The discrepancy between FDG uptake and myocardial fibrosis in patients with pulmonary arterial hypertension – PET/MRI study

Background

Inflammatory processes play an important role in pulmonary arterial hypertension (PAH) pathophysiology. We previously confirmed that in case of right ventricle (RV) failure, changes of cytokines' levels are correlated with myocardial metabolic and hemodynamic alterations observed in PET/MRI hybrid imaging. Presence of late gadolinium enhancement (LGE) in RV insertion points (RVIPs) has been found in majority of PAH patients and is often recognized as evidence of myocardial fibrosis due to RV pressure overload. As qualitative and/or quantitative assessments of LGE may vary due to natural PAH progression or specific therapy, we hypothesized that simple presence of LGE at RVIPs is not unequivocal to fibrotic tissue (without metabolic activity).

Purpose

To check the relationship between LGE mass and 18F-fluorodexyglucose uptake in RV insertion points in PAH patients using PET/MRI hybrid imaging.

Methods

Twenty-eight clinically stable PAH patients (49.9±15.9 years) had simultaneous PET/MRI scans during baseline and follow up (FU) visits, Figure. 18F-fluorodexyglucose (FDG) was used as a tracer and its cardiac uptake was presented as a maximum standardized uptake value (SUV) for RV insertion points (SUV in RVIPS). Septal delayed enhancement mass was quantified in RVIPs and presented as LGE mass. Occurrences of clinical end-points (CEP, defined as death or clinical deterioration) were assessed during 24 months observation.

Results

LGE was found in RVIPs of all PAH patients. Mean LGE mass was 6.32±4.41 g and mean SUV in RVIPS was 7.28±5.36. Follow up values were 8.01±7.75g (p=0.4) and 5.80±3.16 (p=0.16), respectively. We observed significant correlation between baseline SUV in RVIPS and mean pulmonary pressure, mPAP (r=0.49, p=0.04) but no correlation was found between LGE mass and SUV in RVIPS (in both baseline and FU scans).

Between baseline and follow up visits, 16 patients had CEP and needed PAH therapy escalation. CEP+ group of PAH patients presented higher baseline LGE mass (7.53±4.75 vs 3.92±2.21, p=0.04) and SUV in RVIPS (7.27±5.42 vs 6.01±4.52, p=0.4). In all CEP patients who initiated prostacycline therapy and survived (n=8, 50%), SUV in RVIPS decreased in FU PET scans together with an increase in LGE mass in MRI. At FU visits we also observed significant improvement of MRI-derived RV ejection fraction (45.1±9.6% to 52.4±12.9%, p=0.01), and mPAP (50.5±18.3 to 42.8±18.6 mmHg, p=0.03).

Conclusions

Effective PAH therapy have an impact on both LGE mass and FDG uptake in cardiac local tissue changes. Since there was no correlation between LGE mass and FDG uptake RV insertion points, the question arises what the cause of these LGE changes is. Increased fibrosis should cause diminished local glucose metabolism. This phenomenon opens new questions concerning pathophysiology processes in RVIPs and requires confirmation on bigger PAH population.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): National Center for Science in Poland

Integrative analysis of hybrid PET/MR improves recovery prediction of left ventricular contractility after percutaneous revascularisation of coronary chronic total occlusions

Background

The clinical indication to revascularization of coronary chronic total occlusions (CTO) is set on viability assessment through well-established imaging methods. Precise characterization of the myocardium allows a better interventional risk stratification before revascularization. Hybrid positron emission tomography/magnetic resonance (PET/MR) scanners allow the combination of image based biomarkers through simultaneous acquisition and may potentially improve the prediction of therapy response. This study aims to evaluate the accuracy in recovery prediction after CTO revascularization by combining information of myocardial viability and wall motion abnormalities (WMA) from PET/MR exams.

Methods

Viability was assessed with PET/MR imaging in 22 patients before percutaneous revascularization of a CTO. A cardiac MR imaging was performed at follow-up after 6 months. Segmental WMA was assessed on the AHA 17-segments model from cine images at baseline and follow-up with a 5-point scale. Recovery of WMA was predicted using six different models combining parameters from fluorodeoxyglucose (FDG) uptake, transmural extent of the scar with late gadolinium enhancement image (LGE), T1 mapping, and WMA at baseline. From 374 myocardial segments, we considered only 82 that were completely characterized and presented at least hypokinetic WMA at baseline. 46 of them were supplied by the treated CTO artery (CTO-subtended), wherefrom 16 segments recovered the contractility.

A Random Forest classifier was applied for recovery prediction. Training and testing data were selected through stratified random sampling (50 iterations) with replacement from the two sets: CTO-subtended and non-CTO-subtended segments. To compensate for class imbalance and limited sample size, SMOTE oversampling was applied to the training data. Prediction assessment was based on balanced accuracy (bAcc), sensitivity, specificity, ROC AUC, and Precision-Recall AUC (PR AUC), while Wilcoxon test with a Bonferroni's correction was used for comparison between the proposed models and LGE+FDG, which has shown the best performance in previous studies [1,2].

