Ultrasonography: basic principles Flashcards
Ultrasound (US) is defined as sound waves with frequencies that…?
And what is the principle of ultrasonography?
exceed the normal hearing range (> 20,000 Hz).
The U/S waves interact with tissues and return as reflected echoes that are detected by the probe and then converted back from mechanical energy into an electrical signal.
The intensity of the signal is based on the number of US waves reflected, and this intensity is then assigned a relative gray scale value that is depicted on the image.
What is the pulse—echo principle?
It forms the basis for image acquisition in U/S. The transducer sends U/S waves into the area of interest approximately 1% of the time and listens for returning echoes 99% of the time. The returning reflected US waves are the basis for image formation.
There are 5 basic interactions of US waves with tissues.
- Reflection
- Refraction
- Scatter
- Attenuation
- Transmission
Reflection is one of the 5 basic interactions of U/S waves with tissues. Explain it.
Reflection occurs at acoustic boundaries, which correspond with anatomic boundaries, based on differences in the acoustic impedance of the bordering tissues.
- Refraction is one of the 5 basic interactions of U/S waves with tissues. Explain it.
Refraction in ultrasound (U/S) is the change in direction of sound waves as they pass from one tissue to another with different acoustic properties, specifically when there’s a difference in acoustic impedance and the angle of incidence is oblique.
This bending of sound waves occurs due to a change in the speed of sound between the two tissues, similar to how light bends when it passes through different media. It can cause artifacts in imaging, like misplaced structures, and affects the accuracy of ultrasound diagnostics.
- Scatter is one of the 5 basic interactions of U/S waves with tissues. Explain it.
Scattering in ultrasound (U/S) occurs when sound waves encounter small structures or irregularities in tissue that are smaller than the wavelength of the sound beam. Instead of reflecting in a single direction, the waves are deflected in multiple directions.
Scattering contributes to the texture of the ultrasound image and provides valuable information about tissue composition. However, excessive scattering can reduce image clarity by dispersing the sound energy, causing a loss of signal strength and introducing noise into the image.
- Attenuation is one of the 5 basic interactions of U/S waves with tissues. Explain it shortly.
Attenuation in ultrasound (U/S) refers to the gradual loss of sound wave energy as it travels through tissue. This energy loss occurs due to three main factors: absorption, scattering, and reflection.
As the ultrasound waves penetrate deeper into the body, they lose intensity, resulting in weaker echoes returning to the transducer. Higher frequency waves experience more attenuation than lower frequency waves, which is why deeper imaging often requires the use of lower-frequency ultrasound.
Attenuation affects image quality by reducing clarity and limiting the depth of tissue that can be effectively visualized.
The US transducers currently used are called (2)
broad bandwidth or multifrequency transducers. These transducers support movement of the center frequency among a multitude of different frequencies.
For example, a transducer with a range between 6 and 10 MHz may allow central frequency placement at 10, 8, and 6 MHz.
higher frequencies result in decreased
capability for penetration;
however, higher frequencies allow better resolution due to the smaller wavelength of the US waves.
The U/S transducer contains
piezoelectric crystals that deform physically when electrically stimulated, thereby creating an oscillating mechanical pulse that is transmitted into tissues.
There are 3 different crystal arrays for US transducers. Name them.
What’s the difference?
curved, linear, and microconvex-curved arrays.
Curved, microconvex-curved, and linear array transducers differ in the configuration of their crystals: the crystals in a linear transducer are arranged in a line whereas the crystals in a curved array transducer are arranged in a convex array.
The higher the number of returning echoes, the ? the area of the image will be; this result is termed ?.
If there is a relatively small number of returning echoes from a location, then a ? will be assigned and the area termed ?.
The higher the number of returning echoes, the brighter (whiter) the area of the image will be; this result is termed hyperechoic.
If there is a relatively small number of returning echoes from a location, then a blacker or darker gray scale value will be assigned and the area termed hypoechoic or anechoic.
Each U/S image has 3 various fields or zones. Name them.
a near field, focal zone, and far field
The near field is
the area closest to the skin surface/transducer interface or coupling.
The focal zone is
the area where the U/S beam is thinnest (several mm) and has the best resolution; this zone is marked by small triangles along the right side of the image, typically adjacent to the depth markers (cm).
The far field is
includes tissues deep to the focal zone. In this area, the beam will start to broaden, diminishing resolution.
Ultrasound artifacts can be divided into
useful and non-useful artifacts (Table).
Useful artifacts provide information regarding the physical properties of the tissue being imaged.
Non-useful artifacts complicate image interpretation and do not generally result in clinically relevant information.
The 2 most common useful artifacts include:
Through transmission
Distal acoustic shadowing (soft tissue–gas or soft tissue–mineral interactions).
Explain useful artifact “through transmission”.
also known as distal acoustic enhancement—occurs when US waves traverse a cystic structure filled with fluid, rather than a soft tissue dense organ. Attenuation occurs as sound waves travel through soft tissue. So when they travel through a fluid filled structure, more U/S waves are available to interact with the tissue deep to the cystic structure.
Through transmission artifacts can also be used to help interpret US images by distinguishing between solid anechoic nodules and cystic structures. For example, anechoic lesions caused by lymphoma do not demonstrate through transmission (ie, there is no area of increased echogenicity distal to the tissue) because these mass lesions, although they are anechoic, are not fluid filled.
Explain useful artifact “Dirty shadowing”
Dirty shadowing is characterized by reverberation artifacts due to gas trapped in air bubbles, resulting in multiple reflections and a straight echogenic line that originates at the gas–soft tissue interface and extends deep into the tissue (Figure 2); also called a comet tail artifact.
