Biomedical subjects

D H Phong

Publications and source records attributed to D H Phong.

7 recordsLinked to original sources

PubMed2004

Delayed isolated tension intraventricular pneumocephalus after ventriculoperitoneal shunt.

OBJECTIVE AND IMPORTANCE: Delayed and isolated tension intraventricular pneumocephalus without air in the pericerebral subarachnoid spaces is an exceptional complication of a ventriculoperitoneal (VP) shunt. We purpose an explanation for the development of such delayed pneumocephalus. CLINICAL PRESENTATION: A 14-year-old boy developed headaches and right-side hearing loss three months after surgical resection of a cerebellar medulloblastoma and insertion of a VP shunt. CT-scan showed isolated intraventricular pneumocephalus without air in the pericerebral subarachnoid spaces. A petrous bone defect was discovered and surgically repaired with clinical and radiographic resolution of pneumocephalus. CONCLUSION: We presume that the brain tIssue plugging the bony defect retracted following the placement of the shunt and eventually there was rupture through the pia-mater into the lateral ventricle. This could explain why isolated intraventricular occurred, as opposed to air in the subarachnoid spaces.

Adolescent↗

PubMed1983

Singular integrals related to the Radon transform and boundary value problems.

Two classes of integral operators are introduced that combine features of Calderón-Zygmund singular integrals, the Hilbert integral, and the Radon transform. Examples and applications arising from integral geometry and several complex variables are discussed.

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PubMed1982

Symplectic geometry and positivity of pseudo-differential operators.

In this paper we establish positivity for pseudo-differential operators under a condition that is essentially also necessary. The proof is based on a microlocalization procedure and a geometric lemma.

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PubMed1980

On the asymptotic eigenvalue distribution of a pseudo-differential operator.

A description of the number N(K) of eigenvalues less than K for a pseudo-differential operator with positive symbol is given in terms of the number of unit cubes canonically imbedded in the subset of phase space where the symbol is less than CK. This gives back in particular the order of magnitude of N(K) for elliptic symbols.

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PubMed1979

On the lowest eigenvalue of a pseudo-differential operator.

Positive lower bounds for pseudo-differential operators with nonnegative symbols are derived; the bounds in particular yield subelliptic estimates for operators arising as sums of squares of vector fields.

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PubMed1979

On integral representations for the Neumann operator.

This paper is devoted to the derivation of an approximate formula for the kernel of the Neumann operator N on strongly pseudo-convex domains. In particular, a new class of singular integral operators arises that are infinitely smoothing in the interior and preserve singular supports at the boundary and whose kernels are products of isotropic kernels with parabolic ones.

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PubMed1978

On positivity of pseudo-differential operators.

In this paper we obtain new lower bounds for pseudo-differential operators with non-negative symbols, thus providing a sharper form of Gårding's inequality.

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