Echocardiography
Assistant
Principles
Ultrasound utilizes high frequency waves generated from the interaction of electrical energy with piezoelectrical crystal. The piezoelectrical crystals are ceramic crystals that deform when the electrical current passes through them, and this causes a rapid (high frequency) change in their shape. A high frequency vibration is created in the form of sound waves in the 1-20MHz range. These waves can propogate through tissues and will either be transmitted or reflected depending on the nature of the tissue. When these echoes return the kinetic energy that they contain is converted back to electrical energy which when energy is relatively high is reflected on the screen as bright signal (white), when intermediate strength it is gray, and when there is no signal it is black.
Tissue Characterization
The energy that returns is a reflection of the nature of the tissue it has interracted with. If the sound wave returns from a particular position with almost none of the energy it left with, then there has been little attenuation of the sound wave and this occurs when the sound wave has passed through a fluid structure or a cyst. The resulting image has no echoes reflected on the screen. If the signal returns with half the original strength then part has been refllected and part transmitted or absorbed, which may for example indicate a soft tissue structure, and an intermediate signal is seen on the screen. If the signal returning has a lot of energy on returning, then most of the signal has been reflected and a bright signal ensues. This occurs when sound interacts with bone or air. In these instances there is very little signal allowed to pass through the more posterior tissue,s preventing the sound waves from interacting with deeper tissues and hence they cannot be imaged.. This phenomenon is called shadowing.
Time
Speed Distance and the Image
In general the speed with which ultrasound travels through tissues is fairly uniform. Hence the time it takes to to travel and return is a reflection of the depth of the tissue. The This is an important principle to understand because it helps create the image
Impedence
Sound propagates through different materials at different velocities. The velocity is dependent upon the density of the material. For soft tissues, pulse velocity is approximately constant and taken to be 1540 m/s; Reflection occurs when the pulse passes between interfaces with differing acoustic impedances; The more dense a structure the greater its acoustic impedence. The larger the difference in impedance between two adjacent structures, the more sound is reflected. Reflection is also optimized when the interface is aligned perpendicular to the direction of pulse.
Cystic vs Solid
One of the most important roles that ultrasound plays is differentiating cystic from solid disease. Solid tissue has internal structures that have differing acoustic characteristics and reflect the sound waves. A simple cyst is a common entity, containing pure fluid, contains no internal structures and therefore the sound waves passing throigh it go unimpeded manifesting as a anechoic, and black structure.
When the pulse reaches the far side of the cyst, there is a fluid-solid interface which reflects brightly and gives rise to posterior or back wall enhancement. Furthermore, because the signal has not been attenuated while passing through fluid, larger cysts will allow a greater amount of sound energy to penetrate deeper. When the sound energy reaches solid tissue deep to the cyst, small amounts of the pulse are reflected back due to internal reflectors, but appear brighter on the image than adjacent solid tissue in which the reflected sound is weaker because less sound energy penetrated as deeply. This phenomenon is called increased through transmission.
In summary, cyst on ultrasound are characterized by:
- Anechoic
- Posterior wall enhancement
- Increased through transmission
Doppler
Doppler ultrasound utilizes the Doppler principle to detect flow. When sound pulses are reflected off moving substance (such as blood), there is a shift in frequency; The shift in frequency is greater if the substance is moving toward the transducer, and lower if it is moving away. On the ultrasound image, this can be displayed as a waveform (pulsed Doppler waveform ) or as colors on the image (one color for motion towards transducer, and another color for motion away from transducer).
Power Doppler refers to detection of motion but not direction. A single color is displayed on the image for all motion, with low brightness for slow movement and high brightness for rapid motion. It is more sensitive than color Doppler but lack the information on direction.
Practical applications of color Doppler include assessing lesions for vascular flow, which may better characterize or diagnose the lesion. Stenoses in vessels or grafts can be detected, since flow velocity increases with narrowing in vessels. Pseudoaneurysms can be differentiated from hematomas, as there is a typical ‘Yin-yang” flow of blood into and out of the lesion.
thrombus floating RV RA embolus from ashley davidoff on Vimeo.
Courtesy Michael Maysky MD
Aim
The aim of diagnostic sonography is to characterize lesions, document anatomy and, where appropriate, function of the tissues being imaged whilst keeping the cumulative sound energy delivered ALARA (as low as reasonable achieveable). The purpose of ultrasound of the gallbladder is to define the size shape position and character of the gallbladder, and also to determine if there is gallbladder tenderness.
Indications
Contraindications
There are no absolute medical contraindications to ultrasound. It is generally considered to be a safe procedure without significant side-effects. There is a small theoretical risk of damage to tissues from heating, which has been demonstrated with extreme doses in animal studies. As such, the examination is performed following the ALARA principle (keeping exposure As Low As Reasonably Achieveable).
Advantages
Ultrasound presents numerous advantages. First, it is fast and requires little preparation. It can be performed in real time, and is useful for visualize moving structures (heart, fetus), as well as visualizing live structures during interventional procedures. The machines used to perform ultrasound are extremely portable, and cheap in comparison to other imaging modalities. It carries few, if any, side-effects, produces no ionizing radiation, and is safe for use in pregnancy, as well as with delicate organs. Lastly imaging can be performed in any plane.
Disadvantages
Ultrasound is not effective in visualizing bone or air-filled structures. In addition, visualizing deep structures is often only possible at the expense of quality and resolution. Finally, the quality of studies produced is highly operator-dependent.
Method
Patient Preparation
Equipment
Ultrasound equipment consists of a monitor, control panel and transducer (probe). The entire unit is typically wheeled and portable. Ultrasound machines typically have a number of different transducers, which differ in frequency of pulses generated and shape of the probe (and hence the shape of the image – sector versus rectangle). Machines routinely used for diagnostic exams are relatively large, but much smaller units are available and are sometimes used in emergency rooms, physician offices and intensive care units for non-diagnostic assessment of, for example, bleeding in early pregnancy, musculoskeletal pain or obtaining intravenous access.
Technique
The transducer is applied firmly against the patient using acoustic gel to improve transmission of the sound waves. The orientation of the transducer determines the imaging plane. The operator will use the images on the monitor to move and rotate the transducer until the organ or tissue of interest is visualized. The image may be optimized on the monitor by altering frequency, gain, focal zone, filed of view and grey-scale curves. Static images are typically saved, though a series of frames may be recorded in certain examinations (echocardiography) in order to evaluate motion of the organ over time.