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Biomedical subjects

D H Brooks

Publications and source records attributed to D H Brooks.

At least 19 recordsLinked to original sources
PubMed2000

A comparison study of linear reconstruction techniques for diffuse optical tomographic imaging of absorption coefficient.

We compare, through simulations, the performance of four linear algorithms for diffuse optical tomographic reconstruction of the three-dimensional distribution of absorption coefficient within a highly scattering medium using the diffuse photon density wave approximation. The simulation geometry consisted of a coplanar array of sources and detectors at the boundary of a half-space medium. The forward solution matrix is both underdetermined, because we estimate many more absorption coefficient voxels than we have measurements, and ill-conditioned, due to the ill-posedness of the inverse problem. We compare two algebraic techniques, ART and SIRT, and two subspace techniques, the truncated SVD and CG algorithms. We compare three-dimensional reconstructions with two-dimensional reconstructions which assume all inhomogeneities are confined to a known horizontal slab, and we consider two 'object-based' error metrics in addition to mean square reconstruction error. We include a comparison using simulated data generated using a different FDFD method with the same inversion algorithms to indicate how our conclusions are affected in a somewhat more realistic scenario. Our results show that the subspace techniques are superior to the algebraic techniques in localization of inhomogeneities and estimation of their amplitude, that two-dimensional reconstructions are sensitive to underestimation of the object depth, and that an error measure based on a location parameter can be a useful complement to mean squared error.

Algorithms↗
PubMed1999

Inverse electrocardiography by simultaneous imposition of multiple constraints.

We describe two new methods for the inverse problem of electrocardiography. Both employ regularization with multiple constraints, rather than the standard single-constraint regularization. In one method, multiple constraints on the spatial behavior of the solution are used simultaneously. In the other, spatial constraints are used simultaneously with constraints on the temporal behavior of the solution. The specific cases of two spatial constraints and one spatial and one temporal constraint are considered in detail. A new method, the L-Surface, is presented to guide the choice of the required pairs of regularization parameters. In the case when both spatial and temporal regularization are used simultaneously, there is an increased computational burden, and two methods are presented to compute solutions efficiently. The methods are verified by simulations using both dipole sources and measured canine epicardial data.

Animals↗
PubMed1998

Successful salvage of a primary total knee arthroplasty infected with Candida parapsilosis.

Fungal infections of total joint arthroplasties are extremely rare with only 21 previous reported cases in the literature. In 19 of these cases, the offending organism has been a member of the candida species. In all of these cases, the patients had no clinical evidence of disseminated fungal infection. All previously reported cases of total joint fungal infections required removal of the primary prosthesis to eradicate the infection. There has also been a great reluctance to reimplant these patients. In fact, reimplantation has been successful in only one reported case. We report the first case of successful salvage of an arthroplasty infected with candida.

Abscess↗
PubMed1996

Temporal and spatial analysis of potential maps via multiresolution decompositions.

Cardiac potentials recorded on the epicardium or the body surface by an array of electrodes are usually analyzed either as spatial distributions or temporal waveforms. Thus, the analysis often involves temporal descriptors (eg. max dV/dt) or spatial descriptors (eg. location of local extrema) only. The best known transform technique that has been applied to these data that combines both spatial and temporal characteristics is the Karhunen-Loeve transform, a global transform applied to temporal and/or spatial bases obtained by statistical analysis of a database. As an alternative, multiresolution decompositions and related wavelet-type transforms have recently seen great development in signal processing and related fields. They offer flexibility, employing transformations onto local (rather than global) and fixed (rather than data-dependent) databases, and allow transformation of distributions, waveforms, or both, as desired. The utility of this method as applied to temporal and spatial segmentation and analysis of map data from both epicardial plaques and body surface potentials recorded during percutaneous transluminal coronary angioplasty is illustrated.

Body Surface Potential Mapping↗
PubMed1994

Analysis of PTCA-induced ischemia using an ECG inverse solution or the wavelet transform.

