Channels in ultrasound

Definition of Channels

Ultrasonic imaging systems are often called PULSE-ECHO systems. A single transmission and reception process usually goes like this: The ultrasound transducer is impacted by a high-voltage pulse (those who don't understand can lean their hand against an electric switch to understand), and after being hit, it vibrates (you would also vibrate if you touched an electric switch, wouldn't you?), emitting ultrasonic waves. During the propagation of ultrasonic waves, they will be reflected or scattered by the propagation medium, such as the human body, and return to the transducer. The transducer converts the returning ultrasonic wave signals into electrical signals. Once the required signals are received, a single transmission/reception process ends, waiting for processing and display.

The earliest transducers were a single unit, and the system did not have the concept of channels. However, influenced by radar technology, since the 1980s, the vast majority of medical ultrasound imaging systems have used array transducers for imaging. A so-called array transducer looks like a single unit on the outside, but internally, it is actually divided into many completely independent units, called elements.

With more elements, there are naturally different circuits to control the transmission and reception of these elements. These different circuits are actually the channels. Since the system needs to both transmit and receive, the channels of the ultrasound imaging system are divided into transmission channels and reception channels. The number of transmission channels is the number of mutually independent transmission circuits; the number of reception channels is the number of mutually independent reception circuits.

The accompanying figure shows a simplest ultrasound system with 4 transmission channels and 4 reception channels. Four transmission channels mean that in a single transmission/reception process, there are up to four different transmission signals in the transmission path. Generally speaking, only up to 4 elements can be 'hit' (there is also a high-voltage switch in the system that selects which element to 'hit'). Currently, most transducers have far more than 4 elements, so other elements that want to be 'hit' have no way; there are not enough channels, so they have to queue up. Their turn may come next time. Each element of the array transducer will receive the returning ultrasonic waves, but only the signals of 4 elements will be selected to return to the receiving circuit for processing. Others have to be temporarily sad.

From the above rambling explanation, the number of transmission and reception channels can also be interpreted as follows: In a single transmission/reception process, the maximum number of elements that can be 'hit' is the number of transmission channels; for the signals returning to each element, the maximum number of element signals that can be processed is the number of reception channels.

In addition, the number of transmission channels and reception channels is the same in most commercial systems. Therefore, when the number of channels is mentioned, it usually includes both the number of transmission channels and the number of reception channels.

Eight Channels

With more channels, if the probe itself does not fail and there are no major errors in system development, the most direct result is the improvement of spatial resolution, especially lateral resolution. The figure below shows an experiment conducted on a system currently under development. The left side shows the effect when 128 channels are open, and the right side shows the effect when the reception channels are controlled within 64. It can be clearly seen that the far-field points in the right image are stretched laterally, meaning the lateral resolution is worse.

According to my definition in the text, if the number of channels can be used as the most important indicator to describe the system level, what is the common number of channels in systems currently available on the market? See this table:

Generally speaking, the higher the number of channels, the higher the grade. As far as I know, the maximum number of channels for commercial ultrasound machines is 256, which are actually 256 independent transmission and reception circuits. The performance of such a system is certainly excellent, but there is also considerable waste. Because currently available probes, there are not many probes with more than 192 elements. That is, 192 elements are controlled using independent transmission and reception channels, and only 192 channels are needed. The extra 64 channels are not used at all. If your hospital has the financial resources to equip such a machine, it is very likely that none of the probes you have equipped have more than 192 elements. But don't worry, I won't tell you anything to upset you.

In addition, the relationship between the number of channels and the grade of the machine is not very strict. For example, the IU22 and IE33, which were the most recognized in China seven or eight years ago, only had 128 channels. However, ZONARE in the United States (now acquired) sold a 64-channel system in the ultra-high-end US market three or four years ago. At least its B-mode images in GI were very well-received.

In addition to these well-known commercial systems, there are some monsters in the scientific research community. Many years ago, Danish ultrasound expert J. A. Jensen built a huge 2048-channel system in the laboratory to study the imaging of two-dimensional phased arrays. The American scientific ultrasound company Cephasonic now also provides customized ultrasound, and it is said that it can also reach 2048 channels, but the Price is astronomical.

How to Scientifically Calculate the Number of Channels

Returning to the beginning, some people may be interested in how to scientifically calculate 131072 channels. Let's try to calculate it:

Assuming that the actual number of channels in the system is 256, and the transmission and reception channels are separate, so we can multiply by 2. In addition, a desktop machine has 4 probe interfaces, which can also be counted as different channels. 256*2*4=2048, which is still quite far off. However, it is much easier if we consider some imaging technologies, such as adding spatial compounding (high-end systems can achieve 9 angles without any problems), and considering frequency compounding (one frame image contains two or more different frequencies for transmission and reception). Also, consider multi-focus technology, such as 4 focal points.

2048*9*2*4=147456. It seems to be easily solved.

To exaggerate a bit more, if it is 4D imaging, a volume of images projected contains at least 60-70 frames of signals. Multiplying them in, exceeding one million is also EASY.

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