Physics 140-167

Stenosis refers to a narrowing of a vessel. Stenosis of an artery typically results in 4 primary alterations in blood flow in and around the stenotic region. Describe these 4 alterations in blood flow.

1)Within the actual region of stenosis the velocity of blood is increased, although flow remains laminar. 2)Downstream from stenosis, blood flow is turbulent. 3)Downstream from the stenosis, blood pressure is lower than the pressure that is upstream from the stenosis. 4)Downstream from the stenosis, the pulsatile and triphasic nature of arterial flow can vanish. Flow becomes steady.

Describe a shadowing artifact.

A shadowing artifact is characterized by either the absence or a severe reduction in the strength of echoes, extending beyond a highly reflective structure. A shadow artifact has essentially the same color as the background image (i.e. hypoechoic or anechoic).

Do all artifacts hinder the diagnostic accuracy of clinical ultrasound?

Absolutely not! The presence of some types of artifacts may be helpful in establishing a diagnosis. Shadowing is one example. Shadowing occurs when sound waves strike a highly reflective or attenuating structure in the body, causing a substantial reduction in the strength of reflections from posterior structures. Some clinical situations, the occurrence of shadowing confirms the presence of pathology, such as gallstones, calcification and certain types of tumors. In other circumstances, shadowing may limit the diagnostic usefulness of an exam. For example, if an echocardiogram is performed on a patient with an artificial heart valve, shadowing caused by the highly reflective components of the valve may preclude visualization of structures behind the prosthesis.

Describe an enhancement artifact.

An enhancement artifact is characterized by the appearance of abnormally bright reflections (i.e. hyperechoic) that begin just beneath a weakly attenuating structure and extend beyond it. Enhancement is the opposite of shadowing.

What does anachoic mean?

Anechoic refers to the complete absence of echoes.

During inspiration, is Venus flow from the lower extremities increased or decreased? During expiration, is venous flow from the lower extremities increased or decreased?

During inspiration, venous flow from the lower extremities is decreased. With inspiration, the diaphragm compresses the abdomen, which increases abdominal pressure. This increasing pressure tends to reduce the flow of venous blood from the lower extremities toward the abdomen. during expiration, venous flow from the lower extremities is increased. With expiration, the diaphragm decompresses the abdomen, resulting in reduced abdominal pressure. This reduction in pressure tends to increase venous blood flow from the lower extremities toward the abdomen.

During inspiration, is venous return to the heart from the upper extremities increased or decreased? During expiration, is venous return to the heart from the upper extremities increased or decreased?

During inspiration, venous return from the upper extremities is increased. Remember that with inspiration, the diaphragm descends, expanding the thorax. As a result of this expansion, intrathoracic pressure is decreased. This reduction in pressure tends to increase the flow of venous blood from the upper extremities toward the heart. During expiration, venous return from the upper extremities is decreased. With expiration, the diaphragm compresses the thorax, which causes an increase in thoracic pressure. Increased intrathoracic pressure tends to decrease the flow of venous blood from the upper extremities toward the heart.

What is energy? What is an energy gradient? With regards to hemodynamics, why is an energy gradient important?

Energy is generally defined as having the capacity to perform work. There are many forms of energy, including electrical, chemical, pressure, and kinetic energy. When the energy at one location is different than the energy at another location, and energy gradient is said to exist. And hemodynamics, an energy gradient is important because it is the reason why blood moves from one region to another. In other words, blood flows when the amount of energy at one location is greater than the magnitude of energy at another location. Note: it is less correct to state that blood flows from regions of higher pressure to regions of lower pressure. The ideal statement is that blood flows from regions of higher energy to regions of lower energy.

Describe a focal banding artifact.

Focal banding artifact is characterized by the appearance of a horizontal band of abnormally bright echoes from structures located at the depth of the focus. Focal banding artifact is a type of enhancement artifact.

What is friction? With regard to hemodynamics, why is friction important?

Friction is defined as "the act of rubbing." When two structures rub against each other, heat or thermal energy is created. In hemodynamics, friction acts to decrease the total energy of blood as it flows through the circulation. As blood flows through a vessel, some of the energy is lost as a result of friction. In normal arteries and veins, the amount of energy lost resulting from friction is quite small. However, when there is a stenosis of a blood vessel, friction results in a greater amount of energy loss.

What effect does hydrostatic pressure have on blood pressure measurements in a supine patient?

Hydrostatic pressure has no effect in a supine patient. When a person is supine, there is little difference in the height of any part of the body compared to the heart. As a result, hydrostatic pressure is negligible. Therefore, any region of the body from which measurements are obtained (e.g. ankle, arm) will accurately represent the blood pressure at the level of the heart

What is hydrostatic pressure?

