Comparison of Myocardial Perfusion 82Rb PET Performed with CT- and Transmission CT–Based Attenuation Correction

Benefits of adding coronary calcium score scan to stress myocardial perfusion positron emission tomography imaging

There have been little and conflicting data regarding the relationship between coronary artery calcification score (CACS) and myocardial ischemia on positron emission tomography myocardial perfusion imaging (PET MPI). The aims of this study were to investigate the relationship between myocardial ischemia on PET MPI and CACS, the frequency and severity of CACS in patients with normal PET MPI, and to determine the optimal CACS cutoff point for abnormal PET. This retrospective study included 363 patients who underwent same-setting stress PET perfusion imaging and CACS scan because of clinically suspected coronary artery disease (CAD). Fifty-five (55%) of the 363 patients had abnormal PET perfusion. There was an association between sex, diabetes mellitus (DM), smoking, and CACS and PET perfusion abnormities with P = 0.003, 0.05, 0.005, and 0.001, respectively. However, there was no association between PET perfusion abnormalities with age, body mass index, hypertension, and hypercholesterolemia. There was association between CACS and age, sex, and DM with P = 0.000, 0.014, and 0.052, respectively, and stepwise increase in the frequency of myocardial ischemia and CACS groups. Receiver-operating characteristic analysis showed that a CACS ≥304 is the optimal cutoff for predicting perfusion abnormalities with sensitivity of 64% and specificity of 69%. In conclusion, the frequency of CAC in patients with normal PET MPI is 49%, it is highly recommended to combine CACS with PET MPI in patients without a history of CAD. PET MPI identifies myocardial ischemia and defines the need for coronary revascularization, but CAC reflects the anatomic burden of coronary atherosclerosis. Combining CACS to PET MPI allows better risk stratification and identifies high-risk patients with PET, and it may change future follow-up recommendations. CACS scan is readily available and easily acquired with modern PET-computed tomography (CT) and single-photon emission CT (SPECT)-CT with modest radiation exposure.

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.

Dual-Gated Motion-Frozen Cardiac PET with Flurpiridaz F 18

A novel PET radiotracer, Flurpiridaz F 18, has undergone phase II clinical trial evaluation as a high-resolution PET cardiac perfusion imaging agent. In a subgroup of patients imaged with this agent, we assessed the feasibility and benefit of simultaneous correction of respiratory and cardiac motion.

METHODS

In 16 patients, PET imaging was performed on a 4-ring scanner in dual cardiac and respiratory gating mode. Four sets of data were reconstructed with high-definition reconstruction (HD•PET): ungated and 8-bin electrocardiography-gated images using 5-min acquisition, optimal respiratory gating (ORG)-as developed for oncologic imaging-using a narrow range of breathing amplitude around end-expiration level with 35% of the counts in a 7-min acquisition, and 4-bin respiration-gated and 8-bin electrocardiography-gated images (32 bins in total) using the 7-min acquisition (dual-gating, using all data). Motion-frozen (MF) registration algorithms were applied to electrocardiography-gated and dual-gated data, creating cardiac-MF and dual-MF images. We computed wall thickness, wall/cavity contrast, and contrast-to-noise ratio for standard, ORG, cardiac-MF, and dual-MF images to assess image quality.

RESULTS

The wall/cavity contrast was similar for ungated (9.3 ± 2.9) and ORG (9.5 ± 3.2) images and improved for cardiac-MF (10.8 ± 3.6) and dual-MF images (14.8 ± 8.0) (P < 0.05). The contrast-to-noise ratio was 22.2 ± 9.1 with ungated, 24.7 ± 12.2 with ORG, 35.5 ± 12.8 with cardiac-MF, and 42.1 ± 13.2 with dual-MF images (all P < 0.05). The wall thickness was significantly decreased (P < 0.05) with dual-MF (11.6 ± 1.9 mm) compared with ungated (13.9 ± 2.8 mm), ORG (13.1 ± 2.9 mm), and cardiac-MF images (12.1 ± 2.7 mm).

CONCLUSION

Dual (respiratory/cardiac)-gated perfusion imaging with Flurpiridaz F 18 is feasible and improves image resolution, contrast, and contrast-to-noise ratio when MF registration methods are applied.

