Identification and differentiation of resting myocardial ischemia and infarction in man with positron computed tomography, 18F-labeled fluorodeoxyglucose and N-13 ammonia

Autoregulation of Coronary Blood Supply in Response to Demand: JACC Review Topic of the Week

Although our coronary circulation evolved to meet demands during marked physical exertion for "fight or flight" survival, complex and multilayered control mechanisms reduce flow during other periods. Understanding homeostasis of resting flow provides essential insights into clinical pathophysiology. Several homeostatic mechanisms (myogenic, metabolic, endothelial, and neural) maintain sufficient baseline flow regardless of driving pressure (in aggregate, "autoregulation"). As a result, ventricular dysfunction does not arise until coronary perfusion pressure decreases to ∼40 mm Hg. Straightforward clinical parameters explain approximately one-half of observed absolute resting perfusion but with wide imprecision. Resting perfusion does not associate with clinical outcomes and remains unaffected by revascularization, recovery after myocardial infarction, and treating severe aortic stenosis, thereby supporting the notion that the heart was designed for peak performance.

Practical Implications of Myocardial Viability Studies

Many non-invasive methods, such as imaging tests, have been developed aiming to add a contribution to existing studies in estimating patients’ prognosis after myocardial injury. This prognosis is proportional to myocardial viability, which is evaluated in coronary artery disease and left ventricular dysfunction patients only.

While myocardial viability represents the likelihood of a dysfunctional muscle (resulting from decreased oxygen supply for coronary artery obstruction), hibernation represents post-interventional functional recovery itself.

This article proposes a review of pathophysiological basis of viability, diagnostic methods, prognosis and future perspectives of myocardial viability. An electronic bibliographic search for articles was performed in PubMed, Lilacs, Cochrane and Scielo databases, according to pre-established criteria.

The studies showed the ability of many imaging techniques in detecting viable tissues in dysfunctional areas of left ventricle resulting from coronary artery injuries. These techniques can identify patients who may benefit from myocardial revascularization and indicate the most appropriate treatment.

Status of F-18 fluorodeoxyglucose uptake in normal and hibernating myocardium after glucose and insulin loading

Objective

F-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) has been increasingly used in myocardial viability imaging. In routine PET viability studies, oral glucose and intravenous insulin loading is commonly utilized. In an optimal study, glucose and insulin loading is expected to cause FDG uptake both in hibernating and normal myocardium. However, in routine studies it is not uncommon to see absent or reduced FDG uptake in normal myocardium. In this retrospective study we further analyzed our PET viability images to evaluate FDG uptake status in myocardium under the oral glucose and intravenous insulin loading protocol that we use in our hospital.

Methods

Patients who had both myocardial perfusion single photon emission computed tomography (SPECT) and FDG PET cardiac viability studies were selected for analysis. FDG uptake status in normal and abnormal myocardial segments on perfusion SPECT was evaluated. Based on SPECT and PET findings, patients were divided into two main groups and four subgroups. Group 1 included PET viable studies and Group 2 included PET-nonviable studies. Subgroups based on FDG uptake in normal myocardium were 1a and 2a (normal uptake) and 1b and 2b (absent or significantly reduced uptake).

Results

Seventy-one patients met the inclusion criteria. Forty-two patients were PET-viable and 29 were PET-nonviable. In 33 of 71 patients (46.4%) there was absent or significantly reduced FDG uptake in one or more normal myocardial segments, which was identified more in PET-viable than PET-nonviable patients (59.5% vs. 27.5%, p = 0.008). This finding was also more frequent in diabetic than nondiabetic patients (53% vs. 31.8%), but the difference was not significant (p = 0.160).

Conclusions

In nearly half of our patients, one or more normal myocardial segments showed absent or significantly reduced FDG uptake. This finding, particularly if it is diffuse, could be from suboptimal study, inadequacy of current glucose and insulin loading protocols, or various other patient-related causes affecting FDG uptake both in the normal and hibernating myocardium. In cases with significantly reduced FDG uptake in normal myocardium, PET images should be interpreted cautiously to prevent false-negative results for viability.

Imaging Techniques for the Assessment of Myocardial Perfusion

One of the most significant causes of heart failure is coronary heart disease and subsequent left ventricular dysfunction. The prognosis and perioperative mortality are influenced by left ventricular function, which is also an important predictor marker following revascularization. The evaluation of myocardial perfusion is of utmost importance in patients who present several symptoms before choosing cardiac catheterization as treatment. The evaluation of myocardial perfusion and myocardial viability leads to superior diagnostic and treatment algorithms, thus resulting in an important improvement in the outcomes of patients with coronary artery disease. Color Doppler myocardial imaging, single-photon emission computed tomography (SPECT), contrast perfusion echocardiography, positron emission computed tomography (PET) and magnetic resonance imaging (MRI) are currently used methods for assessing myocardial perfusion. This review aims to summarize the benefits and disadvantages of each of these techniques.

Is it Feasible to Use the Commercially Available Autoquantitation Software for the Evaluation of Myocardial Viability on Small-Animal Cardiac F-18 FDG PET Scan?

PURPOSE

To evaluate the reliability of quantitation of myocardial viability on cardiac F-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) scans with three different methods of visual scoring system, autoquantitation using commercially available autoquantitation software, and infarct-size measurement using histogram-based maximum pixel threshold identification on polar-map in rat hearts.

METHODS

A myocardial infarct (MI) model was made by left anterior descending artery (LAD) ligation in rat hearts. Eighteen MI rats underwent cardiac FDG-PET-computed tomography (CT) twice within a 4-week interval. Myocardium was partitioned into 20 segments for the comparison, and then we quantitated non-viable myocardium on cardiac FDG PET-CT with three different methods: method A-infarct-size measurement using histogram-based maximum pixel threshold identification on polar-map; method B-summed MI score (SMS) by a four-point visual scoring system; method C-metabolic non-viable values by commercially available autoquantitation software. Changes of non-viable myocardium on serial PET-CT scans with three different methods were calculated by the change of each parameter. Correlation and reproducibility were evaluated between the different methods.

RESULTS

Infarct-size measurement, visual SMS, and non-viable values by autoquantitation software presented proportional relationship to each other. All the parameters of methods A, B, and C showed relatively good correlation between each other. Among them, infarct-size measurement (method A) and autoquantitation software (method C) showed the best correlation (r = 0.87, p < 0.001). When we evaluated the changes of non-viable myocardium on the serial FDG-PET-CT- however, autoquantitation program showed less correlation with the other methods. Visual assessment (method B) and those of infarct size (method A) showed the best correlation (r = 0.54, p = 0.02) for the assessment of interval changes.

CONCLUSIONS

Commercially available quantitation software could be applied to measure the myocardial viability on small animal cardiac FDG-PET-CT scan. This kind of quantitation showed good correlation with infarct size measurement by histogram-based maximum pixel threshold identification. However, this method showed the weak correlation when applied in the measuring the changes of non-viable myocardium on the serial scans, which means that the caution will be needed to evaluate the changes on the serial monitoring.

