Can black and white color ultrasound still be called color ultrasound?

Produced by: Science Popularization China

Author: Zeng Qiuyuchen (Ultrasonics Laboratory)

Producer: China Science Expo

Imagine this: you go to the hospital for a physical examination, walk into the color Doppler ultrasound room, and the doctor fiddles with a mysterious instrument on your stomach for a long time, and then issues a diagnosis report. You take the report, but you clearly see the following image printed on the image section of the report:

Image source: Document 1

You look at the report, puzzled, and ask the doctor: Why did I have a "color ultrasound", but the image in the report is black and white?

The doctor glanced at the report and said, "Yes, this is the color ultrasound report," and then went to call the next patient.

You are full of doubts, wondering what the heck is color Doppler ultrasound, and why can a "color" Doppler ultrasound be non-color? So after returning home, you try to search online for "what is color Doppler ultrasound" and get this answer:

Color Doppler flow imaging (CDFI) is a color ultrasound. It processes the Doppler information obtained, uses color grayscale coding to identify the direction and speed of blood flow with different colors and brightness, and superimposes them on the B-mode ultrasound image.

Faced with this series of descriptions in which you seem to understand every word, but have no idea what they mean when put together, you seem to be even more confused about why the color ultrasound report is in black and white.

In order to intuitively understand what color ultrasound is, why we need to do color ultrasound, and the question of "why color ultrasound can be black and white", we need to start with the history of medical ultrasound testing.

The earliest form of ultrasound - A-ultrasound

When a person stands on a mountain and shouts at the opposite cliff, the sound waves will be reflected by the opposite cliff and produce an echo. If you know some physics, you can also calculate the distance between you and the opposite cliff by measuring the time difference from shouting to hearing the echo.

Image source: askiitians

Medical ultrasound examinations work in a similar way. When ultrasound first appeared in the field of medical examinations, doctors did not use the long and flat ultrasound probes that we commonly see today, but rather probes that were more like cylinders. This type of probe can produce a long and thin ultrasound beam.

Image source: Photographed by the author

When a thin ultrasonic beam propagates in the human body, if it hits different tissues, or the boundaries between tissues and organs, part of the ultrasonic wave will be reflected due to slight differences in the physical properties of different tissues in the human body.

Image source: Made by the author

We use a probe to record these reflected echoes and display them on an oscilloscope in the form of time-amplitude.

Since the propagation speed of ultrasound in the human body is approximately 1540m/s, we can easily match the echo time recorded by the ultrasound probe with the distance from the human tissue boundary to the probe.

This is the earliest form of ultrasonic medical examination: amplitude ultrasound examination, referred to as A-ultrasound.

Image source: Made by the author

The function of A-ultrasound examination is equivalent to opening a small virtual "window" in the human body using an ultrasound probe without harming the human body. Doctors can use this window to know at what depth changes have occurred in human tissue in a certain part of the human body, such as the heart in the picture above, and make some diagnoses based on existing medical knowledge.

Ultrasound has many advantages over other medical examination methods. It is non-invasive, radiation-free, and the examination equipment is small, portable and easy to operate.

However, the structure of human tissue is very complex. Inferring changes in tissue structure by looking at oscilloscope waveforms is a very unintuitive and error-prone way of examining the body.

Moving towards B-ultrasound

In order to solve the above problems, scientists have made a series of improvements to ultrasound examination based on A-ultrasound.

First, scientists have made clever improvements to ultrasound probes, allowing them to transmit ultrasound beams to multiple directions and targets through mechanical rotation or electronic focusing, the latter of which is called beamforming. In the most common ultrasound systems, these beams are combined into a virtual fan-shaped or rectangular plane as needed.

Image source: Document 2

Image source: Document 3

Secondly, based on the experience of radar, scientists have improved the display method of ultrasonic echoes. In the echoes generated by a single beam, the echo height corresponding to the presence or absence of targets at different depths is converted into brightness information at different depths on a scanning line.

