONE); (ii) manual correction (MANUAL); (iii) automated 6-parameter rigid correction (AUTO); and (iv) targeted use of automated correction only where PET-CTAC alignment was initially judged as incorrect on either stress or rest scan (AUTO for misalignment only). Initial and final registration quality was graded (1-3) by an experienced radiologist (1: satisfactory alignment (<2 mm misalignment), 2: slight misalignment (2-5 mm in any direction), or 3: poor (>5 mm misalignment in any direction). Total perfusion deficit (TPD) and ischemic TPD (ITPD) were computed automatically, and their diagnostic accuracy to detect significant coronary artery disease with each realignment technique was assessed using receiver operating characteristic analysis.

RESULTS

The diagnostic accuracy of ITPD, expressed as area under curve, was .81 ± .03 with no alignment correction (NONE), .83 ± .03 with MANUAL correction, .85 ± .03 with AUTO correction (P < .05 vs. NONE and MANUAL), and .87 ± .03 with the targeted use of AUTO correction (P < .05 vs. NONE, MANUAL and AUTO). Both manual and software corrections increased the percentage of cases with satisfactory PET-CTAC map alignment (P < .05 for all) at rest (from 55% for NONE to 80% for MANUAL and 92% for AUTO) and at stress (from 51% for NONE to 78% for MANUAL and 84% for AUTO).

CONCLUSION

The diagnostic accuracy of (82)Rb PET/CT MPI with automated rigid alignment is improved compared to data with no CTAC scan alignment or with manual alignment. The optimal strategy for diagnostic performance is to apply automatic alignment only in cases which are visually identified as misaligned.

MR-based attenuation correction for cardiac FDG PET on a hybrid PET/MRI scanner: comparison with standard CT attenuation correction

PURPOSE

The aim of this study was to evaluate the feasibility of attenuation correction (AC) for cardiac (18)F-labelled fluorodeoxyglucose (FDG) positron emission tomography (PET) using MR-based attenuation maps.

METHODS

We included 23 patients with no known cardiac history undergoing whole-body FDG PET/CT imaging for oncological indications on a PET/CT scanner using time-of-flight (TOF) and subsequent whole-body PET/MR imaging on an investigational hybrid PET/MRI scanner. Data sets from PET/MRI (with and without TOF) were reconstructed using MR AC and semi-quantitative segmental (20-segment model) myocardial tracer uptake (per cent of maximum) and compared to PET/CT which was reconstructed using CT AC and served as standard of reference.

RESULTS

Excellent correlations were found for regional uptake values between PET/CT and PET/MRI with TOF (n = 460 segments in 23 patients; r = 0.913; p < 0.0001) with narrow Bland-Altman limits of agreement (-8.5 to +12.6 %). Correlation coefficients were slightly lower between PET/CT and PET/MRI without TOF (n = 460 segments in 23 patients; r = 0.851; p < 0.0001) with broader Bland-Altman limits of agreement (-12.5 to +15.0 %). PET/MRI with and without TOF showed minimal underestimation of tracer uptake (-2.08 and -1.29 %, respectively), compared to PET/CT.

CONCLUSION

Relative myocardial FDG uptake obtained from MR-based attenuation corrected FDG PET is highly comparable to standard CT-based attenuation corrected FDG PET, suggesting interchangeability of both AC techniques.

Influence of SPECT attenuation correction on the quantification of hibernating myocardium as derived from combined myocardial perfusion SPECT and ¹⁸F-FDG PET

BACKGROUND

To evaluate the influence of SPECT attenuation correction on the quantification of hibernating myocardium derived from perfusion SPECT and (18)F-FDG PET.