Advances in SPECT and PET Hardware

There have been significant recent advances in single photon emission computed tomography (SPECT) and positron emission tomography (PET) hardware. Novel collimator designs, such as multi-pinhole and locally focusing collimators arranged in geometries that are optimized for cardiac imaging have been implemented to reduce imaging time and radiation dose. These new collimators have been coupled with solid state photon detectors to further improve image quality and reduce scanner size. The new SPECT scanners demonstrate up to a 7-fold increase in photon sensitivity and up to 2 times improvement in image resolution. Although PET scanners are used primarily for oncological imaging, cardiac imaging can benefit from the improved PET sensitivity of 3D systems without inter-plane septa and implementation of the time-of-flight reconstruction. Additionally, resolution recovery techniques are now implemented by all major PET vendors. These new methods improve image contrast, image resolution, and reduce image noise. Simultaneous PET/magnetic resonance (MR) hybrid systems have been developed. Solid state detectors with avalanche photodiodes or digital silicon photomultipliers have also been utilized in PET. These new detectors allow improved image resolution, higher count rate, as well as a reduced sensitivity to electromagnetic MR fields.

CFR and FFR assessment with PET and CTA: strengths and limitations

Positron emission tomography (PET) myocardial perfusion imaging (MPI) has high diagnostic accuracy and prognostic value. PET-MPI can also be used to quantitatively evaluate regional myocardial blood flow (MBF). This technique also allows the calculation of the coronary flow reserve (CFR)/myocardial flow reserve (MFR), which is the ratio of MBF at peak hyperemia to resting MBF. Coronary computed tomography angiography (CTA) is a non-invasive method for accurate detection and exclusion of high-grade coronary stenoses, when compared to an invasive coronary angiography reference standard. However, CTA assessment of coronary stenoses tends toward overestimation, and CTA cannot determine physiologic significance of lesions. Recent advances in computational fluid dynamics and image-based modeling permit calculation of non-invasive fractional flow reserve derived from CT (FFRCT), without the need for additional imaging, modification of acquisition protocols, or administration of medications. In this review, we cover the CFR/MFR assessment by PET and FFR assessment by CT.

Myocardial perfusion imaging with PET

PET-myocardial perfusion imaging (MPI) allows accurate measurement of myocardial perfusion, absolute myocardial blood flow and function at stress and rest in a single study session performed in approximately 30 min. Various PET tracers are available for MPI, and rubidium-82 or nitrogen-13-ammonia is most commonly used. In addition, a new fluorine-18-based PET-MPI tracer is currently being evaluated. Relative quantification of PET perfusion images shows very high diagnostic accuracy for detection of obstructive coronary artery disease. Dynamic myocardial blood flow analysis has demonstrated additional prognostic value beyond relative perfusion imaging. Patient radiation dose can be reduced and image quality can be improved with latest advances in PET/CT equipment. Simultaneous assessment of both anatomy and perfusion by hybrid PET/CT can result in improved diagnostic accuracy. Compared with SPECT-MPI, PET-MPI provides higher diagnostic accuracy, using lower radiation doses during a shorter examination time period for the detection of coronary artery disease.

Evaluation of the endothelial function in hypertensive patients with 13N-ammonia PET

BACKGROUND

Essential hypertension is one of the main risk factors for the development of coronary artery disease (CAD). Hypertension causes endothelial dysfunction which is considered an early sign for the development of CAD. Positron emission tomography is a non-invasive imaging technique that measures myocardial blood flow (MBF), allowing us to identify patients with endothelial dysfunction.

METHODS AND RESULTS

19 patients without comorbidities recently diagnosed hypertensive, as well as 21 healthy volunteers were studied. A three-phase (rest, cold pressor test, and adenosine-induced hyperemia) (13)N-ammonia PET was performed, and MBF was measured. Endothelial-Dependent Vasodilation Index, ΔMBF, and coronary flow reserve (CFR) were calculated for each patient. Hypertensive patients had a significantly higher systolic and diastolic blood pressures compared with the control group (134.6 ± 11.7/86.4 ± 10.6 mm Hg and 106.0 ± 11.8/71.4 ± 6.6 mm Hg, respectively, P < .001). The ENDEVI (1.28 ± 0.26 vs 1.79 ± 0.30, P < .001), the ΔMBF (0.81 ± 0.50 vs 0.25 ± 0.21, P < .001) and the CFR (2.18 ± 0.88 vs 3.17 ± 0.68, P = .001) were significantly lower in the hypertensive patients compared to the control group, 84% of the former group had endothelial dysfunction i.e., ENDEVI < 1.5 and 58% had vasomotor abnormalities, i.e., CFR < 2.5.

