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Ultrasound in surgery KAROLINA WÜRTZ History of ultrasound imaging 1794: Lazzaro Spallanzani – Italian physiologist. First to study ultrasound physics by looking at how bats using ultrasound to navigate by echolocation 1826: Jean Daniel Colladon – Swiss physicist used under-water church bell (“transducer”) to calculate speed of sound through water. He proved sound travelled faster through water than air. 1880: Pierre & Jacques Curie – Break through in ultrasound technology, they discovered the piezoelectric effect. This led to the development of the ultrasound transducer. 1942: Karl Dussik – Austrian neurologist and psychiatrist, the first physician to use ultrasound for medical diagnosis (of brain tumors) 1948: George Ludwig – First describe the use of ultrasound to diagnose gallstones 1958: Ian Donald – used OB-GYN Ultrasound 1950s: Douglas Howry & Joseph Holmes – developed 2D B-mode Ultrasound, echocardiography. 1970s: doppler, duplex scanning 1990s: 3D and 4D images Physics of ultrasound imaging Ultrasound is sound with frequency above 20kHz Medical ultrasound use frequencies in the range from 2-15 MHz The ultrasound beam originates from mechanical oscillations of numerous crystals in a transducer which are excited by electrical pulses (piezoelectric effect). The transducer converts one type of energy into another (electrical to sound). The ultrasound waves are sent from the transducer, propagate through different tissues, and then return to the transducer as reflected echoes. The returned echoes are converted back into electrical impulses by the transducer and are further processed to form the ultrasound image. Ultrasound waves are reflected at the surfaces between the tissues of different density, the reflection being proportional to the difference in impedance. Echoes are not produced if there is no difference in a tissue or between tissues. Homogenous fluids like blood, bile, urine and pleural effusion are seen as echofree structures. Transducer An ultrasonic transducer is the device that converts electrical impulses into ultrasound, and the reverse to form the ultrasound image. Inside the core of the transducer are a number of peizoelectric crystals that have the ability to vibrate and produce sound of a particular frequency when electricity is passed through them. This is how ultrasound waves are formed. The transducer also act as a receiver for the reflected echoes as they generate a small electric signal when a sound wave is received. As the ultrasound beam travels through tissue layers, the amplitude of the original signal becomes attenuated as the depth of penetration increases. Attenuation (energy loss) is due to: Absorption (conversion of acoustic energy to heat) Reflection Scattering at interfaces Refraction - change in the direction of a sound wave. Attenuation and absorption In soft tissue, 80% of the attenuation of the sound wave is caused by absorption. Attenuation is measured in decibels per centimeter of tissue and is represented by the attenuation coefficient of the specific tissue type. The degree of attenuation also varies directly with the frequency of the ultrasound and the distance traveled. Frequency: Number of oscillating cycles per second Measured in Hertz, Hz. Wavelength: The length of one complete cycle A measurable distance Transducer frequency A high frequency wave have shorter wavelength and is associated with high attenuation and lower tissue penetration, whereas a low frequency wave is associated with low tissue attenuation and deep tissue penetration. High frequencies are used for the superficial body structures and low frequencies are used for those that are deeper. Medical ultrasound transducers contain more than one operating frequency. Frequencies typically used for ultrasound examination: 2.5 MHz: deep abdomen, ob and gyn imaging 3.5 MHz: general abdomen, ob and gyn imaging 5 MHz: vascular, breast, pelvic imaging 7.5 MHz: breast, thyroid imaging 10.0 MHz: superficial structures. Reflection and scattering Attenuation also results from reflection and scattering of the ultrasound wave. The extent of reflection depends on the difference in acoustic impedances of the two tissues at the interface Acoustic impedance is the resistance of a tissue to the ultrasound. The higher the degree of impedance difference, the greater the amount of reflection The degree of reflection for air is high because air has an extremely low acoustic impedance relative to other body tissues. Important with gel!!!! Bone also produces a strong reflection because its acoustic impedance is extremely high relative to other body tissues. Tissue echogenecity When an echo returns to the transducer, its amplitude is represented by the degree of brightness of a dot on the display. Combination of all the dots forms the final image. Strong reflections give rise to bright dots = hyperechoic (diaphragm, gallstone, bone, pericardium) Weaker reflections produce grey dots = hypoechoic (solid organs) No reflection produces dark dots = anechoic (fluid and blood filled structures) Also, deep structures often appear hypoechoic because attenuation weakens beam transmission to reach the structures, resulting in a weak returning echo. Contrast media in ultrasound Air = extremely low acustic impedance relative to other body tissues = high reflection Contrast-enhanced ultrasound (CEUS) involves the administration of intravenous contrast agents containing microbubbles of perfluorocarbon