Splenic switch-off as a predictor for coronary adenosine response: validation against 13N-ammonia during co-injection myocardial perfusion imaging on a hybrid PET/CMR scanner

Splenic switch-off in [15O]H2O-positron emission tomography myocardial perfusion imaging using parametric blood flow images

BACKGROUND

Evaluation of sufficient adenosine response constitutes a significant challenge in myocardial perfusion imaging (MPI). Splenic switch-off in MPI studies denotes a visually (qualitatively) reduced splenic radiotracer signal during adenosine stress and is considered indicative of sufficient cardiac vasodilation. In this study, we examined semi-quantitative and quantitative approaches to splenic switch-off assessment using [15O]H2O-PET with either summed activity images or calculated parametric splenic blood flow images.

METHODS

Cohort 1: 90 clinical patients undergoing [15O]H2O MPI in whom adenosine response was considered clinically adequate were identified to characterize the corresponding splenic switch-off. Spleen stress/rest-ratio (SSR-ratio) was calculated as spleen stress signal intensity/spleen rest signal intensity on both summed activity and parametric blood flow images. Cohort 2: Twenty-five patients with repeat MPI due to suspected insufficient adenosine response were identified to observe if splenic switch-off on the initial MPI could predict the outcome of the repeat MPI. Cohort 3: Fifty-four patients who were considered adenosine responders on MPI and who had a coronary angiogram (CAG) follow-up within 3 months after MPI served as a separate validation group.

RESULTS

Splenic switch-off was present in most patients with a clinically sufficient adenosine response (Cohort 1), illustrated by both visual (74.4%-86.7%), semi-quantitative (summed activity images) (85.6%), and quantitative (parametric blood flow images) (92.2%) evaluation, which corresponds to the distribution in patients with sufficient adenosine response and follow-up CAG (Cohort 3). In patients suspected of insufficient adenosine response on the initial MPI (Cohort 2), the repeat MPI only yielded different myocardial blood flow (MBF) results if the initial SSR-ratio was >0.90 on splenic parametric blood flow images.

CONCLUSION

quantitative splenic switch-off assessment on parametric blood flow images was superior to the semi-quantitative splenic switch-off approach. Patients with a suspected insufficient initial adenosine response and SSR-ratio >0.90 can benefit from a repeat MPI. Thus, the integration of quantitative splenic switch-off using parametric blood flow images in the evaluation of adenosine response may support future clinical decision-making.

How to assess nonresponsiveness to vasodilator stress

Myocardial perfusion imaging (MPI) is a powerful tool for the functional assessment of ischemia in patients with suspected or known coronary artery disease (CAD). Given that the diagnostic accuracy and prognostic value of MPI and post-test management are highly dependent on achieving an adequate stress vasodilatory response, it is critical to identify those who may not have adequately responded to vasodilator pharmacological stress agents such as adenosine, dipyridamole, and regadenoson. Caffeine, a potent inhibitor of the adenosine receptor, is a compound that can affect vasodilatory hemodynamics, result in false negative studies, and potentially alter management in cases of inaccurate test results. Vasodilator non-responsiveness can be suspected by examining hemodynamics, quantitative positron emission tomography (PET) metrics such as myocardial flow reserve (MFR), and splenic response to stress. Quantitative MFR values of 1-1.2 should raise suspicion for nonresponsiveness in the setting of normal perfusion, along with the absence of a splenic switch off. Newer metrics, such as splenic response ratio, can be used to aid in the identification of potential nonresponders to pharmacologic vasodilators.

Adenosine-induced splenic switch-off on [15O]H2O PET perfusion for the assessment of vascular vasodilatation

BACKGROUND

Splenic switch-off (SSO) is a marker of adequate adenosine-induced vasodilatation on cardiac magnetic resonance perfusion imaging. We evaluate the feasibility of quantitative assessment of SSO in myocardial positron emission tomography (PET) perfusion imaging using [15O]H2O.

METHODS

Thirty patients underwent [15O]H2O PET perfusion with adenosine stress. Time-activity curves, as averaged standardized uptake values (SUVavg), were extracted from dynamic PET for spleen and liver. Maximum SUVavg, stress and rest spleen-to-liver ratio (SLR), and the splenic activity concentration ratio (SAR) were computed. Optimal cut-off values for SSO assessment were estimated from receiver operating characteristics (ROC) curve for maximum SUVavg and SLR. Also, differences between coronary artery disease, myocardial ischemia, beta-blockers, and diabetes were assessed. Data are presented as median [interquartile range].

