An examination of hyperconjugative and electrostatic effects in the hydride reductions of 2-substituted-4-tert-butylcyclohexanones.

To better understand electronic effects on the diastereoselectivity of nucleophilic additions to the carbonyl group, a series of 2-X-4-tert-butylcyclohexanones (X = H, CH(3), OCH(3), F, Cl, Br) were reacted with LiAlH(4). Reduction of ketones with equatorial substituents yields increasing amounts of axial alcohol in the series for X [H < CH(3) < Br < Cl < F << OCH(3)]. These data cannot be explained by steric or chelation effects or by the theories of Felkin-Anh or Cieplak. Instead, an electrostatic argument is introduced: due to repulsion between the nucleophile and the X group, axial approach becomes energetically less favorable with an increase in the component of the dipole moment anti to the hydride approach trajectory. The ab initio calculated diastereoselectivities were close to the experimental values but did not reproduce the relative selectivity ordering among substituents. For reduction of ketones with axial substituents, increasing amounts of axial alcohol are seen in the series for X [Cl < Br < CH(3) < OCH(3) < H < F]. After some minor adjustments are made, this ordering is consistent with both the electrostatic model and Felkin-Anh theory. Cieplak theory cannot account for these data regardless of adjustments. Ab initio calculated diastereoselectivities were reasonably accurate for the nonpolar substituents but were poor for the polar substituents.

Physicians' recommendations to patients for use of antibiotic prophylaxis to prevent endocarditis.

CONTEXT: The American Heart Association recommendations for infectious endocarditis (IE) prophylaxis, published in June 1997, sought to improve patient and physician compliance by simplifying the dosing regimen and clarifying endocarditis risk. Adherence to these updated recommendations in patients with echocardiographic verification of their endocarditis risk profile is unknown. OBJECTIVE: To determine the recommended and actual use of IE prophylaxis as reported by patients undergoing echocardiography. DESIGN, SETTING, AND PARTICIPANTS: All patients who underwent outpatient transthoracic echocardiography at a university-based tertiary hospital in Boston, Mass, during December 1997 were contacted 6 to 9 months later to respond to a survey, completed by 218 (80%) eligible subjects. MAIN OUTCOME MEASURE: Patients' report of their physicians' instructions on actual use of IE prophylaxis in accordance with patient risk category, determined by echocardiographic data. RESULTS: One hundred eight patients (49.5%) had clinical or echocardiographic findings for which prophylaxis was indicated. Of these 108 patients, 71 (65.7%) reported that they were instructed to take IE prophylaxis. Sixteen high-risk patients (88. 9%) but only 55 moderate-risk patients (61.1%) reported that they were instructed to take prophylaxis. Among the 110 negligible-risk patients, 29 (26.4%) reported that they had been instructed to take IE prophylaxis. Overall, 100 patients (45.9%) reported that they received physician instructions to take IE prophylaxis. Of those who subsequently underwent a procedure for which IE prophylaxis was indicated (n=68), 9 (13.2%) elected not to follow their physician's advice to take prophylaxis. CONCLUSIONS: We found that although most patients reported receiving instructions for IE prophylaxis use consistent with American Heart Association guidelines, IE prophylaxis overuse among negligible-risk patients and underuse among moderate-risk patients was common. Continued physician and patient education may lead to improved adherence to the current American Heart Association recommendations. JAMA. 2000;284:68-71

Preliminary experimental evaluation of an inverse source imaging procedure using a decoupled coil detector array in magnetic resonance imaging.

Recently, we have discussed several of the aspects involved in the detector array concept in magnetic resonance imaging where an image is obtained by applying inverse source procedures to the data assembled by an array of coil detectors surrounding the object. In this work we describe an experimental setup, where a detector array was constructed of 9 coils, that gives a coarse resolution of 3 x 3 pixels. By measuring the induced current signals over this array of coils, a relationship is established between the set of signals and the structure of the body under investigation. Through matrix inversion, reconstruction of the original source from the detected signals is possible. In this preliminary experimental setup, we did not use an actual nuclear magnetic resonance signal. Instead, a miniature solenoid was used as a source, to simulate a precessing magnetic moment.

Calculation of the mutual induction between coplanar circular surface coils in magnetic resonance imaging.

