Introduction

Cardiovascular diseases are the main cause of about one-third of deaths in men and women across the world. It is estimated that more than 19 million patients worldwide suffer from cardiovascular disease annually. One common cause of cardiac events is the buildup of fats, cholesterol, and any other compounds in the arterial wall leading to the formation of plaques, known as vulnerable atherosclerotic plaques (VAPs) (1,2).

VAP formation in coronary arteries is one of the common causes of cardiac disease, which can lead to cardiac events and death. These plaques develop gradually and mild plaques usually have no complications. The complications of atherosclerosis start when the artery becomes occluded due to plaque formation, leading to inadequate blood supply to the tissues. A VAP may restrict coronary blood flow leading, leading myocardial infarction and death. A VAP consists of a fine fibrous cap and a vast lipid core, resulting in the accumulation of activated macrophages. The activated macrophages may rupture the VAP, resulting in sudden cardiac events and death. Given the importance of this issue, the detection of early changes in the coronary walls is of enormous importance (3,4). VAP, which is prone to rupture, usually does not cause marked stenosis; therefore, it is not diagnosed by myocardial perfusion imaging (MPI), the procedure commonly used for ischemia evaluation. Many preclinical and clinical studies have been conducted to understand VAP recently (5).

The standard methods for diagnosis of VAP include physical examination, laboratory tests, ankle/brachial index, electrocardiogram, and ultrasound examination, as well as invasive methods including intravascular coronary ultrasound, angiography, and angioscopy (6,7). All these procedures have differences in terms of sensitivity, specificity, availability, and reproducibility. Nowadays, molecular and cellular imaging has made it possible to evaluate early changes in the arterial walls toward VAP formation, opening new windows in detecting VAP in subclinical and clinical stages.

As mentioned earlier, VAPs result in the accumulation of activated macrophages, therefore imaging macrophages can be used as a potential target for the characterization and diagnosis of VAP. It has been indicated that somatostatin receptor-2 (SSTR-2) is overexpressed by activated macrophages (8). Several radiotracers have an affinity for somatostatin receptors, including SSTR-2, and are used for imaging and management of patients with neuroendocrine tumors (NETs) in multiple centers around the world, such as ⁶⁸Ga-DOTATATE positron emission tomography (PET) and Tc-99m-octreotide single-photon emission computed tomography (SPECT) (9,10,11,12).

The potential role of several radiotracers such as ⁶⁸Ga-DOTATATE and ¹⁸F sodium florine PET in the evaluation of inflammatory processes and detection of VAP has been investigated in several studies (4,13,14,15,16). Therefore, this study was conducted to detect atherosclerotic plaques by somatostatin receptor scintigraphy (SRS) using Tc-99m-octreotide SPECT and to compare SRS with coronary angiography for detection of atherosclerotic plaques and its association with MPI using Tc-99m-MIBI SPECT finding.

Materials and Methods

Study Population

In this study, VAP was evaluated using Tc-99m-octreotide SPECT in the following patients at Bushehr Nuclear Medicine Department:

1. Patients who were referred for MPI were asked to perform an additional Tc-99m-octreotide SPECT for evaluation of VAP within 3 to 7 days following MPI.

2. Patients who were referred for evaluation of NETs by Tc-99m-octreotide and had cardiac risk factors underwent cardiac Tc-99m-octreotide SPECT in addition to the whole-body scan for evaluation of VAP (Figure 1).

A detailed history including age, hypertension, angina, diabetes, hyperlipidemia, smoking, family history of cardiac diseases, obesity and history of cardiac attack was taken from all patients for evaluation of cardiac risk factors. The inclusion criteria were age >18 and the presence of cardiac risk factors. In addition, because of the possibility of false-positive results, patients with a history of interventional coronary therapy such as CABG or stent were excluded from the study. The images were evaluated by nuclear medicine physicians. Patients who showed significant uptake of Tc-99m-octreotide in the heart and had cardiac risk factors were asked to undergo angiography for the evaluation of VAP in coronary arteries. This study was approved by a local ethics committee, and written consent was obtained from all patients before each stage of the study. The Ethical Committee of Bushehr University of Medical Sciences approved this study (IR.BPUMS.REC.1401.175).

