Abstract: A sensing device includes a sensor housing mounting sensor elements and an attachment member, the attachment member including first and second components. The first component is configured to receive the sensor housing therein and to permit rotational movement between the attachment member and the sensor housing and to restrict relative movement between the attachment member and the sensor housing in the axial direction of the sensor housing. The second component includes a flexible membrane that is attachable to an underside of the first component attached to an underside of the first component, the flexible membrane extending at least in part beyond an outer edge of the first component. The flexible membrane allows for attachment of the attachment member to a patient, and the attachment member is adapted to allow passage of non-ionising radiation from the sensor housing to the patient.