THE BASIC PRINCIPLES OF ULTRASONOGRAPHY (USG)

THE BASIC PRINCIPLES OF ULTRASONOGRAPHY (USG)
By : Sumarsono


Diagnostic Ultrasonography sometimes called “diagnostic medical sonography” has become a clinically valuable imaging technique. It differs from diagnostic radiology in that it uses nonionizing, high-frequency sound waves to generate an image of a particular structure. Ultrasound is employed in the visualization of muscles, tendons, and many internal organs, their size, structure and any pathological lesions with real time tomographic images.. Blood velocities may be calculated in vascular and cardiac structures with the Doppler technique.The Ultrasound equipment may be easily moved into the operating room, special care nursery, or intencive care unit or may be manually transported by means of a mobile van service to provide ultrasound service for smaller hospital and clinics. Ultrasound is cost-effective compared with computed tomography (CT), magnetic resonance imaging (MRI), or angiography.Further development in high-frequency, millimeter size tranducers mounted on the tip of an angiographic catheter (IVUS; Intra Vascular Ultrasound) have great potential.


History

Physical Principles
Ultrasound is sound waves greater than 20,000 Hertz (greater than the upper limit of human hearing). The audible sound frequencies are below 15 000 to 20 000 Hz, while frequency ranges used in medical ultrasound imaging are 2 -15 MHz. Audible sound travels around corners, the human can hear sounds around a corner (sound diffraction). With higher frequencies the sound tend to move more in straight lines like electromagnetic beams, and will be reflected like light beams. They will be reflected by much smaller objects (because of sorter wavelengths), and does not propagate easily in gaseous media. At higher frequencies the ultrasound behaves more like electromagnetic radiation. The wavelength is inversely related to the frequency f by the sound velocity c:
c = λf
Meaning that the velocity equals the wavelength times the number of oscillations per second, and thus:

λ =c/f

The sound velocity i a given material is constant (at a given temperature), but varies in different materials :



The speed of sound is different in different materials, and is dependent on the acoustical impedance of the material. However, the sonographic instrument assumes that the acoustic velocity is constant at 1540 m/s. An effect of this assumption is that in a real body with non-uniform tissues, the beam becomes somewhat de-focused and image resolution is reduced.

Basically, all ultrasound imaging is performed by emitting a pulse, which is partly reflected from a boundary between two tissue structures, and partially transmitted. The reflection depends on the difference in impedance of the two tissues.
The ratio of the amplitude (energy) of the reflected pulse and the incident is called the reflection coefficient. The ratio of the amplitude of the incident pulse and the transmitted pulse is called the transmission coefficient. Both are dependent on the differences in acoustic impedance of the two materials. The acoustic impedance of a medium is the speed of sound in the material × the density:
Z = c ×

The reflecting structures does not only reflect directly back to the transmitter, but scatters the ultrasound in more directions. Thus, the reflecting structures are usually termed scatterers.


The time lag, , between emitting and receiving a pulse is the time it takes for sound to travel the distance to the scatterer and back, i.e. twice the range, r, to the scatterer at the speed of sound, c, in the tissue. Thus:

r = cτ / 2

The pulse is thus emitted, and the system is set to await the reflected signals, calculating the depth of the scatterer on the basis of the time from emission to reception of the signal. The total time for awaiting the refelcted ultrasound is determined by the preset depth desired in the image.
Piezoelectric effect
Ultrasound is generated by piezoelectric crystals that vibrates when compressed and decompressed by an alternating current applied across the crystal, the same crystals can act as receivers of reflected ultrasound, the vibrations induced by the ultrasound pulse .
Piezoelectric effect , voltage produced between surfaces of a solid dielectric (nonconducting substance) when a mechanical stress is applied to it. A small current may be produced as well. The effect, discovered by Pierre Curie in 1883, is exhibited by certain crystals, e.g., quartz and Rochelle salt, and ceramic materials. When a voltage is applied across certain surfaces of a solid that exhibits the piezoelectric effect, the solid undergoes a mechanical distortion. Piezoelectric materials are used in transducers , e.g., phonograph cartridges, microphones, and strain gauges, which produce an electrical output from a mechanical input, and in earphones and ultrasonic radiators, which produce a mechanical output from an electrical input. Piezoelectric solids typically resonate within narrowly defined frequency ranges; when suitably mounted they can be used in electric circuits as components of highly selective filters or as frequency-control devices for very stable oscillators .
Transducer
A sound wave is typically produced by a piezoelectric transducer encased in a probe. Strong, short electrical pulses from the ultrasound machine make the transducer ring at the desired frequency.The sound is focused either by the shape of the transducer, a lens in front of the transducer, or a complex set of control pulses from the ultrasound scanner machine. This focusing produces an arc-shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth. A probe containing one or more acoustic transducers to send pulses of sound into the body. Whenever a sound wave encounters a material with a different density (acoustical impedance), part of the sound wave is reflected back to the probe and is detected as an echo. 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 impedances, the larger the echo is. If the pulse hits gases or solids, the density difference is so great that most of the acoustic energy is reflected and it becomes impossible to see deeper.
Older technology transducers focus their beam with physical lenses. Newer technology transducers use phased array techniques to enable the sonographic machine to change the direction and depth of focus. Almost all piezoelectric transducers are made of ceramic.
Materials on the face of the transducer enable the sound to be transmitted efficiently into the body (usually seeming to be a rubbery coating, a form of impedance matching). In addition, a water-based gel is placed between the patient's skin and the probe.
The sound wave is partially reflected from the layers between different tissues. Specifically, sound is reflected anywhere there are density changes in the body: e.g. blood cells in blood plasma, small structures in organs, etc. Some of the reflections return to the transducer.



