Resolution refers to image quality and is not a component of attenuation. Recall that as a sound beam passes through the body, the intensity is diminished (as are the amplitude and power). Three components contribute to the attenuation of a sound beam: reflection, scattering, and absorption. Reflection occurs if a part of the sound beam is sent back toward the transducer. Specular and diffuse are two types of reflections. Specular reflection takes place when there is a smooth boundary such as looking in a mirror. This portion of the beam, however, does not return directly back to the transducer but at an angle. Diffuse reflections take place at a boundary that is not smooth and tends to reflect in various directions known as backscatter. Scattering is the second component of attenuation that occurs when the energy tends to travel in many directions. One cannot predict where scattering will take place, and an ultrasound beam with a higher frequency will demonstrate more scatter. Absorption is the third component of attenuation; it describes when the energy of the ultrasound wave is changed to heat.

Sound waves are longitudinal mechanical waves. Longitudinal waves are characterized by the motion of particles in the same direction as the wave propagation. In the case of sound waves, the particles of the medium (such as air, water, or solids) vibrate back and forth in the same direction as the wave travels. This vibration creates compressions and rarefactions, resulting in the transmission of sound energy. Therefore, sound waves are classified as longitudinal mechanical waves.

A vibrating string or fluid pump is used to test Doppler velocities of an ultrasound system. These Doppler phantoms enable the evaluation of particles in motion that mimic blood flowing through blood vessels to determine the accuracy of velocities as well as the resolution at various depths. The dead zone is the region within the first centimeter of the transducer. Contrast resolution and grayscale sensitivity can both be evaluated with a tissue-equivalent phantom.

Attenuation is a term that describes the weakening of the ultrasound beam as it passes through tissues. This weakening applies to the power, amplitude, and intensity of the beam. The fundamental cause of attenuation is absorption because the energy of the beam is changed to heat. Cavitation refers to the possibility of bioeffects occurring during an exam. Obstruction is not a term that is related to attenuation, and acoustic impedance is the hindrance of sound as it passes through a medium, which affects the reflection of a sound wave.

Impedance can be defined as the interference of the transmission of sound as it moves through tissue. Impedance is the product of the medium's density and propagation speed of the medium, so if the density and speed increase, impedance will also increase. Acoustic impedance is a calculation that has an impact on the amount of reflection that occurs. If two tissues have the same impedance, all of the sound will be transmitted. If two tissue types have vastly different impedances, the majority of the sound will be reflected. Stiffness has a direct relationship to speed, therefore also influencing impedance. The frequency of the transducer does not influence the impedance because impedance is related only to the medium.

This statement is not true because waves with identical frequencies can interfere constructively or destructively depending on their phase. If the waves are in phase, they will interfere constructively, but if they are out of phase, they will interfere destructively.

The spatial pulse temporal average (SPTA) is linked with the greatest amount of temperature elevation within tissues. Bioeffects have not been noted when the SPTA intensity was less than 100 mW/cm², when the ultrasound beam was unfocused. If the ultrasound beam is focused, then the same principle applies if the intensity is less than 1 W/cm². If there is an inquiry regarding the smallest intensity, the answer would be the spatial average temporal average.

Reflection takes places when some of the energy of the sound wave is averted back toward the transducer. Reflection will occur only if the borders of two different tissues have different impedances; otherwise, transmission occurs. Recall that the incident intensity is the intensity of a sound beam as it leaves the transducer prior to coming into contact with tissue. The transmitted intensity is the forward propagation of the intensity of the incident beam after hitting that boundary. Normal incidence is when the incident sound beam comes into contact with the boundary at a 90-degree angle. During

diagnostic imaging, there are few differences in the impedance of soft-tissue

boundaries; therefore, more than 99% of the incident beam will be transmitted. To summarize, with normal incidence and identical impedances of the media, all of the incident beam's intensity will be transmitted. Keep in mind that the incident and transmitted intensities must always equal 100%. Oblique incidence and reflection (or transmission) are hard to conclude because they tend to be complex.

The lowest level of tissue heating occurs when the output intensity of the equipment being used is at its lowest numerical value. Generally, grayscale imaging is the method in which tissue heating will be the lowest. It is typically the highest when pulsed Doppler is being used. M-mode and color flow Doppler tend to fall in the middle of these intensities.

