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Biomedical subjects

A L Williams

Publications and source records attributed to A L Williams.

At least 19 recordsLinked to original sources
PubMed1992

Generation of a substructure library for the description and classification of protein secondary structure. I. Overview of the methods and results.

Protein secondary structure has been typically classified into four major classes--alpha-helices, extended strands, reverse turns, and loops. Available methods for secondary structure analysis utilize predefined structure templates to search for structural matches among proteins. By this approach a significant portion of a proteins backbone conformation is assigned to one of a limited number of conformations or, if unassigned, to random coil. To expand our ability to describe protein secondary structure, we have developed an algorithm that operates independently of a predefined structure template. The procedure uses two geometric descriptors, the linear distance and the backbone dihedral angle, to represent the conformation form the alpha-carbon coordinates. The algorithm functions by searching for conformationally equivalent, contiguous fragments without regard to secondary structural classification and is thus independent of the complexity of the backbone fold. The result is a library of conformationally equivalent structure fragments that exhibit some novel characteristics. The library contains features that reproduce the major secondary structure classes as well as defining conformations previously described only as random or undefined conformations. Additionally, the library defines several subclassifications of beta-strands. We present here a validation of this method and a presentation and discussion of the most significant results. In a second study, we report the results of application of this method to spectra-structure correlations in Fourier transform infrared spectroscopy.

Algorithms↗
PubMed1992

Three-dimensional time-of-flight MR angiography with a surface coil: evaluation in 12 subjects.

We compared three-dimensional time-of-flight MR angiograms obtained with head coils and then with surface coils in five patients with intracranial vascular lesions and in seven normal volunteers to determine if imaging of intracranial vascular anatomy could be improved with the use of a surface coil. Visualization of small peripheral vessels was consistently better with a surface coil than with a head coil at identical small fields of view (FOVs). The surface-coil technique allowed small-FOV imaging of peripheral vascular lesions with higher spatial resolution and signal-to-noise ratio similar to that of large-FOV head-coil images. The use of a surface coil introduced the problem of signal falloff; centrally located vessels were visualized as well or better when a standard head coil was used. We conclude that surface-coil MR angiography can serve as a useful adjunct to routine head-coil MR angiography in the evaluation of peripheral vascular abnormalities.

Angiography↗
PubMed1991

Women's experiences of sexual abuse in childhood.

This study reports the results of questioning 1,253 women about experiences of sexual abuse during childhood. An incidence of 3.5% was identified; many of those affected reported benefit from the opportunity to discuss what had happened, an aspect which should be facilitated during routine consultations. The study identified a substantial burden on emotional health presenting into adult life. The wide range of places where abuse occurred and of relationships between the abusers/victims has important implications for educating children.

Adolescent↗
PubMed1991

Generalization patterns associated with training least phonological knowledge.

This study examined the relationship between productive phonological knowledge and generalization learning patterns in phonologically disordered children. Nine functionally misarticulating children (ages 3:8-5:9) were trained on aspects of their phonological systems that were characterized as inventory constraints that constituted "least phonological knowledge" in relation to the adult sound system (Elbert & Gierut, 1986). The misarticulated sounds were trained in the context of consonant clusters. Although all the subjects exhibited equivalent levels of phonological knowledge on the same misarticulated sounds and identical training was provided, three different learning patterns were observed. Results are discussed with regard to the characterization of phonological knowledge in underlying representations that are depicted as "non-adult-like" or incorrect relative to the target sound system. Implications for clinical assessment are discussed.

Articulation Disorders↗
PubMed1990

MR imaging of optic nerve lesions: value of gadopentetate dimeglumine and fat-suppression technique.

Eleven patients with known or suspected optic nerve lesions and eight normal subjects were examined with spin-echo technique at 1.5 T with unenhanced T1-weighted imaging, IV gadopentetate-dimeglumine-enhanced T1-weighted imaging, and enhanced T1-weighted imaging with fat suppression. Two pathologically proved and four presumed optic nerve meningiomas demonstrated significant enhancement and were best seen with the fat-suppression technique. None of the three presumed optic nerve gliomas nor the optic nerves of normal subjects demonstrated qualitative enhancement. We conclude that the use of a fat-suppression technique with gadopentetate dimeglumine enhancement improves delineation of enhancing optic nerve lesions. This technique should be useful for evaluating other anatomic regions where enhancing tissue marginates fat.

