Ultrasound
Ultrasound interaction with matter
Reflection-boundaries of acoustic impedance mismatch Refraction-change in direction with nonperpendicular incidence on a boundary Scattering-by reflection & refraction with small features Attenuation-loss of intensity of the ultrasound beam from absorption and scattering in the medium Absorption-loss through conversion to heat
Doppler shift
The Doppler shift is the difference between the incident frequency and reflected frequency. f_d = f_r - f_i = (2*(f_i)*v)/(v+c) where c is speed of sound and v is the reflector velocity. The angle between the direction of blood flow and the direction of the sound is called the Doppler angle. The general doppler shift formula is f_d = (2*(f_i)*v*costheta)/(c) The preferred Doppler angle ranges from 30 to 60 degrees. At too large an angle (greater than 60 degrees), the apparent Doppler shift is small, and minor errors in angle accuracy can result in large errors in velocity. At too small an angle (e.g., less than 20 degrees), refraction and critical angle interactions can cause problems, as can aliasing of the signal in pulsed Doppler studies.
Reverberation Artifact
This is caused by multiple echoes generated between two closely spaced interfaces reflecting ultrasound energy back and forth during the acquisition of signal and the next pulse. Only the first reflection is spatially correct. These are commonly found between a highly reflective surface and a transducer OR between reflective interfaces such a metallic objects, calcified tissues, or air pocket/ partial liquid areas of the anatomy. Reverberation echoes are typically manifested as multiple, equally spaced boundaries with decreasing amplitude along a straight line from the transducer. Reverberation artifacts can be improved by changing the angle of insonation so that reverberation between strong parallel reflectors cannot occur. Comet tail is a form of reverberation artifact
Aliasing Artifact
- Produces wrap-around effect where peaks are cut off and appear on the opposite side of the baseline. - Color Doppler: shows as region of reversed flow within region of highest velocity. Aliasing may be a useful indicator of higher velocity flow
Underlying assumptions of artifacts
- Straight line propagation from transducer to reflector and back. - Attenuation invariant with tissue type. - Beam dimensions are small in both lateral and elevational directions. - All detected echoes originate from the axis of the main beam alone. - All detected echoes produced from the last transmitted pulse. - Propagation at a constant 1540 m/s in all tissues. - Each reflector contributes a single echo for a given scan line.
Gain Setting Errors
- There are two ways to get into trouble with this. One is the overall gain setting, and the other is incorrect setting for TCG. - In either case, the outcome is the generation of artifactual echoes.
Total reflection
A situation called total reflection occurs when c2 > c1 and the angle of incidence of the sound beam with the boundary between two media exceeds an angle called the critical angle. In this case, the sound beam does not penetrate the second medium at all but travels along the boundary. Critical angle is calculate by setting theta t = 90 in snells law. Therefore sin theta c = c1/c2
Using c = sqrt(B/rho), explain why the speed of sound is faster in bone than air
B represent resistance to compression. Bone has higher resistance to compression. So the higher the B, the higher the c. Vice versa for air.
Receive Focus
Echoes received at the edge of the element array travel a slightly longer distance than those received at the center of the array, particularly at shallow depths. Signals from individual transducer elements therefore must be rephased to avoid a loss of resolution when the individual signals are synthesized to make an image Dynamic receive focusing is a method to rephase the signals by dynamically introducing electronic delays as function of depth (time).
In a single transducer element
From superficial to deep, we have Metal shield Acoustic absorber Backing block PZT Matching layer
Intensity
Intensity, I, is the amount of power (energy per unit time) per unit area and is proportional to the square of the pressure amplitude. Medical diagnostic ultrasound intensity levels are described in units of milliwatts/cm2
Transducer
Intracavitary (all of them) transducers for high frequency/high resolution imaging of structures near cavity walls (e.g. prostate). Intravascular ultrasound (IVUS) transducers on catheters for vessel imaging.
acoustic impedance
Resistance a material provides to the passage of sound waves. The acoustic impedance can be likened to the stiffness and flexibility of a compressible medium Z = density * c Minor differences in Z allow continued wave propagation with little reflection at the boundary Large difference in Z (stiffness) results in large reflection of energy (e.g. a spring attached to a wall)
Speckle Artifact
Speckle artifact may be encountered in ultrasound. It is caused by the scattering of waves from the surface of small structures within a certain tissue. The artifact produces a textured appearance.
Speed of sound
The speed of sound is the distance traveled by the wave per unit time and is equal to the wavelength divided by the period
Transmit/Receive Switch
The transmit/receive switch, synchronized with the pulser, isolates the high voltage associated with pulsing (~150 V) from the sensitive amplification stages during receive mode, with induced voltages ranging from approximately 1 V to 2 μV from the returning echoes. Higher intensity = better SNR, but more heat deposition in tissue
Summary of three doppler displays
-Color: velocity & direction color coded. Black corresponds to zero velocity. Red/yellow if flow towards transducer, blue/green is away. Brighter colors = higher velocity. - Spectrum: plot of velocity vs time. V>0 is towards transducer. - Duplex: Combines time series with grayscale.
