Physics Notecard (1) PDF

Lesson 1-2 Transducer Part 1 & Part 2 Lead zirconate titanate (PZT) = Ultrasound imaging transducers Resonate frequency = designed to operate most efficiently & Operating frequency is what it actually vibrates at Piezoelectric & Reverse Piezoelectric Effect: Piezoelectric Effect: Pressure ⇒ Energy &...

Lesson 1-2 Transducer Part 1 & Part 2 Lead zirconate titanate (PZT) = Ultrasound imaging transducers Resonate frequency = designed to operate most efficiently & Operating frequency is what it actually vibrates at Piezoelectric & Reverse Piezoelectric Effect: Piezoelectric Effect: Pressure ⇒ Energy & Reverse Piezoeletric Effect: Energy ⇒ Pressure **ultrasound** Transducer Components: Cable, Damping Material, Casing, PZT crystal and matching layers Damping material: # of cycles ○ PURPOSE absorb energy entering (impendence similar to crystal) and reduce “ringing” of crystals (short pulses = better axial resolution) DISADVANTAGE reduces sensitivity, wide bandwidth, low Q factor Matching Layer: Positioned between the crystal and skin, reducing impedance and allowing better sound transmission. Crystal Thickness: thin crystal = short λ, increased frequency/speed and thick crystal long λ, decreased frequency/speed Bandwith: range of frequencies wide bandwidth: short pulse, increased axial resolution, decreased SPL (resonate frequency & low Q factor) narrow bandwidth: long pulse, decreased axial resolution, increased SPL (high Q factor) Q-Factor: Q Factor is a measurement of the acoustic characteristic of sound (tone) Relationships Frequency ∝ Speed of Sound Frequency ⇏ Crystal Thickness Q- Factor ∝ Pulse length Q Factor ⇏ Bandwidth Lesson 3-4 Sound Beams Part 1 & Part 2 1. Huygens’ Principle Interference & Spherical Wavelets Sound waves emanate from multiple point sources ⇒ wavefront combines small wavelets creating an hourglass-shaped beam due to interference. 2. Anatomy of a Sound Beam Focus: narrowest part of the beam is ½ transducer diameter. Near Zone (Fresnel Zone): transducer to focus point ** highest intensity and best lateral resolution ** Focal Length (NZL): Distance from transducer to the beam’s narrowest point. ○ NZL ∝ diameter squared Doubling diameter quadruples NZL. ○ NZL ∝ frequency ○ NZL ⇏ λ Far Zone (Fraunhofer Zone): focus point with beam divergence ** poor lateral resolution ** ○ Sound Beam Divergence: where the beam gradually spreads Beam divergence ⇏ transducer diameter Beam divergence ⇏ frequency Focal Zone: Region around the focus with the narrowest beam and optimal image detail. Lesson 5-6 SPATIAL RESOLUTION 2 Characteristics of pulse that improve image quality: (1) shorter pulse (axial) & narrow beam diameter (lateral) Lateral Resolution: perpendicular (side by side) (pulse width changes with depth) Lateral, Angular, Transverse, Azimuthal (LATA) smaller value means better to see (2 structures can be close and seen as 2 separate structures) 2. Determinants of Lateral Resolution Beam Diameter: A smaller beam width means better resolution (skinny beam) Depth: Beam diverges with depth, worsening lateral resolution. Focus: Optimal lateral resolution is at the beam’s focus. 3. Frequency and Transducer Effects Higher Frequency: Improves lateral resolution by narrowing the beam width = reduces penetration. Transducer Diameter: Larger diameters increase NZL, affecting resolution at different depths. 4. Focusing Methods Mechanical (Fixed): Uses a shaped element or lens to focus at a specific depth. ○ Internal focus: shaped and thicker & external focus: curved & thinner Electronic (Phased Array): Adjusts the focus electronically within the NZL. Axial Resolution: parallel to beam (deeper/shallower) Generally, axial resolution is better than lateral (pulse length doesn’t change with depth) Longitudinal, Axial, Range, Radial, Depth (LARRD) 2. Determinants of Axial Resolution Spatial Pulse Length (SPL): ½ SPL (A shorter SPL improves resolution) ○ >/= ½ SPL (no overlap) 2 distinct structures & < ½ SPL (will overlap) 1 structure ○ SPL: distance over which pulse occurs (λ ∙ # of cycles) Influenced by: ○ Pulse Duration: Shorter pulses improve resolution ○ Frequency: Higher frequency yields shorter λ & SPL, improving resolution. Higher frequency may limit penetration depth due to attenuation. ○ Number of Cycles per Pulse: Fewer cycles improve resolution (damping effect). 4. Practical Examples Smaller Axial Resolution Value: Indicates better ability to distinguish closely spaced objects. ○ Example: 1 mm axial resolution is better than 3 mm. Minimum Distance: Reflectors closer than ½ SPL will appear as a single structure