Ultrasound: physics and knobology

Nature of Sound Waves

• How far the sound waves can travel depend on
  • how strong the signal is
  • the medium that the waves have to travel though; underwater is greater medium compared to brick wall.

Wave Properties

• Penetration
• Attenuation
• Reflection

Penetration

• Ultrasound waves, depending on amount of energy, will pass through a medium to a certain depth.

Attenuation

• As ultrasound waves travel through medium they continually losing energy to medium that they pass.

Reflection

• The rest of waves' energy after travelling through medium is reflected back toward their source.
• Equals to "echo" of submarine sonar.
• Enable image generation on ultrasound machine.
• Eventually energy will not be enough to reflect back to machine, therefore no image.

Effect of Different Tissue Densities

• Ultrasound waves have different abilities to travel through different mediums.
• This quality of mediums or tissues is called resistance or impedance.
• Travel well though liquid: blood, urine.
• Travel less well through solid organs: liver, spleen.
• Not at all though bone (high resistance).
• The more a substance reflect energy back towards the probe the brighter the image appears on screen. On the other hand the less it reflects, the darker the image.
• If the probe is not at right angle to surface of interest scatter (deflection) of ultrasound waves occurs, especially when scanning aorta.

Liquid

• Appears black. No resistance therefore no echo and no energy.
• Blood filled structures: heart or blood vessels.
• Urine filled bladder.
• Cystic structures: gallbladder.
• In normal location fluid is contained and surrounded by echogenic membrane.
• Free fluid (internal haemorrhage or ascites) has no membrane and has irregular shapes.

Solid Organs

• Can function as "acoustic window", which allow deeper penetration into the body. Fluid filled bladder has the same function as well.

Bone

• Bone reflects back almost 100% of the ultrasound waves. It appears as bright white.
• No waves traverse beyond bone therefore no energy after that. Areas behind the bone appears black.
• Bone can impede the view. Also it can be useful serving as landmark, i.e.spine.

Gas

• Almost always bowel gas.
• Subcutaneous emphysema and pathologic gas-forming process in structure can also be the case.
• Appear as grey, "snow storm" appearance, which provide no information.
• Gas or air between the probe and the body can cause problem. Apply generous amount of gel to create better acoustic contact.

Tissue Interface

• The greater the different in echogenicity the easier it is to identify structures.
  • Fluid in pericardial space.
  • Free fluid in hepatorenal or splenorenal pouch.
  • Gestational sac in uterus.
  • Blood-filled vessels.

Modes of Transmission

• B: Brightness, most commonly used.
• A: Amplitude: not used anymore.
• M: Motion, often used for cardiac.
• D: Doppler; "Colour" and "Pulse Wave" functions.

Frequency

• Different types of probes have different range of frequency.
• Frequency is inversely related to penetration.
• Frequency is directly related to axial resolution (the ability to distinguish between two objects at different depth).
• Low frequency probe is good for looking deep into the body cavity (better penetration).
• High frequency probe is good for look at small objects near the surface.

Probes or Transducers

• Format is the field of view produced by the probe which appear on the screen. There are two formats; linear and sector.
  • Linear format has rectangular field of view. Usually used for looking at objects close to the surface.
  • Sector format has pie-wedge shape field of view. Usually used for looking at deep objects.
• Array is the way crystals are arranged in the probe head. There are three types which are commonly used; convex- or curved-linear, flat-linear and phased.
  • Curved-linear array probe
    • The basic all purpose abdominal probe.
    • Low frequency, 2-5 MHz, allow deep penetration into the body.
    • Has large footprint therefore hard to fit into small areas and hard to make a full contact of the entire probe head with the skin.
    • Generate a pie, wedge-shape field of view with a quarter-circle cut out of the top part.
  • Flat-linear probe
    • High frequency, 5-18 MHz.
    • Generate rectangular filed of view.
    • Great axial resolution therefore is the one usually used looking at small objects close to the surface.
    • Does not allow deep penetration into the body, approximately reliable at 6-8 cm depth.
    • Available in large and small footprint. Chose small if you need to get into small area, i.e eyeball, ultrasound-guided central line insertion.
  • Phased-array probe
    • Low frequency like curved-linear array probe but with smaller footprint and flat surface.
    • Crystals are layer and packed closely. They both send and receive signal through computer control phasing.
    • Easier to get into small area. Disadvantages are small near field and poor resolution in the far field when comparing with curved-linear array probe.
  • Other types of probes
    • Microconvex probe
      • Combination of curved-linear array and phased-array probes. Smaller footprint with greater curvature.
      • Easier to get into small area. Same downside as phased array probe; has small near field and poor resolution in the far field.
    • Endocavitary probe
      • Mild frequency microconvex probe with long handle.
      • Filed of view is fanning wider than curved-linear array probe because of greater curvature of probe head.
      • Commonly used for transvaginal scanning.