The collateral circulation of the heart in coronary total arterial occlusions in man: systematic review of assessment and pathophysiology

BACKGROUND

Anatomical and functional assessment of the collateral circulation of the heart in total arterial occlusions is challenging, and this is particularly true of the microcirculation. The pathophysiology of the collateral circulation has historically been and remains of considerable research focus but with diverging and sometimes conflicting results. Our purpose was to conduct a systematic review on the assessment and pathophysiology of the collateral circulation of the heart in total coronary arterial occlusions.

METHODS

We extracted data from Pubmed, Ovid, EMBASE, and Cochrane database from 1966 to December 2012. Two investigators independently reviewed the identified articles for eligibility and extracted the data.

RESULTS

Seventy-seven studies met inclusion criterion. An invasive assessment of the collateral circulation with pressure and/or Doppler wires is the gold standard in the assessment of collateral physiology and anatomy, although this can only be undertaken after successful passage of the sensor in the true lumen of the occluded vessel. A collateral circulation can provide resting metabolic requirements for the heart but invariably cannot meet demands on stress irrespective of the degree of collateralization as assessed by coronary angiography. In the case of myocardium subtending a totally occluded epicardial artery coronary collateral grading systems or physiological assessment of collateral flow is only moderately sensitive and poorly specific at predicting viability. Regression of collaterals seems more profound in totally occluded arteries versus nonoccluded lesions postrevascularization.

CONCLUSIONS

Key controversies in the assessment and pathophysiology of the collateral circulation of the heart in total coronary arterial occlusions are systematically evaluated.

Imaging atherosclerosis with hybrid [18F]fluorodeoxyglucose positron emission tomography/computed tomography imaging: what Leonardo da Vinci could not see

Prodigious efforts and landmark discoveries have led toward significant advances in our understanding of atherosclerosis. Despite significant efforts, atherosclerosis continues globally to be a leading cause of mortality and reduced quality of life. With surges in the prevalence of obesity and diabetes, atherosclerosis is expected to have an even more pronounced impact upon the global burden of disease. It is imperative to develop strategies for the early detection of disease. Positron emission tomography (PET) imaging utilizing [(18)F]fluorodeoxyglucose (FDG) may provide a non-invasive means of characterizing inflammatory activity within atherosclerotic plaque, thus serving as a surrogate biomarker for detecting vulnerable plaque. The aim of this review is to explore the rationale for performing FDG imaging, provide an overview into the mechanism of action, and summarize findings from the early application of FDG PET imaging in the clinical setting to evaluate vascular disease. Alternative imaging biomarkers and approaches are briefly discussed.

Magnetocardiography for the diagnosis of coronary artery disease: a systematic review and meta-analysis

BACKGROUND

Coronary artery disease (CAD) has a significant disease burden making early diagnosis and management imperative. Magnetocardiography (MCG) is a relatively new noninvasive technique that allows diagnosis of CAD by recording the magnetic fields generated by the electrical activity of the heart.

METHODS

We searched MEDLINE and the Cochrane Central Register of Controlled Trials for prospective studies that evaluated the test characteristics (e.g., sensitivity, specificity, likelihood ratios) of MCG for detection of CAD. Studies were included if they evaluated either patients with stable CAD documented by angiogram or patients presenting initially with acute coronary syndrome and subsequently diagnosed with CAD. The quality of included studies was assessed using an adaptation of the Quality Assessment of Diagnostic Accuracy Studies (QUADAS) tool. We performed meta-analyses of sensitivity, specificity and positive and negative likelihood ratios using Meta-DiSc software.

RESULTS

Screening of titles and abstracts followed by full-text review yielded seven studies that met our inclusion criteria. Meta-analyses yielded a pooled sensitivity of 83% (95% confidence interval [CI] 80% to 86%) and a specificity of 77% (95% CI 73% to 81%). The pooled positive likelihood ratio was 3.92 (95% CI 2.30 to 6.66) and negative likelihood ratio was 0.20 (95% CI 0.12 to 0.35). Significant heterogeneity was present in all meta-analyses.

CONCLUSIONS

The pooled test characteristics for MCG are similar to those of existing noninvasive modalities for diagnosing CAD. Our results suggest that MCG is a potential complementary or alternative tool for noninvasive detection of CAD.

F-18 fluorodeoxyglucose uptake and water-perfusable tissue fraction in assessment of myocardial viability

Objectives

¹⁵O-water-perfusable tissue fraction (PTF) has been shown to be a potential index for assessing myocardial viability in PET, an alternative to ¹⁸F-fluorodeoxyglucose (FDG). This study aimed to directly compare these two independent methods in assessing myocardial viability in patients with abnormal wall motion.

Methods

PET study was performed on 16 patients with previous myocardial infarction, before coronary artery bypass graft operation (CABG). The protocol included a ¹⁵O-carbonmonoxide static, a ¹⁵O-water dynamic and an ¹⁸F-FDG dynamic scan, during the euglycemic hyperinsulinemic clamp. Echocardiography was performed at the time of PET and 5–12 months after the CABG, and the wall motion recovery was evaluated on segmental and global bases. Consistency between PTF and ¹⁸F-FDG was evaluated visually and also in a quantitative manner. Predictive values for the wall motion recovery were also compared between the two approaches.

Results

The image quality of ¹⁸F-FDG was superior to that of ¹⁵O-water. The qualitative PTF showed significantly smaller defects than ¹⁸F-FDG, and the quantitative PTF showed slightly greater values than ¹⁸F-FDG in the infarcted region. The two methods were, however, consistent visually and also quantitatively. The predictive values of the wall motion recovery were almost equal between the two approaches. The absolute ¹⁸F-FDG uptake was varied in normal segments, and predictive values for the wall motion recovery by the absolute ¹⁸F-FDG was less (accuracy: 80 %) compared with those by the relative ¹⁸F-FDG (accuracy: 87 %) and the quantitative PTF (accuracy: 89 %).

Conclusion

Despite the small sample size, PTF appears to give consistent results with the ¹⁸F-FDG approach, and might be an alternative viability assessment.

Enhanced myocardial fluorodeoxyglucose uptake following Adriamycin-based therapy: Evidence of early chemotherapeutic cardiotoxicity?

AIM: To analyze changes in myocardial glucose metabolism using fluorodeoxyglucose (FDG)-positron emission tomography (PET) in patients treated with adriamycin and to investigate the clinical significance of these changes.

METHODS: Considering that FDG-PET scanning has the ability to show changes in glucose metabolism in the myocardium, we retrospectively analyzed the FDG-PET studies of 18 lymphoma patients treated with adriamycin-based chemotherapy in both the pre- and post-therapy setting. Cardiac contractile parameters such as left ventricular ejection fraction were not available for correlation in all patients due to the short duration and the level of cumulative dose administered in these patients during the time of the follow-up FDG-PET study. The change in myocardial glucose utilization was estimated by change in standard uptake values (SUV) in the myocardium.

