header


Physics and Technical Facts for the Beginner

Quick Image Reference

Illustration 1:  The illustration shows a schematic drawing of wave length, pressure and amplitude.

Illustration 2: Hearing us animals and humans.

Illustration 3: Absorption, reflection, refraction and scatter.

Figure 1: Sample of B-Mode image.

Figure 2:  M-Mode (lower portion of the image) combined with B-Mode image.  In this still image the M-mode captures the movement of a particular part of the heart.

Figure 3:  Color Doppler image.

Figure 4:  Power Doppler image.

Figure 5:  Sample image of Pulsed Wave Doppler showing arterial flow.

Figure 6: Attenuation (shadowing) artifact caused by gallstones.

Figure 7:  Posterior enhancement, side lobe and mirror artifact.

Figure 8: Edge artifact.

Figure 9:  Sample of reverberation artifact.

Figure 10: Comet tail artifact.

Figure 11: Samples of probes commonly used in the emergency department.

Illustration 4: Spatial orientation.

Beatrice Hoffmann, M.D., Ph.D., RDMS, C. Heather Rumsey, M.D.,
Matthew S. Nixon, MA, MS.

This chapter is designed to introduce the ultrasound beginner to basic concepts in ultrasound physics and managing and manipulating a machine. Further reading material can be found in a variety of great ultrasound books, publications and on the internet.
Only some basic and universal ultrasound machine functions will be explained – every novice sonographer should spend time with the specific ultrasound machine used during his or her rotation in the emergency department.

I.  Basic Ultrasound Physics
Sound is a mechanical wave, which requires a medium in which to travel.  More accurately, it is a series of pressure waves propagating through a medium.
One cycle of the acoustic wave is composed of a complete positive and negative pressure change.  The wavelength is the distance traveled during one cycle, the frequency of the wave is measured in cycles per second or Hertz (cycles/s, Hz), (illustration 1).

Illustration 1:  The illustration shows a schematic drawing of wave length, pressure and amplitude.

For humans audible sound ranges between 16 Hz and 20.000 Hz (20 kHz).  The hearing range of other species can be much higher than 20 kHz and is inaudible for us.  These higher wave frequencies are referred to as “ultrasound” (Illustration 2).


Illustration 2:  Hearing range in various animals and humans.

The speed with which an acoustic wave travels through a medium is determined by the density and stiffness of the medium.  The greater the stiffness, the faster the wave will travel.  This means that sound waves travel faster in solids than liquids or gases.  Acoustic waves are calculated to travel through human tissue at body temperature at approximately 1540 m/s (about one mile per second).
When traveling through a medium the sound waves intensity and amplitude reduces.  This is called attenuation and is the reason why echoes from deeper structures are weaker than echoes from superficial areas.  The major source of attenuation in soft tissue is absorption, which is the conversion of acoustic energy into heat.  Other mechanisms are reflection, refraction and scatter.
The sound wave encounters a boundary between two different media.  Some of the wave bounces back towards the source as an echo (reflection).  The angle of incidence is identical to the angle of the reflection. The remaining sound wave travels through the second medium (or tissue), but is “bent” from its path.  The angle of incidence will be different from the angle of transmission.  The amount of deflection is proportional to the difference in the two tissues ‘stiffness’.  Scatter occurs when ultrasound waves encounter a medium with a nonhomogeneous surface.  A small portion of the sound wave is scattered in random directions while most of the original wave continues to travel in its original path.



Illustration 3:  Absorption, reflection, refraction. Scatter between the unhomogeneos border of two different mediums.

The production of ultrasound waves is based on the so-called ‘pulse-echo-principle’.  The source of the ultrasound wave is the piezoelectric crystal, which is placed in the transducer.  This crystal has the ability to transform an electrical current into mechanical pressure waves (ultrasound waves) and vice versa.  Once the ultrasound wave is generated and travels through the medium, the crystal switches from ‘sending’ into ‘listening’ mode and awaits returning ultrasound echoes.  Actually over 99% of the time is spent “listening”. This cycle is repeated several million times per second. This principle is called “pulsed-echo” principle.  Returning sound waves are converted into images on the ultrasound monitor.

Diagnostic ultrasound used for common medical imaging uses frequencies between 2 and 20 million Hertz (Megahertz, MHz).
Lower frequencies are able to penetrate deeper into tissue but show poorer resolution.  In contrary higher frequency ultrasound will display more detail with a higher resolution in exchange for less depth penetration.  This is a very important principle when choosing your probes and frequencies.1-8

II.  Ultrasound Modes
The most important mode for the ultrasound-beginner is the “B-mode”.  B-mode stands for ‘brightness mode’ and provides structural information utilizing different shades of gray (or different ‘brightness’) in a two-dimensional image (Figure 1).

Illustration 3

Figure 1:  Sample of B-Mode image.

M-mode stands for ‘motion mode’.  It captures returning echoes in only one line of the B-mode image but displays them over a time axis.  Movement of structures positioned in that line can now be visualized.  Often M-mode and B-mode are displayed together on the ultrasound monitor. (Figure 2)

Illustration 3

Figure 2:
  M-Mode (lower portion of the image) combined with B-Mode image.  In this still image the M-mode captures the movement of a particular part of the heart.

