A common artifact that is seen on MR angiography (MRA) is the susceptibility artifact. This results from the presence of previously implanted metallic material (Fig. 14.5). Magnetic susceptibility effects on spins near a metal object distort the image either by loss of signal in the object or by distorting its spatial geometry (Lee et al. 2007). Stents, clips, prosthesis, and inferior vena cava (IVC) filters are some of the metallic devices that produce susceptibility artifacts. These often cause signal loss and influence our ability to comment, for instance, on the presence of in-stent stenosis or on other occasions produce apparent lesions, which are not confirmed during subsequent investigations. Catheters can mimic pseudo-filling defects and dissections, and artifacts caused by surgical clips can also mimic pseudo-stenoses or occlusions. In contrast-enhanced studies, timing of gadolinium injection contributes to pitfalls in image acquisition. Ringing artifacts and venous contamination are usually related to improper timing apart from suboptimal images for diagnosis (Lee et al. 2007). In vessels with turbulent flow, significant dephasing leads to signal dropout. While this effect can be reduced by the use of gadolinium-enhanced MRA, all MRI techniques are affected by this phenomenon. As a result, there is a tendency for MRI to overestimate the severity of vessel stenosis, and this affects all vascular territories to some extent.

Errors that occur in evaluation of abnormalities on duplex imaging include those which are operator dependent, patient related, and inherent to all ultrasound imaging. Many of these examples are dealt with elsewhere in this textbook (see Chap. 1). With regard to the imaging of vascular structures, there are a certain number of pitfalls that merit specific attention. Patient-related factors include the inability to image certain vessels due to physical habitus and related organs. Examples of this include failure to image parts of the common iliac arteries due to their deep position and overlying bowel gas. Although the presence of more proximal disease can often be inferred from flow signal data, it is not always possible to demonstrate the causative lesions. Similarly, imaging of the mesenteric vessels may be difficult due to overlying bowel gas. High bifurcation of the carotid arteries can pose significant practical problems in duplex imaging. Acoustic shadowing from calcified plaques may impair detection and grading of stenoses, especially in small arteries such as the tibial vessels. Careful insonation of flow is vital in quantitative pulsed or continuous wave Doppler studies. Proper angulation of the ultrasound beam can be technically difficult, in spite of changing the probe angulation. An example of this is seen in the arched segment of the common iliac artery at its bifurcation. Aliasing might occur within arteries interrogated with a small Doppler angle, which could result in underestimation of peak flow velocity. An appreciation of the possible under- or overestimation of flow velocities is important, if operators are to avoid making errors. Appreciation of appropriate machine settings should help to avoid many potential problems, but careful attention with regard to gain settings is important in the measurement of vessel size.

DSA has replaced cut-film angiography and currently remains as the gold standard for angiographic diagnosis. However, in common with other imaging techniques, it is subject to certain inherent limitations. Perhaps the most common problem in clinical practice relates to the occurrence of misregistration artifacts. These occur due to a failure of accurate superimposition of the mask image and subsequently imaged anatomic structures. They occur due to uncontrollable movement of organs such as bowel or heart or involuntary movements such as poor breath holding and swallowing during image acquisition. Re-masking (choosing an alternate image of the sequence as a mask) and pixel shifting (reregistering an image) are used to reduce the misregistration artifact (Fig. 14.6). Other patient-related factors such as the presence of foreign bodies, dental fillings, and prosthetic joints can adversely affect DSA imaging (Fig. 14.7). Improper catheter selection and engagement can significantly influence the ability to demonstrate branch vessel anatomy and lead to potentially significant lesions being overlooked. Proper selection of contrast strength, volume, and flow rate is also essential, if reliable diagnostic imaging is to be obtained. Careful timing of bolus injections is needed for delineation of vessels distal to long segments of vessel occlusion, an important example being the demonstration of crural vessels and foot arches.