RESULTS: We observed a significant change in SUVmean values in the myocardium (defined as more than ± 20% change in cardiac SUVmean between pre- and post-chemotherapy PET) in 12 patients, whereas 6 patients did not show any significant cardiac FDG uptake in both pre- and post-therapy PET scans. Patients were divided into three groups based on the changes observed in myocardial tracer uptake on the follow-up ¹⁸F-FDG-PET study. Group A (n = 8): showed an increase in cardiac ¹⁸F-FDG uptake in the post-therapy scan compared to the baseline scan carried out prior to starting adriamycin-based chemotherapy. Group B (n = 6): showed no significant cardiac ¹⁸F-FDG uptake in post-therapy and baseline PET scans, and group C (n = 4): showed a fall in cardiac ¹⁸F-FDG uptake in the post-therapy scan compared to the baseline scan. Mean cumulative adriamycin dose (in mg/m²) received during the time of the follow-up FDG-PET study was 256.25, 250 and 137.5, respectively.

CONCLUSION: Our study shows three different trends in the change in myocardial glucose metabolism in patients undergoing adriamycin-based chemotherapy. A further prospective study with prolonged follow-up of ventricular function is warranted to explore the significance of enhanced FDG uptake as a marker of early identification of adriamycin-induced cardiotoxicity.

Acipimox-enhanced ¹⁸F-fluorodeoxyglucose positron emission tomography for characterizing and predicting early remodeling in the rat infarct model

The rat myocardial infarction (MI) model is widely used to study left ventricular (LV) remodeling. In this study, acipimox-enhanced (18)F-Fluorodeoxyglucose (FDG) gated-positron emission tomography (PET) was assessed for characterizing and predicting early remodeling in the rat infarct model. Nineteen Wistar rats had surgical occlusion of the left anterior descending coronary artery and 7 were sham-operated. PET was scheduled 48 h and 2 weeks later for quantifying MI area and LV function. Segments with <50% of FDG uptake had histological evidence of MI (74 ± 9% decrease in parietal thickness, fibrosis development). At 48 h, MI area was large (>35% of LV) in 6 rats, moderate (15-35% of LV) in 8 rats, limited (<15% of LV) in 5 rats and absent in the 7 sham rats. LV remodeling, assessed through the 2 weeks increase in end-diastolic volume, increased between rats with limited, moderate and large MI (+72 ± 25, +109 ± 56, +190 ± 69 μl, respectively, P = 0.007). This 3-groups classification allowed predicting 44% of the 2 weeks increase in end-diastolic volume, and additional 34% were predicted by heart rate at 48 h. The acipimox-enhanced FDG gated-PET technique provides efficient characterization and prediction of early remodeling in the rat infarct model.

The role of cardiac PET in translating basic science into the clinical arena

Non-invasive imaging has become fundamental in translating findings from basic science research into clinical applications. In this aspect, positron-emission tomography (PET) offers important advantages over other common imaging modalities like single-photon emission computed tomography, computed tomography, and magnetic resonance imaging (MRI), since PET provides superior detection sensitivity in the evaluation of different cardiovascular targets and pathways at the cellular and subcellular level, and because it is a well-established technique for absolute image quantification. The development and the introduction of dedicated small animal PET systems have greatly facilitated and contributed to advancements in the translation of novel radio-labeled compounds from experimental to clinical practice. The scope of the present article is to review the most relevant and successful PET applications in cardiovascular translational research.

Practical assessment of myocardial viability with a positron coincidence gamma camera using 18F-fluorodeoxyglucose in acute myocardial infarction: comparison with dedicated positron emission tomography and 201Tl single photon emission computed tomography

OBJECTIVE

2-[(18)F] fluoro-2-deoxy-D-glucose ((18)F-FDG) began to be supplied commercially to our hospital, which does not have a cyclotron, in autumn of 2005. The purpose of this study was to compare the utility of a dual-head positron coincidence detection (PCD) gamma camera in the detection of myocardial viability using (18)F-FDG with that of dedicated positron emission tomography (PET) and with that of thallium-201 ((201)Tl) single photon emission computed tomography (SPECT).

METHODS

A total of 15 patients (14 men and 1 woman, mean age: 60+/-7 years, range: 46-73) with a large acute myocardial infarction (AMI) underwent (18)F-FDG PET, (18)F-FDG PCD imaging after oral glucose loading (75 g) and (201)Tl SPECT imaging. We divided the SPECT and PET images into a total of 20 segments, and semiquantitative visual analysis was performed by assessing regional tracer activities on a 4-point scoring system (DS): 0=normal uptake, 1=mildly reduced uptake, 2=severely reduced uptake, and 3=no uptake. We summed the DS in each patient as the total DS (TDS).

RESULTS

The TDS of the (18)F-FDG PET image was 14.4+/-7.7. The TDS of the (18)F-FDG PCD image was 18.7+/-7.7. The TDS of the (201)Tl SPECT image was 24.1+/-11.5. The TDS of the (18)F-FDG PET image was significantly smaller than that of the (18)F-FDG PCD image. The TDS of the (18)F-FDG PET image was significantly smaller than that of the (201)Tl SPECT image. The TDS of the (18)F-FDG PCD image was significantly smaller than that of the (201)Tl SPECT image.

CONCLUSION

The findings of the project suggest that (18)F-FDG PCD is a good modality based on its accuracy, convenience, and cost-performance for detecting myocardial viability in hospitals that do not have a PET system.

Prognostic performance of quantitative PET tools for stratification of patients with ischemic cardiomyopathy undergoing myocardial viability assessment

OBJECTIVES

This study was performed to determine the prognostic performance of quantitative PET tools in the stratification of patients with ischemic cardiomyopathy undergoing myocardial viability assessment.

METHODS

We applied four different quantitative tools to 104 consecutive patients with coronary artery disease and previous myocardial infarction who had undergone rest Rb/gated F-fluorodeoxyglucose (FDG) PET, to assess myocardial viability for potential revascularization. One of these tools was based on the FDG study alone and the other three tools assessed the extent of match/mismatch defects using FDG in comparison with a perfusion reference database. The four quantitative tools used in this research to define viability were (i) FDG alone, which calculates the percentage of left ventricular myocardium (LVM) that is above the 50% of the maximum LVM FDG counts, (ii) low flow match/mismatch, which determines the area with a 5% increase in normalized FDG counts in relation to defined resting perfusion defects as compared with a reference database, (iii) all regions match/mismatch, which computes the area with a 10% increase in normalized FDG counts in relation to the left ventricle resting perfusion distribution, and (iv) percentage max FDG match/mismatch, which defines the area with FDG uptake greater than 60% of the maximum LVM FDG counts within defined perfusion defects as determined by the reference database. The primary endpoint for this analysis was cardiac death.

