Isotope Counting Research Articles

Determination of 21 elements by INAA for certification of SRM 1570a, Spinach

Analyses for certification have been made by instrumental neutron activation analysis (INAA) for the determination of 21 elements in the National Institute of Standards and Technology (NIST) Spinach renewal reference material, SRM 1570a. Elements determined included ones with short halflife products (Al, V, Ca, Mg), intermediate halflife products (Mn, Na, K, La), and long halflife products (Ba, Co, Cr, Cs, Eu, Fe, Rb, Sb, Sc, Se, Sr, Th, and Zn). For the first time a new robotic sample changer was used in the counting of long halflife indicator isotopes for certification of an SRM. Uncertainties obtained averaged±1.80% for the four major and minor constituents (Ca, K, Mg, Na); ±3.14% for elements with concentrations from 1 to 400 mg/kg (Al, Ba, Cr, Fe, Mn, Rb, Sr, and Zn); and±8.31% for the ultra trace elements (<1 mg/kg) (Co, Cs, Eu, La, Sb, Sc, Se, Th, and V).

EXPERIMENTAL VOLUME REPLACEMENT THROUGH LOWER EXTREMITY VEINS

Hypovolemic shock was produced in anesthetized pigs by removal of 40% of blood volume over 10 minutes. Following blood loss, the inferior vena cava (IVC) was occluded below the renal veins to simulate the hemodynamics of emergency surgical treatment. Control animals were not treated. Experimental animals received intravenous lactated Ringer's solution equal to three times the blood loss given through catheters either in the IVC or the superior vena cava (SVC) to determine if lower extremity access would be efficacious in this model. To define the path taken by the resuscitation fluids, an additional group of animals received technetium-99m-labelled crystalloid through lower extremity catheters with continuous recording of isotope counts in the IVC and right atrium. The treated animals in all experimental groups had significant improvements in mean arterial pressure, cardiac output, and pH compared with controls. There was no significant difference in hemodynamic response in animals receiving volume replacement through the IVC compared with the SVC. When fluid was infused below a clamped IVC, the arrival of isotope in the right atrium was delayed only 1.5 seconds. We conclude that in a model simulating emergency control of potentially lethal hemorrhage, the beneficial effects of fluid resuscitation are unrelated to the site of venous access. Lower extremity veins provide a valuable site for volume replacement even with IVC occlusion. These findings should have direct application to resuscitation and surgical care of seriously injured patients.

Triple-label β liquid scintillation counting

The detection of radioactive compounds by liquid scintillation has revolutionized modern biology, yet few investigators make full use of the power of this technique. Even though multiple isotope counting is considerably more difficult than single isotope counting, many experimental designs would benefit from using more than one isotope. The development of accurate isotope counting techniques enabling the simultaneous use of three β-emitting tracers has facilitated studies in our laboratory using the multiple tracer indicator dilution technique for assessing rates of transmembrane transport and cellular metabolism. The details of sample preparation, and of stabilizing the liquid scintillation spectra of the tracers, are critical to obtaining good accuracy. Reproducibility is enhanced by obtaining detailed efficiency/quench curves for each particular set of tracers and solvent media. The numerical methods for multiple-isotope quantitation depend on avoiding error propagation (inherent to successive subtraction techniques) by using matrix inversion. Experimental data obtained from triple-label β counting illustrate reproducibility and good accuracy even when the relative amounts of different tracers in samples of protein/electrolyte solutions, plasma, and blood are changed.

Towards a relatively inexpensive, noninvasive, accurate test for colonic motility disorders

Currently available clinical tests of colonic transit, such as the radiopaque marker method, are useful to detect delayed transit but may be less sensitive for rapid transit. The aim of this study was to develop a relatively inexpensive, noninvasive, accurate test of colonic transit using selected scintigraphic observations within the first 24 hours after ingestion of a pH-sensitive, methacrylate-coated, delayed-release capsule containing 111In-labeled resin pellets. The authors' previously published colonic transit data on 22 healthy subjects, 9 patients with diarrhea-predominant irritable bowel syndrome, and 7 patients with idiopathic constipation and previously unpublished data on 4 patients with carcinoid diarrhea were analyzed. A logistic discriminant analysis was used to estimate the sensitivity and specificity of selected combinations or simple summaries of transit. Among combined transit summaries, the emptying rate of the proximal colon was significantly different between healthy and constipation groups; the geometric center of isotope in the colon at 4 hours was significantly greater in the diarrhea group than in healthy controls; the geometric center at 24 hours was significantly lower in the constipation group than in the other two groups. From the logistic discriminant analysis, simple summaries of transit also had significant discriminant value; these included the isotopic contents in the ascending, transverse, and descending colon at 4 hours and the counts in the ascending and transverse colon and stool at 24 hours. At 90% sensitivity, the specificity of the transverse colon counts at 4 hours was 79%, which is identical to the specificity of the proximal colon emptying rate, both adjusted for age. Thus, quantitation of isotopic counts in colonic regions on scans taken at 4 and 24 hours provides an accurate summary of colonic transit, with acceptable specificity at a high sensitivity in the detection of motility disorders of the colon.

