Ultrasound

1. Ultrasound Deniz Nevşehirli

2. Ultrasound is a medical imaging technique that uses high frequency sound waves and their reflections.

3. A basic ultrasound machine consists of the following parts: • Transducer Probe - the part that sends and receives the sound pulses. • Central Processing Unit (CPU) - computer that does all of the calculations and contains the electrical power supplies for itself and the transducer probe. • Transducer Pulse Controls - changes the amplitude, frequency and duration of the pulses emitted from the transducer probe. • Monitor - displays the image from the ultrasound data processed by the CPU.

4. How it works? • The machine transmits high-frequency sound pulses ranging from 1 to 5 Megahertz into the body using a probe. • As the sound waves travel into the body they encounter a boundary between tissues (e.g. soft tissue and bone). • During the transition between two materials with different physical properties, some of the waves get reflected back to the probe, while some get refracted and travel further until they reach another boundary and get reflected.

5. Behavior of an ultrasound beam within tissue • In part A we see the exponential attenuation of ultrasound beam intensity within an homogenous material • In Part B we see reflection and refraction occurring at interface between two materials with different physical properties.

6. How it works? • The probe detects the reflected waves and transfers to the machine. • The distance from the probe to the tissue or boundaries is calculated by the machine using the speed of sound in tissue (1,540 m/s) and the time elapsed until the detection each echo (normally on the order millions per second). • The machine displays the distances and intensities of the echoes on the screen.

7. An Example of an Ultrasound Image

8. Major Uses of Ultrasound • Obstetrics and Gynecology • Urology • Cardiology • To observe structures or functions of the hearth to identify abnormalities. • To measure blood flow through the heart and major blood vessels. • Lungs filled with air and ribs limits the application.

9. Excellent resolution for short penetration distances using high frequencies. (5-15 MHz) 1 dimensional images are obtained. Small size transducer is an advantage. A-Mode Applications

10. Light intensity versus time is used to generate images. Stronger reflections cause brighter line. Mechanical Scanning Transducer is moved and placed on different locations on the patient. From each position an image is obtained and combined to form the display. Electronic Scanning Linear array of transducers are used. (64 or more) Faster operation. Several hundreds of images per second. B-Mode Applications

11. Through transmission imaging is used. Separate transducers are used. By moving transducers, 2d images are obtained. Two tissue properties are derived: Total attenuation is calculated using absorption, scattering and reflection losses. Index of refraction along the path using the time delay which is inversely proportional to the velocity. C-Mode Applications

12. Quantitative and qualitative analysis of heart valve motion are obtained. Transducer is stationary. Positions of heart valve leaflets are displayed at varying times using echoes. Vertical deflection is scanned at a rate of 2-3 cardiac cycles to form a single display. And the vertical axis is given in units of mm/sec. M-Mode Applications

13. Doppler Ultrasound • Doppler ultrasound is based upon the Doppler Effect. • When the object reflecting the ultrasound waves is moving, it changes the frequency of the echoes. • If the object is moving toward the probe, this will result in an increase in the frequency. • If the object is moving away from the probe, this will result in a decrease in the frequency. • The change in frequency depends on how fast the object is moving. Doppler ultrasound measures the change in frequency of the echoes to calculate how fast an object is moving. • Reflected by moving red blood cells, doppler ultrasound has been used mostly to measure the rate of blood flow through the heart and major arteries.

14. Doppler Ultrasound • Pulsed Wave Doppler Imaging • The time interval between transmission and reception is used to calculate distance. So it is depth selective. • Gives precise information about the location of target area and the flow. • Minimum range problem. • Aliasing problem • Continuous Wave Doppler Imaging • Receives information about all moving reflectors along the path of the beam. • Is not depth selective because there is no basis for the measurement of the time delay. • Maximum velocity is calculated. • Color Doppler Imaging • Pulsed Doppler Imaging is used to obtain a static image of blood flow velocity waveforms. • Different flow directions and rates are coded with different colors.

15. Future of Ultrasound • Biomedical engineers at Duke University's Pratt School of Engineering have created a new three-dimensional ultrasound cardiac imaging probe. • Inserted inside the esophagus, the probe creates a picture of the whole heart in the time it takes for current ultrasound technology to image a single heart cross section. • Transesophageal echocardiography, (TEE) is one form of ultrasound cardiac imaging. In this technique a probe is inserted down the patient's throat and behind the heart to capture ultrasound heart images.

16. Future of Ultrasound • Current TEE systems can quickly generate only two-dimensional cross-sectional images. This limitation makes it impractical for use in guiding therapeutic treatment devices such as ablation probes. 2-D probe must repeatedly repositioned during treatments so, instead, fluoroscopy (X-ray) is used. • But the use of X-ray imaging results in radiation exposure for patients and requires lead-shielding for clinicians. In addition, such procedures take up to seven hours to complete. • Since 3-D imaging requires significantly more sensors than 2-D imaging. The new Duke 3-D probe has the size of normal TE probes but contains an array of 504 individual ultrasound sensors. (~8 times the usual number: 64) • The probe generates ultrasound at 5 million vibrations per second. Having 504 sensors, it provides great sensitivity and a sharp image. And because the image is large enough to map the whole volume of the heart, fewer images need to be taken, reducing the required time.

17. Dangers of Ultrasound • Since ultrasound is energy, the question is what is this energy doing to the tissues. • There are two major possibilities of problem with ultrasound: • Tissues or water absorb the ultrasound energy and that increases local temperature. • Solubility of gases decrease with increasing temperature. Dissolved gases can form bubbles due to local heat caused by ultrasound. • Although, there are no records of bad effects of ultrasound in studies in either humans or animals, ultrasound should still be used only when necessary.

18. References • Lectures on Ultrasound by Prof. Yekta Ülgen, Institute of Biomedical Engineering BOĞAZİÇİ UNIVERSITY • http://www.online-medical-dictionary.org • http://electronics.howstuffworks.com/ultrasound.htm • http://ocw.mit.edu/OcwWeb/index.htm • http://www.cardiovascularultrasound.com/articles/browse.asp • http://www.physorg.com/news4277.html • http://www.mgdinc.com/pdfs/Comparison%20of%20Continuous%20Wave%20Doppler%20vs%20Pulse%20Doppler%20Profiling%20Technology.pdf • http://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htm