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Biomedical subjects

D N Levin

Publications and source records attributed to D N Levin.

At least 37 records · Page 2Linked to original sources
PubMed1993

A frameless stereotactic approach to neurosurgical planning based on retrospective patient-image registration. Technical note.

A frameless stereotactic device interfacing an electromagnetic three-dimensional (3-D) digitizer to a computer workstation is described. The patient-image coordinate transformation was found by retrospectively registering a digitizer-derived model of the patient's scalp with a magnetic resonance (MR) imaging-derived model of the same surface. This procedure was performed with routine imaging data, eliminating the need to obtain special-purpose MR images with fiducial markers in place. After patient-image fusion was achieved, a hand-held digitizing stylus was moved over the scalp and tracked in real time on cross-sectional and 3-D brain images on the computer screen. This device was used for presurgical localization of lesions in 10 patients with meningeal and superficial brain tumors. The results suggest that the system is accurate enough (typical error range 3 to 8 mm) to enable the surgeon to reduce the craniotomy to one-half the size advisable with conventional qualitative presurgical planning.

Adult↗
PubMed1990

Three-dimensional magnetic resonance images of the brain: application to neurosurgical planning.

Data from single 10-minute magnetic resonance scans were used to create three-dimensional (3-D) views of the surfaces of the brain and skin of 12 patients. In each case, these views were used to make a preoperative assessment of the relationship of lesions to brain surface structures associated with movement, sensation, hearing, and speech. Interactive software was written so that the user could "slice" through the 3-D computer model and inspect cross-sectional images at any level. A surgery simulation program was written so that surgeons were able to "rehearse" craniotomies on 3-D computer models before performing the actual operations. In each case, the qualitative accuracy of the 3-D views was confirmed by intraoperative inspection of the brain surface and by intraoperative electrophysiological mapping, when available.

Adolescent↗
PubMed1990

Image correlation in oncology.

Image correlation techniques can provide objective spatial registration between multimodality data sets acquired during the planning and follow-up phases of radiation therapy. Correlation of pre-CT/MRI with follow-up CT/MRI and 3D dose matrices may provide insights into normal tissue tolerance. Correlation of SPECT and planar scintigraphs with anatomical maps derived from CT/MRI may be useful in the precise localization of disease and in the evaluation of new modalities, such as radiolabeled monoclonal antibodies, in the diagnosis and treatment of cancer. Correlation of PET and MRI may lead to a more precise understanding of structure-function relationships of the brain. The development and refinement of multimodality image-correlation techniques is a logical step in the evolving role of imaging in radiation therapy.

Brain Neoplasms↗
PubMed1989

Surface of the brain: three-dimensional MR images created with volume rendering.

Image processing and volume rendering have been applied to magnetic resonance (MR) images to produce three-dimensional views of the surface of the brain. Four healthy volunteers and 17 patients with a variety of disorders underwent MR imaging of the head, and these images were processed and subjected to volume rendering. The resulting three-dimensional views depict abnormalities of the brain surface, as well as important landmarks of normal brain surface anatomy such as precentral, postcentral, superior temporal, and inferior frontal gyri, which are difficult to identify on cross-sectional images.

Adult↗
PubMed1989

The brain: integrated three-dimensional display of MR and PET images.

Three patients with intractable epilepsy, two with brain tumors, and one with encephalitis were imaged with magnetic resonance (MR) and positron emission tomography (PET). MR data were used to construct a three-dimensional (3D) computer model of the brain surface depicting the precentral (movement), postcentral (sensation), left inferior frontal (speech), and left superior temporal (hearing) gyri. PET-derived measurements of average surface metabolism were encoded as colors and mapped onto the 3D model by means of a retrospective technique for registering the two scans. The integrated 3D model depicted the location of PET-detected metabolic abnormalities with respect to the gyral anatomy visualized with MR. In each case, the predicted relationships were confirmed intraoperatively by means of inspection of the brain and electroencephalography. Multimodality 3D displays are likely to be particularly valuable for interpreting PET studies of epileptic patients and others with normal MR anatomy.

Brain↗
PubMed1988

SLIM: spectral localization by imaging.

