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E Atalar

Publications and source records attributed to E Atalar.

At least 19 recordsLinked to original sources
PubMed2000

RF transmit power limit for the barewire loopless catheter antenna.

The safety of the barewire loopless catheter antenna in transmit mode is addressed with respect to radiofrequency (RF) heating. Analytical expressions for electric field and specific absorption rate (SAR) distributions surrounding the antenna are postulated and experimentally verified. Limiting RF transmit power to 40-70 mW time-averaged power, depending on the specific antenna design, will ensure that the current regulatory guideline of SAR of 8 W/kg in any gram of tissue is not exceeded. These limits can act as guidelines for the design of RF pulses for use with this device. Further study is required to examine the safety of the antenna in receive mode.

Absorption↗
PubMed2000

Assessment of autonomic nervous system functions in patients with vitamin B12 deficiency by power spectral analysis of heart rate variability.

The purpose of this study was to test the autonomic nervous system function of patients with vitamin B12 deficiency (megaloblastic anemia) by measuring heart rate variability (HRV). The study population consisted of 17 vitamin B12 deficient patients and 15 age- and sex-matched normal volunteers. HRV was measured by power spectral analysis from which power of the low frequency (LF) peak (0.04-0.15 Hz), normalized units of the LF peak (LFNU), power of the high frequency (HF) peak (0.15-0.4 Hz), normalized units of the HF (HFNU), and ratio of power of LF to power of HF (LF:HF) were calculated. Vitamin B12 deficient patients had lower LF, LFNU, HF, HFNU, and LF:HF ratio than normal volunteers (P < 0.05). Decreases in sympathetic indices (LF and LFNU) were greater than those measured in parasympathetic indices (HF and HFNU). All HRV parameters correlated positively with the level of vitamin B12 (P < 0.001) and negatively with the duration of disease (P < 0.001). After vitamin B12 replacement the HRV parameters of patients and controls became comparable (P > 0.05). Our data suggest that autonomic sympathetic and parasympathetic nervous activities are decreased in patients with vitamin B12 deficiency, an abnormality that can be corrected by vitamin B12 replacement therapy.

Autonomic Nervous System↗
PubMed2000

P wave dispersion on 12-lead electrocardiography in patients with paroxysmal atrial fibrillation.

The prolongation of intraatrial and interatrial conduction time and the inhomogeneous propagation of sinus impulses have been shown in patients with atrial fibrillation. Recently P wave dispersion (PWD), which is believed to reflect inhomogeneous atrial conduction, has been proposed as being useful for the prediction of paroxysmal atrial fibrillation (PAF). Ninety consecutive patients (46 men, 44 women; aged 55 +/- 13 years) with a history of idiopathic PAF and 70 healthy subjects (42 men, 28 women; mean age 53 +/- 14 years) were studied. The P wave duration was calculated in all 12 leads of the surface ECG. The difference between the maximum and minimum P wave duration was calculated and this difference was defined as P wave dispersion (PWD = Pmax-Pmin). All patients and controls were also evaluated by echocardiography to measure the left atrial diameter and left ventricular ejection fraction (LVEF). There was no difference between patients and controls in gender (P = 0.26), age (P = 0.12), LVEF (66 +/- 4% vs 67 +/- 5%, P = 0.8) and left atrial diameter (36 +/- 4 mm vs 34 +/- 6 mm, P = 0.13). P maximum duration was found to be significantly higher in patients with a history of PAF (116 +/- 17 ms) than controls (101 +/- 11 ms, P < 0.001). P wave dispersion was also significantly higher in patients than in controls (44 +/- 15 ms vs 27 +/- 10 ms, P < 0.001). There was a weak correlation between age and P wave dispersion (r = 0.27, P < 0.001). A P maximum value of 106 ms separated patients with PAF from control subjects with a sensitivity of 83%, a specificity of 72%, and a positive predictive accuracy of 79%. A P wave dispersion value of 36 ms separated patients from control subjects with a sensitivity of 77%, a specificity of 82%, and a positive predictive accuracy of 85%. In conclusion, P maximum duration and P wave dispersion calculated on a standard surface ECG are simple ECG markers that could be used to identify the patients with idiopathic paroxysmal atrial fibrillation.