Quantitative studies of tubular immune complex formation and clearance in rats

Tubular antibody deposition and clearance was quantitatively studied using affinity-purified rabbit antibodies to rat Tamm-Horsfall protein (TH), a surface membrane glycoprotein of the tubular cells of the thick ascending limb of the loop of Henle. Immune complexes are formed in situ at the base of these cells in rats injected with antisera to TH. The renal binding of I125-anti-TH was determined in pair label studies. Kidneys and other organs were removed from groups of rats for isotope counting at four hours to 14 days after an injection of I125-anti-TH and I131-normal rabbit IgG. The greatest total renal anti-TH binding after injection of 500 micrograms of anti-TH was observed at 24 hours in normal rats (18.55 +/- 1.6 micrograms). During the period of most rapid clearance (day 2 to day 7) the half life of renal anti-TH binding (84.2 hours) and the half life of anti-TH in the serum (68.5 hours) were shorter than that of IgG in the serum (117.8 hours). There was no substantial uptake of anti-TH by other organs. A close relationship between serum levels and renal uptake of anti-TH at 24 hours was also observed in rats given from 50 to 6000 micrograms of anti-TH; renal saturation was evident only at the highest dose. This close relationship was also present during the clearance phase in rats injected with 3700 micrograms of anti-TH; the half life of anti-TH was 96.2 hours in kidneys and 110 hours in serum while the half life of rabbit IgG in serum was 151.8 hours. Markedly increased renal uptake of anti-TH was observed in protein-uric rats with passive Heymann nephritis. In very proteinuric rats, 14.1% of the injected dose was bound to kidneys at 24 hours. In these rats, serum anti-TH levels decreased very rapidly to 4% of control serum levels by five days. Throughout the period of study, the serum levels of anti-TH determined by direct radiometric assay corresponded very closely to those obtained by enzyme-linked immunosorbent assay (ELISA). Urinary excretion was a major mechanism for the clearance of anti-TH in proteinuric rats; more than 10% of the injected I125-anti-TH was recovered intact (that is, protein bound) during the first day after injection. During the clearance phase for renal deposits, urinary clearance of anti-TH exceeded urinary clearance of IgG due to release of renal bound antibody into urine.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular and particulate depositions for regional myocardial flows in sheep.

The deposition of microspheres in small tissue regions is not strictly flow dependent. In comparison with the soluble flow marker 2-iododesmethylimipramine (IDMI), deposition of 16.5-microns microspheres was mildly but systematically biased into high flow regions of rabbit hearts (Bassingthwaighte JB, Malone MA, Moffett T-C, King RB, Little SE, Link JM, Krohn KA. Am J Physiol 1987;253 (Heart Circ Physiol 22):H184-H193). To examine the possibility of bias in larger hearts, a similar study was undertaken in sheep. 141Ce- and 103Ru-labeled 16.5-microns microspheres in one syringe and 125I- and 131I-DMI in another syringe were injected simultaneously into the left atrium of five open-chest sheep while obtaining reference blood samples from the femoral artery. In six other sheep, one microsphere type and one IDMI were used. Hearts were removed 1 minute after injection, cut into approximately 254 pieces averaging 217 mg, and regional deposition densities calculated for each tracer from the isotopic counts. Correlations in the five animals between the two differently labeled IDMIs and between the two microspheres were both greater than or equal to 0.98. In all 11 sheep, scatter plots of microsphere deposition densities versus IDMI densities showed that differences between microspheres and IDMI had substantially more scatter (0.84 less than r less than 0.98) but were not random. Microsphere depositions tended to be lower than IDMI depositions in low flow regions and higher in high flow regions, in accord with the expected bias that at a bifurcation a microsphere is most likely to enter the branch with higher flow. There was less bias ascribable to endomyocardial/epicardial maldistribution. Thus, while microsphere depositions appear to err systematically with respect to flow when the regions of interest are small enough that the diameters of their arterioles are only a few times those of the microspheres, microspheres are, in sheep as in rabbits, adequate for estimating regional flows.

