A detailed analysis is presented of a method to eliminate transverse magnetization prior to each rf excitation in pulse sequences with TR less than T2. It is shown that artifact-free images with high T1 contrast can be obtained only if a phase shift that is incremented during each TR interval is applied to the transverse magnetization. Computer simulations are used to show that when this phase increment is 117 degrees, the steady-state transverse magnetization prior to each rf pulse is nulled over a wide range of T1, T2, and rf tip angles, resulting in optimal T1 contrast. Such nulling of steady-state transverse magnetization cannot be obtained by using large gradient pulses, or gradients of random or linearly incremented amplitude. Images of phantoms and human subjects confirm the theoretical predictions.

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The aims of the study were to use functional magnetic resonance imaging (fMRI) to 1) locate pain-related regions in the anterior cingulate cortex (ACC) of normal human subjects and 2) determine whether each subject's pain-related activation is congruent with ACC regions involved in attention-demanding cognitive processes. Ten normal subjects underwent fMRI with a 1.5-T standard commercial MRI scanner. A conventional gradient echo technique was used to obtain data from a single 4-mm sagittal slice of the left ACC, approximately 3.5 mm from midline. For each subject, interleaved sets of 6 images were obtained during a pain task, an attention-demanding task, and at rest, for a total of 36 images per task. Pain of different intensities was evoked via electrical stimulation of the right median nerve. The attention-demanding task consisted of silent word generation (verbal fluency). Additional experiments obtained data from the right ACC. A pixel-by-pixel statistical analysis of task versus rest images was used to determine task-related activated regions. The pain task resulted in a 1.6-4.0% increase in mean signal intensity within a small region of the ACC. The exact location of this activation varied from subject to subject, but was typically in the posterior part of area 24. The signal intensity changes within this region correlated with pain intensity reported by the subject. The attention-demanding tasks increased the mean signal intensity by 1.3-3.3% in a region anterior and/or superior to the pain-related activation in each subject. The activated region was typically larger than the pain-related activation. In some cases this activation was at or superior to the ACC border, near the supplementary motor area. These regions did not show any pain-intensity-related activation. In one subject both right and left ACC were imaged, revealing bilateral ACC activation during the attention task but only contralateral pain-related activation. These findings shed light on pain- and attention-related cognitive processes. The results provide evidence for a region in the posterior part of the ACC that is involved in pain and a more anterior region involved in other attention-demanding cognitive tasks.

Linear birefringence and optical activity are two common optical polarization effects present in biological tissue, and determination of these properties has useful biomedical applications. However, measurement and unique interpretation of these parameters in tissue is hindered by strong multiple scattering effects and by the fact that these and other polarization effects are often present simultaneously. We have investigated the efficacy of a Mueller matrix decomposition methodology to extract the individual intrinsic polarimetry characteristics (linear retardance delta and optical rotation psi, in particular) from a multiply scattering medium exhibiting simultaneous linear birefringence and optical activity. In the experimental studies, a photoelastic modulation polarimeter was used to record Mueller matrices from polyacrylamide phantoms having strain-induced birefringence, sucrose-induced optical activity, and polystyrene microspheres-induced scattering. Decomposition of the Mueller matrices recorded in the forward detection geometry from these phantoms with controlled polarization properties yielded reasonable estimates for delta and psi parameters. The confounding effects of scattering, the propagation path of multiple scattered photons, and detection geometry on the estimated values for delta and psi were further investigated using polarization-sensitive Monte Carlo simulations. The results show that in the forward detection geometry, the effects of scattering induced linear retardance and diattenuation are weak, and the decomposition of the Mueller matrix can retrieve the intrinsic values for delta and psi with reasonable accuracy. The ability of this approach to extract the individual intrinsic polarimetry characteristics should prove valuable in diagnostic photomedicine, for example, in quantifying the small optical rotations due to the presence of glucose in tissue and for monitoring changes in tissue birefringence as a signature of tissue abnormality.