Introduction to Color Doppler Ultrasound of the Skin

Abstract

High-resolution gray scale and color Doppler ultrasound can be a useful tool in dermatology. Technical requirements and useful sonographic concepts are discussed in this chapter. An ultrasound glossary, tips and teaching points are provided to improve the understanding of this technique.

A comprehensive review of the basics of ultrasound focused on the ­requisites for skin, nail and scalp examinations

Glossary

Transducer

Device that converts one form of energy into another. In ultrasound, the transducer converts electric energy provided by the transmitter to mechanical energy (acoustic pulses) and vice versa, reflected echoes to electric signals. Ultrasound transducers are made of thin piezoelectric materials that expand and contract to generate acoustic vibrations (frequencies).

Resolution

Capacity to separate two objects in close proximity that are along the path of the ultrasound beam. Axial resolution is the smallest thickness while lateral resolution is the smallest width that can be resolved. Higher ultrasound frequencies raise the spatial resolution but lower the depth of penetration into the tissues.

Artifacts

Source of imaging pitfalls, suggesting the presence of structures that are not actually present or obscuring real findings. Examples of artifacts suggesting structures that are not actually present are reverberation, refraction, and side lobes artifacts. Reverberation artifacts are a result of repeated reflection of the ultrasound signal between highly reflective interfaces usually near the transducer. Refraction artifacts are a result of bending of the sound beam causing targets that are not situated along the axis of the transducer to appear in a misleading location. Side lobes artifacts result from a strong out-of-plane reflector, generating confusing echoes. An example of artifact-obscuring findings is posterior acoustic shadowing. Acoustic shadow is a result of total reflection of the ultrasound waves by a strong reflector, causing loss of information on the tissues deep to the reflecting structure.

Harmonics

Multiples or harmonic echoes of the transmitted fundamental frequency generated by the acoustic pulse as it travels within tissues.

Compounding

Image resulting from summing ultrasound images obtained from different scanning angles.

Extended field of view (EFOV)

Panoramic image ­generated by manual movement of a real-time probe in the direction of the transducer array. Imaging processing technology estimates translation and rotation of the probe by comparing successive images. The EFOV image buffer combines the images to produce a panoramic image.

3D Ultrasound

Tissue volume acquired by dedicated 3D transducers employing hardware-based image registration, high density 2D arrays or software registration of scan planes.

Hypoechoic

Reflected signals (echoes) of intermediate intensity or amplitude, generating images of low brightness in different shades of gray. Examples of hypoechoic tissues are the parenchymal organs such as the liver, spleen, renal parenchyma, and muscles.

Hyperechoic

Reflected signals (echoes) of high intensity or amplitude, generating images of high brightness (white). Examples of hyperechoic tissues are fat, fibrous tissues, and bones. Foreign bodies and stones are examples of hyperechoic structures.

Echofree, sonolucent, or anechoic

Absence of reflected signals (echoes), generating images in black. Examples of anechoic targets are normal vessels, gallbladder, urinary bladder, and cysts.

Doppler effect

The result of the change in frequency of the sound when scattered by a moving target. The Doppler frequency shift is described by the Doppler equation: $\Delta F=\left(\text{FR}-\text{FT}\right)=(2\ \text{FT}\ \text v/c)\mathrm{cos}\theta.$

ΔF: Doppler frequency shift

FR: frequency of sound reflected from the moving target

FT: frequency of sound emitted from the transducer

v; target velocity

c: sound velocity in the medium

θ: angle between the flow axis and the incident ultrasound beam (angle of insonation). This angle must be kept at 60° or less.

Pulsatility index (PI)

PI considers the tissue resistance to flow at each complete cardiac cycle. It is calculated as PI = PSV  −  EDV/mean velocity.

Resistive index (RI)

RI considers the tissue resistance to blood flow at systole and diastole. It is calculated as RI = PSV  −  EDV/PSV

Contrast enhanced ultrasound agents (CEUS)

Intravascular blood-pool agents comprised of encapsulated microbubbles of gas, smaller than red blood cells, capable of circulating freely. CEUS seek to enhance the echo amplitude by increasing the backscatter from moving red cells, while increasing attenuation from the static tissue.