Explain useful artifact “Clean shadowing”.
Clean shadowing is characterized by a mineral–soft tissue interface in which all U/S waves are reflected at the interface, which results in a bright echogenic line or curve (based on physical shape of the mineral, stone, or bone) and no U/S interactions (black) deep to the echogenic line (Figure 3).
Non-useful artifacts include (5)
side or grating lobe artifact
refraction artifact
transmission speed error artefact
mirror image artifact
contact artifact
Describe non-useful side or grating lobe artifact.
refers to weak, unwanted sound waves that are emitted at angles away from the main ultrasound beam. These secondary lobes can cause echoes to appear from structures located outside the primary beam’s path, leading to the display of false or misplaced images.
The most common place this effect is seen is inside the urinary bladder. In this case, focal echoes appear inside the urinary bladder that can be mistaken for pathology, such as debris or other echogenic material. In order to remove the side lobe artifact, reposition the probe so that the side lobe no longer interacts with a reflector of US waves.
You can also bounce the probe on the patient and see if the echoes swirl as in the case of echogenic material in the urinary bladder. Side lobes will not create this swirling pattern.
Describe non-useful artefact, refraction artifact.
Refraction occurs as the US beam impacts on a curved surface, resulting in US waves that are not reflected directly back to the transducer, instead bending away from the curved surface.
These US waves are not interpreted as a returning echo by the machine, which results in a zone that lacks US waves deep to the curve and, therefore, no information is provided about that area (Figure 5).
Creation of a hole in the cranial border of the urinary bladder is a unique refraction artifact seen in the urinary bladder of a cat or dog with an abdominal effusion.
Describe non-useful artifact, transmission speed error artifact.
Speed propagation errors are calculation errors for placement of images (Figure 6) that occur because the average speed of sound is used by the machine to calculate the depth from which the reflected US wave originated.
US waves travel more slowly through fat compared with the transmission speed through soft tissue structures.
If US waves are traveling the same distance toward a deeper object through fat or through soft tissue, this object is registered by the machine at 2 different distances.
The object is registered disproportionately deeper if it is below fat.
Describe non-useful artifact,
mirror image artifact.
Mirror image artifacts result from US waves interacting with highly reflective boundaries at depth (Figure 7). The waves are bounced back and forth between the highly reflective interface and tissues; then return to the surface for registration with the US transducer.
Because the bouncing increases the length of time the US waves are in the tissues, the image generated appears to be deeper in the tissues, beyond the highly reflective boundary.
The diaphragm–lung interface is the most common place for US waves to travel to the highly reflective interface, begin a return trip to the transducer, but then interact with tissue again, reflecting back toward the diaphragm.
There, the US wave is reflected again, but this time travels all the way to the transducer. Due to the longer travel time, the US reflection point is interpreted by the US machine as deep to the diaphragm–lung reflective boundary.
Describe non-useful artifact,
contact artifact.
Contact artifacts occur when the transducer surface does not contact the skin adequately, resulting in a dark band originating at the near field surface between the skin and probe (Figure 8).
Moving the transducer, or applying more gel for better contact between the surface of the skin and transducer, can easily resolve this artifact.
False assumptions are errors made by
the US machine when placing information about the scan area on the screen. These errors can result in misinterpretation of US images, misdiagnosis, or a missed diagnosis.
ULTRASOUND MACHINE CONTROLS
There are 7 basic controls the sonographer
must understand when doing an abdominal U/S examination:
- On/off or power switch
- Probe adjustment
- Frequency adjustment
- Depth adjustment
- Focal zone adjustment
- Gain adjustment + time gain compensation (TGC) or depth gain compensation (DGC)
- Image contrast settings (ie, dynamic range or log compression).
A preset is
a specific combination of control settings
that have been adjusted and set by the sonographer.
The machine comes with factory presets, but the basic and advanced settings are usually changed to reflect the personal preference of the sonographer.
Most patients can initially be imaged with
a insert probe type
microconvex curved array transducer.
Review Guide to U/S controls -table.
focal zone adjustment - depth at which the image has the highest resolution
B color map - shades of a color are used rather than grayscale
Every US beam (primary) is narrowest at the focal zone and then broadens or diverges deep to the focal zone.
Describe The focal zone:
- Provides the best detail in the x-direction because this is the thinnest section of the US beam.
- Should be set at or just below the area or organ of interest (Figure 2).
- Is typically displayed as a triangle or arrowhead that can be moved between the near and far field.
Note that each transducer has a marker that signifies..
the direction of the US beam relative to the image.
Each machine’s marker is different,
but convention states that, when the transducer is oriented parallel to the long axis of the dog or cat, the marker should be positioned cranially.
Use the Gain adjustment + time gain compensation (TGC) or depth gain compensation (DGC) controls to adjust
overall image brightness (Figure 3).
However, remember that this is a postprocessing technique that increases the overall whiteness of the screen.
It has no effect on the production,
transmission, or processing of US waves.
Dynamic range—or log compression or contrast—controls..
the overall grayscale of the image.
- A short dynamic range results in an image that is black and white with very little grayscale, as one would use for an
echocardiogram. - A long dynamic range has little contrast and a long grayscale, which results in many different shades of gray; subtle lesions
may not be apparent.
An intermediate dynamic range is preferable for
the abdomen.
To increase lesion conspicuity, the sonographer can (2)
decrease the dynamic range (increase contrast) or use a B color map, wherein shades of a color (blue, yellow, magenta) are used rather than grayscale.
