Ultrasound Spatial Resolution: Tools for Maintaining Ultrasound Image Clarity
©Walter Rasmussen, R.D.C.S.
The spatial resolution of two-dimensional images is based-upon the display of distinct echoes as a grainy collection dots (AKA speckle), of varying brightness dependent upon the strength of the signal from the individual reflector it represents. These dots are comprised of one or more pixels within the digital image, and approximate the tissue texture, helping to convey pathological conditions such as scarring, vegetations, tumors, and various types of cardiomyopathy.
Spatial resolution refers to the machine’s ability to distinguish or resolve distinct structures from each other and is primarily affected by the ultrasound beam frequency, beam width and its focal zone. Frequency affects the overall resolution because higher frequency vibrations are packed closer together, producing images with more detail, while beam width and focal zone affect lateral (horizontal) resolution and are influenced by the transducer design.
The other factor that affects spatial resolution is signal processing. On current ultrasound machines, there are many adjustments that can have beneficial and detrimental effects upon the image. Justification for using them depends upon what the sonographer is trying to achieve.
Processing adjustments that decrease resolution of small or thin parts and tissue texture while enhancing the perception of larger structures or walls and are some times useful, but should not be used as a default setting. Some processing adjustments will decrease texture to the point that tissue looks gelatinous and heart valves appear thicker, leading to potential misdiagnosis.
Below, is a list of adjustments that reduce image detail and texture by blurring or averaging speckle or color Doppler. All of these adjustments should be used minimally and with caution and in most situations, are unnecessary and act to reduce resolution. They are primarily useful when the sonographer needs to compensate for poor image detail in difficult to image patients.
Line Density: Removes lines of information, allowing for higher frame rates and thus improved temporal resolution as the line density is decreased. Progressive degradation of the image is seen, similar in effect of decreasing the transducer frequency as this option is adjusted. This can aid in improving the visualization of walls when there is a lot of noise in the image but if the goal is to find potentially small lesions like a vegetation, the effect of line density should be taken into consideration. Line density can be very useful for increasing temporal resolution while using color Doppler as long as it does not produce a significant loss of color Doppler information.
Above: Original Image with minimal processing.
Above: Too much UD Clarity applied to image above distorts the image.
Data Dependent Processing: DDP reduces random noise, but can also remove important image texture. DDP should be used only to a limited degree in echocardiography because it reduces spatial resolution, producing a softer image which makes recognizing smaller lesions more difficult.
Lateral Averaging: Reduces lateral resolution, blending speckle in the horizontal plane, causing a loss of overall resolution.
Radial Averaging: Reduces axial resolution, blending speckle in the vertical plane, causing a loss of overall resolution.
Color Smooth: A type of frame averaging, mixes distinct velocities, reduces velocity resolution and acts to blur and expand the jet area. Removes important color mosaic information that conveys the presence of turbulence.
UD Clarity: Reduces image speckle, softening wall borders, making excessively grainy images easier to observe. If overused, important tissue characteristics will be masked or obscured. At higher levels, the image will become distorted and can take-on a gelatinous or crumpled appearance.
Persistence: Blends consecutive frames together, giving the image a softer, aqueous appearance. Over-adjustment reduces spatial resolution.
SPD (on the Philips iE33): increases frame rates at the expense of line density and spatial resolution.
Edge Enhance: Improves the visualization of structure borders while decreasing overall resolution.
The uses of the above adjustments will be discussed in following chapters.
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