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Non-ionising radiation quantities and principles of measurement


Non-ionizing radiation includes visible and non-visible (ultraviolet and infrared) light, lasers, ultrasound, and radiofrequency radiation.  Each of these has a unique set of requirements for accurate measurement and each requires specially designed measurement tools.  Some of the terminology and techniques can be confusing, e.g., luminance versus illuminance with each of these requiring a specific dosimeter for measurement purposes.

Important Principles

Luminance and illuminance can be characterized by wavelength and colour temperature.  The units of these measurements may be photometric (weighted by the response of the human eye) or radiometric.  As noted above, two different types of detectors are needed to properly measure luminance (the amount of light emitted from a surface) and illuminance (the amount of light falling on a surface).  In addition to the spectrum visible to the human eye, the term light is often associated with ultraviolet and infrared radiation which are not visible to the eye.  The measurement tools for UV and IR are different from those used for visible light.  Measurement of visible light is important for the characterization of displays and viewboxes for viewing images, and for characterizing the viewing conditions.

Lasers serve many purposes in medicine from alignment devices to surgical tools.  The principles of lasers and laser safety (eye protection and procedures for safe use) are an essential part of medical physics education and training.  The measurement of laser light has different requirements to that of visible light.  It is important to understand laser power, power density, energy, and energy density.  In addition, it is necessary to understand the laser classification system and appropriate warning signs.

Ultrasound is created by pressure waves that makes its measurement different to that of light and lasers.  The generation of ultrasound, transducers, and beam patterns must be understood.  In addition, one must have an appreciation for continuous versus pulsed ultrasound, and an understanding of the principles of Doppler ultrasound and imaging.  Other areas of importance include ultrasound biological effects, relevant quantities for ultrasound measurement including spatial and temporal peak and average measurements, thermal index, and mechanical index in order to properly characterize the ultrasound beam.

Magnetic resonance imaging (MRI) uses radiofrequency (RF) radiation which is, again, different from light, lasers or ultrasound.  The biological effects are different as well as the quantities used for measurement, i.e., power, and specific absorption rate (SAR).  It is essential to understand the sources and generation of RF radiation in medicine in addition to those in MRI.  The tools necessary to measure RF radiation and static magnetic fields are significantly different from those mentioned above and must be understood by the medical physicist.

Introduction to References

The Essential References provide a relatively broad coverage of non-ionizing radiation.  The Idaho State University website provides many links to outstanding websites on both ionizing and non-ionizing radiation.  The University of South Florida site provides a Basic Laser Safety Training PowerPoint presentation which will be useful to the medical physicist learning about laser safety.  However, the presentation will probably have to be edited in order to use it as a teaching tool for healthcare staff.

The NCRP series on ultrasound including NCRP Report #74 (266 pages), #113 (278 pages) and #140 (574 pages) should be considered the basic texts and references on ultrasound bioeffects.