Imaging With Ultrasound I PDF

IMAGING WITH ULTRASOUNDS I Dr Anastasia Hadjiconstanti Prof Vered Aharonson Acknowledgements: Dr. Constantinos Zervides LECTURE LOB’S 28. DESCRIBE THE PRODUCTION OF AN ULTRASOUND BEAM. 29. EXPLAIN THE DETECTION OF ECHOES WITH A TRANSDUCER. HOW IS THE IMAGE PRODUCED I Ultrasound images display differ...

IMAGING WITH ULTRASOUNDS I Dr Anastasia Hadjiconstanti Prof Vered Aharonson Acknowledgements: Dr. Constantinos Zervides LECTURE LOB’S 28. DESCRIBE THE PRODUCTION OF AN ULTRASOUND BEAM. 29. EXPLAIN THE DETECTION OF ECHOES WITH A TRANSDUCER. HOW IS THE IMAGE PRODUCED I Ultrasound images display differences in the acoustic impedance (Z) of different tissues. These are caused by variations in density & compressibility of different tissues, e.g., bone versus liver. In its simplest form, an ultrasonic pressure wave is transmitted from the transducer along a single line of sight into the body. As the pressure wave propagates, echoes are reflected from objects along the line of sight. HOW IS THE IMAGE PRODUCED II The returning echoes are received by the transducer. Image is displayed by: mapping the echo magnitude as brightness in the image, and mapping the arrival time as distance along the line of sight. This is repeated along many lines of sight to produce a two-dimensional image. HOW IS THE IMAGE PRODUCED III RANGE EQUATION I To create an anatomic image, a sound pulse must travel to a reflector located in the body and return to the transducer. The travel time of this journey allows us to accurately position the reflector. Ultrasound systems determine reflector depth by measuring a pulse’s time-of-flight. The elapsed time from pulse creation to pulse reception is called the go-return time or timeof-flight. When a reflector is located superficially, the time from pulse creation to reception is brief, whereas trips to and from deeper reflectors require more time. This is the basis of measuring distance in diagnostic imaging. RANGE EQUATION II Ultrasound systems’ computers are programmed with the average speed of sound in tissue. Since the average speed of sound in soft tissue is know, the time-of-flight and distance that the pulse travels are directly related. The depth of a reflector can be accurately calculated from the go-return time using: 𝒎𝒎 𝟏. 𝟓𝟒 ×𝒈𝒐 − 𝒓𝒆𝒕𝒖𝒓𝒏 𝒕𝒊𝒎𝒆 (𝝁𝒔) 𝝁𝒔 𝒅𝒆𝒑𝒕𝒉 𝒎𝒎 = 𝟐 The 13 μs rule always applies when sound travels through soft tissue. For every 13 μs of go-return time, the object creating a reflection is 1 cm deeper in the body. CLASS EXAMPLE 1: Assume ultrasound echoes are received from two interfaces of an organ in the path of the ultrasound pulse. The time between sending the original pulse and receiving echoes are 4.0𝑥10!"𝑠 and 8.0𝑥10!"𝑠. Assume that the speed of sound is 1540 m/s on average along the pathway. (a) How far from the transducer are the interfaces? (b) How wide across is the organ at this point? CLASS EXAMPLE 1 - Solution (a) How far from the transducer are the interfaces? 𝒅𝒆𝒑𝒕𝒉 𝒎𝒎 = 𝟏. 𝟓𝟒 𝒎𝒎 ×𝒈𝒐 − 𝒓𝒆𝒕𝒖𝒓𝒏 𝒕𝒊𝒎𝒆 (𝝁𝒔) 𝝁𝒔 𝟐 𝑚 1540 𝑥 4.0𝑥10"# 𝑠 𝑠 𝐷! = = 0.031 𝑚 = 3.1 𝑐𝑚 2 𝐷$ = 1540 𝑚 𝑥 8.0𝑥10"# 𝑠 𝑠 = 0.062 𝑚 = 6.2 𝑐𝑚 2 CLASS EXAMPLE 1 - Solution (b) How wide across is the organ at this point? D1 and D2 are the distances to the front and rear interfaces of the organ. The difference between these two numbers gives the dimension along the ultrasound pulse’s path: 6.2 𝑐𝑚 − 3.1 𝑐𝑚 = 3.1 𝑐𝑚 The organ is 3.1 cm wide along this direction. TRANSDUCER I A transducer (probe) is any device that converts one form of energy to another. The ultrasound transducer has a two functions: it is a transmitter that produces ultrasound by converting electrical energy into acoustic energy and It is a receiver, detecting returning ultrasound waves by converting acoustic energy into electrical energy. There are many different transducer shapes and configurations depending on which body part is being imaged. TRANSDUCER II The shape of the transducers determines its field of view. Transducers are generally described by the size and shape of their footprint. TRANSMITTER The transducer is itself the transmitter of the ultrasound pulses into the body. Before pulse transmission, precisely timed, high-amplitude voltage need to be applied to the transducer. The length of an ultrasound pulse is determined by the number of alternating voltage changes applied to the transducer. Transducers have a range of frequencies which they are able to produce. This is known as the bandwidth. PIEZOELECTRIC EFFECT I Certain crystals change their physical dimensions when subjected to an electric field and change back when the field is removed. When compressed, they also have the property of generating electric potentials. Piezo comes from the Greek word, πιέζω, which means “to apply pressure”. Changes in polarity (+ or -) of a voltage applied to a transducer cause the transducer to change in thickness. Polarity changes causes expanding and contracting of the transducer. This results in increases and decreases in pressure. PIEZOELECTRIC EFFECT II This produces ultrasound waves that can be transmitted into the body. Pressure changes caused by the returning ultrasound echoes are converted back into electrical energy signals. These are transferred to a computer and create an ultrasound image. These small events are the source of all ultrasound images. Note that the wavelength of the emitted ultrasound is a function of the crystal size. PIEZOELECTRIC MATERIAL I Transducers are made of a synthetic ceramic (piezoceramic) such as: lead-zirconate-titanate (PZT) or polyvinylidene difluoride (PVDF) or a composite. The piezoelectric properties of PZT do not occur spontaneously. These properties are created by exposing the material to a strong electric field while being heated to a substantial temperature. This is called polarization. PIEZOELECTRIC MATERIAL II The temperature at which PZT is polarized is called the Curie temperature or the Curie point and it is generally around 360 oC. If the polarized PZT is heated above the Curie temperature, the crystal’s piezoelectric properties are destroyed. The loss of piezoelectric properties is called depolarization. HOW DOES THE SPEED OF SOUND IN PZT AFFECT FREQUENCY In a pulsed wave transducer, the speed of sound in PZT and the frequency are directly related. When the speed of sound in PZT is faster, the frequency of sound created is higher and vise versa. The speed of sound in most piezoelectric material ranges from 4 to 6 mm/μs. HOW DOES THE THICKNESS OF THE PZT CRYSTAL AFFECT FREQUENCY For a pulsed wave transducer, thinner active elements, create higher frequency sound pulses. PZT thickness and frequency are inversely related. The mathematical relationship is: 𝒇𝒓𝒆𝒒𝒖𝒆𝒏𝒄𝒚 𝑴𝑯𝒛 = 𝒎𝒎 ) 𝝁𝒔 𝟐 ×𝒕𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔 (𝒎𝒎) 𝒔𝒐𝒖𝒏𝒅 𝒔𝒑𝒆𝒆𝒅 𝒊𝒏 𝑷𝒁𝑻 ( Transducer manufacturers create PZT crystals of varying thickness, thus designing transducers of different frequencies. The thickness of PZT crystals is diagnostic imaging transducers ranges from 0.2 to 0.1 mm. SBA 1 The time interval from the transmission of an ultrasound pulse until the return of the echo is 0.05 ms. The velocity of sound in the medium is 1540 m/s. What is the depth of the interface that generated echo? A. 3.85 cm B. 5.35 cm C. 6.45 cm D. 7.70 cm E. 8.80 cm SBA 1 - Solution The time interval from the transmission of an ultrasound pulse until the return of the echo is 0.05 ms. The velocity of sound in the medium is 1540 m/s. What is the depth of the interface that generated echo? 𝒎𝒎 𝒔𝒐𝒖𝒏𝒅 𝒔𝒑𝒆𝒆𝒅 𝒊𝒏 𝑷𝒁𝑻 ( 𝝁𝒔 ) 𝒇𝒓𝒆𝒒𝒖𝒆𝒏𝒄𝒚 𝑴𝑯𝒛 = 𝟐 ×𝒕𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔 (𝒎𝒎) 1 1 𝑓= = = 20,000 𝐻𝑧 !# 𝑇 0.05 ×10 𝑠 𝑚 1540 𝑣$ 𝑠 𝐷= = = 0.0385 𝑚 !% 2𝑓 2×20,000𝑠 = 3.85 𝑐𝑚 EXERCISE FOR HOME - SBA By changing the image depth, which of the following does the operator also change? A. B. C. D. E. F. Pulse repetition frequency Duty factor Propagation speed Pulse repetition period Amplitude Spatial pulse length EXERCISE FOR HOME A sound wave is created by a transducer, reflects off an object and returns to the transducer. The go-return time is 26 μs. What is the total distance that the pulse travelled? A. B. C. D. 1 cm 2 cm 3 cm 4 cm SUMMARY I Image is displayed by: mapping the echo magnitude as brightness in the image, and mapping the arrival time as distance along the line of sight. All ultrasound scanners consist of similar components that perform key functions. SUMMARY II Ultrasound transducers are made from piezoelectric crystals. These crystals, have the property that an applied electric voltage, changes the thickness of the crystal. Alternating voltage cause the crystal to vibrate generating a sound wave. If a sound wave causes the crystal to vibrate, and alternating voltage is produced. The thickness and acoustic velocity of a piezoelectric crystal determine the resonant frequency of the transducer. REFERENCES Authors Title Edition Publisher Year ISBN R.K.Hobbie and B.J.Roth Intermediate Physics for Medicine and Biology 5th Edition Springer 2015 9783319126814 M.A. Haidekker Medical Imaging Technology 1st Edition Springer 2013 9781461470724 A.B. Wolbarst, P. Capasso and A.R. Wyant Medical Imaging: Essentials for Physicians 1st Edition Wiley-Blackwell 2013 9780470505700 Sidney K. Edekman Understanding Ultrasound Physics 4th Edition E.S.P. Ultrasound 2012 9780962644450 Suzanne Amador Kane Introduction to Physics in Modern Medicine 2nd Edition CRC Press 2009 9781584889434 V. Gibbs, D. Cole, A. Sassano Ultrasound Physics and Technology. How, Why and When 1st Edition Churchill Livingstone 2009 9780702030413