METHODS AND RESULTS

20 patients underwent rest (99m)Tc-tetrofosmin perfusion SPECT/CT and (18)F-FDG PET/CT. Perfusion images were reconstructed without attenuation correction (NC), with attenuation correction based on the CT from the SPECT/CT (AC_SPECT), and with attenuation correction based on the CT from the PET/CT (AC_PET). Another 56 patients had rest (99m)Tc-tetrofosmin perfusion SPECT and (18)F-FDG PET/CT. Perfusion images were reconstructed as NC and AC_PET. The amounts of hibernating myocardium and scar were quantified with QPS and corresponding AC and NC normative databases. In both cohorts, perfusion in the inferior wall was higher in the AC scans than without AC. Global and regional values for total perfusion deficit (TPD), hibernation and scar areas did not differ between NC, AC_SPECT, and AC_PET scans. In a retrospective evaluation with 7% cut-off of hibernating myocardium as a condition for revascularization, the therapeutic approach would have been altered in 5 of 56 patients, if the AC_PET approach had been used.

CONCLUSIONS

AC of SPECT perfusion scans with an attenuation map derived from PET/CT scans is feasible. If AC is unavailable, perfusion scans should be compared to NC normative databases for assessing TPD, hibernation, and mismatch. It should be taken into account that in approximately 10% of the patients, a therapeutic recommendation based on published thresholds for hibernating myocardium would be altered if NC scans were used as compared to AC scans.

Use of measured scatter data for the attenuation correction of single photon emission tomography without transmission scanning

PURPOSE

Attenuation correction is essential for reliable interpretation of emission tomography, however, the use of transmission measurements to generate attenuation maps is limited by availability of equipment and potential mismatches between the transmission and emission measurements. The authors present a first step toward a method of estimating an attenuation map from measured scatter data without a transmission scan.

METHODS

A scatter model has been developed that accurately predicts the distribution of photons which have been scattered once. The scatter model has been used as the basis of a maximum likelihood gradient ascent method to estimate an attenuation map from measured scatter data. In order to estimate both the attenuation map and activity distribution, iterations of the derived scatter based algorithm have been alternated with the maximum likelihood expectation maximization algorithm in a joint estimation process. For each iteration of the attenuation map estimation, the activity distribution is fixed at the values estimated during the previous activity iteration, and in each iteration of the activity distribution estimation the attenuation map is fixed at the values estimated during the previous attenuation iteration. The use of photopeak data to enhance the estimation of the attenuation map compared to the use of scatter data alone has also been considered. The algorithm derived has been used to reconstruct data simulated for an idealized two-dimensional situation and using a physical phantom.

RESULTS

The reconstruction of idealized data demonstrated good reconstruction of both the activity distribution and attenuation map. The inclusion of information recorded in the photopeak energy window in the attenuation map estimation step demonstrated an improvement in the accuracy of the reconstruction, enabling an accurate attenuation map to be recovered. Validation of the results with physical phantom data demonstrated that different regions of attenuation could be distinguished in a real situation and produces results that represent a promising first step toward the use of scatter data to estimate the activity distribution and attenuation map from single photon emission tomography (SPECT) data without a transmission scan.

CONCLUSIONS

The technique presented shows promise as a method of attenuation correction for SPECT data without the need for a separate transmission scan. Further work is required to further improve the method and to compensate for the assumptions used in the scatter model.

Molecular hybrid positron emission tomography/computed tomography imaging of cardiac angiotensin II type 1 receptors

OBJECTIVES

The goal of this study was to explore the feasibility of targeted imaging of the angiotensin II type 1 receptor (AT1R) in cardiac tissue, using clinical hybrid positron emission tomography/computed tomography (PET/CT).

BACKGROUND

AT1R is an attractive imaging target due to its key role in various cardiac pathologies, including post-infarct left ventricular remodeling.

METHODS

Using the novel AT1R ligand [(11)C]-KR31173, dynamic PET/CT was performed in young farm pigs under healthy conditions (n = 4) and 3 to 4 weeks after experimental myocardial infarction (n = 5). Ex vivo validation was carried out by immunohistochemistry and polymerase chain reaction. First-in-man application was performed in 4 healthy volunteers at baseline and under AT1R blocking.

