Echography

Principle

Echography, also called medical (ultra)sonography is an ultrasound-based diagnostic imaging technique.

Echography uses a probe containing acoustic transducers (generally of piezo crystals, see piezoelectricity) to send sound into a material (here tissue). Whenever a sound wave encounters a material with a different acoustical impedance, part of the sound wave is reflected (see Waves) which the probe detects as an echo, see Fig. 1.

Fig. 1

The time it takes for the echo to travel back to the probe is measured and used to calculate the depth of the tissue interface causing the echo. The greater the difference between acoustic impedences, the larger the echo is. The reflectioncoefficient R is:

(1)

with A_r and A_i the amplitudes of reflected and impinging wave and Z₁ and Z₂ the acoustic impedences of medium 1 and 2 respectively. When these media are respectively water (Z=152.000 rayl) and air (Z=400 rayl) then R=0.999474, which is equivalent to a transmission loss of 20log(1-R)=65.6 dB. When medium 1 is air and 2 is water, the same holds (see (1)). Consequently: with a large ratio of the both impedances, the reflection is large.

The above consideration does not take in account scatter from the object, which diminishes the reflectance and disturbes imaging. Taking water as substitute for blood, R of blood-muscle interface is only 0.034 (see for values Acoustic impedance), which asks for highly sofisticated hardware and software to obtain a good image (noise reduction). When bone is involved, R is some 2-20 times higher.

A water-based gel ensures good acoustic coupling between skin and the ultrasound scan head.

A 2D- image can be obtained by a probe with many transducers and a 3-D images can be constructed with a specialized probe.

From the amount of energy in each echo, the difference in acoustic impedance can be calculated and a colour is then assigned accordingly.

Limitations

The spatial resolution in the axial direction (the depth) is directly related to the wavelength λ of a pure ultrasound frequency. In the lateral direction the resolution is determined by the width (the aperture angle) of the beam due to divergence.

Further, there is an ambiguity in depth position. This occurs when the time lapse between sending and receiving a wave is larger than the period time t_per, The reflected wave of the objects at all distances n times 0.5λ are superimposed at the reflected pulse of the object itself. Mathematically: depth = (0.5t_recieve/t_per, − integer{0.5t_recieve/t_per, −})c + nλ. This problem is solved by sending short pulses and adjusting the pulse interval time such that any reflection from boundaries to at least the depth of interest are arriving within the pulse interval time. Since a pulse comprises many frequencies (see Signal Analysis and Fourier) the received signal needs some complicated computation (deconvolution) to reconstruct the echo-image.

The echographic modes

In the A-mode the strength, i.e. amplitude, of the reflected wave is indicated on the vertical axis and time at the horizontal one, with time zero at the origin.

In the B-mode the strength is indicated by the brightness (grayscale) of a dot. With the B-scan, along the vertical axis the penetration depth is indicated. The beam of the ultrasound changes slightly its angle of incidence every time a new sound pulse is emitted. In this way a kind of section of the anatomical object is obtained. However, the less depth, so the closer to the sound source, the more compressed is the image in the horizontal direction (so parallel to the surface). Such a scan is made many times a second and in this way a kind of movie is made. This is very helpful for moving structures, especially the heart (Fig. 2).

Fig. 2 Abnormal echocardiogram showing a mid-muscular ventricular septum defect in a new-born child. The red line in the ECG mark the time instant that the image was captured. Colors indicate blood velocity measured with the combined Doppler apparatus.

In the M-mode, movement is visualized by displaying time along the horizontal axis and an image is made for a single beam direction. When for instance this beam impinges on a mitralis valve, the image shows opening and closing of the valve (Fig. 3).

Fig. 3 Echocardiogram in M-mode

Technical strengths

Ø It images soft tissues very well and is particularly useful for delineating the interfaces between solid and fluid-filled spaces.

Ø It renders "live" images.

Ø It shows the structure as well as functional aspects.

Ø Widely available and comparatively flexible.

Ø Small, cheap, easily carried scanners (bedside) available.

Technical weaknesses

Ø Classical ultrasound devices have trouble penetrating bone but current research on ultrasound bone imaging will make it possible with dedicated devices in the future.

Ø Performs very poorly when there is a gas between the scan head and the organ of interest, due to the extreme differences in acoustical impedance.

