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E-ACA: ECHO TUTORIALS Table of Contents   
Year : 2009  |  Volume : 12  |  Issue : 2  |  Page : 173
How do I get an optimal image?


Department of Anaesthesia, Narayana Hrudayalaya, No. 258/A, Bommasandra Industrial Area, Anekal Taluk, Bangalore - 560 099, India

Click here for correspondence address and email

Date of Web Publication21-Jul-2009
 

   Abstract 

Trans-esophageal echocardiography (TEE) is fast becoming an indispensable monitoring and diagnostic modality in cardiac operation rooms. Its convenience and dependability in making important and crucial decisions intra-operatively, during cardiac operative procedures, makes it one of the most useful weapons in a cardiac anesthesiologist's armory. But to make reliable inferences based on intra-operative TEE, creation and development of a proper image is one of the fundamental requirements. The image quality can be affected by factors like patient anatomy, quality of the ultrasound system, and skill of the echocardiographer. Since the first two cannot be changed, in most of cases, we will have to work on the third factor to optimize image quality. A working knowledge of the physics of ultrasound imaging and a sufficient familiarity with the various knobs and controls on the machine will go a long way in helping one acquire an optimum image.

Keywords: Trans-esophageal echocardiography, image quality, physics

How to cite this article:
Anil Kumar H R. How do I get an optimal image?. Ann Card Anaesth 2009;12:173

How to cite this URL:
Anil Kumar H R. How do I get an optimal image?. Ann Card Anaesth [serial online] 2009 [cited 2019 Sep 18];12:173. Available from: http://www.annals.in/text.asp?2009/12/2/173/53435


TEE examination should be started by satisfying the following checklist first.

  1. No contra-indications for the procedure. In fact there are not many, but any significant and symptomatic esophageal pathology is an absolute contra-indication.
  2. Informed consent must be obtained.
  3. The probe must be cleaned and sterilized in glutaraldehyde (for about 20 minutes) and must be thoroughly washed in clean running water.
  4. Patient is anesthetized (for all intra-operative examinations).
  5. Bite guard is placed (to protect the probe).
  6. Apply generous amounts of ultrasound lubrication before inserting the probe (to maintain airless contact between the probe tip and the mucosa).
  7. A naso-gastric tube is inserted and removed after aspirating all the gastric contents.
  8. Patient details must be entered into the machine and the probe must be calibrated. In most of the newer machines, the probe gets calibrated automatically as soon as it is plugged into the machine.
  9. Electrocardiogram is displayed on the monitor of the TEE machine.


At the outset, it is important to be aware and accept a couple of facts about TEE examination. Firstly, ultrasound imaging is mostly a subjective modality. Hence, what constitutes an optimum image varies between different examiners. Secondly, one has to accept that it may not be always possible to obtain an ideal image of all the suggested views in all the patients.

The mission of getting an optimum image begins with familiarizing oneself with the various knobs and controls of the machine and on the probe. The layout of the controls on the machine varies between different makes and models and the nomenclature also may vary at times. So, it helps to spend some time in studying the control panel of the machine and discovering the location of various knobs.

Of all the multitude of knobs and buttons on the machine, we will be using only a few of them more frequently to optimize our image. They are as follows:

  1. Gain control
  2. Focusing of the ultrasound beam
  3. Depth control and sector size control
  4. Flexion control (on the probe)
  5. Zoom


Gain setting is a pre-processing maneuver where strength of the returning echoes is modified. Any change made during pre-processing affects the information the scanner will access to create an image. Increasing the gain of the imaging system compensates for signal loss due to attenuation. It can be done for all the returning echoes or selectively at various depths, as in Time Gain Compensation (TGC), or at individual scan lines from side to side, as in Lateral Gain Compensation.[1] Optimum gain is achieved when blood inside the chambers appears black and the myocardial walls appear uniformly bright throughout the imaging sector. If the gain is too low, it may result in missed diagnostic information. Only bright signals, such as those from the pericardium, are visible and very low amplitude signals, such as the signal from an LV thrombus is lost. If the gain is too much, it may result in image over-saturation and obscured detail with loss of ability to differentiate structures [Figure 1]. The bright ambient lighting of the operating room often misleads the echocardiographer to use excessive gain settings. It is, therefore, preferable to eliminate the operating room lights briefly during the examination or the screen can be shielded with a custom made hood. [2] TGC can be used to amplify the weaker signals returning from the far field more than the signals returning from the near field [Figure 2]. TGC controls are set lower in the near field and higher in the far field except when imaging objects with low echogenicity in the near field (e.g., thrombus in LA).

Resolution is the ability of an imaging system to distinguish two points in space. It has two components, spatial and temporal. Spatial resolution is the distance that two targets need, to be separated, for the system to distinguish between them. If the spatial resolution of a system is not optimal, we find that the images being created are fuzzy and ill defined. Spatial resolution again has two components, axial and lateral. Axial resolution is the ability to distinguish two structures close to each other, along the direction of beam propagation, as two separate structures. It can be improved by using ultrasound with higher frequency. However, it decreases the wavelength at the cost of decreased depth of penetration. Lateral resolution is the ability of the beam to detect single small objects across the width of the beam, perpendicular to the direction of beam propagation. [3] It is most optimal when the beam width is narrow. Hence, the quality of image tends to deteriorate in the far-field region of the imaging sector where the beam diverges. [3] It can be improved by adjusting the focus of the transducer electronically with controls on the machine. Focusing, however, limits near-field depth where most of the useful imaging is done, since the ultrasound beam is parallel, better reflected here. [4]

Temporal resolution is the ability of the imaging system to display rapidly moving structures and distinguish closely spaced events in time. [5] It depends on the time required to generate one complete frame. Hence it is closely related to the frame rate. A frame is made of multiple scan lines with a scan line density necessary for preserving required spatial resolution. Temporal resolution is an important consideration for real time two-dimensional imaging of the heart since it is continuously moving. There is a need for a higher frame rate, which can be attained if the depth of the scan lines is lower and sector size smaller.

