Sonography is defined as a diagnostic medical procedure, which uses high frequency sound waves to produce dynamic visual images of organs, blood flow, and tissues. At this point, Sonography is increasingly being used in the detection and treatment of heart attack, heart diseases, and vascular diseases that can lead to stroke. Vascular sonography is, therefore defined as the process of using high-pitched sound waves to study the blood vessels in the body. As a matter of fact, an ultrasound image provides an essential way of evaluating the circulatory system of the body. The images are captured in real-time thus helping radiologists to monitor the blood flow to organs/tissues all over the body. In addition, the ultrasound sound images help radiologists to locate and identify blockages and abnormalities like blood clot, emboli, and plaque thereby facilitating a plan for effective treatment.
Vascular refers to the blood vessels that carry blood from the heart to the other body organs and vice versa. Vascular diseases are unhealthy changes that occur in blood vessels. The circulatory system is very complex in function and structure. The flow of blood is influenced by a number of factors among them are elasticity of the vessel walls, vascular injuries, and the tone of vascular smooth muscle. Individuals with vascular injuries as a result of penetrating or blunt trauma can be divided into those which have direct clinical signs of arterial/venous injury and those with indirect signs. Trauma without direct signs represents a particular challenge as a result of the undetected vascular injury. In the past century, there was a conflict regarding the management of vascular trauma because many people advocated for aggressive surgical exploration. The approach led to a high rate of unnecessary surgery thus generating the need for selective diagnostic imaging to establish if operative intervention is required.
The potential for using the reflection of sonography in the visualization of the internal organs of the human body started in the late 1930s. Austrian neurologist Dussik K.T developed a sonographic transmission technique in order to visualize cerebral ventricles. When a particle is activated to vibrate in its equilibrium position, the vibration is transmitted to a neighboring molecule in the medium.
In this manner, kinetic energy is propagated from one molecule to the other thereby spreading through the medium in what is similar to sine wave pattern. The sound waves compresses and expands the medium as it travels through. An ultrasound image is created thus revealing any abnormality in the blood.
Sonography guided vascular access
Vascular access is an essential procedure that clinicians have to master. Injuries, obesity, intravenous drug use, and chronic medical conditions can make placements of vascular catheters in both peripheral and central veins time-consuming and challenging. In the recent years, there have been dramatic improvements in portable sonar technology, which includes the development of relatively inexpensive machines with adequate resolution to guide needle placement through tissues.
Transducer characteristics such as shape and frequency determine sonar image quality. For the purpose of vascular access, it is essential to use high frequency as well as small footprint transducers. As a matter of fact, the high-frequency linear array transducer offers a higher resolution of the superficial areas of soft tissues that includes veins and arteries.
The color Doppler and B-mode are the main ultrasound modes that can used to access venous tissues. The B-mode produces recognizable 2D gray scale images. Color Doppler can applied to characterize blood flow. The mode detects optimal flow of blood when the transducer is parallel to the flow. However, when the transducer is perpendicular to the vessel, the detection of flow is worst.
Optimizing image quality
Best visualization of target vessels calls for an optimal machine setting. Generally, proper transducer selection and the selection of pre-programmed vascular sonography settings offer acceptable quality of images. Moreover, other controls that can further enhance the quality of the image are focus, depth, gain, and frequency.
Proper depth adjustment offers a better target vessel imaging. Furthermore, it facilitates the tracking of equipment used through the tissue. When the depth setting is increased, the target vessel becomes smaller. Contrary, when the depth is too shallow, significant structures that surround the target vessels may be lost. That is to say, it is necessary to select the appropriate depth for the target vessel.
The brightness of an image on the screen is directly controlled by the gain setting of the sonar machine. Furthermore, it depends on the selected gains. By increasing the gain of the machine, the image is made brighter thereby easy to study. However, when the gain is decreased, the image becomes darker thus very difficult to analyze. Actually, the highest resolution of any image displayed is at the focal zone. With the use of sonar machines, it is essential to put the focus at the level of the target vessel of interest.
Physical Principles of Sonography: Doppler Effect
During the examination of blood vessels, the moving blood cells act as the reflectors. To specific, the red blood cells act as the reflectors as a result of their great majority in the blood cells. The difference between the frequency of the reflected and transmitted sound is known as Doppler-frequency-shift. If the direction of the blood flows to the transducer, then Doppler-shift is positive. Nevertheless, if the direction of blood flow is away from the probe, the Doppler-shift is negative.