Atrial Fibrillation↗
PubMed2000

Inappropriate shocks diagnosed by stored electrograms of implantable cardioverter defibrillators--two case reports.

An implantable cardioverter defibrillator is an important therapeutic option for patients with high risk of life-threatening ventricular arrhythmias. However, their use is also associated with several complications including inappropriate shock. Although the most frequent cause of inappropriate shock is supraventricular tachyarrhythmias, lead fracture can also be associated with inappropriate shock. Diagnosis of lead fracture can be made by chest x-ray radiography, fluoroscopic examination, interrogation of the device, and intracardiac electrograms. In this report, the authors present two cases of inappropriate shock due to lead fractures in the costoclavicular region that could only be diagnosed by the help of stored intracardiac electrograms. Methods for diagnosis of lead fractures and modalities to avoid recurrences are also discussed.

Artifacts↗
PubMed1999

A novel object-independent "balanced" reference scan for echo-planar imaging.

Interleaved echo-planar imaging (EPI) is susceptible to significant ghosting artifacts, resulting primarily from system time delays that cause data matrix misregistration. Most EPI applications rely on "reference scans" to measure delays, and post-processing algorithms are used to correct these errors. Unfortunately, delay estimates made with most reference scan techniques are object dependent, since they are biased by magnetic field inhomogeneities and chemical shift. The current work describes the effects of field inhomogeneities and their influence on system time delay estimation. Subsequently, a new, object-independent "balanced" reference method using two readout echo trains is proposed for time delay measurements.

Algorithms↗
PubMed1999

Minimizing dead-periods in flow-encoded or -compensated pulse sequences while imaging in oblique planes.

A technique is developed to minimize magnetic resonance pulse sequence dead-periods, with first and higher moment requirements, while imaging in oblique planes. Dead-period requirements of starting amplitude, ending amplitudes, area, and other moment requirements are transformed from the image coordinate system to the physical gradient coordinate system, where the waveforms are then designed. With this technique, the full capabilities of the gradient hardware are utilized. An online algorithm is presented to perform dead-period minimization and gradient waveform design when the first moment and area of the dead-period are specified. The algorithm is then used to implement three-axis flow-compensation in a fast spoiled gradient-echo sequence. This results in a minimal increase in TE and TR over an equivalent non-flow-compensated sequence, and little variation in the minimum TR over the entire range of oblique slice orientations. Applications of this algorithm extend to the optimization of any pulse sequence in which the first moment is important and oblique imaging is required. J. Magn. Reson. Imaging 1999;10:183-192.

Algorithms↗
PubMed1999

Referenceless interleaved echo-planar imaging.

Interleaved echo-planar imaging (EPI) is an ultrafast imaging technique important for applications that require high time resolution or short total acquisition times. Unfortunately, EPI is prone to significant ghosting artifacts, resulting primarily from system time delays that cause data matrix misregistration. In this work, it is shown mathematically and experimentally that system time delays are orientation dependent, resulting from anisotropic physical gradient delays. This analysis characterizes the behavior of time delays in oblique coordinates, and a new ghosting artifact caused by anisotropic delays is described. "Compensation blips" are proposed for time delay correction. These blips are shown to remove the effects of anisotropic gradient delays, eliminating the need for repeated reference scans and postprocessing corrections. Examples of phantom and in vivo images are shown.

Algorithms↗
PubMed1999

Multi-echo segmented k-space imaging: an optimized hybrid sequence for ultrafast cardiac imaging.

