Ultrasound in Medical Imaging. Doppler Ultrasound


Medical physics is described as the application of concepts found in the field of physics in medicine. The application has greatly improved the provision of healthcare services in many hospitals today. Physical concepts applied in such cases are of great importance in the detection, as well as the treatment, of various illnesses (Bates, 2004). Ultrasound-based techniques, for example, have gained widespread popularity in the field of medicine in the recent past. In addition, the technique is used in medical imaging, which helps in diagnosing various health conditions.

In this paper, the various physical principles applied in both pulsed and continuous Doppler ultrasound systems will be addressed. In addition, the author will point out the medical applications of the two systems in medical imaging. The Doppler ultrasound equipment is named after the Doppler Effect, which is the principle behind its functioning. The effect can be described as “the shift in frequency (of) a wave” (Cosans 2007, 34) in relation to the source. The Doppler Effect can be explained by the sound of a vehicle in motion. The sound changes as the car approach the individual, passes and recedes from them (Cosans 2007, 44). If the epicenter of the wave happens to be moving towards the individual, some changes in the frequency of that wave are discernible. The changes are brought about by the fact that “successive wave crests originate from a source closer to the individual” (Cosans 2007, 45). As the source moves away from the individual, the frequency is reduced. The diagram below explains this clearly:

Change in Wavelength as a Result of Motion.
Diagram 1: Change in Wavelength as a Result of Motion. Source: Salvesen 2011, 270.

Different arrangements are made to achieve the desired functionality in the two Doppler ultrasound techniques (Cosans 2007, 95). In the case of the continuous wave Doppler ultrasound, both the source and the receiver of the waves are stationary. Pulsed Doppler ultrasound uses a single transducer which acts as both the transmitter and the receptor.

An Analysis of the 2 Doppler Systems

Physical Principles behind Pulsed and Continuous Doppler Ultrasound

Continuous Wave Doppler Ultrasound

The technique is more ancient as compared to the pulsed Doppler ultrasound technique. In this technique, both the transmitter and the receiver are fixed. The continuous wave Doppler system is also simpler and easier to operate as compared to the one used in the pulsed wave Doppler technique (Rumack 2005, 34). The system is operated using an uninterrupted flow of ultrasound waves. Two crystal transducers are used in operating this system. One of them performs the role of a transmitter. The other one takes the role of a receiver. The diagram below explains this vividly:

CW Doppler.
Diagram 2: CW Doppler. Source: Haved 2011, 23.

The continuous wave Doppler system operates on a time-sharing principle. To this end, the system shifts back and forth very rapidly. It shifts from one form of examination to the other (Heikkilä 2011, 640). However, the switching is so rapid that the system operator may get the impression that the two examinations are being carried out at the same time. Under normal circumstances, no Doppler data is collected during the imaging period. As a result, estimations are made using the previous data to determine the form of imaging carried out on each of the examinations.

Pulsed Wave Doppler Ultrasound

The system is significantly different from the one discussed above. The pulsed wave Doppler technique operates on a transducer. The latter alters the flow of ultrasound waves (Salvesen 2011, 269). The ultrasonograph is an important instrument that is used in the pulsed-wave Doppler system in medical imaging. The instrument projects the Doppler data from the sample volume into a real-time motion display automatically. The diagram below is an illustration of a sonograph machine:

Sonograph Machine.
Diagram 3: Sonograph Machine. Source: Salvesen 2011, 270.

Periodic cycles are used in this technique. The cycles involve periodic transmission and reception of ultrasound waves to ensure that the results obtained are consistent. Using a short burst of ultrasound is very important. It helps in achieving range resolution in the system (Kiserud 2004, 1050). The received signal is combined with a delayed version of the just-transmitted ultrasound burst. Such a combination makes it possible to selectively monitor the flow in vessels at different depths.