Results

The best model for the prediction of recovery after revascularisation was the combination of LGE+FDG+WMA+T1mapping (bAcc 0.62±0.11, sensitivity 0.72±0.18, specificity 0.70±0.19, ROC AUC 0.67±0.12, PR AUC 0.60±0.12), followed by LGE+FDG+WMA, LGE+FDG+T1mapping, LGE+FDG, LGE, and FDG, respectively. The first model statistically significantly (p-value<0.01) outperformed the reference model LGE+FDG (bAcc 0.54±0.11, sensitivity: 0.67±0.18, specificity 0.55±0.19, ROC AUC 0.56±0.13, PR AUC 0.47±0.10).

Conclusion

This study documents the benefits of a hybrid PET/MR multiparametric assessment beyond the standard analysis based on FDG uptake and LGE for the prediction of contractility recovery after revascularization of CTO of the coronary arteries.

Funding Acknowledgement

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 764458 PETMR protocolROC AUC comparsion

5964Hybrid PET/MR imaging for the prediction of left ventricular (LV) recovery after revascularisation of chronic total occluded coronaries

Background

Percutaneous coronary intervention (PCI) of chronic total occlusion (CTO) represents one of the major challenges in interventional cardiology. Physicians are still reluctant in referring for PCI, assuming non viability of the myocardium subtended by the CTO. Data are controversial in assessing the improvement of left ventricular (LV) wall motion after revascularisation and the prognostic value of viability testing to guide patient selection.

Purpose

The aim of this study was to determine, whether hybrid fluorodeoxyglucose positron emission tomography/magnetic resonance (FDG PET/MR) imaging allows a more accurate prediction of LV regional wall motion recovery after successful PCI of CTOs in comparison to PET or MR alone.

Methods

We enrolled 49 consecutive symptomatic patients with CTO and evidence of wall motion abnormality in the corresponding CTO-territory. All patients underwent hybrid FDG PET/MR imaging as semi-quantitative assessment of myocardial viability - glucose metabolism in PET and late gadolinium enhancement (LGE) transmurality in MR – prior of PCI of the CTO. Follow-up MRI was performed in 23 patients 3–6 months after successful revascularisation to evaluate wall motion changes.

Results

We assessed viability in 124 myocardial segments subtended by a CTO in 23 patients with successful PCI who underwent serial imaging. Segments with wall motion abnormality at baseline (n=80) were analysed. Most of these segments (n=54, 68%) were concordantly assessed viable by PET and MR, conversely only 2 (2%) segments were assessed non-viable by both imaging techniques. However, almost one third of the segments showed discordant patterns of viability either PET not viable/ MR viable (3 (4%) segments) or PET viable/ MR not viable (21 (26%) segments): particularly the latter revealed a significant wall motion improvement (p=0.033).

The combination of PET and MR showed a fair accuracy in predicting myocardial segments with wall motion improvement after CTO revascularisation (PET/MR area under ROC curve (AUC) 0.72, SE 0.07, p=0.002), which was superior to MR-LGE (AUC=0.66, SE 0.09) and FDG-PET (AUC=0.58, SE 0.10) alone (Figure).

Comparisons of ROC curves

Conclusion

Hybrid PET/MR imaging prior to successful CTO showed a better performance than PET or MR alone in predicting regional improvement of disturbed wall motion.

The complimentary information derived from both modalities may particularly help to identify small amounts of viable epicardial myocardium within large scars which can improve contractility after CTO-revascularisation.

Attenuation correction for myocardial perfusion imaging

Aim: We investigated the impact of photon attenuation in myocardial perfusion imaging with SPECT and PET in patients with coronary artery disease. In fact, the regional tracer distribution can be quantitatively assessed by polar map analysis if the effects of photon attenuation are accounted for. PET imaging permits accurate measurement of and correction for photon attenuation, whereas results of attenuation correction in SPECT imaging have been inconsistent. Patients, methods: We compared photon attenuation in resting perfusion imaging studies with SPECT (99mTc-sestamibi) and PET (13N-ammonia) from 21 patients. Transaxial images were reconstructed with and without attenuation correction and reoriented into short axis images. Polar map analysis was utilized to generate regional tracer uptake in six anatomical segments. Results: Average segmental photon attenuation calculated as the ratio of counts in corrected and uncorrected images was 7.2 ± 1.4 in SPECT and 14.0 ± 3.1 in PET imaging (p <0.01). This attenuation factor was significantly related to body mass index for both methods (p <0.001). While attenuation correction for SPECT imaging did compensate for attenuation effects in the inferior wall (from –15% to +6% vs. PET), relative tracer uptake in the anterior wall in SPECT images was significantly reduced after attenuation correction (from –2% to –18% vs. PET, p <0.01). Conclusion: Differential effects of attenuation correction for myocardial SPECT perfusion imaging need to be considered when algorithms designed to compensate effects of photon attenuation in SPECT imaging are employed in clinical practice.

