Imaging Exam 3 - Ultrasound
improvement of elevational resolution
"1.5 D array" reduces the slice thickness profile over an extended depth and allows for multiple transmit focal zones 5-7 discrete arrays replace the single array phase delay timing provides focusing in the elevational plane (similar to lateral transmit and receive focusing)
Linear Phased Arrays
(a) Voltage pulses are applied simultaneously to elements 1 and 5; after a time delay the same pulses are applied to elements 2 and 4, and after a second time delay a voltage pulse is applied to element 3. This sequence of pulses produces a curved wavefront, with a focal distance governed by the values of the time delays. (b) A reduction in the value of the time delays produces a longer focal distance. (c) and (d) Asymmetric time delays, with respect to the individual elements of the array, are used for beam steering.
•The fraction of the impinging energy reflected from an interface depends on ___.
... the difference in acoustic impedance between the materials on the two sides of the interface.
dynamic receiver filtering results in improved ___.
...SNR in the image because the noise level is proportional to the square root of the receiver bandwidth.
elevational resolution profile with an acoustic lens across the transducer array produces ___.
...a focal zone in the slice thickness direction
The high-frequency content of the backscattered signal decreases with depth due to __-.
...greater attenuation in tissue at higher frequency.
For a fixed curvature, a larger diameter lens (or crystal) leads to ___.
...higher lateral resolution.
The smaller the wavelength, the ___.
...higher the lateral resolution.
For a fixed lens, decreasing the radius of curvature will ___.
...improve the lateral resolution.
With a large impedance mismatch at an interface, much of the energy of an ultrasound wave is ___.
...reflected, and only a small amount is transmitted across the interface.
The brightness of each pixel in the image represents ___.
...the amount of energy backscattered at each point.
when a sound beam encounters an obstacle, its behavior depends on ___.
...the size of the obstacle compared with the wavelength of the sound.
Ultrasound attenuation in a medium composed of layers of different materials is the sum of ___.
...thte attenuation in each layer.
attenuation coefficient for fat
0.7*f^1.5 dB cm^-1
Example: for PZT, V ≈ 4000 m/s What is the thickness of the crystal to have a fundamental frequency of 2 MHz?
1 mm thick
Equation for speed of sound wave
1/sqrt(kp) k = compressibility of material p = density
What is the velocity of ultrasound in soft tissues?
1540 m/s
a typical attenuation coefficient for soft tissue
1dB cm^-1 MHz^-1 so for an ultrasound beam at 3MHz^-1, attenuation coefficient is 3dB cm^-1
What range of frequencies includes sound waves that are acoustic or audible to the human ear?
20-20,000 Hz
Dynamic receiver filtering
A progressive reduction in receiver frequency bandwidth as a function of time after pulse transmission.
M Mode detailed
A schematic of M-mode ultrasound scanning. (left) A transducer is placed over the area of interest, which consists of several stationary tissues and one which is moving periodically. (right) The M-mode scan comprises a time-series of A-mode scans, allowing the degree of movement of the individual tissues to be seen.
A-Mode (Amplitude Mode)
An A-mode scan plots the amplitude of the backscattered echoes versus the time after transmission of the ultrasound pulse. With the assumption of a constant speed of sound, time on the x-axis can be presented as distance from the ultrasound transducers
piezoelectric crystal principle
Application of a voltage across a piezoelectric crystal causes the crystal either contract or expand depending upon the polarity of the voltage.
Class absorption coefficient equation:
B(class) = A*f^2 f = ultrasound frequency A = coefficient including coefficient of viscosity, shear viscosity, and thermal conductivity
Focusing Ultrasound
Because the velocity of ultrasound generally is greater in a lens than in the surrounding medium, concave ultrasound lenses are focusing, and convex ultrasound lenses are defocusing.
Relaxation absorption coefficient equation:
Br = (B0*f^2)/(1+(f/fR)^2) max is at fR = 1/(2*pi*tau) demonstrates frequency dependence of relaxation absorption coefficient.