Adenylate cyclase activities of vascular smooth muscle in early and established DOCA/SALT hypertensive rats.

It has been demonstrated that the adenylate cyclase activity of vascular smooth muscle regulates its tonus. The present study was undertaken to examine adenylate cyclase activity in early and established deoxycorticosterone acetate (DOCA)/salt hypertensive rats. Early and established DOCA/salt hypertensive rats were prepared by injecting 30 mg of DOCA weekly for 3 and 10 weeks, respectively, into male Wistar rats given drinking water with 1% saline. The membrane protein fraction medium containing the protein, 50 microM isoproterenol, 100 microM GTP, 50 microM forskolin or 25 microM calmodulin was applied. The adenylate cyclase activity was determined by a modified method developed in our laboratory using double isotope counting. The adenylate cyclase activity in the early DOCA/salt hypertensive rats was significantly higher (p less than 0.05) than that in the control rats in the basal condition, which was unaffected by additions of isoproterenol, GTP or forskolin. There was no significant difference in basal adenylate cyclase activity between the established DOCA/salt hypertensive and control rats. The adenylate cyclase activities in the established DOCA/salt hypertensive rats were significantly lower with GTP (p less than 0.02) and forskolin (p less than 0.01) as compared with the control rats. Calmodulin elevated the adenylate cyclase activity significantly (p less than 0.05) in the established DOCA/salt hypertensive rats as well as in the control rats. However, enzyme activity with calmodulin in the established DOCA/salt hypertensive rats was significantly lower (p less than 0.05) than that in the control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of 5-Methyl tetrahydrofolate (5-Methyl THF) by high performance liquid chromatography (HPLC)

Measurement of serum and red cell folate is one of the commonest tests in Haematology. The methods in current use are microbiological (L.casei) and radioisotope dilution assay (RDA). Microbiological assays are slow and subject to interference from antibiotics and other drugs. RDA are expensive and require isotope counting equipment. In addition, although the assays have been clinically useful, quality assurance trials have shown poor precision and poor agreement between different laboratories. Neither assay is specific for 5-Methyl THF in serum by HPLC and that measurement of whole blood and red cell 5-Methyl THF is even easier by this method. Our preliminary results show precision comparable with existing methods and correlate well with these methods. Another benefit is that it is possible to measure other folate species, which may provide further insight into folate metabolism.

Role of the calcium-calmodulin system in the adenylate cyclase activity of vascular smooth muscle.

In order to evaluate the relationship between the calcium-calmodulin system and the adenylate cyclase activity of vascular smooth muscle, we examined the effects of several calcium effectors on the basal and stimulated adenylate cyclase activity. Thoracic aortae were removed from Wistar rats and the tissues were homogenated with cold homogenizing buffer containing 1 nM EDTA. Membrane protein fraction of the smooth muscle was prepared by centrifugation at 37,000 g. In this procedure, endogenous guanine nucleotides and contractile proteins remained. The protein fraction was incubated with 2 mM EGTA, 50 microM trifluoperazine, 0.1 microM A23187 or 25 microM calmodulin under basal and stimulated (50 microM isoproterenol, 100 microM GTP and 50 microM forskolin) conditions. The adenylate cyclase activity was determined by a method modified in our laboratory using double isotope counting. Trifluoperazine reduced the basal adenylate cyclase activity significantly (p less than 0.01) as well as the stimulated enzyme activities. A23187 did not affect the basal enzyme activity, but elevated the isoproterenol stimulated enzyme activity significantly (p less than 0.05). EGTA did not affect the basal and stimulated adenylate cyclase activities. Calmodulin elevated the basal enzyme activity significantly (p less than 0.02), but did not affect the stimulated enzyme activities. These results suggest that the calcium-calmodulin system is necessary for maintenance of the adenylate cyclase activity of vascular smooth muscle cells. The calmodulin acting site is considered to be the catalytic subunit, and stimulation of the enzyme is accelerated by calcium ion.

Fate of labeled albumin and erythrocytes following injection into amniotic cavity of sheep.

