By Kimela Hardy, MA, RT(R), RDMS
Available literature states the fetal heart beat begins its lifelong work at approximately six weeks, and depending on the sonographer’s skills, ultrasound system, and maternal body habitus, the heart beating may be visualized at this time. There are several factors that can be used to not only see this little miracle at work, but also improve general images.
Back to the Basics of Ultrasound
Thermal Index is the heating of tissue as ultrasound is absorbed by tissue, measured by ratio of power used to produce a temperature increase of 1°C. This is measured in soft tissue (TIS), bone (TIB), and in the cranium (TIC).
The Mechanical Index is an ultrasound measurement used as estimation of the risk of non thermal effects and the degree of bio-effects a given set of ultrasound parameters will induce; Higher MI means a larger bio-effect. These can include cavitation, the formation of transient or stable bubbles, which can damage tissues. The current Federal Drug Administration has set the maximum MI at 1.9
MI = PNP Peak Negative Pressure of the ultrasound wave
Before a specific organ, for example the fetal heart, image can be improved on, first obtain the best image possible. To begin any ultrasound study, but especially in Obstetrical scanning, the correct manufacturer’s Preset must be selected. Presets are essentially a “recipe” set for the ultrasound system. These parameters may include depth, gain, frequency, and focus among other factors. Using the OB Preset sets the Thermal Index (TI) and Mechanical Index (MI) which are generally lower for obstetric ultrasound examinations. In general, the TI and MI are not deliberately manipulated during routine ultrasound examinations.
Which Knobs Can Improve Your Picture?
Once the Preset is selected, consider the overall gain in the image on the monitor. Is it all black, all white, or a combination with many grays? Adjust the overall gain, often a large dial easily accessible, so it is easiest to identify the landmarks and in general is appealing to one’s eye and interpretation. This may differ somewhat with each sonographer, but not to an extreme.
The importance of correctly interpreting the landmarks cannot be over stressed, know the anatomy well.
Be sure the size of your image, or depth, allow demonstration of the area of interest. On some machines, this is either a dial knob or toggle switch labeled Depth, Size, or a combination of these. There is a scale on either side of the image that registers this depth in either centimeters or millimeters, and changes as the dial/toggle is adjusted.
Most transducers/probes are multi-herz, which means they offer more than one frequency, usually 2, 4, and 6 MHz. Once the landmarks have been identified and the overall gain is satisfactory, try each frequency with a simple adjustment and determine which provides the best penetration and resolution.
- The lower the frequency, the higher the penetration but lower the resolution.
- The higher the frequency, the less the penetration but the better the resolution.
This means images of a patient with Large Maternal Body Habitus (LMBH) most often improves with the lowest frequency, and our smaller, more athletic patients can use the higher frequency for better resolution images. The frequency is often displayed at the top of the image where the TI and MI are located.
The optimal area of the ultrasound beam is the focus, demonstrated by a triangle or karat along the depth scale. Place this at the area of interest at the correct depth. On some systems, the focus makes a significant difference in clarity, but in other systems, there does not appear to be much change.
After the above have been set to optimize the image, the slide pods or TGC/STC can be used to fine tune the image even more. These are a step alteration in the gain, with the slides on the top affecting the top of the image and vice versa. Most often the “slope” is a gradual downward slope to the right.
Manufacturers frequently have specific image enhancing features under proprietary names which reduce haze, clutter, and artifacts allowing for improved clarity of images. These harmonic features may allow for increased penetration without details lost. Simply turning this feature on and determining its benefit (or not) is required.
Looking at the Heart
Once the optimal image has been achieved by using the features discussed above, there are additional tips to see that small fetal heart.
Some systems have a Field of View (FOV) which has the effect of “coning down” and creating a smaller field visible and increases image clarification. This is the consequence of taking only a portion of the available area to scan instead of the entire area seen prior to using this option. Often, a pie-shaped icon is on the image top to illustrate and highlight the FOV area.
Using the Zoom option will increase the image size, which also can make it easier to visualize the fetal heart. In addition, most of the Zoom also has a feature which allows the size of the area, or box, to be increased/decreased. Another key to using a zoom option is to be certain the item of interest is directly in the center of the box.
When viewing the small fetal heart, another gain adjustment making the image brighter aids in recognizing the wave form during Motion-mode (M-mode). This gain is sometimes located by turning the M-mode dial. The brighter the image, the more likely the wave form is visualized. Also, the wave form will be in direct relationship to the location of the heart in the 2 Dimensional (2 D) image. For example, if the heart is in the center, the q, r, s, etc. waves will be in the center of the strip. If the heart is at the bottom of the image, the wave form will be at the bottom of the strip.
Oftentimes, maternal respirations interfere with achieving a well demonstrated strip. To overcome this, ask the patient/client to suspend breathing or hold her breath. Be aware, if she takes in a deep breath, the fetal heart may move out of the image, and you will need to make the necessary adjustments.
All of these discussed options to improve ultrasound images pertain to both Transabdominal and Transvaginal imaging. However, it is reasonable to anticipate that Transvaginal images will be larger and therefore improve the ability to obtain a fetal heart rate.
Using these tips should increase the skill set and confidence for the nurse sonographer and show this little miracle to his or her maximum potential. The tips prior to the “M-mode” can be used for general imaging as well.