Ultrasound is a sound wave with a frequency not in the audible range of human beings. Nowadays ultrasound is a name of technique in which high-frequency ultrasound waves are used for diagnostic purposes. The frequency is in the range of 2 to 15 MHz.
Ultrasound is used for a variety of purposes which are linked with examining the internal organs. In order to understand the reason behind any swelling, pain, or infection, doctors recommend ultrasound tests. It is profoundly used to check the fetus’s state as well during pregnancy.
The basic principle behind the test is that sound waves are transmitted by mechanical oscillations produced from crystals lying in the transducer. The transducers are excited by short electrical pulses by a phenomenon piezoelectric effect.
The transducer converts electrical energy into mechanical or sound energy. The produced ultrasound waves then penetrate through the tissues. The waves are reflected and scattered back by striking several internal structures and return as echoes. The reflected waves are then converted into electrical impulses by crystals in transducers. The pulses are then processed to form an image in the computer attached to them.
The range of frequencies produced by ultrasound crystals is called bandwidth. It includes a 2.5 to 3.5 MHz range of frequencies for abdominal diagnosis imaging and a 5.0 to 7.5 MHz of range for superficial imaging.
This is the effect lying behind the principle of Ultrasound. The waves are generated using a probe which produces a piezoelectric effect.
Artificially made crystals are utilized for modern transducers. The setup is put in high temperature and strong electric field that generates piezoelectric effect essential to produce ultrasound waves. It works on the principle that some material generates a voltage by applying pressure or generates pressure by applying voltage.
The crystal face to which voltage is applied contracts or expands depending upon the voltage polarity applied. The resonation of the crystal then converts electricity to ultrasound waves. However, the frequency of the waves depends upon the thickness of the crystal used. Conversely, the crystal produces a voltage when it is deformed by the incoming echo or sound wave. The voltage received is then analyzed by the system which generates an image.
So, ultrasound waves are produced when the crystal expands and contracts more than 20000 times per second. The waves are generated continuously until the voltage applied is discontinued. However, in ultrasound tests very short pulses are required for diagnostic purposes. To produce a short pulse, voltage is applied for a very short period such that the crystal resonates at its own frequency for a short time which decays gradually. The resonation of the crystal is stopped by a damped material that absorbs all the vibrations.
The image formation is produced by generating short pulses of sound waves through the narrow beam to the body organ. The returning echoes are then received as the waves strike the structures in their way. The instrument that produces and receives the short pulses is called a transducer. It is an essential component of the image quality that depends upon it.
A transducer comprises three parts mentioned below.
- A Case
- A Crystal
- A Damping material
These components are connected by electrical wires.
Some Multi-element Transducers used these days have many small crystal elements each producing short pulses. They are arranged in a line and such transducers are referred to as linear array transducers. The wavefronts generated from each crystal element combine and a single wavefront is generated by the Huygens principle. Every single element is electrically and acoustically isolated for allowing flexible beam formation.
The shape of the beam plays an important role in image quality. It consists of three parts
- Fresnel zone (near field)
- Fraunhofer zone (far field)
- Transition point
The transition point is the place where the near the field terminates, and divergence starts. The near field is important for imaging but can be large according to the diameter of the crystal. A narrow beam is required for the high-resolution diagnosis. Many focusing strategies are employed to make the beam narrow like placing a curved lens in front of the crystal or simply curving the crystal. Focusing can also be done by electric means.
The beam profile obtained by the transducer reflects that the energy is not restricted to a single lobe. It emits at different angles to the face of the transducer as off-axis energy called side lobes. The array transducers produce grating lobes which are the weaker beams moving in different directions. Both these lobes are minimized to produce less artifactual echoes.
Resolution is an important phenomenon in ultrasound. It is the ability to distinguish between echoes and therefore high resolution is important for high-quality photos.
1. CONTRAST RESOLUTION
The contrast resolution is useful for differentiating between the tissues with different characteristics like the liver or spleen. It tells the degree of similarity between two tissues (echogenic appearance) yet is still distinguishable.
- TEMPORAL RESOLUTION
Temporal resolution is useful for identifying the malformations in the underlying anatomy. It is utilized profoundly in echocardiography.
- SPATIAL RESOLUTION
Spatial resolution is useful for detecting and displaying structures that are very close together. The image of ultrasound displays the depth of the patient’s tissues or organs as well as the width of the cross-section. Hence it is essential to have both axial and lateral resolutions.
- AXIAL RESOLUTION
It is the ability to distinguish the small targets as separate structures present along the beam path.
- LATERAL RESOLUTION
The lateral resolution identifies the distance between two structures when placed side-by-side.
Higher frequency is useful for higher spatial resolution and lower penetration while lower frequency is useful for decreased spatial resolution and increased penetration. The lower frequency is important for visualizing deep structures.
The waves reflected from the structures between different tissues are proportional to the amount of impedance. The more the density of different tissues more the proportion of reflected waves and the less the proportion of transmitted waves.
However, if there is a big difference in densities of tissues it will produce acoustic shadowing referred for complete reflection of waves. It is most commonly seen in bones, kidney stones, other calculi, or intestinal gas.
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