TITLE: Basic Ultrasonography & Sonographic Artifacts

Title: Basic Ultrasonography & Sonographic Artifacts
Author(s): George Henry, DVM, DACVR
Address (URL): http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3201&O=VIN
Objectives
Present basic physical principles required to understand diagnostic ultrasound imaging.
Discuss the basic controls of an ultrasound scanner and their function in producing a diagnostic quality image.
Review basic descriptive terminology used for describing ultrasound images and findings.
Review and discuss the cause and recognition of common ultrasound artifacts relevant to the interpretation of ultrasound images.
General Key Points
What is diagnostic ultrasound?
Diagnostic ultrasound is the use of sound waves to image the soft tissues and fluids of the body.
Sound is transmitted through tissues by areas of compression and rarefaction at an average speed of 1540 meters per second.
Image production relies on reflective surfaces within the tissues that reflect sound waves back to the transducer!
Utilizes sound waves in the 2-15 MHz (Million Hertz (cycles per second)) range.
Reflective surfaces occur at the interface between tissues/substances with different acoustic properties (acoustic impedance).
Acoustic impedance = velocity of sound in that tissue x tissue density
Air: 0.0004
Fat: 1.38
Water: 1.54
Brain: 1.58
Blood: 1.61
Kidney: 1.62
Muscle: 1.70
Bone: 7.8
Note that bone and air have very different acoustic impedance than the normal soft tissues and fluid of the body.
The percent of sound reflection of a reflective surface is directly proportional to the magnitude of the difference in acoustic impedance's of the tissues at an interface!
Interface: % Reflected
Blood-brain: 0.3
Kidney-liver: 0.6
Liver-muscle: 1.8
Blood-fat: 7.9
Liver-fat: 10.0
Muscle-fat: 10.0
Muscle-bone: 64.6
Soft-tissue-gas: 99.0
Interfaces between bone or air and soft tissues produce highly reflective interfaces that prevent imaging of tissues deep to the interface. An air - soft tissue interface is essentially like looking in a mirror. The ultrasound probe cannot see through that interface because practically all of the sound is reflected back.
Image on the monitor is made of multiple scan lines of gray scale dots.
Depth, y axis, is determined by 1540 m/s times round-trip time of echo divided by 2.
Brightness of dot on display directly related to intensity of the echo.
Scan line determines x axis position.
Sound wave is produced as a pulse of sound.
Pulse = approx. 2-3 wavelengths.
Sound pulse produced approximately 1% of time.
Listening for returning echoes approximately 99% of time.
Resolution
Axial Resolution
Directly related to frequency of transducer.
Related to wavelength / spatial pulse length.
Increase frequency = Increase resolution.
Lateral Resolution
Determined by beam diameter or width.
Attenuation of the Sound beam produced by the following factors:
Reflection loss
Absorption loss
Scattering loss
Refraction loss
Increase frequency = Increased attenuation = Decreased penetration.
Transducer Selection
Select highest frequency transducer that will allow sufficient penetration to image organ of interest!
Multiple frequencies used for many exams.
Ultrasound Image Display
A-mode (Amplitude Mode)
Line graph type display showing depth along x axis and y axis indicates intensity (Amplitude).
No anatomical image.
First ultrasound was A-mode.
Used in some high resolution ocular exams.
B-mode (Brightness Mode)
Line of dots.
Depth along x-axis.
Brightness of dot indicates intensity of the echo.
M-mode (Motion Mode)
Initially was B-mode line imprinted on moving paper so the lines produced represented movement of structures such as ventricular walls of the heart.
Present equipment sweeps the B-mode line across the monitor.
Still used for many cardiac measurements.
Usually will come as part of the cardiac package of a scanner.
Static B-mode
Articulated probe was used to sweep through anatomy and "paint" an anatomical image of the soft tissues.
First ultrasound scanners to produce anatomical images.
Movement during the scanning seriously degraded images.
No one uses these scanners any more!
Real-time B-mode
Current scanners are of this type.
Image made from multiple B-mode scan lines placed together to form an anatomical image of the tissues.
Image is updated frequently (frame rate) to give the appearance of real time motion.
Ultrasound Transducers
A transducer is a device that changes one kind of energy into another.
Ultrasound transducers change electrical energy into mechanical energy (sound) and then change the returning mechanical sound energy into electrical energy for processing by the scanner.