Image Acquisition

SPECT Images

Patients fasted overnight and all cardiovascular medications were discontinued at least 2 days before the study. An intravenous line of normal saline solution was connected to the antecubital vein using a 20-gauge cannula. Dipyridamole (0.56 mg/kg) was infused above 4 min. Patients’ symptoms and three-lead electrocardiography were monitored continuously. A dose of 740 MBq of Tc-99m-sestamibi as a compact bolus was injected 4 min after the initiation of the infusion. Sixty minutes later, the patients were asked to eat a fatty meal to accelerate hepatobiliary clearance of Tc-99m sestamibi, and imaging was performed 90 min after the initial infusion of dipyridamole. The rest phase was performed next day.

The one-day stress-rest protocol was used for MPI. Patients fasted overnight and all cardiovascular drugs were stopped at least 2 days before the examination. In the stress phase, the patients underwent treadmill testing or pharmacologic stress with the infusion of 0.56 mg/kg dipyridamole over 5 min. Then 370-555 MBq (10-15 mCi) Tc-99m-sestamibi (ParsIsotope Co., Iran) was administered at the peak of stress. SPECT images were acquired 30-45 minutes after stress. Four hours after the stress phase, the rest phase was performed with an injection of 740-925 MBq (20-25 mCi) Tc-99m-sestamibi. SPECT images were acquired 60-90 minutes after the radiotracer injection.

For SRS, 60 to 90 min following injection of 740-925 MBq (20-25 mCi) Tc-99m-octreotide, SPECT imaging of the heart was performed in addition to a whole-body scan.

The scans were acquired using a dual-head gamma camera (ADAC-Pegasys) equipped with a low-energy high-resolution collimator, a 140 keV photopeak with a 20% energy window, and a matrix size of 64x64. The SPECT images were acquired with 32 projections (20 seconds/projection) 180° rotations from RAO to LPO for MPI and with 64 projections (20 seconds/projection) and 360° rotations for SRS. Image reconstruction was performed using the filtered back projection method (Wiener filter; cut-off: 0.66, order: 5). The images were evaluated by a nuclear medicine specialist.

Coronary Angiography

Angiography was performed using the standard Judkins method. A cardiologist who was not aware of the scintigraphic results analyzed the angiograms.

Image Analysis

In this study, a blinded cardiologist evaluated the angiographic data and a blinded nuclear medicine specialist evaluated the scintigraphic data. The coronary arteries are divided into three main arteries, including the left anterior descending (LAD), right coronary artery (RCA), and left circumflex artery (LCx), according to angiographic standards. In scintigraphic images, the left ventricular wall was mapped to the standard 17-segment model.

For MPI, a 17-segment model was used for interpretation. According to this model, the left ventricle was divided into three main coronary arteries. For the evaluation of each segment, changes in radiotracer uptake in stress and rest phases were evaluated. A decreased radiotracer uptake (defect) observed in the stress phase that improved on rest was considered an ischemic defect, and a stress defect that did not improve was considered a fixed defect. In this study, any myocardial defect observed in the region of interest was considered abnormal.

For SRS, similar to MPI, the left ventricle was mapped into 17 segments and then divided into three main coronary arteries. For interpretation, any marked radiotracer uptake in these 17 regions as compared to background tissue was considered VAP in that region.

Statistical Analysis

Statistical analysis was performed using SPSS software version 21 for Windows. The continuous variables are expressed as median ± standard error of mean.

Results

Of 783 patients with suspected coronary artery disease who presented to Bushehr Nuclear Medicine Department for MPI, 52 consented to perform additional SRS and were included in the final study cohort. In addition, 43 patients who underwent Tc-99m-octreotide scan for NET also underwent cardiac SPECT. In total, 95 cases had cardiac Tc-99m-octreotide SPECT for evaluation of VAP. Of these 95 patients, 19 patients, including 12 women and 7 men aged 28 to 84 (58±8.04) years) who showed marked radiotracer uptake in SRS, underwent coronary angiography. An example of such a case is presented in Figure 2. Four out of 19 patients were NET patients who did not undergo MPI. Angiography was abnormal in all 19 patients, while MPI was normal in one of them. The baseline characteristics of the patients are shown in Table 1.