RESULTS

During the follow-up period (22+/-14 months), 19 patients (18%) died; in 17 of these the cause of death was cardiac. Using univariate analysis, none of the methods were predictive of cardiac death. Receiver operating characteristic analysis defined the optimal thresholds for the extent of myocardial viability for the four tools in the prediction of cardiac death: FDG alone=20%, low flow match/mismatch=15%, all regions match/mismatch=35%, and percentage max FDG match/mismatch=20%. A censored survival analysis using a Kaplan-Meier method showed a statistically significant difference between patients with cardiac death and those with no cardiac death using only the low flow match/mismatch (hazard ratio=0.29, P=0.01) and percentage max FDG match/mismatch criteria (hazard ratio=0.23, P=0.005) tools.

CONCLUSION

The low flow match/mismatch and percentage max FDG match/mismatch quantitative PET tools are useful for prognostic stratification of patients with ischemic cardiomyopathy undergoing myocardial viability assessment.

Low-dose dobutamine radionuclide ventriculography for prediction of myocardial viability: quantitative analysis of regional left ventricular function

BACKGROUND

It is important to distinguish viable myocardium from necrotic tissue in order to decide upon therapy in patients with ischemic heart disease.

HYPOTHESIS

We verified the hypothesis that quantitative analysis of regional left ventricular function using low-dose dobutamine radionuclide ventriculography (RNV) can sensitively predict myocardial viability and compared its usefulness with thallium-201 (201Tl) single-photon emission computed tomography (201Tl-SPECT).

METHODS

Radionuclide ventriculography at rest and during low-dose dobutamine infusion (5 micrograms/kg/min), 201Tl-SPECT, and coronary angiography were performed in 51 subjects with severe ischemia-related stenosis of coronary arteries and 3 subjects without coronary artery disease. 201Tl uptake was assessed as normal (control), low perfusion (LP), or defect. We compared the response of regional function to dobutamine with the regional 201Tl uptake. The accuracy of both methods for identifying viable myocardium was investigated in 17 patients who underwent successful coronary revascularization, with a resulting improvement in wall motion.

RESULTS

The increase in regional ejection fraction (delta r-EF) in response to dobutamine was significantly greater in the control (12 +/- 6%) and LP (16 +/- 11%) regions than in the defect (5 +/- 10%) regions. The increase in one-third regional ejection fraction (delta r-1/3EF) was also significantly higher in the control (14 +/- 7%) and LP (10 +/- 8%) regions than in the defect regions (5 +/- 6%). We defined myocardial viability as a delta r-EF > 5% or a delta r-1/3EF > 2%. The sensitivity and specificity of the delta r-EF for identification of myocardial viability were 91.4 and 55.5%, respectively. The sensitivity and specificity of the delta r-1/3EF were 91.4 and 66.6%, respectively; the corresponding values for 201Tl SPECT were 74.2 and 77.8%.

CONCLUSION

Low-dose dobutamine RNV with quantitative analysis of regional left ventricular function was more sensitive for identification of viable myocardium than 201Tl-SPECT.

Ischemia-induced activation of AMPK does not increase glucose uptake in glycogen-replete isolated working rat hearts

Alterations in myocardial glucose metabolism are a key determinant of ischemia-induced depression of left ventricular mechanical function. Since myocardial glycogen is an important source of endogenous glucose, we compared the effects of ischemia on glucose uptake and utilization in isolated working rat hearts in which glycogen content was either replete (G replete, 114 micromol/g dry wt) or partially depleted (G depleted, 71 mumol/g dry wt). The effects of low-flow ischemia (LFI, 0.5 ml/min) on glucose uptake, glycogen turnover (glycogenolysis and glycogen synthesis), glycolysis, adenosine 5'-monophosphate-activated protein kinase (AMPK) activity, and GLUT4 translocation were measured. Relative to preischemic values, LFI caused a time-dependent reduction in glycogen content in both G-replete and G-depleted groups due to an acceleration of glycogenolysis (by 12-fold and 6-fold, respectively). In G-replete hearts, LFI (15 min) decreased glucose uptake (by 59%) and did not affect GLUT4 translocation. In G-depleted hearts, LFI also decreased initially glucose uptake (by 90%) and glycogen synthesis, but after 15 min, when glycogenolysis slowed due to exhaustion of glycogen content, glucose uptake increased (by 31%) in association with an increase in GLUT4 translocation. After 60 min of LFI, glucose uptake, glycogenolysis, and glycolysis recovered to near-preischemic values in both groups. LFI increased AMPK activity in a time-dependent manner in both groups (by 6-fold and 4-fold, respectively). Thus, when glycogen stores are replete before ischemia, ischemia-induced AMPK activation is not sufficient to increase glucose uptake. Under these conditions, an acceleration of glycogen degradation provides sufficient endogenous substrate for glycolysis during ischemia.

Attenuation correction in cardiac PET/CT with three different CT protocols: a comparison with conventional PET

PURPOSE

CT-based attenuation correction may influence cardiac PET owing to its higher susceptibility to misalignment compared with conventional (68)Ge transmission scans. The aims of this study were to evaluate whether CT attenuation correction leads to changes in tracer distribution compared with conventional cardiac PET and to determine a suitable CT protocol.

METHODS

A total of 27 patients underwent PET/CT and subsequently a PET scan. Twenty patients received a low-dose CT (LDCT group; 120 kV, 26 mA, 8-s scan time), seven patients a slow CT (SCT group; 120 kV, 99 mA, 46-s scan time) and ten patients an ultra-low-dose CT (ULDCT group; 80 kV, 13 mA, 5-s scan time) as the transmission scan in PET/CT. Polar maps were divided into 17 segments and regression analysis was computed in every scan pair (CT attenuation corrected-(68)Ge attenuation corrected). Correlation coefficient (r), the slope (s) and the offset (os) of the regression line were determined. Visual assessment of misalignment between the transmission and emission data was performed. The effective dose of the different transmission scans was calculated.

RESULTS

Overall, there was a moderate correlation between the mean values measured in all segments on PET/CT and on PET when using LDCT (r=0.78, p<0.0001), SCT (r=0.79, p<0.0001) and ULDCT (r=0.82, p<0.0001). No differences were observed when comparing the scores assigned in the visual misalignment assessment in the three groups (p=0.12). The differences between the results from the regression analysis observed in the respective groups were not statistically significant (Kruskal-Wallis p=0.11 for r, p=0.67 for s and p=0.27 for os). The effective dose was lowest for the ULDCT.

CONCLUSION

Our study shows that CT-based attenuation correction is feasible for cardiac PET imaging. The results indicate that ultra-low-dose CT is the preferable choice for transmission scanning.

