Имеется в виду, что объемное образование (опухоль, киста и т.п.) оказывает на окружающие ткани определенное воздействие, результаты которого (смещение, сдавление и т.п.) можно наблюдать

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Note added at 6 hrs (2022-01-04 19:58:31 GMT)
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Чтобы было понятнее, о чем речь:
Analysis of Mass Effect
CHAPTER 4
Analysis of Mass Effect

Ellen E. Parker

Lesions or processes that cause compression, distortion, and/or displacement of intracranial contents may be said to have “mass effect.” One important concept to understand is that mass effect is a manifestation on imaging of various intracranial processes (including tumor, hemorrhage, ischemia, and trauma) and not a diagnosis in itself. An analogy to clinical medicine is that vertigo is not a diagnosis in itself but rather a symptom with multiple possible underlying causes (such as posterior fossa infarct or vestibular abnormality). Detection and characterization of mass effect is a fundamental skill of neuroradiology. Accurate characterization of mass effect helps to precisely define the location of a lesion, which is crucial to forming an accurate differential diagnosis. In addition to diagnostic value, detection and prompt reporting of mass effect is also of great importance to the care of patients, especially those with life-threatening herniation. The goal of this chapter is to introduce key concepts and methods of analysis of mass effect to the beginning radiologist.
https://radiologykey.com/analysis-of-mass-effect/