Prediction of glioma recurrence using dynamic ¹⁸F-fluoroethyltyrosine PET

BACKGROUND AND PURPOSE

Inter- and intratumor heterogeneity and the variable course of disease in patients with glioma motivate the investigation of new prognostic factors to optimize individual treatment. Here we explore the usefulness of standard static and more sophisticated dynamic (18)F-fluoroethyltyrosine-PET imaging for the assessment of patient prognosis.

MATERIALS AND METHODS

Thirty-four consecutive patients with untreated, first-diagnosed, histologically proved glioma were included in this retrospective study. All patients underwent dynamic PET scans before surgery (± standard treatment) and were followed up clinically and by MR imaging. Static and dynamic tumor-to-background ratio, TTP, and slope-to-peak were obtained and correlated with progression-free survival.

RESULTS

Twenty of 34 patients experienced progression, with a median progression-free survival of 28.0 ± 11.1 months. Dynamic TTP was highly prognostic for recurrent disease, showing a strong correlation with progression-free survival (hazard ratio, 6.050; 95% CI, 2.11-17.37; P < .001). Most interesting, this correlation also proved significant in the subgroup of low-grade glioma (hazard ratio, 5.347; 95% CI, 1.05-27.20; P = .044), but not when using established static imaging parameters, such as maximum tumor-to-background ratio and mean tumor-to-background ratio. In the high-grade glioma subgroup, both dynamic and static parameters correlated with progression-free survival. The best results were achieved by defining ROIs around "hot spots" in earlier timeframes, underlining the concept of intratumor heterogeneity.

CONCLUSIONS

(18)F-fluoroethyltyrosine-PET can predict recurrence in patients with glioma, with dynamic analysis showing advantages over static imaging, especially in the low-grade subgroup.

Attenuation correction for myocardial perfusion imaging. A comparison between SPECT and PET imaging by polar map analysis

AIM

We investigated the impact of photon attenuation in myocardial perfusion imaging with SPECT and PET in patients with coronary artery disease. In fact, the regional tracer distribution can be quantitatively assessed by polar map analysis if the effects of photon attenuation are accounted for. PET imaging permits accurate measurement of and correction for photon attenuation, whereas results of attenuation correction in SPECT imaging have been inconsistent.

PATIENTS, METHODS

We compared photon attenuation in resting perfusion imaging studies with SPECT ((99m)Tc-sestamibi) and PET ((13)N-ammonia) from 21 patients. Transaxial images were reconstructed with and without attenuation correction and reoriented into short axis images. Polar map analysis was utilized to generate regional tracer uptake in six anatomical segments.

RESULTS

Average segmental photon attenuation calculated as the ratio of counts in corrected and uncorrected images was 7.2 +/- 1.4 in SPECT and 14.0 +/- 3.1 in PET imaging (p < 0.01). This attenuation factor was significantly related to body mass index for both methods (p < 0.001). While attenuation correction for SPECT imaging did compensate for attenuation effects in the inferior wall (from -15% to +6% vs. PET), relative tracer uptake in the anterior wall in SPECT images was significantly reduced after attenuation correction (from -2% to -18% vs. PET, p < 0.01).

CONCLUSION

Differential effects of attenuation correction for myocardial SPECT perfusion imaging need to be considered when algorithms designed to compensate effects of photon attenuation in SPECT imaging are employed in clinical practice.

13N-ammonia rest/stress PET

Aim: Hyperhomocysteinaemia (Hhcy) is known to be an independent risk factor for vascular disease. Coronary flow reserve (CFR) measured by positron emission tomography (PET) is a sensitive method to monitor the effects of pharmacologic interventions in Hhcy. We assessed coronary vascular reactivity by PET in patients with coronary artery disease (CAD) dependent on their homocysteine (Hcy) levels before and under high dose folic acid supplementation therapy (FAST). Patients, methods: Twelve patients with CAD underwent rest/adenosine 13N-ammonia PET for quantification of myocardial blood flow (MBF) and CFR before and after nine weeks FAST (10 mg/day). Results: Folate levels increased from 21 ± 6 to 210 ± 34 μg/l (+900%, p <0.0001) while Hcy levels decreased from 12.1 ± 3.6 to 9.1 ± 3.1 μmol/l (–25%; p <0.01). Global resting MBF remained nearly unchanged after FAST, while stress MBF (from 2.61 ± 0.93 to 3.25 ± 1.15 ml/ g/min; p = 0.05) and CFR (from 3.00 ± 0.76 to 3.72 ± 0.93 ml/g/min; p <0.05; +24%) significantly increased in patients with normal and elevated Hcy levels (cut off 12 μmol/l). An inverse relation was found between Hcy and CFR (R = –0.53; p = 0.08) and between Hcy and MBF at rest (R = –0.62; p < 0.05) at baseline conditions, not persisting after FAST. Conclusion: Coronary vascular reactivity can be improved by FAST in patients with CAD and normal or elevated Hcy levels. FAST might lower an increased cardiovascular risk in CAD patients possibly by mechanisms that are not related to Hcy.

