Carl Selin

N-13 ammonia as an indicator of myocardial blood flow.

We have characterized N-13 ammonia as a myocardial blood flow imaging agent suitable for positron-emission computed tomography. However, the mechanisms of uptake and retention of this agent in myocardium are not known, and effects of altered metabolism were not considered. Therefore, we studied the uptake and retention of N-13 ammonia in myocardium under various hemodynamic and metabolic conditions in open-chest dogs. N-13 ammonia was extracted nearly 100% during its initial capillary transit, followed by metabolic trapping that competed with flow-dependent back diffusion. At control flows, the first capillary transit extraction fraction (E) of N-13 ammonia averaged 0.82 ± 0.06. It fell with higher flows by E = 1 − 0.607 exp − 125/F. Myocardial N-13 tissue clearance half-times were similarly inversely related to blood flow, and ranged from 110–642 minutes. Cardiac work and changes in the myocardial inotropic state induced by isoproterenol and propranolol did not affect E or the tissue clearance half-times. Low plasma pH reduced E by an average of 20%, while elevated plasma pH had no effect. Decreases in flow below control also were associated with a fall in E. Inhibition of glutamine synthetase with L-methionine sulfoximine impaired metabolic trapping of N-13 ammonia and implicates the glutamic acid-glutamine reaction as the primary mechanism for ammonia fixation. The product of E times flow predicts the myocardial N-13 tissue concentrations, which increased by 70% when flow was doubled. Thus, blood flow and metabolic trapping are the primary determinants of myocardial uptake and retention of N-13 ammonia. The relative constancy of metabolic trapping over a wide range of hemodynamic and metabolic conditions demonstrates the value of N-13 ammonia as a myocardial blood flow imaging agent.

Sustained regional abnormalities in cardiac metabolism after transient ischemia in the chronic dog model

Positron emission tomography allows noninvasive assessment of myocardial blood flow and metabolism, and may aid in defining the extent and severity of an ischemic injury. This hypothesis was tested by studying, in chronically instrumented dogs, regional blood flow and metabolism during and after a 3 hour balloon occlusion of the left anterior descending coronary artery. The metabolic findings after ischemia were compared with the recovery of regional function over a 4 week period. N-13 ammonia was used as a blood flow tracer, and C-11 palmitic acid and F-18 deoxyglucose as tracers of fatty acid and glucose metabolism, respectively. Regional myocardial function was monitored with ultrasonic crystals implanted subendocardially. Regional function improved most between 24 hours and 1 week after reperfusion, but was still attenuated at 4 weeks. The slow functional recovery was paralleled by sustained metabolic abnormalities, reflected by segmentally delayed clearance of C-11 activity from myocardium and increased uptake of F-18 deoxyglucose. Absence of blood flow and C-11 palmitic acid uptake at 24 hours of reperfusion correlated with extensive necrosis as evidenced by histologic examination. Conversely, uptake of C-11 palmitic acid with delayed C-11 clearance and increased F-18 deoxyglucose accumulation identified reversibly injured tissue that subsequently recovered functionally and revealed little necrosis. Thus, recovery of metabolism after 3 hours of ischemia is slow in canine myocardium and paralleled by slow recovery of function. Metabolic indexes by positron tomography early after reperfusion can identify necrotic and reversibly injured tissue. Positron tomography may therefore aid in defining the extent and prognosis of an ischemic injury in patients undergoing reperfusion during evolving myocardial infarction.

Regional myocardial perfusion assessed with N-13 labeled ammonia and positron emission computerized axial tomography.

The usefulness of N-13 ammonia as an indicator of regional myocardial perfusion suitable for positron emission tomography has been suggested. However, the relation between myocardial blood flow and uptake of N-13 ammonia has not been examined quantitatively. To quantitate the relation of myocardial N-13 ammonia tissue concentration to myocardial blood flow and examine its suitability for positron emission tomographic imaging, 12 open chest dogs were studied. In eight of the dogs, 25 imaging procedures with N-13 ammonia and positron emission tomography were performed; in the remaining four dogs the relation between flow and myocardial N-13 ammonia concentration was assessed with in vitro techniques. Positron emission tomography provided high quality cross-sectional images of the distribution of N-13 ammonia in left ventricular myocardium. No significant redistribution of N-13 ammonia in the myocardium occurred with time. Alterations in regional myocardial blood flow resulted in changes in the regional distribution of N-13 ammonia that were visible in the cross-sectional images. Myocardial N-13 ammonia concentrations measured with positron emission tomography were closely related to myocardial blood flow, although in a nonlinear fashion. The nonlinear relation determined with positron emission tomography was identical to that observed with in vitro techniques. The nonlinearity of this relation of flow to N-13 ammonia concentration was probably due to the inverse relation between flow and the N-13 ammonia single pass extraction fraction. The uptake of N-13 ammonia was studied at myocardial blood flows ranging from 0 to 500 ml/min per 100 g, whereas physiologically occurring flows in man range only from near 0 to approximately 300 ml/min per 100 g. Within this physiologic flow range, the relation between flow and N-13 ammonia concentration was relatively linear, and a 100 percent increase in flow produced an approximately 70 percent increase in N-13 ammonia concentration. It is concluded that N-13 ammonia is a good indicator of myocardial blood flow and compares favorably with conventional monovalent perfusion imaging agents. Positron emission computerized axial tomography permits quantitative measurements of the distribution of this agent in myocardium and therefore should prove useful in measuring regional myocardial perfusion and blood flow.

