Thieme Clinical Companions: Ultrasound (eBook)

1 Basic Physical and Technical Principles

1.1 Physics of Ultrasound

Properties of Sound Waves

Propagation characteristics: Sound waves have several essential properties:

• Propagation of ultrasound waves: Sound waves travel through air, fluids, and human tissue almost exclusively as longitudinal waves. These are zones in which the molecules that make up the medium are alternately rarefied and condensed. Thus, sound waves must propagate through matter and cannot exist in a vacuum.

• Propagation speed: The speed of sound is relatively slow in all materials (in tissue about 1540 m/s). Consequently, its transit time can be accurately determined by electronic measurements and correlated with the distance traveled by applying the time–distance principle.

• Reflection (partial or complete) of sound waves at interfaces: The degree of reflection of incident sound waves at an interface depends on the acoustic resistance (“impedance”) of the medium:

– Impedance = the ratio of the incident sound intensity to the portion that is transmitted.

– Acoustic resistance = the product of the density times the speed of sound.

Doppler effect: The Doppler effect states that the frequency of the returning (received) sound waves changes when the source of the sound is moving toward or away from the receiver. According to the time–distance law, the product of time and velocity equals the distance traveled. Thus, the frequency changes in the sound waves reflected from moving red blood cells can be analyzed to determine the direction and velocity of blood flowing through vessels and in the heart.

Resolution

Ultrasound frequency: The quality of an ultrasound examination depends on two criteria relating to the properties of the sound waves:

• The highest possible resolution (high transducer frequency).

• An adequate depth of sound penetration (low transducer frequency).

• Rule: Shorter wavelengths improve resolution but decrease the penetration depth of the ultrasound beam.

• Tradeoff: The optimum frequency range for diagnostic ultrasound is 1–10 MHz. The optimum range of wavelengths is 0.15–1.5 mm (Table 1).

Fig. 1 Ultrasound beam shape and electronic focusing (after Röthlin, Bouillon, and Klotter)

Velocity of sound propagation: This depends on the density of the medium (approximately 1500–1600 m/s in soft tissues and fluids, 331 m/s in air, and 3500 m/s in bone). Ultrasound instruments are calibrated to a mean sound velocity of 1540 m/s.

Axial resolution: A sound pulse composed preferably of two (or three) wavelengths is emitted in the longitudinal (axial) direction. The maximum ability to resolve two separate points in the longitudinal direction is equal to one-half the pulse length, or approximately one wavelength. For example, the resolution at an operating frequency of 3.5 MHz is approximately equal to 0.5(–1) mm.

Lateral resolution: The ultrasound beam initially converges with increasing depth, and then widens out again with decreasing intensity and resolution. The focal zone (“waist”) of the beam is 3–4 wavelengths wide and is the area where lateral resolution is highest (Fig. 1). The lateral resolution at a frequency of 3.5 MHz is approximately 2 mm, meaning that two adjacent points can be distinguished as separate points when they are at least 2 mm apart.

Focusing: The purpose of beam focusing in sonography is to achieve maximum resolution and improve the ability to recognize fine details.

• Technical options:

– Make the transducer face concave to produce a convergent beam (concave mirror effect).

– Use a collecting lens.

• Mechanical focusing: This creates a fixed focal zone that cannot be moved (fixed-focus system), although it can be modified somewhat by scanning through a fluid offset.

• Electronic focusing: With this option, the focal zone can be set to any desired depth (Fig. 1). For example, the focal zone can be positioned to give a sharp image of the gallbladder, or it can be extended over the full depth of the image field.

• Adjusting the focus during an ultrasound examination: This is the hallmark of a proficient examiner. One feature of a high-quality ultrasound system is that a definite change in resolution is seen as the focal zone is moved.

Propagation Characteristics of Sound Waves

The propagation of ultrasound waves obeys the laws of wave physics. The following terms have been adopted from radiation optics and wave optics.

Reflection: Sound waves are partially reflected and partially transmitted in biological tissues. An image of an organ is generated from the returning echo signals by analyzing the impedance differences at acoustic interfaces. The higher the acoustic impedance, the greater the degree of reflection, with total reflection occurring at interfaces with a very high impedance mismatch (e.g., between soft tissue and bone, calcium, or air, producing a high-amplitude echo). Interfaces with a high acoustic impedance (e.g., gallstones) reflect all of the incident sound and cast an acoustic shadow.

Scattering: This consists of randomly directed reflections that occur at tissue interfaces and rough surfaces. The echoes generated by scattering centers contribute significantly to medical imaging (e.g., the imaging of rounded organ contours).

Refraction: This phenomenon is most pronounced at smooth interfaces with a high acoustic impedance. The sound waves are deflected at an oblique angle relative to the direction of the main beam.

Absorption and attenuation: These describe the “loss” of sound waves due to their spatial distribution in the tissue and the conversion of sound energy to heat. According to the findings of a WHO commission, the conversion of sound energy to heat is within safe limits at the low energy levels used in diagnostic ultrasound. Even so, it is prudent to use the lowest possible ultrasound energy when scanning children and pregnant women. Sound waves are also attenuated in tissues as a result of reflection, scattering, and refraction. This leads to a significant energy loss, which is offset by adjusting the time gain compensation (TGC) on the scanner.

1.2 Ultrasound Techniques

A-Mode, B-Mode, and M-Mode Scanning

A-mode scanning (Fig. 2a): In this technique the amplitudes (A-mode) of the echo signals returned from tissue interfaces are displayed as a series of amplitude deflections along a horizontal axis, as on an oscilloscope.

B-mode scanning (brightness mode, Fig. 2b):

• Principle: Reflected ultrasound pulses are displayed on the monitor as spots of varying brightness in proportion to their intensity. The sound waves are transmitted into the tissue in a parallel scan or a fan-shaped beam, and the echoes are reflected back to the transducer and assembled line-by-line according to their arrival time.

• Signal display and image reconstruction: Approximately 120 image lines are assembled to make a two-dimensional sectional image. The various echo intensities are converted by electronic processing into image spots of varying density or shades of gray (gray-scale display, brightness modulation).

M-mode scanning (time–motion): This technique generates a time–motion trace that records the motion of acoustic reflectors such as heart valves and myocardial walls over time.

Fig. 2a, b A-mode and B-mode scans, illustrated for the maxillary sinus. a A-mode signal. b B-mode display: echo amplitudes are converted to spots of varying brightness. E = entry echoes (bone), E′ = entry echo (bony ridge or polyp), M1 = mucosa, M2 = thickened mucosa, F = fluid, EE = exit echo

Doppler and Duplex Sonography

Continuous-wave (CW) Doppler:

• Principle: Two piezoelectric crystals are used, one for the continuous transmission of ultrasound pulses (continuous wave) and one for the reception of reflected ultrasound signals.

• Signal display: The frequency spectra of returning echoes are displayed acoustically and also visually if desired. The frequency shifts can be used to calculate the direction and velocity of blood flow. This technique does not, however, provide information on the depth or range of the echo source.

Pulsed Doppler:

• Principle: This technique employs one piezoelectric crystal that functions alternately as a transmitter and receiver (pulsed wave).

• Signal display: Echo signals are recorded from a designated sample volume during the receiving phase of the scan. This makes it possible to determine the depth and width of the sample volume and investigate blood flow within a circumscribed area.

Duplex sonography:

• Principle: CW or pulsed Doppler is combined with B-mode imaging, providing visual feedback for positioning the Doppler beam and the sample volume.

Power Doppler: This technique demonstrates the spatial distribution of blood flow but cannot determine flow direction. It is most useful in establishing the presence or absence...