Ultrasound
Ultrasound Technology
Ultrasound wave is produced when an electric current is applied to an array of piezoelectric crystals. This causes distortion of the crystals, makes them vibrate and produce this acoustic wave. The summation of the waves produces an ultrasound beam.
The ultrasound waves are produced in pulses. Each pulse is 2-3 cycles of the same frequency. The pulse length is the distance each pulse travels. The pulse repetition frequency is the rate at which the transducer emits the pulses. The pulses have to be spaced. This allows enough time between pulses so the beam has enough time to reach the target and return to the transducer before the next pulse is generated.
Ultrasound image is produced when the pulse wave that is generated travels through the body, reflects of the tissue interface (echo0 and returns to the transducer. When the wave is transmitted back to the transducer its amplitude is represented by its brightness or echogenicity. It is represented as a dot. The final image is produced by the combination of these dots. Strong reflections produce bright dots (hyperechoic images) e.g. bone, weaker reflections produce grey dots (hypoechoic images) e.g. solid organs. No reflection produces anechoic images, e.g. blood vessels
Frequency and Resolution
Ultrasound frequency is above 20,000 Hz or 20 KHz. Medical ultrasound is in the range of 3 -15 MHz. Average speed of sound through most soft human tissues is 1,540 meters per second. This can be calculated multiplying the wavelength with frequency. The higher frequency wavelength will have shorter wavelength whereas lower frequency wavelength will have longer wavelength. The wavelength for the 2.5 MHz is 0.77 mm whereas that for 15 MHz is 0.1 mm
Image resolution determines the clarity of the image. Such spatial resolution is dependent of axial and lateral resolution. Both of these are dependent on the frequency of the ultrasound. Axial resolution is the ability to see the two structures that are side by side as separate and distinct when parallel to the beam. So a higher frequency and short pulse length will provide a better axial image. Lateral resolution is the image generated when the two structures lying side by side are perpendicular to the beam. This is directly related to the width of the ultrasound beam. Narrower the beam better is the resolution. The width of the beam is inversely related to the frequency. Higher the frequency narrower is the beam. If the beam is wide the echoes from the two adjacent structures will overlap and the image will appear as one.
Attenuation
When the ultrasound beam travels through the tissues there is some energy loss and this is called attenuation. Attenuation of the signal is due to absorption, reflection and scattering. Attenuation is represented by the attenuation coefficient and each tissue ahs its own coefficient. Blood has the lowest coefficient and bone ahs the highest coefficient. In the soft tissue 80% of the attenuation is because of absorption. Attenuation is also results from reflection and scattering. Reflection depends on the difference in acoustic impedances of the tissues at the interface. Acoustic impedance is the resistance offered by the tissues to the transmission of the sound. Higher the difference in impedance greater is the reflection of the wave. Scattering occurs when the ultrasound wave encounters an interface that is not perfectly smooth. Scattering provides most of the information for diagnostic imaging.
Color Doppler
Color Doppler characterizes the flow. There is a moving target and a stationary transducer. When the transducer is positioned on a blood vessel the red cells are moving. They cause a change in the returning echoes. If the cells are moving towards the transducer it is perceived as a higher frequency and is displayed in red. If it is moving away form the transducer it is a lower frequency and is displayed as blue. So depending how the transducer is angled on the blood vessel the color could be blue or red. This color Doppler detection is worst when the transducer is placed 90 degrees to the blood vessel.
Transducers
The transducers (probes have a bandwidth of 7-15 MHz. For superficial structures like supraclavicular block it is ideal to use a probe that has a frequency greater than 7 MHz. These give best resolution if the structures are only 2-3 cm below the skin. When the structures are deeper than 4com it is advisable to use the lower frequency transducer (probe). The image resolution is poor compared to higher frequency transducer. The probes are liner and curvilinear. They can be with a large foot print or small foot print to accommodate for the anatomy. So it is important to remember
Higher frequency - High resolution poor penetration
Lower frequency - Poor resolution deeper penetration
