Head segmentation

Abstract: Several factors have to be considered for implementing an accurate attenuation-correction (AC) method in a combined MR-PET scanner. In this work, some of these challenges were investigated, and an AC method based entirely on the MRI data obtained with a single dedicated sequence was developed and used for neurologic studies performed with the MR-PET human brain scanner prototype. Methods: The focus was on the problem of bone–air segmentation, selection of the linear attenuation coefficient for bone, and positioning of the radiofrequency coil. The impact of these factors on PET data quantification was studied in simulations and experimental measurements performed on the combined MR-PET scanner. A novel dual-echo ultrashort echo time (DUTE) MRI sequence was proposed for head imaging. Simultaneous MR-PET data were acquired, and the PET images reconstructed using the proposed DUTE MRI–based AC method were compared with the PET images that had been reconstructed using a CT-based AC method. Results: Our data suggest that incorrectly accounting for the bone tissue attenuation can lead to large underestimations (>20%) of the radiotracer concentration in the cortex. Assigning a linear attenuation coefficient of 0.143 or 0.151 cm−1 to bone tissue appears to give the best trade-off between bias and variability in the resulting images. Not identifying the internal air cavities introduces large overestimations (>20%) in adjacent structures. On the basis of these results, the segmented CT AC method was established as the silver standard for the segmented MRI-based AC method. For an integrated MR-PET scanner, in particular, ignoring the radiofrequency coil attenuation can cause large underestimations (i.e., ≤50%) in the reconstructed images. Furthermore, the coil location in the PET field of view has to be accurately known. High-quality bone–air segmentation can be performed using the DUTE data. The PET images obtained using the DUTE MRI– and CT-based AC methods compare favorably in most of the brain structures. Conclusion: A DUTE MRI–based AC method considering all these factors was implemented. Preliminary results suggest that this method could potentially be as accurate as the segmented CT method and could be used for quantitative neurologic MR-PET studies.

Abstract: THE first phase of embryonic development in Drosophila consists of the elaboration and interpretation of maternally encoded information that specifies spatial pattern in the embryo1. The product of the maternal gene bicoid (bcd) is thought to organize the anterior pattern of the embryo2. Although the bed transcript is localized at the anterior pole of the egg3,4 the bed protein forms a stable concentration gradient through the anterior two thirds of the embryo5. The graded distribution of bed protein defines position along the anterior-posterior axis of the embryo6–8 through the spatially restricted activation of subordinate targets such as the gap gene hunchback(hb)7–11. In vitro manipulation of specific bcd protein binding sites has shown that the gradient of bed protein can in principle define more than one discrete domain of spatially restricted gene activation in the head of the embryo, depending on the affinity of the available binding sites for the bcd protein8.12. Genetic analysis has indicated the need for at least one additional zygotic segmentation gene to mediate bcd function in portions of the head that lie anterior to the hb domain2,12–14. The missing gene activity is expected to be activated in response to higher levels of bed protein than are required for hb activation12. We report here that three previously identified zygotic genes buttonhead (btd), empty spiracles (ems) and orthodenticle (otd) l5–17 may behave like gap genes that mediate bed function in the embryonic head.

Abstract: Understanding the head is one of the great challenges in the fields of comparative anatomy, developmental biology, and palaeontology of arthropods. Numerous conflicting views and interpretations are based on an enormous variety of descriptive and experimental approaches. The interpretation of the head influences views on phylogenetic relationships within the Arthropoda as well as outgroup relationships. Here, we review current hypotheses about head segmentation and the nature of head structures from various perspectives, which we try to combine to gain a deeper understanding of the arthropod head. Though discussion about arthropod heads shows some progress, unquestioned concepts (e.g., a presegmental acron) are still a source of bias. Several interpretations are no longer tenable based on recent results from comparative molecular developmental studies, improved morphological investigations, and new fossils. Current data indicate that the anterior arthropod head comprises three elements: the protocerebral/ocular region, the deutocerebral/antennal/cheliceral segment, and the tritocerebral/pedipalpal/second antennal/intercalary segment. The labrum and the mouth are part of the protocerebral/ocular region. Whether the labrum derives from a former pair of limbs remains an open question, but a majority of data support its broad homology across the Euarthropoda. From the alignment of head segments between onychophorans and euarthropods, we develop the concept of "primary" and "secondary antennae" in Recent and fossil arthropods, posit that "primary antennae" are retained in some fossil euarthropods below the crown group level, and propose that Trilobita are stem lineage representatives of the Mandibulata.