3 Ultrasound

DR. Muhammad Bin Zulfiqar PGR-FCPS III SIMS/SHL

GRAINGER & ALLISON’S DIAGNOSTIC RADIOLOGY

• FIGURE 3-1 ■Harmonic imaging. (A) Conventional grey-scale imaging through the liver at the porta hepatis. (B) In harmonic mode (note the diff logo on the left) the image has much higher contrast and the portal vein is more clearly delineated. PV = portal vein.

• FIGURE 3-2 The A-scan. The A-scan is a trace indicating ■echo intensity of tissue with depth. In this example, there is a fluid space from 6 to 10 cm, from which no echoes arise. Tissues superficial and deep to this produce echoes of varying intensities and there is a particularly strong echo from the skin (0–5 cm). The time gain compensation (TGC) curve is also shown.

• FIGURE 3-3 M-mode trace. The echo intensity is displayed as ■brightness and the trace is swept across the screen so that the x-axis represents time. This is an M-mode echocardiogram showing the rapid movement of the valve apparatus with thicker proximal and distal moving bands representing the ventricular walls. There is a small pericardial effusion separating the epicardium of the right ventricle from the chest wall.

• FIGURE 3-4 Mechanical transducer. (A) Diagram of a ■mechanical endoprobe transducer. A motor rotates the transducer assembly to sweep out a circular path. (B) An intravascular image of a coronary artery showing the endothelium as a bright inner layer interrupted by an atheromatous plaque (arrowhead). T = transducer. (Figure B courtesy of Professor Ton van der Steen, Erasmus Medical College, Rotterdam, the Netherlands.)

• FIGURE 3-5 Phased array. (A) Diagram illustrating steering an ■ultrasound beam with a phased array transducer. The delay lines introduce small timing differences in the pulses driving the elements so that those at one end are fired earlier than those further along the array. This has the effect of steering the beam away from the centre line, rather as if the transducer face had been tilted. (B) Right lobe of liver taken with a phased array showing the advantage of the small footprint in accessing the portions that lie high under the diaphragm, especially segment 8. 7 and 8 = segments 7 and 8, D = diaphragm.

• FIGURE 3-5 Phased array. (A) Diagram illustrating steering an ■ultrasound beam with a phased array transducer. The delay lines introduce small timing differences in the pulses driving the elements so that those at one end are fired earlier than those further along the array. This has the effect of steering the beam away from the centre line, rather as if the transducer face had been tilted. (B) Right lobe of liver taken with a phased array showing the advantage of the small footprint in accessing the portions that lie high under the diaphragm, especially segment 8. 7 and 8 = segments 7 and 8, D = diaphragm.

• FIGURE 3-6 Linear array. Diagram of a linear array ■showing the formation of one ultrasound beam by triggering a set of elements at one end of the probe. The next beam would be formed by the adjacent or partially overlapping set of elements so that the beam is swept along the transducer face to give a rectangular image.

• FIGURE 3-7 Breast carcinoma. A heterogeneous ■mass (arrowheads) with irregular margins and distal shadowing are characteristics of a breast cancer. This image was taken with an 18-MHz linear array probe. The rectangular format is particularly suitable for superficial structures.

• FIGURE 3-8 Convex array. ■(A) Sagittal image of a normal upper abdomen taken with a 6-MHz curved array. (B) Transvaginal image of a uterus with a fibroid (arrowheads) taken with a tightly curved 9-MHz array. The convex format is a compromise between the footprint requirements of sector and linear probes and has the advantage of giving a wide field of view for deeper structures. E = endometrium, GB = gallbladder, IVC = inferior vena cava, L = liver, PV = portal vein.

