Doppler sonography (2. LF UK)

Introduction
Doppler Sonography is a non-invasive imagery method used in medicine to assess the movement velocity of moving components of the body, i.e tissue and blood.

The doppler sonography technique, as its name indicates, is based on two physical principles: ultrasound and the doppler effect.

The term ultrasound refers to a high frequency sound wave outside the human hearing range. Consecutives ultrasound pulses are transmitted by a transducer towards a target before they are, in turn, reflected by this target. The reflected waves are detected by the transducer which acts as the transmitter and recipient of the ultrasound waves.

However, the emitted and reflected waves differ in wavelength and frequency. This phenomenon is described by the doppler frequency shift, characteristic for the doppler effect.

The frequency of sound waves, hence the sound pitch, changes according to the movement of the source which emits the waves. If the movements effects towards the detector, the frequency is higher -  from the detector the frequency is lower.

Therefore the emitted waves have a higher frequency than the reflected ones, detected by the transducer.

The recorded signal is then translated into a graph which can be analyzed under different aspects.

Importance in clinical medicine
In clinical medicine doppler sonography mainly serves a diagnostic and preventive role and is used in almost all fields.

An analysis of the doppler spectrum, i.e the recorded signal, enables to detect any potential abnormalities. In vascular medicine, it is used to determine the state of the vessels, which allows to diagnose anomalies including stenosis, a peripheral occlusion of arteries and veins. In obstetrics, doppler sonography is used for prenatal screening. This analysis is not only crucial to monitor the development of the fetus, but mostly to detect risk factors, especially congenital vascular malformations such as abnormal connections of veins and arteries. Neurologists utilize sonography to prevent brain damage by recognizing cerebral occlusions, clots, bleeds and blockages of arteries supplying the brain which can cause ischemic strokes.

Changes in perfusion of tissue can indicate to an oncologist, that a tumor is present. In this case doppler sonography is used to assess the extent of spread and whether the tumor is malicious or benign. By analyzing the blood flow of an artery in terms of blood flow velocity, resistance and flow volume heart valve defects and congenital disorders in fetuses can be diagnosed by cardiologists.

Literature Review
Doppler Sonography uses reflected waves to examine the blood velocity in arteries and veins. When using the method an ultrasound sending-and-receiving device called transducer is held in an angle, on top of the skin and over the blood vessels. When the sound waves hit the moving blood cells they bounce back and causes a change in the pitch of  the reflected sound waves. The assessments of the blood vessels tell the examiner about velocity, blood flow and if there is reduced flow in arteries or veins. In our case the graph of velocity is important.

The advantages are the quick data samples about the flow direction and velocity, in form of real time assessments. Furthermore it is non-invasive and pain free to the patient’s body. Also it is exposing a very limited amount of radiation, compared to other diagnostic methods like X-ray and CT scan.

Fat and motions can cause difficulties in obtaining good signal and therefore information about the blood velocity. In addition the patient should remove jewelries to prevent disturbances in the transmittance of signal

Doppler sonography is generally considered safe, even though it is not completely risk free. Due to absorption of the ultrasound energy in the body tissue, heat is produced on and inside the skin of the patient. The longer the same region is exposed the higher the temperature raises. This may have a biological effect when examining blood flow in newborn’s brains.

From an ethical aspect, risks must be put in relation to obstetrics, where Doppler sonography is part of the standard examination, and used to assess the fetus’ health status. “Negative” results obtained from the scan can lead to a dilemma regarding maternal and fetal health interests.

Two types of Doppler sonography can be described:

Continuous-wave system (CW):
Technically simple method for determination of blood flow in the limbs.This Doppler Sonography device uses two crystals as transducers - one sends ultrasound waves and the other one receives the reflected signals from blood cells. Due to the obtuse angle of the transducers an overlapping of sent and received signals occur in a relatively large area. The disadvantages of the device are the lacking of axial resolution and the disability to determine blood vessel placement.

Pulsed-wave system (PW)
In this type of Doppler device, the same transducer emits and receives the ultrasound as pulses. The signal is received between the transmitted and received pulses. Doppler pulses are relatively long and the repeating frequency high compared to the ultrasound imaging pulses. The technique is used to determine blood-vessel depth, blood flow, velocity and direction, but lacks the ability to correctly measure higher velocities above 1,5-1,7 m/s.

Doppler-Sonography device
The main device is the ultrasound machine with its transducer, which will be slightly passed over the skin above the organ to explore. The hand-held transducer is connected to the scanner by a cord. The transducer sends and receives sound waves that are amplified through a microphone. These inaudible sound waves have a frequency around 4 MHz.

Sterile ultrasound gel
The gel is a conductive medium  that enables a tight bond between the skin and the transducer. It  is used to increase the conductivity of the ultrasound waves and without it most of the waves would be reflected. Usually the ultrasonic gel is composed of propylene glycol and water.

BiDop operating software
The software is installed on a computer and records data and displays the graph. With it the blood flow velocity in time, heart rate, ratio of systole and diastole duration can be examined.

The sonographic device is connected to the computer with a cord.

Methodology

 * 1) Turn on device and operating software
 * 2) Palpate optimal spot for examination
 * 3) on wrist/forearm: arteria radialis
 * 4) Apply ultrasound gel to the area to be examined
 * 5) Choose a measuring angle (either 30°, 45° or 60°) and look for the optimal signal (all the selected categories should display data, systole and diastole should be clearly recognisable)
 * 6) Start or finish measurement by pressing the Start/Stop button
 * 7) Modification of the displayed graph can be achieved by
 * 8) scaling the vertical axis
 * 9) changing the angle
 * 10) reversing the displayed direction of blood flow
 * 11) Analyse the obtained graph under different parameters
 * 12) Maximum/minimum blood velocity during systole and diastole
 * 13) Use Cursor option: Horizontal cursor (blood velocity in cm/s), vertical cursor (time)
 * 14) Place both cursors on peak of systole and diastole respectively
 * 15) Duration of one heart cycle = one systole and consecutive diastole
 * 16) Place vertical cursor on beginning of systole and end of diastole (= beginning of next systole
 * 17) Turn off device and clean it when finished

Conclusion
Over the course of the last half decade the development on the field of diagnostic ultrasound has been rapid, the newest technology being 3D-imaging.

The development has not come to an end, au contraire; new techniques for volume blood flow estimation have emerged. Based on Gauss’s theorem, an angle-independent measurement is now possible; the theorem is based on the relation of the integrated flux of a vector field through a surface, and the divergence of the vector field in the closed surface, i.e the inside of the surface.

Doppler sonography improves diagnosis due to provision of immediate clinical information through imagery, hence the reduction of wrong diagnosis consequently a reduction of harm to the patient which could be caused by wrong treatments. On the financial side the immediate result reduce overall healthcare costs.