Contrast resolution is highest for:
1. Variables which maximize the strength of an acoustic echo: Objects with very different acoustic impedance to their surroundings 2. Variables which reduce partial volume averaging: Higher spatial resolution Higher temporal resolution - minimizing impact of motion 3. Variables which maximize signal-to-noise ratio: Use of higher ultrasound power requiring less electronic signal amplification Superficial objects subject to less attenuation and returning scatter
Relative intensity (dB)
10 log (I2/I1) When the intensity ratio is greater than one (e.g., the incident ultrasound intensity to the detected echo intensity), the dB values are positive; when less than one, the dB values are negative. A loss of 3 dB (−3 dB) represents a 50% loss of signal intensity. The tissue thickness that reduces the ultrasound intensity by 3 dB is considered the "half-value" thickness (HVT)
What are the speed of sounds for soft tissue, fatty tissue and air
1540, 1450, 330m/s
Infrasound range Audible range Ultrasoung range Medical ultrasound range
<15 Hz 20 Hz to 20 kHz Ultrasound >20 kHz 2 MHz to 10MHz, with specialized ultrasound up to 50 MHz
Highest temporal resolution is achieved with:
A low number of A-lines A low depth of scan
Transducer Phased Array
A phased-array transducer is usually comprised of 64 to 128 individual elements in a smaller package than a linear array transducer. All transducer elements are activated nearly simultaneously to produce a single ultrasound beam. By using time delays in the electrical activation of the discrete elements across the face of the transducer, the ultrasound beam can be steered and focused electronically without physically moving the transducer on the patient. During ultrasound signal reception, all of the transducer elements detect the returning echoes from the beam path, and sophisticated detection algorithms synthesize the data to form the image.
A-mode
A-mode (A for amplitude) is the display of the processed information from the receiver versus time. A-mode and A-line information is currently used in ophthalmology applications for precise distance measurements of the eye
Axial resolution
Ability to discern two closely spaced objects in the direction of the beam. Achieving good axial resolution requires that the returning echoes be distinct without overlap. The minimal required separation distance between two reflectors is one-half of the SPL to avoid the overlap of returning echoes. Objects spaced closer than ½ SPL will not be resolved. Axial resolution remains constant with depth. Shorter pulses (i.e., shorter SPL) means better axial resolution
Scattering
Acoustic scattering arises from objects and interfaces within a tissue that are about the size of the wavelength or smaller and represent a rough or nonspecular reflector surface. A specular reflector is a smooth boundary between two media, where the dimensions of the boundary (i.e., acoustic impedance) are much larger than the wavelength of the incident ultrasound energy. Hyperechoic (higher scatter amplitude) and hypoechoic (lower scatter amplitude). Acoustic scattering from nonspecular reflectors increases with frequency, while specular reflection is relatively independent of frequency.
Velocity aliasing
Aliasing is an error caused by an insufficient sampling rate (PRF) relative to the high-frequency Doppler signals generated by fast-moving blood. Need minimum of two samples per cycle at Δf for highest velocity to avoid aliasing. Many scanners adjust PRF automatically when operator selects velocity range. If at max PRF, can shift baseline (v=0) to take advantage of asymmetric velocity profile.
Range Ambiguity Artifact
Ambiguity artifacts are created when a high PRF limits the amount of time spent listening for echoes during the PRP. As the PRF increases, the PRP decreases, with returning echoes still arriving from a greater depth after the next pulse is initiated. Mis-mapping of very deep echoes to shallow positions can occur in the image.
B-mode
B-mode (B for brightness) is the electronic conversion of the A-mode and A-line information into brightness-modulated dots along the A-line trajectory. In general, the brightness of the dot is proportional to the echo signal amplitude
Doppler spectral interpretation
Blood flow can exhibit laminar, blunt, or turbulent flow patterns, depending upon the vessel wall characteristics, the size and shape of the vessel, and the flow rate. Fast, laminar flow exists in the center of large, smooth wall vessels, while slower blood flow occurs near the vessel walls, due to frictional forces. Turbulent flow occurs at disruptions in the vessel wall caused by plaque buildup and stenosis. For pulsatile flow, spectral content is a function of time. Fourier transform used to decompose frequency content to produce Doppler spectrum. Spectrum continuously updated and displayed with 2D B-mode image as moving trace with v on the vertical axis. Disturbed and turbulent flow produce Doppler spectra that are correlated with disease processes. Pulsative index (PI) = (V_max - V_min) / (V_avg) Resistive index (RI) = (V_max - V_min) / (V_max) RI = 0: Continuous flow. RI = 1: Forward (systolic) flow only. RI > 1: Reverse (diastolic) flow present (e.g. heart valve regurgitation). For renal artery, RI = 0.7 considered upper limit for normal range (typical is 0.6). PI normal range is 1.36 - 1.56.
