Equipment and Knobology:
These are the true workhorses of the ultrasound machine. Each probe contains piezoelectric crystals which vibrate under electric current. It also acts as a receiver for returning echo, which in turn, cause vibration of the crystals that the probe converts to current, that is later interpreted as data by the ultrasound machine.
While there are many types of probes available for POC (point-of-care) ultrasound, there are 4 that are encountered most commonly.
Curvilinear or Curved Array Probe: often referred to as the ‘abdominal probe’. Also low frequency (2-5MHz). Functions very similarly to phased array probe except with a larger footprint, which allows for better lateral resolution.
Endocavitary Probe: Moderate frequency (8-15MHz). Curved array. Ideal for Intraoral or trans -vaginal use.
Phased array: often referred to as the ‘cardiac probe’ this is a low frequency (1-5MHz) probe which is scanning the abdomen, retroperitoneal and even soft tissue structures. Given the low frequency, you can expect this to have good penetration, at the expense of resolution. Not great for visualizing small superficial structures. The phased array probe also has a small footprint (area of probe surface) which makes it ideal for getting between the ribs in cardiac studies.
Linear array probe: High frequency probe (5-15MHz). Unlike the above probes, the array is parallel, allowing for high lateral resolution, as well as maintaining resolution with depth. Due to this, will only see structures directly below probe. Ideal for superficial soft tissue evaluation, evaluation of neurovascular structures, and procedural use.
The Machines/ ‘Knob-ology’
Know thy Machine!
The University Hospital ED currently employs 3 different machines
A-mode: Rarely used. Plots echo as function of depth where x-axis is depth and y-axis is amplitude. Primarily useful in ophthalmologic/retinal ultrasound and for use of ultrasound beam therapy
2D or B-mode: Standard scanning mode. Spatially oriented- Structures are seen as function of brighness (echoic vs anechoic), depth, and width. Provides a cross sectional view of underlying structures. Able to see structures in real time.
B-mode used as guidance for attempted vessel cannulation
M-mode: takes a small section of horizontal beam (as would be seen in B-mode) and displays it over time. Useful to track motion/dynamic changes in a specific point of an image over a period of time. Vertical axis remains depth, while horizontal axis is time. (used in cardiac valve imaging and fetal heart rate measurements)
M-mode used to calculate TAPSE (tricuspid annular plane systolic excursion)
Color Doppler: Uses ‘doppler shift’, an induced change in frequency of returning echo to determine movement. Superimposes color to image to show flow either toward or away from probe, as indicated by a color key. By convention, movement towards probe is displayed red, and movement away as blue.
Color doppler used to show flow across mitral valve
Power Doppler: Similar to color doppler, but without assigning color to direction. Instead utilizes shades of color to represent level of flow. Useful for low flow states.
Power doppler used to show low flow state in IVC/hepatic vein
Spectral doppler: Either via pulse-wave (alternating) or continuous (simultaneous transmission and reception). Allows for assessment of flow velocity at a specific point in the image which is then displayed as a function of time. Useful for echocardiography.
Spectral doppler used to quantify flow velocity across mitral valve
Getting ready to scan
Scanning Planes: Ultrasound images are a 2-D representation of the underlying structure. Just as in other imaging modalities such as CT scanning or MRI, the image is displayed as a cross section in a particular plane. However, ultrasound can be more complicated in that the plane of section is determined by the sonographer. Just as one can obtain axial images, one can also obtain images in oblique planes.
Transverse plane- AKA axial plane. Perpendicular to the long axis of the body. Separates top from bottom.
Sagittal plane- Parallel to the long axis of the body, separating left from right. Midsagittal plane refers to the plane in the midline as opposed to parasagittal which is either left or right from midline
Coronal plane- Parallel to long-axis of the body, perpendicular to sagittal plane. Separates anterior from posterior.
Oblique plane- Not at right angles to any of the above
Probe indicator: There is always a dot, ridge or groove on one side of the probe which functions to assist with orientation. This allows one to determine which side of the probe corresponds to which side of the screen. By convention the indicator on the screen is a blue dot located on the top left of the screen image. By convention, the probe indicator has a specific position based on the imaging plane:
Transverse plane: Probe indicator towards the patient’s right side
Sagittal and Coronal planes: Probe indicator towards the patient’s head
Handling the probe:
It is important to have stable grasp on the probe, as the acquisition of a good image is often dependent on minute adjustments. In order to be able to make fine adjustments with the probe hand, it is best to hold the probe in the dominant hand with the thumb and index finger, using the other 3 fingers and ulnar aspect of the palm as a sort of ‘tripod’ to brace against the patient’s body.
Sliding: performed by sliding the entire probe over the surface of the body- generally used to find a good imaging window
Fanning (or Pitch): performed by adjusting the angle of the probe body to the skin in anterior posterior dimension. This allows the ultrasonographer to scan through different planes of an organ from one imaging window
Yaw: similar to fanning, but in the left-right dimension. Allows sonographer to extend imaging area to desired target not immediately below probe
Twisting: performed by turning the probe on its long axis. This can allow the provider to obtain an image in an area with small imaging window (i.e ribs), or to obtain appropriate plane of section given variances in anatomy (i.e cardiac views)
Modes: Machines often preloaded with imaging modes for particular organs/indications which optimize settings for study of interest (e.g cardiac, vascular, abdominal, etc.)
Depth: Adjust depth of field presented on screen
Focus: Improve resolution at a particular depth in image (not available on all machines)
Gain: Adjusts the strength of the signal returning to the probe- visualized on screen as contrast
Time gain compensation: Adjusts the gain for a specific depth in the image
AutoGain: allows the machine to calculate optimal gain settings
Calipers: used for measurement of distance
Written by: Aalap Shah, MD
Peer Reviewed by:
All images obtained by Pattie Smith, RDMS and Aalap Shah, MD except as otherwise noted