Hydrostatic pressure is related to the force exerted by a static column of blood. While diving into a swimming pool, you may have experienced a sensation of pressure on your ears, especially if you descended to the bottom the pool. This "hydrostatic" pressure results from the height of the water pressing against your ears.

How can a sonographer offset some of the effects of focal banding artifacts?

If a phased-array transducer is being used, the sonographer can attempt to decrease the amount of beam focusing. This action may diminish the degree of focal banding.

Describe a mirror image artifact.

Mirror image artifact is the appearance of a second copy of a single anatomical structure in a different location. When a strong, smooth reflector is located in between the transducer and another anatomical structure, a mirror image artifact of the anatomical structure may appear on the display at an incorrect location and at a deeper depth.

What produces mirror image artifact?

Mirror image artifact may occur under the following circumstances. A strong, smooth boundary (the mirror) is struck by a pulse at an oblique angle and the pulse then reflects back to the mirror, and finally back to the transducer. The resulting image will show a true echo of the structure in its correct anatomic location together with a mirror artifact of the structure in an incorrect, deeper position. Speckle from the liver that appears to be located within the thoracic cavity is a commonly observed artifact.

How does velocity differ from flow?

Picture 2 cars, a red car @ 5 mph and a blue car @ 100 mph. In this example, the velocity of the two cars is quite different. The red car has a low velocity, while the blue car has high velocity. This is b/c velocity relates to how fast the cars are moving. In summary, flow refers to how much, velocity refers to how fast.

What is refraction?

Refraction is a change or bending in the direction of an acoustic pulse as it travels from one medium to another. For example, sound may change direction as it propagates from muscle to fat.

What causes refraction artifact?

Refraction occurs at the boundary between two media when two conditions occur. First, the incidence of the sound beam must be oblique to the boundary. Second, the propagation speed of the two media on either side of the boundary must be different. If these conditions are met, then the acoustic pulse will change its direction as it propagates from one medium to the other.

Describe the appearance of a reverberation artifact.

Reverberation artifact appears as multiple, equally spaced echoes displayed on the image at ever-increasing depths.

What causes reverberation artifact?

Reverberations occur when two or more strong reflectors are aligned with the main access of the ultrasound pulse. The pulse may actually "ping pong" back and forth between the two reflectors, sending multiple echoes back to the transducer. The first and second echoes are genuine, that is, they are reflections from true anatomic structures in the correct position. The subsequent echoes are caused by the bouncing of the ultrasound pulse between the two reflectors, and they do not represent actual anatomic structures.

What is an ultrasonic imaging artifact? List 4 possible causes of ultrasonic imaging artifacts.

Simply defined, an ultrasonic imaging artifact is an imaging error. Several types of artifacts may occur, including the display of spurious signals; the failure of structures to be displayed; and the display of the structures who shape, size, brightness, or location is incorrect. 4 possible causes of ultrasonic imaging artifacts include the following: 1. Violations of certain assumptions about the physics of ultrasound. 2. Equipment malfunction or incorrect design. 3.The inhere tr physical limitations of ultrasonic imaging. 4. Operator error.

What is viscosity? What are its units? What is viscous loss? With regard to hemodynamics, why is viscous loss important?

Simply put, viscosity describes the thickness of a fluid. Alcohol has low viscosity, while honey has a high viscosity. Viscosity is measured with units of Poise. Viscous loss refers to the loss of fluid energy from internal friction between molecules of the fluid as they slide by each other. In hemodynamics, viscous loss acts to decrease the total energy of blood as it flows through the circulation. Energy is lost due to viscous effects generated by red blood cells rubbing against each other during flow. Persons with anemia (lower than normal red blood cell concentrations) have less viscous loss than persons with normal red blood cell counts. In contrast, persons with erythrocytosis (higher than normal red blood cell concentrations) have a greater viscous loss.

What can a sonographer do to eliminate reverberations?

Since reverberations are caused by the presence of two or more strong reflectors parallel to, or along, the main access of the sound beam, the sonographer may be able to eliminate these artifacts by changing the orientation of the transducer. When the reflectors are no longer in the path of the sound pulse, the reverberations may vanish.

Image acquisition by ultrasonic imaging systems is based on certain assumptions about the physics of ultrasound. Artifacts occur when one or more of these assumptions are either not correct or are violated. Identify the six basic assumptions incorporated into the design of ultrasound systems to bring about accurate image acquisition.

Six basic assumptions include the following: 1. Ultrasound travels in a straight line. 2. Ultrasound reflections are created only from structures along the main axis of the sound beam. 3. The strength of a reflection is related to the scattering characteristics of the anatomic structure that produces it. 4. Ultrasound always travels through soft tissue with an exact speed of 1,540 m/s. 5. The ultrasonic image plane is extremely thin. 6. Ultrasound pulses travel directly to a reflector and then back to the transducer.