The Doppler mode follows very sophisticated and complex laws of physics.
It utilizes a phenomenon called ‘Doppler shift’, which is a change in frequency from the sent to the returning sound wave.  These changes or ‘shifts’ are generated by sound waves reaching moving particles.  The change of frequency/amount of shift correlates with the velocity and direction of particle motion.
In simplified terms, the Doppler mode examines the characteristics of direction and speed of tissue motion and blood flow and presents it in audible, color or spectral displays.

Color Doppler ultrasound is also called color-flow ultrasound.  It is able to show blood flow or tissue motion in a selected two-dimensional area.  Direction and velocity of tissue motion and blood flow are color coded and superimposed on the corresponding B-mode image (Figure 3).

Illustration 3

Figure 3:  Color Doppler image.

Power Doppler:  Unlike color Doppler, common power Doppler does not examine flow velocity or the direction of flow.  It looks at the amplitudes of the returning frequency shifts and is able to detect even states of very low flow (Figure 4).  This is of use when examining vascular emergencies such as testicular or ovarian torsion.

Illustration 3

Figure 4:  Power Doppler image.

Spectral Doppler consists of a continuous and pulsed-wave form. Continuous wave Doppler is often available as a separate small hand-held unit containing discrete transmitting and receiving piezo-electric crystals.  This allows for simultaneous transmitting of ultrasound waves and receiving of returning Doppler shift signals, which are converted to audible frequencies over a loudspeaker.  No image is produced.  This technique is often utilized at the bedside to demonstrate patent vessels or fetal heart tones in pregnancy. Pulsed-wave spectral Doppler shows the “spectrum” of the returned Doppler frequencies in a characteristic two-dimensional display. Venous flow demonstrates a more continuous, band like shape.  Arterial flow shows a more triangular shape (Figure 5).1-8

Illustration 3

Figure 5:  Sample image of Pulsed Wave Doppler showing arterial flow.

III.  Artifacts
Artifacts refer to something seen on the ultrasound image that does not exist in reality.  An artifact can be helpful interpreting the image or it can confuse the examiner.  Several commonly encountered artifacts are mentioned below.

Attenuation Artifacts:
Shadowing:
This artifact is caused by partial or total reflection or absorption of the sound energy.  A much weaker signal returns from behind a strong reflector (air) or sound-absorbing structure (gallstone, kidney stone, bone, figure 6).



Illustration 3


Figure 6:  Attenuation (shadowing) artifact caused by gallstones.

Posterior Enhancement:
In posterior enhancement, the area behind an echo-weak or echo-free structure appears brighter (more echogenic) than its surrounding structures.  This occurs because neighboring signals had to pass through more attenuating structures and return with weaker echoes (Figure 7).

Illustration 3

Figure 7:  Posterior enhancement, side lobe and mirror artifact.

Edge Shadowing:
The lateral edge shadow is a thin acoustic shadow that appears behind edges of cystic structures.  Sound waves encountering a cystic wall or a curved surface at a tangential angle are scattered and refracted, leading to energy loss and the formation of a shadow.

Illustration 3

Figure 8:  Edge artifact.

Propagation Artifacts:
Reverberation:
Reverberation occurs when sound encounters two highly reflective layers.  The sound is bounced back and forth between the two layers before traveling back.  The probe will detect a prolonged traveling time and assume a longer traveling distance and display additional ‘reverberated’ images in a deeper tissue layer (Figure 9).



Illustration 3

Figure 9:  Sample of reverberation artifact.

Comet Tail:
A comet tail artifact is similar to reverberation.  It is produced by the front and back of a very strong reflector (air bubble, BB gun pellet).  The reverberations are spaced very narrowly and blend into a small band (Figure 10).

Illustration 3

Figure 10:  Comet tail artifact.

Mirror Imaging:
If a structure is located close to a highly reflective interface (such as the diaphragm),  it is detected and displayed in its normal position.  However, the strong reflector causes additional sound waves to bend towards the neighboring anatomy, from where they are bounced back towards the strong reflector and return to the transducer.  These sound waves have a longer travel time and are perceived as an additional anatomic structure.  The image is duplicated on the other side of the strong reflector (see figure 7).

Miscellaneous Artifacts:
Ring Down:
The artifact is caused by a resonance phenomenon from a collection of gas bubbles.  A continuous emission of sound occurs from the ‘resonating’ structure causing a long and uninterrupted echo.  It appears very similar to the comet tail artifact.

Side Lobe:
This artifact is caused by low energy ‘side lobes’ of the main ultrasound beam.  When an echo from such a side lobe beam becomes strong enough and returns to the receiver, it is ‘assigned’ to the main beam and displayed at a false location.  Side-lobe artifacts are usually seen in hypoechoic or echo-free structures and appear as bright and rounded lines (see figure 7).