Adipose Tissue↗
PubMed1989

Gradient recalled echo MR imaging of superior sagittal sinus occlusion.

With T1-weighted gradient recalled echo (GRE) MR images and flow compensation, we studied the superior sagittal sinus in 3 normal volunteers and 3 patients with sinus occlusion. In these images, sites of patency of the superior sagittal sinus were identified due to the high signal intensity of the normal sinus. Tumor invading the sinus was nearly isointense with cerebral gray matter. T1-weighted GRE imaging proves to be an effective technique to evaluate sinus blood flow.

Cerebrovascular Circulation↗
PubMed1989

MR imaging of facial nerve enhancement in Bell palsy or after temporal bone surgery.

The authors evaluated magnetic resonance (MR) images obtained with intravenously administered gadolinium in ten patients who had facial paralysis and no facial nerve tumor. In patients with either Bell palsy (four patients) or facial paralysis after temporal bone surgery (six patients), intratemporal facial nerve enhancement was seen. Facial nerve enhancement on MR images proved to be a nonspecific finding.

Facial Nerve↗
PubMed1989

Contrast enhancement in spinal MR imaging.

We evaluated 44 patients with suspected spinal tumors or previous laminectomies with gadolinium-DTPA MR imaging in order to characterize the enhancement in normal, postoperative, and neoplastic intraspinal tissue. Using the signal intensity of CSF as an internal control, we calculated the percentage increase in signal intensity from pre- to postgadolinium studies. Tumors (astrocytoma, ependymoma, schwannoma) enhanced 70-350%; epidural scar, normal epidural venous plexus, and dorsal root ganglion enhanced up to 200%. Contrast enhancement does not per se distinguish neoplastic from normal tissue. Enhancement with gadolinium-DTPA appeared to increase the conspicuousness of intramedullary tumors but not intraosseous metastases. We believe that gadolinium-enhanced MR imaging is a valuable adjunct to routine MR imaging in the evaluation of intraspinal neoplastic processes and may be useful in delineating normal and postoperative structures in the spinal canal.

Contrast Media↗
PubMed1988

The effect of patient positioning on MR imaging of the internal auditory canal.

Identification of individual cranial nerves and complete exclusion of tumor in the internal auditory canal may be difficult with MR, especially in imperfectly positioned patients. MR studies of the temporal bones in patients and in normal volunteers positioned non-rotated or canted were correlated with corresponding cryomicrotomic sections. Especially in axial images, oblique sectioning of cranial nerves VII and VIII may cause difficulty in identifying individual nerves. A combination of axial and coronal short TR and TE images can be used to confidently exclude intracanalicular tumor in most cases.

Facial Nerve↗
PubMed1988

Ratio of serum aspartate to alanine aminotransferase in chronic hepatitis. Relationship to cirrhosis.

The ratio of the serum aspartate to alanine amino-transferase levels (AST/ALT) is often used as a clue to the etiology of the underlying liver disease. This ratio is usually greater than 2.0 in alcoholic liver disease and less than 1.0 in patients with chronic hepatitis and chronic cholestatic syndromes. We analyzed the AST/ALT ratio in 177 patients with various forms of nonalcoholic chronic liver disease who underwent medical evaluation and percutaneous liver biopsy. In the majority of cases of chronic viral hepatitis, the AST/ALT ratio was less than 1.0. However, there was a statistically significant correlation between the AST/ALT ratio and the presence of cirrhosis. Among 100 patients with chronic type B hepatitis, the mean AST/ALT ratio was 0.59 in those without cirrhosis and 1.02 in those with cirrhosis. Furthermore, the AST/ALT ratio often rose to greater than 1.0 when cirrhosis first became manifest. Thus, the finding of an AST/ALT ratio of greater than 1.0 in a patient with nonalcoholic liver disease should suggest the presence of cirrhosis. In addition, the use of the AST/ALT ratio as a means of separating alcoholic and nonalcoholic liver disease must be tempered with the knowledge that this ratio may be less helpful in the presence of cirrhosis.

Adult↗
PubMed1988

Special advisory service for immunisation.

