CT Assessment of the Aortic Valve for Transcatheter Procedures
Multidetector computed tomography (MDCT) is considered crucial for the assessment of aortic valve defects (stenosis and regurgitation) and the preprocedural planning of TAVI
[15]. On the technical side, TAVI CT assessment comprises two scans: an EKG-synchronized computed tomographic angiography (CTA) of the heart and aortic root, followed by a high-pitch non-EKG-synchronized CT multislice (CTM) for the evaluation of the access vessels. The two scans are combined into a complete protocol with a single contrast bolus through a peripheral vein. The amount of contrast medium should be calibrated to its concentration, patient’s BMI, and kidney function (three thresholds should be considered:
≥60 mL/min/body surface area (BSA), 30–59 mL/min/BSA and <30 mL/min/BSA). Furthermore, for high-quality studies, according to the Cardiovascular Computed Tomography Society (SCCT), imaging should cover the entire cardiac cycle, and ALARA principles (as low as reasonably achievable) should be followed
[16]. This means that the radiation dose that a patient could receive will vary depending on their weight or body mass index (BMI): 100 kV for patients with a BMI of 30 or less (or weight of 90 kg or less) to 120 kV for patients with a BMI of more than 30 kg (or weight more than 90 kg). Three scans are necessary to obtain useful parameters: the first is a basal one without contrast employed to calculate calcium scoring; a second one is used for cardiac evaluation with the administration of iodine contrast agent (after retrospective ECG synchronization); the last one should be extended from clavicles to the thigh’s proximal portion in order to evaluate femoral and axillary accesses
[16][17].
CT is almost universally accepted as a standard of care since its use has demonstrated a positive impact on clinical decision-making, favoring better post-TAVI clinical outcomes
[18]. Therefore, Corcione et al. proposed a CT-based score called TAVI CT score, including the presence of nodular subvalvular calcium, the elliptical index (defined as the ratio of minimum aortic valve annulus diameter to maximum aortic valve annulus diameter), the aortic isthmus angle, the aorta ventricle angle, the presence of bicuspid valve, the coronary height, the detection of iliofemoral calcification, the access size, and the type of planned access, to predict outcomes. Notably, an increased risk of vascular complications was associated with higher scores
[19]. Additionally, in a subanalysis of the WIN-TAVI registry, Spaziano et al. found that moderate or severe left ventricle outflow tract (LVOT) calcification detected by CT scan was an independent predictor of 1-year mortality or stroke
[20]. The authors also reported that new pacemaker implantation (PMI) was independently associated with calcium volume of the right coronary cusp. In contrast, a protective effect on the same outcome was observed with the calcium volume of the noncoronary cusp. Lastly, according to this research, severe calcification of the noncoronary/right-coronary commissure can independently predict new atrial fibrillation onset
[20]. In a study by Gama et al., it was demonstrated that membranous septum length measured at CT showed strong discriminatory ability for PMI
[21]. Furthermore, Gegenava et al. highlighted that MDCT-derived global left ventricular longitudinal strain was a powerful predictor of all-cause mortality. As shown with the Kaplan–Meier curves, MDCT-derived LV GLS > −14% was related to higher cumulative rates of all-cause mortality than MDCT-derived LV GLS ≤−14%
[22]. In the following paragraphs, the researchers discuss all specific parameters obtained using CT that are considered of interest in the preprocedural planning for aortic valve replacement.
Peripheral Access
Femoral access is the preferred access for TAVI. MDCT imaging identifies patients with complex vasculature anatomies, which may impact the decision to use an alternative vascular approach (i.e., transapical, transaxillary, or direct aortic)
[23]. Vessel size and diameters, degree of calcification, vessel tortuosity, and high-risk features can be properly identified by CT through multiple projections (
Figure 1 a,b).
Figure 1. (a) Preoperative measurements of peripheral accesses for TAVI feasibility. (b) Preoperative femoral and iliac courses evaluation.
