Introduction
- Cardiac MRI (cMRI) creates both still and moving pictures of the heart and major blood vessels.
- Application of MRI to cardiac function is hampered by motion artifacts from cardiac and respiratory activities.
- A number of techniques have been developed using MRI to image the heart.
- Images can be obtained in real-time and on video, but many of the techniques require the patient to hold their breath and may also necessitate cardiac gating to minimize motion artifacts.
Techniques Used in cMRI
“Black blood” anatomical imaging: this technique is also referred to as “spin-echo”. In this technique, the flowing blood gives no signal to the sensors, while other structures appear as bright. This imaging method is used specifically to examine structural abnormalities of the ventricles, and is also a preferred technique to examine the status of the pericardium.
Figure 1 shows an arrhythmogenic right ventricular dysplasia via “Black blood” imaging. The arrow indicates presence of a fatty infiltration (arrow) of the right ventricular (RV) wall.
Image reproduced with permission from Lin EC, Klepac SR and Samett EJ.
Cardiac MRI, Technical Aspects Primer. Medscape.
http://emedicine.medscape.com/article/352250-overview - aw2aab6b7
Accessed September 30, 2013.
Cardiac Cine with Bright Blood Technique: this technique is standard for functional cMRI imaging. In this technique, blood appears bright while structures appear dark. It is a useful method for obtaining accurate measurements of ventricular volume and myocardial mass.
Figure 2 shows an example image from a “Bright blood” cMRI.
Image reproduced with permission from Lin EC, Klepac SR and Samett EJ. Cardiac MRI, Technical Aspects Primer. Medscape. http://emedicine.medscape.com/article/352250-overview - aw2aab6b7
Accessed September 30, 2013.
Examination of PAH Using cMRI
When pressure overload is induced in the RV by elevated pulmonary artery pressures, the RV will hypertrophy. This is followed by a progressive loss of contractile function, dilation of the RV, and ultimately decompensation of the RV. During decompensation the filling pressure of the RV rises and there is diminished cardiac output.
Since RV function is one of the most important aspects of survival in PAH patients, it is important to quantify the pressure and flow parameters in the right heart. While the gold standard for these measurements is right heart catheterization, cMRI can also provide estimates of these values. There are several important right heart aspects which can be determined by cMRI. These include:
Interventricular septum (IVS)
- In a normal heart, IVS is bowed to the right, the left ventricle retains a circular shape, and the RV is crescent shaped (in cross sectional view).
- When the RV becomes overloaded, as in PAH, the IVS flattens or bulges to the left in early diastole resulting in a crescent shaped left ventricle.
Estimation of RV Systolic Pressure and Mean Pulmonary Artery Pressure (mPAP)
- In the cardiac cycle, the position of the IVS is determined by the pressure gradient between the RV and the LV. Thus any change in the transseptal pressure will affect the radius of curvature of the IVS, which can be used to estimate right ventricular systolic pressure (RVSP).
- Based on cMRI images, this IVS curvature can be measured and RVSP algorithmically calculated.
- Similarly, mPAP can be estimated by computer algorithm based on cMRI estimates of physical parameters (e.g. cross-sectional area of pulmonary artery, blood flow velocity).
- While these measurements are of value and the cMRI technique is non-invasive (as opposed to right heart catheterization), the technique does not provide accuracy across patients and so should not be considered as an alternative to right heart catheterization.
Using cMRI to Monitor PAH Treatment Efficacy
Because it is non-invasive, cMRI may provide a convenient means to monitor patient response to PAH treatment. In one example, a 14 year-old male was diagnosed with idiopathic PH and placed on oral bosentan and anticoagulation therapies. From the images depicted in Figure 3, one can clearly see the
left-bowed IVS in the initial image (A) and the increased size of the RV, indicating RV dilatation. In panel B, the IVS is right-bowed, and the RV has reduced in size, suggesting a significant improvement in right heart function.
Images reproduced with permission from McLure LR and Peacock AJ. Eur Resp J 2009;33:1454-1466.
cMRI can also provide a measure of the right ventricular size expressed at the right ventricular end diastolic volume (RVEDV). Also an accurate RVSP can be measured.
Limitations of cMRI
cMRI is an expensive technique that is not covered by all insurance companies. It is not widely or readily available, and certain contraindications my preclude its use in some patients. Patient-focused contraindications include:
- Claustrophobia (patient must lie in an enclosed space)
- Patient is unable to hold their breath for sufficient time for image capture
- Patient has MRI-incompatible metallic implants (cochlear implants, aneurysm clips, neurostimulators, etc.)
Finally, cMRI may not adequately visualize peripheral areas of the lung vasculature and so may not detect smaller or older thrombi, for example.
References
Lin EC, Klepac SR and Samett EJ. Cardiac MRI, Technical Aspects Primer. Medscape. http://emedicine.medscape.com/article/352250-overview - aw2aab6b7 Accessed September 30, 2013.
Desai R, Torres F, Gupta H, Role of cardiac MRI in pulmonary hypertension. PHA Online University-Advances in PH Journal. Vol 7(4), Winter 2008. http://www.phaonlineuniv.org/Journal/Article.cfm?ItemNumber=739
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