Computing the B1 field of the toroidal MRI coil.

We present an analytic solution for the B1 field produced in a gapped toroidal cavity resonator designed as a probe for high field MRI. This resonator supports standing TEM waves, so its electric and magnetic fields are identical to those produced by a stationary planar current source with the same (constant) cross-section multiplied by a complex exponential propagation factor. An explicit expression for the field may therefore be found by solving Laplace's equation for the static potential, which is accomplished with a two-dimensional logarithmic conformal transformation algorithm. The equipotential curves are also the contours of the field strength B, and the B (vector) field at any point is directed along the contour passing through that point. With this information, we construct the solution by computing the angle made by the equipotential curve with the horizontal axis at each point, using this angle to analyze the B field into its x and y components, and adding the contributions from the current sources to obtain the magnitude and direction of B at each point in the region of interest. Some proposed extensions of this algorithm are also discussed.

Prosopagnosia as an impairment to face-specific mechanisms: Elimination of the alternative hypotheses in a developmental case.

For more than 35 years, researchers have debated whether face recognition is carried out by face-specific mechanisms or whether it involves more general mechanisms that are also used for objects. Prosopagnosic patients have furnished powerful evidence for face-specific mechanisms. Yet for each case that has been tested there have always been several untested alternative explanations that could account for the case. As such, each of these individuals has not been sufficiently tested to provide conclusive evidence for face-specific processes. Here we make a stronger argument with a single case of severe developmental prosopagnosia by exhaustively addressing all extant alternatives. We reject each in turn and thus eliminate all alternative accounts. Because this case is developmental in etiology the results also indicate that face recognition involves developmental mechanisms different from those producing other visual recognition mechanisms.

PCr overshoot': a study of the duration in canine myocardium.

The phosphocreatine (PCr) overshoot is a well-documented phenomenon and is readily observable by 31P MRS. In addition, a second 31P MRS observation during ischemia with reperfusion is a diminution in ATP levels. Combining these two as the 'PCr Overshoot' the PCr/ATP ratio may provide an index of viability. However little information is available regarding the duration of this 'overshoot'. For this approach to be useful clinically, the duration of this phenomenon must be ascertained. An open chest canine model of 12 min of ischemia followed by reperfusion (6h) was used. A 2 cm surface coil was sutured to the myocardium and spectra were acquired at 4.7 T. Gated spectra were acquired in <2.5 min with an interpulse delay of 5 s. Integrals of the PCr and ATP (beta) resonances were analyzed using a line-fitting routine. Overall, the PCr signal increased from 22.0+/-0.8 to 25.5+/-0.9 and ATP decreased from 11.7+/-0.4 to 10.0+/-0.4 (arbitrary units). The PCr remained elevated for the entire 6h period and the percentage increase was 15.9%. The ATP remained depleted for the entire 6h period and the percentage decrease was 17.0%. Thus, the clinically relevant and readily observable PCr/ATP is a product of both an increase in PCr and a decrease in ATP for a calculated net increase in PCr/ATP of 39.6%. The PCr/ATP ratio of the ischemia group for baseline, ischemia, 6h reflow, were: 2.33+/-0.18, 1.04+/-0.29 and 3.22+/-0.21. We demonstrate that the 'PCr overshoot' is readily observable and can be monitored noninvasively and nondestructively for 6h. Therefore, the 'PCr overshoot' may be a viable marker of reversible injury in this model and may prove to be applicable for detecting myocardial viability in patients.