CONCLUSIONS

In this study, we showed that recently diagnosed hypertensive patients have coronary endothelial dysfunction and vasomotor disturbances which are early signs for the development of CAD.

Quantification of regional myocardial blood flow estimation with three-dimensional dynamic rubidium-82 PET and modified spillover correction model

PURPOSE

Myocardial blood flow (MBF) estimation with (82)Rubidium ((82)Rb) positron emission tomography (PET) is technically difficult because of the high spillover between regions of interest, especially due to the long positron range. We sought to develop a new algorithm to reduce the spillover in image-derived blood activity curves, using non-uniform weighted least-squares fitting.

METHODS

Fourteen volunteers underwent imaging with both 3-dimensional (3D) (82)Rb and (15)O-water PET at rest and during pharmacological stress. Whole left ventricular (LV) (82)Rb MBF was estimated using a one-compartment model, including a myocardium-to-blood spillover correction to estimate the corresponding blood input function Ca(t)(whole). Regional K1 values were calculated using this uniform global input function, which simplifies equations and enables robust estimation of MBF. To assess the robustness of the modified algorithm, inter-operator repeatability of 3D (82)Rb MBF was compared with a previously established method.

RESULTS

Whole LV correlation of (82)Rb MBF with (15)O-water MBF was better (P < .01) with the modified spillover correction method (r = 0.92 vs r = 0.60). The modified method also yielded significantly improved inter-operator repeatability of regional MBF quantification (r = 0.89) versus the established method (r = 0.82) (P < .01).

CONCLUSION

A uniform global input function can suppress LV spillover into the image-derived blood input function, resulting in improved precision for MBF quantification with 3D (82)Rb PET.

Coronary arterial 18F-FDG uptake by fusion of PET and coronary CT angiography at sites of percutaneous stenting for acute myocardial infarction and stable coronary artery disease

Whether (18)F-FDG PET can detect inflammation in the coronary arteries remains controversial. We examined (18)F-FDG uptake at the culprit sites of acute myocardial infarction (AMI) after percutaneous coronary stenting (PCS) by coregistering PET and coronary CT angiography (CTA).

METHODS

Twenty nondiabetic patients with AMI (median age, 62 y; 16 men and 4 women) and 7 nondiabetic patients with stable coronary artery disease (CAD; median age, 67 y; 4 men and 3 women) underwent (18)F-FDG PET and coronary CTA 1-6 d after PCS of culprit stenoses. After a low-carbohydrate dietary preparation and more than 12 h of fasting, 480 MBq of (18)F-FDG were injected, and PET images were acquired 3 h later. Helical CTA was performed on a dual-source scanner. Stent position on attenuation-correction noncontrast CT and CTA was used to fuse PET and CTA. Two experienced readers masked to patient data independently quantified maximum target-to-background ratio (maxTBR) at each PCS site. A maxTBR greater than 2.0 was the criterion for significant uptake.

RESULTS

Compared with stable CAD patients, more AMI patients exhibited a PCS site maxTBR greater than 2.0 (12/20 vs. 1/7, P = 0.04). More AMI patients were active smokers (9/20 vs. 0/7 in stable CAD, P = 0.03). After adjusting for baseline demographic differences, stent-myocardium distance, and myocardial (18)F-FDG uptake, presentation of AMI was positively associated with a PCS site maxTBR greater than 2.0 (odds ratio, 31.6; P = 0.044). Prevalence of excess myocardial (18)F-FDG uptake was similar in both populations (8/20 AMI vs. 3/7 stable CAD, P = 0.89).

CONCLUSION

Systematic fusion of (18)F-FDG PET and coronary CTA demonstrated increased culprit site (18)F-FDG uptake more commonly in patients with AMI than in patients with stable CAD. However, this approach failed to detect increased signal at the culprit site in nearly half of AMI patients, highlighting the challenging nature of in vivo coronary artery plaque metabolic imaging. Nonetheless, our findings suggest that imaging of coronary artery inflammation is feasible, and further work evaluating (18)F-FDG uptake in high-risk coronary plaques prior to rupture would be of great interest.

