Biomedical subjects

Peter Lorenzen

Publications and source records attributed to Peter Lorenzen.

4 recordsLinked to original sources

PubMed2006

Multi-modal image set registration and atlas formation.

In this paper, we present a Bayesian framework for both generating inter-subject large deformation transformations between two multi-modal image sets of the brain and for forming multi-class brain atlases. In this framework, the estimated transformations are generated using maximal information about the underlying neuroanatomy present in each of the different modalities. This modality independent registration framework is achieved by jointly estimating the posterior probabilities associated with the multi-modal image sets and the high-dimensional registration transformations mapping these posteriors. To maximally use the information present in all the modalities for registration, Kullback-Leibler divergence between the estimated posteriors is minimized. Registration results for image sets composed of multi-modal MR images of healthy adult human brains are presented. Atlas formation results are presented for a population of five infant human brains.

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PubMed2005

Unbiased atlas formation via large deformations metric mapping.

The construction of population atlases is a key issue in medical image analysis, and particularly in brain mapping. Large sets of images are mapped into a common coordinate system to study intra-population variability and inter-population differences, to provide voxel-wise mapping of functional sites, and to facilitate tissue and object segmentation via registration of anatomical labels. We formulate the unbiased atlas construction problem as a Fréchet mean estimation in the space of diffeomorphisms via large deformations metric mapping. A novel method for computing constant speed velocity fields and an analysis of atlas stability and robustness using entropy are presented. We address the question: how many images are required to build a stable brain atlas?

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PubMed2004

Emergency endovascular stent-grafting for life-threatening acute type B aortic dissections.

BACKGROUND: There is still a considerable controversy regarding optimal treatment for patients with acute type B aortic dissection. Patients with complicated disease are particularly challenging for cardiovascular treatment. Early surgery for acute dissections of the descending aorta with life-threatening complications is known to carry a high mortality. Endovascular stent grafting is developing as an alternative treatment mainly for chronic stages of type B aortic dissection. It is not clear whether endovascular stent grafting is safe and effective in emergency treatment of acute type B aortic dissection. METHODS: In 10 patients (7 men, 3 women; mean age, 59.2 years; range, 46 to 65 years), endovascular stent grafting was performed within 11.0 +/- 5.9 hours (range, 4 to 24 hours) of diagnosis of complications. Indications for acute intervention included contained rupture, hematothorax, life-threatening malperfusion, and refractory pain. Using a retrograde endovascular route after surgical exposure of the femoral artery, self-expanding stent prostheses consisting of polyester-covered Nitinol (Talent, World Medical; mean diameter, 40 +/- 4 mm; length, 10 cm) were placed into the descending aorta distal to the subclavian artery. Before discharge and on follow-up visits, imaging of the aorta was performed using computed tomography. RESULTS: In 9 of 10 patients (90%), the primary entry could be completely occluded with the endovascular stent. Early mortality was 20% (2 of 10): 1 patient died after disruption of the intimal layer distal to the stent, and 1 patient died in hemorrhagic shock after surgical fenestration of the abdominal aorta for persistent malperfusion. Three patients (30%) required consecutive surgical treatment: indications included acute development of retrograde type A aortic dissection, acute stent dislocation by fractured wires and secondary leakage, and late formation of an aneurysm of the descending aorta 6 months after endovascular stent grafting. There were no surgical or late deaths. CONCLUSIONS: Our experience provides some evidence that early mortality of life-threatening acute type B aortic dissection may be reduced by emergency endovascular stent grafting and that this form of treatment is a promising therapeutic option. Refinements, especially in stent design and application, may further improve the prognosis of patients in the life-threatening situation of complicated acute type B aortic dissection.

Acute Disease↗

PubMed2003

Structural and radiometric asymmetry in brain images.

This paper presents a general framework for analyzing structural and radiometric asymmetry in brain images. In a healthy brain, the left and right hemispheres are largely symmetric across the mid-sagittal plane. Brain tumors may belong to one or both of the following categories: mass-effect, in which the diseased tissue displaces healthy tissue; and infiltrating, in which healthy tissue has become diseased. Mass-effect brain tumors cause structural asymmetry by displacing healthy tissue, and may cause radiometric asymmetry in adjacent normal structures due to edema. Infiltrating tumors have a different radiometric response from healthy tissue. Thus, structural and radiometric asymmetries across the mid-sagittal plane in brain images provide important cues that tumors may be present. We have developed a framework that registers images with their reflections across the mid-sagittal plane. The registration process accounts for tissue displacement through large deformation image warping. Radiometric differences are taken into account through an additive intensity field. We present an efficient multi-scale algorithm for the joint estimation of structural and radiometric asymmetry. Results for nine MR images of patients with tumors and four normal control subjects are presented.

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