Two major mechanisms of absorption
Classical absorption and relaxation
Linear Sequential Arrays
Consists of a large number, typically 64-512, of rectangular piezoelectric crystals, each having a width of the order of the ultrasound wavelength. •Each crystal is unfocused and physically and electrically isolated from its neighbors. •Almost twice as many scan lines as there are transducer elements can be formed.
Piezoelectric crystals main materials
Crystal Plastic matching layer Damping material See L18 Slide 14
How are 3d ultrasound scans formed>
Currently, 3D ultrasound scans are produced by mechanically or manually scanning a phase-array transducer in a direction perpendicular to the plane of each B-mode scan.
=FOV =Depth
Decreased N decreased LD
What type of ultrasound imaging describes the use of ultrasound to measure blood flow?
Doppler imaging
Why does intensity of reflected waves vary in clinical applications?
Due to interaction with different interfaces of tissues
FWHM
For a spherical focusing lens, the FWHM at the focal point is FWHM~= 1.1*y*R/2a = 1.1*R*V / 2af where R is the radius of the curvature and a is the radius of the lens.
Angle of incidence = Angle of reflection
For any angle of incidence, the angle at which the reflected ultrasound energy leaves the interface equals the angle of incidence of the ultrasound beam;
What range of ulatrsound frequencies is needed for diagnostic imaging?
Frequencies of 1MHz + are needed to furnish wavelengths suitable for imaging. 1MHz = 1,000,000 Hz
Intensity equation
I = power/a I = E/t*a
Ultrasound + sinusoidal motion: Equation for Intensity
Iavg = 1/2*pm*um Where p = pressure, u = particle velocity uz(t) = um*sinwt p(t) = pm*sinwt where w = ultrasound frequency
Processing Ultrasound Echo Signals
It is the presence and degree of acoustic impedance mismatch at interfaces that we wish to image. •The amplitude of an ultrasound echo is governed by the acoustic impedance mismatch at the interface where the echoes originated, the attenuation of intervening tissues, and the amplitude of the ultrasonic pulse that is sent out from the transducer.
When did diagnostic applications of ultrasound begin?
Late 1940's
What is a disadvantage to ultrasound?
Limited acoustic window
What type of wave is ultrasound?
Longitudinal, mechanical wave
What economical advantage does ultrasound have?
Low cost and widely used
What risk does ultrasound pose?
Non ionizing method means no risk of radiation exposure
Beam Geometry of a Single Transducer
Plane-piston- piezoelectric crystal has a flat face; unfocused transducer. • Ultrasound sources may be considered to be a collection of point sources, each radiating spherical wavelets into the medium. Interference of the spherical wavelets establishes a characteristic pattern for the resulting net wavefronts.
Intensity I (watt/cm^2)
Power per unit area
•The power Prof the backscattered signals from the contrast agents received by a transducer is
Pr = IiN(sigma)a^2/4R^2 where R is the distance between the scattererand the transducer, ais the radius of the transducer, N is the number of scatterers, Ii is the intensity of the incident ultrasound beam, and Sigma is the scattering cross section.
The quality factor Q,
Q = 2*pi*f0/BW For a well damped transducer, Q values are between 1 and 2.
What does ultrasound measure
Reflectivity of tissue to sound waves Velocity of moving objects
Ultrasound Image Formation
Scanning Single-element Transducer To get a 2D image, 1D scanning is needed. To get a 3D image, 2D scanning is needed.
SNR ratio
Signal intensity of the echo signals: •The higher the intensity of the ultrasound pulse transmitted by the transducer, the higher the echo signals. •The higher the frequency, the greater is the tissue attenuation, and therefore the lower the signal. •The stronger the focusing at a particular point, the higher is the energy per unit area of the ultrasound wave, and the higher is the echo signal from that point.
Annular Arrays
The annular array consists of a series of piezoelectric elements in the shape of concentric rings or annuli. The beam may be focused at various distances from the transducer face by varying the time delays among excitations of the rings. (left) A side-view of an annular array transducer. (right) Two-dimensional lateral focusing can be performed using an annular array, but the focal point always lies along the principle axis of the beam.
time gain compensation (TGC) circuit.