What kind of anatomic structure is in the diaphragm?

The diaphragm is a large muscle located below the lungs that plays an important role in breathing. When the diaphragm contracts, it flattens and moves downward. When the diaphragm relaxes, it becomes dome shaped and moves upward.

Assume that water is flowing through a tube at a constant velocity. The pressure gradient from one end of the tube to the other end is then doubled. What will happen to the flow of the water?

The flow of the water will also double. Whenever the pressure gradient across a constant resistance changes, the flow will also change. The changes in pressure gradient and flow are directly related. Therefore, when a pressure gradient changes by a certain percentage, flow changes by an equal percentage. Example: if the pressure gradient decreases by 75%, then flow will also decrease by 75%.

What produces a focal banding artifact?

The focal zone of a sound beam has the greatest intensity and, as a result, may produce reflections of unusually large magnitude. Consequently, these high intensity reflections from the focal zone may be displayed on the image with an abnormal degree of brightness (i.e., hyperechoic).

Assume that water is flowing through a tube at a constant velocity. The flow is then doubled. What will happen to the pressure gradient?

The pressure gradient will also double. Whenever the flow across a constant resistance changes, the pressure gradient will also change. The changes in flow and pressure gradient are directly related. Therefore, when flow changes by a certain percentage, pressure gradient changes by an equal percentage. Example: if there is a fourfold increase in flow, there will also be a four fold increase in the pressure gradient.

What is inertial loss? With regard to hemodynamics, why is inertia important?

The renowned Physicist sir Isaac newton stated "objects at rest tend to stay at rest; and objects in motion, tend to stay in motion." When the speed of an object changes, there is a loss of energy. This energy loss is called inertial loss. In hemodynamics, energy is lost as blood speeds up and slows down. This occurs with normal arterial flow, which is pulsatile. The amount of inertial loss during normal arterial flow is generally moderate. However, when blood speeds up and slows down as it flows through a stenotic region of a vessel, the amount of inertial loss is much greater

How can the sonographer offset some of the effects of shadowing artifact?

The sonographer can offset some of the effects of shadowing by changing views. The transducer may be moved to a different location relative to the reflective structure to avoid shadowing.

How can the sonographer offset some of the effects of enhancement artifact?

The sonographer can offset some other effects of enhancement by changing views. The transducer may be moved to a different location to avoid enhancement.

What term would be used to describe a region of tissue displayed on an ultrasound image that appears to show variations in brightness?

The term heterogeneous would be used to describe an area of tissue that shows variations in brightness.

What term would be used to describe a region of tissue displayed on an ultrasound image that appears to have uniform brightness?

The term homogeneous would be used to describe an area of tissue with uniform brightness.

What does hyperechoic mean?

The term hyperechoic is used when describing a structure that produces echoes that are brighter than echoes from surrounding tissues or structures. Alternatively, the term may be used when the brightness of an echo is greater than normal.

With regard to ultrasonic images what does hypoechoic mean?

The term hypoechoic is used when describing a structure that produces echoes that are less bright than echoes from surrounding tissues or structures. Alternatively, the term may be used when the brightness of an echo is less than normal.

What does isoechoic mean?

The term isoechoic is used when describing a structure that produces echoes having a brightness that is equal to that of echoes from surrounding tissues or structures. Alternatively, the term may be used when the brightness of an echo is normal.

What is laminar flow? With laminar flow, what is the location of highest velocity?

The term laminar is derived from the Latin word lamina, which means thin plate or layer. Laminar flow is characterized by an orderly, layered pattern of movement; the flow of blood cells is linear, smooth, and uniform. Normal blood flow is considered laminar. The highest velocity is found in the center of the lumen.

Name the two cavities that are separated by the diaphragm

The two body cavities that are separated by the diaphragm are the thorax and the abdomen.

What does velocity of a fluid refer to? What are the units of velocity?

The velocity of a fluid, such as blood, refers to the speed at which the fluid is moving. The units of velocity are meters/second. The following analogy should help explain the difference between velocity and flow. Picture a single car travelling on an otherwise empty road at 90 mph in contrast to 100 cars all traveling on a road at 90 mph. In both scenarios, the velocity(speed of the cars) is the same. However, the 2 scenarios differ in terms of flow, defined as the # of cars travelling down the road in a specified time period. The single car represents low flow, whereas the 100 cars represent hugh flow. In physiologic terms, flow refers to the amount of blood that passes through a vessel in a specified time.

Is the venous circulation considered a high resistance circuit or a low resistance circuit?

The venous circulation is considered a low-resistance circuit. In the presence of a low-resistance circuit, the pressure difference (or gradient) from the beginning of the vascular bed to the end of the vascular bed is relatively small. In the venous circulatory system, there is only a small pressure gradient between the veins and the right atrium. Hence, the venous circulation is a low-resistance circuit.