In a prospective study of 130 patients undergoing TAVI, Hayashida et al. found that vascular complications and 30-day mortality were associated with a sheath-to-femoral artery ratio (SFAR) of 1.05 or higher
[24]. Later, Okuyama et al. proposed a change in this cut-off value, believing that this was too strict, providing high sensitivity but poor specificity, and 1.12 may be more appropriate
[25].
Interestingly, the most recent valve delivery systems have a size ranging between 14 and 20 Fr, depending on the specific prosthesis valve type and size. While some years ago, a minimal lumen of femoral arteries of 6.0–6.5 mm was required, currently, with reduced delivery profiles, TAVI might also be indicated in patients with peripheral vessels as small as 5.0 mm
[26].
Aortic Annulus
The aortic annulus for implanting percutaneous prostheses is not directly visible. It is a virtual ring that passes over a plane via the basal insertions (nadir) of the aortic valve cusps. It has an oval shape that varies during the cardiac cycle, even in patients with severe calcific AS. While the size of the aortic annulus was initially determined almost exclusively via two-dimensional transesophageal echocardiography (TEE), it has been proven that the aortic annulus is a dynamic measure during the cardiac cycle. Thus, 2D imaging could not be precise enough, and only 3D imaging can display a correct measurement. Furthermore, compared with a two-dimensional assessment of the annulus, MDCT-based measures have been proven to be highly reproducible and provide a deeper understanding of annular geometry
[27]. Jurenkak et al. found that, during the cardiac cycle, the aortic root varies its shape, suggesting that an MDCT evaluation during the early systole phase is the best choice for TAVI planning
[28].
Annular dimension can be quantified with multiple methods: cubic spline interpolation, polygon, attenuation/Hounsfield-unit-based contour detection, and freehand contour. ECG-synchronized, ideally multiphasic, dataset should be used for proper measurement, identifying the reconstruction phase with the largest annular dimensions, thus ensuring accurate device sizing
[16]. MDCT sizes the aortic annulus using annular area and perimeter measurements. In the Pivotal Trial of Medtronic Corevalve and the PARTNER 3 trial, it was demonstrated that annulus sizing based on CT measurements reduces the risk of complications such as paravalvular leakages. Moreover, the aortic annulus perimeter or cross-sectional area is superior to the annulus diameter for reducing the likelihood of paravalvular aortic regurgitation (PAR)
[29][30] (
Figure 2).
Figure 2. CT measurements of perimeter and area of aortic annulus.
Another goal of current CT sizing algorithms is to calculate a certain degree of oversizing of the transcatheter heart valve (THV) to avoid the post-implantation occurrence of paravalvular leaks (PVLs). It depends on the type of THV and the measurement used (perimeter/area). Self-expandable devices need more oversizing than balloon-expandable prostheses. Conversely, severe oversizing using balloon-expandable devices might increase the annular damage. Additionally, a 10 per cent perimeter oversize does not equal a 10 per cent area oversize, but it is closer to 20 per cent
[31].
Aortic Leaflets
Three major parameters regarding aortic leaflets should be carefully considered during the preprocedural planning for TAVI: the number of cusps, the degree of leaflet calcification, and the height of the leaflets to the coronary ostia. MDCT has been proven to be superior to trans thoracic echocardiogram to discriminate aortic valvular morphology and the exact location and quantification of calcifications
[32] (
Figure 3 a,b).
Figure 3. (a) Calcium degree and morphology evaluation of the aortic valve by CT. (b) LAO–cranial view for calcium location assessment.
The bicuspid aortic valve (BAV) is an important risk factor for premature aortic stenosis and challenging anatomy for percutaneous interventions. Thus, patients with bicuspid valves were excluded in the largest trials regarding TAVI due to complex anatomic features (i.e., more calcified, and asymmetrical leaflets, annular eccentricity, and associated aortic dilation). However, in recent years, the interest in this topic has rapidly grown, and the first data appear to be quite satisfactory
[33]. BAV has historically been diagnosed with echocardiography that, compared with pathologic analyses obtained from cardiac surgery, can detect this kind of valve morphology in a range between 54 and 93%. Nevertheless, CT was found to be superior to transthoracic echocardiography, especially in patients with extensively calcified aortic valves
[34] (
Figure 4). In fact, taking into account echocardiographic and tomographic images of fifty patients with AS plus probable bicuspid aortic valve and comparing with surgical findings, Tanaka et al. found that CT data were not significantly different from the intraoperative ones, but the echocardiographic were
[34].
