Preamble (disclaimer)

Transcription

1 Preamble (disclaimer) PHYSICS AND PRINCIPLES OF HEAD/NECK ULTRASOUND Joseph C. Sniezek, MD FACS LTC, MC, USA Otolaryngology/H&N Surgery Tripler Army Medical Center 1. I am not a physicist 2. ACS has recommended slides 3. Practical focus Ultrasound basics: Waves Important principle: frequency Transducer sends out waves of sound energy They bounce off of tissue and return to transducer image is created Measured in hertz (hz) 1 cycle/second = 1Hz 1,000 c/s = 1kHz 1,000,000 c/s = 1MHz FREQUENCYvs.WAVELENGTH (distance per cycle)

2 THE SOUND SPECTRUM Hz khz Infrasound Audible Sound Ultrasound SOUND VELOCITY Material Air Water Fat Soft Tissue Muscle Bone Velocity (m/s) Principle of Acoustic impedance Property of all substances Equal to product of tissue density and speed of sound Substances with a greater acoustic impedance produce stronger echoes, or reflected waves REFLECTION - due to impedance mismatch - tissue interface is a reflector Soft Tissue Fat Soft Tissue Bone

3 TRANSDUCER Sends out waves and receives echoes Anatomy of a transducer How does the transducer work? PIEZOELECTRIC EFFECT -crystal changes shape and vibrates when voltage is applied to it (lead zirconate titanate, PZT) - Conversion of electrical energy to accoustical energy

4 Sector Mechanical Transducer types Electronic (curved or linear array) Sector Transducer Ultrasound waves move through fanshaped sector Good for cardiac (see through ribs) Mechanical Transducer Single element rotates in circle Good for translumenal studies

5 ARRAY TRANSDUCER Linear array transducer Crystals arranged spatially or functionally Can be linear or curved linear array (convex) Results in rectangular image good for shallow structures Head and neck Curved linear array (convex) Image similar to sector transducers but wider in nearfield Advantagecompromise between sector & linear

6 Transducer frequencies Megahertz Use abd, trauma, cardiac, ob, cardiac rectal, transvag, prostate, pediatric thyroid/parathyroid, vascular, breast RESOLUTION TRANSDUCER FOCUSING (focus is best where beam is narrowest) Clarity/quality of image Ability of equipment to detect 2 separate reflectors in tissue and to display them as 2 separate reflectors on the monitor without merging them

7 LATERAL RESOLUTION- deals with reflectors that lie perpendicular to the axis of US beam. LR is inversely related to beam width. Focal length (narrowest point) gives best LR. AXIAL RESOLUTION- reflectors lie along axis of reflection. Improved by high frequency. CHARACTERISTICS of SOUND FREQUENCY Sound Sound Axial Frequency Penetration Resolution High Low appearance of tissue on US image relative to their ability to reflect US wave brightness Anechoic isoechoic hypoechoic hyperechoic Echogenicity Anechoic

8 ISOECHOIC HYPOECHOIC HYPERECHOIC Attenuation- as sound waves travel through a medium, they lose energy (intensity and amplitude decrease) TIME - GAIN COMPENSATION -signals from deeper reflectors will be weaker than shallow reflectors due to attenuation. Transducer Transducer Distance Distance

9 TIME - GAIN COMPENSATION Increasing TGC amplifies signals more from deeper structures (ie, those that are delayed, TIME GAIN) than from shallow structures *essentially a way to compensate for depth, thus focusing on deeper structures IMAGE ARTIFACTS Echo signals whose displayed position on image does not correspond to actual position in the body Can lead to diagnostic errors or misinterpretation Examples: Enhancement Shadowing Reverberation Refraction/edge artifact Enhancement artifact Shadowing artifact Requires region of low attenuation Tissue distal to cyst appears enhanced Relative higher amplitude of distal reflected echos compared with adjacent tissue Requires region (mass) of high attenuation Reduces amplitude of transmitted signal and echo

10 Reverberation artifact Refraction Artifact Due to interfaces with accoustic impedance mismatches Large reflected echo is reflected back by transducer, re-reflected, etc Progressively fainter rings at equidistant intervals Incident Beam Medium 1 Medium 2 Reflected Beam Transmitted Beam Snell s Law- when a wave hits an interface at other than 90 deg, it s direction is altered (eg- FNA needle), *edge artifact Ultrasound-guided FNA