Understanding Ultrasound Images

• 2-D image of 3-D structure.
• Planes of view: the way the probe is placed along the line in different direction on the body.
  • Longitudinal view: the probe is placed along the line which is running from head to toe (along the body). The left side of the screen correlate with cephalic direction.
  • Transverse view: the probe is placed along the line which is running from left to right (across the body). The left side of the screen correlate with patients' right (patient's head is away form you, like a CT image).

Probe Placement

• The part of the body closest to the probe is displayed at the top of the screen no matter what orientation.
  • Near field: area closer to the probe.
  • Far field: area further away from the probe.
• Be consistent in the way the probe is applied to the body with correct orientation which generate proper longitudinal and transverse views.

Probe Orientation

• Identify indicator, which is a small knob/bar/protrusion on the side of the probe.
• Indicator should point toward patient's head in longitudinal plane and point toward patient's right in transverse plane.
• Echocardiographers orient the probes in the opposite way. So don't be confused if you find different orientation in some literatures.

Centering the Image

• Sliding the probe towards the indicator will move the structure on the left side of the screen toward the centre and the opposite if sliding the probe away from the indicator.

Probe Manipulation and Motion

• Slow hand movement.
• Slide: moving the probe while maintaining same orientation of the probe indicator (point towards the heard or patient's right).
  • There are two ways: slide the probe across the body or slide the probe along the body.
• Sweep: moving the probe while maintaining the same point of contact on the skin (changing the aim of the beam while looking at the are area of interest).
• Rotate: turning the probe on its long axis while maintaining the same aim of the beam at the area of interest.
  • Changing from long-axis to short axis of the area of interest, i.e gallbladder.
  • Moving away obstacles that impede the view on screen, i.e. ribs.
  • Optimize the view of the area of interest, i.e. renal or blood vessels.
• Heeling: rocking the probe back and forth along its length while maintaining the same probe location on the skin.
  • Used for centering the image when it is slightly of the centre.
  • Used for bringing the image on screen when it is slightly of the edge, i.e. visualising diaphragm.

Knobs, Buttons and Dials

Frequency Range

• Each type of probe has its own frequency range that can be set.
• Frequency is usually set at mid range as a default when turning on.
• It can be adjusted according to purpose of scanning.
• Toggle up allow better resolution, i.e. when using flat-linear array probe for placing intravenous peripheral line.
• Toggle down allow better penetration, i.e. when performing DVT scanning in obese patient.
• Some ultrasound companies referring to the frequency range indicating the higher end first, i.e. 12-6 MHz.

Cine Loop

• Function to be able to scroll though several saved images that occurred when hitting freeze button.
• Trackball or touchpad are used for reviewing theses images and select the best one available.

Calipers

• This button allow you to take measurement on the image.
• Freeze the image first before measuring on the page with calliper function.
• A crosshair will appear on the screen when hitting the calliper button.
• Use trackball or touchpad to move a crosshair to the point that you want to begin measuring. Hitting a select button to fix the first crosshair and the second crosshair will appear automatically.
• Move the second crosshair to the other endpoint then hit a select button again to fix the second crosshair.
• The machine will automatically generate measurement that you want on the screen, i.e. distance.

Zoom

• Useful for magnifying a small object that locate deep into the body.
• It enlarge the object that you are interested in but the image will become grainier.

Time Gain Compensation (TGC)

• The darker image at the deeper level occurs due to energy attenuation by surrounding tissue.
• The ultrasound machine auto compensated for this by automatically increasing the gain at deeper levels.
• TGC allows you to manually adjust the amount of gain (brightness) at different depths of the image.

See Also

• Ultrasound (main)