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P M Robitaille

Publications and source records attributed to P M Robitaille.

At least 19 recordsLinked to original sources
PubMed2001

B1 field homogeneity and SAR calculations for the birdcage coil.

The finite-difference time-domain (FDTD) method is used to model a birdcage resonator. All the coil components, including the wires, lumped capacitors and the source, are geometrically modelled together. As such, the coupling effects within the birdcage, including the interactions of coil, source and human head, are accurately computed. A study of the transverse magnetic (B1) field homogeneity and the specific absorption rate (SAR) is presented on an anatomically detailed human head model at 64 and 200 MHz representing 1.5 and 4.7 T MRI systems respectively. Unlike that at 64 MHz, the B1 field distribution is found to be inhomogeneous at 200 MHz. Also, high local SAR values are observed in the tissue near the source due to the coupling between the source and the head at 200 MHz.

Biophysical Phenomena↗
PubMed2000

Computational analysis of the high pass birdcage resonator: finite difference time domain simulations for high-field MRI.

In this work, a finite difference time domain (FDTD) algorithm is validated at 1.5 tesla using the standard GE Signa quadrature head coil and a muscle phantom. The electrical characteristics of the birdcage head coil are then calculated for the linear and quadrature cases. Unlike previous computational analysis which assume idealized currents on the end rings and the struts of the resonator, a complete computational analysis is provided. This treatment considers the coupling between the resonator and the sample and includes a real coil excitation, a complete current derivation, and a thorough description of both B(1) fields and RF radiation. With this improvement, electromagnetic phenomena such as radiation, standing wave currents on the wires, and field inhomogeneities due to interactions between the coil and the load inside the coil are observed. At 200 MHz, it is demonstrated that this particular coil does not work well due to radiation and non-uniformities on the struts of the device. Also, at this frequency magnetic field inhomogeneities become large when the coil is loaded with a phantom.

Computer Simulation↗
PubMed2000

Left ventricular myocardial adenosine triphosphate changes during reperfusion of ventricular fibrillation: the influence of flow and epinephrine.

OBJECTIVE: To determine whether epinephrine in combination with high flow worsens left ventricular (LV) myocardial high-energy phosphate stores during reperfusion of ischemic ventricular fibrillation (VF). DESIGN: Blinded, prospective block randomized, placebo controlled study. SETTING: University medical center research laboratory. SUBJECTS: A total of 22 mixed breed swine weighing 22.0+/-3.3 kg (SD). INTERVENTIONS: Open-chest swine, anesthetized with alpha-chloralose, underwent 10 mins of nonperfused VF followed by reperfusion with cardiopulmonary bypass for 90 mins and then defibrillation. Animals were block randomized to four groups for reperfusion: Group 1 (n = 5), high flow (100 mL/kg/min) and epinephrine (2.5 microg/kg/min); Group 2 (n = 5), high flow and placebo; Group 3 (n = 6), low flow (30 mL/kg/min) and epinephrine; and Group 4 (n = 6), low flow and placebo. MEASUREMENTS AND MAIN RESULTS: In vivo LV creatine phosphate (CP) and adenosine triphosphate (ATP) were determined using whole wall and spatially localized 31P NMR spectroscopy at 4.7 Tesla. During perfusion of the fibrillating myocardium, epinephrine significantly increased aortic pressure (p < .05) and improved defibrillation rates (p < .01). ATP levels during reperfusion were significantly decreased within all groups compared with baseline. There were no differences in ATP levels between groups. High flow, independent of epinephrine, was associated with increased preservation of ATP (p < .05), increased CP/ATP ratios (p < .02) in all layers of the LV wall, and decreased aortic and cardiac vein lactates (p < .001). CONCLUSIONS: Epinephrine, in combination with flow higher than standard cardiopulmonary resuscitation flows, increased perfusion pressure and defibrillation rates, but did not significantly alter myocardial ATP during VF reperfusion in the in vivo heart Reperfusion flow, independent of epinephrine, is a critical determinant of myocardial ATP preservation.

Adenosine Triphosphate↗
PubMed2000

Theoretical model for an MRI radio frequency resonator.