Cardiac magnetic resonance imaging requires high temporal resolution to resolve motion and contrast uptake with low total scan times to avoid breathing artifacts. While spoiled gradient echo (SPGR) imaging is robust and reproducible, it is relatively inefficient and requires long breath-holds to acquire high time resolution movies of the heart. Echo planar imaging (EPI) is highly efficient with excellent signal-to-noise ratio (SNR) behavior; however, it is particularly difficult to use in the heart because of its sensitivity to chemical shift, susceptibility, and motion. EPI may also require reference scans, which are used to measure hardware delays and phase offsets that cause ghosting artifacts; these reference scans are more difficult and less reliable in the heart. Consequently, a hybrid EPI/SPGR sequence is proposed for application to rapid cardiac imaging. A detailed optimization of SNR and echo train length for multi-echo sequences is presented. It is shown that significant reductions in total scan time are possible while maintaining good image quality. This will allow complete motion sampling of the entire heart in one to three breath-holds, necessary for MR cardiac dobutamine stress testing. Improved speed performance also permits sampling of three to six slices every heartbeat for bolus injection perfusion studies.

Echo-Planar Imaging↗
PubMed1999

Switchable multicoil array for MR micro-imaging of breast lesions.

High-resolution breast imaging may improve differentiation between benign and malignant lesions and may be important for refining treatment strategy. This article presents a new, flexible design of a breast-imaging coil capable of providing breast images of a high level of spatial resolution. Referred to as a switchable coil array, the design uses small-diameter surface coils that provide high sensitivity of detection, which, combined with a relatively small field of view, affords a high degree of spatial resolution (up to 200 microm). Remote selection of the coil pair closest to the position of the lesion in the breast permits coverage of the whole breast without changing the position of the coils or the patient. High-resolution MR images of phantom and volunteer patients with benign and malignant breast lesions are presented.

Biopsy, Needle↗
PubMed1999

Transesophageal magnetic resonance imaging.

The purpose of this study was to develop a non-invasive method of imaging the thoracic aorta that would provide both morphological detail within the aortic wall and information about regional aortic wall motion. An esophageal probe is described that allows transesophageal MR imaging (TEMRI) of the thoracic aorta and has several potential advantages over the competing non-vasculoinvasive techniques of transesophageal echocardiography (TEE) or standard MRI. The probe consists of a loopless antenna housed inside a modified Levin gastric tube, with external matching and tuning circuitry. Using this probe, the thoracic aorta has been imaged in longitudinal and cross-sectional views. Details of the aortic wall were readily seen. Tissue tagging for measurement of focal stress/strain relationships was demonstrated to be feasible. TEMRI avoids the risks inherent in intravascular MRI yet provides comparable image quality. Potential applications of the device are discussed.

Animals↗
PubMed1999

Circadian variations of QTc dispersion: is it a clue to morning increase of sudden cardiac death?

BACKGROUND: Several studies related to cardiac events including sudden death have shown a peak incidence in the early morning hours. It has also been known that acute ischemia is a potent stimulus to increased dispersion of repolarization and development of malignant arrhythmias. HYPOTHESIS: The purpose of the present study was to investigate diurnal variations of corrected QT dispersion (QTcD) in patients with coronary artery disease (CAD) (Group 1) compared with controls with normal coronary angiograms (Group 2). METHODS: We investigated a total of 110 patients who had been referred for coronary angiography, of whom 62 (42 men, 20 women; age 55 +/- 7 years) had double- or triple-vessel disease, and of whom 48 (31 men, 17 women; age 54 +/- 9 years) had normal coronary angiograms. QTcD measurements were calculated from a 12-lead resting electrocardiogram (ECG) during sinus rhythm. These ECGs were obtained for each patient in the morning, at noon, in the evening, and at night on the day after performance of coronary angiography. QTcD was significantly greater in patients with abnormal coronary angiograms (Group 1) than in patients with angiographically documented normal coronary arteries (Group 2). This difference appeared to be more prominent in the morning hours (p < 0.001) than at other times. QTcD in the evening and night hours was not statistically different (p > 0.05) between both groups. We also compared intragroup QTcD values: QTcD values were significantly increased in the morning hours and were more prominent in Group 1 than in Group 2. CONCLUSIONS: Our data suggest that QTcD has a circadian variation with an increase in the morning hours, especially in patients with coronary artery disease. This finding was thought to be an explanation for the role played by sympathetic nervous system in the occurrence of acute cardiac events and sudden death during these hours.

