In ultrasonography, a signal generator is combined with a transducer. Piezoelectric crystals in the signal generator convert electricity into high-frequency sound waves, which are sent into tissues. The tissues scatter, reflect, and absorb the sound waves to various degrees. The sound waves that are reflected back (echoes) are converted into electric signals. A computer analyzes the signals and displays the information on a screen.

Frequency used ranges from 1 to 10 MHz
Ultrasonography is portable, widely available, and safe. No radiation is used.
Different modes:
  1. A-mode

  2. B-mode (gray-scale): 

    This mode is most often used in diagnostic imaging; signals are displayed as a 2-dimensional anatomic image. B-mode is commonly used to evaluate the developing fetus and to evaluate organs, including the liver, spleen, kidneys, thyroid gland, testes, breasts, and prostate gland. B-mode ultrasonography is fast enough to show real-time motion, such as the motion of the beating heart or pulsating blood vessels.

  3. M-mode

  4. Doppler: 

    This type of ultrasonography is used to assess blood flow. Doppler ultrasonography uses the Doppler effect (alteration of sound frequency by reflection off a moving object). The moving objects are RBCs in blood.

ECHOGENECITY

Echogenicity of the tissue refers to the ability to reflect or transmit US waves in the context of surrounding tissues.Whenever there is an interface of structures with different echogenicities, a visible difference in contrast will be apparent on the screen. Based on echogenicity, a structure can be characterized as hyperechoic (white on the screen), hypoechoic (gray on the screen) and anechoic (black on the screen).

  • Fluids are anechoic. (Reflects none of the sound back)
  • Solids may be hypo or hyper-echoic (Reflects some to many of the sound back)
  • Air/Gas is hyper-echoic/pan-echoic (Reflects all of the sound back)

Normal echogenecities of body tissues:

  • Bone: Anechoic (Black) with bright hyperchoic rim. Because the US beam cannot penetrate bone, it casts an acoustic shadow beyond it. 
  • Cartilage appears hypoechoic, and is more penetrable by US than bone.
  • Blood vessels also appear black or anechoic due to presence of fluid (blood) within them. (Veins are usually easily collapsible upon external pressure by the transducer, while arteries are pulsatile and do not collapse with moderate pressure.)
  • Lymph nodes appear anechoic or hypoechoic.
  • Nerves appear variably, depending on the proximity to the neuraxium. Proximal nerves are hypo-anechoic, and distal nerves are hyperechoic, with a stippled (“honeycomb”) structure (with hypo-anechoic fascicles on the hyperechoic background of connective tissue surrounding them).
  • Fat (Adipose tissue): Almost anechoic.
  • Fascia and other connective tissue strands and fascicles appear as hyperechoic lines.
  • Tendons have characteristic striation in the long-axis view, and are more anisotropic than nerves.
  • Lung has a very distinct appearance, with a “shimmering”, hyperechoic pleura sliding in rhythm with each breath, as well as comet tail artifacts. –> They are only seen in Live USG. (Loss of these appearance occur in Pneumothorax)

Abnormal echogenecities of body lesions:

  1. Renal stones: Hyperechoic lesion with posterior acoustic shadow
  2. Gall stones: Hyperechoic lesion with posterior acoustic shadow. (When ultrasound hits a dense object such as a gallstone and is completely reflected, a posterior acoustical shadow is formed. The gallstone is bright and echogenic. Because no ultrasound energy is left to go beyond the stone, an echo void is created, which appears as a wedge-shaped dark area posterior to the dense object.)
  3. Cysts: Smooth, hypoechoic lesion
  4. Mass/tumors: May have any echotexture (Hypo-hyperechoic)
  5. Calcifications: Hyperechoic lesions
  6. Acute inflammations (Eg: Appendicitis): Nonperforated, inflamed appendix that is characterized by an aperistaltic, noncompressible, blind-ended, tubular structure with a laminated wall that arises from the base of the cecum. Presence of periappendicular fluid/ Echogenic prominent pericecal fat/ Appendicolith

References:
  • Principles of ultrasonography. – S. Hassani (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2609158)
  • Ultrasound: Basic understanding and learning the language – Barys Ihnatsenka and André Pierre Boezaart (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3063344)
  • http://www.merckmanuals.com/professional/special_subjects/principles_of_radiologic_imaging/ultrasonography.html

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