RESULTS

In concordance with the SSO phenomenon, both the spleen maximum SUVavg and SLR were lower in adenosine stress when compared to rest perfusion (8.1 [6.5, 9.2] versus 16.4 [13.4, 19.0], p < 0.001) and (0.81 [0.63, 1.08] versus 1.86 [1.73, 2.06], p < 0.001), respectively. During adenosine stress, the SSO effect was most prominent 40-160 s after radiotracer injection. Cut-off values of 12.6 and 1.57 for maximum SUVavg and SLR, respectively, were found based on ROC analysis. No differences in SAR, SLRRest, or SLRStress were observed in patients with coronary artery disease, myocardial ischemia, or diabetes.

CONCLUSIONS

SSO can be quantified from [15O]H2O PET perfusion and used as a marker for adequate adenosine-induced vasodilatation response. In contrary to other PET perfusion tracers, adenosine-induced SSO is time dependent with [15O]H2O.

Extracardiac findings with increased perfusion during clinical O-15-H2O PET/CT myocardial perfusion imaging: A case series

BACKGROUND

Coincidental extracardiac findings with increased perfusion were reported during myocardial perfusion imaging (MPI) with various retention radiotracers. Clinical parametric O-15-H2O PET MPI yielding quantitative measures of myocardial blood flow (MBF) was recently implemented at our facility. We aim to explore whether similar extracardiac findings are observed using O-15-H2O.

METHODS AND RESULTS

All patients (2963) were scanned with O-15-H2O PET MPI according to international guidelines and extracardiac findings were collected. In contrast to parametric O-15-H2O MBF images, extracardiac perfusion was assessed using summed images. Biopsy histopathology and other imaging modalities served as reference standards. Various malignant lesions with increased perfusion were detected, including lymphomas, large-celled neuroendocrine tumour, breast, and lung cancer plus metastases from colonic and renal cell carcinomas. Furthermore, inflammatory and hyperplastic benign conditions with increased perfusion were observed: rib fractures, gynecomastia, atelectasis, sarcoidosis, pneumonia, chronic lung inflammation and fibrosis, benign lung nodule, chronic diffuse lung infiltrates, pleural plaques and COVID-19 infiltrates.

CONCLUSIONS

Malignant and benign extracardiac coincidental findings with increased perfusion are readily visible and frequently seen on O-15-H2O PET MPI. We recommend evaluating the summed O-15-H2O PET images in addition to the low-dose CT attenuation images.

The Role of Cardiac PET in Diagnosis and Prognosis of Ischemic Heart Disease: Optimal Modality Across Different Patient Populations

PURPOSE OF REVIEW

Despite single-photon emission computerized tomography (SPECT) being the most used nuclear imaging technique for diagnosis of coronary artery disease (CAD), many now consider positron emission tomography (PET) as a superior modality. This review will focus on the advances of cardiac PET in recent years and its advantages compared to SPECT in diagnosis and prognosis of CAD.

RECENT FINDINGS

PET's higher resolution and enhanced diagnostic accuracy, as well as lower radiation exposure, all help explain the rationale for its wider spread and use. PET also allows for measurement of myocardial blood flow (MBF) and myocardial flow reserve (MFR), which aids in several different clinical scenarios, such as diagnosing multivessel disease or identifying non-responders. PET has also been shown to be useful in diagnosing CAD in various specific populations, such as patients with prior COVID-19 infection, cardiac transplant, and other comorbidities.

Image-derived and physiological markers to predict adequate adenosine-induced hyperemic response in Rubidium-82 myocardial perfusion imaging

AIMS

This study aimed to investigate the potential of different markers to identify adequate stressing in subjects with and without caffeine intake prior to Rubidium-82 myocardial imaging.

METHODS AND RESULTS

This study comprised 40 healthy subjects who underwent four serial Rubidium-82 rest/adenosine stress MPI; two with 0mg caffeine consumption (baseline MPIs) and two with controlled consumption of caffeine (arm 1: 100 and 300mg, or arm 2: 200 and 400mg). We report the sensitivity and specificity of seven markers ability to predict adequate adenosine-induced hyperemic response: (1) the splenic response ratio (SRR); (2) splenic stress-to-rest intensity ratios (SIR); (3) changes in heart rate (ΔHR); (4) percentwise change in heart rate (Δ%HR); (5) changes in the rate pressure product (ΔRPP); (6) changes in the systolic blood pressure (ΔSBP); and (7) changes in the cardiovascular resistance (ΔCVR). Adequate stressing was determined as stress myocardial blood flow > 3ml/g/min and a corresponding myocardial flow reserve >68% of the individual maximum myocardial flow reserve obtained in the baseline MPIs.

