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Module 1 Section 3 |
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Module 1: |
Basic A-scan Biometry | ||
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Section 3: |
Instrumentation | ||
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The new generation of A-scan instrumentation is making IOL measurement easier, faster, and more accurate. A-scan manufacturers know that these machines are not typically operated by expert ultrasonographers. They are building in features that make the instrument easy to learn and operate for the beginner. They are also building in features that reduce errors and improve accuracy. This section discusses features that are common to many of the newer instruments. Use this section to become more familiar with your current instrument and to help guide you in the purchase of a new or used instrument.
The better units employ a four gate (light) measuring system. This means that the biometer does individual velocity measurements for the aqueous, the lens, and the vitreous. The first gate measures the position of the cornea, the second gate measures the position of the anterior lens spike, the third measures the position of the posterior lens spike, and the fourth gate measures the the position of the retina spike. The better units have visible and moveable gates. Not all gate systems are adaptable to the immersion technique.
Automatic acquisition means the instrument decides when you have a good scan. The manual mode allows the operator to decide. All newer instruments have automatic acquisition. Make sure the instrument has a manual mode to use in case you are suspicious of the accuracy of the auto mode. Automatic acquisition is not new. Most older machines have it. The difference is in how it works — the algorithm. A typical older machine has an automatic mode that simply grabs any spike that ventures over a threshold value in a prescribed area along the baseline. The newer machines use more sophisticated pattern recognition algorithms that take more factors into consideration when deciding if the scan is good.
It is best to have a graphic, or "waveform" display; meaning you can see the classic elements of a good A-scan. This allows you to visually confirm that you are getting good measurements. This is very important; do not purchase a machine, new or used, that does not have this type of display. Some displays are easier to read than others. The image in the paragraph above depicts a waveform display. The gain control varies the sensitivity of the instrument. A properly set gain increases the accuracy of the measurement. It is important to have a manual gain control for use in the manual mode. In the newer instruments look for a unit that automatically adjusts the gain when in the automatic mode. Not all instruments with an automatic mode have this feature. Go to Section 1 and Section 2 for more information. Use the back button on your browser to return to this page. This is a very nice feature. The instrument gives you a beep, or a series of beeps, to guide you in probe alignment. This means you don’t have to look back and forth between the eye and the scan. The best audible systems tell you when you have contact with the cornea, when you have axial alignment, and when a measurement has been acquired. This is worth paying extra for.Corneal compression recognition Some instruments can recognize when the probe is indenting the cornea by monitoring anterior chamber depth. The better machines automatically reject these measurements in the automatic mode. For more information on corneal compression, go to Section 1 and Section 2. Use the back button on your browser to return to this page. Newer instruments store multiple measurements and give you an average. Some give you other statistical data such as the high, the low, and the standard deviation. It is preferable for the instrument to display all the measurements and allow you to delete measurements that are far from the norm. A foot switch is needed for the manual mode to freeze the scan. Some instruments can acquire multiple measurements in the automatic mode without the use of a foot switch. This feature can speed up the measurement process. Newer instruments have solid tips. Do not buy an older unit that uses a water filled tip, unless you are getting a very good deal. Water filled probes are tedious to maintain and waste time.Try to work your way through the menu operation of an instrument before you buy it. Try to do it without the help of the manual or a salesperson. You want a machine that you can operate without referring to the manual, or at least operate with a very short "cheat sheet". The newer, more advanced biometers will allow you to choose different protocols for performing pseudophakic scans on eyes with PMMA, silicone, and acrylic IOLs. All biometers will have an aphakic mode. It is desirable to have a biometer that will allow you to adjust the velocity of sound settings for pseudophakic scans and eyes with silicone oil in the vitreous. Of course, new machines will have a printer for documentation. If you are buying a used machine, make sure the printer works. Although not necessary for accurate measurements, they improve documentation and eliminate transcription errors. A printout of the waveform and the calculations may be required documentation in the future. It is nice to have an instrument that automatically calculates the power using several up-to-date formulas. Check out the menu operation for this feature. Is it easy to use? For more information on IOL formulas, see Module 2: Advanced A-scan Biometry. |
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This instrument has a
simple auto-acquisition protocol. Measuring lights are attached to spikes A, C, and
D because they are the first three spikes (beyond the probe/cornea spike) to venture above
the threshold line (T). Spike B is ignored because it is below the threshold line.