Echo Image Optimization

[Sebastian Thorndike]

Inorder to obtain optimal ultrasound images, it is necessary to adjust several parameters continuously during the examination. Typically, examinations in each echocardiographic view (also called window) are initialized by identifying an overview image. Starting in the overview image the depth is reduced as much as possible. Reducing the depth results in increased frame rate and thus better image resolution. If possible, the width of the image is also reduced, which likewise results in increased image-resolution. It is also possible to zoom in on regions of interest; e.g the aortic valve can be zoomed in to study its anatomy and function. Zooming in improves the resolution in a particular area. Alternatively, it is possible to place the focus at the level of the region of interest. The difference between zooming in and shifting focus is that the zoom encloses a specific region of the image, whereas shifting focus simply adjusts the location (along the ultrasound beam) with the best resolution. If the ultrasound image is too dark, it is possible to increase the gain. This amplifies the incoming (reflected) ultrasound waves such that each object appears whiter on the image. Increasing gain excessively results in lower resolution and difficulties discerning tissue borders. These are the main adjustments made to improve image quality.

Examinations in each echocardiographic view are initialized by identifying an overview image. Starting in the overview image the depth is reduced as much as possible, without excluding regions of interest. Reducing the depth results in an increased frame rate and thus enhanced image resolution. If a particular region is of interest, that region can be zoomed in. Note that the image becomes more grainy as the zoom is increased.

The ultrasound machine amplifies all incoming (reflected) ultrasound waves. However, the examiner can further increase the amount of gain applied to incoming sound waves. This is done using gain control or time-gain compensation (TGC).

Gain control regulates the global (overall) gain. Increasing the overall gain will increase the gain for all reflected sound waves, making all objects in the image whiter. This may clarify some tissue borders but excessive use of gain results in deterioration of image quality.

Time-gain control / compensation (TGC) adjusts the gain at specific levels along the ultrasound field. The purpose of TGC is to gradually increase the amount of gain as the depth increases; this compensates for the attenuation that occurs with increasing depth. TGC is adjusted using multiple controls that each represent a specific depth in the image (Figure 1). The bottom control adjusts gain at the bottom of the image etc. TGC is generally increased at the bottom of the image since the ultrasound lines have the lowest density there (and thus the lowest image resolution). TGC at the top of the image is usually kept at low levels.

Low ultrasound wave frequency provides high tissue penetration and low image resolution. High-frequency waves provide good image resolution but worse penetration. Visualizing objects located in close proximity to the transducer, therefore, is done using high-frequency waves. The frequency of the ultrasound wave must generally be reduced in order to visualize objects located far away from the transducer. Hence, using low-frequency waves to visualize distant objects is motivated by the advantages of greater tissue penetration of such waves.

The direction and focus of ultrasound waves can be adjusted by varying the sequence of activation of the piezoelectric crystals (Figure 2). If the activation starts at the lateral crystals and proceeds towards the center, then the ultrasound beam will be focused (Figure 2C). The focus can be placed anywhere along the ultrasound field.

Temporal image resolution is the ability to describe the movement of objects over time. Echocardiography requires high temporal resolution to study the detailed movements of relatively small objects. In order to produce recordings with high temporal resolution, it is critical to produce images rapidly. The more images that can be produced and presented per unit of time (i.e frame rate), the greater the temporal resolution.

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Functional magnetic resonance imaging (fMRI) is most commonly based on echo-planar imaging (EPI). With higher field strengths, gradient performance, and computational power, real-time fMRI has become feasible; that is, brain activation can be monitored during the ongoing scan. However, EPI suffers from geometric distortions due to inhomogeneities of the magnetic field, especially close to air-tissue interfaces. Thus, functional activations might be mislocalized and assigned to the wrong anatomical structures. Several techniques have been reported which reduce geometric distortions, for example, mapping of the static magnetic field B(0) or the point spread function for all voxels. Yet these techniques require additional reference scans and in some cases extensive computational time. Moreover, only static field inhomogeneities can be corrected, because the correction is based on a static reference scan. We present an approach which allows for simultaneous acquisition and distortion correction of a functional image without a reference scan. The technique is based on a modified multi-echo EPI data acquisition scheme using a phase-encoding (PE) gradient with alternating polarity. The images exhibit opposite distortions due to the inverted PE gradient. After adjusting the contrast of the images acquired at different echo times, this information is used for the distortion correction. We present the theory, implementation, and applications of this single-shot distortion correction. Significant reduction in geometric distortion is shown both for phantom images and human fMRI data. Moreover, sensitivity to the blood oxygen level-dependent (BOLD) effect is increased by weighted summation of the undistorted images.

This guide is for physicians and clinicians who often do not receive formal training for their ultrasound machinery. By simply testing a few controls, you can quickly gain a basic understanding of ultrasounds and become a master at image optimization.

Every user manual has a section in the front which outlines the location of different controls. Find that page and leave it open. If you can't find a hard copy of the manual, press the F1 key on your keyboard, this will typically open the electronic manual. As a reference, take a picture of this page with your phone or tablet to refer back later.

One of the most important steps, particularly with CRT (non flat-panel LCD) monitors is balancing the brightness and/or contrast to get the clearest images. On machines with CRT monitors, user manuals will list these recommended steps in enabling these imaging specifications. Most manufacturers recommend starting with 90-100% contrast, then adjusting the brightness if necessary.

The brightness setting is the only consistent adjustment that remains with every new exam. Unless you go through specific steps to save a custom preset, the remaining settings following will not be saved to the ultrasound. So feel free to experiment because you can start over by reselecting the preset.

When starting up your new ultrasound techs typically adjust these to left-of-center for nearfield (top) imaging, and gradually move to right-of-center as quality decreases deeper in the image. The goal to getting a crisper image is to have a lower gain in the nearfield while having a higher gain deeper in the image, where image quality is fainter.

Some machines show a specific frequency or range of frequencies. In this case, use lower frequencies when you need penetration when imaging. Lower frequencies provide the best penetration, but at the expense of image resolution. Higher frequencies provide the best resolution when looking at superficial imaging, although you will lose penetration.

Over the past 20 years, three new mainstream technologies have been introduced that have dramatically impacted ultrasound image quality. These imaging features would be the Tissue Harmonics Imaging, Compound Imaging, and Speckle Reduction Imaging. It is rare for a machine not to have these three core technologies today.

How: Most ultrasounds offer varying levels of Speckle Reduction. The lowest level will only lightly enhance tissue and reduce small amounts of artifact, while the strongest levels can make the image look over-processed. The standard way is to have it set near mid-level. This technology is meant to be used post-processing, meaning you can adjust its level once your image is frozen. For easy access imaging, take your image, freeze it, then adjust its Speckle Reduction levels to see how the image is affected.

Dynamic Range (or Compression) lets you tell the ultrasound how you want the echo intensity displayed as different shades of gray. A wide/broad range displays more shades of gray and a smoother image overall. A narrow/smaller range displays less shades of gray that appear at a higher contrast with a more black-and-white image.

Although different, changing the gray maps on your image will have a similar effect on an ultrasound image as it would be when changing the dynamic range. A gray map decides how light or dark you prefer to show each level of white, gray, or black based on the strength of the ultrasound signal, whereas Dynamic Range adjusts the number of shades of gray overall.

These settings are quite different, although they may appear to have similar effects on your image. It usually helps to adjust them in combination with each other to find an image that works best for you.

Line density adjusts the number of scan lines on your ultrasound image. A higher level will provide better image resolution (more scan lines), but will reduce the frame rate. This feature will get you the best possible image with an acceptable frame rate.

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