Nonspectroscopic magnetic resonance (MR) imaging often shows that a slice is composed of several compartments, each of which can be assumed to have a spatially homogeneous magnetic resonance spectrum, e.g., a limb composed of fat, muscle, bone marrow, and tumor. We show how to use structural information from such a nonspectroscopic image in order to increase the efficiency of subsequent localized spectroscopic measurements. Specifically, knowledge of the boundaries of N compartments makes it possible to reconstruct compartmental spectra from spectroscopic signals from an entire cross section with N or more different degrees of phase encoding. Experimental studies of a two-compartment phantom show that this method (SLIM) can be used to derive regional hydrogen spectra of a single slice from signals with as few as 2 phase-encoding steps, although Fourier transform chemical-shift imaging requires 64 steps to achieve a result of comparable accuracy. SLIM required only 16 phase-encoding steps to obtain accurate regional single slice spectra in a human limb with three compartments. Spectra of similar quality, obtained by Fourier transform chemical-shift imaging, required 256 to 1024 steps.

Humans↗
PubMed1988

Retrospective geometric correlation of MR, CT, and PET images.

Magnetic resonance imaging, computed tomographic, and positron emission tomographic studies of the brain provide complementary information, and many patients undergo more than one of these studies during the course of their diagnostic workup and treatment. A new technique for quantitative geometric correlation of such studies makes it possible to create integrated multimodality images by mapping features from one image onto an image obtained with another modality. The coordinate transformation between any pair of images is found by a semiautomatic algorithm for matching models of the patient's external surface as depicted in the two data sets. The resultant hybrid images, which combine complementary features of different studies, are often more useful for diagnosis and treatment planning than are the original single-modality images. The algorithm can also be used for spatial registration of baseline studies with follow-up images created with the same modality, which allows tracking of a lesion to detect subtle interval changes in size and shape. This technique can be applied to images acquired in routine clinical practice, since it is completely retrospective and does not necessitate special positioning or landmarking of the patient.

Adolescent↗
PubMed1987

Musculoskeletal tumors: improved depiction with linear combinations of MR images.

With postprocessing, the authors produced a single hybrid image that combines the complementary information in conventional T1- and T2-weighted magnetic resonance (MR) images. A 1-T MR imager was used to produce T1- and T2-weighted images (repetition time, msec/echo time, msec: 500/30, 1,500/120) of patients with various primary bone tumors. Various weighted sums and differences of these images were then formed. Weighted subtraction allowed formation of hybrid images with high contrast between tumor and all adjacent normal tissues (muscle, fat, bone marrow), unlike the original T1- and T2-weighted images. Certain weighted sums of the acquired images simultaneously display the high signal-to-noise and clear anatomic detail of the T1-weighted technique along with the high contrast between extraosseous tumor and muscle of the T2-weighted image. A single hybrid MR image can contain useful characteristics both of T1- and T2-weighted images, making it easier for one to detect the extent of an abnormality.

Bone Neoplasms↗
PubMed1985

Public attitudes toward cancer pain.

This study examined public attitudes toward pain associated with cancer and its treatment. A variety of issues were assessed, including the extent to which pain is associated with cancer, fear of pain as a factor in the decision to delay seeking treatment, and concerns about the use of narcotic analgesics for pain control. A statewide sample of 496 adult Wisconsin residents completed a telephone interview. It was found that cancer was perceived as an extremely painful disease relative to other medical conditions. Eighteen percent of the respondents reported that they might be reluctant to seek medical attention due to fear of pain. Concerns about disease- and treatment-related pain were rated as significant psychosocial concerns. There were multiple concerns expressed about the consequences of using narcotic analgesics for pain control. Implications for public education and patient management are discussed.

Analgesics, Opioid↗
PubMed1983

Calcium flow-independent actions of calcium channel blockers in toad urinary bladder.

A role for transmembrane calcium movement in vasopressin stimulation of its target cell has been postulated based on studies with calcium entry blockers such as verapamil. We examined the effect of three sets of structurally different calcium blockers--D600 (an analogue of verapamil), diltiazem, and nifedipine--on water flow in toad bladder. D600 (200 microM), diltiazem (200 microM), and nifedipine (60 microM) inhibited vasopressin-induced water flow but enhanced adenosine 3',5'-cyclic monophosphate (cAMP)-induced water flow, suggesting that the drugs inhibit cAMP generation in response to vasopressin but enhance the response to exogenous cAMP by inhibiting phosphodiesterase activity. In the case of vasopressin stimulation, inhibition of cAMP generation appears to be the overriding effect. This was confirmed by measurements of cAMP content and the protein kinase ratio (-cAMP/+cAMP), which were significantly lower in bladders receiving both D600 and vasopressin than in those receiving vasopressin alone. Furthermore the drugs inhibited activation of adenylate cyclase by vasopressin in cell homogenates and inhibited phosphodiesterase in both homogenates and membrane-free supernatants. Thus these "calcium channel blockers" can directly alter cAMP metabolism in settings where movement of calcium should be irrelevant. The close correlation between the biochemical and transport effects of these agents suggests that their effect on water flow may occur by a direct effect on cellular enzymes or the membranes in which they reside and not by altering local calcium concentrations.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1981

Surface capacity of electrically syncytial tissues.