or nitrogen gas. The bubbles greatly affect ultrasound backscatter and increase vascular contrast. It has the advantage over contrast-enhanced MRI and CT in patients with contraindications such as renal failure or contrast allergy. It also allows for dynamic and repeated examinations. Types of transducers The ultrasound transducers differ in construction according to: Piezoelectric crystal arrangement Footprint Operating frequency (penetration depth) The most popular ultrasound transducers Sector transducer: crystal arrangement: phased-array (most commonly used) footprint size: small operating frequency: 1-5 MHz use: echocardiography, gyn ultrasound, upper body ultrasound Linear transducer: crystal arrangement: linear footprint size: usually big operating frequency: 3-12 MHz use: Superficial structures Convex transducer: crystal arrangement: curvilinear, along the aperture footprint size: big operating frequency: 1-5 MHz useful in all ultrasound types except echocardiography. Typically used abdominal, pelvic and lung ultrasound Other transducers Transvaginal transducer – uterus and ovary imaging Transesophageal transducer – transesophageal echocardiography Transrectal transducer – prostate imaging Ultrasound presentation Each ultrasound transducer is marked on one of the sides with a notch, groove or diode known as an index. The index is reflected by a special sign at the ultrasound screen (trademark, square, dot). Marking of the sides simplifies spatial orientation. The index is usually placed on the left side of the image (except echocardiography). Modes of ultrasound A-mode (Amplitude) The simplest type of ultrasound. The amplitude of reflected ultrasound is displayed on a screen. The A-mode is now used only in ophthalmology. M-mode (Motion) Reflects a motion over time and records a short video of ultrasound. M-mode is extremely valuable for accurate evaluation of rapid movements eg, valvular function. B-mode (Brightness) This is the most commonly used. An amplitude of the reflected ultrasound is converted into a gray scale 2D image. Dense tissue becomes brighter due to higher reflection. Owing to the wide gray scale (256 shades of gray) even very small differences in echogenicity are possible to visualize. D-mode (Doppler) Doppler ultrasound Is based on the Doppler effect that occurs when there is a moving source (blood flow of red blood cells) and a stationary listener (ultrasound transducer). There is an apparent change in the returning echoes due to the relative motion between the sound source and the receiver. This allows us to detect and measure blood flow Different types of Doppler: Color Doppler Power Doppler Pulse Doppler Color doppler ultrasound Can detect and measure flow direction and velocity If the source (RBC) is moving towards the receiver (transducer), the perceived requency is higher and display as red and when the source (RBC) is moving away from the receiver, the perceived frequency is lower than the actual and display as blue. It is important to note that Color Doppler detection of flow and flow direction is worst when the transducer is perpendicular (90 degrees) to the vessel and best when the transducer is parallel (0 degrees) to the blood flow. Power doppler This device give a picture of the amplitude of Doppler signals rather than the frequency shift. This allows detection of a larger range of Doppler signals and better visualization of small vessels. No information about direction or velocity. Pulse doppler This method allows a measurement of volume or "gate" in a vessel. It is visualized on the gray-scale image, and displays a graph of the full range of blood velocities within the gate versus time. The amplitude of the signal is approximately proportional to the number of red blood cells. Intraoperative ultrasound Intraoperative ultrasound (IOUS) can provide various diagnostic information that is otherwise not available, and can guide or assist various surgical procedures in real time. Because the transducer is in direct contact with the organ being examined, high-resolution images can be obtained that are not degraded by air, bone, or overlying soft tissues. It can facilitate and optimize surgical procedures in several important ways. It can accurately localize pathology, guide intraoperative biopsies, limit the extent of surgical resection, improve surgical staging and help the surgeon choose the optimal procedure. F.A.S.T Focused assessment of sonography in trauma The chief aim is to identify intra-abdominal free fluid (blood) allowing for an immediate transfer to operation, CT or other. Technique: Patient in supine position Using a 3.5-5.0 MHz convex transducer five regions may be scanned: Pericardial: subxiphoid transverse view - assess for pericardial effusion and left lobe liver injuries Perihepatic: longitudinal view of the right upper quadrant - assess for right liver injuries, right kidney injury, and Morison pouch Perisplenic: longitudinal view of the left upper quadrant - assess for splenic injury and left kidney injury Pelvic: suprapubic region - assess the bladder and pouch of Douglas Left and right thoracic views to assess for pneumothorax (in extended-fast) Examples of ultrasound images Appendicitis Cholecystitis Normal gallbladder Liver mass Liver cyst Liver hydatid cyst (Echinococcus) Pancreas normal Pancreatitis Kidney normal Polycystic kidney Ascites Urinary bladder with stone Sources http://www.criticalusg.pl/en/ http://radiopaedia.org/ http://www.usra.ca/doppler.php