Accuracy of 3D acquisition mode for myocardial FDG PET studies using a BGO-based scanner

PURPOSE

The aim of the present study was to evaluate the quantitative and qualitative accuracy of 3D PET acquisitions for myocardial FDG studies.

METHODS

Phantom studies were performed with both a homogeneous and an inhomogeneous phantom. Activity profiles were generated along the phantoms using 2D and several 3D reconstructions, varying the 3D scaling value to adjust the scatter correction algorithm. Furthermore, ten patients underwent a dynamic myocardial FDG PET scan, using an interleaved protocol consisting of frames with alternating 2D and 3D acquisition. For each myocardial study, 13 volumes of interest were defined, representing 13 myocardial segments. First, the optimal scaling value for the scatter correction algorithm was determined using data from the phantom and four patient studies. This scaling value was then applied to all ten patients. 2D and 3D acquisitions were compared for both static (i.e. activity concentrations in the last 2D and 3D frames) and dynamic imaging (calculation of the metabolic rate of glucose).

RESULTS

For both phantom and patient studies, suboptimal results were obtained when the default scaling value for the scatter correction algorithm was used. After adjusting the scaling value, for all ten myocardial FDG studies, a very good correlation (r2=0.99) was obtained between 2D and 3D data. With the present protocol no significant differences were observed in qualitative interpretation.

CONCLUSION

The 3D FDG acquisition mode is accurate and has clear advantages over the 2D mode for myocardial FDG studies. A prerequisite is, however, optimisation of the 3D scatter correction algorithm.

Mechanisms leading to reversible mechanical dysfunction in severe CAD: alternatives to myocardial stunning

Patients with severe chronic coronary artery disease (CAD) exhibit a highly altered myocardial pattern of perfusion, metabolism, and mechanical performance. In this context, the diagnosis of stunning remains elusive not only because of methodological and logistic considerations, but also because of the pathophysiological characteristics of the myocardium of these patients. In addition, a number of alternative pathophysiological mechanisms may act by mimicking the functional manifestations usually attributed to stunning. The present review describes three mechanisms that could theoretically lead to reversible mechanical dysfunction in these patients: myocardial wall stress, the tethering effect, and myocardial expression and release of auto- and paracrine agents. Attention is focused on the role of these mechanisms in scintigraphically “normal” regions (i.e., regions usually showing normal perfusion, glucose metabolism, and cellular integrity as assessed by nuclear imaging techniques), in which stunning is usually considered, but these mechanisms could also operate throughout the viable myocardium. We hypothesize that reversion of these three mechanisms could partially explain the unexpected functional benefit after reperfusion recently highlighted by high-spatial-resolution imaging techniques.

Diagnostic and imaging considerations: role of viability

Left ventricular systolic dysfunction is a recognised feature of heart failure. In developed nations, the leading cause of left ventricular systolic dysfunction is coronary artery disease. Revascularisation is a treatment strategy for patients with predominant symptoms of heart failure and significant left ventricular dysfunction. Presence or absence of myocardial viability has been shown to affect outcome after revascularisation. There are various techniques to assess myocardial viability. However, limitations of current literature, lack of completed randomised trials and high peri-procedural trials create significant uncertainty about the optimal strategy. This review focuses on the role of non-invasive testing for myocardial viability in patients with left ventricular systolic dysfunction and heart failure and also outlines the pros and cons of each technique.

Single-shot inversion recovery TrueFISP for assessment of myocardial infarction

OBJECTIVE

The aim of the study was to assess the diagnostic accuracy of imaging the myocardium with a fast multislice inversion recovery 2D single-shot true fast imaging with steady-state precession (trueFISP) sequence during a single breath-hold in comparison with an established segmented inversion recovery turbo fast low-angle shot (turboFLASH) sequence.

SUBJECTS AND METHODS

Forty-three patients with myocardial infarction were examined on a 1.5-T MR system 10 min after administration of contrast material (gadodiamide, 0.2 mmol/kg) with a single-shot 2D multislice technique (single-shot inversion recovery trueFISP) that allows one to image the entire short axis during one breath-hold (18 heartbeats) and with a segmented 2D single-slice technique (inversion recovery turboFLASH) that requires one breath-hold per slice (12 heartbeats). Signal intensity was determined in normal myocardium, in infarcted myocardium, and in the left ventricle. The contrast-to-noise ratio (CNR) of normal and infarcted myocardium was determined. The areas of hyperintense infarctions on selected slices and the entire volumes were compared for both sequence techniques.

RESULTS

The inversion recovery trueFISP sequence has a lower CNR than the inversion recovery turboFLASH sequence (mean values, 10.0 vs 12.9, respectively; p = 0.005) for differentiation of viable from nonviable myocardium. The CNR of injured myocardium and blood in the left ventricular cavity also has a lower value for the multislice technique compared with the single-slice technique (0.6 vs 1.2, respectively; p = 0.045). Assessment of the area of infarction within one slice (r = 0.97, p < 0.002) and of the volume of the entire infarction (r = 0.96, p < 0.003) is possible with excellent correlation of both techniques.

CONCLUSION

Despite having a lower CNR, the inversion recovery 2D single-shot trueFISP sequence allows fast and accurate identification of the area and volume of infarction with high spatial resolution within a single breath-hold.

Evidence of myocardial hibernation in the septic heart*

OBJECTIVE

Myocardial hibernation is an adaptive response to ischemia and hypoxia. Hibernating cardiomyocytes are reversibly hypocontractile and demonstrate characteristic metabolic and ultrastructural changes. These include a switch in primary substrate utilization from fatty acids to glucose, up-regulation of the myocardial specific glucose transporters (GLUT1 and GLUT4), and glycogen deposition within and between cardiomyocytes. We hypothesized that myocardial hibernation may underlie sepsis-associated myocardial depression.

DESIGN

Prospective observational study aimed at identifying the characteristic changes of hibernation in the septic heart.

SETTING

University hospital-based laboratory.

SUBJECTS

Forty-three C57Bl6 male mice.

INTERVENTIONS

Mice underwent cecal ligation and double puncture, sham operation, or no operation and were evaluated 48 hrs after the procedure.

MEASUREMENTS AND MAIN RESULTS

Using novel, clinically relevant technology such as magnetic resonance imaging, positron emission tomography, and single photon emission computed tomography imaging, we found septic mice to have diminished cardiac performance, increased myocardial glucose uptake, increased steady-state levels of myocardial GLUT4, and increased deposits of glycogen, recapitulating the changes during hibernation. Importantly, these changes occurred in the setting of preserved arterial oxygen tension and myocardial perfusion.

CONCLUSIONS

Sepsis-associated cardiac dysfunction may reflect hibernation. Furthermore, such down-regulation of cellular function may underlie sepsis-induced dysfunction in other organ systems.