Image source: Made by the author

Combined with the beamforming technology mentioned above, many scan lines in multiple directions and positions in the human body can be obtained. When these scan lines are stitched together, an ultrasound image reflecting the cross-sectional structure of a certain position in the human body is obtained.

Image source: Made by the author

Since the structure information of the human body is displayed through bright spots of different brightness at different positions in this ultrasound examination technology, it is called brightness type ultrasound examination, or B-ultrasound.

Color Doppler ultrasound is finally here!

So what is color Doppler ultrasound? Simply put, scientists have discovered that with the advancement of technology, it is not only possible to extract the structural position information of the human body from ultrasound echoes, but also to extract Doppler motion information.

Everyone must have seen this scene in person or on camera: a racing car is coming towards you with a sharp roar, and the sound becomes low after passing you. This is the Doppler effect discovered by Austrian physicist Doppler in 1842.

When a moving object is moving towards the observer, the sound waves it emits will be compressed due to the relative motion between the two, so that the frequency increases and sounds sharper. Conversely, when an object moves away from the observer, the sound waves will be stretched, the frequency decreases, and it sounds deeper.

Image source: Document 4

Scientists have realized that based on the B-ultrasound mentioned above, they can further determine whether there is moving human tissue in the area, such as whether there is blood flow, by judging whether the echo corresponding to a certain position is compressed, stretched or remains the same.

Image source: Photographed by the author

For example, when a doctor examines a patient's neck in a posture like the one shown above, he or she can see a clear cross-sectional image of the neck tissue.

Image source: Photographed by the author

On this basis, when the color Doppler ultrasound function is turned on, it can be seen that since the blood in the carotid artery flows from the heart to the brain, that is, toward the direction of the ultrasound probe, the carotid artery area will be displayed in red in the image, indicating that the movement direction is positive. The blood in the jugular vein flows away from the ultrasound probe, and the movement direction is negative, which is displayed in blue.

Other tissues such as blood vessels and muscles do not move relative to the probe, so there is no Doppler frequency shift and they are not displayed on the screen.

From the above explanation, we can see that the color in color ultrasound images is merely an artificial regulation to make the tissue movement information more clearly distinguishable from the tissue structure information. It is not the essential difference between color ultrasound and ordinary B-ultrasound.

Based on the principles and purposes of color ultrasound imaging, it is not difficult to imagine that for many color ultrasound application scenarios, such as checking whether the blood flow in a certain area is smooth, we only need to know whether there is tissue movement, and there is no need for information on the direction of tissue movement.

In this case, black and white images can be used to display the motion information detected by the ultrasound system.

The white patches and movement speed in the imaging area in the figure correspond to the Doppler motion information of blood flow detected by the ultrasound system. Image source: Reference 1

In other words, the difference between color ultrasound and ordinary B-ultrasound is not whether or not the display color is increased, but whether the movement information of human tissue can be obtained from the image, which is the content in the picture above.

So, the case has been solved here. Color ultrasound can indeed display different colors, but not every image needs to display color.

Editor: Wang Tingting

References

Cantisani, V., et al., Color-Doppler ultrasound with Superb Microvascular Imaging (SMI) compared to Contrast Enhanced Ultrasound (CEUS) and CT angiography to identify and classify endoleaks in patients undergoing EVAR. 2016

Kurjak, A. Arenas, J., Donald School Textbook of Transvaginal Sonography. 2018.

Jensen, J., Linear description of ultrasound imaging systems: Notes for the international summer school on advanced ultrasound imaging at the technical university of denmark. 1999

Maulik, D., Doppler Ultrasound in Obstetrics and Gynecology. 2005.

Wan Mingxi, Biomedical Ultrasound (Volumes 1 and 2). 2010.

Shi Keren, Guo Yu, Phased array ultrasonic imaging detection. 2010.

Amsterdam, TLS, DIAGNOSTIC ULTRASOUND IMAGING: INSIDE OUT. 2004.

Woo, J., A short history of the development of ultrasound in obstetrics and gynecology. History of Ultrasound in Obstetrics and Gynecology, 2002. 3: p. 1-25.