Don's Back Room Forum

Think Porthole: Challenges to imaging the heart with ultrasound

May 16, 2019 Don 6

There is no doubt that 2-dimensional (2D) ultrasound imaging can be a challenging task to learn. Performing ultrasound requires some imagination. A simple turn of the transducer can change the orientation of anatomy. Therefore, learning anatomy in this way is not always straightforward as with 3-dimensional fixed images. To further complicate matters, the human body presents obstacles at nearly every turn of the transducer. 

A basic review of how ultrasound waves (pressure waves) react in the human tissue is essential to understanding how to overcome many of these barriers. Looking at the interaction between sound waves entering the body and moving back to the ultrasound transducer, we can see that various changes to the soundwaves will occur.


Soundwaves_blog-post.png#asset:1389


The lower shades of black (represented by the arrows pointing left), are demonstrating a weakening of the pressure wave as it returns to the transducer. There are multiple interactions between tissue and sound waves as they travel into the body and back again to produce the final image. Presented here is an oversimplification. 

Speed of Sound

Sound speed is dependent on the medium through which it travels. The interactions of the pressure wave (returning sound) to the ultrasound transducer is what ultimately produces the image. Example:

(Approximately)

Fluids and soft tissue (blood, urine, muscle, and fat) ---------------1540 m/s

Solids (bone and calcified plaque and gallstones) -----------------4000 m/s

Air (Lungs and bowel gas) --------------------------------------------------300 m/s 

The ultrasound machine (generally speaking), looks for speeds of around 1540 m/s. Call this the “sweet spot” for good image quality. Image data is disturbed in substances like solids and gases because of the drastic change from 1540 m/s in soft tissue. These substances of varying sound speed cause a roadblock due to a dramatic change in sound speed. In elements other than the “sweet spot”, the sound speed is either way too fast or way too slow for the ultrasound system to produce a clear image. The drastic changes in sound speed produce something called artifacts.

Below are examples of artifacts created by both solids (bone) and gases (air)


Artifact_blog-post.png#asset:1388


Although these artifacts can provide useful information in a specific context, both will limit the ability to obtain clear information when imaging cardiac structures.

From the examples above, we have a rudimentary idea of what can happen to sound waves as they travel through different materials.  Artifacts can be generated by the change in sound speed. While attempting to image certain cardiac structures, we will specifically address problems encountered while performing the apical 4 and 2-chamber views. Imaging in the apical views can often present a real challenge.

Ultrasound Windows

While imaging with ultrasound, there is a term called a “window”. window is an area that allows a good pathway for the sound waves to pass in and out of the human body. The window allows for limited interference from changing sound speeds. A good window will promote sound transmission without much interference from obstacles like bone (speed too fast) and air (speed too slow). For example; the liver is such a window where the speed of sound is closely matched to the“sweet spot” of the ultrasound machine.

The liver is a large organ and is filled with blood and composed of very homogeneous tissue. As a result, sound waves produced by the transducer move well in and out of the liver. This allows the sonographer to obtain a great look at the liver and the anatomical structures near or close to the liver.


liver-window.png#asset:1402The liver is a large window that allows access to the gallbladder, pancreatic head, the right kidney and more


The liver is a good window because the organ is usually large and close to the skin. As a result, there is not much interference from bone and air (a few artifacts). Before the advent of intracavitary ultrasound, the urinary bladder was filled to provide this kind of a window into the pelvis. In many scanning situations, a full bladder is still often used for pelvic imaging.

There is a significant difference between the window into the gallbladder, right kidney, and pelvis as compared to the window into the cardiac structures. I call the cardiac window, acardiac porthole”


Porthole_v3.png#asset:1401


As seen in the images above and below, it should be obvious that the cardiac window, like a porthole, has a very limited view. To see through a porthole, you must keep your head in nearly the same location while looking up, looking down, looking to the left or to the right. A view that encompasses all the surroundings is difficult to realize in a porthole. On the other hand, while looking around in a larger window, one can appreciate the surroundings. The comparison is the liver being a larger window, while viewing the heart is more like looking through a porthole

Limitations to the cardiac window

Cardiac-Window.png#asset:1403


BoneSpace (room to move around) and Air

Bone, air, and limited space are all factors that reduce the ability to access the heart from a transducer outside of the body. When performing a transthoracic echocardiogram, the examiner will have the patient (when possible) role onto their left side. Along with the slight shifting of the heart’s apex, the left lateral position will often reduce the air gap between the transducer and the heart. In some cases, acceptable images can only be obtained with respiratory expiration.  

The examiner must always consider the location of the apex of the heart in relation to the rib spaces and the left lung. Since returning soundwaves will be blocked by bone, the transducer must fall between the correct rib space. Depending on the individual, this usually is the 4th, 5th or 6th rib space. In taller individuals, it will more likely be the 5th or 6th rib space. In shorter individuals, it may likely be the 4th or 5th rib space. Keep in mind, that there is usually just one best rib space in which to demonstrate the cardiac structures in the apical views. Once the ideal space is located, this will be the cardiac portholeMoving the transducer to another rib space, will generally not help improve the image.