Cerebral glucose metabolic rates in normal human females versus normal males

Sex-related differences have been reported for some brain neuroanatomical structures and several measures of brain function. We studied the cerebral glucose metabolic rates of normal men (n = 7) and women (n = 7) with positron emission tomography and the fluorodeoxyglucose method. Women were studied between days 5 and 15 of the menstrual cycle. Women had whole brain glucose metabolic rates that were 19% higher than those of men. All neuroanatomical structures surveyed showed significant female greater than male rates, with no particular regions being outstanding. The higher cerebral glucose metabolic rates we observed in women may have been related to the effects of the high estrogen levels that can obtain in the phase of the menstrual cycle during which we tested our female subjects.

Measurement of regional myocardial blood flow with N-13 ammonia and positron-emission tomography in intact dogs.

N-13 ammonia mimics certain properties of microspheres. It rapidly clears from blood into myocardium where it becomes fixed in proportion to myocardial blood flow. Used with positron emission tomography as a means for quantifying in vivo myocardial indicator concentrations, N-13 ammonia may be useful for noninvasive determination of myocardial blood flow with the arterial reference sampling technique. This possibility was examined in 27 experiments in 10 chronically instrumented dogs at control, high and low blood flows. Myocardial blood flow was calculated in vivo from the myocardial N-13 tissue activity concentrations derived from serial cross-sectional images of the heart, the 2 minute arterial input function and the withdrawal rate of arterial blood. These calculations were compared with blood flow determined by the standard microsphere technique. Blood flow determined in vivo with N-13 ammonia and positron emission tomography correlated with microsphere blood flow by y = -36.2 + 1.53x -0.0027x2 (r = 0.94 with a standard error of the estimate of 16 ml/min per 100 g). For flows from 44 to 200 ml/min per 100 g, the relation between in vivo and in vitro measured myocardial blood flow was nearly linear but reached a plateau at flows higher than 200 ml/min per 100 g. These results indicate that in dogs, blood flow in the physiologic range can be quantified in vivo with N-13 ammonia and positron emission tomography.

Cardiac emission computed tomography: underestimation of regional tracer concentrations due to wall motion abnormalities

Possible effects of regional wall motion abnormalities on apparent regional myocardial tracer concentrations on emission tomographic images were evaluated in six open chest dogs. Each dog was studied twice: In Run 1, 13N ammonia and microspheres were injected during a 6 min coronary occlusion, and serial images acquired by positron emission tomography during occlusion and reperfusion. In Run 2, 1 h later, 13N ammonia and microspheres were reinjected at control, and serial images recorded at control, during a repeat 6 min coronary occlusion, and after reperfusion. Segmental function was monitored with ultrasonic crystals, and 13N tissue concentrations determined in vivo from the tomographic images and postmortem by well counting. In Run 1, fractional shortening in ischemic segments fell by 89 ± 16% SD from control. The ischemic versus control segment ratio for 13N activity concentrations averaged 0.29 ± 0.08 and for microspheres 0.20 ± 0.15. In Run 2 the ischemic versus control segment ratio was at control 0.77 ± 0.12 for 13N tissue activity and 0.85 ± 0.07 for microspheres. Fractional shortening fell during occlusion by 131 ± 29% from control, returned to control early, and fell again by 11 ± 16% late during reperfusion. These changes were paralleled by changes in apparent regional 13N tissue concentrations of the prelabeled myocardium. Compared with control, they were 37 ± 9% lower during occlusion and rose to 94 ± 20% early and to 89 ± 16% at control late during reperfusion. In vitro determined tissue concentration ratios of ischemic to control myocardium were similar for 13N and microsphere activity (0.83 and 0.85), which ruled out loss of 13N ammonia from tissue during occlusion or reperfusion. Our results indicate that regional wall motion abnormalities cause artifactual segmental defects in tracer concentrations on emission tomographic images of the heart, which must be considered for qualitative and quantitative analysis of regional tracer tissue concentrations.

In vivo quantitation of regional myocardial blood flow by positron-emission computed tomography.

