penumbra

(redirected from penumbras)
Also found in: Dictionary, Thesaurus, Legal, Encyclopedia.

penumbra

 [pĕ-num´brah]
1. the part of a shadow in which there is a small amount of illumination from a light source.
2. blurring at the edges of a structure on a radiograph.

pe·num·bra

(pe-nŭm'bră),
The region of partial illumination or radiation caused by light or x-rays not originating from a point source; also called geometric unsharpness.
[Mod. L., fr. L. paene, almost, + umbra, shadow]

pe·num·bra

(pĕ-nŭm'bră)
1. radiology The blurred margin of an image.
2. radiation physics The region at the edges of a radiation beam over which a rapid change in dosage rate occurs.
Synonym(s): geometric unsharpness.
[Mod. L., fr. L. paene, almost, + umbra, shadow]

penumbra 

1. Region of very low illumination on a dark background.
2. Zone in which the brightness varies from some illumination to zero (umbra) in the shadow cast by an opaque object intercepting light from an extensive light source. See shadow.

pe·num·bra

(pĕ-nŭm'bră)
1. radiology the blurred margin of an image.
2. radiation physics the region at the edges of a radiation beam over which a rapid change in dosage rate occurs.
Synonym(s): geometric unsharpness.
[Mod. L., fr. L. paene, almost, + umbra, shadow]

penumbra

a blurred edge to an image, a halo effect, in an x-ray film caused usually by an overlarge focal spot exacerbated by a long object-to-film distance.
References in periodicals archive ?
It might be used to detect ischemic penumbra in patients with AIS, identify the type of a cerebral tumor, classify the stage of a malignant tumor, or evaluate the effects of chemotherapy or radiotherapy in patients with a cerebral tumor.
Based on the above literature, we hypothesized that fast ADC would decrease and slow ADC would remain the same in the ischemic penumbra as compared with contralateral normal cerebral tissue.
Image data were included in this study if: (1) The patient presented with symptoms of AIS; (2) Cerebral hemorrhage was excluded by CT examination; (3) Baseline MRI was performed within 6 h of symptom onset in patients with contraindication for intravenous thrombolysis, or > 6 h but within 24 h; (4) Both ASLs and multi b -value DWIs were performed; (5) Obvious ischemic penumbra was observed; the area of decreased ASL CBF was 1.
Regions of interest (ROIs) were delineated as large as possible in the area of ischemic penumbra and the contralateral normal brain tissue of the same size.
sup][20],[22],[23] Recent studies have demonstrated that ASL can be used to quantify CBF values in the ischemic core and ischemic penumbra in patients with AIS.
En particular, las manchas que poseen mas de un 50% de probabilidad de estar asociadas a destellos [mayor que o igual a] 1 en 24 horas son aquellas que: pertenecen a los grupos de Zurich E y F, o las que poseen una mancha principal con penumbra grande y asimetrica, o su poblacion es compacta, o tienen mas de 50 poros, o mas de 700 millonesimas de area, o su clasificacion magnetica es del tipo delta ([delta]).
Among the results are the fact that 87% of flares occur on spot groups with asymmetric main penumbra, and also the more explosive character of the bipolar groups with intermediate of compact populations, specially when a great magnetic unbalance is implied because the leaders of followers ate significantly bigger.
Frecuencias esperadas versus observadas segun tipo de penumbra de la mancha mayor.