Biomedical subjects

N Gelman

Publications and source records attributed to N Gelman.

5 recordsLinked to original sources

PubMed1999

Concentration dependence of 19F nuclear magnetic resonance decay shapes and second moments in bone mineral.

19F nuclear magnetic resonance (NMR) spin-echoes and free induction decays (FIDs) have been observed from samples of fluoridated trabecular canine bone powder, with fluoride concentrations ([F]) ranging from approximately 10 to 33 mg F/g Ca. Curve fitting of echo envelopes and FIDs was performed using a two-component model function, where one of the components incorporates the effects of one-dimensional dipolar coupling. This function provides a good match for both echo envelopes and FIDs. Estimates of the total second moment and its homonuclear (F-F coupling) component were obtained from the fitting procedure. Based on the second moment measurements, it is argued that 19F spins in bone mineral typically experience weaker heteronuclear dipolar coupling than those in the mineral hydroxyapatite (HAP), which is often considered to be a prototype for bone mineral.

Animals↗

PubMed1999

MR imaging of human brain at 3.0 T: preliminary report on transverse relaxation rates and relation to estimated iron content.

PURPOSE: To determine the transverse relaxation rates R2 and R2' from several gray matter regions and from frontal cortical white matter in healthy human brains in vivo and to determine the relationship between relaxation rates and iron concentration [Fe]. MATERIALS AND METHODS: Six healthy adults aged 19-42 years underwent thin-section gradient-echo sampling of free induction decay and echo magnetic resonance (MR) imaging at 3.0 T. Imaging covered the mesencephalon and basal ganglia. RESULTS: Relaxation rates (mean +/- SD) were highest in globus pallidus (R2 = 25.8 seconds-1 +/- 1.1, R2' = 12.0 seconds-1 +/- 2.1) and lowest in prefrontal cortex (R2 = 14.4 seconds-1 +/- 1.8, R2' = 3.4 seconds-1 +/- 1.1). Frontal white matter measurements were as follows: R2 = 18.0 seconds-1 +/- 1.2 and R2' = 3.9 seconds-1 +/- 1.2. For gray matter, both R2 and R2' showed a strong correlation (r = 0.92, P < .001 and r = 0.90, P < .001, respectively) with [Fe]. Although the slopes of the regression lines for R2' versus [Fe] and for R2 versus [Fe] were similar, the iron-independent component of R2' (2.2 seconds-1 +/- 0.6), the value when [Fe] = 0, was much less than that of R2 (12.7 seconds-1 +/- 0.7). CONCLUSION: The small iron-independent component R2', as compared with that of R2, is consistent with the hypothesis that R2' has higher iron-related specificity.

Adult↗

PubMed1998

Wavelet encoding for improved SNR and retrospective slice thickness adjustment.

A method to encode the slice-select direction in magnetic resonance imaging through the use of a discrete wavelet transform is presented. The paper reports the first implementation of wavelet encoding using the Haar basis. Theoretically, the signal-to-noise ratio (SNR) for two levels of wavelet encoding is expected to be approximately 1.6 times higher than for conventional multislice imaging in which each slice is excited separately. A slightly lower SNR is encountered experimentally because of slice interference. The design of radiofrequency (RF) pulses to excite slices approximating Haar wavelet basis functions is discussed. Wavelet encoding also provides an efficient method for balancing slice thickness and SNR after an image has been acquired. The experiments have been conducted at 1.5 Tesla. Wavelet encoding of slices should be especially suitable for low-field MRI, in which the SNR is lower and the characteristically lower T1 values favor the use of shorter TR, which sometimes forces the slices to be acquired in multiple passes.

Algorithms↗

PubMed1996

Wavelet encoding for 3D gradient-echo MR imaging.

An implementation of wavelet encoding to resolve one spatial direction of a three-dimensional gradient-echo magnetic resonance image is described. This is the first report of the use of wavelet encoding with a relatively short repetition time for rapid image acquisition. An important feature of this implementation is that it allows wavelet-encoded images to be reconstructed without phase correction. The images show no discernible degradation compared with images acquired with phase encoding or scaling-function encoding instead of wavelet encoding. In addition to describing the implementation, typical sources of phase errors are investigated both theoretically and experimentally.

Magnetic Resonance Imaging↗

PubMed1990

Field dependence of 19F NMR in rat bone powders.

The 19F NMR properties of fluoridated rat bone powder samples have been studied in several external magnetic fields. The results show a characteristic field dependence, related to the strength of chemical shift interactions compared to dipole-dipole interactions. While the free induction decay shape is relatively insensitive to the 19F bone mineral concentration, the spin-lattice relaxation time decreases with increasing 19F concentration. Multi-exponential spin-lattice relaxation processes indicate that there are at least two chemically inequivalent incorporation sites for fluorine in bone mineral. Evidence for clustering of 19F fluoride impurities in bone mineral is seen in Hahn echo experiments. Sample preparation and handling methods are shown to affect the values of some of the observed NMR parameters.

Animals↗