Effects of heart valve prostheses on phase contrast flow measurements in Cardiovascular Magnetic Resonance – a phantom study
This phantom study investigated the influence of three different metal containing heart valve prostheses on PC flow measurements at 1.5 T. The main finding is that the flow measurements were corrupted in the vicinity of the tested valve prostheses. The extent of flow miscalculation depended on the composition of the prosthesis and on the distance of the measurement slice to the prosthesis. In contrast, PC measurements even in close proximity to a synthetic metal-free aortic valve model or a native aortic valve yielded nearly constant flow values.
PC measurements are expected to be B0-insensitive, as two images containing the same flow-unrelated phase errors are subtracted from each other [27]. In this study, we could reproduce this finding in measurements where no flow was present – notwithstanding all tested metal containing prostheses markedly altered the local magnetic field. Beyond that, imaging and flow encoding gradients induce eddy currents in the metal stent that evoke superimposing local magnetic field distortions.
In case of no flow, i.e. the pump was switched off, it could be shown that the phase contrast measurement yielded a net phase of zero after subtraction of the flow-compensated from the flow-encoded measurement even in the presence of these additional local magnetic field distortions.
In case of flow, however, the results showed that in the proximity of the metal stented prostheses severe flow measurement errors occurred. As for the static magnetic field related distortions and the imaging gradients related distortions, the flow-encoded measurement and the flow-compensated measurement experience identical field alterations. However, as the flow-encoding gradient scheme and the flow-compensating gradient scheme are dissimilar, these gradient schemes also induce different local and temporal magnetic field distortions. Analysis of the B0 maps in the proximity of the heart valve prostheses revealed that the severity of local magnetic field inhomogeneity was coincident with the severity of flow miscalculations. Coherence between the extent of the flow miscalculation and the material composition of the prosthesis seems to be likely as different material compositions inherit different magnetic susceptibilities and thus induce different magnetic field distortions. Analogous to the fact that the interaction with more susceptible material leads to stronger B0 inhomogeneity, the interaction of the gradients with the more susceptible material also leads to stronger net phase errors. Beyond that, also flow velocity influences the amount of additionally accumulated phase and thus the extent of flow miscalculation.
The strongest local magnetic field inhomogeneity and maximum flow miscalculation were observed in prosthesis II, with a maximum flow overestimation of around 150 ml/s in phantom, which is 53% deviant to the reference flow. In-vivo around 65 ml/s flow deviation was seen.
Our findings are related to the dedicated material of these prostheses. The influence on the magnetic field depends on the material. The induced local magnetic field distortions have different impact on flow measurements [19, 28].
The impact of heart-valve-prostheses-related magnetic field distortions depended on the distance of the measurement slice to the level of the prostheses. Maximum miscalculation is seen in the immediate proximity of the prostheses. For the investigated prostheses, the flow values converged in-vitro beyond a distance of about 20 mm and in-vivo about 30 mm distal to the level of the prosthesis. We used a range of equivalence of 15% as a marker for significant deviation from reference flow. The background phase offset errors are within this range; so we deduce a flow-related impact of the metal components.
Although it could be shown that heart valve prostheses are MR-safe up to 4.7 T, the magnetically induced electric currents can hamper the diagnostic performance [29–31]. Both biological prostheses cause magnetic field distortions and signal loss to various extents as seen in Fig. 3. Prosthesis I is constructed onto a flexible, tender stent, whereas prosthesis II comprises a semi-rigid, three-base frame. The extent of magnetic field distortion is related to the amount of metal. Another trial demonstrated metal-stented aortic valve prostheses inducing artefacts in CMR in terms of signal loss in the whole left-ventricular outflow tract. In contrast, the stent-less valve prostheses did not cause artefacts [32].
Among the research on heart valve prostheses, magnetic field distortions due to metallic compounds were investigated in several studies, but flow-measurement-related data are rare [33]. In our study, prosthesis II consists of Elgiloy, an alloy recommended for biomedical implants. As seen in Figs. 3 and 4, it evokes local magnetic field distortions. The mechanical prosthesis III, composed mainly of pyrolythic carbon, was deemed to be nonmagnetic, but showed magnetic field distortions.
The interaction between both mechanical and biological valve prostheses and magnetically induced electric currents has been demonstrated [34].
It has already been described that elements from stainless steel cause stronger artefacts than Nitinol-based products [19, 28]. Our in vitro results are supported by single case examples, but have to be evaluated in a larger cohort.
Assessment of valvular prosthesis is of high impact in clinical cardiology and CMR has an increasing influence. Therefore these results could have an impact on applicability of flow quantification in this patient group.
Strengths and limitations
Our study has the strength that we faced a potential obstacle for clinical decision-making when using phase-contrast techniques in patients with valve prostheses. Our results are limited by the fact that our setting did not include MR-independent flow quantification as a reference. However, the setting has been validated previously [35] and the flow was kept constant to meet the requirements for this study.
In our study, we investigated the behaviour of PC based flow quantification using a flow-encoded and a flow-compensated measurement. As other techniques exist – such as the measurement with two flow-encoded measurements – our findings cannot be generalized for all existing flow measurement techniques in the same manner.
We have investigated three often-used prostheses. Our findings cannot be directly extrapolated to all other types of prosthesis. Especially the recommended distance to achieve correct flow measurements needs careful studies covering all types of prostheses. Further investigation and analysis of all different valve prostheses have to be conducted to address this need.