RESULTS

In healthy pigs, myocardial KR31173 retention was detectable, regionally homogeneous, and specific for AT1R, as confirmed by blocking experiments. Metabolism in plasma was low (85 ± 2% of intact tracer after 60 min). After myocardial infarction, KR31173 retention, corrected for regional perfusion, revealed AT1R up-regulation in the infarct area relative to remote myocardium, whereas retention was elevated in both regions when compared with myocardium of healthy controls (8.7 ± 0.8% and 7.1 ± 0.3%/min vs. 5.8 ± 0.4%/min for infarct and remote, respectively, vs. healthy controls; p < 0.01 each). Postmortem analysis confirmed AT1R up-regulation in remote and infarct tissue. First-in-man application was safe, and showed detectable and specific myocardial KR31173 retention, albeit at a lower level than pigs (left ventricular average retention: 1.2 ± 0.1%/min vs. 4.4 ± 1.2%/min for humans vs. pigs; p = 0.04).

CONCLUSIONS

Noninvasive imaging of cardiac AT1R expression is feasible using clinical PET/CT technology. Results provide a rationale for broader clinical testing of AT1R-targeted molecular imaging.

Cardiac PET/CT misregistration causes significant changes in estimated myocardial blood flow

Misregistration of cardiac PET/CT data can lead to misinterpretation of regional myocardial perfusion. However, the effect of misregistration on the quantification of myocardial blood flow (MBF) has not been studied.

METHODS

Cardiac (82)Rb-PET/CT scans of 10 patients with normal regional myocardial perfusion were analyzed. Realignment was done for the baseline and stress PET/CT images as necessary, and MBF was obtained from dynamic data. Then, the stress images were misregistered by 5 mm along the x-axis (left) and z-axis (cranial) and again by 10 mm. A 10-mm misregistration in the opposite direction (-10 mm along the x-axis [right] and z-axis [caudal]) was also tested. Stress MBF was recalculated for 5-, 10-, and -10-mm misregistrations.

RESULTS

Stress MBF of the left ventricle decreased by 10% ± 6% (P = 0.005) after 5-mm misregistration and by 24% ± 15% (P = 0.001) after 10-mm misregistration. In descending order, the most important stress MBF changes occurred in the anterior (39% ± 9%), lateral (34% ± 9%), apical (20% ± 16%), inferior (12% ± 10%), and septal (10% ± 12%) walls after 10-mm misregistration. Lesser changes were observed after 5-mm misregistration, with the same wall distribution. In contrast, -10-mm misregistration increased global MBF by 9% ± 6% (P = 0.004). In descending order, the overestimation of estimated MBF after -10-mm misregistration occurred in the lateral (15% ± 8%), apical (15% ± 18%), anterior (9% ± 5%), and inferior (9% ± 11%) walls.

CONCLUSION

Misregistration of the stress PET/CT dataset leads to significant global and regional artifactual alterations in the estimated MBF. Quantitative error was observed throughout the myocardium and was not confined to those heart regions that extended into the lung on misregistered CT.

Quantitative cardiac positron emission tomography: the time is coming!

In the last 20 years, the use of positron emission tomography (PET) has grown dramatically because of its oncological applications, and PET facilities are now easily accessible. At the same time, various groups have explored the specific advantages of PET in heart disease and demonstrated the major diagnostic and prognostic role of quantitation in cardiac PET. Nowadays, different approaches for the measurement of myocardial blood flow (MBF) have been developed and implemented in user-friendly programs. There is large evidence that MBF at rest and under stress together with the calculation of coronary flow reserve are able to improve the detection and prognostication of coronary artery disease. Moreover, quantitative PET makes possible to assess the presence of microvascular dysfunction, which is involved in various cardiac diseases, including the early stages of coronary atherosclerosis, hypertrophic and dilated cardiomyopathy, and hypertensive heart disease. Therefore, it is probably time to consider the routine use of quantitative cardiac PET and to work for defining its place in the clinical scenario of modern cardiology.

