Positron emission tomography in the assessment of left ventricular function in healthy rats: a comparison of four imaging methods

Detection of apoptosis by [18F]ML-10 after cardiac ischemia-reperfusion injury in mice

OBJECTIVE

Myocardial infarction leads to ischemic heart disease and cell death, which is still a major obstacle in western society. In vivo imaging of apoptosis, a defined cascade of cell death, could identify myocardial tissue at risk.

METHODS

Using 2-(5-[18F]fluoropentyl)-2-methyl-malonic acid ([18F]ML-10) in autoradiography and positron emission tomography (PET) visualized apoptosis in a mouse model of transient ligation of the left anterior descending (LAD) artery. 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) PET imaging indicated the defect area. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) histology stain indicated cardiac apoptosis.

RESULTS

[18F]ML-10 uptake was evident in the ischemic area after transient LAD ligation in ex vivo autoradiography and in vivo PET imaging. Detection of [18F]ML-10 is in line with the defect visualized by [18F]FDG and the histological approach of TUNEL staining.

CONCLUSION

The tracer [18F]ML-10 is suitable for detecting apoptosis after transient LAD ligation in mice.

Detection of cardiac apoptosis by [18F]ML-10 in a mouse model of permanent LAD ligation

PURPOSE

The loss of viable cardiac cells and cell death by myocardial infarction (MI) is still a significant obstacle in preventing deteriorating heart failure. Imaging of apoptosis, a defined cascade to cell death, could identify areas at risk.

PROCEDURES

Using 2-(5-[18F]fluoropentyl)-2-methyl-malonic acid ([18F]ML-10) in autoradiography and positron emission tomography (PET) visualized apoptosis in murine hearts after permanent ligation of the left anterior descending artery (LAD) inducing myocardial infarction (MI). 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) PET imaging localized the infarct area after MI. Histology by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining validated apoptosis in the heart.

RESULTS

Accumulation of [18F]ML-10 was evident in the infarct area after permanent ligation of the LAD in autoradiography and PET imaging. Detection of apoptosis by [18F]ML-10 is in line with the defect visualized by [18F]FDG and the histological approach.

CONCLUSION

[18F]ML-10 could be a suitable tracer for apoptosis imaging in a mouse model of permanent LAD ligation.

Assessment of left and right ventricular functional parameters using dynamic dual-tracer [13N]NH3 and [18F]FDG PET/MRI

BACKGROUND

Cardiac positron emission tomography/magnetic resonance imaging (PET/MRI) can assess various cardiovascular diseases. In this study, we intra-individually compared right (RV) and left ventricular (LV) parameters obtained from dual-tracer PET/MRI scan.

METHODS

In 22 patients with coronary heart disease (69 ± 9 years) dynamic [13N]NH3 (NH3) and [18F]FDG (FDG) PET scans were acquired. The first 2 minutes were used to calculate LV and RV first-pass ejection fraction (FPEF). Additionally, LV end-systolic (LVESV) and end-diastolic (LVEDV) volume and ejection fraction (LVEF) were calculated from the early (EP) and late-myocardial phases (LP). MRI served as a reference.

RESULTS

RVFPEF and LVFPEF from FDG and NH3 as well as RVEF and LVEF from MRI were (28 ± 11%, 32 ± 15%), (32 ± 11%, 41 ± 14%) and (42 ± 16%, 45 ± 19%), respectively. LVESV, LVEDV and LVEF from EP FDG and NH3 in 8 and 16 gates were [71 (15 to 213 mL), 98 (16 to 241 mL), 32 ± 17%] and [50 (17 to 206 mL), 93 (13 to 219 mL), 36 ± 17%] as well as [60 (19 to 360 mL), 109 (56 to 384 mL), 41 ± 22%] and [54 (16 to 371 mL), 116 (57 to 431 mL), 46 ± 24%], respectively. Moreover, LVESV, LVEDV and LVEF acquired from LP FDG and NH3 were (85 ± 63 mL, 138 ± 63 mL, 47 ± 19%) and (79 ± 56 mL, 137 ± 63 mL, 47 ± 20%), respectively. The LVESV, LVEDV from MRI were 93 ± 66 mL and 153 ± 71 mL, respectively. Significant correlations were observed for RVFPEF and LVFPEF between FDG and MRI (R = .51, P = .01; R = .64, P = .001), respectively. LVESV, LVEDV, and LVEF revealed moderate to strong correlations to MRI when they acquired from EP FDG and NH3 in 16 gates (all R > .7, P = .000). Similarly, all LV parameters from LP FDG and NH3 correlated good to strongly positive with MRI (all R > .7, and P < .001), except EDV from NH3 weakly correlated to EDV of MRI (R = .54, P < .05). Generally, Bland-Altman plots showed good agreements between PET and MRI.