IV.  Probes
Several different types of probes are commonly used in emergency departments.  These transducers consist of the active element (the piezoelectric crystal), damping material and a matching layer.  Different arrangements and forms of activation of the active element have lead to a variance of probes.  The most common transducers utilized in the emergency department are listed below:

Large Convex Probe:
Main ED utilization is transabdominal sonography.
Produces a sector shaped image with a large curved top
The active element is arranged in a large curved line, also called large curved probe or transducer.

Microconvex Probe:
Utilized for transabdominal or transthoracic sonography.
Produces a sector shaped image with a small curved top.
The active element is arranged in a small curved or “convex” line, the probe can be called small curved transducer.

Linear Probe:
Main utilization is vascular sonography or evaluation of superficial soft tissue structures.
It produces a rectangular image.  The active element is arranged in a straight line.

Intracavity Probe:
Basically a microconvex probe on a large handle, it’s main utilization is endovaginal ultrasound.

Sector Probe:
Other probes utilized in emergency departments, especially for transthoracic sonography are sector probes. They produce a pie-shaped image with an angulated top. The active element is arranged in a circle and only parts of it are activated at a time and steered into the direction needed.  This arrangement provides the sector probe with an overall lower resolution as fewer “crystals” are activated at one time.  It has the advantage of requiring only very minimal skin contact or a very small sonographic window to obtain an image.(1-8)

physicsfig11

Figure 11:  Samples of probes commonly used in the emergency department.

V. Common Terminology
**The beginner needs to be familiar with a few commonly used terms:

Image Interpretation:

Image Acquisition / Probe Positions:

 

Illustration 4: Spatial orientation.

 

VI.  Your Machine Functions
This section will list several important machine functions.  They are more or less universal to all ultrasound equipment.  Information is kept as general as possible to make it applicable to most machines.  Make every effort to be as familiar as possible with most of these functions.

On / Off - Remember where to switch the machine on before going into the patient’s room!

Select / Change Probes - Select a specific probe.

Set - Press to select from an activated menu; press to select/fixate a measurement point.

Preset Menu - Select from preset menus such as general abdominal / vascular / procedures / OB or others.  Use the scroll ball to navigate the menu.

Scroll - Use the scroll ball (or in some machines a touch pad) to move the curser within the image or navigate through menus.  After freezing an image-moving the scroll ball will display the last few images just right before you pushed the freeze button (these images are called cine-loops)

Gain
- Changes overall strength of returning echoes, functions as an amplifier.

TGC - Changes strength of returning echoes in a certain depth.

Depth Adjustment - Increases or decreases the depth of the ultrasound beam.

Freeze - Push to freeze the current image.

Print / Save - Will print a frozen image and/or save an image to a hard drive.

Measurement / Cursor
- After activating the first measurement button – a marker will appear on the screen.  Use the scroll button to place it over the desired area.  By pushing ‘set’ or ‘mark’ - the first cursor will be placed there and second cursor will appear.  Use the scroll ball and the set button to complete the process.
Additional measurements can be obtained by pushing the cursor button again.  Some machines will have extra measurement buttons.

Change Mode - Pushing the M-mode button will change the machine to M-mode, Doppler button to Doppler mode, color Doppler to color, etc.  Most machines are set up so that a “dual” screen appears when certain modes are selected (B-Mode combined with Doppler or M-Mode etc.).

Focus - Will change or add focal zones to the image.

Change Paper - Most printers are designed in a very similar way:  Push ‘open’ button on printer and insert new roll as shown inside the printer door, manually close the printer.  Start printing!

VII. References

  1. Block B.
    The Practice of Ultrasound, A Step by Step Guide to Abdominal Scanning.Thieme, New York,2004.

  2. Nielsen TJ, Lambert MJ.
    Physics and instrumentation. In: Ma OJ, Mateer JR., eds., Emergency Ultrasound. McGraw-Hill, New York,2003:45-66.

  3. Heller M, Jehle D.
    Fundamentals. In: Heller M , Jehle D, eds., Ultrasound in Emergency Medicine. Center Page: West Seneca, NY, 2nd edition,2002:1-40.

  4. Hofer M.
    In: Hofer M, eds., Sono-Grundkurs. Ein Arbeitsbuch für den Einstieg. 2nd edition, Thieme: Stuttgart,1997:6-10.

  5. Müsgen D.
    Physikalische und technische Grundlagen. In: Fürst G, Koischwitz D, eds., Moderne Sonographie. Thieme, Stuttgart,2000:1-23.

  6. Odwin CS, Dubinsky T, Fleischer AC.
    Appleton & Lange’s Review for the Ultrasonography Examination. 2nd edition, Appleton & Lange Reviews: McGraw-Hill, New York, 1997.

  7. Kremkrau FW.
    Diagnostic Ultrasound. 6th edition, W. B. Saunders Company, New York,2002.

  8. Smith RS, Fry WR.
    Ultrasound instrumentation. Surg Clin N Am.2004;84:953-971.

       

About Us | Site Map | Privacy Policy | Contact Us | Ultrasound Guide for Emergency Physicians © 2008