In pulse sequencing, a predefined pulse repetition frequency is used to acquire the echo signals. The echo signals are used to determine and measure motion in the direction of the beam propagation (Hendrick 2005, 33). To acquire the repetition frequency, the echo signals are subjected to demodulation to produce in-phase and quadrature signals. Such signals are then passed through a wall filter, which helps to remove the strong and slow motions (Papageorghiou 2006, 599). The signals are then used to produce a spectrum that is important in medical imaging. It is important because the movement is displayed on an ultrasonograph machine.

A Doppler system has audio outputs that are of great importance in medical examination. Changes in the velocity of the flow are processed and converted into an audible sound. The sound is emitted through speakers located within the pulsed Doppler system (Haved 2011, 23). High pitched sound is an indication of massive Doppler shifts, which is associated with high velocities. A low-pitched sound, on the other hand, is an indication of lesser Doppler shifts, as well as lower velocities.

The pulsed wave Doppler ultrasound system is designed in a way that makes it possible to determine the direction of flow. Determining the direction of the flow is done through a special form of audio output that ensures that sounds signifying flow towards, or away from the transducer, are emitted from different speakers in the pulsed wave Doppler system.

The important thing about PW Doppler is that the user can acquire information from a specific point, which they can select. The user can determine the depth the Doppler information is coming from. With PW, the user can measure the time the pulses of sound take to come back, and hence work out the depth. That is not possible with CW.

Medical Applications of the Doppler Systems

Medical Applications of the Pulsed Doppler Ultrasound

Pulsed Doppler Ultrasound is applied in various medical processes. One of the most common fields where it is applied involves determining and detecting human fetal circulation. Recent research has confirmed that there are significant similarities between human fetal circulation and circulation in experimental animals (Caruana 2008, 139). Given that the technique has worked on these experimental animals, health practitioners and researchers have started using it to determine the velocity of circulation in human fetuses.

The technique helps in the detection of placenta vascular diseases associated with pregnancy complications. Such a development has seen a reduction of up to 38% in prenatal mortality rates among high-risk patients. One of the most common placenta vascular disorders is intrauterine growth restriction. The disorder is characterized by changes in the ‘blood velocity waveform’ (Salvesen 2011, 270). The velocity of the waveforms decreases and eventually disappears (Salvesen 2011, 270). Absence of reverse flow is fatal and may lead to death of the fetus. If detected, early delivery can be induced to ensure that the fetus is born alive and incubated to maturity.

The age of the fetus is an important consideration. Pregnancy can be prolonged as the patient is monitored closely using the Doppler system. The practice helps in reducing morbidity, since an immature fetus has decreased chances of survival as compared to a highly developed fetus, which has almost attained the appropriate age of birth. Uterine artery Doppler is also vital in detecting high-risk pregnancies (Hendrick 2005, 87).

Fetal intracranial circulation can be detected and determined as early as the eighth week of pregnancy using the pulsed wave Doppler system. A decreasing pulse signifies decreased circulation in the fetus’ brain (Hendrick 2005, 89). Pulsed wave system is also used in detecting retarded growth when analyzing the development of the fetus. Ability to detect retarded growth is attributed to the fact that fetal cranial Doppler shows varying frequencies in the various developmental stages of a fetus. Fetal blood circulation velocities are used to detect cases of fetal anemia, which is very important in detecting high-risk pregnancies. The diagram below is an illustration of a Doppler scan carried out on a fetus:

Foetal Doppler Scan.
Diagram 4: Foetal Doppler Scan. Source: Caruana 2008, 134.

Medical Applications of Continuous Wave Doppler Ultrasound

Just like its pulsed-wave Doppler ultrasound counterpart, continuous-wave Doppler ultrasound is a major milestone in the world of medical physics. The continuous wave Doppler is used to detecting the rise and fall of blood flow velocities in blood vessels, especially in the aorta (Salvesen 2011, 269). By comparing the velocity of the patient’s blood with that of a normal and healthy individual, the medical practitioner can detect cases of high and low blood pressure. As a result of this, corrective measures are taken on time to treat the patient.

