Several Potential Sources Of Error Research Articles

MR imaging of the knee: expanded classification and pitfalls to interpretation of meniscal tears.

Findings from magnetic resonance (MR) imaging examinations of 194 knees were interpreted retrospectively with a new classification, including not only signal intensity but morphologic abnormalities. All findings were correlated with prospectively recorded results from arthroscopic examinations performed by a knee subspecialist. Meniscal MR imaging findings were expanded to eight types in this study. Positive predictive values for each type of finding and overall accuracy were calculated. Several potential sources of error that could be differentiated through pathologic change were recognized. Sensitivity and specificity of MR imaging findings were, respectively, 95% and 74% for the medial meniscus and 86% and 90% for the lateral meniscus. As the classification grade increased, the likelihood of meniscal tear also increased. Medial meniscal tears tended to occur in a predictable pattern, extending from the posterior to the anterior horn. Thus, when a definite tear of the body or anterior horn of the medial meniscus is present, a tear of the posterior horn should be suspected, even if the findings are not convincing. This expanded classification may be useful for radiologists in expressing their level of confidence in a particular MR imaging finding suggestive of meniscal injury.

Single energy quantitative computed tomography: the effects of phantom calibration material and kVp on QCT bone densitometry.

Bone mineral densitometry by quantitative computed tomography (QCT) was introduced in 1976 (Isherwood et al 1976), and is now a clinically established and useful technique. It is usually performed with a reference phantom scanned with the patient. The reference phantom is required to correct for variations that may occur in measured CT numbers due to CT scale drifts or changes in effective beam energy (Cann and Genant 1980). The measured CT number in a region of interest is expressed in mineral density equivalents of phantom reference material. A saturated solution of dipotassium hydrogen phosphate (K2HPO4) in water has been used in dosimetry as a cortical bone equivalent material for many years (Witt and Cameron, 1969) and dilutions of this material are popular for QCT reference phantoms used in trabecular bone mineral densitometry (Cann and Genant, 1980). Several potential sources of error have been identified with reference phantoms containing aqueous solutions of K2HPO4, including changes in concentration of the solute with time and displacement effects (Rao et al, 1987). These problems have stimulated searches for alternative reference materials, the most popular of which has been calcium hydroxyapatite. This has been regarded as preferable to K2HPO4 as calcium hydroxyapatite is a true constituent of bone. Calcium hydroxyapatite is not water soluble and has to be embedded in a water equivalent plastic. The attenuations of K2HPO4 and calcium hydroxyapatite alter with X-ray energy in similar, but not identical, rates to that of bone (Hubbell, 1969; ICRU, 1989).

Theory and simulation of chain-molecule fluid structure

Results of NVT molecular-dynamics simulations are reported for modelled n-butane and n-pentane over a range of densities at temperatures of about 430 K. The model potential is the repulsive part of the Lennard-Jones potential with bond angles constrained at 109·47° and bond lengths of 0·153 nm, where σ = 0·3923 nm and ε/k = 72 K. Simulation results for average site-site distribution functions are compared with results of an approximate RISM theory. Similarly to the case of diatomic fluids, the accuracy of the approximate theory is only qualitative, but trends of key features of the fluid structure are well represented. The softness of the Lennard-Jones potential appears to play a significant role in smoothing away several potential sources of error in the approximate theory.

Porometry and baseline analysis: The case for compatibility

This paper explores the suggestion by Idso and his colleagues that the response of stomata to saturation vapour pressure deficit may prove to be an artefact of porometry. Re-examination of their analysis reveals several potential sources of error leading to anomalously small ratios of aerodynamic to stomatal resistance both for leaves and for canopies. Arguments are advanced for adopting values of this ratio an order of magnitude larger than those used by Idso. The saturation deficit ( D 0) estimated within the canopies explored by Idso is then less than 1 kPa even when the deficit in ambient air approaches 30 kPa. As Idso's own porometer measurements suggest that stomatal closure in water hyacinth did not occur until D 0 exceeded 1 kPa, his observations of stomatal resistance and radiative surface temperature appear to be fully compatible and his “apparent discrepancy” disappears.