13N-ammonia rest/stress PET: folic acid improves global coronary vasoreactivity in coronary artery disease patients with normal or elevated homocysteine levels

AIM

Hyperhomocysteinaemia (Hhcy) is known to be an independent risk factor for vascular disease. Coronary flow reserve (CFR) measured by positron emission tomography (PET) is a sensitive method to monitor the effects of pharmacologic interventions in Hhcy. We assessed coronary vascular reactivity by PET in patients with coronary artery disease (CAD) dependent on their homocysteine (Hcy) levels before and under high dose folic acid supplementation therapy (FAST).

PATIENTS, METHODS

Twelve patients with CAD underwent rest/adenosine (13) N-ammonia PET for quantification of myocardial blood flow (MBF) and CFR before and after nine weeks FAST (10 mg/day).

RESULTS

Folate levels increased from 21 +/- 6 to 210 +/- 34 microg/l (+900%, p < 0.0001) while Hcy levels decreased from 12.1 +/- 3.6 to 9.1 +/- 3.1 micromol/l ( - 25%; p < 0.01). Global resting MBF remained nearly unchanged after FAST, while stress MBF (from 2.61 +/- 0.93 to 3.25 +/- 1.15 ml/g/min; p = 0.05) and CFR (from 3.00 +/- 0.76 to 3.72 +/- 0.93 ml/g/min; p < 0.05; +24%) significantly increased in patients with normal and elevated Hcy levels (cut off 12 micromol/l). An inverse relation was found between Hcy and CFR (R = - 0.53; p = 0.08) and between Hcy and MBF at rest (R = - 0.62; p < 0.05) at baseline conditions, not persisting after FAST.

CONCLUSION

Coronary vascular reactivity can be improved by FAST in patients with CAD and normal or elevated Hcy levels. FAST might lower an increased cardiovascular risk in CAD patients possibly by mechanisms that are not related to Hcy.

Myocardial blood flow and coronary flow reserve in children with "normal" epicardial coronary arteries after the onset of Kawasaki disease assessed by positron emission tomography

Myocardial blood flow (MBF) was investigated in children (14.2 +/- 5.01 years) with "resolved" coronary involvement after the onset of Kawasaki disease and angiographically normal epicardial coronary arteries. Ten asymptomatic children with a history of Kawasaki disease had electrocardiography, echocardiography, and positron emission tomography (PET) 10.3 +/- 6.01 years after onset of the acute illness. Myocardial perfusion was assessed by NH(3)- PET at rest and after vasodilatation and compared with that of 10 healthy volunteers (26.1 +/- 6.3 years). No patient had signs of myocardial ischemia; on echocardiography ventricular function was normal without dyskinetic areas or signs of enlargement or stenosis of the proximal coronary arteries. There was no statistical significant difference between patients and volunteers in MBF at rest (0.86 +/- 0.27 vs 0.77 +/- 0.17 ml/g/min), whereas MBF after vasodilatation (2.42 +/- 0.81 vs 3.10 +/- 0.8 ml/g/min) and coronary flow reserve (CFR) (2.89 +/- 0.26 vs 4.09 +/- 1.01 mmHg/ml/g/min) were significantly attenuated in the Kawasaki group. No stress-induced perfusion defects could be detected. In children with a history of Kawasaki disease and angiographically normal epicardial coronary arteries, there is a significant attenuation of MBF after vasodilatation and a significant reduction of CFR. Impairment of vasoreactive ability may indicate residual damage of the coronary arteries and may be a risk factor for atherosclerosis in adulthood.

Time course and extent of improvement of dysfunctioning myocardium in patients with coronary artery disease and severely depressed left ventricular function after revascularization: correlation with positron emission tomographic findings

OBJECTIVES

This study was performed to evaluate the prevalence, time course of recovery and extent of improvement of segments with a positron emission tomographic (PET) flow-metabolism mismatch and match pattern, as well as of PET segments with normal perfusion but with impaired myocardial function.

BACKGROUND

Previous studies have shown that scintigraphic techniques evaluating myocardial viability provide predictive information about the improvement of regional wall motion. However, there are little data concerning the time course and extent of improvement of segments according to preoperative scintigraphic patterns.

METHODS

Twenty-nine patients with ischemic cardiomyopathy (ejection fraction 18% to 35%) underwent preoperative PET viability assessment and were functionally assessed by two-dimensional echocardiography preoperatively and at 11 days, 14 weeks and >12 months after coronary artery bypass graft surgery.

RESULTS

In 168 (70%) of 240 dysfunctional segments, a "normal" scintigraphic pattern was present, whereas a "mismatch" pattern was observed in 24% (p<0.01). Mismatch areas were associated with more severe preoperative wall motion abnormalities and incomplete postoperative recovery. After one year, 31% of normal scintigraphic segments, compared with only 18% of mismatch segments, showed complete functional restoration (p<0.05).

CONCLUSIONS

These data suggest that in patients with severe left ventricular dysfunction, a scintigraphic pattern of normal perfusion and normal metabolism is more prevalent than a flow-metabolism mismatch pattern. Functional recovery is more frequent in normal scintigraphic segments, whereas in mismatch segments, postoperative recovery remains incomplete even after one year.

