A CSF ghost artifact was evident in segmented IR sequences used to image myocardial infarction on both GE and Siemens (not shown) systems. The B1-weighted phased-array combining adequately suppressed the artifact in both cases. The Siemens implementation used RF spoiling. Analyses of the transient approach to steady state for RF spoiling are typically given for continuous RF pulses, whereas in this segmented acquisition there is considerable interruption of the steady state between segments, as well as inversion pulses applied every two segments. The use of RF phase spoiling in this segmented acquisition with T1 on the order of two consecutive phase encodes (segments) was found to reduce, but not eliminate, the CSF artifact. Therefore, the artifact suppression provided by the B1-weighted phased-array combining was important.
Since the ghosts appear in the phase-encode direction, one could also resolve the artifact by swapping the frequency and phase-encode directions or using an in-plane rotation. While this approach may be useful during the scan (at the expense of additional breath-hold), it is only effective if the artifact is immediately identified, which depends on the skill of the operator. The CSF artifact is rather small, unlike larger breathing or motion-related artifacts, and is less well recognized as an artifact. In the example shown in , the patient had a history of sarcoidosis. In this case a hyperenhanced region, which is not subendocardial and resembles the CSF artifact, might suggest cardiac involvement. Failure to resolve this case could have resulted in a misdiagnosis.
CSF ghost artifacts were not evident in the in vivo B1-map estimates, for several reasons. The reference images are acquired with a reduced flip angle readout after the magnetization is almost recovered, so that the intensities are almost proton density-weighted with little T1 contrast. Furthermore, there is a significantly reduced transient during the approach to steady state. Therefore, the blood and myocardial signals are stronger than the CSF signal. Additionally, the B1-map estimates use spatial smoothing, which further reduces any contribution due to a small artifact structure.
The effectiveness of B1-weighted phased-array combining in suppressing artifacts depends on the coil sensitivity roll-off, with greater suppression for smaller coils. The cardiac surface coil arrays used for this application were well suited for this purpose. Larger torso coils may not have enough roll-off to provide adequate artifact suppression for FOV/2 ghosts.