• FIGURE 3-9 Ultrasound beam shapes. The shapes of the ultrasound ■ beam from four transducers are indicated. All are of the same frequency (3.5 MHz). The beam from the 10-mm probe in (A) has a complex region close to the transducer face, then a mid-portion with near parallel sides, before the beam spreads out in the far field. The white lines indicate the half-power limits. Increasing the probe diameter (B) improves the overall beam width. In (C), weak focusing has been added by concave shaping of the crystal. This further improves the beam shape in the focal zone but causes it to spread more severely further out. This

would be a useful compromise for general abdominal imaging Stronger focusing (D) exaggerates these effects, producing a fine beam but only over a short distance. This would be useful for imaging superficial structures.

• FIGURE 3-10 Electronic ■focusing. Diagram to illustrate the principle of electronic focusing. The delay lines are set to send the pulses from the outer elements fractionally ahead of those from more central elements. The resulting interference patterns accentuate the central part of the beam and cancel the off-axis portions. The effect is greatest at the focal zone.

• FIGURE 3-11 Ultrasound beam plot. The complexity and ■marked deviation from the ideal of a practical diagnostic ultrasound beam is shown in this intensity plot. The beam is very complex for the first few millimetres from the transducer face (on the left of the figure) but improves towards the focal zone where it reaches an effective diameter of a few millimetres before spreading out again in the far field. Unfortunately, some ultrasound energy is also sent out as side lobes at angles to the main beam, further complicating the effective beam shape. These divergences from the ideal narrow shape limit both the spatial and contrast resolution of ultrasound images. (Beam plot kindly supplied by Dr Adam Shaw of the National Physical Laboratory, Teddington.)

• FIGURE 3-12 Beam width artefact. In this ■ultrasound examination of the bladder, the intense echoes from gas in a loop of bowel (arrow) have spread across into the urine (arrowhead). This artefact results from the finite width of the ultrasound beam.

• FIGURE 3-13 Velocity artefact from a silicone prosthesis. This ■unfortunate young man developed a second teratoma having had an orchiectomy on the right with insertion of a prosthesis. The depth of the prosthesis is depicted as being greater than that of the tumour-bearing testis, in conflict with the clinical impression, which was the reverse; this geometric distortion is the result of the lower speed of sound in the prosthetic material that delays the echoes so that they are plotted as lying deeper than they really are. P = prosthesis, T = teratoma.

• FIGURE 3-14 Beam dispersion by fat. Deep ■to this fatty renalhilum, the retroperitoneal tissue layers (arrowheads) are less clear than adjacent tissue planes because the hilar fat has defocused the beam. S = renal sinus.

• FIGURE 3-15 Reverberation ■artefacts. (A) Multiple internal reflections within tissue layers give rise to false repeated signals, which are most obvious where they fall over echo-free fluid spaces such as the gallbladder (arrowhead). (B) Whereas the intima–media layer is well delineated at the deeper surface of this normal common carotid artery (arrow), the superficial laye is partly obscured by reverberation artefact (arrowheads). C = common carotid artery.

• FIGURE 3-16 Mirror image artefact. One of the ■structures in the liver such as this cyst is mirrored at the air–pleura surface and appears in the position of the lower lobe of the lung, producing the ‘percentage sign’ artefact (arrowhead). When this surface is absent, for example when a pleural effusion is present, the effect does not occur. C = cyst.

• FIGURE 3-17 Acoustic shadowing. The dark ■band (arrowheads) deep to the gallstones (arrow) is an example of shadowing produced by a combination of high absorption and reflection. GB = gallbladder, K = kidney.

• FIGURE 3-18 Increased sound transmission. ■The echoes (arrowheads) deep to this liver cyst (arrow) appear brighter than those from the rest of the liver; this is because the cyst fluid attenuates less than the solid liver and so signals from beyond it are relatively overamplified.

• FIGURE 3-19 Congested liver. In heart failure, the ■liver may become congested with extra fluid. The separation of the reflectors reduces the liver echoes so that it becomes less echogenic than the kidney. In addition, the vascular markings are accentuated because they are not affected. GB = gallbladder, K = kidney.