Know that the person’s BMI will play a role here. In the low BMI individual that is tall and thin, the transducer approach will usually be more anterior and inferior. Additionally, an increased BMI with a shorter individual, the exam will likely require a more posterior and lateral approach. 

The importance of turning the patient onto their left side cannot be stressed enough. Turning the patient can be a real challenge for the very ill and for those who should not be moved from the supine position. In these situations, there are other options for obtaining the required information. However, the discussion is limited to the apical views in this forum. 

Beginning the exam, the transducer should be placed at the maximum impulse (PMI). In most ultrasound system protocols, the orientation of the transducer will be with the image orientation indicator directly facing the table if the system is set to adult cardiac mode (Remember that orientation in most ultrasound systems will change on the screen between abdominal mode and cardiac mode). For the apical 2-chamber view, the transducer orientation indicator will be rotated towards the patient’s left shoulder.

probe-orientation_v2.png#asset:1404


Why use the apical view if it is so difficult to obtain?

Apical-4-video-image.png#asset:1407Click image to view animation


Although challenging at times, the apical 4-chamber and apical 2-chamber views provide for great global transthoracic assessment for left ventricular wall motion. Click here or the image above to view an animation of the two views. The parasternal short axis view is a close second. From these 2 apical views, volume valuation in systole and diastole (modified Simpson’s rule) allow for quantification of ejection fraction (EF). A normal EF should be in the low to mid 50% to 70%. Left ventricular wall motion from these views should not demonstrate areas of decreased wall motion or the absence of motion. Either of these findings may indicate an infarct in that region of the myocardium. If a decrease in wall motion is demonstrated, a reduction in the normal EF will likely occur.

Obstacles to an Accurate Measurement

Speaking from experience, human error is a major factor in obtaining accurate and trustworthy numbers for the ejection EF utilizing the Modified Simpson’s Method. In our echo lab some years ago, while training a new echocardiographer, we would compare EF measurements obtained by at least two echocardiographers. At times there would be a significant variance.

Errors in the positioning the transducer related to the ideal 4-chamber and 2-chamber tracings are summarized below.

  1. An error in transducer rotation may narrow the LV chamber
  2. Transducer tilted too far anterior or too far posterior will cause the LV to be foreshortened
  3. Tracing too much of the myocardium and not the blood-tissue interface. Not measuring the true endocardium
  4. Inconsistent LV length measurement from the mitral annulus to the apex of the LV
  5. An improper angle of the LV in the ultrasound image. Usually tilted too far to the right

What constitutes a good LV volume trace?

LV-Volume_v2.png#asset:1406


(some recommendations from the ASE in orange)

  1.  A focused view of the LV in both diastole and systole
  2. Trace occurs between the blood pool and the compacted myocardium
  3. Trace occurs at end diastole & end systole – ECG can improve accuracy
  4. Must be a clear endocardial border definition
  5. Exclude the papillary muscles
  6. Exclude the endocardial trabecula in the trace (compacted myocardium)
  7. The tracing of the LV is concluded by connecting the base of the LV trace with a straight line at the MV annulus
  8. The length of the LV is measured from the center of the line in the previous step to the apical point trace


Biplane-disk-summation_v2.png#asset:1405Image courtesy of the American Society of Echocardiography (ASE)


Tips & Tricks

  1. Once the apex of the heart is clearly visualized – stay there – changing the rib space will rarely help. 
  2. Within that rib space – slide the transducer to the location of least interference from air in the lungs (expiration may help) – Keep in mind the previous discussion on patient body habitus.
  3. The transducer will only require slight movements in various angles within that same location (think porthole). Again – big changes in location will usually make things worse.
  4. Transducer orientation should be towards the table (if an echo setting is available on the ultrasound system). Otherwise, it may be the opposite if set in “abdominal mode”.
  5. Rotate the transducer in a clockwise and counterclockwise direction to bring the 4 chambers into view – Some angling of the transducer may also be necessary. Very slight changes in angle here make a big difference. Large changes in angling will usually make things worse and orientation can become confusing.
  6. From the 4-chamber location – rotate transducer orientation counterclockwise towards the left shoulder to obtain the apical 2 chamber view – Once again, small variations in rotation and angle will bring the LV and LA into view.


Note: The most significant mistakes are made when too much movement is applied to the transducer in attempting to fix the image. Remember, this is a “porthole”. When first learning how to acquire this view, making large movements with the transducer is a difficult habit to break.         


Think Porthole!


References: 

  1. American Society of Echocardiography (ASE)
  2. American Heart Association
  3. Cleveland Clinic
  4. https://asecho.org/wp-content/uploads/2018/08/WFTF-Chamber-Quantification-Summary-Doc-Final-July-18.pdf
  5. https://www.heart.org/en/health-topics/heart-failure/diagnosing-heart-failure/ejection-fraction-heart-failure-measurement
  6. https://my.clevelandclinic.org...

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