Non-invasive estimation of myocardial efficiency using positron emission tomography and carbon-11 acetate--comparison between the normal and failing human heart

The clearance kinetics of carbon-11 acetate, assessed by positron emission tomography (PET), can be combined with measurements of ventricular function for non-invasive estimation of myocardial oxygen consumption and efficiency. In the present study, this approach was applied to gain further insights into alterations in the failing heart by comparison with results obtained in normals. We studied ten patients with idiopathic dilated cardiomyopathy (DCM) and 11 healthy normals by dynamic PET with 11C-acetate and either tomographic radionuclide ventriculography or cine magnetic resonance imaging. A "stroke work index" (SWI) was calculated by: SWI = systolic blood pressure x stroke volume/body surface area. To estimate myocardial efficiency, a "work-metabolic index" (WMI) was then obtained as follows: WMI = SWI x heart rate/k(mono), where k(mono) is the washout constant for 11C-acetate derived from monoexponential fitting. In DCM patients, left ventricular ejection fraction was 19%+/-10% and end-diastolic volume was 92+/-28 ml/m2 (vs 64%+/-7% and 55+/-8 ml/m2 in normals, P<0.001). Myocardial oxidative metabolism, reflected by k(mono), was significantly lower compared with that in normals (0.040+/-0.011/min vs 0.060+/-0.015/min; P<0.003). The SWI (1674+/-761 vs 4736+/-895 mmHg x ml/m2; P<0.001) and the WMI as an estimate of efficiency (2.98+/-1.30 vs 6.20+/-2.25 x 10(6) mmHg x ml/m2; P<0.001) were lower in DCM patients, too. Overall, the WMI correlated positively with ejection parameters (r=0.73, P<0.001 for ejection fraction; r=0.93, P<0.001 for stroke volume), and inversely with systemic vascular resistance (r=-0.77; P<0.001). There was a weak positive correlation between WMI and end-diastolic volume in normals (r=0.45; P=0.17), while in DCM patients, a non-significant negative correlation coefficient (r=-0.21; P=0.57) was obtained. In conclusion non-invasive estimates of oxygen consumption and efficiency in the failing heart were reduced compared with those in normals. Estimates of efficiency increased with increasing contractile performance, and decreased with increasing ventricular afterload. In contrast to normals, the failing heart was not able to respond with an increase in efficiency to increasing ventricular volume. The present data support the usefulness of the WMI for non-invasive characterization of cardiac efficiency and may serve as a background for improved evaluation of medical therapy for heart failure.

Evaluation of sympathetic nerve terminals with [(11)C]epinephrine and [(11)C]hydroxyephedrine and positron emission tomography

BACKGROUND

The goal of the present study was to directly compare the new radiopharmaceutical agent [(11)C]epinephrine (EPI) with [(11)C]hydroxyephedrine (HED) through the use of PET.

METHODS AND RESULTS

Seven healthy volunteers and 10 patients were investigated after heart transplantation. PET images of both tracers were of excellent quality in the volunteers. Values for radiolabeled metabolites (measured in percent of blood activity) at 5, 20, and 60 minutes after injection were approximately 35%, approximately 82%, and approximately 86% for EPI and approximately 13%, approximately 47%, and approximately 78% for HED, respectively. At 35 minutes, metabolite-corrected mean myocardial retention fraction of EPI (0. 235+/-0.022 min(-1)) was significantly greater (P<0.01) than that of HED (0.142+/-0.012 min(-1)). Corrected tracer retention fractions of both EPI and HED were significantly reduced in transplant recipients (0.055+/-0.004 min(-1), P<0.0001; and 0.050+/-0.006 min(-1), P<0. 0001, respectively) compared with volunteers. Normalization of retention fractions of patients with transplantation within 1 year to volunteers resulted in a value (ratio expressed in percent) of 20. 6+/-1.8% for EPI, significantly (P<0.03) smaller than 27.8+/-0.8% for HED. In patients with transplantation later than 1 year, the values were 26.0+/-2.9% for EPI compared with 44.2+/-5.6% for HED (P<0.014).

CONCLUSIONS

Both tracers showed high selectivity for neuronal uptake in the heart, with a significant reduction in tracer retention in transplant recipients compared with volunteers. Compared with HED, EPI showed greater retention in volunteers and a lower retention ratio in transplant recipients, suggesting that EPI may be the superior tracer with higher sensitivity to neuronal abnormalities. Because EPI reflects neuronal uptake, metabolism, and storage, it may be more suitable for the study of neuronal integrity than HED, which primarily traces uptake-1 capacity.

99mTc-tetrofosmin SPECT for prediction of functional recovery defined by MRI in patients with severe left ventricular dysfunction: additional value of gated SPECT

This study was designed to evaluate gated 99mTc-tetrofosmin SPECT for prediction of functional recovery proven by sequential MRI. 99mTc-labeled tetrofosmin is a recently introduced tracer for myocardial perfusion. However, its role for viability assessment is still under investigation.

