Blood Flow And Metabolism By Pet

Assessment of Tumor Vascularization: Immunohistochemical and Non-Invasive Methods

Growth of solid tumors beyond a certain mass is dependent on the vascular bed from pre-existing host vasculature. The process of angiogenesis is essential not only for primary tumor growth but also for metastasis. The number of microvessels within the invasive component of a primary tumor reflects the degree of tumor angiogenesis. At present the most widely used method to assess neovascularization is the quantitation of intratumoral microvessel density (IMD) by immunohistochemical methods in which specific markers for endothelial cells are employed. In this paper we analyze the different methods used to assess IMD, as well as their advantages and potential methodological pitfalls. Several studies have shown a close correlation between IMD, tumor growth and the occurrence of metastasis, suggesting that IMD is a prognostic indicator of clinical relevance. Furthermore, preliminary studies suggest that determination of angiogenesis may predict responsiveness to some forms of conventional anticancer therapy. Although the histological microvessel density technique is the current gold standard to characterize tumor angiogenesis, it may not be the ideal tool for clinical purposes because it needs to be performed on biopsy material and does not assess the functional pathways involved in the angiogenic activity of tumors. Non-invasive assessment of tumor vascularity is possible in vivo by means of Doppler sonography, dynamic contrast-enhanced magnetic resonance imaging (MRI) and positron emission tomography (PET). These methods may be preferable to histological assay because they are non-invasive, survey the entire tumor, reflect both anatomic and physiologic characteristics, and may be useful to monitor the activity of antiangiogenic therapies.

Quantifying the changes in oxygen extraction fraction and cerebral activity caused by caffeine and acetazolamide

A quantitative estimate of cerebral blood oxygen saturation is of critical importance in the investigation of cerebrovascular disease. We aimed to measure the change in venous oxygen saturation (Y_v) before and after the intake of the vaso-dynamic agents caffeine and acetazolamide with high spatial resolution using susceptibility mapping. Caffeine and acetazolamide were administered on separate days to five healthy volunteers to measure the change in oxygen extraction fraction. The internal streaking artifacts in the susceptibility maps were reduced by giving an initial susceptibility value uniformly to the structure-of-interest, based on a priori information. Using this technique, Y_v for normal physiological conditions, post-caffeine and post-acetazolamide was measured inside the internal cerebral veins as Y_Normal = 69.1 ± 3.3%, Y_Caffeine = 60.5 ± 2.8%, and Y_Acet = 79.1 ± 4.0%. This suggests that susceptibility mapping can serve as a sensitive biomarker for measuring reductions in cerebro-vascular reserve through abnormal vascular response. The percentage change in oxygen extraction fraction for caffeine and acetazolamide were found to be +27.0 ± 3.8% and -32.6 ± 2.1%, respectively. Similarly, the relative changes in cerebral blood flow in the presence of caffeine and acetazolamide were found to be -30.3% and + 31.5%, suggesting that the cerebral metabolic rate of oxygen remains stable between normal and challenged brain states for healthy subjects.

Evaluating the Role of Reduced Oxygen Saturation and Vascular Damage in Traumatic Brain Injury Using Magnetic Resonance Perfusion-Weighted Imaging and Susceptibility-Weighted Imaging and Mapping

The cerebral vasculature, along with neurons and axons, is vulnerable to biomechanical insult during traumatic brain injury (TBI). Trauma-induced vascular injury is still an underinvestigated area in TBI research. Cerebral blood flow and metabolism could be important future treatment targets in neural critical care. Magnetic resonance imaging offers a number of key methods to probe vascular injury and its relationship with traumatic hemorrhage, perfusion deficits, venous blood oxygen saturation changes, and resultant tissue damage. They make it possible to image the hemodynamics of the brain, monitor regional damage, and potentially show changes induced in the brain's function not only acutely but also longitudinally following treatment. These methods have recently been used to show that even mild TBI (mTBI) subjects can have vascular abnormalities, and thus they provide a major step forward in better diagnosing mTBI patients.

Interventional therapy in heart failure management

The incidence and prevalence of congestive heart failure are rapidly increasing because of the progressive decrease in age-adjusted mortality rates for coronary artery disease and hypertensive heart disease, together with the progressive aging of the US population. Despite great advances in maximal medical therapy, most patients with symptomatic congestive heart failure can expect functional impairment, interludes of worsening symptomatology, and a shortened life span. Thus, it is appropriate to ask whether the interventional revolution that is under way for the management of ischemic cardiovascular disease can be applied with benefit to the management of congestive heart failure. The use of interventional therapies for the treatment of elderly patients with congestive heart failure caused by coronary artery disease, valvular heart disease, or renal vascular disease is addressed.

The viable myocardium: epidemiology, detection, and clinical implications

The success of fibrinolytic and other therapies has reduced the mortality of myocardial infarction. However, many survivors develop congestive heart failure. Medical treatment of this disorder has limited efficacy, and cardiac transplantation has limited availability. Contrary to previous teaching about ischaemic injury, roughly 40% of segments involved in myocardial infarction may subsequently recover, either spontaneously or after revascularisation. The persistence of such viable myocardium means that previous approaches to treatment of myocardial infarction must be reappraised. This review examines the pathogenesis of this response, the techniques that may be used to identify the salvageable tissue, and the clinical implications. Myocardial revascularisation may improve symptom status, exercise capacity, and prognosis in selected patients with viable myocardium.