A theoretical model is described for a magnetic resonance imaging (MRI) radio-frequency resonator (an MRI "coil") that is useful at ultrahigh frequencies. The device is a "TEM resonator," which is based on a concept originally proposed by Röschmann (1988). The coil comprises a circular cavity-like structure containing several coaxial transmission lines operating in a transverse electromagnetic (TEM) mode. The model developed herein treats the empty coil and is based on multiconductor transmission line theory. This work generalizes and extends similar analyses of the device by Röschmann (1995) and Chingas and Zhang (1996). The model employs explicit calculation of per-unit-length parameters for TEM lines having arbitrary geometries. Calculations of the resonator's frequency response are found to compare well with measurements. Fields produced by linear (single-point) and quadrature drive are also computed and compared to images of low-permittivity phantoms.

Electromagnetic Fields↗
PubMed1999

Human rapid acquisition with relaxation enhancement imaging at 8 T without specific absorption rate violation.

A standard fast imaging sequence, rapid acquisition with relaxation enhancement (RARE), has been applied to human magnetic resonance at 8 T. RARE is known for its speed, good contrast and high RF power content. Highly T2 weighted images, the hallmark of RARE imaging, were acquired from the human brain. It is demonstrated that while T2 values may be reduced at 8 T, high quality RARE images could still be acquired at this field strength. Most importantly however, it is demonstrated that RARE images could be acquired without violating specific absorption rate (SAR) guidelines. Since it is well known that T2 weighted images are of significant value in clinical diagnosis, the implementation of RARE at this field strength will provide ultra high field MRI (UHFMRI) with a valuable imaging protocol at this field strength without exceeding SAR limitations.

Brain↗
PubMed1999

MDEFT imaging of the human brain at 8 T.

T1-weighted images of the human brain obtained with the MDEFT sequence at 8 T are presented. These images are characterized by an excellent contrast and good signal to noise ratio. Importantly, results were obtained with adiabatic spin inversion and demonstrate that such pulses can be used even in the ultra high frequency (> 300 MHz) range. It is thus possible to obtain high quality results at this field strength without violating SAR guidelines.

Brain↗
PubMed1999

Flow requirements in ventricular fibrillation: An in vivo nuclear magnetic resonance analysis of the left ventricular high-energy phosphate pool.

STUDY OBJECTIVE: We sought to determine whether flow rates of approximately 60% of normal values are sufficient to preserve the left ventricular myocardial high-energy phosphate pool during ventricular fibrillation (VF). METHODS: Mixed-breed swine (weight 22. 4+/-2.5 kg) were anesthetized with alpha-chloralose, placed in a state of VF, and perfused with extracorporeal circulation at a target flow of 50 mL.kg(-1).min(-1). In vivo whole-wall (average of left ventricular wall) and spatially localized phosphorous-31 nuclear magnetic resonance (NMR) spectra were acquired at baseline and during VF. RESULTS: Mean flow during VF was 58+/-20 mL.kg(-1). min(-1) (+/-SD; 95% confidence interval, 44 to 71) or about 60% of baseline cardiac output (n=13). Whole-wall adenosine triphosphate (ATP) decreased during perfused VF (P <.05), whereas creatine phosphate (CP) remained unchanged from baseline. With spatially localized NMR, the ratios of CP/ATP were similar at baseline in all layers (endocardium --> epicardium) of the left ventricular wall. However, during perfused VF, subepicardial CP/ATP ratios increased by 14% to 40% compared with baseline values, whereas subendocardial CP/ATP ratios remained unchanged (1% to 3% increase). An additional 4 animals perfused at 72+/-10 mL.kg(-1).min(-1) (+/-SD; 95% confidence interval, 56 to 92) during VF had preservation of CP and ATP levels. CONCLUSION: Flow levels equivalent to 60% of baseline cardiac output were insufficient to maintain normal high-energy phosphate levels in the in vivo fibrillating myocardium. At this level of flow, myocardial high-energy phosphate loss is nonhomogeneous within the left ventricular wall.

Adenosine Triphosphate↗
PubMed1999

High signal-to-noise FLASH imaging at 8 Tesla.

