Ultrasound Principles Flashcards
WAVE Soft Tissue Level 1
Rad vs US
RAD Advantages
* gas, bones
* larger region at same time
RAD Disadvantages
* ionizing radiation
* fluid = soft tissue
US Advantages
* no ionizating radiation
* differentiation fluid vs soft tissue structures
* organ internal morphology
* real-time evaluation
* guided interventions: FNA, bx,
drainage, etc
US Disadvantages
* impaired by the presence
of gas and bone
* one region at a time
* equipment dependent
* operator dependent * cost * artifact
Limitations of Ultrasound
-Inaccurate large organ measurement
- Limitations in visualizing bone and gas-filled structure
- Poor determination of neoplastic origin in some case
- small field of view
- inferiors for surgical planning compared to CT/MRI
FNA and biopsy consideration
coagulopathies, anemia, panting(movement), patient demeanor, sedation vs anesthesia, location of target, risks
Sound Waves
Mechanical waves of pressure that travel longitudinally through a medium. Molecules along the line of sound are compressed and expanded(Rarefaction).
Sound Wave Cycle
1 repetitive periodic oscillation
Sound Wave Frequency
Number of times wave is repeated(cycles) per second. 1 Herts= 1 cycle per second.
Wavelength
Distance a wave travels in 1 cycle
Sound Wave Velocity
velocity (M/sec) = Frequency(cycle/sec) X Wavelength(M)
Soft Tissue Velocity
1540 M/sec
Ultrasound Frequency
> 20,000 Hz, diagnostic ultrasound uses 2 to 15 MHz.
1 HZ, 1 kHz, 1 MHz
one cycle, 1000 cycles, 1 million cycles
Crystals
Transmit and receive
Piezoelectric Effect
Crystals with in the transducer hear are electrically stimulated and produce sound waves.
Pulse-Echo Principle
Sound produced by transducer in pulses instead of continuously, transmitting sound into the body less than 1% of the time when the machine is on. Soundwaves propagate through tisssues and are reflected back to the transducer.
Speed of Sound
It depends on density and elasticity of the medium(tissue) it is traveling through. Not effected by frequency of transducer.
Velocity of sound in Body mediums
Air 331 M/sec
Fat 1450 M/sec
Water/Fluid 1540 M/sec
Soft Tissue(avg) 1540 M/sec
Liver 1549 M/sec
Kidney 1570 M/sec
Bone 4080 M/sec
Transducer
Receives reflected waves that vibrate the crystals, turning the signal back into an electronic signal that the computer detects and amplifies(compensating for attenuation) and generates an image of pixels representing the depth and intensity of the returning echo.
Attenuation
The loss of energy that occurs as ultrasound waves travel through a medium.
Depth
Time back to the transducer divided by the speed of sound in tissues(1540 M/sec) equals the depth on image
Strength of Returning Echo
Dependent on degree of attenuation, acoustic impedance, angle of incidence
High frequency transducer attenuation
High frequency transducers have greater attenuation. More cycles/sec = greater tissue interaction. More reflection, absorption, and scattering. Less depth penetration
Scatter
Change in direction, caused when encounters rough surfaces. Speckle echotexture.
Absorption
Change to heat
Refraction
Change in direction, different speed
Speckle
Caused by scatter when the beam encouters small, uneven interfaces in the parenchyma of an organ. Contributes to the texture seen in abdominal organs.
Reflection
What returns to transducer to create image.
Acoustic Impedance
The reflection or transmission characteristics of tissue. Inherent propery of tissue based on density and compressibility, independent of sound frequency. High acoustic impedance doesn;t easily allow sound to travel through. AI(z)= speed of sound in tissue(v) X Tissue Density(p)
Angle of Incidence
The angle at which a sound strikes a tissue. Perpendicular yields the greatest reflections and therefore strength.
Specular Reflectors
Large smooth, rounded surfaces(Diaphragm, bladder, kidney, myocardium, cysts, gestational sacs) that strongly reflects back from one direction. Large angle if incidence will make parts of these structures invisible.
A Mode
Amplitude.
Simple. The height of spikes represents the amplitude of returning echos.
Used for optho
B Mode
Brightness.
Strength of returning echos displayed as different degrees of “brightness” on screen.
Commonly used mode
M Mode
Motion.
Time on horizontal axis, depth on vertical axis.
Cardio applications
Doppler
Measures changes in frequency from baseline when reflected from moving targets.