The attenuation by intervening tissues is usually considered an undesirable factor because it produces falloff of signal intensity with depth without yielding any useful information. This attenuation can be compensated for in the image by use of ___.
Suppose that a block of tissue consists of 2 cm fat, 3 cm muscle (ultrasound propagated parallel to the fibers), and 4 cm liver. What percentage is the total energy loss of an ultrasonic beam at 1 MHz passing through the block of tissue?
The attenuation coefficients at this frequency for fat, muscle and liver are 0.6, 1.2, and 0.9 dB cm-1, respectively. •Total energy loss = (Energy loss in fat) + (Energy loss in muscle) + (Energy loss in liver) = (0.6 dB/cm) (2 cm) + (1.2 dB/cm) (3 cm) + (0.9 dB/cm) (4 cm) = 1.2 dB + 3.6 dB + 3.6 dB = 8.4 dB I = 0.145 I0, i.e., 85.5% of the energy is lost for the ultrasound beam passing through the block of tissue.
Lateral Resolution
The definition of the lateral resolution in terms of the FWHM of the ultrasound beamwidth. (left) The separation between the two shaded objects, Δx, is greaterthan the FWHM of the beam. In this case separate echoes are recorded from the two objects, and they can be resolved. (right) If the beamwidthis wider, then the two objects cannot be resolved since the backscattered echoes from each object are superimposed.
f# and focal distance F
The f# of a lens is defined as where R is the radius of curvature and a is the radius of the lens. • The focal distance F is f# = R/2a where R is the radius of curvature and a is the radius of the lens. • The focal distance F is F~= R / 1- 1/f#
Spatial resolution: axial
The longer the ultrasound pulse, the coarser is the axial resolution of the image. The pulse duration is determined by the degree of damping of the transducer and the operating frequency of the transducer.
Multidimensional Arrays
The operation of a two-dimensional array. The black squares represent the individual crystals in the array. (left) Two-dimensional focusing can be achieved using appropriate time delays applied symmetrically with respect to the individual elements of the array. (right) Electronic beam steering can be performed in two dimensions.
Detection with a homodyne demodulator
The signal detected by the second transducer can be represented as The signal is then mixed with the output of the oscillator by multiplying the two signals together. This signal then passes through a low-pass filter with a cutoff frequency The output from this filter is S(Dop) = 1/2*A*cos(2*pi*deltaf*t) See L21 Slide 17
Detection with a homodyne demodulator (cont.) detail
The signal passes through a high-pass filter to remove high-intensity reflected signals from the relatively slow movement of vessel walls during the cardiac cycle. • Typical values for the cutoff frequency of the high-pass filter are 50-1000 Hz. • Fourier transformation of the time-domain signal gives the frequency spectrum, corresponding to the range of blood velocities. Spectral Doppler shifts from an area encompassing the carotid artery. Large frequency shifts, corresponding to high blood flow rates, are measured during the systolic part of the heart cycle, with lower frequency shifts measured during diastole.
Spatial resolution: lateral
The stronger the focusing, the higher is the spatial resolution at the focal spot. Using the technique of dynamic focusing and beam forming with phased array transducers, one can minimize the depth dependence of the lateral resolution.
Frame rate equation
The time delay between the transmission pulse and the detection of the echo is directly related to the depth of the interface: Time(us) = 2d(cm)/0.154(cm/us) For a 2D image, if the number of A lines is N, the frame rate is Frame Rate = 1 / N*T*10^-6 (s/cm*d(cm))
What was the medical use of ultrasound though the 1930s?
Therapeutic applications
Piezoelectric crystal (detailed) See L18 Slide 16
Time-domain and frequency-domain characteristics of damped transducers. A short pulse of alternating voltage is applied to the face of the crystal. With little mechanical damping, the crystal oscillates for a long time, producing a very sharp frequency spectrum with low bandwidth and high Q. With heavy damping, the crystal oscillations die out quickly, producing a much shorter pulse in the time-domain, and much broader bandwidth in the frequency domain.
How does an ultrasound wave transmit through tissue?