The potential of positron-emission computed tomography (PCT) for external quantitation of myocardial indicator concentrations and regonal myocardial blood flow (RMBF) and the effect of left ventricular wall thickness on tracer concentration recovery by PCT was examined in seven open-chest dogs. RMBF was determined by the arterial reference technique in vivo and in vitro. Together with gamma-emitting Ce-141 microspheres, positron-emitting Ga-68-labeled microspheres were injected into the left atrium and their myocardial concentrations determined in vivo from gated and ungated cross-sectional PCT images. It is concluded that (1) myocardial indicator tissue concentrations, and thus, RMBF, can be accurately measured by PCT provided corrections are made for the effect of wall thickness on count recovery; (2) these corrections can be made using in vivo echocardiography; and (3) gated PCT imaging can be used to evaluate regional myocardial systolic wall thickening as an index of regional function and combined with measurements of RMBF or regional metabolism. The results represent a framework for the noninvasive measurement of RMBF and metabolism by PCT in the experimental animal and in man.

Retention and clearance of C-11 palmitic acid in ischemic and reperfused canine myocardium

Free fatty acids are the major energy source for cardiac muscle. Oxidation of fatty acid decreases or even ceases during ischemia. Its recovery after transient ischemia remains largely unexplored. Using intracoronary carbon-11 palmitic acid as a tracer of myocardial fatty acid metabolism in an open chest dog model, retention and clearance of tracer in myocardium were evaluated at control, during ischemia and after reperfusion following a 20 minute occlusion of the left anterior descending coronary artery. Myocardial C-11 time-activity curves were analyzed with biexponential curve-fitting routines yielding fractional distribution and clearance half-times of C-11 palmitic acid in myocardial tissue. In animals with permanent occlusion and intracoronary injection of C-11 palmitic acid distal to the occlusion site, the relative size and half-time of the early clearance curve component differed markedly from control values and did not change with ongoing ischemia. Conversely, in animals with only 20 minutes of coronary occlusion, the relative size of the early C-11 clearance phase was still significantly depressed at 20 and 90 minutes of reperfusion but returned to control level at 180 minutes. Tissue C-11 clearance half-times remained significantly prolonged throughout the reperfusion period. Regional function in reperfused myocardium monitored with ultrasonic crystals recovered slowly and was still less than control after 3 hours of reperfusion. The data indicate that after transient ischemia, myocardial fatty acid metabolism fails to recover immediately. Because the metabolic recovery occurs in parallel with recovery of regional function, C-11 palmitic acid in conjunction with positron tomography may be useful for studying regional fatty acid metabolism noninvasively after an ischemic injury, and may be helpful in identifying reversible tissue injury.

Relationship between TI-201, Tc-99m (Sn) pyrophosphate and F-18 2-deoxyglucose uptake in ischemically injured dog myocardium

We have previously demonstrated that enhanced glucose utilization in reperfused myocardium as assessed by F-18 2-deoxyglucose (FDG) and positron tomography predicts functional recovery. In this study, we compared segmental uptake of F-18 FDG with that of Tl-201 and Tc-99m (Sn) pyrophosphate (Tc-99m PPi) as conventional markers of tissue viability in seven dogs after a 3-hour intracoronary balloon occlusion and 20 hours of reperfusion. Myocardial blood flow was determined with microspheres. Regional retention fractions were calculated from tracer tissue concentrations, the arterial input function, and blood flow. Ischemic injury was assessed by triphenyltetrazolium chloride (TTC) staining and histologic analysis. At 24 hours, blood flow was 22% lower in reperfused than in control myocardium (p less than 0.05). Uptake of Tl-201 was related linearly to blood flow (r = 0.92), while glucose utilization and Tc-99m PPi were 2.9 (p less than 0.01) and 4.7 (p less than 0.05) times higher in reperfused than in control myocardium. Retention fractions of Tc-99m PPi increased with the degree of ischemic injury, while F-18 FDG uptake was highest in segments with mild cell injury. Thus, in ischemically injured myocardium, Tl-201 primarily reflects blood flow. F-18 FDG as a marker of glucose utilization identifies ischemically injured but viable tissue. The admixture of necrotic cells can be determined with Tc-99m PPi. Our results indicate that a dual tracer approach might best characterize the presence and extent of reversibly and of irreversibly injured tissue in a given myocardial region.

Trazodone Treatment Response in Obsessive-Compulsive Disorder – Correlated with Shifts in Glucose Metabolism in the Caudate Nuclei

Obsessive-compulsive disorder (OCD) is a severe psychiatric illness that is difficult to treat. The effects of trazodone hydrochloride treatment were studied, both with and without the addition of a monoamine oxidase inhibitor, in OCD patients. Changes in symptoms correlated with changes in local cerebral metabolic rates for glucose (LCMRGlc), as measured by positron emission tomography and the 18F-fluorodeoxyglucose method. All patients whose OCD responded favorably to drug treatment showed a relative increase in glucose metabolism in the heads of the caudate nuclei compared with the metabolic rate in the ipsilateral hemisphere as a whole (ratio LCMRGlc caudate/LCMRGlc hemisphere). Patients who did not respond to treatment did not show an increase in this ratio, and the difference between responders and nonresponders was significant (p less than 0.03). Changes in the ratio LCMRGlc caudate/LCMRGlc hemisphere correlated with changes on OCD and depression rating scales.