Myocardial blood flow and glucose metabolism in diabetes mellitus

Recent studies at our institution using positron emission tomography (PET) provide evidence that both myocardial blood flow (MBF) and glucose metabolism may be affected in patients with diabetes mellitus. A retrospective study revealed inadequate myocardial glucose uptake as assessed by 2-[18F]fluoro-2-deoxyglucose (18FDG) in 64% of type I (insulin-dependent diabetes mellitus, IDDM) and 36% of type II (non-insulin-dependent diabetes mellitus, NIDDM) patients. However, a study in 7 patients with IDDM and 9 controls showed that metabolic standardization using hyperinsulinemic-euglycemic clamp is associated with similar myocardial glucose uptake in both groups (0.43 +/- 0.16 vs 0.44 +/- 0.12 micromol/g per min; p = nonsignificant). Furthermore, we studied MBF as assessed by [13N]ammonia in 15 IDDM patients without coronary artery disease. We found an impairment in flow reserve in diabetic patients as compared with a control group of 13 healthy volunteers (2.6 +/- 1.3 vs 4.0 +/- 0.6; p <0.01), which was primarily due to a significantly higher resting MBF (95.3 +/- 27.7 vs 69.1 +/- 8.1 mL/100 g per min; p <0.01). Hyperemic flow during adenosine infusion tended to be lower in diabetics, but was not significantly different (236.3 +/- 105.7 vs 273.0 +/- 26.0 mL/100 g per min; p = nonsignificant). Morphologic and functional abnormalities of the coronary microcirculation have been reported in diabetic animals and humans. Furthermore, there is an ongoing controversy regarding the existence of a specific diabetic cardiomyopathy that is not related to epicardial coronary disease. However, few studies have explored the effect of diabetes, hyperinsulinemia, or hyperglycemia on MBF and glucose metabolism in humans. With PET it is possible to perform comprehensive noninvasive studies of various aspects of cardiac function in patients with diabetes mellitus.

Comparison of dobutamine echocardiography and positron emission tomography in patients with chronic ischemic left ventricular dysfunction

OBJECTIVES

The aim of this study was to correlate dobutamine-induced contractile reserve as detected by echocardiography with findings on positron emission tomography in patients with chronic ischemic left ventricular dysfunction.

BACKGROUND

Contractile reserve induced by low dose dobutamine infusion has been proposed as a marker of myocardial viability.

METHODS

Sixty patients with stable coronary artery disease and left ventricular dysfunction (mean ejection fraction [+/- SD] 29 +/- 10%) underwent transthoracic echocardiography with dobutamine infusion (up to 10 micrograms/kg body weight per min) and positron emission tomography with nitrogen-13 ammonia and fluorine-18 (F-18) fluorodeoxyglucose as a perfusion and a metabolic tracer, respectively. Regional wall motion, perfusion and metabolism were analyzed semiquantitatively by using a 16-segment model. Segments with F-18 fluorodeoxyglucose uptake > 50% were considered viable on positron emission tomography.

RESULTS

After dobutamine infusion, hemodynamic variables changed significantly, and myocardial ischemia was evident in 17 patients. All 60 patients had dysfunctional myocardium considered viable on positron emission tomography (8 +/- 4 segments/patient), whereas 52 patients had dysfunctional myocardium with contractile enhancement by dobutamine echocardiography (4 +/- 2 segments/patient, p = 0.01). The extent of dysfunctional myocardium with contractile reserve appeared to correlate less closely with the total extent of viable dysfunctional myocardium identified by positron emission tomography than with the number of such segments associated with a pattern of perfusion-metabolism mismatch.

CONCLUSIONS

In patients with chronic ischemic left ventricular dysfunction, echocardiography can be used to identify enhancement in the contractile function of viable dysfunctional myocardium after infusion of low dose dobutamine. In this study, the presence and extent of such enhancement were relatively less than the values obtained from positron emission tomography.

Should all patients undergo cardiac catheterization after a myocardial infarction?

Between one half and two thirds of patients who survive an acute myocardial infarction (AMI) may be at low risk for future complications and hence can be managed with medical therapy. However, the remaining patients are prone to future complications, which by and large occur within the subsequent 3 months and include cardiac death, recurrent AMI, unstable angina, and congestive heart failure. Current available methods for risk stratification include a good clinical evaluation, rest and stress electrocardiograms (preferentially combined with radionuclide imaging), and possibly two-dimensional stress echocardiography. In patients unable to exercise, pharmacologic perfusion scintigraphy affords a powerful means to identify high-risk patients. Patients deemed to be at high risk should be referred for cardiac catheterization and myocardial revascularization. The practice of performing routine cardiac catheterization after an AMI has led to an over use of resources in the United States. Such a practice is not based on any scientific evidence of enhanced benefit. In fact, in other Western world countries where only selected patients are referred for cardiac catheterization, patient survival appears to be similar to that in the United States. In conclusion, most patients after AMI, whether treated with thrombolytic therapy or not, can be managed conservatively and risk stratified on the basis of noninvasive testing, after which patients deemed to be at high risk should undergo invasive evaluation.