Ultrasound should be used only for clinical exams in which the benefits outweigh the risks. One risk during an exam is an elevation of temperature in tissues exposed to the ultrasound beam. The thermal index (TI) will be highest during an exam with a high- frequency, high-intensity beam. This heating depends on the exposure time and the temperature. Typically, the greatest increase in temperature is witnessed with spectral Doppler exams.

A ring-down artifact (also known as a comet-tail artifact) occurs when the reflectors are parallel to the ultrasound beam; it looks like an echogenic line. An edge shadow is due to the refraction of the sound beam; it appears like a dark shadow that extends deep to a curved structure. Crosstalk occurs only with spectral Doppler as a mirror image artifact in which blood flow looks as if it is moving in two directions. A slice-thickness artifact can degrade image details if the slice thickness of the ultrasound beam is wider than the target. When this takes place, fluid-filled components may not be visualized at all or they may look like they are filled in because the return signal is including soft tissue on either side of the cyst at the same depth.

deca -- deci -- centi -- micro

4 Variables include: Pressure, Density, Particle Motion (Distance), Temperature

The amount of attenuation will be decreased if the sonographer decreases the frequency of the transducer. Absorption is one of the main causes of attenuation, and attenuation increases when the sonographer increases the frequency of the transducer, making this choice incorrect. The distance that the sound wave travels has a direct impact on attenuation, so increasing the imaging depth will increase attenuation (not decrease as asked in this instance). Decreasing the output power creates more attenuation because the ultrasound wave is not as strong.

A period is the length of time it takes for one single cycle to occur. Unit of time microsecond (us)

Time gain compensation (TGC) is a function of the ultrasound system that allows the sonographer to compensate for the weaker signals (attenuation) as the sound waves travel further into the body. This control will allow for uniform brightness when set correctly because the user can adjust at specific depths when necessary. Receiver gain will implement a change to the entire image and not just at certain depths. Focusing will create a beam that is narrower at the focal point, enhancing the resolution at this point. Adjusting the output power will either increase or decrease the energy of the ultrasound beam, creating an entire image that is brighter or darker and affects patient exposure.

Soft tissue has certain properties that affect the propagation of ultrasound waves. These properties are referred to as acoustic propagation properties. This includes the ability of soft tissue to transmit, absorb, and scatter ultrasound waves. Understanding these properties is important in medical imaging as it helps in interpreting ultrasound images and diagnosing various conditions.

Sonographers should use correct ergonomics to care for their own bodies to prevent work-related musculoskeletal disorders. Roughly 80% of sonographers report having some sort of work injury. Ergonomics is a combination of setting up the room appropriately so the patient and machine can be reached easily. Properly using equipment that reinforces the desired body mechanics to prevent any injuries is also key. This may be something as simple as adjusting the keyboard or monitor to the correct height. One of the most common complaints that a sonographer makes is pain in the shoulder while scanning. It is imperative that the patient is close enough so that the sonographer's arm does not have to be abducted more than 30 degrees. The other choices would not protect the sonographer because they are all angles that are more than 30 degrees. 

Explanation: Sensitivity refers to the ability of an ultrasound machine to demonstrate weaker signals. Accuracy is the ability of an exam to diagnose outcomes that are positive and negative. Specificity is when an imaging exam can correctly diagnose results that are normal. Reliability refers to the ability of an exam to produce results that are dependable and consistent.

Cavitation is discussed when bioeffects are considered with the interaction of the ultrasound beam and gas bubbles in the human body. Higher temperatures and pressures may cause the gas bubbles within the body to burst; this is known as transient cavitation, which is extremely concerning for harmful bioeffects. When these gas bubbles expand and contract with fluctuations in pressure without rupturing, it is referred to as stable cavitation. There are only two types of cavitation, so absorption and thermal are not viable answers to this question. 

The frequency closest to the lower limit of the ultrasound is 15.000 Hz. Ultrasound refers to sound waves with a frequency higher than the upper limit of human hearing, which is typically around 20,000 Hz. A frequency of 15,000 Hz is lower than the upper limit of human hearing, making it the closest frequency to the lower limit of ultrasound.

