An analysis of fast imaging sequences with steady-state transverse magnetization refocusing

Recently, several groups have proposed and demonstrated the use of rapid imaging methods, using short pulse repetition times and gradient-reversal echoes. Here, we analyze the behavior of the magnetization and the resulting image contrasts in such sequences for the case where the pulse repetition time TR is of the order of, or shorter than, the transverse relaxation time T2, and the transverse magnetization is not destroyed between phase-encoding cycles. Exact analytical expressions describing the signal evolution between the pulses are derived, taking into account the effects of resonance offsets and flip angles, and examining the influence of constant-phase or alternate-phase RF pulse trains. It is shown that for typical imaging sequences two distinct echo signals will develop between pulses, which may have a detrimental effect on image quality if they partially overlap within the sampling window. It is shown that artifact-free images can be obtained only if the two echo signals overlap precisely, which seems technically close to impossible to achieve, or if they are sufficiently separated in time to allow sampling of only one of the signals.     

Design of adiabatic pulses for fat-suppression using analytic solutions of the bloch equation

Discrimination between signals produced by fat and by water is an important issue in MRI. One efficient approach is to perform fat-suppression by selective inversion. This technique exploits the transition region of a selective RF pulse to invert the longitudinal lipid magnetization while leaving the magnetization of the water protons untouched. The damaging effects of RF field inhomogeneity may be overcome by using pulses based on the adiabatic fast passage principle (AFP). In particular, the well-known sech/tanh adiabatic pulse is a robust and efficient pulse that is obtained as an analytic solution of the Bloch equation. In this paper, a wider class of analytic solutions of the Bloch equation is presented of which the sech/tanh driving function is merely a particular case. The new pulse exhibits an asymmetric distribution of magnetization with one transition sharper than the other. The sharper transition can be used to perform the required selective discrimination between signals. The resulting pulse features excellent adiabatic behavior. Moreover, the transition width of the new pulse can be reduced by a factor of about 2/3 with respect to an equal-duration sech/tanh pulse. The performance of the new pulse is compared with a similar sech/tanh pulse with the aid of a practical design example.     

Splicing factor Tra2-beta1 is specifically induced in breast cancer and regulates alternative splicing of the CD44 gene

The human CD44 gene undergoes extensive alternative splicing of multiple variable exons positioned in a cassette in the middle of the gene. Expression of alternative exons is often restricted to certain tissues and could be associated with tumor progression and metastasis of several human malignancies, including breast cancer. Exon v4 contains multiple copies of a C/A-rich exon enhancer sequence required for optimal inclusion of the exon and binding to the nucleic acid-binding proteins YB-1 and human Tra2-beta1. Here, we show that hTra2-beta1, a member of the extended family of serine/arginine-rich (SR) splicing factors, enhances the in vivo inclusion of CD44 exons v4 and v5. It increased inclusion of exons v4 and v5 and acted synergistically with YB-1. Activation required the C/A-rich enhancer within exon v4. Several other SR proteins had none or only a slight effect on CD44 exon inclusion. In contrast, SC35 inhibited exon usage and antagonized the effects of Tra2 or YB-1. In a matched pair analysis of human breast cancers and their corresponding nonpathologic tissue controls, we found a significant induction of Tra2-beta1 in invasive breast cancer, both on the RNA and protein levels. Together with our functional data, these results suggest an important role for Tra2-beta1 in breast cancer. Induction of this splicing factor might be responsible for splicing of CD44 isoforms associated with tumor progression and metastasis.     

A sexual history-taking curriculum for second year medical students

The purpose of this study is to describe the evaluation of a sexual history-taking curriculum and correlates of student performance during a Clinical Skills Assessment. Reading assignments, small group discussions, a Saturday Sex workshop and performance on a Clinical Skills Assessment were evaluated. Students most favorably rated the workshop and least favorably rated the reading assignments. Eighty-four percent of students asked at least one sexual history question on the Clinical Skills Assessment. We were unable to identify any independent predictors of sexual history-taking behavior.     

Prophylactic activity in rodents of trypanocides complexed with dextran

The prophylactic activity of prepared complexes of each of several trypanocidal drugs with dextran sulphate (M. W. 5 × 10⁵) was determined in rodents at the same time as that of the corresponding uncomplexed drug, and also the suramin complex in some cases. All the dextran complexes tested showed some degree of enhanced prophylaxis over the uncomplexed drug but in most cases a significant extension of prophylaxis was demonstrated only at dose levels well in excess of those used in the field. The dextran complex of isometamidium, however, showed very considerable enhancement effects over a range of dose levels. An amount of this complex containing an equivalent of the therapeutic dose of uncomplexed drug extended the period of effective prophylaxis by several months in both rats and mice. The suramin complex of isometamidium, on the other hand, was almost invariably less effective than the uncomplexed drug.