Transducers

The transducer is the link between the anesthetist and patient.  All transducers require several key components to generate an image.  Elements located in the transducer use the piezoelectric effect to create sound.  The piezoelectric principle enables the elements to convert the electrical voltage sent from the ultrasound system into mechanical energy so it can be safely introduced into the body.  They also receive echoes returning to the transducer and convert them into electrical energy that is processed by a microcomputer to create an image.  Linear transducers, like those commonly used for regional anesthesia and central venous access, contain as many as 120 individual ceramic elements aligned side-by-side along the face of the transducer.  The thickness of the piezoelectric crystals determines the inherent frequency of the transducer.  This determines the depth of the near (Fresnel), focus and far (Fraunhofer) zones.

Transducer basics diagram II

Picture resolution describes the smallest possible distance between two points that allows them to be discriminated.  Resolution is best in the focal zone (A), where the ultrasound beams are the narrowest and most concentrated.  It separates the near and far zones.  Lateral resolution(C), the ability of the ultrasound system to display two objects side-by-side as separate structures, is best in the focal zone.  Lateral resolution depends on the distance between the individual crystals rather than the distance between the objects being viewed.  Resolution diminishes in the far zone as the beam begins to diverge and is attenuated by tissue.  Axial resolution (B) relates to the ultrasound systems ability to differentiate objects in-line with the axis of the sound wave.   It is dependent on the length of the sound impulse and the ultrasound frequency.  A backing layer located behind the crystals dampens the generated pulse reducing the pulse duration and amount of scatter.  This improves axial resolution.  Higher frequency transducers have better axial resolution than lower frequency transducers.

Sound is used to create images of structures within the body using a pulse-echo technique.  The transducer emits brief impulses at a fixed rate and then “listens” in-between the pulses for returning echoes.  Sound waves are transmitted by numerous crystals (elements) aligned in succession along the face of the transducer, creating the rectangular image seen on the screen.  The echoes created by the different acoustic impedances between the tissues are sent back to the transducer and converted into electrical energy that is analyzed to create an image.  Based on the time and strength of these echoes, a microprocessor within the ultrasound system assigns each one a position and color in a grey scale, ultimately forming the image known as B-Mode (brightness Mode). Modern ultrasound systems are commonly referred to as “real-time” imaging devices because they have the ability to rapidly display B-Mode images so that any motion that occurs is visualized as it happens.  Modern systems have the ability to show between 15 and 50 images per second.  In order to produce the effect of continuous movement, at least 16 frames/sec need to be displayed.


References

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Sites B D, Brull R, Chan V W, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med. 2007;32(5):412-418.

Falyar CR. Ultrasound in anesthesia: applying scientific principles to clinical practice. AANA J. 2010 Aug; 78(4):332-40.

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