Artifacts covered:

. Motion

- Respiratory motion

- Cardiac motion

. Gibbs ringing

. Chemical shift

. B0 field inhomogeneity

This presentation is primarily aimed at clinicians but also any scientist interested in learning about frequently found artifacts in cardiovascular MR.

Common cardiovascular MR artifacts are presented with a short description of the mechanisms behind them, and possible solutions and trade-offs. The material of this talk is covered in more detail in the following review:

Ferreira et al.: Cardiovascular magnetic resonance artifacts. Journal of Cardiovascular Magnetic Resonance 2013 15:41.

The objective of this presentation is to summarize, in a language accessible to a clinical readership, some of the most frequent artifacts found in cardiovascular MR applications. It will expose the audience to 1) some of the challenges of MR imaging in the chest; 2) the physics behind typical artifacts; 3) diagnostic pitfalls and 4) possible solutions and trade-offs.

Motion

In general, a moving object will change both the phase and magnitude of its k-space; motion during image acquisition will therefore introduce artifacts. Additionally, motion artifacts can also be created by movement between different components of the sequence.

Cardiac motion has been reasonably well controlled over the years by detecting the QRS complex of the ECG and triggering the acquisition at a certain delay following this. Evidently ECG triggering works best when there is low variation between beats; arrhythmias and ectopic heart-beats will therefore potentially cause artifacts.

Respiratory motion is relatively unpredictable and can vary considerably from person to person and from time to time. Acquiring data over the period of a breath-hold has in recent years largely controlled it, although this can translate into a long acquisition window within the cardiac cycle, thus potentially including periods of more rapid cardiac motion. Respiratory gating is another technique that allows the removal of gross respiratory motion artifacts by restricting data acquisition to the expiratory pause, thus enabling longer scans with shorter acquisition windows within the cardiac cycle, which in turn reduces cardiac motion problems.

Motion artifacts examples in more detail:

. Breathing motion

- Phase-encode order considerations (Figure 1)

. Cardiac motion

- Phase-encode order considerations

- Arrhythmias and poor ECG triggering

. Complex blood flow signal loss (Figure 2)

Gibbs ringing

Gibbs ringing, also known as a truncation artifact, is present in every unfiltered MRI image and results from the fact that there is only enough time to acquire a finite region of k-space. When the sampled signal is truncated at the k-space edge and then this k-space is inverse Fourier transformed into the image, ringing will unavoidably be present at high-contrast sharp edges of structures on the image. The ringing is a known mathematical limitation of the Fourier transform.

Gibbs ringing artifact examples:

. First-pass myocardial perfusion (Figure 3)

Aliasing

This artifact occurs whenever the size of the object being imaged exceeds the FOV in the phase-encode direction; the outside regions are wrapped into the opposite edge of the FOV. Due to the need of large FOVs to cover the chest, aliasing is a common problem. If the region of interest is small, for example the heart only, then some wraparound can be acceptable as long as it does not superimpose on the heart. This keeps imaging time short without sacrificing diagnosis and experienced technologists commonly make careful use of this approach. (Figure 4)

Chemical shift

The resonance frequency of water and fat differs by approximately 210 Hz at 1.5 T (420 Hz at 3 T), which causes a number of effects. Firstly, it causes a misregistration between fat and water based tissues along the frequency-encode direction and more so along the perpendicular phase blip direction for EPI sequences. Secondly, it will result in a slice excitation offset between water and fat. Finally, for gradient-echo sequences only, a possible pixel cancellation effect at water-fat boundaries can occur.

Chemical shift artifacts examples:

. TSE frequency encode shift (Figure 4)

. Fat-water signal cancellation on an SSFP sequence.

B0 field inhomogeneity

The main magnetic field is never completely homogeneous over the volume of the heart. B0 field distortions are common at boundaries between tissues, and particularly those between tissue and air such as between the heart and lungs. These B0 field inhomogeneities cause resonance frequency offsets, where the local resonance frequency deviates from the scanner’s reference frequency, leading to off-resonance effects. Depending on the local geometry, and the sequence being used, these field distortions may sometimes be more intense causing artifacts such as signal loss or spatial distortion. In general, to minimize B0-inhomogeneities careful shimming and especially localized scanner frequency adjustments are advised prior to imaging.

B0 field inhomogeneity artifact examples:

. SSFP sequence (Figure 5)

. Medical devices

. EPI sequence

1.5T VS 3T

Cardiac imaging at 3T is becoming increasingly popular due to the potentially higher SNR and CNR (Contrast to Noise Ratio). Although, at higher fields, some of the above mentioned artifacts become even more problematic such as those caused by B0 and B1 field inhomogeneities or chemical shift. There is also a quadruple increase in RF power absorption. Some sequences, such as bSSFP, may have to be used with a reduced flip-angle and higher TR than at 1.5T, which increases B0 inhomogeneity sensitivity. Cardiac gating can also be more challenging at 3T. The higher field strength creates more magnetohydrodynamic distortion of the ECG signal, which can prevent the scanner from gating properly leading to additional artifacts.