A radio frequency (RF) and gradient spoiled fast low angle shot technique was used to acquire images from the human brain at 8 Tesla. The resulting FLASH images, obtained with a 17 degrees nutation, a 70 ms repetition time, and a 17 ms echo time, displayed an average signal-to-noise ratio (SNR) of 220:1 (slice thickness 2.2 mm, field-of-view 24 cm, matrix 256 x 128). These images were compared with images obtained at 1.5 Tesla using identical parameters yielding a signal-to-noise of less than 10:1. As such, the 8 Tesla images display a remarkable improvement in SNR with increasing field strength. The images also show little evidence of susceptibility distortion, chemical shift, or RF penetration limitations.

Brain↗
PubMed1999

Cognitive, cardiac, and physiological safety studies in ultra high field magnetic resonance imaging.

A systematic analysis of the effect of an 8.0 tesla static magnetic field on physiological and/or cognitive function is presented in the normal volunteer and in the swine. A study of ten human subjects revealed no evidence of detectable changes in body temperature, heart rate, respiratory rate, systolic pressure, and diastolic blood pressure after 1 hour of exposure. In addition, no cognitive changes were detected. Important ECG changes were noted which were related both to the position of the subject in the magnet and to the absolute strength of the magnetic field. As such, the ECG tracing at 8 tesla was not diagnostically useful. Nonetheless, all subjects exhibited normal ECG readings both before and following exposure to the 8 tesla field. Cardiac function was also examined in detail in the swine. No significant changes in body temperature, heart rate, left ventricular pressure, left ventricular end diastollic pressure, time rate of change of left ventricular pressure, myocardial stiffness index, cardiac output, systolic volume, troponin, and potassium levels could be detected following 3 h of exposure to a field strength of 8.0 tesla. It is concluded that no short term cardiac or cognitive effects are observed following significant exposure to a magnetic field of up to 8.0 tesla.

Animals↗
PubMed1999

In-vivo myocardial substrate alteration during perfused ventricular fibrillation.

OBJECTIVES: Earlier work suggests the in-vivo heart alters its substrate utilization as a function of cardiac work. Previous work has also demonstrated the high oxygen requirements of the heart during ventricular fibrillation (VF). The authors hypothesized that myocardial substrate utilization during VF with perfusion is similar to the normal beating heart under conditions of increased workload. METHODS: Myocardial substrate selection was studied in the in-vivo porcine myocardium using 13carbon nuclear magnetic resonance (13C NMR) under conditions of increased cardiac work (dobutamine group) and VF with extracorporeal perfusion (VF group). Once the animal preparation was completed, metabolic steady state was achieved with the infusion of unlabeled acetate into the left anterior descending (LAD) coronary artery. The infused substrate was then changed to [2-13C] acetate and glutamate pool labeling was monitored by 13C NMR. The glutamate C4 resonance areas at baseline and after intervention of either increased workload (dobutamine group) or perfused VF (VF group) were compared within groups using paired t-tests. RESULTS: Baseline aortic and great cardiac vein lactates, glucose levels, blood gases, hemoglobin levels, and temperatures were similar between groups. In both groups, there was a significant decrease from baseline in the labeling of C4 glutamate peaks (dobutamine group: 20.2+/-14.9 vs 84.7+/-32.7, p = 0.002; and VF group: 49.8+/-24.4 vs 83.9+/-24.4, p = 0.02), indicating selection against acetate oxidation in favor of other endogenous substrates. CONCLUSIONS: In the in-vivo heart, despite the absence of functional contractions, changes in substrate utilization during perfused VF are similar to changes that occur with increased workload in the normal beating heart.

Acetates↗
PubMed1998

Human magnetic resonance imaging at 8 T.

In this work, we present the first human magnetic resonance image (MRI) obtained at ultrahigh field strengths (8 T). We demonstrate that clinical imaging will be possible at 8 T and that reasonable quality head images can be obtained at this field strength. Most importantly, we emphasize that the power required to excite the spins at 8 T is much lower than had previously been predicted by the nuclear magnetic resonance theory. A 90 degree pulse in the head at 8 T requires only approximately 0.085 J of energy (90 W for a 2-lobe 4 ms sinc pulse). Based on measurements at 4 T, 1-2 J of energy should have been utilized to achieve a 90 degree excitation at 8 T. The fact that the energy required for spin excitation at 8 T is much lower than predicted by the NMR theory, will be extremely important to the viability of ultrahigh field imaging, since concerns related to power absorption and specific absorption rate (SAR) violations at ultrahigh field are alleviated. As such, it will be possible to utilize RF intensive pulse sequences and adiabatic spin excitation at 8 T without significant risk to the subject.