Acute Disease↗
PubMed1999

Methylene tetrahydrofolate reductase genotype and the risk and extent of coronary artery disease in a population with low plasma folate.

OBJECTIVE: To determine the effects of the thermolabile methylene tetrahydrofolate reductase (MTHFR) mutation on the presence and extent of coronary atherosclerosis in a population with low plasma folate. METHODS: 242 consecutive patients undergoing coronary angiography were prospectively evaluated for conventional risk factors, plasma homocysteine, vitamin B-12, and folate, and MTHFR genotype. The severity of coronary atherosclerosis was determined by the Leaman score. RESULTS: Mean (SD) plasma homocysteine was 15.6 (10) micromol/l in controls and 18.5 (11) micromol/l in patients with coronary artery disease (p > 0.05). Plasma homocysteine concentrations above 15 micromol/l were a risk factor for coronary artery disease (p = 0.03, risk ratio 2.1, 95% confidence interval (CI) 1.07 to 4.4). Homocysteine remained an independent risk factor on multivariate analysis when conventional risk factors were taken into account (p = 0.04). Homocysteine concentrations above 15 micromol/l were correlated with the extent of atherosclerosis (p = 0. 04, risk ratio 3.2, 95% CI 1.3 to 8.2). Homocysteine had no effect on other lipid variables (p > 0.05). Plasma folate was 15.8 (7.2) nmol/l in controls and 11.5 (2.9) nmol/l in patients with coronary artery disease. Plasma folate concentrations below 12.9 nmol/l (5.7 ng/ml) conferred a risk for coronary artery disease (p = 0.03, risk ratio 2.42, 95% CI 1.05 to 5.59). When the MTHFR genotype was determined, the TT genotype was present in 7.4% of patients and 5.2% of controls (p > 0.05). The prevalence of alleles was within the Hardy-Weinberg equilibrium (TT 7, CT 40, CC 53, chi2 = 2.3, p = 0.3). The highest homocysteine concentrations were found in patients with the TT genotype and folate below the median of the population (p = 0. 01). The extent of coronary atherosclerosis judged by the Leaman score was significantly higher in patients with the TT genotype (p = 0.03). CONCLUSIONS: Plasma homocysteine over 15 micromol/l was a significant risk factor for the presence and extent of coronary artery disease. The mean plasma folate of the population was low and correlated negatively with homocysteine. Although TT genotype was not an independent predictor of coronary artery disease, it was an important predictor of the extent of coronary atherosclerosis and plasma homocysteine, especially in the presence of plasma folate values below the median of the population. These findings may have important implications for folate replacement in patients with the TT genotype.

Case-Control Studies↗
PubMed1998

Ultimate intrinsic signal-to-noise ratio in MRI.

A method to calculate the ultimate intrinsic signal-to-noise ratio (SNR) in a magnetic resonance experiment for a point inside an arbitrarily shaped object is presented. The ultimate intrinsic SNR is determined by body noise. A solution is obtained by optimizing the electromagnetic field to minimize total power deposition while maintaining a constant right-hand circularly polarized component of the magnetic field at the point of interest. A numerical approximation for the optimal field is found by assuming a superposition of a large number of plane waves. This simulation allowed estimation of the ultimate intrinsic SNR attainable in a human torso model. The performance of six coil configurations was evaluated by comparing the SNR of images obtained by the coils with the ultimate values. In addition, the behavior of ultimate intrinsic SNR was investigated as a function of main field strength. It was found that the ultimate intrinsic SNR increases better than linearly with the main magnetic field up to 10 T for our model. It was observed that for field strengths of 4 T or higher, focusing is required to reach the ultimate intrinsic SNR.

Abdomen↗
PubMed1998

Catheter-tracking FOV MR fluoroscopy.