The induced current in a coil due to the presence of alternating currents in an adjacent coil is calculated here for a case of two similar and coplanar, circular coils. By using an overlapping configuration, the mutual inductance is nullified as is predicted theoretically and confirmed experimentally.

A decoupled coil detector array for fast image acquisition in magnetic resonance imaging.

A method for magnetic resonance imaging (MRI) is investigated here, whereby an object is put under a homogeneous magnetic field, and the image is obtained by applying inverse source procedures to the data collected in an array of coil detectors surrounding the object. The induced current in each coil due to the precession of the magnetic dipole in each voxel depends on the characteristics of both the magnetic dipole frequency and strength, together with its distance from the coil, the coil direction in space, and the electrical properties of the coils. By calculating the induced current signals over an array of coil detectors, a relationship is established between the set of signals and the structure of the body under investigation. The linear relation can then be represented in matrix notation, and inversion of this matrix will produce an image of the body. Important problems which must be considered in the proposed method are signal-to-noise ratio (SNR) and coupling between adjacent coils. Solutions to these problems will provide a new method for obtaining an instantaneous image by NMR, with no need for gradient switching for encoding. A general algorithm for decoupling of the coils is presented and fast sampling of the signal, instead of filtering, is used in order to reduce both noise and numerical roundoff errors at the same time. Sensitivity considerations are made with respect to the number of coils that is required and its connection with coil radius and SNR. A computer simulation demonstrates the feasibility of this new modality. Based on the solutions presented here for the problems involved in the use of a large number of coils for a simultaneous recording of the signal, an improved method of multicoil recording is suggested, whereby it is combined with the conventional zeugmatographic method with read and phase gradients, to result in a novel method of magnetic resonance imaging. In the combined method, there are no phase-encoding gradients. Only a single slice-selecting gradient, to be followed by a single read-gradient. Instead of phase-encoding gradients, use is made of an equivalent number of coils. The number of coils now is reduced significantly. This method suggests a single slice image taken within a single echo time, and where a 128 x 128 resolution is possible with only 128 coils. The applicability of the method is based on a successful decoupling procedure for the detectors (coils and cables) and the availability of highly accurate, high-gain, low-noise amplifiers with a broad dynamic range.

Possible detection of turbulent blood flow using multiparametric encoding gradients in MRI.

Magnetic resonance imaging (MRI) has significant potential as a highly accurate noninvasive flow measurement technique. Presented here is an approach to the imaging of turbulence in the velocity profiles. Recent publications have presented multiparametric encoding gradient methods, which are based on a deterministic rather than a statistical approach to spin motion. Following these methods of building up a general gradient from a basic set of gradients, the effects of turbulence and undetermined moments of motion on the image are discussed. Image blurring due to the nondeterministic behavior of spins (e.g., diffusion, turbulence) is not removed by these techniques, and this fact may be useful in identifying regions of turbulent flow, which are of importance in the clinical observation of cardiac and vascular haemodynamics. Due to its random nature, the elimination of turbulent effects by the use of a deterministic method is bound to fail. On the other hand, regions of turbulent flow may be identified due to signal decrease from those regions, provided one is careful to remove all the causes of nonturbulent signal reduction present due to nonturbulent flow moments. In the multiparametric phase encoding method, gradient amplitude tends to increase with higher moments of motion. The temporal behavior of these gradients is discussed, and it is suggested that the increase in their duration will enhance the encoding of higher motion terms at the expense of imaging time.

A magnetic resonance imaging method of flow by successive excitation of a moving slice.

A new magnetic resonance imaging of flow by a method of successive excitation of a moving slice is presented here. The method, which in general belongs to the class of time-of-flight methods, is based upon selectively changing the frequencies in a sequence of rf pulses in such a way that a selected slice moves through the imaged volume in a preselected direction and velocity, and is repeatedly excited. It is claimed that most of the measured signal is contributed by only those parts of the imaged substance that move with the selected slice. The activation procedure of the flowing spins, which is presented here, enhances the imaging of the flowing matter and reduces the signal from static matter. A specially tailored selective sequence of a rf alpha-pulses has been developed. During this sequence, a set of rephasing 180 degrees selective rf pulses is applied. At the end of this sequence an additional 180 degrees rf pulse is applied to eliminate redundant signals from the static matter in the last activated slice.