Among steps to improve immunisation rates in Clwyd an advisory service was introduced for children with immunisation problems. We report on setting up the service and the first two years of running it; during this time 191 children attended. Influencing parental and professional attitudes through education, counselling, and the demonstration of good practice is emphasised.

Adolescent↗
PubMed1988

Sensitivity of Gd-DTPA-enhanced MR imaging of benign extraaxial tumors.

The effect of gadolinium diethylenetriaminepentaacetic acid (DTPA) on the sensitivity of cranial magnetic resonance (MR) imaging was measured in a prospective blinded study. Twenty-two consecutive patients with benign extraaxial tumors underwent MR imaging on a 1.5-T system without and with intravenous administration of Gd-DTPA. Readers independently interpreted the unenhanced and enhanced images without clinical information. The interpretations were compared with the anatomically verified diagnoses. Gd-DTPA improved the sensitivity of MR imaging for benign extraaxial tumors, especially in cases of residual or recurrent acoustic neuromas, multiple tumors (e.g., neurofibromatosis), or inconclusive unenhanced MR images. Enhancement with Gd-DTPA impaired the identification of a skull base tumor.

Adenoma↗
PubMed1988

Cervical neural foramina: correlative anatomic and MR imaging study.

Accurate diagnosis of diseases affecting the cervical neural foramina with magnetic resonance (MR) imaging requires an appreciation of the normal anatomic appearance of the foramen. The MR appearance of the foramen was studied in cadavers and healthy volunteers and was correlated with cadaver cryomicrotome sections. With gradient-echo techniques, foraminal soft tissues are highlighted relative to adjacent bone, making gradient-echo images valuable for evaluating the overall size and contents of the neural foramina. Intravenously administered gadolinium DTPA produces enhancement of all foraminal soft tissues, including the dorsal root ganglion. The nerve roots do not enhance with Gd-DTPA. Gradient-echo pulse sequences and intravenous use of Gd-DTPA represent promising techniques for the evaluation of the cervical foramina.

Cervical Vertebrae↗
PubMed1988

MR imaging of the cavernous sinus: value of spin echo and gradient recalled echo images.

A detailed evaluation of the MR appearance of the pituitary gland-cavernous sinus junction has not been described. In a series of coronal T1-weighted spin echo images without and with IV gadolinium, we noted the variable size and signal intensity of cavernous venous spaces adjacent to the pituitary gland and the inconsistent visualization of the dural membrane just lateral to the gland. Correlation of coronal T1-weighted spin echo and gradient recalled echo images (the latter with high-signal-intensity vascular structures) proved to be an effective means of identifying cavernous venous spaces, connective tissue and cranial nerves, and the lateral margins of the pituitary gland, and of differentiating tumor tissue from cavernous venous spaces. Further work is needed to develop criteria to distinguish cavernous sinus compression from actual tumor invasion.

Adenoma↗
PubMed1987

Benign extraaxial tumors: contrast enhancement with Gd-DTPA.

Meningiomas, acoustic neuromas, and other benign extraaxial tumors have little contrast with adjacent brain tissue on conventional magnetic resonance (MR) images. The contrast enhancement produced by intravenous administration of 0.1 mmol/kg of gadolinium-DTPA in these tumors was measured on T1 MR images. Acoustic neuromas showed the greatest enhancement (average, 310%), meningiomas the next greatest (average, 180%), and neurofibromas, glomus tumors, and pituitary microadenomas the least enhancement. The degree of enhancement was almost always greater at 3 minutes than at 25 or 55 minutes. Contrast between the tumor and adjacent tissue resulted from tumor enhancement in neuromas, meningiomas, and neurofibromas and from enhancement of the surrounding tissue in pituitary microadenomas.

Adenoma↗
PubMed1987

MR detection of tumor in the internal auditory canal.

The MR appearance of 15 tumors within or near the internal auditory canal was analyzed in detail and compared with the subsequent surgical observations. In most cases, nonenhanced MR showed the precise extent of the intracanalicular and extracanalicular tumor despite minor variations in appearance. In a case of facial nerve neurinoma, the extent to which the tumor invaded the canal was underestimated by MR. Gadolinium-DTPA enhancement is expected to eliminate uncertainties in MR imaging of the internal auditory canal.

Cerebellar Neoplasms↗