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SLIDE 7 They used a through-transmission technique with two transducers placed on either side of the head, and producing what they called "ventriculograms", or echo images of the ventricles of the brain. Pulses of 1/10th second were produced at 1.2 MHz. Coupling was obtained by immersing the upper part of the patient's head and both transducers in a water bath and the variations in the amount of ultrasonic power passing between the transducers was recorded photographically on heat-sensitive paper as light spots (not on a cathode-ray screen). It was an earliest attempt at the concept of 'scanning' a human organ. Although their apparatus appeared elaborate with the transducers mounted on poles and railings, the images produced were very rudimentary 2-dimensional rows of mosaic light intensity points. They had also reasoned that if imaging the ventricles was possible, then the technique was also feasible for detecting brain tumors and low-intensity ultrasonic waves could be used to visualize the interior of the human body. SLIDE 12 Discovered by the Curie brothers in 1880 when they realized certain crystals cycha s quartz undergo mechanical deformation a potential difference develops across the tow surface crystals. A piezoelectric crystal has an alternating current applied across it. The piezoelectric crystal grows and shrinks depending on the voltage run through it. Running an alternating current through it causes it to vibrate at a high speed and to produce an ultrasound. This conversion of electrical energy to mechanical energy is known as the piezoelectric effect To produce an ultrasound, a piezoelectric crystal has an alternating current applied across it. The piezoelectric crystal grows and shrinks depending on the voltage run through it. Running an alternating current through it causes it to vibrate at a high speed and to produce an ultrasound. This conversion of electrical energy to mechanical energy is known as the piezoelectric effect. The sound then bounces back off the object under investigation. The sound hits the piezoelectric crystal and then has the reverse effect - causing the mechanical energy produced from the sound vibrating the crystal to be converted into electrical energy. By measuring the time between when the sound was sent and received, the amplitude of the sound and the pitch of the sound, a computer can produce images, calculate depths and calculate speeds. Whenever a sound wave moving in air hits a solid surface, it reflects off it. This reflected sound is called an echo. The same applies to a sound wave moving through water and hitting an obstacle. If we know the speed of sound in the air or water, we can calculate the distance to the obstacle. To do this we must measure the time taken for a pulse of sound to travel to the object and back again: The distance to the object and back is given by distance=speed x time As this is the total distance that the sound has traveled to the object and back, we must divide by 2 to find the one-way distance. This use of echoes is the basis of sonar (sound navigation and ranging). The pulse of sound that is used should be short, and high frequencies are usually used, as they travel further without being absorbed. Sounds with a frequency above 20 kiloHertz (20 kHz) are called ultrasonic (beyond the range of human hearing). The sounds used for sonar are well into the ultrasonic range, with frequencies of 1 - 20 megaHertz (MHz). In solving problems on sonar, remember that the speed of sound itself varies from one material to another. The speed also depends on temperature, pressure and other factors. Typical speeds are approximately 330 m/s in air, 1500 m/s in water and 5000 m/s in a metal. The piezoelectric effect also works in reverse. If the crystal is squeezed or stretched, an electric field is produced across it. So if ultrasound hits the crystal from outside, it will cause the crystal to vibrate in and out, and this will produce an alternating electric field. The resulting electrical signal can be amplified and processed in a number of ways (see questions on A-scan and B-scan). So a second crystal can be used to detect any returning ultrasound which has been reflected from an obstacle. Normally the transmitting and receiving crystals are built into the same hand-held unit, which is a called an ultrasonic transducer (generally, a transducer is any device to convert energy from one form to another, usually to or from electrical energy.) the transducer incorporates a piezoelectric element, which converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode SLIDE 13 By measuring the time between when the sound was sent and received, the amplitude of the sound and the pitch of the sound, a computer can produce images, calculate depths and calculate speeds. SLIDE 15 Ultrasound is produced and detected using an ultrasound transducer. Ultrasound transducers are capable of sending an ultrasound and then the same transducer can detect the sound and convert it to an electrical signal to be diagnosed. To produce an ultrasound, a piezoelectric crystal has an alternating current applied across it. The piezoelectric crystal grows and shrinks depending on the voltage run through it. Running an alternating current through it causes it to vibrate at a high speed and to produce an ultrasound. This conversion of electrical energy to mechanical energy is known as the piezoelectric effect. The sound then bounces back off the object under investigation. The sound hits the piezoelectric crystal and then has the reverse effect - causing the mechanical energy produced from the sound vibrating the crystal to be converted into electrical energy. By measuring the time between when the sound was sent and received, the amplitude of the sound and the pitch of the sound, a computer can produce images, calculate depths and calculate speeds. SLIDE 16 Advantages compared with other techniques 1. Ultrasound examinations are non-invasive i.e. they do not require the body to be opened up, or anything to be inserted into the body. This is a major advantage compared to fibre-optic endoscopy, for example, which may involve much more patient discomfort as the probe is inserted. 2. Ultrasound methods are relatively inexpensive, quick and convenient, compared to techniques such as X-rays or MRI scans. The equipment can be made portable, and the images can be stored electronically. 3. No harmful effects have been detected, at the intensity levels used for examinations and imaging. This contrasts with methods based on X-rays or on radioactive isotopes, which have known risks associated with them, and ultrasound methods are preferred whenever possible. This is particularly relevant to examination of expectant mothers. 