Brain↗
PubMed1998

31P-NMR analysis of congestive heart failure in the SHHF/Mcc-facp rat heart.

31P-NMR was used to monitor myocardial bioenergetics in compensated and failing SHHF/MCC-fa(cp) (SHF) rat hearts. The SHHF/Mcc-fa(cp) (spontaneous hypertension and heart failure) rat is a relatively new genetic model in which all individuals spontaneously develop congestive heart failure, most during the second year of life. Failing SHF rat hearts displayed a pronounced decrease in resting PCr:ATP ratios (P<0.001), which was explained by a significant (P<0. 0001) drop in total creatine (47.2+/-3.1 nmol/mg protein) v age matched controls (106+/-3 nmol/mg protein). In end stage failure, NMR determined PCr was 2.9+/-0.1 micro mol/g wet weight under basal conditions. In contrast, 6- and 20-month-old controls and compensated SHFs had PCr values of 5.3+/-0.1, and 5.1+/-0.5 and 5. 1+/-0.2 micro mol/g wet weight. Both compensated and failing SHF hearts were metabolically compromised when the rate pressure product (RPP) was increased, as evidenced by an increase in Pi and a drop in PCr. Compensated SHF hearts, however, were able to increase rate pressure products (RRP, mmHg X beats/min) from 44.5+/-1.4 to 66.6+/-3. 4 K with dobutamine infusion, whereas hearts in end-stage failure were able to increase their RPP from baseline values of 27+/-4 K to only 37+/-7 K. The data indicate that a pronounced decline in PCr and total creatine signals the transition from compensatory hypertrophy to decompensation and failure in the SHF rat model of hypertensive cardiomyopathy.

Adenine Nucleotides↗
PubMed1997

Dynamic 13C NMR analysis of pyruvate and lactate oxidation in the in vivo canine myocardium: evidence of reduced utilization with increased work.

In this work, substrate selection was monitored in the left ventricle of the canine myocardium by following pyruvate and lactate oxidation under in vivo conditions at basal and elevated workloads. These studies were conducted in the open chest model using dynamic 13C NMR techniques in the presence and absence of dichloroacetic acid (DCA), a well-known activator of pyruvate dehydrogenase (PDH). Following the infusion of (3-(13)C) pyruvate or (3-(13)C) lactate into the left anterior descending artery, highly variable 13C enrichments of glutamate, alanine, aspartate, and citrate were noted under low (RPP < 14,500 mmHg/min), intermediate (RPP = 15,000-25,000 mmHg/min), and high (RPP > 25,500 mmHg/min) rate pressure products (RPP). At low workloads, the myocardium typically oxidized the infused (3-(13)C) pyruvate or (3-(13)C) lactate and incorporated the labeled carbon into the glutamate pool as expected. However, in a few notable instances (n = 3), 13C-enriched pyruvate and lactate were unable to label the glutamate pool under in vivo conditions even at the lowest RPPs, indicating a lack of selection for these substrates by the tricarboxylic acid (TCA) cycle. Nonetheless, the levels of glutamate C4 enrichment observed at low workloads could usually be enhanced by infusion of DCA. Importantly, 13C NMR extract analysis revealed that (3-(13)C) pyruvate or (3-(13)C) lactate labeling of the glutamate pool was reduced (< 20%) at high workloads in spite of increased DCA concentrations.

Animals↗
PubMed1997

Phosphorus-31 magnetic resonance spectroscopy studies of pig spinal cord injury. Myelin changes, intracellular pH, and bioenergetics.

RATIONALE AND OBJECTIVES: Phosphorus-31 (31P) nuclear magnetic resonance (NMR) spectroscopy was used to monitor changes in phosphocreatine (PCr), adenosine triphosphate (ATP), inorganic phosphate (Pi), intracellular pH (pHi), and free magnesium in the in vivo pig spinal cord after injury. METHODS: Phosphorus-31 NMR spectra were acquired from healthy (n = 4) and injured pig spinal cords (n = 8) under in vivo conditions using a 4.7-tesla spectrometer. Spinal cords were injured by dropping a 20-g weight from 20 cm onto the surgically exposed cord surface. RESULTS: In vivo spectra of injured cords revealed a reduction in ATP, PCr, pHi, and an increase in Pi. In addition, a broad resonance that is likely to arise from myelin phospholipids was reduced significantly after injury. CONCLUSIONS: Phosphorus-31 NMR can be used to follow in vivo changes in high energy phosphates after injury and may have the potential to follow changes in myelin structure. This technique may prove important in the study of myelin breakdown after secondary, nonreversible spinal cord injury. Changes in high energy phosphates and pHi did not seem to parallel these putative changes in myelin structure.

