Unit 5 Ultrasound

Unit 5 Medical ultrasound      

the nature of ultrasound the generation and detection of ultrasound the factors that affect reflection of ultrasound Ultrasound scan ultrasound diagnostic and imaging Doppler ultrasound blood flow measurement

The nature of ultrasound and its use in medical Ultrasound – sound with a frequency above 20 000 Hz (20 kHz) is defined to be ultrasound. Ultrasounds have frequencies and wavelengths, just like all other types of waves. The wavelength of ultrasound limits the fineness of details that it can detect. Details significantly smaller than the wavelength of a probe cannot be detected. This is true for all types of waves. Because the wavelength of visible light is significantly longer than atoms, we can not see atoms with visible light.

The nature of ultrasound and its use in medical The medical applications of ultrasound are very broad. • In therapy: pulverization of tumor, diathermy; • In diagnostic: • ultrasound scan for the determination of organs and bones thickness, • ultrasound scan for imaging, • Doppler ultrasound for blood flow rate, blood pressure measurement

Production of ultrasound An ultrasound is produced by ultrasound transducers. An ultrasound transducer works like both a speaker and a microphone.

Reflection of ultrasound waves

When an ultrasound is incident on a boundary between two media, reflection and refraction occur, the fraction of reflected ultrasound depends on the acoustic impedance (Z) of the media on either side of the boundary.

• If there is large difference of the acoustic impedances of the two media • Strong reflected of ultrasound. • Weak transmitted signal. • In order that large portion of ultrasound penetrate into the body from transducer when making ultrasound imaging, gel is applied on the skin to eliminate air between transducer and skin.

Pulse echo measurement Ultrasound can be used to detect the thickness of a medium much like sonar is used to detect depth. The total distance traveled by the pulse =(sound speed x the echoes time)/2

Ultrasound scan – A scan

A – scan of an ultrasound pulse

When ultrasound is sent into a patient’s body, the reflections from the boundaries are detected by a transducer. Both the outgoing, and reflected ultrasounds are amplified and graphed against time on a cathode ray oscilloscope (CRO).

Peak 1 comes from the reflection from boundary one. Peak 2 comes from the reflection from boundary two. The time interval between peak one and peak two is the time it took for the ultrasound to travel from boundary one to boundary two and back to boundary one. ‘A scans’ are commonly used to measure the thickness of the lens in the eye before surgery.

Ultrasound scan – B scan

For the B scan, the intensity of the reflected signals is represented as the brightness of a spot.

The transducer can be placed in different angles to obtain a series of spots .

In modern B scans, a series of transducers are arranged in a row and send out ultrasound pulses one at a time .

Ultrasound in medical diagnostics

(a) An ultrasonic image is produced by sweeping the ultrasonic beam across the area of interest, in this case the woman’s abdomen. Data is recorded and analyzed in a computer, providing a two-dimensional image.

How much detail can ultrasound reveal Abdominal scan : 7 MHz Speed of sound in tissue : ~ 1540 m/s λ = v / f = (1540 m/s) / (7×106Hz) = 0.22 mm In practice 1 mm resolution is obtainable. The penetration depth is proportional to the wavelength. Typical Penetration depth in tissue : ~ 500λ So penetration depth = 500 × 0.22 mm = 0.11 m

Ultrasound in medical therapy The tip of this small probe oscillates at 23 kHz with such a large amplitude that it pulverizes tissue on contact.

Focused to intensities of 103 to 105 W/m2, ultrasound can shatter gallstones ( 胆结石 ) or pulverize cancerous tissue.

Ultrasound diathermy (deep-heat treatment) – energy converts from sound to thermal energy. Intensities – 103 to 104 W/m2 Frequencies – 0.8 to 1 MHz Ultrasound diathermy is applied to overworked or damaged muscles in athletes and in physical therapy to relieve pain and to improve flexibility .

Doppler ultrasound The Doppler effect is change in the perceived sound frequency due to movement of the source or the observer .

(a) x: fobs =fs y: fobs =fs

(b) x: fobs fs

(c) x: fobs fs

Doppler ultrasound The frequency shift of the reflected ultrasound :

2 fv cos θ ∆f = vw Where f is the source frequency, v the speed of the moving blood, and vw is the speed of sound in the tissue. By measuring the frequency shift ∆ f, the average speed of the blood can be calculate.