Ultrasound Imaging

Overview

Ultrasound is an essential tool in modern medical practice. Its ability to provide real-time, dynamic imaging without the use of ionizing radiation makes it valuable in a wide range of clinical scenarios. It is commonly used in both outpatient and hospital settings for diagnostic, monitoring, and procedural guidance. Ultrasound is portable, cost-effective, and can be used at the bedside, making it particularly useful in emergency medicine, intensive care, and rural healthcare settings.

Because of its safety, it is the imaging modality of choice during pregnancy. It is routinely used for guiding biopsies, draining fluid collections, and placing central lines.

Common Clinical Uses

Obstetrics: Fetal monitoring and pregnancy assessments
Cardiology: Echocardiography for heart function and structure
Abdominal imaging: Liver, gallbladder, kidneys, pancreas, and bladder
Vascular: Evaluation of blood flow and vessel patency (e.g., DVT studies)
Musculoskeletal: Tendon and soft tissue injury evaluation

Basic Principle

Video Overview: Clarius: Fundamentals of Ultrasound 1 (Physics)

Video Overview: Fundamentals of Ultrasound 2 (Equipment & Usage)

Ultrasound imaging, also known as sonography, uses high-frequency sound waves to create images of structures inside the body. A device called a transducer sends sound waves into the body. These waves bounce off tissues and are reflected back to the transducer. The reflected sound waves (or echoes) are processed by a computer to form real-time images on a screen. No ionization radiation is used.

How Ultrasound Works

Generation of Sound Waves: The transducer (ultrasound probe) contains piezoelectric crystals that vibrate when an electric current is applied. This vibration generates high-frequency sound waves.
Propagation and Reflection: These waves travel through the body. When they encounter a boundary between two tissues (e.g., fluid and soft tissue or bone), some waves are reflected back while the rest pass through the tissue.
Detection of Echoes: The returning, reflected sound waves cause the crystals to vibrate again, generating electrical signals.
Image Formation: A computer interprets these signals and constructs a grayscale image in real time.

Advantages of Ultrasound

Real-time imaging
Safe and non-invasive
Portable and relatively inexpensive
No ionizing radiation

Limitations

Image quality depends on the skill of the operator
Limited penetration in obese patients
Poor visualization of air-filled or bone-obscured structures

Descriptive Terms in Ultrasound Interpretation

Hyperechoic

Definition: Structures that reflect a high amount of ultrasound waves, appearing bright or white on the image.
Examples: Bone, calcifications, fat, tendons.
Interpretation: Indicates dense or fibrous tissue; often associated with posterior acoustic shadowing due to high reflectivity.

Hypoechoic

Definition: Structures that reflect fewer sound waves than surrounding tissue, appearing darker or gray.
Examples: Solid organs (e.g., liver, spleen), soft tissue tumors.
Interpretation: Suggests a solid composition; not necessarily abnormal. Clinical context is essential.

Anechoic

Definition: Structures that do not produce internal echoes, appearing completely black on ultrasound.
Examples: Simple cysts, urinary bladder, blood vessels, pleural effusions.
Interpretation: Typically indicates fluid-filled spaces and is associated with posterior acoustic enhancement.

Isoechoic

Definition: Structures with similar echogenicity to adjacent tissues, blending into the background.
Example: A liver lesion that matches the surrounding parenchyma.
Interpretation: Detection may require attention to contour, shape, or vascular flow.

Ultrasound Artifact Phenomena

Posterior Acoustic Enhancement

Definition: Increased brightness behind an anechoic structure.
Seen With: Cysts, bladder, blood vessels.
Cause: Ultrasound waves travel easily through fluid, transmitting more energy to deeper tissues.

Acoustic Shadowing

Definition: A dark band behind highly reflective or attenuating structures.
Seen With: Bone, gallstones, kidney stones, calcified lesions.
Cause: Sound is blocked or absorbed, preventing imaging of deeper structures.

Table: Echogenicity of Common Body Structures on Ultrasound

Structure / Tissue	Echogenicity	Ultrasound Appearance	Notes
Fluid (e.g., cyst, urine, bile, amniotic fluid)	Anechoic	Black	No internal echoes; sound passes freely; posterior acoustic enhancement.
Blood (flowing)	Anechoic / Hypoechoic	Black or dark gray	Appears black if flowing freely; can vary with clotting or slow flow.
Liver	Homogeneous, Moderate Echogenicity	Medium gray	Used as a reference for comparing other abdominal structures.
Renal Cortex	Slightly Hypoechoic to liver	Slightly darker than liver	Normal kidney tissue.
Renal Medulla (Pyramids)	Hypoechoic / Anechoic	Darker areas within kidney	Less echogenic than cortex.
Spleen	Slightly Hyperechoic to liver	Slightly brighter than liver	More homogeneous.
Pancreas	Isoechoic or Hyperechoic to liver	Same or slightly brighter than liver	Can vary with age and body habitus.
Gallbladder	Anechoic (lumen), Echogenic wall	Black lumen with bright walls	Stones appear hyperechoic with posterior shadowing.
Fat	Hyperechoic (but variable)	Bright, irregular echoes	Often appears mixed or coarse.
Muscle	Hypoechoic with echogenic striations	Dark background with bright lines	Fibrous septa create striated appearance.
Tendons	Hyperechoic, Fibrillar	Bright linear striations	Highly reflective in longitudinal plane; anisotropic.
Bone (cortex)	Highly Hyperechoic	Bright white line with shadowing	Reflects nearly all sound waves—creates posterior shadow.
Lungs (normal)	Hyperechoic line with reverberation	Bright pleural line with artifacts	Normal lung shows “A-lines” due to air reflection.
Pleural Effusion	Anechoic or Hypoechoic	Black or dark area above diaphragm	Fluid above lung base; may show internal debris if infected.
Thyroid	Homogeneous, Moderate Echogenicity	Medium gray	More echogenic than muscle; homogeneous texture.
Testicle	Homogeneous, Low to Medium Echogenicity	Medium gray with fine echoes	Epididymis is slightly less echogenic.
Prostate	Heterogeneous, varies with age	Variable brightness	May become more hyperechoic with age or calcification.
Uterus (Myometrium)	Isoechoic to Hypoechoic	Uniform medium-dark gray	May contain hyperechoic fibroids.
Endometrium	Hyperechoic (bright line)	Bright stripe within uterus	Changes with menstrual cycle phase.
Ovary	Isoechoic or slightly hypoechoic	Slightly darker than uterus	Follicles appear as round anechoic structures.

References 1.Radiology Key. Physics of ultrasound. https://radiologykey.com/physics-of-ultrasound-2/ 2.American College of Emergency Physicians. Ultrasound physics and technical facts for the beginner. https://www.acep.org/sonoguide/basic/ultrasound-physics-and-technical-facts-for-the-beginner/ 3.EchOpen. Introduction to clinical ultrasound – Glossary of the most common terms. https://www.echopen.com/en/ressources/articles/sinitier-a-lechographie-clinique-glossaire-des-termes-les-plus-frequents