Tissue is modeled as a lattice of particle held together by elastic forces. Energy is trasnmitted through the tissue using a transducer. The tissue expands and contracts (rarefaction and compression) in thickness with a sinusoidal motion. The particles move very short distances (1/10s of nm) about a fixed mean position. Meanwhile the ultrasonic energy propagates over much larger distances.
Describe ultrasound propagation
Ultraouns is propagated by displacement of molecules of a medium into regions of compression and rarefaction This displacement requires energy that is provided to the medium by the source of ultrasound
What do resolution and imaging depth depend on?
Ultrasonic frequency used
Impedance Match • The characteristic acoustic impedance of PZT is about 15 times that of skin. • Assume the ultrasound has a normal incidence to the skin. How many percent of the ultrasound energy is delivered into the skin?
Use TI = 4Z1Z2/(Z1+Z2)^2 TI = 4*15*Zs*Zs/(15Zs+Zs)^2 = 23%
How is intensity measured in acoustics?
Using the decibel (dB) - most appropriate for recording data over a range of many orders of magnitude
con: limit on the highest velocity that can be measured with pulsed-mode doppler measurements
V(rbc,max) = (PRR)V/4*fi PRR = pulse repetition rate
process for piezoelectric crystal
Voltage pulses are applied to the crystal to generate ultrasound pulses. •At the end of each voltage pulse, the crystal has a finite "ring-down time" before it physically comes to rest. •If an appropriate material is used to damp the crystal, the duration of the ultrasound pulse is minimized to be close to the duration of the voltage pulse. •Typically an epoxy substrate impregnated with metal powder can be used as the damping material.
Acoustic Impedance for materials
Z = p*V = 1/sqrt(k*p) --->(kg m^-2 s^-2 or Rayls) See L18 Slide 8 for Z for different materials
acoustic impedance Z
Z = p/ u(z) --->(kg m^-2 s^-2 ,or Rayls) Where P is the pressure (N/m2), u(z) is the particle velocity (m/s). Z = p/u(z) =p*V = sqrt(p/k) --->(kg m^-2 s^-2 or Rayls) V = 1/sqrt(k*p) V is the velocity of the ultrasound wave, k is the compressibility of the medium and p is the density.
Example: What is the length of the Fresnel zone for a 5-mmdiameter, 3-MHz unfocused ultrasound transducer?
Z(nfb) = a^2/y
Digital beam-forming
a reverse process of dynamic focusing during signal transmission. •During the time required for the backscattered echoes to return, incremental delays are introduced to the voltages recorded by each element of the transducer before the signals are passed to the time-gain compensation unit. ------------------------------------------------ The process of dynamic beam-forming. (left) Three backscattered ultrasound waves, E1, E2and E3, arrive at the transducer surface at different times. (top right) As the first echo (E1) reaches the transducer, time delays for the voltages from elements 1-5 are introduced to produce the best lateral resolution at the depth at which E1was formed. (center and bottom right) Values of the time delays are dynamically varied to optimize the lateral resolution for echoes E2and E3.
ultrasonic
a sound wave above 20,000Hz - not audible
infrasonic
a sound wave below 20Hz - not audible
Energy E (J)
ability to do work
What are the three mechanisms for ultrasound attenuation?
absorption, scattering, reflection
specular reflection
allows for visualization of the boundaries between organs
Phased array transducer ultrasound beam axial resolution
along the direction of the beam independent of depth
reflection
an orderly deflection of all or part of the beam
attenuation definition
any mechanism that removes energy from the ultrasound beam
relationship between attenuation coefficient and frequency for most tissues:
approximately linear attenuation of ultrasound energy n biological tissues increases with ultrasound frequency. (despite complexity of frequency dependence of different mechanisms and overall inhomogeneity of tissue)
The bandwidth (BW) of a transducer is usually stated _____.
at the 3-dB level.
resolution at the focal distance
axial > lateral > elevational
Absorption loss
conversion of ultrasound mechanical energy into heat
con: maximum depth that can be studied
d(max) = V/2(PRR)
Equation for decibel
dB = - log(I/I0)
Example: the ultrasound frequency lunched from the transducer is 5 MHz.The RBC flows at a velocity of 50 cm s-1and the flowing direction has an angle of 45 degrees relative to the ultrasound beam. What is the Doppler shift?
data f = fi - fr = 2fi*V(rbc)*cos(theta)/V The Doppler shift is 2.26 kHz.