RESULTS

129 MPI sessions (obtained in 39 subjects) were considered for this study. The following sensitivities were obtained: SSR = 72.7%, SIR = 63.6%, ΔHR = 45.5%, Δ%HR = 77.3%, ΔRPP = 54.5%, ΔSBP = 47.7%, and ΔCVR =40.9%, while the specificities were SSR = 80.9%, SIR = 85.0%, ΔHR = 90.4%, Δ%HR = 81.6%, ΔRPP=81.1%, ΔSBP = 86.4%, and ΔCVR =90.4%.

CONCLUSION

The image-derived and physiological markers all provide acceptable sensitivities and specificities when patients follow the caffeine pausation before MPI. However, their use warrants great care when caffeine consumption cannot be ruled out.

[Ischemic heart disease : More than just chronic CAD]

CLINICAL/METHODOLOGICAL ISSUE

Myocardial ischemia as a reduction in perfusion with therefore oxygen deficiency of vital cardiomyocytes. Thus primary and secondary prophylaxis of myocardial infarction and it's complications.

STANDARD RADIOLOGICAL METHODS

Adenosine-regadenoson stress magnetic resonance imaging (AR-stress MRI), computed tomography coronary angiography (CTCA).

METHODOLOGICAL INNOVATIONS

Non-invasive stress testing using AR-stress MRI to exclude relevant obstructive coronary artery disease (CAD).

PERFORMANCE

Meta-analysis: The diagnosis of obstructive CAD at the coronary artery level has a pooled sensitivity of 87.7% and a specificity of 88.6%. Diagnostic accuracy is better than single photon emission computed tomography (SPECT; AUC 0.89 vs. 0.74).

ACHIEVEMENTS

AR-stress MRI can be used to assess myocardial ischemia in the setting of obstructive CAD. Current clinical guidelines for myocardial revascularization have strengthened the use of stress MRI in patients with intermediate risk of CAD and stable symptoms. Cardiac MR imaging using late gadolinium enhancement (LGE) is considered gold standard for myocardial viability assessment in vivo. Both viability and ischemia are considered prognostic factors for major adverse cardiac events.

PRACTICAL RECOMMENDATIONS

AR-stress MRI is used to diagnose myocardial ischemia in combination with viability imaging (LGE). Dobutamine-atropine (DoA) stress MRI is an alternative in the setting of contraindications for AR or specific clinical questions.

Assessing splenic switch-off in Adenosine stress CMR for patients with atrial fibrillation: a propensity-matched study

OBJECTIVES

Splenic switch-off (SSO) is a validated indicator of adequate vasodilator stress unique to adenosine stress cardiac MR (CMR). Patients in atrial fibrillation (AF) may have a reduced adenosine response due to lower hyperaemic coronary flow reserve and may achieve SSO less frequently versus sinus rhythm (SR).

METHODS

1100 stress CMR studies were identified from a clinical CMR database (2016-2021). 70 patients in AF were propensity score matched to a SR group for age, sex, and body mass index. The adenosine dose administered, symptoms, heart-rate change and scan result were recorded. SSO was evaluated subjectively and semi-quantitatively via changes in splenic and myocardial signal intensity (SI) from rest to stress.

RESULTS

SSO occurred significantly less frequently in AF than SR (34/70 [49%] vs 53/70 [76%], p = 0.003). Semi-quantitative assessment supported this, with a smaller splenic SI difference between stress and rest in AF vs SR (median splenic stress:rest peak SI ratio 0.92 [IQR:0.61-1.11] vs 0.56 [IQR:0.45-0.75], p < 0.001). A heart-rate increase >10 bpm predicted visual SSO in SR but not AF. Fewer patients in AF than SR had inducible ischaemia (9/70 [13%] vs 17/69 [25%], p = 0.058). This difference was not driven by inducible ischaemia rates in patients who did not achieve SSO (6/36 [17%] AF vs 4/17 [24%] SR, p = 0.403).

CONCLUSIONS

SSO occurs significantly less frequently with AF. This may risk the under diagnosis of inducible ischaemia and requires further assessment.

ADVANCES IN KNOWLEDGE

SSO, a validated marker of adequate stress in CMR, occurs significantly less frequently in the presence of AF, risking a suboptimal functional assessment of coronary disease.