Automated quantitative Rb-82 3D PET/CT myocardial perfusion imaging: normal limits and correlation with invasive coronary angiography

BACKGROUND

We aimed to characterize normal limits and to determine the diagnostic accuracy for an automated quantification of 3D 82-Rubidium (Rb-82) PET/CT myocardial perfusion imaging (MPI).

METHODS

We studied 125 consecutive patients undergoing Rb-82 PET/CT MPI, including patients with suspected coronary artery disease (CAD) and invasive coronary angiography, and 42 patients with a low likelihood (LLk) of CAD. Normal limits for perfusion and function were derived from LLk patients. QPET software was used to quantify perfusion abnormality at rest and stress expressed as total perfusion deficit (TPD).

RESULTS

Relative perfusion databases did not differ in any of the 17 segments between males and females. The areas under the receiver operating characteristic curve for detection of CAD were 0.86 for identification of ≥50% and ≥70% stenosis. The sensitivity/specificity was 86%/86% for detecting ≥50% stenosis and 93%/77% for ≥70% stenosis, respectively. In regard to normal limits, mean rest and stress left ventricular ejection fraction (LVEF) were 67% ± 10% and 75% ± 9%, respectively. Mean transient ischemic dilation ratio was 1.06 ± 0.14 and mean increase in LVEF with stress was 7.4% ± 6.1% (95th percentile of 0%).

CONCLUSION

Normal limits have been established for 3D Rb-82 PET/CT analysis with QPET software. Fully automated quantification of myocardial perfusion PET data shows high diagnostic accuracy for detecting obstructive CAD.

Comparison of clinical tools for measurements of regional stress and rest myocardial blood flow assessed with 13N-ammonia PET/CT

Several models for the quantitative analysis of myocardial blood flow (MBF) at stress and rest and myocardial flow reserve (MFR) with (13)N-ammonia myocardial perfusion PET have been implemented for clinical use. We aimed to compare quantitative results obtained from 3 software tools (QPET, syngo MBF, and PMOD), which perform PET MBF quantification with either a 2-compartment model (QPET and syngo MBF) or a 1-compartment model (PMOD).

METHODS

We considered 33 adenosine stress and rest (13)N-ammonia studies (22 men and 11 women). Average age was 54.5 ± 15 y, and average body mass index was 26 ± 4.2. Eighteen patients had a very low likelihood of disease, with no chest pain, normal relative perfusion results, and normal function. All data were obtained on a PET/CT scanner in list mode with CT attenuation maps. Sixteen dynamic frames were reconstructed (twelve 10-s, two 30-s, one 1-min, and one 6-min frames). Global and regional stress and rest MBF and MFR values were obtained with each tool. Left ventricular contours and input function region were obtained automatically in system QPET and syngo MBF and manually in PMOD.

RESULTS

The flow values and MFR values were highly correlated among the 3 packages (R(2) ranging from 0.88 to 0.92 for global values and from 0.78 to 0.94 for regional values. Mean reference MFR values were similar for QPET, syngo MBF, and PMOD (3.39 ± 1.22, 3.41 ± 0.76, and 3.66 ± 1.19, respectively) by 1-way ANOVA (P = 0.74). The lowest MFR in very low likelihood patients in any given vascular territory was 2.25 for QPET, 2.13 for syngo MBF, and 2.23 for PMOD.

CONCLUSION

Different implementations of 1- and 2-compartment models demonstrate an excellent correlation in MFR for each vascular territory, with similar mean MFR values.

Epicardial fat volume and concurrent presence of both myocardial ischemia and obstructive coronary artery disease

OBJECTIVE

Epicardial fat volume (EFV) is linked to cardiovascular event risk. The aim of this study was to evaluate whether EFV is independently related to concurrent presence of both myocardial ischemia and obstructive coronary stenosis.

METHODS

We studied 92 consecutive patients without known coronary artery disease (CAD) who underwent Rb-82 PET, coronary calcium scoring (CCS) and invasive coronary angiography (ICA) within 6 months. EFV was computed from non-contrast CT by validated software and indexed to body surface-area (EFVi, cm(3)/m(2)). Ischemia was defined by ≥ 5% difference of total perfusion deficit (quantified by validated software) between stress and rest. Obstructive stenosis was defined ≥ 50% luminal diameter stenosis.

