Biomedical Instrumentation Unit 14 ReviewMedical Imaging: Ultrasound & Doppler

Basics of Ultrasound

• Uses high-frequency sound waves to create images of internal body structures
• Sound waves are emitted by a transducer, which also detects the returning echoes
• Frequencies used in medical ultrasound typically range from 1-20 MHz
• Higher frequencies provide better resolution but have less tissue penetration
• Ultrasound is non-invasive, real-time, and does not use ionizing radiation
• Commonly used for imaging soft tissues, such as organs and blood vessels
• Can be used for diagnostic and therapeutic purposes (focused ultrasound)

Physics Behind Ultrasound Imaging

• Sound waves are mechanical waves that require a medium to propagate
• Ultrasound waves are longitudinal waves, with particle motion parallel to the direction of wave propagation
• Wave properties include frequency, wavelength, amplitude, and speed
• Speed of sound varies in different tissues, averaging around 1540 m/s in soft tissue
• Acoustic impedance is the product of a material's density and the speed of sound in that material
• Differences in acoustic impedance between tissues cause reflections at boundaries
• Attenuation is the loss of wave intensity as it travels through a medium
  • Caused by absorption, scattering, and reflection
  • Increases with higher frequencies and greater tissue depth
• Reflection, refraction, and scattering of sound waves influence image formation

Ultrasound Transducers and Equipment

• Transducers convert electrical energy into sound waves and vice versa
• Piezoelectric crystals in the transducer expand and contract when an electric field is applied
• Array transducers contain multiple piezoelectric elements for electronic beam steering and focusing
• Linear array transducers produce rectangular images and are used for small, superficial structures
• Curvilinear array transducers produce fan-shaped images and are used for abdominal and obstetric imaging
• Phased array transducers use small piezoelectric elements to steer the beam and are used for cardiac imaging
• Ultrasound machines include a pulse generator, receiver, and display
• Gel is applied between the transducer and skin to reduce air gaps and improve coupling

Image Formation and Processing

• Pulses of sound waves are sent into the body, and the returning echoes are detected by the transducer
• Time delay between pulse emission and echo reception determines the depth of the reflecting structure
• Echo amplitude is represented by pixel brightness on the display (B-mode)
• Scan converters process the received signals to create 2D images
• Image resolution is determined by the transducer frequency, focusing, and signal processing
  • Axial resolution depends on pulse length and frequency
  • Lateral resolution depends on beam width and focusing
• Contrast resolution is the ability to distinguish between tissues with similar acoustic properties
• Image artifacts can arise from equipment limitations, patient factors, or physical properties of ultrasound
  • Examples include shadowing, enhancement, reverberation, and speckle
• Post-processing techniques, such as filtering and compression, can improve image quality

Doppler Effect and Its Applications

• Doppler effect is the change in frequency of a wave when there is relative motion between the source and the observer
• In ultrasound, the Doppler effect is used to measure blood flow velocity
• Doppler shift is the difference between the transmitted and received frequencies
• Doppler shift depends on the blood flow velocity, the angle between the ultrasound beam and the blood vessel, and the transducer frequency
• Continuous wave Doppler uses two piezoelectric elements for continuous transmission and reception
  • Provides velocity information but no depth discrimination
• Pulsed wave Doppler uses a single piezoelectric element for alternating transmission and reception
  • Allows depth discrimination but has a limited maximum detectable velocity (Nyquist limit)
• Color Doppler imaging displays blood flow velocity and direction as color-coded pixels superimposed on a B-mode image
• Power Doppler imaging displays the intensity of the Doppler signal, which is related to the number of moving blood cells

Types of Ultrasound Scans

• 2D imaging (B-mode) provides a cross-sectional view of the body
• M-mode imaging displays the motion of structures along a single ultrasound beam over time
  • Used for evaluating heart valve movement and fetal heart rate
• 3D imaging combines multiple 2D images to create a volumetric representation
  • Requires specialized transducers and processing software
• 4D imaging is 3D imaging in real-time, showing motion and changes over time
• Doppler imaging techniques (color, power, pulsed wave, continuous wave) assess blood flow
• Elastography measures tissue stiffness by applying compression or using shear waves
  • Used for evaluating liver fibrosis, thyroid nodules, and breast lesions
• Contrast-enhanced ultrasound uses gas-filled microbubbles to improve visualization of blood flow and tissue perfusion

Clinical Applications and Interpretation

• Abdominal ultrasound evaluates organs such as the liver, gallbladder, pancreas, and kidneys
  • Can detect cysts, tumors, stones, and inflammation
• Obstetric ultrasound monitors fetal development, growth, and well-being
  • Used for dating pregnancy, detecting abnormalities, and guiding interventions
• Cardiac ultrasound (echocardiography) assesses heart structure and function
  • Evaluates valve morphology, chamber sizes, wall motion, and blood flow
• Vascular ultrasound examines blood vessels for stenosis, thrombosis, and aneurysms
• Musculoskeletal ultrasound visualizes muscles, tendons, ligaments, and joints
  • Detects tears, inflammation, and fluid collections
• Thyroid ultrasound evaluates nodules, cysts, and gland enlargement
• Ultrasound-guided procedures, such as biopsies and injections, improve accuracy and safety

Safety and Limitations

• Ultrasound is generally considered safe, as it does not use ionizing radiation
• Thermal effects can occur due to absorption of ultrasound energy, particularly in bone and at air-tissue interfaces
• Mechanical effects, such as cavitation, can cause tissue damage at high intensities
• The ALARA (As Low As Reasonably Achievable) principle is used to minimize exposure
• Ultrasound has limited penetration in bone, air-filled structures, and obese patients
• Image quality can be affected by patient factors, such as body habitus and motion
• Operator dependence requires proper training and experience for optimal image acquisition and interpretation
• Artifacts can mimic or obscure pathology, leading to misdiagnosis if not recognized
• Complementary imaging modalities, such as CT or MRI, may be necessary for further evaluation