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Ultrasound physics
| Question | Answer |
|---|---|
| Reflections from long smooth surfaces are called ______ | Specular reflectors |
| Reflections from long smooth surfaces (specular reflections) are highly ____ dependent | Angular |
| Irregular surfaces reflect fewer echoes straight back. These surfaces are called | Diffuse reflectors |
| Reflectors in normal liver and other soft tissues that provide echotexture are called ______ | Acoustic scatters |
| The tissue property that determines the strength of surface reflections is _____ | Impedence (Z) |
| The strength of a surface reflection depends on the absolute value of difference in ______ | Impedence between two tissues |
| What is impedence dependent upon? | Density of tissue and speed of sound in that tissue |
| Reflection coefficients tell you what? | The percentage of sound reflected. They are calculated by squaring the impedence difference between two tissues and dividing it by their sum. |
| What is reverberation? | For example in a gallbladder filled with air, the sound bounces around between the bubbles and is therefore mapped deeper in the image. |
| What is refraction? | If the incident sound is non-perpendicular and the speed of sound differs on each side of an interface, the sound wave will bend. |
| In which kind of tissue is the speed of sound fastest? | Noncompressible tissues, like bone. |
| Where is refraction seen? | Commonly along the lateral edges of fluid filled structures |
| As sound penetrates tissue it diminishes in strength, a process called _____ | Attenuation |
| How are deeper structures still seen well despite attenuation? | Time gain compensation (also known as depth gain compensation). This increases the amplification of deeper echoes |
| A transducer produces sound by converting between electrical and vibrational energy via the ______ | Piezoelectric effect |
| What are the crystals in an ultrasound transducer? | Piezoelectric crystals |
| Transducer crystals vibrate with a preferred wavelength equal to ______ | Twice the thickness of the crystal |
| A damping layer in the transducer is (superficial or deep) to the piezoelectric crystal and serves to ____ | Deep Lessen vibrations and shorten the pulse |
| The near field region is the narrowest portion of the beam. What produces a longer near field region (NFL)? | INCREASED FREQUENCY Increased crystal width |
| What is a focal zone? | Region where beam width is narrowest |
| The ultrasound beam is narrowest in the near or far field? | Near field (always). The beam fans out the farther away it goes. |
| The range of frequencies in a pulse is called the _____ | Bandwidth |
| Define wave period, pulse period, and pulse duration. | Wave period: The wavelength of outgoing sound Pulse period: All of the wave periods of the outgoing sound Pulse duration: Combination of the pulse period and the time "listening" for the reflecting sound |
| How are frequency and pulse period related? | Increasing frequency means both the wave period and pulse period are shorter |
| Shortening a pulse results in _____ bandwidth. | Greater |
| Linear transducers are used for _____ while curved arrays are used for ____. | superficial structures deeper structures |
| What are phased array transducers? | Really small footprint but super wide field of view the deeper you go. |
| Why are higher frequency transducers also higher resolution? | Because of their longer NFL. This creates narrower beam widths, so the lateral resolution is much better. |
| What increases axial resolution? (i.e. resolving structures along the axis of the ultrasound beam) | Shorter pulses (successive short pulses can distinguish two object close to each other along the axis of the sound waves) |
| What enables shorter pulses? | Higher frequencies (shorter pulse period) Broad bandwidth |
| What is the range equation? | Tells you at what depth the echo signal is formed (D). D = c*t/2, where c = speed of sound and t = time it takes to receive the echo again. |
| What is the assumed speed of sound in soft tissues? | 1540 m/s |
| When the sound propagation speed for a tissue is lower than 1540 m/s, what happens to the distance calculated by the range equation? | It's erroneously long; the structure will be mapped deeper than it really is. |
| Which ultrasound mode is now commonly used? A, M, or B? | B-mode |
| The time between successive individual ultrasound pulse emissions is called ____ | Pulse repetition period |
| Compared to pulse duration, pulse repetition period is longer or shorter? | Longer. The next pulse cannot be emitted until the collection of echo signals of the previous line is completed. |
| What's the equation for the minimum pulse repetition period? | 2D/c |
| What is the PRF (pulse repetition frequency)? | PRF is the inverse of the pulse repetition period, hence = c/2D |
| Newer transducers are called array transducers. What is their advantage over older single element transducers? | Array transducers can focus acoustic energies to various depths and hence you can change your focal zone. |
| What's the frame rate and what factors influence it? | Frequency of refreshing the 2D image. Factors include: speed of sound, maximum depth of the field of view, number of scan lines, and number of focal zones. |
| What are the three kinds of resolution and what improves each? | Axial (deep plane; better with shorter pulses), Lateral (better with narrower beam width/higher frequency), and Elevational (slice thickness, improves with decreased slice thickness) |
