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Ростовский Государственный Медицинский Университет

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Медицина общая

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T.A. Hameed

size for transcatheter replacement is of critical importance as a larger than appropriate size may cause annulus rupture during valve implantation or a small valve-size may lead to valve embolization or post-procedure para-valvular leak. Although echocardiography (including trans- esophageal technique) and catheter angiography are also used for the assessment of aortic valve size, CT is able to provide more accurate estimate of size as the aortic valve annulus is more commonly oval or elliptical than circular, which is better evaluated with 3-D capability of CT imaging compared to 2-D techniques.

CT is also utilized for evaluation of other features of aortic root morphology such as the presence of tri-leaﬂet versus bicuspid valve, which may make the procedure challenging for appropriate alignment of the prosthetic valve during implantation. Assessment of the amount and distribution of calciﬁcations in aortic valve is important as asymmetric distribution or inferior extent of calciﬁcations into the ventricular outﬂow tract may lead to higher risk of post-procedure aortic valve regurgitation or para-valvular leak. CT evaluation of the height of coronary artery ostia from the level of annulus plane and the size of

native valve leaﬂets is important to assess the risk of coronary artery occlusion due to displaced leaﬂets of native valve or potential superimposition of prosthetic valve. As the CoreValve is longer and extends from the left ventricular outﬂow tract (LVOT) to ascending aorta, minimum diameter and height of aortic sinus to accommodate the valve size is assessed by CT and the size of ascending aorta and sinotubular junction (STJ) is evaluated to exclude an aneurysm. CT images of aortic root are also used to predict appropriate ﬂuoroscopy tube angle for visualization of aortic annulus plane for TAVR procedure.

The caliber, calciﬁcations, and tortuosity of access vessels are assessed by CT for feasibility of transfemoral or, if necessary, subclavian artery approach. The amount and irregularity of atherosclerotic plaque or the presence of thrombi in aortic arch is evaluated for potential risk of cerebral embolization during catheter manipulation. CT is also used to assess the length of ascending aorta and calciﬁcations in its wall for feasibility of direct trans-aortic approach via minithoracotomy as short ascending aorta or large amount of calciﬁcations may preclude safe access.

14 Transcatheter Aortic Valve Replacement Planning

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14.1.1 CT Imaging for Pre-TAVR Planning

CT angiography (CTA) examination of the thorax, abdomen, and pelvis is performed. This includes helical ECG-gated CTA data acquisition of the thorax in the arterial phase following intravenous (I.V.) contrast administration (using bolus triggering or timing run in the ascending aorta) for the assessment of aortic root and thoracic aorta. CTA examination of the abdomen and pelvis for abdominal aorta and iliofemoral arteries is usually obtained by a separate helical acquisition without ECG gating. The total intravenous contrast dose is variable for the two examinations depending on the scan duration, usually 100–120 mL.

14.1.2 Image Reconstructions

CT images of chest with ECG synchronization are reconstructed in the cardiac systolic phase (20–35% R-R interval) to obtain maximum size of the aortic valve annulus. The maximum annulus size has been reported to correlate with images at 20% R-R interval. Using the acquired dataset, multiplanar reformats and 3-D images are obtained for pre-procedure assessment of TAVR (Cases 1–5).

14.1.3 CT Angiography with low

contrast dose

Many patients with severe aortic stenosis being evaluated for TAVR have comorbidities including reduced renal function which places them at high risk of contrast induced nephropathy (CIN). In these patients, CT techniques can be utilized to reduce their exposure to iodinated contrast and CTA may be performed with very small doses of I.V. contrast. The diagnostic image quality with small contrast doses is achieved by optimizing different aspects of CT imaging and higher density of enhancement is primarily achieved with the use of 80 Kilovoltage Peak (KVp) technique as the mean energy of 80

KVp X-ray beam matches closely with the absorption k-edge of iodine, thereby producing higher attenuation for a given concentration and volume of iodinated contrast (Fig. 14.4a, b). Higher image noise associated with 80 KVp imaging may be reduced by corresponding increase in tube current or by the use of newer image reconstruction techniques such as model-based iterative reconstruction (Fig. 14.4c). Timing bolus run to determine the arrival of contrast in the aorta is then used to plan for peak contrast enhancement at the time of imaging. CTA examination of chest is performed with ECG synchronization and the imaging can be continued to include abdomen and pelvis with the same small volume contrast injection or a separate non-gated examination may be obtained with a second injection of small contrast dose (Cases 6–8).

