Ultrasounds Physics and Artifacts Basics
Alain Giroux, DVM, MSc, DACVR
a) Sound wave: propagating variation of acoustic variables in a medium.
-Acoustic Variables: pressure; density; temperature; particle motion
b) Sound properties:
-Source (transducer) determined:
–Amplitude (US console) and intensity (US console)
-Source and medium determined:
-Unit: Hertz (Hz): one cycle per second
-Megahertz (MHz): 1 million Hertz
-Infrasound: less than 20 Hz
-Ultrasound: above 20000 Hz
-Veterinary medical ultrasound: 3 to 12 MHz
-Length in between wave cycles (mm)
e) Propagation speed:
-Speed of sound wave: meter per second (m/s)
-Different in Air, liquid, solid.
-Speed in soft tissue: 1540 m/s
-Why important? Because US equipment assumes this speed in calculating depth location of structures.
-Propagation Speed X Density = Impedance
-High reflection occurs when large difference in tissue impedance and propagation speed.
2-Diagnostics Ultrasound Characteristics
a)Pulse-echo: Ultrasound is sent as pulses.
Each pulse contains small number of waves
As illustrated on figure.
distance and location
b) Attenuation (dB):
–Decreased in intensity as it travels in soft tissues
–Absorption (conversion into heat)
–Scattering: Image generation (specular scattering)
-Converts bi-directionnaly electric energy into ultrasound energy.
-Piezoelectricity: PZT materials (ceramics, quartz) are deformed by applied voltage and causes changes of pressure that propagates as a sound pulses.
-Transducers construction: Single element or linear-array made of piezocomposites.
b) Type of transducers:
–MechanicalRotating or oscillating single or array of elementsin acoustic coupling liquid.
–Sequenced and phased array
a) Gain: sometimes identified as decibel dB.
b) Time gain compensation (TGC)-also called: Near field gain, far field gain; multiple field gain; STC: surface time compensation
-Depth/range; zoom; magnification
-Brightness (B); Motion (M); Doppler (D); various combination of modes
-Mark, Trace; enter; Set
f) Probe; transducers
g) Focus: in phased array-Focal Zones; manual focus
a) Acoustic shadowing:
-Reduction of the echo amplitude from reflectors that is located beyond a strongly reflecting or attenuating structure.
b) Acoustic enhancement:
-Increased brightness (echo amplitude) of structures (reflectors) located beyond a low-attenuating structure.
c) Refraction (edge shadowing):
-Change in direction of sound when it encounters a round surface. Echoes from the structure beyond the surface are decreased in amplitude.
d) Mirror image:
-Structures of one side of strong reflector presented on other side of the reflectors. Decreased brightness and synchronized movement of structures.
e) Improper gain:
-Part of a structure and of the image is falsely decreased in amplitude due to erroneous TGC setting.Video segment
f) Section thickness:
-Portion of a round shape structure wall or outer lining is projected falsely inside its lumen.