Ø Even in the absence of bone or air, the depth penetration of ultrasound is limited, making it difficult to image structures that are far removed from the body surface, especially in obese patients.

Ø The method is operator-dependent. A high level of skill and experience is needed.

Applications

Echography is widely utilized, often with a hand-held probe. It is especially practiced in cardiology, gynecology and obstetrics, urology (kidney, prostate), vascular medicine, gastroenterology (also liver), endocrinology and ophthalmology.

More info

There exist several types of echograpy, mostly combined with a Doppler application.

Doppler echography

Echography can be enhanced with Doppler measurements, which employ the Doppler effect (see Doppler principle) to assess whether structures (usually blood) are moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a particular sample volume, for example a jet of blood flow over a heart valve, its velocity and direction can be determined and visualized. This is particularly useful in cardiovascular studies and vascular examinations of other organs (e.g. liver portal system). The Doppler signal is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, sound. Doppler echography can by distinguished in several modifications. The most common ones are here discussed.

The duplex (Doppler) scanner

The duplex scanner detects in a selected part of the image the moving blood by using the Doppler effect. The scanner calculates the actual velocity of the blood provided the angle between the direction of the ultrasonic beam and the direction of movement is known. The operator therefore aligns a marker along the direction of flow in the blood vessel and positions a cursor at the height of the peak systolic blood velocity.

In the common and superficial femoral arteries, the waveform normally has a forward component followed by a reverse component and a second smaller forward component. This is called a triphasic waveform because of the three phases. More distally in the superficial femoral artery, the second forward component may be absent, giving a biphasic waveform with two phases.

Fig. 4 Duplex scan of superficial femoral artery with a triphasic blood velocity waveform Horizontal axis: time (large ticks 1 s); vertical axis velocity.

The frequency shift is normally in the audio range (due to the ”ultra” frequencies), so most duplex scanners send the signal to a pair of audio speakers, and this enables the operator to hear the signal in addition to seeing the display (as in Doppler echocardiography).

Color Doppler scanners

Color Doppler (CD) scanners detect and display moving structures by superimposing color onto the grey-scale image. Color is superimposed wherever the scanner detects a moving structure, usually blood. The hue of the color shows the direction and magnitude of the blood velocity. In this image, red and yellow indicate flow away from the probe, with dark red representing low velocities and orange and yellow indicating higher velocities. Flow towards the probe is indicated in blue and green, with green indicating higher velocities. The hue can therefore be used to identify sites where the artery becomes narrower and the blood has to move faster to achieve the same volume flow rate. When the blood velocity exceeds the limit of the color scale, aliasing occurs (high velocity in one direction interprets as lower velocity in the other, wrong, direction). Color Doppler can also be used to display venous blood flow.

CD and Duplex sonography are often combined to Duplex/CD sonography, especially for assessing stenoses.

Power Doppler (PD)

Duplex/CD sonography is not an effective technique when the artery under study is almost perpendicular to the ultrasonic beam or by other poor conditions as bowel gas, breathing movements and obesity. Power Doppler (PD) has improved diagnostic capabilities of vascular Duplex/CD sonography, mainly because it is independent from the angle of insonation and has more sensitivity. PD generates an intravascular color map reflecting the integrated power in the Doppler signal, which essentially depends on the density of red blood cells within the sample volume. However, due to its intrinsic limitations, PD cannot replace conventional sonographic techniques and especially CD. So, PD is used as an adjunctive tool in vascular sonography.

Tissue Doppler Imaging (TDI)

Tissue Doppler Imaging (TDI) measures and displays peak velocities high temporal resolution (ca. 8 ms) at any point of the ventricular wall during the cardiac cycle. The mean velocities can be calculated with time velocity maps and displayed as color encoded velocity maps in either an M-mode or a two-dimensional format. Indeed, since all the points within the ventricular walls are velocity-encoded in real-time, the color-coded display should provide a huge amount of information which could form the basis for the application of accurate, reproducible quantitative evaluation.

Literature

http://en.wikipedia.org/wiki/Ultrasonography

http://www.worldwidewounds.com/2000/sept/Michael-Lunt/Doppler-Imaging.html#Colour%20Doppler%20scanners