ITE Keywords US physics - bushman

ITE Keywords US physics - bushman

Transcript

1. ITE Keywords: Ultrasound Physics MATT BUSHMAN CA3
2. US terminology  Wavelength – distance between successive wave crests  Frequency – numbers of cycles per second  1 Hertz = 1 cycle / second  Velocity – wavelength x frequency  Velocity of sound is dependent upon density and compressibility of the medium
3. US image generation  US waves are generated by piezoelectric crystals  Convert mechanical energy of returning echoes into electrical current that can be processed by a machine to produce 2-D image
4. Reflection  A reflection of a beam is called ECHO  Occurs at interface of different types of tissue  Dependent upon acoustic impedance  Specular Reflections  Occurs at large change in impedance producing large reflection, decreased continuing amplitude  Ex) bone  Diffuse Reflections  Occurs at soft tissue  Ex )liver
5. Refraction  Occurs when sounds strikes a boundary at an oblique angle  Dependent on difference of velocities of two mediums  Reflections do not return directly to transducer  Results in unclear image  Can cause artifact that occurs beneath large vessels
6. Absorption / Attenuation  Direct conversion of sound energy to heat  Higher frequencies absorbed faster  Better axial resolution  Lower frequency US may be needed to increase penetration for deeper structures  Lower resoluton
7. Resolution  Axial Resolution  Minimum separation of above-below places along the beam axis  Axial resolution = Wavelength λ × Number of cycles per pulse n ÷ 2  Determined by transducer specific characteristics  High frequency US can detect smaller objects and provide better resolution  Axial resolution is typically between 0.05-0.5
8. Resolution  Lateral Resolution  Parameter of sharpness to describe minimum side by side distance between two objects  Determined by frequency and beam width  High frequency transducers = narrow focus  Better lateral resolution
9. US modes  A Mode (amplitude)  Oldest (1930)  Simple 1-D image generated as series of vertical peaks corresponding to depth of structures
10. US modes  B-mode (brightness)  2-D images  Provides cross-sectional image through the area of interest  Horizontal and vertical directions represent real distances  Intensity of grey scale indicated echo strength
11. US modes  M-mode (motion)  1-D  US pulses emitted in quick succession and recorded over time  Used to determine velocity of specific organ structures  Ex) heart valves
12. Doppler  Measures the shift in frequency between incident and the reflected wave after hitting a moving object  Color flow  Blood flowing TOWARDS the probe = RED  Blood flowing AWAY from probe = BLUE
13. Echogenicity  Hyperechoic  areas that produce great amount of energy from returning echoes -> bright/white  Hypoechoic  Areas that produce less energy from returning echoes -> dark gray / black  Anechoic  Areas without returning echoes -> black
14. Echogenicity
15. Tissue Specific Echogenicity  Veins = anechoic / hypoechoic, non pulsatile, compressible, may vary w/ respiration  Arteries = round in cross section, pulsatile, not easily compressible  Fat = Hypoechoic, compressible  Muscle = Heterogeneous (hyperechoic borders w/ hypoechoic tissue background)  Bone = very hyperechoic w/ acoustic shadowing  Nerves = Honeycomb (hypoechoic fascicles surrounded by hyperechoic sheath)  Air = Hyperechoic  Pleura = hyperechoic line  Local Anesthetic = expanding hypoechoic region
16. Machine Controls  Gain – controls brightness of image  ratio of output to input electric power  Amplifies returning signals as well as background noise  Depth – start deep and decrease as needed  Leave small area behind to observe useful artifacts  Low frequency probes = increased depth w/ lower resolution  High frequency probes = decreased depth w/ higher resolution
17. Transducer Types  Linear –rectangular image  High frequency, high resolution, low penetrance  Ex) vascular access , regional anesthesia  Curvilinear –arc shaped  Lower frequency, lower resolution, increased penetrance  Ex) abdominal exams, FAST  Phased – small faced  Low frequency, low resolution, high penetrance,  Ex)transthoracic cardiac imaging