Biomedical subjects

T E Skinner

Publications and source records attributed to T E Skinner.

7 recordsLinked to original sources

PubMed1999

Novel methods for characterizing a decoupler channel using "undetectable" quantum coherences.

Exact solutions for the effect of time-independent RF pulses on any initial configuration of an IS J-coupled system demonstrate that on-resonance CW decoupling yields signals whose frequency depends on RF field strength and homogeneity. These signals are enhanced starting with "undetectable" antiphase and multiple quantum coherences, which can also produce centerband intensity to mimic the signal from decoupled Sx. Conversely, these coherences can be generated from Sx using a low-power pulse, B1 = J/2, of length (2J)-1, dubbed a "90J pulse" since it is the selective equivalent of {(2J)-1-90[I]}. Utilizing 90J pulses, new characterization-of-decoupler (COD) pulse sequences can determine the performance of an insensitive I-spin channel by observing large signals from either antiphase or multiple quantum coherences with the S-spin channel, allowing, in minutes rather than hours: (i) frequency calibration to an accuracy of 0.1 Hz; (ii) measurement of RF amplitudes over a 500-fold variation; and (iii) mapping of RF homogeneity along the sample axis with a single 1D B1 spectrum. These 90J coherence transfer pulses are of potential general use for selective spectroscopy.

Magnetic Resonance Spectroscopy↗

PubMed1999

J pulses for multiplet-selective NMR.

Exact product operator solutions have been obtained for the evolution of weakly coupled spin-(1/2) I(m)S(n) systems during arbitrary RF irradiation of one spin. These solutions, which completely characterize the nature of J-coupling modulation during RF pulses, show that significant exchange occurs between single-spin magnetization and two-spin product operator states when the RF field strength is comparable to the coupling. In particular, a long (t(p) = [2J](-1) s), low-power (B(1) = J/2 Hz), constant amplitude pulse applied on resonance to one spin in an IS system completely interconverts the spinstates S(z) <--> 2S(x)I(z) and S(x) <--> 2S(z)I(z) when the RF is applied to the S spins, and interconverts S(x) <--> 2S(y)I(y) in 100% yield when the RF is applied to the I spins. Thus, these "J pulses," which select a bandwidth approximately equal to J Hz, may replace any combination of a (2J)(-1) delay period and a consecutive hard 90 degrees pulse in any polarization transfer or multiple quantum sequence. Although these rectangular pulses are highly frequency selective, in general they increase the replaced (2J)(-1) period by only a modest 40%, a time saving of a factor of 5 compared to existing pulses exhibiting the same selectivity. In favorable cases, there is no increase in duration of a pulse sequence using a particular type of J pulse, the 90(J) variety, which accomplishes the third spin state transformation listed above. J pulses will be advantageous for systems subject to rapid signal loss from relaxation and more generally for the enhanced operation of pulse sequences via the use of J modulation during RF irradiation.

Magnetic Resonance Spectroscopy↗

PubMed1999

Exact product operator evolution of weakly coupled spin-12 I(m)S(n) systems during arbitrary RF irradiation of the I spins.

In this article, we consider the evolution of weakly coupled I(m)S(n) systems of spin-12 nuclei during arbitrary RF irradiation of the I spins. Exact solutions are presented for the time dependence of the density operator in terms of its constituent product operator components for a complete set of initial states derived from polarization of either the I or the S spin. The solutions extend the range of applications that are accessible to the product operator formalism and its associated vector picture of nuclear spin evolution. This marriage of quantum mechanics and a literal vector description of spin dynamics during RF irradiation supports physical intuition and has led to simple pulses for selective coherence transfer, among other new applications. The evolution of initial states that are free of transverse S-spin components can be described by classical precession of the I-spin components about effective fields defined by the interaction between the coupling and RF fields. Although there is no analogue involving classical rotations for the evolution of initial states composed of S(x) or S(y), a vector description is still possible, and the solutions completely characterize the nature of J-coupling modulation during RF pulses. We emphasize the Cartesian product operator basis in the present treatment, but the solutions are readily obtained in any other basis that might prove suitable in analyzing an experiment. For a system of N coupled spins, standard exact methods involving diagonalization and multiplication of the 2(N) x 2(N) matrices that represent the system require on the order of (2(N))(3) operations to calculate the system response to a general RF waveform at each point in the time domain. By contrast, the efficiency of the present method scales linearly with the number of spins. Since the formalism presented also accommodates the absence of either RF irradiation or the coupling, the solutions provide an efficient means of general pulse sequence simulation, encompassing any combination of arbitrary RF waveforms, delays, and coherence gradients.