⁸²Rb PET myocardial perfusion imaging is superior to ⁹⁹mTc-labelled agent SPECT in patients with known or suspected coronary artery disease

PURPOSE

We compared the quality, interpretive confidence and interreader agreement between SPECT and PET myocardial perfusion imaging (MPI) in the same group of patients.

METHODS

The study group comprised 27 patients (age 55 ± 8.5 years, 12 men) with known or suspected coronary artery disease (CAD) who had undergone gated rest/stress MPI with (99m)Tc-labelled agent SPECT (with and without attenuation correction, AC), and subsequent clinical confirmation with (82)Rb PET. Three experienced readers blinded to the clinical information interpreted all MPI studies.

RESULTS

Interreader agreement was significantly superior for PET studies than for SPECT studies. Following consensus interpretation, the quality of 22 % of the non-AC SPECT studies, 33 % of the AC SPECT studies and 63 % of the PET studies was assessed as excellent or good (p = 0.016). Interpretations were definitely normal or abnormal in 7 % of non-AC SPECT studies, 30 % of AC SPECT studies and 85 % of PET studies (p = 0.046). In 13 patients who had received either invasive coronary angiography or CT angiography with no significant CAD, the true-positive rate for significant CAD was higher for PET, and the true-negative rate was equal for PET and AC SPECT, and lower for non-AC SPECT.

CONCLUSION

(82)Rb PET MPI, used as a confirmatory test after SPECT, offers improved image quality, interpretive confidence and interreader agreement.

Impaired global myocardial flow dynamics despite normal left ventricular function and regional perfusion in chronic kidney disease: a quantitative analysis of clinical 82Rb PET/CT studies

Impaired global myocardial flow reserve (MFR) may be associated with increased risk for cardiac events and coronary artery disease progression. Chronic kidney disease (CKD) is also considered a risk factor for cardiovascular disease. We sought to investigate the effect of CKD on the myocardial microcirculation in patients referred for clinical (82)Rb PET/CT, who had normal left ventricular (LV) function and no flow-limiting coronary artery disease.

METHODS

Estimated glomerular filtration rate (eGFR) was available for 230 patients who had undergone rest and pharmacologic stress (82)Rb PET/CT for suspected coronary artery disease. CKD was defined as an eGFR less than 60 mL/min/1.73 m(2). After patients with hemodialysis, a renal transplant, abnormal regional perfusion (summed stress score > 4), or reduced LV function (LV ejection fraction < 45%) were excluded, 40 CKD patients remained. Those were compared with a control group without CKD, which was matched for age, sex, coronary risk factors, and systemic hemodynamics (n = 42). List-mode acquisition of PET enabled quantification of myocardial blood flow (MBF) and MFR using a previously validated retention model with correction for (82)Rb extraction. Rest MBF was normalized to rate-pressure product.

RESULTS

Mean eGFR in the CKD group was reduced (44 ± 14 vs. 99 ± 28 mL/min/1.73 m(2); P < 0.0001), and creatinine was significantly elevated, compared with controls (1.9 ± 1.1 vs. 0.8 ± 0.2 mg/dL; P < 0.0001). MFR was significantly reduced in CKD (2.2 ± 1.0 vs. 3.0 ± 1.2 for controls; P = 0.027). This reduction was mainly due to increased rest MBF (1.1 ± 0.4 in CKD vs. 0.8 ± 0.2 mL/min/g in controls; P = 0.007). Stress myocardial flow was comparable between both groups (2.3 ± 0.9 vs. 2.3 ± 0.8 mL/min/g; P = 0.08). Overall, MFR was significantly correlated with eGFR (r = 0.41; P = 0.0005). Stress MBF did not correlate with eGFR (r = 0.002; P = 0.45), but rest MBF showed an inverse correlation (r = -0.49; P < 0.0001). Rest MBF was also inversely correlated with hemoglobin (r = -0.28; P = 0.014), but only eGFR was an independent correlate at multivariate analysis.

CONCLUSION

MFR is impaired in patients with renal insufficiency with normal regional perfusion and LV function, mostly because of elevated rest flow. Absolute quantification of flow may be useful to identify microvascular dysfunction as a precursor of clinically overt coronary disease in this specific risk group.