Speed Displacement Artifact
Can be identified as an area of focal discontinuity and displacement of an echo deeper than that its actual position in an imaged structure. If there is differential variation in tissue composition (i.e fatty tissue with 1450m/s) of the tissues under the same ultrasound beam, then different return times to the transducer will be processed as different depths of tissue as opposed to differences in propagation velocity between the tissues. This may result in discontinuity in the displayed ultrasound image, and as such is referred to as a propagation velocity misrepresentation. - If actual c > 1540 m/s, reflector position will be closer to transducer than it should be. - If actual c < 1540 m/s, reflector will appear deeper than it should.
Color flow imaging
Color flow imaging provides a 2D visual display of moving blood in the vasculature, superimposed upon the conventional gray-scale image. Shades of red for blood moving toward the transducer, and shades of blue for blood moving away from the transducer. Two dimensional color flow systems do not use the full Doppler shift information because of a lack of time and/or a lack of parallel channels necessary for real-time imaging. Instead, phase-shift autocorrelation or time domain correlation techniques are used. Phase-shift autocorrelation is a technique to measure the similarity of one scan line measurement to another when the maximum correlation (overlap) occurs. Use 4 - 8 traces to determine motion along one Aline. Interleave with B-mode image acquisition. Smaller FOV allows faster frame rate but limits region available for flow evaluation Time domain correlation is an alternate method for color flow imaging. It is based upon the measurement that a reflector has moved over a time delta-t between consecutive pulse-echo acquisitions. Overlap signals with time shifts to find greatest correlation Displacement of the reflector; delta_x = (c*delta_t) / 2 Measured velocity; V_m = delta_x /PRP Correction for the doppler angle; V = (V_m) / cos theta With time domain correlation methods, shorter pulses can be used as compared to Doppler processing methods where narrow bandwidth is necessary. Short pulses allow for better axial resolution. Time domain is less prone to aliasing effects since greater time shifts can be tolerated, allowing for higher Vmax There are several limitations with color flow imaging 1. Noise/clutter from low v structures can overwhelm small echoes from blood cells 2. Low spatial resolution compared to reference images 3. Not as accurate as full Doppler process 4. Some aliasing still possible for very high v (stenosis)
Ultrasound Transducer
Comprised of ceramic element which converts electrical energy into mechanical energy to produce ultrasound and mechanical energy into electrical energy for ultrasound detection.
Angle of refraction
Determined by Snell's law sin theta t / sin theta i = c2/c1 When c2 > c1, the angle of transmission is greater than the angle of incidence. And vice versa
Doppler
Doppler ultrasound is based on the shift of frequency in an ultrasound wave caused by a moving reflector, such as blood cells in the vasculature. The moving reflectors in the body are the blood cells. By comparing the incident ultrasound frequency with the reflected ultrasound frequency from the blood cells, it is possible to discern the velocity of the blood. Quantitative for both direction and magnitude of velocity. Information usually presented as color maps showing direction and velocity
Duplex scanning
Duplex scanning refers to the combination of 2D B-mode imaging and pulsed Doppler data acquisition. Without visual guidance to the vessel of interest, pulsed Doppler systems would be of little use. Doppler angle, distance to gate region determined from image geometry. Volume flow rate (cm3/s) determined from vessel area and linear flow rate. Possible sources of error include vessel diameter, Doppler angle, vessel axis out of scan plane, misplacement of gate boundaries
Ultrasound contrast agents
Encapsulated microbubbles containing air, nitrogen or insoluble gases (i.e perfluorocarbon). Encapsulation materials, such as human albumin, provide a container for the gas to maintain stability for a reasonable time in the vasculature after injection. Ultrasound contrast agent are used for vascular(evaluate blood vessel) and perfusion imaging (evaluate blood flow) - X-ray contrast agents modify attenuation since attenuation differences produce contrast. - MRI contrast agents modify signal decay rates (T1, T2, T2*) since signal decay rates produce contrast. - Ultrasound contrast agents modify acoustic impedance. The basis for generating an ultrasound signal is the large difference in acoustic impedance between the gas and the fluids and tissues, as well as the compressibility of the bubbles compared to the incompressible materials that are displaced. Bubbles are small compared to λ and represent point sources reflecting in all directions. Contrast agents require use of acquisitions other than standard B-mode, e.g. pulse inversion harmonic imaging.
