08: Ultrasound

Acoustic impedence

The ability of material to propagate sound

What is acoustic impedance related to

• Ability of the substance to be compressed

• The velocity of propagation through the substance

Does a hard or a soft substance have higher acoustic impedence

Harder

What substances don’t show up well on US

Bone and air

What color is bone on US

Black

What color is air on US

White

What physical property of sound is the basis for US imaging

Reflection of sound waves

Why does bone or air not image well on US

Bone has such a drastically higher acoustic impedance than most tissues, and air has such a lower acoustic impedance. At the interface of tissues with bone/air almost 99% of the sound waves are reflected, and not enough keeps going to image the bone/air

Imaging artifact that results from a highly reflective interface

Acoustic shadowing

Echogenicity

The ability of a tissue to reflect or transmit US in the context of surrounding tissues

Anechoic

No echo, shows up black

What material is anechoic

Air

Hypoechoic

Smaller echo, shows up gray

What material is hypoechoic

Soft tissue

Hyperechoic

Greater echo, shows up brighter

What material is hyperechoic

Bone and air

What type of probe gives you a higher resolution but lower penetration

High frequency probe

What type of probe gives you a lower resolution but penetrates deeper

Low frequency probe

What type of probe is more useful in large animals

Low frequency probe

What type of probe is more useful in small animals

High frequency probe

How should the probe be oriented in relation to the patient

Notch/line facing cranial or the patient’s right

How should you position the probe if you want transverse images

Face the notch to the patient’s right

How should you position the probe if you want sagittal images

Face the notch to the patient’s head

US function that affects amplitude (brightness/contrast)

Gain

US function that affects how far you are imaging

Depth

Which way do you swipe on the US screen for gain

R to L

Which way do you swipe on the US screen for depth

Up and down

What US artifact do you get when you go through fluid

Acoustic enhancement

Why does acoustic enhancement occur

Increased echoes from structures that are deep to material that transmits exceptionally well (like fluid)

What US artifact do you get when you encounter gas and fluid

Reverberation

Why does reverberation occur

When an US beam encounters 2 strong parallel reflectors (gas and fluid)

Function of color doppler

Helps you assess speed and direction of blood flow

What color shows up on a color doppler when the blood is flowing towards the probe

Red

What color shows up on the color doppler when the blood is flowing towards the probe

Blue

Pros of US

• Low or no sedation

• Comparatively inexpensive

Cons of US

• Doesn’t work through gas or bone

• Can’t diagnose masses

• Requires advanced training to be proficient