As for concordance between scintigraphic data and angiography in three main arteries, the results of SRS and angiography were concordant in the LAD territory in 15/19 (79%) patients, while only 7/15 (46%) cases had concordant MPI and angiography results. In the RCA territory, the results of SRS and angiography were concordant in 16/19 (84%) cases, while MPI and angiography results were concordant in 11/15 (73%) cases. In the LCx territory, the results of SRS and angiography were concordant in 15/19 (79%) cases, while MPI and angiography were concordant in 6/15 (40%) cases. Table 2 shows the results in detail. A false positive and false negative case is presented in Figure 3.

As for the scintigraphic data, the results of SRS and MPI were concordant in 11/15 (73%), 10/15 (66%), and 10/15 (66%) patients in the LAD, RCA, and LCx territories, respectively (Table 3).

Cardiac events were not observed in a follow-up period of 2-11 months (7.52±2.71) in any of the remaining 76 patients who did not undergo coronary angiography based on the cardiovascular profile. Interestingly, they did not show a remarkable uptake on SRS.

Discussion

This study was conducted to evaluate coronary VAP using SRS and compare it with MPS and angiography for the first time. Several studies have evaluated PET radiotracers such as ⁶⁸Ga-DOTATATE, ¹⁸F-fluorodeoxyglucos (FDG), and ¹⁸F sodium florine for VAP (4,7,15,16,17); however, no study has investigated SPECT radiotracers. We previously reported the role of SRS in a case of myocarditis (18).

It has been shown that about 50% of cardiovascular-related mortality is due to acute myocardial infarction. About 75% of myocardial infarctions are due to an acute thrombotic event because of the rupture of a VAP, and plaque erosion is responsible for the remaining 25% (19). Because of the importance of this subject, early detection of VAP is of great significance.

Clinically, according to the guidelines (20,21), invasive coronary angiography is considered the gold standard investigation in patients presenting with symptoms of CAD and angina. MPI is performed when the diagnosis is equivocal or to evaluate the functional significance of a familiar coronary lesion. CT coronary angiography (CTa) is another modality that may help to avoid invasive methods. Except for CTa, other above-mentioned modalities are only used for detection of obstructive intraluminal stenosis and therefore provide little information on the nature of plaques and cannon determine the vulnerability of plaques (22). It has been shown that multicontrast magnetic resonance imaging (MRI) has the highest potential for evaluation of carotid plaque and interrogation of the fibrous cap, but its high cost and lengthy procedure are its major limitations (23,24).

Molecular imaging procedures have attracted attention for the evaluation of VAP, which provide additional information about the process of VAP formation such as calcification, apoptosis, hypoxia, and neovascularization as a non-invasive method. Moreover, these procedures can lead to early diagnosis of high-risk patients with prospective culprit lesions and help to prevent subsequent cardiovascular events with proper management (22). Clinically, many studies have investigated two molecular imaging protocols including ultrasmall superparamagnetic iron oxide-contrast-enhanced MRI and ¹⁸F-FDG-PET (25). Although ¹⁸F-FDG-PET is the most common radiotracer for nuclear plaque imaging, some other PET radiotracers, including 11C and ¹⁸F-choline, 11C-PK11195, 11C-acetate, ¹⁸F-sodium fluoride, and ⁶⁸Ga-DOTATATE, have also been studied for plaque imaging (4,13,14).

Biologically, VAP formation occurs following endothelial injury and accumulation of low-density lipoprotein cholesterol in the intimal layer of the coronary wall. Monocytes are recruited into the damaged wall in response to damage and transform into macrophages (26). This inflammatory process following the accumulation of activated macrophages on the coronary wall can lead to destabilization, progression, initiation, and finally rupture of the VAP, which causes the cardiac event. Therefore, this inflammatory process can be used for imaging and diagnosis of VAP (17).