In patients without significant collaterals, percutaneous transluminal coronary angioplasty (PTCA) produces acute transient ischemia that is detectable in both standard electrocardiograms (ECG) and body surface potential maps (BSPMs). Control recordings made before or between inflations provide personalized baselines, which isolate the effects of ischemia from interpatient differences, such as torso shape and electrode location. In this study, two methods of evaluating PTCA-induced ischemia from BSPM recordings are presented. In the first method, an ECG inverse solution that estimates epicardial potentials from body surface signals using a realistic model of torso geometry is applied. The strength of this method lies in its potential ability to localize areas of cardiac ischemia on the epicardial surface. In the second approach, wavelet transforms were used to perform a multiresolution decomposition of the BSPM data into different frequency bands. The basis functions of the wavelet transform are time-limited and narrow band and hence can be expected to be sensitive to features of the BSPM that originate in discrete electrophysiologic events, such as intrusion of the activation front onto regions of ischemia or arrhythmias due to local conduction abnormalities. The method also offers a means of temporal and frequency localization of cardiac events related to the initiation of injury currents and abnormal conduction due to PTCA-induced ischemia. The inverse solution and the wavelet transform each offer new views of the spatial and temporal courses of acute ischemia potentially leading to new diagnostic insights in ECG patient examination.

Angioplasty, Balloon, Coronary↗
PubMed1993

Transplantation of Y79 cells into rat eyes: an in vivo model of human retinoblastomas.

PURPOSE: To develop an in vivo model of human retinoblastoma by returning cultured Y79 retinoblastoma cells to the retinal environment of a widely available laboratory animal. In so doing, to study the survival, integration, and invasive characteristics expressed by tumoral cells grafted into an intraocular milieu from which these progenitor cells originated more than 20 years ago. METHODS: Using the retinal grafting method of Lazar and del Cerro, Y79 cells were injected under direct visualization into the subretinal space of Fischer 344 rats. The host rats included 36 animals that received daily injections of cyclosporin A and 4 that did not. All hosts were sacrificed 30 to 60 days after transplantation. RESULTS: Clinical examination showed vitreal invasion by masses of flocculent white material or the intravitreal formation of solid tumors. Histologic examination showed these formations to be outgrowths of grafted tumoral cells into the host retina and vitreal cavity. Highly anaplastic tumoral cells were also found lodged in subretinal and intraretinal locations. There were signs of continued and intense cell division within the grafts, with no indication of cell-mediated host reaction against the grafted cells. CONCLUSIONS: After intraretinal xenografting, human Y79 retinoblastoma cells retain a highly tumoral nature despite many years of in vitro propagation. When xenografted, these cells survive, grow, and express their malignancy within the retina of the common laboratory rat protected by a moderate immunosuppressive regimen. This partial immunosuppression is a requirement for the xenografts to prosper. This model offers a valuable opportunity to study in vivo the cellular and molecular biology of this and other human retinoblastomas, and it may facilitate the evaluation of antitumoral treatments.

Animals↗
PubMed1990

Comparison and testing of least-squares time domain inverse solutions in electrocardiography.

The use of several mathematical methods for estimating epicardial ECG potentials from arrays of body surface potentials has been reported in the literature; most of these methods are based on least-squares reconstruction principles and operate in the time-space domain. In this paper we introduce a general Bayesian maximum a posteriori (MAP) framework for time domain inverse solutions in the presence of noise. The two most popular previously applied least-squares methods, constrained (regularized) least-squares and low-rank approximation through the singular value decomposition, are placed in this framework, each of them requiring the a priori knowledge of a 'regularization parameter', which defines the degree of smoothing to be applied to the inversion. Results of simulations using these two methods are presented; they compare the ability of each method to reconstruct epicardial potentials. We used the geometric configuration of the torso and internal organs of an individual subject as reconstructed from CT scans. The accuracy of each method at each epicardial location was tested as a function of measurement noise, the size and shape of the subarray of torso sensors, and the regularization parameter. We paid particular attention to an assessment of the potential of these methods for clinical use by testing the effect of using compact, small-size subarrays of torso potentials while maintaining a high degree of resolution on the epicardium.