Strontium Isotopic Stratigraphy Utilizing Authigenic Dolomites in Hemipelagic Sediments: ABSTRACT

Authigenic dolomites commonly occur in organic-rich, continental margin marine sediments. These dolomites play a key role in the age dating of stratigraphic sections. The dolomites often are the only lithology amenable to paleomagnetic stratigraphy; they preserve siliceous microfossils against diagenetic; recrystallization, and provide useful strontium isotopic stratigraphic ages. Several potential sources of error frequently are unique to the use of authigenic dolomites in the strontium isotope methods. (1) The dolomites occur as cements of the host lithology, hence, they are not a pure phase. Potentially important contaminants during analysis include gypsum clay minerals, feldspars, and iron and manganese oxides. Strontium may occur as a structural substituent ion in these minerals or as a surface-adsorbed ion. Various leaching techniques have been tested to isolate dolomitic strontium. Purer dolomites and strontium-enriched dolomites often can be selected to ease these problems. (2) The dolomites form after the deposition of the host sediment, therefore, they record the diagenetic age not the depositional age. The stable isotopic composition of the dolomites can aid in selection of early formed samples. (3) The dolomites record pore-water strontium isotope compositions, not seawater isotopic compositions. This problem is also minimized by choosing dolomites formed near the sediment-water interface. (4)more » The dolomites formed near the sediment-water interface originated as rotodolomites and undergo subsequent burial diagenesis, creating a potential for later strontium isotope exchange. This problem is minimized by selecting fresh samples from the interior of nearly impermeable beds and nodules. Results from the Miocene Monterey Formation of California and from the Eocene through Pliocene Pisco basin of Peru show that authigenic dolomites can provide useful strontium isotopic age estimates.« less

Comparison of Different Pulse Sequences for in Vivo Determination of T1 Relaxation Times in the Human Brain

Quantitative in vivo determination of T1 relaxation times by magnetic resonance imaging (MRI) is hampered by several potential sources of error. This study focused on the influence of the radiofrequency pulse sequences applied with special attention to the significance of the repetition time (TR). T1 measurements were performed on the human brain using a whole body MR scanner operating at 1.5 tesla. Three different pulse sequences were compared including two 6-points inversion recovery (IR) sequences with TR = 2.0 s and 4.0, respectively, and a 12-points partial saturation inversion recovery (PSIR) sequence with TR varying between 0.24 and 8.0 s. The median T1 relaxation times obtained in cortical grey matter and cerebrospinal fluid were significantly shorter in the IR experiments at TR = 2 s than in those carried out at TR = 4 s. Concerning white matter the discrepancy was much less pronounced, but still statistically significant. Supplementary phantom measurements indicated that the higher T1 values are increasingly underestimated when TR is reduced to 2 s. The results suggest that the PSIR sequence or IR sequences with a TR greater than 2 X the T1 level of the tissue type investigated should be employed for accurate T1 determination by MRI in clinical work.

Non-invasive measurement of respiratory tidal volume in aquatic, air-breathing animals.

ABSTRACT In recent years, a method which allows breathing patterns to be fully quantified in aquatic, air-breathing species while free-ranging and undisturbed, has come into common use (Brett & Shelton, 1979; Glass, Boutilier & Heisler, 1983, 1985; Butler, Milsom & Woakes, 1984; Silver & Jackson, 1985; Milsom & Chan, 1986). Use of this method in our laboratory, however, has revealed several potential sources of error in the normal procedures used for calibrating this system for the measurement of tidal volume (VT). In the present study we have systematically studied these potential sources of error under a variety of conditions and describe a technique for eliminating these errors in routine measurements.