=LD =Depth
decreased FOV decreased N
=FOV =N =LD
decreased depth
Depth of Focus (DOF)
defined to be the distance over which a standard reflector produces an echo reduced in intensity by 50% from that at the focal point.
Intensity
described as intensity of ultrasound waves sent into the body vs that of the ultrasound reflected back to the surface
What was the industrial use of ultrasound beginning in 1928?
detecting hidden flaws in materials
Peirre and Jacques Curie (French physicists)
discovered the piezoelectric effect in 1880
Resonant frequency of piezoelectric crystal
f0 = V(crystal)/2d There are also resonant frequencies at the odd harmonics of f0, that is 3 f0, 5 f0, 7 f0, etc.
Disadvantage of a strongly focused transducer:
faster diverging beam. • A compromise has to be made between lateral resolution and depth of focus.
Rayleigh scattering equation
generally scattering is extremely complicated and there is no fact mathematical expression however for particles much smaller than the ultrasound wavelength, see L17 Slide 16
Improve Axial Resolution
increase transducer damping increase ultrasound frequency
Ultrasound attenuation coefficiente is the sum of __.
individual coefficients for scatter and absoprtion
The axial resolution
is defined as the closest separation, in the direction of the propagating ultrasound wave, of two scatterers that results in resolvable backscattered signal. t = 2d/v axial resolution = pulse duration*v/2
attenuation in water
little attenuation medium is a very good transmitter of ulatrsound
How to find inverse log?
logb(x) = y b^y = x
Classical Absoprtion
occurs due to friction between particles as they are displaced by the wave
absorption
part of the beam's energy is converted into other forms of energy, such as increase in temperature
scatter
part of the ultrasound beam changes direction in a less orderly fashion
nonsecular reflection
permits visualization of tissue parenchyma
scattering cross-section (sigma(s)) definition
power scattered per unit incident intensity
Power (watt) or (Joules/sec)
rate at which energy is trasnported
Ultrasound Contrast Agent (cont.) •Rayleigh scattering:
sigma(s) = .... where ks is the adiabatic compressibility of the scattererand kis that of the surrounding tissue. ps is the density of the scatterer and p is that of the tissue. •For a gas-filled microsphere, ks>> k and ps<< p, resulting in a very large scattering cross section. •Gas-filled microspheres can act as harmonic oscillators, producing increases in scattering cross section three orders of magnitude greater than their actual geometric cross section.
What was the use of ultrasound during WWII?
sound navigation and ranging
Phased array transducer ultrasound beam lateral resolution & elevational resolution
strongly dependent on depth lateral resolution is determined by transmit and receive focus electronics elevational resolution is determined by the height of the transducer elements
What was the proposed use of ultrasound during WWI?
submarine detection
Explain how the pressure wave and relaxation mechanism lead to the maximum absorption of energy:
the actions are opposite to one another - if the relaxation time of the molecule is of the same order as the period of the ultrasound wave, then at the time of the next positive pressure, the relaxation mechanism is acting to return the articles to their equilibrium position.
Parenchyma
the functional tissue of an organ as distinguished from the connective and supportive tissue
IF the obstacle is large, compared with th wavelength of sound ...
the obstacle is relatively smooth, and the beam retains integrity as it changes direction
If the obstacle size is comparable to or smaller than the wavelength of sound...
the obstacle will scatter energy in various directions
Compression of the particle corresponds to:
the passage of the positive half of the pressure wave, which forces the particles together
relaxation time, tau
time taken for a molecule to return to its original position after being displaced by a wave
How do ultrasound imaging work? Basic principle
ultrasound transducer transmits mechanical energy into body. energy backscatters from tissue boundaries and small structures and is detected by the ultrasound the body is scanned with several adjacent ultrasound beams to form an image.
