Phy Ultrasound Reading Material

Ultrasound Basics

• Sonography employs ultrasound waves for imaging organs, vessels, and soft tissues, it is non-invasive and painless
• Sonography and ultrasound are often interchangeable terms
• The image generated by ultrasound is known as a sonogram
• Ultrasound waves used in medical applications range from 1 to 20 MHz
• The upper limit of human hearing extends to approximately 20 kHz

The Basic Ultrasound Principle

• The ultrasound transducer emits an ultrasound signal
• The transducer detects echoes from structures the wave encounters
• Echoes are converted into an image based on time, amplitude, and frequency

How Ultrasound Waves Interact With Tissues

• Reflection involves waves reflecting back to the transducer
• Absorption occurs when waves are absorbed by the tissue, converting energy to heat
• Scattering involves waves reflecting in multiple directions
• Refraction means the direction of waves changes

Tissue Impedance

• Tissue impedance influences reflection magnitude
• Each tissue type has impedance, which gauges resistance to sound propagation dependent on tissue density and sound speed

Hyperechoic, Hypoechoic and Isoechoic Structures

• Structures that produce strong echoes appear bright, and are called hyperechoic
• Structures that produce weak echoes appear dark, and are called hypoechoic
• Structures that produce echoes similar to surrounding tissues are called isoechoic.

Signal to Image Production

• Ultrasound equipment generates an electrical signal which then travels to a transducer
• Piezoelectric crystals translate the electrical signal into sound waves, which then transmit outward from the probe
• The crystals convert the returning ultrasound echo into an electrical signal and is converted to an image

Transducer Types and Their Applications

• Different transducers offer varied strengths, limitations, and applications
• Curved Transducers (1-5 MHz): Good penetration and a wide field of view, suited for abdominal and deep structure imaging, but has low resolution
• Linear Transducers (3-12 MHz): Great resolution, and poor penetration, best for superficial structures, musculoskeletal imaging, and neck imaging
• Phased Array Transducers (1-5 MHz): Wide field of view and also small transducer surface, use for echocardiography and imaging intercostal views, but low resolution

Image Optimization Parameters

• Modern ultrasound systems automatically optimize images
• Users can adjust specific parameters for image refinement
• Gain: Increases image brightness, but can amplify noise
• Frequency: Higher frequency improves quality, but reduces penetration
• Depth: Greater depth gives a wider view, but reduces visibility of details
• Focus: Sharpens the ultrasound image at the set focal depth

Artifacts

• Interactions between the equipment and the body causes artifacts
• Some artifacts can be used to glean information about scanned anatomy

Common Artifacts

• Acoustic Shadowing: Results in low signal behind structures that absorb or reflect heavily, like gallstones
• Enhancement: Increased signal appears below structures that transmit sound well, like fluid filled structures like the bladder
• Anisotropy: Mostly found in musculoskeletal ultrasound with fibrillar structures like tendons, where sound waves reflect away from the transducer resulting in a hypoechoic appearance
• Changing transducer angle can help alleviate the effects of anisotropy

The Doppler Effect

• The Doppler effect is used to detect and measure movement, especially blood flow
• Approaching objects lead to frequency increases, whereas receding objects lower frequency

Medical Ultrasound and The Doppler Effect

• The frequency of the echo alters when reflecting tissue moves relative to the transducer
• Frequency elevations during tissue movements towards the transducer and reduced as it moves away

Colour Doppler Overlay

• Colour Doppler overlay is used to determine flow direction
• Positive Doppler shifts appear as red to indicate movement towards the transducer
• Negative Doppler shifts appear as blue to indicate movement away from the transducer

Doppler Effect Applications

• Qualitative: Assessing increased gallbladder wall flow as inflammation signs or reduced blood flow for testicular torsion evidence
• Quantitative: Measuring flow rates through heart valves or carotid arteries to check stenosis evidence

Contrast-Enhanced Ultrasound (CEUS)

• This employs a distinct contrast agent from CT or MRI
• Contrast agents consist of gas-containing microbubble solutions that are strictly intravascular
• A half life is 5-15 minutes, with effects being rare

CEUS Indications

• Characterization of liver masses
• Visualization of targets in ablation
• Characterization of masses found in various other organs

CEUS Contraindications

• CEUS is generally very safe
• Contraindications include sensitivities to active substances, known right-to-left cardiac shunts, severe pulmonary hypertension, acute respiratory distress, and egg allergies

Strengths of Ultrasound

• Low cost
• High availability
• High portability
• Safe and non-invasive
• Fast
• Dynamic

Limitations of Ultrasound

• Highly operator dependant
• Difficult to reproduce
• Highly patient dependant
• Poor penetration in air and bone

Key Takeaways Messages

• Ultrasound waves are sound waves with a high frequency
• Captured echoes are analyzed to compose an image on screen
• Different transducers are used for different applications
• Recognize and understand the importance of artifacts, as they may influence the diagnosis
• Doppler effect is used to visualize movement and blood flow
• CEUS is a safe for liver lesion characterization and other applications
• Evaluate the strengths and limitations before performing an ultrasound