Potential Role of Vocalizations in West Indian Manatees

Although vocalizations have been relatively well studied in several groups of marine mammals, very little is known regarding the context and function of vocalizations of West Indian manatees (Trichechus manatus). Manatee vocalizations have been described by Schevill and Watkins (1965), Evans and Herald (1970), Sonoda and Takemura (1973), and Hartman (1979). Most of these authors presumed that the calls were primarily used for social reasons, such as maintaining cow/calf contact, rather than for navigation. While studying the ecology and behavior of manatees at Blue Spring Run (Volusia Co., Florida) and in the St. Johns River, Florida (Bengtson, 1981), manatees were heard vocalizing in a variety of behavioral contexts other than cow/calf interactions. In order to examine the importance of vocalizations during certain behaviors, we measured call rates during different activities. Recordings of manatees vocalizations were made opportunistically in Blue Spring Run and the St. Johns River using an Inter-Oceans Systems, Inc. R-130 hydrophone (frequency response range 20 Hz to 8 kHz) and a Panasonic RQ-309AS tape recorder. This equipment was well within the range of manatee vocalizations described by Schevill and Watkins (1965). The behavioral situations in which recordings were made included: 1) Natural feeding: grazing on submerged, rooted vegetation while moving slowly along the bottom; 2) Swimming: traveling in the St. Johns River while passing our anchored boat (not recorded in Blue Spring Run); 3) Bottom resting: resting or sleeping on the bottom of the waterway, rising periodically to the surface to breathe; 4) Artificial feeding: experimental presentation of plants to manatees by lowering a tray holding various plant species into Blue Spring Run. Sometimes two or more manatees jostled each other while attempting to gain access to the tray (Bengtson, 1983); 5) Milling: manatees grouped together but not cavorting (see below) as they moved slowly about; and 6) Cavorting: rolling, pushing, nuzzling, and embracing each other, sometimes with much chasing and splashing. There were several potential sources of error in the recordings and counts of manatees. Both the number of manatees present and the classification of different behaviors may have been misidentified outside the clear waters of Blue Spring (78% of the recordings were made in Blue Spring Run). In the St. Johns River, the number of manatees was estimated by counting their noses as they breathed. At night our estimates were poorer, based on the number of radio-tagged individuals and the breathing frequency. Another possible source of error was the vocalizations of nearby manatees involved in a different behavior. For example, in some cases when recordings were being made of individuals cavorting, others nearby might have been bottom resting or feeding. However, most recordings were made when an individual was alone or when individuals in a group of manatees were behaving similarly. The presence of cow/calf pairs (present in 18% of the samples) may have raised calling rates in our recordings. Rather than attempt to differentiate between calls (whistle-squeaks and chirps), all vocalizations were lumped together and reported as number of calls per unit time. Although both sexes were present during recordings, no differentiation was made between their vocalizations. A total of 3,744 calls was tallied during 21.5 h of recording on 19 days in February through April, 1979. Of these, 2,124 calls were selected for analysis on the basis of knowing manatees' behavior. Calls recorded during periods when the manatees' behavior was uncertain were discarded. Recordings were analyzed for average number of calls (discrete pulses) per 5-min interval for each taping session within a behavioral type. Because the number of manatees per session ranged between 1 and 20 and the length of observation period varied between sampling sessions, the mean number of calls per manatee per unit time was calculated for each behavioral category. These data were normalized using a square root transformation, which stabilized the variances among the different behaviors. A one-way analysis of variance and Tukey's Q test indicated significantly different call rates among behaviors (Fig. 1). Rates of vocalizations 816 Vol. 66, No. 4

Identification and quantitation of fatty acid ethyl esters in biological specimens