CONCLUSIONS

Deriving LV and RV functional values from various phases of dynamic NH3 and FDG PET is feasible. These results could open a new perspective for further clinical applications of the PET examinations.

Comparison of transient and permanent LAD ligation in mice using 18F-FDG PET imaging

Objective

Animal models for myocardial injuries represent important cornerstones in cardiovascular research to monitor the pathological processes and therapeutic approaches. We investigated the association of 18F-FDG derived left ventricular metabolic volume (LVMV), defect area and cardiac function in mice after permanent or transient ligation of the left anterior descending artery (LAD).

Methods

Serial non-invasive ECG-gated 2-deoxy-2-[18F]fluoro-d-glucose positron emission tomography (18F-FDG PET) after permanent or transient LAD ligation enabled a longitudinal in vivo correlation of 18F-FDG derived left ventricular metabolic volume to functional parameters and myocardial defect.

Results

The LVMV shows a more prominent drop after permanent than transient LAD ligation and recovers after 30 days. The loss of LVMV correlates with the defect area assessed by QPS software. Cardiac function parameters (e.g., EDV, ESV, SV) by the QGS software positively correlate with LVMV after permanent and transient LAD ligation.

Conclusions

This study provides novel insight into 18F-FDG derived LVMV after permanent and transient LAD ligation by longitudinal in 18F-FDG PET imaging and underlines the associations of the FDG derived parameter and cardiac function.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12149-022-01734-8.

The Number of Frames on ECG-Gated 18F-FDG Small Animal PET Has a Significant Impact on LV Systolic and Diastolic Functional Parameters

Objectives

This study is aimed at investigating the impact of frame numbers in preclinical electrocardiogram- (ECG-) gated ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) on systolic and diastolic left ventricular (LV) parameters in rats.

Methods

¹⁸F-FDG PET imaging using a dedicated small animal PET system with list mode data acquisition and continuous ECG recording was performed in diabetic and control rats. The list-mode data was sorted and reconstructed with different numbers of frames (4, 8, 12, and 16) per cardiac cycle into tomographic images. Using an automatic ventricular edge detection software, left ventricular (LV) functional parameters, including ejection fraction (EF), end-diastolic (EDV), and end-systolic volume (ESV), were calculated. Diastolic variables (time to peak filling (TPF), first third mean filling rate (1/3 FR), and peak filling rate (PFR)) were also assessed.

Results

Significant differences in multiple parameters were observed among the reconstructions with different frames per cardiac cycle. EDV significantly increased by numbers of frames (353.8 ± 57.7 μl^∗, 380.8 ± 57.2 μl^∗, 398.0 ± 63.1 μl^∗, and 444.8 ± 75.3 μl at 4, 8, 12, and 16 frames, respectively; ^∗P < 0.0001 vs. 16 frames), while systolic (EF) and diastolic (TPF, 1/3 FR and PFR) parameters were not significantly different between 12 and 16 frames. In addition, significant differences between diabetic and control animals in 1/3 FR and PFR in 16 frames per cardiac cycle were observed (P < 0.005), but not for 4, 8, and 12 frames.

Conclusions

Using ECG-gated PET in rats, measurements of cardiac function are significantly affected by the frames per cardiac cycle. Therefore, if you are going to compare those functional parameters, a consistent number of frames should be used.

Cardiac 18F-FDG Positron Emission Tomography: An Accurate Tool to Monitor In vivo Metabolic and Functional Alterations in Murine Myocardial Infarction

Cardiac monitoring after murine myocardial infarction, using serial non-invasive cardiac 18F-FDG positron emissions tomography (PET) represents a suitable and accurate tool for in vivo studies. Cardiac PET imaging enables tracking metabolic alterations, heart function parameters and provides correlations of the infarct size to histology. ECG-gated 18F-FDG PET scans using a dedicated small-animal PET scanner were performed in mice at baseline, 3, 14, and 30 days after myocardial infarct (MI) by permanent ligation of the left anterior descending (LAD) artery. The percentage of the injected dose per gram (%ID/g) in the heart, left ventricular metabolic volume (LVMV), myocardial defect, and left ventricular function parameters: end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and the ejection fraction (EF%) were estimated. PET assessment of the defect positively correlates with post-infarct histology at 3 and 30 days. Infarcted murine hearts show an immediate decrease in LVMV and an increase in %ID/g early after infarction, diminishing in the remodeling process. This study of serial cardiac PET scans provides insight for murine myocardial infarction models by novel infarct surrogate parameters. It depicts that serial PET imaging is a valid, accurate, and multimodal non-invasive assessment.

