Multimodal Diffuse Optical Imaging - Biomedical Optical Imaging Technologies: Design and Applications - page 356

to avoid systematic bias. They demonstrated superior results in terms of minimized
interparameter cross talk based on their approach compared to techniques not taking
into account spectra uncertainties.

10.3.5 Safety/Regulatory Considerations

The diffuse optical imaging field is maturing, and various optical imaging systems
are commercially available to the clinicians. Optical imaging instruments provide
diagnostic information impacting patient health and are therefore subject to govern-
mental regulatory oversight. Thus, it is critical to identify the regulatory issues that
impact the design of optical clinical products. As stated by the first requirement of
the “essential Principles of Safety and Performances of Medical devices” [ 47 ]:

Medical devices should be designed and manufactured in such a way that, when used
under the conditions and for the purposes intended and, where applicable, by virtue of
the technical knowledge, experience, education or training of intended users, they will not
compromise the clinical condition or the safety of patients, or the safety and health of users
or, where applicable, other persons, provided that any risks which may be associated with
their use constitute acceptable risks when weighted against the benefits to the patient and
are compatible with a high level of protection of health and safety.

Therefore, patient protection concerns are essential in the design of clinical optical
imager.
The different safety hazards of interest during the design of the optical imaging
platform include tissue trauma, electrical shock, material toxicity, disease transmis-
sion, electromagnetic compatibility, and software malfunction. These considerations
are common to a large variety of medical devices. In addition, optical imaging
devices need to consider the potential hazard originating from the interaction of light
with biological tissue. The recommended exposure limits for protecting patients and
device operators from these safety risks are listed in international standards.
Optical hazards can be divided in three main categories: photochemical, pho-
tothermal, and photomechanical [ 48 ]. Diffuse optical imaging techniques are
operating in the NIR window, and thus, the primary optical hazard of concern
is the photothermal damage. The absorption of light by the tissue leads to local
increase of temperature to levels that may injure the tissue. Therefore, diffuse optical
instruments should be designed to shine light below the maximum permissible
exposure (MPE) limit for the skin. The MPE is the highest power or energy density
(in W=cm 2 or J=cm 2 ) of a light source that is considered safe, i.e., that has a
negligible probability for creating damage. It is usually about 10 % of the dose that
has a 50 % chance of creating damage [ 49 ] under worst-case conditions. The MPE
is organ and wavelength dependent. However, thanks to the increase in detector
sensitivity, numerous optical imaging systems are based on class I or class II light
sources that are safe for the patient, even in the case of starved photon techniques
such as time domain approaches.