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Virtually all prosthetic heart valves (PHV) are considered safe in the magnetic resonance (MR) environment at field strengths of up to 1.5 T (Figure 1 A-H).
(Figure 1) - Examples of different types of PHV in aortic position (except for H) as imaged by CMR (balanced SSFP sequence). Virtually all PHV are safe in the MRI environment at field strengths up to 1.5T.
There is no evidence to justify withholding a cardiac or extracardiac MR study at 1.5T in such a patient just due to the presence of a PHV. Most of the PHV are also safe at 3T, but testing is less widely available at 3T. In case of any doubt or for documentation purposes, online resources (e.g., www.mrisafety.com) are available for checking on safety of any devices for MR imaging.
The quality of images and therefore the diagnostic capability of a cardiovascular magnetic resonance (CMR) study in a patient with PHV depend on artefacts from the metallic components of the PHV. Cine sequences and phase contrast velocity mapping sequence are essential for assessment of valve function and quantification of velocity and flow. These sequences are gradient echo sequences and thus susceptible to artefacts caused by ferromagnetic metals. Spin echo sequences are less prone to these artefacts but provide only static images.
The daily workhorse cine sequence in CMR is balanced steady state free precession (SSFP) which is very sensitive to artefacts from ferromagnetic objects. The severity of artefacts in cine images caused by PHV may range from no artefact to severe artefact obscuring not only the valve but also the adjacent structures. Therefore from a CMR perspective the ferromagnetic metallic component of the PHV is the main factor determining cine image quality. While one may think that mechanical PHV will cause the more severe artefacts and bioprosthetic PHV will be friendlier with the image quality, this is not always the case. The metallic component of a mechanical valve may not be highly ferromagnetic whereas the very small amount of metal used in the supporting stent of a bioprosthetic valve may cause significant image distortion (Figure 2 A, B).
Even the annuloplasty rings can cause significant image distortion when they contain metal in their structure (Figure 3).
(Figure 3) - Even the annuloplasty rings cause significant image artefacts when metals are included in their structure. An annuloplasty ring implanted for functional mitral regurgitation causing significant artefact (LVOT and short axis views).
Stentless bioprosthetic valves are essentially metal free and do not cause any artefacts, hence their appearance is not much different than native valves (Figure 4).
(Figure 4) - Stentless bioprosthetic valves are not different than native valves from an imaging perspective and can be assessed reliably by CMR. A Toronto stentless valve is shown, the images are devoid of any artefact and the bioprosthetic valve appears almost like a native aortic valve.
The image quality of CMR scans for the new percutaneous valves are reasonably good as well (Figure 1 F, G, H). When the cine images with balanced SSFP are significantly deteriorated due to artefacts from the PHV, a spoiled gradient echo sequence which is less prone to these artefacts can be used to acquire cine images, albeit at the expense of a lower signal to noise ratio (Figure 5).
(Figure 5) - Balanced SSFP sequence is very susceptible to artefacts caused by ferromagnetic metals. A spoiled gradient echo sequence can also be used to acquire cine images which is relatively less susceptible to such artefacts and may yield better image quality.
Cine images are also essential for reliable assessment and monitoring of ventricular size and function after valve replacement (Figure 6 A, B).
(Figure 6) - CMR is a robust technique for assessment of ventricular volumes and ejection fraction. Accurate calculation of ventricular volumes is not only important for timing of valve replacement but also for follow-up after surgery.
These images are essential to acquire velocity and flow data. Depending on the type of the PHV, there may be a signal void region below and above the valve. Imaging slice for through plane images is positioned just above this signal void level to avoid the artefact. It is easier to use through plane images to quantify flow and velocity for the aortic and pulmonary valves as the borders of flow through these valves (the ascending aorta and pulmonary artery) can be traced (Figure 7 A, B, C and D).
(Figure 7) - PC sequence is used to derive velocity and flow data through any valve or vessel. For the aortic and pulmonary valves a through plane PC image just above the valve is most reliable to detect maximum velocity. This should be just above the level of the metallic artefact. For mitral and tricuspid valves through plane images are less useful and therefore not commonly acquired, however in-plane PC images can still provide important information on valve function (see Figure-15 C).
For the mitral and tricuspid valves however phase contrast images are mainly used for qualitative assessment.
These are least prone to artefacts from ferromagnetic objects, but they can only be used to acquire static images. As such they cannot be used to assess function of the PHV; however when any structural abnormality related to a PHV is suspected spin echo images can be very useful (Figure 8 A, B).
(Figure 8) - Spin echo sequences are less effected from artefacts caused metallic structures of the valves but can only provide static images. Nevertheless these may be valuable when structural abnormalities are assessed.
No established guidelines or recommendations exist about when to use CMR for the assessment of PHV. Doppler echocardiography is essentially the first line of imaging modality for assessment of patients with PHV. However, CMR can provide invaluable information in most of the patients with PHV. This may include information on one or more of the following:
Quantitative data that can be derived from CMR for assessment of valvular heart disease are summarized below (see relevant native valve disease sections). All these approaches are essentially valid for the assessment of PHV function, and planning of the images are the same except for minor alterations to avoid artefacts. The reliability of the assessments, however, will depend finally on the quality of images that can be acquired.
Valve area planimetry can be calculated from the cine images and the maximum velocity through the valve can be derived from PC velocity mapping.
(Figure 10) - Another example of a bioprosthetic valve with moderate stenosis.A pulmonary tissue valve in a 27 years-old female patient. RVOT cine and in-plane PC images (a and b) suggest stenosis of the valve, the maximum velocity was 3.4 m/s. A cross cut through the valve leaflets level (c) showing the opening of the valve leaflet. Valve area by planimetry was 1.2 cm2.
Regurgitation volume and fraction can be calculated from PC velocity mapping of aortic and pulmonary flow (for aortic and pulmonary regurgitation, respectively), from difference between left and right ventricular strokes volumes or from difference between ventricular stroke volumes and the corresponding outflow (such as difference between LV stroke volume and aortic flow derived from aortic PC image to calculate mitral regurgitant volume). Also secondary findings of valvular regurgitation can be imaged (e.g., appearance of regurgitation jet severity, diastolic flow reversal in descending aorta in aortic regurgitation)
(Figure 11) - An aortic bioprosthetic valve with degeneration, central coaptation defect and severe aortic regurgitation.
(Figure 13) - A 45 years-old male who had Ross procedure for bicuspid aortic valve. He then had mechanical aortic valve replacement for failing aortic homograft. Patient was referred for a CMR study for assessment of mechanical prosthetic aortic valve and pulmonary homograft valve.
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