Types of Ultrasound Transducers
Linear-Array Transducer
Produces rectangular image.
Advantage-wide image in near field (close to transducer).
Advantage-simple electronics (less expensive).
Disadvantage-not able to see larger image beyond a small superficial acoustic window.
Mechanical Sector Transducer
Produces sector type image.
Advantage-relatively simple electronics (less expensive).
Advantage-small near field image that widens allows observation of structures through a small acoustic window-e.g., cardiac ultrasound.
Disadvantage-only a small portion of structures in near field are displayed.
Disadvantage-usually has fixed focal zone.
Disadvantage-moving parts.
Phased-array, Curved-array Transducers
Produces sector type image.
Most common type used by newer higher quality equipment.
Advantage-allows dynamic changing of focal zones including multiple focal zones to improve image quality.
Advantage-no moving parts.
Disadvantage-more expensive.
Annular Array Transducers
Similar to phased/curved array transducers except they use circular crystals to produce a cone shaped sound beam.
Knowledge of cross-sectional anatomy is essential for recognition of structures and abnormalities!
Consistent orientation of image planes important to decrease confusion and prevent misinterpretation of relational abnormalities.
Example: For sagittal and parasagittal images of the abdomen orient cranial to the left of the screen and caudal to the right of the screen.
Terminology is related to position, echoic intensity and echo texture.
Hyperechoic / echogenic / echo rich / high intensity (bright areas).
Anechoic / echo free (black areas).
Hypoechoic / echo poor (dark to medium gray scale areas).
Focal, multifocal, or diffuse describe physical extent of lesion or lesions in an organ.
Fine, medium, or coarse parenchymal texture refers to small or large "dots."
Uniform (homogeneous) or nonuniform (heterogeneous) can refer to texture and echogenicity. Therefore, the terms should be combined to indicate what is being described. "Heterogeneous appearance" is confusing as it does not indicate what is heterogeneous-texture, echogenicity, or both.
General Scanner Controls
There are many controls on most ultrasound scanners that vary from machine to machine and should be understood to produce the maximum image quality and information. A number of control settings are stored in "presets" for various different ultrasound examinations. These allow optimum beginning settings for different exams such as cardiac, vascular, abdominal, small parts, extremities etc. The following list concerns the most frequently used controls and their basic impact on the image produced.
Power Switch (on/off)-always best to turn equipment off before unplugging the equipment.
Power / Intensity / Output Control-Some scanners have a knob labeled power, intensity or output. This controls the power or intensity of the sound beam produced by the transducers. It is basically like a volume control on a speaker. Most of the time this will be set to the highest value depending on the equipment.
Gain (Amplification)-This is the overall gain control that controls the amount of gain or amplification of the sound detected by the transducer. Increasing this control will brighten the entire image. Setting the gain too high will brighten the image but will also increase the "noise." Setting the gain too high will decrease observation of subtle changes in echogenicity and echo texture.
Reject-Some scanners use the term "filter." This control allows "filtering out" selected intensities from the image. This can be used to filter out some noise from the image. Setting this control incorrectly can cause loss of useful information and should be used with caution.
Time-Gain (Depth-Gain) Compensation
Individual controls for amount of gain/amplification of individual depth zones in the image. This allows adjustment of the image so that there is a uniform appearance/brightness of a structure.
A common example is a sagittal image of the liver will necessitate increased gain/amplification of the deeper portions and decreased gain of the near field to give a uniform medium echoic parenchyma.
Depth Control-This controls the depth of view indicated in mm or cm. Start with deeper depth views to get orientation and then decrease depth as needed to view superficial organs. Depth of view must be adjusted frequently to optimism the image for interpretation.
Focal Zone Control
Available only on scanners that allow dynamic focus control. This control allows the sonographer to change the depth of the focal zone for optimal imaging of target structures. Most machines with this feature also allow multiple focal zones to further enhance the image quality. However, using multiple focal zones will usually significantly decrease the frame rate of the scanner. The "frame rate" is the rate at which the image on the screen is updated. Frame rate is critical for faster moving structures such as the heart. Dropping below 15 frames per second will produce a "jerky" image if any movement is present.