Algorithms↗

PubMed1997

An extended two-point Dixon algorithm for calculating separate water, fat, and B0 images.

A new algorithm is presented that provides separate water, fat, and B0 images utilizing the in-phase and opposed-phase acquisitions of the two-point Dixon (2PD) method. The accuracy of the extended method (E2PD) compares favorably with the three-point Dixon (3PD) method, and the acquisition requires 2/3 the 3PD scan time. Slightly increased mismapping may occur in pixels containing an admixture of water and fat due to reduced SNR in the B0 field map compared with the 3PD method.

Algorithms↗

PubMed1994

Water and fat MR imaging with chemical shift selective 3D steady state methods.

A new 3D acquisition regimen that enables the collection of conventional, water-suppressed, and fat-suppressed images with no increase in scan times compared with currently implemented 3D sequences is presented. The method is based on conventional 3D steady state with interleaved selective excitation of the fat resonance resulting in acquisition of a fat-based image during the TR period experienced by the water spins. This new sequence is relatively tolerant to susceptibility artifacts and results in excellent water-based images. Because the idea, which we propose, is independent of the type of steady-state imaging protocol utilized, it can be easily applied with regimens that are specifically tailored to enhance contrast.

Fats↗

PubMed1994

Proton magnetic resonance spectroscopy (1H MRS) of the hippocampal formation in schizophrenia: a pilot study.

BACKGROUND: Recent post-mortem and magnetic resonance imaging (MRI) studies strongly suggest a decrease in the volume of the hippocampus and other limbic temporal structures in schizophrenia. Therefore, we hypothesised that N-acetyl aspartate (NAA) which is found mainly in neurons and which can be measured by proton magnetic resonance spectroscopy (1H MRS) would be decreased in the limbic temporal region in schizophrenia. METHOD: Consenting subjects fulfilling DSM-III-R criteria for schizophrenia (n = 11) and matched healthy volunteers (n = 11) who were recruited in a tertiary university referral centre, participated in a 1H MRS brain study. Proton MRS spectra were obtained from a 12 cm3 voxel (2 x 2 x 3 cm) in the right and left hippocampus/amygdala region. A researcher blind to the source of the spectra, measured the NAA intensity in all subjects, which were then statistically compared across the two groups. RESULTS: NAA intensities were significantly reduced in the right hippocampus/amygdala region of schizophrenic patients (P = 0.038). The difference of the left side did not reach significance at the 95% confidence level. CONCLUSIONS: The findings of decreased NAA in this study suggest that there may be a decrement in neuronal number or tissue volume of the right hippocampal/amygdala region in schizophrenia. Biochemical alterations in the metabolism of NAA in schizophrenia may be an alternative explanation. The findings are consistent with other types of post-mortem and in vivo evidence for hypoplasia of the limbic temporal structures in schizophrenia, postulated to be of neurodevelopmental pathogenesis.

Adult↗

PubMed1993

Transaminase reaction rates, transport activities and TCA cycle analysis by post-steady state 13C NMR.

In this work, we present the post-steady state analysis of the TCA cycle and a closed form solution to the rate of label washout from the C4 carbon of glutamic acid through the transaminases and the malate-aspartate shuttle and then through alpha-ketoglutarate dehydrogenase. We demonstrate using a model of this problem that the rate of label washout depends not only on the flux through alpha-ketoglutarate dehydrogenase, but most importantly on the activity of the malate-aspartate shuttle as determined by the forward and reverse fluxes through the transaminases and by the rate of transport of glutamate and alpha-ketoglutarate across the mitochondrial membrane.

Animals↗