Algorithms↗
PubMed1989

Evaluation and clinical significance of appendicular skeletal assessment by radiographic photodensitometry.

"Senile" or age-related bone loss affects cortical and trabecular bone and may be detected in the appendicular as well as the axial skeleton. This study presents radiographic photodensitometry as a precise and sensitive technique for evaluating appendicular skeletal status. Data from reproducibility studies indicate that the coefficient of variation for the technology is between 1.9% and 4.5% for the three bones analyzed. Evaluation of age-related changes in bone mass for over 800 subjects demonstrated a decline in mass vs. age after the fourth decade in women, with the slope of the decline being very similar to that seen in CT longitudinal studies. Applied serially to a patient over time, the technology identifies changes in bone mass and may be used to evaluate the response to intervention therapy.

Absorptiometry, Photon↗
PubMed1981

Expanded polytetrafluoroethylene femoropopliteal grafts: forty-eight-month follow-up.

Ninety-eight expanded polytetrafluoroethylene (PTFE) grafts were used for femoropopliteal reconstruction in 81 patients. Forty-eight-month follow-up is now available for 20 grafts and 36-month follow-up is available for 51 grafts. Seventy-four percent of the patients were men. Thirty-one percent had diabetes mellitus, 38% had hypertension, 36% had atherosclerotic heart disease, 18% had prior myocardial infarction, 17% had a previous operation for aortoiliac disease, and 14% had a previous ipsilateral femoropopliteal bypass procedure. The indication for operation was claudication in 47%, rest pain in 20%, and ischemic pregangrene or gangrene in 32% of patients. Distal runoff was angiographically graded as good (76%) or poor (24%). Seventy-three grafts were anastomosed to the proximal or midpopliteal artery (above the knee); 25 grafts were anastomosed to the distal popliteal artery (below the knee). Graft occlusion was determined by the return of ischemic symptoms, disappearance of previously palpable pulses, or by angiographic or Doppler assessment. There were no operative deaths. Nonocclusive causes of graft loss were death (7), amputation (2), infection (2), and aneurysm (5). The overall cumulative patency rate calculated by the life-table method, according to the criterion of occlusion alone, was 75% at 6 months, 68% at 2 years, 58% at 3 years, and 48% at 4 years. Preoperative symptoms, the number of patent outflow vessels, popliteal anastomosis placed above or below the knee, or hypertension did not adversely affect graft patency. Diabetes mellitus was associated with significantly increased graft failure. The PTFE graft is an acceptable alternative for femoropopliteal reconstruction for the patient without a suitable autologous saphenous vein.

Aged↗
PubMed1979

Expanded microporous polytetrafluoroethylene as a vascular substitute: a two year follow-up.

Since 1974, 131 femoropopliteal, distal popliteal, and tibial bypasses have been performed using expanded microporous polytetrafluoroethylene (PTFE). Forty patients were operated on for limb salvage, and 21 had had previous bypass procedures. The overall patency rate was 82%. Early occlusions possibly were related to technical error, but most probably were due to severity of disease and poor runoff. Late occlusions were related to progressive atherosclerosis in the proximal or distal arterial tree. A 75.7% cumulative patency rate was noted at 28 months. In man the PTFE prosthesis demonstrates a smooth intimal lining with fibroblastic ingrowth into the interstices of the graft. These results are considered to be excellent in this high-risk patient population. The patency rates achieved with PTFE are better than those accomplished with alternative conduits and approach the patency rates reported with autogenous saphenous vein. Expanded microporous polytetrafluoroethylene with its high patency, pliability, and tissue incorporation is an excellent arterial substitute. Only with continued use of this material and a more uniform patient selection can more equitable comparisons be made between expanded PTFE and the autogenous vein.