The Extent of Perfusion–F18-Fluorodeoxyglucose Positron Emission Tomography Mismatch Determines Mortality in Medically Treated Patients With Chronic Ischemic Left Ventricular Dysfunction

OBJECTIVES

The purpose of this study was to assess the determinants of mortality in a large group of patients with ischemic cardiomyopathy who are treated medically and the impact of the extent of viable tissue on prognosis.

BACKGROUND

Whether the presence of viability drives mortality in patients with ischemic cardiomyopathy who are treated medically and whether the extent of viability is important are issues that are currently unclear.

METHODS

Two hundred sixty-one patients with ischemic cardiomyopathy underwent positron emission tomography (PET) for assessment of viability. Prospective follow-up was obtained.

RESULTS

Ninety-four patients were revascularized and 167 were not. The cardiac death rate was significantly less in the revascularized patients as compared with medically treated patients (13% vs. 24%, p < 0.05). In the revascularized patients, there was a trend toward better survival in patients with viable myocardium as compared with nonviable myocardium (3.5-year survival, 85% and 75% respectively, p = NS). In the medically treated group, age (hazard ratio [HR] 2.1, 95% confidence interval [CI] 1.2 to 3.7), presence of left bundle branch block (HR 3.4, 95% CI 1.6 to 7.2) and extent of perfusion-metabolism mismatch on PET (HR 1.36, 95% CI 1.1 to 1.6) predicted cardiac death during a median follow-up period of 2.1 years. The risk of cardiac death was not significantly increased when the extent of mismatch was < or =20% (HR 0.97, 95% CI 0.46 to 2.05) but was significantly increased when the extent of mismatch was >20% (HR 3.21, 95% CI 1.38 to 7.49).

CONCLUSIONS

Medically treated patients with ischemic cardiomyopathy and large areas of viable myocardium on PET are at high risk for cardiac death.

Quantitative analysis of myocardial glucose utilization in patients with left ventricular dysfunction by means of 18F-FDG dynamic positron tomography and three-compartment analysis

PURPOSE

Myocardial glucose utilization (MGU) is altered in various heart diseases. The aim of this study was to quantitatively assess regional myocardial glucose utilization in patients with left ventricular (LV) dysfunction by dynamic( 18)F-fluorodeoxyglucose positron emission tomography (FDG PET).

METHODS

A total of 18 subjects were studied, including ten with LV dysfunction (seven with idiopathic dilated cardiomyopathy and three with aortic regurgitation; NYHA II in 8 and III in 2) and eight healthy normal volunteers. Patients with diabetes mellitus were excluded. A dynamic PET study was performed for 40 min following the injection of 370 MBq of FDG after 50-g glucose loading. On the basis of a three-compartment model, MGU, K1, k2, and k3 were computed on a pixel by pixel basis to generate LV myocardial parametric maps. FDG standardized uptake value (SUV) was also calculated using static images obtained 40 min after FDG injection. These metabolic values were compared with myocardial flow distribution (%Flow), LVEF, LV volumes, and LV wall thickening (WT) determined by gated myocardial single-photon emission computed tomography using QGS software in eight myocardial segments.

RESULTS

MGU correlated positively with LV volumes and negatively with LVEF. K(1) was significantly higher in the segments of the patients than in those of the normal volunteers (0.082+/-0.055 vs 0.041+/-0.017 ml min(-1) g(-1), p<0.05), although there was no difference in MGU between the groups. On the other hand, SUV, k2, and k3 did not differ significantly between the groups. Among the patients, the K1 values were significantly higher in the areas with impaired WT (%WT<17%) (0.109+/-0.063 vs 0.069+/-0.062 ml min(-1) g(-1), p<0.05) and in the areas with flow reduction (%Flow<71%) (0.112+/-0.076 vs 0.071+/-0.046 ml min(-1) g(-1), p<0.05).

CONCLUSION

These results indicate that glucose utilization was preserved in the patients with LV dysfunction, mainly due to an increase in glucose transport, particularly in the regions with severely impaired LV function. Thus, the quantitative assessment of myocardial glucose utilization by FDG dynamic PET may provide useful information for assessing the regional myocardial metabolic status in patients with LV dysfunction.

Myocardial glucose metabolism assessed by positron emission tomography and the histopathologic findings of microvessels in syndrome X

BACKGROUND

Syndrome X has been recognized as a disease that is primarily reflected in the cardiac microvasculature. Myocardial metabolism in this condition has been studied, but not in relation to small vessel pathology.

METHODS AND RESULTS

In order to examine the relationship between myocardial metabolism and small vessel pathology, 24 consecutive patients with syndrome X (7 men, 17 women; mean age 58 years) were evaluated by the thallium exercise stress test, positron emission tomography using F-18 fluoro-deoxyglucose (FDG), and an endomyocardial biopsy. All patients showed either diffuse or focal increase in the myocardial uptake of FDG, but only 17 patients (71%) showed hypoperfused areas with partial or complete redistribution in the thallium study. Quantification of myocardial FDG uptake revealed that the value in syndrome X patients was 10-fold higher than in controls (p<0.0001). Histopathological examination revealed that in syndrome X there is an extensive increase in smooth muscle cells and thickening of the vascular wall, even in capillary vessels, and the small vessel lumen was markedly narrowed. There was a significant inverse correlation between FDG myocardial uptake and the microvessel luminal area.

CONCLUSIONS

In syndrome X patients, myocardial FDG uptake is increased extensively, which is strongly associated with narrowed myocardial microvasculature.

Contrast-enhanced MR imaging of the heart: overview of the literature

The use of magnetic resonance (MR) imaging for cardiac diagnosis is expanding, aided by the administration of paramagnetic contrast agents for a growing number of clinical applications. This overview of the literature considers the principles and applications of cardiac MR imaging with an emphasis on the use of contrast media. Clinical applications of contrast material-enhanced MR imaging include the detection and characterization of intracardiac masses, thrombi, myocarditis, and sarcoidosis. Suspected myocardial ischemia and infarction, respectively, are diagnosed by using dynamic first-pass and delayed contrast enhancement. Promising new developments include blood pool contrast media, labeling of myocardial precursor cells, and contrast-enhanced imaging at very high fields.

Myocardial viability evaluation using magnetocardiography in patients with coronary artery disease

OBJECTIVE

Magnetocardiography (MCG) has been used to risk stratify patients in terms of sudden death or to detect ischemia. We evaluated the potential of this technique to assess myocardial viability in coronary artery disease.

METHODS

Fifteen patients aged 36-75 (median, 59) years with stable single-vessel disease (> or =70% diameter stenosis) and corresponding regional wall-motion abnormality underwent (1) echocardiography to evaluate wall motion, (2) Tl dipyridamole single-photon emission computed tomography to document perfusion and (3) quantitative F-fluorodeoxyglucose positron emission tomography to assess viability in 16 left-ventricular wall segments. MCG was performed in each patient using a shielded prototype 49-channel low-temperature superconducting quantum interference device (SQUID) system. Multiple time and area parameters were extracted automatically from each baseline-corrected data set.

