Turbo spin-echo sequences in magnetic resonance imaging of the brain: Physics and applications

Fast SE imaging provides considerable measure time reduction, high signal-to-noise ratios as well as similar contrast behavior compared to conventional SE sequences. Besides TR and TE_eff, echo train length (ETL), interecho time , and-space trajectory determine image contrast and image quality in fast SE sequences. True proton density contrast (CSF hypointense) and not too strong T2 contrast are essential requirements in routine brain MRI. A Turbo SE sequence with very short echo train length (ETL=3), short TE_eff and short interecho time (17 ms), and TR=2000 ms was selected for proton density contrast; a Turbo SE sequence with ETL=7, TE_eff=90 ms, =22 ms, and TR=3250 ms was selected for T₂-weighted images. Using both single-echo Turbo SE sequences yielded 50% measure time reduction compared to the conventional SE technique. Conventional SE and optimized Turbo SE sequences were compared in 150 patients resulting in very similar signal and contrast behavior. Furthermore, reduced flow artifacts in proton density—and especially in T₂-weighted Turbo SE images—and better contrast of high-intensity lesions in proton density-weighted Turbo SE images were found. Slightly reduced edge sharpness—mainly in T₂-weighted Turbo SE images—did not reduce diagnostic reliability. Differences between conventional and Turbo SE images concerning image contrast and quality are explained regarding special features of fast SE technique.Address for correspondence: Institut für Röntgendiagnostik, Klinikum der Universität Regensburg, Franz-Josef-Strauß-Allee 11, 93042 Regensburg, Germany. Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Venn Plaza, New York, NY 10119.     

Shift-invariant interpattern association neural network

A shift-invariant neural network that uses the translation-invariant property of the modulus Fourier spectra with the heteroassociation interpattern association memory is proposed. A binary encoding of a spectral sampling of the training set is used to preserve the main features. Computer simulations and experimental demonstrations are provided that show the shift-invariant property of the proposed optical neural network.     

Infrared optical constants of polycrystalline boron nitride: reply to comment

This reply is to the comment by C. G. Ribbing [Appl. Opt. 33, 8055 (1994)] on our paper on IR optical constants of polycrystalline boron nitride.     

Object illumination angle measurement in speckle interferometry

Errors in measurement of the object illumination angle affect fringe interpretation accuracies in speckle interferometry. To measure the object illumination angle we propose a method based on the moiré effect. The technique is easy to implement and was found to yield fast and accurate measurements.     

White-light reconstruction setup for shearograms using optical-fiber waveguides

A white-light reconstruction setup for shearograms using optical-fiber waveguides is presented. The setup is compact, is not susceptible to dust and scratches, and enables the two-dimensional fringe pattern on shearograms to be viewed directly under uniform and enhanced illumination with minimal image distortion.     

Compact, high-gain pulsed dye amplifier for weak cw sources

A compact two-cuvette dye amplifier capable of pulse amplifying weak cw visible sources by factors in excess of 10(9) has been demonstrated. Seeded with a 300-μW single-frequency He-Ne laser and without the need for a Faraday isolator, the preamplifier yields >40-μJ pulses of 4.5-ns duration and 1.4× transform-limited linewidths around 140 MHz. Subsequent power amplification yields 4-ns FWHM, 2-mJ pulses with excellent pulse-to-pulse stability and linewidths around 170 MHz. Scaling of the operational envelope and extension to pulse amplification of cw single-mode tunable diode lasers is discussed.     

Experimental tests of a novel Raman cell for vacuum ultraviolet generation to below Lyman-α

A novel-type Raman cell especially suitable for the generation of tunable vacuum ultraviolet (VUV) radiation with pump radiation from a frequency-doubled dye laser is described. This hydrogen-filled Raman cell permits the generation of narrow-bandwidth radiation to below 114 nm. Absolute VUV energies in the various anti-Stokes orders and measurements of pulse durations and pressure dependences are given.