Transient ischemic dilation ratio in 82Rb PET myocardial perfusion imaging: normal values and significance as a diagnostic and prognostic marker

In myocardial perfusion SPECT, transient ischemic dilation ratio (TID) is a well-established marker of severe ischemia and adverse outcome. However, its role in the setting of (82)Rb PET is less well defined.

METHODS

We analyzed 265 subjects who underwent clinical rest-dipyridamole (82)Rb PET/CT. Sixty-two subjects without a prior history of cardiac disease and with a normal myocardial perfusion study had either a low or a very low pretest likelihood of coronary artery disease or negative CT angiography. These subjects were used to establish a reference range of TID. In the remaining 203 patients with an intermediate or high pretest likelihood, subgroups with normal and abnormal TID were established and compared with respect to clinical variables, perfusion defect scores, left ventricular function, and absolute myocardial flow reserve. Follow-up was obtained for 969 ± 328 d to determine mortality by review of the social security death index.

RESULTS

In the reference group, TID ratio was 0.98 ± 0.06. Accordingly, a threshold for abnormal TID was set at greater than 1.13 (0.98 + 2.5 SDs). In the study group, 19 of 203 patients (9%) had an elevated TID ratio. Significant differences between subgroups with normal and abnormal TID ratio were observed for ejection fraction reserve (5.0 ± 6.4 vs. 1.8 ± 7.9; P < 0.05), difference between end-systolic volume (ESV) at rest and stress (ΔESV[stress-rest]; 1.8 ± 7.4 vs. 12.3 ± 13.0 mL; P < 0.0001), difference between end-diastolic volume (EDV) at rest and stress (ΔEDV[stress-rest]; 10.8 ± 11.5 vs. 23.8 ± 14.6 mL; P < 0.0001), summed rest score (1.8 ± 3.8 vs. 3.8 ± 7.6; P < 0.05), summed stress score (3.0 ± 5.4 vs. 7.5 ± 9.8; P < 0.002), summed difference score (1.3 ± 2.6 vs. 3.7 ± 5.3; P < 0.02), and global myocardial flow reserve (2.1 ± 0.8 vs. 1.7 ± 0.6; P < 0.02). Additionally, TID-positive patients had a significantly lower overall survival probability (P < 0.05). In a subgroup analysis of patients without regional perfusion abnormalities, TID-positive patients' overall survival probability was significantly smaller (P < 0.03), and TID was an independent predictor (exponentiation of the B coefficients [Exp(b)] = 6.22; P < 0.009) together with an ejection fraction below 45% (Exp[b] = 6.16; P < 0.002).

CONCLUSION

The present study suggests a reference range of TID for (82)Rb PET myocardial perfusion imaging that is in the range of previously established values for SPECT. Abnormal TID in (82)Rb PET is associated with more extensive left ventricular dysfunction, ischemic compromise, and reduced global flow reserve. Preliminary outcome analysis suggests that TID-positive subjects have a lower overall survival probability.

Effect of patient arm motion in whole-body PET/CT

Arm motion during whole-body PET/CT acquisition is not uncommon and can give rise to striking cold artifacts on PET images. We investigated the mechanisms that underlie these artifacts and proposed a potential solution.

METHODS

A phantom experiment based on 5 clinical cases of suspected arm motion was designed. The experiment involved a central 20-cm-diameter (68)Ge/(68)Ga cylinder simulating the neck and 2 peripheral 10-cm-diameter (18)F cylinders simulating arms. After motion-free CT and PET on a whole-body PET/CT system, the position of the arms was altered so as to introduce different amounts of misalignment. Twenty sequential PET scans were acquired in this position, alternating between 2-dimensional (2D) and 3-dimensional (3D) acquisition, as the (18)F decayed. Decay of (18)F in the arms, while the activity in the (68)Ge/(68)Ga cylinder remained approximately constant, allowed the relative impact of scatter and attenuation-correction errors to be determined.

