Anthropomorphic cardiac phantom for dynamic SPECT.

BACKGROUND: As myocardial blood flow measurement (MBF) in SPECT systems became recently available, significant effort has been devoted to its validation. For that purpose, we have developed a cardiac phantom that is able to mimic physiological radiotracer variation in the left ventricle cavity and in the myocardium, while performing beating-like motion. The new phantom is integrated inside a standard anthropomorphic torso allowing a realistic tissue attenuation and gamma-ray scattering METHODS AND RESULTS: A mechanical cardiac phantom was integrated in a commercially available anthropomorphic torso. Using a GE Discovery 530c SPECT, measurements were performed. It was found that gamma-ray attenuation effects are significant and limit the MBF measurements to global/three-vessel resolution. Dynamic SPECT experiments were performed to validate MBF accuracy and showed mean relative error of 14%. Finally, the effect of varying radiotracer dose on the accuracy of dynamic SPECT was studied CONCLUSIONS: A dynamic cardiac phantom has been developed and successfully integrated in a standard SPECT torso. A good agreement was found between SPECT-reported MBF values and the expected results. Despite increased noise-to-signal ratio when radiotracer doses were reduced, MBF uncertainty did not increase significantly down to very low doses, thanks to the temporal integration of the activity during the measurement.

A new cardiac phantom for dynamic SPECT.

BACKGROUND: In recent years, with the advance of myocardial blood flow (MBF) measurement capability in dynamic single photon emission computerized tomography (SPECT) systems, significant effort has been devoted to validation of the new capability. Unfortunately, the mechanical phantoms available for the validation process lack essential features-they either have a constant radiotracer concentration or they have rigid (static) walls unable to simulate cardiac beating. METHODS AND RESULTS: We have developed a mechanical cardiac phantom that is able to mimic physiological radiotracer variation in the left ventricle (LV) cavity and in the myocardium (M), while performing beating-like motion. We have also developed a mathematical model of the phantom, allowing a description of the radiotracer concentrations in both regions (LV, M) as a function of time, which served as a tool for experiment planning and to accurately mimic physiological-like time-activity curves (TACs). A net retention model for the phantom was also developed, which served to compute the theoretical (i.e., expected) MBF of the phantom from measured quantities only, and thus validate the MBF reported by the SPECT system. In this paper, phantom experiments were performed on a GE Discovery NM 530c SPECT system. CONCLUSIONS: A novel dynamic cardiac phantom for emission tomography has been developed. The new phantom is capable of producing a wide range of TACs that can mimic physiological (and potentially in the future, pathological) curves, similar to those observed in dynamic SPECT systems. SPECT-reported MBF values were validated against known (measured) activity of the injected radiotracer from phantom experiments, which allowed to determine the accuracy of the GE Discovery 530c SPECT system.

In vitro simulation of placental transport: part II. Glucose transfer across the placental barrier model.

INTRODUCTION: In utero fetal development and fetal programming for adulthood life are strongly associated with maternal-to-fetal transfer of nutrient and other substances. Gestational diabetes mellitus (GDM) is a major problem and associated with abnormal fetal development, but the mechanisms underlying glucose transport across the placenta barrier (PB) are not completely understood. METHODS: We developed a placenta simulator that can mimic feto-maternal blood circulations along with real transfer across the in vitro biological model of the PB, which is made of a co-culture of endothelial cells (EC) and trophoblast cells (TC) on both sides of a denuded amniotic membrane (AM). Maternal-to-fetal transfer of glucose was monitored over 24 h. RESULTS: The AM is highly permeable to glucose compared to the cellular structures and can serve as a substrate for the co-culture model. The transfer characteristics for glucose are independent of its initial concentration in the maternal compartment, but strongly dependent on the cellular components of the PB. The EC are more resistive to glucose transfer than the TC. The in vitro PB model is the most resistive to glucose transfer. DISCUSSION AND CONCLUSION: The good correlation between the present in vitro results with existing in vivo data demonstrated the potential of this new approach, which can be extended to study various aspects of transplacental transfer, including medications, relevant to GDM or any problem related to in utero programing.

In vitro simulation of placental transport: part I. Biological model of the placental barrier.

INTRODUCTION: The placental barrier (PB) is the thin biological membrane made of endothelial cells (EC), trophoblast cells (TC) and basal membrane that separates between maternal and fetal blood circulations within the placenta and facilitates feto-maternal transport characteristics, which are not completely understood. METHODS: An in vitro biological model of the PB model was co-cultured of human TC (HTR8) and human umbilical vein EC (HUVEC) on both sides of a denuded amniotic membrane (AM) using custom designed wells. RESULTS: Confocal and transmission electron microscopy (TEM) imaging confirmed the morphology expressions of human EC and TC. Further support on the integrity of the new PB model was obtained from the existence of tight junctions and permeability experiments with fluorescence markers of small and large molecules. The monolayer of EC demonstrated the limiting layer for the transport resistance across this complex barrier. DISCUSSION AND CONCLUSION: This new in vitro viable model mimics the architecture of the human PB and can be used in in vitro simulations of transplacental transport studies.

