Chapter 10 Common CMR artefacts
Introduction
CMR can provide excellent static and dynamic images but some knowledge of artefacts is important when unusual findings occur or suboptimal images are obtained.
Motion artefact
Patient motion artefacts are common, and are also referred to as phase mismapping or ghosting (Figure 10.1). Two important reasons for these artefacts are respiratory and cardiac motion. Other causes are flow and actual patient bodily movement on the table. Image distortion is due to anatomical movement between the application of the phase and frequency encoding gradients, leading to within-view errors, and anatomical motion between each application of the phase encoding gradient, causing view-to-view errors. Motion artefacts always occur along the direction of the phase encoding gradient, the phase encode axis, and appear as blurring across an image. Periodic motion will be located at regular intervals along the phase encode axis with the shape of the ghost reflecting the moving structure. The false images usually have increased signal intensity at the expense of the causal moving structure, from which signal is reduced. There are several ways to reduce motion artefact. General measures include swapping the direction of the phase and frequency encoding gradients so that the ghosting falls outside the area of interest, and vendor-specific gradient moment rephasing methods which can automatically correct altered phases back to their original values. More specific measures directed at respiratory and cardiac motion are discussed below.
Respiratory motion
Respiratory motion artefacts are usually eliminated by instructing patients to hold their breath at end expiration throughout the duration of scanner noise (Figure 10.2). If this is unsuccessful, then the operator should firstly reiterate the importance of total suspension of breathing during image acquisition and repeat the scan (Figure 10.3). Following failure of repeated instruction, breath-holding can be tried at maximal inspiration or the scan acquisition time shortened by the addition of parallel acquisition. These methods employ computational techniques and arrays of coils wherein each coil independently and simultaneously images a given volume. Parallel imaging can be used to either reduce the total acquisition time or increase the resolution of a scan. There will be some loss of image quality in return for reduced scan duration. Respiratory navigator techniques can also be tried as in coronary MRA. Additionally, a prepulse RF signal can be directed across the chest wall to reduce or eliminate signal coming from it. Such prepulses are either spatially selective or chemically selective. Chemically selective prepulses tend to remove signal from methylene (CH2) protons in adipose tissue and are therefore means of fat suppression.
Cardiac motion
This is generally reduced using ECG-gating, which synchronizes data acquisition with the phases of the cardiac cycle which are identified relative to the R-R interval. R-R signal is detected using externally placed ECG electrodes as part of patient preparation for CMR. Problems from inadequate R-R signal necessitate alternative ECG electrode selection, placement, or further patient skin preparation to increase electrode adhesion, while problems with varying R-R interval are more troublesome. This occurs in arrhythmias such as atrial fibrillation, frequent ventricular ectopics, and ventricular bigeminy (Figure 10.4). Atrial fibrillation requires the use of a variation of the ECG-gating process known as prospective gating. This is as opposed to retrospective gating methods, which acquire data continuously during the cardiac cycle. Retrospective gating is suitable when the R-R interval is regular since the same part of the data is acquired at the same point. When the R-R interval becomes erratic then the shortest interval period is chosen and data are obtained only during that period for each subsequent cycle until the imaging sequence is complete. With frequent ventricular ectopy, a specific arrhythmia rejection program can be instituted to recognize and eliminate the unwanted data. Ventricular bigeminy causes the greatest disruption to image quality and can be counteracted by attempting to exclusively acquire data from the ‘normal’ cardiac cycles, which will prolong scan duration, or pharmacological methods of arrhythmia suppression such as short-acting prior beta-blocker treatment. Parallel acquisition is also useful for reducing cardiac motion artefacts when used to reduce scan duration.