Transmit Focus
For a single transducer or group of simultaneously fired transducers in a linear array, the focal distance is a function of the transducer diameter, the center operating frequency, and the presence of any acoustic lenses attached to the element surface. This focal depth is unchangeable. Phased array transducers and many linear array transducers allow a selectable focal distance by applying specific timing delays between transducer elements that cause the beam to converge at a specified distance. A shallow focal zone (close to the transducer surface) is produced by firing outer transducers in the array before the inner transducers in a symmetrical pattern, as shown. Greater focal distances are achieved by reducing the delay time differences amongst the transducer elements, resulting in more distal beam convergence
Which of the following changes with media properties? frequency, wavelength or velocity
Frequency does not change with media properties, only wavelength and velocity change. The spatial resolution of the ultrasound image depend on the wavelength. The attenuation of the ultrasound beam energy depend on the frequency. A high-frequency ultrasound beam (small wavelength) provides better resolution and image detail than a low-frequency beam; however, the depth of beam penetration is significantly reduced at higher frequency. For body parts requiring greater travel distance of the sound waves (e.g., abdominal imaging), lower frequency ultrasound is used (3.5 to 5 MHz) to image structures at significant depths. For small body parts or organs close to the skin surface (e.g., thyroid, breast), higher frequency ultrasound is selected (7.5 to 10 MHz).
Harmonic imaging
Harmonic frequencies are integral multiples of the frequencies contained in an ultrasound pulse. Harmonic imaging enhances contrast agent imaging by using a low frequency incident pulse and tuning the receiver (using a multifrequency transducer) to higher frequency harmonics. These higher frequencies arise through the vibration of encapsulated gas bubbles used as ultrasound contrast agents or with the nonlinear propagation of the ultrasound as it travels through tissues. Even though the returning harmonic signals have higher attenuation (e.g., the first harmonic will have approximately twice the attenuation coefficient compared to the fundamental frequency), the echoes have to only travel half the distance as the originating ultrasound pulse and thus have a relatively large signal. Useful in abdominal imaging where a low transmit frequency is used for adequate penetration with higher frequency receive for less surface clutter. The returning echoes comprising the harmonics travel only slightly greater than one-half the distance to the transducer and, despite the higher attenuation, have less but substantial amplitude compared to the fundamental frequency. The first harmonic (twice the fundamental frequency) is commonly used because it suffers less attenuation than higher order harmonics and because higher order harmonics are likely to exceed the transducer's bandwidth. Generally use long SPL for narrow bandwidth and easier separation of transmit frequency echoes from harmonics. Some degradation of axial resolution with long SPL, but benefits of unambiguous detection of contrast agent overcome the resolution issue. Improved lateral spatial resolution (a majority of the echoes are produced in the central area of the beam), reduced side lobe artifacts, and removal of multiple reverberation artifacts caused by anatomy adjacent to the transducer are some advantages of tissue harmonic imaging.
Image data aquisition
Image formation using the pulse-echo approach requires a number of hardware components: the beam former, pulser, receiver, amplifier, scan converter/image memory, and display system
Preamplification and Analog-to-Digital Conversion
In multielement array transducers, all preprocessing steps are performed in parallel. Each transducer element produces a small voltage proportional to the pressure amplitude of the returning echoes. An initial preamplification increases the detected voltages to useful signal levels. This is combined with a fixed swept gain, to compensate for the exponential attenuation occurring with distance (time) traveled. Fixed swept gain is also know as the Time Gain Compensation (TGC) In state-of-the-art ultrasound units, each piezoelectric element has its own preamplifier and ADC. Sampling rates run 20-40MHz with 8 to 12 bits of precision
Pulse-echo operation
In the pulse-echo mode of transducer operation, the ultrasound beam is intermittently transmitted, with a majority of the time occupied by listening for echoes. The time delay between the transmission pulse and the detection of the echo is time = (2*distance) / 1540 where d = distance from the transducer to the reflector. One pulse-echo sequence produces one amplitude- modulated (A-line) of image data. The number of times the transducer is pulsed per second is known as the pulse repetition frequency (PRF). The time between pulses is the pulse repetition period (PRP), equal to the inverse of the PRF. An increase in PRF results in a decrease in echo listening time. The maximum PRF is determined by the time required for echoes from the most distant structures to reach the transducer The maximal range a pulse can travel is determined from the product of the speed of sound and the PRP divided by 2; Maximal range = 1540 * PRP *(1/2) The ultrasound frequency is calibrated in MHz, whereas PRF is in kHz, and the ultrasound period is measured in microseconds compared to milliseconds for the PRP Pulse duration is the ratio of the number of cycles in the pulse to the transducer frequency and is equal to the instantaneous "on" time Pulse duration = (no of cycles in the pulse / transducer frequency) Duty cycle, the fraction of "on" time, is equal to the pulse duration divided by the PRP. Duty cycle = (Pulse duration / PRP)
Transmission coefficient
Intensity transmission coefficient is defined as the fraction of the incident intensity that is transmitted across an interface. Transmission coefficient = 1-reflection coefficient
Piezoelectric materials
It converts electrical energy into mechanical (sound) energy by physical deformation of the crystal structure. Piezoelectric materials are characterized by a well-defined molecular arrangement of electrical dipoles. When mechanically compressed by an externally applied pressure, the alignment of the dipoles is disturbed from the equilibrium position to cause an imbalance of the charge distribution. A potential difference (voltage) is created across the element with one surface maintaining a net positive charge and one surface a net negative charge. Surface electrodes measure the magnitude of voltage, which is proportional to the incident mechanical pressure amplitude. Example of natural PZT material is quartz. Ultrasound transducers for medical imaging applications employ a synthetic piezoelectric ceramic, most often lead-zirconate-titanate (PZT)—a compound with the structure of molecular dipoles
Electronic Scanning and Real-Time Display
Linear and curvilinear array transducers produce rectangular and trapezoidal images, respectively. They are typically composed of 256 to 512 discrete transducer elements of ½ to 1 wavelength width each in an enclosure from about 6 to 8 cm wide Advantages of the linear array are the wide FOV for regions close to the transducer and uniform, rectangular sampling across the image. Phased-array transducers are typically comprised of a tightly grouped array of 64, 128, or 256 transducer elements in a 3- to 5-cm-wide enclosure Spatial compounding is a method in which ultrasound information is obtained from several different angles of insonation and combined to produce a single image. The resultant compound image improves image quality. Speckle noise, a random source of image variation, is reduced by the averaging process of forming the compound image, with a corresponding increase in signal-to-noise ratio.