Adenosine Triphosphate↗
PubMed1996

Alternate k-space sampling in EPI: compensation for T2* and adjustable T2 weighting.

In this work, preliminary results are described for a modification of the MBEST sampling scheme such that image resolution can be increased while preserving image contrast. In this new approach, a single spin-echo is used in sampling k-space. The basic idea relies on acquiring a conventional EPI image from the center of k-space and applying a pi pulse to permit the acquisition of the two outer edges of k-space. Using this new approach, it is possible to obtain an enhancement in EPI image resolution, while reducing the extent of T2* weighting. As a result, the resulting images possess reduced T2* contrast and suffer less signal loss from T2* effects such as spatial variations in susceptibility and field inhomogeneity.

Brain↗
PubMed1996

Muscle torque in young and older untrained and endurance-trained men.

Plantar flexor torque was measured in 24 young (25 +/- 1.4 y) and older (62 +/- 2 y) untrained and endurance-trained men to test the hypothesis that age-associated declines in muscle function would be attenuated in older men who also endurance trained. Endurance-trained subjects averaged 7-9 h/wk of aerobic activity for 10-12 years. These subjects had not engaged in resistance training previously in the past 10 years. Plantar flexor torque was measured at velocities between 0 and 5.23 rads. s-1. In absolute terms, maximal isometric torque was 23% lower in older men compared to young men, regardless of their training status. On the other hand, relative measures of isometric strength (i.e., torque.muscle cross-sectional area-1 and torque.muscle volume-1) were similar in young and older men but were higher in trained than in untrained men. Isokinetic torque.muscle cross-sectional area-1 and torque.muscle volume-1 was greater at contraction velocities of 0.26-2.09 rads.s-1 for trained subjects. These data suggest that endurance training does not attenuate the age-associated loss of muscle mass or absolute strength. However, endurance training might reduce the extent of loss of relative strength because torque-muscle cross-sectional area-1 and torque.muscle volume-1 are greater in endurance-trained older men than in untrained older men.

Adult↗
PubMed1996

Spatial localization with modified Fourier series windows. Application to the transmural 13C-nuclear magnetic resonance analysis of the in vivo myocardium.

RATIONALE AND OBJECTIVES: A modified Fourier Series Window (FSW) method is introduced that provides a simple, reasonably accurate, solution to spatial localization for layers in nuclear magnetic resonance (NMR) spectroscopy. This method was developed because signal originating from spins immediately adjacent to the coil plane can leak into standard FSW localized spectra obtained from more distal layers. METHODS: The B1 profile of the surface coil and a priori sample knowledge were used to generate modified FSW coefficients that largely compensate for contamination from proximal layers. These modified coefficients were used to acquire spatially localized spectra from a three-layered phantom containing inorganic phosphate, phosphocreatine, and pyrophosphate. Spatially localized spectra also were obtained from the open-chest canine myocardium. RESULTS: The modified Fourier Series Window approach was validated using a quality assurance phantom. This method was then applied to the transmural analysis of 13C-containing metabolites in the in vivo canine myocardium during 3-(13)C sodium pyruvate infusion (n = 10). In vivo NMR spectra were characterized by resonances corresponding to the C2, C3, and C4 carbons of glutamic acid and to endogenous triacyglycerides and surface lipids. A transmural trend in metabolites could be observed under these conditions. This trend, however, was likely to result from the interference of surface lipids in the epicardial layer. CONCLUSIONS: The authors demonstrate that the modified Fourier Series Window method can be applied in situations where signal-to-noise considerations limit the applicability of more sophisticated spatial localization methods. In addition, the authors report a slight gradient in the endogenous triacylglyceride resonance, which is likely to originate from the presence of surface lipids.

Animals↗