Ultrasound Sector Image Width: How it affects Frame Rate and Temporal Resolution

©Walter Rasmussen, R.D.C.S.

Ultrasound sector image width is an adjustment that determines how many degrees through which an ultrasound beam is swept. Frame rate refers to how many times per second one complete sweep of the imaging sector is made. Temporal resolution refers to how quickly images are updated and therefore how accurately movement is depicted.

image width

The ultrasound beam is only a few millimeters in diameter and if held stationary, would reveal limited tissue information therefore, the ultrasound beam is swept through a 90 degree or less sector that can be narrowed for the purpose of increasing the number of frames (sweeps) per second. Some current ultrasound machines can scan a sector area greater than 90 degrees while still maintaining acceptable frame rates.

wide 2 d sector fnl

Above: Greater than 90 Degree Sector Image.

low parasternal

Above:  Standard 90 Degree Sector.

Because cardiac structures and blood are moving at a high velocity, it is desirable to sweep the ultrasound beam through the sector at a rate that faithfully reproduces motion. Frame rates are generally not an issue with two-dimensional imaging alone however the sonographer should be able to recognize the appearance of low frame rates which make the heart appear hypokinetic or blurred. This type of temporal distortion can also come about by improper use of various image processing features such as Persistence, Color Doppler Smoothing, Three-Dimensional Imaging, and some proprietary features offered by various ultrasound machine manufacturers which affect speckle, softening the image while sacrificing resolution.   These will be discussed in a later section.

When two-dimensional imaging is combined with other modalities such as color Doppler, severe drops in frame rates occur. The best way to maintain frame rates is by decreasing the 2-D image width, and the depth and the size of the color Doppler box, thereby decreasing the distance required for the ultrasound pulse to travel and the time it takes for it to complete a single sweep. The sonographer can learn to recognize low color Doppler frame rates by its flashing or choppy appearance and by glancing at the data section of the monitor which gives an exact frame rate value such as, “FR: 24 Hz”.   For Echocardiography, color Doppler frame rates should be kept 17 Hz or above and preferably in the 20-30 Hz range.  Every ultrasound machine has a numeric display of the current frame rate, which can range from less than 10 Hz to more than 60 Hz.

FPS fnlAbove:  Image data area with FPS (Frames Per Second) and other adjustments.

Many current ultrasound machines have a direct frame rate adjustment when color Doppler is in operation however, the sonographer should be aware that changing these adjustments is less desirable than first narrowing the sector image and color Doppler box because they sacrifice resolution of the color Doppler data itself, which can result in an expanded jet area, and loss of the separation of velocities within the jet. Such sacrifices in data quality may be acceptable for other ultrasound specialties, but should not be routinely used for echocardiography. The direct frame rate adjustment should be placed in the middle position as a default setting. Increasing the frame rate setting is suitable for high velocity blood flow and decreasing it improves the mapping of low velocity blood flow.

The following table gives examples of how far an object will have moved before it is re-scanned by two-dimensional imaging or color Doppler at progressively higher frame rates:

 

@.5 M/sec        @1.0 M/sec         @2.0 M/sec        @3.0 M/sec       @4.0 M/sec

10 Hz 5.0 mm 10.0 mm 20.0 mm 30.0 mm 40.0 mm
11 Hz 4.5 mm 9.1 mm 18.2 mm 27.3 mm 36.4 mm
12 Hz 4.2 mm 8.3 mm 16.7 mm 25.0 mm 33.3 mm
13 Hz 3.8 mm 7.7 mm 15.4 mm 23.1 mm 30.8 mm
14 Hz 3.6 mm 7.1 mm 14.3 mm 21.4 mm 28.6 mm
15 Hz 3.3 mm 6.7 mm 13.3 mm 20.0 mm 26.7 mm
16 Hz 3.1 mm 6.3 mm 12.5 mm 18.8 mm 25.0 mm
17 Hz 2.9 mm 5.9 mm 11.8 mm 17.6 mm 23.5 mm
18 Hz 2.8 mm 5.6 mm 11.1 mm 16.7 mm 22.2 mm
19 Hz 2.6 mm 5.3 mm 10.5 mm 15.8 mm 21.1 mm

From the above table, it is readily apparent that low frame rates will omit a considerable amount of tracking information and the sonographer should be constantly monitoring frame rate throughout the Echocardiography exam and make adjustments to keep it as high as possible.  Unfortunately, in day-to-day practice, the cardiac sonographer is often forced to hover near the lower border of acceptable limits and because this is undesirable, needs to know how to push the frame rates higher under various conditions. Noticeable improvement in temporal resolution can be seen when frame rates are 22-30 Hz. At higher heart rates, higher frame rates are required in order to faithfully track motion.

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