Time-frequency analysis of breathing signals: in vitro airway model.

Sound signals of respiratory airflow represent summations of acoustic waves of various frequencies, which basically depend on the characteristics of the flow and on those of the surrounding tissue. This study was designed to examine the capability of time-frequency distribution (TFD) of respiratory signals in order to differentiate between unobstructed and obstructed upper airways. In order to investigate the TFD characteristics of defined upper airway geometry we conducted a controlled basic study in a laboratory system with an in vitro isolated airway model, which was either unobstructed or had concentric obstructions of various degrees at different locations along the tube. Pressure fluctuations were acquired with a microphone proximal to the airway opening. A short-term Fourier transform was used to study the TFDs of these signals. The results of the in vitro study showed that the energy of the higher frequencies increased for relatively small incremental changes in: i) reduction of the lumen cross-section, ii) decrease of distance from measurement site to obstruction, and iii) increase of breathing effort. Further development of this method may lead to noninvasive clinical techniques for early diagnosis of upper airway obstructions.

In vitro model of intravenous fluid administration: analysis of vein resistance to rapid fluid delivery.

Rapid fluid administration is the cornerstone of successful trauma resuscitation of patients in a state of shock. Intravenous (IV) fluid delivery is a physical intrusion into a vein which results in a complex interaction between the rigid catheter and the compliant vein. We present an experimental model of IV infusion into a vein-like compliant tube that (a) demonstrated the interdependence between fluid administration and blood flow in a compliant tube and (b) allowed investigation of the contribution of the central venous system (between the infusion site and the heart) to the total resistance to infusion flow rate. The results show that in cases with very high resistance in the central venous system a significant increase of infusion flow rate cannot be achieved just by increasing the infusion pressure. Similarly, in cases of small veins when only small catheters can be used, infusate flow rate may be increased only by using two independent infusion ports. In cases with increased tissue pressure due to edema, gravity-driven infusion may not produce sufficient perfusion of the vascular compartments. It was also shown that the vein valves do not always close, and that peripheral blood flow may continue together with the infusate fluid (e.g., when there is a small downstream resistance and infusion with a small catheter).

A technique for global assessment of respiratory muscle performance at different lung volumes.

A system for noninvasive assessment of an all-inclusive function of respiratory muscles at different lung volumes is presented. The apparatus was based on the interrupter technique and facilitated simultaneous measurements of mouth pressure and airflow rate during dynamic or quasistatic manoeuvres. In this study, mouth pressure values were continuously acquired during and after interruption of a forced inspiratory or expiratory manoeuvre for as long as the subject could sustain an elevated mouth pressure against the obstructed opening. These measurements provided information on both muscle strength and power. A total of 420 forced maximal inspiratory and expiratory manoeuvres performed by six healthy subjects were monitored at different lung volumes. The pattern of maximal pressure-time curves was consistent for the same subject regardless of lung volume. Similar values of maximal mouth pressure can be generated by healthy subjects by using either a flange-style mouthpiece or facial mask. For both methods mouth pressure shows a significant (p < 0.05) second order dependency on lung volume for both inspiration and expiration. The standard deviation of measurements from a single subject about a second order curve is of the order of 5-15%. The findings of interchangeability between methods of measurement may be useful in allegedly non-compliant patients.

Hemodynamic changes determine the efficacy of thrombolysis: results from an in-vitro flow model.

BACKGROUND: The efficacy of thrombolytic therapy might be influenced by changes in the hemodynamic status. The aim of the study was to examine whether changes in perfusion pressure could affect the time to reflow in an in-vitro model. METHODS: The in-vitro flow system comprised flexible plastic tubes that enclosed a blood clot formed in one of the circuit arms. Streptokinase (125,000 U) or saline (control) was injected from a proximal side branch to induce thrombolysis. The protocol comprised four treatments: A, perfusion pressure 150/90 mmHg with streptokinase infusion; B, perfusion pressure 150/90 mmHg with saline; C, perfusion pressure 120/60 mmHg with streptokinase; D, perfusion pressure 120/60 mmHg with saline. Reflow was defined as flow restoration determined by the ultrasonic flowmeter. RESULTS: Successful recanalization was obtained in six of six samples subjected to treatment A (100%), two of seven samples subjected to treatment B (28%), three of five samples subjected to treatment C (60%) and none of six samples subjected to treatment D (0%). Time to reflow was 23 +/- 11 min with treatment A, 76 +/- 24 min with treatment B, 66 +/- 25 min with treatment C, and > 90 min with treatment D. CONCLUSIONS: Our data suggest that the hemodynamic status determines the efficacy of streptokinase-induced thrombolysis, and that spontaneous clot lysis is more likely to occur at greater perfusion pressures. It is conceivable that the hypotensive reaction induced by streptokinase in the clinical setting may adversely affect angiographic patency, compared with that observed with other lytic agents such as tissue-type plasminogen activator.

