Ultrasound Imaging: Principles & Physics

• Ultrasound imaging, also known as sonography, uses high-frequency sound waves to create images of internal body structures
• It is a non-invasive diagnostic technique widely used in medicine

Ultrasound Physics

• Ultrasound waves are mechanical pressure waves with frequencies beyond the range of human hearing (typically >20 kHz)
• Diagnostic ultrasound commonly uses frequencies between 2-18 MHz
• The ultrasound transducer emits sound waves into the body and receives echoes reflected from tissues
• The time it takes for the echo to return and the intensity of the echo determine the location and characteristics of the tissue
• Acoustic impedance is the resistance of a material to the propagation of sound waves
• Differences in acoustic impedance between tissues cause reflection and refraction of ultrasound waves, which are used to create the image
• Reflection occurs when ultrasound waves encounter a boundary between two media with different acoustic impedances
• The amount of reflection depends on the difference in acoustic impedance; a large difference results in strong reflection
• Refraction is the bending of ultrasound waves as they pass through a boundary between two media with different speeds of sound
• Attenuation refers to the weakening of the ultrasound beam as it travels through tissue due to absorption, scattering, and reflection
• Higher frequency ultrasound provides better resolution but has greater attenuation and thus poorer penetration
• Lower frequency ultrasound provides deeper penetration but has lower resolution
• The Piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress
• Ultrasound transducers use piezoelectric crystals to convert electrical energy into mechanical energy (sound waves) and vice versa

Diagnostic Imaging Techniques

• A-mode (Amplitude Mode) displays echoes as spikes on a graph, with the height of the spike representing the amplitude of the echo and the position representing the depth
• Used in ophthalmology to measure distances within the eye
• B-mode (Brightness Mode) displays echoes as dots on a screen, with the brightness of the dot proportional to the amplitude of the echo
• Forms the basis for real-time 2D ultrasound imaging
• M-mode (Motion Mode) displays the movement of structures over time as a line on a graph
• Used in echocardiography to assess the motion of heart valves and walls
• Doppler ultrasound measures the change in frequency of the reflected ultrasound waves to determine the velocity of moving structures, such as blood flow
• Color Doppler displays the direction and velocity of blood flow as different colors on the screen
• Power Doppler displays the amplitude of the Doppler signal, which is related to the concentration of moving blood cells
• Provides greater sensitivity to slow flow but does not provide directional information
• Elastography assesses the stiffness of tissues by measuring their deformation in response to an applied force
• Used to differentiate between benign and malignant lesions in organs such as the liver, breast, and thyroid
• Contrast-enhanced ultrasound (CEUS) involves injecting microbubble contrast agents into the bloodstream to enhance the visibility of blood vessels and tissues
• 3D and 4D ultrasound generate three-dimensional images of structures and allow for real-time visualization of movement in three dimensions
• Commonly used in obstetrics to visualize the fetus

Applications in Medicine

• Obstetrics and Gynecology:
• Monitoring fetal development, detecting fetal abnormalities, and guiding procedures such as amniocentesis
• Assessing the uterus, ovaries, and other pelvic structures
• Cardiology:
• Assessing heart structure and function, detecting valve abnormalities, and evaluating blood flow
• Abdomen:
• Imaging the liver, gallbladder, pancreas, spleen, and kidneys
• Detecting gallstones, tumors, and other abnormalities
• Musculoskeletal:
• Imaging muscles, tendons, ligaments, and joints
• Diagnosing tears, strains, and other injuries
• Vascular:
• Assessing blood flow in arteries and veins
• Detecting blood clots, stenosis, and aneurysms
• Small Parts:
• Imaging the thyroid, breast, and testicles
• Detecting nodules, cysts, and tumors
• Emergency Medicine:
• Focused Assessment with Sonography for Trauma (FAST) exam to detect internal bleeding
• Guiding procedures such as central line placement
• Ultrasound-guided biopsies and aspirations:
• Guiding needles to specific locations for tissue sampling or fluid drainage
• Therapeutic ultrasound:
• Using high-intensity focused ultrasound (HIFU) to destroy tumors
• Using ultrasound to promote tissue healing

Training in Ultrasound

• Medical professionals who perform ultrasound examinations require specialized training
• This typically includes completing a sonography program or receiving training from experienced sonographers or radiologists
• Training programs cover ultrasound physics, anatomy, scanning techniques, and image interpretation
• Certification is available through organizations such as the American Registry for Diagnostic Medical Sonography (ARDMS)

Safety and Guidelines

• Ultrasound is generally considered a safe imaging modality because it does not use ionizing radiation
• However, ultrasound energy can cause thermal and mechanical effects in tissues
• The ALARA (As Low As Reasonably Achievable) principle should be followed to minimize exposure
• Output power and exposure time should be kept as low as possible while still obtaining diagnostic-quality images
• Guidelines for safe ultrasound practice are provided by organizations such as the American Institute of Ultrasound in Medicine (AIUM)

Vessels

• Ultrasound is used to look at arteries and veins in the body
• It is often used to assess for blood clots, stenosis (narrowing of the vessel), and aneurysms (bulging of the vessel wall)
• Doppler ultrasound is crucial in vascular studies to evaluate blood flow direction and velocity
• Carotid arteries are commonly assessed to detect plaque buildup that could lead to stroke
• Deep vein thrombosis (DVT) in the legs is a frequent indication for venous ultrasound
• Abdominal aorta aneurysms (AAA) can be monitored using ultrasound to prevent rupture
• Ultrasound can guide the placement of catheters and needles in vessels

Abdomen/Body Anatomy

• Ultrasound can visualize many organs in the abdomen including liver, gallbladder, pancreas, spleen, kidneys, and bladder
• It is often used to detect gallstones, liver masses, kidney stones, and abnormalities in the pancreas and spleen
• The liver appears homogeneous in texture on ultrasound, masses can be cystic or solid
• The gallbladder is an anechoic (black) structure when filled with bile, gallstones appear as bright echoes with shadowing
• The pancreas can be difficult to visualize due to bowel gas, abnormalities can be identified with careful scanning
• The spleen is typically crescent shaped and homogeneous, it is frequently measured to assess for enlargement
• Kidneys can be readily visualized, hydronephrosis (swelling due to urine backup) and stones can be identified
• The bladder is an anechoic (black) structure when filled with urine, masses within the bladder can be identified
• Bowel gas can obscure the visualization of abdominal organs with ultrasound

OBGYN

• Ultrasound is essential in obstetrics for monitoring fetal development, assessing gestational age, and detecting abnormalities
• Transabdominal and transvaginal approaches are used depending on the stage of pregnancy and the information needed
• Early pregnancy can be visualized with transvaginal ultrasound to confirm intrauterine pregnancy
• Fetal heart rate can be detected as early as 6 weeks gestation
• Nuchal translucency is measured between 11 and 14 weeks to assess the risk of chromosomal abnormalities
• Anatomy scans are done around 20 weeks to assess fetal organs
• Fetal growth and amniotic fluid are monitored throughout pregnancy
• Ultrasound can also be used to diagnose ectopic pregnancies and other gynecological conditions