To generate a 2D-image, the ultrasonic beam is swept. A transducer may be swept mechanically by rotating or swinging. Or a 1D phased array transducer may be use to sweep the beam electronically. The received data is processed and used to construct the image. The image is then a 2D representation of the slice into the body.
3D images can be generated by acquiring a series of adjacent 2D images. Commonly a specialised probe that mechanically scans a conventional 2D-image transducer is used. However, since the mechanical scanning is slow, it is difficult to make 3D images of moving tissues. Recently, 2D phased array transducers that can sweep the beam in 3D have been developed. These can image faster and can even be used to make live 3D images of a beating heart.
Doppler ultrasonography is used to study blood flow and muscle motion. The different detected speeds are represented in color for ease of interpretation, for example leaky heart valves: the leak shows up as a flash of unique color. Colors may alternatively be used to represent the amplitudes of the received echoes.
Display Modes
Four different modes of ultrasound are used in medical imaging. These are:
• A-mode (amplitude modulation) : A-mode is the simplest type of ultrasound. The received energy at a certain time, i.e. from a certain depth, can be displayed as energy amplitude. The greater the reflection at the interface, the larger the signal amplitude will appear on the A-mode screen.


• B-mode (Brightness) : The amplitude can also be displayed as the brightness of the certain point representing the scatterer. In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen.

• M-mode (motion mode) : if some of the scatterers are moving, the motion curve can be traced In m-mode a rapid sequence of B-mode scans whose images follow each other in sequence on screen enables to see and measure range of motion, as the organ boundaries that produce reflections move relative to the probe.

• D Mode or Doppler mode: This mode makes use of the Doppler effect. The Doppler information is displayed graphically using spectral Doppler, or as an image using color Doppler (directional Doppler) or power Doppler (non directional Doppler). This Doppler shift falls in the audible range and is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, pulsing sound.

Artifacts
Artifacts is Portions of the display which are not a “true” representation of the tissue imaged. Medical Diagnostic Ultrasound imaging utilizes certain artifacts to characterize tissue.The ability to differentiate solid vs. cystic tissue is the hallmark of ultrasound imaging. Acoustic shadowing and acoustic enhancement are the two artifacts that provide the most useful diagnostic information. Acoustic shadowing diminished sound or loss of sound posterior to a strongly reflecting (e.g.,large calcifications, bone) or strongly attenuating structure (solid tissue, significantly dense or malignant masses).

Acoustic enhancement is the increased of transmission of the sound wave posterior to a weakly attenuating structure (e.g., simple cysts or weakly attenuating masses). Gain curve expected a certain loss or attenuating with depth of travel.

Diagnostic applications
A general-purpose sonographic machine may be able to be used for most imaging purposes. Usually specialty applications may be served only by use of a specialty transducer. The dynamic nature of many studies generally requires specialized features in a sonographic machine for it to be effective; such as endovaginal, endorectal, or transesophageal transducers.
Obstetrical ultrasound is commonly used during pregnancy to check on the development of the fetus. In a pelvic sonogram, organs of the pelvic region are imaged. This includes the uterus and ovaries or urinary bladder. Men are sometimes given a pelvic sonogram to check on the health of their bladder and prostate. There are two methods of performing a pelvic sonography - externally or internally. The internal pelvic sonogram is performed either transvaginally (in a woman) or transrectally (in a man). Sonographic imaging of the pelvic floor can produce important diagnostic information regarding the precise relationship of abnormal structures with other pelvic organs and it represents a useful hint to treat patients with symptoms related to pelvic prolapse, double incontinence and obstructed defecation.
In abdominal sonography, the solid organs of the abdomen such as the pancreas, aorta, inferior vena cava, liver, gall bladder, bile ducts, kidneys, and spleen are imaged. Sound waves are blocked by gas in the bowel, therefore there are limited diagnostic capabilities in this area. The appendix can sometimes be seen when inflamed eg: appendicitis