An exact geometry-independent formula is derived that gives the total surface membrane capacity of an electrical syncytium in terms of its input resistance (RIN) and the phase angle (phi) of its complex admittance. The formula strips off the effects of resistance in the extracellular space and exposes the true capacity of the external surface of preparations such as skeletal muscle fibers, cardiac Purkinje fibers, or spherical cardiac aggregates. The shape, extent, and resistivity of the extracellular space may be arbitrary and need not be measured. The medium in this space may have an arbitrary and nonuniform resistivity. It is assumed that the tissue is impaled with current and voltage electrodes, so that the intracellular resistance between the electrodes and membranes is negligible or can de dealth with by theoretical calculations. Under these circumstances the total surface membrane capacity at high frequency is determined exactly by RIN and a frequency domain integral over phi. The method is tested with synthetic data for RIN and phi generated by the "disk" model of skeletal muscle fibers and the "pie" model of cardiac Purkinje fibers. The formula allows the "inversion" of these data and the deduction of the correct value of the total surface membrane capacity.

Cell Aggregation↗
PubMed1981

Unit membrane parameters of electrically syncytial tissues.

A change in the holding voltage, exposure to channel-blocking agents, and similar interventions will induce changes in the membrane properties of electrically syncytial tissues. The altered membrane characteristics will produce changes in the input resistance (RIN) and the phase angle (phi) of the complex admittance of the whole preparation. Exact geometry-independent formulas are derived that give the intervention-induced changes in the membrane capacitance and conductance in terms of the measured changes in RIN and phi. The formulas automatically account for the effects of extracellular resistance in tissues such as skeletal muscle fibers, cardiac Purkinje fibers, and small cardiac "aggregates." The size, shape, and resistance of the extracellular space may be arbitrary and need not be measured. The surface (invaginated) membranes, which face the bath (extracellular space), are assumed to be characterized by an RC circuit with specific capacity Cme (Cmi) and specific conductivity gme (gmi). It is assumed that the intracellular voltage gradient between the electrodes and the membranes is negligible or reliably calculable. The intervention is assumed to leave the geometry and resistivity of the extracellular space unchanged. Under these circumstances the intervention-induced changes in Cme, Cmi, gme, and gmi are determined exactly in terms of the corresponding changes in RIN and certain frequency domain integrals over phi. The technique is illustrated by synthetic data for RIN and phi generated by the "disk" model of a skeletal muscle fiber in which Cme and Cmi depend upon holding voltage. The corresponding voltage dependence of RIN and phi is successfully "inverted" to expose the underlying voltage dependence of Cme and Cmi. These computations suggest that the formulas for Cme and Cmi will be useful in realistic situations, since they are not too sensitive to experimental error in the data for RIN and phi. This method makes it possible to detect voltage-dependent capacity changes due to unit membrane processes (e.g., charge movement) as long as the intrinsic time constant of that process is very small (e.g., less than 1/30 ms). As a second example I consider a disk model that is exposed to increasing concentrations of a channel-blocking agent. The drug dependence of RIN and phi is used to calculate the drug dependence of the total membrane conductivity (the sum of gme and gmi, weighted by the areas of surface and invaginated membranes, respectively).

Cell Membrane↗
PubMed1981

A cleft model for cardiac Purkinje strands.

Conduction of the action potential in cardiac muscle is complicated by its multicellular structure, with narrow intercellular clefts and cell-to-cell coupling. A model is developed from anatomical data to describe cardiac Purkinje strands of variable diameter and different internal arrangements of cells. The admittance of the model is solved analytically and fit to results of cable analysis. Using the extracted specific membrane and cell electrical parameters (Rm = 13 K omega cm2, Cm = 1.5 mu F/cm2, Ri = 100 mu cm, and Re = 50 omega cm), the model correctly predicted conduction velocity and filling of capacitance at the onset of a voltage step. The analysis permits more complete studies of the factors controlling conduction velocity; for instance, the effect on conduction velocity of a capacity in the longitudinal current circuit is discussed. Predictions of the impedance and phase angle were also made. Measurements of the frequency dependence of phase angle may provide a basis for separating cleft membrane properties from those of the surface membrane and may aid the measurement of nonlinear membrane properties in muscle.

Action Potentials↗