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W L Davis

Publications and source records attributed to W L Davis.

134 records · Page 8Linked to original sources
PubMedDate not supplied

Vessel enhancement filtering in three-dimensional MR angiography.

For a variety of reasons, small vessels have low signal intensity in magnetic resonance angiography. When the vessel signal intensity is lower than the signal intensity of background tissues, these vessels tend not to be visible on maximum-intensity-projection images. The authors developed a nonlinear second-difference spatial filtering technique that enhances the details of small vessels while suppressing both noise and uniform background tissue. Two similar nonlinear second-difference filters are presented and compared with the linear Laplacian second-difference filter. To evaluate the performance of these filters, they were applied to intracranial three-dimensional time-of-flight MR angiographic data and the results compared with the vessel enhancement obtained with a simple second-difference Laplacian filter and with magnetization transfer contrast (MTC) techniques. The comparisons demonstrated that nonlinear filtering and MTC techniques result in similar improvement in small-vessel visibility and apparent continuity. A quantitative comparison demonstrated that the improvement in the contrast-to-noise ratio is much greater with the nonlinear filters than the Laplacian filter.

Circle of Willis↗
PubMedDate not supplied

Experimental and theoretical studies of vessel contrast-to-noise ratio in intracranial time-of-flight MR angiography.

CNR studies were performed for human intracranial vessels in 3D MRA data sets. The CNR dependency of different imaging parameters, such as flip angle, field of view, echo time, repetition time, and echo readout symmetry, was studied for vessels in the region of the circle of Willis. A theoretical model was developed for MR vascular imaging based on the Bloch equations and Fourier imaging theory. This model predicts the MR image intensity of vessels from basic subject parameters, such as the relaxation times of blood and stationary tissue, vessel dimension, and flow velocity, and the parameters of the imaging technique, such as flip angle, voxel volume, repetition time, and echo time. For most experiments, the model was found to fit the experimental results well. The validity of this model allows the optimization of imaging parameters to maximize vessel CNR in MR angiography.

Adult↗
PubMedDate not supplied

MR angiography for preoperative evaluation of vascularized fibular grafts.

PURPOSE: Two magnetic resonance (MR) angiography pulse sequences, two-dimensional (2D) time of flight (TOF) and multislab three-dimensional (3D) TOF, were compared for ease of application and capability of depicting the arterial trifurcation in candidates for vascularized fibular grafts. PATIENTS AND METHODS: Both 2D TOF and multislab 3D TOF MR angiography procedures were performed to image the lower legs of 15 healthy volunteers and six patients. Three radiologists evaluated each study for the number of trifurcation vessels depicted at the knee and at the ankle, the corresponding degree of confidence, and the presence of anatomic variants. RESULTS: All trifurcation vessels were identified at the level of the knee with a high degree of confidence. Usually two vessels could be identified at the ankle with a slightly lower degree of confidence. A higher number of vessels were identified at the ankle with the 2D TOF technique than with the 3D TOF technique. A number of anatomic variants were identified. Anatomy of all six patients was identified correctly and confirmed at surgery. CONCLUSION: Both 2D TOF and multislab 3D TOF are useful techniques to define the arterial anatomy of the lower leg and have proved valuable in preoperative planning for vascularized grafts. Vessel visualization at the level of the ankle was superior with the 2D TOF technique.

Fibula↗
PubMedDate not supplied

Changes in cytosolic calcium, bleb formation, and cell death in neural crest cells treated with isotretinoin and 4-oxo-isotretinoin.

Information regarding the cytopathologic mechanism of action of the retinoids [isotretinoin (IR) and 4-oxo-isotretinoin (4-OIR)] on neural crest cells (NCCs) in culture was sought. Those pathophysiologic alterations in cell metabolism studied were: cell blebbing (xieosis), free radical formation, cell viability, and cellular calcium homeostasis. Cells were treated with IR or 4-OIR in the presence of high (1.4 mM) and low (5.0 microM) levels of extracellular calcium ions. Recently developed techniques utilizing fluorescent molecular probes for calcium analyses, i.e., Fura 2AM, were used to study the effects of these drugs on the cytosolic calcium concentration of NCCs. The effects of IR and 4-OIR on NCC viability, [Ca++]int, were contrasted with the effects of certain sulfhydryl drugs (HgCl2, NEM, PCMBS) and calcium ionophores (ionomycin, A23187), agents known to perturb cell membranes, increase cytosolic calcium loads, and induce cell injury and subsequent cell death. Both retinoids were shown to induce an increase in the generation of superoxide radicals (SO) and increase the influx of calcium ions by the NCCs, thus increasing [Ca++]int by several hundred percent within 5 to 10 min. The liberation of SO was calcium dependent. These early effects were accompanied by an increase in cell blebbing activity. Also, a significant decrease in NCC viability was seen as early as 10 min after the addition of IR or 4-OIR to the incubation medium. 4-OIR proved to be the more potent of the two retinoids tested. The severity of these effects on NCC metabolism was dependent on medium calcium concentration with all changes being increased in the presence of the higher extracellular calcium levels. From the data presented it appears as though the retinoids cause a rapid elevation in cytosolic [Ca++]int possibly by purturbing the integrity of the cell membrane, denaturing membrane Ca-ATPase activity, or both. Retinoid-induced changes in membrane activity are evidenced by increased surface blebbing and superoxide formation. The prolonged elevation of intracellular [Ca++] may be directly related to depressed NCC viability and thus explain the known teratogenic effects of these drugs and their relationship to ectomesenchymal cell hypoplasia and craniofacial dysmorphogenesis.

Animals↗
PubMedDate not supplied

Intracranial MRA: single volume vs. multiple thin slab 3D time-of-flight acquisition.

Single volume three-dimensional (3D) time-of-flight (TOF) MR angiography is the most commonly used noninvasive method for evaluating the intracranial vasculature. The sensitivity of this technique to signal loss from flow saturation limits its utility. A recently developed multislab 3D TOF technique, MOTSA, is less affected by flow saturation and would therefore be expected to yield improved vessel visualization. To study this hypothesis, intracranial MR angiograms were obtained on 10 volunteers using three techniques: MOTSA, single volume 3D TOF using a standard 4.9 ms TE (3D TOFA), and single volume 3D TOF using a 6.8 ms TE (3D TOFB). All three sets of axial source images and maximum intensity projection (MIP) images were reviewed. Each exam was evaluated for the number of intracranial vessels visualized. A total of 502 vessel segments were studied with each technique. With use of the MIP images, 86% of selected vessels were visualized with MOTSA, 64% with 3D TOFA (TE = 4.9 ms), and 67% with TOFB (TE = 6.8 ms). Similarly, with the axial source images, 91% of selected vessels were visualized with MOTSA, 77% with 3D TOFA (TE = 4.9 ms), and 82% with 3D TOFB (TE = 6.8 ms). There is improved visualization of selected intracranial vessels in normal volunteers with MOTSA as compared with single volume 3D TOF. These improvements are believed to be primarily a result of decreased sensitivity to flow saturation seen with the MOTSA technique. No difference in overall vessel visualization was noted for the two single volume 3D TOF techniques.

Adult↗