Doppler sonography/medical applications

Introduction
Doppler Sonography also called ultrasonography is the practice of using high frequency waves to produce an image for medical analysis. These high frequency waves are called ultrasound. A Doppler ultrasound is a noninvasive test that can be used to estimate the blood flow through blood vessels by bouncing high frequency sound waves off circulating red blood cells.

Ultrasounds are sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound devices operate with frequencies from 20KHz upto several gigahertz.

History
Although discovered 12 years before the X ray, the ultrasound is a much later found application in medicine. The ultrasound was first used in medicine in the fifties of last century. Ultrasound was first introduced in obstetrics, and after that in almost all the fields of medicine. The Doppler ultrasound became the widely accepted method soon after.

Advantages of Doppler Ultrasound
The medical ultrasound provides several advantages some of them including:

•	Non invasion

•	Good visualization characteristics

•	Easy management

Working
The ultrasound device consists of a transducer, transmitter pulse generator, compensating amplifiers, the control unit for focusing, digital processors and systems for display. Sonographers typically use a hand held probe (called a transducer) that is placed directly on and moved over the patient. A water-based gel is used to couple the ultrasound between the transducer and patient.

The ultrasound diagnostic can be differed two techniques: Transmission and Reflection.

Transmission technology is based on distinguishing the tissues with different absorbance of ultrasound, Due to uneven absorption of ultrasound images provides internal structure that consists of a mosaic of light and darker places. This technology is now abandoned.

Reflection technology (echo) registers the pulse reflects from the boundary of two tissues with a different acoustic resistance. This technique is based on the principle of functioning sonar.

Importance in Clinical Medicine
Doppler ultrasound is widely used in the medical field to obtain images of different parts of the body for differential diagnosis. Being a noninvasive test, medical ultrasound is a widely preferred method for diagnosis of medical conditions. It is used in the cases of abdominal, cardiac, maternity, gynaecological, urological, and cerebrovascular examination, breast examination and small pieces of tissue as well as in pediatric and operational review.

Disadvantages in the ultrasonic device
Discrimination of an ultrasonic device can be defined as the minimum distance of two reflectors in the body that is on the screen can be recognized as separate. Resolution can be divided into:

•	Lateral (sideways)

•	Axial (depth)

Lateral resolution depends on the thickness of the beam. At higher frequencies it is easier to achieve narrow beam, but the penetration is reduced. In examination of the children are used frequency 5-7 MHz, while in adults 3-5 MHz. If we work with the reduced sensitivity of the device, then the weak reflectors (parenchyma) lose the pictures, but the lateral resolution for the remaining, stronger, reflectors is better.

Axial resolution is much better than regular lateral also for display of thin structures (e.g. thin blood vessels) the probe should be always oriented to the vessels so that blood flow across the ultrasound beam. At present conventional ultrasonic device to create images we are using only the amplitude (intensity) response.

Different modes of ultrasonic devices
There are 3 different modes of ultrasonic devices:

A-mode
A-mode is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumor or calculus is also A-mode, to allow for pinpoint accurate focus of the destructive wave energy.

B-Mode
In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen.

M-Mode
M stands for motion. In m-mode a rapid sequence of B-mode scans whose images follow each other in sequence on screen enables doctors to see and measure range of motion, as the organ boundaries that produce reflections move relative to the probe.

Color Doppler
Color Doppler (also known as Color Flow) imaging utilises the Doppler shifted frequencies generated by moving red blood cells (RBCs). When red blood cells pass through the Doppler beam, the movement of the cells causes a shift in the frequency of the transmitted sound beam to a higher or lower frequency, depending on whether the cells are moving towards or away from the sound beam’s axis. The difference between the transmitted frequency and the received frequency is the Doppler shift frequency, which is usually in the kilohertz range. Once the ultrasound system has determined the Doppler frequency shifts, various color maps can be used to color-code the Doppler signals and display them as flow in relation to the axis of the Doppler beam. Most machines will overlay the color flow display on a B-mode image of the anatomy. Various shades of red and blue are the most common colors displayed on a color Doppler image. However, considerable controversy exists as to what color should be used to denote flow toward and away from the beam axis. Unlike astronomy where stars moving away from the observer are “red shifted” and stars moving toward the observer are “blue shifted”, in medicine there are no standards for the color display. Some use red to represent arteries and blue to represent veins, others use red to represent flow toward the transducer and blue to represent flow away from the transducer, and still others choose to follow the lead of the 150 years of astronomy literature. In today’s medicine, there is no standard for the Doppler color display and direction; one must understand the anatomy, physiology, and the sonographic instrumentation and display in order to accurately interpret the images.

Conclusion
The future of Doppler sonography is strong. The addition of micro-bubble contrast agents, which are injected into the blood stream, enhances the returning Doppler signals, which creates much better images of flow in the heart and small blood vessels. This new technique also carries the possibility of using encapsulated cytotoxins for delivering cancer chemotherapy directly to tumors. The operator watches with Doppler for the bubble’s arrival in the tumor, then bursts them within the tumor using specific harmonic pulses.

Another new therapeutic use of Doppler ultrasound is thrombolysis of blood clots in the brain. While thrombolysis does not rely upon the Doppler effect, color Doppler is used to guide the application of the ultrasonic energy to specific blood vessels within the brain. The sonic vibrations appear to enhance the thrombolytic actions of anticoagulants. The first 50 years of the application of ultrasound through sonography and Doppler imaging has been an amazing period in modern medicine. As the Baby Boomers start turning geriatric, there will only be an expansion of these techniques to improve the health of all people from before birth to old age.