Non-contrast MRI protocol for TAVI guidance: quiescent-interval single-shot angiography in comparison with contrast-enhanced CT

The standard pre-imaging workup of transcatheter aortic valve intervention (TAVI) currently consists of a combination of transthoracic and transesophageal echocardiography as well as contrast-enhanced aortoiliacal CTA for the exact determination of valve size and implantation route [1]. However, the administration of a contrast agent may be hindered by acute or chronic kidney disease (CKD), with reported prevalence rates up to 41% for acute kidney injuries and up to 70% for CKD in patients undergoing TAVI assessment [2, 3]. Therefore, cross-sectional imaging techniques to reduce patient exposure to iodinated contrast agents are desirable [4]. Our study group recently presented a contrast-enhanced MRI protocol to provide measurements of aortic annulus and access routes with good to excellent agreement in comparison with CTA resulting in accordant decisions for prosthesis size as well as transfemoral TAVI access capability [5]. However, in patients with stage 4 and 5 renal impairment (estimated glomerular filtration rate (eGFR) < 30 ml/min/1.73 m²), rare cases of nephrogenic systemic fibrosis (NSF), a severe and potentially fatal condition without known effective treatment, were reported [6]. Moreover, recent reports of intracranial gadolinium deposition with currently unclear clinical relevance have fostered efforts to develop unenhanced MRI protocols [7, 8].

The QISS technique represents a clinically feasible and efficient non-contrast MRA, however, until now mainly evaluated in peripheral arterial disease (PAD) [8‐10]. Hence, we sought to compare a comprehensive non-contrast-enhanced MRI protocol, including three-dimensional (3D) “whole heart” acquisition and QISS-MRA with CTA for TAVI planning using aortic root and access vessel measurements as well as decision for transfemoral accessibility.

Twenty-six patients (mean age 83 ± 5 years) referred for TAVI evaluation underwent non-contrast MRI and additional CTA within 1 day (interquartile range (IQR) 0–2 days). In addition to the 26 patients who received both MRI and CTA examinations, nine additional patients underwent a MRI scan exclusively. This results in a MRI cohort of a total of 35 patients for inter-observer comparison and image quality evaluation of QISS-MRA.

Local ethics committee approval was provided prior to inclusion of the first patient and written informed consent was obtained from each participant.

General inclusion criteria were severe aortic stenosis according to recent guidelines (aortic valve area ≤ 1.0 cm² or aortic valve index ≤ 0.6 cm²/m²) with the decision for TAVI procedure by an interdisciplinary heart team and typical symptoms of severe aortic stenosis like shortness of breath, angina, or syncope [11, 12].

Exclusion criteria were contraindications to perform MRI or CTA; contraindications for TAVI and Killip class ≥ 3.

For a successful TAVI procedure, it is crucial to accurately size the valve prosthesis dictated by aortic annulus dimensions as well as to evaluate access capability of aortoiliofemoral vessels, usually driven by a contrast-enhanced CTA. However, in patients undergoing TAVI, the reported prevalence of acute kidney injury ranges between 8.3 and 41% with an associated increase in 30-day mortality, 1-year mortality, and length of hospital stay [2, 3]. In line with published observations, patients with CKD (eGFR < 60 ml/min/1.73 m²) constituted 69% of the overall population of the present study [13]. Moreover, patients with chronic and acute kidney injury, especially at Kidney Disease Outcomes Quality Initiative (KDOQI) stages 4 and 5, are at higher risk to suffer NSF after administration of gadolinium-based MR contrast agents [6]. However, the rapidly growing interest in more sophisticated, non-contrast MR imaging is also driven by reports of intra- and extracranial gadolinium depositions with currently unclear clinical significance [14].

Regarding contrast-free imaging in the context of TAVI, Renker and colleagues presented a rapid, non-contrast, free-breathing, self-navigated three-dimensional MR sequence to study aortic root and vascular access route [15]. However, this feasibility study was conducted in 10 healthy volunteers only. Our MRI evaluation of aortoiliofemoral access routes in 26 patients referred for TAVI planning was based on QISS-MRA measurements. This unenhanced MRA technique was first introduced in 2010 by Edelman et al and until now has been evaluated in a large field of clinical applications at 1.5 T and 3 T [16‐19], mainly for the detection and characterization of lower extremity arterial disease [20‐24], and also in the demonstration of the pulmonary embolism [19], for coronary imaging [25], and for intracranial MRA [26].