Elastography is a recent ultrasound technology that uses sound waves to test the stiffness of tissue or lesions that are being evaluated. Elastography is used most often when evaluating the liver, breast, or prostate or for thyroid disease. Contrast-enhanced ultrasound involves injecting (or giving orally) contrast to help visualize tumors or to evaluate blood vessels. Fusion imaging involves concurrent ultrasound imaging with a previous computed tomography or magnetic resonance imaging scan on a split screen to enhance diagnostic confidence that an area previously mentioned is the same area being evaluated. This can be extremely helpful during ultrasound-guided biopsies. Harmonics are signals that are produced by the patient's body (not the ultrasound system). This technology has been in use for several years.

The speed of ultrasound in soft tissue is closest to 1.540 km/sec (kilometers per second). This is the typical speed of ultrasound in soft biological tissues, and it's often used as a reference value in medical imaging and diagnostic procedures.

Sonographers should always consider ways to practice the ALARA principle. ALARA stands for as low as reasonably achievable, and sonographers can achieve this by using the lowest possible output power that enables a diagnostic image. Keeping the exposure time to a minimum is another effective way of practicing ALARA. The gain can change the brightness (or darkness) of the image, but it has no effect on patient exposure, so the power should always be reduced, if possible, before the gain to reduce exposure to the patient. On the other hand, if an image is too dark, it is best to increase the gain because it will not affect exposure to the patient. Time gain compensation (TGC) enables the sonographer to adjust the amplification of reflections that are located at a greater depth in the body, but it has no effect on the exposure. 

Duty factor (DF) is the percentage of time that an ultrasound system is imparting sound waves. For continuous wave (CW) systems, the DF is 100% because a signal is always being sent (but is unable to generate an ultrasound image because images can be created only with pulsed waves). A DF of 0% means that a system is not generating a pulse. Recall that the maximum value for DF of a CW sound beam is 100% and the minimum value is 0%. Pulses are necessary to create images, and the typical DF for imaging is 0.2%, which is for pulsed wave systems. 

A tissue-equivalent phantom (also referred to as a grayscale or tissue-mimicking phantom) is important because it evaluates the sensitivity of the system's grayscale capabilities. This type of phantom can distinguish between shades of gray that are even marginally different, such as in contrast resolution. Axial resolution can also be evaluated with a tissue-equivalent phantom because separate structures that are parallel to the ultrasound wave can be seen individually as one in front of the other. Horizontal resolution is the ability to determine that reflectors are in the proper locations when they are perpendicular and in a horizontal position in relation to the ultrasound beam. Horizontal resolution can also be assessed in a tissue-equivalent phantom. Temporal resolution depends on the frame rate and is not tested with a tissue- equivalent phantom.

In this image, a gallstone (cholelithiasis) is the filling defect that is visualized within the lumen of the gallbladder. Artifacts can offer important information such as shadowing from something that is calcified such as a gallstone, as seen in this instance, or even a kidney stone. This filling defect attenuates a great deal and has created a shadowing artifact, which is the hypoechoic area that is seen underneath the gallstone. The regions deep to the filling defect are obscured due to the shadowing. The opposite of a shadowing artifact is one that is known as an enhancement artifact. This type of artifact can also be advantageous for sonographers and interpreting physicians because it appears as a hyperechoic region underneath a structure that demonstrates a lower attenuation rate such as a cyst or the gallbladder.

The attenuation coefficient describes how the intensity of the ultrasound beam decreases when considering the distance and frequency of the ultrasound wave. The attenuation coefficient units are decibels per centimeter (dB/cm). Attenuation occurs because of reflection, scattering, and absorption of the sound beam's energy. Frequency is directly related to scattering and absorption, so we can assume that frequency is also directly related to the attenuation coefficient. The equation that can be used to calculate this is as follows:

Attenuation coefficient (dB/cm) = frequency (Mhz)/2 = 12/2 = 6 dB/cm.

The mechanical index (MI) is a parameter that the user should monitor to help determine if cavitation is expected. If a sonographer notices that the MI is too high during an ultrasound, it is important to decrease the acoustic power of the machine. Another step the sonographer can do to keep the MI lower is to increase the frequency of the transducer. The receiver gain has no effect on patient exposure. The ultrasound operator is not able to adjust the period unless a different transducer is chosen.