We have previously described a method to measure the amniotic fluid (AF) volume and fetal swallowing rate in near-term sheep by use of 125I-labeled albumin (RISA) and 51Cr-labeled red blood cells (51Cr-RBC). However, when we measured these volumes on consecutive days, reentry of the radionuclides into the amniotic cavity from fetal urine affected the calculated values of AF volume and swallowing rates. In an attempt to clarify the recirculation problem, we injected RISA and Cr-RBC daily for 9 days into the AF of five chronically catheterized pregnant sheep (124 days gestation on the 1st experimental day). We calculated AF volume and fetal swallowing rate, comparing those values to the values corrected for fetal urine isotopic counts. The mean AF volume and fetal swallowing rate measured by RISA were 808 +/- 48 ml (mean +/- SE) and 559 +/- 29 ml/day, respectively. These values were only slightly different from the corrected volumes, 808 +/- 48 ml and 561 +/- 29 ml/day, respectively, because fetal urine 125I activity reached only 4.8% of AF activity even on the 9th day. In contrast, 51Cr-activity in fetal urine on the 9th day showed 47% of the activity of AF. The mean uncorrected AF volume (785 +/- 44 ml) and swallowing rate (561 +/- 31 ml/day) measured by Cr-RBC were different from the corrected values (790 ml and 570 ml/day, respectively). After the 4th day these differences were particularly conspicuous.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of the [formula omitted] method to dating Pleistocene carbonates and comparison with other dating methods

The 227Th 230Th dating method is described in detail and its usefulness investigated by comparing ages of sixteen Pleistocene carbonates (mainly cave deposits) with those determined by the 231Pa 235U and 230Th 234U methods. The 227Th 230Th ages are found to be critically dependent on corrections for decay of 227Th prior to alpha counting and ingrowth of daughter isotopes of 232Th derived from clastic detritus. Of nineteen sets of ages determined for the sixteen samples, good agreement is found for only seven sets. Differences are attributed to low U content of some samples and the possibility of excess 227Th in the calcite of samples younger than ~50 ky, possibly due to the coprecipitation of 231Pa during formation. Calculated “negative” 227Th 230Th ages may be a direct result of this process and the fact that, unlike the other methods, the activity ratio is non-zero at zero age. Nevertheless, the 227Th 230Th is found to be a useful alternative dating technique for carbonates which are between ~50 and 300 ky, because no spiking is required. It also serves as a check for partial concordancy with ages dated by the other methods.

Limitations of standard filtration techniques in measuring specific binding of transported substrates to renal plasma membranes.

Under equilibrium conditions, the specific binding of 14C-alpha-methyl-D-glucoside and 14C-p-aminohippurate to brushborder and basolateral membranes of rabbit kidney cortex could not be quantitated using standard filtration techniques. It was concluded that the low receptor affinities of these transported solutes and the masking of bound isotopic counts by vesicular uptake prevented accurate measurement of specific binding activities.

The thromboxane antagonist, 13-azaprostanoic acid, inhibits arachidonic acid-induced Ca 2+ release from isolated platelet membrane vesicles

In the present study we investigated the ability of the arachidonic acid metabolites, prostaglandin H 2 and thromboxane A 2, to release Ca 2+ from isolated platelet vesicles. The vesicles were prepared through modification of previously described procedures. 45Ca uptake and release were determined by Millipore filtration and isotope counting of the filter paper. Incubation of the vesicles (25°C) with 50 μM CaCl 2 (plus 45Ca) resulted in the accumulation of 13 nmol Ca 2+ per mg of protein under steady-state conditions. Addition of arachidonic acid (25 μM) resulted in a 42% release of the accumulated Ca 2+ and the production of 150 ng thromboxane B 2/mg protein. Pretreatment of the vesicles with indomethacin (4 μM) completely inhibited arachidonic acid-induced Ca 2+ release and reduced thromboxane B 2 synthesis by 82%. Pretreatment of the vesicles with the specific thromboxane A 2/prostaglandin H 2 antagonist, 13-azaprostanoic acid (20 μM), also resulted in complete inhibition of Ca 2+ release but no inhibition of thromboxane B 2 production. Addition of prostaglandin H 2 (0.3 μM) to the platelet vesicles produced a significant release of Ca 2+ only in the presence of the adenylate cyclase inhibitor, 2′,5′-dideoxyadenosine (100 μM). This Ca 2+ release was totally blocked by 13-azaprostanoic acid (20 μM). The thromboxane synthetase inhibitor 9,11-azoprosta-5,13-dienoic acid (azo analog I 3.6 μM), in the presence of 2′,5′-dideoxyadenosine, only slightly inhibited Ca 2+ release in response to added prostaglandin H 2, even though thromboxane B 2 production was blocked by 95%.