Evaluation of domiciliary long-term oxygen therapy with oxygen concentrators.

Domiciliary long-term oxygen therapy (LTOT) is usually supplied by means of oxygen concentrators (OCs). Various factors that determine the efficacy of such a treatment were evaluated. Sixty-three patients, arbitrarily selected from lists of health care providers, were visited at home by a biomedical engineer and a pulmonary function technician. The evaluation consisted of: i) responses to a directed questionnaire, ii) assessment of the OC output characteristics, and iii) measurement of the patient's oxygen saturation (SaO2) at rest with and without oxygen supplement. Only 33% of patients received oxygen treatment for the recommended 12-24 hours/day and 5% of patients waited the recommended 10 minutes of OC warm-up before connection. Filters were cleaned weekly by only 30% of patients and the concentrator was serviced 3-4 times a year in 25% of cases. The OC was thought to be unduly noisy by 24% of patients and connecting tubing of less than 6 meters was fitted to 90% of OCs, thereby limiting patient mobility. Most of the OCs did not yield the recommended oxygen concentration and the flow rate meters on them tended to underread. Therefore, only 22% of patients received the prescribed oxygen supplement. Whilst breathing room air, a substantial proportion of patients had an SaO2 >90%. Improvements are clearly required in terms of medical indications for LTOT, patient education and supervision, supply and maintenance of concentrators and related equipment.

Hydrodynamic evaluation of intravenous infusion systems.

STUDY OBJECTIVE: We investigated the hydrodynamic characteristics of IV infusion sets for rapid fluid resuscitation. A simple technique has been devised for quantitative evaluation of the hydrodynamic characteristics of IV sets, including their components, for a range of infusion pressures. SETTING AND METHODS: Previous investigations have measured the overall flow rate of infusion sets with and without IV catheters. This study presents a quantitative technique for measuring the resistance to flow of the IV delivery set as a whole as well as its components. An infusion set was measured with 14- and 18-gauge IV catheters while delivering fluid at infusion pressures between 50 (gravity) and 400 mm Hg. MEASUREMENTS AND MAIN RESULTS: At gravity-driven infusion, the drip chamber imposes a resistance to flow of the same order as that of the catheter. At pressurized infusion with small-bore catheters, the catheter consumes the majority of the overall pressure drop. At pressurized infusion with a large-bore catheter or tubing, the standard drip chamber becomes the limiting component and imposes the largest resistance to flow. CONCLUSION: At gravity-delivered pressures (50 and 100 mm Hg), the only effective way of increasing flow rate (more than twofold) is to use a low-resistance drip chamber or to use two infusion sites. At pressurized delivery pressures (more than 200 mm Hg), increasing catheter size from 18 to 14 gauge would be more effective than doubling the number of infusion sets. Also, a more efficient drip chamber adds an important advantage. Finally, increasing the tubing diameter adds only minimal benefit.

Performance characteristics of pressure infusors.

Pressure infusion devices (PID) are used in trauma hospitals and in military applications to enhance rapid fluid replacement of hypovolemic shocked patients. This work presents a simple inexpensive technique for evaluation of the performance characteristics of pressure infusors. Pressure-volume and volume-time curves of pneumatic and spring-activated infusors have been derived for pressurized infusion with various output resistances. Three indices of performance are suggested for evaluation of pressure infusors: the energy to discharge fluid from the bag, efficiency index and time required to empty 50% of the bag.

Performance characteristics of pressure infusors.

Pressure infusion devices (PID) are used in trauma hospitals and in military applications to enhance rapid fluid replacement of hypovolemic shocked patients. This work presents a simple inexpensive technique for evaluation of the performance characteristics of pressure infusors. Pressure-volume and volume-time curves of pneumatic and spring-activated infusors have been derived for pressurized infusion with various output resistances. Three indices of performance are suggested for evaluation of pressure infusors: the energy of discharge fluid from the bag, efficiency index and time required to empty 50% of the bag.