Physical Agents: Ultrasound, Laser, and Diathermy

Vocabulary practice flashcards covering physical agents, therapeutic ultrasound, photobiomodulation (laser), and shortwave diathermy concepts, dosages, and application principles.

Transducer (Sound Head)

A device containing a crystal that converts electrical energy into acoustic energy via expansion and contraction during alternating current flow.

Attenuation (Ultrasound)

The progressive decrease in ultrasound intensity as the sound wave travels through biological tissues, primarily caused by absorption, reflection, and refraction.

Effective Radiating Area (ERA)

The specific area of the ultrasound transducer face from which the acoustic energy radiates; it is typically slightly smaller than the total surface area of the transducer face.

Beam Non-Uniformity Ratio (BNR)

The ratio that compares the spatial peak intensity of the ultrasound beam to the spatial average intensity across the transducer face, with safe clinical ratios falling between $< 6:1$ and $8:1$.

Duty Cycle

The proportion of total treatment time that ultrasound is actively being delivered, calculated as $\frac{\text{On time}}{\text{Total time}}$ and expressed as a percentage or ratio (such as $20\%$ or $1:5$).

Ultrasound Frequency ($1\,\text{MHz}$ vs. $3\,\text{MHz}$)

The number of compression-rarefaction cycles per second; $1\,\text{MHz}$ targets deep tissue up to $5\,\text{cm}$ ($2\,\text{in}$), whereas $3\,\text{MHz}$ targets superficial tissue ($< 2.5\,\text{cm}$ or $1\,\text{in}$) and heats approximately $3\times$ faster.

Vigorous Heating (Ultrasound)

A tissue temperature elevation of $\ge 4\,^\circ\text{C}$ used clinically to stretch collagen and alter viscoelastic properties of restricted structures.

Stretching Window

The time period immediately following vigorous heating during which target tissue achieves maximum elongation ($3.3\,\text{minutes}$ post-ultrasound for muscle; $5\,\text{minutes}$ for tendon and ligament).

Phonophoresis

The therapeutic application of ultrasound to enhance the transdermal delivery and local uptake of topically applied medications, such as dexamethasone.

Exogen® Home US Unit

A home-use device delivering preset, low-intensity pulsed ultrasound ($1.5\,\text{MHz}$, $20\%$ duty cycle, $0.15\,\text{W/cm}^2$ for $20\text{--}30\,\text{min}$ daily) to promote healing of non-union bone fractures.

Photobiomodulation (PBM)

The therapeutic application of electromagnetic radiation with wavelengths in or near the visible and infrared light spectrum to modulate cellular function and stimulate tissue repair.

LASER

Light Amplification by Stimulated Emission of Radiation; light that is monochromatic (single wavelength), coherent (in same phase), and directional (minimal divergence).

Class 3B Laser

A laser classification with power output between $5\,\text{mW}$ and $500\,\text{mW}$; used clinically in rehabilitation, it poses an eye hazard from brief exposure and requires protective eyewear.

Energy Density (Photobiomodulation)

The clinical dosage parameter for phototherapy measured in Joules per square centimeter ($\text{J/cm}^2$), calculated by multiplying power by time divided by the beam surface area.

Arndt-Schultz Principle

A biological principle stating that an optimal dose of energy produces a maximal therapeutic effect, whereas too small a dose yields no effect and excessive energy causes inhibitory or negative effects.

Cluster Probe

A light therapy applicator that contains multiple diodes (such as laser diodes, LEDs, or SLDs) designed to deliver treatment over a larger area.

Diathermy

The clinical application of high-frequency electromagnetic energy (shortwave or microwave) that produces heat $3\text{--}5\,\text{cm}$ deep via molecular agitation, predominantly in tissues with high water content.

Capacitive Plates (Diathermy)

Shortwave diathermy applicators containing metal encased in plastic that produce an electric field between two opposing plates, concentrating heat in superficial, low-conductivity tissues such as fat.

Inductive Coil (Diathermy)

A shortwave diathermy applicator that passes alternating current through a coil, producing a perpendicular magnetic field that induces circular eddy currents to heat both deep and superficial vascular tissues.

Spatial Average Intensity

The average intensity of an ultrasound beam over the face of the transducer during the beam-on period, expressed in units of $\text{W/cm}^2$.