Left ventricle function assessment using gated first-pass 18F-FDG PET: Validation against equilibrium radionuclide angiography

PURPOSE

We appraised the feasibility of left ventricle (LV) function assessment using gated first-pass 18F-FDG PET, and assessed the concordance of the produced measurements with equilibrium radionuclide angiography (ERNA).

MATERIALS AND METHODS

Twenty-four oncologic patients benefited from 99mTc-labeled red-blood-cell ERNA, in planar mode (all patients) and using SPECT (22 patients). All patients underwent gated first-pass 18F-FDG cardiac PET. Gated dynamic PET images were reconstructed over 1 minute during tracer first-pass inside the LV and post-processed using in-house software (TomPool). After re-orientation into cardiac canonical axes and adjustment of the valves plane using a phase image, pseudo-planar PET images obtained by re-projection were automatically segmented using thresholded region growing and gradient-based delineation to produce an LV ejection fraction (EF) estimate. PET images were also post-processed in fully-tomographic mode to produce LV end diastole volume (EDV), end systole volume (ESV), and EF estimates. Concordance was assessed using Lin's concordance (ccc) and Bland-Altman analysis. Reproducibility was assessed using the coefficient of variation (CoV) and intra-class correlation (ICC).

RESULTS

Pseudo-planar PET EF estimates were concordant with planar ERNA (ccc = 0.81, P < .001) with a bias of 0% (95% CI [- 2%; 3%], limits of agreement [- 11%; 12%]). Reproducibility was excellent and similar for both methods (CoV = 2 ± 1% and 3 ± 2%, P = NS; ICC = 0.97 and 0.92, for PET and ERNA, respectively). Measurements obtained in fully-tomographic mode were concordant with SPECT ERNA: ccc = 0.83 and bias = - 3 mL for LV EDV, ccc = 0.92 and bias = 0 mL for LV ESV, ccc = 0.89 and bias = - 1% for LV EF (all P values < .001 for ccc, all biases not significant).

CONCLUSIONS

Gated first-pass 18F-FDG PET might stand as a relevant alternative to ERNA for LV function assessment, enabling a joint evaluation of both therapeutic response and cardiac toxicity in oncologic patients receiving cardiotoxic chemotherapy.

Left Ventricular Diastolic Dysfunction in a Rat Model of Diabetic Cardiomyopathy using ECG-gated 18F-FDG PET

In diabetic cardiomyopathy, left ventricular (LV) diastolic dysfunction is one of the earliest signs of cardiac involvement prior to the definitive development of heart failure (HF). We aimed to explore the LV diastolic function using electrocardiography (ECG)-gated ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET) imaging beyond the assessment of cardiac glucose utilization in a diabetic rat model. ECG-gated ¹⁸F-FDG PET imaging was performed in a rat model of type 2 diabetes (ZDF fa/fa) and ZL control rats at age of 13 weeks (n = 6, respectively). Under hyperinsulinemic-euglycemic clamp to enhance cardiac activity, ¹⁸F-FDG was administered and subsequently, list-mode imaging using a dedicated small animal PET system with ECG signal recording was performed. List-mode data were sorted and reconstructed into tomographic images of 16 frames per cardiac cycle. Left ventricular functional parameters (systolic: LV ejection fraction (EF), heart rate (HR) vs. diastolic: peak filling rate (PFR)) were obtained using an automatic ventricular edge detection software. No significant difference in systolic function could be obtained (ZL controls vs. ZDF rats: LVEF, 62.5 ± 4.2 vs. 59.4 ± 4.5%; HR: 331 ± 35 vs. 309 ± 24 bpm; n.s., respectively). On the contrary, ECG-gated PET imaging showed a mild but significant decrease of PFR in the diabetic rats (ZL controls vs. ZDF rats: 12.1 ± 0.8 vs. 10.2 ± 1 Enddiastolic Volume/sec, P < 0.01). Investigating a diabetic rat model, ECG-gated ¹⁸F-FDG PET imaging detected LV diastolic dysfunction while systolic function was still preserved. This might open avenues for an early detection of HF onset in high-risk type 2 diabetes before cardiac symptoms become apparent.