Transducer Linear Arrays
Linear array transducers typically contain 256 to 512 elements. In operation, the simultaneous firing of a small group of approximately 20 adjacent elements produces the ultrasound beam. Echoes are detected in the receive mode by acquiring signals from most of the transducer elements. A rectangular field of view (FOV) is produced with this transducer arrangement. For a curvilinear array, a trapezoidal FOV is produced.
Nonresonance (Broad bandwidth) "Multifrequency" Transducers
Modern transducer design coupled with digital signal processing enables "multifrequency" or "multihertz" transducer operation, whereby the center frequency can be adjusted in the transmit mode. Unlike the resonance transducer design, the piezoelectric element is intricately machined into a large number of small "rods" and then filled with an epoxy resin to create a smooth surface. Provides greater transmission frequency. Broadband multifrequency transducers have bandwidths that exceed 80% of the center frequency. Excitation of the multifrequency transducer is accomplished with a short square wave burst of approximately 150 V with one to three cycles, unlike the voltage spike used for resonance transducers. Likewise, the broad bandwidth response permits the reception of echoes within a wide range of frequencies.
Multipath Reflection and Mirror Image
Near highly reflective surfaces, multiple beam reflections and refractions can find their way back to the transducer. A common example is the interface of the liver and the diaphragm in abdominal imaging. The pulse from the transducer generates echoes from a mass in the liver and continues to the diaphragm, where a very strong echo is produced. This echo travels from the diaphragm back to the mass, producing another set of echoes now directed back to the diaphragm. These echoes are reflected from the diaphragm to the transducer. The back and forth travel distance of the second echo set from the mass produces an artifact in the image that resembles a mirror image of the mass, placed beyond the diaphragm A multipath artefact is an ultrasound beam artefact in which the primary beam reflects off anatomy at an angle, resulting in a portion of the beam returning to the transducer, whilst another portion takes a longer duration as it reflects a second structure. This phenomenon results in a propagation path error in which the transducer will interpret a structured to be deeper than it is. This artifact causes image degradation and does not appear as a discrete artefact. Common example involves the liver and diaphragm
Pressure amplitude is defined
Pressure amplitude is defined as the peak maximum or peak minimum value from the average pressure on the medium in the absence of a sound wave
Pulsed doppler operation
Pulsed Doppler ultrasound combines the velocity determination of continuous wave Doppler systems and the range discrimination of pulse-echo imaging. SPL is longer (5 - 25 cycles per pulse) for higher Q to improve accuracy (smaller bandwidth) at the expense of axial resolution. Depth selection is achieved with an electronic time gate circuit to reject all echo signals except those falling within the gate window, as determined by the operator. Single pulses do not provide adequate information to compute fd. Multiple pulses (at PRF) used to sample phase over time. Must have sampling rate (PRF) at least twice the maximum f_d to prevent aliasing The maximum Doppler shift f_max that is unambiguously determined in the pulsed Doppler acquisition is f_max = PRF/2 =(2*(f_0)*v_max*costheta)/(c) For Doppler shift frequencies exceeding one-half the PRF, aliasing will occur, causing a potentially significant error in the velocity estimation of the blood. Thus, a 1.6-kHz Doppler shift requires a minimum PRF of 2 * 1.6 kHz =3.2 kHz.