What is turbulent flow?

The word turbulent comes from a Latin term meaning confusion. Turbulent blood flow is chaotic, with blood cells travelling in many direction and at different speeds. Murmurs (also called bruits) are the audible manifestations of turbulent flow.

How can the sonographer eliminate a mirror artifact if it occurs?

To eliminate mirror artifact, the sonographer should change the orientation of the ultrasound beam. If the orientation of the us beam in relation to the highly reflective boundary (the mirror) and other nearby structures is changed, the artifact is likely to disappear.

What type of cardiovascular pathology is turbulent flow associated with?

Turbulent flow is associated with cardiovascular pathology that impedes the movement of blood across an obstruction. Examples of such obstructive lesions include stenosis of heart valves or arteries and coarctation of the aorta.

A patient is standing. When measured at the level of the heart, the patient's systolic blood pressure is 100 mmHg. What will the systolic pressure be if measurement is obtained at the level of the patient's ankle? What will the systolic pressure be if a measurement is obtained at the level of the patient's ear?

When a patient is standing, there is a large height difference between the heart and the ankle. The degree of hydrostatic pressure at the ankles is equal to the weight of the blood at the level of the heart pressing on the ankles. In the avg person, blood pressure measured at the level of the ankle is about 95 mmHg higher than that measured at the level of the heart. In this case, the ankle-level systolic blood pressure will be approx. 195 mmHg. When a measurement of arterial blood pressure is obtained above the level of the heart, hydrostatic pressure exerts a negative effect. That is, arterial blood pressure will be lower than the pressure at the level of the heart. In a typical person, systolic blood pressure at the level of the ear is about 30 mmHg less than at the level of the heart. In this particular patient, the systolic pressure measured at the level of the ear will be roughly 70 mmHg (100mmHg-30mmHg).

What produces a shadowing artifact?

When an ultrasound pulse travels through a structure that is highly reflective or absorbent, the sound beam becomes weakened. The amount of attenuation that occurs is considerably greater than normal. Consequently, structures that lie behind the highly attenuating structure will produce weak or faint echoes on the image, and sometimes no echoes at all. This results in a "shadow" on the image. (Think of the shadow that you cast when standing in strong sunlight.)

What produces enhancement artifact?

When an ultrasound pulse travels through an anatomical structure that has abnormally low attenuation, the sound beam retains a typical strength or intensity. Consequently the strength of echoes from deeper structures is excessive and they are displayed on the image with an abnormal degree of brightness (i.e. enhancement).

What is the effect on an image when refraction occurs?

When refraction occurs, the US system does not recognize that the sound beam has changed direction, and assumes that the pulse has traveled in a straight line. The result is that a copy or duplicate of the anatomic structure is displayed on the image. This artifact or false echo is located alongside the true reflector at the same depth.

Assume that you obtain measurements of the venous blood pressure from a standing patient. When measured at the level of the heart, the patient's venous blood pressure is 12 mmHg. What will the venous pressure be if a measurement is obtained at the level of the patient's ear?

When venous blood pressure is obtained above the level of the heart, hydrostatic pressure exerts a negative effect. That is venous blood pressure will be lower than pressure at the level of the heart. In the avg person, venous blood pressure at the level of the ear is about 30 mmHg less than at the level of the heart.. In this patient, the venous pressure measured at the level of the ear will be 0mmHg. Notes: BP cannot have a negative value. When negative hydrostatic pressure exceeds positive bp, the blood vessel will simply collapse, and bp equals 0 mmHg rather than a negative bp. The term "coaptation' describes a collapsed vessel when the opposing vessel walls come together and the lumen is obliterated.

The veins have flexible, thin walls. Venous volume changes as a result of venous pressure. What happens to the volume of blood in the veins when venous pressure is very low? What happens to the shape of the cross section of veins when venous pressure is very low? What happens to the volume of blood in the veins when venous pressure increases? What happens to the shape of the cross-section of veins when venous pressure increases?

When venous pressure is very low, the veins are only partially filled with blood (volume is reduced). Consequently, the veins partially collapse and their shape becomes flattened. When venous pressure increases, the veins fill completely with blood (volume is increased) and their shape becomes rounded. Note: since the veins are very flexible, pressure needs to increase by only a few millimeters of mercury in order for their shape to change from flattened to round.

What effect does the diaphragm have on these two body cavities when we inhale or take in a breath of air?

When we breathe in (inhalation/inspiration) the diaphragm contracts and moves downward, resulting in compression of the abdomen. Simultaneously, the thorax expands and is decompressed.

What effect does the diaphragm have on these two body cavities when we exhale or let out. breath of air?

When we breathe out (exhalation/expiration) the diaphragm relaxes and moves upward, resulting in compression of the thorax. At the same time, the abdomen expands and is decompressed.