RESULTS

Image artifacts were largely confined to the local region of motion in 2D but extended throughout the affected slices in 3D, where they manifested as a striking underestimation of radiotracer concentration that became more significant with increasing misalignment. For 3D, scatter-correction error depended on activity in the arms, but for typical activity concentrations scatter-correction error was more significant than attenuation-correction error. 3D image reconstruction without scatter correction substantially eliminated these artifacts in both phantom and patient images.

CONCLUSION

Reconstruction artifacts due to patient arm motion can be substantial and should be recognized because they can affect both qualitative and quantitative assessment of PET.

Positron emission tomography for the evaluation and treatment of cardiomyopathy

Congestive heart failure accounts for tremendous morbidity and mortality worldwide. There are numerous causes of cardiomyopathy, the most common of which is coronary artery disease. Positron emission tomography (PET) has an established and expanding role in the evaluation of patients with cardiomyopathy. The specific application of PET to hypertrophic cardiomyopathy, cardiac sarcoidosis, and diabetic cardiomyopathy has been studied extensively and promises to be a useful tool for managing these patients. Furthermore, evaluating the efficacy of standard treatments for congestive heart failure is important as health care costs continue to rise. Recently, there have been significant developments in the field of cardiovascular stem cell research. Familiarity with the mechanisms by which stem cells benefit patients with cardiovascular disease is the key to understanding these advances. Molecular imaging techniques including PET/CT imaging play an important role in monitoring stem cell therapy in both animals and humans. These noninvasive imaging techniques will be highlighted in this paper.

Comparison of the myocardial blood flow response to regadenoson and dipyridamole: a quantitative analysis in patients referred for clinical 82Rb myocardial perfusion PET

BACKGROUND

Regadenoson is a novel selective A2A adenosine receptor agonist, which is administered as an intravenous bolus at a fixed dose. It is currently not clear if the absolute flow increase in response to this fixed dose is a function of distribution volume in individual patients or if it is generally comparable to the previous standard agents dipyridamole or adenosine, which are dosed based on weight. We used quantitative analysis of clinical 82Rb PET/CT studies to obtain further insights.

METHODS

A total of 104 subjects with normal clinical rest/stress 82Rb perfusion PET/CT were included in a retrospective analysis. To rule out confounding factors, none had evidence of prior cardiac disease, ischaemia or infarction, cardiomyopathy, diabetes with insulin use, calcium score>400, renal disease or other significant systemic disease. A group of 52 patients stressed with regadenoson were compared with a group of 52 patients stressed with dipyridamole before regadenoson became available. The groups were matched for clinical characteristics, risk factors and baseline haemodynamics. Myocardial blood flow (MBF) and myocardial flow reserve (MFR) were quantified using a previously validated retention model, after resampling of dynamic studies from list-mode 82Rb datasets.

RESULTS

At rest, heart rate, blood pressure and MBF were comparable between the groups. Regadenoson resulted in a significantly higher heart rate (34±14 vs. 23±10 beats per minute increase from baseline; p<0.01) and rate-pressure product. Patients in the regadenoson group reported less severe symptoms and required less aminophylline. Stress MBF and MFR were not different between the groups (2.2±0.6 vs. 2.1±0.6 ml/min/g, p=0.39, and 2.9±0.8 vs. 2.8±0.7, p=0.31, respectively). In the regadenoson group, there was no correlation between stress flow or MFR and body weight or BMI.

CONCLUSION

Despite its administration at a fixed dose, regadenoson results in an absolute increase in MBF which is comparable to that following dipyridamole administration and is independent of patient distribution volume. This further supports its usefulness as a clinical stress agent.