RESULTS

Fifty three patients had both ischemia and stenosis. Compared to those without, patients with both having ischemia and stenosis had significantly higher CCS (1125 ± 1230 vs. 626 ± 690, p = 0.02) and EFVi (64.6 ± 20.6 vs. 49.7 ± 14.2 cm(3)/m(2), p=0.0002). On multivariable analysis after adjusting age, gender, cardiovascular risk factors, chest pain, and CCS (≥ 400), only elevated EFVi (>68.1cm(3)/m(2)) significantly predicted concurrent presence of both ischemia and stenosis (odds ratio 6.18, 95% confidence interval 1.73-22.01, p = 0.005). Area under the receiver-operator-characteristic analysis demonstrated a trend towards improved incremental prediction of concurrent myocardial ischemia and obstructive stenosis over age, gender, chest pain, and high CCS (0.73 vs. 0.65, p = 0.09).

CONCLUSIONS

Our study suggests that elevated EFVi measured using non-contrast CT may be related to concurrent presence of both ischemia and stenosis.

Automatic 3D registration of dynamic stress and rest (82)Rb and flurpiridaz F 18 myocardial perfusion PET data for patient motion detection and correction

PURPOSE

The authors aimed to develop an image-based registration scheme to detect and correct patient motion in stress and rest cardiac positron emission tomography (PET)/CT images. The patient motion correction was of primary interest and the effects of patient motion with the use of flurpiridaz F 18 and (82)Rb were demonstrated.

METHODS

The authors evaluated stress/rest PET myocardial perfusion imaging datasets in 30 patients (60 datasets in total, 21 male and 9 female) using a new perfusion agent (flurpiridaz F 18) (n = 16) and (82)Rb (n = 14), acquired on a Siemens Biograph-64 scanner in list mode. Stress and rest images were reconstructed into 4 ((82)Rb) or 10 (flurpiridaz F 18) dynamic frames (60 s each) using standard reconstruction (2D attenuation weighted ordered subsets expectation maximization). Patient motion correction was achieved by an image-based registration scheme optimizing a cost function using modified normalized cross-correlation that combined global and local features. For comparison, visual scoring of motion was performed on the scale of 0 to 2 (no motion, moderate motion, and large motion) by two experienced observers.

RESULTS

The proposed registration technique had a 93% success rate in removing left ventricular motion, as visually assessed. The maximum detected motion extent for stress and rest were 5.2 mm and 4.9 mm for flurpiridaz F 18 perfusion and 3.0 mm and 4.3 mm for (82)Rb perfusion studies, respectively. Motion extent (maximum frame-to-frame displacement) obtained for stress and rest were (2.2 ± 1.1, 1.4 ± 0.7, 1.9 ± 1.3) mm and (2.0 ± 1.1, 1.2 ±0 .9, 1.9 ± 0.9) mm for flurpiridaz F 18 perfusion studies and (1.9 ± 0.7, 0.7 ± 0.6, 1.3 ± 0.6) mm and (2.0 ± 0.9, 0.6 ± 0.4, 1.2 ± 1.2) mm for (82)Rb perfusion studies, respectively. A visually detectable patient motion threshold was established to be ≥2.2 mm, corresponding to visual user scores of 1 and 2. After motion correction, the average increases in contrast-to-noise ratio (CNR) from all frames for larger than the motion threshold were 16.2% in stress flurpiridaz F 18 and 12.2% in rest flurpiridaz F 18 studies. The average increases in CNR were 4.6% in stress (82)Rb studies and 4.3% in rest (82)Rb studies.

CONCLUSIONS

Fully automatic motion correction of dynamic PET frames can be performed accurately, potentially allowing improved image quantification of cardiac PET data.

Cardiac PET/CT for the evaluation of known or suspected coronary artery disease

Positron emission tomography (PET) is increasingly being applied in the evaluation of myocardial perfusion. Cardiac PET can be performed with an increasing variety of cyclotron- and generator-produced radiotracers. Compared with single photon emission computed tomography, PET offers lower radiation exposure, fewer artifacts, improved spatial resolution, and, most important, improved diagnostic performance. With its capacity to quantify rest-peak stress left ventricular systolic function as well as coronary flow reserve, PET is superior to other methods for the detection of multivessel coronary artery disease and, potentially, for risk stratification. Coronary artery calcium scoring may be included for further risk stratification in patients with normal perfusion imaging findings. Furthermore, PET allows quantification of absolute myocardial perfusion, which also carries substantial prognostic value. Hybrid PET-computed tomography scanners allow functional evaluation of myocardial perfusion combined with anatomic characterization of the epicardial coronary arteries, thereby offering great potential for both diagnosis and management. Additional studies to further validate the prognostic value and cost effectiveness of PET are warranted.