The Doppler-shift can be displayed in a number of ways, which depend on the Doppler technique. The use of spectral mode is popular because Doppler tracing can be seen. Moreover, color/Doppler mode can be used. This mode displays the Doppler-shift as shades of one or more color inside the color box.
Spectral Doppler Sonography
Spectral Doppler techniques display consists of two types: pulsed and continuous wave Doppler modes. Pulsed wave Doppler mode is used in peripheral/abdominal vascular studies while continuous wave Doppler is to measure high velocities. A pulsed wave transducer contains one piezoelectric crystal.
In connection the above point, the crystal transmits pulses at regular intervals. The same crystal receives reflected signal and compares it with the transmitted reference. Pulsing the waves allow Doppler measurements to be taken from a specific region within the image field thereby allowing velocity measurements from the selected vessels. A pulsed wave allows precise localization of the volume of tissue from, which the Doppler blood flow signal is sampled in contrast with the continuous wave technique.
Duplex imaging uses pulsed wave Doppler with a two-dimensional real-time image. Normally, the location of the target volume is displayed on the B-scan tomogram. At this point, the sampling gate can be moved to the lumen of the vessel as highlighted on the real-time image. The velocity changes, which occur in each cardiac cycle, can be displayed graphically. The running time is placed on the horizontal axis. If the cursor is aligned parallel to the blood vessel, the velocity of the moving cells can be seen on the vertical axis.
During a duplex Doppler examination, the Doppler-shift can be displayed graphically or in audible form. The arteries have swish-like sounds while veins have continuous wind-blow-like sounds. The intensity of the audible sounds is directly proportional to the quantity of moving blood cells. The higher of the velocity of the flowing blood, the higher the audible sound is. In effect, pulsed wave Doppler sonography promotes the assessment of the direction, presence, and velocity of blood flow in the sample volume.
The most commonly used methods for measuring the blood flow velocity in a vessel are the maximum velocity method and uniform insonation method. In the uniform insonation method, the entire lumen of the vessel is incorporated into the gate. Nevertheless, maximum velocity method is where a small sample volume is placed in the placed in the centre of the vessel.
Color Doppler imaging
The basics of color Doppler imaging are almost similar to pulsed wave Doppler mode; however, it has a number of multiple sample volumes inside a circumscribed region known as a color box rather a visible one. The position and size of the color box on the B-mode image is determined by the operator. This type of sonography displays the two-dimensional flow information in color superimposed on the B-mode image of the vessel as well as the surrounding tissue.
Inside the color box, all the points are in a shade of red or blue rather than a shade of gray. The direction of flow relative to the transducer is illustrated on a color bar adjacent to the image. By convection, the flow the move towards the transducer is red whereas the flow away from the transducer is blue.
Color Doppler sonogram technique offers movement-information regarding a large part of the image. In point of fact, color Doppler flow imaging promotes the assessment of the presence, quality, and direction of blood flow more quickly than it does in other noninvasive technique.
Planes and Views
For the purposes of vascular access, two types of planes are used: longitudinal and transverse views. In the transverse view, the transducer plane is placed in cross section of the target vessel and the vessel is displayed on the screen as a circle. However, in a longitudinal view, the transducer plane is placed parallel to the one another and the vessel is displayed on the screen as a long tube running across the screen. On the whole, a longitudinal view allows visualization of the entire vessel of interest but requires that needle, transducer beam, and the target vessel to be held parallel to one another.
Differentiating vein and artery
Differentiating between vein and artery is important to safely perform sonography guided vascular access. The simplest way to differentiate between artery and vein is the compressibility of veins. Basically, veins compress with minimal pressure while arteries retain much of their original shape and appearance despite a heavy pressure. While performing an internal jugular vein placement, it is necessary to visualize the influence of respiratory variation on the vein diameter. Trendelenberg positioning and valsalva maneuvers make vein larger; however, it has a minimal impact on the carotid artery.
The guidance of vascular access using sonogram can be grouped as dynamic or static. In the static use of sonogram, providers apply sonogram to localize the vein and mark the site of the needle insertion on the skin. The dynamic guidance entails the use of sonogram in real-time with continuous visualization of the needle insertion throughput the procedure. The success rate for dynamic guidance is higher than those of static technique.
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