(<13 G iodine or less than 35 mL I.V. contrast with 370 mg/100 mL iodine concentration):

ab c

Fig. 14.4 Attenuation of iodinated contrast with different kilovoltage (a) 120 KVp image (HU) (b) 80 KVp image with higher attenuation and increased noise (SD)

using hybrid image reconstruction (c) 80 KVp image (same as b) with reduced noise using model-based image reconstruction

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T.A. Hameed

14.2 Case 1

14.2.1 History

An 88-year-old with severe aortic stenosis for pre-TAVR evaluation.

14.2.2 Findings

Standard helical cardiac/chest CT imaging with intravenous contrast dose of 57 mL was performed on a 256-slice scanner with a bolus triggering at 150 HU with ROI in ascending aorta.

Images were reconstructed at 20% and 25% RR interval. Images in true transverse plane at select levels demonstrate the size of aortic valve annulus, sinus of Valsalva, STJ, and LVOT (Figs. 14.5 and 14.6). Image at the level of valve leaﬂets shows tri-leaﬂet conﬁguration with moderate calciﬁcation. Coronal images with the left and right coronary arteries in proﬁle at origin show the height of ostia from the level of aortic annulus (Fig. 14.7). Figure 14.8 shows reference levels for the transaxial images. The mean diameter of the annulus, and the effective diameter calculated from perimeter as well as area is 28.6 mm.

Fig. 14.5 Transverse images (left-right) of annulus, sinus of Valsalva, STJ, and LVOT

14 Transcatheter Aortic Valve Replacement Planning

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Fig. 14.6 Transverse images (left-right) of annulus, sinus of Valsalva, STJ, and LVOT with markings

ab c

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Fig. 14.7 (a) Axial image of ascending aorta and coronal images (b, c) showing coronary arteries

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Fig. 14.8 Reference coronal image depicting levels of transaxial images

14.2.3 Discussion

The size of aortic annulus is measured on true transverse image which is acquired orthogonal to the long axis through the aortic annulus by ﬁrst obtaining oblique coronal (Fig. 14.9) and oblique sagittal (Fig. 14.10) images passing through the center of the annulus. The measurement is made at the level of inferior attachment of the valve leaﬂets (Fig. 14.11). The long and

T.A. Hameed

short axis diameters of the annulus are measured at this level to obtain mean diameter. Effective diameter may be calculated from the perimeter or by obtaining the area of the lumen at the level of annulus by planimetry. Since the annulus is usually oval or elliptical, the diameter derived from area has been shown to match more closely with the appropriate valve size selection for the procedure. Similarly, transaxial images are obtained at other levels orthogonal to the long axis derived from oblique sagittal and coronal images (Figs. 14.12, 14.13, 14.14, and 14.15). For coronary ostium height from the annulus, a coronal image passing through the ostium of coronary artery is obtained using the transverse image at the level of coronary artery and the distance is measured from annulus plane (Fig. 14.16, 14.17, 14.18, and 14.19). During the THV placement, optimal angiographic projections perpendicular to the plane of native aortic valve are needed. CT can be used to predict the optimal angiographic angle. Figures 14.20 and 14.21 show the reference image of aorta and curve generated by computer software using the annulus plane to predict the appropriate ﬂuoroscopic tube angle during angiography for TAVR procedure for projection perpendicular to the valve plane for appropriate valve positioning thereby reducing the contrast use and procedure time. This patient was treated with transfemoral implantation of 29 mm Sapien 3 valve (Fig. 14.22).