Diagnostic value of SPECT, PET and PET/CT in the diagnosis of coronary artery disease: A systematic review

PURPOSE

The purpose of the study was to investigate the diagnostic value of SPECT, PET and PET/CT in the diagnosis of coronary artery disease, based on a systematic review.

MATERIAL AND METHODS

A search of PubMed/Medline and Sciencedirect databases in the English-language literature published over the last 24 years was performed. Only studies with at least 10 patients comparing SPECT, PET or combined PET/CT with invasive coronary angiography in the diagnosis of coronary artery disease (50% stenosis) were included for analysis. Sensitivities and specificities estimates pooled across studies were analysed using a Chi-square test.

RESULTS

Twenty-five studies met the selection criteria and were included for the analysis. Ten studies were performed with SPECT alone; while another six studies were performed with PET alone. Five studies were carried out with both PET and SPECT modalities, and the remaining four studies were investigated with integrated PET-CT. The mean value of sensitivity, specificity and accuracy of these imaging modalities for the diagnosis of coronary artery disease was 82% (95%CI: 76 to 88), 76% (95%CI: 70 to 82) and 83% (95%CI: 77 to 89) for SPECT; 91% (95%CI: 85 to 97), 89% (95%CI: 83 to 95) and 89% (95%CI: 83 to 95) for PET; and 85% (95%CI: 79 to 90), 83% (95%CI: 77 to 89) and 88% (95%CI: 82 to 94) for PET/CT, respectively. The diagnostic accuracy of these imaging modalities was dependent on the radiotracers used in these studies, with ammonia resulting in the highest diagnostic value.

CONCLUSION

Our review shows that PET has high diagnostic value for diagnosing coronary artery disease, and this indicates that it is a valuable technique for both detection and prediction of coronary artery disease.

Agreement of visual estimation of coronary artery calcium from low-dose CT attenuation correction scans in hybrid PET/CT and SPECT/CT with standard Agatston score

OBJECTIVES

We sought to evaluate the accuracy and reproducibility of visual estimation of coronary artery calcium (CAC) from computed tomography attenuation correction (CTAC) scans performed for hybrid positron emission tomography (PET)/computed tomography (CT) and single-photon emission computed tomography (SPECT)/CT myocardial perfusion imaging (MPI).

BACKGROUND

At the time of MPI, hybrid systems obtain a low-dose, non-electrocardiogram (ECG)-gated CT scan that is used to perform attenuation correction. Utility of this CTAC scan in estimating actual CAC as measured by Agatston score (AS) on standard ECG-gated scans has not been previously studied.

METHODS

A total of 492 patients, from 3 centers, receiving both MPI with CTAC and a standard CAC scan were studied. At each site, experienced readers blinded to AS reviewed CTAC images, visually estimating CAC on a 6-level scale: classifying patients as estimated AS of 0, 1 to 9, 10 to 99, 100 to 300, 400 to 999, or ≥1,000. Agreement between visually estimated coronary artery calcium (VECAC) on CTAC and AS, measured standardly and converted to the same scale, was evaluated, as was inter-reader agreement.

RESULTS

Although CTAC images are low dose and nongated, a high degree of association was observed between VECAC and AS, with 63% of VECACs in the same category as the AS category and 93% within 1 category. Weighted kappa was 0.89 (95% confidence interval: 0.88 to 0.91, p < 0.0001). High weighted kappa statistics were observed for each site, scanner type, and sex. Readers reported identical scores in 65% of cases and scores within 1 category in 93%.

CONCLUSIONS

CAC can be visually assessed from low-dose CTAC scans with high agreement with AS. CTAC scans should be routinely assessed for VECAC.

