Cardiac MRI

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

Advantages

In comparison to other techniques, cardiac MRI offers:

• improved soft tissue definition
• protocol can be tailored to likely differential diagnoses
  • a large number of sequences are available
  • dynamic imaging provides functional assessment
• no ionising radiation
  • MRI safety still requires consideration

Limitations

MRI is generally inferior to cardiac CT for evaluation of the coronary arteries.

Cardiac MRI can be technically challenging. In particular, a comprehensive understanding of cardiac imaging planes is required for scan planning.

Imaging

Dark blood Imaging

Dark blood imaging may be based on fast spin echo or double inversion recovery sequencessequence. The fast acquisition time of the sequences minimises respiratory and cardiac movement artifacts. However, a low signal/noise ratio results in inferior 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 the 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, improve 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 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.

See also

• Cardiac MRI (an approach)