| Assuming the speed of sound is 1540 m/s and a reflector is positioned 1 cm from the transducer, how long does it take for the sound to travel to the reflector and return? | 2D/c = 2 x 0.01m/1540 = 12.9 us |
| Transmit power increased results in what? | Increased amplitude of the pulse emitted from the transducer |
| More powerful pulses yield | Stronger returned echoes, improved signal-to-noise ration, and increased maximum depth of pentration |
| The major disadvantage of high transmit power is what? | Potential increased bioeffects |
| Gain is analogous to what? Does it affect depth of penetration? | Volume control. Doesn't affect penetration depth. |
| What is the dynamic range? What is it expressed in? What's the CT correlate? | Ratio of the largest to smallest signals (largest is "saturated" and smallest is noise/threshold). Expressed in decibels: 10log10 (lmax/lmin)dB. Think of this like a window width in CT; a narrow dynamic range is like a narrow liver window. |
| What's the difference between write zoom and read zoom? | Write zoom is the acquisition of new images in a magnified field of view. Read zoom is zooming up on the image after it has been acquired. Write zoom hence has higher resolution. |
| What does compound imaging do? | Decreases inherent artifacts |
| How frequently do ultrasound machines need to be quality checked? | Semiannually |
| What's the deadzone? | Minimum distance from the surface of the transducer to the first discernable echo. Can be no greater than 3 mm at 7 Mhz. |
| How do you calculate the maximum available frame rate? | max = PRF/N (where N = number of scan lines per frame); hence max = c/2DN |
| What is the unit of acoustic impedance? What causes the formation of an echo? | Rayls (physical density of the tissue in kg/m3 multiplied by the speed of sound in that tissue in m/s). Echoes are formed at the interfaces of tissues that have different impedence values. |
| What technically causes acoustic shadowing? | Attenuation of the beam. |
| What is the Doppler Effect? Describe the frequency of reflecting sound waves when blood is flowing towards or away from the transducer. | Change in frequency of a wave due to relative motion of the source (think about the change in pitch of a train's whistle as it passes). Blood moving towards the transducer will reflect sound at a higher frequency relative to the incoming frequency. |
| What is Doppler shift? How is it used in radiology? | Difference in the transmitted frequency and the frequency reflected by a moving object. This can tell you both the direction and velocity of blood flow. |
| On color Doppler, how will aliasing manifest? | It will be display the highest positive frequency shift in the color that represents the highest negative frequency shift. |
| On spectral Doppler, how will aliasing manifest? | It will appear as if the top of the spectrum is cut off and wrapped around the baseline, reappearing in the opposite region of the spectrum. |
| Aliasing occurs when the Doppler shift is higher than what threshold? | The Nyquist limit, which is equal to 1/2 x PRF. |
| What should your PRF be set at to ensure lack of aliasing? | At least twice the Doppler shift. |
| How can you reduce aliasing? | Increase PRF, selecting a lower frequency transducer (decreases Doppler shift), selecting a sample volume at a lesser depth (PRF increases as depth decreases), and choosing a Doppler angle closer to 90 degrees (decreases Doppler shift). |
| What does the non-vascular color in soft tissues represent often times? | Perivascular vibration. |
| What is twinkle artifact? | Characteristic noisy spectrum caused by strongly reflecting surfaces that results in rapid fluctuations and a mixture of red and blue colors without actual movement. |
| What is a flash artifact? | Sudden burst of color that fills the color box causes by transducer or patient motion. |
| What is color bleed and how do you decrease it? | Appearance of mottled color beyond the vessel wall, which can be minimized by decreasing color gain. |
| What is the relationship between transducer frequency and sensitivity of detecting blood flow? | Higher frequency transducers have increased sensitivities. |
| What is gain? | The technique by which sound or Doppler signal is amplified. |
| What is the basic definition of PRF? What are high and low PRFs good for? | The number of sound pulses transmitted per second. High PRF allows display of high velocity flow without aliasing. Low PRF increases sensitivity to low velocity blood flow. |
| What is a wall filter? | Think of it as a threshold below which no Doppler signal will be detected. A high wall filter reduces artifacts from soft tissue movement or vessel wall pulsation, but it can also obscure low-velocity flow from being detected. |
| What does color Doppler measure, peak or average velocity? | Average velocity. Peak velocity can be measured with spectral Doppler. |
| What is the axial resolution of a transducer as determined by pulse length? How do you increase axial resolution. | Axial resolution = spatial pulse length / 2. You can increase the axial resolution by increasing the frequency of the transducer, but the tradeoff is tissue penetration. |
| What's the relationship between PRF and frequency when trying to visualize very deep structures? | PRF is the number of sound pulses transmitted per second. If the frequency of the transducer decreases, then the sound wave takes longer to get back to the transducer and the PRF is limited. |
Created by:
radiologyresident