Quantitative Study of Rigid-Body and Respiratory Motion of Patients Undergoing Stress and Rest Cardiac SPECT Imaging

We report patient motion in 110 Tl-201 cardiac perfusion SPECT studies in 66 patients. The imaging consisted of emission followed by sequential transmission imaging during which motion tracking with a visual tracking system (VTS) was performed. We investigated the extent, time, and frequency of respiratory and rigid-body motion in these patients. We also determined whether the motion occurred gradually or in sudden jumps, whether it was sustained, and if it occurred along one or more axes predominantly. We then studied the differences in respiratory and body motion (BM), if any, between stress versus rest imaging groups, male versus female subjects, and exercise versus pharmacological stress groups. We found that 23% of the studies had sustained motion (> 4min.) of between 3-6 mm, and 5% had sustained motion larger than 6 mm during emission imaging. In terms of respiratory motion, 13% showed a downward trend of the respiratory baseline of more than 6 mm during emission imaging. Also, in 9% of the studies, the average position of patients was displaced by more than 3 mm between emission and transmission imaging phases. Both of these motions may lead to misalignment of the attenuation map. In hypothesis testing of grouped studies, it was determined that stress and rest imaging did not show any significant differences in body motion but did in respiratory motion associated with a change in respiration following stress. Exercise-stress studies showed a larger extent of respiratory motion than the pharmacologically induced stress studies. Significant differences in body and respiratory motion of male and female groups were also observed. A visual assessment of the reconstructed slices in the studies with measured motion was made to investigate the impact of the motion. Illustrative example studies are included.

Low-dose quantitative myocardial blood flow imaging using 15O-water and PET without attenuation correction

Misalignment between PET and low-dose CT (LD-CT) can cause severe artifacts in cardiac PET/CT because of attenuation-correction errors, even when using slow or cine LD-CT. Myocardial blood flow (MBF), as measured by (15)O-water, can be determined from the rate of (15)O-water washout from myocardial tissue, which is independent of tissue attenuation. The purpose of the present study was to assess the accuracy of these MBF measurements in the absence of attenuation correction.

METHODS

Twenty-five patients referred for evaluation of myocardial perfusion underwent 6-min rest and adenosine stress PET scans after the administration of 370 MBq of (15)O-water; both scans were followed by slow LD-CT. Data were acquired on a PET/CT scanner and reconstructed by a 3-dimensional row-action maximum likelihood algorithm both with (CTAC) and without (NAC) attenuation correction. An ascending aorta volume of interest was used as input function. MBF and coronary flow reserve (CFR) were calculated for 17 myocardial segments using nonlinear regression of the standard single-tissue-compartment model with corrections for left and right ventricular spillover and perfusable tissue fraction.

RESULTS

High correlation (r(2) = 0.99 and 0.97, with slopes of 0.96 and 0.91 for rest and stress, respectively) and excellent agreement (intraclass correlation coefficient [ICC], 1.00 and 0.98) between NAC- and CTAC-based MBF values were found. Absolute rest and stress MBF values were 3% and 8%, respectively, lower for NAC scans. The correlation coefficient between all NAC and CTAC CFR values was 0.95 (ICC, 0.95; slope, 0.92) and 0.97 (ICC, 0.99; slope, 1.01) when only CFR values below 2 were considered. Deviations between CTAC and NAC values were smallest for basal segments and increased toward the apex.

CONCLUSION

MBF and CFR can be measured accurately using (15)O-water and PET without correcting for attenuation, reducing the effective dose to the patient to 0.8 mSv for a complete rest-stress protocol. This dose is an order of magnitude lower than typical values for (82)Rb, (99m)Tc-methoxyisobutylisonitrile, or CT perfusion scans.

Integration of coronary anatomy and myocardial perfusion imaging

Advances in cardiovascular imaging have resulted in the development of multiple noninvasive techniques to evaluate myocardial perfusion and coronary anatomy, each of which has unique strengths and limitations. For example, CT angiography can directly visualize the presence of atherosclerosis, but the hemodynamic effect of many lesions identified by this technique is unknown. Alternatively, myocardial perfusion imaging enables a physiological assessment, but it may underestimate the extent of atherosclerosis in patients with multivessel disease. Dual-modality simultaneous imaging or multimodal sequential imaging techniques facilitate integration of information on both myocardial perfusion and coronary anatomy, and thus have the potential to improve diagnostic and prognostic evaluation, which could translate into improved care of patients. This Review discusses the strengths and limitations of the currently available individual noninvasive techniques for imaging coronary anatomy and myocardial perfusion. Approaches to integration of these imaging modalities are described, followed by an exploration of the clinical utility and future directions of hybrid imaging.