Unit 5 Medical Ultrasound

Unit 5: Medical ultrasound   

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the nature of ultrasound the generation and detection of ultrasound the factors that affect reflection of ultrasound ultrasound scan ultrasound diagnostic 超音波診斷 and imaging Doppler ultrasound blood flow measurement

1. Nature of ultrasound Ultrasound is defined as a sound with a frequency of over 20,000 Hz (20 kHz). 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. 2. The production of ultrasound

Ultrasound can be generated by ultrasound transducers 傳感 器. Ultrasound transducers can work as both a speaker (generate ultrasound) and a microphone (detect ultrasound). 3. Reflection of ultrasound waves When ultrasound reaches a boundary between two different kinds of media, part of the ultrasound is refracted, and the other part is reflected. The amount that is refracted and reflected depends on the acoustic impedance ( 聲阻抗 Z) of the media on both sides of the boundary. (Acoustic impedance is the product of the density of the medium and the sound speed in the medium) ● a larger difference between the acoustic impedance of the two media, will lead to more ultrasound being reflected and less being refracted. ● when an ultrasound image is taken of the body, gel is applied to the skin, to eliminate the air between the skin and the transducer so that a larger portion of the ultrasound will enter the body Ultrasound is better at detecting density than x-rays. The amount of ultrasound reflected depends on the acoustic impedance changes, and acoustic impedance changes depend on density differences. The largest amount of ultrasound is reflected at the places of the greatest density changes.

4. Pulse echo measurement 超音波厚度量測 Ultrasound can be used to detect the thickness of a medium much like sonar is used to detect depth. The time it takes for the ultrasound to travel the length of the medium, and bounce back is measured. From this, the thickness traveled by the ultrasound can be calculated. Ultrasound scan – A scan 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 (示波器 CRO). Peak one comes from the reflection from boundary one, and peak two 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. 5. Ultrasound in medical diagnostics Ultrasound usage in the medical community has many benefits, and no known side effects, unlike the x-ray. The thermal heat caused by the ultrasound is too low to cause any harm. The intensity of ultrasound used for diagnostic purposes is around 10-2 W/m2. Formation of ultrasound images: ultrasound B scans In the B scan, the intensity The transducer can be of the reflected signals is placed in different angles represented as the brightness to obtain a series of spots. of a spot.

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

In the figure, the ultrasound scanner is swept across the abdomen to produce a 2D image of the stomach.

How much details can ultrasound reveal? Abdominal scans use 7 MHz ultrasound waves. The speed of sound in tissue: ~ 1540 m/s. λ = vw / f = (1540 m/s) / (7×106Hz) = 0.22 mm The maximum spatial resolution 空間取樣能力 is limited by wavelength. The penetration depth is proportional to the wavelength. Typically penetration depth in tissue: ~ 500λ . So For 7 MHz : penetration depth = 500 × 0.22 mm = 0.11 m 6. Ultrasound in medical therapy The tip of this probe oscillates at 23 kHz with a very large amplitude, and can pulverize tissue on contact. Ultrasound with intensities of 103 to 105 W/m2 can shatter gallstones or pulverize cancerous tissue. Ultrasound diathermy 透 熱 療 法 (deep-heat treatment) – sound energy is converted 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.

7 Doppler effect 都卜勒效應 The Doppler effect: change in the perceived sound frequency due to movement of the source or the observer.

Doppler shifts in frequency. Case (a): Case (b):

The sound waves heard by X have the same frequency heard by observer y since they are both stationary. Observer X hears sound wave with a longer wavelength and lower frequency, while observer Y hears sound wave with a shorter wavelength and a higher frequency.

Case (c):

X and Y are moving instead of the car. However since the relative motion is the same, the same effect is produced. A sound wave’s frequency increases when you move towards it, and decreases when you move away from it. The following equation calculates the frequency perceived by the observer when he/she is stationary and the source is moving. .  vw   f obs = f s   vw vs  fs = frequency of the source vs =speed of the source along a line joining the source and observer vw =the speed of sound. The minus sign is used when the source is moving toward to observer, and the plus sign is used when the source is moving away from the observer. The following equation is used when the source is stationary and the observer is moving.  v vobs f obs = f s  w  vw

  

vobs = speed of the observer along a line joining the source and observer. . The plus sign is used when the observer is moving towards to source. The minus sign is used when the observer is moving away from the source. 7. Doppler-shifted ultrasound The magnitude of the Doppler shift observed in an echo is directly proportional to the velocity of the object reflecting the echo. Echo – double shift blood flow measurement. ∆f =

2 fv cos θ vw

f =source frequency v =the speed of the moving blood vw =is the speed of sound in the tissue. By calculating the frequency shift ∆ f, we can find the average speed of the blood.

Checklist • recall the ultrasound frequency range • recall the typical frequency and intensities use in medical therapy • recall the principle of ultrasound imaging and its advantages • recall the typical frequency and intensities use in ultrasound imaging

• calculation involving penetration depth of ultrasound in tissue • principle of Doppler-shifted ultrasound technique in blood velocity measurement

Bone density studies (optional reading) Osteoporosis, defined as low bone mass leading to an increased risk of fragility fractures, is an extremely common disease in the elderly due to age-related bone loss in both sexes and menopauserelated bone loss in women. Current practice guidelines published by the National Osteoporosis Foundation (NOF) recommend that measurement of bone mineral density (BMD) be performed in all women over the age of 65 and in postmenopausal women with additional risk factors. Additional risk factors include a personal history of fracture as an adult, history of fracture in a first-degree relative, current cigarette smoking, and low body weight (<127 lbs). Patients receiving glucocorticoid therapy are also at risk for bone loss, no matter what the age. Therefore, BMD measurements are often performed prior to initiating therapy. BMD is one of the key determinants of the need for pharmacologic therapy. BMD is typically expressed in terms of the number of standard deviations (SD) the BMD falls below the mean for young, healthy adults. This number is termed the T score. The NOF guidelines recommend that pharmacologic therapy be initiated in women with T scores below –2 in the absence of other risk factors, and in women with BMD T scores below –1.5 if other risk factors are present. Current pharmacologic options include hormone replacement therapy, bisphosphonates such as alendronate (Fosamax), selective estrogen receptor modulators (SERMs) such as raloxifene (Evista), and calcitonin. While BMD measurements are typically used to determine the need for pharmacologic therapy, serial monitoring of BMD to determine treatment response is also performed. BMD can be measured with a variety of techniques in a variety of sites. Sites are broadly subdivided into central sites (e.g. hip or spine) and peripheral sites (e.g. wrist, finger, heel). While BMD measurements are predictive of fragility fractures at all sites, central measurements of the hip and spine are the most predictive. Additionally, fractures of the hip and spine (e.g. vertebral fractures) are the most clinically relevant. Ultrasound densitometry is a relatively new technique for measuring BMD at peripheral sites, typically the heel, but also the tibia and phalanges. Compared to osteoporotic bone, normal bone demonstrates higher attenuation of the ultrasound wave and is associated with a greater velocity of the wave passing through bone. Ultrasound densitometry has no radiation exposure, and machines may be purchased for use in an office setting.

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