ESOPHAGEAL CLEARANCE OF ACID IN NORMAL SUBJECTS

Previous studies of esophageal acid clearance have not distinguished clearance of acid volume from restoration of pH. In this study we compared pH clearance with simultaneously measured clearance of acid volume. In 5 supine normal subjects an electrode monitored esophageal pH while a transducer assembly recorded esophageal-body motor activity. Volume clearance was monitored by scintiscan. A 15 ml bolus of 0.1N HC1 with 100 uCi 99mTc sulfur colloid was injected into the esophagus and the subject then swallowed (Sw) q 30s until pH returned to 5. We obtained 3 sequences in each subject. Before each study 100 uCi of isotope, sealed in a tube, was counted with the tube in the esophagus. This method allowed conversion of count rate to volume. In 5 other subjects volume clearance was assessed using 2, 5 and 15 ml boluses with 100 uCi isotope. Results: Most injected boluses elicited secondary peristalsis (2°P) resulting in 80–90% volume clearance in <20s. Volume not cleared by 2°P was 90–95% cleared about 30s later by the first swallow-induced 1°P sequence. For the next 5 min residual isotope counts persisted despite Sw q 30s. Studies with 2 ml isotope boluses showed <0.2 ml residual volume after 1 or 2 peristaltic sequences. Despite nearly total volume clearance of a 15 ml bolus after 1 or 2 peristaltic sequences, esophageal pH did not begin to rise until 20–200s later and 303 ± 67 SE s elapsed before pH returned to 5. The pH rise usually occurred in steps, each associated with a Sw. We conclude: 1) Esophageal clearance of acid volume and restoration of pH, albeit related, are not equivalent events, 2) Nearly complete clearance of acid volume usually occurs after 1 or 2 peristaltic sequences, whereas pH restoration does not begin until a subsequent swallow and requires multiple additional swallows for completion, 3) After initial volume clearance by 1 or 2 peristaltic sequences, a residual film of acid on the esophageal mucosa sustains a low intraluminal pH, 4) Step increases in pH that occur during acid clearance cannot be accounted for by volume clearance alone because volume reduction per se would not change pH. When intra-esophageal acid volume is reduced to a minimal volume, swallowing of saliva appears to play a major role in restoring esophageal pH to normal.

A new assay for cytotoxic lymphocytes, based on a radioautographic readout of 111In release, suitable for rapid, semi-automated assessment of limit-dilution cultures

A new assay for cytotoxic T lymphocytes is described, of general application, but particularly suitable for rapid, semi-automated assessment of multiple microculture tests. Target cells are labelled with high efficiency and to high specific activity with the oxine chelate of 111indium. After a 3–4 h incubation of test cells with 5 × 10 3 labelled target cells in V wells of microtitre trays, samples of the supernatant are spotted on paper (5 μl) or transferred to soft-plastic U wells (25–50 μl) and the 111In release assessed by radio-autography. Overnight exposure of X-ray film with intensifying screens at −70°C gives an image which is an intense dark spot for maximum release, a barely visible darkening with the low spontaneous release, and a definite positive with 10% specific lysis. The degree of film darkening, which can be quantitated by microdensitometry, shows a linear relationship with cytotoxic T lymphocyte dose up to the 40% lysis level. The labelling intensity and sensitivity can be adjusted over a wide range, allowing a single batch of the short half-life isotope to serve for 2 weeks. The 96 assays from a single tray are developed simultaneously on a single small sheet of film. Many trays can be processed together, and handling is rapid if 96-channel automatic pipettors are used. The method allows rapid visual scanning for positive and negative limit dilution cultures in cytotoxic T cell precursor frequency and specificity studies. In addition, in conjunction with an automated densitometer designed to scan microtitre trays, the method provides an efficient alternative to isotope counting in routine cytotoxic assays.

Recurrent Apathetic Hyperthyroidism

A patient experienced three separate episodes of apathetic hyperthyroidism. Each time, this clinical picture appeared after the patient had been euthyroid for one to three months and within one to three weeks of discontinuing antithyroid drug therapy. This rapid recurrence of apathetic hyperthyroidism represents evidence against the concept that this disorder represents "burnt-out," or exhaustion of, body reserves in long-standing, untreated hyperthyroidism. Initial recognition and diagnosis of thyrotoxicosis was delayed by contamination of her serum from therapeutic radioactive iodine and possible interference with thyroxine radioimmunoassay. The solution to this problem with double isotope counting is discussed.