Feasibility of biventricular volume and function assessment using first-pass gated 15O-water PET

Background

We investigated the feasibility of left ventricular (LV) and right ventricular (RV) volume and function estimation using a first-pass gated ¹⁵O-water PET. This prospective study included 19 patients addressed for myocardial perfusion reserve assessment using ¹⁵O-water PET. PET data were acquired at rest and after regadenoson stress, and gated first-pass images were reconstructed over the time range corresponding to tracer first-pass through the cardiac cavities and post-processed using TomPool software; LV and RV were segmented using a semi-automated 4D immersion algorithm. LV volumes were computed using a count-based model and a fixed threshold at 30% of the maximal activity. RV volumes were computed using a geometrical model and an adjustable threshold that was set so as to fit LV and RV stroke volumes. Ejection curves were fitted using a deformable reference curve model. LV results were compared to those obtained using ^99mTc-sestamibi gated myocardial SPECT in terms of end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and ejection fraction (EF).

Results

There was an excellent concordance between rest and stress PET in terms of EDV and ESV (Lin’s coefficient ~ 0.85–0.90), SV (~ 0.80), and EF (~ 0.75) for both ventricles. Correlation with myocardial SPECT was high for LV EDV (Pearson’s R = 0.89, p < 0.001) and ESV (R = 0.87, p < 0.001) and satisfying for LV SV (R = 0.67, p < 0.001) and EF (R = 0.67, p < 0.001). Minimal LV ESV overestimation (+ 4 mL, p = 0.03) and EF underestimation (− 4%, p = 0.01) were observed using PET.

Conclusions

Biventricular volume and function assessment are achievable using the first-pass PET, and LV parameters correlate well with those derived from gated myocardial SPECT.

Differential Effects of Isoproterenol on Regional Myocardial Mechanics in Rat using 3D cine DENSE Cardiovascular Magnetic Resonance

The present study assessed the acute effects of isoproterenol on left ventricular (LV) mechanics in healthy rats with the hypothesis that ß-adrenergic stimulation influences the mechanics of different myocardial regions of the LV wall in different ways. To accomplish this, magnetic resonance images were obtained in the LV of healthy rats with or without isoproterenol infusion. The LV contours were divided into basal, mid-ventricular, and apical regions. Additionally, the mid-ventricular myocardium was divided into three transmural layers with each layer partitioned into four segments (i.e., septal, inferior, lateral, and anterior). Peak systolic strains and torsion were quantified for each region. Isoproterenol significantly increased peak systolic radial strain and circumferential-longitudinal shear strain, as well as ventricular torsion, throughout the basal, mid-ventricle, and apical regions. In the mid-ventricle, isoproterenol significantly increased peak systolic radial strain, and induced significant increases in peak systolic circumferential strain and longitudinal strain in the septum. Isoproterenol consistently increased peak systolic circumferential-longitudinal shear strain in all mid-ventricular segments. Ventricular torsion was significantly increased in nearly all segments except the inferior sub-endocardium. The effects of isoproterenol on LV systolic mechanics (i.e., 3D strains and torsion) in healthy rats depend on the region. This region-dependency is also strain component-specific. These results provide insight into the regional response of LV mechanics to ß-adrenergic stimulation in rats, and could act as a baseline for future studies on subclinical abnormalities associated with the inotropic response in heart disease.

Practical identifiability and uncertainty quantification of a pulsatile cardiovascular model

Mathematical models are essential tools to study how the cardiovascular system maintains homeostasis. The utility of such models is limited by the accuracy of their predictions, which can be determined by uncertainty quantification (UQ). A challenge associated with the use of UQ is that many published methods assume that the underlying model is identifiable (e.g. that a one-to-one mapping exists from the parameter space to the model output). In this study we present a novel workflow to calibrate a lumped-parameter model to left ventricular pressure and volume time series data. Key steps include using (1) literature and available data to determine nominal parameter values; (2) sensitivity analysis and subset selection to determine a set of identifiable parameters; (3) optimization to find a point estimate for identifiable parameters; and (4) frequentist and Bayesian UQ calculations to assess the predictive capability of the model. Our results show that it is possible to determine 5 identifiable model parameters that can be estimated to our experimental data from three rats, and that computed UQ intervals capture the measurement and model error.