Lateral resolution or azimuthal resolution
Refers to the ability to discern as separate two closely spaced objects perpendicular to the beam direction. The beam diameter determines the lateral resolution. The lateral resolution is depth dependent. The best lateral resolution occurs at the near field-far field interface. At this depth, the effective beam diameter is approximately equal to ½ the transducer diameter. In the far field, the beam diverges and substantially reduces the lateral resolution. Lateral resolution of linear & curvilinear arrays can be varied by selecting # of elements used for transmit
Resonant Transducers
Resonance transducers transmit and receive preferentially at a single "center frequency". Resonance transducers for pulse-echo ultrasound imaging operate in a "resonance" mode, whereby a voltage (usually 150 V) of very short duration (a voltage spike of ~1 micro s) is applied, causing the piezoelectric material to initially contract and then subsequently vibrate at a natural resonance frequency. The natural resonance frequency is determined by the thickness of the transducer thickness equal to 1/2 wavelength. Higher frequencies are achieved with thinner elements, and lower frequencies with thicker elements.
Grating lobe
Result is low energy coherent wave at a large angle away from the main beam. This misdirected energy of relatively low amplitude can result in the appearance of highly reflective objects in the main beam.
Ringdown Artifact
Ring down artifact is a special type of resonance artifact. Its appearance is similar to the ladder-like reverberation of comet-tail artifact, but it is produced by a completely different mechanism. The artifact is only associated with gas bubbles. This artifact can be eliminated by angling the ultrasound probe.
Shadowing and Enhancing Artifact
Shadowing is a hypo-intense signal caused by high attenuation objects such as bones and kidney stones. These high attenuating objects reduce the intensity of the transmitted beam which returns low signal which causes shadowing artifacts. Enhancement artifact is a hyper-intense signal caused by low attenuating objects such as fluid filled cyst. These low attenuating objects increase the intensity of the transmitted beam which returns high signal which causes artifacts.
Side Lobes and Grating Lobes
Side lobes are emissions of the ultrasound energy that occur in a direction slightly off axis from the main beam and arise from the expansion of the piezoelectric elements orthogonal to the main beam. Grating lobes occur with multielement array transducers and result from the division of a smooth transducer surface into a large number of small elements. This misdirected energy can create ghost images of off-axis high-contrast objects. Grating lobes are similar but are due to interactions in the transducer elements when the size of the elements is greater than 1/2 the wavelength (so these can be removed by keeping elements small) Linear array transducers are more prone to grating lobe artifacts than phased-array transducers, chiefly due to the larger width and spacing of the individual elements. Can be reduced by reducing excitation voltage to outer elements in arrays, variation of transducer angle, element size.
Side lobe
Side lobes are unwanted emissions of ultrasound energy directed away from the main pulse, caused by the radial expansion and contraction of the transducer element during thickness contraction and expansion. In continuous mode operation, the narrow frequency bandwidth of the transducer (high Q) causes the side lobe energy to be a significant fraction of the total beam. In pulsed mode operation, the low Q, broadband ultrasound beam produces a spectrum of acoustic wavelengths that reduce the emission of side lobe energy. By keeping the individual transducer element widths small (less than ½ wavelength), the side lobe emissions are reduced. Another method to minimize side lobes with array transducers is to reduce the amplitude of the peripheral transducer element excitations relative to the central element excitations.
Real-Time Ultrasound Imaging
The 2D image (a single frame) is created from a number of A-lines, N (typically 100 or more), acquired across the FOV. Larger N improves image quality, but limits temporal resolution (frame rate). Frame rate is the frequency at which frames in a television picture, film, or video sequence are displayed. Larger D requires increased PRP, which for given N reduces temporal resolution. This is because the larger the distance between the transducer and the reflector, the longer the time it takes to take a round trip, the longer the time between pulses. Acquisition time for each line T_line = 13(microsec/cm) * D(cm) Time per frame N * T_line = N * 13(microsec/cm) Frame rate per second Frame rate = 1/T_frame In general, to get higher frame rates, need to reduce N and/or D Spatial sampling decreases with increasing D for sector and trapezoidal scans, and remains constant for rectangular format scans Another factor that affects frame rate is transmit focusing, whereby the ultrasound beam (each A-line) is focused at multiple depths for improved lateral resolution. The frame rate will be decreased by a factor approximately equal to the number of transmit focal zones placed on the image, since the beam former electronics must transmit an independent set of pulses for each focal zone
Spatial Pulse Length (SPL)
The SPL is the number of cycles emitted per pulse by the transducer multiplied by the wavelength
Spatial resolution
The axial, lateral, and elevational (slicethickness) dimensions determine the minimal volume element. Each dimension has an effect on the resolvability of objects in the image.
Capacitive Micromachined Ultrasonic Transducers (CMUTs)
The basic element of a CMUT is a capacitor cell with a fixed electrode (backplate) and a free electrode (membrane). The principle of operation is electrostatic transduction, whereby an alternating voltage is applied between the membrane and the backplate, and the modulation of the electrostatic force results in membrane vibration with the generation of ultrasound. Conversely, when the membrane is subject to an incident ultrasound wave, the capacitance change can be detected as a current or a voltage signal. The main advantages of CMUT arrays compared to PZT are better acoustic matching with the propagation medium, which allows wider bandwidth capabilities, improved resolution, potentially lower costs with easier fabrication, and the ability to have integrated circuits on the same "wafer". CMUT arrays feature improved axial resolution;
Beam Former
The beam former is responsible for generating the electronic delays for individual transducer elements in an array to achieve transmit and receive focusing and, in phased arrays, beam steering.