• FIGURE 3-20 Fatty liver. The multiple interfaces ■between fatladen liver lobules and the surrounding watery tissues give these fatty liver high-intensity echoes which can be seen as increased contrast with the adjacent renal cortex (compare Fig 3-19). K = kidney, L = liver.

• FIGURE 3-21 Laminar flow. Diagrammatic ■representation of the concentric layers of blood flowing at different velocities, with the highest velocity in the centre of the vessel.

• FIGURE 3-22 Parabolic velocity profile. The ■fastest flow is in the centre of the vessel, with a progressive reduction in velocity towards the vessel wall.

• FIGURE 3-23 Plug flow. The flow velocities ■are almost equal across the whole vessel diameter.

• FIGURE 3-24 Normal common femoral ■artery flow. There is triphasic flow with early diastolic flow reversal.

• FIGURE 3-25 Low-resistance flow. The flow is ■continuously forward throughout the cardiac cycle, with moderately high flow throughout diastole. There is very little low-velocity flow throughout most of the cardiac cycle (compare with Figs 3-26 and 3-27).

• FIGURE 3-26 Moderate spectral ■broadening. The spectrum throughout late systole and most of diastole has been filled in by an increased range of blood flow velocities.

• FIGURE 3-27 Complete spectral ■broadening. There is now complete filling of the spectrum throughout the cardiac cycle, with brief periods of simultaneous reverse flow during systole.

• FIGURE 3-28 Turbulence beyond a stenosis. ■There is highvelocity flow beyond this carotid artery stenosis, with simultaneous forward and reverse velocities. The internal : common carotid peak systolic velocity ratio is 2.14, indicating a haemodynamically significant stenosis.

• FIGURE 3-29 Close-up view of the spectral trace. The spectral ■trace is composed of increments on both the vertical and horizontal axes. The horizontal increments indicate the individual time intervals during which Doppler sampling occurs, 20 ms in this example. The vertical increments indicate increasing frequency. The brightness of the trace within each pixel indicates the number of blood cells moving with that velocity at that time.

• FIGURE 3-30 Dependence of RI on heart ■rate. The value of enddiastole is relatively high if the heart rate is rapid (1). A slower heart rate allows a greater time for diastolic deceleration, leading to a lower end-diastolic velocity (2) and a higher resistance index. V = velocity, T = time.

• FIGURE 3-31 (A) ■Spectral distribution in plug flow. A very narrow band of velocities is present throughout the cardiac cycle in this artery. (B) Spectral distribution in parabolic flow. The slower flow in this wide portal vein gives rise to a wide range of velocities during each time interval.

• FIGURE 3-32 Colour Doppler. Fetal circle of Willis. The flow in ■the right middle cerebral artery, the left anterior communicating artery, the left posterior communicating artery and the right posterior cerebral artery is displayed in red as the flow is towards the transducer. The flow in the other vessels is passing away from the transducer and is therefore represented by blue.

• FIGURE 3-33 Power Doppler display of a right ■kidney. This extended field of view shows the flow in the inferior vena cava, main renal vessels and the intrarenal vessels right out to the capsule. The loss of directional information prevents the differentiation of arteries from veins.

• FIGURE 3-34 Vessel wall definition. Normal ■carotid artery. The power Doppler display colour intensity decreases near the vessel wall owing to the volume elements lying partly outside the vessel. This gives an apparent improvement in the definition of the intimal surface.

• FIGURE 3-35 (A) Colour ■Doppler with 90° beam-to-vessel angle. There is poor flow detection and direction ambiguity throughout the vessel. (B) Power Doppler display with 90° beam-tovessel angle. As direction information is not used in the display, uniform flow detection is achieved throughout the vessel segment.

• FIGURE 3-36 Calculation of time-averaged ■mean velocity. The weighted mean velocity during each time interval has been calculated. The average of these throughout the trace is a close estimate of the true mean velocity.