Equation for particle velocity of z direction
uz = dW/dt where W = particle displacement (1/10s of nm) uz usually 1-10 cm/s
attenuation in bone
very high attenuation also a large reflection coefficient at the tissue bone interface difficult to visualize structure lying behind bone
What is the solution to the high coefficient of ultrasound reflection at the air-tissue interface?
water or and various creams are used during ultrasound examinations to remove air pockets (i.e., to obtain good acoustic coupling) between the ultrasound transducer and the patient's skin.
Advantage to 3d imaging
• 3D information gives more accurate measures of, for example, tumor volume or tissue malformations.
Lateral Resolution (cont.)
• Because a single-crystal transducer typically has a diameter of between 1 and 5 cm, the intrinsic lateral resolution is poor. • Beam focusing is needed to improve lateral resolution. • Either using a concave lens or fabricating a crystal with a curved face.
Intensity Transmission Coefficient
• For an ultrasound wave incident perpendicularly upon an interface, the fraction of the incident energy that is transmitted across an interface is described by the intensity transmission coefficient, TI = (4*Z1*Z2) / (Z2+Z1)^2 where Z1 and Z2 are the acoustic impedances of the two media. TI + RI = 1
Doppler Effect
• The "apparent" frequency fRBC of the transmitted ultrasound beam as seen by the RBC is given by The wavelength of the ultrasound is independent of the velocity of the RBC: Because the transmission and backscattered paths must both be considered, the overall Doppler shift Δf of the received signal is given by See L21 Slide 13
Snell's law
• The relationship between incident and refraction angles: sin thetai = V1 ____________________ sin thetat = V2
Time gain compensation detailed process
•(left) The dynamic range of backscattered echoes is larger than can be amplified linearly. •(center) A time-dependent amplification factor is applied to the backscattered echoes. •(right) The dynamic range of the amplified signals has been reduced to a level appropriate for further processing.
•Example: Muscle has an impedance of 1.70 MRaylsand air has an impedance of 400 Rayls. What percentage of the ultrasound energy is reflected at the air-liver interface?
•99.91% of the ultrasound energy is reflected at the air-liver interface, and only 0.09% of the energy is transmitted.
Artifacts in Ultrasonic Imaging shadowing and enhancement
•Acoustic shadowing occurs when either a very strong reflector such as a gas/tissue boundary or a highly attenuating medium "shadows" a deeper lying organ. •Accousticshadowing results a dark area or "hole" in the image. •The opposite phenomenon, known as acoustic enhancement, occurs when a region of low attenuation, such as a gallbladder, is present within an otherwise homogeneous medium. •The areas behind such tissues have higher than expected intensity.
Ultrasound instrumentation
•An ultrasound transducer converts electrical energy into mechanical (ultrasound) energy for transmitting ultrasound signals. •An ultrasound transducer can also converts mechanical (ultrasound) energy into electrical energy for receiving ultrasound echoes. •An ultrasound transducer is based on piezoelectric materials, e.g. lead zirconatetitanate(PZT) crystal.
Frame Rate
•B-mode imaging normally can be acquired rapidly. •For example, in the case of an image with a 10-cm depth-of-view, it takes 130 μs after transmission of the ultrasound pulse for the most distant echo to return to the transducer. •If the image consists of 120 lines, then the total time to acquire one frame is 15.6 ms and the frame rate is 64 Hz. •At image frequencies greater than approximately 24 Hz, the motion of moving structures seems continuous, even though it may appear that the images are flickering. Images that are refreshed at frequencies greater than approximately 48 Hz are free of flicker.
Pulsed-Mode Doppler Measurements (cont.)
•Calculation of the blood velocity relies on the fact the time delay between the transmitted pulse and the backscattered signal decreases if the blood is flowing toward the transducer and increases if the blood is flowing away from the transducer. After each pulse in a pulse-train a backscattered signal (S1, S2....S5) is recorded. The signal amplitude at a particular depth, corresponding to the dotted line in the top figure, is plotted as a function of the time after the initial pulse. Fourier transformation of this plot results in the Doppler frequency, and hence blood velocity, distribution at the chosen location.