Damping Block
The damping block, layered on the back of the piezoelectric element, absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing. This component also dampens the transducer vibration to create an ultrasound pulse with a short spatial pulse length (SPL), which is necessary to preserve detail along the beam axis (axial resolution). Dampening of the vibration lessens the purity of the resonance frequency and introduces a broadband frequency spectrum. With dampening, With ring-down, an increase in the bandwidth (range of frequencies) of the ultrasound pulse occurs by introducing higher and lower frequencies above and below the center (resonance) frequency. The "Q factor" describes the bandwidth of the sound emanating from a transducer as. Q = (f_0 / BW) A "high Q" transducer has a narrow bandwidth (i.e., very little damping) and a corresponding long SPL. A "low Q" transducer has a wide bandwidth and short SPL. The higher the Q value, the higher the SPL
Elevational resolution or slice thickness
The elevational or slice-thickness dimension of the ultrasound beam is perpendicular to the image plane. Elevational resolution is dependent on the transducer element height. Slice thickness is typically the weakest measure of resolution for array transducers. Elevational resolution reaches a minimum value in the focal zone, away from which partial volume effects increase due to increasing effective slice thickness. As with lateral resolution, can acquire with multiple elevational phase shifting to minimize slice thickness across a range of depth. Penalty is increased image acquisition time
Frequency
The frequency is the number of times the wave oscillates through one cycle each second.
Dynamic aperture
The lateral spatial resolution of the linear array beam varies with depth, dependent on the linear dimension of the transducer width (aperture). A process termed dynamic aperture increases the number of active receiving elements in the array with reflector depth, so that the lateral resolution does not degrade with depth of propagation.
Ultrasound Attenuation
The loss of acoustic energy with distance traveled, is caused chiefly by scattering and tissue absorption of the incident beam. The attenuation coefficient, μ, expressed in units of dB/cm, is the relative intensity loss per centimeter of travel for a given medium. An approximate rule of thumb for "soft tissue" 0.5 (dB/cm)/MHz. HVT = 3dB/μ For soft tissues; HVT = 3dB / (((0.5dB/cm)/MHz)*f)
Matching Layer
The matching layer provides the interface between the raw transducer element and the tissue and minimizes the acoustic impedance differences between the transducer and the patient. It consists of layers of materials with acoustic impedances that are intermediate to soft tissue and the transducer material. The thickness of each layer is equal to ¼ wavelength. For example, the wavelength of sound in a matching layer with a speed of sound of 2,000 m/s for a 5-MHz ultrasound beam is 0.4 mm. The optimal matching layer thickness is equal to ¼ wavelength = ¼ * 0.4 mm = 0.1 mm. In addition to the matching layer, acoustic coupling gel (with acoustic impedance similar to soft tissue) is used between the transducer and the skin of the patient to eliminate air pockets that could attenuate and reflect the ultrasound beam.
The Near Field (Fresnel zone)
The near field, also known as the Fresnel zone, is adjacent to the transducer face and has a converging beam profile. Beam convergence in the near field occurs because of multiple constructive and destructive interference patterns of the ultrasound waves from the transducer surface. Near field length = (r^2 / lambda). Lateral resolution (the ability of the system to resolve objects in a direction perpendicular to the beam direction) is dependent on the beam diameter and is best at the end of the near field for a single-element transducer. Peak ultrasound pressure occurs at the end of the near field, corresponding to the minimum beam diameter for a single-element transducer
Period
The period is the time duration of one wave cycle
Pulser
The pulser (also known as the transmitter) provides the electrical voltage for exciting the piezoelectric transducer elements and controls the output transmit power by adjustment of the applied voltage
Receiver
The receiver accepts data from the beam former during the PRP, which represents echo information as a function of time (depth). Subsequent signal processing occurs in the following sequence 1. Gain adjustments and dynamic frequency tuning: TGC is a user-adjustable amplification of the returning echo signals as a function of time, to further compensate for beam attenuation. The ideal TGC curve makes all equally reflective boundaries equal in signal amplitude, regardless of the depth of the boundary. The TGC amplification effectively reduces the maximum to minimum range of the echo voltages as a function of time. Dynamic frequency tuning is a feature of some broadband receivers that changes the sensitivity of the tuner bandwidth with time, so that echoes from shallow depths are tuned to a higher frequency range, while echoes from deeper structures are tuned to lower frequencies. Accounts for beam softening 2. Dynamic range (logarithmic) compression: After TGC, the signals must be reduced to 20 to 30 dB, which is accomplished by compression using logarithmic amplification to increase the smallest echo amplitudes and to decrease the largest amplitudes. 3. Rectification, demodulation, and envelope detection: Rectification inverts the negative amplitude signals of the echo to positive values. Demodulation and envelope detection convert the rectified amplitudes of the echo into a smoothed, single pulse. Demodulation recovers the basic envelope shape of the waveform. Envelope detection converts the demodulated waveform to a smooth single pulse 4. Rejection level adjustment sets the threshold of signal amplitudes allowed to pass to the digitization and display subsystems. This removes a significant amount of undesirable low-level noise and clutter generated from scattered sound or by the electronics. 5. Processed images are optimized for gray-scale range and viewing on the limited dynamic range monitors, so that subsequent adjustments to the images are unnecessary
Reflection coefficient
The reflection coefficient describes the fraction of sound intensity incident on an interface that is reflected. R_I = (I_R/I_I) = (((Z_2 - Z_1)/(Z_2 + Z_1))^2)
Ultrasound Beam Properties
The ultrasound beam propagates as a longitudinal wave from the transducer surface into the propagation medium, and exhibits two distinct beam patterns: a slightly converging beam out to a distance determined by the geometry and frequency of the transducer (the near field), and a diverging beam beyond that point (the far field).