• FIGURE 3-37 Superior ■mesenteric arterial trace. (A) Normal: the vessel has remained within the range gate throughout the cardiac cycle, giving a satisfactory Doppler trace. (B) Artefactually abnormal SMA trace: the gate is too small and is misplaced so that the vessel moves out of the range gate during diastole, giving rise to the false appearance of increased vascular resistance.

• FIGURE 3-38 Dependence of velocity error on beam-■to-vessel angle. The error in velocity calculation is less than 10% for angles of less than 50°. There is a rapid and unacceptable rise in the error rate for angles greater than 70°.

• FIGURE 3-39 Mean peak velocity calculation. ■Automatic software has calculated the peak instantaneous velocity for each time interval throughout the cardiac cycle. The mean of these values is the mean peak velocity.

• FIGURE 3-40 Aliasing. ■(A) The frequencies above the Nyquist limit have appeared on the wrong side of the baseline. (Note that no beam-to-vessel angle has been set so the velocity values are uncorrected and therefore meaningless.) (B) The aliased peaks have been electronically transposed to their correct locations.

• FIGURE 3-41 Inappropriately high wall ■filter. The wall filter has been set at 200 Hz and has removed all the low-velocity information in this venous study.

• FIGURE 3-42 Artefactual flow reversal on colour flow ■imaging. Flow within the splenic vein (arrowheads) is from left to right. In this transverse epigastric image, on the left side of the image the flow is towards the curvilinear array probe and is therefore displayed as red. On the right side, the flow is away from the probe and is therefore displayed as blue. There is a thin black area at the point where the colour changes, indicating a true flow direction reversal, rather than aliasing, as the cause of the colour change. A = aorta, I = inferior vena cava.

• FIGURE 3-43 True and artefactual colour flow reversal. The ■ high-velocity jet through this portal vein stenosis has given rise to aliasing. The aliased green signal passes through yellow to red in a continuous gradation (in the vertical limb of the vessel in this display). The coarse vortex within the poststenotic dilatation gives rise to a true area of flow reversal, colour-coded blue, which is separated from the forward flow red component by a black margin.

• FIGURE 3-44 Contrast-enhanced ultrasound (CEUS) of a ■haemangioma. The baseline transverse section through the right lobe of the liver (A) shows a subtle lesion (arrowheads). The system was then reset to display the contrast image on the left (using contrast pulse sequences) and the B-mode image on the right, both with low mechanical indices. SonoVue (2.4 mL) was given IV and the haemodynamics of the flow through the lesion observed in real time. At 11 s after injection (B), the lesion showed peripheral nodular enhancement (arrowhead). By 22 s (C), the lesion shows centripetal filling and by 41 s (D) it had almost completely filled, a pattern characteristic of a haemangioma. The liver and the kidney also show enhancement. The ability to provide a firm diagnosis of a benign mass as soon as it was detected is a benefit of CEUS. K = kidney.

• FIGURE 3-44 Contrast-enhanced ultrasound (CEUS) of a ■haemangioma. The baseline transverse section through the right lobe of the liver (A) shows a subtle lesion (arrowheads). The system was then reset to display the contrast image on the left (using contrast pulse sequences) and the B-mode image on the right, both with low mechanical indices. SonoVue (2.4 mL) was given IV and the haemodynamics of the flow through the lesion observed in real time. At 11 s after injection (B), the lesion showed peripheral nodular enhancement (arrowhead). By 22 s (C), the lesion shows centripetal filling and by 41 s (D) it had almost completely filled, a pattern characteristic of a haemangioma. The liver and the kidney also show enhancement. The ability to provide a firm diagnosis of a benign mass as soon as it was detected is a benefit of CEUS. K = kidney.

• FIGURE 3-45 Elastography. The echogenic ■lesion in the left pane has the appearances of a haemangioma (arrowhead). In the elastogram in the right pane, it is seen as a blue region against the liver’s mainly green coloration; this indicates that the lesion is stiffer than the liver.