RESULTS

Eleven patients had prior myocardial infarction. In each patient, four to 12 (median, seven) segments were lesion dependent, totalling up to 117 out of 240 segments. A total of 88 segments (75%) were viable and 29 segments (25%) represented scar. Patients were divided into three categories: (a) no scar segments (five patients), (b) scar in one to three segments (six patients) and (c) scar in > or = four segments (four patients). The three MCG parameters with the best selectivity were identified using linear discriminant analysis with forward inclusion (P<0.10). The corresponding Fisher's discriminant functions classified all patients correctly (Wilks' lambda=0.079).

CONCLUSION

Selected MCG parameters yielded accurate patient classification with regard to the extension of myocardial scar within the viable tissue in retrospect. These findings indicate that MCG may contribute to the assessment of myocardial viability. Further evaluation in a comprehensive multicenter study is warranted.

Myocardial perfusion and glucose uptake coupling in CAD patients

PURPOSE

To evaluate coronary artery disease (CAD) patients regarding to their perfusion-glucose uptake relationship at rest for all myocardial regions and to determine whether this evaluation could typify patients with different positron emission tomography (PET)-pattern proportions and pathophysiological characteristics.

METHODS

Rest/dipyridamole H(15)2O and 18FDG PET studies were performed in 23 patients with left ventricular dysfunction. Regional index (relative perfusion, %H(15)2O; relative glucose uptake, %18FDG) allowed to detect PERFUSION-metabolism mismatch (i.e. hibernation) and dipyridamole-induced reversible stress defects (RSD).

RESULTS

The correlation (r) between %H(15)2O and % 18FDG at rest allowed definition of three groups: correlated (CORR; r > 0.7; n = 10), semicorrelated (SEMI; 0.5 < r < or = 0.7; n = 6) and uncorrelated (UNCO; r < or = 0.5; n = 7). In UNCO, 96% of regions had a %H(15)2O > or = 55% (p < 0.01 vs. 89 and 82% in SEMI and CORR) and 95% of regions had a %18FDG > or = 55% (p < 0.01 vs. 78 and 71% in SEMI and CORR). Mismatch proportions increased from CORR to SEMI and UNCO (11, 19 and 27%; p < 0.02) and proportion of regions with RSD was higher in UNCO and SEMI (25 and 24 vs. 6% in CORR; p < 0.01). Proportion of mismatch with RSD was at least three fold higher in UNCO (17/58) (p < 0.01 vs. 3/33 and 1/16 in SEMI and CORR).

CONCLUSIONS

Analysis of perfusion and glucose uptake at rest allowed to typify three categories of CAD patients with different PET-patterns proportions, distinctive ranges of perfusion and glucose uptake and distinctive hyperemic response. Our results suggest that myocardial hibernation associated with defective hyperemic response is specific of patients with preserved perfusion and glucose uptake.

Detection of scarred and viable myocardium using a new magnetic resonance imaging technique: blood oxygen level dependent (BOLD) MRI

BACKGROUND

The identification of viable myocardium in patients with impaired left ventricular contraction secondary to coronary heart disease is important clinically as such myocardium is likely to benefit from revascularisation. Blood oxygen level dependent (BOLD) magnetic resonance imaging (MRI) relies on changes in deoxyhaemoglobin concentration under stress for signal generation and could be used for the differentiation between scarred and viable myocardium.

AIM

To assess the signal change on BOLD MRI in viable and scarred myocardium as identified by positron emission tomography (PET).

METHOD

19 patients with impaired left ventricular contraction and at least one akinetic area were enrolled. They underwent rest and dipyridamole stress MRI, using a double breath hold T2* weighted, ECG gated sequence to produce BOLD contrast images, and cine-MRI for wall thickening assessment. Dynamic perfusion and metabolic PET images followed the MRI. Signal change on BOLD MRI and the wall thickening were compared between rest and stress images in hibernating and scarred segments identified by PET on two short axis slices of mid ventricle, with eight segments each.

RESULTS

Using PET, 68 segments were identified as hibernating and 42 as scarred. The hibernating segments were found on BOLD MRI to have an average signal change between rest and stress of -9.53%, compared with -2.15% in the scarred segments (p = 0.008). The average wall thickening was 8.7 mm in the hibernating segments compared with 5.9 mm in the scarred segments (p < 0.0001).

CONCLUSIONS

BOLD MRI with wall thickening may differentiate scarred and viable myocardium and help identify suitable patients for revascularisation. Further larger studies are needed to establish a threshold for detection, sensitivity, and specificity.

AMP-activated protein kinase (AMPK) control of fatty acid and glucose metabolism in the ischemic heart

Myocardial ischemia is the leading cause of all cardiovascular deaths in North America. Myocardial ischemia is accompanied by profound changes in metabolism including alterations in glucose and fatty acid metabolism, increased uncoupling of glucose oxidation from glycolysis and accumulation of protons within the myocardium. These changes can contribute to a poor functional recovery of the heart. One key player in the ischemia-induced alteration in fatty acid and glucose metabolism is 5'AMP-activated protein kinase (AMPK). Accumulating evidence suggest that activation of AMPK during myocardial ischemia both increases glucose uptake and glycolysis while also increasing fatty acid oxidation during reperfusion. Gain-of-function mutations of AMPK in cardiac muscle may also be causally related to the development of hypertrophic cardiomyopathies. Therefore, a better understanding of role of AMPK in cardiac metabolism is necessary to appropriately modulate its activity as a potential therapeutic target in treating ischemia reperfusion injuries. This review attempts to update some of the recent findings that delineate various pathways through which AMPK regulates glucose and fatty acid metabolism in the ischemic myocardium.

Assessment of myocardial viability with a positron coincidence gamma camera using fluorodeoxyglucose in comparison with dedicated PET

The use of dual-head gamma camera modified positron coincidence detection (PCD) is a new, alternative method of 2-[18F]fluoro-2-deoxy-D-glucose (FDG) imaging. This study investigated the potential ability of evaluating myocardial viability in patients with ischaemic heart disease by FDG imaging using PCD. A total of 21 patients (18 male, three female; mean age 59.7+/-8.5 years) with a history of previous myocardial infarction and confirmed coronary angiography underwent FDG PCD and FDG PET after oral glucose loading (75 g). Quantitative analysis was compared between images of FDG PCD and FDG PET. A significant linear correlation between the segmental percentage of FDG uptake obtained by PCD and PET was observed (r=0.63, P<0.001). By receiver operating characteristic (ROC) analysis, using FDG PET as the 'gold standard', at the 50% threshold value in PET, FDG PCD showed a sensitivity of 92% and specificity of 63% in detecting myocardial viability. Regional analysis showed lower agreement of FDG PCD and FDG PET in the inferior (79%) and septal (70%) walls compared with the other walls. Quantitative evaluation of myocardial viability using FDG PCD yielded comparable clinical results in apex, anterior and lateral walls to that of FDG PET. However, the agreement was lower in the inferior and septal walls. Therefore, results of FDG PCD should be carefully interpreted in evaluating myocardial viability in the inferior and septal walls. The application of a measured attenuation correction and scatter correction are needed to improve the detectability of myocardial viability in FDG imaging by coincidence gamma camera.