Depth setting selects the maximal distance from the probe to be displayed. The depth at which imaging is done is adjusted in such a way that the structure of interest is centered in the image and profiled in its entirety. If this is not done, not only will the temporal resolution be reduced but the structure will also look smaller, since, a larger area of cardiac anatomy has to be displayed on a screen of fixed size. Also, lateral resolution of the ultrasound system is inversely proportional to the depth, it makes sense to position the probe as close as possible to the structure of interest. For example, when the leaflets of the aortic valve are being evaluated, mid-esophageal aortic valve short-axis view is preferable to the deep transgastric long-axis view because the probe is closer to the aortic valve thereby improving lateral resolution. [2] Similarly, reducing the image field by decreasing the sector width so that it is just enough to display the structure of interest will also increase the frame rate and hence the temporal resolution. This will become important when rapidly moving structures like valves are being evaluated.

Color gain setting is similar to that of two-dimensional imaging and should be set appropriately. If the gain is set too low, smaller jets, as in small atrial septal defect or PFO, can be missed. If it is set too high, the size of a regurgitant jet will be overestimated. Optimum color gain is adjusted by increasing the gain until the color appears outside the chambers and also on the myocardium. Then it is decreased until it disappears. [2] The color scale is the range of velocities displayed. To optimize this, one should be aware of the usual velocities of blood flow being evaluated. For low velocity flows (e.g., pulmonary veins and flow across atrial septal defect) one should decrease the color scale (0.3 to 0.4 m/sec). For high velocity flows, it is set at a higher level (0.6 to 0.7 m/sec). One should remember that adjusting the color scale will affect the Nyquist limit. If set at lower velocities, aliasing (flow mosaic or variance color flow map) will occur at lower velocities.

The tip of the probe can be flexed anteriorly or retrogradely using the circular knob on the probe to change the plane of imaging. [6] This maneuver can be used to correct some of the imaging artifacts (like fore-shortening) and optimize profiling in the transgastric views.

Cardiac structures can be studied better by selectively magnifying an outlined area inside the imaging sector using the zoom option. Planimetry measurements will be more accurate if done on a zoomed image. [6]

Respiratory excursions move the heart inside the pericardial cavity. This motion of the heart can adversely affect quality of the image and also make Doppler measurements of hemodynamic parameters difficult. So, respiration may need to be stopped while taking those measurements. Doppler measurements will be accurate only if the Doppler beam can be aligned parallel to the direction of the flow velocity that is being measured. Therefore, knowledge of various views suitable for making flow velocity measurements is necessary (e.g., for the aortic valve and left ventricular outflow tract, deep transgastric long axis, and transgastric long axis views).

Most of the machines will have pre-loaded software to make hemodynamic calculations, which can be accomplished either on a two-dimensional image or Doppler spectrum. Some of the hemodynamic parameters commonly measured are pressure gradients, chamber volumes in different phases of the cardiac cycle, ejection fraction, etc. There will be an option on all the machines to store and archive the studies on either an optical media or on flash drives. It is important to familiarize oneself with the various options available on a particular machine. One should remember to hide the patient details on still pictures and cine loops while storing the study.

Obtaining an optimum image is crucial in useful diagnosis. With TEE, it is an art that has to be cultivated with persistent practice. It requires a reasonable understanding of ultrasound physical principles and echocardiographic technology, in addition to an appreciation of cardiac anatomy and physiology.

 
   References Top

1.Zagebski J. Pulse-echo ultrasound instrumentation. In: Zagebski J, editor. Essentials of ultrasound physics. St. Louis: Mosby; 1996. p. 46-68.  Back to cited text no. 1    
2.Herbert W, Dyal II, Frith MD, Reeves ST. Techniques and tricks for optimizing Transesophageal images. In: Perrino AC, Reeves ST, editors. A practical approach to Transesophageal echocardiography. 2 nd edition Wolters Kluwer Health; 2007. p. 435-50.  Back to cited text no. 2    
3.Hedrick W, Hykes D, Starchman D. Basic ultrasound instrumentation. In: Hedrick W, Hykes D, Starchman D, editors. Ultrasound physics and instrumentation. 3 rd ed. St. Louis: Mosby; 1995. p. 31-70.  Back to cited text no. 3    
4.Weyman A. Physical principles of ultrasound. In: Weyman A, editor. Principles and practice of echocardiography. 2 nd ed. Philadelphia: Lea and Febiger; 1994. p. 3-28.  Back to cited text no. 4    
5.Feigenbaum H. Instrumentation. In: Feigenbaum H, editor. Echocardiography. 5 th ed. Baltimore: Williams and Wilkins; 1993. p.1-67.  Back to cited text no. 5    
6.Shernan SK. Optimizing Two-Dimensional echocardiographic imaging. In: Savage RM, editor. Comprehensive textbook of intraoperative transesophageal echocardiography.  Back to cited text no. 6    

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Correspondence Address:
H R Anil Kumar
Department of Anaesthesiology, Narayana Hrudayalaya, No. 258/A, Bommasandra Industrial Area, Anekal Taluk, Bangalore - 560 099
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-9784.53435

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