Adult↗
PubMed1979

Extra-anatomic bypass with expanded polytetrafluoroethylene.

Expanded polytetrafluoroethylene has been used successfully for femoropopliteal bypass, aortopulmonary bypass and as a venous substitute. Thirteen patients with impending limb loss had extra-anatomic bypasses with expanded polytetrafluoroethylene. Five patients with unilateral iliac disease had femorofemoral bypass for impending limb loss, and five debilitated patients underwent axillofemoral and bifemoral bypass for limb salvage. Nine of ten patients had salvage of the extremities. Three patients had extended profundoplasty, two combined with polytetrafluoroethylene femoropopliteal bypass to isolated popliteal artery segments. Two of these patients had limb salvage. The patency rate is 92 per cent, and the limb salvage rate is 85 per cent in this difficult group of patients. The follow-up period extends to 28 months, and 12 patients are beyond one year. In six patients, polytetrafluoroethylene carotid subclavian bypass was performed for the subclavian steal syndrome; all patients had relief of the symptoms. One patient underwent axillary-axillary bypass with excellent results. Expanded microporous polytetrafluoroethylene with its high patency, shortened operating time, biocompatibility and excellent tissue incorporation is an excellent arterial substitute. The pliability and no requirement for preclotting make polytetrafluoroethylene ideally suited for patch angioplasty and suturing in areas difficult to expose.

Adult↗
PubMed1977

Primary pulmonary paraganglioma: report of a case and review of the literature.

A case of primary paraganglioma of the lung in a 63-year-old woman is presented. The tumor arose from a segmental pulmonary artery and had typical epithelioid cell nests in a "Zellballen" pattern surrounded by reticulum. An electron microscopic study revealed cytoplasmic neurosecretory granules. As in eight previously reported cases, in the English literature, the tumor was benign. The tumor has occurred in seven women and two men with an age range of 43 to 69 years. All patients were asymptomatic, and the diagnosis has never been made prior to operation.

Cytoplasmic Granules↗
PubMed1976

The use of expanded microporous polytetrafluoroethylene for limb salvage: a preliminary report.

Initial laboratory and clinical evaluations of a new prosthetic material, expanded microporous polytetrafluoroethylene (PTFE), for small vessel replacement is promising and encourages further clinical trial. Frequently the autogenous saphenous vein is not available for bypass procedures, and alternative arterial substitutes have not proved reliable for replacement of small vessels. In this study, 15 patients with impending loss of limb and no available saphenous vein underwent revascularization of the lower extremity with expanded microporous PTFE grafts. Thirteen of 15 patients now demonstrate viable extremities with a resulting over-all early patency and limb salvage rate of 87 percent for this series. Follow-up ranges from one to 8 months. Seven patients had diabetes mellitus and eight had atherosclerotic heart disease. Nine grafts crossed the knee joint. In all patients arterial runoff was poor. Six patients had previous femoropopliteal bypasses, five with autogenous veins and one with Dacron velour. Two patients had multiple previous operations that failed, first with autogenous vein and later with fabric grafts. The current limb salvage and patency rate of 87 percent in high-risk patients suggests that expanded PTFE may be the prosthesis of choice when an autogenous vein is not available and possibly an equally good substitute when the venous autograft is available.

Aged↗
PubMed1975

Sensory and meaning features in stimulus recognition and associative retrieval.

Three experiments addressed the problem of isolating the effects of sensory similarity on subprocesses involved in coding paired associates. In the first, the standard recognition-recall procedure was used and stimulus similarity, concreteness, and frequency were varied. However, because of concern with the validity of this recognition procedure as a measure of functional stimulus contact, an alternative was developed. This alternative led to the second study in which only stimulus similarity was manipulated. In the third experiment, similarity was varied, and the pairs were either associatively compatible, unrelated, or incompatible. The results using the new procedure indicated that similarity consistently disrupted functional stimulus contact but not associative retrieval. By contrast, associative relatedness facilitated both subprocesses.

Adolescent↗