Abdominal ultrasonography

image of Abdominal ultrasonography
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Ultrasound equipment can be considered in two parts: the transducer, which sends the ultrasound waves out and receives the returning echoes; and the computer which analyses the data, orders the ultrasound waves emission/reception sequence, allows modifications and displays the information as an image. Ultrasound waves are produced by crystals with piezoelectric properties. These crystals vibrate when subjected to an electrical voltage. The vibration amplitude of the crystals, and therefore of the emitted ultrasound waves, is dependent on the natural resonance of the crystals. The drystals are made of natural quartz or a synthetic ceramic, and are located inside the ultrasound transducer. This chapter considers the Basic physics of ultrasound waves; Ultrasound equipment; Ultrasound image; Doppler ultrasonography; and Abdominal examination.

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3.1 The piezoelectric effect: the electrical current induces vibration of the crystals, which in turn generates ultrasound waves. The sinusoidal ultrasound wave has an amplitude (A) and describes a cycle, which is characterized by its wavelength (λ) (distance of a cycle) and its frequency (number of cycles per unit of time).
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3.2 Ultrasound wave reaching an interface between tissues may become refracted or reflected.
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3.3 The narrowest part of the ultrasound beam is known as the focal zone. It affects the sharpness (S) of the object’s borders as well as its echogenicity (E).
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3.4 Types of transducer. (1) Mechanical without shell. (2) Linear array. (3) Diagram of a curvilinear array. (4) Curvilinear array. (5) Phased array.
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3.5 Effects of total gain. (i) Insufficient total gain. (ii) Excessive total gain. Effects of time gain compensation (TGC). (i) Insufficient proximal TGC. (ii) Correct TGC and total gain.
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3.6 Echogenicity scale. Note that the liver (L) is hypoechoic to the spleen (S).
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3.7 Nodules and foci. (1) Hepatic nodules (N) are so-called because they distort the architecture of the liver and have a mass effect on the contour of the gallbladder. The echogenicity of the nodules is mixed (both hypoechoic and hyperechoic) in a ‘target-like’ distribution. The artefactual mirror image (MN) of the real nodule is also visible. (2) Numerous hypoechoic well defined foci (F) in the prostate gland. There is no distortion of the architecture of the prostate gland. This is an unusual site of lymphoma infiltrate. (3) Hyperechoic well defined perivascular focus (F) in the spleen. There is no distortion of the architecture of the spleen. This is a site of benign and frequent myelolipoma.
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3.8 A curvilinear array transducer used at different frequency ranges, for the same patient, with an identical number and location of focal zones (triangles). The liver parenchyma has a finer texture (more detail is visible) in the resolution mode (right image), particularly in the first 3 cm of depth, but the deeper parts of the liver parenchyma are better seen in the penetration mode (left image).
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3.9 Strong acoustic shadows cast by (1) multiple calculi lying within the bladder (rectangular image obtained with a linear transducer) and (2) a cloth foreign body within a jejunal intussusception.
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3.10 Refractive (edge) shadowing is visible on the edge of various organs as a narrow black cone (arrowed). C = Cone of reflective shadowing; L = Organ of interest; R = Ultrasound wave passing through the organ. (1) Refraction (arrowed) helps to identify the ovary. (2) The presence of a refraction artefact on the edge of renal pelvis fat (arrowed) may be mistaken for acoustic shadowing (with the corollary erroneous diagnosis of renal calcification). (3) When the bladder (B) is surrounded by abdominal effusion, refraction of the tangential echo (black arrows) results in an artefactual bladder wall interruption (white arrow).
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3.11 Multiple streams of reverberation (arrowed) arising from pockets of gas within the jejunum. More specifically, ring down (arrows pointing to the right) reverberations appear as repeated hyperechoic lines.
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3.12 Distal enhancement is seen deep to the gallbladder (G) as a hyperechoic cone (white arrows). The edge of the gallbladder also creates edge shadowing (black arrows). The part of the ultrasound beam which passes through the gallbladder is less attenuated (and therefore stronger) than the part of the ultrasound beam which passes to one side of the gallbladder.
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3.13 The areas of the image beyond the diaphragm (D) between the white arrows (mirror image 1) and delineated by black arrows (mirror image 2) are artefacts mimicking the presence of pleural effusion. Reverberation artefacts due to air present in the lungs can be seen deep to the black arrows.
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3.14 The band of echoes lying in the dependant portion of the bladder lumen (arrowed) is artefactual and due to side-lobe artefacts associated with the nearby colon (in this case, in the same image plane). A = Accessory ultrasound beam; P = Primary ultrasound beam.
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3.15 Equation for evaluating blood flow using Doppler ultrasonography. α = Angle between the ultrasound beam and the blood flow; ΔF = Frequency change (Doppler shift) between incoming and outgoing sound; c = Velocity of sound in the medium; f = Frequency of the initial ultrasound beam; V = Blood velocity. If the object (e.g. blood constituents) is moving towards the transducer, the reflected ultrasound waves are compressed and the frequency increases. Conversely, if the object is moving away from the transducer, the reflected ultrasound waves are stretched and the frequency decreases.
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3.16 PW Doppler ultrasonography allows quick differentiation of the pulsed signal of the aorta (bloodstream moving towards the transducer, positive signal) from the wavy continuous signal of the portal vein (bloodstream moving away from the transducer, negative signal) when searching for a portosytemic shunt in the liver hilus. The more parallel is the sample volume to the blood flow, the more accurate is the velocity.
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3.17 Colour flow ultrasonography. (1) The lack of colour flow in one tubular structure within the sample volume box is diagnostic for an intrahepatic biliary dilatation. (2) The presence of colour flow in the sample volume box represents a normal vesicoureteral urinary jet in the bladder. (3) The lack of complete colour flow within the sample volume box is diagnostic for a <6-hour-old thrombus in the caudal vena cava. (4) (a) Transverse section of the liver hilus. (b) Colour Doppler ultrasonogram of the same area demonstrating a portocaval shunt (S). Ao = Aorta; CVC = Caudal vena cava.
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3.18 Power Doppler ultrasonography generally does not give directional information but detects flow from small vessels and vessels with slow flow, such as in this 2.5 kg cat’s kidney.
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3.19 Sagittal view of the liver (L) in three different patients, which led to the identification of thoracic lesions beyond the diaphragm (D). Identification of small intestine (SI) cranial to the diaphragm allowed diagnosis of a diaphragmatic hernia. Pleural effusion (Eff) and pulmonary consolidation (PC) are visible. The mass was confirmed as a pulmonary carcinoma (diagnosis achieved via aspiration from a thoracic window). Thoracic mass (M). Analysis of the movement of the mass compared with the surrounding lungs is essential to differentiate a pulmonary from extrapulmonary mass. S = Stomach.
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