Value and limitation of myocardial fluorodeoxyglucose single photon emission computed tomography using ultra-high energy collimators for assessing myocardial viability

Single photon emission computed tomography (SPECT) using ultra-high energy collimators permits wide clinical application of (18)F-fluorodeoxyglucose (FDG) imaging without the use of expensive positron emission tomography (PET) cameras. This study was designed to evaluate the value of FDG SPECT using ultra-high energy collimators in assessing myocardial viability compared with FDG PET on a regional basis. We prospectively studied 33 patients with ischaemic heart disease. The patients were injected with 555 MBq of FDG under a hyperinsulinaemic glucose clamp, and FDG PET was performed 40 min later. FDG SPECT using ultra-high energy collimators was performed immediately after FDG PET. The images of the left ventricular myocardium were divided into nine segments and the regional defect score was assessed visually using a four-point scale (0=normal to 3=defect). Regional FDG uptake (%uptake) was quantitatively analysed using polar maps. In 297 segments of all the 33 patients, agreement between the defect scores based on FDG SPECT images and those based on FDG PET images was 70%, and agreement within one rank was 96% (kappa value=0.52). The %uptake based on FDG SPECT images significantly correlated with that based on FDG PET images (r=0.77, P<0.01). However, the defect scores in the inferior wall based on FDG SPECT images were higher (1.41+/-1.14) than those based on FDG PET images (1.06+/-1.12, P<0.01). When the viable region is defined as %uptake > or =50% in FDG PET studies, the optimal cut-off level of %uptake based on FDG SPECT images was 60% in the anterior wall, apex, septum and lateral wall (accuracies, 97%, 93%, 96% and 99%, respectively), and 45% in the inferior wall (accuracy, 99%). It is concluded that FDG SPECT using ultra-high energy collimators can be used for the assessment of myocardial viability as accurately as FDG PET. However, a slight difference was observed in the defect scores mainly due to attenuation in the inferior wall. Therefore, a slightly different cut-off level for assessing myocardial viability should be applied to the inferior wall when using FDG SPECT.

Myocardial viability assessment with gated SPECT Tc-99m tetrofosmin % wall thickening: comparison with F-18 FDG-PET

OBJECT

This study was designed to assess the value of gated SPECT Tc-99m-tetrofosmin (TF) wall thickening (WT) in addition to TF exercise (Ex)/rest myocardial SPECT, in comparison with F-18 fluorodeoxyglucose (FDG)-PET.

METHODS

The study population consisted of 33 patients with old myocardial infarction (27 men and 6 women; mean age, 62 +/- 8 years old). All patients underwent Ex/rest TF SPECT and glucose loading FDG-PET. Polar map images of Ex/rest TF were generated and divided into 24 segments for further analysis. We classified LV segments according to the exercise-rest perfusion scintigraphy. LV segments with less than 70% of the maximum TF activity on the exercise image were defined as stress-induced defects. Among these, the segments whose TF activity increased by 10% from exercise to rest images or exceeded 70% of the maximum uptake were defined as reversible (viable) defects. The remaining defects on the rest image were irreversible (non-viable) defect segments, and were considered for viability study on the basis of %WT. %WT was calculated according to the standard method: [(counts ES - counts ED)/counts ED] x 100. A viable segment on gated SPECT was defined as a segment whose %WT exceeded the lower limit of the normal value (mean - SD). PET viability was defined as FDG uptake exceeding 50% of the maximum count.

RESULTS

Among the 792 segments evaluated in the 33 patients studied, there were 689 PET viable segments. Of the 689 segments analyzed, 198 (29%) were identified as having defects on Ex images. Among these defects, 55 (8%) were reversible or partially reversible, as evidenced by rest images, and 143 (21%) were irreversible. Of the irreversible segments on Ex/rest images, 106 (15%) demonstrated no apparent WT by gated TF SPECT, whereas 37 (6%) segments with irreversible defects did have apparent WT. Overall, the sensitivity of Ex/rest TF perfusion imaging was 79%. Sensitivity was improved from 79% to 85% by combining %WT and perfusion data, but specificity was reduced from 70% to 56%.

CONCLUSION

%WT evaluated from gated TF imaging enhanced myocardial viability assessment in comparison with FDG-PET.

18F-deoxyglucose and the assessment of myocardial viability

The glucose analogue 18F-deoxyglucose allowed for the first time the ability to noninvasively probe and characterize the regional metabolism of glucose as a major fuel substrate of the human heart. Used with positron emission tomography, it became the tool for demonstrating the operation of metabolic processes, long before established in invasive or destructive experiments in animals, directly in the human myocardium. Clinical investigations with 18F-deoxy-glucose, combined with other radiotracers of the myocardium's substrate metabolism, showed the dependency of the heart's substrate selection on circulating levels of glucose, free fatty acid and insulin, and the operation of Randle's cycle in the human myocardium. Regional responses in substrate metabolism to the myocardial ischemia were now visualized entirely noninvasively as, for example, decreases in fatty acid usage and oxidation and oxygen consumption, but foremost as an increase in glucose use. Regional 18F-deoxyglucose uptake markedly in excess of myocardial blood flow in dysfunctional myocardium of patients after a myocardial infarction, with chronic coronary artery disease or with ischemic cardiomyopathy, soon became recognized as a hallmark of myocardial viability or potentially reversible contractile dysfunction. Defined as blood flow metabolism mismatch, this particular regional glucose uptake pattern identifies patients to be at high risk for cardiac events and, at the same time, to benefit most from surgical revascularization. The patterns predict a postrevascularization improvement in global left ventricular function and, even more important, in symptoms related to congestive heart failure and in long-term survival. 18F-deoxyglucose is now widely used with positron emission tomography and, more recently, with single photon emission computed tomography and radiotracers of myocardial perfursion for stratifying ischemic cardiomyopathy patients to the most efficacious treatment.

Neuro-fuzzy systems for computer-aided myocardial viability assessment

This paper describes a multimodality framework for computer-aided myocardial viability assessment based on neuro-fuzzy techniques. The proposed approach distinguishes two main levels: the modality-independent inference level and the modality-dependent application level. This two-level distinction releases the hard constraint of multimodality image registration. An abstract description template is used to describe the different myocardial functions (contractile function, perfusion, metabolism). Parameters extracted from different image modalities are combined to derive a diagnostic image. The neuro-fuzzy techniques make our system transparent, adaptive and easily extendable. Its effectiveness and robustness are demonstrated in a positron emission tomography/magnetic resonance imaging data fusion application.