Coronary calcium score scan-based attenuation correction in cardiovascular PET imaging

OBJECTIVE

Cardiac positron emission tomography (PET)/CT imaging is a noninvasive procedure allowing the assessment of coronary artery disease (CAD). CT-based attenuation correction of PET data is essential for accurate quantitative analysis in PET/CT imaging. Coronary artery calcium scoring CT (CaScCT) is used as a noninvasive tool for the diagnosis of atherosclerosis in patients with medium risk for CAD. In addition to the CaScCT examination, current cardiac rest/stress NH3 or ¹⁸F-fluorodeoxyglucose viability PET/CT protocols incorporate a correlated low-dose CT scan for attenuation correction purposes (ACCT). As a result, the patient receives a non-negligible radiation dose. The aim of this study is to evaluate the possibility of using CaScCT images for AC of myocardial rest/stress/viability PET data with the aim of reducing patient dose.

METHODS

Since in cardiac PET/CT protocols, the CaScCT examination is usually reconstructed using a small field-of-view, the CaScCT data were reconstructed again with extended field-of-view (ExCaScCT) and used for AC of the corresponding PET data. The feasibility study was performed using 10 patients including four NH3 perfusion and six ¹⁸F-fluorodeoxyglucose viability examinations acquired on the Biograph TP 64 PET/CT scanner. The assessment of PET images corrected using both ACCT and ExCaScCT images was carried out through qualitative assessment performed by an expert nuclear medicine specialist in addition to the regression analysis and the Bland-Altman plots, and 20-segment myocardial bull's eye view analysis.

RESULTS

Despite the good agreement between PET images corrected using ACCT and ExCaScCT images as expressed by the correlation coefficient and slope of the regression line in viability (0.949 ± 0.041 and 0.994 ± 0.124) and stress perfusion examinations (0.944 ± 0.008 and 0.968 ± 0.055), the rest perfusion examinations had weak correlation (0.454 ± 0.203 and 0.757 ± 0.193). This is attributed to the fact that the CaScCT scan is performed immediately after the stress/viability ACCT in our protocol that leads to a small misalignment between the CaScCT and stress/viability ACCT images, whereas there is a large misalignment between the CaScCT and rest ACCT images. The bull's eye view analysis showed that the difference between the uptake values was larger in the inferior wall because of diaphragm motion.

CONCLUSION

Our preliminary results seem to suggest that the calcium score study could be used for attenuation correction of cardiac PET images, thus allowing the elimination of ACCT in viability and stress perfusion studies and as such reduce patient dose.

Reference ranges for LVEF and LV volumes from electrocardiographically gated 82Rb cardiac PET/CT using commercially available software

Electrocardiographic gating is increasingly used for (82)Rb cardiac PET/CT, but reference ranges for global functional parameters are not well defined. We sought to establish reference values for left ventricular ejection fraction (LVEF), end systolic volume (ESV), and end diastolic volume (EDV) using 4 different commercial software packages. Additionally, we compared 2 different approaches for the definition of a healthy individual.

METHODS

Sixty-two subjects (mean age +/- SD, 49 +/- 9 y; 85% women; mean body mass index +/- SD, 34 +/- 10 kg/m(2)) who underwent (82)Rb-gated myocardial perfusion PET/CT were evaluated. All subjects had normal myocardial perfusion and no history of coronary artery disease (CAD) or cardiomyopathy. Subgroup 1 consisted of 34 individuals with low pretest probability of CAD (<10%), and subgroup 2 comprised 28 subjects who had no atherosclerosis on a coronary CT angiogram obtained concurrently during the PET/CT session. LVEF, ESV, and EDV were calculated at rest and during dipyridamole-induced stress, using CardIQ Physio (a dedicated PET software) and the 3 major SPECT software packages (Emory Cardiac Toolbox, Quantitative Gated SPECT, and 4DM-SPECT).

RESULTS

Mean LVEF was significantly different among all 4 software packages. LVEF was most comparable between CardIQ Physio (62% +/- 6% and 54% +/- 7% at stress and rest, respectively) and 4DM-SPECT (64% +/- 7% and 56% +/- 8%, respectively), whereas Emory Cardiac Toolbox yielded higher values (71% +/- 6% and 65% +/- 6%, respectively, P < 0.001) and Quantitated Gated SPECT lower values (56% +/- 8% and 50% +/- 8%, respectively, P < 0.001). Subgroup 1 (low likelihood) demonstrated higher LVEF values than did subgroup 2 (normal CT angiography findings), using all software packages (P < 0.05). However, mean ESV and EDV at stress and rest were comparable between both subgroups (p = NS). Intra- and interobserver agreement were excellent for all methods.