Because of the overexpression of SSTR-2 on activated macrophages, ⁶⁸Ga-DOTATATE PET and Tc-99m-octreotide, which have a high affinity for SSTR-2 and an unremarkable uptake in the normal myocardium, can be used for the evaluation of VAP. The role of ⁶⁸Ga-DOTATATE in the evaluation of VAP has been demonstrated in several studies. Rominger et al. (13) studied the uptake of ⁶⁸Ga-DOTATATE in the LAD and showed a significantly increased ⁶⁸Ga-DOTATATE uptake in the LAD, suggesting the potential role of this tracer for plaque imaging in the coronary arteries. Mojtahedi et al. (4) evaluated the uptake of ⁶⁸Ga-DOTATATE in the coronary arteries for the detection of VAP. They reported a significantly increased uptake in fibrotic and VAPs compared with normal coronary arteries (4). In addition, previous studies have shown the superiority of ⁶⁸Ga-DOTATATE for detecting VAP compared to ¹⁸F-FDG (14,17).

In this study, all patients with significant uptake of the radiotracer in SRS had abnormal coronary angiography. The results showed a high concordance between SRS and coronary angiography in three main coronary arteries; therefore, SRS can be used as a non-invasive and widely available modality for the assessment of VAP. The higher concordance of SRS with coronary angiography compared to MPI may be due to the inability of MPI to detect plaques without remarkable coronary stenosis, which can be detected by SRS. Therefore, early VAPs can be detected using SRS, which prevents future cardiac events. In conclusion, SRS and MPI complete each other in the assessment of coronary artery diseases.

Finally, it should be mentioned that PET radiotracers, including ⁶⁸Ga-DOTATATE and ¹⁸F-FDG, are the best radiotracers for nuclear plaque imaging because in comparison between PET and SPECT, PET has a higher spatial resolution and sensitivity. Moreover, absolute quantification of radionuclide uptake can be performed by PET. However, SPECT has advantages that cannot be ignored including lower costs, wider availability, and better physical properties of its radionuclides such as longer half-life. Therefore, the SPECT protocol is more applicable than PET for evaluating VAP, especially in regions where PET is less available.

Study Limitations

The limitations of this study include its small sample size and lack of PET/CT and SPECT/CT for anatomical assessment. Therefore, studies with larger populations are warranted for the validation of the clinical use of SPECT with somatostatin receptor imaging for evaluating VAP. In addition, due to ethical considerations, coronary angiography was only performed for patients according to cardiovascular guidelines, which may limit the accuracy of the findings due to the lack of angiography results in all SRS participants. It is necessary to perform accurate diagnostic tests, e.g. IVUS, accompanied by histological examinations for all patients undergoing SRS to improve the results. Another limitation of the study is that not all ischemic lesions diagnosed on MPI are due to plaque formation in the coronary arteries; therefore, plaque detection is not a good measure for comparing these two tests. Further well-designed studies are required to verify these preliminary findings.

Conclusion

The results of Tc-99m-octreotide uptake were more concordant with coronary plaques than Tc-99m-MIBI SPECT, suggesting a potential role for Tc-99m-octreotide in the evaluation of atherosclerosis. In addition, coronary uptake may provide a molecular guide for coronary atherosclerotic lesions. Specific regional uptake should be ascertained by histology.

Ethics

Ethics Committee Approval: The Ethical Committee of Bushehr University of Medical Sciences approved this study (IR.BPUMS.REC.1401.175).

Informed Consent: All participants provided written informed consent before participation

Peer-review: Externally and internally peer-reviewed.

Authorship Contributions

Surgical and Medical Practices: A.A., M.R.P., D.I., H.A., M.A., Concept: A.A., M.R.P., R.N., H.A., M.A., Design: E.J., D.I., R.N., H.A., M.A., Data Collection or Processing: A.A., E.J., M.R.P., D.I., H.A., M.A., Analysis or Interpretation: E.J., R.N., H.A., M.A., Literature Search: E.J., Writing: E.J., H.A., M.A.

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The authors declared that this study has received no financial support.