So far, the only study integrating the QISS technique for the assessment of vascular access anatomy showed technical feasibility for TAVI planning but was clearly limited by a very small sample size (5 patients and 10 healthy volunteers) [27]. Our comparatively larger cohort of 26 patients significantly expands these findings by presenting excellent inter-method agreement for the decision of transfemoral TAVI accessibility between QISS-MRA and CTA measurements. Of note, while minimal vessel diameters throughout the access routes correlated strongly between QISS-MRA and CTA measurements, the inter-method bias of minimal infrarenal aorta and iliofemoral vessel diameters was, although in the size range of up to only 1 mm, statistically significant. This observation can be related to several studies reporting limited image quality in the abdominopelvic region owing to motion artifacts as well as flow-related artifacts based on high systolic and diastolic flow velocities and multiple directions of flow [10, 16, 23, 28]. QISS-MRA in general is relatively insensitive to patient motion due to 2D acquisition in the transverse plane. However, the overall longer acquisition time compared with that of CTA and scanning of the pelvic segment at the end of the examination when patient motion is usually observed may render this station more prone to motion artifacts. Moreover, remaining immobile in supine position for an extended period can be challenging for TAVI candidates that are often multimorbid and high-aged. Similar to CTA, the scan direction of QISS-MRA was perpendicular to the mainly vertically oriented descending aorta. Transverse plane imaging of the anatomically more curved, relatively narrow pelvic axis may decrease accuracy in quantification of both anterior-posterior and left-right diameters and may therefore be another explanation for the significant bias in this scan region.

A recent study specifically addressed QISS imaging of abdominopelvic arteries and presented two prototype free-breathing fast low-angle shot-based (FLASH) sequences at 3 T [29]. FLASH QISS-MRA provided improved image quality and sensitivity for detection of stenosis > 50% when compared with SSFP-based QISS-MRA, while specificity was comparable. Especially in patients with metallic implants or artifacts by gas-filled bowel loops, image quality improved with slightly longer image acquisition time of the FLASH QISS technique.

In contrast with our study, where we compared arterial diameters for TAVI planning between QISS and CT angiography, former QISS studies evaluating lower extremity PAD did not quantify artery diameters and considered dichotomous stenosis grading (< 50% and ≥ 50% stenosis) only [9, 10, 16, 17, 22, 30]. Regarding dissecting flaps, both observers reliably detected three unilateral iliacofemoral dissections in both modalities. QISS-MRA provided sufficient arterial signal in both the true and the false lumens where flow is expected to be particularly slow. This is in concordance to the consistent and robust QISS signal behavior across a wide range of flow velocities raging from approximately 5 to > 50 cm/s as shown in a phantom setting [9]. However, QISS-MRA described a single additional dissection of the common femoral artery without evidence of a corresponding CTA correlate. It has to be mentioned that the intraluminal QISS signal, due to its slightly granular aspect, may simulate subtle linear signal attenuations, which requires a QISS-trained eye to avoid false positive dissection findings. However, in general, QISS-MRA showed an excellent subjective image quality. Of note, QISS-MRA lacks visualization of calcified plaque burden since it depicts exclusively the vessel lumen and not the wall. However, calcification in CTA, particularly in small vessels, results in blooming artifact and partial volume averaging with underestimation of the minimal luminal diameter [31]. This may explain in part the significant bias regarding the mean effective minimal vessel diameter between the modalities showing a larger diameter for MRA than CTA.

Moreover, the single inter-method mismatch regarding transfemoral TAVI accessibility in this study may be owed to this problem and is in line with previous reports demonstrating superior performance of QISS-MRA over CTA in heavily calcified vessels [22].

In accordance with a previously published study by our group [5], this study again confirmed high reproducibility of aortic root parameters combined with very low inter-observer variability evaluated by 3D whole heart MR. Strong correlations between CTA and MRI measurements regarding diastolic aortic annulus dimensions are in good concordance with previous studies using this sequence [32].

Comparable prosthesis sizing between CTA and MRA in combination with sufficient access route evaluation highlights the feasibility of this comprehensive unenhanced MRI protocol for pre-procedural TAVI imaging, resulting in reduction of both total radiation dose and amount of applied contrast media. Acute kidney injury (AKI) induced by iodinated contrast media has been shown to mostly occur in the course of pre-procedural TAVI imaging, increasing in-hospital mortality [33]. Especially in chronic kidney disease, in our study concerning 69% of all subjects, as a risk factor for AKI, patients may benefit from contrast-free TAVI imaging since injected contrast medium volumes more than 100 ml and repeated injection of iodinated contrast media within a short interval are associated with an additional increase in AKI risk [34].

As a limitation of the study, the relatively small patient cohort has to be addressed; therefore, more patients referred for TAVR evaluation should be included in future studies.

This study presents an easy-to-implement contrast-free MRI approach for aortoiliofemoral accessibility evaluation in patients scheduled for TAVI. Comparison with CTA as “gold standard” for minimal vessel diameter showed strong correlations with QISS-MRA measurements, resulting in comparable decisions for transfemoral access capability. The combination of QISS-MRA with an unenhanced 3D sequence for aortic root measurements facilitates TAVI planning without contrast media. Given the high prevalence of renal function impairment in patients referred for TAVI, the presented data highlights this MRI protocol as a reliable, contrast-free guidance alternative.