The rabbit as an animal model to study pharmacokinetics of levonorgestrel in women

In an attempt to develop an animal model to facilitate investigations on the effects of various nutritional and environmental factors on the metabolism of the contraceptive steroid levonorgestrel, the pharmacokinetics of levonorgestrel were investigated in rabbits. Six adult female rabbits received an intravenous bolus dose of 3H-levonorgestrel, 8–20 μCi, with 15–20 μg of carrier cold levonorgestrel per kg body weight. Blood samples were drawn at different intervals of time after the dose and the plasma drug concentration was determined both by radioisotopic dilution method and by radioimmunoassay. Plasma concentrations of levonorgestrel as assayed by radioimmunoassay agreed very closely with those obtained by isotope counting. The decline in plasma radioactivity attributable to unchanged levonorgestrel followed a triexponential pattern. Plasma radioactivity data were therefore analysed according to a three-compartment open model by graphical and regression analysis. The half-lives of the three phases π, α and β representing the shallow, deep peripheral and the terminal elimination compartment, respectively, were in close agreement with those reported in women. The metabolic clearance rate (MCR) and volume of distribution (v b) of levonorgestrel expressed per unit of body weight were comparable to values reported in women. The overall observed trend of pharmacokinetics of levonorgestrel in rabbits resemble closely those seen in women. It would therefore appear that the rabbit is a suitable animal model for studying the effects of nutritional factors on the pharmacokinetics of the drug.

Use of indium-111-labeled cells in measurement of cellular dynamics of experimental cardiac allograft rejection.

This study evaluates the kinetics and utility of infused indium-111-labeled cells in detecting rejection in ACI to Lewis rat heart allografts. Syngeneic leukocytes, lymph node lymphocytes, and platelets were isolated and labeled with indium-111 (111In) oxine, respectively, and were infused i.v. into Lewis rats carrying beating ACI or syngeneic hearts from post-transplant days 0 to 6. Recipients were imaged serially at 24 hr after infusion of labeled cells followed by excision of both native and transplanted hearts for direct isotope count. Labeled leukocytes accumulative progressively in the allograft with the scan becoming positive by post-transplant day 4. The ratio of allograft to native heart isotope counts rose from 1.25 on day 1 to 10.07 (P less than 0.0001) on day 7. The Lewis recipients infused with labeled lymphocytes showed a positive scan on days 6 and 7 whereas the allograft to native heart isotope count ratio rose from 0.97 on day 1 to 5.33 (P less than 0.001) on day 7. Recipients infused with 111In-labeled platelets showed a positive scan on days 5 to 7 and the allograft to native heart isotope count ratio rose sharply from 2.56 on day 4 to 16.98 (P less than 0.005) on day 7. Syngeneic heart grafts failed to demonstrate significant accumulation of any of the labeled cell population. These studies confirm the importance of nonlymphocytic cells in cellular rejection, evaluate the kinetics of graft invasion by the various cell types, and suggest that the techniques used afford a method for a safe and an early detection of allograft rejection.

Radioisotope Dating Using an en-Tandem Accelerator

In the last few years, progress in radioisotope dating has been made by direct counting of radioactive isotopes rather than measuring the radioactive decay. The single atom detection can be achieved by using an electrostatic accelerator as a component of an ultra-sensitive mass spectrometer (1). For such applications the existing EN-Tandem accelerator facility at the Swiss Federal Institute of Technology, Zurich (ETH) is being modified in order to measure the isotopic composition of all elements of interest with Z ⩽ 53.

Evaluation of thallium-201 imaging in nontransmural ischemia and infarction.

To assess the validity of thallium-201 myocardial imaging in the diagnosis of nontransmural ischemia and infarction, the proximal left anterior descending coronary artery was partially occluded for 60 minutes with a balloon-tip catheter in intact anesthetized dogs in a basal state or during atrial pacing. In vivo scintigrams of myocardium were compared with those obtained in the isolated heart and in the incised ventricle spread flat. None of the animals with partial occlusion with or without pacing demonstrated abnormal scintiscans in vivo. Removal of background by isolating the heart increased positive images to 30%; positive images were associated with an isotope count ratio between ischemic and normal muscle of less than 0.67. Removal of superimposed nonischemic muscle in the heart enface increased image detection after pacing to 11 of 15. Since animals with subendocardial scar failed to demonstrate a "cold area" in vivo, unfavorable geometry as well as extent and degree of ischemia appear to be important. Thus, thallium radioactivity in superimposed and adjacent myocardium, as well as background, may limit the detection of nontransmural ischemia and scar.