超声仪器的工作原理-2

摘要：When we look at things with our eyes, there are various ways in which we “look”. At times, we might choose to look only straight a

When we look at things with our eyes, there are various ways in which we “look”. At times, we might choose to look only straight ahead like when we read a notice on a wall.

Or we might look horizontally when scanning the sea.

Or we might scan the whole area, up and down, left and right, in many dimensions when absorbing scenery such as the one below in Sri Lanka.

In a similar way, there are many different ways a ultrasound probe can “look” at things. These ways are called “modes” and these will be described below. The modes are named with letters and may sound very confusing. However, we will discuss each in turn and you will, at the end, understand the basics of them.

The A mode is the simplest form of ultrasound imaging and is not frequently used. The ultrasound wave that comes out of the probe travels in a narrow pencil like straight path.

One use of the A scan is to measure length. For an example, ophthalmologists can use it to measure the diameter of the eye ball. Imagine that the red circle below is the eye ball and you want to measure the diameter of it.

An ultrasound machine scanning in "A scan" mode can be used. The probe is placed on one end of the eye ball.

An ultrasound wave is sent from the probe and at the same instance, a line from the left of the screen starts to be drawn. This line moves horizontally measuring time.

As the wave reaches the first wall of the eye, some of the ultrasound is reflected back into the probe. The returned wave is recorded on the line as a bump. The stronger is the returned wave, higher the height of the bump. The height of the bump is called Amplitude which is what the "A" of "A scan" stands for.

The ultrasound wave continues further in the eye.

The wave then meets the wall that is furthest away. Again some of the ultrasound wave is reflected back into the probe and another bump is drawn.

The time difference between the first bump and the second bump represents how long the ultrasound wave took to travel between the two walls. Longer the length, longer is the time difference. The speed of ultrasound in the eye is known to be 1500 meters per second (yes, that is fast). So if you know the time difference (given by the interval between the two bumps), you can calculate how far the wave traveled between the two walls of the eye, giving you the eyeball length.

In its simplest form, the B scan mode is very similar to the A scan mode. Just like the A scan, a wave of ultrasound is sent out in a pencil like narrow path. And again like the A scan, the horizontal line represents the time since the wave was released.

Again using the eye ball as an example, the probe is placed on one end. Like in the A scan, when the wave meets the first wall, a part of the wave is reflected back into the probe. However, this time, instead of a bump, the strength of the returning wave is recorded by a bright dot. The brightness of the dot represents the strength of the returning wave. The brighter the dot, the stronger is the returning wave. The letter "B" of "B scan" represents Brightness.

The wave continues.

And when the wave reaches the other wall, again part of it is reflected back into the probe. This returning wave, like the returning wave from the previous wall, is represented as a bright dot on the screen.

The B scan in the form discussed doesn't amount to much .... just a few dots of different brightness along a line. However, if a B scan is done at different levels of the object, you will get a two dimensional image on the screen as shown below. First a B scan is done at the top of the structure chosen, e.g. the eye.

The first B scan line is kept on the screen. Then at a slightly different level, the B scan is repeated.

This B scan result is also kept on the screen.Then, again at a slightly different level, the B scan is repeated.

In this way, a two dimensional (2 D) image of the object is formed on the screen.

In real life, the process happens very quickly. The structures are scanned and the image redrawn many times a second.

Since the image is redrawn so rapidly, one can see size changes (e.g. pulsations of carotid artery) in "real time" (i.e. as it happens) .The B scan is the commonest mode of ultrasound that we see in anesthesia. The complete description of the mode is "real time , 2 dimensional (2 D), B scan". The 2 dimensional (2D) refers to the fact that the image has two dimensions; horizontal (X axis) and vertical ( Y axis).

This mode is mostly used in cardiology and will not be described.

Frequency, wavelength, resolution, and depth are interlinked. These seem to be complex physics stuff to grasp, but it is worth understanding them because it can help you to get the best images from your ultrasound machine.This website will simply it all for you so don't worry.

The first step is to understand what the terms mean. Each of the terms will be described to you, starting with "frequency".

As you may recall, ultrasound waves are created by a vibrating piezo electric crystal. The vibration creates alternate high pressure and low pressure areas which travel forwards.