Away=lower than base line
Towards=higher than baseline
Doppler Angle
Angle between the flow direction and the sound wave direction. Optimal <60 degrees relative to angle of beam(30-60 preffered).
Types of Dopple
Color flow, Power Doppler, Pulsed Wave, Continuous wave
Color Flow Doppler
Commonly used. Superimposes doppler on grayscale image.
Evaluate wide area and determine flow direction.
Only displays mean velocity, not precise, angle dependent.
Transducer types
Singe or Multifrequency(High or low frequency). Mechanical or electronic sector scanners(single crystal, oscillated or rotated). Multiple Element Arrays(Linear, curvilinear, phased)
High Frequency Probes
7.5- 15 MHz
Shorter wavelength, more tissue interaction, greater resolution, less penetration, good for shallow structures.
Low Frequency Probes
2-5 MHz
Longer wavelength, less tissue interaction, less resolution, greater penetration.
Transducer Applications: Mid-frequency Microconvex
Small animal Abdomen
Transducer Applications: Higher frequency, Linear
Small animal muskuloskeletal, superficial structures(ex ocular, masses), Large animal muskuloskeletal
Transducer Applications: Lower Frequency, Large Curvilinear
Big Dog Abdomen, Large animal abdomen or thorax
Phased-Array Sector
True pie shaped image, small near field footprint.
Tighter spaces(ex in-between ribs) and can do CW doppler.
Lateral dropout.
Used in cardiac and small animal imaging,(echo, lung, pleura).
1.5 - 4 Mhz, good penetration and resolution
Linear Array
Rectangular shaped image, wide near and far field, large “foot print”, no lateral “drop out”.
Use in very small animal abdominal, neck(thyroid) high resolution scanning, tendons, vascular.
5-12 MHz, very good resolution poor penetration.
Curvilinear Array
Best of both linear and sector technology, modified pie-shape, small, “foot print”, similar enhanced resolution of linear, less artifact, no CW doppler.
Use in small animals and cardiac, abdomen.
2-5 MHz, good/average resolution, good penetration.
Resolution
Ability to resolve 2 closely spaced objects or reflectors.
Axial Resolution
Ability to display two reflectors as distinctly separate along the axis of the beam. Minimal distance has to be atleast 1/2 spatial pulse length(SPL) to avoid overlap of returning echos.
SPL
Spatial Pulse Length. Number of cycles emitted per second by the transducer multiplied by the wave length.
Lateral resolution
Ability to display two adjacent reflectors as distinct when located perpendicular to the axis of the beam.
Two objects at the same depth need to be wider apart than the sound beam. Related to width of sound beam.
Can be altered with focusing.
Beam width narrower for higher frequency transducers.
Elevational Resolution
Determined by the thickness of the imaging plane. Measured in the direction perpindicular to the imaging plane. Reults in “Filling-in” of anechoic sturctures with echoes- slice thickness artifact.
Contrast Resolution
Ability to discriminate between Gray scale objects.
Long Gray Scale
Wide latitude, low contrast, softer images, “Lots of gray”
Short Gray Scale
Short Latitude, high contrast, very “Black and white”
Sagittal plane
The reference marker is pointed cranially.
Transverse Plane
The reference marker is pointed to the patients right side.
Slide
To move the transducer along the skin without changing the transducer orientation with respect to the reference mark or angle.
Fan/Tilting
Allows for other planes in the same axis to come into view without sliding the transducer along the body.
Rock/Point
Rocking or pointinh the transducer toward or away from the indicator. In plane motion.
Rotate or Twist
90 degree rotation for transverse to sagital or vise versa while staying in the same spot.
Pressure
Applying pressure on the transducer and compressing tissues.
Homogenous
Smooth, even echo pattern throughout the structure
Heterogenous
Uneven or dissimilar echo pattern. Patchy, mottled, lacey or swiss cheese appearence.
Hypoechoic
Less echogenic, dark echos, blacker. Tissue contains poorly reflective internal echoes.
Hyperechoic
More echogenic, brighter, whiter. Tissue or structure contains highly reflective properties.
Anechoic
No Echoes, Black, lacks any tissue surface to reflect sound.
Isoechoic
Echos are the same. Similar acoustic properties to surrounding tissues.
Order of Increasing Echogenicity of Tissues
Bile, blood, urine
Renal medulla
Muscle
Renal cortex
Liver
Storage/falciform fat
Prostate
Renal sinus
Structural fat, vessel walls
Bone, gas, organ capsules