METHODS

99mTc-tetrofosmin uptake in 19 patients with coronary artery disease and severe left ventricular dysfunction was correlated to regional wall thickening before and 4.5 +/- 0.8 mo after successful coronary artery bypass grafting, as derived from corresponding gated short-axis MRI. Preoperative wall thickening determined by gated SPECT was used as an additional parameter for prediction of functional outcome. Optimal threshold cutoffs to separate reversible from irreversible dysfunction were determined by receiver operator characteristic (ROC) analysis.

RESULTS

The sensitivity and specificity of regional 99mTc-tetrofosmin for prediction of functional recovery was 87% and 42%, respectively (cutoff: 50% of maximum tracer retention). The area under ROC curves for prediction of functional recovery measured 0.66 +/- 0.01. Segments with > or =50% uptake and impaired but detectable wall thickening determined by gated SPECT had a significantly higher likelihood for functional improvement compared with segments with absent wall thickening (P < 0.05). There was no difference in segments with <50% tracer retention. There was good agreement for ejection fraction measurements by MRI and gated SPECT (mean ejection fraction 32 +/- 12 versus 34 +/- 11; r = 0.71, P < 0.001).

CONCLUSION

Regional 99mTc-tetrofosmin uptake provided high sensitivity but limited specificity for prediction of functional recovery after revascularization, leading to fair overall accuracy. Wall thickening assessment derived from gated SPECT may improve the specificity of 99mTc-tetrofosmin uptake for prediction of functional recovery but not the sensitivity in low-flow areas. In addition to the assessment of global function, gated data acquisition can be helpful to improve the overall accuracy of 99mTc-tetrofosmin SPECT for prediction of functional recovery after bypass surgery.

Oxidative metabolism of the transplanted human heart assessed by positron emission tomography using C-11 acetate

For investigation of the metabolic profile of the transplanted human heart, positron emission tomography with C-11 acetate was performed to compare global and regional oxidative metabolism in 14 transplant patients with that in 10 healthy volunteers. Because no difference between transplants and normals could be observed, the results suggest that oxidative metabolism remains stable after transplantation, whereas cardiac work remains the major determinant.

Serial assessment of sympathetic reinnervation after orthotopic heart transplantation. A longitudinal study using PET and C-11 hydroxyephedrine

BACKGROUND

Little is known about the progressiveness of sympathetic reinnervation late after cardiac transplantation (HTX). The aim of the present study was to describe individual growth of sympathetic terminals after HTX by a longitudinal quantitative assessment.

METHODS AND RESULTS

In 20 patients after HTX, dynamic PET with C-11 hydroxyephedrine (HED) was performed twice within 3.0+/-0.5 years. According to the time interval between HTX and first PET, subgroups of patients early (group A, <1.5 years; n=7), intermediate (group B, 1.5 to 7 years; n=7) and late (group C, >7 years; n=6) after HTX were defined. At the time of first HED PET, 10 patients were completely denervated (7 in group A, 2 in group B, and 1 in group C). Only 3 remained denervated at second PET. A significant increase of reinnervated myocardium between first and second PET was found in all 3 groups (0% to 9+/-9% of left ventricle for group A, P<0.05; 13+/-12% to 23+/-17% for group B, P<0.05; 21+/-21% to 37+/-23% for group C, P<0.05). The magnitude of increase was similar between groups. Reinnervation was first surveyed in the basal anterior region, then toward apex, septal, and lateral wall. Inferior wall remained denervated. The largest reinnervated area surveyed in an individuum was 66% of the left ventricle.

CONCLUSIONS

The present data confirm the low likelihood of sympathetic reinnervation within 18 months after HTX. Once the reinnervation process is initiated, a continuous growth is observed even late after HTX, suggesting a progressive nature of reinnervation. Reinnervation, however, remained regionally heterogeneous, and a complete restoration was not found until 15 years after HTX.

Regional patterns of myocardial sympathetic denervation in dilated cardiomyopathy: an analysis using carbon-11 hydroxyephedrine and positron emission tomography

OBJECTIVE

To assess presynaptic function of cardiac autonomic innervation in patients with advanced congestive heart failure using positron emission tomography (PET) and the recently developed radiolabelled catecholamine analogue carbon-11 hydroxyephedrine (HED) as a marker for neuronal catecholamine uptake function.

DESIGN AND PATIENTS

29 patients suffering from dilated cardiomyopathy with moderate to severe heart failure were compared with eight healthy controls. Perfusion scan was followed by HED dynamic PET imaging of cardiac sympathetic innervation. The scintigraphic results were compared with markers of disease severity and the degree of sympathetic dysfunction assessed by means of heart rate variability.

RESULTS

In contrast to nearly normal perfusions, mean (SD) HED retention in dilated cardiomyopathy patients was abnormal in 64 (32)% of the left ventricle. Absolute myocardial HED retention was 10.7 (1.0)%/min in controls v 6.2 (1.6)%/min in dilated cardiomyopathy patients (p < 0.001). Moreover, significant regional reduction of HED retention was demonstrated in apical and inferoapical segments. HED retention was significantly correlated with New York Heart Association functional class (r = -0.55, p = 0. 002) and ejection fraction (r = 0.63, p < 0.001), but not, however, with plasma noradrenaline concentrations as well as parameters of heart rate variability.

