Cardiac MRI

Cardiac MRI consists of using MRI to study heart anatomy, physiology and pathology.

Advantages

The main advantages of cardiac MRI in comparison with other techniques are:

• better definition of soft tissues
• use of different types of sequences improves diagnostic accuracy
• no ionising radiation
  • some safety issues still require consideration

Limitations

The main limitation of MRI, compared to cardiac CT, is the poorer evaluation of the coronary arteries.

In addition, cardiac MRI employs some particular imaging planes.

Imaging

Dark blood Imaging

Dark blood imaging involves spin echo sequences. Its main advantage is a fast acquisition that minimizesminimises respiratory and cardiac movement artefacts. It’s the Its main issue is a low signal/noise ratio and, therefore, a deficient spatial resolution. 

These can be T1, T2, or proton density weighted sequences:

• T1 weighted sequences achieve better anatomic definition
• T2 and PD weighted sequences reach better tissue characterization

White blood Imaging

White blood imaging involves gradient echo sequences and steady-state free precession MRI (SSFP). In practice, the difference between boththe two is that SSFP is less vulnerable to the T2* effect.

The main advantage of white blood imaging is its fast acquisition. It can obtain movement sequences and allows studying cardiac function and movement.

Flux quantification sequences

The most usual sequence of this group is phase contrast imaging. It encodes flux direction and speed, similarly to CSF flow studies.

Inversion Recovery sequences

These imaging techniques use additional 180º pulses to null signal from blood and other tissues, and, therefore, improvingimprove contrast.

The most used sequence is STIR.

Contrast-enhanced techniques

Perfusion imaging (also known as first-pass images)

These are T1 weighted, gradient-echo sequences. Image acquisition is performed 3 minutes after gadolinium contrast administration. If there is a hypoenhanced area, this implies a zone of myocardial infarction that is non-viable.

Viability study delayed (also known as myocardial enhancement study)

These are T1 weighted, gradient-echo sequences. Image acquisition is performed 10 minutes after gadolinium contrast administration. 

Focal myocardial fibrosis has a delayed gadolinium contrast wash out. So an hyperenhancement indicates a myocardial scar, thus an evolved myocardial infarction.

Usually, an extra inversion pulse is used to improve contrast between fibrosis and the surrounding myocardium.