penumbra

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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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

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]
Farlex Partner Medical Dictionary © Farlex 2012

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]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

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.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann

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]
Medical Dictionary for the Dental Professions © Farlex 2012
References in periodicals archive ?
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 1.5 times larger than the area of the restricted diffusion.
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.
ASL is a noncontrast perfusion imaging method that relies on the magnetic labeling of arterial water.[sup][19],[20] Radiofrequency pulses are used to magnetically alter the status of water protons in arterial blood with respect to those in stationary tissue, thereby generating an endogenous intravascular tracer.[sup][20],[21] Presuming that the magnetically labeled arterial blood exchanges with tissue water at the capillary level, CBF can, in principle, be quantified from a labeling experiment and a control experiment without magnetically labeled water protons, based on the theory of diffusible tracer kinetics.[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.[sup][5],[6],[7],[8]
The ischemic penumbra is functionally impaired, yet still viable, tissue surrounding the ischemic core.[sup][24],[25] Several studies have indicated that for the purpose of defining the tissue at risk of infarction, the ischemic penumbra can be operationally defined as the mismatch between the lesion volumes detected with PWI and that detected with DWI.[sup][3],[26] With PWI, perfusion decreases in the ischemic penumbra but is normal in DWI.
A linear relationship between IVIM perfusion parameters and classic perfusion parameters was derived under given assumptions by Le Bihan et al .[sup][28] In brief, a fast ADC is inversely proportional to mean transit time (MTT), which is consistent with the result that fast ADCs decreased in ischemic penumbra in this study.
Second, no CBF threshold other than visual assessment was used to evaluate the penumbra in ASL perfusion.
Evolving paradigms in neuroimaging of the ischemic penumbra. Stroke 2004;35 11 Suppl 1:2662-5.