Assessment of Splenic Switch-Off With Arterial Spin Labeling in Adenosine Perfusion Cardiac MRI

BACKGROUND

In patients with suspected coronary artery disease (CAD), myocardial perfusion is assessed under rest and pharmacological stress to identify ischemia. Splenic switch-off, defined as the stress to rest splenic perfusion attenuation in response to adenosine, has been proposed as an indicator of stress adequacy. Its occurrence has been previously assessed in first-pass perfusion images, but the use of noncontrast techniques would be highly beneficial.

PURPOSE

To explore the ability of pseudo-continuous arterial spin labeling (PCASL) to identify splenic switch-off in patients with suspected CAD.

STUDY TYPE

Prospective.

POPULATION

Five healthy volunteers (age 24.8 ± 3.8 years) and 32 patients (age 66.4 ± 8.2 years) with suspected CAD.

FIELD STRENGTH/SEQUENCE

A 1.5-T/PCASL (spin-echo) and first-pass imaging (gradient-echo).

ASSESSMENT

In healthy subjects, multi-delay PCASL data (500-2000 msec) were acquired to quantify splenic blood flow (SBF) and determine the adequate postlabeling delay (PLD) for single-delay acquisitions (PLD > arterial transit time). In patients, single-delay PCASL (1200 msec) and first-pass perfusion images were acquired under rest and adenosine conditions. PCASL data were used to compute SBF maps and SBF stress-to-rest ratios. Three observers classified patients into "switch-off" and "failed switch-off" groups by visually comparing rest-stress perfusion data acquired with PCASL and first-pass, independently. First-pass categories were used as reference to evaluate the accuracy of quantitative classification.

STATISTICAL TESTS

Wilcoxon signed-rank, Pearson correlation, kappa, percentage agreement, Generalized Linear Mixed Model, Mann-Whitney, Pearson Chi-squared, receiver operating characteristic, area-under-the-curve (AUC) and confusion matrix.

SIGNIFICANCE

P value < 0.05.

RESULTS

A total of 27 patients (84.4%) experienced splenic switch-off according to first-pass categories. Comparison of PCASL-derived SBF maps during stress and rest allowed assessment of splenic switch-off, reflected in a reduction of SBF values during stress. SBF stress-to-rest ratios showed a 97% accuracy (sensitivity = 80%, specificity = 100%, AUC = 85.2%).

DATA CONCLUSION

This study could demonstrate the feasibility of PCASL to identify splenic switch-off during adenosine perfusion MRI, both by qualitative and quantitative assessments.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: 2.

Spleen Perfusion as an Index of Gender Impact on Sympathetic Nervous System Response to Exercise

Objective: Sympathetic nervous system (SNS) reaction to exercise is gender dependent. Nevertheless, clinically applicable methods to identify this difference are still missing. An organ largely sensitive to SNS is the spleen whose response to exercise can be easily evaluated, being included in the field of view of myocardial perfusion imaging (MPI). Here, we aimed to verify whether gender interferes with the spleen perfusion and its response to exercise. Methods: For this purpose, we evaluated 286 original scans of consecutive patients submitted to MPI in the course of 2019. Our standard procedure implies a single-day stress-rest sequence with a gap of ≥2 h between the administrations of 180 and 500 MBq of 99mTc-Sestamibi, respectively. Imaging is performed 30 min after radiotracer administration, with scan duration set at 25 and 35 s per view, respectively. Non-gated scans were reconstructed with the filtered back-projection method. A volume of interest was drawn on the spleen and heart to estimate the dose-normalized average counting rate that was expressed in normalized counts per seconds (NCPS). Results: In all subjects submitted to exercise MPI (n = 228), NCPS were higher during stress than at rest (3.52 ± 2.03 vs. 2.78 ± 2.07, respectively; p < 0.01). This effect was not detected in the 58 patients submitted to dipyridamole-stress. The response to exercise selectively involved the spleen, since NCPS in heart were unchanged irrespective of the used stressor. This same response was dependent upon gender, indeed spleen NCPS during stress were significantly higher in the 75 women than in the 153 men (3.86 ± 1.8 vs. 3.23 ± 1.6, respectively, p < 0.01). Again, this variance was not reproduced by heart. Finally, spleen NCPS were lower in the 173 patients with myocardial reversible perfusion defects (summed difference score ≥3) than in the remaining 55, despite similar values of rate pressure product at tracer injection. Conclusion: Thus, exercise interference on spleen perfusion can be detected during MPI. This effect is dependent upon gender and ischemia confirming the high sensitivity of this organ to SNS activation.