4. Ultrasound is particularly suited to imaging soft tissues such as the eye, heart and other internal organs, and examining blood vessels. Disadvantages of ultrasound compared with other techniques 1. The major disadvantage is that the resolution of images is often limited. This is being overcome as time passes, but there are still many situations where X-rays produce a much higher resolution. 2. Ultrasound is reflected very strongly on passing from tissue to gas, or vice versa. This means that ultrasound cannot be used for examinations of areas of the body containing gas, such as the lung and the digestive system. 3. Ultrasound also does not pass well through bone, so that the method is of limited use in diagnosing fractures. It is possible to obtain quite good ultrasound scans of the brain, but much greater detail is obtained by an MRI scan. SLIDE 17 Disadvantages of ultrasound compared with other techniques 1. The major disadvantage is that the resolution of images is often limited. This is being overcome as time passes, but there are still many situations where X-rays produce a much higher resolution. 2. Ultrasound is reflected very strongly on passing from tissue to gas, or vice versa. This means that ultrasound cannot be used for examinations of areas of the body containing gas, such as the lung and the digestive system. 3. Ultrasound also does not pass well through bone, so that the method is of limited use in diagnosing fractures. It is possible to obtain quite good ultrasound scans of the brain, but much greater detail is obtained by an MRI scan. SLIDE 20 Color Doppler ultrasound of normal femoral vein at the level of the bifurcation of the deep and superficial femoral veins. Main differences between Ultrasound and X-rays Diagnostic Ultrasound X-rays (radiology) wave type longitudinal waves electromagnetic waves transmission requirements elastic medium No medium generation stressing the medium accelerating charges velocity depends on the medium through which it propagates It is relatively constant: 299,792.456.2 m/s similar waves seismic, acoustic radio, light mechanical electric Velocity of sound in some Biological Materials Velocity of sound in some Biological Materials Material Velocity of Sound (m/s) Impedance (Rayl x 10 -6) Air 330 0.0004 Fat 1450 1.38 Water 1480 1.48 Average Human Soft Tissue 1540 1.63 Brain 1540 NA Liver 1550 1.65 Kidney 1560 1.62 Blood 1570 1.61 Muscle 1580 1.7 Lens of eye 1620 NA Skull Bone 4080 7.8 Main differences between Ultrasound and X-rays Diagnostic Ultrasound X-rays (radiology) wave type longitudinal waves mechanical electromagnetic waves transmission requirements elastic medium No medium generation stressing the medium accelerating charges velocity depends on the medium through which it propagates similar waves acoustic electric radio, light Velocity of sound in some Biological Materials Velocity of sound in some Biological Materials Material Velocity of Sound (m/s) Impedance (Rayl x 10 -6) Air 330 0.0004 Fat 1450 1.38 Water 1480 1.48 Average Human Soft Tissue 1540 1.63 Brain 1540 NA Liver 1550 1.65 Kidney 1560 1.62 Blood 1570 1.61 Muscle 1580 1.7 Lens of eye 1620 NA Skull Bone 4080 7.8 Let’s begin by breaking the meaning of Ultrasound. Can anyone break down the word Ultrasonography? Answer: Ultra stands for ultra high frequency. Sonic means sound. Graphy is a permanent record. So now put it all together. What is the process of Sonar? Answer: It is a technique of sending sound waves thru water and observing the returning echoes to identify submerged objects. Frequencies of sound that are heard by the human ear are called audible sound. What is the difference in the sound waves in ultrasound and audible sound frequencies? Answer: Sound waves have a higher frequency than audible sound waves. In ~ 1947 patients with suspected ventricular disease, tumors or other intracranial lesions were sent to get an US. Top part of the patients head was immersed in water and two transducers were placed on opposite sides of the head. The images were recorded on paper (photographic) as rudimentary 2-dimensional rows of light spots. Present day patients are not submerged in water and the transducer are much smaller as are the machines. The water has been replaced by gel. Piezoelectric effect: Defined as “pressure electric” Curie brothers discovered when AC is applied to certain crystals they expand and contract, causing the crystals to vibrate, in response to the electrical field. The electric energy (AC) is converted to mechanical energy in the form of an ultrasound wave. The sound bounces off the anatomy and is returned to the crystals and they start to vibrate again. This is when the mechanical energy is converted back into electrical energy. The transmitting and receiving crystals are built into the same handheld device. What is this handheld device called? Answer: transducer Once US waves are produced they are directed into the body. They travel thru the body until striking a tissue barrier that reflects the sound waves back to the transducer: they are called echoes. Acoustic impedance: sound impedance Impedance delays or prevents the progress of the acoustic waves from being transmitted thru a medium. Ultrasound has properties very similar to light: 1) Focused with a smaller transducer 2) Refracted: US signal is deflected from a straight path and the angle of the deflection is away from the transducer 3) Reflected: US signal is reflected back toward the transducer 4) Scatter: tissue boundaries are less than the wavelength of US wavelengths. This is the most common cause of scattering. Blood is the most common anatomy that causes scatter. This is because blood has a wavelength 20 times less than US wavelength. What is the velocity of sound? Answer: Sound is vibration that travels thru a medium (anatomy) as a wave. Velocity describes how far the wave will travel in a given amount of time. The way in which the US responds when it comes into contact with anatomy is dependent on the density of the anatomy. US travels faster or has an increased velocity when traveling through water in comparison to air. If we know US travels faster in water what can we deduce when we are trying to determine whether US travels faster thru fat or bone? Explain? Answer: The higher the atomic number of the material the faster US passes thru a given medium. The sound waves returned are picked up by the computer to make images. The intensity and pitch images produces images. The computer uses this data to calculate depth and speed. There are many types of transducers with varying frequencies and shapes depending on the body part to be imaged. First lets do a review on the realationship between wavelength and frequency. How are frequency and wavelength related in x-ray? Answer: Frequency increases wavelength decreases.