An example of a procedure that uses the continuous wave Doppler ultrasound technique is the Doppler echocardiography (Papageorghiou 2006, 595). The Continuous-wave Doppler technique is used by medical practitioners to examine valves, chambers, and blood vessels found in the heart. As a result of this, the speed, as well as the direction of blood flow, is established. The continuous wave Doppler system is known to produce more accurate results concerning the velocity and direction of blood flow as compared to the pulsed wave Doppler.

The continuous wave Doppler is also known to help in detecting cases of abnormal communication between the right and the left chambers of the heart. As such, it is an important technique in the diagnosis of cardiac disorders (Caruana 2008, 134). The information gathered using this technique is used to detect such defects as leakages in blood vessels and the chambers of the heart. The technique is quite effective given that there are no invasive procedures required to determine the velocity of the blood. What this means is that the technique does not require surgical procedures. As a result, the dangers that the patient is exposed to during diagnosis are greatly reduced.

To determine the velocity of blood, medical practitioners prefer to use phase shifts as opposed to frequency shifts. The procedure can be used on a wide range of patients, given that there are no age, size, and gender requirements that may limit the application of the technique (Papageorghiou 2006, 600). The technique helps in monitoring the functioning of the cardiac valves and heart chambers. As a result, it is possible to detect cardiac disorders early. Early detection enables the medical practitioners to apply medical procedures necessary in correcting the disorder on time. The diagram below illustrates a Doppler scan performed on the heart:

Doppler Scan.
Diagram 5: Doppler Scan. Source: Caruana 2008, 139.


Medical physics entails the application of concepts found in the field of physics in medicine. Techniques used in medical physics have been of great importance in the field of medicine since they help in the early detection of illnesses and other health disorders. The Doppler Effect is one of the most widely used techniques in medical physics. It helps in detecting the velocity of the blood. Two techniques can be used concerning Doppler ultrasound. The techniques are the pulsed wave Doppler ultrasound technique and the continuous wave Doppler ultrasound technique (Caruana 2008, 140). Though the two are used to detect the velocity of blood, pulsed wave Doppler ultrasound can be used to conduct more complicated medical assessments, such as the mapping of fetal circulation, as compared to the continuous wave technique. The reason for this is that the former is an improved version of the latter. The Doppler ultrasound technique is important in medical imaging, given that the Doppler information obtained can be displayed on an ultrasonograph machine.

Reference List

Bates, John. Abdominal Ultrasound: How, Why and When. London: Churchill Livingstone, 2004.

Caruana, Jackson. “The Role of the Biomedical Physicist in the Education of the Healthcare Professions: An EFOMP Project of Physica Medica.” European Journal of Medical Physics 25, no. 3 (2008): 133-140.

Cosans, Richard. Applied Physics Medical Imaging. New York: Free Press, 2007.

Haved, Thomas. “Doppler Assessment of the Foetal Venous System.” Semin Perinatol 25, no. 1 (2011): 21–31.

Heikkilä, Kemppainen. “Handedness in the Helsinki Ultrasound Trial.”Ultrasound in Obstetrics & Gynecology 37, no. 4 (2011): 638-642.

Hendrick, William. Ultrasound Physics and Instrumentation. New York: Elsevier-Mosby, 2005.

Kiserud, Thomas. “The Foetal Circulation.” Prenat Diagn 24, no. 6 (2004): 1049–1059.

Papageorghiou, Campbell. “First Trimester Screening for Preeclampsia.” Curr Opin Obstet Gynecol 5, no. 7 (2006): 594–600.

Rumack, Cirus. Diagnostic Ultrasound. New York: Mosby, 2005.

Salvesen, Kelvin. “Ultrasound in Pregnancy and Non-right Handedness: Meta-analysis of Randomized Trials.” Ultrasound in Obstetrics & Gynecology 38, no. 5 (2011): 267–271.

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