CONCLUSION

The reference range of LVEF and LV volumes from gated (82)Rb PET/CT varies significantly among available software programs and therefore cannot be used interchangeably. LVEF results were higher when healthy subjects were defined by a low pretest probability of CAD than by normal CT angiography results.

Attenuation-emission alignment in cardiac PET/CT based on consistency conditions

PURPOSE

In cardiac PET and PET/CT imaging, misaligned transmission and emission images are a common problem due to respiratory and cardiac motion. This misalignment leads to erroneous attenuation correction and can cause errors in perfusion mapping and quantification. This study develops and tests a method for automated alignment of attenuation and emission data.

METHODS

The CT-based attenuation map is iteratively transformed until the attenuation corrected emission data minimize an objective function based on the Radon consistency conditions. The alignment process is derived from previous work by Welch et al. ["Attenuation correction in PET using consistency information," IEEE Trans. Nucl. Sci. 45, 3134-3141 (1998)] for stand-alone PET imaging. The process was evaluated with the simulated data and measured patient data from multiple cardiac ammonia PET/CT exams. The alignment procedure was applied to simulations of five different noise levels with three different initial attenuation maps. For the measured patient data, the alignment procedure was applied to eight attenuation-emission combinations with initially acceptable alignment and eight combinations with unacceptable alignment. The initially acceptable alignment studies were forced out of alignment a known amount and quantitatively evaluated for alignment and perfusion accuracy. The initially unacceptable studies were compared to the proposed aligned images in a blinded side-by-side review.

RESULTS

The proposed automatic alignment procedure reduced errors in the simulated data and iteratively approaches global minimum solutions with the patient data. In simulations, the alignment procedure reduced the root mean square error to less than 5 mm and reduces the axial translation error to less than 1 mm. In patient studies, the procedure reduced the translation error by > 50% and resolved perfusion artifacts after a known misalignment for the eight initially acceptable patient combinations. The side-by-side review of the proposed aligned attenuation-emission maps and initially misaligned attenuation-emission maps revealed that reviewers preferred the proposed aligned maps in all cases, except one inconclusive case.

CONCLUSIONS

The proposed alignment procedure offers an automatic method to reduce attenuation correction artifacts in cardiac PET/CT and provides a viable supplement to subjective manual realignment tools.

Cardiac positron emission tomography

Positron emission tomography (PET) is a powerful, quantitative imaging modality that has been used for decades to noninvasively investigate cardiovascular biology and physiology. Due to limited availability, methodologic complexity, and high costs, it has long been seen as a research tool and as a reference method for validation of other diagnostic approaches. This perception, fortunately, has changed significantly within recent years. Increasing diversity of therapeutic options for coronary artery disease, and increasing specificity of novel therapies for certain biologic pathways, has resulted in a clinical need for more accurate and specific diagnostic techniques. At the same time, the number of PET centers continues to grow, stimulated by PET's success in oncology. Methodologic advances as well as improved radiotracer availability have further contributed to more widespread use. Evidence for diagnostic and prognostic usefulness of myocardial perfusion and viability assessment by PET is increasing. Some studies suggest overall cost-effectiveness of the technique despite higher costs of a single study, because unnecessary follow-up procedures can be avoided. The advent of hybrid PET-computed tomography (CT), which enables integration of PET-derived biologic information with multislice CT-derived morphologic information, and the key role of PET in the development and translation of novel molecular-targeted imaging compounds, have further contributed to more widespread acceptance. Today, PET promises to play a leading diagnostic role on the pathway toward a future of high-powered, comprehensive, personalized, cardiovascular medicine. This review summarizes the state-of-the-art in current imaging methodology and clinical application, and outlines novel developments and future directions.