Think Porthole: Challenges to imaging the heart with ultrasound

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.

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.

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”. A 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.

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, a“cardiac porthole”

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.

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 porthole. Moving 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.

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