Fatty acid ethyl esters, recently described as enzymatic products of nonoxidative ethanol metabolism in the heart, may represent a mediator or marker of ethanol-induced organ pathology such as alcoholic cardiomyopathy. This study was designed to develop a method for the extraction, quantitation, and definitive identification of fatty acid ethyl esters formed both in biological specimens and during enzymatic incubations. First, several potential sources of error were identified and characterized. Tissue extraction with alcohols led to the time, temperature, and concentration-dependent nonenzymatic formation of fatty acid alcohol esters. Contamination of both substrates, [ 14C]ethanol and 14C-fatty acid, used to measure enzymatically mediated fatty acid ethyl ester synthesis, could be removed by purification. Accurate quantitation of fatty acid ethyl esters in tissue was achieved using acetone as an extraction solvent, after which isolated lipids were thinlayer chromatographed on silica gel developed with an apolar solvent system (petroleum ether:diethyl ether:acetic acid, 75:5:1). Gas chromatography and mass spectroscopy identified individual fatty acid ethyl esters. The reproducibility of this assay was high, as assessed by quintuplicate determinations of fatty acid ethyl esters formed in liver and heart homogenates, a method with standard deviations 4 to 11% of the mean.

Interstitial water sampling by dialysis: Methodological notes

The dialysis technique for sampling interstitial waters is subject to several potential sources of error. Cellulose‐based membranes can lead to the underestimation or overestimation of porewater solutes. Polycarbonate was found unsuitable for dialyzer construction because of iron precipitation problems. The initial presence of dissolved O2 in the compartments can significantly affect sample composition. Equilibration times of 15–20 days were found adequate for most sediments. An efficient technique for pH measurement in small volumes of porewater is described.

Decomposition in Reliability Analysis of Fault-Tolerant Systems

Two important problems which arise in modeling fault-tolerant systems with ultra-high reliability requirements are discussed. 1) Any analytic model of such a system has a large number of states, making the solution computationally intractable. This leads to the need for decomposition techniques. 2) The common assumption of exponential holding times in the states is intolerable while modeling such systems. Approaches to solving this problem are reviewed. A major notion described in the attempt to deal with reliability models with a large number of states is that of behavioral decomposition followed by aggregation. Models of the fault-handling processes are either semi-Markov or simulative in nature, thus removing the usual restrictions of exponential holding times within the coverage model. The aggregate fault-occurrence model is a non-homogeneous Markov chain, thus allowing the times to failure to possess Weibull-like distributions. There are several potential sources of error in this approach to reliability modeling. The decomposition/aggregation process involves the error in estimating the transition parameters. The numerical integration involves discretization and round-off errors. Analysis of these errors and questions of sensitivity of the output (R(t)) to the inputs (failure rates and recovery model parameters) and to the initial system state acquire extreme importance when dealing with ultra-high reliability requirements.

The development and validation of radioallergosorbent tests for the detection of specific human IgE antibody directed against laboratory animal urinary proteins.

Radioallergosorbent tests (RAST) using urinary proteins from mice, rats, guinea pigs and rabbits have been developed and used in the diagnosis of laboratory animal allergy (LAA). Of the 273 subjects tested, 15 had been previously diagnosed as laboratory animal allergic and 8 of these (53%) gave one or more positive RAST results. Of the 258 symptom-free individuals, only 9 (3.5%) had one or more positive RAST. Of these 9, 7 had previously worked with animals or had occupational exposure to the appropriate species; the remaining 2 individuals had only some pet exposure. RAST was, therefore, of value in the diagnosis of LAA. During the development of these RAST assays, several sources of potential error were identified. Modest titres of total IgE (600 IU/ml and above) were found to influence the specific RAST index observed and lead to false positive results. The presence of human IgG antibody specific for rabbit serum proteins was also identified in four sera, and was responsible for interference in the rabbit urinary protein RAST system.

