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Biomedical subjects

J L Ackerman

Publications and source records attributed to J L Ackerman.

9 recordsLinked to original sources
PubMed1992

Phosphorus-31 magnetic resonance imaging of hydroxyapatite: a model for bone imaging.

One-dimensional 31P nuclear magnetic resonance images (projections) of synthetic calcium hydroxyapatite, Ca10(OH)2(PO4)6, have been obtained for samples on the order of 0.5 to 1.0 cm in linear extent at 7.4 T magnetic field strength. Because of the solid state nature of these samples, short 31P spin-spin relaxation times under 1 ms occur, necessitating echo times of 1 ms and phase-encoding magnetic field gradient pulses shorter than 500 microseconds. Optimal projection quality and shortest acquisition times result from pulsed gradient phase-encoding of the spatial dimension, using a compensating gradient pulse to cancel the distorting effects of gradient waveform transients. The exceedingly long 31P spin-lattice relaxation times could lead to potentially intolerable image acquisition times; these have been reduced with a flipback pulse technique. In addition to holding great potential as a novel research tool in the study of biomineralization of those organisms containing calcium phosphate solid phases, these methods should be of general utility in the multinuclear imaging of a wide variety of solids of interest in biophysics and materials science.

Bone and Bones↗
PubMed1991

MR contrast due to microscopically heterogeneous magnetic susceptibility: numerical simulations and applications to cerebral physiology.

We calculate the effects of subvoxel variations in magnetic susceptibility on MR image intensity for spin-echo (SE) and gradient-echo (GE) experiments for a range of microscopic physical parameters. The model used neglects the overlap of gradients from one magnetic inclusion to the next, and so is valid for low volume fractions and weak perturbations of the magnetic field. Transverse relaxation is predicted to deviate significantly from linear exponential decay in both SE and GE at a particle radius of 2.5 microns. Calculated changes in transverse relaxation rates for SE and GE increase linearly with volume fraction of high-susceptibility regions of 5 microns diameter, but increase with about the 3/2 power of volume fraction of regions with 15 micron spacing between centers. This sensitivity to the actual size and spacing of magnetized regions may allow them to be measured on the basis of contrast. without being resolved in images. GE and SE decay rates are approximately twice as sensitive to long cylinders of 5 microns diameter than to spheres of the same size, for diffusion constants of 2.5 micron 2/ms. Calculated changes in transverse decay rates increase with approximately the square of field and susceptibility variation for 5-microns spheres and a diffusion constant of 2.5 microns 2/ms. This exponent is smaller for cylindrical magnetized regions of the same size, and also depends on the diffusion constant. We discuss possible applications of our theoretical results to the analysis of the effects of high-susceptibility contrast agents in brain. Experimental data from the literature are compared with calculated signal changes according to the model. The monotonic dependence of decay rates on the volume of distribution of the contrast agent suggests that cerebral blood volume and flow could be measured using MR contrast.

Brain↗
PubMed1991

Porous block hydroxyapatite in orthognathic surgery.

Seventy-six nonconsecutive patients undergoing orthognathic surgery, in whom blocks of porous hydroxyapatite were implanted into osteotomy gaps in lieu of autogenous bone grafts, are the subjects of this report. Surgical procedures include inferior maxillary repositioning (10 patients), maxillary advancement (24 patients), transverse maxillary expansions (17 patients) and inferior repositioning of the chin (25 patients). A total of 140 anatomic sites were implanted. Eleven patients later consented to open biopsy of the implant material at a mean 10.2 months following implantation. At the time of follow-up, mean 16.3 months, excellent osseous stability was observed. Three patients developed complications relative to the presence of the implant. Twenty-one of 24 biopsy specimens demonstrated an osseous union of implant to bone with osseous deposition within the implant pores. Radiographic follow-up revealed implant blocks to maintain their volume with no change in density or discreteness. The biological behavior and biomechanical properties of porous block hydroxyapatite are discussed. These implant characteristics make it a feasible bone graft substitute in orthognathic surgery and justify its continued use in this context.

Adolescent↗
PubMed1991

Nuclear magnetic resonance microscopy of atheroma in human coronary arteries.

The purpose of this study was to use direct nuclear magnetic resonance (NMR) microscopy to quantitate and image accumulations of atheroma lipids in human coronary arteries and to validate the results by comparison with histologic preparations. NMR microscopy was performed on a superconducting experimental NMR imaging system operating at 2 Tesla with a probe designed for short echo time (TE), strong B1 field strength, and small samples. Data acquisition used multiple-offset chemical encoding with offsets based on the thermotropic spectral signature of atheroma lipids within the human arterial vessel wall. Three separate channels of image data yielded color axis display of atheroma within the vessel walls. Atheroma location by histology was identified by rarefaction of stroma, as the lipids are extracted in the process of embedding in paraffin. Perimeters, areas, and a shape index (perimeter2:4 pi area) of lumen, atheroma, and outer wall were determined and compared for NMR vs histology. There was no significant difference in the measurements with the exception of luminal shape indices, which were uniformly larger by histology, attributable to flattening of the vessels during histologic preparation. NMR measurement of atheroma content of coronary artery walls agreed well with histology (r = 0.996). NMR microscopy with color axis display proved able to quantitate and image atheroma in coronary arteries, obviating the distortions and lipid removal associated with fixation, embedding, and sectioning for histology.

Adult↗
PubMed1975

A systematic modification of edgewise therapy.

This article describes a highly systematic modifcation of the edgewise technique that incorporates the principle of light continuous force application. It is a re-evaluation of our earlier article in view of the experience gained in applying this treatment approach in a teaching clinic. The classification of cases according to retraction requirements, the appliance designs, and the use of four basic phases of treatment remains largely the same as previously described. Some of the variations and modifications that have been necessary because of new information and pedogogical considerations are discussed. The treatment of typical orthodontic problems with differing retraction requirements is illustrated.

Humans↗
PubMedDate not supplied

Oligodontia: a study of its prevalence and variation in 4032 children.

The frequency of oligodontia in the study group was 4.34 percent; 79.82 percent of those with congenitally missing teeth lacked not more than two teeth. In terms of number of teeth missing, there was no significant difference between the maxilla and mandible. When considering the whole dentition, bilateral agenesis was more common than unilateral agenesis. The findings of this study largely confirm the results of previous studies of oligodontia.

Adolescent↗