Technical issues

The chest has a relatively poor SNR as the proton density of the lung fields is low. In addition, there is little fat to give good contrast. The implementation of a volume array coil helps maintain SNR. This is especially useful when thinner slices and a finer matrix are required. Chest imaging can be performed with a number of RF coil designs. Although the body coil is notorious for lower SNR, it can be utilized for large areas of coverage. Also, the body coil is acceptable in cases where contrast is the main determining factor in image contrast, such as MRA of the aorta or the pulmonary arteries. For the most part therefore, the body coil will produce optimum images for chest imaging. For higher SNR, higher resolution torso array coils should be used (see Heart and great vessels).

The use of multiple NEX/NSA not only reduces some respiratory and cardiac artefact because of signal averaging, but also improves the SNR due to increased data collection. The disadvantage of this strategy is the associated increase in scan time, although this can be compensated for, to some degree, by the implementation of a coarser matrix or a rectangular/asymmetric FOV. SE T1-weighted sequences are traditionally used to show anatomy and black blood. Dark-blood images can be acquired by using the double inversion recovery (DIR) technique to null the signal from blood. DIR sequences can be FSE, in which case each image is acquired in a separate breath-hold, or single shot, in which case multiple images are acquired in one breath-hold. DIR images are always gated to the ECG so that cardiac motion artefact is reduced from the images. GRE sequences are useful for the evaluation of flow, and T2-weighted sequences demonstrate pathology and free fluid. On some systems, FSE is not compatible with RC techniques that use phase reordering. Under these circumstances, SE sequences can be substituted. Fast imaging employing T2 steady-state acquisition sequences should also be considered instead of SS-FSE to image fluid-filled structures in very short acquisition times.

Artefact problems

Respiratory, cardiac and flow motion are the most obvious artefacts in the chest. Image quality can be compromised by physiologic motion from the lungs and heart, from flowing blood within vessels and, in some cases, from the oesophagus and stomach. Compensation for physiologic motion artefacts in the chest can be reduced by a number of imaging options including respiratory compensation, breath-hold techniques, cardiac gating, cardiac triggering or other imaging options (saturation pulses or gradient moment nulling). Breath-hold techniques are achieved by the acquisition of rapid imaging sequences (20 s or less) and instructing the patient to hold their breath during the image acquisition.

Motion artefact will always occur along the phase encoding axis, and the degree to which they interfere with the image is mostly due to the proficiency of the respiratory compensation techniques used and ECG gating. For respiratory gating and/or respiratory triggering, a respiratory monitoring device, known as a bellows, is placed around the patient’s chest or abdomen. Placement can be determined by the patient’s breathing style. If, for example, patient respiratory motion is generated by motion in the chest, then the bellows should be placed there. However, if the patient’s motion occurs more within the abdomen, the bellows should be located in the abdominal region. (For more information about bellows placement, see Respiratory compensation techniques in Part 1.) Alternatively, breath-hold techniques may be used to suspend respiration. Check that the ECG leads are correctly attached and that the ECG trace has good amplitude and is triggering correctly (see Gating and respiratory compensation techniques in Part 1). When implemented properly, ECG gating effectively reduces cardiac motion artefact. However, if ECG gating is inefficient, image quality is compromised.

The phase encoding direction is generally prescribed by the system and thus defaults along a given direction. For the most part, the phase direction defaults along the short axis of the anatomy. Therefore, the phase encoding axis usually lies A to P on axial images and R to L on coronals. Swapping the phase axis to R to L on the axial scans and S to I on coronals is occasionally beneficial to remove artefact from the area of interest. For example, the phase direction for axial chest imaging generally defaults to a position anterior to posterior direction. However, if the motion artefact ‘streaks’ across anatomy or pathology of interest, swapping phase and frequency directions can be selected to ‘relocate’ the motion artefact to a position right to left across the image (Figures 10.7 and 10.8). As this strategy positions the long axis of the anatomy along the phase axis, oversampling is necessary to avoid aliasing. Furthermore, caution should be used if rectangular FOV has also been selected as the short axis of the rectangle is located along the phase direction. Therefore, as the phase direction is swapped, so is the direction of the rectangle.

Spatial pre-saturation pulses are also important to reduce flow artefact further. They are placed S and I to the FOV to decrease artefact from the aorta and IVC. R and L spatial pre-saturation pulses are beneficial in coronal images to decrease artefact from venous flow entering the chest from the subclavian vessels. In addition, when used in conjunction with SE or FSE sequences, spatial pre-saturation pulses produce black blood (see Figure 10.19). If signal persists in a vessel, it may indicate either slow flow or occlusion. GMN reduces flow artefact further, but as it also increases the signal in vessels and the minimum TE available, it is not usually beneficial in T1-weighted sequences unless contrast agents have been administered. When used in conjunction with GRE sequences, GMN produces bright blood (see Figure 10.18). If low signal is seen within the vessel, it may indicate either slow flow or occlusion. In addition, consideration should be given to the fact that when a structure is bright and it moves during MR image acquisition, it can ‘streak’ across the MR image in the phase direction.