• Using tissue harmonics to reduce artefact (Fig. 1.5). This technique uses the second harmonic frequency using pulse inversion.¹ This results in a higher signal to noise ratio, which demonstrates particular benefits in many difficult scanning situations, including obese or gassy abdomens.

Fig. 1.1 • The effect of changing frequency.

(A) At 2.7 MHz the wires are poorly resolved and the background ‘texture’ of the test object looks coarse.

(B) The same transducer is switched to a resonant frequency of 5.1 MHz. Without changing any other settings, the six wires are now resolved and the background texture appears finer.

(C) A small nodule in the anterior portion of the left lobe of liver demonstrated with a 5.0 MHz transducer.

(D) Using 7.5 MHz, the nodule in (C) has improved detail, and a further small nodule (calipers B), not seen on the lower frequency, is detected near the anterior surface.

Fig. 1.2 • The effect of line density.

(A) 76 frames per second (FPS).

(B) 36 FPS – the resulting higher line density improves the image, making it sharper.

(C) The gallbladder is displayed with a low line density, as the scanning area is large.

(D) By reducing the field of view, the line density is increased, clarifying the small stone in the gallbladder.

Fig. 1.3 • The effect of focal zone placement.

(A) With the focal zone in the near field, structures in the far field are poorly resolved.

(B) Correct focal zone placement improves both axial and lateral resolution of the wires.

(C) The focal zone incorrectly set in the near field (arrowhead) makes it difficult to demonstrate small gallstones.

(D) With the focal zone correctly set (arrowhead), the stones are resolved with a clear, diagnostic band of posterior shadowing.

Fig. 1.4 • The effect of using post-processing options.

(A) A small nodule in a cirrhotic liver merges into the background and is difficult to detect.

(B) A post-processing option that allocates the range of grey shades in a non-linear manner enhances contrast resolution and improves the lesion’s conspicuity.

Fig. 1.5 • The effect of tissue harmonic imaging: the left image shows a liver containing cysts. The right image has tissue harmonic imaging applied, which reduces artefact and clarifies the structures.

The bottom line is, it is far better to have a scan performed properly on a low-tech piece of equipment by a knowledgeable and well-trained operator than to have a poorly performed scan on the latest high-tech machine (Fig. 1.6). A good operator will get the best out of even the lowliest scanning device and produce a result that will promote the correct patient management. A misleading result from a top-of-the-range scanner can be highly damaging and at best, delay the correct treatment or at worst promote incorrect management.

Fig. 1.6 • The importance of using correct equipment settings:

(A) Incorrect use of equipment settings makes it difficult to appreciate the structures in the right kidney.

(B) By increasing the resonant frequency, decreasing the frame rate (increasing line density) and adjusting the focal zone, structures in the kidney are clarified.

The operator should know the limitations of the scan in terms of equipment capabilities, operator skills, clinical problems and patient limitations, take those limitations into account and communicate them where necessary.

The use of doppler

Many pathological processes in the abdomen affect the haemodynamics of relevant organs and the judicial use of Doppler is an essential part of the diagnostic procedure. This is discussed in more detail in subsequent chapters.

Colour Doppler is used to assess the patency and direction of flow of vessels in the abdomen, to establish the vascularity of masses or lesions and to identify vascular disturbances such as stenoses. Flow information is colour coded (usually red towards and blue away from the transducer) and superimposed on the image. This gives the operator an immediate impression of a vascular ‘map’ of the area (Fig. 1.7). This Doppler information is obtained simultaneously, often from a relatively large area of the image, at the expense of the grey-scale image quality. The extra time taken to obtain the Doppler information for each line results in a reduction in frame rate and line density, which worsens as the colour Doppler area is enlarged. It is advisable, therefore, to use a compact colour ‘box’ to maintain image quality.

Fig. 1.7 • Colour Doppler of the hepatic vein confluence. Flow is coloured blue to indicate a direction away from the transducer.

Power Doppler also superimposes Doppler information on the grey-scale image, but without any directional information. It displays only the amount of energy (Fig. 1.8). It has the advantage of a stronger signal, allowing identification of smaller vessels with lower velocity flow than colour Doppler. As it is less angle-dependent than colour Doppler it is particularly useful for vessels which run perpendicular to the beam – such as the inferior vena cava (IVC).

Fig. 1.8 • Power Doppler of the hepatic vein confluence. Directional information is lost, but power Doppler can be superior to colour in demonstrating low velocity flow.

Pulsed Doppler uses pulses of Doppler from individual elements or small groups of elements within the array. This allows the operator to select a specific vessel, which has been identified on the grey-scale or colour Doppler image, from which to obtain a spectrum. This gives further information regarding the flow envelope, variance, velocity and downstream resistance of the blood flow (Fig. 1.9).

Fig. 1.9 • Flow velocity waveforms.

(A) Low resistance flow towards the transducer from a normal hepatic artery. Good end diastolic flow throughout the cycle with a ‘filled in’ waveform indicating variance in flow.

(B) In contrast, this hepatic vein trace with flow away from the transducer is triphasic, with a clear ‘envelope’ consistent with less variance. The pulsatile nature of the flow incorporates brief flow towards the transducer (arrows) at the end of each cycle.

Getting the best out of doppler

Familiarity with Doppler controls is essential in order to avoid the pitfalls and increase confidence in the results. It is relatively straightforward to demonstrate flow in major vessels and to assess the relevant spectral waveform; most problems arise when trying to diagnose the lack of flow in a suspected thrombosed vessel, and in displaying low velocity flow in difficult-to-access vessels.

Doppler is known to produce false positive results for vessel occlusion (Fig. 1.10) and the operator must avoid the pitfalls. It is essential that the Doppler settings are sensitive enough to detect the velocity of flow in the vessel (Box 1.2). This means that the angle of insonation to the direction of flow must be as close to 0° as possible (i.e. the vessel must be flowing towards or away from the beam, not perpendicular to it), the pulse repetition frequency (PRF) must be set to detect slow flow and the Doppler gain must be turned up sufficiently.