Intensity Reflection Coefficient:
•For an ultrasound wave incident perpendicularly upon an interface, the fraction RI of the incident energy that is reflected RI = [ (Z2-Z1)/(Z2+Z1) ]^2 where Z1 and Z2 are the acoustic impedances of the two media.
relationship between damping, Q, BW, pule duration, axial resolution
•Higher damping-->Lower Q -->larger BW -->shorter pulses -->higher axial resolution.
B (Brightness)-mode
•In B-mode presentation of pulse echo images the location of echo-producing interfaces is displayed in two dimensions. •Each line in the image consists of an A-mode scan with the brightness of the signal being proportional to the amplitude of the backscattered echo. •Regions in the patient that are more echogenic correspond to regions in the image that are brighter.
Pros and Cons of Continuous Wave Doppler Measurements
•No limitation on the maximum velocity that can be measured. •Low depth selectivity: The overlapped region of the sensitive regions of the two transducers defines the area in which blood flow is detected. This area is typically quite large. •Spectral broadening of the frequencies occurs with a large sample area across the vessel profile. •Problems in interpretation can occur when there is more than one blood vessel within this region.
Blood Velocity Measurements Using Ultrasound
•Noninvasive, localized blood velocity measurements are vital in the diagnosis of a number of diseases. •Based on the backscattered echoes from red blood cells. -Scattered at all different angles -Weak signals -Signals intensity is proportional to the fourth power of the ultrasound frequency.
•At a muscle (Z = 1.70 MRayls)-liver (Z = 1.65 MRayls) interface, what percentage of the ultrasound energy is reflected?
•Only 0.02% of the incident energy is reflected, and about 99.98% of the energy is transmitted across the interface.
Pulsed-Mode Doppler Measurements
•Only one transducer is used. •It transmits pulses and receives backscattered signals a number of times in order to estimate the blood velocity.
Artifacts in Ultrasonic Imaging reverb and reflection
•Reverberations occur if there is a very strong reflector close to the transducer surface. •Multiple reflections occur between the surface of the transducer and the reflector. •These reflections appear as a series of repeating lines in the image. •Typically, they occur when ultrasound interacts with either bone or air.
Limitation of Doppler imaging
•The Doppler shift can be increased by using higher ultrasound frequencies, but in this case the maximum depth at which vessels can be measured decreases.
M (motion)-mode
•The M-mode presentation of ultrasound images is designed specifically to depict moving structures. In an M-mode display, the position of each echo-producing interface is presented as a function of time. •The most frequent application of M-mode scanning is echocardiography, where the motion of various interfaces in the heart is imaged.
Limitations of time gain compensation
•The attenuation of different body parts and variations from one patient to another render a single factory setting of the TGC impractical. The TGC must be adjusted for each patient and readjusted during the scan as different tissues are encountered.
Ultrasound Contrast Agent
•The detection of blood flow in small vessels deep in tissue is very difficult due to the low SNR of the Doppler signal. •Contrast agents usually consist of gas-filled microspheres with diameter less than 10 μm.
Noise floor:
•The electronic noise of the detection system, especially due to the large amplification of weak signals. •Speckle due to coherent wave interference in tissue. Speckle gives a granular appearance to what should appear as a homogeneous tissue. •Clutter due to signals arising from side lobes, grating lobes, multipath reverberation, tissue motion, etc.
Pros and Cons of Pulsed-Mode Doppler Measurements
•The major advantage of pulsed-mode over CW Doppler is the ability to measure Doppler shifts in a specific region. •The detection volume can be chosen by: -Transducer diameter and focusing scheme; -The time delay between the pulse is transmitted from the transducer and the time starting to acquire backscattered signals; -The time duration for which the signal is acquired.
Continuous Wave Doppler Measurements
•The transducers are fabricated with only a small degree of mechanical damping. •A continuous wave of ultrasound is transmitted by one transducer and the backscattered signal is detected by a second one. small area of overlap
Diagnostic Scanning Modes
•Three basic modes: A-mode, B-mode, and M-mode.
Ultrasound imaging limitations
•Ultrasound is not suitable for imaging lung or tissues behind a bone.