Figure 4. CT image of bicuspid valve.
MDCT images should assess several anatomical aspects in the setting of bicuspid aortic valves: the location of raphe by reviewing the commissures symmetry from the basal plane to the sinotubular junction, the specific morphology according to the Sievert classification, the extent of leaflet asymmetry, the degree of calcification, and the length and width of the raphe into the lumen. Indeed, more paravalvular regurgitation due to incomplete valve expansion could be associated with longer calcified raphe and calcium impingement > 4 mm
[35].
Quantifying aortic valve calcification by noncontrast CT (CT-AVC) has demonstrated good accuracy in defining the valvular calcification burden (location, extent, and severity), demonstrating satisfactory concordance with echocardiography, and providing incremental prognostic information. CT-AVC has been reported to correlate significantly with peak velocity, mean gradient, and the aortic valvular area measured by echocardiography
[36]. AVC is typically assessed by analyzing 1.5–3 mm slice thicknesses CT scans using prospective or retrospective electrocardiogram (ECG) gating and at lower tube current than contrast-enhanced CT studies, thus resulting in a low radiation dose of approximately 1 mSv. AVC can be measured by numerous methods, including Agatston, mass, and volumetric scores. Among these, the first method is more frequently employed.
Electron-beam and multidetector row CT studies demonstrate that the severity of AVC is a robust prognostic predictor in asymptomatic patients with severe AS
[37]. Therefore, worse outcomes have been experienced by asymptomatic patients with severe AS and high calcium scores compared with those with low scores
[38]. Moreover, preoperatively AVC assessment is crucial for TAVI because it has been demonstrated to negatively affect procedural success and outcomes given the greater presence of paravalvular regurgitation, conduction abnormalities, aortic annulus rupture, coronary ostia occlusion, and stroke. Furthermore, Leber et al. found that, in an analysis of 68 patients undergoing TAVI, calcium score significantly correlated with 30-day major adverse cardiac events (MACE) and 1-year mortality and with the incidence and severity of post-procedural aortic regurgitation. Patients with calcium scores > 750 experienced a significantly lower 1-year survival rate than those with <750
[39]. Haensig et al. confirmed these findings in a series of 120 patients receiving an Edwards SAPIEN prosthesis via a transapical approach
[40].
Extensive calcification of the aortic cusps also represents a risk factor for obstruction of the left main coronary artery during THV deployment
[23]. Additionally, Lee et al. found that subjects with at least mild–moderate post-procedural aortic regurgitation had higher AVC scores than patients with no PAR on follow-up
[41]. Unbehaun et al. evaluated the amount of calcifications of the aortic landing zone, including the left ventricle outflow tract (LVOT), aortic annulus, and valvular cusps in 307 patients undergoing transapical TAVI, using a semiquantitative (4-point scale) and quantitative (Agatston calcium score) CT method. They found that an increased risk of PAR was strongly related to each 100-unit increment of the Agatston calcium score. Furthermore, a higher risk of AR was reported in patients with asymmetric cusp calcification and severe calcification of the landing zone
[42]. Interestingly, Feuchtner et al. recently showed that paravalvular AR increased with the protruding rather than the “adherent” calcifications of the aortic valve and root
[43].
Coronary Ostia
MDCT is crucial to identify the distance from the annulus/leaflet hinge point to the left main and right coronary ostia and the length of the corresponding coronary cusp (
Figure 5 a,b). Multiplanar, three-dimensional techniques allow better visualization and assessment of these complex structures and their relationships. For this reason, CT represents the preprocedural imaging gold standard for the evaluation of the risk of coronary occlusion. Even if there is no threshold value contraindicating the procedure, an increased risk of coronary occlusion was reported in patients with a coronary ostial height from the annulus < 12 mm and a sinus of Valsalva mean diameter < 30 mm. To ensure reproducibility, the height of the coronary ostia perpendicular to the plane of the annulus should be measured with an electronic caliper from the plane of the ring to the bottom of the ostium. No recommendations exist as to whether these measurements should be performed in systole or diastole
[16].
