A helmet adapted to be worn by a patient laying on an operating table, comprising: a lower half-shell and an upper half-shell; and the lower half-shell being anatomically shaped to receive and support the nape and a rear part of the neck or torso leaving free a front part of the neck or torso and being anatomically shaped so as the head of the laying patient is accommodated on the lower half-shell, said upper half-shell being anatomically shaped to cover said front part of the neck or torso of the laying patient leaving free said rear part of the neck or torso, wherein said upper half-shell and said lower half-shell having fastening means for fastening one to the other, said upper half-shell and said lower half-shell being configured to be fitted one to the other and to the neck or torso of the patient to form a substantially airtight enclosure for enclosing the head of the patient; and wherein at least one inlet port for supplying gas and at least one outlet port for evacuating gas are defined on said lower half-shell and/or said upper half-shell; wherein the helmet is configured to assume either a closed position, when edges of said upper half-shell are brought to match onto corresponding edges of said lower half-shell, or an opened position when said edges of said upper half-shell are detached from said corresponding edges of said lower half-shell; and wherein said upper half-shell and said lower half-shell are shaped to enclose together the head of the patient in said enclosure surrounding the neck or torso of the laying patient when in the closed position, and are configured to free the neck or torso of the laying patient in the opened position allowing the laying patient to exit from the lower half-shell and to remove the helmet by raising the head. a lower half-shell and an upper half-shell; and the lower half-shell being anatomically shaped to receive and support the nape and a rear part of the neck or torso leaving free a front part of the neck or torso and being anatomically shaped so as the head of the laying patient is accommodated on the lower half-shell, said upper half-shell being anatomically shaped to cover said front part of the neck or torso of the laying patient leaving free said rear part of the neck or torso, wherein said upper half-shell and said lower half-shell having fastening means for fastening one to the other, said upper half-shell and said lower half-shell being configured to be fitted one to the other and to the neck or torso of the patient to form a substantially airtight enclosure for enclosing the head of the patient; and wherein at least one inlet port for supplying gas and at least one outlet port for evacuating gas are defined on said lower half-shell and/or said upper half-shell; wherein the helmet is configured to assume either a closed position, when edges of said upper half-shell are brought to match onto corresponding edges of said lower half-shell, or an opened position when said edges of said upper half-shell are detached from said corresponding edges of said lower half-shell; and wherein said upper half-shell and said lower half-shell are shaped to enclose together the head of the patient in said enclosure surrounding the neck or torso of the laying patient when in the closed position, and are configured to free the neck or torso of the laying patient in the opened position allowing the laying patient to exit from the lower half-shell and to remove the helmet by raising the head.