Derivation of a respiration trigger signal in small animal list-mode PET based on respiration-induced variations of the ECG signal

BACKGROUND

Raw PET list-mode data contains motion artifacts causing image blurring and decreased spatial resolution. Unless corrected, this leads to underestimation of the tracer uptake and overestimation of the lesion size, as well as inaccuracies with regard to left ventricular volume and ejection fraction (LVEF), especially in small animal imaging.

METHODS AND RESULTS

A respiratory trigger signal from respiration-induced variations in the electro-cardiogram (ECG) was detected. Original and revised list-mode PET data were used for calculation of left ventricular function parameters using both respiratory gating techniques. For adequately triggered datasets we saw no difference in mean respiratory cycle period between the reference standard (RRS) and the ECG-based (ERS) methods (1120 ± 159 ms vs 1120 ± 159 ms; P = n.s.). While the ECG-based method showed somewhat higher signal noise (66 ± 22 ms vs 51 ± 29 ms; P < .001), both respiratory triggering techniques yielded similar estimates for EDV, ESV, LVEF (RRS: 387 ± 56 µL, 162 ± 34 µL, 59 ± 5%; ERS: 389 ± 59 µL, 163 ± 35 µL, 59 ± 4%; P = n.s.).

CONCLUSIONS

This study showed that respiratory gating signals can be accurately derived from cardiac trigger information alone, without the additional requirement for dedicated measurement of the respiratory motion in rats.

Synthesis of fluorine-18 radio-labeled serum albumins for PET blood pool imaging

We sought to develop a practical, reproducible and clinically translatable method of radiolabeling serum albumins with fluorine-18 for use as a PET blood pool imaging agent in animals and man. Fluorine-18 radiolabeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester, [(18)F]F-Py-TFP was prepared first by the reaction of its quaternary ammonium triflate precursor with [(18)F]tetrabutylammonium fluoride ([(18)F]TBAF) according to a previously published method for peptides, with minor modifications. The incubation of [(18)F]F-Py-TFP with rat serum albumin (RSA) in phosphate buffer (pH9) for 15 min at 37-40 °C produced fluorine-18-radiolabeled RSA and the product was purified using a mini-PD MiniTrap G-25 column. The overall radiochemical yield of the reaction was 18-35% (n=30, uncorrected) in a 90-min synthesis. This procedure, repeated with human serum albumin (HSA), yielded similar results. Fluorine-18-radiolabeled RSA demonstrated prolonged blood retention (biological half-life of 4.8 hours) in healthy awake rats. The distribution of major organ radioactivity remained relatively unchanged during the 4 hour observation periods either by direct tissue counting or by dynamic PET whole-body imaging except for a gradual accumulation of labeled metabolic products in the bladder. This manual method for synthesizing radiolabeled serum albumins uses fluorine-18, a widely available PET radionuclide, and natural protein available in both pure and recombinant forms which could be scaled up for widespread clinical applications. These preclinical biodistribution and PET imaging results indicate that [(18)F]RSA is an effective blood pool imaging agent in rats and might, as [(18)F]HSA, prove similarly useful as a clinical imaging agent.

In-vivo monitoring of erythropoietin treatment after myocardial infarction in mice with [⁶⁸Ga]Annexin A5 and [¹⁸F]FDG PET

BACKGROUND

Several studies substantiate the cardioprotective effects of erythropoietin (EPO). Our goal was to quantify the effects of EPO treatment on the early expression of the apoptosis marker phosphatidylserine as well as on the left ventricular volumes and function by means of small animal PET.

METHODS AND RESULTS

Myocardial infarction (MI) was induced in C57BL/6 mice. Animals were assigned to saline or EPO groups and underwent Annexin PET (day 2) and gated FDG PET (days 6 and 30). Annexin uptake was significantly higher in the infarction than in remote myocardium, with no differences between treatment groups. Infarct size showed a slight decrease in the EPO group and a slight increase in the controls, which did not reach statistical significance. Follow-up analyses revealed a significant increase of end-diastolic and end-systolic volumes in the EPO group, in which a stable left ventricular ejection fraction (LVEF) was maintained.

CONCLUSION

We find that deleterious effects of EPO can outweigh cardioprotective effects. The present EPO treatment did not significantly reduce apoptosis after MI, but seemingly provoked significant myocardial dilation while maintaining a stable LVEF. Molecular mechanisms of EPO treatment may need further elucidation to optimize therapy regimens.