Figure 5. (a) Right and (b) left coronary ostia height assessment by CT images.
Ribeiro et al. found that coronary occlusion most commonly affects the left coronary (88.6 per cent) and occurs more commonly in women and following balloon expandable TAVI. In a systematic review, several parameters were found to be possible predictors of procedural complications: a low-lying coronary ostium < 10 to 12 mm from the basal leaflet insertion to the coronary ostium as measured by MDCT, mean sinus of Valsalva diameter of <30 mm, and sinus of Valsalva diameter/annular diameter ratio of <1.25. Furthermore, according to this analysis, women appear to be more subjected to coronary occlusion because of smaller aortic root dimensions and lower coronary ostial height
[44]. Probably, significantly oversized THV or aortic root dissection, the displacement of native bulky aortic leaflets, impingement of the coronary ostia by the THV support structure, the embolization of calcium, thrombus, etc., are the mechanisms underlying coronary obstruction.
Sinotubular Junction (STJ) and Proximal Aorta
An inaccurate measure of the sinotubular junction and its degree of calcification may be associated with the incidence of prosthesis–patient mismatch (PPM)
[45]. Small and severely calcified sinotubular junctions may also predispose to several complications: THV embolization, aortic root rupture during implantation of the THV, and balloon migration. In addition, using longer balloon-expandable valves, the sinotubular junction could be damaged with a transcatheter device.
The accuracy of MDCT imaging of the sinotubular junction depends on the measurement method. Compared with axial methods, double-oblique imaging yields similar findings to planimetry, and measurements obtained by double-oblique imaging are generally smaller, as was demonstrated in a study of patients with thoracic aortic aneurysms
[46]. The height of the sinotubular junction should be measured perpendicular to the ring’s plane using an electronic caliper from the annulus to its lowest point
[16] (
Figure 6 a,b).
Figure 6. (a) Sinotubular junction and (b) proximal ascending aorta evaluated by CT for TAVI.
Left Ventricular Outflow Tract (LVOT) and Septum
Both self-expandable and balloon-expandable THVs extend their lower extremity into the left ventricular outflow tract (LVOT). A careful assessment of this region should be performed to minimize complications (Figure 7 a,b).
Figure 7. (a) LVOT calcium degree evaluation. (b) Preoperative LVOT measurements.
The proper seating of the THV and the risk of spontaneous repositioning after deployment can be affected by marked upper septal hypertrophy protruding in LVOT. Furthermore, prominent septal hypertrophy is associated with atrioventricular blocks and the need for post-TAVI PMI
[47]. Accordingly, Jilaihawi et al. showed that MDCT-based measurements of septal thickness predict the post-TAVI occurrence of AV blocks
[48]. Septal hypertrophy causing significant LVOT obstruction is a contraindication to TAVI; however, as demonstrated by Moreno et al., proper modifications to THVs can allow successful implantation in this kind of patients
[49]. Notably, Stolzmann et al. found that echocardiography tends to overestimate interventricular septum thickness compared with MDCT (up to 2.2 mm)
[50].
Furthermore, the risk of significative residual aortic regurgitation and second valve implantation can be affected by moderate or severe LVOT calcification. In this regard, Okuno et al. proposed a specific classification based on the severity of LVOT calcification: mild was detected in the presence of one nodule extending <5 mm in any dimension and covering <10% of the perimeter of the LVOT; moderate was documented in the presence of two nodules or one extending >5 mm in any direction or covering >10% of the perimeter of the LVOT; and severe was considered in the case of multiple nodules of single focus extending >10 mm in length or covering >20% of the perimeter of the LVOT. Additionally, patients with moderate or severe LVOT calcification and treated with earlier-generation THVs suffered from an increased risk of annular rupture. The difference was not statistically significant among patients treated with newer generation prosthetic valves. Concordantly, it was observed that patients with moderate or severe LVOT calcification have an increased risk of death for all-cause mortality at one year
[51].