Coma Image

The term coma is derived from the comet-like appearance of the image of a point object that is located off the axis. In spherical aberration, various zones of a lens produce a longitudinal difference of focus for rays parallel to the 

Larger zones producing larger comatic circles. 

Bands of light move to different focal points, depending on their orientation, (a) Tangential focus, (b) Radial focus. 

---

Coma" Image size (magnification) varies with radial position in the focal region. Off-axis flaring. 

So far, only defocus and spherical aberration have been considered as aberrations affecting the image contrast. Both depend only on the magnitude of the spatial frequency g[ but not on the diffraction direction, thus resulting in rotationally symmetric phase shifts (Equation 11). However, the objective lens may exhibit further aberrations resulting in additional phase shifts, which are not necessarily rotational symmetric. The most important of these additional aberrations are astigmatism and coma. 

Coma is due to the electron beam being tilted away from the optical axis of the objective lens, the coma-free axis. The coma results in lattice fringes related to +g and -g being shifted by the objective lens. Lattice fringes belonging to different beam pairs, +g and -g, are shifted differently resulting in an asymmetry of the HRTEM image. 

Figure 10. Reconstructed exit wave function (phase) of the same area as shown in Figure 5. The left image shows the phase of the exit wave function before aberration correction and the right image shows the phase after correction for residual 2-, 3-fold astigmatism, and coma. The difference clearly illustrates that numerical correction of residual aberrations is crucial for...

Fig. 16.7. A 15-day-old child with cardiac arrest and coma. The DWI images show bilateral hyperintensities in the cortex and white matter (top row), associated with decreased ADC values (bottom...

Christophe C, Fonteyne C, Ziereisen F, Christiaens F, Deltenre P, de Maertelaer V et al (2002) Value of MR imaging of the brain in children with hypoxic coma. Am J Neuroradiol 23 716-723... 

Phencyclidine use results in excitation with marked paranoid or aggressive behavior, which is often selfdestructive. Effects include distortion of body image, diminished pain perception, illusions, and delusions, including a perception of superhuman strength and invulnerability. Miosis and nystagmus (horizontal, vertical, and rotary) may be seen in association with ataxia, bizarre behavior, and hallucinations. Tachycardia, hypertension, hyperreflexia, seizures, respiratory depression, and coma are reported with high doses. Dystonias and dyskinesias have been reported. 

On en conclut comae dans 5.3 que 1 image de 0 est un ouvert U de Aut, (H), et que muni de la structure induite, U represents le faisceau quotient Norm,(h)/Centr,(H). Ce dernier est done 6tale et sdpard sur S puisque... 

As mentioned earlier, the COMA positive resists tend to have higher optical absorption at 193 nm than polymethacrylate and COBRA systems, which would produce a tapered image profile. To overcome this potential problem, the T-top formation by absorption of base into the top layer (see above) has been intentionally incorporated in the lithographic process (amine gradient process) [281]. Poly(acrylic acid-co-methyl acrylate) and L-proline were dissolved in water and spin-cast on a COMA resist. During PEB the amine in the overcoat diffuses into the COMA resist layer and compensates for the acid gradient caused by illumination, providing a vertical profile. 

Where coma of cerebrovascular origin is suspected a C.SF examination (Fig. 3) will show the pre.sence of blood and will help to differentiaie a subarachnoid haemorrhage from cerebral infarction. When available, imaging with a CT scan is prefeniblc. 

---