Wave speed
The wave speed is determined by the ratio of the bulk modulus (a measure of the stiffness of a medium and its resistance to being compressed) and the density of the medium c = sqrt(B/rho)
Wavelength
The wavelength of the ultrasound energy is the distance (usually in micrometers) between compressions or rarefactions, or between any two points that repeat on the sinusoidal wave of pressure amplitude
Refraction artifacts
This is the change in transmission direction as a result of non-perpendicular media when the speed of sounds are different between the two media. This displacement can be changed by changing the position of the transducer and angle of incidence with the tissue boundary. Types of refraction error: - Misregistration: geometric distortion, improper placement in the image plane. - Defocusing: loss of coherence, shadowing at the edge of large curved structures. - Ghost image: duplication of structures with significant Z mismatch.
Twinkling Artifact
Twinkling artifact is seen with color flow Doppler ultrasound. The twinkling artifact is represented as a rapidly changing mixture of colors, is typically seen distal to a strong reflector such as a calculus, and is often mistaken for an aneurysm when evaluating vessels. This artifactual appearance is possibly due to echoes from the strong reflector with frequency changes due to the wide bandwidth of the initial pulse and the narrow band "ringing" caused by the structure. Twinkling artifact may be used to identify small renal stones, as shown in, and differentiate echogenic foci from calcifications within kidney, gall bladder, and liver. -Arises from strong reflection with frequency shift due to wide bandwidth of incident pulse combined with narrowband "ringing" produced by structure.
Continuous doppler operation
Two transducers used for transmit and receive. An oscillator produces a resonant frequency to drive the transmit transducer and provides the same frequency signal to the demodulator, which compares the returning frequency to the incident frequency. The receiver amplifies the returning signal and extracts the residual information containing the Doppler shift frequency by using a "low-pass" filter, which removes the superimposed high-frequency oscillations. The Doppler signal contains very low-frequency signals from vessel walls and other moving specular reflectors that a wall filter selectively removes. An audio amplifier amplifies the Doppler signal to an audible sound level, and a recorder tracks spectrum changes as a function of time for analysis of transient pulsatile flow. Continuous wave Doppler suffers from depth selectivity with accuracy affected by object motion within the beam path. Multiple overlying vessels will result in superimposition, making it difficult to distinguish a specific Doppler signal. Advantages are high accuracy (very narrow bandwidth) and no aliasing.
Image display
Ultrasound data usually reconstructed to 512x512 or 640x480 matrices. Normally 8-bit depth (256 gray scale values). With no compression, about ¼MB per image frame. If real time data at 10 - 30 fps, can run well past 100MB for a scan. If there is color information (e.g. Doppler) than storage requirement is tripled. Color data represented as RGB which each color channel at 8-bit depth (256 levels for each color) for 24 bits (3 bytes) per pixel.
Safety concerns
Ultrasound does not use ionizing radiation so radiation dose is not a concern. However, the sound waves can damage tissue if they are too powerful (in ultrasound the Mechanical Index (MI) is a measure of power). Therefore, there is a regulatory limit of the MI in US imaging which is given as 1.9.
Far field (Fraunhofer zone)
Where the beam diverges. The angle of ultrasound beam divergence, theta, for the far field is given by sin theta = 1.22 * (lambda / d) ultrasound intensity in the far field decreases monotonically with distance
Sound propagates through tissue by
compression and rarefaction Compression is caused by a mechanical deformation induced by an external force, with a resultant increase in the pressure of the medium. Rarefaction occurs following the compression event—as the backward motion, the compressed particles transfer their energy to adjacent particles with a subsequent reduction in the local pressure amplitude
M-mode
motion mode; used to display motion of the reflectors
Comet Tail Artifact
Comet-tail artifact is a specific type of reverberation artifact. This results a short train of reverberations from an echogenic focus which has strong parallel reflectors within it. Appears as a single long hyperechoic echo.