Freeze Button
This button allows the current image to be "frozen" to allow printing, saving and/or measurements of the image. When the freeze button is active, the transducer is not active. The freeze button should be activated when no active imaging is being performed for a long time. When the probe is active and not interfaced with tissue, the transducer may heat up and shorten the life of the transducer. Some machines will automatically go into freeze mode if there is no use of controls or buttons for a set period of time.
Ultrasound Artifacts-What you see is not necessarily what or where you think!
Acoustic Shadowing
Occurs at interfaces that reflect and/or absorb a significant portion of the ultrasound beam.
Clean Shadows-Echogenic surface with dark acoustic shadow deep to the structure. Commonly seen with mineral interfaces with tissues and fluid such as bone surfaces, calculi, and foreign bodies.
Dirty Shadows-Usually a highly echogenic surface with shadow containing "noise" that appears to indicate echoes are coming from deep to the shadowing interface. These are usually produced by gas interfaces with tissues and fluid.
Size matters! Smaller mineral objects and small air bubbles may not show the typical type of shadow! The sonographer must recognize that the "echoes" apparently distal to these interfaces do NOT represent actual structures in the area of the shadow!
Higher frequency transducers usually show better shadows!
Acoustic shadows are more distinct if the object is in the focal zone.
A small diameter calculus that is not as large as the ultrasound beam width may not produce a shadow.
The "dirty" versus "clean" acoustic shadowing is not always definitive for mineral versus gas. In some situations gas can produce a clean shadow.
Gas and fecal mater in the colon frequently produce a mixed type of acoustic shadowing.
Reverberation Artifact
Due to highly reflective surface. Especially very smooth reflectors perpendicular to the ultrasound beam. Produces dirty acoustic shadow with multiple parallel echoic lines deep to the interface.
Gas such as in the lung, GI bubbles or free air within a body cavity are the most frequent cause.
Metal such as metal sutures, bone plates, sewing needles, etc. will also produce reverberation artifact.
Smaller width strong reverberation artifacts called "Comet tails" are commonly seen with irregular gas interfaces.
"Ring down" artifact due to resonance in air bubbles appears similar to comet tails.
Mirror Image Artifact
Type of "multipath" artifact.
Associated with highly reflective curved surfaces
Produces duplicate of structure deep to the reflective surface
Liver appearing on the thoracic side of the diaphragm (intrapulmonary air and soft tissue interface) is the most commonly recognized example.
Beware of a false diagnosis of diaphragmatic hernia due to this artifact!!
Through Transmission Enhancement
Increased echogenicity of tissue deep to an area or structure.
Most often seen with fluid filled structures.
Due to improved penetration of sound through fluid because of decreased attenuation of the sound beam. Therefore the sound deep to the fluid filled structure will appear brighter than adjacent tissue at the same depth. Sound adjacent to the fluid filled structure passed through tissue that attenuated the sound beam more than occurred to the sound beam passing through the fluid. Since echoes from a given depth all the way across the image are amplified equally, the area deep to the fluid filled structure will appear brighter.
Useful in identifying fluid within structures.
Slice thickness
Echoes from two structures within the width of the sound beam are combined.
Commonly causes pseudosludge in the gall bladder
Side-lobe or grating-lobe artifact
Weak echoes produced by the transducer outside of the main sound beam produced weak reflections from echogenic interfaces out side of the main beam. These weak echoes can be reflected back to the transducer and detected. The ultrasound scanner assumes echoes occur along a straight line and places the echoes along the main scan line. These low intensity echoes are usually not observed in relatively echoic tissue as the main echoes are of higher intensity and "cover" up the weak echoes. However, when anechoic or very hypoechoic areas are present in the image, these weak echoes will show as echoes from within the anechoic structure.
Commonly seen as pseudoechoes within the urinary bladder from echoes produced by echogenic gas interface within the colon.
Edge Shadowing
Most common with round structures.
Occurs at the edge parallel to the sound beam.
Causes dark shadow distal to the edge.
Due to refraction and reflection factors
Summary
Ultrasound imaging requires a good knowledge of the physical principles of sound/tissue interactions that produce the image in order to understand the normal and abnormal tissue appearances.
The sonographer must understand that ultrasound is significantly different than other diagnostic imaging modalities such as x-rays. Radiographs record transmission of x-rays through tissues with different absorption properties (density) whereas ultrasound is looking at reflected sound waves returning to the transducer that are not necessarily directly related to "density" of the tissues.