CONCLUSIONS

In this study, using PET in combination with HED in patients with dilated cardiomyopathy, not only global reduction but also regional abnormalities of cardiac sympathetic tracer uptake were demonstrated. The degree of abnormality was positively correlated to markers of severity of heart failure. The pathogenetic mechanisms leading to the regional differences of neuronal damage as well as the prognostic significance of these findings remain to be defined.

Regional myocardial wall thickening and global ejection fraction in patients with low angiographic left ventricular ejection fraction assessed by visual and quantitative resting ECG-gated 99mTc-tetrofosmin single-photon emission tomography and magnetic resonance imaging

We investigated the use of visual and quantitative technetium 99m tetrofosmin ECG-gated single-photon emission tomography (SPET) for the assessment of regional myocardial wall thickening (WT) and left ventricular (LV) ejection fraction (EF) in comparison with gated magnetic resonance imaging (MRI) in patients with a low angiographic LVEF. Gated SPET using 99mTc-labelled flow tracers offers potential for simultaneous assessment of myocardial perfusion and LV function. Few data are available on the use of visual and quantitative gated SPET in patients with low LVEF. In this study 21 patients with low angiographic LVEF (mean 37%+/-5%) were studied. Resting gated 99mTc-tetrofosmin SPET and gated MRI were performed within 48 h. WT was assessed by visual interpretation (five point score) and quantitative analysis based on count increase. There was good agreement for EF measurements by MRI and gated SPET (mean EF: 33%+/-12% vs 35%+/-11%, r = 0.86, P<0.001). Areas under receiver operator characteristic curves (AUC) for differentiation between MRI WT score points ranged from 0.60 to 0.66 for visual SPET WT analysis, from 0.59 to 0.71 for delta count increase values and from 0.46 to 0.60 for % WT, indicating substantial overlap between WT categories. Absolute agreement for visual WT between MRI and gated SPET ranged from 25% to 57% (kappa 0.03-0.25) depending on tracer uptake, and was limited in areas with moderate to severe perfusion defects (kappa 0.03-0.13). It is concluded that gated SPET provided reliable estimates of regional WT and global function in patients with low angiographic LVEF.

Use of nuclear magnetic resonance imaging angiography to follow-up arterial remodeling in an animal model

Appropriately sized arteries in small animals may be possible models for studying the remodeling process as occurs after arterial balloon injury in humans. Magnetic resonance imaging (MRI) is able to noninvasively image tissue in vivo. To date, small animal angiography models have mostly used research-dedicated instruments and resolution, which are not universally available. Experiments were carried out on a rat aorta model of remodeling in vivo (n=40). Arteries were injured by oversized balloon dilation; control arteries were uninjured. Angiography imaging was performed immediately before sacrifice with an unmodified clinical MRI unit, a 1.5 Tesla MR tomograph with a 20-cm-diameter coil. Longitudinal MRI pictures of the aorta and morphometry of tissue sections to measure luminal and arterial wall areas were analyzed with use of computer-assisted techniques. Comparison of dimensions demonstrated correlation between MRI and histology measurements of the lumen. MRI and morphometry showed a gradual increase in mean luminal area over 6 weeks following injury. The lumen increase correlated with total arterial area and thickness. In this rat aorta model, remodeling documented at histology was followed-up in vivo. The use of such clinical MRI scanners has potential to reduce animal numbers needed to follow-up the remodeling process after therapeutic intervention.

Preoperative positron emission tomographic viability assessment and perioperative and postoperative risk in patients with advanced ischemic heart disease

OBJECTIVES

This study sought to investigate whether determination of tissue viability by means of positron emission tomography (PET) before coronary artery bypass graft surgery (CABG) affects clinical outcome with respect to both in-hospital mortality and 1-year survival rate.

BACKGROUND

Patients with coronary artery disease (CAD) and severe left ventricular (LV) dysfunction are at higher risk for perioperative complications associated with CABG. Therefore, the selection of patients who will benefit from CABG is an important clinical issue.

METHODS

This study retrospectively evaluated 76 patients with advanced CAD and LV dysfunction (LV ejection fraction < or = 0.35) who were considered candidates for CABG. Thirty-five patients were selected for CABG on the basis of clinical presentation and angiographic data (group A), and 34 of 41 patients were selected according to extent of viable tissue determined by PET (group B) in addition to clinical presentation and angiographic data.

RESULTS

There were four in-hospital deaths (11.4%) in group A and none in group B (p = 0.04). After 12 months, the survival rate was 79% in group A and 97% in group B (p = 0.01). Postoperatively, group B patients had a less complicated recovery (p = 0.05). They required lower doses of catecholamines (p = 0.002) and demonstrated a significantly decreased incidence of low output syndrome (p = 0.05).

CONCLUSIONS

This retrospective data analysis suggests that selection of patients with impaired LV function on the basis of extent of viability supplementary to clinical and angiographic data may lead to postoperative recovery with a low early mortality and promising short-term survival. Therefore, viability studies permit selection of patients who are at low risk for serious perioperative complications.

