Ultrasound

Ultrasound is a radiological technique that involves sending soundwaves with very high frequencies (∼ 2–20 MHz for diagnostic imaging) through the body and receiving their echoes to visualize internal structures and organs. The soundwaves are produced by piezoelectric transducers encased in a probe that also detects the echoes. The denser a material is, the stronger the echo and, accordingly, the intensity (brightness) of the structure visualized will be; the further the material is away from the probe, the longer the echo will take to return to the probe, plotting accordingly on the screen. Ultrasound can also be used endoscopically (e.g., transrectally, transvaginally, transesophageally, or transgastrically), to better evaluate internal organs and otherwise difficult to evaluate structures such as the prostate, ovaries, heart valves, and pancreas. The use of ultrasound contrast agents, which have very few side effects compared to radiocontrast agents, further expands the diagnostic applications of the modality. Ultrasound machines offer a variety of modes for different diagnostic purposes. Amplitude mode (A mode) is the most basic mode and employs a single transducer to plot echoing structures on a single axis as a series of peaks whose height reflects the depth of those structures; brightness-modulated mode (B mode or 2D mode) employs an array of transducers, producing a two-dimensional, grayscale image of the echoing structures that reflects density as well as the depth; motion mode (M mode) employs a sequence of pulses in rapid succession to generate a series of A or B mode images visualizing the motion of the echoing structures (e.g., the heart); and doppler or duplex mode takes advantage of the doppler effect, produced by shifting the frequencies of the ultrasound waves used, to visualize motion within structures, e.g., the velocity and direction of blood flow. Due to the safety of ultrasound waves and the portability ultrasound machines, low cost, and high availability of ultrasound machines, the modality is often used as a complementary tool in clinical diagnosis, especially in children and pregnant women. Another advantage is that ultrasound offers quick diagnosis for certain diseases (e.g., cholecystitis and acute right ventricular load). The main disadvantages of ultrasound are the training and skill required of the examiner, low resolution, and the common appearance of artifacts.

Generation of ultrasound waves

• Ultrasound waves are high-frequency sound waves above the limit of human hearing (∼ 20 kHz); medical ultrasound employs frequencies ∼ 2–20 MHz)
• In medical devices, ultrasound waves are generated by piezoelectric transducers.
  • Piezoelectric effect: the accumulation of an electric charge in certain solid materials (e.g., crystals) in response to deformation by mechanical stress; inversely, the application of an electric charge to such materials will induce their deformation.
  • Medical ultrasound uses the inverse piezoelectric effect on lead zirconate titanate or polyvinylidene fluoride crystals to generate and record ultrasound waves.

Recording of ultrasound waves

• The rate at which tissue absorbs ultrasound waves is proportional to the frequency employed.

• The greater the density of a medium, the greater the intensity of the echo. The contrasting details of an ultrasound picture are created by the varying echo intensities created by different degrees of tissue density.
• The ultrasound waves generated by the transducer are also received by the transducer and then converted into electrical signals, which are then translated into an image and recorded.
• Spatial resolution is achieved by measuring the time intervals between echoing ultrasound waves.
• The reflected waves can be used to visualize morphology or assess motion (mostly the velocity of blood flow) using different recording techniques.

Assessment of morphology

The reflection of ultrasound waves at the interfaces of tissues of different densities allows for the measurement of tissue echogenicity (corresponds to tissue density), while the return time allows for the determination of the distance between the structures visualized and the transducer (spatial distribution).

• A-mode (amplitude mode)
  • A one-dimensional visualization of structures based on the representation of echo amplitude (echogenicity) along the y axis and echo delay (depth) along the x axis.
  • Original ultrasound modality that is rarely used today
• B-mode/2D mode (brightness mode)
  • A two-dimensional grayscale visualization of structures based on the representation of echo intensity by white dots of varying brightness depending on echogenicity of the structure. Image depth is represented by distance from transducer (top of the screen) based on the echo delay, while image width is represented by the combination of a series of ∼ 120 parallel images.
• M-mode (motion mode)
  • A B mode visualization of structures that represents motion within structures (e.g., heart valves) on a vertical axis
  • Useful in assessing rapidly moving structures (used especially in echocardiography)

Assessment of direction of flow and velocity

• Doppler mode
  • Doppler ultrasound takes advantage of the doppler effect to visualize flow (e.g., blood) within structures by representing the compression and distention of soundwaves by the echoing structure rather than the echo delay.
  • The higher the flow velocity (relative to the transducer), the stronger the compression or extension of the waves.
  • Flow velocity is specified in m/s and visualized as changes in flow velocity (y axis) through time (x axis).
• Pulsed wave Doppler (PW doppler)
  • As in A-mode, B-mode, and M-mode, a piezoelectric element emits and receives ultrasound waves in pulses.
    • Advantage of pulsatile, intermittent recordings: visualization of structure depth
    • Disadvantage: limitations to temporal resolution
• Continuous wave Doppler (CW doppler)
  • Two piezoelectric elements are used, in which one continuously emits and the other continuously receives.
    • Advantage: High flow velocity (e.g., valve stenosis) can also be measured.
    • Disadvantage: Spatial distribution of the reflecting structures is not possible.

Combined method: duplex ultrasonography

• Duplex ultrasonography is a combination of PW-Doppler and B-mode imaging that allows for visualization of morphology (e.g., vessels) and flow direction/velocity of moving structures (e.g., blood).
• Color duplex ultrasonography (also known as color flow mapping) provides color-coding of flow direction (red = towards the transducer, blue = away from the transducer) and flow rate (the brighter the color, the faster the flow rate). Motionless structures are represented in grayscale according to echogenicity.

• Advantages
  • Ultrasound waves have no known harmful effects (compared to modalities using ionizing radiation)
    • Can be employed with no known risk to children and pregnant women as a potential alternative to x-ray and CT
  • Noninvasive
  • Enables real-time morphological and functional (moving image) diagnostics in various clinical settings
• Disadvantages
  • The quality and diagnostic value of an ultrasound image depend strongly on the skill of the examiner.
  • Artifacts
    • Acoustic shadow
      • If ultrasound waves are strongly absorbed and echoed at the surface, the waves will fail to penetrate the tissue. All structures behind the surface will appear black.
      • Demarcation of the structures is not possible.
      • Especially relevant in the depiction of structures superimposed by bones (spleen or kidney posterior to the ribs) or air (intestinal loops, pancreas)
    • Acoustic enhancement: Because ultrasound waves are hardly weakened in fluids, structures that are located behind fluid-filled spaces will appear hyperechoic (brighter) in B-mode.
    • Reverberation: reverberation artifacts
      • Multiple echoes that arise at strongly reflecting surfaces that are reflected back and forth at regular intervals, also behind the surface

Ultrasound modalities

Conventional ultrasound

CNS (neonatology)

Thyroid

Thoracic organs

Abdominal organs

Kidney and urinary tract

Female sex organs and pregnancy

Musculoskeletal system

Duplex ultrasonography

Endoscopic ultrasound