Recent improvements in intravascular magnetic resonance imaging techniques mandate an accurate method of monitoring the introduction of MR catheter probes into the vessel of interest. For this purpose, a novel imaging protocol and a display method have been designed. First, a roadmap 3D image data set with standard pulse sequences is obtained using an external imaging coil. Subsequently, using very narrow rectangular-FOV fast-spoiled gradient recalled (SPGR), a movie of the percutaneous placement procedure of an MR catheter probe is acquired at a rate of 7.3 frames/second. In this protocol, the probe is used to transmit RF pulses and receive MR signal. A computer program was written for image unwrapping and for displaying the unwrapped movie frames on the roadmap image. In an alternative protocol, the movie frames in two projection angles were acquired in an interleaved fashion. Frames were unwrapped and combined with a 3D roadmap and displayed on a Silicon Graphics workstation equipped with stereovision goggles. Using these methods, percutaneous catheter placement in a phantom and a dog was examined. In conclusion, a new visualization technique for MR catheter placement is proposed. Combining this technique with high resolution intravascular MRI techniques may result in a very useful diagnostic tool for the evaluation of atherosclerosis and other vessel diseases.

Algorithms↗
PubMed1998

Precision of myocardial contour estimation from tagged MR images with a "black-blood" technique.

RATIONALE AND OBJECTIVES: The authors determined whether blood presaturation of tagged magnetic resonance (MR) images affects identification of left ventricular endocardial borders. MATERIALS AND METHODS: Three healthy volunteers underwent MR imaging performed with a breath-hold segmented spoiled gradient-recalled-echo sequence with tissue tagging. Two saturation pulses (in the basal and apical regions) were used to generate black-blood images. Manual segmentation of endocardial contours on black-blood and white-blood images was performed independently by five observers. RESULTS: Endocardial borders were better identified on black-blood images compared with white-blood images, especially in the early systolic phases. Interobserver variability in contour estimation was significantly higher for white-blood images (P < .001) and was twice that for corresponding black-blood images during early systole. Contour variability appeared to be affected mainly by tag-to-myocardium contrast (P = .009) and myocardium-to-chamber contrast (P = .05). CONCLUSION: Blood presaturation of tagged MR images improves reliability of contour segmentation.

Adult↗
PubMed1998

Paradoxical hypotension during dobutamine infusion for myocardial perfusion scintigraphy.

AIM: Dobutamine as a predominant beta-1 agonist increases heart rate and myocardial contractility and at sufficient high doses, it also increases systolic blood pressure. This study was undertaken to describe instances of paradoxical hypotension during dobutamine infusion for TI-201 myocardial perfusion SPECT study and the relationship between scintigraphic findings and hypotension occurred during dobutamine infusion. METHODS: In 201 consecutive patients unable to perform adequate exercise, dobutamine TI-201 myocardial SPECT was performed. Dobutamine was infused starting from 10 micrograms/kg/min increasing to 40 micrograms/kg/min. Paradoxical hypotension was defined as a decrease in systolic blood pressure > or = 20 mmHg compared with baseline study. RESULTS: Paradoxical hypotension was observed in 40 patients (Group A) out of 201 (19.9%) while no significant change in systolic blood pressure was detected in the remaining 161 patients (Group B). Mean maximum fall in systolic blood pressure was 39 +/- 18 mmHg (range: 20-90). In 33 of 40 patients (83%) with paradoxical hypotension, scintigraphy was normal compared to 131 (81%) of the remaining 161 patients. In patients of Group A, angiography, echocardiography and tilt table tests were performed in 13, 11 and 6 patients respectively. Nine of 13 angiographic evaluations (69%), 10 of 11 echocardiographic evaluations (91%), all of the tilt table tests were normal. Additionally, all of the patients of Group A were clinically followed up at least 6 months after the myocardial perfusion scintigraphy. None of the patients had a cardiac event except one patient during the follow-up period. CONCLUSION: Paradoxical hypotension during dobutamine infusion for myocardial scintigraphy is not an uncommon finding and up to 19.9% patients may develop such hypotension. To maximize test safety, precautions should be taken during dobutamine myocardial stress test, since remarkable decrease in systolic blood pressure may occur. Unlike hypotension occurring with exercise test, hypotension response to dobutamine is not always a marker for coronary artery disease.

Adrenergic beta-Agonists↗