Evaluation of the Cd/hemoglobin affinity assay for the rapid determination of metallothionein in biologival tissues

A modification of the Cd/hemoglobin affinity assay described by S. Onosaka, T. Keiichi, M. Doi, and O. Kunio (1978, Eisei Kagaku 24, 128–131) was evaluated for its ease, sensitivity, and specificity for quantifying tissue levels of metallothionein (MT). The five-step method involves (1) preparation of the heat-denatured tissue homogenate supernatant fraction, (2) incubation of the supernatant fraction with 1 μg/ml 109Cd solution, (3) addition of 2% bovine hemoglobin solution, (4) heat precipitation of hemoglobin, and (5) analysis of the supernatant fraction for Cd remaining in the fraction. After purification of MT by gel permeation and ionexchange chromatography and analysis of each fraction for Cd-binding potential, Cd remained in the supernatant fraction only in those samples where MT characteristically elutes. The assay was capable of detecting 40 ng of MT, equivalent to a tissue concentration of 0.8 μg MT/g. The relative standard deviation for replicate assays was less than 4% of the mean. Treatment of rats with CdCl 2 demonstrated the ability of this assay to detect both large and small changes in tissue content of MT after Cd treatment in a variety of tissues. Althogh several potential sources of error are recognized, the ease and sensitivity of this assay should provide a useful means for estimating MT content in most biological tissues and should be particularly useful for large-scale studies where time and expense are major considerations.

The actions of tubocurarine at the frog neuromuscular junction.

1. The action of tubocurarine on voltage-clamped frog muscle end-plates has been re-examined by means (a) equilibrium dose-ratio measurements, (b) current fluctuation measurements and (c) voltage-jump relaxation measurements. 2. The equilibrium measurements can be interpreted as implying that tubocurarine has (a) a competitive blocking action, with a dissociation constant of 0.34 microM, which is not dependent on membrane potential, and (b) an additional voltage-dependent blocking action. 3. In the presence of tubocurarine two kinetic components can be seen. The faster one is similar to, but rather faster than, the normal ion channel closing rate. The other is much slower (1--3 sec), and, in relaxation experiments it is in the opposite direction to the fast relaxation. 4. A number of alternative explanations for the results are discussed. The mechanism that fits them best appears to be a combination of competitive block (or block of shut channels), with a strongly voltage-dependent block of open ion channels by tubocurarine. Estimates of the rate constants for channel blocking (and their voltage dependence) are derived. From these estimates the dissociation constant for the binding of tubocurarine to open channels appears to be roughly 0.12 microM at --70 mV and 0.02 microM at --12 mV. 5. Several potential sources of error in the experiments, and in their interpretation, are discussed. The most serious of these are problems associated with diffusion in the small volume of the synaptic cleft, viz. (a) changes in cleft concentration consequent on changes in binding, and (b) ionophoretic flux of antagonist and agonist into the synaptic cleft.

On the calculation of sugar concentration in flower nectar.

There are several sources of potential error in calculating the concentration or energy value of floral nectar. Errors resulting from confusing data become substantial with increasing concentration. The different methods of expressing sugar concentration are here clarified, and the correct methods of converting from one to the other are provided. Refractometers in use in field studies usually read on a weight per total weight basis; this is recommended as the mode of statement. The perils of oversimplifying conversions from this mode, as is often done, are pointed out.

Prediction of solar irradiance distribution in a wheat canopy using a laser technique

A new measurement technique, the “laser technique”, is proposed for providing timely information concerning the solar irradiance distribution in a vegetative canopy. The method is conceptually similar to the point quadrat method and involves measuring the height where a collimated light beam of very small cross section, aimed at the canopy, first hits foliage. To demonstrate its use, the technique is used to determine power and energy budgets of radiation intercepted by the various foliage components of a wheat ( Triticum turgidum L.) canopy. It was found for the wheat canopy in the dough maturity stage that the middle and upper canopy layers intercept the bulk of the solar irradiance during the middle hours of the day, and that the soil and leaves intercept no sunlight for low sun angles. In comparison with other available measuring methods the laser technique is shown to have two major advantages. First, it is amenable to automation, and, using such an automated system, to rapid acquisition of large data sets. Second, it is applicable to canopies of various heights to about 30 m. Several potential sources of error in application of the technique, methods of error minimization, and improvements to the technique are discussed.