Assessment of myocardial reinnervation in cardiac transplants by positron emission tomography: functional significance tested by heart rate variability

While it is known that scintigraphically measured uptake of [C-11] hydroxyephedrine (HED) correlates well with the catecholamine uptake and storage in the nerve terminals of the myocardium, the functional significance of this morphologic marker remains unclear. It had been shown that normal, innervated myocardium exhibits pronounced heart rate variability (HRV). In this study, parallel to the scintigraphic determination of regional HED uptake, the functional sympathetic innervation of the left ventricle was measured by spectral analysis of HRV. Prior to PET scanning, changes in the low frequency HRV after tilt were determined as validated marker of sympathetic function in 12 patients with cardiac transplants (mean time after transplantation 4.7 years). Dynamic PET data acquisition allowed the definition of a retention index as regional marker for sympathetic nerve terminals. Intensity as well as regional extent of this marker were then compared to the results of HRV studies. Two groups were defined based on the scintigraphic findings: 8 of 12 patients displayed regionally increased HED retention (11.5 +/- 3.1%/min.) in the anteroseptal region of the left ventricle, while 4 patients showed little HED uptake (4.3 +/- 0.5%/min.). Uptake values tended to increase for longer times after surgery. Average HRV in the first group (7.57 +/- 7.47 ms2/Hz) was significantly higher (p < 0.05) compared to the low HED uptake group (1.17 +/- 1.08 ms2/Hz). HRV was lower than normal values even in the first group. These independent electrophysiological measurements prove the significance of HED retention for the functional characterization of reinnervation in cardiac transplants.

Noise analysis of ion current through the open and the sugar-induced closed state of the LamB channel of Escherichia coli outer membrane: evaluation of the sugar binding kinetics to the channel interior

LamB, a sugar-specific channel of Escherichia coli outer membrane was reconstituted into lipid bilayer membranes and the current noise was investigated using fast Fourier transformation. The current noise through the open channels had a rather small spectral density, which was a function of the inverse frequency up to about 100 Hz. The spectral density of the noise of the open LamB channels was a quadratic function of the applied voltage. Its magnitude was not correlated to the number of channels in the lipid bilayer membrane. Upon addition of sugars to the aqueous phase the current decreased in a dose-dependent manner. Simultaneously, the spectral density of the current noise increased drastically, which indicated interaction of the sugars with the binding site inside the channel. The frequency dependence of the spectral density was of Lorentzian type, although the power of its frequency dependence was not identical to -2. Analysis of the power density spectra using a previously proposed simple model (Benz, R., A. Schmid, and G. H. Vos-Scheperkeuter. 1987. J. Membr. Biol. 100: 12-29), allowed the evaluation of the on- and the off-rate constants for the maltopentaose binding to the binding site inside the LamB channels. This means also that the maltopentaose flux through the LamB channel could be estimated by assuming a simple one-site, two-barrier model for the sugar transport from the results of the noise analysis.

High-resolution one- and two-dimensional 1H MRS of human brain tumor and normal glial cells

Astrocytoma (WHO grade II, III), glioblastoma, malignant melanoma, and normal glial cell cultures, established from biopsies, were investigated by 1H MRS. At a 1H resonance frequency of 500 MHz (11.75 T) a high spectral resolution was achieved in 1D 1H spectra; in conjunction with 2D shift-correlated (COSY) MRS, resonances of alanine, aspartate, choline, creatine, glutamate, glutamine, hypotaurine, myo-inositol, phosphocreatine, phosphoryl-ethanolamine, phosphoryl-choline, lactate, lysine, N-acetylaspartate, taurine, threonine and valine could be identified. T1 relaxation times for the most prominent compounds are presented. T1 values of lactate ranged between 450 ms and 850 ms. The intensity of the lactate signal revealed differences between individual spectra, but exhibited no correlation between different tumor specimens or degree of malignancy. It was shown that the lactate signal at 1.3 ppm is covered by peaks arising from threonine and fatty acids. The choline signal level varied among spectra of different tumors, among tumors with similar degree of malignancy, and within the same tumor. Further preliminary differences due to aspartate, inositol and glutamine/glutamate were found in 1D and 2D COSY spectra between normal glial cells as well as different tumors. These results indicate that some differences observed in in vivo spectra may be attributable to secondary macroscopic structural changes (hypoxia, necrosis) and not to tumor inherent characteristics. Further correlation between in vivo and in vitro spectroscopy is therefore required.

T1 maps by K-space reduced snapshot-FLASH MRI

The T1 maps evaluated from k-space reduced Snapshot fast low angle shot (FLASH) images provide high contrast parameter images for tissue characterization in vivo of any body region. An algorithm for computing T1 values that allows a fast and reliable evaluation of T1 maps and yields reproducible values of tissue parameters in MR imaging is presented. The algorithm combined with the Snapshot FLASH inversion recovery imaging sequence permits a precise determination of T1 values, even for T1 times as low as 50 ms. Comparison with a spectroscopical inversion recovery method on identical phantoms demonstrates the accuracy of this technique. With its total acquisition time of approximately 2 s, IR Snapshot FLASH is fast enough to be used in monitoring fast T1 dynamics.