Biomedical subjects

M A Fernández-Seara

Publications and source records attributed to M A Fernández-Seara.

3 recordsLinked to original sources

PubMed2003

Multipoint mapping for imaging of semi-solid materials.

Multipoint k-space mapping is a hybrid between constant-time (single-point mapping) and spin-warp imaging, involving sampling of a k-line segment of r points per TR cycle. In this work the method was implemented for NMR imaging of semi-solid materials on a 400 MHz micro-imaging system and two different k-space sampling strategies were investigated to minimize the adverse effects from relaxation-induced k-space signal modulation. Signal attenuation from T(2) decay results in artifacts whose nature depends on the k-space sampling strategy. The artifacts can be minimized by increasing the readout gradient amplitude, by PSF deconvolution or by oversampling in readout direction. Finally, implementation of a T(2) selective RF excitation demonstrates the feasibility of obtaining short-T(2) contrast even in the presence of tissues with long-T(2). The method's potential is illustrated with 3D proton images of short-T(2) materials such as synthetic polymers and bone.

Animals↗

PubMed2001

Trabecular bone volume fraction mapping by low-resolution MRI.

Trabecular bone volume fraction (TBVF) is highly associated with the mechanical competence of trabecular bone. TBVF is ordinarily measured by histomorphometry from bone biopsies or, noninvasively, by means of high-resolution microcomputed tomography and, more recently, by micro-MRI. The latter methods require spatial resolution sufficient to resolve trabeculae, along with segmentation techniques that allow unambiguous assignment of the signal to bone or bone marrow. In this article it is shown that TBVF can be measured under low-resolution conditions by exploiting the attenuation of the MR signal resulting from fractional occupancy of the imaging voxel by bone and bone marrow, provided that a reference signal is available from a marrow volume devoid of trabeculation. The method requires accurate measurement of apparent proton density, which entails correction for various sources of error. Key among these are the spatial nonuniformity in the RF field amplitude and effects of the slice profile, which are determined by B(1) field mapping and numerical integration of the Bloch equations, respectively. By contrast, errors from variations in bone marrow composition (hematopoietic vs. fatty) between trabecular and reference site are predicted to be small and usually negligible. The method was evaluated in phantoms and in vivo in the distal radius and found to be accurate to 1% in marrow volume fraction. Finally, in a group of 12 patients of varying skeletal status, TBVF in the calcaneus was found to strongly correlate with integral bone mineral density of the lumbar vertebrae (r(2) = 0.83, p < 0.0001). The method may fail in large imaging objects such as the human trunk at high magnetic field where standing wave and RF penetration effects cause intensity variations that cannot be corrected. Magn Reson Med 46:103-113, 2001.

Animals↗

PubMed2000

Postprocessing technique to correct for background gradients in image-based R*(2) measurements.

Background static magnetic field gradients are a source of signal loss in gradient-echo imaging, as they typically result from discontinuity in the magnetic susceptibility at air-tissue boundaries. Moreover, these induced gradients severely compromise the measurement of R*(2), the effective transverse relaxation rate, which is of interest in many biomedical applications of MRI. Since the slice thickness is usually larger than the in-plane pixel dimensions, gradients parallel to the slice-select direction are of particular concern. In this work, a post-processing technique is introduced which attempts to correct the signal on the assumption that the background gradients are approximately linear across the voxel and the signal decay in the absence of these gradients is exponential. In this case, the time-domain signal is weighted by a sinc function characterized by the amplitude G(b) of the background gradient, which is typically not known a priori. The algorithm searches for the estimate of G(b) which yields the optimum fit of the corrected experimental data to an exponential. It is shown to be effective as long as this gradient is below a critical threshold. Evaluation in a phantom and in the human brain at 1.5 and 4 T demonstrates that this method can restore R(2)* in spite of the apparent rate constant exceeding the true value by up to 100%. Contrary to prospective correction techniques, the approach presented in this study does not prolong scan time.

Adult↗