Potential sources of error in the use of peritoneal lavage as a diagnostic tool

Peritoneal lavage has been found to be of value in the early diagnosis of intraperitoneal injury, and has been of particular help in cases of multiple trauma, or when the patient is unable to cooperate with the examiner. There are, however, several potential sources of error which may lead to unnecessary laparotomy. A series of trauma patients who were seen in the University of Virginia Emergency Department and underwent peritoneal lavage is presented. Of the 48 patients in this study, 5% had false positive lavage (ie, no intraperitoneal injury at the time of exploration). Of the 28 who had a negative lavage, none required exploration during the period of observation. Potential and real sources of misinterpretation, particularly with regard to false positive results, are presented. Methods of avoiding this occurrence are also discussed.

Evaluation of the Eye Lens Technique for Aging Columbian Black-Tailed Deer

In order to evaluate the eye lens technique for aging Columbian black-tailed deer (Odocoileus hemionus columbianus), lenses from 83 known-age animals were fixed, desiccated, and weighed in a standardized manner. Several potential sources of error in the preparation and weighing of lenses were explored, including length of desiccating time, length of cooling period before weighing, removal of lenses from eyeballs before fixing, and freezing prior to fixing. The effects of freezing and removal of the lens before fixing were investigated on a series of domestic sheep (Ovis aries) eyes. Freezing had no significant effect, but it was determined that fresh lenses were easily damaged with consequent loss of weight. For statistical comparisons of the influence of sex, nutrition, body weight, and geographical variation, a mathematical expression of the lens weight-age relationship was necessary, and the equation Y = a + b log X was found to give a satisfactory representation of the data. A comparison was made with weights of lenses from 20 known-age Rocky Mountain mule deer (Odocoileus hemionus hemionus) from Utah, and significant differences were found. Lenses from male and female deer differed significantly in weight. The effects of nutrition on lens weight are not clear cut, and even though there is a good general correlation with body weight, trends vary with increasing age. Using the regression, a test was made of the usefulness of the lens technique by estimating the ages of the knownage deer. The margin of error was found to be directly proportional to age, but did not bear a constant relationship and increased rapidly in deer older than 5 years. Accuracy was improved by comparing males and females with their appropriate regressions. In general, this technique is considered to be satisfactory for aging deer at least through 5 years, especially if only year classes are desired. Since Lord (1959) described the use of the dry weight of the eye lens for aging cottontail rabbits (Sylvilagus floridanus), this technique has been applied to a number of other mammals and birds with varying degrees of success. Lord (1961) has also investigated aging of the gray fox (Urocyon cinereoargenteus) by this means, and Sanderson (1961) reported on a study of the raccoon (Procyon lotor). Other applications of this technique have been published by Beale (1962) on fox squirrels (Sciurus niger); Martinson et al. (1961) on swamp rabbits (Sylvilagus aquaticus); Wight and Conaway (1962) on cottontails; and Kolenosky and Miller (1962) for pronghorn antelope (Antilocapra americana). Lord (1962) also applied this method to white-tailed deer (Odocoileus virginianus) and described the relationship between lens weight and nutritional status of the deer. In this paper he also mentioned unpublished studies on the deer mouse (Peromyscus maniculatus) and the Norway rat (Rattus norvegicus). Montgomery (1963) has studied the effects of freezing and decomposition on the weights of raccoon lenses and attributed some of the variability in Sanderson's data (1961) to these causes. In Australia, Dudzinski and Mykytowycz (1961) have also used this method for aging wild rabbits (Oryctolagus cuniculus). Various investigators have attempted to use the lenses of birds for aging purposes. Payne (1961) reported this method to be unsatisfactory for house sparrows (Passer domesticus) and cited unsuccessful studies with other birds. Campbell and Tomlinson (1962) attempted to age chukar partridges (Alectoris graeca) with similarly discouraging results. Generally birds appear to have too much individual variation and overlap in weights between age-classes for successful use of this technique. The principal objective of the present